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Cr(VI) Reduction and Immobilization by Magnetite under Alkaline pH Conditions: The Role of Passivation
Abstract
This study investigated Cr(VI) reduction and immobilization by magnetite under alkaline pH conditions similar to those present at the Hanford site. Compared to acidic and neutral pH, chromium(VI) reduction by magnetite at high pH conditions is limited (<20% of potential reduction capacity), and the extent of reduction does not vary significantly with increasing NaOH concentration. This is due to the formation of maghemite, goethite, and/or Fe1-xCrxOOH, which may form a passivation layer on the magnetite surface, stopping further chromate reduction. Maghemite is formed in lower NaOH concentrations. The extent of goethite formation increases with NaOH concentration. Goethite may be formed through two mechanisms: (i) dissolution of magnetite leads to the precipitation of goethite and/or (ii) dissolution of newly formed maghemite intermediate, followed by precipitation of goethite. Extended X-ray absorption fine structure spectroscopy shows that Cr has a similar structural environment at alkaline pH as at acidic and circumneutral conditions.
... Notably, certain factors such as particle size , pH value (Katsoyiannis et al., 2008;Mamindy-Pajany et al., 2011), co-existing ions (Vilardi et al., 2017a(Vilardi et al., , 2017c, hydrodynamic filed (Vilardi et al., 2019c) and contaminant concentration were restricted performance of iron (Shi et al., 2011). The passivation layer on the surface of the iron particle formed under alkaline conditions could sequester the electron derived from iron, wherein the passivation layer was mainly contained non-conductive hydroxide of iron and Cr (He et al., 2005). Research efforts have been done on impairing the effect of the passivation layer. ...
... Magnetite or ferrosoferric oxide (Fe 3 O 4 ) is commonly found in nature and characterized by properties like conductivity, magnetism, high surface area, and reducibility. Its importance in literature has been recognized in the elimination of targeted contaminants (Crean et al., 2012;Gorski et al., 2010;He et al., 2005;Petrova et al., 2011;Su, 2017;Wiatrowski et al., 2009;Yuan et al., 2009Yuan et al., , 2010. It was reported that Cr (VI) could directly reduce by magnetite . ...
... Besides, the surface area of iron increased after the sulfidation, which helped in the adsorption of Cr(VI) and succeeding reduction. Corresponding to the passivation of ZVI, the virgin magnetite was also expected to be passivated with maghemite, goethite, and/or Cr 1-x Fe x OOH under alkaline pH during reaction with Cr (VI) which inhibited the reduction of Cr(VI), subsequently (He et al., 2005). Similarly, a research study implied that the removal efficiency of Cr(VI) on magnetite-ZVI composite was 96.4%, while about 18.8 and 48.8% were noticed by ZVI and Fe 3 O 4 , respectively (Lv et al., 2012). ...
In recent years, zero-valent iron (ZVI) has been extensively employed for the elimination of organic and inorganic contaminants. However, the performance of ZVI was restrained due to the inherent properties in the process of pollutants sequestration like agglomeration, surface passivation, and sensitivity to the pH and dissolved oxygen (DO) in the environment. To combat these issues, ZVI-based materials were utilized to attenuate the drawbacks of ZVI. Therefore, in this review, the representative hazardous hexavalent chromium (Cr(VI)) was chosen as the target pollutant to discuss the performance, limitations, and future of ZVI-based materials. The prevailing preparation methods of ZVI-based materials could be classified into aqueous reduction and mechanical procedures. Further, the conventional ZVI-based materials were mainly encompassed carbon-ZVI, sulfur-ZVI, bimetallic materials of ZVI, and magnetite-ZVI composites. A new insight into the co-effect of pH and DO on Cr(VI) removal by ZVI through five pathways was also proposed. The mechanism of Cr(VI) elimination by ZVI-based materials was dominant through the combination of reduction, adsorption, and co-precipitation, wherein the enhanced reduction capability of ZVI-based materials compared to their monometallic counterpart was critically scrutinized. Besides, some field applications of ZVI-based materials such as ZVI incorporation into the permeable reactive barrier (PRB) to remediate groundwater have also been examined. Finally, barriers in market penetration of ZVI-based materials in removing Cr(VI) have been highlighted which would open a new window for the researcher to accomplish the research gaps for shifting applications of ZVI-based materials from lab-scale to real or commercial implementations.
... In the following period, the content of Cr(VI) in a single magnetite system decreased slowly and, at last, resulted in a steady-state. According to previous studies, it was assumed that a passive layer was formed on the surface of magnetite, preventing further reduction (Peterson et al., 1997;He et al., 2005;Jung et al., 2007;Pinakidou et al., 2016). The Cr(VI) concentration declined steadily in single Lysinibacillus sp. ...
... These results implied that Fe(II) in the PC was oxidized to Fe(III) in the Cr(VI) removal process. It has been previously reported that changes in the valence state of Fe are observed during the reduction of Cr(VI) by magnetite (Peterson et al., 1997;He et al., 2005;Jung et al., 2007;Pinakidou et al., 2016). In addition to the Fe(III) in FeOOH, Fe(II) corresponding to the peaks at binding energies of 710.6 eV and 723.7 eV were observed on the surface of the Lysinibacillus sp.-magnetite system, indicating that Lysinibacillus sp. could reduce the produced Fe(III) to Fe(II) (Masaoki et al., 1976). ...
... Without Lysinibacillus sp., it has bee studied in various situations that removal of Cr(VI) by magnetite through a combined adsorption-reduction process. For example, with X-ray absorption fine structure (XAFS) and TEM, Peterson et al. (1997) and He et al. (2005) found that magnetite can immobilize Cr(VI) on its surface directly and reduce Cr(VI) to Cr(III). After the reaction, hemite and goethite were formed and covered on the surface of magnetite, which prevents further Cr(VI) reduction. ...
In this work, Lysinibacillus sp. JLT12 was used to remove the Cr(VI)-induced passive layer on the magnetite. Mechanism study via dynamic kinetics, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy analyses revealed that Lysinibacillus sp. JLT12 could remove the passive layer (lepidocrocite and goethite) to facilitate the further Cr(VI) reduction by magnetite. For large-scale applications, porous ceramsite (PC) was prepared with magnetite, kaolin, and fallen leaves. Lysinibacillus sp. was then immobilized on the holes in PC. Slow-released nutrients were added to immobilized porous ceramsite (IM-PC) at a ratio of 1.5:10 (g/g) to supply carbon, nitrogen, and phosphorus to Lysinibacillus sp. JLT12 with low secondary pollution. The performance of IM-PC was evaluated via a column experiment. The results indicate that, in the presence of Lysinibacillus, the break-through time and maximum adsorption ability of IM-PC were 11.67 h and 121.47 mg/g, respectively. These values are higher than those of PC. Additionally, break-through curves detected at 5, 10, and 15 days demonstrated that the usage life of IM-PC was significantly longer than that of PC.
... The potential of sulfide minerals such as pyrite or reduced sulfur in redox-sensitive contaminant remediation has attracted great interest in recent years (He and Traina, 2005). However, the pyrite surface is usually passivated due to the formation of Fe-Cr compounds, resulting pyrite utilization being blocked and thus inhibiting further Cr(VI) reduction (He and Traina, 2005;Gong et al., 2017). ...
... The potential of sulfide minerals such as pyrite or reduced sulfur in redox-sensitive contaminant remediation has attracted great interest in recent years (He and Traina, 2005). However, the pyrite surface is usually passivated due to the formation of Fe-Cr compounds, resulting pyrite utilization being blocked and thus inhibiting further Cr(VI) reduction (He and Traina, 2005;Gong et al., 2017). Strengthening measures that have been applied in pyrite or iron-containing material-based Cr(VI) reduction mainly focus on thermal modification, mechanical crushing, and organic acid/chelating agent addition, etc. (Doyle et al., 2004). ...
... Fe decreased rapidly with an increase in pH and may have been precipitated in secondary iron minerals such as jarosite (Zhu et al., 2013;Gan et al., 2015). The deposition of secondary iron minerals on the pyrite surface would have been unfavorable for pyrite dissolution (Stott et al., 2000;He and Traina, 2005). Basically, the TFe concentration in the biological system was lower than that in both the pH-independent and -stable chemical treatments. ...
Cr(VI) is considered as a priority pollutant, and its remediation has attracted increasing attention in the environmental area. In this study, the driving of pyrite-based Cr(VI) reduction by Acidithiobacillus ferrooxidans was systematically investigated. The results showed that pyrite-based Cr(VI) reduction was a highly proton-dependent process and that pH influenced the biological activity. The passivation effect became more significant with an increase in pH, and there was a decrease in Cr(VI) reduction efficiency. However, Cr(VI) reduction efficiency was enhanced by inoculation with A. ferrooxidans. The highest reduction efficiency was achieved in the biological system with a pH range of 1–1.5. Pyrite dissolution and reactive site regeneration were promoted by A. ferrooxidans, which resulted in the enhanced effect in Cr(VI) reduction. The low linear relevancy between pH and Cr(VI) dosage in the biological system indicated a complex interaction between bacteria and pyrite. Secondary iron mineral formation in an unfavorable pH environment inhibited pyrite dissolution, but the passivation effect was relieved under the activity of A. ferrooxidans due to S/Fe oxidization. The balance between Cr(VI) reduction and biological activity was critical for sustainable Cr(VI) reduction. Pyrite-based Cr(VI) remediation driven by chemoautotrophic acidophilic bacteria is shown to be an economical and efficient method of Cr(VI) reduction.
... However, a long-time exposure to substantial Cr(III) is still harmful. The combination of reduction with adsorption or precipitation is usually adopted in Cr-contaminated water treatment to avoid the secondary pollution [6,8]. ...
... Several studies have been devoted to exploring the mechanism of Cr (VI) adsorption by magnetite [8,22,23], but there are still insufficient data about the reaction of Cr(VI) with magnetite during the electrochemical reduction process. As a matter of fact, the electrochemical reaction system containing Cr(VI) is extremely complicated, and the results reported from different literatures are sometimes disputable. ...
... Journal of Hazardous Materials 374 (2019) [26][27][28][29][30][31][32][33][34] goethite when exposed to Cr(VI) solution [8,13]. In this work, no obvious changes were observed in the peak position and lattice parameter in XRD patterns after the electroreduction (Fig. S5 and Table. ...
Aqueous hexavalent chromium (Cr(VI)) poses serious threats to ecological environments. Magnetite is a potential adsorbent for Cr(VI). However, its adsorption capacity is limited due to the formation of Fe(III) oxide coating on magnetite surface. Herein, constant potential reduction was conducted to improve the Cr(VI) removal capacity of magnetite, and the influence of pH, potential, and supporting electrolytes including KNO 3 , KCl, and K 2 SO 4 on the adsorption capacity was also investigated. The results showed that the highest Cr(VI) reduction percentage reached 93.7% with a total Cr removal capacity of 514.7 mg g ⁻¹ at optimized pH 2 and −0.2 V (vs. SCE) in supporting electrolyte of KNO 3 . Cr(VI) was reduced to Cr(III) on the surface of magnetite due to the direct electrochemical reduction at low potentials and reduction by Fe ²⁺aq electrochemically generated from magnetite. The Cr(III) was subsequently removed and easily separated due to the formation of Cr(OH) 3 precipitate on magnetite surface when KNO 3 and KCl were used as supporting electrolyte; however, when K 2 SO 4 was used instead, Cr(OH) 3 precipitate was not observed. The decrease in pH and electrical potential was found to facilitate the reduction and removal of Cr(VI). This work proposes a facile method to enhance Cr(VI) removal by iron oxides.
... CaCO 3 could increase the soil pH and reduce the availability of heavy metals [54]. Such an alkaline condition was conducive to the transport of Cr(VI), as the adsorption and reduction processes were inhibited [55]. However, an obvious decrease in soil pH values can be observed and shows a significant negative relationship with Cr(VI) concentration (r = −0.817, ...
... The content of soil organic matter, another main reduction substance, had relatively narrow gaps among these soil layers, so the effect of the organic matter on the difference between the soil layers was minimal. Furthermore, soil environmental variables, such as pH [6,55], redox potential (ORP) [57,69] and metal stress [21,24], may affect chromium behaviors in soil. In this study, pH played a non-negligible role in Cr(VI) distribution. ...
Hexavalent chromium (Cr(VI)) waste produced by chrome plating activities pollutes the surrounding environment and harms human health. However, information about the chromium (Cr) pollution characteristics of actual electroplating sites is still lacking. In this study, the concentration, leachability and speciation of Cr in soils from a typical chrome plating site were analyzed. Our results showed that this site was severely contaminated by Cr (7.2 to 7735.2 mg/kg) and Cr(VI) reached the mean concentration of 138.7 mg/kg. The spatial distribution of Cr(VI) was related to the plating processes. Chrome plating and sewage treatment areas could be considered as the hot spots of contaminated sites. The vertical distribution of Cr(VI) was mainly affected by soil properties, where the loam layer retained and reduced a large amount of Cr(VI) due to its high content of iron minerals and finer particle fractions. Additionally, the chemical extraction results showed that Cr was mainly in non-residual fractions and the existence of Cr(VI) led to a high leaching toxicity based on the toxicity characteristic leaching procedure (TCLP) results. Moreover, X-ray photoelectron spectroscopy (XPS) results revealed the speciation of Cr in the long-term contaminated soils. A large amount of Cr(VI) was reduced into Cr(III) and mainly existed as Cr(OH)3 and Cr2O3. Furthermore, Cr(VI) tended to precipitate as CaCrO4 and persisted in soils. Therefore, it is necessary to find appropriate strategies to remediate these contaminated soils. Overall, these findings strengthen our understanding of Cr(VI) behaviors and lay a foundation for the future pollution investigation, ecological remediation and risk assessment of sites contaminated by electroplating.
... Oxidation of magnetite generally results in the formation of a maghemite surface layer. However, while this maghemite layer can protect the bulk of the underlying magnetite from further oxidation for larger sized (non-nano) particles, nanoparticulate magnetite is more vulnerable to oxidation (He and Traina, 2005;Khan et al., 2015;Rebodos and Vikesland, 2010). In this case, oxidation can lead to significant amounts of maghemite in the near-surface region (core-shell structure) or even to complete transformation (Kuhn et al., 2002;Sharifi Dehsari et al., 2018;Signorini et al., 2003). ...
... Furthermore, XPS analysis and the comparison of the Fe(II)/Fe total ratio of solids before and after adsorption of selenite or selenate yielded comparable results (Table S3). The observation that the oxidation of magnetite to maghemite is not progressing significantly during the oxic adsorption experiments can be explained by the formation of an initial maghemite oxidation layer that slows down aerial oxidation of the underlying magnetite core (He and Traina, 2005;Khan et al., 2015;Rebodos and Vikesland, 2010). ...
Magnetite nanoparticles are a promising cost-effective material for the remediation of polluted wastewaters. Due to their magnetic properties and their high adsorption and reduction potential, they are particularly suitable for the decontamination of oxyanion-forming contaminants, including the highly mobile selenium oxyanions selenite and selenate. However, little is known how in field applications the remediation efficiency of magnetite nanoparticles is affected by partial oxidation and the formation of magnetite/maghemite phases. Here we characterize the retention mechanisms and capacity of partially oxidized nanoparticulate magnetite for selenite and selenate in an oxic system at different pH conditions and ionic strengths. Data from adsorption experiments showed that retention of selenate is extremely limited except for acidic conditions and strongly influenced by competing chloride anions, indicating outer-sphere adsorption. By contrast, although selenite adsorption capacity of oxidized magnetite is also adversely affected by increasing pH, considerable selenite quantities are retained even at alkaline conditions. Using spectroscopic analyses (XPS, XAFS), both mononuclear edge-sharing (²E) and binuclear corner-sharing (²C) inner-sphere selenite surface complexes were detected, while reduction to Se(0) or Se(-II) species could be excluded. Under favourable adsorption conditions, up to ∼pH 8, the affinity of selenite to form ²C surface complexes is higher, whereas at alkaline pH values and less favourable adsorption conditions ²E complexes become more dominant. Our results demonstrate that magnetite can be used as a suitable reactant for the immobilization of selenite in remediation applications, even under (sub)oxic conditions and without the involvement of reduction processes.
... The soluble and toxic Cr(VI) can be reduced to insoluble Cr(III) upon sorption to Fe(II,III)oxy(hydr)oxides such as magnetite (He and Traina, 2005;Kendelewicz et al., 2000;Peterson et al., 1997;Fendorf and Li, 1996) and green rust (Williams and Scherer, 2001;Loyaux-Lawniczak et al., 2000), but also Fe(II)-sulfides (Doyle et al., 2004;Kim et al., 2002a;Patterson et al., 1997), Fe(II)-bearing clay minerals (Taylor et al., 2000;Bishop et al., 2014;2019;Joe-Wong et al., 2017;Qafoku et al., 2017) and organic matter (Szulczewski et al., 2001). On the opposite, direct oxidation of Cr(III) to Cr(VI) by dissolved O 2 in natural systems is possible but limited because of its very low kinetic (Eary and Rai, 1987). ...
... Un apport direct de chrome sous forme oxydée Cr(VI) ne peut pas être exclu car des études précédentes ont montré la présence de ces formes du chrome dans les massifs ultrabasiques de Nouvelle-Calédonie (Fandeur et al., 2009a ;2009b). Cependant, la présence d'(oxyhydr)oxydes mixtes Fe(II)/Fe(III), de phyllosilicates Fe(II) et de sulfures Fe(II) ne serait pas favorable à la stabilisation de ces formes oxydées du chrome car ces minéraux sont connus pour leur capacité à réduire Cr(VI) en Cr(III) (Bishop et al., 2019;Qafoku et al., 2017 ;Joe-Wong et al., 2017;He and Traina, 2005;Doyle et al., 2004;Kim et al., 2002a;Kendelewicz et al., 2000;Taylor et al., 2000;Peterson et al., 1997;Patterson et al., 1997 ;Fendorf and Li, 1996). De plus, la matière organique présente également cette capacité de réduction des formes oxydées du chrome (Szulczewski et al., 2001). ...
La Nouvelle-Calédonie est formée sur 33% de son territoire de roches ultrabasiques enrichies en éléments traces métalliques (ETM) (Fe, Ni, Cr, Co et Mn). L’érosion de ces massifs représente une source importante de ces ETM vers le lagon, partiellement inscrit au patrimoine mondial par l’UNESCO pour sa biodiversité. La compréhension des cycles géochimiques de ces éléments toxiques apparaît donc essentielle pour évaluer les impacts possibles sur la biodiversité de cet écosystème. La spectroscopie d’absorption des rayons X a montré que la contribution des sulfures de fer est très faible, Ni et Fe sont portés par la goethite et les argiles. Ces derniers étant la phase majeure de ces deux éléments ont été identifiés comme du chrysotile, et des greens clays : une smectite (nontronite), un mica (glauconite) et une serpentine de type greenalite/berthierine. Le cycle des argiles joue donc un rôle majeur dans le cycle de Fe et de Ni mais aussi de Mn et dans une moindre mesure de Cr. La spéciation de Mn se partage entre les argiles et les carbonates alors que Cr est surtout porté par la goethite et la chromite hérité des massifs et en dernier par les argiles. Il est important de noter que Cr se trouve sous sa forme réduite correspondant à sa forme la moins toxique. L’absence de Cr(VI) est liée à l’absence des oxydes de Mn dans les sédiments, permettant de ne pas oxyder Cr(III) en Cr(VI). Enfin, les extractions chimiques montrent une biodisponibilité relativement faible de ces métaux à l’exception de Mn impliquant un piégeage efficace, limitant ainsi les impacts sur l’environnement même si les concentrations extraites sont loin d’être négligeables pour la biodiversité du lagon.
... Aqueous solutions of calcium nitrate tetrahydrate, Ca(NO 3 ) 2 ·4H 2 O (> 99.0%, Merck ACS) and diammonium hydrogen phosphate, (NH 4 ) 2 HPO 4 (> 98.0%, Sigma-Aldrich) were used as precursors for the synthesis of hydroxyapatite, ammonium hydroxide solution, NH 4 Tin-functionalized hydroxyapatite (Sn/HAP, 10 wt% Sn loading) was prepared from an acidic tin chloride solution (pH ~ 2) by using a flash deposition technique already validated to deposit Sn 2+ species onto HAP 31 . ...
Our group recently proposed an innovative sustainable reductant-adsorbent material, tin(II)-hydroxyapatite (Sn/HAP, ca. 10 wt% Sn) for the interfacial Cr(VI) reductive adsorption process. In this study, Cr(VI) removal capacity was evaluated in multi-component solutions containing representative background ions (i.e., CaCl2, Ca(NO3)2, MgSO4, Na2SO4, Fe(NO3)3, AlCl3, Zn(NO3)2, or Mn(NO3)2). Sn/HAP was able to reduce Cr(VI) with complete Cr³⁺ adsorption on HAP surface, except in the presence of Fe³⁺ and Al³⁺ ions. Some metal ions co-existing in solution, such as Fe³⁺, Al³⁺, Zn²⁺, and Mn²⁺, were also adsorbed on HAP surface. Reuse experiments of the Sn/HAP sample, up to 7 runs, resulted in a total amount of reduced Cr(VI) of ca. 15–18 mg g⁻¹. Fast kinetics of Cr(VI) reductive adsorption at 25 °C in a multi-metal component solution was observed. The pseudo-second order model was in excellent agreement with the experimental kinetic data, leading to a rate constant (k25°C) value of ca. 30 M⁻¹ s⁻¹. The collection of adsorption isotherms of Cr³⁺ and Fe³⁺, together with TEM–EDX analysis permitted the unveiling of competitive adsorption phenomena between metal ions. The obtained results demonstrate that Sn/HAP could be an efficient material for the removal of hexavalent chromium in aqueous solutions containing high concentrations of inorganic impurities.
... 25,28 The change of the stoichiometry in MNPs can greatly improve the remediation capacity of magnetite, which was previously shown for Cr. 29,30 Conversely, it has also been shown that microbial activity decreased the reactivity of MNPs toward As(V) 22 and that magnetite surface passivation can occur through chromium reduction to Cr(III), resulting in a surface layer maghemitization. 31 Studies have shown that increase of Fe 2+ led to greater reduction of nitroaromatic compounds 32 and that an increased stoichiometry in magnetite enhanced the capacity to bind antibiotics. ...
Heavy metal pollutants in the environment are of global concern due to their risk of contaminating drinking water and food supplies. Removal of these metals can be achieved by adsorption to mixed-valent magnetite nanoparticles (MNPs) due to their high surface area, reactivity, and ability for magnetic recovery. The adsorption capacity and overall efficiency of MNPs are influenced by redox state as well as surface charge, the latter of which is directly related to solution pH. However, the influence of microbial redox cycling of iron (Fe) in magnetite alongside the change of pH on the metal adsorption process by MNPs remains an open question. Here we investigated adsorption of Cd²⁺ and Cu²⁺ by MNPs at different pH values that were modified by microbial Fe(II) oxidation or Fe(III) reduction. We found that the maximum adsorption capacity increased with pH for Cd²⁺ from 256 μmol/g Fe at pH 5.0 to 478 μmol/g Fe at pH 7.3 and for Cu²⁺ from 229 μmol/g Fe at pH 5.0 to 274 μmol/g Fe at pH 5.5. Microbially reduced MNPs exhibited the greatest adsorption for both Cu²⁺ and Cd²⁺ (632 μmol/g Fe at pH 7.3 for Cd²⁺ and 530 μmol/g Fe at pH 5.5 for Cu²⁺). Magnetite oxidation also enhanced adsorption of Cu²⁺ but inhibited Cd²⁺. Our results show that microbial modification of MNPs has an important impact on the (im-)mobilization of aqueous contaminations like Cu²⁺ and Cd²⁺ and that a change in stoichiometry of the MNPs can have a greater influence than a change of pH.
... This is understandable because more iron oxyhydroxide was formed with longer-oxygenated FeS, increasing sorption (Guo et al., 2020;Jeong et al., 2010). There was, however, a little sorption with FeS oxygenated for ≤240 min because a passivation layer of Cr(III)-Fe(III) hydroxide and Cr (III) hydroxide could form upon Cr(VI) reduction by Fe(II) and S(-II) on the surface of the oxygenated FeS, which could cover the surface of γ-FeOOH, hinder the interaction with Cr(VI) and thus inhibit Cr(VI) sorption (He and Traina, 2005;Peterson et al., 1997). Taken together, short-time (≤60 min) oxygenated FeS was much more effective for Cr(VI) removal than more oxygenated FeS, and reductive immobilization was the dominant removal mechanism for Cr(VI) removal from the aqueous phase. ...
Iron sulfide (FeS) can reductively convert soluble Cr(VI) into insoluble Cr(III) under anoxic conditions. However, the fate and transformation of FeS and the stability of immobilized Cr under various oxic environmental conditions are poorly understood. The results show that FeS transforms into pyrrhotite and pyrite intermediates principally and finally lepidocrocite and elemental sulfur, accordingly accounting for 66.1 % and 33.9 %. Temperature, fulvic acid as natural organic matter and coexisted ions of nitrate, bicarbonate, and calcium affect the evolution of FeS insignificantly. Transformation of FeS involves surface-mediated oxidation of FeS solids, and minor proton-promoted dissolution and oxidation, accompanying synergistic oxidation of Fe(II) and S(-II). Cr(VI) removal performances of oxygenated FeS with increasing duration showed a rise-fall trend. Reduction dominates Cr(VI) uptake first and finally, sorption prevails with the gradual FeS oxygenation. Cr(VI) removal correlates linearly with Cr(VI) reduction, and the reduced Cr species can be predicted based on the known Cr(VI) removal performance. As the FeS oxygenation time increases, newly generated pyrite improves Cr(VI) reduction and removal, and then a decreasing ability to reduce Cr(VI) causes a drop in Cr(VI) removal. These findings provide new insight into the oxidative transformation of FeS in oxic aquatic environments and its impact on Cr(VI) levels.
... Several processes involved in Cr(VI) immobilization by FeS after oxygenation are significantly affected by pH, including reduction by iron sulfides, adsorption on, co-precipitation with and incorporation into iron oxides, and the Fenton-like mobilization of Cr(III) to Cr(VI). For the minerals formed from the oxygenation of FeS, acidic conditions favored not only the Cr(VI) reduction by FeS, pyrite and magnetite (He and Traina, 2005;Wang et al., 2019aWang et al., , 2019b, but the removal of Cr(VI) using ferrihydrite, α-FeOOH, and γ-FeOOH through adsorption, co-precipitation, and incorporation (Johnston and Chrysochoou, 2012;Mamun et al., 2017;Wu et al., 2016;Xie et al., 2015). In our recent work, increasing H + concentration could contribute to the oxidation of Cr(III) to Cr(VI) in the oxic FeS systems because it increased the higher yield of hydroxyl radicals (•OH) as the main responsible oxidant through Fenton-like reactions (Wang et al., 2022). ...
Iron sulfide (FeS) has been widely used to reduce toxic Cr(VI) into Cr(III) in anoxic aquatic environments, where pH could strongly influence Cr(VI) removal. However, it remains unclear how pH regulates the fate and transformation of FeS under oxic conditions and the immobilization of Cr(VI). The results of this study showed that typical pH conditions of natural aquatic environment significantly affected the mineral transformation of FeS. Under acidic conditions, FeS was principally transformed to goethite, amarantite, and elemental sulfur with minor lepidocrocite through proton-promoted dissolution and oxidation. Instead, under basic conditions, the main products were lepidocrocite and elemental sulfur via surface-mediated oxidation. In typical acidic or basic aquatic environment, the pronounced pathway for the oxygenation of FeS solids may alter their ability to remove Cr(VI). Longer oxygenation impeded Cr(VI) removal at acidic pH, and a decreasing ability to reduce Cr(VI) caused a drop in Cr(VI) removal performance. Cr(VI) removal decreased from 733.16 to 36.82 mg g−1 with the duration of FeS oxygenation increasing to 5760 min at pH 5.0. In contrast, newly generated pyrite from brief oxygenation of FeS improved Cr(VI) reduction at basic pH, followed by a drop in Cr(VI) removal performance due to the impaired reduction capacity with increasing to the complete oxygenation. Cr(VI) removal increased from 669.58 to 804.83 mg g−1 with increasing oxygenation time to 5 min and then decreased to 26.27 mg g−1 after the full oxygenation for 5760 min at pH 9.0. These findings provide insight into the dynamic transformation of FeS in oxic aquatic environments with various pHs and the impact on Cr(VI) immobilization.
... For the Cr(III) distribution in Fe minerals, two underlying binding ways, including the formation of Fe mineral-Cr(III)-OM and/or Fe mineral-OM-Cr(III) ternary composites and incorporation into the lattice structure of newly formed Fe minerals ( Fig. 6b and 6c), could attribute for Cr(III) sequestration [30]. The local coordination environment of Cr(III) in the Cr-bearing Fe minerals has already been reported [17,19,34,54,63]. ...
Coupled reactions among chromium (Cr), organic matter (OM), and iron (Fe) minerals play significant roles in Cr and carbon (C) cycling in Cr-contaminated soils. Although the inhibitory effects of Cr or polysaccharides acid (PGA) on ferrihydrite transformation have been widely studied, mechanistic insights into detoxification of Cr(VI) and immobilization of Cr and C during the microbially mediated reductive transformation of ferrihydrite remain unclear. In this study, underlying sequestration mechanisms of Cr and C during dissimilatory Fe reduction at various Cr/Fe ratios were investigated. Solid-phase analysis showed that reductive transformation rates of ferrihydrite were impeded by high Cr/Fe ratio and more magnetite was found at low Cr loadings. Microscopic analysis showed that formed Cr(III) was immobilized by magnetite and goethite through isomorphous substitution, whereas PGA was adsorbed on the crystalline Fe mineral surface. Spectroscopic results uncovered that binding of Fe minerals and PGA was achieved by surface complexation of structural Fe with carboxyl functional groups, and that the adhesion order of PGA functional groups and Fe minerals was influenced by the Cr/Fe ratios. These findings have significant implications for remediating Cr contaminants, realizing C fixation, and developing a quantitative model for Cr and C cycling by coupling reductive transformation in Cr-contaminated environments.
... It is reported that Fe(II) in magnetite nanoparticles reduces toxic Cr(VI) to the non-toxic form of Cr(III). 34 As the reaction progresses, the Cr(III) hydroxide or Cr(III)−Fe(III) complex is formed (i). 35 Sorption of Cr(VI) follows a two-step process: (i) electrostatic attraction of the Cr(VI) ion and then (ii) electron transfer between Cr(VI) and Fe(II). ...
The triphenyl group (trityl radical) possessing three-phenyl rings, self-assembled through aromatic π–π stacking interactions, can form interesting crystalline organic nano-flowers. In this work, we have synthesized a hybrid material of 1,2-bis(tritylthio)ethane and magnetite, which reduces toxic Cr(VI) to non-toxic Cr(III). We validated the efficacy of the hybrid in reducing toxic Cr(VI) along with three other adsorbent systems. Among the five adsorbent systems tested, we observed that human hair has higher Cr removal efficiency, which prompted us to explore further using different mechanical forms of human hair. Pulverized hair (PH), hair powder (HP), and raw hair (RH) were evaluated by employing different reaction factors such as the adsorbent dose, pH, initial Cr(VI) concentration, and contact time. The comparative evaluation showed that PH has greater adsorption capacity (15.14 mg/g), followed by RH (13.27 mg/g) and HP (10.5 mg/g). While investigating the adsorption mechanism, we observed that it follows pseudo-second-order kinetics suggesting chemisorption. The Freundlich isotherm model fitted well for Cr(VI) adsorption by human hair, suggesting a multi-layered adsorption process. Overall, this study promises a cost-effective and eco-friendly bio-adsorbent for Cr(VI), which may be scaled up to design automated industrial waste disposal systems.
... In particular, the higher toxicity of Cr(VI) species than Cr(III) has made the remediation of Cr(VI) in the environment an imperative task in the ever-growing Cr-related industries. Conversion of Cr(VI) to its less toxic form of Cr(III) via the adsorption-reduction method has thereby appeared and rapidly developed to become a feasible solution for Cr remediation (Eyvazi et al., 2019;Gao et al., 2018;He and Traina, 2005;Wang et al., 2021a;Mpouras et al., 2021). ...
Efficient nano-scale chromium (Cr) remediating agents used in the water industry may find their application in soil difficult because of the strong aggregation effect. In this study, a millimeter-sized PANI/PVA/SA composite (PPS) was synthesized by embedding polyaniline (PANI) into polyvinyl alcohol (PVA)/sodium alginate (SA) gel beads. Additionally, the PPS was used to recover hexavalent chromium (Cr(VI)) contaminated water and soil to study the remediation impacts and mechanism. Results showed that the PPS was an irregular sphere with a pore size of 24.24 nm and exhibited strong adsorption capacity (83.1 mg/g) for removing Cr(VI) in water. The Cr(VI) adsorption by PPS could be well described with the pseudo-second-order kinetics and the Redlich-Peterson isotherm model, indicating that the chemical reactions were the controlling step in the Cr(VI) adsorption process. PPS also exhibited excellent physicochemical properties (< 13 mg/L TOC release) and reusability (efficiency of 95.25% after four runs) for Cr(VI) removal. Soil incubation results showed that the 5% PPS (5PPS) treatment could efficiently remove 24.17% of total Cr and 52.47% of Cr(VI) in the contaminated soil after 30 days. Meanwhile, the water-soluble and the leaching Cr contents were decreased by 43.37% and 61.78% in the 5PPS group, respectively. Elemental speciation by XPS revealed that Cr(VI) removal from solution and soil proceeded mainly by electrostatic attraction, reduction, and complexation/chelation. The study implied that PPS could be a useful amendment to remediate both the Cr(VI)-contaminated water and soil.
... In this study, alkaline pH values indicated the predominant negative charges on mineral surfaces, which would lead to appreciable electrostatic repulsion of the chromate oxyanions (He and Traina 2005;Gu et al. 2017). However, it was also noted that Cr(VI) would be firmly retained by Fe(III)/Cr(III) hydroxides, likely due to the formation of surface precipitates or complexes between Cr(VI) and Cr(III) sites, which could hardly be substituted by OHduring the pH increase (Tzou et al. 2003). ...
Phytoremediation of metal-contaminated soil can be an eco-friendly technology. However, relatively long cultivation times impedes its popularization on a commercial scale. This study evaluated the effectiveness of lavender plants (Lavandula dentata L.) to remediate a highly chromium (Cr)-contaminated site through a pot experiment. The lavender growing soil was mixed both with and without biochar (2.5% w/w) + oyster shell waste (2.5% w/w) and biochar (2.5% w/w) + citrus peel waste (2.5% w/w). The results indicated that Cr(VI) accounted for 19.0% to 4.7% of the total soil Cr, while Cr(III) accounted for 81.0% to 95.3%, from the beginning to the end of the cultivation. The water-soluble Cr concentration decreased from 44.6 mg/kg to 7.5 mg/kg. The biomass of the lavender growing in the contaminated soil decreased by factors in the range between 4-fold and 6-found.The addition of soil amendments significantly reduced the (potential) bioavailable Cr (p < 0.05) in the range of 2 to 3 fold, consequently improving the growth of lavender in the highly toxic soil. In addition, the soil amendments significantly reduced the Cr bioaccumulation and the translocation from the roots to the shoots. These results showed that the cultivation of lavender with suitable amendments can effectively be used for phytomanagement techniques in highly contaminated soil.
... On the contrary, the adsorption of Cr(VI) by aluminum oxides, such as boehmite and gibbsite, is mainly dominated by outer-sphere complexes by electrostatic attraction adsorption and ion exchange adsorption Chrysochoou 2015, 2016). When Fe(II) is contained in the iron oxide, the iron oxide has a reducing ability on Cr(VI), such as magnetite (He and Traina 2005;Jiang et al. 2014). Cr(VI) anions are generally weakly adsorbed on clay minerals because the surface is usually negatively charged (Bradl 2004). ...
Chromite ore processing residue (COPR) storage sites are widely distributed all over the world, causing serious soil and groundwater pollution. However, the differences in soil constituents and properties in different regions are significant, and the dynamic migration and transformation of Cr(VI) in different types of soil under alkaline condition of the COPR site is still unclear. In this study, typical black soil, red soil and loess in different regions of China were chosen to investigate the adsorption kinetics and thermodynamics of Cr(VI) under the original pH conditions of the soil, and then the alkaline Cr(VI) solution was introduced into the soil column to simulate the dynamic migration and transformation process of Cr(VI) at COPR sites. According to the results, the Cr(VI) breakthrough curve predicted by the solid–liquid distribution coefficient Kd based on the static isotherm adsorption experiments significantly underestimated and overestimated the retention effect of black soil and red soil on Cr(VI) dynamic migration, respectively. For the black soil, the retention of Cr(VI) was dominated by Cr(VI) reduction, which was a slow reaction compared with Cr(VI) adsorption. Therefore, the reduction kinetics process during the column experiment cannot be neglected. With respect to the red soil, the outlet Cr(VI) concentration turned to be higher than the inlet concentration with the soil alkalization, which indicated that the adsorbed Cr(VI) desorbed again, and this was the main reason for the overestimation of Cr(VI) retention effect by the red soil. This study shows that the environmental risks of Cr in different types of soil are quite different, mainly related to the valence and occurrence form of Cr governed by the soil constituents and properties. In addition, the stable form of Cr in the black soil column after the reaction indicates that the soil organic matter can be used as a potential environmentally friendly remediation material for Cr(VI) contaminated soils at COPR sites.
... In particular, the higher toxicity of Cr(VI) species than Cr(III) has made the remediation of Cr(VI) in the environment an imperative task in the ever-growing Cr-related industries. Conversion of Cr(VI) to its less toxic form of Cr(III) via the adsorption-reduction method has thereby appeared and rapidly developed to become a feasible solution for Cr remediation (Eyvazi et al., 2019;Gao et al., 2018;He and Traina, 2005;Wang et al., 2021a;Mpouras et al., 2021). ...
Chromium (Cr) pollution in water has become an environmental and social problem because of the highly toxic nature of Cr(VI). Biochar has been widely used in Cr-containing wastewater treatment due to its adsorption advantage and intrinsic electron-donating ability. In this paper, Cr(VI) was taken as the target pollutant, and corn-straw derived biochar (BC) and its iron-modified counterpart (BC–Fe) were taken as the main adsorbents. The effects of fulvic acid (FA) and lactic acid (LA) on the adsorption efficiency of BC and BC-Fe in aqueous solution were discussed, and the internal reaction mechanism was revealed by SEM, FTIR, XPS, and Zeta potential analysis. The results showed that the BC-Fe pyrolyzed at 600 °C (i.e., BC-Fe600) had good magnetic property and adsorption effect across a wide pH range (pH 3–9) (the maximum removal efficiency was 96%). At the same time, LA had a concentration-dependent promoting effect on Cr(VI) adsorption in the BC600. However, the addition of FA and LA both inhibited the adsorption of Cr(VI) by BC-Fe600 at pH = 5 and 7, with LA showing a more inhibiting effect on Cr(VI) removal (decreased by 16.09% at pH 5) than FA (decreased by 2.09% at pH 5). The addition of FA and LA caused the surface potential of BC-Fe600 to drop, resulting in an increasing electrostatic repulsion between Cr(VI) and the material. However, LA increased the reduction of Cr(VI) on BC-Fe600, possibly through the combined effects of the electron-donating ability of LA and the photolysis of Fe(III)-lactate complexes.
... In addition, several reactions have already demonstrated their capacity to modify the redox state of Cr in natural systems. The soluble and toxic Cr (VI) can be reduced to insoluble Cr(III) upon sorption to Fe(II,III)-(hydr) oxides such as magnetite (He and Traina, 2005;Kendelewicz et al., 2000;Peterson et al., 1997;Fendorf and Li, 1996) and green rust (Williams and Scherer, 2001;Loyaux-Lawniczak et al., 2000), but also Fe(II)sulfides (Doyle et al., 2004;Kim et al., 2002a;Patterson et al., 1997), Fe (II)-bearing clay minerals (Taylor et al., 2000;Bishop et al., 2014Bishop et al., , 2019Joe-Wong et al., 2017;Qafoku et al., 2017) and organic matter (Szulczewski et al., 2001). On the opposite, direct oxidation of Cr(III) to Cr(VI) by dissolved O 2 in natural systems is possible but limited because of its very low kinetics (Eary and Rai, 1987). ...
Ferralsols upon ultramafic rocks are among the most Cr-enriched soils at the Earth surface. Weathering and erosion of these soils represents a major source of Cr for coastal sediments downstream of ultramafic settings. Although Cr mainly occurs as Cr(III)-bearing chromite and Fe-(hydr) oxides in Ferralsols upon ultramafic rocks, several evidences of oxidized Cr(VI) in relation with Mn-oxides have been reported. Regarding the high solubility and toxicity of this latter Cr species, a thorough characterization of Cr and Mn crystal-chemistry in tropical sedimentary settings downstream of Ferralsols upon ultramafic rocks is needed to evaluate the potential threat towards the biodiversity of these coastal environments. In this study, we determined Cr and Mn speciation across a shore-to-reef gradient in lagoon sediments downstream of one of the largest lateritized ultramafic regolith in New Caledonia that contains up to 5wt% Cr2O3. Chromium K-edge XANES data emphasized the absence of Cr(VI) and indicated a major hosting of Cr by chromite and clay minerals close to the shore, whereas Cr-bearing goethite dominated Cr speciation close to the reef. Manganese K-edge XANES data indicated a major hosting of Mn by clay minerals close to the shore, whereas Mn-carbonates dominated Mn speciation close to the reef. The lack of Mn-oxides detection was considered to explain the absence of Cr(VI) in the studied sediments. This result thus suggests that, despite their shallow character that can favor occasional re-oxidation of the top-layer sediments upon re-suspension events, lagoon sedimentary settings downstream of Cr-rich Ferralsols upon ultramafic rocks appear rather favorable to Cr sequestration as the less mobile and less toxic Cr(III) form. However, the reverse trends observed from the shore to the reef between the chromite and goethite contributions to Cr speciation, as well as the decrease of the Cr/Ti ratio, suggest that a fraction of Cr could have been released towards the water column upon partial weathering of chromite to goethite during sediments transport across the shore-to-reef gradient. This latter point emphasizes the potential hazard that could still represent Cr for the exceptional biodiversity of tropical lagoon ecosystems downstream of Cr-rich Ferralsols, despite the absence of detectable Cr(VI). It thus calls for further studies aimed at better evaluating the stability of Cr(III)-bearing mineral phases upon early diagenesis in these shallow sedimentary settings.
... (Regazzoni et al., 1983). An increase to alkaline pH (pH 12) shifts the surface site speciation to a majority of negatively charged sites, yielding electrostatic repulsion between chromate oxyanions and the magnetite surface, thus inhibiting the sorption of Cr to the surface of magnetite and therefore the reduction of Cr(VI) by Fe(II) (He and Traina, 2005). (Note: No obvious changes in the surface morphology of magnetite are evident under oxic conditions at neutral pH (Fig. S7 c+d). ...
Magnetite nanoparticles are promising materials for treating toxic Cr(VI), but safe handling is challenging due to their small size. We prepared flow-through columns containing 10% or 100% (v/v) magnetite-coated sand. Cr(VI) removal efficiency was determined for different Cr(VI) concentrations (0.1 or 1.0 mM), neutral or alkaline pH, and oxic/anoxic conditions. We formulated a reactive-transport model that accurately predicted total Cr removal, accounting for reversible and irreversible (chemi)sorption reactions. Our results show that the material removes and irreversibly sequesters Cr(VI). For the concentration range used 10 and 100% (v/v) -packed columns removed >99% and 72% of influent Cr(VI), respectively. Two distinct parameter sets were necessary to fit the identical model formulation to the 10 or 100% (v/v) columns (e.g., maximum sorption capacities (qmax) of 1.37 µmol Cr/g sand and 2.48 µmol Cr/g, respectively), which we attributed to abrasion-driven magnetite micro-particle detachment during packing yielding an increase in reactive surface area. Furthermore, experiments under oxic conditions showed that, even when handled in the presence of O2, the magnetite-coated sand maintained a high removal capacity (47%). Our coupled experimental and modelling analyses indicates that magnetite-coated sand is a promising and suitable medium for treating Cr(VI)-contaminated water in fixed-bed reactors or permeable reactive barriers.
... The particle sizes were estimated using the TEM scale bars by measuring a number of particles (n = [15][16][17][18][19][20]. DLS analysis showed that size ranges of MNPs increased with increasing pH (Fig. S2); this is likely attributed to their agglomeration at higher pH, as shown in previous studies [5][6][7] . SEM images of MNP-BC composites (Fig. S3) showed MNPs are widely distributed on the surface of BC, evidencing that BC prevents MNP agglomeration. ...
Abstract Biochar (BC) and magnetite (Fe3O4) nanoparticles (MNP) have both received considerable recent attention in part due to their potential use in water treatment. While both are effective independently in the removal of a range of anionic metals from aqueous solution, the efficacy of these materials is reduced considerably at neutral pH due to decreased metal adsorption and MNP aggregation. In addition to synthetic metal oxide–biochar composites for use in treatment and remediation technologies, aggregates may also occur in nature when pyrolytic carbon is deposited in soils. In this study, we tested whether magnetite synthesized in the presence of biochar leads to increased removal efficiency of hexavalent chromium, Cr(VI), at the mildly acidic to neutral pH values characteristic of most natural and contaminated aqueous environments. To do so, magnetite nanoparticles and biochar produced from ground willow were synthesized to form composites (MNP–BC). Batch studies showed that MNP–BC markedly enhanced both adsorption and reduction of Cr(VI) from aqueous solution at acidic to neutral pH as compared to MNP and BC separately, suggesting a strong synergetic effect of hybridizing Fe3O4 with BC. Mechanistically, the Cr(VI) removal processes occurred through both adsorption and intraparticle diffusion followed by reduction to Cr(III). Synchrotron-based X-ray absorption spectroscopy analyses confirmed that Cr(VI) was reduced at the surface of MNP–BC, with electrons derived directly from both biochar and magnetite at low pH, while at near-neutral pH, biochar increased Cr(VI) reduction by inhibiting MNP aggregation. Extended X-ray absorption fine structure fitting results confirmed that the Cr(III) precipitates consist of Cr(OH)3 and chromite (Cr2FeO4) nanoparticles. Our results demonstrate that MNP–BC composites have great potential as a material for the treatment of chromate-containing aqueous solutions across a wide range of pH values, and provide information valuable broadly relevant to soils and sediments that contain biochar.
... As for GR SO4 experiments, some amorphous Cr,Fe phases may have also formed in S-nZVI reactions, but could not be identified with XRD here. These oxidised Fe phases have lower zero point of charge (lepidocrocite: 7.1 [66] and magnetite: 6.5 [67]) compared to goethite (which is the oxidation product . A control experiment with no added reducing agent is also shown. ...
Abstract Chromate, Cr(VI), contamination in soil and groundwater poses serious threat to living organisms and environmental health worldwide. Sulphate green rust (GRSO4), a naturally occurring mixed-valent iron layered double hydroxide has shown to be highly effective in the reduction of Cr(VI) to poorly soluble Cr(III), giving promise for its use as reactant for in situ remedial applications. However, little is known about its immobilization efficiency inside porous geological media, such as soils and sediments, where this reactant would ultimately be applied. In this study, we tested the removal of Cr(VI) by GRSO4 in quartz sand fixed-bed column systems (diameter × length = 1.4 cm × 11 cm), under anoxic conditions. Cr(VI) removal efficiency (relative to the available reducing equivalents in the added GRSO4) was determined by evaluating breakthrough curves performed at different inlet Cr(VI) concentrations (0.125–1 mM) which are representative of Cr(VI) concentrations found at contaminated sites, different flow rates (0.25–3 ml/min) and solution pH (4.5, 7 and 9.5). Results showed that (i) increasing Cr(VI) inlet concentration substantially decreased Cr(VI) removal efficiency of GRSO4, (ii) flow rates had a lower impact on removal efficiencies, although values tended to be lower at higher flow rates, and (iii) Cr(VI) removal was enhanced at acidic pH conditions compared to neutral and alkaline conditions. For comparison, Cr(VI) removal by sulphidized nanoscale zerovalent iron (S-nZVI) in identical column experiments was substantially lower, indicating that S-nZVI reactivity with Cr(VI) is much slower compared to GRSO4. Overall, GRSO4 performed reasonably well, even at the highest tested flow rate, showing its versatility and suitability for Cr(VI) remediation applications in high flow environments.
... At pH that is low, the primary species of Cr (VI) are Cr 2 O 7 2− , and HCrO 4− and at basic pH, CrO 4 2− is the stable form of Cr (VI). Cr (VI) to Cr (III) reduction under different conditions are shown below [61,62]. ...
It is essential to utilize the adsorption-coupled reduction mechanism when using adsorption techniques for heavy metal ions remediation. This particular work deals with the synthesis of ZnO functionalized MWCNTs (ZC) where ZnO nanoparticles are grafted on the functionalized MWCNTs by a facile hydrothermal approach for Cr (VI) removal. The metal ion adsorption behavior of ZC nanocomposites has been examined for removal of Cr (VI), and it was noticed that ZC-10 (10 wt % functionalized MWCNT content) exhibited excellent adsorption capacity and removal efficiency of 94% was accomplished at pH 2. Equilibrium data have been analyzed using various isotherm models and were found to be represented best by the Langmuir adsorption model. Various kinetic models have been used to understand the adsorption process, and the kinetics of adsorption best followed the pseudo-second-order kinetic model. The influence of several operational parameters such as initial pH, Cr (VI) concentration, dosage, agitation speed, and the presence of competing anions were well elucidated and optimized. Thermodynamic parameters revealed that the adsorption of Cr (VI) on ZC-10 adsorbent was spontaneous and endothermic. Additionally, ZC-10 nanocomposite exhibited improved photocatalytic activity under UV–Vis light irradiation in contrast to ZnO as a result of reduced electron-hole pair recombination and improved anti-photocorrosion.
... 14 Compared to acidic (pH = 1) and neutral condition (pH = 7), Cr(VI) reduction by magnetite at high pH conditions (pH = 13) was limited to <20% reduction capacity. 43 Cr Distribution and Oxidation States in Soils. Although the absolute concentration of Cr(VI) was high, 96.7% or more of Cr was present as Cr(III) species (Table 1). ...
Chromium speciation in naturally contaminated soils appears more complex than spiked studies have shown. This study characterized Cr speciation (total content; oxidation states; availability; molecular geometry) intended to highlight the genesis of immobile Cr(VI) species in long-term tannery waste contaminated soils. In a series of samples obtained from Shuitou in China, chemical extraction methods determined Cr(III) was the dominant species, with Cr(VI) concentration up to 144 mg kg-1. Of the total Cr(VI) present, immobile Cr(VI) represents > 90 %. Synchrotron-based x-ray near edge structure (XANES) showed the occurrence of Cr(VI), which was not removed by phosphate buffer extraction, confirming significant amount of immobile Cr(VI) fractions in soils. X-ray fluorescence maps exhibited a strong spatial relationship of Cr and Mn in soils, suggesting Mn could oxidize Cr(III) to Cr(VI). This hypothesis is further supported by density functional theory calculations showing both CrFeO3 and CrOOH lose electrons to attached MnO2. Linear combination fitting of theoretical modelling and experimental XANES demonstrated fractional weights (%) in samples were CrFeO3 (49.3 – 53.6), CrOOH (22.3 – 29.5) and CaCrO4 (13.2 – 25.3). Our results suggest i) Cr(VI) is immobilized in soils; ii) mechanisms for Cr(VI) immobilization is via precipitation as CaCrO4 and via recrystallization with Fe oxides.
... Ferrous iron (Fe 2þ ) plays an important role in initiating the reduction of Cr 6þ to Cr 3þ in groundwater, but the reduction process can be inhibited due to pH condition and adsorption of organic matter on the surface of IBSMs (Choi et al. 2008;Kendelewicz et al. 2000). The literature has reported that removal of Cr 6þ by Fe 3 O 4 could be inhibited under alkaline condition (e.g., pH 8-10) due to the oxidation of Fe 2þ to a ferric ion (Fe 3þ ) (He and Traina 2005). The formation of Fe 3þ decreased the reactive surface of Fe 3 O 4 and inhibited the removal kinetic of Cr 6þ . ...
Chromium (Cr) is a toxic contaminant and ubiquitously present in the environment. This study was conducted to investigate the removal of Cr by nano-magnetite (nano-Fe3O4) in different biogeochemical environments of groundwater. Size of nano-Fe3O4 used was in the range of 50–100 nm, and it contained Fe (58.88%) and O (31.13%). Removal rates of total Cr (CrT) by nano-Fe3O4 were significantly improved (0.40–0.84 min−1) as the concentrations of nano-Fe3O4 were increased (0.25–3.75 g/L). However, removal rates of CrT by nano-Fe3O4 were decreased (1.52–0.66 min−1) as the concentrations of CrT increased (0.5–1.5 mg/L). In addition, the removal rate of CrT by nano-Fe3O4 was significantly increased (0.56–1.63 min−1) as the pH increased (pH 5.5–pH 9). Removal rates of CrT by nano-Fe3O4 were significantly decreased (0.93–0.28 min−1) as the concentrations of humic acid (HA) increased (0.25–3.75 g/L). This study revealed that the reactive surface of nano-Fe3O4 plays an important role to enhance maximum removal of CrT. The surface charge of nano-Fe3O4 with or without CrT is directly related to the reactive surface area of nano-Fe3O4, which has not been well reported in the literature. Chemisorption reaction mechanisms may occur during the removal kinetics of CrT by nano-Fe3O4. Experimental findings from this study proved that nano-Fe3O4 has the capability to remove CrT and provided fundamental knowledge on the potential reaction mechanisms of CrT removal by nano-Fe3O4.
... Unfortunately, these materials provide only very low adsorption capacities and those succeeded at acidic conditions and extremely high concentrations not applicable for drinking water treatment [16,17]. Recent studies have introduced granular adsorbents with high reducing potentials as a class of materials capable of capturing Cr(VI) by reducing it to the insoluble forms of Cr(III) [18,19]. The efficiency of these absorbents is proportional to their ability to operate as electron donors as long as possible. ...
Despite significant risks to human health due to elevated Cr(VI) concentrations in drinking water, a selective adsorbent capable of purifying water before consumption is still not commercially available. This work introduces an integrated household water filtration setup, for point-of-use applications, loaded with a tin-based Cr(VI)-oriented adsorbent that was tested under various contact times, pH values and Cr(VI) concentrations. The adsorbent comprises a chloride-substituted stannous oxy-hydroxide with a structure resembling that of the mineral abhurite. It demonstrated high reducing capacity that triggered the formation of insoluble Cr(III) hydroxides and the complete removal of Cr(VI) in considerably high volumes of polluted water. Test operation of the filtration system verified its ability to produce Cr(VI)-free water in compliance with the impending drinking water regulation, even for extreme initial concentrations (1000 μg/L). Apart from its high efficiency, the potential of the studied material is enhanced by its minimal-cost synthesis method carried out in a continuous-flow reactor by tin chloride precipitation under acidic conditions.
... This phenomenon may partly be explained by partial dissolution of INP at lower pH as well as formation of nanoparticle aggregates (Baalousha et al. 2008). He and Traina (2005) reported inefficiency of INP in Cr(VI) removal under basic conditions. Similar results has also been reported in other studies where repulsive electrostatic interaction at high pH did not inhibit Cr(VI) removal (Jiang et al. 2014). ...
Presence of carcinogenic chromium, i.e., Cr(VI), in different industrial effluents necessitates design and development of effective abatement technologies. Nanosorbent consisting of iron oxide nanoparticles functionalized with soil-derived humic acid was employed for removal of Cr(VI). The point of zero charge for both humic acid and nanoparticles as estimated from pH shift experiments was between pH 8 and 9. Adsorption isotherm from batch experiments at neutral pH followed Langmuir model with projected maximum adsorption capacities for humic acid coated nanoparticles (24.13 mg/g) much higher than its uncoated counterpart (2.82 mg/g). Adsorption was process very fast and kinetics could be described with pseudo-second-order model (R2 > 0.98), for both nanoparticles. High E4/E6 ratio of extracted humic acid and Fourier transform infrared spectroscopy of coated nanoparticles (20–100 nm) indicated enrichment of hydroxyl, carboxylic, and aliphatic groups on surface leading for the better adsorption. Humic acid coated and uncoated nanoparticles regenerated with EDTA, NaOH, urea, Na2CO3, and NaCl treatments retained 35.90–59.67 and 26.37–36.28% of their initial adsorption capacities, respectively, in 2nd cycle. Experimental controls (virgin nanoparticles subjected to an identical regenerating environment) revealed irreversible surface modification as the cause for loss of their adsorption capacities.
A novel nanocomposite consisting of Fe3O4-loaded tin oxychloride is demonstrated as an efficient adsorbent for the removal of hexavalent chromium in compliance to the new drinking water regulation. This study introduces a continuous-flow production of the nanocomposite through the separate synthesis of (i) 40 nm Fe3O4 nanoparticles and (ii) multilayered spherical arrangements of a tin hydroxy-chloride identified as abhurite, before the application of a wet-blending process. The homogeneous distribution of Fe3O4 nanoparticles on the abhurite’s morphology, feature nanocomposite with magnetic response whereas the 10 % loaded nanocomposite preserves a Cr(VI) uptake capacity of 7.2 mg/g for residual concentrations below 25 μg/L. Kinetic and thermodynamic examination of the uptake evolution indicates a relative rapid Cr(VI) capture dominated by interparticle diffusion and a spontaneous endothermic process mediated by reduction to Cr(III). The efficiency of the optimized nanocomposite was validated in a pilot unit operating in a sequence of a stirring reactor and a rotary magnetic separator showing an alternative and competitive application path than typical fixed-bed filtration, which is supported by the absence of any acute cellular toxicity according to human kidney cell viability tests.
Monitoring the extent of heavy metal remediation on reactive materials employed in remediation schemes relies on downflow concentration sampling. The latter implies that a decreasing barrier remediation efficiency only becomes apparent once a contaminant breakthrough has occurred and thus contamination past the barrier. Spectral induced polarization (SIP), a noninvasive geophysical technique sensitive to sorption-induced changes in the surface charging properties of mineral surfaces in porous media, offers a potentially powerful monitoring alternative. We conducted a flow-through column experiment fitted with retracted electrodes and packed with magnetite-coated sand as a bench-scale reactive barrier analogue for monitoring Cr(VI) sorption using SIP. Our SIP responses measured during the Cr(VI)-injection pulse highlight that adsorption of Cr(VI) onto the magnetite coating led to a short-lived increase in imaginary conductivity, followed by a strong continuous decrease. Via reactive transport modeling, we simulated the redox-dependent sorption of Cr(VI) and linked the decrease in the SIP-derived imaginary conductivity to the gradual reduction in the remaining sorption capacity on the magnetite coating. The excellent agreement between our geochemical results and SIP signals suggests that noninvasive geophysical methods can function as an early warning tool for monitoring the finite lifetime of reactive barriers in situ.
The current levels of emerging and traditional pollutants in water are remarkably increasing,nwhich is an enormous concern. Adsorption is acknowledged as a potential method for treating
various kinds of pollutants in water and wastewater. This method has an operational cost, a fast kinetic removal ability, a minimal generation of secondary pollutants (i.e., sludge), and a high
efficiency rate to remove different pollutants even under low initial concentrations of pollutants. However, the adsorption efficiency and affinity of pollutants are strongly dependent on the properties
of adsorbents, the nature of adsorbates, and adsorption conditions. These can be reflected through adsorption mechanisms. Natural adsorbents often exhibit a high adsorption capacity to target pollutants. For example, activated carbon is an excellent material used for adsorbing organic pollutants (i.e., dye) in water. Some synthetic materials (suitable surfactant-modified zeolite) can serve as dual-electronic and amphiphilic adsorbents used for removing potentially toxic metals (cationic and oxyanionic ions) and organic compounds with low and high water solubility.
This reprint is expected to provide innovative and outstanding reference materials for water decontamination and bring more updated and advanced theories on science and technology related to adsorption for internal and national scientific communities. The Guest Editor would like to thank
all authors and colleagues for their great contributions.
In this study, the iron-based carbon composite (hereafter FCN-x, x = 0, 400, 500, and 600 calcination) was synthesized by a simple high-temperature pyrolysis method using iron-containing sludge coagulant generated from wastewater treatment settling ponds in chemical plants. The FCN-x was used for the adsorptive reduction of aqueous phase Cr(VI) effectively. The FCN-x was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier infrared spectrometer (FT-IR), X-ray photoelectron spectrometer (XPS), and Brunauer-Emmett-Teller theory (BET). FCN-x adsorption of Cr(VI) was examined in batch experiments using CrO42− as a function of physicochemical parameters. The chemical kinetics of Cr(VI) adsorption by FCN-500 were modeled by 1st and 2nd order empirical pseudo kinetics. Based on these experiments, FCN-500 has been selected for further studies on Cr(VI) adsorptive reduction. The maximum Cr(VI) adsorption by FCN-500 was 52.63 mg/g showing the highest removal efficiency. The Cr(VI) adsorption by the FCN-500 was quantified by the Langmuir isotherm. XPS result confirmed the reduction of Cr(VI) to Cr(III) by the FCN-500. The iron-based carbon composites have high reusability and application potential in water treatment. The electroplating wastewater with 117 mg/L Cr(VI) was treated with FCN-500, and 99.93% Cr(VI) was removed within 120 min, which is lower than the national chromium emission standard of the People’s Republic of China. This work illustrates the value-added role of sludge generated from dye chemical plants to ensure environmental sustainability.
Cr(VI) detoxification and organic matter (OM) stabilization are usually influenced by the biological transformation of iron (Fe) minerals; however, the underlying mechanisms of metal-reducing bacteria on the coupled kinetics of Fe minerals, Cr, and OM remain unclear. Here, the reductive sequestration of Cr(VI) and immobilization of fulvic acid (FA) during the microbially mediated phase transformation of ferrihydrite with varying Cr/Fe ratios were investigated. No phase transformation occurred until Cr(VI) was completely reduced, and the ferrihydrite transformation rate decreased as the Cr/Fe ratio increased. Microscopic analysis was uncovered, which revealed that the resulting Cr(III) was incorporated into the lattice structure of magnetite and goethite, whereas OM was mainly adsorbed on goethite and magnetite surfaces and located within pore spaces. Fine line scan profiles showed that OM adsorbed on the Fe mineral surface had a lower oxidation state than that within nanopores, and C adsorbed on the magnetite surface had the highest oxidation state. During reductive transformation, the immobilization of FA by Fe minerals was predominantly via surface complexation, and OM with highly aromatic and unsaturated structures and low H/C ratios was easily adsorbed by Fe minerals or decomposed by bacteria, whereas Cr/Fe ratios had little effect on the binding of Fe minerals and OM and the variations in OM components. Owing to the inhibition of crystalline Fe minerals and nanopore formation in the presence of Cr, Cr sequestration and C immobilization can be synchronously favored at low Cr/Fe ratios. These findings provide a profound theoretical basis for Cr detoxification and synchronous sequestration of Cr and C in anoxic soils and sediments.
Iron-modified biochar (Fe-biochar) has been widely developed to attenuate Cr(VI) pollution in both acid and alkaline environments. However, there are few comprehensive studies on how the iron speciation in Fe-biochar and chromium speciation in solution influencing the removal of Cr(VI) and Cr(III) under varying pH. Here, multiple Fe-biochar containing Fe3O4 or Fe(0) were prepared and applied to remove aqueous Cr(VI). Kinetics and isotherms suggested that all Fe-biochar could efficiently remove Cr(VI) and Cr(III) via adsorption-reduction-adsorption. The Fe3O4-biochar immobilized Cr(III) by forming FeCr2O4, while amorphous Fe-Cr coprecipitate and Cr(OH)3 was formed when using Fe(0)-biochar. Density functional theory (DFT) analysis further indicated that pH increase caused more negative adsorption energies between Fe(0)-biochar and the pH-dependent Cr(VI)/Cr(III) species. Consequently, the adsorption and immobilization of Cr(VI) and Cr(III) species by Fe(0)-biochar was more favored at higher pH. In comparison, Fe3O4-biochar exhibited weaker adsorption abilities for Cr(VI) and Cr(III), which were in consistent with their less negative adsorption energies. Nonetheless, Fe(0)-biochar merely reduced ∼70% of adsorbed Cr(VI), while ∼90% of adsorbed Cr(VI) was reduced by Fe3O4-biochar. These results unveiled the importance of iron and chromium speciation for chromium removal under varying pH, and might guide the application-oriented design of multifunctional Fe-biochar for broad environmental remediation.
The coprecipitation of magnetite (Fe3O4) with arsenic (As) is a potential remediation technique for As-contaminated groundwater that can be applied to meet increasingly stringent As drinking water limits. However, knowledge of the fate of As coprecipitated with magnetite during aging for extended periods is lacking, which is critical to predict the long-term efficiency of this As treatment strategy. In this work, I combined aqueous As measurements with solid-phase characterization by synchrotron-based Fe and As K-edge X-ray absorption spectroscopy (XAS) to track the transformation of magnetite and the speciation of coprecipitated As(V) or As(III) for up to a year in oxic or anoxic conditions. It was determined that the initial magnetite particle increased in crystallinity for all aging experiments, but some differences in solid-phase Fe speciation were detected depending on aging conditions. For the anoxic aging samples with initial As(V), a significant fraction (15% of the total Fe) of maghemite (a magnetic Fe oxide spinel with formulγ-Fe2O3) was identified, which was coupled to As(V) reduction [As(III) was â∼30% of the total sorbed As], suggesting electron transfer between magnetite and particle-bound As(V). In the oxic aging experiments, the initial particle crystallized, with a large fraction of Fe(III) (oxyhydr)oxides (i.e., maghemite and lepidocrocite, γ-FeOOH) in the final products. Despite increased crystallinity suggested by Fe XAS analysis, sorbed As was not released from the particles in any experiment (aqueous As never exceeded 1 μg/L). This remarkable stability of As coprecipitated with magnetite was revealed by As K-edge XAS to be largely due to the formation of distinct multinuclear As uptake modes [i.e., As(V) incorporation; hexanuclear 3C As(III) complexes]. These results demonstrate the unique potential of magnetite for long-term As sequestration.
Chromite ore processing residue (COPR) has been a severe environmental contaminant which is worthy of attention. In this study, we developed an eco-friendly and practical technology for effectively stabilizing and recovering Cr(VI) in COPR via combining FeSO4 reducing agent and the hydrothermal treatment. A stable spinel phase product was formed during detoxification. In addition, the ferrochrome resources in the treated COPR can be obtained by magnetic separation. As we studied, the hydrothermal environment promoted the release of unstable Na2CrO4 from COPR into the solution, and the released CrO4²⁻ was reduced to Cr(III) by FeSO4. Subsequently, Cr(III), Fe(II) and Fe(III) were hydrothermally mineralized to form the magnetic spinel phase Fe²⁺(Cr³⁺X, Fe³⁺2-x)O4 (FeCr spinel substance), which was conducive to the magnetic separation of ferrochrome resources. Under the optimal hydrothermal conditions (0.15 g FeSO4/2 g COPR, treatment at 180 °C for 8 h), the total Cr leaching concentration of treated COPR (COPR-HT) was decreased from 120.51 mg L⁻¹ to 0.23 mg L⁻¹, well below the regulatory limit of 1.5 mg L⁻¹ (HJ/T 301–2007, China EPA). After 300 days aging under atmospheric conditions, the total Cr leaching concentration of COPR-HT was still below 1.5 mg L⁻¹. Besides, the COPR-HT after magnetic separation contained 11.52 wt% Cr2O3 and 53.44 wt% Fe2O3, which can be used as the raw material for steel industry. The underlying mechanism of COPR stabilization was explained by XRD, XPS and SEM-EDS analysis. This work converted the toxic and unstable Cr(VI) in COPR into the long-term stable FeCr spinel substance that is easy to magnetically separate. It has important reference for the harmless disposal and resource utilization of other chromium-containing hazardous wastes including chromium slag and electroplating sludge.
Magnetite (Fe3O4) is considered a promising catalyst for the removal of hexavalent chromium (Cr VI) in wastewater. To optimize its adsorptive capacity, bivalent copper ions (Cu II) were added to the oxide matrix. Structural and electronic properties, as well as chromate adsorption processes (CrO4²⁻), were investigated by the density functional theory method with incorporating the Hubbard correction term (DFT+U). Adsorption energy data indicate that the doped magnetite, Cu/Fe3O4, performs better in removing CrO4²⁻ than the oxide in its original form. This strategy can be extended to other materials to design active catalysts in environmental applications.
At room temperature, a facile approach has been utilized for preparing novel CdS-attapulgite (CdS-ATP) composites and the composites were applied in photocatalytic reduction ofp-nitrophenol and Cr(vi). The effect of ATP on the photocatalytic activity of the CdS-ATP composites were studied by controlling the mass ratio of attapulgite. The results showed that the CdS-20%ATP composite has an excellent photocatalytic activity. In order to figure out the key to improve the photocatalytic efficiency, the prepared composites were characterized by Brunauer-Emmett-Teller (BET) specific surface area, UV-vis diffuse reflectance spectroscopy (DRS) and electrochemical impedance spectroscopy (EIS). The superior photocatalytic performance of the CdS-20%ATP composite can be ascribed to the existence of the ATP which can fix the CdS and prevent agglomeration. The interaction between ATP and CdS in the composites facilitates the electron transfer and also promoted their photocatalytic performance. This work provides us with some significant guidance in the development of CdS-ATP composite photocatalysts.
We investigated the feasibility of using FeS-coated alumina and silica for permeable reactive barrier (PRB) applications. By both coated materials, Cr(VI) was reduced to Cr(III), which was immobilized via surface complexation/precipitation at acidic pH, and bulk precipitation at neutral to basic pH. Both pH and surface coating density (the amount of FeS deposits per unit surface area of a supporting matrix) controlled Cr(VI) reduction capacity and [Cr,Fe](OH)3 composition. The reduction was higher at acidic pH due to lower passivation, as evidenced by the increased production of Fe(III) (oxyhydr)oxides over Fe(II)-Fe(III) phases. The coated alumina, despite the lower amount of FeS deposits than the coated silica, showed greater reduction capacities due to its higher surface coating density, which made Fe(III) closer together to favor Fe(III) (oxyhydr)oxide formation. Since Cr(III) was preferentially substituted for Fe(III) in Fe(III) (oxyhydr)oxides, lower pH and higher surface coating density led to lower Cr fractions in [Cr,Fe](OH)3 because of the increased production of Fe(III) (oxyhydr)oxides. Given that Cr-poor [Cr,Fe](OH)3 is resistant to re-oxidation, FeS-coated alumina is better for PRB applications. This study reveals the significance of the surface coating density when evaluating the effectiveness of coated materials in redox-based treatments.
In a number of laboratory studies, sulfidated nanoscale zero-valent iron (S-nZVI) particles showed increased reactivity, reducing capacity, and electron selectivity for Cr(VI) removal from contaminated waters. In our study, core-shell S-nZVI particles were successfully injected into an aquifer contaminated with Cr(VI) at a former chrome plating facility. S-nZVI migrated towards monitoring wells, resulting in a rapid decrease in Cr(VI) and Crtot concentrations and a long-term decrease in groundwater redox potential observed even 35 m downstream the nearest injection well. Characterization of materials recovered from the injection and monitoring wells confirmed the presence of nZVI particles, together with iron corrosion products. Chromium was identified on the surface of the recovered iron particles as Cr(III), and its occurrence was linked to the formation of insoluble chromium-iron (oxyhydr)oxides such as CrxFe(1−x)(OH)3(s). Injected S-nZVI particles formed aggregates, which were slowly transformed into iron (oxyhydr)oxides and carbonate green rust. Elevated contents of Fe⁰ were detected even several months after injection, indicating good S-nZVI longevity. The sulfide shell was gradually disintegrated and/or dissolved. Geochemical modelling confirmed the overall stability of the resulting Cr(III) phase at field conditions. This study demonstrates the applicability of S-nZVI for the remediation of a Cr(VI)-contaminated aquifer.
Magnetite is a common mixed ferrous–ferric iron oxide in soils and sediments, which has both adsorption and reduction capacities for Cr(VI). Previous studies mainly focused on the application of magnetite in Cr(VI) removal from aqueous phase by adsorption, while few efforts has been devoted to the reduction process of adsorbed Cr(VI) on the solid phase. In this study, the kinetic and thermodynamic experiments were simultaneously conducted to verify the relationship between Cr(VI) adsorption and reduction, and SEM-EDS, XPS and Raman spectroscopies were utilized to reveal the mechanisms of Cr(VI) reduction and magnetite transformation. According to the results, Cr(VI) was found to be quickly adsorbed onto magnetite surface with a very high adsorption rate constant of 11.369 d⁻¹, and then the adsorbed Cr(VI) as an intermediate was directly reduced to Cr(III) by the active Fe(II) on magnetite surface with a fast reaction rate constant of 1.606 d⁻¹. Notably, the reduction rate constant gradually decreased as low as 0.032 d⁻¹ with time, which might be induced by the maghemite passive layer formed on the outer-sphere of magnetite, where the adsorbed Cr(VI) could only be reduced indirectly by the Fe(II) located at the inner-sphere of magnetite, and thus the electron transfer rate from internal Fe(II) to adsorbed Cr(VI) became the rate-limiting step of Cr(VI) reduction. Accordingly, a coupling mechanism of “fast adsorption-direct reduction-indirect reduction” was proposed, and a multi-step kinetic model was developed based on that, which had a much better fitting effect (0.936 < R² < 0.996) than the classical first-order or second-order kinetic models (0.885 < R² < 0.956). This work highlighted the multi-step reaction mechanism of Cr(VI) with magnetite, especially the distinguished Cr(VI) reduction schemes by magnetite with different passivation extents, and these findings may favor the understanding of the interfacial processes of Cr(VI) on Fe(II) containing minerals in subsurface environment, where the minerals may exist in various passivation degrees due to natural oxidation.
Biogenic Fe(II) is a dominant natural reductant to convert carcinogenic Cr(VI) to less toxic Cr(III). Field-applied biochar could promote microbial production of Fe(II) and form iron-biochar composites. Although there have been mounting research on the interactions of biochar or Fe(II) with Cr(VI), their coupling effects on Cr(VI) immobilization have been largely neglected. Here, iron mineral-biochar composite (IMBC) was prepared via biochar-mediated dissimilatory reduction of ferrihydrite or goethite by Shewanella oneidensis MR-1, and its reaction with Cr(VI) was investigated. IMBC was able to effectively remove aqueous Cr(VI) via reductive transformation by adsorbed Fe(II). The removal process nicely followed pseudo-second-order kinetics and Langmuir isotherm model. The removal ability of IMBC decreased with increasing pH (5.5–8.0) but was independent of ionic strength changes (0–100 mM). After reaction, the Fe–Cr coprecipitates formed on IMBC exhibited slightly higher Fe/Cr ratios (0.93–0.96) than those on corresponding iron mineral controls (0.88–0.94). For IMBC, while the presence of biochar decreased the reactivity of adsorbed Fe(II), their removal capacities were ~30% higher than those of iron minerals alone, due to the enhanced yields of adsorbed Fe(II). These findings improved our knowledge of interactions among biochar, iron mineral and iron-reducing bacteria and their contribution to chromium immobilization.
In this study, well-defined sludge-based magnetic polydopamine (SMP) was synthesized by a facile and green approach. Pristine activated sludge (PAS) was recycled to be a carbonaceous matrix, represented as sludge-based support (SBS) and used to support bare magnetic Fe3O4 microspheres (BM), which finally gave sludge-based magnetic Fe3O4 microspheres (SM). A lot of BM were further transformed to core–shell structured magnetic Fe3O4@polydopamine microspheres (MP) by polymerization of dopamine. The specific surface areas of SMP were 30.9 m² g⁻¹, far larger than those of the BM (5.5 m² g⁻¹) and SBS (2.1 m² g⁻¹). The maximum adsorption capacity was calculated to be 118 mg g⁻¹ at 323 K. More significantly, reduction-stabilization process of Cr(VI) was found to occur on the surface nanolayered coating. Specifically, after being protonated, the nitrogen atoms contained in SMP were confirmed to attract negatively charged Cr(VI) by electrostatic attraction. The redox system promoted electron transfer from polydopamine (PDA) to Cr(VI), and then the non-protonated nitrogen atoms in-situ chelated the reduction product Cr(III) as coordination atoms. In short, BM supported by the SBS and coated by PDA possessed excellent capability for magnetic separation and reduction–stabilization of Cr(VI), and proved to be a stable and green adsorbent.
The remediation of hexavalent chromium [Cr(VI)] contaminated water has been a challenge due to its high toxicity and mobility in the environment. Adsorption has so far been the most promising method for the treatment of chromium containing water due to the availability of low cost materials capable of chromium sequestration. The mechanism for hexavalent chromium removal has however not been fully understood with many studies pointing to a reduction-coupled mechanism. In this study, a composite material consisting of agricultural waste material (pine cone) and magnetite nanoparticles was used for the remediation of hexavalent chromium contaminated water with a focus on the removal mechanism and adsorbent regeneration studies. The mechanism was identified to be adsorption-coupled reduction where Cr(VI) was reduced on the adsorbent surface to the less toxic trivalent chromium [Cr(III)] before being adsorbed. The adsorbed chromium was successfully desorbed using NaOH allowing for concentration and chromium recovery. The adsorbent material was applied in three adsorption-desorption cycles without a significant loss in adsorption capacity.
Employing biochar for environmental remediation has been widely practiced. Nonetheless, the reduction mechanisms of hexavalent chromium (Cr(VI)) in the presence of biochar have not been fully elucidated (i.e., direct or indirect reduction of Cr(VI) by biochar). In particular, the effect of light on Cr(VI) reduction by biochar was rarely reported. Thus, to clarify the reduction mechanisms of Cr(VI) by biochar at the fundamental level, this study laid great emphasis on the photo-induced reduction of Cr(VI) in the application of biochar. Biochar releases dissolved organic matter (DOM), the DOM can extract Fe(III) from soil by complexation, and the complexes can be photo-reacted under the light. In these respects, Fe(II) formed by the photo-induced reaction of DOM-Fe(III) was particularly evaluated in this study. To evaluate that, three biomass samples (rice straw, granular sludge from an up-flow anaerobic sludge blanket, and spent coffee ground) were torrefied to biochar. To circumvent the adsorption of Cr(VI) onto biochar, biochar extractives (served as a source for DOM) and Fe(III) solution were tested with/without UV light to prove Fe(II) formation. This study experimentally proved that the more Fe(II) under the UV radiation was formed in the co-existence with biochar extractives and Fe(III). All experimental data from three biochar samples were indeed very similar. Cr(VI) reduction by Fe(II) from GB, RB, and CB reached up to 96, 79, and 100%, respectively. The different reduction efficiency signified that the low molecular weight of organic acids, such as oxalate, were more sensitive to the UV light, thereby resulting in the enhanced Fe(II) formation. Such Fe(II) formation subsequently led to the high reduction efficiency of Cr(VI).
The reaction mechanisms and rates of reaction of a number of the common rock-forming silicates with synthetic cement pore fluids have been evaluated in a series of laboratory experiments at 70°C. Mass transfer is dominated by the dissolution of the primary silicate and the precipitation of a range of Na-K-Al substituted calcium silicate hydrates, and a possible zeolite. Calcium was lost from, and silicon gained by, the fluid phase as a result of the reactions. Secondary solids formed thick layers on primary silicates, but dissolution of the silicates was not diffusion-limited. The rate of dissolution of the silicates was determined to be 2–3 orders of magnitude greater at pH 12–13 than at neutral pH, and confirm measurements by other authors. The rate of growth of calcium silicate hydrates was limited by the rate of supply of silicon from the primary silicates. Although the results of the laboratory experiments were dominated by the loss of calcium from the fluid and the precipitation of calcium silicate hydrates, thermodynamic modelling suggests that these may be replaced by zeolites and/or feldspars when groundwater residence times are considered.
The central tenet of this program is that a fundamental understanding of specific mineral surface-site reactivities will substantially improve reactive transport models of contaminants in geologic systems, and will allow more effective remediation schemes to be devised. To this end, we carried out a program of research that focuses on the fundamental mechanisms of redox chemistry of contaminants on mineral surfaces. As much of this chemistry in sediments involves the Fe(III)/Fe(II) couples, we focused on mineral phases containing these species. Our approach was to conduct carefully controlled experiments on model, single-crystal Fe oxide mineral surfaces grown by molecular beam epitaxy, natural Fe oxide single crystals, and synthetic mineral powders. We used the results from the model surfaces, which were very well defined in terms of surface composition, structure, and defect densities, to understand the results obtained on more complex mineral specimens. We used a variety of experimental probes, along with molecular modeling theory, to determine clean mineral surface structure, details of the chemisorption and decomposition of water, and the interface structure and redox chemistry of important contaminants such as CrO4 -2 on these surfaces.
A hybrid chemical kinetic and equilibrium model has been developed to quantitatively assess the dynamic oxidation-reduction (redox) transformations of chromium in natural waters. Simulations are conducted to examine the effects of 11 chemical and environmental parameters on the redox cycle between trivalent [Cr(III)] and hexavalent [Cr(VI)] chromium. Based on the model results, it is found that natural water serves as a reductive environment for chromium under typical conditions, since the common oxidants of Cr(III) (e.g., hydroxyl radical and manganese) do not convert chromium to Cr(VI) at significant rates because of their extremely low levels or slow oxidation kinetics. At low pH (6.0), ferrous iron [Fe(II)] becomes the predominant reductant. However, at pH greater than 8.0 and in the presence of dissolved oxygen (DO), Cr(VI) reduction by Fe(II) is greatly suppressed due to the rapid oxidation of Fe(II) by DO. Other reactive species such as hydroperoxyl radical (HO2
/O2
-) and hydrogen peroxide (H2O2) present in sunlit waters can indirectly affect the chromium redox cycle through their reactions with iron and S(IV). pH appears to be an important parameter, as it affects both chemical speciation and reaction kinetics. Chelating agents in natural waters should not significantly affect chromium redox transformations due to their usually low concentrations and generally weak interactions with Fe(II).
The chemical compositions of the pore solutions extracted from seven different cement pastes (one Swedish and one French standard Portland cement, sulfate resistant, blast-furnace slag, fly ash, silica, and high alumina cement) have been determined. Analyses covered Na, K, Ca, Mg, Al, Fe, and Si as well as pH and Eh (redox potential). Ionic strengths in the range 0.03–0.29 M and pH-values in the range 12.4–13.5 were obtained. Only the pore solutions from the slag cement and the French Portland cement were reducing (negative Eh). The dominating cations of standard Portland, sulfate resistant, slag, silica and fly ash cement pore solutions were Na and K, and in sulfate resistant cement also Ca. The main components in the pore solutions of aluminate cement were Na and Al. The Si and Fe concentrations were low in all pore solutions.
Batch sorption experiments performed on Cr(VI) species sorption showed a significantly enhanced removal of inorganic hexavalent chromium anionic species from aqueous solution by montmorillonite clays modified with quaternary amine, hexadecyltrimethylammonium (HDTMA) bromide. Unmodified clay had no affinity for chromium(VI) species. The sorption of Cr(VI) species has been carried out as a function of pH, contact time, adsorbate concentration (4.14x10(-5) to 8.62x10(-3) M), and temperature (5-45 degrees C). The surfactant-modified clay surface was stable when exposed to extremes in pH. The optimum pH for maximum sorption of Cr(VI) species was found to be at pH 1 and was constant between pH 2 and pH 6. The sorption data obtained was well described by DKR and Langmuir sorption isotherms. Sorption energy (E) for (i) surfactant sorption by montmorillonite clay and (ii) sorption of chromium(VI) species by surfactant modified clay have been computed from the DKR equation. Sorption energy evaluated for the sorption of both surfactant and Cr(VI) species showed that an ion-exchange mechanism was operative. The mechanism of retention appears to be replacement of counterion of the surfactant by Cr(VI) anionic species. Adsorbent capacity for the sorption of Cr(VI) species has been evaluated from the Langmuir sorption isotherm data. Thermodynamic parameters (Delta H degrees, Delta S degrees and Delta G degrees ) for surfactant sorption on montmorillonite clay and Cr(VI) sorption by modified clay have been evaluated. The specific rate constant for sorption of Cr(VI) species on modified montmorillonite was rapid during the first 10 min and equilibrium was found to be attained within 30 min. The sorption of Cr(VI) species onto modified montmorillonite clay followed first-order rate kinetics. Copyright 2000 Academic Press.
A consequence of the dissolution of the green rust when in contact with concentrations below the critical value suggests that the process of washing to remove salts will inevitably cause breakdown, even if oxygen is excluded. The balance between the rate of dissolution of the green rust and the rate of oxidation of Fe 2+ in the solution governs whether or not a plateau occurs in the plot of base required to maintain pH constant. This critical concentration is influenced by pH as well as the type of anion incorporated into the interlayer of the green rust. X-ray diffraction data showed that the anions sulphate, perchlorate and nitrate gave products which can accomodate water in the interlayer, giving spacings dependent on the relative humidity. This contrasts with all other anions studied.
In alkaline media and at 70°C dilute suspensions of ferrihydrite transformed to goethite between pH 11.2 and 14 and to a mixture of goethite and hematite above and below this pH range. Increasing the temperature of the transformation or the concentration of the suspension reduced the pH range in which goethite alone formed. The morphology of goethite was chiefly a function of the pH of the system. Acicular crystals formed at all pHs and exclusively above pH 12.2. Epitaxial twinned crystals predominated at pHs below 11, and twins free from hematite formed at higher pHs. Increasing the suspension concentration, ionic strength, or temperature extended the pH range over which twinned crystals formed. Electron micrographs showed that twins formed mainly during the initial stage of the transformation, whereas acicular crystals formed over a longer period. Thus, the twins appeared to nucleate in the ferrihydrite; nucleation of acicular particles took place in solution.
Chromium(VI) is a priority pollutant of some soils and natural waters in industrial areas. Iron(II), an important natural reductant of Cr(VI), is an option in remediation of contaminated sites, transforming Cr(VI) to essentially nontoxic Cr(III). After kinetics and pathways of this redox reaction had been reported to depend strongly on pH and organic ligands, this study investigated the influence of mineral surfaces. Kinetic measurements with UV-vis in mineral and soil suspensions at pH 5 showed that all minerals tested, except Al{sub 2}O{sub 3}, accelerated the Cr(VI) reduction by Fe(II), in the order of {alpha}-FeOOH {approximately} {gamma}-FeOOH {much{underscore}gt} montmorillonite {gt}kaolinite {approximately} Sio{sub 2} {much{underscore}gt} Al{sub 2}O{sub 3}. Similar kinetics were observed with soil from the E and B{sub Fe} horizons of a Podzol. The reactions appear to be driven by the high reactivity of adsorbed Fe-(II). Whereas adsorbed Cr(VI) was reducible by Fe(II), the sparingly soluble BaCrO{sub 4} was largely protected from reduction. This is of environmental relevance since in many polluted soils, Cr(VI) is partly present as Ba, Ca, Fe, or Pb salts. Kinetic data and reaction pathways are important in the optimization of Fe(II)/mineral mixtures as reluctant of Cr(VI) in technical systems, in the evaluation of in-situ remediation of Cr(VI)-contaminated waters and soils by Fe(II), and in qualitative predictions and modeling of Cr-(VI) in natural systems.
The rust layers formed on weathering and mild steels by atmospheric corrosion in an industrial region for a quarter of a century have been characterized using various analytical techniques. In particular, analysis of a local portion of the rust layers by means of Raman spectroscopy gives important information on the structure of the layers. It is elucidated that the inner stable and protective rust layer which covered the surface of weathering steel mainly consists of nano-particles of α-FeOOH containing a considerable amount of Cr. The relative change of the amount of rust constituents for low alloy steels supports well a newly proposed schematic progress of long term alteration in stable and protective rust layer formed on a weathering steel in an industrial environment, i.e. the γ-FeOOH, as an initial rust layer, is transformed into a final stable rust layer of α-FeOOH, probably via an amorphous oxyhydroxide substance, during the long term atmospheric corrosion of a weathering steel.
The optimum conditions were studied for the formation of Fe3O4 by the air oxidation of Fe(OH)2. The suspensions obtained by mixing NaOH and FeSO4 solutions in various values of R(=2NaOH/FeSO4) were subjected to oxidation with air at various temperatures. The oxidation products were then examined by X-ray powder diffraction, chemical analysis, electron microscopic observation, and BET surface-area determination. Fe3O4 is formed at higher temperatures than is FeOOH. The temperature of formation becomes low as R approaches 1.0. In neutral suspensions (R<1), Fe3O4 is formed via green rust II or a mixture of green rust II and Fe(OH)2. By further oxidation, the Fe3O4 formed gradually changes to γ-Fe2O3. A mixture of α-FeOOH and either NaFe3(OH)6(SO4)2 or α-Fe2O3 is formed as the final oxidation product. In alkaline suspensions (R>1), Fe3O4 is formed directly. The morphology and ferrous-ion content of Fe3O4 powder change considerably with the presence of green rust II before the formation of Fe3O4. It is suggested that Fe3O4 is formed near the surface of the particles of Fe(OH)2 and green rust II by the coprecipitation of ferrous ions with ferric hydroxo-complexes.
A study of the reaction of alkaline chemicals with minerals constituting reservoir rock is presented. Static tests were conducted with high concentrations of NaOH and orthosilicate solutions and minerals (montmorillonite, kaolinite, illite, and quartz sand). The reaction time varied from 10 minutes to 2 months. Solutions were analyzed for hydroxide, Si, and Al, and solids were analyzed by X-ray diffraction (XRD) and other spectroscopic techniques. The loss of useful alkalinity was the highest with kaolinite and the least with quartz sand for high alkali concentration (5 wt%) or temperature (180°F [82 °C]). A detailed study of kaolinite/alkali reaction kinetics and quartz/alkali equilibrium is presented.
The radionuclide technetium is a common surface and groundwater contaminant at many nuclear fuels processing facilities. This research investigated a new method for removing pertechnetate from contaminated waters based on the low aqueous solubility of reduced technetium species. The removal method involved electrostatic adsorption of pertechnetate at an anodically polarized magnetite electrode, followed by reduction of the adsorbed Tc(VII). This method was capable of reducing technetium associated β activity below the 900 pCi/L drinking water maximum contaminant level set by the US EPA for manmade β activity. Upon termination of the applied polarization, the reduced technetium species remained adhered to the magnetite electrode under anaerobic conditions. Under aerobic conditions, the technetium was slowly released back into solution, indicating that the reduced technetium is afforded a degree of cathodic protection due to preferential oxidation of the magnetite. The advantages of this electrosorption/reduction technique over direct cathodic reduction are an increase in the stability of the reduced technetium, removal to lower aqueous concentrations, and greatly increased removal kinetics.
Permeable-reactive redox walls, placed below the ground surface in the path of flowing groundwater, provide an alternative remediation approach for removing electroactive chemicals from contaminated groundwater. Four types of Fe-bearing solids, siderite [FeCO3], pyrite [FeS2], coarse-grained elemental iron [Fe0], and fine-grained Fe0, were assessed for their ability to remove dissolved Cr(VI) from solution at flow rates typical of those encountered at sites of remediation. Batch studies show that the rate of Cr(VI) removal by fine-grained Fe0 is greater than that for pyrite and coarse-grained Fe0. Results from column studies suggest that partial removal of Cr(VI) by pyrite and coarse-grained Fe0 and quantitative removal of Cr(VI) by fine-grained Fe0 occur at rapid groundwater flow velocities. The removal mechanism for Cr(VI) by fine-grained Fe0 and coarse-grained Fe0 is through the reduction of Cr(VI) to Cr(III), coupled with the oxidation of Fe0 to Fe(II) and Fe(III), and the subsequent precipitation of a sparingly soluble Fe(III)−Cr(III) (oxy)hydroxide phase. Mineralogical analysis of the reactive material used in the batch tests indicates that Cr is associated with goethite (α-FeOOH). These results suggest that Cr(III) is removed either through the formation of a solid solution or by adsorption of Cr(III) onto the goethite surface. The effective removal of Cr(VI) by Fe0 under dynamic flow conditions suggests porous-reactive walls containing Fe0 may be a viable alternative for treating groundwater contaminated by Cr(VI).
The reduction of Cr(VI) to Cr(III) decreases the toxicity and mobility of chromium contaminants in soils and water. In addition, the formation of a highly insoluble Cr(III) product would decrease the likelihood of future Cr(III) re-oxidation. Amorphous iron sulfide minerals like mackinawite (FeS1-x) have the potential to reduce large quantities of Cr(VI) and in the process form very stable [Cr, Fe](OH)3 solids. In this study, we examine the effectiveness of amorphous FeS as a reductant of Cr(VI) by identifying the solution and solid-phase products of the reaction between FeS suspensions and chromate. Iron sulfide suspensions at pH 5.0, 7.0, and 8.0 were reacted with a range of Cr(VI) solutions from 50 to 5000 μM in a N2 atmosphere glovebox for 3 d. Solutions were analyzed using ICP−AES, IC, and colorimetric methods; solids were analyzed using XRD, TEM, EDS, and XANES spectroscopy. Iron sulfide removed all of the added Cr(VI) from solution for the reaction conditions studied and reduced between 85% and 100% of the Cr(VI) to Cr(III). Chromate reduction occurred dominantly at the FeS surface and resulted in [Cr0.75,Fe0.25](OH)3; while less extensive, reduction of Cr(VI) by Fe(II) (aq) was noted and produced a solid with the opposite Cr:Fe ratio, [Cr0.25,Fe0.75](OH)3.
Recent studies have shown promising results for subsurface remediation of dissolved chromate using permeable-reactive redox walls. Chromate reduction in the presence of iron filings and quartz grains was studied to determine the fate of reduced chromium in proposed wall material. Using a flow-through column apparatus, iron filings mixed with quartz grains were reacted with solutions that contained about 20 mg/L dissolved Cr(VI). Reacted iron filings developed coatings comprised of goethite with chromium concentrated in the outermost edges. Surface analysis showed all detectable chromium occurred as Cr(III) species. In addition, in regions of increased chromium concentration, goethite acquired chemical and structural characteristics similar to Fe2O3 and Cr2O3. Results of the study show that complete reduction of Cr(VI) to Cr(III) occurred and that Cr(III) was incorporated into sparingly soluble solid species.
The purpose of this investigation was (i) to test the effectiveness of a barrier engineered to remove Cr(VI) from leachates of higher pH and salinity typical of coal burning ashes and (ii) to determine which geochemical processes control Cr immobilization. Laboratory column and batch desorption experiments show that a barrier composed of sand, Fe(0), and bentonite irreversibly immobilizes Cr. Concentrations fall from 25 mg Cr L-1 in the leachate to below detection limits (0.0025 mg Cr L-1) and solution pH increases by about two units. Solid-phase analytical techniques such as SEM, EDS, XPS, and TOF-SIMS were used to characterize the barrier material prior to and after exposure to the Cr leachate. In the barrier material, Cr(III) was found associated with Fe(III)-oxides, as separate Cr oxides and as a Ca,Cr phase, probably Ca-chromite, CaCr2O4. The attenuating barrier can be an alternative to traditional liners and leachate collection systems at coal ash storage and disposal sites.
In alkaline solution ferrihydrite adsorbs OH, the amount initially adsorbed being dependent on [OH]. At high pH values ferrihydrite transforms rapidly to goethite with the rate of transformation also depending on [OH]. With time, goethite crystals grow, the total surface area of the solid decreases and OH previously adsorbed is progressively desorbed and reappears in the supernatant liquid. At any stage during the transformation the amount of OH adsorbed onto the solid is linearly related to both the amount of residual ferrihydrite and to the specific surface area of the solid. Regression equations indicate that the OH-adsorption capacity per unit surface area is similar for the initial ferrihydrite and the goethite produced (6 to 7 μmole OH m−2). X-Ray diffraction suggests that the goethite crystallites formed at high [OH] are smaller in both the crystallographic a and b dimensions. At 25°C the goethite crystals develop their final small thickness along the crystallographic a axis after a very short time. Subsequently they grow almost exclusively in width (along b axis) and length (c axis) until the ferrihydrite is completely transformed.
The interfacial properties of ZrO2 and Fe3O4 inmersed in water were studied through measurements of electrophoretic mobilities and surface charge densities obtained by potentiometric titrations. The pH(pzc) values were also determined by the addition method. The results are critically compared with previous data from the literature, and interpreted through the model of Davis, James, and Leckie [J. Colloid Interface Sci.63, 480 (1978); 67, 90 (1978); 74, 32 (1980)]. Values of the constants , and are derived for both oxides in KNO3 solutions. The relative contribution of the two mechanisms (potential-determining ions exchange with solvent and complex pairs formation with swamping electrolyte) to charge development is very different in both cases. This behavior is discussed in terms of the DJL model.
This report summarizes environmental information for the Hanford Site in Washington State for the calendar year 2001.
Experimental studies demonstrate that structural Fe(II) in magnetite and ilmenite heterogeneously reduce aqueous ferric, cupric, vanadate, and chromate ions at the oxide surfaces over a pH range of 1–7 at 25°C. For an aqueous transition metal m, such reactions are and where z is the valance state and n is the charge transfer number. The half cell potential range for solid state oxidation [Fe(II)] → [Fe(III)] is −0.34 to −0.65 V, making structural Fe(II) a stronger reducing agent than aqueous Fe2+ (−0.77 V). Reduction rates for aqueous metal species are linear with time (up to 36 h), decrease with pH, and have rate constants between 0.1 and 3.3 × 10−10 mol m−2 s−1. Iron is released to solution both from the above reactions and from dissolution of the oxide surface. In the presence of chromate, Fe2+ is oxidized homogeneously in solution to Fe3+.X-ray photoelectron spectroscopy (XPS) denotes a Fe(III) oxide surface containing reduced Cr(III) and V(IV) species. Magnetite and ilmenite electrode potentials are insensitive to increases in divalent transition metals including Zn(II), Co(II), Mn(II), and Ni(II) and reduced V(IV) and Cr(III) but exhibit a log-linear concentration-potential response to Fe(III) and Cu(II). Complex positive electrode responses occur with increasing Cr(VI) and V(V) concentrations. Potential dynamic scans indicate that the high oxidation potential of dichromate is capable of suppressing the cathodic reductive dissolution of magnetite. Oxide electrode potentials are determined by the Fe(II)/Fe(III) composition of the oxide surface and respond to aqueous ion potentials which accelerate this oxidation process.Natural magnetite sands weathered under anoxic conditions are electrochemically reactive as demonstrated by rapid chromate reduction and the release of aqueous Fe(III) to experimental solution. In contrast, magnetite weathered under oxidizing vadose conditions show minimum reactivity toward chromate ions. The ability of Fe(II) oxides to reduce transition metals in soils and groundwaters will be strongly dependent on the redox environment.
Synchrotron-based X-ray absorption fine structure (XAFS) spectroscopy was used to investigate the reduction of aqueous Cr(VI) to Cr(III) in magnetite-bearing soils from Cr-contaminated sites. Soils from two field sites were examined, showing that mixed-valence Cr(III/VI) effluent is reduced to Cr(III) when associated with the magnetite fraction of the soil, whereas the Cr effluent associated with non-Fe(II) -bearing minerals results in mixed Cr(III/VI) adsorbates or precipitated phases. The Fe2+ in magnetite, Fe2+Fe23+O4, may act as an electron source for heterogeneous Cr(VI)-to-Cr(III) reduction, converting magnetite topotactically to maghemite, γ-Fe23++O3. The ratio of Cr(VI)/total Cr was determined by the height of the Cr(VI) XAFS pre-edge feature, which is due to a is to 3d electronic transition. This pre-edge feature was calibrated as a function of Cr(VI)Cr(III) using mixtures of Cr(III) and Cr(VI) model compounds. Environmental remediation of Cr-contaminated sites requires knowledge of chromium oxidation and speciation, and XAFS spectroscopy may be used to supply both types of information with minimal sample processing or data analysis.
The mobility and toxicity of Cr within surface and subsurface environments is diminished by the reduction of Cr(VI) to Cr(III). The reduction of hexavalent chromium can proceed via chemical or biological means. Coupled processes may also occur including reduction via the production of microbial metabolites, including aqueous Fe(II). The ultimate pathway of Cr(VI) reduction will dictate the reaction products and hence the solubility of Cr(III). Here, we investigate the fate of Cr following a coupled biotic–abiotic reduction pathway of chromate under iron-reducing conditions. Dissimilatory bacterial reduction of two-line ferrihydrite indirectly stimulates reduction of Cr(VI) by producing aqueous Fe(II). The product of this reaction is a mixed Fe(III)-Cr(III) hydroxide of the general formula Fe1−xCrx(OH)3 · nH2O, having an α/β-FeOOH local order. As the reaction proceeds, Fe within the system is cycled (i.e., Fe(III) within the hydroxide reaction product is further reduced by dissimilatory iron-reducing bacteria to Fe(II) and available for continued Cr reduction) and the hydroxide products become enriched in Cr relative to Fe, ultimately approaching a pure Cr(OH)3 · nH2O phase. This Cr purification process appreciably increases the solubility of the hydroxide phases, although even the pure-phase chromium hydroxide is relatively insoluble.
Hexavalent chromium is a highly toxic, carcinogenic, and mobile contaminant present in wastewaters from mining and industrial operations. Its reduction to trivalent chromium, both less toxic and less soluble over the pH range of most natural waters, has previously been observed in solutions in contact with the redox-sensitive iron oxide magnetite (Fe2+Fe23+O4), and occurs via electron transfer from Fe2+ in the magnetite structure. This study presents direct in situ X-ray absorption fine structure (XAFS) evidence for the presence of Cr(III), initially resulting from the reduction of Cr(VI)aq in solution, at the surface of synthetic magnetite at near-neutral pH. Cr(VI) reacts with freshly-synthesized magnetite at a surface coverage of 4.5 μmol m−2 to be entirely reduced to Cr(III), as evidenced by the Cr absorption edge position and by the absence of a 1s → 3d pre-edge peak. XAFS spectra of Cr on progressively oxidized magnetite surfaces, however, show increasing pre-edge peak height indicating the presence of Cr(VI), i.e. a decrease in the Cr-reducing capacity of altered (maghemite-coated) magnetite grains. As expected, Cr(VI) sorbed to a ferric oxide, synthetic maghemite (γ-Fe23+O3), is not significantly reduced. The Cr pre-edge peak height for Cr(VI) reacted with maghemite is comparable to pre-edge peak heights of Cr(VI) model compounds. XAFS spectra for Cr model compounds compare well with theoretical XAFS spectra for the same compounds calculated using the ab initio, single- and multiple-scattering code FEFF. Fit parameters derived from FEFF models of Cr(III)- and Cr(VI)-containing compounds were used to determine the coordination environment (number and chemical identity of neighboring shells of atoms and their interatomic distances) of Cr sorbed to magnetite and maghemite samples.
The rates of the reduction of Cr(VI) with Fe(II) were measured in NaCl, NaClO4, and natural seawater as a function of pH (1.5–8.7), temperature (5–40°C) and ionic strength (I = 0.01–2 M). The pseudo first-order rate constant (log k1) showed a parabolic dependence on pH decreasing from 1.5 to 4.5 and increasing from 5.5 to 8.7. The kinetics of the reaction in these two regions of pH also showed different influences of temperature, ionic strength, and reductant concentration. The rate of Cr(VI) reduction is described by the general expression −d[Cr(VI)]/dt = k [Cr(VI)] [Fe(II)] where k (M−1 min−1) can be determined from the for the pH range 1.5–4.5 (σ = 0.2) and for the pH range 5–8.7 (σ = 0.2) from 5 to 40°C and 0.01 to 2 M ionic strength. The effect of pH, temperature, and ionic strength on the reaction indicates that the reactions at low pH are due to While the reactions at high pH are due to The overall rate expression over the entire pH range can be determined from (H2A = H2CrO4) where kH2A−Fe = 5 x 106, kHA−FeOH = 1 x 106, kHA−Fe (OH)2= 5 x 1011. In oxic aqueous systems Cr(VI) competes with O2 in the oxidation of Fe(II) and an extension of the rate law for Cr(VI) reduction with Fe(II) in oxygenated solutions is proposed. The application of this extended rate law to environmental conditions suggests that this reaction influences the distribution of oxidized and reduced species of chromium in oxic and anoxic waters.
We have used soft X-ray core-level photoemission and adsorption spectroscopies to study the reaction of aqueous sodium chromate solutions (50 μM Na2CrO4, pH 6 and 8.5) with clean surfaces of magnetite (111) prepared under UHV conditions. Chromium 2p photoemission and Cr L-edge absorption spectra indicate that tetrahedrally coordinated Cr(VI)aq, which reacts with magnetite (111), is reduced by a heterogeneous redox process to octahedrally coordinated Cr(III) and incorporated in an overlayer on the magnetite. The thickness of the reacted overlayer at pH 6 increases with increasing immersion time in the Na2CrO4 solution for up to ≈10 min but remains unchanged for longer immersion times. The reaction rate is initially fast and follows a logarithmic law. When the passivated magnetite can no longer reduce Cr(VI)aq, the passivating overlayer is 15±5 Å thick and consists of a chromium oxyhydroxide or hydroxide phase with only trace amounts of iron. Evidence for extensive hydroxylation in the overlayer is provided by a chemically shifted component in both the O 1s photoemission and O K-edge absorption spectra. The overlayer appears to lack long-range order based on loss of the first EXAFS-like feature in the O K-edge spectrum with increasing immersion time. Clear evidence for oxidation of Fe(II) to Fe(III) in the magnetite surface during reduction of Cr(VI)aq to Cr(III) is provided by Fe 2p photoemission and Fe L-edge absorption spectra. Strong attenuation of the Fe 2p signal during the first 10 min of the redox reaction indicates that iron does not outdiffuse significantly into the overlayer. At pH 8.5, the reaction follows a similar path, but its rate is lower, and Cr(VI)aq reduction continues for immersion times of up to 1 h. The results of this study are compared with results from an earlier XPS study of the passivation of zero-valent iron by chromate solutions [E. McCafferty, M.K. Bernett, J.S. Murday, Corros. Sci. 28 (1988) 559]. The resulting overlayer compositions, thicknesses, and reaction rates are very similar for the two systems.
For a better understanding of the atmospheric rusting of iron and steels, the present work is aimed to explore the mechanism of formation of green rusts, Fe3O4, α-FeOOH, β-FeOOH, γ-FeOOH, δ-FeOOH and amorphous ferric oxyhydroxide in aqueous solution at room temperature. The formation processes on which end products are determined are strongly affected by the oxidation rate, pH and the structure and composition of initial and intermediate species of iron. The systematic diagram of formation processes of iron oxide and oxyhydroxides has been presented, in which both dissolved and solid species of iron are included.
The available information regarding the pathways of processes leading to precipitation of iron (hydrous) oxides in aqueous salt solutions is reviewed. The importance of the early hydrolysis stages in determining the nature and the morphology of the solid phases is discussed. In the second part, the phase transformation between various oxides and oxohydroxides in aqueous suspensions is described with emphasis on mechanistic considerations. Such phase transformations may proceed under milder conditions (e.g., at lower temperatures) by a dissolution-recrystallization mechanism than in dry powders, which can also influence the morphology and the size of the resulting particles. A proper control of experimental parameters has resulted in a number of well defined colloidal iron (hydrous) oxides with respect to their composition, structure, morphology, and size.
A thin-film continuous flow-through reactor was used to investigate reactions between aqueous Cr(VI) and two iron oxides, geothite and magnetite. Delayed effluent breakthrough of Cr(VI) indicated significant uptake by both oxides. Accumulation and remobilization of Cr(VI) depends on pH and the redox properties of the surface. For geothite the surface was quickly saturated and no further adsorption observed. Chromate anion (CrO42−) exhibited Langmuir-type adsorption. For magnetite, a significant slow steady-state rate of Cr(VI) uptake was observed. We propose two different mechanisms of chromium uptake: surface complexation of Cr(VI) species on geothite, and reductive precipitation of Cr(VI) at Fe(II) sites on magnetite.
Radioactive core samples containing elevated concentrations of Cr from a high level nuclear waste plume in the Hanford vadose zone were studied to asses the future mobility of Cr. Cr(VI) is an important subsurface contaminant at the Hanford Site. The plume originated in 1969 by leakage of self-boiling supernate from a tank containing REDOX process waste. The supernate contained high concentrations of alkali (NaOH ≈ 5.25 mol/L), salt (NaNO3/NaNO2 >10 mol/L), aluminate [Al(OH)4− = 3.36 mol/L], Cr(VI) (0.413 mol/L), and 137Cs+ (6.51 × 10−5 mol/L). Water and acid extraction of the oxidized subsurface sediments indicated that a significant portion of the total Cr was associated with the solid phase. Mineralogic analyses, Cr valence speciation measurements by X-ray adsorption near edge structure (XANES) spectroscopy, and small column leaching studies were performed to identify the chemical retardation mechanism and leachability of Cr. While X-ray diffraction detected little mineralogic change to the sediments from waste reaction, scanning electron microscopy (SEM) showed that mineral particles within 5 m of the point of tank failure were coated with secondary, sodium aluminosilicate precipitates. The density of these precipitates decreased with distance from the source (e.g., beyond 10 m). The XANES and column studies demonstrated the reduction of 29–75% of the total Cr to insoluble Cr(III), and the apparent precipitation of up to 43% of the Cr(VI) as an unidentified, non-leachable phase. Both Cr(VI) reduction and Cr(VI) precipitation were greater in sediments closer to the leak source where significant mineral alteration was noted by SEM. These and other observations imply that basic mineral hydrolysis driven by large concentrations of OH− in the waste stream liberated Fe(II) from the otherwise oxidizing sediments that served as a reductant for CrO42−. The coarse-textured Hanford sediments contain silt-sized mineral phases (biotite, clinochlore, magnetite, and ilmenite) that are sources of Fe(II). Other dissolution products (e.g., Ba2+) or Al(OH)4− present in the waste stream may have induced Cr(VI) precipitation as pH moderated through mineral reaction. The results demonstrate that a minimum of 42% of the total Cr inventory in all of the samples was immobilized as Cr(III) and Cr(VI) precipitates that are unlikely to dissolve and migrate to groundwater under the low recharge conditions of the Hanford vadose zone.
Chromate adsorption on amorphous iron oxyhydroxide was investigated in dilute iron suspensions as a single solute and in solutions of increasing complexity containing CO2(g), SO4S (aq), H4SiO4(aq), and cations (K , MgS , CaS (aq)). In paired-solute systems (e.g., CrO4S -H2CO3*), anionic cosolutes markedly reduce CrO4S adsorption through a combination of competitive and electrostatic effects, but cations exert no appreciable influence. Additionally, H4SiO4 exhibits a strong time-dependent effect: CrO4S adsorption is greatly decreased with increasing H4SiO4 contact time. In multiple-ion mixtures, each anion added to the mixture decreases CrO4S adsorption further. Adsorption constants for the individual reactive solutes were used in the triple-layer model. The model calculations are in good agreement with the CrO4S adsorption data for paired- and multiple-solute systems. However, the model calculations underestimate CrO4S adsorption when surface site saturation is approached. Questions remain regarding the surface interactions of both CO2(aq) and H4SiO4. The results have major implications for the adsorption behavior of CrO4S and other oxyanions in subsurface waters.
The kinetics of Cr(VI) reduction to Cr(III) by carbonate green rust were studied for a range of reactant concentrations and pH values. Carbonate green rust, [FeII4FeIII2(OH)12][4H2O x CO3], was synthesized by induced hydrolysis (i.e., coprecipitation) of an Fe(ll)/Fe(III) solution held at a constant pH of 8. An average specific surface area of 47 +/- 7 m2 g(-1) was measured for five separate batches of freeze-dried green rust precipitate. Heterogeneous reduction by Fe(II) associated with the carbonate green rust appears to be the dominant pathway controlling Cr(VI) loss from solution. The apparent stoichiometry of the reaction between ferrous iron associated with green rust ([Fe(II)GR]) and Cr(VI) was slightly higherthan the expected 3:1 ratio, possibly due to the presence of other oxidants, such as oxygen, protons, or interlayer carbonate ions. The rate of Cr(VI) reduction was proportional to the green rust surface area concentration, and psuedo-first-order rate coefficients (kobs) ranging from 1.2 x 10(-3) to 11.2 x 10(-3) s(-1) were determined. The effect of pH was small with a 5-fold decrease in rate with increasing pH (from 5.0 to 9.0). At low Cr(VI) concentrations (<200 microM), the rate of reaction was first order with respect to Cr(VI) concentration, whereas, at high Cr(VI) concentrations, rates appearto deviate from first-order kinetics and approach a constant value. Estimated amounts of surface Fe(II) and total Fe(II) suggest that the deviation from first-order kinetics observed at higher Cr(VI) concentrations and the 50-fold decrease in rate observed upon three sequential exposures to Cr(VI) is due to exhaustion of available Fe(II).
The kinetics of nitrate, nitrite, and Cr(VI) reduction by three types of iron metal (Fe0) were studied in batch reactors for a range of Fe0 surface area concentrations and solution pH values (5.5-9.0). At pH 7.0, there was only a modest difference (2-4x) in first-order rate coefficients (k(obs)) for each contaminant among the three Fe0 types investigated (Fisher, Peerless, and Connelly). The k(obs) values at pH 7.0 for both nitrite and Cr(VI) reduction were first-order with respect to Fe0 surface area concentration, and average surface area normalized rate coefficients (kSA) of 9.0 x 10(-3) and 2.2 x 10(-1) L m(-2) h(-1) were determined for nitrite and Cr(VI), respectively. Unlike nitrite and Cr(VI), Fe0 surface area concentration had little effect on rates of nitrate reduction (with the exception of Connelly Fe0, which reduced nitrate at slower rates at higher Fe0 surface areas). The rates of nitrate, nitrite, and Cr(VI) reduction by Fisher Fe0 decreased with increasing pH with apparent reaction orders of 0.49 +/- 0.04 for nitrate, 0.61 +/- 0.02 for nitrite, and 0.72 +/- 0.07 for Cr(VI). Buffer type had minimal effects on reduction rates, indicating that pH was primarily responsible for the differences in rate. At high pH values, Cr(VI) reduction ceased after a short time period, and negligible nitrite reduction was observed over 48 h.
This study investigated Cr(VI) reduction by dissolved Fe(II) in hyperalkaline pH conditions as found in fluid wastes associated with the U.S. nuclear weapons program. The results show that Cr(VI) reduction by Fe(II) at alkaline pH solutions proceeds very quickly. The amount of Cr(VI) removed from solution and the amount reduced increases with Fe(II):Cr(VI) ratio. However, the Cr(VI) reduction under alkaline pH condition is nonstoichiometric, probably due to Fe(II) precipitation and mixed iron(III)-chromium-(III) (oxy)hydroxides blocking Fe(II) surface sites, as well as removing Fe(II) from solution through O2 oxidation. After Cr(VI) was reduced to Cr(III), it precipitated out as mixed Fe(x)Cr1-xO3(solids) and various Fe(III) precipitates with an overall Cr:Fe ratio of 1:3; all Cr remaining in the solution phase was unreduced Cr(VI). EXAFS data showed that Cr-O and Cr-Cr distances in the precipitates equal to 1.98 and 3.01 A, respectively, consistent with the spinel-type structure as chromite.
Waste tank summary report for month ending
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Rate and mechanism of the reaction of silicates with cement pore fluids
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