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Direct speciation of chromium in coal combustion by-products by X-ray absorption fine-structure spectroscopy
Abstract
Chromium X-ray absorption fine-structure (XAFS) spectroscopy has been used to determine the relative amounts of Cr(VI) and Cr(III) in coal combustion by-products, principally fly-ash and bottom ash. The method, which is based on the relative heights of the pre-edge peaks for the different Cr oxidation states in X-ray absorption near-edge structure (XANES) spectra, can be used to speciate as little as 50 ppm of chromium in ash. The results indicate that the fraction of Cr(VI) oxidation state present in combustion ash samples from most commercial combustion plants is typically below or close to the detection limit (approximately 3–5% of the total chromium). There are exceptions, however, that have been found to reach as high as 20%. Such findings provide justification for a reappraisal of whether or not chromium should be considered a significant hazardous air pollutant in coal combustion.
... It has been noted for fly ash derived from U.S. coals that the Cr(VI) content of Class C fly ash, which is generally derived from low-sulfur, lower-rank, Western U.S. coals, is often higher than that of Class F fly ash, usually derived from combustion of high-sulfur, higher-rank Eastern U.S. coals. Alkaline earth elements, which are more abundant in Class C fly ash, may play a role in stabilizing the Cr(VI) oxidation state due to formation of stable chromates such as CaCrO 4 or BaCrO 4 (Dindarloo and Hower, 2015;Shoji et al., 2002;Huggins et al., 1999). Alternatively, based on theoretical and experimental studies, the greater presence of SO x in the combustion environment for higher sulfur coals may serve to impede the oxidation of Cr(III) species (Linak et al., 1996). ...
... Cr(VI) percentages were quantified from the chromium XANES spectra by fitting the region between 5980 and 6000 eV to two Lorentzian-Gaussian peaks located at about 5991 and 5993 eV and an arctangent function representing the background chromium absorption. As established by a number of authors, the peak at 5993 eV derives primarily from Cr(VI) and its intensity can be used to determine the percentage of hexavalent chromium in solid materials (Huggins et al., 1999;Lytle et al., 1995;Bajt et al., 1993). Three factors in the current study greatly enhance the precision of the Cr(VI)/Cr(III) determination: (i) the six spectra were obtained from the same synchrotron beam-line under identical conditions (hence, the major uncertainty of comparing measurements from different synchrotron beam-lines was eliminated); (ii) the material was essentially the same fly-ash (i.e. ...
... The spectra show relatively similar features, with the exception of the pre-edge region highlighted in the insets to the figures. The original and leached amounts of Cr(VI) listed in Table 2 were estimated from the heights of the two pre-edge peaks using the method previously described specifically for coal-derived fly ash (Huggins et al., 1999). For the coarse sample (B2), about 4.6% (6 mg/kg) of the total chromium was present initially as Cr(VI), compared to about 8% (14 mg/kg) for the fine sample (B18). ...
... It has been noted for fly ash derived from U.S. coals that the Cr(VI) content of Class C fly ash, which is generally derived from low-sulfur, lower-rank, Western U.S. coals, is often higher than that of Class F fly ash, usually derived from combustion of high-sulfur, higher-rank Eastern U.S. coals. Alkaline earth elements, which are more abundant in Class C fly ash, may play a role in stabilizing the Cr(VI) oxidation state due to formation of stable chromates such as CaCrO 4 or BaCrO 4 (Dindarloo and Hower, 2015; Shoji et al., 2002; Huggins et al., 1999). Alternatively, based on theoretical and experimental studies, the greater presence of SO x in the combustion environment for higher sulfur coals may serve to impede the oxidation of Cr(III) species (Linak et al., 1996). ...
... Cr(VI) percentages were quantified from the chromium XANES spectra by fitting the region between 5980 and 6000 eV to two Lorentzian-Gaussian peaks located at about 5991 and 5993 eV and an arctangent function representing the background chromium absorption. As established by a number of authors, the peak at 5993 eV derives primarily from Cr(VI) and its intensity can be used to determine the percentage of hexavalent chromium in solid materials (Huggins et al., 1999; Lytle et al., 1995; Bajt et al., 1993 ). Three factors in the current study greatly enhance the precision of the Cr(VI)/Cr(III) determination: (i) the six spectra were obtained from the same synchrotron beam-line under identical conditions (hence, the major uncertainty of comparing measurements from different synchrotron beam-lines was eliminated); (ii) the material was essentially the same fly-ash (i.e. ...
... The spectra show relatively similar features, with the exception of the pre-edge region highlighted in the insets to the figures. The original and leached amounts of Cr(VI) listed in Table 2were estimated from the heights of the two pre-edge peaks using the method previously described specifically for coal-derived fly ash (Huggins et al., 1999). For the coarse sample (B2), about 4.6% (6 mg/kg) of the total chromium was present initially as Cr(VI), compared to about 8% (14 mg/kg) for the fine sample (B18). ...
... It has been noted for fly ash derived from U.S. coals that the Cr(VI) content of Class C fly ash, which is generally derived from low-sulfur, lower-rank, Western U.S. coals, is often higher than that of Class F fly ash, usually derived from combustion of high-sulfur, higher-rank Eastern U.S. coals. Alkaline earth elements, which are more abundant in Class C fly ash, may play a role in stabilizing the Cr(VI) oxidation state due to formation of stable chromates such as CaCrO 4 or BaCrO 4 (Dindarloo and Hower, 2015;Shoji et al., 2002;Huggins et al., 1999). Alternatively, based on theoretical and experimental studies, the greater presence of SO x in the combustion environment for higher sulfur coals may serve to impede the oxidation of Cr(III) species (Linak et al., 1996). ...
... Cr(VI) percentages were quantified from the chromium XANES spectra by fitting the region between 5980 and 6000 eV to two Lorentzian-Gaussian peaks located at about 5991 and 5993 eV and an arctangent function representing the background chromium absorption. As established by a number of authors, the peak at 5993 eV derives primarily from Cr(VI) and its intensity can be used to determine the percentage of hexavalent chromium in solid materials (Huggins et al., 1999;Lytle et al., 1995;Bajt et al., 1993). Three factors in the current study greatly enhance the precision of the Cr(VI)/Cr(III) determination: (i) the six spectra were obtained from the same synchrotron beam-line under identical conditions (hence, the major uncertainty of comparing measurements from different synchrotron beam-lines was eliminated); (ii) the material was essentially the same fly-ash (i.e. ...
... The spectra show relatively similar features, with the exception of the pre-edge region highlighted in the insets to the figures. The original and leached amounts of Cr(VI) listed in Table 2 were estimated from the heights of the two pre-edge peaks using the method previously described specifically for coal-derived fly ash (Huggins et al., 1999). For the coarse sample (B2), about 4.6% (6 mg/kg) of the total chromium was present initially as Cr(VI), compared to about 8% (14 mg/kg) for the fine sample (B18). ...
Coarse and fine samples of a Class F fly ash obtained from commercial combustion of Illinois bituminous coal have been exposed to two long-term leaching tests designed to simulate conditions in waste impoundments. ICP-AES analysis indicated that the coarse and fine fly ash samples contained 135 and 171 mg/kg Cr, respectively. Measurements by XAFS spectroscopy showed that the ash samples originally contained 5 and 8% of the chromium, respectively, in the hexavalent oxidation state, Cr(VI). After exposure to water for more than four months, the percentage of chromium as Cr(VI) in the fly-ash decreased significantly for the coarse and fine fly-ash in both tests. Combining the XAFS data with ICP-AES data on the concentration of chromium in the leachates indicated that, after the nineteen-week-long, more aggressive, kinetic test on the coarse fly ash, approximately 60% of the Cr(VI) had been leached, 20% had been reduced to Cr(III) and retained in the ash, and 20% remained as Cr(VI) in the ash. In contrast, during the six-month-long baseline test, very little Cr was actually leached from either the coarse or the fine fly-ash (<0.1 mg/kg); rather, about 66% and 20%, respectively, of the original Cr(VI) in the coarse and fine fly-ash was retained in the ash in that form, while the remainder, 34% and 80%, respectively, was reduced and retained in the ash as Cr(III). The results are interpreted as indicating that Cr(VI) present in Class F fly-ash can be reduced to Cr(III) when in contact with water and that such chemical
reduction can compete with physical removal of Cr(VI) from the ash by aqueous leaching.
... Peterson et al. [36,37] measured the Cr 3+ :Cr 6+ ratio in complex oxide samples using several methods and suggested that either peak height or peak area may be used to compute the Cr 6+ /total Cr content. Huggins et al. [35] reached similar model-fitting findings on Cr pre-edge information, valency, and symmetry (Pseudo-Voigt fitting). Although several methodologies have been introduced by many researchers that pre-edge features relate to dipolar and quadrupolar transitions from the metal 1s core state to metal 3d with some contribution from 4p depending on the local symmetry, their intensity and energy are likely dependent on the metal cation valency and site symmetry [13,32,33,35,[52][53][54][55][56][57]. ...
... Huggins et al. [35] reached similar model-fitting findings on Cr pre-edge information, valency, and symmetry (Pseudo-Voigt fitting). Although several methodologies have been introduced by many researchers that pre-edge features relate to dipolar and quadrupolar transitions from the metal 1s core state to metal 3d with some contribution from 4p depending on the local symmetry, their intensity and energy are likely dependent on the metal cation valency and site symmetry [13,32,33,35,[52][53][54][55][56][57]. Cr oxidation and site symmetry could simply be determined by Peterson's initial method since the characteristics of Cr 3+ and Cr 6+ are distinct. ...
Natural spinel (MgAl2O44) usually contains some minor and trace elements (e.g., Cr, Co, Fe, V) that may cause various hues. The ratios of these chromophores directly affect the color composition. The red color in spinel is attributed to the combination of significant Cr and V. Magenta and purple to blue and green colors in spinels are affected by the significant Fe concentration, whereas orange color in spinel shows the contribution of significant V content compared to Cr and Fe. After the heating experiment, advanced gemological investigation reveals some noteworthy characteristic features. X-ray absorption spectroscopy (XAS) indicates a greater change in oxidation state, as well as disordering of Fe and V. Broadening of the dominant peak at around 406 cm−1 with occurrences of additional small peaks at around 715–719 cm−1 in Raman spectra, as well as broadening of the 685 nm (R-line) and poorly defined structure of additional peaks (N-lines) in photoluminescence spectra should be significant indicators of spinel undergone heat treatment.
... In addition to quantitative analysis of Cr speciation, the pre-edge region of Cr K-edge XANES spectra was also used to determine the redox state of Cr, since tetrahedral Cr(VI) is characterized by a strong absorption peak at 5993 eV in the pre-edge region (Huggins et al., 1999), whereas octahedral Cr(III) yields only two weak peaks at 5990 and 5993 eV (Gaudry et al., 2007;Juhin et al., 2008). For that purpose, the preedge region (5985-6000 eV) of the Cr K-edge XANES spectra was extracted by fitting the background of the main edge to an arctangent function (Huggins et al., 1999). ...
... In addition to quantitative analysis of Cr speciation, the pre-edge region of Cr K-edge XANES spectra was also used to determine the redox state of Cr, since tetrahedral Cr(VI) is characterized by a strong absorption peak at 5993 eV in the pre-edge region (Huggins et al., 1999), whereas octahedral Cr(III) yields only two weak peaks at 5990 and 5993 eV (Gaudry et al., 2007;Juhin et al., 2008). For that purpose, the preedge region (5985-6000 eV) of the Cr K-edge XANES spectra was extracted by fitting the background of the main edge to an arctangent function (Huggins et al., 1999). The resulting data were then compared either to those from Cr VI O 2 /Cr III 2 O 3 synthetic mixtures with 0 or 5% Cr (VI) (see Fandeur et al., 2009a for further details on the preparation procedure of these synthetic samples) or from the model compounds used for the LC-LS fits of experimental Cr K-edge XANES data. ...
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.
... During coal combustion process, inorganic chromium primarily stays in ash and is difficult to release. On the contrary, organic chromium mostly combined with coal matrix is easily decomposed to flue gas [24,25]. Then, the enriched chromium in fly ash or gaseous chromium containing species in flue gas would flow through SCR catalyst. ...
... Simultaneously, transition of O 2 to surface O α also determined the proceeding of reaction (19) [33,34,39,45]. Additionally, apart from the weak disproportionation of NO via reactions (22)-(23)-(24) [67], oxidation of NO to NO 2 at high temperature as demonstrated in Fig. S5 (c) would facilitate the NH 3 -NSCR of NO 2 through Eley-Rideal mechanism between NO 2(g) and NH 2(ad) as reaction (25) [35,[68][69][70]. Moreover, the oxidation of NO to NO 2 also needed the participation of O 2 . ...
Poisoning effect of Cr in flue gas on the remarkable generation of greenhouse gas N2O over γ-Fe2O3 catalyst during selective catalytic reduction of NOx with ammonia was firstly observed in this work. At low temperature, N2O generation over Cr poisoned γ-Fe2O3 catalyst was attributed to non-selective catalytic reduction (NSCR) of NOx. At high temperature, NSCR of NOx and ammonia peroxidation accompanying by weak NO disproportionation induced a more generation of N2O. The seeming promotion effect of Cr on NOx conversion of γ-Fe2O3 catalyst at low temperature was partly resulted from the NSCR of NO which also contributed to NOx conversion. At high temperature, the reduction of NOx to innocent N2 was far inhibited after chromium poisoning in view of the low selectivity of NO to N2. The formed FeCr2O4 in chromium poisoned γ-Fe2O3 catalyst was the induction factor for large N2O production, which enhanced the acidity and reducibility of catalyst. Transient DRIFTS results indicated that SCR and NSCR of NOx over chromium poisoned γ-Fe2O3 catalyst predominantly happened via Eley-Rideal mechanism. Moreover, O2 in flue gas determined the activation of adsorbed NH3 and which dictated the NSCR of NOx and ammonia peroxidation.
... Target Transformation (TT) was then used to select the model compounds spectra that were further used in the linear combination least-square fitting (LC-LSF) of the sediment spectra. LC-LSF analysis was realized with the ATHENA software (Ravel and Newville, 2005) In addition to quantitative analysis of Cr speciation, the pre-edge region of Cr K-edge XANES spectra was also used to determine the redox state of Cr, since tetrahedral Cr(VI) is characterized by a strong absorption peak at 5993 eV in the pre-edge region (Huggins et al., 1999), whereas octahedral Cr(III) yields only two weak peaks at 5990 and 5993 eV (Gaudry et al., 2007;Juhin et al., 2008). For that purpose, the pre-edge region (5985-6000 eV) of the Cr K-edge XANES spectra was extracted by fitting the background of the main edge to an arctangent function (Huggins et al., 1999). ...
... LC-LSF analysis was realized with the ATHENA software (Ravel and Newville, 2005) In addition to quantitative analysis of Cr speciation, the pre-edge region of Cr K-edge XANES spectra was also used to determine the redox state of Cr, since tetrahedral Cr(VI) is characterized by a strong absorption peak at 5993 eV in the pre-edge region (Huggins et al., 1999), whereas octahedral Cr(III) yields only two weak peaks at 5990 and 5993 eV (Gaudry et al., 2007;Juhin et al., 2008). For that purpose, the pre-edge region (5985-6000 eV) of the Cr K-edge XANES spectra was extracted by fitting the background of the main edge to an arctangent function (Huggins et al., 1999). The resulting data were then compared either to those from Cr VI O2/Cr III 2O3 synthetic mixtures with 0 or 5% Cr(VI) (see Fandeur et al., 2009a for further details on the preparation procedure of these synthetic samples) or from the model compounds used for the LC-LS fits of experimental Cr K-edge XANES data. . ...
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.
... Chromium is more likely to occur primarily as Cr(III) in most bituminous coals (114,(116)(117)(118). There is limited presence of Cr(VI) (119). Researchers have found that Cr(VI) in coal is reduced in the flue gas by sulfur dioxide (SO 2 ), during the combustion process. ...
... Researchers have found that Cr(VI) in coal is reduced in the flue gas by sulfur dioxide (SO 2 ), during the combustion process. It is estimated that the fraction of Cr(VI) in fly ash is less than 5% (116,119). However, Huggins et al. reported that some samples of fly ash may contain up to 20% of Cr(VI) (119). ...
Throughout the world, coal is responsible for generating approximately 38% of power. Coal ash, a waste product, generated from the combustion of coal, consists of fly ash, bottom ash, boiler slag, and flue gas desulfurization material. Fly ash, which is the main component of coal ash, is composed of spherical particulate matter with diameters that range from 0.1 μm to >100 μm. Fly ash is predominately composed of silica, aluminum, iron, calcium, and oxygen, but the particles may also contain heavy metals such as arsenic and lead at trace levels. Most nations throughout the world do not consider fly ash a hazardous waste and therefore regulations on its disposal and storage are lacking. Fly ash that is not beneficially reused in products such as concrete is stored in landfills and surface impoundments. Fugitive dust emissions and leaching of metals into groundwater from landfills and surface impoundments may put people at risk for exposure. There are limited epidemiological studies regarding the health effects of fly ash exposure. In this article, the authors provide an overview of fly ash, its chemical composition, the regulations from nations generating the greatest amount of fly ash, and epidemiological evidence regarding the health impacts associated with exposure to fly ash.
... Cr leaching is usually reaching the lowest values at near neutral pH (in average 0.02 mg/kg) and showing a leachability plateau from pH 8 to 12 (typically around 5 mg/kg). Most of the data in the literature reports predominant where maximum leachability of chromium is stated to be lowest at normal environmental conditions (Huggins et al. 1999;Stam et al. 2011;Izquierdo and Querol 2012 The values in italics shows the highest amount of chromium extracted from different coal ash samples and bottom ash (Huggins et al. 1999;Stam et al. 2011). Narukawa et al. (2007 showed that only Cr(IV) is leached from coal fly ash at alkaline pH. ...
... Cr leaching is usually reaching the lowest values at near neutral pH (in average 0.02 mg/kg) and showing a leachability plateau from pH 8 to 12 (typically around 5 mg/kg). Most of the data in the literature reports predominant where maximum leachability of chromium is stated to be lowest at normal environmental conditions (Huggins et al. 1999;Stam et al. 2011;Izquierdo and Querol 2012 The values in italics shows the highest amount of chromium extracted from different coal ash samples and bottom ash (Huggins et al. 1999;Stam et al. 2011). Narukawa et al. (2007 showed that only Cr(IV) is leached from coal fly ash at alkaline pH. ...
... L'intensité normalisée du « prépic » est proche de 1, soit environ 20 fois plus que pour les « prépics » de Cr III (Villain et al. 2007). La présence de ce « prépic » a été beaucoup utilisée pour doser le Cr VI dans divers systèmes d'intérêt environnemental (Manceau et Charlet 1992 ; Bajt et al. 1993 ; Peterson et Gordon E. Brown 1997 ; Arcon et al. 1998 ; Huggins et al. 1999 ; Shaffer et al. 2001 ; Parsons et al. 2002 ; Pantelouris et al. 2004 ; Sawalha et al. 2005 ; Chaurand 2006), avec différentes méthodes (intensité du prépic ou aire). Toutefois, les spectres XANES de verres au Cr sont très rares (Choi et al. 2000). ...
... Le seuil se caractérise par l'absence de « ligne blanche ». La présence de ce « prépic » a été beaucoup utilisée pour doser le Cr VI dans divers systèmes d'intérêt environnemental (Manceau et Charlet 1992 ; Bajt et al. 1993 ; Peterson et Gordon E. Brown 1997 ; Arcon et al. 1998 ; Huggins et al. 1999 ; Shaffer et al. 2001 ; Parsons et al. 2002 ; Pantelouris et al. 2004 ; Sawalha et al. 2005 ; Chaurand 2006), avec différentes méthodes. Les informations sur le degré d'oxydation du chrome obtenues à partir des prépics de XANES sont de plusieurs ordres, puisque tant l'intensité que l'aire et la position des structures du préseuil dépendent du degré d'oxydation. ...
The aim of the thesis is to study the structural environment of chromium in glasses, mainly by X-Ray absorption spectroscopy and optical absorption spectroscopy (NIR-visible-UV). An innovative method has been developped for the determination of the relative proportion of CrIII and CrVI by XANES. The major part of the results concerns the evolution of the local structure around CrIII. EXAFS shows no significant modification of both the coordinence and the Cr-O distance with respect to glass composition. In the same time, optical absorption properties (crystal field splitting, Racah parameter, bandwidth and band intensity, additional interference structures) depend strongly on the nature of network formers and other cations, thus pointing out a modification of Cr site distribution, of Cr site distorsion and of the covalency of the Cr-O bond. In silicate glasses, optical parameters are sensitive to the nature of alkaline cations (and secondarily to alkaline earth cations). This preference of chromium for alkaline cations can be explained by a bond valence model and is indicative of a non random environment of chromium within glass structure. Consequently, chromium cannot constitute a suitable probe of the structure of silicate glasses. In borate glasses, optical absorption properties are sensitive to Na/B ratio and are therfore related to the change of boron coordination (3B/4B). In borosilicate glasses, optical properties depend on both Na/B and Si/B ratios. The use of optical absorption spectroscopy both at high and low temperatures shows that the thermal expansion of Cr site does not follow macroscopic dilatation measured on the same samples.
... The chemical ion exchange mechanism was also attributed to the sorption of Cr(VI) by carbon nanotubes [33]. The region above the edge generally indicates multiple scattering diffusion, and the white line describes the transitions to continuum or delocalized states, as reported by Berry et al [52]. ...
Single-walled carbon nanotubes (SWCNTs) were used for the sorption of monovalent K(I), divalent Pb(II), Zn(II) and Cd(II) in addition to hexavalent Cr(VI) metal elements from aqueous solutions. X-ray absorption spectroscopy (XAS) was employed for the preliminary characterization of the metals sorbed SWCNTs materials. The adsorption capacities of the investigated metals are associated with the functional groups involved in the sorption mechanism. The sorption and XAS results revealed that hexavalent Cr(VI) was highly adsorbed by SWCNTs. The sorption mechanism of Cr(VI) is associated with the reduction of Cr(VI) to trivalent Cr(III). XAS also suggested that in the environment nearest to the sorbed metals Cr, Pb, Zn and K, the existing binding interactions are Cr-O, Pb-O, Zn-O and K-O respectively.
... When Cr (III) and Cr(VI) are both present in a sample, the height of the pre-edge peak is diminished compared to pure Cr (VI). The Cr(VI)/Cr total ratio is linearly related to the height or area of the normalized pre-edge peak, which can be used to determine the relative percentage of Cr(VI) in samples with an uncertainty of $5% (Peterson et al., 1997;Huggins et al., 1999;Farges, 2009). ...
Chromium (Cr) isotope compositions of marine carbonates have widely been used to trace the evolution of oxygen levels in the atmosphere and ocean during geological history. Recent studies have indicated that Cr found in natural carbonates is dominated by trivalent Cr, also known as Cr(III). However, the incorporation behavior of Cr(III) into carbonates remains undefined. Here, we conducted coprecipitation experiments for Cr(III) with calcium carbonate to constrain the behavior of Cr(III) in carbonate deposition environments. Extended X-ray absorption fine structure (EXAFS) results suggest that Cr(III) is incompatible with the calcite crystal lattice. There are likely to be several Cr species in the coprecipitation samples. Most Cr is adsorbed on the crystal surface of calcite or exists as amorphous Cr hydroxide [Cr(OH)3]. A small fraction of Cr can reside in the calcite crystal, but Cr³⁺ cannot directly substitute for Ca²⁺. It may exist in interstitial voids in crystals or occupy the position of Ca²⁺ in the form of divalent Cr hydroxide cation [Cr(OH)²⁺], leading to a distorted structure. Moreover, we find that Cr(III) can stimulate polymorph selection of vaterite [a metastable polymorph of calcium carbonate (CaCO3)] during CaCO3 precipitation, likely due to the formation of Cr hydroxide on the surface of vaterite crystals, hindering its transformation to calcite. Because Cr(III) is incompatible with the carbonate crystal lattice, we suggest that most Cr in natural carbonates is adsorbed on the crystal surface and may be acquired at water-sediment interfaces after carbonate precipitation. Therefore, Cr isotope compositions of carbonates may easily be affected by post-depositional diagenetic processes, and Cr isotope variations in sedimentary carbonates should not exclusively correspond to changes in oxygen levels in the atmosphere and ocean. Paleoenvironment reconstruction based on Cr isotope compositions of carbonates should take post-depositional processes into consideration.
... XANES analysis revealed that these features comprised two distinct components, with maxima at ∼5990 eV and ∼5993 eV. The normalised intensity of the peak at ∼5993 eV is known to be an excellent indicator of the presence of chromium (VI) (e.g., Peterson et al., 1997;Huggins et al., 1999). The relative intensities of these two peaks vary, to a small degree, depending on the deportment of chromium (III) between chromite, goethite, and haematite, with the latter two producing pre-edge features comprising two peaks of equal intensity (Fandeur et al., 2009). ...
Limonitic layers of the regolith, which are often stockpiled as waste materials at laterite mines, commonly contain significant concentrations of valuable base metals, such as nickel, cobalt, and manganese. There is currently considerable demand for these transition metals, and this is projected to continue to increase (alongside their commodity values) during the next few decades, due in the most part to their use in battery and renewable technologies. Limonite bioprocessing is an emerging technology that often uses acidophilic prokaryotes to catalyse the oxidation of zero-valent sulphur coupled to the reduction of Fe (III) and Mn (IV) minerals, resulting in the release of target metals. Chromium-bearing minerals, such as chromite, where the metal is present as Cr (III), are widespread in laterite deposits. However, there are also reports that the more oxidised and more biotoxic form of this metal [Cr (VI)] may be present in some limonites, formed by the oxidation of Cr (III) by manganese (IV) oxides. Bioleaching experiments carried out in laboratory-scale reactors using limonites from a laterite mine in New Caledonia found that solid densities of ∼10% w/v resulted in complete inhibition of iron reduction by acidophiles, which is a critical reaction in the reductive dissolution process. Further investigations found this to be due to the release of Cr (VI) in the acidic liquors. X-ray absorption near edge structure (XANES) spectroscopy analysis of the limonites used found that between 3.1 and 8.0% of the total chromium in the three limonite samples used in experiments was present in the raw materials as Cr (VI). Microbial inhibition due to Cr (VI) could be eliminated either by adding limonite incrementally or by the addition of ferrous iron, which reduces Cr (VI) to less toxic Cr (III), resulting in rates of extraction of cobalt (the main target metal in the experiments) of >90%.
... Typical applications of XANES include ''fingerprint analysis" and the determination of the oxidation state of the analyte. Fingerprinting essentially means that one measures the XANES of known reference samples and subsequently compares the shape and structure of the spectrum with the XANES of an unknown system [142]. Using a more advanced analytic approach one can also perform a fit with a linear-combination of known references to derive the relative contributions in mixed systems. ...
X-ray spectroscopy is an important tool for scientific analysis. While the earliest demonstration experiments were realised in the laboratory, with the advent of synchrotron light sources most of the experiments shifted to large scale synchrotron facilities. In the recent past there is an increased interest to perform X-ray experiments also with in-house laboratory sources, to simplify access to X-ray absorption and X-ray emission spectroscopy, in particular for routine measurements. Here we summarise the recent developments and comment on the most representative example experiments in the field of in-house laboratory X-ray spectroscopy. We first give an introduction and some historic background on X-ray spectroscopy. This is followed by an overview of the detection techniques used for X-ray absorption and X-ray emission measurements. A short paragraph also puts related high energy resolution and resonant techniques into context, though they are not yet feasible in the laboratory. At the end of this section the opportunities using wavelength dispersive X-ray spectroscopy in the laboratory are discussed. Then we summarise the relevant details of the recent experimental laboratory setups split into two separate sections, one for the recent von Hamos setups, and one for the recent Johann/Johansson type setups. Following that, focussing on chemistry and catalysis, we then summarise some of the notable X-ray absorption and X-ray emission experiments and the results accomplished with in-house setups. In a third part we then discuss some applications of laboratory X-ray spectroscopy with a particular focus on chemistry and catalysis.
... The spectra in Fig. S5 show that the pre-edge absorption peak of Cr(VI) at 5993 eV was indiscernible for all samples, suggesting that Cr speciation on the solid portions of all heat-treated soils was dominated by Cr(III). Given that XANES analyses could only accurately determine the Cr species that accounted for more than 3% of the total Cr (Hsu et al., 2015;Huggins et al., 1999), the speciation for sorbed Cr derived from XANES analyses was in line with that obtained from the chemical analyses (Fig. 3a). ...
... Its leachability does not obviously change with pH in leachates of the NN and WX fly ashes, but follows an amphoteric trend for the SD, LBA, and LBB fly ashes (Fig. 4). Both Cr (III) and Cr (VI) occur in coal combustion products with the former being dominant in fly ashes (Huggins et al., 1999;Rivera et al., 2017). Trivalent Cr tends to be amphoteric (concentrations increasing towards the acid and alkaline ends of the studied pH range) and this amphoteric trend exists in the leaching behavior of the SD, LBA, and LBB fly ashes. ...
The leaching behavior of trace elements in five fly ashes from five Chinese coal-fired power plants were obtained by conducting the new US EPA leaching test methods (LEAF) and the conventional EPA Toxicity Characteristic Leaching Procedure (TCLP) test method. These were done under a range of pH values and liquid to solid ratios. Despite different elemental and mineralogical compositions of the ashes and different ranks of the respective feed coals, the majority of elements show similar liquid/solid partitioning (LSP) curve trends among the different ash samples in the pH-dependent leaching tests. This indicates that most elements are sensitive to changes in pH. Different LSP curves exist for Cr from pH-dependent tests, probably due to different valence states of this element in the unleached fly ashes. Although most elements show similar LSP curves for different fly ash samples from the liquid/solid (L/S) dependent tests, variation exists for LSP curves of some individual elements among different ash samples. Caution should be taken when generating general L/S dependent leaching characteristics for individual elements. The leachate concentrations of the highly soluble elements such as B and Se, as well as oxyanions under alkaline conditions, are dependent on their initial concentrations in the fly ashes. The conventional EPA method TCLP overestimates the leachability of most hazardous elements, such as Be, B, Co, Ni, Cu, Zn, Cd, Sb, and U, at least in the studied alkaline fly ashes. LEAF tests have the advantage of being a comprehensive evaluation of the leaching characteristics, with a range of pH and L/S allowing for the examination of leachability under various conditions at different disposal or beneficial use scenarios.
... As reported in Huggins and Huffman (2004), the hexavalent Cr(VI), including chromates and dichromates, is highly soluble in water, whereas trivalent Cr(III) is less soluble. In most bituminous coals, chromium appears to be Cr(III) in organic association and inorganic minerals such as oxides, carbonates, monosulfides, and silicates (Dale et al., 1999;Huggins et al., 1999;Huggins and Huffman, 2004;Goodarzi et al., 2008;. ...
The modes of occurrence of trace elements in coal reveal essential information about environmental impact during coal exploitation and utilization. Utilizing improvements on a recently published sequential leaching protocol, this study investigated the modes of occurrence of Cr, Co, Ni, Cu, Cd, and Pb (water-soluble, ion-exchangeable, carbonate-bound, sulfide-bound, organic-bound, and silicate-bound) in coal samples collected from the Nantong coalfield in southwestern China. Results suggest the improved method favorably affected the sequential leaching experiment. The ashing temperature did not exceed 390°C and did not damage the structure of the silicate minerals, facilitating the effective extraction of trace elements within the organic matter. The case study in the Nantong coalfield demonstrated that the water-soluble and ion-exchangeable fractions account for only a small portion of the six elements. Specifically, chromium exhibited affinities with carbonates and silicates. The percentages of different target fractions for cobalt varied significantly in different samples. Nickel was almost evenly distributed in the fractions of carbonates, sulfides, organic matter, and silicates. For cadmium, silicate was the main fraction, followed by sulfides. Lead predominantly appeared in carbonates and sulfides. The correlativity of different elements in the corresponding fractions revealed carbonates and sulfides as the primary carriers for all six elements, followed by silicates and organic matter. Overall, Cr, Co, Ni, and Cu exhibited very close relationships and similar distribution patterns in different coal samples. Organic-bound Cr, Co, and Ni were noticeable in the No. 4 and 6 coal seams, though they were not the dominant fractions. The affinity between Pb and Cd indicated they have similar modes of occurrence.
... The absorbing chromium atom of the chromite reference is trivalent occupying octahedral sites (Peterson et al., 1996;Huggins et al., 1999;Eeckhout et al., 2007;Farges, 2009) and this is confirmed by the occurrence of a weak pre-edge peak appearing at ca. 5990.3 eV in the preedge region of the normalized Cr K-edge XANES spectrum. By contrast, the hexavalent Cr-containing synthetic compound (K 2 Cr 2 O 7 ) shows a distinct intense pre-edge peak at 5993.2 eV prior to the main Cr absorption edge. ...
Chromium-bearing clays occurring at Crommyonia (Soussaki), Greece, are studied in detail. In this area Mesozoic ultramafic ophiolitic rocks were affected by a Pliocene volcanic activity continuing, nowadays, with intense post-volcanic phenomena. Particularly, the Cr-bearing clays were formed at the margins of the ophiolite into highly altered serpentinized peridotite. These clays were studied by means of x-ray diffraction (powder-XRD), microscopic (SEM-EDS), nanoscopic (TEM-EDS/STEM-HAADF), and spectroscopic (FTIR, XANES) techniques. The XRD and SEM-EDS studies confirmed smectites and halloysites, with average Cr2O3 content ca 6.2 wt.% and 4.3 wt.%, respectively. The mineralogy and mineral-chemistry of the above unique Cr-clays was also confirmed by TEM-EDS/STEM-HAADF measurements. Subsequent FT-IR investigation indicated the relation of chromium ions with structural hydroxyls whereas a XANES study showed the presence of only Cr(III) in both phases and the absence of Cr(VI). According to the above data, Cr(III) ions should occupy octahedral sites in the structure of the studied clays. Geochemical data indicated high enrichment in Cr, and also in As, Se, Ni, V, Sc, Tl, when compared to Upper Continental Crust (UCC), and in Cr when compared to average global peridotite and primitive mantle. The formation of Cr-bearing clays is attributed to additional alteration of serpentinized peridotite minerals (such as Cr-spinels), due to hydrothermal fluids leaching different elements from the volcanic, sedimentary, and ophiolitic rocks of the area.
... More than 20 % total Cr in MSWI-FA was removed. Certain fly ash samples exhibit up to 20 % Cr as Cr 6? (Huggins et al. 1999). It was obvious that the pH of influent solution did not significantly change the leachability of Cr in MSWI-FA. ...
To determine the potential release of heavy metals from municipal solid waste incineration fly ash (MSWI-FA), the leaching behaviors of MSWI-FA from Likeng waste incineration power plant in China were characterized according to the US EPA method 1314. Heavy metals (i.e. Pb, Cr, Cd, Cu and Zn) leaching characteristics were investigated over a wide range of liquid-to-solid ratios (L/S 0.2–10 mL/g-dry) and different pH conditions (2, 4 and 6). The test results revealed that eluent with high pH of about 12 increased a little with increase of influent solutions pH. Similar concentration patterns of a heavy metal were observed at different pH influent solutions, and different concentration patterns for different heavy metals were showed at different liquid-to-solid ratio (L/S) range. Cumulative maximum release of heavy metals followed the pattern of Cr > Pb > Cu > Cd > Zn. The release of Pb and Cd decreased with pH of influent solutions while Cu and Zn changed oppositely. However, concentration of Cr did not significantly change and cumulative release linearly increased with L/S due to the leaching process was controlled by solubility. The effect of L/S ratio on Pb, Cd and Cu release in MSWI-FA indicated the prevalence of the dominant mechanism was either solubility or diffusion controlled. Zn extract concentrations were observed to decrease initially with L/S increase to 5 followed by a slight increase due to the presence of soluble zinc compounds in strong basic conditions.
... Consideration of the environmental samples finds that the aquatic grain XANES spectra display similar profiles. The lack of an intense peak in the pre-edge region coupled with two small pre-edge resonances indicates that Cr in such samples is present mainly in the octahedrally coordinated trivalent form, and that the contribution of tetrahedrally coordinated Cr(VI) is estimated to be ≤5 % (Huggins et al. 1999). Comparison of the pre-edge and edge position energies of the two aquatic samples spectra show comparability to the XANES spectra of the Cr(III) model compounds. ...
Purpose
Chromium, a potentially harmful element, occurs commonly within the urban sediment cascade as a result of abundant industrial and transport-related sources. The risks that Cr-bearing particles pose to ecosystems and humans depend on the solid-phase chemical speciation of Cr and its environmental mobility. In this study, we adopt an integrated geochemical approach to investigate and determine the long-term fate of Cr in the urban sediment cascade.
Materials and methods
We use bulk chemical digests, sequential chemical extraction analysis, electron microscopy, electron microprobe and microfocus XANES analysis to describe the solid-phase speciation, geochemical characteristics and potential long-term behaviour of Cr in urban particulate matter from both aquatic sediment and road dust sediment (RDS) in Manchester, UK.
Results and discussion
Cr-bearing grains within RDS and aquatic sediment are predominantly iron oxides and alumino-silicate glass grains. Electron microprobe analysis indicates Cr concentrations up to 3300 and 133,400 μg g⁻¹ in the RDS and aquatic grains, respectively. XANES analysis indicates that Cr(III) is the dominant oxidation state, with only trace amounts of Cr(VI). Importantly, Cr speciation does not appear to have changed between sedimentary environments and the dominance of Cr(III) suggests limited bioavailability or toxicity under predominant environmental (anoxic and neutral pH) conditions in the aquatic sediment sink. Furthermore, geochemical analyses suggest the environmental mobility of Cr in the aquatic sediment sink is low (compared to other toxic metals) due to its association mainly with alumino-silicate glass grains and its inclusion as an integral part of the glass structure.
Conclusions
Industrial glass grains are a major component of urban sediment worldwide. The speciation and geochemical investigations performed in this study suggest most Cr within the urban sediment cascade may be resistant to environmental processes that could mobilise other toxic metals.
... As a hazardous metal that is subject to potential bioaccumulation, chromium (Cr) can enter the body, including the digestive tract, respiratory tract, and the skin (Agree and Toffoli, 2009;Huggins et al., 1999). Its hexavalent state, Cr(VI), is highly water-soluble and toxic, and can accumulate in the body and damage the liver, kidney, and other organs. ...
... Energy calibration was executed via an in-line pure Cr 0 standard, with its first inflection point being set to 5989 eV. The fitting method and parameters used to quantify the proportion of hazardous Cr 6+ is described in detail by Low et al. 8 This method is in accordance with that of Huggins et al., 6 which quantifies the relative abundance of Cr species based on the relative heights of Cr preedge peaks. ...
We use in-situ high-temperature X-ray diffraction (HT-XRD), ex-situ XRD and synchrotron X-ray absorption near edge structure spectroscopy (XANES) to derive fundamental insights into mechanisms of chromium oxidation during combustion of solid fuels. To mimic the real combustion environment, mixtures of pure eskolaite (Cr3+2O3), lime (CaO) and/or kaolinite [Al2Si2O5(OH)4] have been annealed at 600-1200ºC in air versus 1% O2 diluted by N2. Our results confirm for the first time that (1) the optimum temperature for Cr6+ formation is 800ºC for the co-existence of lime and eskolaite; (2) upon addition of kaolinite into oxide mixture, the temperature required to produce chromatite shifts to 1000ºC with a remarkable reduction in the fraction of Cr6+. Beyond 1000ºC, transient phases are formed that bear Cr in intermediate valence states, which convert to different species other than Cr6+ in the cooling stage; (3) of significance to Cr mobility from the waste products generated by combustion, chromatite formed at >1000ºC has a glassy disposition that prevents its water-based leaching; and 4) Increasing temperature facilitates the migration of eskolaite particles into bulk lime and enhances the extent to which Cr3+ is oxidized, thereby completing the oxidation of Cr3+ to Cr6+ within 10 min. .
In the context of global warming, wildfires are expected to increase in both frequency and intensity in the forthcoming decades. Among the environmental and ecological wildfires-induced impacts, the risk of freshwater pollution by soilborne trace metals deserves a more extensive and accurate assessment because of its potential threat to human health. This study aims to contribute to this evaluation by investigating the influence of laboratory soil heating on chromium solid speciation (including redox state) and mobility in Ferralsols, Cambisols, Vertisols and Regosols holding varying amounts of this trace metal in New Caledonia (South Pacific). A first result is that soil heating from 400 °C induces a partial Cr(III) oxidation to Cr(VI) in all the studied soils, confirming recent findings and extending them to a wider range of soil types. A second result is that the rate of heat-induced Cr(III) oxidation to Cr(VI) appears to depend on chromium speciation. This latter parameter might thus represent the primary driver of chromium reactivity in burned soils. Finally, a third result is the confirmation that most of the Cr(VI) formed in the heated soils is highly mobile. Heat-induced Cr(III) oxidation to Cr(VI) in burned soils thus represents a significant risk towards freshwater quality. At the local scale, this risk might concern a large range of drinking water catchments since the soils investigated in this study encompass a wide portion of the pedological diversity in New Caledonia. At a broader scale, considering the wide occurrence of Cr-bearing pedological settings worldwide, the potential threats to freshwater systems resulting from the occurrence of highly toxic Cr(VI) in burned soils should be considered as a global emerging risk towards water quality that requires further assessment.
X-ray absorption spectroscopy (XAS) was used to characterize the volcanic ash (VA) materials as a low-cost and natural environmental adsorbent for the sorption of Cr(VI). The batch adsorption method was employed to synthesize the sample of chromium(VI) sorbed by VA. The results showed that the Cr-VA material is in the chromium oxidation state(III) and can be described as a composition of Cr2O3 (47.9%), FeCr2O7 (45.2%), and Na2Cr2O7 (6.9%). The promotion of the 1s Cr core electron implies the electronic transitions 1𝑠→3𝑑(𝑡2𝑔) and 1𝑠→3𝑑(𝑒𝑔). The first and second shells in the Cr-VA material were coordinated to the oxygen and chromium atoms in a distorted geometrical arrangement. The adsorption mechanism was associated with the chemical reduction of Cr(VI) to Cr(III) via an ion-exchange process. The XAS technique significantly contributes to the characterization of environmental VA, which can be proposed as an economical and alternative adsorbent for the sorption of Cr from contaminated wastewater.
Traditional wet and dry Chromite Ore Processing Residue (COPR) detoxification methods consume acid for pH adjustment and produce greenhouse gasses (CO2), yielding potential secondary pollution. It is imperative to develop an environmentally-friendly, efficient, and effective method for COPR treatment. This study reported the one-step natural pyrite-assisted mechanochemical detoxification of Cr(VI) in COPR. Under optimal conditions (i.e., the pyrite/COPR mass ratio of 5% and mechanochemical treatment at 600 rev./min for 2.5 h), Cr(VI) in COPR was efficiently reduced and immobilized, and the leaching concentration of total Cr met the regulatory limit of 5 mg/L. The X-ray absorption near-edge structure (XANES) analysis further verified that the Cr(VI) in treated COPR was 100% reduced to Cr(III). Potential mechanism could be that the enclosed non-exchangeable Cr(VI) in the chromite matrix was exposed to the COPR particle surface under the mechanical action; then, the redox reaction between pyrite and Cr(VI) were induced and enhanced through accumulated crystal defects and generated new surfaces with dangling bonds under mechanical forces. Life cycle assessment (LCA) results highlight that the high energy input is the major contributor eliciting environmental impacts for this current technology. More efficient and cleaner energy supplies should be sought alternatively to overcome the overall sustainability.
The sluggish kinetics associated with the oxygen evolution reaction (OER) limits the sustainability of fuel production and chemical synthesis. Developing catalysts based on Earth abundant elements with a reasonable strategy could solve the challenge. Here, we present a heterostructure built from CrOx and CuS whose interface gives rise to the advent of new functionalities in catalytic activity. Using X-ray photoelectron and absorption spectroscopies, we identified the multiple oxidation states and low coordination number of Cr metal in CrOx-CuS heterostructure. Benefitting from these features, CrOx-CuS generates oxygen gas through water splitting with a low over potential of 190 mV vs RHE at a current density of 10 mA cm⁻². The catalyst shows no evident deactivation after a 36-hours operation in alkaline medium. The high catalytic activity, inspired by first principles calculations, and long-time durability make it one of the most effective OER electrocatalysts.
Coal combustion mainly releases chromium (Cr) as particulates and may oxidize Cr into toxic hexavalent forms. A series of combustion experiments combined with thermodynamic equilibrium calculations were used to study the speciation transformation behavior of Cr during the burning of a typical bituminous coal. Iron-bearing minerals and the combustion temperature were found to be vital for enriching Cr in ash and preventing Cr oxidation at high-temperature combustion process. During coal combustion, an increasing release trend of Cr versus the combustion temperature was observed. However, the degree of Cr oxidation didn't increase with the increased temperature. The fraction of Cr(VI) reached a maximum at 800°C, but gradually decreased as the combustion temperature rose to 1000°C. Further speciation study revealed that Cr was mostly presented in illite and pyrite in coal, which can be transformed into aluminosilicate glass and hematite. The ion-exchangeable, carbonate, and organic-bound Cr were easily released and also partly converted into less soluble residual form. At high combustion temperature (> 800°C), Fe2O3 exhibited a significant effect on Cr transformation and diminished the Cr(VI) ratios in coal ash. This was beneficial to the ash disposal or reutilization since Cr was retained as stable Cr (III) compounds in ash.
Arsenic (As), selenium (Se), and chromium (Cr) are harmful to humans at certain concentrations, and can possibly be eluted from coal ash (fly ash) discharged from coal-fired power plants. Insolubilization can be employed to treat As, Se, and Cr eluted from fly ash. Arsenic, selenium, and chromium are known to have different environmental toxicities and susceptibilities to insolubilization depending on their chemical forms. Therefore, the speciation analysis of these elements eluted from fly ash is crucial from the viewpoint of environmental science and waste management. Thus, As, Se, and Cr in different fly ash samples were analyzed using X-ray absorption near-edge structure (XANES) spectroscopy and liquid chromatography combined with inductively coupled plasma mass spectrometry (LC–ICP-MS). Compared with As(III) and Se(VI), As and Se were eluted in the form of As(V) and Se(IV), which could be readily insolubilized, and Cr was eluted in the form of carcinogenic Cr(VI). The results confirmed that As, Se, and Cr can be eluted, and insolubilization should be recommended as a treatment method for these elements that elute from fly ash.
Metal-bearing brown coal and coal ash can have a practical importance as the alternate sources of metals. Brown coal of Sutarsk and Ushumunsk deposits, as well as other coal deposits of the Far East, contain rare and rare-earth, precious and ferrous metals. At a comprehensive approach they can be sequentially extracted from ashes of coals in a production string. The possibility of metal extraction heavily depends on mode of their occurrence and grain size. In coals and coal ash of the researched deposits metals are localized in various mineral phases: oxides, sulfides, carbonates, phosphates, in a native mode of occurrence and as a part of intermetallic compounds. Particles are usually fine (1-30 microns). Gold, silver, copper, bismuth, iron, titanium, tungsten, zinc, barium, alloy of rare earths (cerium - lanthanum - neodymium) are found in native mode of occurrence. The interesting fact is that mode of occurrence of metals typical both for the reduction and oxidizing conditions are observed in one sample. It is necessary to account for the processing and extraction of metals from coal ashes that the most mineral particles are small and characterized by various morphology and composition.
Quantum chemistry combined with kinetic simulation and drop tube furnace (DTF) experiments were conducted to reveal the transformation behavior of chromium at the presence of steam and HCl under oxy-coal combustion. A completed kinetic system Cr-O-H-Cl containing 107 elementary reactions was firstly proposed. The unknown microcosmic reaction paths and corresponding Arrhenius parameters were calculated via quantum chemistry. Kinetic simulations on the basis of Cr-O-H-Cl system clarified that HCl promoted the oxidation of chromium to hexavalent CrO2Cl2. Coexistence of HCl and steam divided the transition of chromium into two stages. At the early stage, reaction rate between chromium and steam was faster than chromium with HCl, chromium mainly transformed to CrO(OH)2. Hereafter, HCl was dominant in the transformation of chromium, then chromium mainly presented as CrO2Cl2. Moreover, DTF experimental results indicated that introduction of HCl into combustion atmosphere induced more chromium release. Presence of steam reinforced the effect of HCl on chromium attributing to the significant transition of CrOx(OH)y to CrO2Cl2. Although CaO and Fe2O3 both exhibited good reactivity with chromium, presence of HCl largely suppressed chromium capture by CaO and Fe2O3. Moreover, the inhibition effect of HCl on Fe2O3 was stronger than CaO for Cr capture.
Coal fly ash (CFA) has gained increasing societal concerns as hazardous trace elements (HTEs) in CFA might pose environmental and public health risks. To develop better management and remediation strategies, it is highly important to obtain an in-depth understanding of the occurrence and mobility of HTEs in CFA. This study systematically characterized the speciation, distribution, and mobility of Cr, Ni, and Cu in class F and class C CFA samples using scanning electron microscopy with energydispersive X-ray spectroscopy (SEM−EDX), X-ray absorption near-edge spectroscopy (XANES), and sequential chemical extraction. SEM−EDX results show that Cr, Ni, and Cu are present in a range of 2−15 μm sized particles either as discrete particles or encapsulated in the glass phase. XANES results suggest that Cr is mainly present in Fe oxides or silicates and spinel minerals; Ni mainly in Fe oxides and as NiO; and Cu mainly as Cu2O/CuO or in Fe oxides. Cr(III) and Cu(I) are the dominant oxidation states for Cr and Cu, respectively. Sequential extraction generally shows higher mobility of trace elements (Cr, Mn, Co, Ni, Cu, Zn, Cd, and Pb) in class C CFA, as compared to the mobility of those in class F CFA, implying potentially higher environmental risks for class C CFA.
Due to the unfavorable combination of its toxicity and high volatility, Cd is contained in most lists of potentially hazardous air pollutants with the greatest environmental and human-health concerns. The review paper evaluates the behavior of Cd during combustion (incineration) processes and its redistribution among condensed fractions (bottom ash/slag, fly ash) and volatilized fractions (that passes through most particulate control devices). The paper addresses all important effects of Cd interactions, such as presence of organic or inorganic chlorides, moisture levels, S, P and Na concentrations, flue gas composition etc. Possibilities of using various adsorbents (either within in-furnace regime or applied in post-combustion zone) are evaluated as well. Special attention is paid to mitigating its emissions factors; decreasing Cd volatility and facilitating Cd retention are discussed with the view of various combustion (incineration) conditions and the feed fuel composition.
Chromium is commonly found in the flue gases and ashes of Municipal Solid Waste Incineration. It has been reported as an active catalyst for the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/F) during de novo tests, yet its specific mode of action has remained unclear. This study aims to identify the effects of chromium chloride on the formation of PCDD/F and other chloro-aromatics and to elucidate the underlying reaction mechanisms. A series of de novo tests, conducted over a wide range of temperature (from 250 to 550 °C) and for four different oxygen contents (0, 5, 10, 20 %), confirmed the promoting effect of CrCl3 on the PCDD/F formation. In situ X-ray Absorption Fine Structure (XAFS) spectroscopy was applied to investigate the behavior of CrCl3 during heating, describing the entire picture of CrCl3-promoted formation pathways of dioxins. The effect of oxygen was studied by measuring XAFS spectra on samples heated at different oxygen concentrations. According to these spectra, chromium compounds play two key roles during dioxins formation: (a) chlorinating carbon, using chlorine derived from conversion of CrCl3 into Cr2O3, and further oxidation to Cr(VI), and (b) facilitating oxidative destruction of the carbon matrix, while reducing Cr(VI) to Cr2O3.
Many trace elements such as chromium and mercury are toxic. The leaching of trace elements from fly ash is usually monitored at ash storage and disposal sites to ensure that they do not leak into the natural environment at elevated concentrations. Preparatory studies are commonly carried out to ensure that the use of fly ash and other coal by-products in agricultural applications such as soil amendment and landfill do not lead to contamination of soil or groundwater. The concentrations of chromium and radioactive trace elements are also limited in cement and concrete applications as a result of recently tightened legislation. Coal fly ash can be the major source of chromium and radioactivity in these applications. This report draws together data on the concentrations of trace elements in fly ash from both conventional and advanced coal-fired power plants. It examines and evaluates the analytical techniques required to determine the concentrations of trace element species in fly ash and to predict their behaviour in applications such as cement and concrete. The legislated limits for trace elements in fly ash for use in the construction industry and other applications are summarised. The report examines the variation in trace element partitioning with different furnace technologies and pollution control systems. New and emerging technologies to reduce emissions of pollutants such as mercury and fine particulates will affect the trace element concentration of ash. The report summarises how these changes may affect the marketing potential of the ash produced.
Very few studies have investigated the leaching characteristics of cement activated fly ash and slag treated soils, although the inclusion of cement significantly enhances the material pH and may alter the leachability of elements. In this study the leaching behavior and mechanisms of chromium (Cr), copper (Cu), iron (Fe) and sulfur (S) from cement activated fly ash and slag stabilized soils were evaluated. An array of synthetic precipitation leaching procedure (SPLP), batch water leach test (WLT), toxicity characteristic leaching procedure (TCLP) and pH-Static leach tests were conducted. A geochemical equilibrium model Visual MINTEQ was implemented to identify the leaching controlling mechanisms of the metals. Results indicated that, the leached concentrations of Cr, Cu, Fe and S in SPLP, WLT and TCLP effluents were in the range of 0.016 to 0.74 mg/L, 0.013 to 0.17 mg/L, 0.019 to 0.27 mg/L and 1.78 to 234 mg/L, respectively. Quantitative comparisons between the standard test procedures suggested the necessity of multiple test methods for a comprehensive leaching assessment. Cr and Cu showed amphoteric leaching behaviors, whereas Fe and S followed cationic leaching patterns. According to the geochemical analyses, amorphous Cr(OH)3; tenorite and Cu(OH)2; ferrihydrite and goethite; gypsum and anhydrite; could control the leaching of Cr, Cu, Fe and S, respectively. The effluent Cr concentrations frequently exceeding the U.S. EPA specified maximum contaminant level of 0.1 mg/L. Yet, the use of cement activated fly ash and slag mixed soils could be beneficial, since less toxic trivalent Cr (III) was identified through geochemical modeling.
Growing public awareness of the environmental impact of coal combustion has raised serious concerns about the various hazardous trace elements produced by coal firing. Arsenic deserves special attention due to its toxicity, volatility, bioaccumulation in the environment, and potential carcinogenic properties. As the main anthropogenic source of arsenic is coal combustion, its behavior in power plants is of concern. Unlike mercury, arsenic behavior in coal combustion has not been subjected to systematic, in-depth research. Different researchers have reached opposing conclusions about the behavior of arsenic in combustion systems and, as yet, there is relatively little research on arsenic removal technologies.
In this paper, the volatilization, transformation, and emission behavior of arsenic and its removal technologies are discussed in depth. Factors affecting the volatilization characteristics of arsenic are summarized, including temperature, pressure, mode of occurrence of arsenic, coal rank, mineral matter, and the sulfur and chlorine content of the fuel. The behavior of arsenic during oxy-fuel combustion and the effect of combustion atmosphere (O2, CO2, SO2 and H2O(g)) are also reviewed in detail.
In order to better understand the pathways of arsenic in a power plant environment, a particular focus in this work is the transformation mechanism of ultra-fine ash particles and the partitioning behavior of arsenic. Finally, the effects of air pollution control devices (APCDs) on arsenic emissions are examined, along with the effectiveness of flue gas arsenic removal technologies with different kinds of adsorbents, including calcium-based adsorbents, metal oxides, activated carbon, and fly ash.
The influence of steam and minerals on Cr transformation during oxy-coal combustion has been examined in a drop tube furnace at 1273 K. Extra 500 ppmV HCl and 1000 ppmV SO2 were also added to flue gas to examine the competition of H2O, HCl and SO2 on Cr behaviors during oxy-coal combustion upon the recirculation of impurities in flue gas. It was found that CaO and Fe2O3 exhibited high capability on capturing Cr-bearing vapors during combustion compared to limestone and kaolin. Changing air to oxy-coal combustion obviously weakened the role of CaO, whereas promoted reaction between Fe2O3 and Cr vapors. The existence of H2O in flue gas facilitated the oxidation of trivalent Cr(III) to high valance Cr vapors in a short second time scale. CaO, particularly in the co-existence of H2O during coal combustion, remarkably enhanced the toxic Cr(VI) fraction in solid ash. H2O affected the behavior of minerals reacted with Cr. CaO was inhibited and Fe2O3 was enhanced by steam to react with Cr during oxy-coal combustion. The retention of Cr in ash reduced with H2O increase from 5% to 20% during oxy-coal combustion. HCl significantly promoted the release of Cr through chlorination reaction. The presence of H2O offset the negative effect of HCl on Cr recovery when H2O was coexistent with HCl in flue gas. SO2 favored the retention of Cr via sulfation reaction to form Cr2(SO4)3 condensed into solid ash. Moreover, it was found Cr vapors preferred to react with HCl than SO2 during combustion.
Coal ash released into the environment may release toxic trace elements into water, sediments, and soils. The objective of this study was to characterize the chemical speciation of As, Se, Cu, Zn, Cr, and U in coal fly ash samples related to the 2008 Kingston ash spill. Three ash samples were analyzed using X-ray absorption spectroscopy (XAS) to determine oxidation states or dominant species of trace-elements, which were previously found to range in concentration from 8 to 20 mg kg⁻¹. Linear combination fitting (LCF) of X-ray Absorption Near Edge Structure (XANES) spectra from ash samples indicated that both reduced and oxidized forms of the trace elements were present in the fly ash samples. We used the mineralogical composition of the fly ash to select the most relevant standards for LCF fitting of XANES spectra, which included metal-doped glasses, trace elements sorbed to iron oxy(hydroxides), and pure mineral phases for each element. Arsenic K-edge XANES spectra were best fit as oxidized As(V) (95–100%) associated with iron phases or aluminosilicate glass, where selenium K-edge XANES spectra were fit as Se(IV) (77–86%) associated with glass, with lesser proportions of Se(VI) and a more reduced Se species [fit as Se(0) or Se(II)S2]. Zinc K-edge XANES spectra were best fit as Zn associated with ferrihydrite (70–77%), franklinite (ZnFe2O4, 8-12%), and ZnO (14-20%). Qualitative assessment of U LIII-edge and Cr K-edge XANES spectra showed dominances of U(VI) and Cr(III) oxidation states. Copper K-edge XANES data indicated the possibility of both reduced and oxidized species, although our analysis could not fully account for all spectral features. Our results revealed multiple species of each trace element in the fly ash samples, which is important for predicting environmental mobility and bioavailability under the range of geochemical conditions found in aquatic and terrestrial environments.
A promising strategy for the detoxification and immobilization of chromite ore processing residue (COPR) in a spinel-based glass-ceramic matrix is reported in this study. In the search for a more chemically durable matrix for COPR, the most critical crystalline phase for Cr immobilization was found to be a spinel solid solution with a chemical composition of MgCr1.32Fe0.19Al0.49O4. Using Rietveld quantitative X-ray diffraction analysis, we identified this final product is with the phases of spinel (3.5 wt.%), diopside (5.2 wt.%), and some amorphous contents (91.2 wt.%). The partitioning ratio of Cr reveals that about 77% of the Cr was incorporated into the more chemically durable spinel phase. The results of Cr K-edge X-ray absorption near-edge spectroscopy show that no Cr(VI) was observed after conversion of COPR into a glass-ceramic, which indicates successful detoxification of Cr(VI) into Cr(III) in the COPR-incorporated glass-ceramic. The leaching performances of Cr2O3 and COPR-incorporated glass-ceramic were compared with a prolonged acid-leaching test, and the results demonstrate the superiority of the COPR-incorporated glass-ceramic matrix in the immobilization of Cr. The overall results suggest that the use of affordable additives has potential in more reliably immobilizing COPR with a spinel-based glass-ceramic for safer disposal of this hazardous waste.
Chromium valence states in yeast biomass are investigated by X-ray absorption near-edge structure spectroscopy (XANES) using an upgraded experimental method. The main advantage of this new XANES method is to record the transmission and fluorescence experimental data simultaneously in a same ion chamber. Such method is very convenient to analyze a series of unknown chromium valence state samples ( e. g. relatively low Cr concentration with fluorescence mode) comparing with a standard sample (Cr2O3) using transmission mode, in order to avoid systematic error associated to the uncertainty of the edge energy induced by the different experimental operating condition. The experimental results show that in yeast biomass almost all Cr(VI) is reduced to Cr(III) addressing the potential use of biomass for chromium reduction.
To determine the potential release of chlorine from municipal solid waste incineration (MSWI) fly ash, the leaching behaviors of MSWI fly ash from Likeng waste incineration power plant in China were characterized according to the US EPA method 1314. Chlorine leaching characteristics were investigated over a wide range of liquid-to-solid ratios (L/S 0.2–10 mL/g-dry) and different pH conditions (2, 4 and 6). The test results revealed that the pH of eluate increased with increasing L/S ratio due to the increased release of CaO in fly ash. An initial fast early chlorine release followed by a slower, final release period was obtained. Chlorine release widely exceeds the limit of 2500 mg/L at initial stage. A maximum extractable amount (43–48 %) was obtained when the cumulative L/S ratio is about 2. The leachability of chlorine was almost pH independent due to the strong solubility. The leaching of chlorine is controlled by solution followed by diffusion of capillary. Exponential decay equation could well fit the cumulative chlorine release curves. X-ray diffraction results showed that the water-soluble chlorides, such as sodium chloride (NaCl), potassium chloride (KCl) and calcium chloride (CaCl2·2H2O), were easy to leach out, while calcium chloride hydroxide [Ca(OH)Cl] was hard to release.
Chromium released during Municipal Solid Waste (MSW) incineration is toxic and carcinogenic, Cr(?) emission should be kept as low as possible as Cr(?) is known to be more toxic than Cr in other forms. In order to investigate the speciation and release of chromium during MSW incineration, simulated MSW based on the typical components of MSW from Chinese cities were thermal treated in a drop tube furnace at the temperature of 1073-1673 K, Results showed that the emission concentration of Cr(VI) was up to 59.28μg/L, constituting about 16.50%-55.05% of the emission of total Cr. The increase in temperature promoted the release of chromium and formation of Cr(VI) effectively by improving the saturated vapor pressure and enhancing the oxidation of Cr(III). For providing oxidizing agent for the oxidation of Cr(III), high O2 content in the air had the same effect. Both organic chlorides and inorganic chlorides enhanced the release of chromium and the formation of Cr(VI) obviously, and organic chlorides had a more effective impact than inorganic chlorides.
A study was conducted to allow the sorting of coal before import using only a small amount of coal by estimating the amount of leachable Cr(VI). The amount of leached Cr(VI) was determined in eight fly-ash samples from a power plant after extraction with dilute hydrochloric acid (pH 5.8 - 6.3). The Cr(VI)/total Cr ratio varied widely in the range of 0.25 to 5.0 % among the coal-ash samples tested. To prepare coal-ash similar to fly-ash from a coal-fired power plant, three coal types were burned in two types of combustion furnaces. One was an experimental furnace with a burner for pulverized coal firing. The leached Cr(VI) concentrations from coal-ash prepared using the experimental furnace were in good agreement with those from a coal-fired power plant, allowing estimates of leached Cr(VI) concentrations from coal-ash prepared in the experimental furnace. The second type was an electric furnace that incinerated pulverized coal on a ceramic dish under an air atmosphere at temperatures from room temperature to a preset value, according to the Japanese Industrial Standard. The Cr(VI) concentration from coal-ash prepared using the electric furnace depended on both the incinerating temperature and type of coal; it was not possible to select an incinerating temperature suitable for all types of coal-ash that had a leached Cr(VI) concentration, in good agreement with that from a coal-fired power plant.
Bottom ash is one of the by-products of coal combustion from circulating fluidized bed boiler which contains 22 ppm of chromium that can be leached and recovered. The study aims to determine the effects of pH and type of leachant on the leaching of chromium using continuous leaching test. Results show that acetic acid at pH 2 has the highest cumulative amount of chromium leached with 3.224 ppm and reddish-orange leachant and followed by sulfuric acid at pH 6 with 0.170 ppm and translucent color leachant. Moreover, the amount of chromium mobilized was 27.11% and 1.43% for acetic acid pH 2 and sulfuric acid pH 6 respectively.
In this paper, the emission, partitioning, and characterisation of twelve trace elements (As, Ba, Be, Co, Cr, Cu, Mn, Ni, Pb, Sr, V, and Zn) in the ash deposits from different sections of an industrial pulverised lignite-fired boiler and in the flue gas have been thoroughly examined. Apart from the combustion of the raw lignite, the combustion of coal mixed with 4 wt% of a silica-based additive for the inhibition of ash slagging and fouling has also been conducted to investigate the influence of silica on the behaviour of the aforementioned trace elements. In terms of relative enrichment (RE) in the combustion zone (i.e. coal flame zone), the vaporisation tendencies of the individual trace elements in the lignite studied showed a remarkable deviation from the conventional classification reported by the International Energy Agency (IEA) Coal Research Centre. Irrespective of the use of the additive, Zn in the classic group II and Ba, Co, Cu, and Ni, which are commonly presented as intermediates between group I and II, have been elevated to group III elements for the lignite tested. On trace element partitioning trends across the boiler, the group encompassing As, Cr, Mn, and V, witnessed a noticeable reduction in enrichment across all boiler positions upon using the silica-based additive. The group; Ba, Co, Ni and Zn, have depleted concentrations irrespective of the use of the silica-based additive. For the rest of the trace elements particularly Be, Cu, and Pb, they have intertwining trends between the different RE sections, and also between the two combustion cases of the raw coal and coal mixed with the silica-based additive. Out of all the trace elements studied, addition of the silica-based additive had an opposite effect on Sr where its RE factors was increased throughout. Upon statistical analysis for the case of raw coal alone, As was found to mainly correlate with Ca and Si; Ba, Cr, and Cu was found to correlate with Fe; and V, to correlate with Ca. Addition of the silica-based additive was shown to provide competing reactions for the heterogeneous transformation of the trace elements and hence altering their relationships. This was further supported by X-ray absorption near-edge structure spectroscopy (XANES) analysis of Cr in ash deposits. The amount of highly hazardous Cr(VI) was found to be in between 0% and 14%, with the addition of the silica-based additive incurring an overall reduced Cr(VI) capture/fixation, partially due to significant Cr contamination effect derived from the silica-based additive itself.
This study investigated the combustion conditions for Cr(VI) formation during pulverized coal combustion using three types of furnaces. Cr(VI) compounds can be highly soluble in water, whereas Cr(III) compounds are less soluble. The amount of Cr(VI) in coal ash was measured using the amount of Cr(VI) leached in a leaching solution, with ICP-AES after the separation of Cr(III) by Fe(OH)3 co-precipitation. The results from laboratory drop tube furnace experiments indicated that the amount of Cr(VI) leached from coal ash depended on the temperature of the furnace, the air ratio (amount of air used in combustion to the theoretical air demand), and the O2 concentration in the carrier gas. Since these dependences were similar to those for NOx emission, a two-stage combustion method used as a NOx reducing method was examined by using an experimental furnace with a burner for pulverized coal firing and a coal-fired power plant. The results showed that the amount of leached Cr(VI) decreased along with NOx as the two-stage combustion ratio increased. Low-NOx combustion conditions likely reduced the Cr(VI) formation in the combustion zone.
Studies of geochemistry and health are exceedingly complex, and it can be difficult to satisfy the need for some trace elements without causing disease through a surfeit of others. The essential macronutrients are first reviewed, then the essential micronutrients including cobalt, chromium, copper, fluorine, iodine, iron, manganese, molybdenum, selenium and zinc.-K.Clayton
Iron K-edge X-ray absorption fine structure (XAFS) studies have been carried out to determine the bulk and surface structures of a nanophase iron oxide catalyst with average particle size of 30 Å. Both the extended X-ray absorption fine structure (EXAFS) and the X-ray absorption near-edge structure (XANES) spectra indicate that the structure of the catalyst is similar to iron oxyhydroxide. The intensity of the 1s → 3d transition peak in the XANES of the 30-Å catalyst is increased by a factor of 1.7 compared with that for iron oxide and iron oxyhydroxide model compounds, in which iron ions are coordinated by six oxygens or oxygen/hydroxyl groups. The increase of the 1s → 3d transition intensity is attributed to coordination unsaturated (CUS) sites on the particle surface produced by dehydroxylation. The absorption edge position of the 30-Å catalyst indicates that the valence state of the iron ions remains + 3.
X-ray absorption spectroscopy (XAS), Fourier transmission infra red spectroscopy (FTIR), X-ray photo-electron spectroscopy (XPS), Auger profiling and microscopy were used to characterize chemically and structurally the surface oxide layer created by the Alodine 1200S process on 2024 and 7075 aluminum alloys. The thickness of the layer was found to range from ~2000 Å to 1 μm for processing times of 1–3 min. The top ~20 Å of the fresh Alodine surfaces were composed of H2O, Cr, Fe and CN. Al and the other elements in the alloys generally were not components of the top ~20 Å. The Fe and CN were introduced from the Alodine bath and were present throughout the surface oxide layer as Fe(CN)63−. The hydration of the Cr was evident in both the FTIR and XAS spectra. Prior to salt spray exposure the Cr was coordinated with oxygen as ~23 ± 2% Cr6+ in tetrahedral coordination, CrO = 1.71 ± 0.03 Å, the balance as Cr3+ in octahedral coordination, CrO = 1.99 ± 0.01 Å. During salt spray testing ~500 Å of the surface was eroded away and a large fraction of the remaining Cr6+ was chemically reduced to Cr3+ resulting in as little as ~7 ± 1% Cr6+ in tetrahedral coordination. The other elements in the alloy, i.e. Al, Cu, Zn, Mn, Ca, Ti and Si were found in the eroded surface. The Cu and Zn were present and buried within the surface oxide layer essentially in the form of the same intermetallic compounds comprising the original bulk alloy. The Alodine process did not change them. The Zn environment changed slightly during salt spray testing.
Cr(III) and Cr(VI) were determined by positive thermal ionisation isotope dilution mass spectrometry (IDMS). From the spiked samples, Cr(VI) was separated from Cr(III) by extraction with the liquid anion exchanger Amberlite LA-2. Re-extraction of Cr(VI) was performed with an ammonia solution. After electrolytic deposition of chromium from both species the isotope ratio 52Cr/53Cr was determined. Detection limits of 1.2 ng g−1 for Cr(III) and of 2.4 ng g−1 for Cr(VI), respectively, were achieved. A HCO−3/CO2 buffer solution of pH 6.4 containing Cr(III) and cr(VI) enriched in 53Cr was used as spike solution. The stability of the chromium species in this solution was tested in model experiments with labelled radioactive 51Cr. The accuracy of the IDMS method could be shown in two interlaboratory comparisons. Several fresh and treated waste water samples with Cr(III) concentrations in the range of 2–55 ng g−1 and Cr(VI) concentrations of less than 2.4 ng g−1 were analysed. The results of the fresh water samples could be explained by the known geochemical behaviour of chromium in natural water systems.
The speciation analysis of Cr was studied using hydraulic high pressure nebulization in combination with inductively coupled plasma mass spectrometry. Ion-pair chromatography with tetrabutylammonium acetate as the ion-pairing reagent and 25% methanol in the eluent was applied for the separation of CrIII and CrVI. Interferences from carbon could be reduced by the addition of oxygen to the aerosol gas. With careful optimization of the operating conditions, a procedure has been established that results in detection limits of down to 1 ng ml–1 for the Cr species.
A method for the determination of chromium(III) and total chromium was studied. Chromium(III) was chelated with 0.1 mol l–1 quinolin-8-ol in methanol, extracted with isobutyl methyl ketone and determined by electrothermal atomic absorption spectrometry. The effects of the pH, extraction and heating time and amounts of reagents required for the extraction were studied. The detection limit of this method was 12 ng l–1. Different modifiers (nitric acid, magnesium nitrate and sodium tungstate) for the determination of total chromium were also investigated and a method for preconcentration on a graphite tube with hot injection using magnesium nitrate was optimized. The detection limit for the determination of total chromium using this procedure was 29 ng l–1. The precision obtained for different amounts of chromium(III) and total chromium were 2.8–6.2 and 2.2–4.3%, respectively. The graphite furnace programmes, the accuracy and the interferences for both procedures were also studied.
This paper describes the development of an ion chromatography system for the separation and determination of Cr(III) and Cr(VI) species in aqueous samples. The separation was carried out on a Dionex AS4A anion exchange column containing a small proportion of cation exchange groups. Very high sensitivity was achieved using a chemiluminescence post-column reaction detector based on the catalytic oxidation of luminol. Linear calibrations were obtained over the range 0.1 to 500 μg l−1. Reproducibility was better than ±5% at the 10 μg l−1 level with detection limits of 0.05 and 0.1 μg l−1 for Cr(III) and Cr(VI) respectively. A preliminary study of chromium speciation in a fresh water standard reference material showed that the conditions necessary for simultaneous determination of the two chromium species was critical, as Cr(VI) was vulnerable to reduction in acidic solutions and Cr(III) was strongly hydrolysed above pH 3.
The authors review the development of extended x-ray absorption fine structure (EXAFS) within the last decade. Advances in experimental techniques have been largely stimulated by the availability of synchrotron radiation. The theory of EXAFS has also matured to the point where quantitative comparison with experiments can be made. The authors review in some detail the analysis of EXAFS data, starting from the treatment of raw data to the extraction of distances and amplitude information, and they also discuss selected examples of applications of EXAFS chosen to illustrate both the strength and limitations of EXAFS as a structural tool.
A method to determine hexavalent chromium in environmental solid waste has been developed by using an ammonium complexation technique in ICP-AES. This method consists of a two-step extraction and the complexation of ammonium ion. An ammonium hydroxide solution was used to extract the chromium(VI) out of the fly ash sample into the solution. Interfering metal ions, such as Pb, Fe, and Zn in the fly ash sample, were effectively removed by extracting them as diethyldithio-carbamate (DDTC) complexes at pH 9.5 into an MIBK (methyl isobutyl ketone) solution. The pH of the aqueous phase of the sample is then transferred and adjusted to 6.0, and the Cr(VI) is extracted as its complex with MIBK prior to the organic ICP analysis. A method detection limit of 50 ng/g or better, as well as spike recoveries of 90 to 110% (at a spike concentration level of 500 ng/g), is obtained when this method is applied to determine chromium(VI) in fly ash samples.
A voltammetric procedure to determine chromium in aqueous solution using cathodic stripping voltammetry (CSV) preceded by adsorptive collection of complex species with diethylenetriaminepentaacetic acid (DTPA) on a hanging mercury drop electrode is optimized for sea water. The optimized conditions include a pH of 5.2 and a DTPA concentration of 2.5 mM. Comparison of the chemical speciation of DTPA in sea water and distilled water with the CSV sensitivity indicated that the adsorptive species is a complex of chromium(III) with H-2DTPA. The sensitivity of the CSV procedure for chromium(VI) in sea water is a decade lower than in distilled water due to major cation competition (of calcium and magnesium) for DTPA in the sea water. The limit of detection for chromium(VI) in sea water is 0.1 nM at a deposition time of 2 min. Chromium(VI) produces a stable peak using the optimized CSV procedures, whereas the peak due to chromium(III) is unstable due to probable conversion of the chromium(III) complex to an electrochemically inert complex over a period of almost-equal-to 30 min. The different behaviours of chromium(VI) and -(III) were utilized to devise a procedure to determine reactive chromium(III) and chromium(VI) in sea water samples. Experiments with an added chelator indicated that the reactive chromium(III) concentration does not include complexes with organic material. The total dissolved chromium concentration was determined after UV-irradiation of the sample at its original (neutral) pH. Storage experiments indicated that neither the redox speciation of chromium nor its total dissolved concentration in sea water could be stored. The CSV method was applied successfully to certified sea water samples and to the ship-board determination of chromium(VI) and -(III) in the water column of the N.W. Mediterranean.
A sensitive procedure to det. Cr(VI) is described. A chelate of Cr with an org. ligand is adsorbed on a hanging Hg drop electrode and the redn. current of the accumulated chelate is measured by differential-pulse voltammetry. The effects of various parameters (pH, ligand concn., potential and collection time) on the response are discussed. Possible interferences by trace metals and org. matter are considered. [on SciFinder(R)]
The environmental impact of specific trace element species in coal utilization and waste disposal depends not only on the abundance but also on the form(s) of occurrence of the element present in coal and coal ash. While there are a number of analytical methods for determining the abundance of trace elements in coal and ash, there are very few methods available for determining the form of occurrence (speciation) of a trace element in such materials at abundances as low as 10 ppm. In this report, the potential of XAFS spectroscopy for trace element speciation is demonstrated by means of measurements on two environmentally important trace elements, arsenic and chromium, in coal and ash. Results indicate that arsenic in coal is most commonly present as arsenical pyrite (in which form it may readily oxidize to arsenate, AsO43-, even under ambient storage conditions), and that arsenic is present as arsenate in ash and slag. Conversely, chromium is present as Cr3+ in coal and remains in that oxidation state in ash and slag.
X-ray absorption near-edge structure (XANES) has been applied to the quantitative analysis of chromate (CrO[sub 4][sup 2[minus]]) content in oxides, silicate glasses, and simulated cementitous waste forms. The method uses the normalized area of a pre-edge peak (bound-state transition) in XANES spectra acquired with the synchrotron X-ray microprobe on beam line X26A at the National Synchrotron Light Source. The estimated accuracy of the technique is [+-] 5% in terms of Cr[sup 6+]/total Cr. The minimum detection limit approaches 10 ppm Cr[sup 6+] using a 150-[mu]m synchrotron X-ray beam. These results demonstrate the value of synchrotron-based microXANES for the nondestructive quantification of the first-row transition metal species present as tetrahedral oxyanions. 12 refs., 3 figs., 3 tabs.
A method for the simultaneous preconcentration of chromium(III) and chromium(VI) in aqueous solution using an anion-exchange resin was developed. After chelating with EDTa, chromium(III) was bound to Dowex 1-X8 anion-exchange resin under the controlled conditions. Chromium(VI) was then enriched on the same rein. Factors affecting the chelate formation and optimum conditions for trapping and desorption were investigated. The proposed method provides a relatively simple and convenient procedure for the simultaneous preconcentration of chromium species by a readily available resin with the advantages of a high recovery and low operating cost.
The iron K-edge absorption features of a series of 28 synthetic iron compounds and a total of 11 different complexes of ovotransferrin, catechol 1,2-dioxygenase, and protocatechuate 3,4-dioxygenase have been compared to determine the coordination number of the protein complexes. The intensity of the 1s to 3d preedge transition measured for the synthetic compounds varies inversely with coordination number. The normalized preedge peak areas averaged 8 units for six-coordinate compounds, 16 units for five-coordinate compounds, and 24 units for four-coordinate compounds. An extended-Hückel molecular orbital calculation showed a good correlation of the preedge peak areas with the total amount of iron 4p atomic orbitals mixed into the predominantly iron 3d molecular orbitals. This correlation is expected for a dipolar transition. The ovotransferrin complexes exhibited preedge peak areas consistent with the coordination numbers 6 and 7, while the dioxygenase complexes exhibited values that range from 8 to 13.4 units, indicating an active site that can be five- or six-coordinate.
High-resolution vanadium $K$-edge absorption spectra have been recorded for a number of selected vanadium compounds of known chemical structure with use of the synchrotron radiation available at the Stanford Synchrotron Radiation Laboratory (SSRL). The compounds studied include the oxides VO, ${\mathrm{V}}_{2}$${\mathrm{O}}_{3}$, ${\mathrm{V}}_{4}$${\mathrm{O}}_{7}$, ${\mathrm{V}}_{2}$${\mathrm{O}}_{4}$, and ${\mathrm{V}}_{2}$${\mathrm{O}}_{5}$; the vanadates ${\mathrm{NH}}_{4}$${\mathrm{VO}}_{3}$, ${\mathrm{CrVO}}_{4}$, and ${\mathrm{Pb}}_{5}$${({\mathrm{VO}}_{4})}_{3}$ Cl; the vanadyl compounds ${\mathrm{VOSO}}_{4}$.${3\mathrm{H}}_{2}$O, vanadyl bis(1-phenyl-1,3-butane) dionate, vanadyl phthalocyanine, and vanadyl tetraphenylporphyrin; the intermetallics VH, ${\mathrm{VB}}_{2}$, VC, VN, VP, and ${\mathrm{VSi}}_{2}$; and ${\mathrm{V}}_{2}$${\mathrm{S}}_{3}$ and a vanadium-bearing mineral, roscoelite. Vanadium in these compounds exhibits a wide range of formal oxidation states (0 to +5) and coordination geometries (octahedral, tetrahedral, square pyramid, etc.) with various ligands. The object of this systematic investigation is to gain further understanding of the details of various absorption features in the vicinity of the $K$ absorption edge of a constituent element in terms of its valence, site symmetry, coordination geometry, ligand type, and bond distances. In particular, the intensity and position of a well-defined pre-edged absorption in some of these compounds have been analyzed semiquantitatively within a molecular-orbital framework and a simple coordination-charge concept.
At low concentrations, recording X-ray absorption spectra in fluorescence excitation mode is more sensitive than transmission mode. For dilute samples, the fluorescence signal is often obscured by scattered X-rays, and matrix and filter fluorescence. To discriminate against this background, while maintaining a large angular acceptance and high count rate capability, we have constructed a new detection system based on an array of intrinsic Ge detectors. The device uses 13 individual 11 mm diameter Ge detectors, clustered in a 1:3:5:3:1 pattern on a common cryostat, combined with Soller slits and filters to reduce the background signals. Pulsed optical feedback preamplifiers are followed by Gaussian-shaping amplifiers having fast discriminators to register the incoming count rate (ICR). Correction for dead time using the ICR signal allowed operation in the vicinity of 75 kHz per channel, with a 1 μs shaping time at 6 keV. For lower count rate applications, an average resolution of 160 eV at 5.9 keV was obtained with 8 μs shaping. Recent experience with this detector at the Stanford Synchrotron Radiation Laboratory is presented. The performance is illustrated using spectra obtained from phosphorus compounds and a thin iridium foil. The performance of this device is compared with previous fluorescence detection schemes, such as those using filter/slit combinations or barrel monochromators.
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.
This work succeeded in developing a reference material for speciation of Cr(III) and Cr(VI) in water. It consisted of a HCO−3/H2CO3 (50 mmol/l) buffer solution containing Cr(III) (40 μg/l) and Cr(VI) (10 μg/l) at pH 6.4. A second reference solution, typical for speciation of Cr(VI) from welding dust, contained Cr(VI) (5 mg/l) at pH 9.6. The parameters to obtain stable solutions were carefully studied. Two formulas were tested: first an aqueous solution kept under a CO2 blanket in sealed quartz ampoules: this later evolved into a lyophilized version to be reconstituted in the buffer solution at the right pH. The stability studies demonstrated the reliability of the materials, which allowed to organize 2 interlaboratory studies between 7 European laboratories. The results of the intercomparisons confirmed the validity of the methodology for preparing such candidate reference materials for Cr(III)/Cr(VI) speciation.
The problem of absorption of soft X-rays by thick Be windows in hard X-ray beam lines is well known. Although the signal at 2.4 keV was reduced by ∼ 103 we have routinely measured the absorption spectra of S (2472 eV) and elements at higher energies including Cl, Ar and K. These spectra were obtained on hard X-ray beam lines at Stanford Synchrotron Radiation Laboratory (SSRL) with Si(111) monochromator crystals and a fluorescent ion chamber detector [1]. Higher energy harmonics were minimized by detuning and the end station was enclosed in a helium bag to prevent absorption by air. Although the diminished X-ray flux and decreasing fluorescent yield were serious negative factors at these low X-ray energies the spectra from thick samples were of excellent quality with sufficient sensitivity to characterize 1% S in coal. Representative spectra are shown comparing data from focused and unfocused beam lines and with S data from JUMBO [2]. Comparison of Ar and KCl data to excellent data found in the older literature [3–5] allow a confirmation of the resolution function (energy bandpass) of the monochromator. A simple new beam line is suggested which would allow a substantial increase in low energy X-ray flux (measurements down to Al and Si) with the sample and detector in a He atmosphere.
A previously described expert system for the voltammetric determination of Cu, Zn, Cd, Pb, In, Ni, Co, Tl, Hg, V and Se (optionally also Te) is enlarged and improved by including methods for speciation of chromium and arsenic. Voltammetric procedures for the quantification of Cr(III), Cr(VI), As(III) and As(V) are considered, Cr(III) and As(V) being determined by the difference between total chromium and Cr(VI), and total arsenic and As(III) respectively. The techniques implemented are differential pulse polarography, differential pulse anodic stripping voltammetry, differential pulse cathodic stripping voltammetry and differential pulse adsorptive stripping voltammetry, using mercury drop electrodes and a rotating gold electrode. The expert system is developed using KES (knowledge engineering system).
Cr-bearing ultramafic rocks from Campo Formoso, Brazil, exhibit various stages of hydrothermal alteration and weathering, each being characterized by distinct Cr parageneses. The Cr-bearing minerals are mainly phyllosilicates (muscovite, smectite, clinochlore) but other phases present include chromiferous opal and hydroxycarbonates as well as chromite. X-ray and optical absorption spectra and K- edge fine structure studies allow the characterization of Cr in these minerals. No change was found in the oxidation state of Cr during alteration but the crystal field stabilization energy of Cr3+ showed important variations. -R.A.H.
Sampling and analytical techniques used for determining trace metal concentrations in atmospheric precipitation waters collected in Hungary are presented. The results of the analyses are briefly discussed and special attention is devoted to chromium speciation. For the preconcentration of the trace metals a chelating cellulose, iminodiacetic acid ethylcellulose (IDAEC) microcolumn was used in a flow-injected system. Cd, Co, Cu, Fe, Mn, Ni, Pb, Ti, V and Zn were determined by ICP-AES. In precipitation water the concentrations of the trace elements were in the 0.1-50 mug/l. range. The two forms of chromium, Cr(III) and Cr(VI) were separated using IDAEC and the anion exchanger diethylamine ethylcellulose, respectively. Cr was determined by GFAAS. In atmospheric precipitation the concentration of Cr(III) was in the range of 0.1-0.4 mug/l. while that of Cr(VI) in the range of 0.04-0.1 mug/l.
Comparatively few valid data are available on the chromium content of foods and on the dietary chromium (Cr) intake of various populations. This is chiefly because of the difficulties encountered in contamination control during sampling, sample pretreatment, and analysis. Moreover, there are several analytical problems involved that are mostly owing to the low concentration level of Cr in foods. However, with the recent establishment of food reference materials with certified low concentrations of Cr, the analytical validity of studies on Cr content of foods and on its dietary intake by various populations can be ascertained. With the exception of herbs and condiments, and certain other special food items with a relatively low average consumption rate, such as tea, coffee, and some candies, most foods contain Cr below 100-mu-g/kg. Staple foods, particularly cereals and milk, are very low (less-than-or-equal-to 10-mu-g/kg) in Cr, showing little or no geographic variation. Food processing may increase food Cr content depending on the process. Processes, such as meat grinding and homogenization using stainless-steel equipment, very strongly increase the Cr content of foods. Also, acidic fruit juices in contact with steel cans are high in Cr, whereas cooking in aluminium vessels reduces the Cr content of foods. Average dietary Cr intake seems to fluctuate considerably among countries. In many developing countries, such as Brazil, the Sudan, and Iran, the dietary intake is high, from 50-100-mu-g/d, whereas in certain developed countries, such as Finland, Sweden, Switzerland, and the US, the intake is 50-mu-g/d or lower and, consequently, at or below the estimated safe and adequate daily dietary intake range of 50-200-mu-g/d established by the US National Academy of Sciences. The average Cr content of human milk is below 0.5-mu-g/L, thus resulting in a very low average intake of 0.3-mu-g Cr/d by exclusively breast-fed infants in the US and Finland.
Chromium was discovered in 1797 by Vauquelin. Numerous industrial applications raised chromium to a very important economic element. At the same time, with the development of its uses, the adverse effects of chromium compounds in human health were being defined. Trivalent chromium is an essential trace element in humans and in animals. Chromium as pure metal has no adverse effect. Little toxic effect is attributed to trivalent chromium when present in very large quantities. Both acute and chronic toxicity of chromium are mainly caused by hexavalent compounds. The most important toxic effects, after contact, inhalation, or ingestion of hexavalent chromium compounds are the following: dermatitis, allergic and eczematous skin reactions, skin and mucous membrane ulcerations, perforation of the nasal septum, allergic asthmatic reactions, bronchial carcinomas, gastro-enteritis, hepatocellular deficiency, and renal oligo anuric deficiency. Prevention of occupational risks, biological monitoring of workers, and treatment of poisoning are also reported.
X-ray absorption spectroscopy
- D C Konigsberger
- R Prins