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Reductive Precipitation of Uranium(VI) by Zero-Valent Iron
Abstract
This study was undertaken to determine the effectiveness of zero-valent iron (Fe0) and several adsorbent materials in removing uranium (U) from contaminated groundwater and to investigate the rates and mechanisms that are involved in the reactions. Fe0 filings were used as reductants, and the adsorbents included peat materials, iron oxides, and a carbon-based sorbent (Cercona Bone-Char). Results indicate that Fe0 filings are much more effective than the adsorbents in removing uranyl (UO22+) from the aqueous solution. Nearly 100% of U was removed through reactions with Fe0 at an initial concentration up to 76 mM (or 18 000 mg of U/L). Results from the batch adsorption and desorption and from spectroscopic studies indicate that reductive precipitation of U on Fe0 is the major reaction pathway. Only a small percentage (<4%) of UO22+ appeared to be adsorbed on the corrosion products of Fe0 and could be desorbed by leaching with a carbonate solution. The study also showed that the reduced U(IV) species on Fe0 surfaces could be reoxidized and potentially remobilized when the reduced system becomes more oxidized. Results of this research support the application of the permeable reactive barrier technology using Fe0 as a reactive media to intercept U and other groundwater contaminants migrating to the tributaries of Bear Creek at the U.S. Department of Energy's Y-12 Plant located in Oak Ridge, TN.
... ]) and negatively charged (e.g. [UO 2 (CO 3 ) 3 ] 4-, [UO 2 (OH) 3 (CO 3 )] -) species [19][20][21]. The experimental results were modelled using the Langmuir and Freundlich equations [3,9,10,21]. ...
... The oxidation of the ZVI in aqueous solutions takes place according to reactions [11,[19][20][21]. ...
... The elucidation of the mechanism of uranium removal from aqueous solutions by Fe 0 nanoparticles is still under discussion in the scientific community [11,25,26]. At first it was claimed that uranium removal was taking place through reductive precipitation [20,27]. The low solubility and mobility of U (IV) enhance the effective and long-term uranium removal from aquatic environment and wastewaters. ...
Your article is protected by copyright and all rights are held exclusively by Akadémiai Kiadó, Budapest, Hungary. This e-offprint is for personal use only and shall not be self-archived in electronic repositories. If you wish to self-archive your article, please use the accepted manuscript version for posting on your own website. You may further deposit the accepted manuscript version in any repository, provided it is only made publicly available 12 months after official publication or later and provided acknowledgement is given to the original source of publication and a link is inserted to the published article on Springer's website. The link must be accompanied by the following text: "The final publication is available at link.springer.com". Abstract Removal of uranium (VI) from aqueous solutions by zero-valent iron nanoparticles (nZVI) was investigated in both acidic and alkaline solutions in absence and presence of counter-ions (0.1 M NaNO 3). Langmuir and Freundlich equations were applied to model the sorption isotherms whereas the data obtained at 15, 25, 35 and 45°C from the kinetic experiments were successfully reproduced by a pseudo-second order equation and used for calculation of thermodynamic parameters. Electron microscopy (SEM/EDS), X-ray photoelectron spectroscopy and UV-Fluorescence were applied to explore possible mechanisms. The U-removal seems to be due mainly to sorption followed by U(VI) reduction.
... Sorption onto surfaces and reduction are both possible removal reactions in iron-bearing walls. To which amount and at what efficiency the distinct processes take place was studied in several articles [59][60][61]. ...
... The plutonium production needed for the Soviet atomic bomb program generated huge amounts of radioactive wastes whose reduction below the permissible level of 3.7 MBq/l was then not feasible. Therefore, huge amounts of Sustainable Remediation Methods for Metals and Radionuclides, Na, 24 Na, 32 P, 35 S, 36 Cl, 54 Mn, 56 Mn, 59 Fe, 60 Co, 82 Br, 89 Sr, 90 Y, 91 Y, 95 Nb, 95 Zr, 105 38 Cl, 55 Fe, 64 Cu, 69 Zn, 71 Ge, 97 Zr, 131 Cs, 136 Cs Low liquid radioactive wastes (110-259 PBq, i.e., 3-7 MCi) were discharged into the Techa-Iset'-Tobol-Irtysh-Ob river system between 1949 and 1956. The amount of radioactive waste accumulated in the Techa River has no equivalent in the world. ...
Sustainability has become the conscientious and future-oriented principle of modern resource management and environmental protection because caring for the future is tantamount to providing manageable and healthy surroundings for ourselves. For the foreseeable future, geotechnical and environmental engineers must therefore be concerned with ensuring a healthy balance between extraction, processing, manufacturing, utilization, recycling, and disposal of materials and products.
... V Removal of U (VI) by the Fe 2+ ion is kinetically a slow proccess when aided by a strong acid, as is the case of our solution, while the adsorption of the UO 2 2+ ion to the Fe oxy-hydroxides takes place at a relatively high range of pH values, generally from 5 to 10. All Fe oxides strongly adsorb uranyl species and, in fact, the adsorption rate on amorphous oxyhydroxides of Fe 3+ is higher than that of crystalline hematite (Fe 2 O 3 ) (Gu et al., 1998). In practice, the removal of U (VI) via Fe oxides can be accomplished through two possible mechanisms. ...
... R.A. Crane et al. examined the separation of uranium from environmentally polluted waters with magnetite and Fe 0 nanoparticles and concluded that the main mechanism of uranium removal in both systems initially involved adsorption followed by reduction of U (VI) to U (IV), which takes place on the surface of the material (Massey et al., 2014). Similarly B. Gu et al., B. Grambow et al., L.J. Matheson et al. in their studies stated that hexavalent uranium is adsorbed on Fe oxidation products and a minor percentage is reduced to tetravalent (Gu et al., 1998). More specifically, through the oxidation course, U may be liberated out of the inner particle core since the formation tends to re-orders due to the large U grain radius against Fe (Shannon, 1976). ...
Various types of plutonic and volcanic rocks and their alteration products from Greece (serpentinite, magnesite and andesite), have been used for sustainable removal of Uranium (U) from the acidic drainage of Kirki mine, as well as for the pH increase of the polluted solutions. In this light, this study aims at the further understanding and improvement of the ecofriendly reuse of sterile, natural raw materials (including those remaining through industrial processing and engineering testing of aggregate rocks), for remediation of acid mine drainage. The selected rocks constitute such residues of sterile materials were used as filters in experimental continuous flow devices in the form of batch-type columns, in order to investigate acidic remediation properties with special focus on U removal. The initial pH of the wastewater was 2.90 and increased after seven (7) days of experimental application and more specifically from the fourth day onwards. Uranium removal became quantitatively significant once pH reached the value of 5.09. The volcanic rocks appeared to be more effective for U removal than the plutonic ones because of microtextural differences. However, optimum U removal was mainly achieved by serpentinite: while the raw materials rich in Mg strongly reacted and remediated the pH of the drainage water waste. Furthermore, the increase of pH values due to the presence of mineral raw materials, provided increased oxidation potential which deactivated the toxic load of metals, particularly U. Consequently, batch-type serpentinite reaction with the tailing fluid caused a drop in U concentration from an initial value of 254 ppb to the one of 8 ppb, which corresponds to 97% of removal. Andesite presented the second best reactant for experimental remediation, especially when it was mixed with magnetically separated mineral fractions. Despite the fact that the proposed methodology is currently at a relatively low Technology Readiness Level (TRL), it carries the potential to become an extremely effective and low-cost alternative to conventional environmental restoration technologies.
... Sohrabi and Arabi (2014) used nZVI particles prepared by liquid phase reduction method to degrade the basic dye methylene blue from aqueous solutions, and the adsorption capacity in terms of monolayer adsorption was 208.33 mg g − 1 . In addition to adsorption and degradation, radionuclide ions with high oxidation states, e.g., ReO 4 − , TcO 4 − and UO 2 2+ can be reduced to sparingly soluble species by nZVI, thereby achieving their in-situ immobilization (Gu et al., 1998;Sihn et al., 2019;Boglaienko et al., 2019). For instance, Boglaienko et al. (2019) evaluated the reductive removal of TcO 4 − from aqueous solutions by eleven commercial ZVI materials manufactured by different methods, which revealed the feasible of TcO 4 − reduction in practical application by ZVI. ...
... The concentration of U(VI) and other metals in the solutions were measured by inductively coupled plasma optical emission spectrometer (ICP-OES, Horiba JY2000-2, Japan). Based on the references (Gu et al., 1998;Yan et al., 2010), it is confirmed that the adsorbed U(VI) can be extracted from solid phase by using Na 2 CO 3 solution, therefore the desorption experiment can be used to quantitatively analyze U(VI)/U(IV) proportion. 0.1 mol L − 1 Na 2 CO 3 solution was adopted to desorb U(VI) from uranium-containing adsorbent for 24 h. ...
A novel composite of zero-valent iron nanoparticles supported on alkalized Ti3C2Tx nanoflakes (nZVI/Alk-Ti3C2Tx) was constructed by an in-situ growth method for simultaneous adsorption and reduction U(VI) from aqueous solution in anoxic conditions. The effect of various factors such as adsorbent dose, pH, ionic strength, contact time, initial U(VI) concentration and environmental media were comprehensively investigated by batch experiments. Benefiting from the good dispersion uniformity of nZVI on MXene substrates, nZVI/Alk-Ti3C2Tx exhibited rapid removal kinetics, excellent selectivity, 100% removal efficiency and up to 1315 mg g⁻¹ uptake capacity for U(VI) capture. In the presence of mimic groundwater, 1.0 mM NaHCO3 and 10 mg L⁻¹ humic acid, the removal percentages of U(VI) by the composites could reach 95.1%, 88.9% and 69.5%, respectively. The reaction mechanism between U(VI) and nZVI/Alk-Ti3C2Tx has been clarified based on FTIR, XANES, XPS and XRD analysis. Depending on the consumption of reactive nZVI in the composites and the solution pH, the elimination of U(VI) could be realized by different pathways including reductive immobilization in the form of UO2, inner-sphere surface complexation and hydrolysis precipitation. The present study illustrates that the nZVI/Alk-Ti3C2Tx composite may be an efficient scavenger for radioactive wastewater purification in environmental remediation.
... 8 Zero-valent iron (ZVI) is a common reductant, effectively used for decontamination of aqueous waste streams from inorganic and organic compounds. [9][10][11][12][13] It has been investigated as a commercially available non-modified material 14,15 and as an engineered material with enhanced efficiency via a silica support or cover. [16][17][18][19][20][21] Supported with silica gel or porous silica, ZVI is less prone to agglomeration, 16,22 and covered with a silica shell, ZVI nanoparticles are characterized by a larger surface area, higher mobility, and reduced oxidation. ...
... 56 Formation of a layer of iron hydroxide/oxyhydroxide leads to depletion of dissolved iron and inhibition of the layer growth, which reoccurs once there is enough substrate, i.e. time-dependent supply of the dissolved iron. A code based on this model 56 simulates a rich variety of patterns, found in nature, throughout different display modes and controllable parameters from eqn (8)- (10). Fig. 7 presents the modelled pattern and its comparison to the dominant morphologies observed in this study. ...
Nano-structural transformation of iron minerals in the natural environment is altered and often retarded in the presence of silica (e.g., impeded transformation of ferrihydrite) resulting in a modulated interaction with constituents or contaminants present in groundwater. This phenomenon can significantly affect molecular mechanisms of reduction, precipitation, and sequestration of pertechnetate (TcO4-), the most prevalent in the environment chemical form of radioactive contaminant technetium-99, by elemental iron Fe0 often referred to as zero valent iron (ZVI). Understanding the role of silica in moderating reactivity of Fe0 toward reduction of TcO4- to Tc4+ and its interaction with in situ formed iron minerals (ferrihydrite, magnetite) is crucial for successful design of a practical separation system and can be related to similar environmental systems. This study was designed to evaluate silica-modified ZVI systems with two commercially available iron materials. The results revealed that the efficiency of TcO4- reduction by Fe0 increased in the presence of silica due to inhibited transformation of iron oxyhydroxide to non-stoichiometric magnetite. Moreover, microscopic evaluation of the newly formed iron mineral phases, both in the presence and absence of silica, revealed unique morphologies related to geological phenomena, such as Orbicular rocks and Liesegang rings, suggesting that iron dissolution/re-precipitation is a rhythmical reaction-diffusion process, which occurs from atomic scale to macro-geological environments and results in layered structures of iron oxidation products.
... Iron-based adsorption studies have received significant, sustained attention for many years now. Many long-term studies have verified that iron-based adsorbents are capable of removing various heavy metals [13][14][15][16][17][18]. They exist in various forms and have varied properties, and they can be classified as iron oxides, iron hydroxides, or iron oxyhydroxides, depending on their characteristics [19,20]. ...
These authors contributed equally to this work. Abstract: Environmental issues related to phosphate and resource depletion have recently emerged as serious problems. This study focuses on solving the problems of phosphate removal and recovery using synthesized granular akaganeite (GAK). This study identified that akaganeite, which possesses an FeOOH structure in iron oxyhydroxide, can be synthesized and used as a reusable material. Immobilization with the core-shell method using polyethersulfone was applied as a strategy to recover phosphate anions from a trace of phosphate solution. GAK was successfully analyzed using SEM/TGA/BET to understand its physical properties. XRD and SAD pattern analyses suggested that the GAK powder form was amorphous in nature. The powdered akaganeite had a surface area of 231 mg 2 /g and a maximum adsorption capacity of 21.27 mg/g. To prevent the dispersion of powder during granulation, polyethersulfone was used as a scaffold since akaganeite particles can be effectively immobilized onto PES polymer scaffolds, as substantiated by the SEM/EDS results. Moreover, a lack of changes in the pore sizes suggested that physical properties remained unchanged. Furthermore, compared to the granular akaganeite, the surface area of powdered akaganeite decreased 4-5-fold. The adsorption kinetic of granular akaganeite fit the pseudo-second-order model. The powdered form displayed high removal efficiency, intimate with phosphate anions, when n > 1.0, instead of lower K F. On the other hand, granular akaganeite showed lower affinity when n < 1.0, but appeared positive for an adsorbate with higher KF. This implies that the granulation of akaganeite with the PES polymer did not change its adsorption property, with the maximum adsorption capacity for granular akaganeite being 3.65 mg/g.
... This means that dissolved U(VI) will migrate further together with other released radionuclides. It is well known that U(VI) is reduced to U(IV) and sorbed and/or precipitated on the surface of metallic iron, both from studies made on ZVI (Zero Valent Iron) barriers [10][11][12][13] and also from studies related to waste disposal [ 14 , 15 ]. For example, the initial concentration of 1 ppm U(VI) decreased 2-3 orders of magnitude in a simplified groundwater solution consisting of 10 mM NaCl and 2 mM NaHCO 3 in contact with pure iron foils [15] . ...
... A variety of natural materials, including zeolites [8][9][10], limestone [6,11], apatite [12], artificially created materials such as cement-based filter media (CBFM) [13], waste products (e.g., fly ash [14]), as well as numerous organic materials and their compositions with minerals [15], can be used to create these barriers. The formation of in situ geochemical barriers using reducing agents, such as zero-valent iron [16,17] and other additives, is another method of treating groundwater. Their injection into the groundwater formation creates local zones for metal immobilization. ...
Groundwater samples contaminated with potentially toxic elements (PTE), including metals and nitrate ions, were collected at a depth of 8-10 m from the Siberian Chemical Plant multicomponent waste storage. The possibility of developing a permeable biogeochemical barrier with zeolite and lightweight expanded clay aggregate (LECA) was investigated. The mass fraction and properties of several metals (Mn, Fe, Co, Ni, Cu, Zn, Cd, Hg and Pb) were determined to investigate their fixation on the chosen materials at the given experimental conditions. It was established that metals in sulfide or phosphate forms can be effectively immobilized via biomineralization on LECA, whereas metals from the non-chalcogen group are primarily retained in the form of phosphates. The formation of biogenic deposits of iron sulfide, which serve as a sorption-precipitation phase during the immobilization of the majority of metals, is an important aspect of the LECA loading process. The use of LECA and zeolite in the form of a two-component barrier is feasible based on the data obtained. It is assumed that metal immobilization processes occur due to sorption mechanisms in the zone of zeolite loading. Microbial nitrate removal and the formation of iron sulfide phases under reducing conditions, which form a geochemical barrier for metals, are expected in the LECA zone.
... Various conventional approaches were applied for reclamation of radioactive industrial liquid effluents, for instance, bioprecipitation [7], reductive precipitation [8], ion change [9,10], and adsorption [11][12][13]. Among all these mentioned techniques, adsorption is a talented method for uranium species illumination from wastewater owing to its simplicity, effectiveness, and feasibility [13,14]. ...
Uranium (as a hazardous and radioactive element) removal from wastewater requires reliable technology and proper functional materials. Carbon fiber species that are produced from agricultural solid waste can be a proper type of low-cost adsorbents for wide uses in wastewater treatment. In this work, two carbon fiber species labeled CF-RH and CF-SCB were synthesized from two different agricultural wastes, namely, rice husk and sugarcane bagasse respectively. The structural properties of carbon fiber were verified by XRD, FTIR, and Raman, spectroscopy. Both nitrogen-adsorption–desorption BET surface area and TEM were performed to figure out the textural characteristics of the presented sorbents. The charges on surfaces of the fibers were detected via zeta potential analysis. The prepared carbon fibers were applied for uranium removal from aqueous solution by adsorption technique. The acquired data display that the equilibrium time was 240 min. The results of adsorption process are nicely fitted with pseudo-second-order-kinetic and Langmuir isotherm models. The maximum sorption capacity was 21.0 and 29.0 mg/g for CF-RH and CF-SCB, respectively. Sorption thermodynamics declare that adsorption of U(VI) is an endothermic, spontaneous, and feasible process. The picked findings of this study could emphasize high reliability of the introduced adsorbents in efficient tackling of water contaminants.
... Several strategies have been developed to decrease the risks posed by radioactive waste leaks, including removing the radioactive substance by solvent extraction (Chiarizia and Horwitz, 1990), chemical precipitation (Gu et al., 1998), photocatalytic/electric reduction (Yang et al., 2021;Yu et al., 2021;Liu et al., 2017;Zhang et al., 2021), or adsorption Imam et al., 2018;Yin et al., 2021;Wang et al., 2020;Yang et al., 2017;Sun et al., 2017;Calì et al., 2018;Xu et al., 2017;Chen et al., 2020aChen et al., , 2020bChen et al., 2016;Chen et al., 2020aChen et al., , 2020bZhuang and Wang, 2019). Adsorption techniques can typically remediate very concentrated radioactive effluents economically, efficiently, and selectively. ...
Chitosan crosslinked with potassium tripolyphosphate (CTPP) and monochloroacetic-acid-modified chitosan crosslinked with potassium tripolyphosphate (MCTPP) were synthesized for removing UO2²⁺ from acidic radioactive effluent. The influential factors, operational requirements, and interactive mechanisms of the adsorption process were systematically investigated. The mesh-structured composites adsorbed UO2²⁺ most effectively at pH 5.0. The maximum adsorption capacities for pure chitosan, CTPP, and MCTPP were 374.93, 780.89, and 1487.72 mg/g, respectively. Batch experiments indicated that the pH and adsorbent dose strongly influenced UO2²⁺ adsorption. MCTPP could adsorb most UO2²⁺ within 15 min, and equilibrium was reached by ~1 h. The adsorption isotherms indicated that UO2²⁺ adsorption by MCTPP may be an endothermic single-layer adsorption process. Moreover, common metal ions in single-metal systems only slightly affected this process. The results of instrumental characterization and natural water application indicated that the highly developed pore structure and abundant tripolyphosphate groups in synthesized composites were dominant adsorption contributors besides amino and hydroxyl groups. Successful development of the novel material for efficiently adsorbing UO2²⁺ and identification of the adsorption mechanism will provide valuable guidance to chitosan modification and further remediation practices of radioactive effluent.
... Zero-valent iron shows much more precipitating power than the other sorbents in eliminating U(VI) ions from water. Precipitation process over iron oxide is more preferred when the external surface of the sorbent has not shielded by corrosion yields (Noubactep et al. 2005;Gu et al. 1998;Zhao et al. 2012). ...
Arsenic is a crystalline metalloid present in the earth’s crust in different minerals. Various natural processes and human interventions have mobilized As into the groundwater system over the years. The dissolved As presence in drinking water is now a significant health concern in India and many other countries. As is a proven carcinogen, and long-term ingestion of low concentrations of As can lead to serious health issues. As in drinking water was first regulated by the United States Public Health Service (USPHS), which set a maximum permissible concentration (MPC) of 50 μg/L. Later, based on lung and bladder cancer risk, the United States Environmental Protection Agency (USEPA) promulgated a maximum contaminant level (MCL) of 10 μg/L. Indian standard for drinking water also recommends a total As concentration of 10 μg/L as the acceptable limit. However, some of the developed countries have As standards lower than 10 μg/L as well. The removal of As in drinking water is inevitable, considering its widespread presence in groundwater, potential human toxicity, and stringent regulation in drinking water. Several point-of-use (PoU) and community-based treatment technologies have been developed to provide As-free water to the affected population. The processes adopted in these technologies include coagulation and filtration, sorptive filtration, ion-exchange, electrocoagulation, and membrane filtration. The present chapter reviews these technologies critically and their suitability in the Indian context. The chapter also provides insight into the As occurrence in the environment, its speciation, toxicity, and health effects.
... Multiple techniques (Alharbi et al. 2020;Hu et al. 2020;Wang et al. 2020) have been developed for uranium removal, including adsorption (Chen et al. 2017b;Wu et al. 2018;Yang et al. 2020), ion change (Semnani et al. 2012), reductive precipitation (Gu et al. 1998), and bio-precipitation (Appukuttan et al. 2006), among which developing uranium adsorption materials has attracted particular attention owning to its high efficiency, ease of operation, reusability, and low generation of secondary waste (Anirudhan et al. 2010;Parth D.Bhalara, and D, K. 2014). Various types of materials, such as minerals (Prikryl et al. 2001), inorganic oxides (Amesh et al. 2020; Hongliang Bao and Chunyu Xie contributed equally to this work. ...
Oxidized carbon foam (oxidized CF) was prepared by using a facile chemical oxidation treatment at relatively low temperature of 450 °C and applied to capture uranyl cation [U(VI)] from aqueous solutions. The effects of pH, contact time, initial U(VI) concentration, and temperature on the U(VI) absorption performance of oxidized CF were investigated by batch experiments. The oxidized CF was illustrated to exhibit fast sorption kinetics (92% removal within 15 min and 98% removal in 2 h) and high sorption capacity (305.77 mg g⁻¹ at pH 5) toward U(VI). Integrated analyses combining energy-dispersive X-ray spectroscopy and Fourier transform infrared spectroscopy were applied on the U(VI)-loaded oxidized CF, showing the introduction of carboxyl groups as U(VI) sorption sites on the surface of CF after oxidation treatment. Furthermore, extended X-ray absorption fine structure spectroscopy was employed to identify the binding modes of U(VI) indicating that each UO2²⁺ cation is coordinated with one or two carboxyl groups on the equatorial plane. Notably, the low content of U(VI) in wastewater can be efficiently immobilized by the oxidized CF, and the immobilized U(VI) can be further concentrated and converted into Na2U2O7 or U3O8 by a simple sintering step. These findings presented in this work suggest the potential of using oxidized CF for further treatment of low concentration wastewater containing U(VI).
... A bicarbonate extraction method was used to determine the valence state of uranium on the reacted nZVI particles. 29 Briefly, a 10 mL nZVI slurry was withdrawn from the nZVI reaction unit of CSTR at the end of the experiments and added to a 25 mL bottle with 10 mL bicarbonate solution (0.2 M) which has been purged with high purity nitrogen gas (>99.999%) for 20 min. ...
The core-shell structure of nanoscale Zero-Valent Iron (nZVI) offers multiplex solution and surface chemistry for reliable and high-efficiency enrichment and separation of uranium from low-level sources such as wastewater, groundwater, even seawater. In this work, the reduction, enrichment and separation of uranium from uranium-tailings wastewater was demonstrated in continuous-flow reactors. Specifically, a two-stage continuous flow stirred tank reactor system was operated for 193 h. 497.7 L radioactive uranium-tailings wastewater containing ~331 g-U/L was treated with a total of 60.22 g iron nanoparticles. It produced reacted nZVI containing ~0.19 wt% uranium, well above typical high-grade uranium ores. The treated wastewater had an average concentration of just 1.47 g U/L. High-resolution elemental mappings performed with aberration corrected scanning transmission electron microscopy (Cs-STEM) indicated that uranium was deposited largely in the core area of the reacted nZVI particles with chemical reduction as the principal enrichment mechanism. Together with our previous work on nZVI, this further offers growing evidence of a cost-effective nanotechnology for simultaneous pollution control and resource recovery.
... In addition, redox transformations of U play an important role in U solubility. 28,29 Oxidized, U(VI) (uranyl cation, U(VI)O 2 2+ ) is readily soluble, while its reduced form, U(IV) (uraninite, U(IV)O 2 ) is highly insoluble. U reductive precipitation to uraninite can be catalyzed by chemical processes [30][31][32] and by biologically-mediated reactions, 29,33,34 in which U(VI) is used as terminal electron acceptor by metal reducing bacteria to support metabolic growth. [35][36][37] However, uraninite is only stable under reducing environments, and U(IV) is readily oxidized to U(VI) under aerobic conditions or in the presence of alternative terminal electron acceptors, such as iron(III) 38,39 and nitrate. ...
We investigated the mechanisms of uranium (U) uptake by Tamarix (Salt Cedars) growing along the Rio Paguate, which flows throughout the Jackpile Mine near Pueblo de Laguna, New Mexico. Tamarix were selected for this study due to the detection of U in the roots and shoots of field collected plants (0.6– 58.9 mg/kg), presenting an average bioconcentration factor greater than 1. Synchrotron-based micro X-ray fluorescence analyses of plant roots collected from the field indicate that the accumulation of U occurs in the cortex of the root. The mechanisms for U accumulation in the roots of Tamarix were further investigated in controlled-laboratory experiments where living roots of field plants were macerated for 24 h or 2 weeks in a solution containing 100 µM U. The U concentration in the solution decreased 36-59% after 24 h, and 49-65% in two weeks. Microscopic and spectroscopic analyses detected U precipitation in the root cell walls near the xylems of the roots, confirming the initial results from the field samples. High-resolution TEM was used to study the U fate inside the root cells, and needle-like U-P nanocrystals, with diameter <7 nm, were found entrapped inside a vacuole in the cell. EXAFS shell-by-shell fitting suggest that U is associated with carbon functional groups. The preferable binding of U to the root cell walls may explain the U retention in the roots of Tamarix, followed by U-P crystal precipitation, and pinocytotic active transport and cellular entrapment. This process resulted in a limited translocation of U to the shoots in Tamarix plants. This study contributes to better understanding of the physicochemical mechanisms affecting the U uptake and accumulation by plants growing near contaminated sites.
... The stripping of uranyl nitrate from TBP requires lower nitrate concentrations due to the presence of nitrates in both the organic phase and the stripping solution [7], which leads to the need for higher aqueous to organic ratios or multiple stripping contacts resulting in increased waste volumes. Usually, the stripping is followed by a recovery step, which can typically entail the chemical precipitation of the target metal, i.e. uranium, from the loaded stripping solution [8][9][10][11] where additional impurities are however commonly introduced. ...
The feasibility of electrolytic stripping of U(VI) using an electrolytic separation cell with anode and cathode compartments, separated by a cation exchange membrane (CEM), was investigated as a possible final step in a traditional solvent extraction process to recover uranium. The reduction mechanism of U(VI)–U(IV) in oxalic acid was compared with the well-known nitric acid and hydrazine mechanism. It was shown that a similar mechanism was followed in both electrolytes, forming a U(V) intermediate followed by disproportionation to U(IV) and U(VI). Measurable amounts of U(IV) were produced and the transfer of U(VI) through the CEM with simultaneous electro-reduction to U(IV) was confirmed.
... Of these materials, ZVI aggregates have been widely tested for reductive immobilization of redox sensitive metals, metalloids and radionuclides [14]. Gu et al. found that ZVI filings provided faster reaction kinetics and higher efficiency for U(VI) removal compared to peat and iron oxides, and the researchers confirmed that U(IV) precipitates were formed on the ZVI surface [15]. More recently, Yan et al. reported that bare ZVI nanoparticles removed U(VI) 5 times faster than conventional iron flings owing to the greater specific surface area and reactivity [13], and the Xray photoelectron spectroscopy (XPS) results confirmed that reductive conversion of U(VI) to U(IV) was the key mechanism through analyses. ...
Uranium is one of the most commonly detected radionuclides in the environment. Of the two most predominant oxidation states, U(VI) is much more soluble, mobile and toxic than U(IV). Consequently, converting U(VI) to U(IV) can facilitate the removal of U from water and reduce its mobility and biological exposure. In this work, stabilized zero-valent iron (ZVI) nanoparticles were prepared using starch or carboxymethyl cellulose (CMC) as stabilizers and then tested for reductive removal of U(VI) from simulated groundwater. Nearly 100% removal of U(VI) (initial U = 25 mg/L) was achieved using CMC-stabilized ZVI (Fe = 35 mg/L) at pH 6. In pH range of 6–9, the lower pH favored the reaction. CMC-ZVI nanoparticles presented better deliverability than starch-ZVI, while bare ZVI nanoparticles was almost trapped in the soil column. CMC-ZVI worked effectively in the presence of a model humic acid (up to 10 mg/L as TOC) and bicarbonate (1 mM), though higher dosages of the ligands inhibited U(VI) removal. After treatment, no re-mobilization of U was detected when aged for 6 months under anoxic conditions and the addition of strong ligands only remobilized U(VI). When exposed to oxic conditions, the immobilized U will be partially oxidized and remobilized due to the ingress of atmospheric O2 and CO2. In terms of toxicity reduction, the ZVI treated U had almost no inhibition for natural bacteria activity, while dissolved U(VI) showed significant inhibitive effects. The CMC-ZVI nanoparticles may serve as effective reactive materials to facilitate immobilization of U(VI) in groundwater, which in turn can greatly mitigate the human exposure and toxic effects of U on biota.
... A valuable alternative is in situ mineral transformation of a metallic iron, Fe 0 , or zero valent iron (ZVI), potentially offering continuum reductive separation and structural incorporation of Tc under ambient conditions from complex aqueous electrolyte mixtures over wide pH and concentration ranges, albeit via poorly understood mechanisms. Overall, ZVI is a readily available strong reductant that has been proposed for decontamination and remediation strategies, including reductive removal of radioactive contaminants, uranyl and pertechnetate, in groundwater [22][23][24] . ZVI (iron powder, nano-iron, and steel coupons) exhibits effective reductive separation of Tc [25][26][27] , and is one of the materials compatible with immobilization and stabilization of the separated Tc for long-term disposal, but its application can be modulated by Tc anticorrosive properties 28,29 , and has not been investigated for high Tc loading. ...
The sequestration of metal ions into the crystal structure of minerals is common in nature. To date, the incorporation of technetium(IV) into iron minerals has been studied predominantly for systems under carefully controlled anaerobic conditions. Mechanisms of the transformation of iron phases leading to incorporation of technetium(IV) under aerobic conditions remain poorly understood. Here we investigate granular metallic iron for reductive sequestration of technetium(VII) at elevated concentrations under ambient conditions. We report the retarded transformation of ferrihydrite to magnetite in the presence of technetium. We observe that quantitative reduction of pertechnetate with a fraction of technetium(IV) structurally incorporated into non-stoichiometric magnetite benefits from concomitant zero valent iron oxidative transformation. An in-depth profile of iron oxide reveals clusters of the incorporated technetium(IV), which account for 32% of the total retained technetium estimated via X-ray absorption and X-ray photoelectron spectroscopies. This corresponds to 1.86 wt.% technetium in magnetite, providing the experimental evidence to theoretical postulations on thermodynamically stable technetium(IV) being incorporated into magnetite under spontaneous aerobic redox conditions.
Recent studies have revealed that a combination of zero-valent iron (ZVI) and pyrite (FeS2) can effectively remove (Cr(VI)) from water, but the reasons behind this synergistic effect are still unclear. Our batch experiments showed that dissolved oxygen (DO) is a critical factor in the improved removal of Cr(VI) by ZVI and pyrite. When 0.08 g/L pyrite was combined with 0.5 g/L ZVI in the presence of DO, total Cr was reduced from 10 mg/L to 0.02 mg/L within 6 h. Conversely, in the absence of DO, total Cr was only reduced to 5.6 mg/L. DO oxidation of pyrite produced protons that promote ZVI corrosion, and mixing pyrite with water creates dissolved sulfide, which also contributes to the improved removal of Cr(VI). Electron microscopy images and X-ray absorption near edge structure analyses revealed that the presence of dissolved sulfide led to the formation of ferrous sulfide precipitates on the ZVI surface, preventing the formation of a passivating layer.
Covalent-organic-framework-based composites (COF-based composites) are novel functional materials constructed by combining COFs and various organic/inorganic materials, which usually exhibit more advantageous physicochemical properties, stability, selectivity, and pollutant removal performance compared with pristine COFs. In this review, we highlighted the detailed categories and the recent advances of binary and ternary COF-based composites. In particular, the progress of COF-based composites for the efficient removal of heavy metals and radionuclides is comprehensively summarized. By comparing the performance with other materials, we found that the functional materials make the composites stand out. In addition, the technologies to investigate the internal mechanism of pollutants removed by COF-based composites, mainly including characterization analysis and theoretical calculations, are discussed and summarized, providing the direction for in-depth understanding of COF-based composites. According to the discussion above, the opportunities and challenges of COF-based composites in the treatment of heavy metals and radionuclides are summarized and prospected. Through this review, we desire to raise the awareness of COF-based composites and advance further studies in this field by proposing the potential research directions and applications.Graphical Abstract
The removal of hazardous metal ions from liquid waste effluents is very important for water as well as environmental safety. In this regard, this article discusses in detail the U(VI) uptake from aquatic environment using biomass-based Soya Bean activated carbon (labeled as AC-SB). XRD, SEM, FTIR, Raman, and BET analysis were used to characterize the synthesized AC-SB sorbent. Batch-type experiments were used to investigate the effect of various parameters on adsorption efficiency, including pH, metal-ion concentration, temperature, and contact time. The sorption experimental data have been described well with pseudo-second-order kinetic mathematical equations. The equilibrium state of the uptake reaction was 120 min. The Langmuir isotherm model accurately described the equilibrium process which declares that the uranium sorption is a monolayer and homogeneous process. The sorption capacity of the prepared AC was 32.7 mg g ⁻¹ . Thermodynamic analysis explore that the U(VI) uptake process is endothermic, feasible and spontenous process. The displayed results demonstrate that the prepared AC-SB sorbent could be used as the proper material for uranium sorption from real matrix samples.
In this work, zero-valent iron (ZVI) combined with anaerobic bacteria was used in the remediation of Cd(II)-polluted soil under the mediation of sulfate (SO42-). Owing to hydrogen-autotrophic sulfate reduction, serious corrosion occurred on sulfate-mediated biotic ZVI in terms of solid phase characterization as massive corrosive products (e.g., goethite, magnetite and green rust) were formed, which were crucial in the immobilization of Cd(II). Thus, this integrated system achieved a 4.9-fold increase in aqueous Cd(II) removal and converted more than 53% of easily available Cd(II)-fractions (acid-extractable and reducible) to stable forms (oxidizable and residual) based on the sequential extraction results as compared to the sulfate-mediated ZVI system. Increasing SO42- concentration and ZVI dosage both demonstrated positive correlation to Cd(II) immobilization, which further reflected that hydrogenotrophic desulfuration acted an essential role in improving Cd(II) immobilization. It indicated that hydrogenotrophic desulfuration could accelerate iron corrosion and promote reactive mineral formation through biomineralization, as well as generate cadmium sulfide precipitates (CdS) to achieve excellent immobilization performance for Cd(II). Besides, this reaction was favorable under neutral pH condition. Our results highlighted the promoted effect of hydrogen-autotrophic desulfuration on ZVI corrosion to immobilize Cd(II) and offered a practicable technique in Cd(II)-polluted soil remediation.
Science denial relates to rejecting well-established views that are no longer questioned by scientists within a given community. This expression is frequently connected with climate change, and evolution. In such cases, prevailing views are built on historical facts and consensus. For water remediation using metallic iron (Fe0), the remediation Fe0/H2O system, a consensus on electrochemical contaminant reduction was established during the 1990s and is still prevailing. Arguments against the reductive transformation concept has been regarded for more than a decade as 'science denial'. However, is it the prevailing concept that had denied the science of aqueous iron corrosion? This communication retraces the path used by our research group to question the reductive transformation concept. It is shown that the validity of the following has been questioned: (i) analytical applications of arsenazo III method for the determination of uranium, (ii) molecular diffusion as sole relevant mass transport process in the vicinity of the Fe0 surface in filtration systems, and (iii) volumetric expansive nature of iron corrosion at pH > 4.5. Item (i) questions the capability of Fe0 to serve as electron donor for UVI reduction under environmental conditions. Items (ii) and (iii) are interrelated as the Fe0 surface is permanently shielded by an oxide scale acting as diffusion barrier to dissolved species and conductive barrier to electrons from Fe0. The net result is that no electron transfer from Fe0 to contaminants is possible under environmental conditions. This conclusion refutes the validity of the reductive transformation concept and call for alternatives.
Effective separation and decontamination of U(VI) from radioactive wastewater is beneficial to both the sustainable development of nuclear energy and the reduction of radioactive pollution. Electrosorption has recently emerged as a promising technology for U(VI) separation from solution. Herein, the biomass-derived carbon/polypyrrole (BC/PPy) electrodes with high hybrid capacitance and hierarchical porous structure were fabricated by the electrodeposition of PPy on the BC surface for using as pseudocapacitive electrodes for enhancing U(VI) electrosorption. The electrochemical measurements showed that the BC/PPy electrodes exhibited a significant capacitance-controlled behavior in which the capacitance contributions could be quantized. The U(VI) electrosorption by BC/PPy was conducted in a capacitive deionization (CDI) system. The results suggested that among different BC/PPy composites, BC/PPy-3 presented best electrosorption performance, owing to the improved electric conductivity, the hierarchical porous structure, and the synergistic effect of hybrid capacitance (EDL and pseudo-capacitance). The electrosorption isotherms and kinetics were found to be well-simulated by Langmuir and PFO models. The BC/PPy-3 showed a maximum electrosorption capacity of 237.9 mg/g for U(VI) at the applied voltage of 0.9 V, it also exhibited good recycling stability after several electrosorption-desorption cycles. This work provides a promising strategy to the fabrication of cost-effective biomass-derived carbon/conductive polymer composite electrodes for the effective CDI treatment of radioactive wastewater.
CdS-based composites as the highly efficient photocatalyst have been extensively investigated in recent years due to the suitable band gap and high photocatalytic efficiency. In this study, the effect of various factors (pH, U(VI) concentration, contents, and types of photocatalyst) on photocatalytic reduction of U(VI) by MoS2/CdS composite was investigated. The optimized experimental conditions (e.g., pH 7.0, 20 mg/g U(VI), and 1.0 g/L photocatalyst) was obtained by batch techniques. Approximately 97.5% of U(VI) was photo-catalytically reduced into U(IV) by 2.5 wt% MoS2/CdS composite within 15 min. After 5 cycles, 2.5 wt% MoS2/CdS composite still exhibited the high removal efficiency of U(VI) under 50-min irradiation, indicating the good stability. The photo-reduction mechanism of U(VI) on MoS2/CdS composite was attributed to the O−2 radicals by quenching experiments, ESR, and XPS analysis. The findings indicate that CdS-based catalyst has a great potential for the photocatalytic reduction of uranyl in actual environmental remediation.
Owing to the abundant uranium reserves in the oceans, the collection of uranium from seawater has aroused the widespread interest. Compared to the uranium extraction from ore, uranium collection from seawater is a more environmentally friendly strategy. The amidoxime (AO) functional group has been considered as one of the most efficient chelating groups for uranium capture. In this work, by drawing upon the photothermal character and antibacterial activity of cuttlefish ink, a cuttlefish ink loaded polyamidoxime (CI-PAO) membrane adsorbent is developed. Under one-sun illumination, the CI-PAO membrane shows a high extraction capacity of 488.76 mg-U/g-Ads in 500 mL 8 ppm uranium spiked simulated seawater, which is 1.24 times higher than PAO membrane. The adsorption rate of CI-PAO membrane is increased by 32.04%. Furthermore, exhibiting roughly 75% bacteriostatic rate in composite marine bacteria, the CI-PAO shows a dramatically antibacterial activity, which effectively prevents the functional sites on the adsorbent surface from being occupied by the biofouling blocks. After immersing in natural seawater for 4 weeks, light-irradiated CI-PAO gave high uranium uptake capacity of 6.17 mg-U/g-Ads. Hence, the CI-PAO membrane adsorbent can be considered as a potential candidate for the practical application for uranium extraction from seawater.
In this study, hydrogen-autotrophic microorganisms and zero-valent iron (Fe⁰) were filled into columns to investigate hydrogenotrophic denitrification effect on cadmium (Cd(II)) removal and column life-span with sand, microorganisms, Fe⁰ and bio-Fe⁰ columns as controls. In terms of the experiment results, the nitrate-mediated bio-Fe⁰ column showed a slow Cd(II) migration rate of 0.04 cm/PV, while the values in the bio-Fe⁰ and Fe⁰ columns were 0.06 cm/PV and 0.14 cm/PV respectively, indicating much higher Cd(II) removal efficiency and longer service life of the nitrate-mediated bio-Fe⁰ column. The XRD and SEM-EDX results implied that this improvement was attributed to hydrogenotrophic denitrification that caused more serious iron corrosion and larger amount of secondary mineral generation (e.g., green rust, lepidocrocite and goethite). These active minerals provided more reaction sites for Cd(II) adsorption and further immobilization. In addition, the decrease of Cd(II) migration front and the increase of removal capacity along the bio-Fe⁰ column mediated by nitrate presented an uneven distribution in reactive zone. The latter half part was identified to be a more active region for Cd(II) immobilization. The above results indicate that the introduction of nitrate and microorganisms will improve the performance of iron-based permeable reactive barriers for the remediation of Cd(II)-containing groundwater.
During the chemical weathering of the uranium mill tailings, released uranium could be immobilized by the newly formed secondary minerals such as oxyhydroxides. A deeper understanding of the interaction between uranium and common oxyhydroxides under environmental conditions is necessary. In this work, uranium sorption behaviors on Al-, Mn- and Fe-oxyhydroxide minerals (boehmite, manganite, goethite, and lepidocrocite) were investigated by batch experiments. Results showed that the uranium sorption on Al-oxyhydroxide behaved significantly differently from the other three minerals. The sorption edge of the Mn- and Fe-oxyhydroxides located around pH 5, while the sorption edge of boehmite shifted about 1.5 pH unit to near neutral. The sorption isotherms of uranium on manganite, goethite and lepidocrocite at pH 5.0 could be well fitted by the Langmuir model. Instead of surface complexation, sorption on boehmite happened mainly by uranium-bearing carbonates and hydroxides precipitation as illustrated by the characterization results. Both carbonate and phosphate strongly affected the uranium sorption behavior. The removal efficiency of uranium by boehmite exceeded 98% after three sorption-desorption cycles, indicating it may be a potential material for uranium removal and recovery.
A N, S, P co-doped and oxidized porous carbon was prepared by the carbonization of poly (cyclotriphosphazene-co-4,4’-sulfonyldiphenol) at 750 °C, followed by KOH activation and HNO3 oxidation. The carbon was used as an adsorbent for uranium(VI) in aqueous solutions. TEM, SEM, XPS and FTIR were used to characterize its microstructure before and after adsorption. Results indicate that there is an optimum pH value of 6 for U(VI) adsorption. The adsorption kinetics and isotherms were fitted well by the pseudo-second-order and the Langmuir models, respectively. The maximum adsorption capacity determined by the Langmuir model at 298 K and a pH value of 6 was 402.9 mg g⁻¹. The carbon has excellent reusability and retains 70% of the capacity of the original value after five adsorption-desorption cycles. The high U(VI) adsorption capacity is mainly attributed to the carboxyl, and P and S groups by the formation of the UO2²⁺(COO⁻)2 complex, and U―O―P and U―O―S bonds.
With the rapid development of nuclear-related industries in the world, the consequent environmental pollution of radionuclides has become an increasingly serious problem due to the great harm to environmental diversity and human health. The photocatalytic removal of radionuclide on CdS-based photocatalysts has been attracted widely attentions due to the suitable band gap and high photocatalytic efficiency. In this study, approximately 97.5% of U(VI) was photo-catalytically reduced into U(IV) by 2.5 wt% MoS 2 /CdS composite within 15 min. After 5 cycles, 2.5 wt% MoS 2 /CdS composite still exhibited the high removal efficiency of U(VI) under 50 min irradiation, indicating the good stability. The findings indicated that CdS-based catalyst has a great potential for the photocatalytic reduction of uranyl in actual environmental remediation.
Nanoscale zero-valent iron (nZVI) is a widely applied nanomaterial in the removal of toxic and radioactive metal ions from groundwater and industrial wastewater. The advancement of nZVI (i.e., small size, large surface area, environmental friendliness, and high reactivity) has made it feasible to separate many toxic metal ions from wastewater while fulfilling its large-scale application. In this chapter, we aim to describe the progress thus far with a view to providing a comprehensive summary of toxic and radioactive metal ion removal using nZVI-based materials. Several synthetic and modification techniques are introduced for pristine nZVI and nZVI-based nanomaterials. Furthermore, we overview the major adsorptive and reductive factors influencing the application of nZVI to toxic and radioactive metal ions, such as nZVI intrinsic characteristics (e.g., surface area, surface passivation, and metallic iron core), different target species (e.g., U(VI), Tc(VII), Se(IV)/Se(VI), Cr(VI), As(III)/As(V), Hg(II), Cu(II), and Ni(II)), and various operating conditions (e.g., pH, coexisting anions and cations, reaction time, temperature, and natural organic matter), and the potential mechanisms involved. The interaction mechanism between respective contaminants and nZVI-based nanomaterials with advanced spectroscopic techniques and the DFT theoretical calculation model are discussed. Moreover, the toxicity and risks of nZVI-based nanomaterial application are also examined. These efforts and attempts can broaden the development of nZVI-based nanomaterials for large-scale water treatment.
The treatment for large amounts of low concentration radioactive wastewater has always been a worldwide problem, just like the Fukushima nuclear power plant is facing the problem of insufficient storage space. In this work, we have assessed the effectiveness of flow electrode capacitive deionization (FCDI) as a recently developed electrochemical technology to concentrate the radioactive wastewater for the first time. Continuous batch experiments demonstrated that uranium (U) can accumulate into the electrolyte and on activated carbon at the same time in the flow electrode, and the removal efficiency of U remained over 99% during each cycle. Combined with XPS analysis, the migration route and valence change of U in the flow electrode were revealed, which helped to understand the high adsorption capacity of U in FCDI. Long-term batch experiments exhibited that the low concentration of feed water (60 mg L⁻¹ U) could be concentrated by 47 times after 48th continuous cycles, achieving a final concentration of 2843 mg L⁻¹ U in the electrolyte, and 40 mL reduced volume of uranium-containing water from 2,400 mL. Under optimized conditions, a charge efficiency of 86% and a low energy consumption of 2.03 mg J⁻¹ were achieved in 360 mg L⁻¹ initial concentration of UO2²⁺. Overall, high removal rate, excellent concentration effect and low energy consumption make FCDI to be a promising way for radioactive wastewater treatment.
MoS4–Ppy was synthesized by a simple oxidative polymerization method and functionalized with the MoS4²⁻ ions. Because of the good stability and excellent ion-exchange properties, it has been considered as one of the promising candidates for metal ions adsorption. In this regard, the as-prepared MoS4–Ppy was used to remove U(VI) and well characterized by XRD, TEM, FT-IR, and XPS techniques. The batch adsorption experiments were systematically studied. It was found that the adsorption of U(VI) on MoS4–Ppy largely depends on the solution pH; the maximum adsorption capacity of MoS4–Ppy was determined to be 333.9 mg/g. Impressively, MoS4–Ppy showed excellent selectivity toward U(VI). The adsorption mechanism indicated that U(VI) mainly depends on the electrostatic interaction of MoS4²⁻ on the surface of MoS4–Ppy. And comprehensive experimental results show that MoS4–Ppy is highly stable under acid conditions, which can effectively remove radionuclides from wastewater.
Although nanoscale zero-valent iron (nZVI) has been intensively investigated for the remediation of uranium contaminated water, few efforts have been dedicated to U(VI) removal and recovery by nZVI from strongly alkaline solutions (e.g., pH 10.7) in the presence of carbonate. This work demonstrates that U(VI) can be removed by nZVI from strongly alkaline solutions in the anaerobic atmosphere. Results show that reductive precipitation is responsible for U(VI) removal with an effective reduction efficiency of nZVI up to 95.5 ± 3.3% (6.09 ± 0.21 g of U/g of Fe). In the aerobic atmosphere, however, the efficiency is no more than 8.3% (0.53 g of U/ g of Fe) due to relatively rapid corrosion reactions of nZVI as well as the re-oxidation of U(IV). The difference in the efficiency originates not only from the variation in the kinetics of corrosion reactions, but also from U(VI) redox and coordinate chemistry. Under the aerobic condition, uranium removal reaches the maximum at 1 h of reaction and then decreases, because uranium delivery from solid phase occurs due to successive processes of U(IV) oxidation, UO2²⁺ complexation with carbonate, and [UO2(CO3)3]⁴⁻ desorption from surfaces. The uranium delivery limits the application of nZVI for uranium removal under the aerobic condition, but provides a candidate approach for the uranium recovery from the spent nZVI. A recovery of 100% can be obtained by sodium carbonate solution in the presence of O2. Both the oxidation and desorption processes control the uranium recovery rate. This study not only offers new insight into the mechanisms of interactions between U(VI) and nZVI especially in strongly alkaline media with high concentration of carbonate, but also provides a reference approach to the issue of U(VI) removal and recover from the wastewater generated in the nuclear industry.
Biogeochemical hotspots are defined as areas where biogeochemical processes
occur with anomalously high reaction rates relative to their surroundings.
Due to their importance in carbon and nutrient cycling, the characterization
of hotspots is critical for predicting carbon budgets accurately in the context of climate change. However, biogeochemical hotspots are difficult
to identify in the environment, as methods for in situ measurements often directly affect the sensitive redox-chemical conditions. Here, we present
imaging results of a geophysical survey using the non-invasive induced
polarization (IP) method to identify biogeochemical hotspots of carbon turnover in a minerotrophic wetland. To interpret the field-scale IP
signatures, geochemical analyses were performed on freeze-core samples
obtained in areas characterized by anomalously high and low IP responses.
Our results reveal large variations in the electrical response, with the
highest IP phase values (> 18 mrad) corresponding to high concentrations of phosphates (> 4000 µM), an indicator of
carbon turnover. Furthermore, we found a strong relationship between the
electrical properties resolved in IP images and the dissolved organic
carbon. Moreover, analysis of the freeze core reveals negligible
concentrations of iron sulfides. The extensive geochemical and geophysical
data presented in our study demonstrate that IP images can track small-scale changes in the biogeochemical activity in peat and can be used to
identify hotspots.
Pollution causes a significant change in the physico-chemical properties of air, water, and soil that may induce harmful impact and make potential hazards to all living beings. Pollution is one of the severe problems observed throughout the world. Water pollution has ascended due to the nearness of toxic environment, which arisen by human activities such as increased agricultural, industrial, and urban area developments. According to the thriving of the human population, the need for energy is increasing for the development of the world and its economy. Coal, oil, and other nonrenewable fossil are the prime sources of energy which have regularly consumed. Now, the world is facing an unexpected energy emergency. Due to the utilization of radioactive nuclides with the progress in human civilization, the atmosphere is facing cumulative radioactive pollution. Natural and anthropogenic radionuclides and its effect on humans, animals, and all other living beings have become a major environmental problem because of their prevalent occurrence in the atmosphere with concentrations that exceed the World Health Organization (WHO) recommended maximum levels. Operative, suitable, effective, and environmental friendly techniques are essential to mitigate this problem. This chapter describes the development of different conventional techniques and their removal performance of uranium(VI) ions from aqueous solution. Moreover, the chapter also investigates the various adsorbent materials used for the removal of radioactive uranium(VI) ions from water.
Due to its reliable performance and low manufacturing cost, biochar has gradually attracted attention in the treatment of uranium-containing wastewater. Herein, Luffa rattan (LR), a biowaste material, was used to synthesize functional biochar (F-LRBC) to adsorb U(VI) from aqueous solutions for the first time. The effect of solution pH, contact time and initial concentration of uranyl ions on uranium(Ⅵ) adsorption by F-LRBC was evaluated. FT-IR and SEM analysis confirmed that there are abundant oxygen-containing functional groups and pores on the surface of F-LRBC. Experiments demonstrated that the adsorption behavior of uranium(Ⅵ) by F-LRBC is spontaneous and efficient. The maximum adsorption capacity of F-LRBC for uranyl ions predicted by the Langmuir model was higher than that of most biochar adsorbent. The adsorption mechanism of uranyl ions on F-LRBC is complicated, including electrostatic interaction, complexation and physical adsorption.
With proper modification, polyamidoxime (PAO) shows extraordinary solubility in aqueous alkaline solution. A hybrid sponge was prepared by freeze-drying an aqueous solution of PAO/alginate. After simple in situ ionic and covalent crosslinking, the sponge showed highly efficient uranium capture performance with a saturated adsorption capacity of 718.38 ± 12.38 mg g-1, as well as excellent chemical and mechanical stability in running seawater for weeks. This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.
Pertinent to the behaviour of carbon steel based nuclear waste packages in saline geological environments, sorption and reduction of U(VI) on real container corrosion products has been studied in an Ar/8%H 2 flushed glove box. A carbon steel was corroded in MgCl 2-brine at elevated temperatures. U(VI) was added and the redox states both of uranium and iron and their distribution among solid and liquid phases were investigated. The corrosion products initially consisted to more than 97% of hydrous Fe(II) oxides containing Cl and Mg 2+, but always some magnetite was present. In the course of the sorption step, the amount of magnetite increased. Reaction products buffer pH values of the system in a very narrow neutral range. Less than 1% of iron or uranium were found in colloidal state and already after one day, more than 98% of initially dissolved uranium was found associated with the immobile phases. A reciprocal relationship of solution concentrations of uranium with the nominal mass of magnetite was found. Behaviour of uranium species was rationalized in terms of Eh/pH diagrams. Reduction of hexavalent to tetravalent uranium was observed but to a much less pronounced extent than expected from thermodynamical considerations.
The rate and selectivity of reductions of various organic functional groups by dissolving metals can be influenced by cementation of a second metal onto the surface of the base metal. Pb, Pd or Ni deposition onto Zn, Al or Fe had a significant effect.
The adsorption of uranyl species onto a well-characterized hematite sol has been studied in the presence of bicarbonate ions. The uptake of uranium decreases abruptly with increasing solution pH. The decrease coincides with a decrease in the proportion of (UO2)2CO3(OH)3− present. Increasing the adsorption temperature from 25 to 60°C alters little the adsorption behavior. Results of electrophoretic mobility studies suggest that more than one uranium species may be involved in the adsorption and that the adsorbed species are negatively charged. Fourier Transform Infrared spectrum of the adsorption products show that carbonate groups are present, indicating that uranium is retained as carbonate complexes.
To date it does not appear to have been demonstrated in the literature that halogenated ethylenes can undergo reductive {beta}-elimination to alkynes under environmental conditions. The purpose of this paper is to provide experimental evidence that such pathways may be involved in the reaction of chloroethylenes with zero-valent metals as well as to speculate on the significance of the products that may result. Calculations indicate that reductive {beta}-elimination reactions of chloroethylenes are in fact comparable energetically to hydrogenolysis at neutral pH. Experiments were therefore initiated to assess whether {beta}-elimination reactions of chlorinated ethylenes could occur in the presence of two zero-valent metals, Fe and Zn. 76 refs., 3 figs., 1 tab.
The interaction of aqueous U(VI) with galena and pyrite surfaces under anoxic conditions has been studied by solution analysis and by spectroscopic methods. The solution data indicate that uranyl uptake is strongly dependent on pH; maximum uptake (>98%) occurs above a pH range of between 4.8 and 5.5, depending on experimental conditions. Increasing the sorbate/sorbent ratio results in a relative decrease in uptake of uranyl and in slower sorption kinetics' Auger electron spectroscopy (AES) analysis indicates an inhomogeneous distribution of sorbed uranium at the surface. In the case of galena, formation of small precipitates (approximately 40 nm wide needles) of a uranium oxide compound are found. Pyrite shows a patchy distribution of uranium, mainly associated with oxidized surface species of sulfur and iron. X-ray photoelectron spectroscopy (XPS) yields insight into possible redox processes indicating, for both sulfides, the concomitant formation of polysulfides and a uranium oxide compound with a mixed oxidation state at a U(VI)/U(IV) ratio of approximately 2. Furthermore, in the case of pyrite, at pH above 6 increased oxidation of sulfur and iron and higher relative amounts of unreduced surface-uranyl are observed. Fourier Transformed Infrared (FTIR) analysis of surface-bound uranyl shows a significant shift of the asymmetric stretching frequency to lower wavenumbers which is consistent with the formation of a U3O8-type compound and thus, independently, confirms the partial reduction of uranyl at the sulfide surface. The combination of AES, XPS, and FTIR provides a powerful approach for identifying mechanisms that govern the interaction of redox sensitive compounds in aqueous systems. Our overall results indicate that sulfide minerals are efficient scavengers of soluble uranyl. Comparing our results with recent field observations, we suggest that thermodynamically metastable U3O8 controls uranium concentrations in many anoxic groundwaters.
The generation and release of acidic drainage containing high concentrations of dissolved metals from decommissioned mine wastes is an environmental problem of international scale. A potential solution to many acid drainage problems is the installation of permeable reactive walls into aquifers affected by drainage water derived from mine waste materials. A permeable reactive wall installed into an aquifer impacted by low-quality mine drainage waters was installed in August 1995 at the Nickel Rim mine site near Sudbury, Ontario. The reactive mixture, containing organic matter, was designed to promote bacterially mediated sulfate reduction and subsequent metal sulfide precipitation. The reactive wall is installed to an average depth of 12 feet (3.6 m) and is 49 feet (15 m) long perpendicular to ground water flow. The wall thickness (flow path length) is 13 feet (4 m). Initial results, collected nine months after installation, indicate that sulfate reduction and metal sulfide precipitation is occurring. The reactive wall has effectively removed the capacity of the ground water to generate acidity on discharge to the surface. Calculations based on comparison to previously run laboratory column experiments indicate that the reactive wall has potential to remain effective for at least 15 years.
An efficient fiberoptic interface system was developed to convert a Gary 14 H spectrophotometer into a fiberoptic instrument. This new spectrophotometer can be used to study the UV-visible and near-IR absorption spectroscopy of radioactive samples and wastes in a confined environment.
The overall goal of this portion of the project was to package one or more unit processes, as modular components in vertical and/or horizontal recirculation wells, for treatment of volatile organic compounds (VOCs) [e.g., trichloroethene (TCE)] and radionuclides [e.g., technetium (Tc){sup 99}] in groundwater. The project was conceived, in part, because the coexistence of chlorinated hydrocarbons and radionuclides has been identified as the predominant combination of groundwater contamination in the US Department of Energy (DOE) complex. Thus, a major component of the project was the development of modules that provide simultaneous treatment of hydrocarbons and radionuclides. The project objectives included: (1) evaluation of horizontal wells for inducing groundwater recirculation, (2) development of below-ground treatment modules for simultaneous removal of VOCs and radionuclides, and (3) demonstration of a coupled system (treatment module with recirculation well) at a DOE field site where both VOCs and radionuclides are present in the groundwater. This report is limited to the innovative treatment aspects of the program. A report on pilot testing of the horizontal recirculation system was the first report of the series (Muck et al. 1996). A comprehensive report that focuses on the engineering, cost and hydrodynamic aspects of the project has also been prepared (Korte et al. 1997a).
High-temperature near-IR absorption spectroscopy was used to study the dissolution of UO2 in molten MgCl2, CaCl2, and AlCl3 melts. The study reveals that UO2 is most soluble in molten AlCl3, followed by the melt of MgCl2. The solubility of UO2 in molten CaCl2 is too small to be measured with optical spectroscopy. This strong dependence of the solubility on the cations of the melts was rationalized by the use of the Flood−Forland−Grjotheim thermodynamic cycle.
Uranyl ion (UO22+) has been doped in silica glasses through a sol−gel process. The photochemistry of UO22+-doped glasses immersed in ethanol solution has been investigated. Our experimental results indicate that UO22+ in this system can be photochemically converted to U(IV) species, and evidence is provided for the possible formation of U(IV) species via a subsequent chemical reduction of the intermediate UO2+ instead of the conventional bimolecular disproportionation reaction. The latter mechanism is known to be a dominant mechanism for the photochemical reactions in homogeneous solutions.
The contaminant of most concern in groundwater at the Oak Ridge Y-12 Plant's Bear Creek Valley Characterization Area is soluble uranium. The removal mechanism of soluble uranium from groundwater by zero-valent iron (ZVI, Fe0) was investigated. X-ray photoelectron spectroscopy (XPS, ESCA) was used to determine the uranium oxidation state at the Fe0 or iron oxide surface. Product speciation and relative reaction kinetics for the removal of soluble uranium under aerobic and anaerobic conditions with ZVI are presented. Under aerobic conditions, U6+ is rapidly and strongly sorbed to hydrous ferric oxide particulates (“rust”), whereas U6+ is slowly and incompletely reduced to U4+ under anaerobic conditions.
Permeable-reactive redox walls, placed below the ground surface in the path of flowing groundwater, provide an alternative remediation approach for removing electroactive chemicals from contaminated groundwater. Four types of Fe-bearing solids, siderite [FeCO3], pyrite [FeS2], coarse-grained elemental iron [Fe0], and fine-grained Fe0, were assessed for their ability to remove dissolved Cr(VI) from solution at flow rates typical of those encountered at sites of remediation. Batch studies show that the rate of Cr(VI) removal by fine-grained Fe0 is greater than that for pyrite and coarse-grained Fe0. Results from column studies suggest that partial removal of Cr(VI) by pyrite and coarse-grained Fe0 and quantitative removal of Cr(VI) by fine-grained Fe0 occur at rapid groundwater flow velocities. The removal mechanism for Cr(VI) by fine-grained Fe0 and coarse-grained Fe0 is through the reduction of Cr(VI) to Cr(III), coupled with the oxidation of Fe0 to Fe(II) and Fe(III), and the subsequent precipitation of a sparingly soluble Fe(III)−Cr(III) (oxy)hydroxide phase. Mineralogical analysis of the reactive material used in the batch tests indicates that Cr is associated with goethite (α-FeOOH). These results suggest that Cr(III) is removed either through the formation of a solid solution or by adsorption of Cr(III) onto the goethite surface. The effective removal of Cr(VI) by Fe0 under dynamic flow conditions suggests porous-reactive walls containing Fe0 may be a viable alternative for treating groundwater contaminated by Cr(VI).
Recent studies have shown promising results for subsurface remediation of dissolved chromate using permeable-reactive redox walls. Chromate reduction in the presence of iron filings and quartz grains was studied to determine the fate of reduced chromium in proposed wall material. Using a flow-through column apparatus, iron filings mixed with quartz grains were reacted with solutions that contained about 20 mg/L dissolved Cr(VI). Reacted iron filings developed coatings comprised of goethite with chromium concentrated in the outermost edges. Surface analysis showed all detectable chromium occurred as Cr(III) species. In addition, in regions of increased chromium concentration, goethite acquired chemical and structural characteristics similar to Fe2O3 and Cr2O3. Results of the study show that complete reduction of Cr(VI) to Cr(III) occurred and that Cr(III) was incorporated into sparingly soluble solid species.
Trichloroethene (TCE) was reduced with zero-valence iron and palladized iron in zero-head-space extractors. Progress of the reaction in these batch studies was monitored with purge-and-trap gas chromatography and a flame ionization detector. When a 5 ppm initial concentration of TCF. reacts with zero-valence iron, approximately 140 ppb of vinyl chloride persists for as long as 73 days. The concentration of vinyl chloride (approximately If) ppb) remaining with palladized iron is approximately an order of magnitude less than when zero-valence iron is the reductant. These data suggest that volatile byproducts may be under-represented in oilier published data regarding reduction with zero-valence metals. These results also demonstrate that the reduction of TCE with palladized iron (0.05 percent palladium) is more than an order of magnitude faster than with zero-valence iron. Wilh a 5:1 solution-to-solid ratio the TCE half-life with zero-valence iron is 7.41 hours. but is only 0.59 hours with the palladized iron.
Gibbs free energies, enthalpies and entropies of 42 dissolved uranium species and 30 uranium-bearing solid phases have been critically evaluated from the literature and estimated when necessary for 25°C. Application of the data shows that the uranium in natural waters is usually complexed. At typical concentrations of chloride, fluoride, phosphate and sulfate, uranous (U4+) fluoride complexes are important in anoxic waters below pH 3–4. An intermediate Ehs (between about +0.2 and −0.1 V) and pH values 1–7, UO2+ ion may predominate. In oxidized waters, uranyl (U22+) fluoride complexes and uranyl ion predominate below pH 5; from about pH 4 to 7.5, UO2(HPO4)22− is the principal species; while at higher pHs, UO2CO30 and the di- and tri-carbonate complexes predominate. Uraninite [UO2-UO2.25], α-U3O8 and schoepite are the stable uranium oxides and hydroxides in water at 25°C. Coffinite, USiO4 (c), is probably stable relative to UO2(c) when dissolved silica exceeds about 60 ppm (as SiO2). At low Ehs and pH 4–6, the solubilities of stoichiometric crystalline uraninite and coffinite are below roughly 10−4 ppb. But at intermediate Ehs and neutral to alkaline pHs in the presence of phosphate or carbonate, the formation of uranyl phosphate or carbonate complexes can increase the solubilities of these minerals by several orders of magnitude. The uranyl minerals carnotite, tyuyamunite, autunite, potassium autunite and uranophane are least soluble at pHs in the range 5–8.5 and, in the case of carnotite and tyuyamunite, have solubilities as low as 0.2 and 1 ppb uranium, respectively. The autunites and uranophane are usually several orders of magnitude more soluble than this, consistent with their natural occurrences. Sorption of uranyl on to natural materials is maximal in the same pH range of 5–8.5.
This work reports a new approach that can effectively separate and recover Tc (as pertechnetate, TcO4−) from contaminated groundwater. Activated carbon was used in both batch adsorption and column leaching studies. The adsorption experiments indicated that activated carbon adsorbs TcO4− selectively and effectivele over a wide range of pH values and from various dilute electrolyte solutions (< 0.01 M). The partitioning coefficient (Kd) of TcO4− exceeded 27 000 ml/g when actual groundwater was used, and exceeded 12000 ml/g when background solutions of 0.01 M CaCl2 and Na2SO4 were used. TcO4− removal efficiency was > 99% under these conditions, except in a 0.01 M NaNO3 background solution. Column studies confirmed a high adsorption capacity and selectivity of activated carbon for TcO4−. Within the detection limit, no Tc breakthrough was observed when more than 14000 pore volumes of contaminated groundwater (containing ∼3000 pCi Tc/1) were passed through a small column (6.6 × 30 mm) with 0.5 g activated carbon. Recovery of TcO4− from activated carbon was studied using various chemical reagents such as salicylate, phthalate, NaNO3, NaCl, and Na2SO4. Salicylate was found to be the most effective in desorbing and recovering the adsorbed TcO4− (as high as 100%). Therefore, the spent carbon can be disposed as low-level radioactive wastes or may be regenerated. Results of this work suggest that the use of activated carbon to remediate Tc-contaminated groundwater can be a promising technology — it is cost-effective and requires minimal installation and maintenance during the pump-and-treat processes.
Different organic compounds or fractions of natural organic matter (NOM) show different adsorption affinities (K) and capacities (qm) on mineral surfaces. We hypothesize that these different organic compounds or fractions compete for adsorption when surface adsorption sites are limited. In this study, competitive adsorption of binary mixtures of Suwannee River NOM (SR-NOM), polyacrylic acid (PAA), phthalic acid, and salicylic acid on iron oxide was investigated at a constant solid:solution ratio, temperature, and pressure, but at varying C weight fractions, pH, and solution concentrations of the mixture. Results revealed that, in general, PAA is the most competitive whereas SR-NOM is more competitive than phthalic and salicylic acids. The competitive adsorption of these organic compounds is pH-dependent. At pH < 4, PAA becomes less competitive than SR-NOM or phthalic and salicylic acids. The competition among these organic compounds may be related to their carboxyl functional groups and their molecular structure. The overall strong competitiveness of PAA at pH > 4 in comparison with other organics is attributed to its high carboxyl density and linear molecular structure, which promote strong surface complexation with iron oxide. Because of the heterogeneity or polydispersity of NOM, this research indicates that NOM partitioning and transport in the subsurface soil environment are influenced by the dynamic competitive interactions between NOM subcomponents (or fractions). This process ultimately influences the distribution, interaction, and cotransport of contaminants and mineral colloids that are associated with NOM.
To better understand the geochernical processes that result in the reduction of hexavalent chromium [Cr(VI)] in natural waters, the reaction of Cr(VI) with Fe(II) has been studied as a function of pH and temperature. Over the concentration range studied [0.3–100 μM Fe(II) and 0.9–600 μM Cr(VI)], the reaction follows first-order kinetics with respect to Fe(H) and Cr(VI). The relationship between the rate coefficient and solution conditions can be explained by considering the reactivity of each of the Fe(II) species that are present at significant concentrations [i.e., Fe2+, FeOH+, and Fe(OH)2]. The effect of pH on the rate of reduction of Cr(VI) can be expressed by the following relationship: where and The value of k can be adjusted for the effects of temperature by correcting the constants using published ΔH values. These results suggest that the reduction of Cr(VI) occurs on the timescale of minutes to months in Fe(II)-containing sediments, soils, and waters.
Zero-valent iron (Fe0), metallic iron, is being evaluated as a permeable reactive barrier material to mitigate the transport of a wide array of highly mobile contaminants in groundwater. Zero-valent iron has previously been shown to destroy effectively numerous chlorinated hydrocarbon compounds via reductive dehalogenation. No references could be found regarding the ability of zero-valent iron to reduce UO2+2, MoO2−4, or TcO−4.A series of kinetic-batch studies was conducted to determine the capability of particulate Fe0 to remove UO2+2, MoO2−4, TcO−4, and CrO2−4 from groundwater. Particulate Fe0 effectively removed each of these contaminants from solution; removal rates decreased as follows: CrO2−4 > TcO−4 > UO2+2 ⪢ MoO2−2. The removal mechanism appears to be reductive precipitation. Thermodynamic equilibrium calculations indicated that the rate of removal of the metals from solution increased as the difference in pe (Δpe) increased between the redox half reaction for the redox couple of interest and the couple. Furthermore, the pe value for a redox couple provided a qualitative indication of the reduction rate by Fe0. These results indicate that the rate of removal of CrO2−4, TcO−4, and UO2+2 from groundwater is rapid, permitting an inexpensive barrier of practical dimensions to be used for in situ remediation purposes.
Reduction of chlorinated solvents by fine-grained iron metal was studied in well-mixed anaerobic batch systems in order to help assess the utility of this reaction in remediation of contaminated groundwater. Iron sequentially dehalogenates carbon tetrachloride via chloroform to methylene chloride. The initial rate of each reaction step was pseudo-first-order in substrate and became substantially slower with each dehalogenation step. Thus, carbon tetrachloride degradation typically occurred in several hours, but no significant reduction of methylene chloride was observed over 1 month. Trichloroethene (TCE) was also dechlorinated by iron, although more slowly than carbon tetrachloride. Increasing the clean surface area of iron greatly increased the rate of carbon tetrachloride dehalogenation, whereas increasing pH decreased the reduction rate slightly. The reduction of chlorinated methanes in batch model systems appears to be coupled with oxidative dissolution (corrosion) of the iron through a largely diffusion-limited surface reaction.
The reduction of chromium from the Cr(VI) to the Cr(III) state by the presence of elemental, or zero-oxidation-state, iron metal was studied to evaluate the feasibility of such a process for subsurface chromate remediation. Reactions were studied in systems of natural aquifer materials with varying geochemistry. Different forms of iron metal had significantly different abilities to reduce chromate, ranging from extremely rapid to essentially no effect. Impure, partially oxidized iron was most effective, with iron quantity being the most important rate factor, followed by aquifer material type and solid:solution ratio. Evidence for chromium-iron hydroxide solid solution (CrXFe1-x)(OH)(3)(ss) formation was obtained by electron probe microanalysis. A cyclic, multiple reaction electrochemical corrosion mechanism, enhanced by the development of an electrical double-layer analogue, is proposed to explain the differing iron reactivities and aquifer material effects.
This study was undertaken to elucidate the interaction mechanism between NOM (natural organic matter) and iron oxide surfaces and to develop a predictive model for NOM adsorption and desorption. Results indicated that ligand exchange between carboxyl/hydroxyl functional groups of NOM and iron oxide surfaces was the dominant interaction mechanism, especially under acidic or slightly acidic pH conditions. This conclusion was supported by the measurements of heat of adsorption (microcalorimetry), FTIR and [sup 18]C NMR analysis, and competitive adsorption between NOM and some specifically adsorbed anions. A modified Langmuir model was proposed in which a surface excess-dependent affinity parameter was defined to account for a decreasing adsorption affinity with surface coverage due to the heterogeneity of NOM and adsorbent surfaces. With three adjustable parameters, the model is capable of describing a variety of adsorption isotherms. A hysteresis coefficient, h, was used to describe the hysteretic effect of adsorption reactions that, at h = 0, the reaction is completely reversible, whereas at h = 1, the reaction is completely irreversible. Fitted values of h for NOM desorption on iron oxide surfaces ranged from 0.72 to 0.92, suggesting that the adsorbed NOM was very difficult to be desorbed at a given pH and ionic composition. 54 refs., 8 figs., 3 tabs.
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