Extractive removal of chromium (VI) from industrial waste solution

Extractive removal of Cr (VI) was carried out from chloride solutions using cyanex 923 mixed with kerosene. The efficiency of this extractant was studied under various experimental conditions, such as concentration of different mineral acids in the aqueous phase, concentration of cyanex 923 and Cr (VI) present in the initial aqueous feed, temperature and time of extraction, organic to aqueous (O/A) phase ratio. Percentage Cr (VI) extraction decreases with the increase in temperature at varying concentration of cyanex 923. The interference of the impurities usually associated with Cr (VI) such as Cr (III), Cu, Ni, Fe (II), Zn, Chloride and sulphate, etc., were examined under the optimized conditions and only Zn was found to interfere. Under the optimum experimental conditions 98.6-99.9% of Cr (VI) was extracted in 3-5 min at O/A of 2 with the initial feed concentration of 1g/L of Cr (VI). The extracted Cr (VI) was quantitatively stripped with 1M NaOH and the organic phase obtained after the stripping of Cr (VI) was washed with dilute HCl solution to neutralize any NaOH trapped/adhered to the solvent and then with distilled water. This regenerated solvent was reused in succeeding extraction of chromium (VI). Finally a few experiments were performed with the synthetic effluent from an electroplating industry.     

Study of Extraction of Palladium from Nitric Acid Solutions with Nitrogen-Containing Compounds,as Applied to Recovery of Fission Palladium from Spent Nuclear Fuel of Nuclear Power Plants: 3. Optimization of Extraction Process for Palladium Recovery and Refining

An extraction process for recovery of fission Pd is proposed. The process parameters are optimized with simulated high-level liquid radioactive waste from nuclear power plants using tri-n-octylamine (TOA) and tricaprylmethylammonium nitrate (Aliquat 336 nitrate) as extractants. The degree of Pd decontamination from fission and corrosion products is estimated and the major interfering components are determined.     

Optimization of process parameters for the extraction of chromium (VI) by emulsion liquid membrane using response surface methodology

The emulsion liquid membrane technique was used for the extraction of hexavalent chromium ions from aqueous solution of waste sodium dichromate recovered from the pharmaceutical industry wastewater. The liquid membrane used was composed of kerosene oil as the solvent, Span-80 as the surfactant and potassium hydroxide as internal reagent. Trioctyl amine and Aliquat-336 were used as carriers. The emulsion stability was carried out at different surfactant concentration, agitation speed and emulsification time. Statistical experimental design was applied for the optimization of process parameters for the extraction of chromium by emulsion liquid membrane. The effects of process parameters namely, agitation speed, membrane to emulsion (M/E) ratio and carrier concentration on the extraction of chromium were optimized using a response surface method. The optimum conditions for the extraction of chromium (VI) using response surface methodology for Trioctyl amine were: agitation speed – 201.369 rpm, M/E ratio – 0.5887% (v/v) and carrier concentration – 4.0932% (v/v) and for Aliquat-336: agitation speed – 202.097 rpm, M/E ratio – 0.5873% (v/v) and carrier concentration – 3.9211% (v/v). At the optimized condition the maximum chromium extraction was found to be 89.2% and 96.15% using Trioctyl amine and Aliquat-336, respectively.     

Effect of formaldehyde on Cu(II) removal from synthetic complexed solutions by solvent extraction

The effect of formaldehyde (HCHO) on the extraction of Cu(II) from an equimolar EDTA (ethylenediaminetetraacetic acid, H4L) solution with Aliquat 336 in kerosene (a quaternary amine) was examined. Experiments were carried out at different initial concentrations of Cu(II) (1.57-5 mol/m3), solution pH (3.0-8.0), HCHO concentrations (0-3 vol.%), Aliquat 336 concentrations (80-400 mol/m3), and temperatures (15-35 degrees C). It was shown that the distribution ratios (D) of Cu(II), which exists in the form of complexed anions CuL2-, increased with increasing equilibrium pH (pHeq), but reached a plateau at pHeq>4 for the system without HCHO and at pHeq>4.5 for the system with 1 vol.% HCHO. The D values increased with increasing HCHO concentration, likely due to the reduction of Cu(II) to Cu(I) by HCHO in solution. A semi-empirical two-parameter model was proposed to describe the extraction equilibrium, in which the non-ideality in organic phases was considered. The thermodynamic parameters were also evaluated and discussed.     

Study of Extraction of Palladium from Nitric Acid Solutions with Nitrogen-Containing Compounds,as Applied to Recovery of Fission Palladium from Spent Nuclear Fuel of Nuclear Power Plants: 2. Effect of Radiation on Palladium Recovery and Condition of Extraction Systems

Extraction of palladium nitrate complexes from 1-3 M HNO₃ with tri-n-octylamine (TOA) and tricaprylmethylammonium nitrate (Aliquat 336 nitrate) in diethylbenzene under irradiation at a dose of up to 100 W h l^{- 1} is studied. In irradiated extraction systems, the acidity of the aqueous and organic phases decreases and HNO₂ is formed. With increasing radiation dose the degree of Pd recovery decreases by virtue of partial degradation of the organic phase. The radiation effects on the condition of extraction systems are discussed.     

Selective preconcentration and determination of chromium(VI) using a flat sheet polymer inclusion sorbent: potential application for Cr(VI) determination in real samples

A method for sorption preconcentration of Cr(VI) from aqueous samples was developed using a polymer inclusion sorbent (PIS). The PIS used in this method was prepared by physical inclusion of Aliquat-336 in the matrix formed by cellulose triacetate and 2-nitrophenyl octyl ether. This sorbent was found to be stable, cost-effective, efficient for preconcentration of Cr(VI) present in the aqueous samples, and amenable to direct quantitative analysis of Cr(VI) held in it by neutron activation analysis and spectrophotometry. The quantifying of Cr(VI) in PIS by spectrophotometry was carried out by developing color directly on the PIS after reacting it with 1,5-diphenylcarbazide. The distinct color developed on the PIS even at very low concentrations of Cr(VI) suggests its possible use for field determination of Cr(VI). The composition of PIS was optimized to obtain maximum uptake of Cr(VI) without sacrificing uniformity in terms of thickness and distribution of ion-exchange sites, stability, and time required for quantitative sorption of Cr(VI) from aqueous samples. The Cr(VI) species held in the PIS, mainly HCrO4- and CrO4(2-), were found to vary as a function of pH of the aqueous samples from which Cr(VI) was preconcentrated. A close agreement was found in the abundances of Cr(VI) species held in the PIS with those reported in the literature for aqueous solutions at different pH. The variation of Cr(VI) species as a function of pH was found to have a significant impact on the tolerance to anions on the uptake of Cr(VI) in the PIS. The high selectivity of PIS toward Cr(VI) from aqueous solution at pH = 2 was explained on the basis of hydration of anions. The uptake of Cr(VI) was found to be fairly constant (88 +/- 3%) up to nearly complete exchange of counterions present in the PIS. The method developed in the present work was successfully used for the preconcentration of Cr(VI) from tap water and seawater samples containing low levels of Cr(VI).     

Cr(VI) sorption behavior from aqueous solutions onto polymeric microcapsules containing a long-chain quaternary ammonium salt: Kinetics and thermodynamics analysis

This work studies the adsorption of Cr(VI) ions from an aqueous acid solution on hydrophobic polymeric microcapsules containing a long-chain quaternary ammonium salt-type extractant immobilized in their pore structure. The microcapsules were synthesized by adding the extractant Aliquat 336 during the in situ radical copolymerization of the monomers styrene (ST) and ethylene glycol dimethacrylate (EGDMA). The microcapsules, which had a spherical shape with a rough surface, behaved as efficient adsorbents for Cr(VI) at the tested temperatures. The results of kinetics experiments carried out at different temperatures showed that the adsorption process fits well to a pseudo-second-order with an activation energy of 82.7 kJ mol⁻¹, confirming that the sorption process is controlled by a chemisorption mechanism. Langmuir's isotherms were found to represent well the experimentally observed sorption data. Thermodynamics parameters, namely, changes in standard free energy (ΔG⁰), enthalpy (ΔH⁰), and entropy (ΔS⁰), are also calculated. The results indicate that the chemisorption process is spontaneous and exothermic. The entropy change value measured in this study shows that metal adsorbed on microcapsules leads to a less chaotic system than a liquid–liquid extraction system.     

Stabilization of chromium ore processing residue (COPR) with nanoscale iron particles

Laboratory batch experiments were conducted on heavily contaminated groundwater and chromium ore processing residue (COPR) samples to determine the rate and extent of hexavalent chromium [Cr(VI)] reduction and immobilization by nanoscale iron particles. Laboratory synthesized nanoscale iron particles (<100 nm, specific surface area 35 m(2)/g) were used for this work. Groundwater ([Cr(VI)]=42.83 +/- 0.52 mg/L, pH 11.0+/-0.5) and COPR samples ([Cr(VI)] = 3280 +/- 90 mg/kg) were collected from an industrial site in New Jersey. Cr(VI) in the water and COPR samples was quickly reduced and precipitated out of the aqueous solution. The surface area normalized reaction rate constant of Cr(VI) reduction by nanoscale iron particles was 0.157 +/- 0.018 mg m(-2) min(-1), about 25 times greater than that by iron powders (100 mesh). One gram of nanoparticles can reduce 84.4-109.3mg Cr(VI) in the groundwater and 69.3-72.7 mg Cr(VI) in the COPR. This reduction capacity is 50-70 times greater than that of iron powders under the same experimental conditions.     

Sorption of Cr(VI) ions on two Lewatit-anion exchange resins and their quantitative determination using UV-visible spectrophotometer

The sorption of Cr(VI) from aqueous solutions with macroporous resins which contain quarternary amine groups (Lewatit MP 64 and Lewatit MP 500) was studied at varying Cr(VI) concentration, adsorbent dose, pH, contact time and temperature. Batch shaking sorption experiments were carried out to evaluate the performance of Lewatit MP 64 and Lewatit MP 500 anion exchange resins in the removal of Cr(VI) from aqueous solutions. The concentration of Cr(VI) in aqueous solution was determined by UV-visible spectrophotometer. The ion exchange process, which is dependent on pH, showed maximum removal of Cr(VI) in the pH range 3-7 for an initial Cr(VI) concentration of 1x10(-3) M. The optimum pH for Cr(VI) adsorption was found as 5.0 for Lewatit MP 64 and 6.0 for Lewatit MP 500. The maximum Cr(VI) adsorption at pH 5.0 is 0.40 and 0.41 mmol/g resin for Lewatit MP 64 and Lewatit MP 500 anion exchangers, respectively. The maximum chromium sorption occurred at approximately 60 min for Lewatit MP 64 and 75 min for Lewatit MP 500. The suitability of the Freundlich and Langmuir adsorption models was also investigated for each chromium-sorbent system. The uptake of Cr(VI) by the anion exchange resins was reversible and so it has good potential for the removal of Cr(VI) from aqueous solutions. Both ion exchangers had high bonding constants but Lewatit MP 500 showed stronger binding. The rise in the temperature caused a slight decrease in the value of the equilibrium constant (K(c)) for the sorption of Cr(VI) ion.     

Application of immobilized nanotubular TiO(2) electrode for photocatalytic hydrogen evolution: reduction of hexavalent chromium (Cr(VI)) in water

In this study, immobilized TiO(2) electrode is applied to reduce toxic Cr(VI) to non-toxic Cr(III) in aqueous solution under UV irradiation. To overcome the limitation of powder TiO(2), a novel technique of immobilization based on anodization was applied and investigated under various experimental conditions. The anodization was performed at 20V-5 degrees C for 45min with 0.5% hydrofluoric acid, and then the anodized samples were annealed under oxygen stream in the range 450-850 degrees C. Based on the results of the experiments, the photocatalytic Cr(VI) reduction was favorable in acidic conditions, with approximately 98% of the Cr(VI) being reduced within 2h at pH 3. Among the samples tested under same anodizing condition, the nanotubular TiO(2) annealed at 450 and 550 degrees C showed highest reduction efficiencies of Cr(VI). In addition, the surface characterizations (zeta potential, XRD, and SEM) of these samples proved that the Cr(VI) reduction efficiency was higher under acidic conditions and at a lower annealing temperature. From this research, it was concluded that the anodized TiO(2) has the potential to be a useful technology for environmental remediation as well as photocatalytic hydrogen production from water.     

A novel method for speciation of Cr(III) and Cr(VI) and individual determination using Duolite C20 modified with active hydrazone

Trivalent and hexavalent chromium have been successfully separated and estimated from different solutions using 1-(3,4-dihydroxybenzaldehyde)-2-acetylpyridiniumchloride hydrazone (DAPCH) loaded on Duolite C20 in batch and column modes. The obtained modified resin [DAPCH-Duolite C20] was identified by C, H and N analyses and infrared spectra. The presence of multi-active chelating sites gives the ability for DAPCH to bind more chromium, Cr(III) by forming stable complex and chromate by forming ion pair molecule [H(2)DAPCH-Duolite C20](2+)[Cr(2)O(7)](2-) (H(2)DAPCH-Duolite C20 is the protonated form in acidic medium). The extraction isotherms were measured at different pH. The pH was found to be the backbone for the separation procedure in which the Cr(VI) and Cr(III) ions are sorbed selectively from aqueous solution at pH 2 and 6, respectively. The sorbed ions can be eluted using different concentrations of HCl. The saturation sorption capacity (41.6 and 20.05 mg g(-1)), the preconcentration factor (150 and 200) and the detection limit (13.3 and 10.0 ppb) were calculated for Cr(III) and (VI). The loaded resin can be regenerated for at least 50 cycles. The utility of the modified resin was tested in aqueous samples and shows R.S.D. value of <4% reflecting its accuracy and reproducibility.     

Removal of chromium from aqueous solution by using oxidized multiwalled carbon nanotubes

The batch removal of hexavalent chromium (Cr(VI)) from aqueous solution by using oxidized multiwalled carbon nanotubes (MWCNTs) was studied under ambient conditions. The effect of pH, initial concentration of Cr(VI), MWCNT content, contact time and ionic strength on the removal of Cr(VI) was also investigated. The removal was favored at low pH with maximum removal at pH <2. The adsorption kinetics was modeled by first-order reversible kinetics, pseudo-first-order kinetics, pseudo-second-order kinetics, and intraparticle diffusion models, respectively. The rate constants for all these kinetic models were calculated, and the results indicate that pseudo-second-order kinetics model was well suitable to model the kinetic adsorption of Cr(VI). The removal of chromium mainly depends on the occurrence of redox reaction of adsorbed Cr(VI) on the surface of oxidized MWCNTs to the formation of Cr(III), and subsequent the sorption of Cr(III) on MWCNTs appears as the leading mechanism for chromium uptake to MWCNTs. The presence of Cr(III) and Cr(VI) on oxidized MWCNTs was confirmed by the X-ray photoelectron spectroscopic analysis. The application of Langmuir and Freundlich isotherms are applied to fit the adsorption data of Cr(VI). Equilibrium data were well described by the typical Langmuir adsorption isotherm. Overall, the study demonstrated that MWCNTs can effectively remove Cr(VI) from aqueous solution under a wide range of experimental conditions, without significant Cr(III) release.     

Separation and subsequent determination of low radioactivity levels of radium by extraction scintillation

A method is developed for separation and subsequent determination of low (226)Ra levels based on its extraction efficiency by various organic extractants followed by direct interaction with mixed scintillation cocktail (SC) in toluene as the most suitable solvent. Three organic extractants including Tricaprylmethyleammonium chloride (Aliquat-336), triphenylphosphine sulphide (TPPS) and tri-n-octylphosphine oxide (TOPO) are individually embedded in 1-phenyl,4-phenyloxazole (PPO) as a primary scintillator and 1,4-di-2-(4-methyl, 5-phenyl oxazoyl) benzene (POPOP) as a second scintillator. Different parameters affecting the extraction process including shaking time, type and concentration of extractant, aqueous/organic volume ratio as well as various levels of (226)Ra are individually investigated. Based on the optimized condition, it was found that the extraction of (226)Ra by Aliquat-336 is highly selective with efficient separation capability from (133)Ba, which is necessary to determine the yield and recovery of (226)Ra in related measurements. The combination of extraction with direct scintillation processes is mainly elaborated to overcome the heterogeneity problem and hence the slow scintillation rate that often exists in conventional liquid scintillation counting especially with low radioactivity levels.     

Adsorption behavior of Cr(VI) on modified natural zeolite by a new bolaform N,N,N,N',N',N'-hexamethyl-1,9-nonanediammonium dibromide reagent

The demand for effective adsorbents is to increase in response to the widespread recognition of the deleterious health effects of Cr(VI)-oxyanions exposure through drinking water. In this study, Cr(VI)-oxyanions uptake from aqueous solutions by a new bolaform N,N,N,N',N',N'-hexamethyl-1,9-nonanediammonium dibromide reagent-modified natural zeolitic materials from Zahedan, Iran, was investigated using batch technique. Spectrophotometry method was used for Cr determination. The Cr(VI)-solution concentration varied between 2 and 104 mg L(-1). It was shown that the Cr(VI) uptake strongly depended on pH. The maximum removal of Cr(VI) occurred in acidic media at pH<1.5. The amounts of Cr(VI) adsorbed increased with increase in dose of both adsorbents and their contact time. Based on results an adsorption mechanism has been suggested. The adsorption data for modified zeolite using the amine was consistent with Langmuir isotherm equation and the equilibrium data was analyzed using the Langmuir isotherm.     

Cloud-point preconcentration and spectrophotometric determination of trace amounts of molybdenum(VI) in steels and water samples

A cloud-point extraction process using micelle of the cationic surfactant CTAB to extract Mo(VI) from aqueous solutions was investigated. The method is based on the color reaction of molybdenum with bromopyrogallol red in the presence of potassium iodide at pH 1.0 glycine/HCl buffer media and micelle-mediated extraction of complex. The optimal extraction and reaction conditions (e.g., surfactant concentration, reagent concentration and effect of time) were studied and the analytical characteristics of the method (e.g., limit of detection, linear range, preconcentration and improvement factors) were obtained. Linearity was obeyed in the range of 0.3-320.0 ng mL(-1) of molybdenum(VI) ion and the detection limit of the method was 0.1 ng mL(-1). The relative standard deviation (R.S.D.) and relative error for five replicate measurements of 65.0 ng mL(-1) Mo(VI) were 1.1% and 1.9%, respectively. The interference effect of some anions and cations was also tested. The method was applied to the determination of molybdenum(VI) in steels and tap water and well water samples.     

Characterization of Cr(VI) binding and reduction to Cr(III) by the agricultural byproducts of Avena monida (oat) biomass

Chromium contamination of the environment has become an important issue due to the potential health threat it poses. Conventional technologies to clean up heavy metal ions from contaminated waters have been utilized, but these technologies are not cost-effective. However, the use of agricultural waste byproducts for the removal of Cr(VI) from contaminated waters may be a new cost-effective alternative. Oat byproducts from the Juarez Valley in Mexico were studied for the ability to bind Cr(VI) under different temperature and time conditions. The metal binding ability of oat byproducts was calculated from experimental data collected at temperatures of 8, 26, and 54°C, and time exposures of 1, 6, 24, 48, and 72 h at each temperature. These results showed that the binding of Cr(VI) to oat biomass increased as time and temperature increased. The bound chromium was recovered from the oat biomass by treatment with 0.2 M HCl. Through the use of X-ray absorption spectroscopy, the reduction of Cr(VI) to Cr(III) was determined to occur by the oat byproducts. These results indicate that the use of agricultural waste byproducts could be a better alternative for the removal and subsequent reduction of Cr(VI) to Cr(III) from contaminated waters.     

Adsorption of chromium(VI) and Rhodamine B by surface modified tannery waste: kinetic, mechanistic and thermodynamic studies

In this study, various activation methods have been employed to examine the potential reuse of tannery residual biomass (TRB) obtained from vegetable tanning process for the removal of Cr(VI) and Rhodamine B (RB) from aqueous solution. The maximum BET surface area (10.42 m(2)/g), honey comb pore distribution and uptake of both Cr(VI) and RB were achieved when only 3-fold volume of HCl was used to activate the biomass. The pH and temperature experiment showed that they have considerable impact on the adsorption capacity of the used adsorbent. The presence of other ions (Na(+), Ca(2+) and NH(4)(+)) significantly reduces the metal uptake but marginal enhancement in the dye removal was observed when Na(+) and NH(4)(+) ions were present in the solution. The equilibrium data fitted satisfactorily with the Langmuir model and monolayer sorption capacity obtained as 177-217 and 213-250 mg/g for Cr(VI) and RB at 30-50°C, respectively. The sorption kinetics was found to follow the pseudo-second-order kinetic model. The increase in adsorption capacity for both metal and dye with increase in temperature indicates that the uptake was endothermic in nature. The results indicate that the HCl modified TRB (A-TRB) could be employed as a low cost adsorbent for the removal of both Cr(VI) and RB from the aqueous solution including industrial wastewater.     

Statistical design of experiments as a tool for optimizing the batch conditions to Cr(VI) biosorption on Araucaria angustifolia wastes

In order to reduce the total number of experiments for achieving the best conditions for Cr(VI) uptake using Araucaria angustifolia (named pinh?o) wastes as a biosorbent, three statistical design of experiments were carried out. A full 2(4) factorial design with two blocks and two central points (20 experiments) was experimented (pH, initial metallic ion concentration-C(o), biosorbent concentration-X and time of contact-t), showing that all the factors were significant; besides, several interactions among the factors were also significant. These results led to the performance of a Box-Behnken surface analysis design with three factors (X, C(o) and t) and three central points and just one block (15 experiments). The performance of these two statistical designs of experiments led to the best conditions for Cr(VI) biosorption on the pinh?o wastes using a batch system, where: pH 2.0; C(o) = 1200 mg l(-1) Cr(VI); X = 1.5 g l(-1) of biosorbent; t = 8 h. The maximum Cr(VI) uptake in these conditions was 125 mg g(-1). After evaluating the best Cr(VI) biosorption conditions on pinh?o wastes, a new Box-Behnken surface analysis design was employed in order to verify the effects of three concomitant ions (Cl(-), NO(3)(-) and PO(4)(3-)) on the biosorption of Cr(VI) as a dichromate on the biosorbent (15 experiments). These results showed that the tested anions did not show any significant effect on the Cr(VI) uptake by pinh?o wastes. In order to evaluate the pinh?o wastes as a biosorbent in dynamic system, a glass column was fulfilled with pinh?o wastes (4.00 g) as biosorbent, and it was fed with 25.0 mg l(-1) Cr(VI) at pH 2.0 and 2.5 ml min(-1). The breakpoint was attained when concentrations of effluent of the column attained the value of 0.05 mg l(-1) Cr(VI) using 5550 ml of the metallic ion solution. In these conditions, the biosorbent was able to remove completely Cr(VI) from aqueous solution with a ratio of Cr(VI) effluent volume/biosorbent volume of 252.3.     

Synthesis of Cr(VI)-imprinted poly(4-vinyl pyridine-co-hydroxyethyl methacrylate) particles: its adsorption propensity to Cr(VI)

The aim of this study is to prepare ion-imprinted polymers, which can be used for the selective removal of Cr(VI) anions from aqueous media. 4-Vinyl pyridine (4-VP) was used as functional monomer. The Cr(VI)-imprinted poly(4-vinyl pyridine-co-2-hydroxyethyl methacrylate), poly(VP-HEMA), particles were prepared by bulk polymerization. The Cr(VI)-imprinted polymer particles were grained from the bulk polymer, and the template ions (i.e., Cr(VI)) were removed using thiourea (0.5%, v/v) in 0.5M HCl. The Cr(VI)-imprinted polymer contained 21.4 μmol 4-VP/g polymers. The specific surface area of the IIP2 particles was found to be 34.5m(2)/g (size range of 75-150 μm), and the swelling ratio was about to 108%. The effect of initial concentration of Cr(VI) anions, the adsorption rate and the pH of the medium on adsorption capacity of Cr(VI)-imprinting polymer were studied. The maximum experimental adsorption capacity was 3.31 mmol Cr(VI)/g polymer. Under competitive condition, the adsorption capacity of Cr(VI)-imprinted particles for Cr(VI) is 13.8 and 11.7 folds greater than that of the Cr(III) and Ni(II) ions, respectively. The first- and second order kinetics models were estimated on the basis of comparative analysis of the corresponding rate parameters, equilibrium capacity and correlation coefficients. The Langmuir adsorption isotherm model was well described the Cr(VI)-imprinted system and the maximum adsorption capacity (Q(max)) was found to be 3.42 mmol/g. Moreover, the reusability of the poly(VP-HEMA) particles was tested for several times and no significant loss in adsorption capacity was observed.     

Effects of pH and dissolved oxygen on Cr(VI) removal in Fe(0)/H2O systems

The effects of pH and dissolved oxygen (DO) on aqueous Cr(VI) removal by micro-scale zero-valent iron (Fe(0)/H(2)O system) were investigated. Batch experiments were conducted at pH 4.0, 5.0 and 6.0 under oxic and anoxic conditions. Column experiments were performed at pH 5.0 and 7.5 under oxic condition. Spectroscopic analyses were applied to explain the mechanism of Cr(VI) removal using X-ray absorption near-edge structure (XANES), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Results showed that the kinetics of Cr(VI) removal were fastest at pH 5 under both oxic and anoxic conditions. As a rule, Cr(VI) removal were faster under oxic conditions than under anoxic conditions. Column experiments showed that Cr(VI) removal was about 1.7-fold higher at pH 5 than at pH 7.5. XANES (X-ray absorption near edge structures) results showed that Fe(0) reduced Cr(VI) to Cr(III) under both oxic and anoxic conditions. X-ray diffraction patterns of the Cr(VI)-Fe(0) reaction products suggested partial formation of chromite (FeCr(2)O(4)) at pH 5 and 6 under oxic conditions. However, nano-sized clusters of Cr(III)/Fe(III) hydroxide/oxyhydroxide were formed on the surface of Fe(0) under anoxic conditions. These results indicate that the presence of oxygen in solution plays an important role in control of the kinetic of Cr(VI) removal and in development of various Cr(VI) reduction products.