Pyridine appended L-methionine: a novel chelating resin for pH dependent Cr speciation with scanning electron microscopic evidence and monitoring of yeast mediated green bio-reduction of Cr(VI) to Cr(III) in environmental samples

Chemical speciation and pH dependent separation of Cr(III) and Cr(VI) species in environmental samples have been achieved by solid phase extraction using a new chelating resin containing pyridine appended L-methionine. Cr(III) is completely sorbed on the resin at pH 8.0 and Cr(VI) at pH 2.0. Hence a pH dependent separation of Cr(III) and Cr(VI) is possible with a limit of detection of 1.6 μg mL(-1) and 0.6 μg mL(-1) respectively. The sorption capacity of the resin for Cr(III) and Cr(VI) is 2.8 mmol g(-1) and 1.3 mmol g(-1) respectively. The sorption of chromium on the resin is supported by scanning electron microscopy (SEM). Complete desorption of Cr(III) and Cr(VI) from 1g of Cr loaded resin was achieved using 10 mL of 2 mol L(-1) HNO(3) and 6 mL of 3 mol L(-1) HNO(3) respectively. Quantitative recoveries of Cr(III) (pH 8.0) and Cr(VI) (pH 2.0) were found to be 96.0% and 98.0% respectively. Reduction efficiency of Rhodotornula mucilaginosa yeast from Cr(VI) to Cr(III) was monitored with this new resin. Concentrations of metal ions were measured by flame atomic absorption spectroscopy (FAAS).     

Cr(VI) and Cr(III) speciation on Bacillus sphaericus loaded diaion SP-850 resin

A speciation procedure for chromium(III) and chromium(VI) in the environmental samples has been established in the presented work. The procedure presented based on quantitative biosorption of chromium(III) on Bacillus sphaericus loaded Diaion SP-850 at pH 5. The Cr(VI) recoveries at pH 5 were below 5% on the biosorbent. After reduction of Cr(VI) by concentrated H(2)SO(4) and ethanol, the system was applied to the total chromium. Cr(VI) was calculated as the difference between the total chromium content and the Cr(III) content. Optimal analytical conditions including pH, amounts of biosorbent, etc. for Cr(III) recoveries were investigated. The influences of the some alkaline and earth alkaline ions and some transition metals on the recoveries were also investigated. The capacity of biosorbent for chromium(III) was 6.95 mgg(-1). The detection limit (3 sigma) of the reagent blank for chromium(III) was 0.50 microgL(-1). The procedure was successfully applied to the speciation of chromium(III) and chromium(VI) in natural water samples (R.S.D. lower than 5%, recoveries greater than 95%).     

Oxidation of Cr(III) in tannery sludge to Cr(VI): field observations and theoretical assessment

Sludge, soil and leachate samples collected from a chromium-contaminated tannery waste dumping site in Kanpur, India, were found to contain considerable amounts of Cr(VI), despite the fresh tannery sludge containing little or no Cr(VI). Literature reports suggested that dry Cr(III) precipitates could be converted to Cr(VI) when heated in the presence of oxygen. Also, Cr(III) in aqueous phase could be oxidized through interaction with manganese dioxide (MnO2) surface to Cr(VI). Measurement of manganese in the sludge samples collected from the site showed concentrations up to 0.6 mg/g. Based on equilibrium calculations, it was determined that both dry phase Cr(III) oxidation by atmospheric oxygen and aqueous phase Cr(III) oxidation by MnO2 surface were thermodynamically feasible. It was further suggested that in aqueous phase, manganese may act effectively as an electron transporter between Cr(III) and dissolved oxygen during Cr(III) oxidation, leading to regeneration of MnO2 solid phase. Further, as dissolved Cr(III) is oxidized, dissolution of Cr(OH3) will take place to maintain the equilibrium between the dissolved and solid phases of Cr(III). In the pH range of 3-10, and at oxygen partial pressure (P(O2)) of 10(-6) atm or higher, equilibrium conditions stipulate nearly complete conversion of Cr(III) to Cr(VI). At P(O2) of 10(-20) atm or lower, very little Cr(VI) is expected to be present under equilibrium conditions. In the intermediate P(O2) regions, incomplete dissolution of the Cr(OH3) solid phase and only partial conversion of chromium from +3 to the +6 oxidation state is expected, especially at lower pH values.     

A novel solid phase extraction procedure on Amberlite XAD-1180 for speciation of Cr(III), Cr(VI) and total chromium in environmental and pharmaceutical samples

Due to the toxicity of chromium, species depend on their chemical properties and bioavailabilities, speciation of chromium is very important in environmental samples. A speciation procedure for chromium(III), chromium(VI) and total chromium in environmental samples is presented in this work, prior to flame atomic absorption spectrometric determination of chromium. The procedure is based on the adsorption of Cr(III)-diphenylcarbazone complex on Amberlite XAD-1180 resin. After oxidation of Cr(III), the developed solid phase extraction system was applied to determinate the total chromium. Cr(III) was calculated as the difference between the total Cr content and the Cr(VI) content. The analytical conditions for the quantitative recoveries of Cr(VI) on Amberlite XAD-1180 resin were investigated. The effects of some alkaline, earth alkaline, metal ions and also some anions were also examined. Preconcentration factor was found to be 75. The detection limits (LOD) based on three times sigma of the blank (N: 21) for Cr(VI) and total chromium were 7.7 and 8.6 microg/L, respectively. Satisfactory results for the analysis of total chromium in the stream sediment (GBW7310) certified reference material for the validation of the presented method was obtained. The procedure was applied to food, water and pharmaceutical samples successfully.     

Adsorption characteristics and separation of Cr(III) and Cr(VI) on hydrous titanium(IV) oxide

A hydrous titanium(IV) oxide was prepared to study the adsorption characteristics and the separation of chromium species. Batch sorption studies have been carried out to determine the effect of pH on the sorption of Cr(III) and Cr(VI) on hydrous TiO2. An excellent separation efficiency of Cr(III) and Cr(VI) was obtained at pH 2. The adsorption percentage of Cr(VI) was above 99%, whereas that of the Cr(III) was less than 1% at this pH. The adsorption isotherm of Cr(VI) on hydrous TiO2 at pH 2 was in good agreement with the Langmuir isotherm. The maximum adsorption capacity of Cr(VI) on TiO2 was 5 mg g(-1). The rate of adsorption of Cr(VI) by hydrous TiO2 with average particle diameter 250 and 500 microm has been studied under particle diffusion controlled conditions. The diffusion coefficients of Cr(VI) for both hydrous TiO2 having average particle diameter of 250 and 500 microm was calculated at pH 2 as 3.84 x 10(-10) m2 s(-1) and 8.86 x 10(-10) m2 s(-1), respectively.     

Speciation of Cr(III) and Cr(VI) in environmental samples by solid phase extraction on Ambersorb 563 resin

A simple and sensitive method for the speciation, separation and preconcentration of Cr(VI) and Cr(III) in natural water, soil and sediment samples was developed. Cr(VI) has been separated from Cr(III) and preconcentrated as Cr(III)-diphenylcarbazone complex by using Ambersorb 563 resin and determined by spectrophotometric method at 540 nm. Effect of analytical parameters such as sulfuric acid concentration, ligand volume, type of elution solution, sample volume, amount of resin and foreign ions were investigated. The presented procedure was successfully applied for the chromium speciation in various environmental samples with successfully results.     

Grape waste as a biosorbent for removing Cr(VI) from aqueous solution

Grape waste generated in wine production is a cellulosic material rich in polyphenolic compounds which exhibits a high affinity for heavy metal ions. An adsorption gel was prepared from grape waste by cross-linking with concentrated sulfuric acid. It was characterized and utilized for the removal of Cr(VI) from synthetic aqueous solution. Adsorption tests were conducted in batch mode to study the effects of pH, contact time and adsorption isotherm of Cr(VI), which followed the Langmuir type adsorption and exhibited a maximum loading capacity of 1.91 mol/kg at pH 4. The adsorption of different metal ions like Cr(VI), Cr(III), Fe(III), Zn(II), Cd(II) and Pb(II) from aqueous solution at different pH values 1-5 has also been investigated. The cross-linked grape waste gel was found to selectively adsorb Cr(VI) over other metal ions tested. The results suggest that cross-linked grape waste gel has high possibility to be used as effective adsorbent for Cr(VI) removal.     

Extent of oxidation of Cr(III) to Cr(VI) under various conditions pertaining to natural environment

Calculations show that oxidation of chromium oxide (Cr2O3) by oxygen and oxidation of chromium hydroxide (Cr(OH)3) by manganese dioxide (MnO2) are thermodynamically feasible in both aerobic and mildly anoxic environments. Experiments were carried out to determine the rate and extent of chromium oxidation under various conditions, i.e., when Cr2O3 was heated in the presence of oxygen, when Cr(OH)3 and MnO2 mixtures were suspended in aerobic or anoxic aqueous media at various pH values, when Cr(OH)3 and MnO2 mixtures interacted in moist aerobic conditions and when chromium assumed to be Cr(OH)3 and manganese assumed to be MnO2 interacted in the presence of competing electron donors/acceptors, as is the case in chromium-contaminated sludge. Results indicate that trivalent chromium in Cr2O3 could be readily converted to hexavalent chromium at a temperature range of 200-300 degrees C, with conversion rates of up to 50% in 12 h. In aqueous media, Cr(OH)3 was slowly converted to dissolved Cr(VI) in the presence of MnO2, both in aerobic and anoxic conditions, with conversion rates of up to 1% in 60 days. In moist aerobic conditions and in the presence of MnO2, Cr(OH)(3) slowly converted to hexavalent chromium, with up to 0.05% conversion observed in 90 days. Chromium oxidation also occurred in sludge samples, especially under aerobic conditions. However, such transformation was found to be transitory, with the Cr(VI) formed being ultimately reduced back to Cr(III) due to the presence of various reducing agents in the sludge. Nevertheless since up to 17% conversion of Cr(III) to Cr(VI) occurred in sludge under aerobic conditions by 30 days, there is real danger under field conditions of spreading Cr(VI) pollution due to possible intervening rainfall, runoff and percolation.     

Iminodiacetic acid functionalized cation exchange resin for adsorptive removal of Cr(VI), Cd(II), Ni(II) and Pb(II) from their aqueous solutions

Iminodiacetic acid functionality has been introduced on styrene-divinyl benzene co-polymeric beads and characterized by FT-IR in order to develop weak acid based cation exchange resin. This resin was evaluated for the removal of different heavy metal ions namely Cd(II), Cr(VI), Ni(II) and Pb(II) from their aqueous solutions. The results showed greater affinity of resin towards Cr(VI) for which 99.7% removal achieved in optimal conditions following the order Ni(II)>Pb(II)>Cd(II) with 65%, 59% and 28% removal. Experiments were also directed towards kinetic studies of adsorption and found to follow first order reversible kinetic model with the overall rate constants 0.3250, 0.2393, 0.4290 and 0.2968 for Cr(VI), Ni(II), Pb(II) and Cd(II) removal respectively. Detailed studies of Cr(VI) removal has been carried out to see the effect of pH, resin dose and metal ion concentration on adsorption and concluded that complexation enhanced the chromium removal efficacy of resin drastically, which is strongly pH dependent. The findings were also supported by the comparison of FT-IR spectra of neat resin with the chromium-adsorbed resin.     

Speciation of Cr(III) and Cr(VI) in environmental samples determined by selective separation and preconcentration on silica gel chemically modified with niobium(V) oxide

In this study a new method for chromium speciation in water using solid phase extraction coupled to a flow injection system and flame atomic absorption spectrometry was developed. The adsorption behavior of Cr(III) and Cr(VI) on Nb2O5-SiO2 allowed the selective separation of Cr(III) from Cr(VI) in the pH range of 6-9. Thus, a method for Cr(III) preconcentration and extraction using Nb2O5-SiO2 was developed. Total chromium was determined after the reduction of Cr(VI) to Cr(III) using sodium sulfite in acidic medium. The operational variables of the preconcentration and reduction procedures were optimized through full factorial and Doehlert designs. The limit of detection for Cr(III) was 0.34microgL(-1) and the precision was below 4.6%. Results for recovery tests using different environmental samples were between 90 and 105%. Certified reference materials (NIST 1640 and NIST 1643e) were analyzed in order to check the accuracy of the proposed method, and the results were in agreement with the certified values.     

Characterization of Cr(VI) removal from aqueous solutions by a surplus agricultural waste--rice straw

The removal of Cr(VI) from aqueous solution by rice straw, a surplus agricultural byproduct was investigated. The optimal pH was 2.0 and Cr(VI) removal rate increased with decreased Cr(VI) concentration and with increased temperature. Decrease in straw particle size led to an increase in Cr(VI) removal. Equilibrium was achieved in about 48 h under standard conditions, and Cr(III), which appeared in the solution and remained stable thereafter, indicating that both reduction and adsorption played a part in the Cr(VI) removal. The increase of the solution pH suggested that protons were needed for the Cr(VI) removal. A relatively high level of NO(3)(-) notably restrained the reduction of Cr(VI) to Cr(III), while high level of SO(4)(2-) supported it. The promotion of the tartaric acid modified rice straw (TARS) and the slight inhibition of the esterified rice straw (ERS) on Cr(VI) removal indicated that carboxyl groups present on the biomass played an important role in chromium remediation even though were not fully responsible for it. Isotherm tests showed that equilibrium sorption data were better represented by Langmuir model and the sorption capacity of rice straw was found to be 3.15 mg/g.     

Speciation of Cr(III) and Cr(VI) after column solid phase extraction on Amberlite XAD-2010

A speciation procedure for Cr(III) and Cr(VI) based on column solid phase extraction on Amberlite XAD-2010 and flame atomic absorption spectrometry combination. Cr(VI) was quantitatively recovered on Amberlite XAD-2010 resin at pH range of 2.0-3.0 as its diethyldithiocarbamate complex, while the recoveries of Cr(III) was below 5%. The influences of the various parameters including amounts of the reagents, eluent type and its volume, sample volume, etc., on the quantitative recoveries were examined. The interference of matrix and coexistent elements for method were studied. The detection limit (corresponding to three times the standard deviation of the blank) and the enrichment factor for Cr(VI) were found to be 1.28 microg/L and 25, respectively. To verify the accuracy of the method, drinking water certified reference material (CRM-TMDW-500) was analyzed and the results obtained were in good agreement with the certified value. The proposed method has been successfully applied to the speciation of Cr(III) and Cr(VI) in water samples and preconcentration of total chromium in environmental samples.     

Mechanism of Cr(VI) adsorption by coir pith studied by ESR and adsorption kinetic

The oxidation state of chromium in coir pith after Cr(VI) adsorption from aqueous solution was investigated using electron spin resonance (ESR). To elucidate the mechanism of chromium adsorption on coir pith, the adsorption studies of Cr(VI) onto lignin, alpha-cellulose and holocellulose extracted from coir pith were also studied. ESR signals of Cr(V) and Cr(III) were observed in coir pith adsorbed Cr(VI) at solution pH 2, while ESR spectra of lignin extracted from coir pith revealed only the Cr(III) signal. In addition, ESR signal of Cr(V) was observed in alpha-cellulose and holocellulose extracted from coir pith adsorbed Cr(VI). These results confirmed that lignin in coir pith reduced Cr(VI) to Cr(III) while alpha-cellulose and holocellulose extracted from coir pith reduced Cr(VI) to Cr(V). The Cr(V) signal exhibited in ESR of alpha-cellulose and holocellulose might be bound with glucose in cellulose part of coir pith. In addition, xylose which is main in pentosan part of coir pith, indicated that it is involved in form complex with Cr(V) on coir pith. The adsorption kinetic of Cr(VI) from aqueous solution on coir pith was also investigated and described well with pseudo second order model. ESR and desorption experiments confirmed that Cr(VI), Cr(V) and Cr(III), exist in coir pith after Cr(VI) adsorption. The desorption data indicated that the percentage of Cr(VI), Cr(V) and Cr(III) in coir pith were 15.63%, 12.89% and 71.48%, respectively.     

The oxidation of Cr(III) to Cr(VI) in the environment by atmospheric oxygen during the bush fires

The presence of Cr(VI) in soils and plants of remote unpolluted areas can be explained by partial oxidation of Cr(III) with atmospheric oxygen during seasonable bush fires, which are rather frequent event in South Africa. Experiments with thermal treatment of a veld grass, Hyperthelia dissoluta, in muffle furnace at high temperature, followed by electrothermal atomic absorption spectrometry (ETAAS) determination of chromium, show a remarkable increase in Cr(VI) concentration from initial 2.5 to 23.2% after the treatment of grass ash at 500 degrees C and to 58.1% at 900 degrees C. Before ETAAS determination, the two chromium species of interest were separated by the treatment of samples with 0.1M Na2CO3. Thermodynamic calculations confirm the possibility of Cr(III) to Cr(VI) oxidation with atmospheric oxygen at high temperature in alkaline media, which is typical for vegetation ash. Analysis of field samples show that percent of Cr(VI), in respect to the total amount of chromium increased from initial 2.5% in grass to 9.3% in ash of grass. Without oxidation the percent of Cr(VI) in grass and ash of grass should be a constant value. After the fire Cr(VI) concentration in top soil (0-3 cm) increased from 0.3+/-0.05 to 1.8+/-0.5 microg g(-1) and the total Cr from 26+/-9 to 69+/-14 microg g(-1). The reason for the appearance of additional amount of Cr on top soil can be explained by condensation of chromium species from flame and shouldering ash on a soil surface. The results of studies demonstrate that Cr(VI) is formed by Cr(III) oxidation with atmospheric oxygen at high temperature during bush fires.     

Optimization of the Simultaneous Determination of Cr(III) and Cr(VI) by Ion Chromatography with Chemiluminescence Detection

An optimized method for the simultaneous determination of Cr(III) and Cr(VI) in aqueous solutions using ion chromatography with chemiluminescence detection is described. Excellent resolution of the two chromium species was obtained using a single mixed-bed ion-exchange column with continuous elution. After postcolumn reduction of Cr(VI) to Cr(III), the light emitted during the Cr(III)-catalyzed oxidation of luminol with hydrogen peroxide was measured. Parameters affecting the postcolumn reactions such as reductant concentration, reductant mixing, point of luminol introduction, and luminol flow rate were optimized. The calibration curves in the range tested (0.01-50 μg L(-)(1)) were linear, and detection limits of 0.002 μg L(-)(1) for both Cr(III) and Cr(VI) were obtained. The results of the analyses of the water reference materials LGC CRM6010 and NIST SRM1643d with certified chromium values of 49 ± 4 and 18.53 ± 0.20 μg L(-)(1) and found to contain only Cr(III) were 49.2 ± 1.8 and 19.0 ± 1.5 μg L(-)(1), respectively. Values of 10.6 ± 0.5 and 10.1 ± 0.5 μg L(-)(1) were obtained when a simulated water sample containing 10 μg L(-)(1) Cr(III) and Cr(VI) was analyzed.     

Photoredox pathways of Cr(III)-tartrate complexes and their impacting factors

In the present study, exposure of Cr(III)-tar to full light of medium pressure mercury lamps and a xenon lamp was conducted in batch reaction systems at 25°C and different pH values to predict the potential for Cr(III) oxidation. The results indicated that the more intense irradiation and higher pH facilitated Cr(III)-tar oxidation. It appears that a ligand-to-metal charge-transfer occurs for Cr(III)-tar after irradiation, leading to the generation of Cr(II) and tar· or ·OH. The accompanying photochemical decomposition of tar·/or tar, together with O(2), further caused the formation of ·OH through multiple pathways, which ultimately converted Cr(II) to Cr(VI) step by step. H(2)O(2), a direct source of ·OH under irradiation, significantly enhanced photooxidation of Cr(III)-tar, but not obviously of aqueous Cr(III) or Cr(III)-tar in dark, implying that Cr(II) acts as a precursor of Cr(III) oxidization to Cr(VI).     

Cr(III)-imprinted polymeric beads: Sorption and preconcentration studies

The purpose of this study is to prepare a novel molecular imprinted adsorbent to Cr(III) analysis with high selectivity. Initially, chromium (III)-methacryloylhistidine (MAH/Cr(III)) complex monomer was synthesized and then Cr(III) ion imprinted ethyleneglycoldimethacrylate-methacryloylhistidine (poly(EDMA-MAH/Cr(III))) was polymerized. Cr(III) ions were removed from the Cr(III)-imprinting polymer. Selective cavity for the chromium (III) ions was obtained in the (poly(EDMA-MAH/Cr(III))) beads. These Cr(III)-imprinted beads were used in the sorption-desorption process. The effect of initial concentration of metal, the sorption rate and the pH of the medium on sorption capacity of Cr(III)-imprinting sorbents were studied. Sorption equilibrium time was achieved in about 30min. The maximum sorption of Cr(III) ions onto imprinted beads was about 69.28mg/g. Sorption studies of Co(II), Ni(II), Cr(III) and Cr(VI) ions were also investigated using Cr(III)-imprinting polymers. The obtained results show that selectively adsorbed amount of Cr(III) ion on Cr(III)-imprinting polymers is higher than all other studied ions. When the beads were used repeatedly, their adsorption capacity was showed feasibility.     

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.     

A comparative study of two chelating ion-exchange resins for the removal of chromium(III) from aqueous solution

Macroporous resins containing iminodiacetic acid (IDA) groups (Lewatit TP 207 and Chelex-100) were investigated as a function of concentration, temperature and pH for their sorption properties towards chromium(III). The chromium(III) ions sorbed onto the resin and in the equilibrium concentration were determined by inductively coupled plasma spectrophotometer. The maximum sorption for chromium ions was observed at pH 4.5. Solution pH had a strong effect on the equilibrium constant of Cr(III). The equilibrium constants were 320 and 7 at pH value 4.5 for Lewatit TP 207 and Chelex-100 resin, respectively. The Langmuir isotherm was used to describe observed sorption phenomena. Both the sorbents had high bonding constants with Lewatit TP 207 showing stronger binding. The equilibrium related to adsorption capacity and energy of adsorption was obtained by using plots of Langmuir adsorption isotherm. It was observed that the maximum adsorption capacity of 0.288 mmol of Cr(III)/g for Chelex-100 and 0.341 mmol of Cr(III)/g for Lewatit TP 207 was achieved at pH of 4.5. The rise in temperature caused a slight increase in the value of the equilibrium constant (K_c) for the sorption of chromium(III) ion.     

Cr(VI) reduction in aqueous solutions by siderite

Hexavalent chromium is a common and toxic pollutant in soils and wastewaters. Reduction of the mobile Cr(VI) to less mobile and less toxic Cr(III) is a solution for decontamination of industrial effluents. In this study, the reduction of hexavalent chromium in aqueous solutions by siderite was investigated. The influences of amount of acid, contact time, siderite dosage, initial Cr(VI) concentration, temperature and particle size of siderite have been tested in batch runs. The process was found to be acid, temperature and concentration dependent. The amount of acid is the most effective parameter affecting the Cr(VI) reduction since carbonaceous gangue minerals consume acid by side reactions. The highest Cr(VI) reduction efficiency (100%) occurred in the 50 mg/l Cr(VI) solution containing two times acid with respect to stoichiometric amount of Cr(VI) and at the conditions of siderite dosage 20 g/l, contact time 120 min and temperature 25 degrees C. Reduction efficiency increased with increase in temperature and decrease in particle size. The reduction capacity of siderite was found to be 17 mg-Cr(VI)/g.