Efficient removal of Cr(VI) from aqueous solution with Fe@Fe2O3 core-shell nanowires

The batch removal of Cr(VI) from simulated wastewater with Fe@Fe2O3 core-shell nanowires (FCSNs) was investigated in this study. Itwas found that each gram of the FCSNs could remove 7.78 mg of Cr(VI) from simulated wastewater containing 8.0 mg L(-1) of Cr(VI) with an initial pH of 6.5 at room temperature. The Freundlich adsorption isotherm was applicable to describe the removal processes. Kinetics of the Cr(VI) removal was found to follow pseudo-second-order rate equation. Furthermore, the as-prepared and Cr(VI)-adsorbed FCSNs were carefully examined by scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX), Fourier transform infrared spectroscopy (FT-IR), and X-ray photoelectron spectroscopic analysis (XPS). The characterization results suggested that the adsorbed Cr(VI) was partially reduced to Cr(lll) in Cr2O3/Cr(OH)3 on the FCSNs. The possible mechanism of removal of Cr(VI) on FCSNs was proposed, which involved the dominant Cr(VI) adsorption, followed by the partial reduction of Cr(VI) to Cr(III) (chromium(III) oxyhydroides) on the surface of FCSNs. These Fe@Fe2O3 core-shell nanowires with high specific surface area and strong magnetic property are very attractive for the removal of Cr(VI) from wastewater.     

Removal of chromium (VI) by acid-washed zero-valent iron under various groundwater geochemistry conditions

The hexavalent chromium (Cr(VI)) removal capacity of acid-washed zerovalent iron (AW-Fe0) was evaluated under different groundwater geochemistry conditions through column experiments. It was found that each gram of the AW-Fe0 could remove 0.65-1.76 mg of Cr(VI) from synthetic groundwater in the absence of bicarbonate (HCO3-), magnesium and/or calcium ions. Groundwater geochemistry was found to exert various degrees of impact on Cr(VI) removal by the AW-Fe0, in which HCO3- alone gave the mildest impact whereas the copresence of calcium and HCO3- exerted the greatest impact In comparison with the unwashed Fe0, the AW-Fe0 showed a poorer Cr(VI) removal capacity and was also more susceptible to the influence of the dissolved groundwater constituents on Cr(VI) removal,thereby indicating the unsuitability of using AW-Fe0 in permeable reactive barriers for remediation of Cr(VI)-contaminated groundwater. On the AW-Fe0 surface, where the indigenous iron precipitates were almost erased, trivalent chromium including chromium (III) oxides, hydroxides, and oxyhydroxides in irregular strip, chick footmark-liked or boulder-liked forms as well as Cr(III)-Cr(VI) mixed oxides were detected.     

改性介孔材料对贝类副产物中重金属离子的脱除性能

为研究改性介孔材料对贝类副产物中重金属离子的脱除性能,对SBA-15介孔材料分别嫁接2-巯基噻唑啉、2-巯基苯并噻唑、吡咯烷二硫代氨基甲酸铵三种不同基团,得到三种材料分别记作MT-SBA-15、MBT-SBA-15、APDC-SBA-15,然后对材料孔结构进行表征,采用三种材料吸附铅(Pb)、铬(Cr)、镉(Cd)、铜(Cu)标准溶液,考察脱除pH、脱除时间、金属离子加标浓度对脱除能力的影响,最后用APDC-SBA-15脱除贝类副产物中重金属离子,考察重金属脱除率以及营养损失率。结果表明,三种材料都具有较大的比表面积和孔容。三种材料对重金属最优脱除pH为6.0,吸附平衡时间均为30 min;当金属离子浓度为1200 mg/L时,三种材料对金属离子吸附量最大,其中APDC-SBA-15的吸附量最高,对Pb、Cr、Cd、Cu吸附量分别为223.6、240.0、259.8、259.0 mg/g。APDC-SBA-15对菲律宾蛤仔蒸煮液中Pb的脱除率为100.00%,总糖损失率为7.32%;对菲律宾蛤仔蒸煮液多糖中Pb脱除率为84.50%,总糖损失率为2.57%;对牡蛎多肽中Pb、Cr的脱除率分别为62.20%和100.00%,对可溶性蛋白质的损失率为14.62%。综上,APDC-SBA-15可以高效脱除贝类副产物中的重金属,对蛋白质、总糖等营养成分的损失率较低。     

Reduction of hexavalent chromium with the brown seaweed Ecklonia biomass

A new type of biomass, protonated brown seaweed Ecklonia sp., was used for the removal of Cr(VI). When synthetic wastewater containing Cr(VI) was placed in contact with the biomass, the Cr(VI) was completely reduced to Cr(III). The converted Cr(III) appeared in the solution phase or was partly bound to the biomass. The Cr(VI) removal efficiency was always 100% in the pH range of this study (pH 1 to approximately 5). Furthermore, the Cr(VI) reduction was independent of the Cr(III) concentration, the reaction product, suggesting that the reaction was an irreversible process under our conditions. Proton ions were consumed in the ratio of 1.15 +/- 0.02 mol of protons/mol of Cr(VI), and the rate of Cr(VI) reduction increased with decreasing the pH. An optimum pH existed for the removal efficiency of total chromium (Cr(VI) plus Cr(III)), but this increased with contact time, eventually reaching approximately pH 4 when the reaction was complete. The electrons required for the Cr(VI) reduction also caused the oxidation of the organic compounds in the biomass. One gram of the biomass could reduce 4.49 +/- 0.12 mmol of Cr(VI). From a practical viewpoint, the abundant and inexpensive Ecklonia biomass could be used for the conversion of toxic Cr(VI) into less toxic or nontoxic Cr(III).     

磺胺化聚丙烯腈纳米纤维膜的制备及其对Cr(VI)和Pb(II)的吸附性能

为更好地吸附水中的Cr(VI)和Pb(II)等重金属离子,并且避免吸附材料对水体的二次污染,利用对甲基苯磺酰胺为功能化试剂,通过水热法对聚丙烯腈(PAN)纳米纤维膜进行化学改性,得到了具有吸附重金属离子功能的磺胺化PAN纳米纤维膜,并研究了该纤维膜对Cr(VI)和Pb(II)的吸附去除性能和机制。结果表明:当水热温度为125 ℃,水热时间为2.5 h时,可得到形貌良好的磺胺化PAN纳米纤维膜;磺胺化PAN纳米纤维膜对Cr(VI)的吸附符合Langmuir模型,且满足二级动力学方程,在质量浓度为50 mg/L的K₂Cr₂O₇溶液中1 h后可达到吸附平衡,吸附量为220.4 mg/g;对Pb(II)的吸附符合Freundlich吸附模型,且满足二级动力学方程,在质量浓度为50 mg/L的Pb(NO₃)₂溶液中1 h后可达到吸附平衡,吸附量为185.6 mg/g。     

Co-removal of hexavalent chromium through copper precipitation in synthetic wastewater

The mechanisms of hexavalent chromium [Cr(VI)] co-removal with copper [Cu(II)] during homogeneous precipitation were studied with batch tests using a synthetic solution containing Cr(VI) and Cu(II). Metal precipitation was induced by adding Na2CO3 stepwise to different pH, and the respective removals of Cu(II) and Cr(VI) were measured. At the same time, the relative quantities of Cu(II) and Cr(VI) in the precipitates were also analyzed to establish their stoichiometric relationship. The results indicated that, in a solution containing 150 mg/L Cu(II) and 60 mg/L Cr(VI), the initial co-removal of Cr(VII with Cu(II) began at pH 5.0 and completed at pH 6.2. At pH 5.0-5.2, coprecipitation took place through the formation of copper-chromium-bearing solids [such as CuCrO4 and/or CuCrO4 x 2Cu(OH)2]. Thereafter, the remaining soluble copper started to react with carbonate in a heterogeneous environment to form the negatively charged basic copper carbonate precipitates [CuCO3 x Cu(OH)2], which subsequently adsorbed additional Cr(VI) (or HCrO4-) at pH 5.2-6.2. The maximum Cr(VI) co-removal took place at pH 6.2. Between the two mechanisms, co-precipitation accounted for about 29% of the total chromium's co-removal while the remaining 71% was attributed to surface adsorption, mainly through electrostatic attraction and ligand exchange. When the solution pH was increased to beyond 7.5, a surface charge reversal took place on the basic copper carbonate solids, and this led to some Cr(VI) desorption. Thus, the extent of Cr(VI) adsorption is highly pH dependent.     

Combined removal of chlorinated ethenes and heavy metals by zerovalent iron in batch and continuous flow column systems

The combined removal of chlorinated ethenes and heavy metals from a simulated groundwater matrix by zerovalent iron (ZVI) was investigated. In batch, Ni (5-100 mg L(-1)) enhanced trichloroethene (TCE, 10 mg L(-1)) reduction by ZVI (100 g L(-1)) due to catalytic hydrodechlorination by bimetallic Fe0/Ni0. Cr(VI) or Zn (5-100 mg L(-1)) lowered TCE degradation rates by a factor of 2 to 13. Cr(VI) (100 mg L(-1)) in combination with Zn or Ni (50-100 mg L(-1)) inhibited TCE degradation. Addition of 20% H2(g) in the headspace, or of Zn (50-100 mg L(-1)), enhanced TCE removal in the presence of Ni and Cr(VI). Sorption of Zn to ZVI alleviated the Cr(VI) induced inhibition of bimetallic Fe0/Ni0 apparently due to release of protons necessary for TCE hydrodechlorination. In continuous ZVI columns treating tetrachloroethene (PCE, 1-2 mg L(-1)) and TCE (10 mg L(-1)), and a mixture of the metals Cr(VI), Zn(II), and Ni(II) (5 mg (L-1)), the PCE removal efficiency decreased from 100% to 90% in columns operated without heavy metals. The PCE degradation efficiency remained above 99% in columns receiving heavy metals as long as Ni was present. The findings of this study indicate the feasibility and limitations of the combined treatment of mixtures of organic and inorganic pollutants by ZVI.     

Effect of coupled dissolution and redox reactions on Cr(VI)aq attenuation during transport in the sediments under hyperalkaline conditions

Aluminum-rich, hyperalkaline (pH > 13.5) and saline high-level nuclear waste (HLW) fluids at elevated temperatures (>50 degrees C), that possibly contained as much as 0.41 mol L(-1) Cr(VI), accidentally leaked to the sediments at the Hanford Site, WA. These extreme conditions promote base-induced dissolution of soil minerals which may affect Cr(VI)aq mobility. Our objective was to investigate Cr(VI)aq transport in sediments leached with HLW simulants at 50 degrees C, under CO2 and O2 free conditions. Results demonstrated that Cr(VI)aq fate was closely related to dissolution, and Cr(VI)aq mass loss was negligible in the first pore volumes but increased significantly thereafter. Similar to dissolution, Cr(VI)aq attenuation increased with increasing fluid residence time and NaOH concentration but decreased with Al concentrations in the leaching solutions. Aqueous Cr(VI) removal rate half-lives varied from 1.2 to 230 h with the fastest at the highest base concentration, lowest Al concentration, greatest reaction time, and lowest Cr(VI) concentration in the leaching solution. The rate of Cr(VI) removal (normalized to 1 kg of solution) varied from 0.83 x 10(-9) (+/-0.44 x 10(-9)) to 9.16 x 10(-9) (+/-1.10 x 10(-9)) mol s(-1). The predominant mechanism responsible for removing Cr(VI) from the aqueous phase appears to be homogeneous Cr(VI) reduction to Cr(III) by Fe(II) released during mineral dissolution. Cr(VI)aq removal was time-limited probably because it was controlled by the rate of Fe(II) release into the soil solution upon mineral dissolution, which was also a time-limited process, and other processes that may act to lower Fe(II)aq activity.     

The physicochemical characterization,equilibrium, and kinetics of heavy metal ions adsorption from aqueous solution by arrowhead plant (Sagittaria trifolia L.) stalk

The arrowhead plant stalk (APS) has been investigated as a novel biosorbent for removal of Cd(II), Pb(II), and Cr(III) ions from aqueous solution. The surface physicochemical properties favorable for metals adsorption were systematically characterized. The Langmuir isotherm fitted well with Cd(II) and Pb(II) adsorption process onto APS while Dubinin–Radushkevich model best described Cr(III) sorption. The maximal adsorption capacities of APS for Cd(II), Pb(II), and Cr(III) were up to 38.2, 97.1, and 23.5 mg·g^?1, respectively. The adsorption kinetic data of individual metal fitted the pseudo‐second order model best. The adsorption of Cd(II) was exothermic, whereas the Pb(II) and Cr(III) underwent endothermic reaction. Overall, this investigation indicated, for the first time, APS is a potentially efficient biosorbent applied in Cd(II), Pb(II), and Cr(III) adsorption. It is also helpful for further utilizing the abundant quantity of APS which were abandoned in dietary arrowhead processing.

Practical applications

The growing food industry around world generates large quantity of by‐products. The high value‐added utilization of food processing by‐products is one of the most important area in food industry. An attempt was made in present study to use the food by‐product, arrowhead plant stalk (APS), as a novel biosorbent for removing Cd(II), Pb(II), and Cr(III) ions. In present study, it was evidenced that the adsorption capacities of APS for Cd(II), Pb(II), and Cr(III) were obviously higher than those of most reported agricultural by‐products. This finding is significant for sustainable utilization of food crop arrowhead plant and elimination of environmental issues arising from the abandoned arrowhead plant stalk.     

One-pot synthesis of magnetic graphene nanocomposites decorated with core@double-shell nanoparticles for fast chromium removal

A facile thermodecomposition process to synthesize magnetic graphene nanocomposites (MGNCs) is reported. High-resolution transmission electron microscopy and energy filtered elemental mapping revealed a core@double-shell structure of the nanoparticles with crystalline iron as the core, iron oxide as the inner shell and amorphous Si-S-O compound as the outer shell. The MGNCs demonstrate an extremely fast Cr(VI) removal from the wastewater with a high removal efficiency and with an almost complete removal of Cr(VI) within 5 min. The adsorption kinetics follows the pseudo-second-order model and the novel MGNC adsorbent exhibits better Cr(VI) removal efficiency in solutions with low pH. The large saturation magnetization (96.3 emu/g) of the synthesized nanoparticles allows fast separation of the MGNCs from liquid suspension. By using a permanent magnet, the recycling process of both the MGNC adsorbents and the adsorbed Cr(VI) is more energetically and economically sustainable. The significantly reduced treatment time required to remove the Cr(VI) and the applicability in treating the solutions with low pH make MGNCs promising for the efficient removal of heavy metals from the wastewater.     

Removal of Cr(VI) from aqueous solutions by modified walnut shells

Walnut shell (WNS) (Juglans regia) has been utilised as an adsorbent for the removal of Cr(VI) ions from aqueous solutions after treatment with citric acid. The modification reaction variables, such as citric acid level (5-10 g), reaction time (0-24 h), and temperature (110-130 °C) were studied in batch experiments. The rate of adsorption was studied under a variety of conditions, including initial Cr(VI) concentration (0.1-1.0 mM), amount of adsorbent (0.02-0.20 g), pH (2-9), temperature and contact time (10-240 min). Adsorption of Cr(VI) is in all cases pH-dependent showing a maximum at equilibrium pH values between 2.0 and 3.0 for citric acid modified walnut shell (CA-WNS). The applicability of the Langmuir, Freundlich and D-R adsorption isotherms has been tested for the equilibrium. Maximum adsorption capacities of CA-WNS and untreated WNS under experimental conditions were 0.596 and 0.154 mmol/g for Cr(VI) ions, respectively.     

Recycling MgOH2 nanoadsorbent during treating the low concentration of CrVI

Investigations about how to recycle the deactivated nanomaterials are well-needed. This work was designed to explore the recycling strategy of Mg(OH)(2) nanoadsorbent during treating low concentration of Cr(VI) solution as an example. It was demonstrated that a reversible route between Cr-adsorbed nano-Mg(OH)(2) and Cr-desorbed bulk-MgCO(3) · 3H(2)O can be established by using CO(2) as a phase transformation medium. In each adsorption-desorption cycle, Cr(VI) solution with initial concentration of 10 mg · L(-1) could be enriched over 40 times. An aggregation-induced rapid phase transformation mechanism from nano-Mg(OH)(2) to bulk-MgCO(3) · 3H(2)O was discovered, which was one of the critical factors to ensure the disposing efficiency of this environmental-friendly Cr(VI) disposal system. A pilot-scale experiment was conducted with this strategy to deal with 50 L Cr(VI)-containing simulated industrial wastewater. The enrichment of Cr(VI) and the recycle of nano-Mg(OH)(2) can be successfully achieved simultaneously.     

果胶/聚间苯二胺凝胶珠的制备和表征及其对铅（II）吸附性能的研究

以低酯果胶和CaCl₂为原料,采用离子交联法制备果胶凝胶珠,然后在果胶凝胶珠表面组装聚间苯二胺,制备新型果胶/聚间苯二胺凝胶珠并用于铅（II）的吸附。采用傅里叶变换红外光谱（FTIR）,扫描电子显微镜（SEM）,X射线衍射（XRD）,热重分析（TGA）,比表面积与孔隙度分析（BET）和能量色散X射线（EDX）对其结构进行了表征,并探究了初始pH、吸附时间、铅（II）初始浓度、吸附剂添加量和共存其它金属离子等条件对铅（II）吸附性能的影响。结果表明,与果胶凝胶珠相比,果胶/聚间苯二胺凝胶珠的比表面积与热稳定性有着显著提高,在相同条件下其对铅（II）的吸附性能更好。吸附过程与朗缪尔等温线模型和准二级动力学模型非常吻合,表明吸附是单分子层并且吸附过程是由化学吸附主导的。果胶/聚间苯二胺凝胶珠对铅（II）的最大吸附容量为352.03 mg/g,远高于果胶凝胶珠（162.99 mg/g）。钠（I）和钙（II）的共存对铅（II）的吸附具有一定程度的抑制作用。在三种重金属离子（铅（II）,铁（II）,铜（II））体系下,果胶/聚间苯二胺凝胶珠对重金属离子的亲和力为铅（II）>铁（II）>铜（II）。吸附铅（II）的机理可能是钙（II）与铅（II）的离子交换,与含氧官能团和含氮官能团的螯合作用以及静电相互作用。在5个吸附/解吸循环后,果胶/聚间苯二胺凝胶珠显示出良好的再生能力（去除率为90%）。果胶/聚间苯二胺凝胶珠可以作为一种去除铅（II）的吸附剂。     

Hexavalent chromium removal by reduction with ferrous sulfate, coagulation, and filtration: a pilot-scale study

A flow-through pilot-scale system was tested for removal of Cr(VI) from contaminated groundwater in Glendale, California. The process consisted of the reduction of Cr(VI) to Cr(lll) using ferrous sulfate followed by coagulation and filtration. Results indicated that the technology could reduce influent Cr(VI) concentrations of 100 microg L(-1) to below detectable levels and also remove total Cr (Cr(VI) plus Cr(lll)) to very low concentrations (< 5 microg L(-1)) under optimized conditions. Complete reduction of Cr(VI) to Cr(lll) was accomplished with Fe(ll) doses of 10-50 times the Cr(Vl) concentration even in the presence of significant dissolved oxygen levels. The overall Cr removal efficiency was largely determined by the filterability of Cr(lll) and Fe(lll) precipitates, of which a relatively high filtration pH (7.5-7.6) and high filter loading rate (6 gpm ft(-2)) had negative impacts. The pilot system was able to operate for an extended time period (23-46 h depending on the Fe:Cr mass ratio) before turbidity breakthrough or high head loss. Backwash water was effectively settled with low doses (0.2-1.0 mg L(-1)) of high molecular weight polymer. Backwash solids were found to be nonhazardous bythe toxicity characteristic leaching procedure but hazardous by the California waste extraction test.     

Ferrate treatment for removing chromium from high-level radioactive tank waste

A method has been developed for removing chromium from alkaline high-level radioactive tank waste. Removing chromium from these wastes is critical in reducing the volume of waste requiring expensive immobilization and deep geologic disposition. The method developed is based on the oxidation of insoluble chromium(III) compounds to soluble chromate using ferrate. This method could be generally applicable to removing chromium from chromium-contaminated solids, when coupled with a subsequent reduction of the separated chromate back to chromium(III). The tests conducted with a simulated Hanford tank sludge indicate that the chromium removal with ferrate is more efficient at 5 M NaOH than at 3 M NaOH. Chromium removal increases with increasing Fe(VI)/Cr(II) molar ratio, but the chromium removal tends to level out for Fe(VI)/ Cr(III) greaterthan 10. Increasingtemperature leadsto better chromium removal, but higher temperatures also led to more rapid ferrate decomposition. Tests with radioactive Hanford tank waste generally confirmed the simulant results. In all cases examined, ferrate enhanced the chromium removal, with a typical removal of around 60-70% of the total chromium present in the washed sludge solids. The ferrate leachate solutions did not contain significant concentrations of transuranic elements, so these solutions could be disposed as low-activity waste.     

Kinetics of Cr(VI) reduction by carbonate green rust

The kinetics of Cr(VI) reduction to Cr(III) by carbonate green rust were studied for a range of reactant concentrations and pH values. Carbonate green rust, [FeII4FeIII2(OH)12][4H2O x CO3], was synthesized by induced hydrolysis (i.e., coprecipitation) of an Fe(ll)/Fe(III) solution held at a constant pH of 8. An average specific surface area of 47 +/- 7 m2 g(-1) was measured for five separate batches of freeze-dried green rust precipitate. Heterogeneous reduction by Fe(II) associated with the carbonate green rust appears to be the dominant pathway controlling Cr(VI) loss from solution. The apparent stoichiometry of the reaction between ferrous iron associated with green rust ([Fe(II)GR]) and Cr(VI) was slightly higherthan the expected 3:1 ratio, possibly due to the presence of other oxidants, such as oxygen, protons, or interlayer carbonate ions. The rate of Cr(VI) reduction was proportional to the green rust surface area concentration, and psuedo-first-order rate coefficients (kobs) ranging from 1.2 x 10(-3) to 11.2 x 10(-3) s(-1) were determined. The effect of pH was small with a 5-fold decrease in rate with increasing pH (from 5.0 to 9.0). At low Cr(VI) concentrations (<200 microM), the rate of reaction was first order with respect to Cr(VI) concentration, whereas, at high Cr(VI) concentrations, rates appearto deviate from first-order kinetics and approach a constant value. Estimated amounts of surface Fe(II) and total Fe(II) suggest that the deviation from first-order kinetics observed at higher Cr(VI) concentrations and the 50-fold decrease in rate observed upon three sequential exposures to Cr(VI) is due to exhaustion of available Fe(II).     

Polyethylenimine-modified fungal biomass as a high-capacity biosorbent for Cr(VI) anions: sorption capacity and uptake mechanisms

Heavy metal pollution in the aqueous environment is a problem of global concern. Biosorption has been considered as a promising technology for the removal of low levels of toxic metals from industrial effluents and natural waters. A modified fungal biomass of Penicillium chrysogenum with positive surface charges was prepared by grafting polyethylenimine (PEI) onto the biomass surface in a two-step reaction. The presence of PEI on the biomass surface was verified by FTIR and X-ray photoelectron spectroscopy (XPS) analyses. Due to the high density of amine groups in the long chains of PEI molecules on the surface, the modified biomass was found to possess positive zeta potential at pH below 10.4 as well as high sorption capacity for anionic Cr(VI). Using the Langmuir adsorption isotherm, the maximum sorption capacity for Cr(VI) at a pH range of 4.3-5.5 was 5.37 mmol/g of biomass dry weight, the highest sorption capacity for Cr(VI) compared to other sorbents reported in the literature. Scanning electronic microscopy (SEM) provided evidence of chromium aggregates formed on the biomass surface. XPS results verified the presence of Cr(III) on the biomass surface in the pH range 2.5-10.5, suggesting that some Cr(VI) anions were reduced to Cr(III) during the sorption. The sorption kinetics indicated that redox reaction occurred on the biomass surface, and whether the converted Cr(III) ions were released to solution or adsorbed on the biomass depended on the solution pH. Sorption mechanisms including electrostatic interaction, chelation, and precipitation were found to be involved in the complex sorption of chromium on the PEI-modified biomass.     

Enhanced redox conversion of chromate and arsenite in ice

The redox transformation of trace elements is critically affected by properties of the environmental media. While the environmentally important redox chemical reactions in aquatic environments have been extensively studied, those in the ice phase have been studied in only a few cases. In this work, chromium and arsenic species were selected as the model inorganic oxyanion contaminants for the study of redox chemical transformation in ice. We investigated (1) the reduction of hexavalent Cr(VI) (as chromate) by model organic acids (e.g., citric and oxalic acid) and (2) the simultaneous removal of Cr(VI) and As(III) (as arsenite) in ice phase in comparison with their counterparts in aqueous solution. The reduction of Cr(VI) by various organic acids (electron donors) was negligible in ambient aqueous solution but was significantly accelerated in ice. The simultaneous reduction of Cr(VI) and oxidation of As(III) in ice phase proceeded stoichiometrically, whereas their mutual conversion was insignificant in aqueous solution. The enhanced redox conversion of Cr(VI)/As(III) in ice is ascribed to the freeze concentration of both electron donors (e.g., organic acids, arsenites) and protons in the ice crystal grain boundaries. When the concentrations of both electron donors and protons were highly raised to an extreme, the removal rates of Cr(VI) in aqueous solution approached to those in ice. This specific combination of Cr(VI)/As(III) redox couple may provide an example that represents innumerable redox conversion reactions that could be greatly accelerated in ice/snow-covered or frozen environments.     

Electrically regenerated ion exchange for removal and recovery of Cr(VI) from wastewater

Ion exchange is widely used for removal and recovery of Cr(VI) from wastewater. Generally, the exhausted ion exchanger is regenerated using chemicals. Although chemical regeneration is efficient, contaminants are introduced, leading to difficulty for the subsequent recovery of Cr(VI). To overcome such a problem, a new regeneration method, namely electrical regeneration, which is carried out on the principle of electrodialysis, is presented in this paper. Experimental results showed that the weak-base resin used could be effectively regenerated electrically. About 93% capacity of the resin was restored under a constant current of 0.25 A over a period of 24 h. The pure chromic acid was recovered in the anode chamber with a concentration of 5.03 g Cr(VI)/L. It was found that the weak-base resin regenerated electrically could remove Cr(VI) from wastewater as effectively as that regenerated chemically. The Cr(VI) concentration was reduced from initial 50 mg/L to lower than the detectable limit, 0.01 mg/L, after treatment.     

Removal of hexavalent chromium with a lignocellulosic substrate extracted from wheat bran

In this paper, a new recovery system of the toxic hexavalent chromium Cr(VI) is proposed that uses a lignocellulosic substrate derived from the industrial treatment process of wheat bran. We studied the adsorption mechanism of Cr(VI) onto the lignocellulosic substrate and showed that the adsorption reaction consumes a large amount of protons goes along the reduction of Cr(VI) into Cr(III). The oxidation of lignin moieties takes place concurrently to the chromium reduction and leads to the formation of hydroxyl and carboxyl functions. The latter contribute to an increase in the number of ion-exchange sites for the reduced chromium. The maximum adsorption capacity for hexavalent chromium was found at about 35 mg g(-1) in an acidic medium. This is comparable to other natural substrates and ordinary adsorbents.