Biosorption of Cr (VI) with Trichoderma viride immobilized fungal biomass and cell free Ca-alginate beads

Ability of Cr (VI) biosorption with immobilized Trichoderma viride biomass and cell free Ca-alginate beads was studied in the present study. Biosorption efficiency in the powdered fungal biomass entrapped in polymeric matric of calcium alginate compared with cell free calcium alginate beads. Effect of pH, initial metal ion concentration, time and biomass dose on the Cr (VI) removal by immobilized and cell free Ca-alginate beads were also determined. Biosorption of Cr (VI) was pH dependent and the maximum adsorption was observed at pH 2.0. The adsorption equilibrium was reached in 90 min. The maximum adsorption capacity of 16.075 mgg(-1) was observed at dose 0.2 mg in 100 ml of Cr (VI) solution. The high value of kinetics rate constant Kad (3.73 x 10(-2)) with immobilized fungal biomass and (3.75 x 10(-2)) with cell free Ca- alginate beads showed that the sorption of Cr (VI) ions on immobilized biomass and cell free Ca-alginate beads followed pseudo first order kinetics. The experimental results were fitted satisfactory to the Langmuir and Freundlich isotherm models. The hydroxyl (-OH) and amino (-NH) functional groups were responsible in biosorption of Cr (VI) with fungal biomass spp. Trichoderma viride analysed using Fourier Transform Infrared (FTIR) Spectrometer.     

Low-cost supports used to immobilize fungi and reliable technique for removal hexavalent chromium in wastewater

The main goal of this study was to exploit low-cost and efficient sorbents for the removal and recovery of Cr(VI) in wastewater. Three supports of sawdust, polyurethane and alginate were applied to immobilize living and dead R. cohnii cells, respectively. There was a distinct increase in the Cr(VI) removal efficiency before and after the HCl-pretreatment. Langmuir adsorption isotherm model was well used to describe the distribution of Cr(VI) between the liquid and solid phases in batch studies. The values of q0 predicted by Thomas model were near to experimental ones in the experiments of packed column. The breakthrough curves calculated with this model were consistent well with experimental ones at a largely extent. Desorption, regeneration and reuse of the packed column were studied. After 5 cycles, adsorption capacity was still kept at higher level, reaching to 91.4, 87.9, 91.4 and 93.3mg/l contrasted with the first cycle (94.1, 90.4, 94.8 and 98.5mg/l) and the desorption efficiency were 85.0%, 96.2%, 93.4% and 91.4% compared with the first cycle (87.6%, 95.4%, 96.7% and 94.3%), corresponding to living cells immobilized with sawdust, polyurethane, and dead cells immobilized with polyurethane and alginate, respectively. The results indicated that the packed columns with the immobilized living and dead R. cohnii cells were the better option to adsorb, desorb and recover Cr(VI) from wastewater.     

木霉生物吸附重金属铬机理的研究

利用木霉（Trichoderma lhd）菌体作为吸附剂,对水体中的六价铬进行生物吸附,借助傅立叶红外变换光谱和拉曼光谱对六价铬的生物吸附机理进行了探讨。实验条件优化结果表明,温度28 ℃以及酸性环境条件（pH 1）有利于Cr （VI） 的生物吸附,12小时内,Cr （VI） 的生物吸附去除效率达99 %。吸附机理实验结果分析表明,相比于对照实验,2350 cm^-1吸收峰的出现为吸附剂表面质子化的氨基如＞NH2^＋, NH^＋, ＞C=NH^＋―等基团吸附Cr （VI）所致。拉曼光谱中吸收峰2097 cm^-1强度显著增强进一步表明,Cr （VI）的生物吸附是吸附剂表面氨基基团在起作用。     

絮凝酵母吸附Cr(VI) 的机理

絮凝酵母SPSC01为酿酒酵母Saccharomyces cerevisiae和粟酒裂殖酵母Schizosaccharomyces pombe的融合菌株,用其吸附水溶液中的重金属Cr(VI),可以大大降低生物吸附的固液分离成本。为了探讨SPSC01菌体絮凝蛋白对Cr(VI) 还原吸附的影响,对SPSC01与其亲本菌株的吸附行为进行了比较。结果表明,SPSC01和其具有絮凝性状的亲本S. pombe的Cr(VI) 去除速率基本同步,远优于无絮凝性状的亲本S. cerevisiae;达到吸附平衡时,S. pombe、SPSC01和S. cerevisiae对总Cr去除率分别达68.8％、48.6％和37.5％;从而证明了絮凝有利于Cr(VI) 的还原、吸附,絮凝蛋白在Cr(VI) 的还原吸附过程中起促进作用。通过化学屏蔽方法和傅立叶变换红外光谱 (FTIR) 分析,对SPSC01菌体表面吸附Cr(VI) 的机理进行了研究,结果表明SPSC01菌体表面吸附Cr(VI) 起主要作用的基团是氨基、羧基和酰胺基。     

Continuous adsorption and recovery of Cr(VI) in different types of reactors

This study reports the results of experiments on continuous adsorption and desorption of Cr(VI) ions by a chemically modified and polysulfone-immobilized biomass of the fungus Rhizopus nigricans. A fixed quantity of polymer-entrapped biomass beads corresponding to 2 g of dry biomass powder was employed in packed bed, fluidized bed, and stirred tank reactor for monitoring the continuous removal and recovery of Cr(VI) ions from aqueous solution and synthetic chrome plating effluent. Parameters such as flow rate (5, 10 and 15 mL/min), inlet concentration of Cr(VI) ions (50, 100, 150 and 250 mg/L) and the depth of biosorbent packing (22.8, 11.2 and 4.9 cm) were evaluated for the packed bed reactor. The breakthrough time and the adsorption rates in the packed bed column were found to decrease with increasing flow rate and higher Cr inlet concentrations and to increase with higher depths of sorbent packing. To have a comparative analysis of Cr adsorption efficiency in different types of reactors, the fluidized bed reactor and stirred tank reactor were operated using the same quantities of biosorbent material. For the fluidized bed reactor, Cr(VI) solution of 100 mg/L was pumped at 5 mL/min and fluidized by compressed air at a flow rate of 0.5 kg/cm.(2) The stirred tank reactor had a working volume of 200 mL capacity and the inlet/outlet flow rate was 5 mL/min. The maximum removal efficiency (mg Cr/g biomass) was obtained for the stirred tank reactor (159.26), followed by the fluidized reactor (153.04) and packed bed reactor (123.33). In comparison to the adsorption rate from pure chromate solution, approximately 16% reduction was monitored for synthetic chrome plating effluent in the packed bed. Continuous desorption of bound Cr ions from the reactors was effective with 0.01 N Na(2)CO(3) and nearly 80-94% recoveries have been obtained for all the reactors.     

Reduction of Cr(VI) by immobilized cells of Desulfovibrio vulgaris NCIMB 8303 and Microbacterium sp. NCIMB 13776

Hexavalent chromium, a carcinogen and mutagen, can be reduced to Cr(III) by Desulfovibrio vulgaris NCIMB 8303 and Microbacterium sp. NCIMB 13776. This study examined Cr(VI) reduction by immobilized cells of the two strains in a common solution matrix using various entrapment matrices. Chitosan and PVA-borate beads did not retain integrity and supported low or no reduction of Cr(VI) by the cells. A commercial preparation (Lentikats) was stable but also did not support Cr(VI) reduction. K-carrageenan beads were stable in batch suspensions but gel integrity was lost after only 5 h in a flow-through system in the presence of 100 microM Cr(VI). The best immobilization matrices were agar and agarose, where the initial rates of reduction of Cr(VI) (from 500 microM solution) for D. vulgaris NCIMB 8303 and Microbacterium sp. NCIMB 13776 were 127 (agar) and 130 (agarose), and 15 (agar) and 12 (agarose) nmol h(-1) mg dry cell wt(-1), respectively. The higher removal of Cr(VI) by D. vulgaris was also seen in 14-mL packed-bed flow-through columns, where, at a flow rate of 2.4 mL h(-1), the percentage removal of Cr(VI) was approximately 95% and 60% for D. vulgaris and Microbacterium sp., respectively (agar-immobilized cells). The Cr(VI) reducing activities of D. vulgaris and Microbacterium sp. were lost after 159 and 140 h, respectively. Examination of the beads for structural integrity within the columns in situ using magnetic resonance imaging after 24 and 100 h of continuous operation against Cr(VI) (with negligible Cr retained within the columns) showed that agar beads were more stable with time. The most appropriate system for development of a continuous bioprocess is thus the use of D. vulgaris NCIMB 8303 immobilized in an agar gel matrix.     

Synthesis of tentacle-type magnetic beads as immobilized metal-chelate affinity support for cytochrome c adsorption

Magnetic poly(2-hydroxyethylmethacrylate) (mPHEMA) beads with an average diameter of 100-140 microm were produced by suspension polymerization in the presence of magnetite particles (i.e. Fe3O4). Specific surface area and average pore size of the magnetic beads was found to be 50 m2/g and 819 nm, respectively. Ester groups in the mPHEMA structure were converted to imine groups by reacting with poly(ethyleneimine) (PEI) in the presence of NaH. Amino (-NH2) content of PEI-attached mPHEMA beads was determined as 102 mg PEI/g. Then, Cu2+ ions were chelated on the magnetic beads in the range of 20-793 micromol Cu2+/g. Cytochrome c (cyt c) adsorption was performed on the metal chelating beads from aqueous solutions containing different amounts of cyt c at different pHs, Cu2+ loadings and temperatures. Cyt c adsorption on the mPHEMA/PEI beads was 4.6 mg/g. Cu2+ chelation increased the cyt c adsorption significantly (40.1 mg/g). Adsorption capacity increased with Cu2+ loading and then reached a saturation value. Cyt c adsorption decreased with increasing temperature. Cyt c molecules could be reversibly adsorbed and eluted ten times with the magnetic adsorbents without noticeable loss in their cyt c adsorption capacity. The applicability of two kinetic models including pseudo-first order and pseudo-second order model was estimated on the basis of comparative analysis of the corresponding rate parameters, equilibrium capacity and correlation coefficients. Results suggest that chemisorption processes could be the rate-limiting step in the adsorption process. In the last part of this article, cyt c adsorption experiments were performed in a magnetically stabilized fluidized bed (MSFB) system at optimum conditions determined from the batch experiments. The adsorption capacity decreased significantly from 46.8 to 15.4 mg/g polymer with the increase of the flow-rate from 0.5 to 4.0 ml/min. The resulting magnetic chelator beads possessed excellent long-term storage stability.     

Studies on chromium(VI) adsorption-desorption using immobilized fungal biomass

The aim of this study was to investigate the Cr(VI) biosorption potential of immobilized Rhizopus nigricans and to screen a variety of non-toxic desorbing agents, in order to find out possible application in multiple sorption-desorption cycles. The biomass was immobilized by various mechanisms and evaluated for removal of Cr(VI) from aqueous solution, mechanical stability to desorbents, and reuse in successive cycles. The finely powdered biomass, entrapped in five different polymeric matrices viz. calcium alginate, polyvinyl alcohol (PVA), polyacrylamide, polyisoprene, and polysulfone was compared for biosorption efficiency and stability to desorbents. Physical immobilization to polyurethane foam and coir fiber was less efficient than polymer entrapment methods. Of the different combinations (%, w/v) of biomass dose compared for each matrix, 8% (calcium alginate), 6% (polyacrylamide and PVA), 12% (polyisoprene), and 10% (polysulfone) were found to be the optimum. The Cr sorption capacity (mg Cr/g sorbent) of all immobilized biomass was lesser than the native, powdered biomass. The Cr sorption capacity decreased in the order of free biomass (119.2) > polysulfone entrapped (101.5) > polyisoprene immobilized (98.76) > PVA immobilized (96.69) > calcium alginate entrapped (84.29) > polyacrylamide (45.56), at 500 mg/l concentration of Cr(VI). The degree of mechanical stability and chemical resistance of the immobilized systems were in the order of polysulfone > polyisoprene > PVA > polyacrylamide > calcium alginate. The bound Cr(VI) could be eluted successfully using 0.01 N NaOH, NaHCO3, and Na2CO3. The adsorption data for the native and the immobilized biomass was evaluated by the Freundlich isotherm model. The successive sorption-desorption studies employing polysulfone entrapped biomass indicated that the biomass beads could be regenerated and reused in more than 25 cycles and the regeneration efficiency was 75-78%.     

Adsorption of Cr(VI) and As(V) ions by modified magnetic chitosan chelating resin

Cross-linked magnetic chitosan anthranilic acid glutaraldehyde Schiff's base (CAGS) was prepared for adsorption of both As(V) and Cr(VI) ions and their determination by ICP-OES. Prepared cross-linked magnetic CAGS was investigated by means of SEM, FTIR, wide angle X-ray diffraction (WAXRD) and TGA analysis. The adsorption properties of cross-linked magnetic CAGS resin toward both As(V) and Cr(VI) were evaluated. Various factors affecting the uptake behavior such as pH, temperature, contact time, initial concentration of metal ions, effect of other ions and desorption were studied. The equilibrium was achieved after about 110 min and 120 min for As(V) and Cr(VI), respectively at pH = 2. The adsorption kinetics followed the mechanism of the pseudo-second order equation for all systems studied, evidencing chemical sorption as the rate-limiting step of adsorption mechanism and not involving a mass transfer in solution. The equilibrium data were analyzed using the Langmuir, Freundlich, and Tempkin isotherm models. The best interpretation for the equilibrium data was given by Langmuir isotherm, and the maximum adsorption capacities were 58.48 and 62.42 mg/g for both Cr(VI) and As(V), respectively. Cross-linked magnetic CAGS displayed higher adsorption capacity for Cr(VI). The adsorption capacity of the metal ions increased with increasing temperature under optimum conditions in case of Cr(VI), but decreased in case of As(V). The metal ion-loaded cross-linked magnetic CAGS were regenerated with an efficiency of greater than 88% using 0.2 M sodium hydroxide (NaOH).     

Treatment of Cr(VI)-containing wastewaters with exopolysaccharide-producing cyanobacteria in pilot flow through and batch systems

Seven exopolysaccharide-producing cyanobacteria were tested with regard to their capability to remove Cr(VI) from the wastewater of a plating industry. The cyanobacterium which showed, under lab conditions, the most promising features with regard to both Cr(VI) removal (about 12 mg of Cr(VI) removed per gram of dry biomass) and growth characteristics (highest growth rate and simplest culture medium) was Nostoc PCC7936. Furthermore, in lab experiments, it was also found that a HCl pretreatment is essential to abate the concentration of Cr(VI) in solution and that the viability of the biomass is not necessary. Subsequently, three pilot devices were tested, one batch (a dialysis cell) and two flow-through systems (a filter press and a column filled with quartz grain). The best performances were obtained with the filter press, where it was observed a sharp decrease in the concentration of Cr(VI), partly due to the adsorption of the metal by the biomass (about 50%) and partly due to its reduction to Cr(III). The results are discussed in terms of the role played by the different components (biomass and polysaccharide) of the cyanobacterial cultures in the removal of Cr(VI).     

Detoxification of hexavalent chromate by Amphibacillus sp. KSUCr3 cells immobilised in silica-coated magnetic alginate beads

Recently isolated Cr(VI)-reducing Amphibacillus KSUCr3 whole cells were immobilised in magnetic gels. Magnetic magnetite (Fe₃O₄) nanoparticles were synthesised with an average particle size of 47 nm and 80 electromagnetic unit (emu)/g saturation magnetisation. Whole cells were immobilised by entrapment in agar, agarose, alginate, or gelatin in the presence or absence of Fe₃O₄ nanoparticles for the preparation of both magnetic and nonmagnetic immobilised cells. Of the gels tested, alginate was selected as the best immobilisation matrix, and following optimisation of the entrapment process, the immobilisation yield reached 92.5%. In addition to the ease of separation and reuse of the magnetic cell-containing alginate beads using an external magnet, the magnetically immobilised cells showed approximately 16% higher Cr(VI) reduction activity compared with nonmagnetic immobilised cells. To improve their physical and mechanical properties, the magnetic alginate beads were successfully coated with a dense silica layer using sol-gel chemistry and Ca(OH)₂, an alkaline catalyst for tetraethyl orthosilicate, to avoid leaching of Ca²⁺ ions. Amphibacillus KSUCr3 cells immobilised in silica-coated magnetic alginate beads showed approximately 1.4- to 3.9-fold enhancement of thermal stability compared with free cells. Furthermore, after seven batch cycles, the Cr(VI) reduction activity of free cells decreased to 48%, whereas immobilised cells still retained 81.1% of their original activity. In addition, the Cr(VI)-reduction rate of immobilised cells was higher relative to free cells, especially at higher Cr(VI) concentrations. These results supported the development of a novel, efficient biocatalysts for Cr(VI) detoxification using a combination of whole cell immobilisation, sol-gel chemistry, and nanotechnology.     

Influence of magnetic field on Cr(VI) adsorption capability of given anaerobic sludge

To provide beneficial guide for the application of the magnetic field in the bio-treatment of the Cr(VI)-contained wastewater, sludge samples from the control bio-system A (absent of magnetic field) and the contrast bio-system B (present of magnetic field) were used to adsorb the synthetic wastewater with 100 mg l⁻¹ Cr(VI). Influences of two adsorption modes, single adsorption and once continuous adsorption, on the Cr(VI) adsorption capacities of both sludge samples were compared. And the influence of regeneration on the Cr(VI) adsorption capacities were also studied. The results of adsorption experiments showed that the Cr(VI) adsorption capacities of the first single adsorption for sludge sample A and B were pretty nearly, which were 9.79 and 9.93 mg, respectively. And after 5 single adsorption periods, the total Cr(VI) adsorption capacity and efficiency of the sample B were 25.88 and 55.66 mg Cr(VI) g⁻¹VSS, while those of the control were 14.95 and 33.98 mg Cr(VI) g⁻¹VSS, respectively. For the sludge sample A and B after a single adsorption, both functions of regeneration were remarkable. But after 13 cycles of the single adsorption-regeneration, the Cr(VI) adsorption capacity and efficiency of the sample B were 110.15 and 189.91 mg Cr(VI) g⁻¹VSS, while those of the control were 70.89 and 140.38 mg Cr(VI) g⁻¹VSS, respectively. Though the Cr(VI) adsorption capacity of a once continuous adsorption period was more than that of a single adsorption period obviously, the Cr(VI) removal rates of the sludge sample A and B in the third period of once continuous adsorption-regeneration were only 8.12 and 33.51%, respectively. It was concluded that the weak magnetic field did improve the Cr(VI) bio-removal efficiency and the sludge stability, the batch treatment was an ideal operation mode for the bio-treatment of the Cr(VI)-contained wastewater, as compared with the continuous operation mode, but regeneration and enough sludge content were two necessary conditions to ensure the efficiency of batch treatment.     

Effect of stable weak magnetic field on Cr(VI) bio-removal in anaerobic SBR system

To study the impact of stable weak magnetic field on the Cr(VI) removal efficiency of predominated strains in ASBR system, the choice of the optimum magnetic density and its effect should be considered chiefly. At different magnetic densities, the growth and propagation rates of predominated strains in solid or liquid mediums and their capabilities of removing Cr(VI) were compared. The results showed that the optimum magnetic density was 6.0 mT. To meet the state first-class standard of effluent discharge, it took 2–5 h more in the plant wastewater treatment than in the synthetic wastewater treatment, but the presence of magnetic field made the reaction time up to par to decrease 1 and 2–3 h, respectively, compared with that of the control. The magnetized magnetic powder could improve the sludge sedimentation capability, turbidity of outflow water and efficiency of bio-system.     

Chromium (VI) uptake by grape stalks wastes encapsulated in calcium alginate beads: equilibrium and kinetics studies

Abstract

The removal of hexavalent chromium from aqueous solution using grape stalks wastes encapsulated in calcium alginate (GS–CA) beads was investigated. Cr(VI) sorption kinetics were evaluated as a function of chromium initial concentration and grape stalks (GS) content in the calcium alginate (CA) beads. The process follows pseudo second-order kinetics. Transport properties of hexavalent chromium on GS–CA beads was characterised by calculating chromium diffusion coefficient using the Linear Absorption Model (LAM). Langmuir isotherms, at pH 3.0 were used to describe sorption equilibrium data as a function of GS percentage in the CAbeads. Maximum uptake obtained was 86.42 mmol of Cr(VI) per L of wet sorbent volume. Results indicated that both kinetic and equilibrium models describe adequately the adsorption process.     

Comparative Study of Cr(VI) Removal by Exiguobacterium sp. in Free and Immobilized Forms

Cr(VI) is a toxic environmental pollutant. To determine the potential role of microbes towards chromate bioremediation, two bacterial strains, E1 and E4, that could tolerate Cr(VI) at levels up to 2250 μg ml^?1 were isolated from the soil of a tannery. They were identified as Exiguobacterium sp. To estimate the removal of Cr(VI) using immobilized bacterial cells, 2% sodium alginate and 2.5% agar were used as immobilizing matrices. In the case of sodium alginate, 89% and 93% of Cr(VI) removal by E1 and E4, respectively, were observed. When agar beads were used as an immobilizing matrix, removal was recorded as 39% and 48% for E1 and E4, respectively. Removal of Cr(VI) was also estimated in sterile and nonsterile tannery effluent. More Cr(VI) removal was noted in the nonsterile effluent than in the sterile effluent. The maximum uptake of Cr(VI) of bound cells of E1 and E4 was found to be 17.54 and 20.04 μg ml^?1, respectively. Fourier transform infrared (FTIR) spectra of cells of E4 with Cr(VI), without Cr(VI), and immobilized cells depicted several absorption peaks, mainly for P?OH group, C?H bending, C?O bond, and amide II groups, reflecting the complex nature of the bacterial cells and the contribution of these functional groups to the Cr(VI) binding process.     

Adsorption of chromium from aqueous solution by activated alumina and activated charcoal

Cr(VI) is considered to be potentially carcinogenic to humans. Removal of Cr(VI) ions from aqueous solution under different conditions was investigated using activated alumina (AA) and activated charcoal (AC) as adsorbents. Batch mode experiments were conducted to study the effects of adsorbent dose, contact time, pH, temperature and initial concentration of Cr(VI). Results showed that the adsorption of Cr(VI) depended significantly on pH and temperature. Equilibrium studies showed that Cr(VI) had a high affinity for AA at pH 4 and AC at pH 2. For AA, maximum adsorption was found at 25 degrees C, indicating exothermic adsorption, while for AC, maximum adsorption was at 40 degrees C. Freundlich and Langmuir adsorption isotherms were also applied and they showed good fits to the experimental data. The results suggest that both AA and AC could be used as effective adsorbents for the removal of Cr(VI) ions.     

Removal of hexavalent chromium by fungal biomass of <Emphasis Type="Italic">Mucor racemosus</Emphasis>: influencing factors and removal mechanism

This study reported the hexavalent chromium removal by untreated Mucor racemosus biomass and the possible mechanism of Cr (VI) removal to the biomass. The optimum pH, biomass dose, initial Cr (VI) concentration and contact time were investigated thoroughly to optimize the removal condition. The metal removal by the biomass was strongly affected by pH and the optimum pH ranged from 0.5 to 1.0. The residual total Cr was determined. It was found that dichromate reduction occurred at a low very low pH value. At biomass dose 6 g/l, almost all the Cr (VI) ions were removed in the optimum condition. Higher removal percentage was observed at lower initial concentrations of Cr (VI) ions, while the removal capacity of the biomass linearly depended on the initial Cr (VI) concentration. More than half of Cr (VI) ions were diminished within 1 h of contact and removal process reached a relative equilibrium in approximately 8 h. Almost all of the Cr (VI) ions were removed in 24 h when initial concentrations were below 100 mg/l. The equilibrium data were fitted in to the Langmuir and the Freundlich isotherm models and the correlated coefficients were gained from the models. A Fourier transform infrared spectra was employed to elucidate clearly the possible biosorption mechanism as well.     

Adsorption of chromium from aqueous solution on treated sawdust

The adsorption of Cr(VI) from aqueous solutions on formaldehyde treated sawdust (SD) and sulphuric acid treated sawdust carbon (SDC) of Indian Rosewood, a timber industry waste, was studied at varying Cr(VI) concentrations, adsorbent dose, pH and agitation time. Similar experiments were conducted with commercially available coconut based activated carbon to compare the results. The Cr(VI) adsorption efficiency on SDC was higher than SD. The adsorption followed first order rate expression and Lagergren equation. An initial pH of 3.0 was most favorable for Cr(VI) removal by both the adsorbents. Maximum Cr(VI) was sequestered from the solution within 60 min after the beginning for every experiment. It is proposed that SDC and SD can be potential adsorbents for Cr(VI) removal from dilute solutions.     

Reductive Immobilization of Chromium in Soils Containing Heterogeneous Fe-Bearing Minerals

Cr(VI) immobilization in systems containing Fe-bearing soil minerals was studied in batch and column systems. Batch experiments showed that water chemistry such as solution pH and Cr(VI) concentration had a pronounced impact on Cr(VI) removal by Fe-bearing soil minerals. Acidic conditions were observed to be more favorable for enhanced Cr(VI) removal. The dependence of Cr(VI) removal on Cr(VI) concentration indicated that there were limited numbers of surface sites on Fe-bearing minerals responsible for Cr(VI) removal. A complexing agent, citrate, significantly enhanced both Cr(VI) removal and total Fe-dissolution from the mineral surfaces relative to non-citrate containing systems, and the iron dissolved from the mineral surfaces was in Fe(III) oxidation form, implying that Cr(VI) removal occurred mainly on mineral surfaces, and the surface Fe(II) sites played an active role in Cr(VI) reduction. The results from column experiments showed that the accumulation of surface precipitates resulted in clogging of pore spaces, thereby creating preferential flow paths within the column. However, the addition of citrate significantly prevented the accumulation of surface precipitates due to the formation of highly soluble Fe–citrate complexes. SEM images revealed that the precipitates accumulated in the column had sponge-like shapes. The energy-dispersive spectroscopy analysis provided further evidence that the surface precipitates formed also contained Cr species as well as Fe. Overall it is clear that Fe-bearing minerals may serve as an effective reducing agent for in-situ reductive immobilization of hexavalent chromium in subsurface systems.     

Biosorption of Cr (VI) from aqueous solution by Rhizopus nigricans

The study was aimed to quantify the Cr sorption ability of powdered biomass of Rhizopus nigricans at the best operating conditions. The influence of solution pH, agitation, Cr (VI) concentration, biomass dosage, contact time, biomass particle size and temperature were studied. The optimum pH for biosorption of Cr (VI) was found to be 2.0. Higher adsorption percentage was noted at lower initial concentrations of Cr ions, while the adsorption capacity of the biomass increased with increasing concentration of ions. Optimum biomass dosage was observed as 0.5% (w/v). More than 75% of the ions were removed within 30 min of contact and maximum removal was obtained after 8 h. Biomass particles of smaller size (90 microm) gave maximum adsorption (99.2%) at 100 mg/l concentration. The adsorption capacity increased with increase in temperature and agitation speed and the optimum were determined as 45 degrees C at 120 rpm. Freundlich and Langmuir isotherms were used to evaluate the data and the regression constants were derived. The adsorption rate constant values (Kad) were calculated for different initial concentration of Cr ions and the sorption was found to be higher at lower concentration (100 mg/l) of metal ion.