XANES study of Cr sorbed by a kitchen waste compost from water

A kitchen waste compost was used to sorb Cr for various times from water containing either Cr(NO3)3 or CrO3 in different concentrations. Scanning electron microscopy (SEM) results show that the composts have been partially oxidized by Cr(VI) during the sorption experiments. X-ray absorption near edge structure (XANES) simulation suggests that about 54.1-61.0% Cr sorbed on the compost is in form of organic Cr(III) through ionic exchange process with the rest being existent as Cr(NO3)3 in the Cr(III) sorption case; no Cr(OH)3 is observed or expected because the solution pH after sorption experiments is or= 5.94. Moreover, organic Cr(III) represents about 51.7-69.0% of the total sorbed Cr, and the rest (6.1-28.5%) is Cr(VI).     

Investigation of the removal mechanism of Cr(VI) in groundwater using activated carbon and cast iron combined system

Zero-valent iron (Fe⁰) has been widely used for Cr(VI) removal; however, the removal mechanisms of Cr(VI) from aqueous solution under complex hydrogeochemical conditions were poorly understood. In this research, the mixed materials containing cast iron and activated carbon were packed in columns for the treatment of aqueous Cr(VI)-Cr(III) in groundwater with high concentration of Ca²⁺, Mg²⁺, HCO₃ ⁻, NO₃ ⁻, and SO₄ ²⁻. We investigate the influences of those ions on Cr(VI) removal, especially emphasizing on the reaction mechanisms and associated precipitations which may lead to porosity loss by using X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) techniques. The results show that the precipitations accumulated on the material surface were (Fe/Cr) (oxy)hydroxide, mixed Fe(III)-Cr(III) (oxy)hydroxides, Fe₂O₃, CaCO₃, and MgCO₃. During these reactions, the Cr(VI) was reduced to Cr(III) coupled with the oxidated Fe⁰ to Fe(II) through the galvanic corrosion formed by the Fe⁰-C and/or the direct electron transfer between Fe⁰ and Cr(VI). In addition, Cr(VI) could be reduced by aqueous Fe(II), which dominated the whole removal efficiency. The primary aqueous Cr(III) was completely removed together with Cr(III) reduced from Cr(VI) even when Cr(VI) was detected in the effluent, which meant that the aqueous Cr(III) could occupy the adsorption sites. In general, the combined system was useful for the Cr(VI)-Cr(III) treatment based on galvanic corrosion, and the hardness ions had a negative effect on Cr(VI) removal by forming the carbonates which might promote the passivation of materials and decrease the removal capacity of the system.

     

Preparation of a novel iron-based biochar composite for removal of hexavalent chromium in water

The chitosan-stabilized ferrous sulfide nanoparticles were loaded on biochar to prepare a composite material FeS-CS-BC for effective removal of hexavalent chromium in water. BC and FeS-CS-BC were characterized by Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) analyses. Batch experiments were employed to evaluate the Cr(VI) removal performance. The experimental results showed that the removal rate of Cr(VI) by FeS-CS-BC(FeS:CS:BC?=?2:2:1) reached 98.34%, which was significantly higher than that of BC (44.58%) and FeS (79.91%). In the pH range of 2–10, the removal of Cr(VI) by FeS-CS-BC was almost independent of pH. The limitation of coexisting anions (Cl^?、SO₄^2?、NO₃^?) on Cr(VI) removal was not too obvious. The removal of Cr(VI) by FeS-CS-BC was fitted with the pseudo-second-order dynamics, which was a hybrid chemical-adsorption reaction. The X-ray photoelectron spectroscopy (XPS) analysis result showed that Cr(VI) was reduced, and the reduced Cr(VI) was fixed on the surface of the material in the form of Cr(VI)–Fe(III).

Removal of hexavalent chromium from wastewater by FeS-CS-BC composite synthesized by impregnation.

     

Effect of sulfate reduction activity on biological treatment of hexavalent chromium [Cr(VI)] contaminated electroplating wastewater under sulfate-rich condition

Electroplating wastewater (EW) containing heavy metals was treated by a two-stage packed-bed reactor system. The EW was highly contaminated with hexavalent chromium and other heavy metals as well as sulfate because sulfuric acid had been mainly used to polish the surface of metals to be electroplated. This acidic EW was effectively neutralized in an alkaline reactor where limestone had been packed. The neutralized wastewater together with organic wastewater from a starch-processing factory (SPW) was fed to a bioreactor packed with waste biomass. The SPW was used to supplement the electron donor in the sulfidogenic bioreactor. During the whole operation, we investigated the stoichiometry of electron to see what could be a major factor to remove Cr in the wastewater. The removal rates of sulfate and Cr(VI) were dependent on the consumption rate of organic materials in the wastewater. The stoichiometric studies also showed that about 63% of electrons from oxidation of organic materials were used to reduce sulfate. When the electrons of sulfide oxidation to elemental sulfur was at least 1.3 times higher than that of Cr(VI) reduction to Cr(III), Cr(VI) was completely removed. This result suggests that Cr(VI) reduction can be expected to take place under sulfate-rich anaerobic conditions, and sulfide produced by sulfate reducing bacteria could be used to immobilize soluble chromium through Cr(VI) reduction.     

Chromium species behaviour in the activated sludge process

The purpose of this research was to compare trivalent chromium (Cr(III)) and hexavalent chromium (Cr(VI)) removal by activated sludge and to investigate whether Cr(VI) reduction and/or Cr(III) oxidation occurs in a wastewater treatment system. Chromium removal by sludge harvested from sequencing batch reactors, determined by a series of batch experiments, generally followed a Freundlich isotherm model. Almost 90% of Cr(III) was adsorbed on the suspended solids while the rest was precipitated at pH 7.0. On the contrary, removal of Cr(VI) was minor and did not exceed 15% in all experiments under the same conditions. Increase of sludge age reduces Cr(III) removal, possibly because of Cr(III) sorption on slime polymers. Moreover, the decrease of suspended solids concentration and the acclimatization of biomass to Cr(VI) reduced the removal efficiency of Cr(III). Batch experiments showed that Cr(III) cannot be oxidized to Cr(VI) by activated sludge. On the contrary, Cr(VI) reduction is possible and is affected mainly by the initial concentration of organic substrate, which acts as electron donor for Cr(VI) reduction. Initial organic substrate concentration equal to or higher than 1000 mgl(-1) chemical oxygen demand permitted the nearly complete reduction of 5 mgl(-1) Cr(VI) in a 24-h batch experiment. Moreover, higher Cr(VI) reduction rates were obtained with higher Cr(VI) initial concentrations, expressed in mg Cr(VI) g(-1) VSS, while decrease of suspended solids concentration enhanced the specific Cr(VI) reduction rate.     

Bioadsorption and bioaccumulation of chromium trivalent in Cr(III)-tolerant microalgae: A mechanisms for chromium resistance

Anthropogenic activity constantly releases heavy metals into the environment. The heavy metal chromium has a wide industrial use and exists in two stable oxidation states: trivalent and hexavalent. While hexavalent chromium uptake in plant cells has been reported that an active process by carrying essential anions, the cation Cr(III) appears to be taken up inactively. Dictyosphaerium chlorelloides (Dc1M), an unicellular green alga is a well-studied cell biological model organism. The present study was carried out to investigate the toxic effect of chromium exposures on wild-type Cr(III)-sensitive (Dc1M^wt) and Cr(III)-tolerant (Dc1M^Cr(III)R30) strains of these green algae, and to determine the potential mechanism of chromium resistance. Using cell growth as endpoint to determine Cr(III)-sensitivity, the IC₅₀₍₇₂₎ values obtained show significant differences of sensitivity between wild type and Cr(III)-tolerant cells. Scanning electron microscopy (SEM) showed significant morphological differences between both strains, such as decrease in cell size or reducing the coefficient of form; and transmission electron microscopy (TEM) revealed ultrastructural changes such as increased vacuolization and cell wall thickening in the Cr(III)-tolerant strain with respect to the wild-type strain. Energy dispersive X-ray analysis (SEM/XEDS) revealed that Cr(III)-tolerant D. chlorelloides cells are able to accumulate considerable amounts of chromium distributed in cell wall (bioadsorption) as well as in cytoplasm, vacuoles, and chloroplast (bio-accumulation). Morphological changes of Cr(III)-tolerant D. chlorelloides cells and the presence of these electron-dense bodies in their cell structures can be understood as a Cr(III) detoxification mechanism.     

Cr(VI) sorption by free and immobilised chromate-reducing bacterial cells in PVA–alginate matrix: equilibrium isotherms and kinetic studies

Chromate-resistant bacterial strain isolated from the soil of tannery was studied for Cr(VI) bioaccumulation in free and immobilised cells to evaluate its applicability in chromium removal from aqueous solution. Based on the comparative analysis of the 16S rRNA gene, and phenotypic and biochemical characterization, this strain was identified as Paenibacillus xylanilyticus MR12. Mechanism of Cr adsorption was also ascertained by chemical modifications of the bacterial biomass followed by Fourier transform infrared spectroscopy analysis of the cell wall constituents. The equilibrium biosorption analysed using isotherms (Langmuir, Freundlich and Dubinin–Redushkevich) and kinetics models (pseudo-first-order, second-order and Weber–Morris) revealed that the Langmuir model best correlated to experimental data, and Weber–Morris equation well described Cr(VI) biosorption kinetics. Polyvinyl alcohol alginate immobilised cells had the highest Cr(VI) removal efficiency than that of free cells and could also be reused four times for Cr(VI) removal. Complete reduction of chromate in simulated effluent containing Cu²⁺, Mg²⁺, Mn²⁺ and Zn²⁺ by immobilised cells, demonstrated potential applications of a novel immobilised bacterial strain MR12, as a vital bioresource in Cr(VI) bioremediation technology.     

Feasibility of using microalgal biomass cultured in domestic wastewater for the removal of chromium pollutants

The feasibility of obtaining and using the biomass of a microalga, Chlorella miniata, from domestic wastewater (DW) cultures for the removal of chromium(III) [Cr(III)] and chromium(VI) [Cr(VI)] was compared with that from commercial Bristol medium (BM). Results showed that Chlorella miniata cultured in DW under 16-8 hours light-dark cycle [DW(16-8)] had similar growth to that in BM [BM(16-8)], but these two biomass had different biochemical compositions, and the former one had lower carbohydrate and higher protein content. When cultured in domestic wastewater, a higher biomass was obtained under continuous illumination [DW(24-0)], and the cells had higher carbohydrate and lower protein concentrations than that of DW(16-8). The spectra of the Fourier transform infrared spectrometer revealed that the functional groups on the surface of the three kinds of biomass--DW(16-8), DW(24-0), and BM(16-8)--were comparable, except an additional peak at 1731 cm(-1) was found in the biomass cultured in domestic wastewater, which was probably the result of bacterial contamination. Although biochemical differences were found among the three kinds of microalgal biomass, similar biosorption performances to chromium pollutants were recorded, with approximately 75% Cr(III) and 100% Cr(VI) removed at equilibrium in Cr(III) and Cr(VI) experiments, respectively, when dead biomass was used as a biosorbent. Therefore, it is possible to culture Chlorella miniata in domestic wastewater and use the biomass for the removal of chromium pollutants.     

Evaluation of Acinetobacter sp. B9 for Cr (VI) resistance and detoxification with potential application in bioremediation of heavy-metals-rich industrial wastewater

Present work demonstrates Cr (VI) detoxification and resistance mechanism of a newly isolated strain (B9) of Acinetobacter sp. Bioremediation potential of the strain B9 is shown by simultaneous removal of major heavy metals including chromium from heavy-metals-rich metal finishing industrial wastewater. Strain B9 tolerate up to 350 mg L^?1 of Cr (VI) and also shows level of tolerance to Ni (II), Zn (II), Pb (II), and Cd (II). The strain was capable of reducing 67 % of initial 7.0 mg L^?1 of Cr (VI) within 24 h of incubation, while in presence of Cu ions 100 % removal of initial 7.0 and 10 mg L^?1 of Cr (VI) was observed with in 24 h. pH in the range of 6.0–8.0 and inoculum size of 2 % (v/v) were determined to be optimum for dichromate reduction. Fourier transform infrared spectroscopy and transmission electron microscopy studies suggested absorption or intracellular accumulation and that might be one of the major mechanisms behind the chromium resistance by strain B9. Scanning electron microscopy showed morphological changes in the strain due to chromium stress. Relevance of the strain for treatment of heavy-metals-rich industrial wastewater resulted in 93.7, 55.4, and 68.94 % removal of initial 30 mg L^?1 Cr (VI), 246 mg L^?1 total Cr, and 51 mg L^?1 Ni, respectively, after 144 h of treatment in a batch mode.     

Kinetics of chromium (VI) reduction by a type strain Shewanella alga under different growth conditions.

We conducted kinetic batch experiments to determine the reduction of Chromium(VI) by a type strain of Shewanella alga (BrY-MT) ATCC 55627. Chromium(VI) was reduced to Chromium(III) by BrY-MT grown in three different substrates: BHIB (brain heart infusion broth), TSB (tryptic soy broth), and M9 (minimum broth). Four different Cr(VI) concentrations 4.836, 10.00, 37.125, and 260.00 mg l-1 were reduced at different rates by BrY-MT in both aerobic and anaerobic conditions. BrY-MT grown in BHIB reduced the maximum amount of Cr(VI) followed by TSB and M9. Carbondioxide produced from bacterial respiration varied with and without Cr(VI) under both aerobic and anaerobic conditions. The Cr(VI) reduction data under anaerobic condition was fitted by a monod model to determine the bacterial kinetic parameters. The kinetic parameters determined by fitting the anaerobic experimental data were used to run a forward simulation for experiments conducted under aerobic conditions. The monod model was modified to account for an inhibition parameter for the Cr(VI) experiment at 260 mg l-1. All the parameters varied within a narrow range, and were distinct for different substrates. Our studies show that, successful in situ bioremediation of Cr(VI) is depended on the type of substrates (electron donors) and the concentration of Cr(VI) in geologic medium.     

Biogeochemical influence on transport of chromium in manganese sediments: experimental and modeling approaches

Hexavalent chromium (Cr(VI)) was reduced to immobile and nontoxic Cr(III) by a dissimilatory metal reducing bacteria, Shewanella alga Simidu (BrY-MT) ATCC 55627. A series of kinetic batch and dynamic column experiments were conducted to provide an understanding of Cr(VI) reduction by the facultative anaerobe BrY-MT. Reduction of Cr(VI) was rapid (within 1 h) in columns packed with quartz sand and bacteria, whereas Cr(VI) reduction by BrY-MT was delayed (57 h) in the presence of beta-MnO2-coated sand. A mathematical model was developed and evaluated against data obtained from column experiments. The model takes into account (1) advective-dispersive transport of Cr(III), Cr(VI), lactate, and protein (mobile and immobile bacteria); (2) first-order kinetic adsorption of Cr(III) and lactate; (3) conversion of solid phase beta-MnO2 to solid phase MnOOH due to oxidation of Cr(III); (4) dual-Monod kinetics, where Cr(VI) is the electron acceptor and lactate is the electron donor. The breakthrough data for Cr(III), Cr(VI), lactate, and protein (mobile and immobile bacteria) were fitted simultaneously. The breakthrough data are well described by the mathematical model that considers the above processes. This result demonstrates the ability of the coupled hydrobiogeochemical model to simulate chromium transport in complex reactive systems.     

Chromium (VI) remediation by a native strain: effect of environmental conditions and removal mechanisms involved

A native bacterial strain with high capability for Cr (VI) removal was isolated from tannery sediments located in Elena (Córdoba Province, Argentina). The strain was characterized by amplification of 16S rRNA gene and identified as Serratia sp. C8. It was able to efficiently remove different Cr (VI) concentrations in a wide range of pHs and temperatures. The addition of different carbon sources as well as initial inoculum concentration were analyzed, demonstrating that Serratia sp. C8 could reduce 80 % of 20 mg/L Cr (VI) in a medium containing glucose 1 g/L, at pH 6–7 and 28 °C as optimal conditions, using 5 % inoculum concentration. The mechanisms involved in Cr (VI) removal were also evaluated. The strain was capable of biosorpting around 7.5–8.5 % of 20 mg/L Cr on its cell surface and to reduce Cr (VI). In addition, approximately a 54 and 46 % of total Cr was detected in the biomass and in the culture medium, respectively, and in the culture medium, Cr (III) was the predominant species. In conclusion, Serratia sp. C8 removed Cr (VI) and the mechanisms involved in decreasing order of contribution were as follows: reduction catalyzed by intracellular enzymes, accumulation into the cells, and biosorption to the microbial biomass. This strain could be a suitable microorganism for Cr (VI) bioremediation of tannery sediments and effluents or even for other environments contaminated with Cr.     

A study on the Cr(VI) removal from aqueous solutions by steel wool

The reduction of Cr(VI) by steel wool and the precipitation of reduced chromium by CaCO(3) powder and NaOH solution were investigated in continuous and batch systems, respectively. The effects of acid and initial Cr(VI) concentrations, volumetric rate and temperature of solution on Cr(VI) reduction were studied. The results showed that the reduction of Cr(VI), to a large extent, depended on, and increased with, acid concentration. The Cr(III) and iron ions in the reduced solution were completely precipitated by using NaOH solution at appropriate alkaline conditions. It was concluded that CaCO(3) powder could be used as a cheap precipitant for Cr(III) ions. But the iron ions in the reduced solution could not be fully removed by using this precipitant.     

Reduction of Cr(VI) by malic acid in aqueous Fe-rich soil suspensions

Detoxification of Cr(VI) through reduction by organic reductants has been regarded as an effective way for remediation of Cr(VI)-polluted soils. However, such remediation strategy would be limited in practical applications due to the low Cr(VI) reduction rate. In this study, the catalytic effect of two Fe-rich soils (Ultisol and Oxisol) on Cr(VI) reduction by malic acid was evaluated. As the results shown, the two soils could obviously accelerate the reduction of Cr(VI) by malic acid at low pH conditions, while such catalytic effect was gradually suppressed as the increase in pH. After reaction for 48 h at pH 3.2, Oxalic acid was found in the supernatant of Ultisol, suggesting the oxidization of hydroxyl in malic acid to carboxyl and breakage of the bond between C₂ and C₃. It was also found that the catalytic reactivity of Ultisol was more significant than that of Oxisol, which could be partly attributed to the fact that the amount of Fe(II) released from the reductive dissolution of Ultisol by malic acid was larger than that of Oxisol. With addition of Al(III), the catalytic effect from Ultisol was inhibited across the pH range examined. On the contrary, the presence of Cu(II) would increase the catalytic effect of Ultisol, which was more pronounced with the increase in pH. This study proposed a potential way for elimination of the environmental risks posed by the Cr(VI) contamination by use of the natural soil surfaces to catalyze Cr(VI) reduction by the organic reductant such as malic acid, a kind of organic reductant originating from soil organic decomposition process or plant excretion.     

Effect of humic substance on thermal treatment of chromium(VI)-containing latosol soil

Latosol soils contaminated with chromium(VI) [Cr(VI)], which is hazardous, can be recycled as raw materials for porcelain and construction sectors if a proper thermal stabilization process is implemented. This study investigates how thermal treatment affects Cr behavior during the sintering of latosol and deorganic latosol samples; both samples are artificially contaminated with CrO3. Approaches including X-ray absorption spectroscopy, scanning electron microscopy, N2-based Brunauer Emmett Teller surface analyzer, thermogravimetric analyzer/differential scanning calorimeter, and the toxicity characteristic leaching procedure promulgated by Taiwan Environmental Protection Administration are used in this study. After drying the Cr(VI)-contaminated latosol (i.e., containing 37,120 mg of Cr/kg sample) at 105 degrees C, approximately 80% of the doped CrO3 is chemically reduced to Cr(OH)3 by a humic substance naturally existing in the soil. In contrast, in the organics-free CrO3-contaminated latosol dried at 105 degrees C, only 9% of the doped CrO3 is reduced to Cr(OH)3. Heating the samples at 500 and 1100 degrees C transforms hazardous Cr(VI) into Cr(III) that is negligibly toxic; Cr2O3, which is insoluble, is detected as the most abundant Cr species. Moreover, formation of Cr2SiOs, which is suggested to relate to low Cr leaching, is only detected in the sample heated at 1100 degrees C. Surface morphology, surface area, and thermogravimetric analyzer/differential scanning calorimeter results demonstrate that thermal treatment at 1100 degrees C can incur considerable soil sintering/ melting if the humic substance in the soil has been heated off previously. Finally, Cr concentrations in the toxicity characteristic leaching procedure leachates collected from the samples thermally treated at 1100 degrees C for 4 hr are < or =0.21 mg of Cr L(-1) that are much less than the Taiwan Environmental Protection Administration regulatory limit (<5 mg of Cr L(-1)); consequently, these two samples are nonhazardous, and they have the potential for resource recycling. Conversely, Cr concentrations in the leachates from all 500 degrees C and 105 degrees C samples are in the 25.6-1279 mg L(-1) range.     

Mobility and recalcitrance of organo-chromium(III) complexes

Hexavalent chromium [Cr(VI)] is a major industrial pollutant. Bioremediation of Cr(VI) to Cr(III) is a viable clean-up approach. However, Cr(VI) bioreduction also produces soluble organo–Cr(III) complexes, and little is known about their behavior in the environment. When tested with soil columns, citrate–Cr(III) showed little sorption to soil; malate–Cr(III) had limited partitioning with soil; and histidine–Cr(III) exhibited significant interaction with soil. It appears that the mobility varies depending on the organic ligand. Further, Ralstonia eutropha JMP 134 and Pseudomonas aeruginosa pAO1 readily degraded malate, citrate, and histidine, but not the corresponding organo–Cr(III) complexes. The recalcitrance is not due to toxicity, but the complexes are likely to cause hindrance to enzymes, as malate dehydrogenase and amino acid oxidase could not use malate–Cr(III) and histidine–Cr(III), respectively. The data are in agreement with the reports of soluble organo–Cr(III) complexes in the environment.     

Biotreatment of chromite ore processing residue by Pannonibacter phragmitetus BB

Chromite ore processing residues (COPR) is the source of the Cr(VI) contamination in the environment. Pannonibacter phragmitetus BB was used to treat two different types of COPRs in this research. The water-soluble Cr(VI) of COPR A and B is 3,982.9 and 1,181.4 mg/kg, respectively. In the column biotreatment process, P. phragmitetus BB can reduce Cr(VI) in the leachate to an undetectable level at the flow rate of 1 and 2 ml/min. In the direct biotreatment process, Cr(VI) in the liquid supernatant of COPR A and B decreased from 265 and 200 mg/l to 145 and 40 mg/kg after 240 h of incubation. In one-step and two-step biotreatment processes, Cr(VI) in the liquid supernatant of both COPRs can be reduced to an undetectable level. The toxicity characteristic leaching procedure results indicate that the Cr(VI) concentration of treated COPR A (3.48 mg/l) is lower than the identification standards for hazardous wastes of China (5 mg/l) (GB 5085.6-2007). The information obtained in this study has significance for the application of P. phragmitetus BB to remediate COPR contamination.     

Immobilised microbial reactor for the biotransformation of hexavalent chromium

Out of an array of bacterial strains isolated from soil contaminated with effluents from electroplating wastewater, Bacillus coagulans exhibited the maximum Cr(VI) reduction potential. The feasibility of an immobilized B. coagulans bioreactor for hexavalent chromium reduction was investigated. Experimental results demonstrated that near complete removal of Cr(VI) was achieved in the reactor with an initial Cr(VI) concentration of 26 mg/l and reactor time of 24 h. The removal efficiency in the bioreactor was significantly affected by the influent Cr(VI) concentration, the Cr(VI) loading rate, the reaction time and the amount of Cr(VI) reduced by the biomass.     

Mechanisms of chromium and arsenite adsorption by amino-functionalized SBA-15

The adsorption of Cr(VI) and As(III) by amino-functionalized SBA-15 (NH₂-SBA-15) from single and binary systems were investigated in this work. The effects of pH and temperature on the adsorption of NH₂-SBA-15 were studied. Adsorption kinetics, isotherm model, and thermodynamics were studied to analyze the experimental data. pH 2 was the optimum condition for the adsorption of Cr(VI) and pH 4 for As(III) adsorption. Increasing temperature had a positive effect on the removal of both Cr(VI) and As(III). The Freundlich isotherm model can depict the adsorption process best. The pseudo-second-order kinetic model fitted well with the kinetic data of Cr(VI) and As(III) in the single-component system. In the binary system, the adsorption of As(III) by NH₂-SBA-15 was slightly enhanced with the presence of Cr(VI); however, As(III) had no obvious effect on the removal of Cr(VI). Regeneration experiments indicated that 0.1 mol/L NaHCO₃ was an efficient desorbent for the recovery of Cr(VI) and As(III) from NH₂-SBA-15; the desorption rates for Cr(VI) and As(III) were 91.6 and 33.59 %, respectively. After five recycling cycles, the removal rates were 88 and 7 % for Cr(VI) and As(III) adsorption by NH₂-SBA-15, respectively.     

Transportation and distribution of chromium in the anaerobic sludge treating the chromium-containing wastewater

The chromium distribution and transportation in the anaerobic activated sludge was investigated using a sequential extraction method. The results showed that Cr(VI) in aqueous solution was reduced by the metabolic product of SRB and form indissoluble Cr(OH)₃ in a solid phase. More than 99% of Cr(III) in the sludge was in a stable residual form (RES) of bio-reduction. The Cr(VI) was mainly present in RES and organic form (OR). With increasing Cr(VI) concentration, Cr(VI) was transferred from RES into OR and even exchangeable form (EXCH). Meanwhile, sulphate and co-existing metal ions affected the occurrence form of Cr(VI).