Equilibria of chromate(VI) species in acid medium and ab initio studies of these species

Metagenomes from microbial populations beneath a chromium waste tip give insight into the mechanism of Cr (VI) reduction

Dumped Chromium Ore Processing Residue (COPR) at legacy sites poses a threat to health through leaching of toxic Cr(VI) into groundwater. Previous work implicates microbial activity in reducing Cr(VI) to less mobile and toxic Cr(III), but the mechanism has not been explored. To address this question a combined metagenomic and geochemical study was undertaken. Soil samples from below the COPR waste were used to establish anaerobic microcosms which were challenged with Cr(VI), with or without acetate as an electron donor, and incubated for 70 days. Cr was rapidly reduced in both systems, which also reduced nitrate, nitrite then sulfate, but this sequence was accelerated in the acetate amended microcosms. 16S rRNA gene sequencing revealed that the original soil sample was diverse but both microcosm systems became less diverse by the end of the experiment. A high proportion of 16S rRNA gene reads and metagenome-assembled genomes (MAGs) with high completeness could not be taxonomically classified, highlighting the distinctiveness of these alkaline Cr impacted systems. Examination of the coding capacity revealed widespread capability for metal tolerance and Fe uptake and storage, and both populations possessed metabolic capability to degrade a wide range of organic molecules. The relative abundance of genes for fatty acid degradation was 4× higher in the unamended compared to the acetate amended system, whereas the capacity for dissimilatory sulfate metabolism was 3× higher in the acetate amended system. We demonstrate that naturally occurring in situ bacterial populations have the metabolic capability to couple acetate oxidation to sequential reduction of electron acceptors which can reduce Cr(VI) to less mobile and toxic Cr(III), and that microbially produced sulfide may be important in reductive precipitation of chromate. This capability could be harnessed to create a Cr(VI) trap-zone beneath COPR tips without the need to disturb the waste.

Interactions between chromium species and DNA in vitro and their potential role in the toxicity of hexavalent chromium

Epidemiological and animal studies have supported the carcinogenicity of hexavalent chromium [Cr(VI)]; however, molecular changes responsible for the induction of cancer by Cr(VI) are not entirely understood. Numerous mechanistic studies suggested the role of oxidative stress and genotoxicity in Cr(VI)-mediated carcinogenesis; however, specific types of DNA damage have not yet been conclusively attributed to specific chromium species or other reactive byproducts generated in biological systems exposed to Cr(VI). Due to the remarkably complex chemistry and biological effects of chromium species generated through the intracellular reduction of Cr(VI), their relevance for Cr(VI)-mediated carcinogenesis has not yet been fully elucidated and continues to be a subject of ongoing discussions in the field. In this report, we describe a complex world of chromium species and their reactivity with DNA and other biologically relevant molecules in vitro to inform a more complete understanding of Cr(VI)-mediated toxicity. In addition, we discuss previous results in the context of in vitro models and analytical methods to reconcile some conflicting findings on the biological role of chromium species.

A novel biosorbent functionalized pillar[5]arene: Synthesis, characterization and effective biosorption of Cr(VI)

Among toxic chemicals, hexavalent chromium (Cr(VI)) is one of the most carcinogenic and toxic pollutants that hostiles to the health of both humans and other living things. Therefore, the removal of Cr(VI) is of great importance to keep our environment clean and tidy. In this study, an easy-make, inexpensive, and natural biosorbent material (Sp-P[5]) was prepared to preserve our environment using a pillar[5]arene based-on sporopollenin microcapsule. The prepared biosorbent was successfully characterized by some techniques such as FTIR, XRD, and SEM. The biosorbent, Sp-P[5], exhibited an open mesoporous structure richly decorated with multi-amine-containing moieties resulting in enhanced Cr(VI) sorption. The sorption behavior of Cr(VI) ions is satisfactorily adapted from the sorption kinetics pseudo-second-order law and the isotherm models to the Langmuir model at different temperatures. The Langmuir model fits at different temperatures (298-328 K) and the maximum sorption capacities of the Cr(VI) ion ranged from 106.38 to 117.26 mg/g. The thermodynamic calculations reveal that the sorption of Cr(VI) ions on the Sp-P[5] is entropy-driven, endothermic, and spontaneous. The prepared biosorbent was also applied to the natural wastewater samples and different ions (chromate and dichromate). The sorption and desorption experiments showed that the sorption efficiency for Cr(VI) ions of the Sp-P[5] decreased to 70.88 % after 8 cycles. As result, the synthesized biosorbent, Sp-P[5], has outstanding potential in the removal of Cr(VI) ions from water bodies and natural wastewater systems.

Removal of hexavalent chromium via biochar-based adsorbents: State-of-the-art, challenges, and future perspectives

Chromium originates from geogenic and extensive anthropogenic activities and significantly impacts natural ecosystems and human health. Various methods have been applied to remove hexavalent chromium (Cr(VI)) from aquatic environmental matrices, including adsorption via different adsorbents, which is considered to be the most common and low-cost approach. Biochar materials have been recognized as renewable carbon sorbents, pyrolyzed from various biomass at different temperatures under limited/no oxygen conditions for heavy metals remediation. This review summarizes the sources, chemical speciation & toxicity of Cr(VI) ions, and raw and modified biochar applications for Cr(VI) remediation from various contaminated matrices. Mechanistic understanding of Cr(VI) adsorption using different biochar-based materials through batch and saturated column adsorption experiments is documented. Electrostatic interaction and ion exchange dominate the Cr(VI) adsorption onto the biochar materials in acidic pH media. Cr(VI) ions tend to break down as HCrO₄^-, CrO₄^2-, and Cr₂O₇^2- ions in aqueous solutions. At low pH (∼1-4), the availability of HCrO₄^- ions attributes the electrostatic forces of attraction due to the available functional groups such as -NH₄⁺, -COOH, and -OH₂⁺, which encourages higher adsorption of Cr(VI). Equilibrium isotherm, kinetic, and thermodynamic models help to understand Cr(VI)-biochar interactions and their adsorption mechanism. The adsorption studies of Cr(VI) are summarized through the fixed-bed saturated column experiments and Cr-contaminated real groundwater analysis using biochar-based sorbents for practical applicability. This review highlights the significant challenges in biochar-based material applications as green, renewable, and cost-effective adsorbents for the remediation of Cr(VI). Further recommendations and future scope for the implications of advanced novel biochar materials for Cr(VI) removal and other heavy metals are elegantly discussed.

Impact of the choice of buffer on the electrochemical reduction of Cr(vi) in water on carbon electrodes

The nature of the buffer influences the PCET step gating Cr(vi) reduction in water at pH 4.75, as well as the extent of deposition on carbon electrodes. Electrode activity is recovered without polishing, through a simple acid wash step.

Preparation of a novel environmentally friendly hydrophobic deep eutectic solvent ChCl-THY and its application in removal of hexavalent chromium from aqueous solution

A liquid-liquid extraction methodology was developed for the removal of Cr(VI) from contaminated water using a novel green hydrophobic deep eutectic solvent (DES) as an efficient sole extracting agent. The hydrophobic DES was obtained by mixing choline chloride and thymol in 1:4 molar ratio at 70°C for 10 min and was denoted as ChCl-THY(1:4). The ChCl-THY(1:4) works efficiently for removal of high (20 mg/L) and low (500 µg/L) concentration of Cr(VI) from artificially contaminated natural water with >95% extraction efficiency (E%) at optimized reaction conditions (pH 2-6, 40°C). The DES was characterized by ¹ H NMR and FTIR spectroscopy, and the data suggest that interaction occurs between Cl^- ion of choline chloride and H atoms of thymol molecules. Physicochemical properties such as density, melting point, moisture, and solubility were studied and discussed. Herein, no sharp melting point was observed for ChCl-THY(1:4) in DSC curve. DES was regenerated using 0.1 M NaOH as stripping agent, and 50%-60% extraction efficiency could be attained in the next cycle. A plausible mechanism of interaction between Cr(VI) species and DES was also explored with the help of FTIR spectroscopy. PRACTITIONER POINTS: A novel hydrophobic DES (ChCl-THY) is prepared by mixing choline chloride and thymol at 1:4 molar ratio. ChCl-THY(1:4) is employed for the first time as sole extracting agent to remove the Cr(VI) from contaminated aqueous solution. >95% extraction efficiency was achieved by ChCl-THY(1:4) in natural water conditions at µg/L and mg/L level of contamination. Both the component used to prepare the DES are naturally abundant; hence, DES is not toxic for biota. The element present in natural water did not show any interference with extraction of Cr(VI).

On the Origins of Some Spectroscopic Properties of "Purple Iron" (the Tetraoxoferrate(VI) Ion) and Its Pourbaix Safe-Space

In this work, the authors attempt to interpret the visible, infrared and Raman spectra of ferrate(VI) by means of theoretical physical-inorganic chemistry and historical highlights in this field of interest. In addition, the sacrificial decomposition of ferrate(VI) during water treatment will also be discussed together with a brief mention of how Rayleigh scattering caused by the decomposition of Fe^VIO₄²⁻ may render absorbance readings erroneous. This work is not a compendium of all the instrumental methods of analysis which have been deployed to identify ferrate(VI) or to study its plethora of reactions, but mention will be made of the relevant techniques (e.g., Mössbauer Spectroscopy amongst others) which support and advance this overall discourse at appropriate junctures, without undue elaboration on the foundational physics of these techniques.

Nature-Inspired Coral-like Layered [Co0.79Al0.21(OH)2(CO3)0.11]·mH2O for Fast Selective ppb Level Capture of Cr(VI) from Contaminated Water

Rapid industrialization has led to the release of hexavalent chromium (Cr(VI)), a "Class A" human carcinogen, mutagen, and teratogen in biological systems. Current adsorbents like anionic exchange resins and metal-organic frameworks can remove harmful heavy metal oxyanions from water but are not stable in a broad pH range, suffer from selectivity, and cannot capture them from trace values below the tolerance limits given by the U.S. EPA (100 ppb) and WHO (50 ppb). Herein, we have synthesized nature-inspired coral-like three-dimensional hierarchical structures of [Co_0.79Al_0.21(OH)₂(CO₃)_0.11]·mH₂O (CoAl-LDH) that sets a new benchmark for sequestering oxyanions of Cr(VI). CoAl-LDH shows a broad pH working range (1.93-12.22), high selectivity toward saturated water samples containing monovalent (Cl^-, F^-, Br^-, and NO₃^-) and divalent (SO₄^2-) anions with fast kinetics (reaches equilibrium within a minute), high capacity (93.4 ± 7.8 mg g^-1), and high distribution coefficient of 1.09 × 10⁶ mL g^-1. Unlike other materials, it can decrease Cr(VI) concentration up to 0.012 ppb. This high selectivity for Cr(VI) is linked to the weak bonding interaction between Cr₂O₇^2- and brucite-like layers, as revealed from thermogravimetric and infrared spectroscopy. With these remarkable features coupled with low cost and an environmentally friendly nature, we have also designed an anion exchange column that can remove >99% Cr(VI) with just 1 wt % of CoAl-LDH and 99 wt % of sand and is a prominent candidate for the elimination of Cr(VI) from industrial effluents.

Efficient Removal of Chromium(VI) Anionic Species and Dye Anions from Water Using MOF-808 Materials Synthesized with the Assistance of Formic Acid

This study presents a simple approach to prepare MOF-808, an ultra-stable Zr-MOF constructed from 6-connected zirconium clusters and 1,3,5-benzene tricarboxylic acid, with tailored particle sizes. Varying the amount of formic acid as a modulator in the range of 200-500 equivalents results in MOF-808 materials with a crystal size from 40 nm to approximately 1000 nm. Apart from the high specific surface area, a combination of a fraction of mesopore and plenty of acidic centers on the Zr-clusters induces a better interaction with the ionic pollutants such as K2Cr2O7 and anionic dyes. MOF-808 shows uptakes of up to 141.2, 642.0, and 731.0 mg/g for K2Cr2O7, sunset yellow, and quinoline yellow, respectively, in aqueous solutions at ambient conditions. The uptakes for the ionic dyes are significantly higher than those of other MOFs reported from the literature. Moreover, the adsorption capacity of MOF-808 remains stable after four cycles. Our results demonstrate that MOF-808 is a promising ideal platform for removing oxometallates and anionic dyes from water.

Electrochemical reduction of Cr(VI) in water: lessons learned from fundamental studies and applications

Converting toxic Cr(vi) to benign Cr(iii) would offer a solution to decontaminate drinking water. Electrochemical methods are ideally suited to carry out this reduction without added external reductants. Achieving this transformation at low overpotentials requires mediating the transfer of protons and electrons to Cr(vi). In this review thermodynamic parameters will be discussed to understand Cr(vi) speciation in water and identify reduction pathways. The electrochemical reduction of Cr(vi) at bare electrodes is reviewed and mechanistic considerations are discussed. Works on modified electrodes are compared to identify key parameters influencing the reduction. An overview of current applications to Cr(vi) reduction is briefly discussed to link fundamental studies to applications.

Treatment of a wastewater from a galvanizing industry containing chromium(VI) and zinc(II) by liquid surfactant membranes technique

Galvanizing industries generate large amounts of effluents rich in toxic and carcinogenic chromium(VI) species. Effective and sustainable treatments are required to comply with environmental regulations. This work focused on the development of innovative treatments for Cr(VI) by its removal from a galvanizing industry wastewater (pH_initial = 5.9) containing Cr (78 mg.L^-1) and Zn (2178 mg.L^-1) using the liquid surfactant membranes technique. The membrane phase carrier was Alamine® 336 in Escaid™ 110. For a synthetic solution (Cr(VI) = 353mg.L^-1, pH_{internal phase} = 1.5), 99.9% of Cr(VI) was extracted in three stages ([KOH]_{internal phase} = 0.27 mol.L^-1). For the galvanizing wastewater, two selective extractions treatments were proposed: (1) 87% of Cr(VI) and 2% of Zn(II) were extracted in a single stage ([HCl]_{feed phase} = 0.03 mol.L^-1, [KOH]_{internal phase} = 0.6 mol.L^-1); (2) 95.6% of Cr(VI) and practically no zinc were extracted in a single stage ([HCl] _{feed phase} = 10^-6mol.L^-1, [HCl] _{internal phase} = 5mol.L^-1). In another treatment condition ([HCl] _{feed phase} = 2mol.L^-1 and [KOH] _{internal phase} = 1.2 mol.L^-1), the simultaneous Cr(VI) and Zn(II) extractions (95% and 70%, respectively) were obtained in a single stage and more than 99% of both metals in three stages. This resulted in a depleted feed phase with 0.01 mg.L^-1 of Cr(VI), that allows its discharge, according to the Brazilian legislation (≤0.1 mg/L).

Combination of Thermal, Hydrometallurgical and Electrochemical Tannery Waste Treatment for Cr(III) Recovery

A combination of thermal (500–750 °C in air) and hydrometallurgical (acidic) treatments have been applied to dried tannery sludge, resulting in the initial conversion of Cr(III) to Cr(VI) and its subsequent leaching as wastewater with high Cr(VI) concentration content (3000–6000 mg/L), presenting an extraction efficiency over 90%. The optimal electrochemical conditions for the subsequent Cr(VI) reduction with respect to acid concentration and acid kind were established by applying appropriate rotating disc electrode (RDE) experiments, using a glassy carbon (GC) electrode, and found to be equal or higher than 0.5 M H2SO4 (for the respective Cr(III) concentration range studied). The result from leaching Cr(VI) wastewater was further treated in small electrochemical bench-scale reactor for its conversion back to Cr(III) form, potentially reusable in the tanning industry. Ti-based anodes and a reticulated vitreous carbon (RVC) cathode were used to treat small (350–800 mL) samples in batch, as well as in batch-recirculation prototype electrochemical reactors, under the application of constant current or appropriately applied potential to achieve Cr(VI) conversion/reduction efficiency over 95%.

A colloid chemistry route for the preparation of hierarchically ordered mesoporous layered double hydroxides using surfactants as sacrificial templates

An efficient synthetic route was developed to prepare hierarchically ordered mesoporous layered double hydroxide (LDH) materials. Sodium dodecyl sulfate (SDS) was used as a sacrificial template to tune the interfacial properties of the LDH materials during the synthetic process. The SDS dose was optimized to obtain stable dispersions of the SDS-LDH composites, which were calcined, then rehydrated to prepare the desired LDH structures. Results of various characterization studies revealed a clear relationship between the colloidal stability of the SDS-LDH precursors and the structural features of the final materials, which was entirely SDS-free. A comparison to the reference LDH prepared by the traditional co-precipitation-calcination-rehydration method in the absence of SDS shed light on a remarkable increase in the specific surface area (one of the highest within the previously reported LDH materials) and pore volume as well as on the formation of a beneficial pore size distribution. As a proof of concept, the mesoporous LDH was applied as adsorbent for removal of nitrate and dichromate anions from aqueous samples, and excellent efficiency was observed in both sorption capacity and recyclability. These results make the obtained LDH a promising candidate as adsorbent in various industrial and environmental processes, wherever the use of mesoporous and organic content-free materials is required.

'Turn-off' fluorescence strategy for determination of hexavalent chromium ions based on copper nanoclusters

Cu nanoclusters (CuNCs) capped by tannic acid (TA) (CuNCs@TA) can be used as a highly sensitive fluorescent probe for Cr(VI) detection. Therefore, a fluorescence detection method for Cr(VI) can be established according to the fluorescence quenching of CuNCs@TA that is caused immediately after the addition of Cr(VI). The fluorescence quenching efficiency of CuNCs@TA was linearly correlated with Cr(VI) concentration within the range 0.03-60 μM, and the detection limit for Cr(VI) was 5 nM. This method was demonstrated to be suitable for detecting Cr(VI) in actual water samples. We found that sodium thiosulfate (ST) can redox with Cr(VI) and therefore restore the fluorescence of CuNCs@TA. The mechanism of CuNCs@TA fluorescence quenching and enhancement by Cr(VI) and ST was investigated in detail. The 'turn-on' fluorescent sensor is of practical significance and has broad application prospects.

Removal of anionic hexavalent chromium and methyl orange pollutants by using imidazolium-based mesoporous poly(ionic liquid)s as efficient adsorbents in column

Poly(ionic liquid)s (PILs) are attractive for their various applications, but the use of porous PILs have rarely been reported in anionic pollutants removal via ion-exchange by column. Herein, we report a serial of crosslinked imidazolium-based mesoporous PILs with Cl^- and Br^- as anions for hexavalent chromium (Cr(VI)) and methyl orange (MO) removal. Among them, PDVIm-Cl-SCD, from the free-radical polymerization of a dicationic monomer (N,N'-methylene-bis(1-(3-vinylimidazolium)) chloride, DVIm-Cl) and further supercritical carbon dioxide drying (SCD), displayed a very high sorption capacity (328.2 mg g^-1 at 25 °C) and excellent utilization of adsorption sites (UOA, 86.2%) towards Cr(VI), and an unprecedentedly high sorption capacity (1615.0 mg g^-1 at 25 °C) with a UOA of 67.4% to MO. Moreover, PDVIm-Cl-SCD also exhibited a broad pH range, excellent regeneration and remarkable reusability. Regarding to Cr(VI) removal, the volume of saturated KCl aqueous used for regenerating the Cr(VI) saturated PDVIm-Cl-SCD column (7.5-9.5 mL) was much less than the volume of treated Cr(VI) solution (160-200 mL). For MO removal, the volume of saturated NaCl solution used for regenerating the MO saturated PDVIm-Cl-SCD column (10.5-13.5 mL) was also much less than the volume of treated MO solution (220-235 mL), implying the great potential of PDVIm-Cl-SCD in sustainable wastewater treatment.

Removal of trace hexavalent chromium from aqueous solutions by ion foam fractionation

Ion foam fractionation is a green and cost-effective technology where separation of molecules exploits the difference in surface affinity. In this work, a batch ion foam fractionation system was designed and optimized for the separation of trace hexavalent chromium (Cr(VI)) from aqueous solutions. The effect of surfactant head groups (collectors) on the adsorption dynamics was analyzed. Cetyl trimethyl ammonium bromide (CTAB), a cationic surfactant showed high efficiency for the removal of Cr(VI) from aqueous solutions. An experimental investigation of the effect of different operational parameters on the separation characteristics is presented. The recovery of Cr(VI) increased with the increase in CTAB/Cr(VI) molar ratio and reached a maximum of 92.5% at optimum operating conditions. However, with CTAB concentrations close to the critical micelle concentration (CMC) wet foams were produced resulting in high liquid hold-up and poor enrichment ratio. The presence of Cr(VI) at the gas-liquid interface significantly improved the drainage characteristics of the foam decreasing the liquid hold-up. Further, a three-stage ion foam fractionation unit was developed with Cr(VI) removal efficiency of more than 99%. The concentration of Cr(VI) in the residue after the three-stage operation was less than 0.02 mg/L which is below the USEPA recommended standards for drinking water.

Abiotic reduction of Cr(VI) by humic acids derived from peat and lignite: kinetics and removal mechanism

Hexavalent chromium contamination of groundwater is a worldwide problem caused by anthropogenic and natural processes. We report the rate of Cr(VI) removal by two humic acids (extracted from Miocene age lignite and younger peat soil) in aqueous suspensions across a pH range likely to be encountered in terrestrial environments. Cr(VI) was reduced to Cr(III) in a first-order reaction with respect Cr(VI) concentration, but exhibited a partial order (~ 0.5) with respect to [H+]. This reaction was more rapid with the peat humic acid, where Cr(VI) reduction was observed at all pH values investigated (3.7 ≤ pH ≤ 10.5). ¹³C NMR and pyrolysis GC-MS spectroscopy indicate that the reaction results in loss of substituted phenolic moieties and hydroxyl groups from the humic acids. X-ray absorption spectroscopy indicated that at all pH values the resulting Cr(III) was associated with the partially degraded humic acid in an inner-sphere adsorption complex. The reaction mechanism is likely to be controlled by ester formation between Cr(VI) and phenolic/hydroxyl moieties, as this initial step is rapid in acidic systems but far less favourable in alkaline conditions. Our findings highlight the potential of humic acid to reduce and remove Cr(VI) from solution in a range of environmental conditions.

Modified APHA closed-tube reflux colorimetric method for TOC determination in water and wastewater

The analysis of total organic carbon (TOC) by the American Public Health Association (APHA) closed-tube reflux colorimetric method requires potassium dichromate (K₂Cr₂O₇), silver sulfate (AgSO₄), and mercury (HgSO₄) sulfate in addition to large volumes of both reagents and samples. The method relies on the release of oxygen from dichromate on heating which is consumed by carbon associated with organic compounds. The method risks environmental pollution by discharging large amounts of chromium (VI) and silver and mercury sulfates. The present method used potassium monochromate (K₂CrO₄) to generate the K₂Cr₂O₇ on demand in the first phase. In addition, miniaturizing the procedure to semi microanalysis decreased the consumption of reagents and samples. In the second phase, mercury sulfate was eliminated as part of the digestion mixture through the introduction of sodium bismuthate (NaBiO₃) for the removal of chlorides from the sample. The modified method, the potassium monochromate closed-tube colorimetry with sodium bismuthate chloride removal (KMCC-Bi), generates the potassium dichromate on demand and eliminates mercury sulfate. The semi microanalysis procedure leads to a 60% reduction in sample volume and ≈ 33.33 and 60% reduction in monochromate and silver sulfate consumption respectively. The LOD and LOQ were 10.17 and 33.90 mg L^-1 for APHA, and 4.95 and 16.95 mg L^-1 for KMCC-Bi. Recovery was between 83 to 98% APHA and 92 to 104% KMCC-Bi, while the RSD (%) ranged between 0.8 to 5.0% APHA and 0.00 to 0.62% KMCC-Bi. The method was applied for the UV-Vis spectrometry determination of COD in water and wastewater. Statistics was done by MINITAB 17 or MS Excel 2016. ᅟ Graphical abstract.

Luminescent metal–organic frameworks as chemical sensors: common pitfalls and proposed best practices

In this review we approach the emerging field of sensors based on luminescent metal–organic frameworks from the perspective of the most commonly encountered pitfalls and we suggest best practices so that they can be avoided.

Cr(VI) reduction and Cr(III) immobilization by resting cells of Pseudomonas aeruginosa CCTCC AB93066: spectroscopic, microscopic, and mass balance analysis

The aim of this study was to investigate the mechanism of Cr(VI) reduction and Cr(III) immobilization by resting cells of Pseudomonas aeruginosa using batch experiments and analysis techniques. Data showed that resting cells of this strain (3.2 g/L dry weight) reduced 10 mg/L of Cr(VI) by 86% in Tris-HCl buffer solution under optimized conditions of 5 g/L of sodium acetate as an electron donor, pH of 7.0 and temperature of 37 °C within 24 h. Cr(VI) was largely converted to nontoxic Cr(III), and both soluble crude cell-free extracts and membrane-associated fractions were responsible for Cr(VI) reduction. While remnant Cr(VI) existed only in the supernatant, the content of resultant Cr(III) in supernatant, on cell surface and inside cells was 2.62, 1.06, and 5.07 mg/L, respectively, which was an indicative of extracellular and intracellular reduction of chromate. Scanning electron microscopy analysis combined with energy dispersive X-ray spectroscopy revealed the adsorption of chromium on the bacterial surface. Interaction between Cr(III) and cell surface functional groups immobilized Cr(III) as indicated by Fourier transform infrared analyses and X-ray photoelectron spectroscopy. Transmission electron microscopy revealed Cr(III) precipitates in bacterial interior suggesting that Cr(II) could also be intracellularly accumulated. Thus, it can be concluded that interior and exterior surfaces of resting P. aeruginosa cells were sites for reduction and immobilization of Cr(VI) and Cr(III), respectively. This is further insight into the underlying mechanisms of microbial chromate reduction.

Adsorptive Removal of Toxic Chromium from Waste-Water Using Wheat Straw and Eupatorium adenophorum

Environmental pollution with heavy metals is a serious issue worldwide posing threats to humans, animals and plants and to the stability of overall ecosystem. Chromium (Cr) is one of most hazardous heavy metals with a high carcinogenic and recalcitrant nature. Aim of the present study was to select low-cost biosorbent using wheat straw and Eupatorium adenophorum through simple carbonization process, capable of removing Cr (VI) efficiently from wastewater. From studied plants a low cost adsorbent was prepared for removing Cr (VI) from aqueous solution following very simple carbonization method excluding activation process. Several factors such as pH, contact time, sorbent dosage and temperature were investigated for attaining ideal condition. For analysis of adsorption equilibrium isotherm data, Langmuir, Freundlich and Temkin models were used while pseudo-first-order, pseudo-second-order, external diffusion and intra-particle diffusion models were used for the analysis of kinetic data. The obtained results revealed that 99.9% of Cr (VI) removal was observed in the solution with a pH of 1.0. Among all the tested models Langmuir model fitted more closely according to the data obtained. Increase in adsorption capacity was observed with increasing temperature revealing endothermic nature of Cr (VI). The maximum Cr (VI) adsorption potential of E. adenophorum and wheat straw was 89.22 mg per 1 gram adsorbent at 308K. Kinetic data of absorption precisely followed pseudo-second-order model. Present study revealed highest potential of E. adenophorum and wheat straw for producing low cost adsorbent and to remove Cr (VI) from contaminated water.

Selective capture of hexavalent chromium from an anion-exchange column of metal organic resin-alginic acid composite

We report an anion exchange composite material based on a protonated amine-functionalized metal-organic framework, denoted Metal Organic Resin-1 (MOR-1), and alginic acid (HA). MOR-1-HA material shows an exceptional capability to rapidly and selectively sorb Cr(vi) under a variety of conditions and in the presence of several competitive ions. The selectivity of MOR-1-HA for Cr(vi) is shown to be the result of strong O3CrVI···NH2 interactions. The composite sorbent can be successfully utilized in an ion-exchange column, in contrast to pristine MOR-1 which forms fine suspensions in water passing through the column. Remarkably, an ion exchange column with only 1% wt MOR-1-HA and 99% wt sand (an inert and inexpensive material) is capable of reducing moderate and trace Cr(vi) concentrations to well below the acceptable safety limits for water. The relatively low cost of MOR-1-HA/sand column and its high regeneration capability and reusability make it particularly attractive for application in the remediation of Cr(vi)-bearing industrial waste.

Rapid, green and inexpensive synthesis of high quality UiO-66 amino-functionalized materials with exceptional capability for removal of hexavalent chromium from industrial waste

UiO-66-NH₂ type materials show excellent capability for removal of Cr(vi) from industrial waste.