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Chromium pollution of soil is a serious environmental problem. The methods for remediating heavy metal contaminated soil include: containment, leaching, microbial remediation and phytoremediation. However, the drawbacks, such as high chemical cost, secondary pollution and lack of long-term stability, have limited the practical application of these methods. In this study, an environmentally friendly method of electrokinetic remediation (EKR) combined with adsorption was used to remediate chromium contaminated soil. Two groups of EKR experiments were conducted in the laboratory to study the factors affecting the electric current, pH, and removal efficiency. First, the variation in electric current and electrolyte pH when using different electrolytes (citric acid, sodium chloride, and deionized water) was compared. Then, the effect of remediation time (3, 5, and 7 days) and potential gradient (2, 1, and 0.5 V/cm) on Cr6+ removal efficiency was studied. Moreover, the mechanisms of heavy metal removal were analysed. The results showed that the citric acid cannot only neutralize hydroxide ions produced by water electrolysis in the cathode but also reduce some of Cr6+ anions to nontoxic Cr3+ cations. The Cr6+ concentration in section S2 and S3 was higher than that in section S1, S4 and S5, because the migration direction of Cr6+ anion was opposite under the functions of electromigration and electroosmosis. The Cr6+ removal efficiency reached up to 72.4% under potential gradient of 2 V/cm and remediation time of 5 days, and no toxic chemicals were added or produced. EKR combined with exchange resin adsorption is environmentally friendly and efficient and can be applied in situ.

Hydroxypropyl chitosan-Cs 3 Bi 2 Cl 9 perovskite quantum dots (HPCS-PQDs) were synthesized by a simple ligand-assisted reprecipitation method via green hydroxypropyl chitosan as the ligand and used as the specific signal of a fluorescence probe to achieve the highly sensitive detection of hexavalent chromium (Cr(VI)) and compared with chitosan-Cs 3 Bi 2 Cl 9 QDs (CS-PQDs). HPCS-PQDs with multiple active hydroxyl passivations were found to enhance the photoluminescence quantum yield (PLQY) by 90%. After being placed in aqueous solution and irradiated with ultraviolet light for 96 h the fluorescence intensity of HPCS-PQDs remained above 60%. The blue emission of HPCS-PQDs has a good selectivity and short response time (30 s) for Cr(VI). A good linear relationship is established between the fluorescence quenching rate of the HPCS-PQDs and concentration of Cr(VI) from 0.8 to 400 µM, with a limit of detection (LOD) of 0.27 µM. The fluorescence quenching mechanism is the static quenching and internal filtration effect caused by HPCS-PQDs forming a non-fluorescent ground-state complex with Cr(VI). The sensor can not only be used to detect Cr(VI) in water samples with high accuracy but can also be prepared as a test paper for the detection for Cr(VI). Graphical Abstract

Hexavalent chromium is very carcinogenic, and it is, therefore, important to remove it from wastewater prior to disposal. This study reports the photoreduction of Cr(VI) under simulated sunlight using graphene-derived TiO2 nanowire (TNW) composites. Electrophoretic deposition (EPD) of graphene oxide (GO) and reduced graphene oxide (rGO) was carried out on rutile phase TNWs. The TNWs were fabricated by thermal oxidation of titanium foil in the presence of 1M potassium hydroxide mist at 750 °C. The TNWs uniformly covered the surface of the titanium foil. EPD of GO or rGO was done as a function of time to produce deposits of different thicknesses. The photocatalytic performances of the GO/TNWs or rGO/TNWs were tested to reduce Cr(VI) under visible light. The performance of rGO/TNWs in reducing Cr(VI) was better than GO/TNWs. A 10-second-deposited rGO on TNW samples can reduce 10 mg/L Cr(VI) within 30 min under visible light, likely as a result of the high electron transfer from rGO to TNWs accelerating the Cr(VI) reduction.

Deteriorating the quality of different parts of the ecosystem due to toxic metals is a serious global issue. Hexavalent chromium is a metal that can cause adverse effects on all living beings, including plants, animals, and microorganisms, on exposure to high concentrations for prolonged periods. Removing hexavalent chromium from various types of wastes is challenging; hence the present study investigated the use of bacteria incorporated with selected natural substrates in removing hexavalent chromium from water. Isolated Staphylococcus edaphicus KCB02A11 has shown higher removal efficiency with a wide hexavalent chromium range (0.025-8.5 mg/L) within 96 h. Incorporating the isolated strain with natural substrates commonly found in the environment (hay and wood husk) showed high removal potential [100% removal with 8.5 mg/L of Cr(VI)], even within less than 72 h, with the formation of biofilms on the used substrates applied for metal removal on a large scale for prolonged periods. This study is the first report investigating hexavalent chromium tolerance and removal by Staphylococcus edaphicus KCB02A11.

Molybdenum disulfide (MoS 2 ) was supported on graphene oxide (GO) by hydrothermal method. The resulting nanocomposite (MoS 2 -rGO) was characterized by X-ray photoelectron spectroscopy, scanning electron microscopy, and transmission electron microscopy. The experiments show that at pH 2, MoS 2 -rGO has a great affinity for adsorption of hexavalent chromium ions while Cr(III) ions remain in aqueous sample. In the adsorption process, the dominant role plays chemisorption. The determined adsorption capacity is 583.5 mg g −1 . Parameters affecting the extraction process, namely sample pH, sample volume, contact time, and matrix ions, were investigated by sequential batch tests. Under optimal conditions (pH 2, sample volume 50 mL, sonication time 10 min, adsorbent mass 1 mg), the calibration curve covers the 1–200 ng mL −1 range with a correlation coefficient ( R 2 ) of 0.998. The recovery of the method is 97 ± 3%. Other data of merit include a relative standard deviation of < 3.5%, enrichment factor of 3350, and detection limit of 0.050 ng mL −1 . The accuracy of the method was confirmed by analysis of the reference materials QC1453 (chromium VI in drinking water) and QC3015 (chromium VI in seawater). The method was successfully applied to chromium speciation in water samples, including high salinity ones. The concentration of Cr(III) was calculated as the difference between the total concentration of chromium (after oxidation of Cr(III) to Cr(VI) with potassium permanganate) and the initial Cr(VI) content. Graphical abstract Schematic presentation of a method for determination of chromium species by energy dispersive X-ray fluorescence spectrometry after preconcentration on molybdenum disulfide supported on reduced graphene oxide.

The current work is designed to search for suitable rice (Oryza sativa L.) cultivars capable of growing on Cr(VI), hexavalent chromium contaminated soils. The study of tolerance and phenotypical changes of three selected rice cultivars like Bina Dhan 11, Kalachampa, and Pratikshya, at the seedling stages, was done under soil Cr(VI) concentrations up to 300 mg.kg-1 of soil. The 7-day seedlings of these rice cultivars growing on Cr(VI) treated soils were found to exhibit a significant reduction in shoot and root growth at p ≤ 0.05. The experimental results support that 7-day seedlings of Bina Dhan 11 were found to be the best among the three cultivars under soil Cr(VI) stress conditions. The present work may help in selecting suitable rice cultivar for paddy cultivation on Cr(VI) contaminated crop lands present in mining and industrial belts. Further work on this aspect may be useful in increasing rice productivity, catering to the increase in demand for food.

Chromium (Cr) pollution is a critical environmental concern posing a considerable danger to the food safety of aquatic products and health of aquatic animals. Developing a reliable and effective strategy for reducing the adverse effects of Cr on aquatic organisms is of great importance. The objective of this study was to evaluate the role of the brown alga Sargassum kjellmanianum in reducing chronic toxicity induced by hexavalent Cr (Cr6+) in yellow catfish, Pelteobagrus fulvidraco. A total of 360 healthy fish were exposed to Cr6+ (3.24 mg L-1) and S.kjellmanianum (5 % in diet), individually or in combination for 28-days. The results revealed that Cr6+ exposure significantly decreased growth performance of fish, and serum immune parameters [total protein and albumin levels, acid phosphatase, alkaline phosphatase, and lysozyme activities]. Also, it significantly increased serum aspartate aminotransferase, alanine aminotransferase, total cholesterol, triglyceride levels and muscle Cr concentration. Additionally, significant reduction in serum total antioxidant capacity, superoxide dismutase, and catalase activities as well as marked increments of malondialdehyde were observed in Cr6+-exposed fish compared to the control group. Importantly, SK supplementation could overcome the negative impacts of Cr6+ and normalise the examined parameters nearly to the control values. In conclusion, dietary S.kjellmanianum supplementation could protect P.fulvidraco against the toxic effects of Cr6+, and therefore, may serve as a beneficial feed additive for application in the aquaculture industry.

Phosphorus (P) fertilizers from secondary resources became increasingly important in the last years. However, these novel P-fertilizers can also contain toxic pollutants such as chromium in its hexavalent state (Cr(VI)). This hazardous form of chromium is therefore regulated with low limit values for agricultural products even though the correct determination of Cr(VI) in these fertilizers may be hampered by redox processes, leading to false results. Thus, we applied the novel diffusive gradients in thin-films (DGT) technique for Cr(VI) in fertilizers and compared the results with the standard wet chemical extraction method (German norm DIN EN 15192) and Cr K-edge X-ray absorption near-edge structure (XANES) spectroscopy. We determined an overall good correlation between the wet chemical extraction and the DGT method. DGT was very sensitive and for most tested materials selective for the analysis of Cr(VI) in P-fertilizers. However, hardly soluble Cr(VI) compounds cannot be detected with the DGT method since only mobile Cr(VI) is analyzed. Furthermore, Cr K-edge XANES spectroscopy showed that the DGT binding layer also adsorbs small amounts of mobile Cr(III) so that Cr(VI) values are overestimated. Since certain types of the P-fertilizers contain mobile Cr(III) or partly immobile Cr(VI), it is necessary to optimize the DGT binding layers to avoid aforementioned over- or underestimation. Furthermore, our investigations showed that the Cr K-edge XANES spectroscopy technique is unsuitable to determine small amounts of Cr(VI) in fertilizers (below approx. 1% of Cr(VI) in relation to total Cr).

Cr(VI) is a ubiquitous pollutant that poses a serious threat to human health. Recently, the use of microorganisms to adsorb heavy metals has attracted research attention. However, there are few studies on the biosorption of Cr(VI) by Shewanella putrefaciens, which is a metal-reducing bacterium. In this paper, single-factor experiments were designed to investigate the effect of hexavalent chromium by Shewanella putrefaciens, and response surface methodology (RSM) based on the Box-Behnken design (BBD) was performed to study the Cr(VI) biosorption behavior of Shewanella putrefaciens. The coefficient of determination (R2 = 0.811 for Cr(VI)) and probability value (P < 0.05) demonstrated significance for the obtained regression model. The results showed that the model was suitable for experimental data, and the maximum Cr(VI) removal efficiency by Shewanella putrefaciens was 85.68% under the optimum conditions of a contact time of 16.57 h, pH value of 8, and biomass dosage of 0.42 g/L, which were verified by additional experiments. ANOVA and 3D response graph analysis showed that the variables with significant influences were pH and temperature. In addition, scanning electron microscopy (SEM) results demonstrated that after biosorption of Cr(VI) by Shewanella putrefaciens, granular complexes attached to rough cell surfaces were observed. Furthermore, Fourier transform infrared spectroscopy (FT-IR) analysis showed that the distribution of Cr(VI) on the cell surface was related to the carboxyl, ether, amide, hydroxyl, and phosphoric acid groups of Shewanella putrefaciens. This study is useful to explore the process and mechanism of heavy metal adsorption by Shewanella putrefaciens and provide new ideas for the microbial remediation of metal pollution in water.

Wastewater from several businesses that produce paints, steel, tannery products, dyes, and chrome-plated items contains hexavalent chromium [Cr(VI)]. The main contributing factor to pollution concentrations in water bodies is the wastewater that tannery businesses dump, which includes organic pollutants and heavy metals, particularly Cr(VI). The harmful consequences of Cr(VI) on humans include eczema, allergies, ulceration, respiratory tract problems, lung cancer, as well as genotoxic and mutagenic effects. Using bacteria to biotransform hexavalent chromium to trivalent chromium [(Cr(III)] is a practical strategy with proven viability in bioremediation. Following their isolation from the tannery industry’s raw effluent, bacterial strains were investigated biochemically and molecularly. Based on this study, it was concluded that the microorganisms resistant to Cr(VI) were Bacillus albus and Bacillus australimaris. In a two-chambered microbial fuel cell reactor, the isolated Cr(VI) resistant bacteria will be employed as electrogenic bacteria with SPEEK (Sulphonated polyether ether ketone) as PEM (proton exchange membrane), which can synergistically aid in the reduction of hexavalent chromium and green energy generation.