Effects of exogenous riboflavin or cytochrome addition on the cathodic reduction of Cr(VI) in microbial fuel cell with Shewanella putrefaciens.

Bioreduction of Cr(VI) is recognized as a cost-effective and environmentally friendly method, attracting widespread interest. However, the slow rate of Cr(VI) bioreduction remains a practical challenge. Additionally, the direct removal efficiency of microbes for high concentrations of Cr(VI) is not ideal due to the toxicity. Therefore, this study investigated the effects of exogenous riboflavin or cytochrome on the cathodic reduction of Cr(VI) in microbial fuel cells. The results demonstrated that the exogenous riboflavin or cytochrome effectively improved the voltage output of the cells, with riboflavin increasing the voltage by 52.08%. Within the first 24 h, the Cr(VI) removal ratio in the normal, cytochrome, and riboflavin groups was 14.3%, 29.3%, and 53.8%, respectively. And the final removal ratio was 55.1%, 69.1%, and 98.0%, respectively. These results showed different enhancement effects of riboflavin and cytochrome on Cr(VI) removal. The analysis of riboflavin and cytochrome contents revealed that the additions did not have a significant impact on the autocrine riboflavin of S. putrefaciens, but affected the autocrine cytochrome. SEM, XPS, and FTIR results confirmed the presence of reduced Cr(III) on the cathode, which formed precipitate and adhered to the cathode surface. The EDS analysis showed that the amount of Cr on the cathode in normal, cytochrome, and riboflavin groups was 4.71%, 6.37%, 7.56%, respectively, which was consistent with the voltage and Cr(VI) removal data. These findings demonstrated the significant enhancement of exogenous riboflavin or cytochrome on Cr(VI) reduction, thereby providing data reference for the future bio-assisted remediation of Cr(VI) pollution.

Integration of adsorption, reduction, and filtration in PANI/PVDF nanofiber composite membrane for removal of Cr(VI).

Here, polyaniline/polyvinylidene fluoride (PANI/PVDF) nanofiber composite membrane was fabricated using electrostatic spinning technology to remove hexavalent chromium Cr(VI). The employment of PANI not only extremely enhanced the hydrophilic property of the nanofiber membrane, but also facilitated the transfer of Cr2O72- from water to the membrane. The PANI/PVDF membrane had an extremely excellent performance in getting rid of Cr(VI) and a quite large flux (250 L/m2 h). The maximum adsorption quantity of the membrane could reach 334.5 mg/g in which adsorption played 52.12% part and reduction played 47.87% part. The removal rate could reach nearly 100% immediately in the permeate solution under filtration while it needed 240 min to reach 100% only by static adsorption. Therefore, the interception of the membrane and the adsorption reduction of PANI had synergistic effect on removal of Cr(VI). Furthermore, the removal rate of Cr(VI) could still reach 95.97% after reused 8 times. The membrane showed a very good reusability and application prospect.

In situ carbothermal synthesis of carbonized bacterial cellulose embedded with nano zero-valent iron for removal of Cr(VI).

Carbonized bacterial cellulose embedded with highly dispersed nano zero-valent iron (nZVI), denoted as nZVI@CBC, was prepared through one-step in situ carbothermal treatment of bacterial cellulose adsorbing iron(III) nitrate. The structure characteristics of nZVI@CBC and its performance in removing hexavalent chromium Cr(VI) were investigated. Results showed the formation of nZVI@CBC with a surface area of 409.61 m2/g at 800 °C, with nZVI particles of mean size 28.2 nm well distributed within the fibrous network of CBC. The stability of nZVI was enhanced by its carbon coating, despite some inevitable oxidation of exposed nZVI. Batch experiments demonstrated that nZVI@CBC exhibited superior removal efficiency compared to bare nZVI and CBC. Under optimal conditions, nZVI@CBC exhibited a high Cr(VI) adsorption capacity of up to 372.42 mg/g. Therefore, nZVI@CBC shows promise as an effective adsorbent for remediating Cr(VI) pollution in water.

Probiotics Pediococcus acidilactici GR-1 promotes the functional strains and remodels gut microbiota to reduce the Cr(VI) toxicity in a dual-chamber simulated intestinal system.

Numerous animal studies have demonstrated the toxicity of hexavalent chromium [Cr(VI)] and the bioremediative effects of probiotics on the composition and functions of gut microbiota. Since the precise mechanisms of Cr(VI) detoxification and its interactions with human gut microbiota were unknown, a novel dual-chamber simulated intestinal (DCSI) system was developed to maintain both the stability of the simulated system and the composition of the gut microbiota. Probiotic GR-1 was found to regulate intestinal gut microbiota, thereby reducing the toxicity of Cr(VI) within the DCSI system. The results indicate that Cr(VI) levels were reduced from 2.260 ± 0.2438 µg/g to 1.7086 ± 0.1950 µg/g in the gut microbiota cell pellet, and Cr(VI) permeability decreased from 0.5521 ± 0.1132 µg/L to 0.3681 ± 0.0178 µg/L after 48 h in simulated gut fluid. Additionally, the removal rate of 1,1-Diphenyl-2-picrylhydrazyl (DPPH), reducibility (Vitamin C), and total antioxidant capacity (T-AOC) increased by 50.83%, 31.70%, and 27.56%, respectively, following probiotic treatment. The increase in antioxidant capacity correlated with total Cr removal (P < 0.05, r from -0.80 to 0.73). 16S rRNA sequencing analysis showed that gut microbiota composition was reshaped by the addition of probiotics, which regulated the recovery of the functional gut microbiota to normal levels, rather than restoring the entire gut microbiota composition for community function. Thus, this study not only demonstrates the feasibility and stability of culturing gut microbiota but also offers a new biotechnological approach to synthesizing functional communities with functional strains for environmental risk management.

Enhancement of bio-promoters on hexavalent chromium inhibited sulfur-driven denitrification: repairing damage, accelerating electron transfer, and reshaping microbial collaboration.

Proposing recovery strategies to recover heavy-metal-inhibited sulfur-driven denitrification, as well as disclosing recovery mechanisms, can provide technical support for the stable operation of bio-systems. This study proposed an effective bio-promoter (mediator-promoter composed of L-cysteine, biotin, cytokinin, and anthraquinone-2,6-disulfonate) to recover Cr(VI) inhibited sulfur-driven denitrification, which effectively reduced the recovery time of NO3--N reduction (18-21 cycles) and NO2--N reduction (27-42 cycles) compared with self-recovery. The mediator-promoter repaired microbial damage by promoting intracellular chromium efflux. Moreover, the mediator-promoter reduced the accumulated reactive oxygen species by stimulating the secretion of antioxidant enzymes, reaching equilibrium in the oxidative-antioxidant system. To improve electron transmission, the mediator-promoter restored S2O32- oxidation to provide adequate electron donors and increased electron transfer rate by increasing cytochrome c levels. Mediator-promoter boosted the abundance of Thiobacillus (sulfur-oxidizing bacterium) and Simplicispira (denitrifying bacterium), which were positively correlated, facilitating the rapid denitrification recovery and the long-term stable operation of recovered systems.

Removal and reduction mechanism of Cr (VI) in Leersia hexandra Swartz constructed wetland-microbial fuel cell coupling system.

Cr (VI) is extremely harmful to both the environment and human health, and it can linger in the environment for a very long period. In this research, the Leersia hexandra Swartz constructed wetland-microbial fuel cell (CW-MFC) system was constructed to purify Cr (VI) wastewater. By comparing with the constructed wetland (CW) system, the system electricity generation, pollutants removal, Cr enrichment, and morphological transformation of the system were discussed. The results demonstrated that the L. hexandra CW-MFC system promoted removal of pollutants and production of electricity of the system. The maximum voltage of the system was 499 mV, the COD and Cr (VI) removal efficiency was 93.73% and 97.00%. At the same time, it enhanced the substrate and L. hexandra ability to absorb Cr and change it morphologically transformation. Additionally, the results of XPS and XANES showed that the majority of the Cr in the L. hexandra and substrate was present as Cr (III). In the L. hexandra CW-MFC system, Geobacter also functioned as the primary metal catabolic reducing and electrogenic bacteria. As a result, L. hexandra CW-MFC system possesses the added benefit of removing Cr (VI) while producing energy compared to the traditional CW system.

Leachability of hexavalent chromium from fly ash-marl mixtures in Sarigiol basin, Western Macedonia, Greece: environmental hazard and potential human health risk.

Hexavalent chromium (Cr (VI)) is an environmental contaminant brining high concerns due to its higher toxicity and mobility in comparison with trivalent chromium Cr(III). Cr (VI) has been linked with several adverse health effects, including respiratory diseases, lung cancer, and skin irritation. The primary sources of it in the environment are industrial activities.Most of the time, fly ash made of lignite can release Cr(VI) when it comes into contact with water in an aquatic environment. The objective of this study is the investigation of Cr (VI) concentration in leachates of fly ash and marl mixtures and the determination of its solubility under different pH conditions. Samples of fly ash were collected from the Power Plant of Agios Dimitrios. Additionally, samples of marl were collected from the mine of South Field, and both samples were mixed and prepared in in different proportions (% w.t.). The leaching experiments were carried out according to the EN-12457/1-4 (2003) standard under different pH conditions and chemical analysis of the leachates were performed by spectrophotometry with diphenylcarbazide (DPC). The environmental footprint of Cr (VI) in the study area was significant, especially in mixtures containing higher concentrations of fly ash. A critical pH range between 6 to 12 is observed. At acidic pH values, a high release of Cr (VI) was observed, while at the mentioned critical values (pH 10-12), a gradual decrease in its leachability was noticed. The high concentrations of Cr (VI) in the industrial area studied require immediate actions in terms of managing and limiting the potential hazardous impacts on the environment and by extension on the public health by developing appropriate prevention strategies.

Extracellular bioreduction is the main Cr(VI) detoxification strategy of Bacillus sp. HL1.

Bacterium with high Cr(VI) detoxification capability belonged to the genus Bacillus have been largely explored, yet their reduction strategies are still in debate. Cr(VI) removal performance and mechanism of Bacillus sp. HL1 isolated from tailings a site was comprehensively investigated in this study. Approximately 88.31% of 100 mg/L Cr(VI) was continuously removed within 72 h, while it could resist up to 300 mg/L Cr(VI). Metal ions Mn2+ and Cu2+ could effectively improve the Cr(VI) removal performance to 14.41% and 3.41% under the optimal conditions, respectively. Cr(VI) removal performances by subcellular extracts showed that nearly 45.28% of 100 mg/L extracellular Cr(VI) was efficaciously reduced to Cr(III), while only 14.27%, 6.40%, and 2.73% of the cell-free extract, resting cells, and cell debris were reduced, respectively. This suggested that extracellular bioreduction was the primary Cr(VI) detoxification strategy despite a small part of Cr(VI) reduction took place intracellularly. In particular, the reduction products of the intracellular and extracellular compounds significantly differed, with organo-Cr(III) complex outside the cell and crystalline Cr(III) precipitate inside. Such observation was also evidenced by the intracellular black precipitate observed in the TEM image. XRD, XPS, and EPR analysis showed different Cr(III) compositions of intracellular and extracellular products. This study deepens our insights into the different fates of microorganisms that reduce Cr(VI) intracellularly and extracellularly.

Crucial roles of soil inherent Fe-bearing minerals in enhanced Cr(VI) reduction by biochar: The electronegativity neutralization and electron transfer mediation.

Biochar has been used for soil Cr(VI) remediation in the last decade due to its enriched redox functional groups and good electrochemical properties. However, the role of soil inherent Fe-bearing minerals during the reduction of Cr(VI) has been largely overlooked. In this study, biochar with different electron-donating capacities (EDCs) was produced at 400 °C (BC400) and 700 °C (BC700), and their performance for Cr(VI) reduction in soils with varied properties (e.g., Fe content) was investigated. The addition of BC400 caused around 14.2-36.0 mg g-1 Cr(VI) reduction after two weeks of incubation in red soil, paddy soil, loess soil, and fluvo-aquic soil, while a less Cr(VI) was reduced by BC700 (2.57-16.7 mg g-1) with smaller EDCs. The Cr(VI) reduction by both biochars in different soils was closely related to Fe content (R2=0.93-0.98), so red soil with the richest Fe (14.8% > 1.79-3.49%) showed the best reduction capability, and the removal of soil free Fe oxides (e.g., hematite) resulted in 71.9% decrease of Cr(VI) reduction by BC400. On one hand, Fe-bearing minerals could increase the soil acidity, neutralize the surface negative charge of biochar, enhance the contact between Cr(VI) and biochar, and thus facilitate the direct Cr(VI) reduction by biochar in soils. On the other hand, Fe-bearing minerals could also facilitate the indirect Cr(VI) reduction by mediating the electron from biochar to Cr(VI) with the cyclic transformation of Fe(II)/Fe(III). This study demonstrates the key role of soil Fe-bearing minerals in Cr(VI) reduction by biochar, which advances our understanding on the biochar-based remediation mechanism of Cr(VI)-contaminated soils.

Simultaneous removal of methylene blue and Cr(VI) in a dual-chamber photocatalytic microbial fuel cell with WO3/MoS2/FTO photocathode.

Carbon felt was used as the anode and WO3/MoS2/FTO (fluorine-doped tin oxide) was used as the photocathode in a photocatalytic microbial fuel cell (PMFC). The photoelectric performance of the WO3/MoS2/FTO photocathode and the removal efficiency of methylene blue (MB) and Cr(VI) mixed pollutants were systematically investigated in the cathode chamber. The results showed that after 12 h of light irradiation in the PMFC with WO3/MoS2/FTO as the photocathode, the removal rates of MB and Cr(VI) were 84.56 and 68.11 %, respectively, which were much higher than those using WO3/FTO as a photocathode (55.57 % and 45.26 %, respectively). The corresponding maximum output power was 33.14 mW/m2, which was 1.85 times that of the WO3/FTO photocathode PMFC. These results can be attributed to the fact that WO3 is an n-type semiconductor and MoS2 is a p-type semiconductor. Analysis of trapping experiments showed that the composite of WO3 and MoS2 formed a Z-scheme heterojunction, which improved the separation efficiency of the photoelectric carriers and enhanced the pollutant removal efficiency of the photocathode. PMFCs are a new and environment-friendly technology for removing pollutants thereby providing an experimental basis for future engineering applications.

Effect of oxide interactions on chromium speciation transformation during simulated municipal solid waste incineration.

Chromium released during municipal solid waste incineration (MSWI) is toxic and carcinogenic. The removal of chromium from simulated MSWI flue gas by four sorbents (CaO, bamboo charcoal (BC), powdered activated carbon (PAC), and Al2O3) and the effects of four oxides (SiO2, Al2O3, Fe2O3, and CaO) on chromium speciation transformation were investigated. The results showed that the removal rates of total Cr by the four sorbents were Al2O3 < CaO < PAC < BC, while the removal rates of Cr(VI) by the four sorbents were Al2O3 < PAC < BC < CaO. CaO had a strong oxidizing effect on Cr(III), while BC and PAC had a better-reducing effect on Cr(VI). SiO2 was better for the reduction of Na2CrO4 and K2CrO4 above 1000°C due to its strong acidity, and the addition of CaO significantly inhibited the reduction of Cr(VI). MgCrO4 decomposed above 700°C to form MgCr2O4, and the reaction between MgCrO4 and oxides also existed in the form of a more stable trivalent spinel. Furthermore, when investigating the effect of oxides on the oxidation of Cr(III) in CrCl3, it was discovered that CaO promoted the conversion of Cr(III) to Cr(VI), while the presence of chlorine caused chromium to exist in the form of Cr(V), and increasing the content of CaO and extending the heating time facilitated the oxidation of Cr(III). In addition, silicate, aluminate, and ferrite were generated after the addition of SiO2, Al2O3, and Fe2O3, which reduced the alkalinity of CaO and had an important role in inhibiting the oxidation of Cr(III). The acidic oxides can not only promote the reduction of Cr(VI) but also have an inhibitory effect on the oxidation of Cr(III) ascribed to alkali metals/alkaline earth metals, and the proportion of acidic oxides can be increased moderately to reduce the generation of harmful substances in the hazardous solid waste heat treatment.

Removal of Cr(VI) by glutaraldehyde-crosslinked chitosan encapsulating microscale zero-valent iron: Synthesis, mechanism, and longevity.

Microscale zero-valent iron (mZVI) has shown great potential for groundwater Cr(VI) remediation. However, low Cr(VI) removal capacity caused by passivation restricted the wide use of mZVI. We prepared mZVI/GCS by encapsulating mZVI in a porous glutaraldehyde-crosslinked chitosan matrix, and the formation of the passivation layer was alleviated by reducing the contact between zero-valent iron particles. The average pore diameter of mZVI/GCS was 8.775 nm, which confirmed the mesoporous characteristic of this material. Results of batch experiments demonstrated that mZVI/GCS exhibited high Cr(VI) removal efficiency in a wide range of pH (2-10) and temperature (5-35°C). Common groundwater coexisting ions slightly affected mZVI/GCS. The material showed great reusability, and the average Cr(VI) removal efficiency was 90.41% during eight cycles. In this study, we also conducted kinetics and isotherms analysis. Pseudo-second-order model was the most matched kinetics model. The Cr(VI) adsorption process was fitted by both Langmuir and Freundlich isotherms models, and the maximum Langmuir adsorption capacity of mZVI/GCS reached 243.63 mg/g, which is higher than the adsorption capacities of materials reported in most of the previous studies. Notably, the column capacity for Cr(VI) removal of a mZVI/GCS-packed column was 6.4 times higher than that of a mZVI-packed column in a 50-day experiment. Therefore, mZVI/GCS with a porous structure effectively relieved passivation problems of mZVI and showed practical application prospects as groundwater Cr(VI) remediation material with practical application prospects.

Two-stage hydrothermal oxygenation for efficient removal of Cr(VI) by starch-based polyporous carbon: Wastewater application and removal mechanism.

Cr(VI) is of concern because of its high mobility and toxicity. In this work, a two-stage hydrothermal strategy was used to activate the O sites of starch, and by inserting K-ion into the pores, starch-based polyporous carbon (S-PC) adsorption sites was synthesized for removal of Cr(VI). Physicochemical characterization revealed that the O content of the S-PC reached 20.66 % after activation, indicating that S-PC has excellent potential for adsorption of Cr(VI). The S-PC removal rate for 100 mg/L Cr(VI) was 96.29 %, and the adsorption capacity was 883.86 mg/g. Moreover, S-PC showed excellent resistance to interference, and an equal concentration of hetero-ions reduced the activity by less than 5 %. After 8 cycles of factory wastewater treatment, the S-PC maintained 81.15 % of its original activity, which indicated the possibility of practical application. Characterization and model analyses showed that the removal of Cr(VI) from wastewater by the S-PC was due to CC, δ-OH, ν-OH, and C-O-C groups, and the synergistic effect of adsorption and reduction was the key to the performance. This study provides a good solution for treatment of Cr(VI) plant wastewater and provides a technical reference for the use of biological macromolecules such as starch in the treatment of heavy metals.

Fine particulate matter PM2.5 and its constituent, hexavalent chromium induce acute cytotoxicity in human airway epithelial cells via inflammasome-mediated pyroptosis.

PM2.5-induced airway injury contributes to an increased rate of respiratory morbidity. However, the relationship between PM2.5 toxicants and acute cytotoxic effects remains poorly understood. This study aimed to investigate the mechanisms of PM2.5- and its constituent-induced cytotoxicity in human airway epithelial cells. Exposure to PM2.5 resulted in dose-dependent cytotoxicity within 24 h. Among the PM2.5 constituents examined, Cr(VI) at the dose found in PM2.5 exhibited cytotoxic effects. Both PM2.5 and Cr(VI) cause necrosis while also upregulating the expression of proinflammatory cytokine transcripts. Interestingly, exposure to the conditioned PM, obtained from adsorption in the Cr(VI)-reducing agents, FeSO4 and EDTA, showed a decrease in cytotoxicity. Furthermore, PM2.5 mechanistically enhances programmed pyroptosis through the activation of NLRP3/caspase-1/Gasdermin D pathway and increase of IL-1ß. These pyroptosis markers were reduced when exposure to conditioned PM. These findings provide a deeper understanding of mechanisms underlying PM2.5 and Cr(VI) in acute airway toxicity.

Selective adsorption of Cr(VI) by nitrogen-doped hydrothermal carbon in binary system.

Selective adsorption of heavy metal ions from industrial effluent is important for healthy ecosystem development. However, the selective adsorption of heavy metal pollutants by biochar using lignin as raw material is still a challenge. In this paper, the lignin carbon material (N-BLC) was synthesized by a one-step hydrothermal carbonization method using paper black liquor (BL) as raw material and triethylene diamine (TEDA) as nitrogen source. N-BLC (2:1) showed excellent selectivity for Cr(VI) in the binary system, and the adsorption amounts of Cr(VI) in the binary system were all greater than 150 mg/g, but the adsorption amounts of Ca(II), Mg(II), and Zn(II) were only 19.3, 25.5, and 6.3 mg/g, respectively. The separation factor (SF) for Cr(VI) adsorption was as high as 120.0. Meanwhile, FTIR, elemental analysis and XPS proved that the surface of N-BLC (2:1) contained many N- and O- containing groups which were favorable for the removal of Cr(VI). The adsorption of N-BLC (2:1) followed the Langmuir model and its maximum theoretical adsorption amount was 618.4 mg/g. After 5th recycling, the adsorption amount of Cr(VI) by N-BLC (2:1) decreased about 15%, showing a good regeneration ability. Therefore, N-BLC (2:1) is a highly efficient, selective and reusable Cr(VI) adsorbent with wide application prospects.

Efficient green Cr(VI) adsorbent from sorghum waste: Eco-designed functionalized mesoporous silica FDU-12.

This paper presents an eco-design approach to the synthesis of a highly efficient Cr(VI) adsorbent, utilizing a positively charged surface mesoporous FDU-12 material (designated as MI-Cl-FDU-12) for the first time. The MI-Cl-FDU-12 anion-exchange adsorbent was synthesized via a facile one-pot synthesis approach using sodium silicate extracted from sorghum waste as a green silica source, 1-methyl-3-(triethoxysilylpropyl) imidazolium chloride as a functionalization agent, triblock copolymer F127 as a templating or pore-directing agent, trimethyl benzene as a swelling agent, KCl as an additive, and water as a solvent. The synthesis method offers a sustainable and environmentally friendly approach to the production of a so-called "green" adsorbent with a bimodal micro-/mesoporous structure and a high surface area comparable with the previous reports regarding FDU-12 synthesis. MI-Cl-FDU-12 was applied as an anion exchanger for the adsorption of toxic Cr(VI) oxyanions from aqueous media and various kinetic and isotherm models were fitted to experimental data to propose the adsorption behavior of Cr(VI) on the adsorbent. Langmuir model revealed the best fit to the experimental data at four different temperatures, indicating a homogeneous surface site affinity. The theoretical maximum adsorption capacities of the adsorbent were found to be 363.5, 385.5, 409.0, and 416.9 mg g-1 at 298, 303, 308, and 313 K, respectively; at optimal conditions (pH=2, adsorbent dose=3.0 mg, and contact time of 30 min), surpassing that of most previously reported Cr(VI) adsorbents in the literature. A regeneration study revealed that this adsorbent possesses outstanding performance even after six consecutive recycling.

Influence of elevated temperature on the species and mobility of chromium in ferrous sulfate-amended contaminated soil.

Ferrous sulfate (FeSO4) combined with acid pretreatment is usually employed to remediate contaminated soils containing Cr(VI). However, the long-term efficiency of this stabilization method is important for its sustainability. In this study, a gradient temperature-elevating exposure test was employed to investigate the stability of Cr in FeSO4-remediated soil when exposed to elevated temperatures (40 °C, 120 °C, and 500 °C), possibly caused by hot weather and/or wildfires. The results of chemical extraction and X-ray absorption near edge structure spectroscopy (XANES) showed that the Cr(VI) in contaminated soil was successfully transformed to Cr(III) after stabilization, resulting in the dramatic decrease of water-leachable Cr(VI). The stabilization efficiency was further improved under 40 °C treatment after 30 days. Subsequently, the 120 °C treatment (7 days) had relatively little effect on the Cr speciation and mobility in soils. However, even one day of 500 °C calcination resulted in the deterioration of stabilization efficiency, and the water-leachable Cr(VI) re-increased and became higher than the Chinese environmental standards (total Cr 15 mg/L, Cr(VI) 5 mg/L) for the classification of hazardous solid wastes. XANES results reflected that heating at 500 °C facilitate the formation of Cr2O3, which was mainly caused by thermal decomposition and dehydration of Cr(OH)3 in the soil. Besides, the transformation of Cr species resulted in the enhanced association of Cr with the most stable residual fraction (88.3%-91.6%) in soil. Based on chemical extraction results, it was suggested that the oxidation of Cr(III) to Cr(VI) contributed to the re-increased mobility of Cr(VI) in soil. However, the XANES results showed that almost no significant re-oxidization of Cr(III) to Cr(VI) happened after heating at 500 °C, which was probably caused by XANES linear combination fits (LCF) uncertainties. Moreover, the changes in soil properties, including a rise in pH to a slightly alkaline range and/or the decomposition of organic matter, possibly contributed to the enhanced mobility of Cr(VI) in soil. This study contributes to clarifying the mobility and transformation of Cr in contaminated soils and provides a support for the sustainable management of remediated soils.

Efficient nitrate and Cr(VI) removal by denitrifier: The mechanism of S. oneidensis MR-1 promoting electron production, transportation and consumption.

When Cr(VI) and nitrate coexist, the efficiency of both bio-denitrification and Cr(VI) bio-reduction is poor because chromate hinders bacterial normal functions (i.e., electron production, transportation and consumption). Moreover, under anaerobic condition, the method about efficient nitrate and Cr(VI) removal remained unclear. In this paper, the addition of Shewanella oneidensis MR-1 to promote the electron production, transportation and consumption of denitrifier and cause an increase in the removal of nitrate and Cr(VI). The efficiency of nitrate and Cr(VI) removal accomplished by P. denitrificans as a used model denitrifier increased respectively from 51.3% to 96.1% and 34.3% to 99.8% after S. oneidensis MR-1 addition. The mechanism investigations revealed that P. denitrificans provided S. oneidensis MR-1 with lactate, which was utilized to secreted riboflavin and phenazine by S. oneidensis MR-1. The riboflavin served as coenzymes of cellular reductants (i.e., thioredoxin and glutathione) in P. denitrificans, which created favorable intracellular microenvironment conditions for electron generation. Meanwhile, phenazine promoted biofilm formation, which increased the adsorption of Cr(VI) on the cell surface and accelerated the Cr(VI) reduction by membrane bound chromate reductases thereby reducing damage to other enzymes respectively. Overall, this strategy reduced the negative effect of chromate, thus improved the generation, transportation, and consumption of electrons. SYNOPSIS: The presence of S. oneidensis MR-1 facilitated nitrate and Cr(VI) removal by P. denitrificans through decreasing the negative effect of chromate due to the metabolites' secretion.

Visual and rapid fluorescence sensing for hexavalent chromium by hydroxypropyl chitosan passivated bismuth-based perovskite quantum dots.

Hydroxypropyl chitosan-Cs3Bi2Cl9 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-Cs3Bi2Cl9 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).

S-scheme towards interfacial charge transfer between POMs and MOFs for efficient visible-light photocatalytic Cr (VI) reduction.

The establishment of heterojunctions was considered as an exceptional strategy to obtain high-efficiency charge separation and enhanced photocatalytic performance. Herein, a series of FePMo/MIL-53(Fe) (FeM-53) heterojunctions were successfully constructed through in-situ growth of FePMo onto MIL-53(Fe) surface and their photocatalytic capacity were examined by visible-light-induced Cr(VI) reduction. Interestingly, the as-fabricated composites offered various photocatalytic activities controllably relying on the mass ratio of FePMo to MIL-53(Fe). Particularly, the one with the 10% ratio displayed the highest Cr(VI) reduction rate (100%) within 75 min, which was respectively over 4 and 2 folds higher than pure FePMo and MIL-53(Fe). The boosted photoactivity might be ascribed to the establishment of S-scheme heterojunction with suitable band alignment between FePMo and MIL-53(Fe), which broadened the light absorption range and improved charge separation. Further mechanism investigations implied both •O2- and e- were the key reactive species for Cr(VI) removal. Besides, the composite preserved excellent stability after 4 consecutive tests, and performed well in the presence of organic dyes. Such a S-scheme heterojunction may promise for highly efficient environmental mitigation.