Sustainable soil rehabilitation with multiple network structures of layered double hydroxide beads: Immobilization of heavy metals, fertilizer release, and water retention.

The remediation of heavy metal-contaminated soils necessitated a holistic approach that encompassed water and fertilizer conservation alongside soil property restoration. This study introduced the synthesis of (poly)acrylamide-layered double hydroxide gel spheres (PAM-LDH beads), which were designed to simultaneously immobilize heavy metals, control the release of fertilizers, and enhance soil water retention. Laboratory soil experiments under diverse conditions highlighted the superior performance of PAM-LDH beads in the immobilization of hexavalent chromium (Cr(VI)). The layered double hydroxide (LDH) component was identified as the key player in Cr(VI) immobilization, with anion exchange being the predominant mechanism. Notably, the encapsulated urea within the beads was released independently of environmental influences, governed by a concentration gradient across the beads surface. This release process was characterized by an initial phase of absorptive swelling followed by a diffusive phase. The impact on plant growth was assessed, revealing that PAM-LDH beads significantly curtailed Cr(VI) accumulation and alleviated its phytotoxic effects. Changes in the carbon (C) and nitrogen (N) content of the plants suggested that the urea encapsulated within the beads served as a nutrient source, contributing to soil fertility. Moreover, the water-holding capacity and soil-water characteristic curves of PAM-LDH beads suggested that these superabsorbent beads could delay soil water evaporation. The observed shifts in microbial community structure provided evidence for the enhancement of soil carbon and nitrogen cycles, indicative of improved soil properties.

Aerobic Fe transformation induced decrease in the adsorption and enhancement in the reduction of Cr(VI) by humic acid-ferric iron coprecipitates.

Humic substance (HS)-ferric iron (Fe(III)) coprecipitates are widespread organo-mineral associations in soils and aquifers and have the capacity to immobilize and detoxify Cr(VI). These coprecipitates undergo transformation owing to their thermodynamic instability; however, the effects of this transformation on their environmental behaviors remain unclear, particularly in aerobic environments. In this study, the aerobic transformation of humic acid (HA)-Fe(III) coprecipitates, a representative of HS-Fe(III) coprecipitates, was simulated. The environmental effect was then evaluated after conducting an adsorption-reduction batch experiment toward Cr(VI). The aerobic transformation characteristics, as well as the adsorption/reduction capacity of HA-Fe(III) coprecipitates, were found to depend strongly on their structures. In ferrihydrite (Fh)-like coprecipitates, amorphous Fh is readily transformed into crystalline hematite and goethite at aerobic environments, leading to a much lower specific surface area and adsorption capacity. However, this increasing degree of crystallization enhanced the inductive reduction ability towards Cr(VI) owing to the more significant shift of electron pairs in the FeOC bond toward the HA direction. In HS-like coprecipitates, Fe(III) always serves as a cation bridge connecting HA molecules, but can be reduced to Fe(II) by the associated HA after aerobic transformation. The produced Fe(II), therefore, drove the reduction of the adsorbed Cr(VI). These findings emphasize the pivotal role of aerobic transformation in enhancing the reduction capacity for Cr(VI), which opens a new avenue for the development of in-situ remediation agents for Cr(VI)-contaminated sites.

Efficient removal of Cr(VI) from contaminated kaolin and anolyte by electrokinetic remediation with foamed iron anode electrode and acetic acid electrolyte.

Combined electrokinetic remediation employing reducing agents represents an extensively utilized approach for the remediation of hexavalent chromium (Cr(VI))-contaminated soil. In this investigation, electrokinetic remediation of artificially contaminated kaolin was conducted utilizing a separate circulation system for the anolyte, with a 0.5M solution of acetic acid (HAc) as the electrolyte and foamed iron serving as the anode. The experimental outcomes demonstrated that employing HAc as the electrolyte enhances the electromigration of Cr(VI) and establishes an acidic milieu conducive to the reduction of Cr(VI) by foamed iron, thereby facilitating the rapid reduction of Cr(VI) accumulated in the anolyte through electrokinetic remediation. In the self-prepared contaminated kaolin, the initial concentration of Cr(VI) was 820.26 mg/L. Following the remediation process under optimal experimental conditions, the concentration was significantly reduced to 11.6 mg/L, achieving a removal efficiency of Cr(VI) in the soil of 98.59%. In the optimal experimental setup, the Cr(VI) concentration in the anolyte was reduced to 0.05 mg/L, which is below the EPA's Safe Drinking Water Act standard for Cr(VI) content of 0.1 mg/L. The removal mechanism of Cr(VI) from the electrolyte primarily involves reduction, precipitation, and co-precipitation, with the foamed iron playing a predominant role. HAc and foamed iron exhibit a synergistic effect. The findings of this study substantiate that the integration of foamed iron with HAc is efficacious for the electrokinetic remediation of soil contaminated with Cr(VI).

Sustainable Biomass Acts as an Electron Donor for Cr(VI) Reduction during the Subcritical Hydrothermal Process: Molecular Insights into the Role of Hydrochar and Liquid Compounds.

Heavy metal pollution is a critical environmental issue that has garnered significant attention from the international community. Subcritical hydrothermal liquefaction (HTL) as an emerging green technology has demonstrated remarkable promise in environmental remediation. However, there is limited research on the remediation of highly toxic Cr(VI) using HTL. This study reveals that the HTL reaction of biomass enables the simultaneous reduction and precipitation of Cr(VI). At 280 °C, the reduction of Cr(VI) was nearly complete, with a high reduction rate of 98.9%. The reduced Cr as Cr(OH)3 and Cr2O3 was primarily enriched in hydrochar, accounting for over 99.9% of the total amount. This effective enrichment resulted in the removal of Cr(VI) from the aqueous phase while simultaneously yielding clean liquid compounds like organic acids and furfural. Furthermore, the elevated temperature facilitated the formation of Cr(III) and enhanced its accumulation within hydrochar. Notably, the resulting hydrochar and small oxygenated compounds, especially aldehyde, served as electron donors for Cr(VI) reduction. Additionally, the dissolved Cr facilitated the depolymerization and deoxygenation processes of macromolecular compounds with lignin-like structures, leading to more small oxygenated compounds and subsequently influencing Cr(VI) reduction. These findings have substantial implications for green and sustainable development.

Hexavalent chromium uptake in rice (Oryza sativa L.) mediated by sulfate and phosphate transporters OsSultr1;2 and OsPht1;1.

Chromium (Cr) soil contamination is a critical global environmental concern, with hexavalent chromium (Cr[VI]) being especially perilous due to its high mobility, bioavailability, and phytotoxicity. This poses a significant threat to the cultivation of crops, particularly rice, where the mechanisms of Cr(VI) absorption remain largely unexplored. This study uncovered a competitive interaction between Cr(VI) and essential nutrients-sulfate and phosphate during the uptake process. Notably, deficiencies in sulfate and phosphate were associated with a marked increase in Cr(VI) accumulation in rice, reaching up to 76.5 % and 77.7 %, respectively. Employing q-PCR, this study identified significant up-regulation of the sulfate transporter gene, OsSultr1;2, and the phosphate transporter gene, OsPht1;1, in response to Cr(VI) stress. Genetic knockout studies have confirmed the crucial role of OsSultr1;2 in Cr(VI) uptake, with its deletion leading to a 36.1 % to 69.6 % decrease in Cr uptake by rice roots. Similarly, the knockout of OsPht1;1 resulted in an 18.1 % to 25.7 % decrease in root Cr accumulation. These findings highlight the key role of the sulfate transporter OsSultr1;2 in Cr(VI) uptake, with phosphate transporters also contributing significantly to the process. These insights are valuable for developing rice varieties with reduced Cr(VI) accumulation, ensuring the safety of rice grain production.

EDAC-modified chitosan/imidazolium-polysulfone composite beads for removal of Cr(VI) from aqueous solution.

To enhance the stability and adsorption performance of chitosan in Cr(VI)-contaminated acidic wastewater, a novel EDAC-modified-EDTA-crosslinked chitosan derivative (CSEC) was synthesized via a one-pot method with chitosan, 1-Ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride (EDAC), and Na2EDTA as raw materials. To further improve the mechanical strength and separation performance of CSEC, a novel composite bead (CSEP) of CSEC and imidazolium-functionalized polysulfone (IMPSF) was prepared through a phase inversion method. The chemical composition and microstructure of CSEC and CSEP were characterized by FESEM, FTIR, NMR and XPS techniques. The maximum adsorption capacities of CSEC and CSEP for Cr(VI) were 145.96 and 135.82 mg g-1 at pH 3, respectively, and the equilibrium time for Cr(VI) adsorption by CSEC and CSEP was 5 min and 8 h, respectively. The adsorption process of Cr(VI) by both CSEC and CSEP was exothermic and spontaneous. Compared to CSEC, CSEP has significantly enhanced resistance to interference from coexisting anions. The removal mechanism of Cr(VI) by CSEP might involve redox reaction as well as electrostatic attraction between Cr(VI) oxyanions and various nitrogen cations, including protonated amino groups, guanidinium groups, protonated tertiary amine groups, and imidazolium cations. The CSEP beads have potential application value in the treatment of acidic wastewater containing Cr(VI).

Optimizing Cr(VI) reduction to Cr(III) using Pd-CNTs nanocatalyst: kinetic Monte Carlo simulation and experimental design insights.

In this investigation, we explored the kinetics of Cr(VI) reduction to Cr(III) on carbon nanotube decorated with palladium (Pd-CNTs) nanocatalyst, using formic acid as the reducing agent. This study has been bone utilizing kinetic Monte Carlo simulation and experimental design methods. The mechanism and kinetic parameters of this reaction are provided. The effect various factors such as reaction time, pH level, dichromate (Cr2O72-) concentration, and formic acid concentration on Cr(VI) reduction was studied. Concentrations of HCOOH and Cr2O72- were identified as the crucial variables, while the HCOOH concentration has the most significant impact. Positive influences on Cr(VI) reduction were observed with increasing pH level and HCOOH concentration. Reaction time positively affects on Cr(VI) reduction efficiency. However, the concentration of Cr2O72- showed an increasing effect up to a threshold, negatively impacting the efficiency. The optimal conditions (Reaction time = 60 min, pH = 4.5, [Cr2O72-] = 5.05 × 10-3 M, and [HCOOH] = 0.95 M) for Cr(VI) reduction. At optimal conditions, the Cr(VI) reduction efficiency was obtained to be 100%.

Development of a continuous electrochemical reactor incorporated with waste-derived activated carbon electrode for the effective removal of hexavalent chromium from industrial effluent.

Being a recognized carcinogen, hexavalent chromium is hazardous to both human and environmental health. Thus, it is imperative to regulate and oversee their levels in a variety of industries, including textiles, dyes, pigments, and metal finishing. This study strives to reduce Cr(VI) in wastewater by using capacitive deionization in conjunction with an activated carbon-based electrode and a continuous electrochemical reactor (CER). Activated carbon derived from rubberwood sawdust demonstrated excellent properties, including a high surface area of 1157 m2 g-1. The electrical conductivity and mechanical stability of the electrode were enhanced by the incorporation of synthesized expanded graphite (EG) into the AC. Key parameters were optimized via systematic batch electroreduction experiments with an optimal response surface design. The efficacy of the fabricated CER was proved when it successfully reduced Cr(VI) in a 5 mg L-1 solution within 15 min under optimized conditions, in contrast to the considerably longer durations anticipated by conventional methods. Validation of these findings was done by treating industrial wastewater of 30 mg L-1 in the CER. The electroreduction of Cr(VI) followed the Langmuir isotherm with a maximum capacity of 13.491 mg g-1 and pseudo-second-order kinetics. These results indicate that the combined use of the modified AC electrode and CER holds potential as a sustainable and economical approach to effectively eliminate Cr(VI) from wastewater.

Novel Ag@NH2-UiO-66(Zr) photocatalyst with controllable charge transfer pathways for efficient Cr(VI) remediation.

Rational fabrication of core-shell photocatalysts to hamper the charge recombination is extraordinarily essential to enhance photocatalytic activity. In this work, core-shell Ag@NH2-UiO-66 (Ag@NU) Schottky heterojunctions with low Ag content (1 wt%) were constructed by a two-step solvothermal method and adopted for Cr(VI) reduction under LED light. Typically, the one with the Ag: NH2-UiO-66 mass ratio (1 : 100) led to 100% Cr(VI) removal within 1 h, superior to bare NH2-UiO-66 and Ag/NH2-UiO-66 (Ag was directly decorated on NH2-UiO-66 surface). The enhanced photocatalytic activity was related to the migration of the electrons on the CB of NH2-UiO-66 to Ag NPs through a Schottky barrier, and thus the undesired charge carriers recombination was avoided. This result was also evidenced by Density functional theory (DFT) calculations. The computational simulations indicate that the introduction of Ag effectively narrowed the band gap of NH2-UiO-66, facilitating the transfer of photo-generated electrons, expanding the light absorption area, and significantly enhancing photocatalytic efficiency. Most importantly, such a core-shell structure can inhibit the formation of •O2-, letting the direct Cr(VI) reduction by photo-excited e-. In addition, this structure can also protect Ag from being oxidized by O2. Ten cyclic tests evidenced the Ag@NU had excellent chemical and structural stability. This research offers a novel strategy for regulating the Cr(VI) reduction by establishing core-shell photocatalytic materials.

Complexing agent-assisted Cr(VI) removal in a continuous fixed-bed system with nanoscale Fe0/NaA molecular sieve membrane supported on nickel foam.

Complexing agents (CAs) can be used for the removal of Cr(VI) via nanoscale Fe0 (nZVI) reduction in cost-effective manner. However, nZVI is prone to aggregation and passivation, and some conventional CAs are toxic and difficult to biodegrade, potentially causing secondary pollution. Therefore, selecting an environmentally friendly CA for assisting in the removal of Cr(VI) via supported nZVI is imperative. Herein, NaA molecular sieve membrane-supported nZVI (nZVI/NaA-NF) was prepared via the secondary growth and liquid-phase reduction method using nickel foam (NF) as the carrier. The physicochemical characteristics of nZVI/NaA-NF before and after reaction were analysed via SEM, EDS, and XPS. A CA-improved nZVI/NaA-NF was used for the effective removal of Cr(VI) in a continuous fixed-bed system. Furthermore, the influences of various experimental factors including the CA type, CA concentration, solution pH, space velocity, and inlet Cr(VI) concentration on Cr(VI) removal were systematically investigated. The results demonstrated that nZVI particles were homogeneously immobilized on the NaA molecular sieve membrane/NF for fresh nZVI/NaA-NF, and tetrasodium iminidisuccinate (IDS-4Na) inhibited the aggregation of Cr(III)/Fe(III) (hydr)oxide precipitates during the reaction. IDS-4Na demonstrated excellent promotive effect on Cr(VI) removal via nZVI/NaA-NF. The breakthrough time for Cr(VI) in the addition of IDS-4Na was considerably longer than that of nZVI/NaA-NF alone. The breakthrough concentration of Cr(VI) only reached 1.1% and 9.9% of the inlet concentration at 220 and 240 min, with an IDS-4Na concentration of 4 mM, a pH of 2.5, and a space velocity of 0.265 min-1. The Bohart-Adams model was appropriate to predict the initial part of Cr(VI) breakthrough curves in the nZVI/NaA-NF fixed bed. The saturated concentration (N0) increased with an increase in inlet Cr(VI) concentration. The Yoon-Nelson model afforded good fitting results for all breakthrough curves of Cr(VI). The k' value increased with an increase in space velocity, and the τ value decreased.

Synergistic effects of sulfur vacancies and internal electric fields in FeS/MoS2 heterojunctions: A new approach to photocatalytic chromium removal.

Molybdenum disulfide (MoS2) is heralded as an exemplary two-dimensional (2D) functional material, largely attributed to its distinctive layered structure. Upon forming heterojunctions with reducing species, MoS2 displays remarkable photocatalytic properties. In this research, we fabricated a novel heterojunction photocatalyst, FeS/MoS2-0.05, through the integration of FeS with hollow MoS2. This composite aims at the efficient photocatalytic reduction of hexavalent chromium (Cr(VI)). A comprehensive array of characterization techniques unveiled that MoS2 flakes, dispersed on FeS, provide numerous active sites for photocatalysis at the heterojunction interface. The inclusion of FeS seemingly promotes the formation of sulfur vacancies on MoS2. Consequently, this heterojunction catalyst exhibits photocatalytic activity surpassing pristine MoS2 by a factor of 3.77. The augmented activity of the FeS/MoS2-0.05 catalyst is attributed chiefly to an internal electric field at the interface. This field enhances the facilitation of charge transfer and separation significantly. Density functional theory (DFT) calculations, coupled with experimental analyses, corroborate this observation. Additionally, DFT calculations indicate that sulfur vacancies act as pivotal sites for Cr(VI) adsorption. Significantly, the adsorption energy of Cr(VI) species shows enhanced favorability under acidic conditions. Our results suggest that the FeS/MoS2-0.05 heterojunction photocatalyst presents substantial potential for the remediation of Cr(VI)-contaminated wastewater.

Phytic acid inhibits Cr(VI) reduction on Fe(II)-bearing clay minerals: Changing reduction sites and electron transfer pathways.

The presence of organic phosphorus may influence the characteristics of Cr(VI) reduction and immobilization on Fe(II)-bearing clay minerals under anoxic conditions, as the organic phosphorus tends to bind strongly to clay minerals in soil. Herein, reduced nontronite (rNAu-2) was used to reduction of Cr(VI) in the presence of phytic acid (IHP) at neutral pH. With IHP concentration from 0 to 500 µM, Cr(VI) reduction decreased obviously (17.8%) within first 5 min, and then preferred to stagnate during 4-12 h (≥50 µM). After that, Cr(VI) was reduced continuously at a slightly faster rate. Density functional theory (DFT) calculations revealed that IHP primarily absorbed at the edge sites of rNAu-2 to form Fe-IHP complexes. X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and Fourier transform infrared spectroscopy (FTIR) results demonstrated that IHP hindered the ingress of CrO42- into the interlayer space of rNAu-2 and impeded their reduction by trioctahedral Fe(II) and Al-Fe(II) at basal plane sites in the initial stage. Additionally, Fe(II) extraction results showed that IHP promoted the electron from interior transfer to near-edge, but hindered it further transfer to surface, resulting in the inhibition on Cr(VI) reduction at edge sites during the later stage. Consequently, IHP inhibits the reduction and immobilization of Cr(VI) by rNAu-2. Our study offers novel insights into electron transfer pathways during the Cr(VI) reduction by rNAu-2 with coexisting IHP, thereby improve the understanding of the geochemical processes of chromium within the iron cycle in soil.

Enhanced Cr(VI) removal via CPBr-modified MIL-88A@amine-functionalized GO: synthesis, performance, and mechanism.

Water contamination by heavy metals, especially chromium (VI), poses a critical environmental issue due to its carcinogenic nature and persistence in the environment. Addressing this, the current study develops an efficient adsorbent, CPBr-MIL-88A@AmGO, which utilizes the synergistic capabilities of Cetylpyridinium bromide-modified MIL-88A and amine-functionalized graphene oxide to enhance Cr(VI) removal from aqueous solutions. The obtained results indicate that CPBr-MIL-88A@AmGO achieves its highest removal efficacy at pH 2, where the interaction of CPBr and AmGO's positively charged centers significantly contributes to the adsorption processes. According to the Langmuir isotherm model, the composite's adsorption capacity reached a maximum of 306.75 mg/g. The adsorption kinetics adhered to a pseudo-second-order model along with the endothermic nature of the process. Although the presence of SO42- ions significantly reduces adsorption capacity, other interfering ions including Na+, K+, Ca2+, Cl-, and NO3- only slightly affect it. Remarkably, the composite maintains high removal efficiency, over 82%, even after 7 recycling tests, underscoring its potential for practical applications in water treatment systems. The proposed mechanism involves the contribution of electrostatic attractions, ion exchange, complexation, and the reduction of Cr(VI) to Cr(III) in the removal process. This study not only offers a potent solution for Cr(VI) remediation but also contributes to sustainable water resource management.

Phycoremediation of As(III) and Cr(VI) by Desmodesmus subspicatus: Impact on growth and biomolecules (carbohydrate, protein, chlorophyll and lipid) - A dual mode investigation.

Microalgae are under research focus for the simultaneous production of biomolecules (e.g., carbohydrates, proteins, pigments and lipids) and bioremediation of toxic substances from wastewater. The current study explores the capability of indigenously isolated microalgae (Desmodesmus subspicatus) for the phycoremediation of As(III) and Cr(VI). Variation of biomolecules (carbohydrate, protein, lipid and chlorophyll) was investigated during phycoremediation. D. subspicatus survived up to the toxicity level of 10 mg/L for As(III) and 0.8 mg/L for Cr(VI). A 70% decline in carbohydrate accumulation was observed at 10 mg/L of As(III). An increased content of proteins (+ 28%) and lipids (+ 32%) within the cells was observed while growing in 0.5 and 0.2 mg/L of As(III) and Cr(VI) respectively. A decrease in carbohydrate accumulation was noted with increasing Cr(VI) concentration, and the lowest (- 44%) was recorded at 0.8 mg/L Cr(VI). D. subspicatus showed an excellent maximum removal efficiency for Cr(VI) and As(III) as 77% and 90% respectively.

Intensification of Cr(VI) adsorption using activated carbon adsorbent modified with ammonium persulfate.

Granular activated carbon has been modified by ammonium persulfate as a new adsorbent for Cr(VI) adsorption from aqueous solutions. The adsorbent was characterized by nitrogen adsorption-desorption isotherm data and infrared spectroscopy. The impact of different factors, such as the initial pH level of the solution, time, temperature, ionic strength, and initial concentration of the Cr(VI) ion, on the adsorption efficiencies of the adsorbent has been studied by batch experiments. Kinetic studies and the adsorption thermodynamics of Cr(VI) with ammonium persulfate-modified activated carbon adsorbent were carefully studied. The results showed that the Cr(VI) adsorption follows a pseudo-second-order kinetic model and the adsorption reaction is endothermic and spontaneous. The adsorption isotherm was scrutinized, and the fitting results showed that the Langmuir model could well represent the adsorption process. The maximum adsorption capacity of Cr(VI) onto persulfate-modified activated carbon was 108.69 mg g-1. The research results showed that using persulfate-modified activated carbon adsorbent can greatly remove Cr(VI) from aqueous solutions.

Disclosing the influence mechanism of facet-dependent pyrite photo-activation and photo-dissolution processes on the reduction of Cr(VI).

The photo-activation and photo-dissolution processes of pyrite (FeS2) can affect the environmental behavior of the co-existing hexavalent chromium (Cr(VI)). But the photochemical performance of FeS2 is intimately dependent on its exposed facets. Herein, FeS2 nanosheets (FeS2 NS) and FeS2 nanocubes (FeS2 NC) with the dominant exposed facets of (001) and (210)/(100) respectively are prepared. The more Fe3+, Fe2+, and SO42- are released in the FeS2 NS system than the other system due to its more excellent generation ability of photogenerated electrons and reactive oxygen species. The higher surface energy on (001) facet leads to the faster dissolution rate of FeS2 NS. Due to the optimal production ability of photogenerated electrons and Fe2+ of (001) facet, the much higher Cr(VI) elimination efficiency in the FeS2 NS system is observed than that in the FeS2 NC (72.8%) system within 120 min. This work could help to unveil the influence of FeS2 on the fate of Cr(VI) in surface environment, and offer a theoretical support to clarify the influence of heavy metal ions on the iron sulfide minerals.

Ferric citrate enhanced bioreduction of Cr(VI) by Bacillus cereus RCr in aqueous solutions: reduction performance and mechanisms.

The bioreduction characteristics and mechanisms of Cr(VI) onto Bacillus cereus RCr enhanced by ferric citrate were investigated. The optimum conditions were initial pH 9, temperature 40 °C, inoculation amount 4%, and glucose 3 g/L, respectively. The addition of 1.5 g/L ferric citrate increased the average reduction rate from 120.43 to 220.61 mg/(L∙h) compared with the control (without ferric citrate). The binding capacity of Cr(III) on the cell surface increased to 21%, in which the precipitates were mainly CrO(OH), Cr2O3, and FeCr2O4. Cell membrane was the main site of reduction, related important functional groups: - COOH, C-H, - NH2, C = C, and P-O. Fe(III) increased the yield of NADH and cytochrome c by approximately 48.51% and 68.63%, which significantly facilitated the electron generation and electron transfer, thus increasing the amount of electrons in the bioreduction of heavy metals by an average of 110%. Among the electrons obtained by Cr(VI), the proportion of indirect reduction mediated by Fe(III)/Fe(II) shuttle was 62% on average, whereas direct reduction mediated by reductase was 38%. These results may provide insights into the bioreduction process by bacteria enhanced by Fe(III) for detoxification of heavy metals with multiple valences, as an important step towards improving microbial remediation.

Uncovering an effecient binary system as a chemosensor for visual and fluorescence detection of chromium (VI) in water samples.

There is an urgent requirement for the development of sensitive and quick sensors to monitor chromium (VI) due to its substantial carcinogenic and mutagenic properties. A coexisting system of coumarin 334 and diphenylcarbazide (C334/DPC) was used in this study as a fluorescent chemosensor to detect Cr(VI) ions. Upon the addition of Cr(VI), a purple chelate complex (Cr(III)-diphenylcarbazone) was produced, which resulted from the quantitative reaction between Cr(VI) ions and diphenylcarbazide (DPC), whereas no interaction between Cr(VI) and coumarin 334 took place. More interestingly, the absorption spectra of purple (Cr(III)-diphenylcarbazone) complex (λmax = 540 nm) were overlapped with emission and excitation spectra of coumarin 334 (λex/em = 453/492), resulting in the efficient quenching of coumarin 334 (C334) via the inner filter effect. Furthermore, the semi-quantitative estimation of Cr(VI) ion concentration may be achieved by visually watching the progressive color transformation of the probe from yellow to red after the addition different concentration of Cr(VI). The calibration plot for determination of Cr(VI) by this method is ranging from 0.048 to 268 µM. DFT calculations were conducted to enrich our understanding about the mechanism of action. This approach demonstrates an excellent selectivity and sensitivity for Cr(VI) including a detection limit of 48 nM. The new sensor was successfully applied to water samples (tap, mineral, and waste waters). The accuracy was confirmed by the atomic absorption spectroscopy.

Efficient removal of Chromium(VI) from wastewater based on magnetic multiwalled carbon nanotubes coupled with deep eutectic solvents.

Industrial wastewater containing heavy metal Cr(VI) seriously affects the health of organisms and may even lead to cancer. Developing efficient adsorbents that can quickly separate heavy metals is crucial for treating wastewater. In this study, magnetic multiwalled carbon nanotubes (MMWCNTs) with moderate particle size and abundant surface active sites were prepared by coating multiwalled carbon nanotubes with magnetic nanoparticles. The results of FTIR, XRD, TG, VSM, BET, and EDS showed MWCNTs completely encapsulated on the surface of the magnetic nanoparticles, with a particle size of approximately 30 nm. Oxygenated groups provided abundant surface active sites and formed numerous mesopores. The response surface methodology was used to optimize the adsorbent dose, adsorption contact time and adsorption temperature, and the removal rate of Cr(VI) was more than 95%. The quasi-second order kinetics and Freundlich adsorption isotherm model explained the adsorption process to Cr(VI). MMWCNTs interacted with Cr(VI) through electrostatic attraction, reduction reactions, complexation, and other means. The extensive hydrogen bonding of the green solvent deep eutectic solvent (DES) was employed to desorb the MMWCNTs and desorption rate exceed 90%. Even after five adsorption-regeneration cycles, the adsorbent maintained a high capacity. In conclusion, these novel MMWCNTs, as efficient adsorbents paired with DES desorption, hold broad potential for application in the treatment of Cr(VI)-contaminated wastewater.

The dominant role of extracellular polymeric substances produced by Achromobacter xylosoxidans BP1 in Cr(VI) microbial reduction.

Extracellular polymeric substances (EPS) have demonstrated significant benefits for reducing multivalent metal contamination. Using Achromobacter xylosoxidans BP1 isolated from a coal chemical site in China, this study elucidated the contribution of EPS production to Cr (VI) reduction and revealed its biological removal mechanism. BP1 grew at an optimum pH of 8 and the lowest inhibitory concentration of Cr(VI) was 300 mg/L. The spent medium completely removed Cr(VI), whereas resting cells were only able to remove 10.47 % and inactivated cells were nearly incapable of Cr(VI) removal. S-EPS and B-EPS reduced Cr(VI) by 98.59 % and 11.64 %, respectively. SEM-EDS analysis showed that the BP1 cells were stimulated to produce EPS under Cr stress. The XPS results showed that 29.63 % of Cr(VI) was enriched by intracellular bioaccumulation or biosorption and 70.37 % of Cr(VI) was reduced by extracellular enzymes to produce Cr(OH)3 and organic Cr(III) complexes. According to FTIR, EPS with -OH, COO-, and amide groups supplied binding sites and electrons for the reductive adsorption of Cr(VI). Genomic studies showed that BP1 primarily produces extracellular polysaccharides, metabolises sulphur and nitrogen, and reduces reactive oxygen species damage as a result of DNA repair proteases.