Remediation Of Cr Research Articles

Synthesis of bimetallic-nanocrystal-decorated rice bowl-shaped polymer particles for catalytic Cr(VI) remediation

Rice-bowl shaped polymer particles with convex and concave surfaces have emerged as promising noble metal catalyst supports. However, their synthesis and integration with catalytic materials using environmentally friendly methods have remained a challenge. This study presents a sustainable and efficient approach for synthesizing rice-bowl-shaped polystyrene (PS) particles and explores their potential as catalyst supports. The key to achieving this morphology lies in the utilization of anisotropic PS seeds, specifically hollow dimpled PS seeds. Uniform rice-bowl-shaped PS particles are obtained by dispersing these seeds in an oil-in-water (O/W) emulsion and allowing them to dry under ambient conditions. Non-toxic reducing agents and environmentally friendly solvents are employed for the in-situ chemical reduction process and galvanic displacement reaction to synthesize bimetallic (Ag/Pd) nanocrystals on the surfaces of the PS particles. The resulting Ag/Pd-coated PS particles exhibit excellent catalytic activity as microreactors for Cr(VI) reduction in water, while demonstrating remarkable stability during repeated use without significant loss of activity. Furthermore, the shape of the rice-bowl particles facilitates efficient packing and stacking, leading to enhanced catalytic performance not only in bulk solutions but also in confined spaces. This study highlights the potential of rice-bowl-shaped polymer particles as efficient and environmentally friendly catalyst supports, paving the way for the development of green catalytic applications.

Enhanced remediation of Cr(VI)-contaminated soil by modified zero-valent iron with oxalic acid on biochar

Hexavalent chromium (Cr(VI)) is carcinogenic and widely presented in soil. In this study, modified zero-valent iron (ZVI) with oxalic acid on biochar (OA-ZVI/BC) was prepared using wet ball milling method for the remediation of Cr(VI)-contaminated soil. Microscopic characterizations showed that ZVI were distributed on the biochar uniformly and confirmed the enhanced interface interaction between biochar and ZVI by wet ball milling. Electrochemical analysis indicated the strong electron transfer ability and enhanced corrosion behavior of OA-ZVI/BC. Moreover, inhibitory efficiencies of Cr(VI) removal with the addition of 1,10-phenanthroline suggested abundant Fe2+ generation in OA-ZVI/BC, which might facilitate the reduction of Cr(VI) to Cr(III). Theory calculation further demonstrated the ZVI modified by oxalic acid was more susceptible to solid–solid interfacial reactions with Cr(VI), and more electrons were transferred to Cr(VI). When applied to Cr(VI)-contaminated soil, OA-ZVI/BC could passivate 96.7 % total Cr(VI) and maintained for 90 days. The toxicity characteristic leaching procedure (TCLP) and simple based extraction test (SBET) were used to evaluate the leaching toxicity and bioaccessibility of Cr(VI), respectively. The TCLP-Cr(VI) decreased to 0.11 mg·L−1 after OA-ZVI/BC treatment, much lower than that of soils with ZVI/BC and OA-ZVI remediation (1.5 mg·L−1 and 4.1 mg·L−1). The bioaccessibility of Cr(VI) reduced by 93.5 % after 3-month remediation. Sequential extraction showed that Cr fractions in the soil after OA-ZVI/BC remediation was converted from acetic acid-extractable (HOAc-extractable) to more stable forms (e.g., residual and oxidizable forms). Benefiting from the synergies of oxalic acid, biochar and wet ball milling, OA-ZVI/BC exhibited an excellent performance on the remediation of Cr(VI)-contaminated soil, whose mechanisms involved adsorption, reduction (Fe0/Fe2+, Fe2+/Fe3+) and co-precipitation. This study herein develops a promising ZVI technology in the remediation of heavy metal-contaminated soil.

Fe3O4 quantum dots anchored on functionalized graphene: A multimodal platform for sensing and remediation of Cr(VI)

Fe3O4 quantum dots anchored on functionalized graphene: A multimodal platform for sensing and remediation of Cr(VI)

Cr(VI) reduction by Agrobacterium sp. Cr-1 and Lysinibacillus sp. Cr-2, novel Cr(VI)-reducing strains isolated from chromium plant soil.

The bioremediation of Cr(VI)-contaminated soil is a promising strategy; however, the performance of Cr(VI)-reducing bacteria is limited by the toxicity of Cr(VI). In this study, two novel Cr(VI)-reducing bacteria were isolated from a Cr salt plant and identified as Agrobacterium sp. and Lysinibacillus sp. The Cr(VI) reduction conditions of the two strains were optimized. At a Cr(VI) concentration of 500mg/L, Agrobacterium sp. Cr-1 reduced Cr(VI) with a removal rate of 96.91%, while that for Lysinibacillus sp. Cr-2 was 92.82%. First-order reaction kinetic equations simulated the positive relationship between time and Cr(VI) concentration during Cr(VI) reduction in these two strains. Agrobacterium sp. Cr-1 was further studied, and the effects of different cell components on Cr(VI) reduction were detected. The extracellular extracts of Agrobacterium sp. Cr-1 played a major role in Cr(VI) reduction, followed by intracellular extracts and cell membranes. The scanning electron microscope-energy dispersive spectrometer (SEM-EDS) images show that the precipitation was Cr. The high Cr(VI) reducing ability of Agrobacterium sp. Cr-1 suggests that this strain is promising for the remediation of Cr(VI)-contaminated sites.

Homogeneous dispersion of amorphous nanoscale zero-valent iron supported on chlorella-derived biochar: In-situ synthesis and application mechanism for Cr(VI) removal over a wide pH range

Novel recyclable chlorella pyrenoidosa-derived biochar (CP-BC) supported nanoscale zero-valent iron composite (nZVI@CP-BC) was developed via an in-situ liquid-phase chemical reduction-deposition method. The obtained adsorbent kept prominent hexavalent chromium (Cr(VI)) removal capacity with a broad pH range (3.0–11.0). Also, these findings revealed that the removal performance of Cr(VI) via nZVI@CP-BC was 1.37, 6.67 and 1.25 times more than that of pristine nZVI, unmodified CP-BC and nZVI + CP-BC, respectively. The exceptional Cr(VI) removal benefited from the combination of high-reactivity nZVI and CP-BC resulting in an effective synergy. The physicochemical property of the nZVI@CP-BC was systematically elucidated utilizing multiple characterizations. It is launched that the Cr(VI) molecules were converted into Cr(Ⅲ) mainly via surface adsorption and chemical reduction. And CP-BC may regulate beneficial conditions for Cr(VI) chemical reduction reaction via largely mediating solution pH and electron transfer, which showed a rational reaction mechanism of promoted removal of Cr(VI) under a broad working pH range. The corrosion mechanism of nZVI in the whole reaction was also studied thoroughly. This work indicates that this novel nZVI@CP-BC would be a prominent material for the preferable remediation of Cr(VI)-bearing wastewater.

Effective Cr(VI) reduction and immobilization in chromite ore processing residue (COPR) contaminated soils by ferrous sulfate and digestate: A comparative investigation with typical reducing agents

Chemical reduction combined with microbial stabilization is a green and efficient method for the remediation of hexavalent chromium (Cr(VI)) contaminated soil. In this study, the combination of ferrous sulfate with kitchen waste digestate was applied to reduce and immobilize Cr(VI) in chromite ore processing residue (COPR) contaminated soils, and systematically evaluated the remediation performance of Cr(VI) compared with several typical reducing agents (i.e., ferrous sulfate, zero valent iron, sodium thiosulfate, ferrous sulfide, and calcium polysulfide). The results showed that the combination of ferrous sulfate and digestate had superior advantages of a lower dosage of reducing agent and a long-term remediation effect compared to other single chemical reductants. Under an Fe(II):Cr(VI) molar ratio of 3:1% and 4% digestate (wt), the content of Cr(VI) in the soil decreased to 5.07 mg/kg after 60 days of remediation. Meanwhile, the leaching concentrations of Cr(VI) were below detection limit, which can meet the hazardous waste toxicity leaching standard. The risk level of Cr pollution was decreased from very high risk to low risk. The X-ray photoelectron spectroscopy (XPS) results further demonstrated that the combined treatments were beneficial to Cr(VI) reduction and stabilization. The abundance of bacteria with Cr(VI) reducing ability was higher than other treatments. Moreover, the high abundance of carbon and nitrogen metabolism in the combined treatments demonstrated that the addition of digestate was beneficial to the recovery and flourishing of Cr(VI)-reducing related microorganisms in COPR contaminated soils. This work provided an alternative way on Cr(VI) remediation in COPR contaminated soils.

The dual effect of disodium anthraquinone-2,6-disulfonate (AQDS) on the Cr(VI) removal by biochar: The enhanced electron transfer and the inhibited adsorption

Due to large specific surface area, abundant surface functional groups, and stable chemical structure, biochar has been widely used in many environmental fields, including the remediation of Cr pollution. Alternatively, electrochemically active organic matter (e-OM), which is prevalent in both natural environments and industrial wastewater, exerts an inevitable influence on the mechanisms underlying Cr(VI) removal by biochar. The synergistic interplay between biochar and e-OM in the context of Cr(VI) remediation remains to be fully elucidated. In this study, disodium anthraquinone-2,6-disulfonate (AQDS) was used as a model for e-OM, characterized by its quinone group's ability to either donate or accept electrons. We found that AQDS sped up the Cr(VI) removal process, but the enhancement effect decreased with the increase in pyrolysis temperature. With the addition of AQDS, the removal amount of Cr(VI) by BC300 and BC600 increased by 160.0% and 49.5%, respectively. AQDS could release more electrons trapped in the lower temperature biochar samples (BC300 and BC600) for Cr(VI) reduction. However, AQDS inhibited the Cr(VI) removal by BC900 due to the adsorption of AQDS on biochar surface. In the presence of the small molecule carbon source lactate, more AQDS was adsorbed onto the biochar surface. This led to an inhibition of the electron transfer between biochar and Cr(VI), resulting in an inhibitory effect. This study has elucidated the electron transfer mechanism involved in the removal of Cr(VI) by biochar, particularly in conjunction with e-OM. Furthermore, it would augment the efficacy of biochar in applications targeting the removal of heavy metals.

MXene@MOF for synergetic removal of Cr(VI) by adsorption and reduction

MOF-based nanomaterials have shown great potential for absorbing Cr(VI) contamination, however, reducing Cr(VI) to Cr(III) with low solubility and toxicity has been challenging. In this work, MXene@MIL-53-NH2 nanocomposite was fabricated and supreme to pristine MOF and MXene in Cr(VI) removal. The hybridization of MOF and MXene increased the distribution coefficient (LgKd=4.8), demonstrating the large binding affinity to Cr(VI) oxyanions and enhanced Cr(VI) adsorption of MXene@MIL-53-NH2. According to batch experiment analysis, 0.6MXene@MIL-53-NH2 showed a high Cr(VI) removal at pH 3. The adsorption kinetics fitted to pseudo-second-order model, and adsorption isotherms followed Langmuir and Temkin models well. The calculated theoretical adsorption capacity was up to 224.46 mg·g−1. The thermodynamics revealed that the reaction process was spontaneous and endothermic. Notably, the adsorption and reduction processes simultaneously determine the Cr(VI) sequestration on MXene@MIL-53-NH2. Spectroscopic and electrochemical analysis showed that MXene@MIL-53-NH2 boosted the conversion of adsorbed Cr(VI) to Cr(III) by promoting electron conversion and elevating the electron-donating potential. The prepared MXene@MIL-53-NH2 showed superior ionic interference resistance and recyclability under experimental conditions, which provided information for better evaluation and design of nanomaterials for Cr(VI) remediation.

Sulfite Poses a Risk of Hexavalent Chromium Rebound in Vadose Zone: A Challenge of the Stability of CrxFe1-x(OH)3.

Cr(VI) rebound is the primary risk associated with the reduction remediation of Cr(VI)-contaminated soil. The potential impact of sulfites, which can be produced by microbial activities or originate from sulfur-containing remediation agents, on the Cr(VI) rebound in the vadose zone has been overlooked. When sulfites are present, the stability of CrxFe1-x(OH)3 is compromised and significantly inferior to that of Cr(OH)3, as demonstrated in this paper. First, Fe acts as a catalyst for the conversion of adsorbed sulfite to SO4·-, which subsequently triggers the oxidation of Cr(III) and results in the rebound of Cr(VI). The heterogeneous catalysis by Fe on the surface of CrxFe1-x(OH)3 plays a predominant role, contributing to 78% of the actual oxidation of Cr(III) among all employed catalytic processes. The presence of ambient Cl- can exacerbate the rebound effect of Cr(VI) by promoting the generation of HOCl. Furthermore, a portion of released Cr(VI) was reduced to Cr(III) by dissolved sulfite in the presence of dissolved Fe as a catalyst, thereby increasing the dissolution and migration risk associated with CrxFe1-x(OH)3. Hence, the presence of sulfites results in a significant increase in the Cr(VI) rebound and Cr(III) release from CrxFe1-x(OH)3. This challenges the conventional understanding of the stability of CrxFe1-x(OH)3.

Biochar Decreases Cr Toxicity and Accumulation in Sunflower Grown in Cr(VI)-Polluted Soil.

Biochar is preferentially recommended for the remediation of heavy metal-polluted soils. Sunflower is an important high-biomass oil crop with a promising potential for phytoremediation of Cr(VI)-polluted soil. However, how biochar affects sunflower growth and Cr accumulation in Cr(VI)-polluted soil needs to be elucidated. Here, a pot culture experiment was conducted to study whether soil amendment with biochar (0, 0.1%, 1%, and 5%, w/w) can mitigate Cr toxicity and accumulation in sunflower seedlings grown in soils artificially polluted with different levels of Cr(VI) (0, 50, and 250 mg Cr(VI)/kg soil). The addition of Cr(VI) exhibited significant phytotoxicity, as evidenced by inhibited plant growth and even the death of seedlings at 250 mg/kg Cr(VI). Overall, biochar amendment showed positive effects on plant growth and Cr immobilization, dependent on both the biochar dose and Cr addition level. When 50 mg/kg Cr(VI) was added, 1% biochar showed positive effects similar to 5% biochar on improving plant growth and mineral nutrition (particularly K), reducing Cr content in shoots and roots, and decreasing Cr availability and Cr(VI) content in the soil. In comparison with non-amendment, 1% and 5% biochar caused 85% and 100% increase in shoot dry weights, and 75% and 86% reduction in shoot Cr concentrations, respectively. When 250 mg/kg Cr(VI) was added, a 5% dose produced much better benefits than 1%, while a 0.1% dose did not help plants to survive. Overall, an appropriate dose of biochar enhanced Cr(VI) immobilization and subsequently decreased its toxicity and accumulation in sunflower seedlings. Our findings confirm that biochar can be used as an efficient amendment for the remediation of Cr(VI)-polluted soils and cleaner production of sunflower oil and biomass.

Permeable reactive composite approaching cathode enhanced Cr removal in soil using the byproduct of electrokinetic technology: emphasized energy utilization efficiency.

Cost-effective techniques with significant removal rates and low energy consumption are urgently required for in-situ Cr-contaminated soil remediation to reduce potential environmental toxicity to the ecosystem and human bodies. Electrokinetic technology is a valuable and promising soil remediation technology; however, the acidic and alkaline fronts evolutioninduced by the electrokinetic byproducts (H+, OH-) has significant hindering characteristics for ion removal. To effectively utilize the byproducts for enhancing Cr elimination, this paper proposed the permeable reactive composite approaching cathode with rhamnolipid-modified biochar as reactive material. Power utilization efficiency (η) was presented to comprehensively evaluate the target species elimination effect, considering removal rate and energy consumption. Results suggested that biosurfactant rhamnolipid stimulated Cr removal in acid and base fronts. Acid front induced rhamnolipid protonation reducing anolyte Cr(VI) to Cr(III), and base front induced rhamnolipid deprotonation complexing with Cr(III) and expediting Cr(VI) dissolution by electrostatic repulsion. Permeable reactive composite approaching cathode induced the maximum removal rate of Cr(VI) and Cr(III) in each section by impelling the alkaline front. Approaching cathode caused increased resistance and energy consumption in the near-anode regions, ultimately decreasing energy utilization efficiency. Optimized moving frequency and applied potential magnitude could adjust power consumption distribution in a single soil layer to obtain better electrokinetic removal performance of contaminates. This work provided essential scientific and practical importance for in-situ electrokinetic remediation of Cr(VI) and Cr(III), considering elimination efficiency and energy consumption in the future.

A detailed insight into the adsorption efficiency of Cr(VI) by a novel polypyrrole modified kendu (Diospyros melanoxylon Roxb) seed biochar

The occurrence of heavy metals in drinking water is a foremost health concern in today’s world. The use of naturally occurring biomass for the removal of toxic pollutants from water is a proficient and inexpensive treatment method. Based on a comprehensive study of the pyrolysis features, pore configuration, chemical functionality, and carbon framework, we report a cost-effective polypyrrole-modified kendu (Diospyros melanoxylon Roxb) seed-derived biochar (PPY-KSC) via oxidative polymerization. The potentiality of hexavalent chromium decontamination from an aqueous solution was assessed by applying PPY-KSC as a bio-adsorbent. Synthetic steps were designed and the reusability of biomass was investigated by taking the minimum toxicity of the environment into account. The 1:1 V/W (volume/weight) ratio of pyrrole and KSC was optimized for the production of PPY-KSC considering the effectiveness of Cr(VI) removal. All the synthesized biochar were characterized following several techniques such as TEM, SEM, FTIR, XRD, Raman, Zeta potential, BET, and TGA analysis. SEM, TEM, and BET analysis proposes that PPY-KSC exhibited a spherical polypyrrole chain with an uneven microporous structure having an elevated surface area (81.55 m2/g) than KSC. PPY-KSC showed efficient Cr(VI) adsorption (98.8%) for 30 mg adsorbent dose in 100 mL of 10 ppm of Cr(VI) solution in 70 min at room temperature. Cr(VI) adsorption was following the pseudo-second-order kinetic model besides obeying the Langmuir isotherm model with R2 = 0.99 and a Langmuir adsorption capacity of 81.96 mg/g. Furthermore, a simultaneous physio-chemical process along with microporous capture governs adsorption. The Intraparticle diffusion model demonstrates the chemical interaction between polypyrrole and chromium through the electrostatic force of attraction. The thermodynamic parameters signify the remediation of Cr(VI) was irreversible, spontaneous, feasible and endothermic. The present approach allows reusability and studies showed that PPY-KSC owes the sorption property being 81.5% efficient up to five consecutive cycles.

Characterization and application of Fe-modified biochar alleviating Cr(VI) stress in pak choi seedling cultivated in Cr-polluted hydroponics

Chromium (Cr) is one of the common environmental pollutants, which causes severe health hazards on human health and environmental security. In this study, we characterized two biochars, a raw biochar (RBC) and a Fe-modified biochar (MBC) made from poplar wood chips and determined the effect of the two biochars on remediation of hexavalent chromium (Cr(VI)) in hydroponic system by monitoring Pak choi growth. Results showed the surface area, pore number and pore volume were significantly higher in MBC than in PBC, but the pore size was larger in PBC than in MBC. When compared to the control, low concentrations of Cr(VI) (≤2 mg L−1) promoted the growth and biomass production of Pak choi by 10–78%. In contrast, the high concentrations of Cr(VI) (≥4 mg L−1) showed a significantly reduction of the growth and biomass production of Pak choi by 10–28%. Fe-modified biochar (MBC) had a more significant impact than RBC on the remediation of Cr in the Cr(VI) pollution and improved growth and biomass production of Pak choi to a greater extent. Our study indicated that MBC has a better effect on degrading Cr(VI) pollution. The findings provide scientific basis and reference for the remediation of heavy metals in aquatic ecosystems by using biochar.

Simultaneous remediation of Cr and Ni from waste water using efficient Schiff-base resin bis(2-methoxy-1-napthaldehy)-O-phenylene diimine as an adsorbent

In this study, we report the synthesis of a Schiff base, bis(2-methoxy-1-napthaldehy)-o-phenylene diimine (BMNOPhen), and a Schiff-base resin, bis-2-methoxy-1-napthaldehy-o-phenylenediimine (PMBMNOPhen). The materials were characterized via different spectroscopic techniques; CHN analysis, FT-IR, TGA, EDS, SEM, and XRD to analyze the structure and surface morphology and crystallographic evolution of the polymeric resin. The synthesized resin was used for uptake studies with Ni(II) and Cr(VI) ions. The optimum conditions were determined by different parameters used to describe the effects of uptake including initial concentration, temperature, time, and pH through batch adsorption methods. Different functional groups like azomethine, OH, and amine present in the Schiff-based resin structure contain donor N and O atoms which can coordinate cationic metal ions, supported by the active adsorption of chromium(VI) and nickel(II). The isotherm data are informative and useful for calculating and modeling adsorption capacity and exploring the mechanism of removal by the adsorbent Different types of adsorption isotherms like Langmuir, Freundlich, and kinetic studies were used on environmental water samples. The polymeric resin PMBMNOPhen was used for elimination of Cr(VI) and Ni(II) ions in real water samples from different agricultural cities of Sindh (Province of Pakistan) such as Sanghar, Tando-Adam, Kotri, and Jamshoro.

The study of removal chromium (VI) ions from aqueous solution by bimetallic ZnO/FeO nanocomposite with Siltstone: Isotherm, kinetics and reusability

In this study, nanocomposites of Baghanwala Siltstone (BSS) with ZnO (BSS/ZnO), FeO (BSS/FeO), and BSS/ZnO/FeO were successfully prepared for the removal of hexavalent chromium [(Cr (VI)] from aqueous solutions via a batch adsorption process. The characterization studies by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy-energy-dispersive X-ray spectroscopy (SEM-EDX) found successful synthesis of the composites and demonstrated the occurrence of different active functional groups that played an active role in Cr ion adsorption. The effect of Cr initial concentrations (25–100 mg/L), adsorbent dose (0.5–2 g/L), pH (2–8), and contact time (0–160 min) on Cr remediation from contaminated water was examined. The order of Cr ion removal was BSS/ZnO/FeO (77-87%) > BSS/ZnO (74-83%) > BSS/FeO (71-77%) > BSS (68-74%). The as-synthesized BSS/ZnO/FeO led to successful Cr removal (87%) at a 25 mg/L Cr concentration at pH 4.0. The Cr ion adsorption by the BSS/ZnO/FeO nanocomposite was well explained by the Langmuir adsorption isotherm model (R2 > 0.99), while the kinetic experimental data was well fitted with the pseudo-second-order model (R2 > 0.99). Among the as-synthesized adsorbents, the BSS/ZnO/FeO nanocomposite showed excellent stability and reusability in seven sorption cycles. The results showed that the as-synthesized BSS/ZnO/FeO nanocomposite had the greatest adsorption potential for removing Cr ions from contaminated water.

Thermodynamic considerations on the combined effect of electron shuttles and iron(III)-bearing clay mineral on Cr(VI) reduction by Shewanella oneidensis MR-1

Electron shuttles (ESs) and Fe-bearing clay minerals are commonly found in subsurface environments and have shown potential in enhancing the bioreduction of Cr(VI). However, the synergistic effect of ESs at different redox potentials and Fe-bearing clay minerals on Cr(VI) bioreduction, as well as the fundamental principles governing this process, remain unclear. In our study, we investigated the role of ESs and Fe(III) in Cr(VI) bioreduction. We found that the acceleration of ESs and Fe(III) are crucial factors in this process. Interestingly, the promotion of ESs on Cr(VI) and Fe(III) showed opposite trends. Electrochemical methods confirmed the limited steps are the extent of reduced ESs and the redox potential difference between ESs and Fe(III), separately. Furthermore, we investigated the combined effect of ESs and NAu-2 on Cr(VI) bioreduction. Our results revealed two segments: in the first segment, the ES (5-HNQ) and NAu-2 did not synergistically enhance Cr(VI) reduction. However, in the second segment, ESs and NAu-2 demonstrated a synergistic effect, significantly increasing Cr(VI) reduction by MR-1. These bioreduction processes all follow linear free energy relationships (LFERs). Overall, our study highlights the fundamental principles governing multivariate systems and presents a promising approach for the remediation of Cr(VI)-contaminated sites.

Cr(VI) Reduction and Fe(II) Regeneration by Penicillium oxalicum SL2-Enhanced Nanoscale Zero-Valent Iron.

Nanoscale zero-valent iron (nZVI) faces significant challenges in Cr(VI) remediation through aggregation and passivation. This study identified a Cr(VI)-resistant filamentous fungus (Penicillium oxalicum SL2) for nZVI activation and elucidated the synergistic mechanism in chromium remediation. P. oxalicum SL2 and nZVI synergistically and effectively removed Cr(VI), mainly by extracellular nonenzymatic reduction (89.1%). P. oxalicum SL2 exhibited marked iron precipitate solubilization and Fe(II) regeneration capabilities. The existence of the Fe(II)-Cr(V)-oxalate complex (HCrFeC4O9) indicated that in addition to directly reducing Cr(VI), iron ions generated by nZVI stimulated Cr(VI) reduction by organic acids secreted by P. oxalicum SL2. RNA sequencing and bioinformatics analysis revealed that P. oxalicum SL2 inhibited phosphate transport channels to suppress Cr(VI) transport, facilitated iron and siderophore transport to store Fe, activated the glyoxylate cycle to survive harsh environments, and enhanced organic acid and riboflavin secretion to reduce Cr(VI). Cr(VI) exposure also stimulated the antioxidative system, promoting catalase activity and maintaining the intracellular thiol/disulfide balance. Cr(VI)/Fe(III) reductases played crucial roles in the intracellular reduction of chromium and iron, while nZVI decreased cellular oxidative stress and alleviated Cr(VI) toxicity to P. oxalicum SL2. Overall, the P. oxalicum SL2-nZVI synergistic system is a promising approach for regenerating Fe(II) while reducing Cr(VI).

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.

Detoxification and resource utilization of soda ash chromite ore processing residue by copper slag

Chromite ore processing residue (COPR) and copper slag (CS) are industrial waste slags that cause environmental pollution and require effective treatment. This study proposes a new strategy for the co-treatment and resource utilization of COPR and CS. Under the optimal conditions (liquid-solid ratio of 9.99; H2SO4 concentration of 0.85 M; the CS/COPR mass ratio of 9.98%; temperature of 88.20 ℃; and duration of 60 min), the reduction efficiency of Cr(VI) reached 99.48%, and the total Cr leaching concentration of residue reached 1.35 mg/L, which was below the USEPA regulatory limit of 5 mg/L. In addition, the detoxicated slag containing 61.44 wt% Fe2O3 can be used as a raw material in the steel industry. Through kinetic, characterization, and DFT analyses, the effective remediation of Cr(VI) in COPR by CS is a combination of adsorption and reduction. The unreacted shrinkage nuclear reaction model under the control of the surface chemical reaction is the most suitable model to describe the process, and when the apparent activation energy is 63.90 kJ/mol, the apparent rate equation is:1-(1-x)1/3= 1·371×107[CS/COPR]2·35004 [H2SO4]2·3945 [L/S]1·95338 exp(–63·90/RT)This is an important study for waste-waste co-treatment and resource utilization of industrial waste.

Comparative Study of Chromium Phytoremediation by Two Aquatic Macrophytes.

Chromium (Cr) occurs in several oxidation states from trivalent to hexavalent. However, hexavalent forms are more toxic and mainly produced by anthropogenic activities. A hydroponic experiment was conducted to analyse the comparative remediation of Cr by Marsilea minuta and Pistia stratiotes. Plants were exposed to four concentrations of Cr (0.5, 1.0, 1.5, and 2.0 mM) for 3 days. The highest accumulation of Cr was seen at the 1.5 mM concentration after 3 days in Marsilea (11.96mg/g) and Pistia (18.78mg/g). Dry weights decreased and malondialdehyde (MDA) levels increased in response to increasing Cr concentrations. Results indicate that both macrophytes are suitable candidates for Cr phytoremediation. Antioxidant-enzyme activity as a function of metal tolerance is imperative for a coherent understanding of plant physiology under metal stress.