Improved removal capacity of magnetite for Cr(VI) by electrochemical reduction

Promoted iron corrosion and subsequent hexavalent chromium removal in zero-valent iron systems by oxidant activation

Zero-valent iron (ZVI), as an effective medium, is widely used to eliminate heavy metal ions in filter tanks. However, it will react with Cr(VI) to generate Fe-Cr precipitates with low conductivity on its surface, resulting in slow iron corrosion and low Cr(VI) removal efficiency. In this study, three oxidants (KMnO4, NaClO, and Na2S2O8) were employed to promote iron corrosion in ZVI systems for enhanced Cr(VI) removal at a concentration of 5 mg/L through batch tests and column experiments. The ZVI/KMnO4, ZVI/NaClO, and ZVI/Na2S2O8 systems achieved significantly higher Cr(VI) removal rates of 31.5%, 52.8%, and 65.9% than the ZVI system (9.8%). Solid phase characterization confirmed that these improvements were attributed to promoted iron corrosion and secondary mineral formation (e.g., lepidocrocite, ferrihydrite, and magnetite) by oxidants. Those minerals offered more reaction sites for Cr(VI) reduction, adsorption, and sequestration. Cycle experiments indicated that ZVI/oxidant systems could stably remove Cr(VI). In long-term column experiment, the ZVI/NaClO column showed a much longer life-span and exhibited a 34.8 times higher Cr(VI) removal capacity than that of the ZVI column. These findings demonstrated that ZVI in combination with a reasonable amount of oxidants was a promising method for removing Cr(VI) in practical filter tanks and provided a new insight to enhance Cr(VI) removal.

Efficient removal of hexavalent chromium by nano-cerium-based adsorbent: The critical role of valence state and oxygen vacancy

Cerium-based adsorbents have been gradually used for the adsorption removal of highly toxic Cr(VI) from wastewater due to their low toxicity and wide working pH. However, the intrinsic properties of adsorbents contribute significantly to their adsorption performance, and the relationship between them needs to be clarified. Herein, series of nano-cerium based adsorbents (Ce@Cs) with different surface defects and Ce(III) content were prepared to explore their effects on the Cr(VI) adsorption capacity. Results showed that the optimal Ce@C performed well over a wide pH range of 2.0-12.0, and the calculated Cr(VI) adsorption capacity reached 302.43 mg/g at 45 ℃. Ce(III) and surface defects in cerium-based adsorbents exhibited an important influence on the Cr(VI) adsorption performance of Ce@Cs, and their contents showed a good positive correlation with the Cr adsorption capacity (R2 =0.988 and 0.827). A series of evidences confirmed that the generated Ce(III) and oxygen vacancies could provide more sufficient coordination number to promote Cr(VI) complexation with Ce@Cs and lower the impedance of Ce@Cs to improve the reduction of Cr(VI) to low-toxic Cr(III). This work provides new insights into the Cr(VI) adsorption using cerium-based adsorbents, which helps to improve their potential in the purification of Cr(VI)-containing wastewater.

Microplastics as an emerging vector of Cr(VI) in water: Correlation of aging properties and adsorption behavior

Microplastics (MPs) are emerging contaminants with growing concerns due to their potential adverse effects on the environment. However, understanding the aging properties and adsorption behavior of MPs is still limited. In this study, we investigated the correlation between the adsorption capacity, aging stages, and aging properties of polyethylene MPs using a correlation equation. Our results revealed that the trends of O/C ratio and contact angle of polyethylene MPs with aging time were fitted to be linear under xenon lamp accelerated aging conditions. Conversely, the trends of other properties such as particle size, crystallinity, and molecular weight with time were fitted to conform to the Boltzmann equation. Moreover, the aging curve data for carbonyl index and molecular weight (Mw) perfectly matched, confirming Mw play a crucial role in verifying the aging process. Additionally, the adsorption amount of polyethylene MPs increased sharply with the increase of aging ages, reaching up to 1.850 mg/g. The adsorption data fit well to the pseudo-second-order kinetics and Langmuir model, suggesting that the adsorption process is dominated by chemisorption. The low pH and low salt concentration is beneficial to the adsorption capacity of MPs onto Cr(VI). Further, a relationship equation was established to predict adsorption risk at different aging stages. These findings provide new insights into the impact of aging on pollutants transport and the fate of MPs, enabling the prediction of adsorption risk of MPs at different aging stages in water environments.

Using the inherent elements in yeast biomass to produce Ni2P/N-doped biocarbon composites for efficient hexavalent chromium reduction

The heterogeneous catalytic reduction of Cr(VI) to Cr(III) is an effective strategy for aqueous Cr(VI) contamination abatement, which requires the development of highly efficient, low-cost, and recyclable catalysts. Herein, Ni2P/N-doped biocarbon composites (Ni2P/N-BC) were fabricated through an anoxic pyrolysis process using NaCl and KCl as activators. A precursor of yeast biomass provided the essential C, N, and P elements for Ni2P/N-BC formation. When adopted for Cr(VI) reduction in the presence of oxalic acid as a reductant, the fabricated Ni2P/N-BC performed superior catalytic activity with a 100% Cr(VI) reduction efficiency within 10 min (Ni2P/N-BC-5 = 0.2 g L-1, oxalic acid = 0.4 g L-1, Cr(VI) = 20 mg L-1). Typical affecting parameters, e.g., catalyst dosage, oxalic acid loading, reaction temperature, initial solution pH, and water matrix, were investigated. Ni2P/N-BC exhibited good applicability in a broad pH range from 3.0 to 9.0 and in actual aquatic systems. Cr(VI) reduction efficiency remained 92.7% after five recycle runs. Such promising catalytic activity may originate from the well-crystallized Ni2P, N-doped biocarbon framework and high specific surface area of the materials. Preliminary reaction mechanism analysis indicated that the favorable charge state of Ni2P, fast hydrogen transfer, affinity of oxalic acid to Cr(VI), and inherent electron transfer in the biocarbon matrix contributed to effective Cr(VI) reduction. This work not only provides a facile and low-cost strategy to construct Ni2P/N-doped biocarbon nanosheet composite using environmentally benign biomass but also brings new insights for the remediation of Cr(VI) contamination.

Chromium (IV) transfer to Amusium pleuronectes by LDPE microplastics: An experimental study

An experiment was carried out to investigate the potential of virgin LDPE microplastics to transfer heavy metals. Desired shapes (fibres, fragments, and films) and sizes (< 5 mm) of virgin LDPE microplastics were immersed in a known concentration (30 µg/l) of chromium (IV). These Cr-coated microplastics were introduced into a culture tank containing edible scallops (Amusium pleuronectes). After the completion of the experiment (5 days), the sediments in the culture tank and edible tissues of A. pleuronectes were tested for the presence of Cr. In the sediments, a maximum concentration of 1.934 µg/g of Cr was accumulated at a rate of R2 = 0.979, while in the tissues, the maximum accumulation concentration was 0.733 µg/g of Cr at a rate of R2 = 0.807. Energy Dispersive X-ray Spectroscopy analysis also confirmed the presence of Cr (2.61 ± 0.44 mass % and 1.80 ± 0.30 atom%) in the tissues of A. pleuronectes, which was absent in the control tissues. The study showed that when exposed to contaminants such as heavy metals, LDPE microplastics can adhere and transfer them to biotic tissues. LDPE showed the potential to transfer adhered contaminants; however, the effects caused by these transferred contaminants on biota must be studied further. Risk assessment study showed that potential ecological risk of Cr is < 40 indicating low risk however, the combined effect of Cr and LDPE can compound its toxicity which needs to be studied further.

Research progress on the magnetite nanoparticles in the fields of water pollution control and detection

Magnetite nanoparticles (MNPs) have shown increasing application in the fields of water pollution control and detection due to their perfect combination of interfacial functionalities and physicochemical properties, such as surface interface adsorption, (synergistic) reduction, catalytic oxidation, and electrical chemistry. This review presents the research advances in the synthesis and modification methods of MNPs in recent years, systematically summarizes the performances of MNPs and their modified materials in terms of three technical systems, including single decontamination system, coupled reaction system, and electrochemical system. In addition, the progress of the key roles played by MNPs in adsorption, reduction, catalytic oxidative degradation and their coupling with zero-valent iron for the reduction of pollutants are described. Moreover, the application prospect of MNPs-based electrochemical working electrodes for detecting micro-pollutants in water were also discussed in detail. This review addresses that the construction of MNPs-based systems for water pollution control and detection should be adapted to the natures of the target pollutants in water. Finally, the following research directions of MNPs and their remaining challenges are outlooked. In general, this review will inspire MNPs researchers in different fields for effective control and detection of a variety of contaminants in water.

Chromium removal at neutral pHs via electrochemical Cr(VI) reduction and subsequent Cr(III) adsorption with MoS2 nanoflowers-modified graphite felt

The operation performance and stability of electrochemical Cr(VI) reduction are strongly restricted at neutral pHs (e.g., drinking water and groundwater) by the high Cr(VI) oxidation potentials and cathode passivation of Cr(OH)₃ precipitates. Herein, we fabricated MoS₂ nanoflowers-modified graphite felt (GF-MoS₂) to construct the electrochemical apparatus (EA) and adsorption column (AC), attempting to stable and effective Cr(VI) removal at neutral pHs via electrochemical Cr(VI) reduction and subsequent Cr(III) adsorption. In EA with a sequential oxidation-reduction process, Cr(VI)-contaminated influent (5 mg/L) at neutral pHs (6.0-8.0) was oxidized first by anode to generate large amounts of H⁺ ions via H₂O oxidation, decreasing the pH of anode-oxidized influent to ∼2.5 at 2.6 V and 1000 L/m²/h. Subsequently, the acidic anode-oxidized influent was further reduced by GF-MoS₂ cathode, promoting significantly Cr(VI) reduction via decreasing Cr(VI) oxidation potentials and alleviating Cr(III) precipitation on cathode. These results enabled the stable and effective operation of GF-MoS₂-based EA with almost Cr(VI) reduction to Cr(III). With further assembling GF-MoS₂-based AC, Cr(III) species in EA effluent were easily adsorbed or intercepted by GF-MoS₂, achieving undetectable Cr species in AC effluent. Combination techniques of GF-MoS₂-based electrochemical reduction and adsorption can be an effective approach for remediating Cr(VI)-contaminated water at neutral pHs.

Preparation of a novel metal-free polypyrrole-red phosphorus adsorbent for efficient removal of Cr(VI) from aqueous solution

The toxicity and carcinogenicity of Cr(VI) makes it a major threat to the health of animals and people. However, how to efficiently remove Cr(VI) still faces important challenges. In this study, a new metal-free polypyrrole-red phosphorus (PPy-RP) composite is successfully synthesized by in-situ oxidation polymerization for Cr(VI) removal from wastewater. The maximum adsorption capacity (q_m) of Cr(VI) on PPy-RP-1 is 513.2 mg/g when the pH value is 2, which is far superior to RP nanosheets (207.8 mg/g) and PPy (294.9 mg/g). The improved q_m can be ascribe to the good dispersion and increased specific surface area of PPy-RP adsorbent. Encouragingly, PPy-RP adsorbent still exhibits excellent stability after 7 cycles tests without a significant decline in removal efficiency, and remain above 81.4%. Based on the fittings of adsorption isotherms and kinetics, the process conforms to the pseudo-first-order kinetic model and the single-layer adsorption of the Langmuir model with an R² value of 0.98533. The adsorption process is chemical and monolayer. The experimental result demonstrates that the PPy-RP can efficient removal Cr(VI) by electrostatic attraction and complexation reaction (formation of N-Cr(VI) bond) through the PPy on the surface. The results of this study indicate that PPy-RP is a promising adsorbent to remove the Cr(IV).

Performance and mechanism of starch-based porous carbon capture of Cr(VI) from water

Cr(VI) pollution has seriously affected the survival of biological organisms and humans, so reducing the harm of Cr(VI) pollution is a significant scientific goal. Natural starch exhibits a low adsorption capacity for Cr(VI); thus, physical or chemical modification is needed to improve the adsorption and regeneration performance of starch. In this study, a novel starch-based porous carbon (SPC) was prepared to remove Cr(VI) from water by using soluble starch as a raw material. The characterization results show that the SPC shows a ratio surface area of 1325.39 m²/g. Kinetics suggest that the adsorption of Cr(VI) on SPC is dominated by chemisorption. The isotherm data demonstrated that the adsorption of Cr(VI) by SPC adhered to the Freundlich model. SPC exhibits a multimolecular layer adsorption structure, and the highest amount of adsorbed Cr(VI) in SPC was 777.89 mg/g (25 °C). Ion competition experiments show that SPC exhibits significant selectivity for Cr(VI) adsorption. In addition, the adsorption cycle experiment shows that SPC maintains a 63 % removal rate after 7 cycles. In this study, starch was transformed into high-quality adsorbent materials by hydrothermal and activation strategies, offering a new innovation for the optimization of starch-based adsorbents.

Sustainable and reagent-free cathodic precipitation for high-efficiency removal of heavy metals from soil leachate

Heavy metal pollution of soils has become a serious environmental problem. Soil washing with degradable reagents is an effective remediation technique of heavy metal pollution, and the generated leachate must be appropriately treated before discharge. However, the existing methods usually have the problems of large consumption of regents, high cost, and secondary pollution. This study proposed a reagent-free electrochemical precipitation method to remove mixed heavy metal ions extracted from soils by citrate using inert electrodes (IrO₂-Ta₂O₅/Ti anode and graphite cathode). The results showed that the low potential of cathode led to the electrodeposition of Cd; the local alkaline environment provided by electro-mediated water reduction caused the hydrolytic precipitation of Zn and Pb; and the precipitation of Fe washed out from Fe-rich soil resulted in the coprecipitation of As on cathode surface. These combined cathodic precipitation processes decreased the concentrations of toxic heavy metals by over 99.4% after 12 h of electrolysis at 26 mA cm^-2. The electrodes exhibited high stability after multiple successive cycles of reuse. The concentrations of As, Zn, Pb and Cd in the leachate decreased to below the limits of industrial wastewater discharge in each cycle, and those in soils could be reduced by 53.8%, 58.8%, 25.5%, and 70.2% at the initial concentrations of 1549, 1016, 310 and 50 mg kg^-1, respectively. The heavy metal removal rate increased with increasing current density in the range of 0-52 mA cm^-2. This work provides an efficient and sustainable method for the remediation of site soils polluted by mixed heavy metals.

Preparation of N-doped cellulose-based hydrothermal carbon using a two-step hydrothermal induction assembly method for the efficient removal of Cr(VI) from wastewater

Cr(VI) pollution is a growing problem that causes the deterioration of the environment and human health. We report the development of an effective adsorbent for the removal of Cr(VI) from wastewater. N-doped cellulose-based hydrothermal carbon (N-CHC) was prepared via a two-step hydrothermal method. The morphology and structural properties of N-CHC were investigated by various techniques. N-CHC has many O and N groups, which are suitable for Cr(VI) adsorption and reduction. Intermittent adsorption experiments showed that N-CHC had an adsorption capacity of 151.05 mg/g for Cr(VI) at pH 2, indicating excellent adsorption performance. Kinetic and thermodynamic analyses indicates that the adsorption of Cr(VI) on N-CHC follows a monolayer uniform adsorption process, which is a spontaneous endothermic process dominated by chemical interaction and limited by diffusion within particles. In a multi-ion system (Pb²⁺, Cd²⁺, Mn⁷⁺, Cl^-, and SO₄^2-), the selectivity of N-CHC toward Cr(VI) was 82.62%. In addition, N-CHC demonstrated excellent reuse performance over seven adsorption-desorption cycles; the Cr(VI) removal rate of N-CHC in 5-20 mg/L wastewater was >99.87%, confirming the potential of N-CHC for large-scale applications. CN/C-OR, pyridinic-N, and pyrrolic-N were found to play a critical role in the adsorption process. This study provides a new technology for Cr(VI) pollution control that could be utilized in large-scale production and other environmental applications.

A sustainable biosorption technique for treatment of industrial wastewater using snail shell dust (Bellamya bengalensis)

Water, an essential commodity available to mankind, is constantly under pollution threat. Industries are one of the major causative factors for its poor quality and therefore all organisms depending upon it, directly or indirectly are affected by various life-threatening problems. Thus, the treatment of discharge waste into the freshwater ecosystem is the dire need of the hour. The objective of the study is valorization of discarded snail shells for treatment of industrial wastewater. In the present study, industrial wastewater was treated using snail shell dust obtained from Bellamya bengalensis to assess change in water quality parameters. Various physico-chemical parameters like pH, total dissolved solids, electric conductivity, dissolved oxygen, biological oxygen demand, chemical oxygen demand, calcium, magnesium, total hardness, chlorides, bicarbonates, orthophosphates, sulfates, nitrates, and ammonia-N were assessed after its treatment with snail shell dust. Based on the present observation, it was concluded that all studied parameters except dissolved oxygen showed a remarkable decline in concentration after treatment with snail shell dust at the rate of 15 g per liter at the end of 4 days. Moreover, increased dissolved oxygen concentration also endorsed an enhancement in water quality. Statistical analysis through Pearson correlation and indices, viz., WQI (Water quality index) as well as Nemerow's Pollution index when applied to the present data, also supported an improvement in the water quality. The findings thus endorsed the utilization of snail shell dust as an eco-friendly technique and can be substituted as a sustainable method for the treatment of industrial wastewater.

Mn-Doped Spinel for Removing Cr(VI) from Aqueous Solutions: Adsorption Characteristics and Mechanisms

In this study, the manganese (Mn) was doped in the MnFe₂O₄ crystal by the solid-phase synthesis method. Under the optimum conditions (pH = 3), the max removal rate and adsorption quantity of Cr(VI) on MnFe₂O₄ adsorbent obtain under pH = 3 were 92.54% and 5.813 mg/g, respectively. The DFT calculation results indicated that the adsorption energy (E_ads) between HCrO₄^- and MnFe₂O₄ is -215.2 KJ/mol. The Cr(VI) is mainly adsorbed on the Mn atoms via chemical bonds in the form of HCrO₄^-. The adsorption of Mn on the MnFe₂O₄ surface belonged to chemisorption and conformed to the Pseudo-second-order equation. The mechanism investigation indicated that the Mn in MnFe₂O₄ has an excellent enhancement effect on the Cr(VI) removal process. The roles of Mn in the Cr(VI) removal process included two parts, providing adsorbing sites and being reductant. Firstly, the Cr(VI) is adsorbed onto the MnFe₂O₄ via chemisorption. The Mn in MnFe₂O₄ can form ionic bonds with the O atoms of HCrO₄^-/CrO₄^2-, thus providing the firm adsorbing sites for the Cr(VI). Subsequently, the dissolved Mn(II) can reduce Cr(VI) to Cr(III). The disproportionation of oxidized Mn(III) produced Mn(II), causing Mn(II) to continue to participate in the Cr(VI) reduction. Finally, the reduced Cr(III) is deposited on the MnFe₂O₄ surface in the form of Cr(OH)₃ colloids, which can be separated by magnetic separation.

Synthesis and characterization of hydrogel-based magnetite nanocomposite adsorbents for the potential removal of Acid Orange 10 dye and Cr(VI) ions from aqueous solution

Magnetic responsive hydrogels (CMX-cl-P4VP/M-NPs) were successfully synthesized through in situ co-precipitation procedure and investigated using various techniques. The surface morphology analysis revealed that the M-NPs were uniformly distributed within the hydrogel matrix and had average sizes ranging from 4.98 to 15.02 nm. The graft copolymer containing nanoparticles exhibited a sensitive magnetic response, and their recovery could be facilitated by applying a magnetic field. The purpose of this research is to study the ability of the prepared magnetic hydrogel to remove AO-10 dye and hexavalent chromium ions (Cr(VI)) from the aqueous solution under various factors, namely contact time, pH, amount of adsorbent, coexisting ions and AO-10 and Cr(VI) concentrations. The outcomes of the batch adsorption demonstrated that the adsorbent hydrogel incorporated with a low percentage (10 %) of M-NPs had a strong affinity for the removal of AO-10 dye and Cr(VI) ions at an optimum pH = 3, and the removal percentage reached 99.3 and 97.4 % for 500 mg L^-1 and 300 mg L^-1 of AO-10 dye and Cr(VI) ions within 90, 50 min, respectively. The data were well-fitted by pseudo-first-order kinetics. The maximum adsorption capacities of AO-10 dye and Cr(VI) ions onto adsorbent were 2448 and 574.7 mg g^-1 at 298 K, calculated from the Langmuir model.

Ultra-fast and ultra-efficient removal of Cr (VI) by the aqueous solutions of monolayer MXene (Ti3C2Tx)

Chromium (VI) removal is crucial to the safety of water resources, but there is still a lack of effective and fast dislodge methods, especially the on-site rapid remediation and emergency removal technology. Herein, the monolayer (ML) Ti₃C₂T_x nanosheets (NSs) were prepared to remove Cr (VI) from water. The solution sample of the two-dimensional (2D) Ti₃C₂T_x NSs demonstrate ultrafast adsorption kinetics (the concentration decreasing from 300 to 3 mg L^-1 in 10 min) and impressively high capacity (1483 mg g^-1), which is several times higher than that of the most reported Cr (VI) adsorption by Ti₃C₂T_x. The analysis of the adsorption kinetics and isotherm models indicates that the adsorption reaction is endothermic, with physical and chemical adsorption coexisting (mainly monolayer chemical adsorption). The joint study of the experimental analysis and theoretical calculation based on the density functional theory (DFT) demonstrates that the extraordinary removal performance of Cr (VI) is due to the synergetic effect of reduction reaction removal and electrostatic adsorption. It is found that the Cr (VI) is mainly reduced by the OH- terminals on Ti₃C₂, and the detoxified Cr (III) is anchored on the surface of Ti₃C₂T_x through complexation reaction with O terminal. The advantages of ultrafast adsorption kinetics and extra-high adsorption capacity of the single-layered Ti₃C₂T_x for Cr (VI) removal make it a remarkable candidate for rapid and urgent removal of Cr (VI) wastewater.

Comparing the influence of humic/fulvic acid and tannic acid on Cr(VI) adsorption onto polystyrene microplastics: Evidence for the formation of Cr(OH)3 colloids

Microplastics (MPs) can act as vectors for various contaminants in the aquatic environment. Although some research has investigated the adsorption characteristics and influencing factors of metals/organic molecules on MPs, the effects of dissolved organic matter (DOM) (which are ubiquitous active species in ecosystems) on metal oxyanions such as Cr(VI) capture by MPs are largely unknown. This study explored the adsorption behaviors and mechanisms of Cr(VI) oxyanions onto polystyrene (PS) MPs using batch adsorption experiments and multiple spectroscopic methods. The effects of representative DOM components (i.e., humic acid (HA), fulvic acid (FA) and tannic acid (TA)) on Cr(VI) capture by PS were particularly studied. Results revealed a significantly enhanced adsorption of Cr(VI) on PS in the presence of TA. The Cr(VI) adsorption capacity was increased from 2876 μg g^-1 to 4259 μg g^-1 and 5135 μg g^-1 when the TA concentrations raised from 0 to 10 and 20 mg L^-1, respectively. Combined microscopic and spectroscopic investigations revealed that Cr(VI) was reduced to Cr(III) by TA and formed stable Cr(OH)₃ colloids on PS surfaces. Contrarily, HA and FA inhibited Cr(VI) adsorption onto PS, especially at pH > 2.0 and higher DOM concentrations, due to site competition and electrostatic repulsion. Increase in pH was found to reduce zeta potentials of MPs, resulting in inhibited Cr(VI) adsorption. The adsorbed Cr(VI) declined with increasing ionic strength, implying that outer-sphere surface complexation affected the adsorption process in the presence of DOM. These new findings improved our fundamental understanding of the fate of Cr(VI) and MPs in DOM-rich environmental matrices.

Study on the Preparation of Nano-FeS Loaded on Fly Ash and Its Cr Removal Performance

Chromium has been considered as one of the most hazardous heavy metals because of its strong and persistent toxicity to the ecosystem and human beings. In this study, fly ash-loaded nano-FeS (nFeS-F) composites were constructed with fly ash as the carrier, and the performance and mechanism of the composites for the removal of Cr(VI) and total chromium from water were investigated. The composite was characterized by X-ray diffraction and transmission electron microscopy. The effects of fly ash size, molarity of FeSO₄, and flow rate of FeSO₄ solution on the removal of Cr(VI) and total chromium were investigated by a single factor experiment. The interaction of various factors was studied by the Box-Behnken response surface methodology. The optimum conditions of removal of Cr(VI)and total chromium by nFeS-F were determined. The results show that ① the optimal preparation conditions for nFeS-F were an FeSO₄ concentration of 0.45 mol/L, a fly ash particle size of 120-150 mesh, and a flow rate of 0.43 mL/s.② The response surface model provides reliable predictions for the removal efficiencies of Cr(VI) and total chromium.③ The removal efficiencies of Cr(VI) and total chromium were 92.87 and 83.53%, respectively, under the optimal preparation conditions by the experimental test. This study provides an effective method for the removal of Cr(VI) and total chromium.

Removal of Chromium (VI) from the Steel Mill Effluents Using the Chemically Modified Leaves of Pteris vittata as Adsorbent

Chromium (Cr), a metal that is released in appreciable amounts from the steel industry into water bodies, is not only the main causative agent of lung cancer in human but also negatively affects the metabolic activities of plants. Keeping in view the hazardous effects of Cr(VI), the present study was aimed to eliminate it from industrial effluents of steel mills installed in Dargai District Malakand, Pakistan, using chemically modified Pteris vittata plant leaves as an adsorbent. The instrumental techniques such as FTIR, surface area analysis, SEM, TGA and EDX were used to evaluate surface functionality, morphology, thermal stability and elemental composition of the modified leaves. To identify the ideal conditions for the biosorption process, batch adsorption tests were carried out under varied conditions of pH, contact time, initial metal concentration, biosorbent dose, as well as temperature. Various models, such as those of Freundlich, Jovanovich, Temkin, Langmuir, and Harkins–Jura, were utilized to explain the isothermal experimental data. The high value of R2 (0.991) was exhibited by the Langmuir model. Pseudo-first-order, power function, pseudo-second-order, intraparticle diffusion, and Natarajan–Khalaf models were employed to obtain an insight into kinetics of the process. The highest R2 value, close to unity was recorded with pseudo-second order. At pH = 2, the best elimination of Cr was observed with maximum uptake capacity qmax(66.6 mg/g) as calculated from the Langmuir isotherm. The thermodynamic analysis, which was conducted at different temperatures, showed that the nature of this sorption process was exothermic and spontaneous. The modified leaves-based biosorbent could be used as an alternative adsorbent for effective Cr elimination from water, and its use could be extended to other heavy metals and organic pollutants as well, and further experimentation are needed in this regard.

Carbon Nanotubes-Based Fuel Cell for Cr(VI) Removal and Electricity Generation

A fuel cell, an energy transducer, can convert chemical energy into electrical energy. In this work, graphite felt (GF) loaded with polypyrrole (PPy) and carboxylic carbon nanotubes (CNTs-COOH) was used as a cathode (GF/PPy/CNTs-COOH) in a double-chamber nonbiofuel cell (D-nBFC) to remove Cr(VI) efficiently. Therein, Na₂S₂O₃ in an alkaline solution and Cr(VI) in a strongly acidic solution were employed as anode and cathode solutions, respectively. An agar salt bridge, consisting of saturated KCl solution, was used to transport ions between the anode and cathode. This system suggested that the removal efficiency of Cr(VI) could reach 99.6%. The maximum current, power, and power density could achieve 136.8 μA, 18.7 μW, and 20.8 mW/m² at 90 min, respectively. Additionally, GF/PPy/CNTs-COOH also had good electrocatalytic stability and reusability after four cycles, which played an important role in the development of the D-nBFC system. Therefore, this study provides an environmentally friendly and efficient method to remove Cr(VI) and generate electricity simultaneously.

Sulfate-accelerated photochemical oxidation of arsenopyrite in acidic systems under oxic conditions: Formation and function of schwertmannite

The weathering of arsenopyrite is closely related to the generation of acid mine drainage (AMD) and arsenic (As) pollution. Solar radiation can accelerate arsenopyrite oxidation, but little is known about the further effect of SO₄^2- on the photochemical process. Here, the photooxidation of arsenopyrite was investigated in the presence of SO₄^2- in simulated AMD environments, and the effects of SO₄^2- concentration, pH and dissolved oxygen on arsenopyrite oxidation were studied as well. SO₄^2- could accelerate the photooxidation of arsenopyrite and As(III) through complexation between nascent schwertmannite and As(III). Fe(II) released from arsenopyrite was oxidized to form schwertmannite in the presence of SO₄^2-, and the photooxidation of arsenopyrite occurred through the ligand-to-metal charge-transfer process in schwertmannite-As(III) complex along with the formation of reactive oxygen species in the presence of O₂. The photooxidation rate of arsenopyrite first rose and then fell with increasing SO₄^2- concentration. In the pH range of 2.0-4.0, the photooxidation rate of arsenopyrite progressively increased in the presence of SO₄^2-. This study reveals how SO₄^2- promotes the photooxidation of arsenopyrite and As release in the AMD environment, and improves the understanding of the transformation and migration of As in mining areas.

The inhibiting effects of organic acids on arsenic immobilization by ferrihydrite: Gallic acid as an example

Organic acids usually compete the immobilization of As by iron (hydro)oxides, but their oxidizing effects are ignored. Therefore, the gallic acid (GA) with strong redox activity was chosen to investigate the influence of arsenite [As(III)] oxidation on As immobilization by ferrihydrite. Our results found that the As amount adsorbed on ferrihydrite decreased with the pH rising from 5 to 9 in the presence of GA, and the adsorption amount (28.8 g kg^-1) at pH 9 was 45.1% lower than that in the absence of GA. Meanwhile, the As adsorption amounts in treatments of GA addition before As (Fh-GA-As(III)) were significantly lower than that in their corresponding simultaneous addition (Fh-As(III)/GA). The proportions of As(V)/As_total on ferrihydrite and in equilibrium suspension were increased as the pH increased in the presence of GA, and the highest oxidation efficiency of As(III) by GA at pH 9 was 90.3%, which was mainly due to the contribution of hydrogen peroxide (H₂O₂, 52.6%) and semiquinone radicals (SQ^-, 27.1%). In general, the oxidation and competition adsorption of As by GA inhibited the As immobilization by ferrihydrite, and the oxidation of As(III) by GA was strongly dependent on pH, while H₂O₂ and SQ^- were demonstrated as the main oxidant at pH 9.

Gas Hydrate-Based Heavy Metal Ion Removal from Industrial Wastewater: A Review

Innovating methods for treating industrial wastewater containing heavy metals frequently incorporate toxicity-reduction technologies to keep up with regulatory requirements. This article reviews the latest advances, benefits, opportunities and drawbacks of several heavy metal removal treatment systems for industrial wastewater in detail. The conventional physicochemical techniques used in heavy metal removal processes with their advantages and limitations are evaluated. A particular focus is given to innovative gas hydrate-based separation of heavy metals from industrial effluent with their comparison, advantages and limitations in the direction of commercialization as well as prospective remedies. Clathrate hydrate-based removal is a potential technology for the treatment of metal-contaminated wastewater. In this work, a complete assessment of the literature is addressed based on removal efficiency, enrichment factor and water recovery, utilizing the gas hydrate approach. It is shown that gas hydrate-based treatment technology may be the way of the future for water management purposes, as the industrial treated water may be utilized for process industries, watering, irrigation and be safe to drink.

Amyloid-Templated Palladium Nanoparticles for Water Purification by Electroreduction

Electrocatalysis offers great promise for water purification but is limited by low active area and high uncontrollability of electrocatalysts. To overcome these constraints, we propose hybrid bulk electrodes by synthesizing and binding a Pd nanocatalyst (nano‐Pd) to the electrodes via amyloid fibrils (AFs). The AFs template is effective for controlling the nucleation, growth, and assembly of nano‐Pd on the electrode. In addition, the three‐dimensional hierarchically porous nanostructure of AFs is beneficial for loading high‐density nano‐Pd with a large active area. The novel hybrid cathodes exhibit superior electroreduction performance for the detoxification of hexavalent chromium (Cr⁶⁺), 4‐chlorophenol, and trichloroacetic acid in wastewater and drinking water. This study provides a proof‐of‐concept design of an AFs‐templated nano‐Pd‐based hybrid electrode, which constitutes a paradigm shift in electrocatalytic water purification, and broadens the horizon of its potential engineered applications.

Rapid and efficient reduction of chromate by novel Pd/Fe@biomass derived from Enterococcus faecalis

Efficient reduction of chromate is highly desirable for its detoxification and remediation of the contaminated environment. This study described a fusion of the concepts of precious metal biorecovery and fabrication of Pd/Fe@biomass derived from simulated wastewater. The effectiveness of Pd/Fe@biomass during reduction process of Cr(VI) was evaluated by comparing with pure nZVI, E. faecalis and Pd@biomass. Results showed that Pd(II) could be recovered by E. faecalis with Fe(II) as the electron donor, and precipitation could yield nZVI anchored onto Pd-loaded E. faecalis. The nano particles (NPs) on Pd/Fe@biomass were well-dispersed, which provided 2.70 folds specific surface area comparing with nZVI. Efficient Cr(VI) reduction could be achieved at a higher catalyst dosage, the most appropriated Pd/Fe molar ratio of 2% and a wide pH range. Typically, 0.5 mM Cr(VI) could be completely reduced in 5 min driven by Pd/Fe@biomass under the conditions of dosage of 1.0 g/L and pH 3. Moreover, the mechanisms of Cr(VI) reduction by Pd/Fe@biomass were proposed, which intimately related to nZVI electron donating capacities, Pd catalysis for hydrogenation and galvanic cell effects between Fe and Pd. Therefore, Pd/Fe@biomass could be an alternative for rapid and complete reduction of Cr(VI).

Enhanced desorption of cationic and anionic metals/metalloids from co-contaminated soil by tetrapolyphosphate washing and followed by ferrous sulfide treatment

In this study, a novel approach was employed for the remediation of cationic and anionic metals/metalloids co-contaminated soil by tetrapolyphosphate enhanced soil washing coupled with ferrous sulfide treatment. Tetrapolyphosphate could simultaneously enhance the desorption of cationic metals (Pb and Zn) and anionic metal/metalloid (Cr and As) from the contaminated soil in the whole tested pH range of 2-10. With addition of 0.15 mol/L tetrapolyphosphate at pH 7.0, the removal ratio of Pb, Zn, As and Cr could achieve 83.1%, 70.4%, 75.7% and 66.4% respectively. The fractionation analysis of heavy metals/metalloids demonstrated the release of exchangeable and Fe/Mn bound forms contributed to most desorption of Pb and Zn. The decreases of non-specifically sorbed form and amorphous and poorly-crystalline hydrous oxides of Fe and Al bound form were responsible for most removal of As. The comparison with other common washing agents (EDTA, oxalate and phosphate) under their respective optimal dosage could confirm that tetrapolyphosphate was superior to simultaneously desorb the cationic and anionic metals/metalloids with higher efficiency. After 12 h, applying 150 mg/L FeS at pH 3.5 could totally remove Pb, Zn, As and Cr from the washing effluent by sulfide precipitation, reduction and adsorption processes. Higher pH would inhibit the removal of As and Cr by FeS. Meanwhile, the residual of tetrapolyphosphate could be totally recovered from the washing effluent by employing anion exchange resin. This study suggests tetrapolyphosphate enhanced soil washing coupled with ferrous sulfide treatment is a promising approach for remediation of cationic and anionic metals/metalloids co-contaminated soil in view of its high efficiency and simple operation.

Selective adsorption and separation of Cr(VI) by surface-imprinted microsphere based on thiosemicarbazide-functionalized sodium alginate

Cr(VI)-imprinting thiosemicarbazide-functionalized sodium alginate (IIPTSC@SA) microspheres were fabricated to achieve the good selective adsorption for Cr(VI) removal from aqueous solution containing multiple ions. The selective adsorption and reusability of IIPTSC@SA microsphere were intensively investigated. The results showed that the grafting conditions of thiosemicarbazide were optimized at 0.2 g of TSC at 60 °C for 5 h and the ion-imprinting conditions at 100 mL of 2 wt % glutaraldehyde at 60 °C for 6 h. IIPTSC@SA microspheres exhibited the maximum adsorption capacity of 252.5 mg/g and good selectivity to remove Cr(VI) from multi-ion coexisting water. The adsorption process complied with a monolayer adsorption by virtue of chemical interactions together with endothermic spontaneously. After eight adsorption-desorption, the adsorption capacity and selectivity of IIPTSC@SA microspheres remained relatively stable.

Indium sulfide deposited MIL-53(Fe) microrods: Efficient visible-light-driven photocatalytic reduction of hexavalent chromium

The ecosystems and human health were seriously threatened by hexavalent chromium (Cr(VI)) in wastewater. In this article, using the idea of the highly matched energy band structure between indium sulfide (In₂S₃) and MIL-53(Fe), a Type-II heterojunction has been constructed by loading In₂S₃ on MIL-53(Fe) microrod to overcome the fault like high recombination rates of photogenerated electron-holes of In₂S₃. The composite with 20:1 mass ratio of In₂S₃ to MIL-53(Fe) (IM-2) was adopted as an optimal sample for efficient photocatalytic Cr(VI) reduction under visible light. Various characterization techniques were used to verify the characteristics of composites and delved into the structure-effect relationship between this heterojunction and its activity. Results showed that the reaction rate constants of the photoreduction process over IM-2 was ~ 4 and 26 times higher than those of pure In₂S₃ and MIL-53(Fe), respectively, and the catalyst could maintain superior removal efficiency (88.6%) and steady crystal structure after four cycles. First-principles calculations further illustrated that the heterostructure formed between In₂S₃ and MIL-53(Fe) could effectively accelerate the separation of photogenerated electrons and holes, thus improving the photocatalytic reduction performance. Moreover, the active species analyses revealed that the superoxide radicals and electrons were mainly involved in the reduction of Cr(VI).

Four Keggin-type polyoxometalate-based complexes derived from bis(pyrazine)–bis(amide) ligands for electrochemical sensing of multiple analytes and adsorbing dye molecules

Four new Keggin-based complexes derived from bis(pyrazine)–bis(amide) ligands are used to detect multiple analytes (BrO3− NO2−, Cr(vi) and Fe(iii) ions) and adsorb organic dye molecules from aqueous solution.

Enhancing Cr(VI) bio-reduction by conductive materials and enrichment of functional microbes under anaerobic conditions

A few studies reported the impact of mineral conductivity properties on contaminant-mineral-microbe interactions and microbial community structure changes in the interaction process. To fill the gap, conductive minerals (magnetite/hematite) and an insulative mineral (quartz) were introduced into Cr(VI) reduction systems to investigate the effect of mineral conductivity properties on Cr(VI) removal. Results showed that conductive minerals enhanced Cr(VI) reduction rate as compared to insulative minerals. Higher reduction percentage (>86%) was observed when both ERB (extracellular respiratory bacteria) and conductive minerals were presence than those with only minerals (<10%) or ERB (<55%), indicating a synergistic effect existed in this bio-remediation system. Moreover, surface elements detection manifested higher Fe-containing groups and Fe(III)-Cr(III) complexes covered on conductive minerals surface when ERB was present. Electrochemical data suggested that ERB facilitated the activity of electron transference on the surface of conductive minerals. Our results indicated that conductive minerals did act as an "electron shuttle" while insulative minerals increased adsorption sites to accelerate Cr(VI) reduction. 16S rRNA sequences results demonstrated that conductive minerals changed the microbial community structure and increased the diversity of the functional microbes including Pseudomonas spp. and Exiguobacterium spp. This work is of deep significance for better understanding the process of elements biogeochemical and elimination of pollutants.

Electrosorption of cadmium and arsenic from wastewaters using nitrogen-doped biochar: Mechanism and application

Biochar has been widely applied as an adsorbent, whose electrochemical capacity and heavy metal adsorption performance can be improved by nitrogen doping. In this work, nitrogen-doped biochar (NBC) was synthesized by calcinating sodium humate with sodium bicarbonate (NaHCO₃) and urea as the activation agent and nitrogen source, respectively. The NBC was then used to electrochemically adsorb Cd(II) and As(III,V) from simulated and actual wastewaters, respectively. The results indicated that NaHCO₃ activation and nitrogen doping could increase the surface area and nitrogen content of the biochar, contributing to the enhancement of adsorption performance for Cd(II) and As(III,V). The electrosorption capacities for Cd(II) and total arsenic (As(T)) increased first and then reached equilibrium with increasing nitrogen content, increased first and then decreased with increasing calcination temperature, and consistently increased with increasing voltage. The Cd(II) electrosorption capacity (79.0 mg g^-1) and As(T) removal ratio (94.0%) at 1.2 V in actual As-contaminated wastewater (1.16 mg L^-1) were about 4 and 2.6 folds of their inorganic adsorption capacities, respectively. After five cycles of reuse, the Cd(II) and As(T) removal ratio could be maintained at 65.8% and 51.7% of the initial electrosorption capacity. This work expands the application of NBC for heavy metal removal.

Biogenic Synthesis of Iron Oxide Nanoparticles Using Enterococcus faecalis: Adsorption of Hexavalent Chromium from Aqueous Solution and In Vitro Cytotoxicity Analysis

The biological synthesis of nanoparticles is emerging as a potential method for nanoparticle synthesis due to its non-toxicity and simplicity. In the present study, a bacterium resistant to heavy metals was isolated from a metal-contaminated site and we aimed to report the synthesis of Fe3O4 nanoparticles via co-precipitation using bacterial exopolysaccharides (EPS) derived from Enterococcus faecalis_RMSN6 strains. A three-variable Box-Behnken design was used for determining the optimal conditions of the Fe3O4 NPs synthesis process. The synthesized Fe3O4 NPs were thoroughly characterized through multiple analytical techniques such as XRD, UV-Visible spectroscopy, FTIR spectroscopy and finally SEM analysis to understand the surface morphology. Fe3O4 NPs were then probed for the Cr(VI) ion adsorption studies. The important parameters such as optimization of initial concentration of Cr(VI) ions, effects of contact time, pH of the solution and contact time on quantity of Cr(VI) adsorbed were studied in detail. The maximum adsorption capacity of the nanoparticles was found to be 98.03 mg/g. The nanoparticles could retain up to 73% of their efficiency of chromium removal for up to 5 cycles. Additionally, prepared Fe3O4 NPs in the concentration were subjected to cytotoxicity studies using an MTT assay. The investigations using Fe3O4 NPs displayed a substantial dose-dependent effect on the A594 cells. The research elucidates that the Fe3O4 NPs synthesized from EPS of E. faecalis_RMSN6 can be used for the removal of heavy metal contaminants from wastewater.

Hexavalent chromium inhibits the formation of neutrophil extracellular traps and promotes the apoptosis of neutrophils via AMPK signaling pathway

As the most common heavy metal pollutant, hexavalent chromium [Cr(VI)] has caused serious environmental pollution and health damage. Although the toxic effect of Cr(VI) has been widely studied, and oxidative stress has been confirmed to be the main mechanism of its cytotoxicity, the toxicity of Cr(VI) to human immune system remains to be elucidated. Neutrophil extracellular traps (NETs) participate in the innate immune response, and the release of NETs is considered to be the most important part of the extracellular killing mechanism. We demonstrated in this study that Cr(VI) inhibited the formation of NETs in rat peripheral blood and induced neutrophils apoptosis by inhibiting the AMP-activated protein kinase (AMPK) signaling pathway. Cr(VI)-induced inhibition of NETs was accompanied by down-regulated myeloperoxidase (MPO)/Histones-3 (H3) protein expressions and decreased NETs-associated intracellular and extracellular DNA levels in the neutrophils. Metformin (Met), as an AMPK activator, triggered autophagy and thus alleviated the inhibitory effect of Cr(VI) on NETs. At the same time, Met can reduce the intracellular reactive oxygen species (ROS) level by activating the AMPK/nuclear factor erythroid-2 related factor 2 (Nrf2) signaling pathway, thus alleviating Cr(VI)-induced neutrophils apoptosis. In conclusion, this study elucidated the mechanism of Cr(VI)-induced neutrophils toxicity and the role of AMPK as a key regulatory signal, which could provide valuable experimental basis for the prevention and treatment of related diseases in Cr(VI)-exposed populations.

Mn-incorporated ferrihydrite for Cr(VI) immobilization: Adsorption behavior and the fate of Cr(VI) during aging

Chromium(VI) (Cr(VI)) is an environmental priority pollutant, and its mobility in natural environment is strongly controlled by ferrihydrite. Ferrihydrite always contains various ions, which may change the properties of ferrihydrite, thereby affecting the behavior of pollutants. This study aims to investigate the adsorption of Cr(VI) by Mn-incorporated ferrihydrite and the mobility behavior of Cr(VI) during aging. Results showed that the incorporation of Mn enhanced the adsorption of Cr(VI) on ferrihydrite, and the adsorption performance increased with the increase of Mn content. The maximum adsorption capacity for Cr(VI) reached to 48.5 mg/g with molar ratio of Mn/Fe 5%, while it was 36.1 mg/g for pure ferrihydrite. After aging for 7 days, ferrihydrite transformed into goethite and hematite. The adsorbed Cr(VI) on the surface of ferrihydrite was released into the solution during aging. The incorporation of Mn retarded the transformation of ferrihydrite, which inhibited the migration of adsorbed Cr(VI). Nevertheless, the incorporation of Mn resulted in the transformation of adsorbed Cr(VI) to non-desorbed Cr(VI), thereby enhancing the retention of Cr(VI). Our results suggest that the incorporation of Mn into ferrihydrite has an important role on the mobility of Cr(VI), which enhances our understanding of the behavior of Cr(VI) in the environment.

Electrochemical recovery and high value-added reutilization of heavy metal ions from wastewater: Recent advances and future trends

Wastewater treatment for heavy metals is currently transitioning from pollution remediation towards resource recovery. As a controllable and environment-friendly method, electrochemical technologies have recently gained significant attention. However, there is a lack of systematic and goal oriented summarize of electrochemical metal recovery techniques, which has inhibited the optimized application of these methods. This review aims at recent advances in electrochemical metal recovery techniques, by comparing different electrochemical recovery methods, attempts to target recycling heavy metal resources with minimize energy consumption, boost recovery efficiency and realize the commercial application. In this review, different electrochemical recovery methods (including E-adsorption recovery, E-oxidation recovery, E-reduction recovery, and E-precipitation recovery) for recovering heavy metals are introduced, followed an analysis of their corresponding mechanisms, influencing factors, and recovery efficiencies. In addition, the mass transfer efficiency can be promoted further through optimizing electrodes and reactors, and multiple technologies (photo-electrochemical and sono-electrochemical) could to be used synergistically improve recovery efficiencies. Finally, the most promising directions for electrochemical recovery of heavy metals are discussed along with the challenges and future opportunities of electrochemical technology in recycling heavy metals from wastewater.

The potential of microplastics as adsorbents of sodium dodecyl benzene sulfonate and chromium in an aqueous environment

Considering the omnipresence of microplastics (MPs) in aquatic environments, they are expected to exert significatn impacts as carriers for diverse waterborne pollutants. In this work, the adsorptive behavior of two ionic components (i.e., sodium dodecyl benzene sulfonate (SDBS) and Cr(VI)) has been explored against the two types of MPs as model adsorbents, namely poly (ethylene terephthalate) (PET) and polystyrene (PS). The influence of key variables (e.g., pH, particle size, and dose of the MPs) on their adsorption behavior is evaluated from various respects. The maximum adsorption capacity values of SDBS on PET and PS are estimated to be 4.80 and 4.65 mg⋅g^-1, respectively, while those of Cr(VI) ions are significantly lower at 0.080 and 0.072 mg⋅g^-1, respectively, The adsorptive equilibrium of SDBS is best described in relation to pH and MP size by a Freundlich isotherm. In contrast, the adsorption behavior of Cr(VI) is best accounted for by a Langmuir isotherm to indicate its adsorption across at least two active surface sites.

Electrochemical reduction of Cr(VI) in the presence of sodium alginate and its application in water purification

Chromium (Cr) is used in many manufacturing processes, and its release into natural waters is a major environmental problem today. Low concentrations of Cr(VI) are toxic to human health and living organisms due to the carcinogenic and mutagenic nature of this mineral. This work examined the conversion of Cr(VI) to Cr(III) via electrochemical reduction using gold electrode in an acidic sodium alginate (SA) solution and subsequent removal of the produced Cr(III)-SA by the polymer-enhanced ultrafiltration (PEUF) technique. A solution of SA in nitric acid was used both as an electrolytic medium during the voltammetric measurements and bulk electrolysis and as an extracting agent during the PEUF technique. The electroanalysis of Cr(VI) was performed by linear sweep voltammetry in the presence of acidic SA solution to study its voltammetric behavior as a function of the Cr(VI) concentration, pH, presence of Cr(III), SA concentration and scan rate. In addition, the quantitative reduction of Cr(VI) to Cr(III) was studied through the bulk electrolysis technique. The results showed efficient reduction with well-defined peaks at approximately 0.3 V vs. Ag/AgCl, using a gold working electrode. As the pH increased, the reduction signal strongly decreased until its disappearance. The optimum SA concentration was 10 mmol/L, and it was observed that the presence of Cr(III) did not interfere in the Cr(VI) electroanalysis. Through the quantitative reduction by bulk electrolysis in the presence of acidic SA solution, it was possible to reduce all Cr(VI) to Cr(III) followed by its removal via PEUF.

Graphene-modified graphite paper cathode for the efficient bioelectrochemical removal of chromium

Metal-free electrocatalysts have been widely used as cathodes for the reduction of hexavalent chromium [Cr(VI)] in microbial fuel cells (MFCs). The electrocatalytic activity of such system needs to be increased due to the low anodic potential provided by bacteria. In this study, graphite paper (GP) was treated by liquid nitrogen to form three-dimensional graphite foam (3DGF), improving the Cr(VI) reduction by 17% and the total Cr removal by 81% at 30 h in MFCs. X-ray absorption spectroscopy confirmed the Cr(VI) reduction product as Cr(OH)₃. Through the spectroscopy characterizations, electrochemical measurements, and density functional theory calculations, the porous structures, edges, and O-doped defects on the 3DGF surface resulted in a higher electroconducting rate and a lower mass transfer rate, which provide more active sites for the Cr(VI) reduction. Additionally, the scrolled graphene-like carbon nanosheets and porous structures on the 3DGF surface might limit the OH^- diffusion and result in a high local pH, which accelerated the Cr(OH)₃ formation. The results of this study are expected to provide a simple method to manipulate the carbon materials and insights into mechanisms of Cr(VI) reduction in MFCs by the 3DGF with in situ exfoliated edges and O-functionalized graphene.

Insight into efficient removal of Cr(VI) by magnetite immobilized with Lysinibacillus sp. JLT12: Mechanism and performance

In this work, Lysinibacillus sp. JLT12 was used to remove the Cr(VI)-induced passive layer on the magnetite. Mechanism study via dynamic kinetics, X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy analyses revealed that Lysinibacillus sp. JLT12 could remove the passive layer (lepidocrocite and goethite) to facilitate the further Cr(VI) reduction by magnetite. For large-scale applications, porous ceramsite (PC) was prepared with magnetite, kaolin, and fallen leaves. Lysinibacillus sp. was then immobilized on the holes in PC. Slow-released nutrients were added to immobilized porous ceramsite (IM-PC) at a ratio of 1.5:10 (g/g) to supply carbon, nitrogen, and phosphorus to Lysinibacillus sp. JLT12 with low secondary pollution. The performance of IM-PC was evaluated via a column experiment. The results indicate that, in the presence of Lysinibacillus, the break-through time and maximum adsorption ability of IM-PC were 11.67 h and 121.47 mg/g, respectively. These values are higher than those of PC. Additionally, break-through curves detected at 5, 10, and 15 days demonstrated that the usage life of IM-PC was significantly longer than that of PC.

Polyethylene glycol-stabilized bimetallic nickel–zero valent iron nanoparticles for efficient removal of Cr(vi)

In order to solve the agglomeration of nanoscale zero-valent iron (nZVI) and improve its performance in pollutant treatment, polyethylene glycol-stabilized nickel modified nZVI (Ni/Fe–PEG) was synthesized by a liquid-phase reduction method and used to treat Cr(vi) solution for the first time.

Hazardous wastes treatment technologies

A review of the literature published in 2019 on topics related to hazardous waste management in water, soils, sediments, and air. The review covered treatment technologies applying physical, chemical, and biological principles for the remediation of contaminated water, soils, sediments, and air. PRACTICAL POINTS: This report provides a review of technologies for the management of waters, wastewaters, air, sediments, and soils contaminated by various hazardous chemicals including inorganic (e.g., oxyanions, salts, and heavy metals), organic (e.g., halogenated, pharmaceuticals and personal care products, pesticides, and persistent organic chemicals) in three scientific areas of physical, chemical, and biological methods. Physical methods for the management of hazardous wastes including general adsorption, sand filtration, coagulation/flocculation, electrodialysis, electrokinetics, electro-sorption ( capacitive deionization, CDI), membrane (RO, NF, MF), photocatalysis, photoelectrochemical oxidation, sonochemical, non-thermal plasma, supercritical fluid, electrochemical oxidation, and electrochemical reduction processes were reviewed. Chemical methods including ozone-based, hydrogen peroxide-based, potassium permanganate processes, and Fenton and Fenton-like process were reviewed. Biological methods such as aerobic, anoxic, anaerobic, bioreactors, constructed wetlands, soil bioremediation and biofilter processes for the management of hazardous wastes, in mode of consortium and pure culture were reviewed. Case histories were reviewed in four areas including contaminated sediments, contaminated soils, mixed industrial solid wastes and radioactive wastes.