Total Cr Removal Research Articles

Efficient photo-assisted reduction of Cr(VI) via activated carbon mediated Z-scheme FeS2/α-FeOOH/C heterojunction: Focusing on molecular oxygen fate and synergetic reduction mechanism

Herein, pyrite (FeS2), goethite (α-FeOOH) and activated carbon (C) were combined to construct FeS2/α-FeOOH/C composites with C mediated Z-scheme heterojunction for efficient photo-assisted Cr(VI) reduction. Characterization tests indicated the strong interaction via Fe-O-C and C-S-C, the increased amount of crystal defects and surface oxygenic functional groups, and the decreased agglomeration degree during combination strengthened electron transport and visible light conversion efficiency, resulting in synergistic effect for enhanced Cr(VI) removal. Additionally, C mediated Z-scheme heterojunction with lower resistance facilitated the transfer and separation efficiency of photo-generated carriers, further accelerating Cr(VI) reduction. Consequently, almost 100 % of Cr(VI) was reduced via FeS2/α-FeOOH/C in 60 min, 4 times that of pristine FeS2. Quenching, EPR and control experiments confirmed photocatalysis made more contribution than single reductive reagents (FeS2) in Cr(VI) reduction. The specific contribution rate of reductive species like e−, Fe2+, S22− and •O2− in this complicated system was firstly detailedly demonstrated and •O2− played a predominant role under oxic condition. Meanwhile, oxidative species like H2O2, 1O2 and •OH ascribed to DO activation or photo-generated h+ preferred to reacted with Fe2+ and S22− with higher reducibility and-only delayed Cr(VI) reduction rate. Hence, the final total Cr(VI) removal efficiency and generated Cr(III) kept stable in oxic solution.

Influence of coexisting anions on the one-step electrochemical reduction and precipitation removal of Cr(VI): Implications for advanced wastewater treatment

Electroreduction of Cr(VI) coupled with in-situ precipitation of Cr(III) on the cathode is a promising method for removing Cr(VI) from wastewaters. However, the influence of coexisting anions in wastewaters on the electrochemical removal process remains unclear. This study investigated the impact of common inorganic anions, including nitrate (NO3−), chloride (Cl−), phosphate (PO43−) and sulfate (SO42−), on the electrochemical removal processes of Cr(VI). The results indicated that HCrO4− was directly electrochemically reduced to Cr3+, and the OH− generated through electro-mediated water reduction could complex with Cr3+, thereby transforming Cr3+ into chromium hydroxide (Cr(OH)3) coated at cathode. Coexisting anions would partially penetrate the alkaline Cr(III) complexes, inhibiting the formation of Cr(OH)3 passivation layer and promoting the electroreduction of Cr(VI), whose penetration ability followed the order of SO42− > PO43− > Cl− > NO3−. Both the inhibitory effect on Cr(III) precipitation and promoting effect on Cr(VI) reduction were intensified with increasing concentrations of these anions in the range of 1–100 mmol L−1. Accordingly, after electrolysis of 10 mg L−1 Cr(VI) at an initial pH of 3.0 and −0.2 V (vs. Ag/AgCl), the highest electrochemical reduction ratio of Cr(VI) (99.9%) was achieved in the presence of 100 mmol L−1 SO42−, while the total Cr removal ratio was minimal (3.3%). In contrast, the presence of NO3− at 1 mmol L−1 resulted in a nearly lowest reduction ratio of Cr(VI) (92.9%), with the maximum total Cr removal ratio (92.8%). These findings provide new insights into the electrochemical removal mechanisms of Cr(VI) in complex solution environments.

Interplay of humic acid and Cr(VI) on green microalgae: Metabolic responses and chromium enrichment

Dissolved organic matter (DOM) present in aquatic environments can significantly influence microalgal metabolism and the enrichment of heavy metals. However, the specific mechanism through which typical DOM affects the enrichment of the heavy metal chromium (Cr) in green algae remains unclear. This study investigates the impacts of varying concentrations of humic acid (HA), selected as a representative DOM in water, on the growth, metabolism, and Cr enrichment in Chlorella vulgaris, a typical green alga. The results indicated that low concentrations of HA were capable of enhancing Cr enrichment in C. vulgaris, with the highest Cr enrichment rate recorded at 41.50 % at TOC = 10 mg/L. The enrichment of Cr in algal cells primarily occurred through cell proliferation and complexation reduction of extracellular polymeric substances (EPS). In the presence of HA, C. vulgaris predominantly removed Cr through extracellular adsorption, accounting for 79.76–85.88 % of the total Cr removal. Furthermore, carboxyl complexation and hydroxyl reduction within EPS facilitated both the enrichment of Cr (18.72–21.49 %) and the reduction of Cr(VI) (63.93–74.10 %). These findings provide valuable insights into strategies for mitigating heavy metal pollution and managing associated risks in aquatic environments.

Fabrication of Active Z-Scheme Sr2MgSi2O7: Eu2+, Dy3+/COF Photocatalyst for Round-the-Clock Efficient Removal of Total Cr.

Photoreduction is recognized as a desirable treatment method for hexavalent chromium (Cr(VI)). However, it has been limited by the intermittent solar flux and limited light absorption. In this work, a novel Z-scheme photocatalyst combining a covalent organic framework (COF) with Eu2+, Dy3+ co-doped Sr2MgSi2O7 (Sr2MgSi2O7:Eu2+, Dy3+) is synthesized, which shows the high spectral conversion efficiency and works efficiently in both light irradiation and dark for Cr(VI) reduction. Sr2MgSi2O7:Eu2+, Dy3+ serves as both an electron transfer station and active sites for COF molecule activation, thus resulting in 100% photoreduction of Cr(VI) (50 mL, 10 mg/L) with high light stability and over 1 h dark activity. Moreover, the XPS and FT-IR analyses reveal the existence of functional groups (Si-OH on Sr2MgSi2O7:Eu2+, Dy3+, and -NH- on COFTP-TTA) on the composited catalyst as active sites to adsorb the resultant Cr(III) species, demonstrating a synergistic effect for total Cr removal. This work provides an alternative method for the design of a round-the-clock photocatalyst for Cr(VI) reduction, allowing a versatile solid surface activation for establishing a more energy efficient and robust photocatalysis process for Cr pollution cleaning.

Highly alkaline electrokinetic extraction: Characteristics of chromium mobilization, conversion and transport in high alkalinity soil

In site chromium (Cr) contaminated soil characterized by high alkalinity and carbonate content, protons are not effectively targeted for Cr(III) mobilization but rather accelerate the reduction of easily transportable Cr(VI) within the acidification electrokinetic (EK) system. As an alternative, the highly alkaline extraction conditions (HAECs) maintained by anolyte regulation are explored owing to the ability to desorb strong binding Cr(VI) and form anionic Cr(III)-hydroxides (Cr(OH)4−, Cr(OH)52−). The results demonstrate that HAECs were more efficient in mobilizing ions in severe alkalinity and electrical conductivity soil compared to organic acid acidifying extraction conditions (OAECs). Simultaneously, a limited amount of soluble Cr(III) was produced; however, its transportation was hindered and more noticeable in the case of Cr(VI), displaying a distinct retention phase within the intermediate soil chamber. The antagonistic interplay between electromigration and electroosmotic flow was considered the main responsible factor. The conversion intensity of Cr(VI) to Cr(III) was inhibited at HAECs. The promising mobilization and low conversion intensity contributed to total Cr removal. At HAECs, enhanced electromigration and electroosmotic flow combined with a favorable oxidation environment may facilitate in situ delivery of oxidants, offering practical implications for the EK detoxification of high alkalinity site soil contaminated with Cr. The practicability of HAECs is likely to be enhanced when the cost-benefit balance of providing a simultaneous energy supply during site treatment is resolved.

Removal of chromium using rhamnolipid biosurfactant, ferrous sulfate and cationic tannin‐based flocculant in a dissolved air floating system using a central composite design (CCD)

AbstractThis research studied the wastewater chromium removal efficiency by dissolved air flotation (DAF) using a rhamnolipid (RL) biosurfactant as a collector. An experimental flotation DAF apparatus with 6 vessels of 2 L containing a coupled saturator injecting compressed air at 5.88 kPa in the vessels was used. The total RL concentration of the broth resulting from fermentation was 9 ± 1.0 g/L. This broth was used in nature in the DAF experiments. A central composite design (CCD) was used to optimize removal of Cr(VI) and total Cr with regards to two independent variables, pH (3.17–8.83) and iron concentration of the medium (0–225.0 mg/L), with a three assays performed at the conditions of the central point of the design. The experimental conditions for DAF were an initial hexavalent Cr concentration of 100 mg/L; RL broth volume of 500 mL; saturated with oxygen water volume of 200 mL; and a rapid mixing time of 6 min through stirring at 120 rpm. The results showed that under acidic pH conditions and with high iron concentrations, both the Cr(VI) and total Cr removal rates were highest. The optimal removal region determination was at a pH of 3.5 and iron concentration of 180 mg/L. Subsequently, cationic tannin‐based flocculant was also evaluated as a collector, and ferrous sulfate was used as a coagulant during Cr(III) removal. The best Cr(III) removal percentage was obtained at cationic polymer concentrations of 300 mg/L with Cr(III) removal of 50.8% and a pH of 5.5.

Biosorption and desorption of chromium using hybrid microalgae-activated sludge treatment system

This study utilised a mixed culture of Chlorella vulgaris and bacteria from sludge to treat synthetic tannery wastewater (STWW) in modified stirred-tank photobioreactors (MSTPBRs). The MSTPBRs were fabricated locally and operated at irradiance value of 580 µmol/m2s supplied by red light-emitting diodes at 12:12 light–dark cycles and 100 ± 1 rpm continuous stirring. In each case, 50, 100 and 150 mg/L concentrations of STWW were inoculated with mixed culture of microalgae and bacteria in three MSTPBRs, with the control MSTPBR operating at 50 mg/L of STWW. Chromium concentrations were measured using colorimeter whilst Fourier transform infrared spectroscopy (FTIR) indicated possible Cr biosorption. Maximum Cr (VI) and total Cr removal efficiencies of 93 and 94% were achieved, with more than 78% total Cr recovery. Results from FTIR suggested involvement of Chlorella vulgaris in the Cr biosorption. The hybrid microalgae-bacteria system efficiently treated tannery wastewater with considerable Cr removal efficiencies. The potentials of the system in treating tannery wastewater in larger scale may require further investigation.

A mechanistic evaluation for total removal of toxic hexavalent and trivalent chromium from water by oxygen vacancy-engineered nanocomposite

Oxygen vacancy-engineered nanocomposites are emerging materials in adsorption and photocatalysis research. This study evaluated total toxic Chromium removal by a nanocomposite of core-shell zeolite A@oxygen-vacant ZnO1−X (ZA@ZnO1−X) from water using its coexisting dual characteristics. ZA@ZnO1−X has removed 98.3% of total Cr, achieving a discharge limit of 0.05 ppm of Cr(VI) after the photoreduction and adsorption simultaneously. The detailed structural and physicochemical properties of synthesized ZA@ZnO1−X nanocomposite show higher BET surface area, oxygen-vacant percentage, and visible light photoactivity than the sole ZnO1−X nanosheet. The photocatalytic reduction efficiency (PRE) effect is systematically studied on varying operational variables like pH, citric acid (CA), Cr(VI) concentration, catalyst load, presence of anions, and radical scavengers. ZA@ZnO1−X removed Cr(VI) and Cr(III) simultaneously by 98.5% PRE with a catalyst load of 4 g/l, CA= 5 mM, pH= 5.06 under halogen light irradiation (300 W, 240 V) for 50 mins. This experimental study evaluates the prominent role of oxygen vacancy of ZA@ZnO1−X for photocatalytic reduction of Cr(VI) and enhanced adsorption of Cr(III). The kinetic study reveals adsorption of both Cr(VI) and Cr(III) follows pseudo-second-order kinetics, whereas photoreduction of Cr(VI) follows pseudo-first-order kinetics. A possible mechanism of total Cr removal is sketched, supporting enhanced adsorption due to unsaturated Zn atoms and unpaired e- at the oxygen defect site, followed by photoreduction by photogenerated e- and CO2-• radical. The ZA@ZnO1−X performance is reduced by only 2.5% after five consecutive runs without deformation. The easy regeneration process makes it suitable for toxic total Cr removal, avoiding any secondary pollution.

Covalent organic framework modified vermiculite for total Cr removal and subsequent recycling for efficient ciprofloxacin and NO photooxidation

Design and fabrication of feasible remediation composites for total Cr (Cr(T)) removal is still challenging but urgently required. Herein, eco-friendly expanded vermiculite (VE) is integrated with a photoactive covalent organic framework (COF) polymer, in which photoinduced electrons of surface anchored COF can freely transfer to Cr(VI) for chemical reduction, and layered expanded VE allows ion exchange between resultant Cr(III) cations and interlayered K+, Ca2+, Mg2+, Na+, etc. The Cr(T) removal capacities of the surface-modified VE with important parameters (solution pH value, initial Cr(VI) concentration, etc.) are discussed extensively to understand how to select the best conditions for optimum Cr(T) removal performance. More interestingly, from a circular economy view point, spent Cr-loading VE-based waste can serve as a photocatalyst towards oxidation conversion of ciprofloxacin and NO gas subsequently. Explanations for different effects on physicochemical properties as well as catalytic activities of the reused Cr-loading waste are given. This strategy could provide valuable and promising contribution towards the development of sustainable low-cost mineral materials for Cr(T) removal. These findings also shed new light on the research of recycling spent photocatalyst for resource and reutilization.

Impact of different anode materials on electro-Fenton process and tannery wastewater treatment using sequential electro-Fenton and electrocoagulation

The influence of anode materials on the electrochemical treatment of tannery wastewater (TWW) was evaluated using Pt, Ti/RuO2–IrO2 (DSA), Ti/SnO2–Sb, Ti/PbO2, and Ti/SnO2–Sb/PbO2 electrodes. The comparison of the degradation mechanism of these electrodes in the electro-Fenton (EF) treatment was evaluated. The Ti/SnO2–Sb/PbO2 anode was efficient, with high electrocatalytic activity, stability, and reproducibility of the degradation results. Further, the study was extended to define the ability of sequential EF and electrocoagulation (EC) processes to clean TWW. The EC treatment was conducted using Al electrodes, and the performance of the combined treatment was evaluated by the removal of chemical oxygen demand (COD), turbidity, total suspended solids (TSS), sulfide, and Cr removal. The role of chlorides and sulfate salts during both treatments was evaluated by monitoring the concentration changes of these anions during the whole treatment using ion chromatography (IC). A sequential 1.5 h EF and 1 h EC treatment were applied to achieve a satisfactory degradation of (81.2 ± 3.9)% COD, >98% Cr, >99% turbidity, TSS, and sulfide removal. Additionally, the combined treatment was found to be more efficient towards the COD removal, achieving about 22.5% higher COD removal consuming almost the same amount of electrical energy.

Household-scale treatment units for reductive removal of hexavalent chromium from groundwater

Legacy chromium-laden hazardous industrial solid waste has been a major reason for severe groundwater pollution in low-to-middle income countries, often impacting marginalized communities who lack access to centralized water treatment solutions for their potable needs. This study aimed to develop low-cost, simple-to-adopt, and robust decentralized treatment units for total Cr removal using methods that do not generate toxic sludge post-treatment. The research involved a systematic comparison of the extents and rates of Cr (VI) reduction in groundwaters using ferrous iron (Fe (II)) for chemical coagulation (CC) with Fe (II) sulphate and for electrocoagulation (EC) with iron electrodes in batch experiments over a range of Cr (VI) (1.5–13 mg L−1) and Fe(II) (3–42 mg L−1) concentrations. While complete Cr removal was achieved by CC within 5 min, the extent of Fe(II)-Cr(VI) reaction depended on initial Fe(II) dosage. In contrast, the reaction kinetics of EC was controlled by initial Cr(VI) concentration and Fe(II) dosing rate, with the extent of reaction determined by the duration of current applied. Time-dependent Cr(VI) and Fe(II) concentrations in groundwater were modelled using rate equations and constants derived from past studies performed on simpler matrices. Subsequently, treatment units that combined either CC- or EC-based Cr(VI) reduction with sand filtration, to remove generated Fe(III)-Cr(III)-oxyhydroxide colloids, were built to obtain potable groundwater. Efficiencies of Cr(VI) removal from spiked and real-contaminated groundwaters by CC- and EC-based treatment units were ~98 % and >99 %, respectively. Although EC-based unit was more efficient than CC-based unit for groundwaters with initial Cr(VI) >4 mg L−1, it exhibited longer treatment time. The costs of treated water for EC and CC were estimated to be USD 0.05 and USD 0.13 per 100 L, respectively. Results indicated that point-of-use decentralized Cr(VI) treatment units could be a promising solution for marginalized communities.

Selective removal of total Cr from a complex water matrix by chitosan and biochar modified-FeS: Kinetics and underlying mechanisms

Cr(VI) is difficult to remove from wastewater via a one-step method because it is a type of oxyanion. Developing ARPs to selectively remove total Cr is critical for Cr(VI) remediation, including Cr(VI) adsorption-reduction and Cr(III) complexation. Hereon, chitosan and biochar modified-FeS (CTS-FeS@BC) was prepared to apply in the selective removal of total Cr from wastewaters. The results showed that the activity of amorphous FeS on CTS-FeS@BC for Cr(VI) removal (110.0 mg/g FeS) was significantly enhanced by CTS and BC, and efficiency was inhibited slightly by many anions and humic acid (HA). Meanwhile, the removal of total Cr by CTS-FeS@BC (99.1 mg/g FeS) via ARPs was improved by 1.2 and 40.3 times when compared with CTS-FeS and raw FeS, respectively. Besides, CTS-FeS@BC exhibited an outstanding selectivity for total Cr removal in metal cations-Cr binary solutions and in a complex water matrix. The mechanism of ARPs on CTS-FeS@BC demonstrated by the results of the 1,10-phenanthroline experiment and the distribution of Cr species was that Cr(VI) was first adsorbed by outer-sphere complexation for reduction, and then adsorbed Cr(III) combined with Fe(III) species to generate Fe(III)-Cr(III) complex for total Cr removal. Overall, this study provides an ARP to effectively solve Cr pollution in wastewaters.

Hydroxyl radical induced Cr flocculation via redox reaction: The extending application of heterogeneous advanced oxidation processes on Cr removal

Flocculation is a traditional and effective method to remove Cr from wastewater, but the addition of flocculants inevitably leads to secondary pollution. In this study, Cr flocculation was induced using hydroxyl radical (•OH) (•OH flocculation) generated in an electro-Fenton-like system, achieving total Cr removal of 98.68% at initial pH = 8 within 40 min. The obtained Cr flocs showed significantly higher Cr content, lower sludge yield, and good settling properties compared to alkali precipitation and polyaluminum chloride flocculation. •OH flocculation behaved like a typical flocculant, introducing electrostatic neutralization and bridging. The mechanism proposed that •OH could overcome the steric hindrance of Cr(H2O)63+ and combine with it as an additional ligand. Then Cr(III) was proved to undergo multi-step oxidation to form Cr(IV) and Cr(V). After these oxidation reactions, •OH flocculation took precedence over Cr(VI) generation. As a result, Cr(VI) didn’t accumulate in solution until •OH flocculation was completed. This work provided a clean and eco-friendly strategy for Cr flocculation instead of flocculants and extended the application of advanced oxidation processes (AOPs), which is expected to enrich existing strategies of AOPs towards Cr removal.

Precise decomplexation of Cr(III)-EDTA and in-situ Cr(III) removal with oxalated zero-valent iron

Cr(III)-organic complexes removal from wastewater is challenging to decompose the complexes without producing toxic Cr(VI). Herein we demonstrated that oxalated zero-valent iron (Ox-ZVI) could one-step decomplex 95.4% of Cr(III)-EDTA with 94.2% removal of total Cr and 60.5% removal of TOC without generating Cr(VI) at pH 6.0. Ox-ZVI with FeC2O4 shell preferred to adsorb and activate O2 to produce more reactive species than ZVI with iron oxide shell, while the generated 1O2 could selectively break the Cr-O and Cr-N bond to precisely decomplex Cr(III)-EDTA without producing Cr(VI). The released Cr(III) was in-situ precipitated on the surface of ZVI in Cr(OH)3 form. Interestingly, •OH preferred the effective degradation of the released EDTA ligand to the oxidation of the chelated Cr(III) at pH 6.0. This study offers a green and facile method for the Cr(III)-organic complexes treatment, and also sheds light on the importance of reactive species adjustment for the precise pollutant control.

High-ratio {100} plane-exposed ZnO nanosheets with dual-active centers for simultaneous photocatalytic Cr(VI) reduction and Cr(III) adsorption from water.

The development of an efficient catalyst for the simultaneous removal of Cr(VI) and Cr(III) from water is required to eliminate the risk of Cr(III) reconversion in the photocatalytic Cr(VI) reduction process. ZnO with large regions of high-energy {001} and {101} surfaces is often used to degrade various pollutants due to its high activity. However, the more readily available low-energy facets have relatively limited its applications. Here, we report a new strategy that employs a high proportion of {100} plane-exposed ZnO nanosheets for simultaneous photocatalytic Cr(VI) reduction and Cr(III) adsorption. The mechanism of Zn-O co-exposed on the {100} plane as the dual-active centers to jointly promote Cr(VI) reduction and Cr(III) adsorption was clarified at the atomic level. ZnO nanosheets with a high exposure ratio of the {100} plane achieve a total Cr removal rate of over 90% within 120min under simulated sunlight irradiation, neutral conditions, and a negligible difference in the band structure.

Production and optimisation of adsorbent materials from teawaste for heavy metal removal from aqueous solution: feasibility of hexavalent chromium removal – kinetics, thermodynamics and isotherm study

Abstract A face-centred central composite experimental design scheme was used to find experimental conditions this would result in producing magnetic bleached teawaste (MBTW) adsorbent material to have the optimum hexavalent chromium (Cr(VI)) removal capacity. The effect of treatment conditions, namely temperature (200 °C, 350 °C, 500 °C), duration of the heat treatment (2 h, 3 h, 4 h), the concentration of the iron chloride (0.02 g/g, 0.055 g/g, 0.09 g/g) added was investigated. The results obtained showed that the pyrolysis temperature and the amount of iron chloride added during treatment had significant effects on the performance of MBTW material in removal of both total chromium and Cr(VI). Increasing temperature from 200 °C to 500 °C, both Cr(VI) and total Cr removal would decrease. The material with optimum removal of Cr(VI) was used for the detailed adsorption study. The results from the detailed adsorption study indicated that the MBTW material has an excellent Langmuir removal capacity of Cr(VI) (269.9 mg/g). The Cr(VI) removal process also involved the reduction of the Cr(VI) to Cr(III). The removal process was shown to be temperature-dependent and endothermic. According to the FTIR analysis and the kinetic modelling results, chemisorption is the main mechanism responsible for the removal of the Cr(VI). The effect of background ions on the reduction of Cr(VI) was also investigated and the results show that presence of carbonate ions caused a huge reduction in removal of Cr(VI) while chloride ions had the least effect.

A novel strategy for treating chromium complex wastewater: The combination of a Fenton-like reaction and adsorption using cobalt/iron-layered double hydroxide as catalyst and adsorbent

Advanced oxidation processes (AOPs) are promising technologies to treat wastewater containing heavy metal complexes, and the main challenge of treating wastewater containing chromium (Cr) complexes by these processes is the formation of more toxic and soluble hexavalent Cr (Cr(VI)). In this study, we proposed a combination process of a Fenton-like reaction and adsorption (Fenton-like reaction/adsorption) using cobalt/iron-layered double hydroxide (Co/Fe-LDH) as catalyst and adsorbent to treat Cr complex-containing wastewater. The results showed that the Cr(VI) formed during treatment process could be adsorbed by the Co/Fe-LDH immediately after its formation. The Co/Fe-LDH dosage was the key factor affecting the total Cr removal, and the initial pH of the solution was the key factor affecting the Co leakage from the material. Under suitable treatment conditions, the total Cr concentration could be reduced to less than 1.5 mg L−1. The superoxide free radical (O2−) is the main radical species in this process. During treatment, part of Cr(III) in Cr(III)-citrate is oxidised into Cr(VI), and the corresponding organic ligand is decomposed to H2O, CO2 or some intermediate products. Then, the released Cr(VI) and undecomposed Cr(III)-citrate are removed via the adsorption of Co/Fe-LDH. This study established a feasible strategy to treat wastewater containing Cr complexes by AOPs and avoid Cr(VI) accumulation.

Application of microbial fuel cell technology to the remediation of compound heavy metal contamination in soil

Exploring the removal rules of MFC on composite heavy metal pollution is very important for the future development and field application of MFC. We constructed a three-chamber soil MFC and the results showed that with the gradual deterioration of soil heavy metal contamination from single heavy metal to metals in different oxidation states (e.g., copper (II), lead (II), and chromium (III) compounds), the internal resistance of the soil MFC increased by 2.16–2.71 times, which significantly inhibited the power production performance of the MFC. After 59 days of remediation, the migration removal efficiencies of total Cu, total Cr and total Pb from the soil under composite conditions were 36.69%, 52.35% and 19.67%, respectively. The main removal mechanisms included both electromigration and diffusion, where electromigration contributed 74.41%, 31.48% and 97.67% to the removal of total Cu, Cr and Pb, respectively. The removal of composite heavy metals was affected by adsorption-desorption competition and synergism. The competition of Pb for specific adsorption sites in soil leads to the increase of mobility of Cr and Cu, which is conducive to migration and removal. The migration of Cu and Pb ions to the cathode inhibited the diffusion of Cr to the anode; however, it drove the synergistic migration of Pb ions to the cathode. For the heavy metals migrated from the soil into the catholyte, only Cu2+ with high redox potential is reduced to copper at the cathode.

Efficient all-in-one removal of total chromium over nonconjugated polymer-inorganic ZnIn2S4 semiconductor hybrid

Chromium (Cr)-containing wastewater has caused a serious threat to the environment due to its high toxicity and mobility. The traditional Cr removal methods are generally based on an inconvenient two-step process with the first transformation of Cr(VI) to Cr(III) and the consecutive removal of Cr(III) by precipitation. Herein, we demonstrate the efficient all-in-one removal of total Cr through the simultaneous photocatalytic reduction of Cr(VI) to Cr(III) and in-situ fixation of Cr(III) over the nonconjugated polymer engineered ZnIn2S4 (P-ZIS) photocatalyst. By in-situ polyvinylpyrrolidone (PVP) modification of ZIS during the preparation process, the resulted P-ZIS can completely reduce Cr(VI) within 60 min under visible light irradiation. The kinetics of Cr(VI) reduction over P-ZIS is 2.8 times as that of pure ZIS, which is proved to be benefited from the enhanced light absorption, uplifted conduction band for strengthening reducibility, and accelerated charge carrier transfer. Moreover, as compared to ZIS, P-ZIS also exhibits significantly improved in-situ adsorption ability for Cr(III), thus resulting in efficient all-in-one elimination of total Cr within a single system. We show that this polymer engineered strategy could be a facile and versatile protocol for modulating the electronic structure and surface chemistry of the semiconductor photocatalysts towards complete, safe, and cost-efficient removal of Cr.

Covering extracellular polymeric substances to enhance the reactivity of sulfidated nanoscale zerovalent iron toward Cr(VI) removal

For effective removal and recovery of carcinogenic hexavalent chromium (Cr(VI)), we synthesized sulfidated nanoscale zerovalent iron@extracellular polymeric substances (SnZVI@EPS) with the aim of combining the respective advantages of EPS and SnZVI. EPS was introduced during the synthesis of SnZVI and successfully covered SnZVI, resulting in a larger specific surface area and better environmental friendliness of SnZVI@EPS over that of SnZVI (108.6362 vs. 10.9331 m2/g, 4.04-log vs. 6.42-log Escherichia coli reduction). SnZVI@EPS was more effective than SnZVI in removing Cr(VI) (64.90 mg/g vs. 43.76 mg/g), also under anaerobic, aerobic, wide pH, and high concentration of coexisting ions. The total Cr removal by SnZVI@EPS was higher than that of SnZVI (64.64 mg/g vs. 41.83 mg/g). The pseudo-second order model described well the adsorption process of SnZVI and SnZVI@EPS with equilibrium removal capacity of 44.21 and 66.18 mg/g for 20 mg/L Cr(VI) removal. Chemisorption and monolayer-based adsorption were dominated during Cr(VI) adsorption by SnZVI@EPS. The covered EPS provided better Cr(VI) removal and Cr(III) precipitation of SnZVI@EPS than SnZVI through the faster electron transfer, stronger solid–liquid reactions between SnZVI@EPS and Cr(VI), and additional binding sites for Cr(III)-Fe(III) coprecipitation. This work proved that environmental friendliness and efficient Cr(VI) removal and recovery of SnZVI can be improved by EPS modification.