Aqueous Hexavalent Chromium Research Articles

Efficient co-removal of aqueous Cr(VI) and ciprofloxacin by alkali lignin-derived carbon supported nanoscale zero-valent iron via adsorption and redox synergistic mechanisms

Developing an efficient co-removal strategy is crucial for the treatment of combined contaminants with heavy metals and antibiotics due to their great threat to sustainable advancement and ecological preservation. Herein, the efficient co-removal of aqueous hexavalent chromium (Cr(VI)) and ciprofloxacin (CIP) is achieved via a newly developed composite of nanoscale zero-valent iron particles embedded into alkali lignin-derived carbon (nZVI/ALC) without external assistance (such as light, advanced oxidants). nZVl/ALC demonstrates superior co-removal efficiencies of Cr(VI) (99.9 %) and CIP (89.9 %) with high removal kinetic rate constants of 1.595 and 0.779 min−1, respectively. The underlying mechanisms involving the co-removal of Cr(VI) and CIP verify that Cr(VI) ions are adsorbed onto the skeleton of nZVI/ALC originating from the electrostatic interaction, and CIP molecules adsorbed on nZVl/ALC act as a bridge, complexing with the Cr(VI) ions to promote Cr(VI) removal. Meanwhile, the CIP adsorption by nZVI/ALC involves hydrogen bonding, π-π interaction, and complexation. The coexisting Cr(VI) ions are transformed into Cr(III) components with abundant electron transfer, leading to the generation of more •O2– radicals, thus the self-generating reactive oxygen species (•OH, •O2– and 1O2) in ambient-air condition promote CIP degradation in the binary system. The CIP molecules’ degradation pathways mainly include piperazine ring cracking, piperazine epoxidation, quinolone ring opening and defluorination according to the analysis of HPLC-MS and FuKui function, along with the gradual reduction in toxicity throughout the degradation process of CIP molecules. This study offers a new insight on the co-removal of Cr(VI) and CIP, which would provide promising guidance for the remediation of compound wastewater with heavy metals and antibiotics.

Green-synthesized, biochar-supported nZVI from mango kernel residue for aqueous hexavalent chromium removal: Performance, mechanism and regeneration

A biochar-supported green nZVI (G-nZVI@MKB) composite was synthesized using mango kernel waste with “dual identity” as reductant and biomass of biochar. The G-nZVI@MKB with a Fe/C mass ratio of 2.0 (G-nZVI@MKB2) was determined as the most favorable composite for hexavalent chromium (Cr(VI)) removal. Distinct influencing parameters were discussed, and 99.0% of Cr(VI) removal occurred within 360 min under these optimized parameters. Pseudo-second order kinetic model and intra particle diffusion model well depicted Cr(VI) removal process. The XRD, FTIR, SEM and XPS analyses verified the key roles of G-nZVI and functional groups, as well as the primary removal mechanisms involving electrostatic attraction, reduction, and complexation. G-nZVI@MKB2 exhibited a good stability and reusability with only a 16.4% decline in Cr(VI) removal after five cycles. This study offered evidence that mango kernel could be recycled as a beneficial resource to synthesize green nZVI-loaded biochar composite for efficient Cr(VI) elimination from water.

Pseudocapacitive deionization of high concentrations of hexavalent chromium using NiFe-layered double hydroxide/polypyrrole asymmetric electrode

The wastewater polluted by high concentrations of aqueous hexavalent chromium (Cr(VI)) has serious adverse effects on ecological environment and human health. As a burgeoning, environmental-friendly, and promising wastewater treatment technology, capacitive deionization (CDI) process for efficient removal of aqueous Cr(VI) needs to be investigated. In this work, pseudocapacitive NiFe-layered double hydroxide/ polypyrrole (NiFe-LDH/PPy, denoted as NiFe/PPy) electrode was synthesized by the in-situ polymerization of PPy on the NiFe-LDH surface. Results of characterizations manifested that NiFe/PPy had the chain spherical structure of PPy. In particular, excellent PPy provides effective reversible capacitance and ion diffusion and improves the conductivity and pseudocapacitance performance of NiFe-LDH. Then, the NiFe/PPy anode and activated carbon (AC) cathode were assembled into the CDI cell for capacitive removal of high concentrations of aqueous Cr(VI). In 20 mL/min of flow velocity, 1.2 V, and 100 mL 100 mg/L of Cr(VI) solution, the Cr(VI) electrosorption capacity and removal ratio attained to 47.95 mg/g and 95.89%, respectively. The electrosorption of Cr(VI) slightly decreased under the common competing anions of Cl−, NO3−, SO42−, HCO3−, and CO32−. Based on Langmuir isotherm model, the maximum theoretical removal capacity of NiFe/PPy for Cr(VI) was 111 mg/g. The electrostatic attraction, reduction, and surface complexation played primary roles in the CDI system of NiFe/PPy//AC asymmetric electrodes. The outstanding electrosorption performance makes this CDI system have a favorable application prospect in the actual wastewater treatment containing high concentrations of Cr(VI).

Molecular-level investigation on removal mechanisms of aqueous hexavalent chromium by pine needle biochar

Pine needle biochar produced via oxygen-limited pyrolysis at 800 °C for 2 h was used for removing aqueous hexavalent chromium [Cr(VI)] from water. The optimality of the pseudo-second order kinetic model indicated the chemisorption dominated the removal process. Strong acid environment (pH = 2.0) was more conductive to Cr(VI) removal. The functional groups (C–O, O–C–O, C = O) on biochar contributed to Cr(VI) elimination from aqueous solution. The governing removal mechanisms involved electrostatic attraction, reduction, and complexation. Especially, confocal micro-X-ray fluorescence (μ-XRF) mapping indicated that Cr ions and functional groups were primarily distributed on the surface of biochar. This study firstly provided a deep interpretation of Cr speciation and spatial distribution on pine needle biochar, which provided important information on removal mechanisms of Cr(VI) by pine needle biochar.

Carbon cloth@SnS2 nanosheet array catalysts for efficient microwave induced catalytic reduction of Cr(VI): Experiment and DFT study

Aqueous hexavalent chromium (Cr(VI)) is an environmental safety problem that seriously threatens ecosystems and human health. Herein, SnS2 nanosheet array catalysts on carbon cloth (CC) for microwave-induced catalytic reduction of Cr(VI) were prepared by a simple hydrothermal method. Benefiting from the unique interfacial and the improved electron transfer effect between the SnS2 nanosheet arrays and the CC substrate, the reduction ratio of the CC@SnS2 nanosheet array catalysts reached 100 % for a Cr(VI) solution (50 mg/L) under 35 min-microwave treatment at a frequency of 2.45 GHz and a power of 300 W. Density functional theory (DFT) calculations reveal that Cr2O72− ions were more easily adsorbed on S atoms than HCrO4– ions in an acidic solution, indicating that the activity of S atoms was evidently superior to that of Sn atoms in CC@SnS2 nanosheet arrays. This work provides an in-depth understanding of the mechanism of microwave-catalyzed removal of Cr(VI) and paves a new avenue for designing highly efficient microwave catalysts for wastewater treatment.

Capacitive deionization of high concentrations of hexavalent chromium using nickel–ferric-layered double hydroxide/molybdenum disulfide asymmetric electrode

To decontaminate wastewater affected by high concentrations of aqueous hexavalent chromium (Cr(VI)) and improve the capability of layered double hydroxide (LDH) as an electrode in the capacitive deionization (CDI) process, nickel–ferric-LDH (NiFe-LDH) and NiFe-LDH/molybdenum disulfide (NiFe/MoS2) were synthesized using a hydrothermal method. Characterization results indicated that the flower-like cluster framework of MoS2 was decorated with the NiFe-LDH. Addition of MoS2 improved the conductivity, capacitance reversibility, charge efficiency, coulombic efficiency, and stability of NiFe/MoS2. The CDI performance of aqueous Cr(VI) was evaluated using NiFe/MoS2 and activated carbon as the anode and cathode, respectively. The process reached equilibrium within 240 min. The deionization capacity and removal ratio for Cr(VI) (100 mg/L, 100 mL) were 49.71 mg/g and 99.42 %, respectively, at 1.2 V and 20 mL/min. The isothermal data were accurately described using the Langmuir model, and the theoretical maximum deionization capacity of NiFe/MoS2 for Cr(VI) was 106.2 mg/g. The interaction mechanisms included electrostatic attraction, surface complexation, and reduction. These findings indicate that NiFe/MoS2 has feasible applications in practical wastewater treatment for Cr(VI) removal.

Selective and Efficient Photoextraction of Aqueous Cr(VI) as a Solid-State Polyhydroxy Cr(V) Complex for Environmental Remediation and Resource Recovery

Aqueous hexavalent chromium (Cr(VI)) treatment and chromium resource recovery toward Cr-containing wastes are of significant importance and necessity to both wastewater remediation and resource recovery. Herein, via mild photoreaction conditions with isopropanol (IPA) as an electron donor, a catalyst-free strategy for aqueous Cr(VI) extraction to form an insoluble polyhydroxy Cr(V) complex is developed for the first time. Aqueous Cr(VI) with concentration from 5 to 150 ppm can be efficiently extracted with high selectivity even in the presence of coexisting ions, and the total Cr concentration in residue solution can be as low as 0.5 ppm. The Cr resource could be efficiently recovered as pure Cr2O3 by calcinating the resulting Cr(V) precipitate. Outstanding extraction efficiency could be realized with various IPA concentrations (1.3-12.0 mol/L) by coordinately tuning the pH value to promote the formation of Cr(VI)-IPA ester. The formed ester undergoes intramolecular electron transition under visible light irradiation, resulting in a polyhydroxy solid-state Cr(V) intermediate complex. The controlled pH value blocks further reduction of Cr(V) to soluble Cr(III); thus the insoluble Cr(V) intermediate complex is stabilized thermodynamically under ambient conditions. Because of its electric neutrality property and the strong intermolecule interaction via hydrogen bonds, a dioxo-bridged di-nuclear Cr(V) complex {Cr2(μ-O)2(OH)4[OCH(CH3)2]2} is finally precipitated as the main product. Satisfactory extraction and recovery of Cr from chromium-plating wastewater and discarded stainless steel verify that this approach is ideal for both one-step purification of Cr(VI)-containing wastewater and selective resource recovery from Cr-containing solid wastes in practical application.

Biochar Nanocomposite as an Inexpensive and Highly Efficient Carbonaceous Adsorbent for Hexavalent Chromium Removal

Biochar is commonly used for soil amendment, due to its excellent water-holding capacity. The Cr(VI) contamination of water is a current environmental issue in industrial regions. Here, we evaluated the effects of two-step modifications on boosting biochar’s performance in terms of the removal of aqueous hexavalent chromium (Cr(VI)), along with investigating the alterations to its surface properties. The first modification step was heat treatment under air at 300 °C, producing hydrophilic biochar (HBC). The resulting HBC was then impregnated with zero-valent iron nanoparticles (nZVI), creating an HBC/nZVI composite, adding a chemical reduction capability to the physical sorption mechanism. Unmodified biochar (BC), HBC, and HBC/nZVI were characterized for their physicochemical properties, including surface morphology and elemental composition, by SEM/EDS, while functional groups were ascertained by FTIR and surface charge by zeta potential. Cr(VI) removal kinetic studies revealed the four-time greater sorption capacity of HBC than BC. Although unmodified BC showed faster initial Cr(VI) uptake, it rapidly worsened and started desorption. After nZVI impregnation, the Cr(VI) removal rate of HBC increased by a factor of 10. FTIR analysis of biochars after Cr(VI) adsorption showed the presence of Cr(III) oxide only on the used HBC/nZVI and demonstrated that the carbonyl and carboxyl groups were the main groups involved in Cr(VI) sorption. Modified biochars could be considered an economical substitute for conventional methods.

Synthesis of conjugated polyvinyl chloride derivative coupled MnFe2O4 nanoparticles as a magnetic visible-light photocatalyst

MnFe2O4 is a potential magnetic visible-light photocatalyst, but its photocatalytic efficiency is badly restricted by the high recombination rate of its photogenerated electrons and holes. This work used conjugated polyvinyl chloride derivative (CPVC) to couple MnFe2O4 to boost its photocatalytic efficiency. CPVC/MnFe2O4 nanocomposites with an n-p heterojunction structure were prepared by an easily implementable three-step approach. When applied to the photocatalytic reduction of aqueous hexavalent chromium (Cr(VI)) with the help of visible-light (λ > 420 nm), the CPVC/MnFe2O4 nanocomposite synthesized under the optimized conditions (CPVC/MnFe2O4-2) demonstrated not only superior photocatalytic activity than MnFe2O4 nanoparticles and many other recently reported visible-light photocatalysts, but also remarkable durability. In addition, CPVC/MnFe2O4-2 also had strong magnetism and could be effectively separated from the solution by attraction with a magnet. According to the results of structure and photoelectrochemical characterization, the superior photocatalytic activity of CPVC/MnFe2O4-2 largely results from its bigger specific surface area and improved photogenerated charge separation and transfer capability. This work suggests that a new cost-effective, efficient and magnetically recoverable visible-light photocatalyst may be developed by coupling MnFe2O4 with CPVC.

Myrica Rubra-like MnFe2O4 Microsphere: A high efficiency microwave reduction catalyst for Cr(VI) removal from water

Aqueous hexavalent chromium (Cr(VI)) is easy to enter the biological system, causing serious threat to human and aquatic environment health. In this paper, a myrica rubra-like MnFe2O4 catalyst with average diameter of 200–300 nm were prepared by a simple solvothermal method. Benefiting from the knobby surface and the considerable microwave absorption ability, the as-prepared MnFe2O4 microsphere exhibited excellent microwave catalytic reduction ability for Cr(VI) in water, and the catalysts dosage, initial Cr(VI) concentration, solution pH value and temperature showed crucial effect on the catalytic reduction process. When the pH value of the solution was 2, the amount of catalysts was 2 g/L, and the temperature was 100 ℃, the myrica rubra-like MnFe2O4 catalysts could remove as high as 96% of Cr(VI) from a 50 mg/L aqueous solution only after 10 min microwave irradiation, which is much higher than the commercial one. The enhanced catalysis performance could be ascribed to the surface “knob” with a size of ∼ 17 nm, which accelerated the generation of e– under microwave irradiation, finally promoted the catalytic reduction performance. The microwave catalytic reduction strategy is a hopeful alternative for ultrafast reduction of Cr(VI) in sewage with high efficiency.

Highly dispersed and stable nano zero-valent iron doped electrospun carbon nanofiber composite for aqueous hexavalent chromium removal.

In this work, a nZVI doped electrospun carbon nanofiber (nZVI-CNF) composite was prepared and applied for aqueous hexavalent chromium (Cr(vi)) removal. Firstly, FeCl3/PAN nanofibers were prepared by a simple electrospinning method; Then, nZVI-CNFs were obtained by carbonization of FeCl3/PAN nanofibers at 800 °C. The surface morphology and internal structure of nZVI-CNFs were characterized by SEM and TEM, showing that the uniformly dispersed nZVI particles were well integrated into the carbon layer structure. The Cr(vi) removal efficiency of nZVI-CNFs was 91.5% with a Cr(vi) concentration of 10 mg L−1 and the mechanism was further studied by XRD and XPS. Meanwhile, the nZVI-CNFs exhibited good stability over a wide range of pH values from 4–8 and a long time placement stability. Furthermore, nZVI-CNFs can be used as a filter membrane for continuous treatment of wastewater, suggesting great potential for practical application.

Enhanced Adsorption Removal of Aqueous Hexavalent Chromium by an Acid-Treated Biochar Coupled with Calcium Chloride Activation

Enhanced Adsorption Removal of Aqueous Hexavalent Chromium by an Acid-Treated Biochar Coupled with Calcium Chloride Activation

Synthesis of calcium–aluminum-layered double hydroxide and a polypyrrole decorated product for efficient removal of high concentrations of aqueous hexavalent chromium

To efficiently remove high concentrations of hexavalent chromium (Cr(VI)), calcium–aluminum-layered double hydroxide (CaAl-LDH, denoted as CAL), and polypyrrole-modified CAL (CAL-PPy) were prepared by hydrothermal and in situ polymerization methods, respectively. The chemical structure, morphology, and elemental results indicated that the chain-like polypyrrole was decorated with hexagonal CAL. The specific surface area of CAL-PPy increased from 8.746 m2/g to 24.24 m2/g. The adsorption performances of CAL and CAL-PPy for aqueous Cr(VI) were investigated using batch equilibrium experiments. The decontamination process of aqueous Cr(VI) (100 mg/L) reached the equilibrium state within 50 min, and the kinetic data met the pseudo-second-order kinetic equation. The Langmuir model described the isothermal data properly, and the obtained theoretical adsorption capacity of CAL for Cr(VI) at 318 K was 34.06 mg/g, while that of CAL-PPy was 66.14 mg/g. The removal mechanisms involved electrostatic attraction, surface complexation, anion exchange, and reduction to low-toxicity Cr(III). Therefore, CAL and CAL-PPy have underlying applications in treating real wastewater containing Cr(VI).

Isotherms and kinetic modelling of mycoremediation of hexavalent chromium contaminated wastewater

Fungal biomass, Aspergillus terricola, was synthesized and employed for the biosorption of aqueous hexavalent chromium via batch technique. Insight into the nature of the surface chemistry and morphology of the fungal biomass was obtained via infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) analyses, respectively. Also, the effect of selected process variables on the Aspergillus terricola biosorption capacity was elucidated. By employing isotherm and kinetic of varying parametric equations, the study investigated the effect of the number of parameters in a given model equation on their modelling performance. Marczewki-Jaroniec (4-parameter) and Fractal-like Pseudo-first order (3-parameter) model emerged as the best fit for isotherm and kinetics studies, respectively. The finding demonstrated the dependence of modelling accuracy on the inherent number of parameters of a given model. Meanwhile, the result of the mechanistic studies highlighted the superiority of the film diffusion mechanism during the hexavalent chromium biosorption, with a Langmuir maximum adsorption capacity (q max) of 87.3 mg. g-1. Notably, the biosorption efficacy of Aspergillus terricola biomass was succinctly demonstrated in the study.

Enhanced microbial reduction of aqueous hexavalent chromium by Shewanella oneidensis MR-1 with biochar as electron shuttle

Biochar, carbonaceous material produced from biomass pyrolysis, has been demonstrated to have electron transfer property (associated with redox active groups and multi condensed aromatic moiety), and to be also involved in biogeochemical redox reactions. In this study, the enhanced removal of Cr(VI) by Shewanella oneidensis MR-1(MR-1) in the presence of biochars with different pyrolysis temperatures (300 to 800 °C) was investigated to understand how biochar interacts with Cr(VI) reducing bacteria under anaerobic condition. The promotion effects of biochar (as high as 1.07~1.47 fold) were discovered in this process, of which the synergistic effect of BMBC700(ball milled biochar) and BMBC800 with MR-1 was noticeable, in contrast, the synergistic effect of BMBCs (300–600 °C) with MR-1 was not recognized. The more enhanced removal effect was observed with the increase of BMBC dosage for BMBC700+MR-1 group. The conductivity and conjugated O-containing functional groups of BMBC700 particles themselves has been proposed to become a dominant factor for the synergistic action with this strain. And, the smallest negative Zeta potential of BMBC700 and BMBC800 is thought to favor decreasing the distance from microbe than other BMBCs. The results are expected to provide some technical considerations and scientific insight for the optimization of bioreduction by useful microbes combining with biochar composites to be newly developed.

Natural pyrite improved steel slag towards environmentally sustainable chromium reclamation from hexavalent chromium-containing wastewater

Currently, varied processes adopted to remove hexavalent chromium from aqueous solution have been realized to cause secondary pollution. As such, this study explored a green method for aqueous hexavalent chromium (Cr(Ⅵ)) reclamation by waste steel slag (SS) enhanced by natural pyrite (NP). Compared with the sole SS or NP, more efficient Cr(Ⅵ) removal was achieved by NP-SS at an initial pH value ranging from 1 to 8, resulting in a final pH value of 7–8. Cr(Ⅵ) in the solution could be initially reduced to Cr(III) by Fe2+ provided by NP, which was then bound with the OH- in the solution and the supersaturated calcium silicate hydrate on the surface of SS. In addition, the stearic acid anions existing on the surface of SS could promote the adsorption of Cr(III) to form chromium stearate. The used adsorbent could be potentially used for chromium smelting. Overall, this study provides a feasible and environmental sustainable solution to chromium reclamation from hexavalent chromium-containing wastewater.

Chemical processes of Cr(VI) removal by Fe-modified biochar under aerobic and anaerobic conditions and mechanism characterization under aerobic conditions using synchrotron-related techniques

Fe-modified biochar (FeBC) has been considered for aqueous hexavalent chromium (Cr(VI)) removal, but a better understanding is needed with respect to the removal behavior, chemical processes, and removal mechanisms under aerobic and anaerobic conditions. Aqueous Cr(VI) removal was evaluated using unmodified (BC) and FeBC. The Cr(VI) was completely removed in a pH range of 2–10. The removal behavior was properly depicted using pseudo-second-order (PSO) and Langmuir models under aerobic conditions, and using PSO and Freundlich models under anaerobic conditions. Removal rate and capacity were enhanced by up to 3.8 times under anaerobic conditions. Desorption experiments indicated removed Cr in FeBC was stable except under strong acid condition. X-ray absorption spectroscopy (XAS) analysis suggested removed Cr in FeBC was 100% in Cr(III) form and bound to Fe with a bond length of 3.01 Å in the stable form of Fe(III)nCr(III)(1−n)(OOH). The removal mechanisms of Cr(VI) under aerobic conditions by FeBC mainly included electrostatic adsorption, chemical reduction, and complex precipitation.

Enhanced simulated sunlight photocatalytic reduction of an aqueous hexavalent chromium over hydroxyl-modified graphitic carbon nitride

Hydroxyl-modified graphitic carbon nitride (HUCN) with interconnected open-framework is fabricated by hydrolysis of urea-derived graphitic carbon nitride (UCN) in an aqueous NaOH solution followed by a self-assembly in dialysis process. Compared with bulk graphitic carbon nitride (UCN), HUCN possesses more advantages such as plentiful exposed active sites, fast photogenerated charge carrier separation rate and more negative conduction band edge potential. These advantages bestow the HUCN remarkably higher simulated sunlight photocatalytic reduction ability towards an aqueous Cr(VI) as compared with UCN. By the combination of the experimental results including photoelectrochemistry and electron scavenger with the DFT calculated electrostatic potential (ESP) distribution, a possible reaction mechanism for the simulated sunlight photocatalytic reduction of highly toxic and carcinogenic Cr(VI) to non-toxic Cr(III) is put forward tentatively.

Efficient removal of aqueous hexavalent chromium by activated carbon derived from Bermuda grass

In this study, a novel raw material, Bermuda grass had been devised for the synthesis of activated carbon (BGAC) and enhanced the pore volume by potassium hydroxide. The effects of different factors on activated carbon products by orthogonal experiment was optimized. The synthesized BGAC was characterized by scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) analysis, energy dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS) and then, Cr(VI) removal batch experiments were conducted to investigate the Cr(VI) removal performance. Kinetic model and Weber-Morris diffusion model were fitted to the Cr(VI) removal process indicated that the chemisorption was the predominant removal mechanism and intraparticle diffusion was the sole rate-controlling mechanism. Langmuir isotherms could fit the experimental date well, which revealed that the adsorption of Cr(VI) ions was monolayer adsorption and the maximum adsorption capacity could be reached at 403.23 mg g−1. The results also promulgated that BGAC had an excellent potential on Cr(VI) removal. The removal processes were considered to comprise adsorption, reduction, precipitation and other ways through the study of the removal mechanism.

Reducing Hexavalent Chromium to Trivalent Chromium with Zero Chemical Footprint: Borohydride Exchange Resin and a Polymer-Supported Base.

Aqueous hexavalent chromium, Cr(VI), is rapidly reduced to trivalent chromium, Cr(III), by exposure to (polystyrylmethyl)trimethylammonium borohydride and with Amberlite-supported mild bases in a heterogeneous environment. Post-reaction removal of the insoluble reagents leaves no remediation-based chemical footprint in the source water. Time dependence with stirred and static conditions is discussed.