Aqueous Cr Research Articles

Sunlight-Driven Direct/Mediated Electron Transfer for Cr(VI) Reductive Sequestration on Dissolved Black Carbon-Ferrihydrite Coprecipitates.

Surface runoff horizontally distributed chromium (Cr) pollution into various surface environments. Sunlight is a vital factor for the Cr cycle in the surface environment, which may be affected by photoactive substances such as ferrihydrite (Fh) and dissolved black carbon (DBC). Herein, sunlight-driven transformation dynamics of Cr species on DBC-Fh coprecipitates were studied. Under sunlight, the removal of aqueous Cr(VI) by DBC-Fh coprecipitates occurred through sunlight-driven reductive sequestration including adsorption, followed by surface reduction (pathway 1) and aqueous reduction, followed by precipitation (pathway 2). Additionally, coprecipitates with a higher DBC content exhibited a more effective reduction of both adsorbed (kapp,S_red) and aqueous Cr(VI) (kapp,A_red). Photoelectrons facilitated Cr(VI) reduction through direct electron transfer; notably, electron donating DBC promoted the production of photoelectrons by consuming photogenerated holes. Photogenerated Fe(II) species (mineral-phase and aqueous Fe(II)) mediated electron transfer for Cr(VI) reduction, which was reinforced via a ligand-to-metal charge transfer (LMCT) process between DBC-organic ligands and mineral Fe(III). Furthermore, ·O2- also mediated Cr(VI) reduction, although this impact was limited. Overall, this study demonstrates that photoelectrons and photogenerated electron mediators play a crucial role in Cr(VI) reductive sequestration on DBC-Fh coprecipitates, providing new insights into the geochemical cycle of Cr pollution in sunlight-influenced surface environments.

Mechanistic insight into the simultaneous removal of Cr(VI) and phosphate by a novel versatile bimetallic material

Industrial wastewater containing hexavalent chromium Cr(VI) and phosphate poses a great threat to human health and the aquatic ecological environment. In this study, a novel La(OH)3- and CaO2-fabricated CNT (La-Ca-CNT) material was developed for the simultaneous and highly effective removal of aqueous Cr(VI) and phosphate. Characterization results showed that synergistic effects existed between the CNT support and La and Ca species. La-Ca-CNT showed high phosphate and Cr(VI) removal rates in acidic conditions with high stability. Kinetic study suggested that the fast adsorption of phosphate on La-Ca-CNT was controlled by intraparticle diffusion. The removal of Cr(VI) and phosphate interacted with each other, and the removal of Cr(VI) went through a slow-fast-slow reaction, reaching a Cr(VI) removal rate of over 97% within 180min. La-Ca-CNT had a high phosphate adsorption capacity (174.3mg/g), and over 70% of the adsorbed phosphate could be recovered. The presence of co-existing anions showed negative effects on phosphate adsorption but negligible effects on Cr(VI) reduction, while organic carbon in natural water showed no effect on phosphate and Cr(VI) removal. A mechanistic study revealed that phosphate was removed by physical-chemical adsorption (outer-sphere complexation and ligand exchange) over La-Ca-CNT, while Cr(VI) was reduced by H2O2 generated from CaO2 or directly by surface CaO2.

Chitosan-based composite featuring dual cross-linking networks for the removal of aqueous Cr(VI)

To avoid the environmental detriment caused by Cr(VI) waste, this study constructs a dual cross-linking network structure using sodium alginate (SA) and polyvinyl alcohol (PVA). Chitosan (CS) is further introduced through electrostatic attraction and hydrogen bonding and SA/PVA/CS (SPC) composite with porous structure is successfully prepared for the removal of Cr(VI) from wastewater. Batch adsorption experiments show that SPC has excellent adsorption capacity and practical usability with a broad pH applicability range. Under optimal adsorption conditions (pH = 2, t = 150 min, T = 35 °C, m/V = 2 g/L, C0 = 10 mg/L), the Cr(VI) removal rate of SPC achieves 89.2 %. Adsorption kinetics and isotherm models indicate that the adsorption process is primarily multi-layer and chemical adsorption. Additionally, FT-IR and XPS reveal that the adsorption mechanism of SPC for Cr(VI) involves a synergy of electrostatic attraction, reduction, ion exchange and complexation. Density functional theory (DFT) simulations also confirm the dominant role of CS in the Cr(VI) adsorption. In summary, SPC shows great potential for Cr(VI) wastewater treatment.

Magnetic iron-based waterworks sludge modified by chitosan and FeS for aqueous Cr(vi) adsorption and reduction.

Heavy metals have been considered an evolving environmental concern due to their harmful and long-lasting impacts. We synthesized a composite of FeS/CS@MIBWS for aqueous Cr(vi) adsorption and reduction utilizing the iron-based waterworks sludge modified by chitosan and FeS. After determining the optimal conditions for the FeS/CS@MIBWS preparation, its Cr(vi) removal capability was evaluated using material characterisation and static Cr(vi) adsorption assays. Cr(vi) elimination by the composite was a pH-dependent process, with pH 2 being the optimum in the range of 2-10. The adsorption process was befitted a pseudo-second-order model, and the equilibrium results agreed well with the Langmuir model. The thermodynamics investigation showed that Cr(vi) removal by the composite has both spontaneous and endothermic nature. Considering the ionic effects, Cl-, SO4 2- and PO4 3- decreased Cr(vi) elimination in the sequence of Cl- < SO4 2- < PO4 3-. The key mechanisms for Cr(vi) elimination were physical and chemical adsorption, chelation, and Cr(vi) reduction into Cr(iii). Furthermore, FeS/CS@MIBWS demonstrated steady reusability (removal effectiveness of 70% after 5 cycles). FeS/CS@MIBWS's rapid, high-performance, reusable, and easily separable adsorption properties make it a promising choice for heavy metal environmental cleaning.

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.

Construction of Pillared-Layer Metal-Organic Frameworks as an All-Visible-Light Switchable Photocatalyst for Aqueous Cr(VI) Reduction.

Recently, two-dimensional metal-organic frameworks that are photoactive have shown great potential for efficiently converting solar energy into chemical energy. In this work, we successfully synthesized and designed two M2-MOFs ([Cu(L1)((CH3)2NH)]n (Cu-MOF) and [Zn(L1)(CH3)2NH)]n (Zn-MOF), H2L1 = 4,4'-(benzo[c][1,2,5]thiadiazole-4,7-diyl)dibenzoic acid). Structural analysis suggests that the five-coordinated M(II) ion is surrounded by four oxygen ions from two ligands and one nitrogen atom from one dimethylamine molecule. The ligand spacer acts as a bridge between two SBUs and forms a 2D layer with rhomboid windows. These moieties are arranged in a staggered ABAB pattern, which likely aids in exfoliation. The UV-vis diffuse reflectance spectra (DRS) test shows that when the metal center in the MOF framework is replaced with Cu(II) ions, the light absorption range covers 200-1100 nm, which is much larger than the light absorption range of Zn-MOF. Moreover, the photoelectric current, electrochemical impedance spectra (EIS), and Mott-Schottky tests all indicate that Cu-MOF has better photoelectric properties. When applied to the photocatalytic reduction of Cr(VI), Cu-MOF and Zn-MOF can completely reduce Cr(VI) within 100 min under 450 nm LED light irradiation. Under sunlight irradiation, Cu-MOF can completely reduce Cr(VI) within 40 min, achieving the removal of Cr(VI) ions, which is much faster than the rate of Cr(VI) removal by Zn-MOF.

Prospective application of eco-friendly banana peel reduced graphene oxide (BRGO) for aqueous Cr (VI) and acid dye adsorption: A waste utilization approach

Banana peels are an environmentally benign alternative to toxic reducing agents. Banana peel-reduced graphene oxide (BRGO) showed high performance in the adsorption of metal and dye ions. Fourier transform infrared (FTIR), scanning electron microscopy (SEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), and a Zeta potential analyzer were used to characterize the BRGO. Different influencing factors such as pH, dose, concentration, duration, and temperature, were studied to evaluate the efficiency of the adsorbent. The Langmuir and Freundlich isotherm models were fitted for Cr (VI) and acid dye adsorption. The maximum adsorption capacity (qm) of BRGO for Cr (VI) and AV54 dye were 135.87 and 110.74 mg/g, respectively. The pseudo-second-order kinetic model was better suited for Cr (VI) and AV54 dye adsorption, which indicated chemical adsorption. Thermodynamically, the adsorption exhibited spontaneity and was endothermic. Regeneration and reuse of adsorbents were also studied for potential use in treating tannery effluent.

The role of cell surface sulfhydryl and amine binding sites in the removal of Cr(VI) from solution by Bacillus subtilis bacterial cells

A range of bacterial species are known to be capable of reducing soluble Cr(VI) to Cr(III), causing the precipitation of Cr(III) phases from solution, but the mechanism responsible for the initial interaction between anionic aqueous Cr(VI) and the negatively charged cell surface has not been identified. Our study examines Cr(VI) removal from solution by the Gram-positive soil bacterial species Bacillus subtilis. We measured the kinetics of Cr(VI) removal as a function of pH in B. subtilis suspensions with different Cr(VI):cell ratios, and we measured the effect of pH on the extent of Cr(VI) removal at a fixed time in experiments with B. subtilis biomass (cells plus exudates) and in experiments involving B. subtilis exudates alone. The roles of sulfhydryl and amine binding sites in Cr(VI) removal were constrained using site–specific blocking molecules, and the impact of the site blocking on Cr(VI) removal was studied as a function of pH. Our results indicate that Cr(VI) removal by B. subtilis cells under the experimental conditions is at least partially non-reversible and is dependent on binding site concentration, pH, and metal loading. Our results are consistent with a two step removal process: Cr(VI) first adsorbs reversibly onto a cell wall binding site, followed by Cr(VI) reduction to Cr(III) likely via electron transfer from cell wall electron transport chain molecules. B. subtilis exudates are capable of removing a relatively small fraction of Cr(VI) from solution, and hence our results indicate that the dominant mechanism of Cr(VI) removal requires interaction with bacterial biomass. The presence of either the sulfhydryl-specific blocking molecule or the amine-specific blocking molecule or both in the experimental systems dramatically reduces the extent of Cr(VI) reduction, especially under circumneutral pH conditions, strongly suggesting that both sulfhydryl and amine binding site types participate in the initial attachment of Cr(VI) onto the cell surface. The experiments with either sulfhydryl or amine sites blocked both exhibited a similar reduction in Cr(VI) removal to the experiments with both types of sites blocked, strongly suggesting that both types of sites are involved simultaneously in binding Cr(VI) species to the cell surface. For example, these sites could both be involved in forming a bidentate bond with Cr(VI) species, or the positive charge of an amine site in close proximity to a sulfhydryl site could be necessary in order to enable the Cr(VI) species to approach the cell wall and to bind to a sulfhydryl site. The results of this study are the first to propose a viable mechanism that can explain the binding of anionic Cr(VI) onto an overall negatively charged cell surface as a first step in the reduction to Cr(III) and subsequent removal of Cr from solution.

Synthesis of polyaniline modified FeWO4 nanoparticles for superior visible-light photocatalysis

This communication reports the development of a new high-efficiency visible-light photocatalyst by coupling p-type FeWO4 with n-type polyaniline (PANI). PANI/FeWO4 nanocomposite was synthesized through a facile composition-controllable solid liquid mixing-evaporation of solvent method. The composition and structure of the synthesized PANI/FeWO4 nanocomposite were characterized by X-ray diffraction, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. The photoabsorption and photogenerated charge separation and transfer performance of PANI/FeWO4 nanocomposite were compared with those of FeWO4 nanoparticles. The photocatalytic properties of PANI/FeWO4 nanocomposite and FeWO4 nanoparticles were tested in the reduction of aqueous Cr(VI) under visible-light (λ > 420 nm). The results showed that the photocatalytic activity of PANI/FeWO4 was significantly elevated compared to FeWO4. Coupling with PANI can increase the adsorption of Cr(VI), enhance the absorption of visible-light, and promote the separation and transfer of photogenerated charges of FeWO4, thus contributing to the better photocatalytic activity of PANI/FeWO4.

Synergistic CdS@CeO2 nanocomposites: Harnessing Z-scheme electron transfer for enhanced photocatalytic CO2 conversion and aqueous Cr(VI) reduction

The present work describes the fabrication of CdS-D@CeO2 Z-scheme heterojunctions as highly stable, efficient and recyclable photocatalysts for the visible light assisted reduction of CO2 to methanol as well as Cr(VI) to Cr(III). The bare CeO2 produced methanol with the rate of 111 μmolh−1g−1 that is much lower in comparison to CdS-D@CeO2 (501 μmolh−1g−1). In addition, the photocatalytic efficiency remained nearly same for three successive runs, indicating the higher stability and recycling ability of the heterojunctions CdS-D@CeO2. It was also observed that under direct sunlight irradiation, CdS-D@CeO2 efficiently reduced 100% Cr(VI) to Cr(III) in an aqueous medium in just 40 minutes. Moreover, >95% removal was maintained even after 5 catalytic cycles. Additionally, for mimicking real contaminated water samples, the influence of the foreign ions on Cr(VI) reduction was evaluated by using lake water sample and no adverse effects on the result were observed. Thus, this work showed a way to develop cost-effective, easily synthesized, durable and recyclable composite photocatalysts for CO2 utilization and proficient removal of waste water pollutants by employing the clean, green, and renewable solar energy.

In situ preparation of two-dimensional mortise and tenon structure g-C3N4/MoO3 photocatalyst for the reduction of aqueous Cr(VI)

In situ solid-state synthesis was employed to prepare the [Formula: see text] (CM) heterojunctions with a two-dimensional mortise and tenon structure. The prepared products were characterized using X-ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy, suggesting that amorphous graphitic carbon was obtained. The photocatalytic performance of the prepared photocatalysts was evaluated in aqueous Cr(VI) reduction reactions under visible-light ([Formula: see text] &gt; 420 nm) irradiation. The results of the experiments showed that the photocatalytic activity of CM is influenced by the composition of heterojunctions. The optimum photocatalytic activity for the photocatalytic reduction of Cr(VI) was found in the compound CM-25, which contains a 25% [Formula: see text] concentration. The Cr(VI) reduction rate on CM-25 was four times that of [Formula: see text] and 14.5 times that of [Formula: see text], the photocatalytic activity of CM-25 only dropped by 7.7% in four cycles. Scavenger investigations and EPR tests indicated that the major photogenerated radicals in the reduction of Cr(VI) were superoxide negative radicals (•[Formula: see text]) and hydroxyl radicals (•[Formula: see text]OH). CM-25 was discovered to be an S-scheme heterojunction that effectively suppressed photogenerated charge carrier recombination and retained photogenerated holes (in the valence band of [Formula: see text]) and electrons (in the conduction band of [Formula: see text]) with higher redox capabilities. CM-25 has potential applications in the photocatalytic reduction of Cr(VI) in wastewater.

Actual Adsorptive Efficiency of Aqueous Cr(VI) Using Modified Banana Pseudostems

A wide application of chromium (Cr) in industries has discharged large quantity of Cr(VI) into the environment. Cr(VI) is a strong oxidizing agent that can react rapidly to produce toxic compounds to humans. Various agricultural products and byproducts have been studied for the sorptive removal of Cr(VI). However, less attention has been given to the presence of Cr(III) in solution after the adsorption with significant effect to actual adsorption capacity of the material. Here the Cr(VI) removal efficiency was investigated by using modified banana pseudostems (m-BP). Banana pseudostems were treated with 0.08 M HCl for 4 h and washed thoroughly. The maximum adsorption capacity of Cr(VI) was found at pH 2, adsorbent dose 34 g/L, contact time of 180 min and 100 mg/L initial Cr(VI) concentration. The adsorption efficiency of Cr(VI) onto m-BP was recorded to be 73.6% while the total conversion of Cr(VI) was 99.6%. The adsorption efficiency was lower than the total conversion of Cr(VI) due to the reduction of Cr(VI) to Cr(III) by the presence of organic groups in banana pseudostems. Owing to relatively high efficiency, m-BP can be used as effective and very low cost adsorbent for Cr(VI) removal from wastewater.

Reaction Thermodynamics of Zerovalent Iron and Hexavalent Chromium at the Solid-Liquid Interface and in Solution.

Zerovalent iron (Fe0) is a promising candidate for remediating hexavalent chromium (Cr(VI)) via adsorption and (or) reduction. Herein, the reaction between Fe0 and Cr(VI) at the solid-liquid interface and in solution under varying pHs was inspected using the methodology of equilibrium thermodynamics. First, species distribution functions of aqueous Cr(VI), Cr(III), Fe(III), and Fe(II) are deduced to illuminate the quantitative distribution of aqueous metal species. Second, the plausible reaction at pH = 0-14 either at the solid-liquid interface or in solution is determined according to the species distribution function. Third, the spontaneity of each reaction is evaluated via a thermodynamic calculation based on the van't Hoff equation. The results present the following. (1) At the solid-liquid interface, the redox reaction 2Cr(VI) + 3Fe0 → 2Cr(III) + 3Fe(II) is spontaneous, inducing complete Cr(VI) → Cr(III) reduction at pH = 0-14. Especially, the high spontaneity of the redox reaction is mainly ascribed to Fe0 oxidation, which serves as a highly spontaneous subreaction. (2) In solution, the redox reaction Cr(VI) + 3Fe(II) → Cr(III) + 3Fe(III) is nonspontaneous at pH = 6 and 7, whereas it is spontaneous at pH = 6-7, 0-5, and 8-14. Accordingly, no Cr(VI) → Cr(III) reduction at pH = 6-7 and complete Cr(VI) → Cr(III) reduction at pH = 0-5 and 8-14 are expected. Particularly, the nonspontaneity of the Cr(VI) reduction at pH = 6-7 is majorly attributed to water ionization, which is involved as a highly nonspontaneous subreaction. On the contrary, the spontaneity of the Cr(VI) reduction at pH = 0-5 and 8-14 is mainly owing to acid-base neutralization, which is involved as a highly spontaneous subreaction. This work may deepen our knowledge about the chemistry involved in hexavalent chromium remediation by the zerovalent iron.

Processes and mechanisms in remediation of aqueous chromium contamination by sulfidated nano-scale zerovalent iron (S-nZVI): Experimental and computational investigations

Sulfidated nano-scale zerovalent iron (S-nZVI) has emerged as an advanced functional nanomaterial for efficiently remediating Cr(VI) contamination in aqueous environments. However, there is an insufficient understanding of its coherent process, removal pathway, and hydrochemical reactive mechanisms, presenting potential challenges for its future environmental applications. To address this gap, this study successfully synthesized S-nZVI through a chemical precipitation method and effectively applied it for the removal of Cr(VI). Additional characterization revealed that the removal of Cr(VI) followed a sequence of rapid chemisorption and intraparticle diffusion processes, concomitant with an increase in pH and a decrease in oxidation-reduction potential. The remediation mechanism encompassed a synergistic reduction of Cr(VI) to Cr(III) and simultaneous immobilization via Cr2FeO4 coprecipitation. The highest Cr(VI) removal capacity of 75 mg/g was attained during dynamic removal experiments in the sand column packed with S-nZVI. Further computational analysis, employing density functional theory calculations based on the experimental data, revealed the involvement of multiple molecular orbitals of Cr(VI) in the removal process. It also elucidated a step-by-step reduction pathway for Cr(VI) characterized by decreasing free energy. These findings provide evidence-based insights into Cr(VI) remediation using S-nZVI and can serve as valuable technical support for future environmental management of heavy metals.

Development of Fe0 biochar composites synergistically activated with MgFe2O4 and Fe2+ through one-step pyrolyzing Fe sludge and MgCl2 for enhanced aqueous Cr(VI) removal

The surface passivation layer on Fe0 severely restricts its reactivity during environmental decontamination. In this study, a new composite—an Fe0 biochar composite (Fe0/BC) coupled with MgFe2O4 and Fe2+—was first developed for enhanced Cr(VI) removal by simple co-pyrolysis of Fe sludge (FS) with MgCl2. The characterization results demonstrated that MgFe2O4 and Fe0 could be formed simultaneously at 700 ℃. However, MgFe2O4 disappeared at temperatures of 800 and 900 ℃. By varying the flow rate of N2 and holding time, Fe0/BC was further optimized based on the determined MgCl2 to FS mass ratio (1.9:10.0) and temperature (700 °C). The optimal Fe0/BC (MBC700120-400–30) achieved a Cr(VI) removal efficiency of 97.67%, which was 2.43 times higher than that of the unmodified Fe0/BC obtained at 700 ℃. This enhanced reactivity was directly related to the electron transfer of Fe0 stimulated by MgFe2O4 as well as the conversion of passive corrosion products into Fe3O4 facilitated by soluble Fe2+ components. The predicted maximum adsorption capacity of MBC700120-400–30 was 85.43 mg/g (pH = 5) and the removal of Cr(VI) was dominated by chemisorption. The reduction of Cr(VI) to Cr(Ⅲ) was identified as the principal removal mechanism. Overall, this study demonstrates a novel strategy for developing functional Fe0-based materials for water contamination treatment.

Modeling the Role in pH on Contaminant Sequestration by Zerovalent Metals: Chromate Reduction by Zerovalent Magnesium.

The role of pH in sequestration of Cr(VI) by zerovalent magnesium (ZVMg) was characterized by global fitting of a kinetic model to time-series data from unbuffered batch experiments with varying initial pH values. At initial pH values ranging from 2.0 to 6.8, ZVMg (0.5 g/L) completely reduced Cr(VI) (18.1 μM) within 24 h, during which time pH rapidly increased to a plateau value of ∼10. Time-series correlation analysis of the pH and aqueous Cr(VI), Cr(III), and Mg(II) concentration data suggested that these conditions are controlled by combinations of reactions (involving Mg0 oxidative dissolution and Cr(VI) sequestration) that evolve over the time course of each experiment. Since this is also likely to occur during any engineering applications of ZVMg for remediation, we developed a kinetic model for dynamic pH changes coupled with ZVMg corrosion processes. Using this model, the synchronous changes in Cr(VI) and Mg(II) concentrations were fully predicted based on the Langmuir-Hinshelwood kinetics and transition-state theory, respectively. The reactivity of ZVMg was different in two pH regimes that were pH-dependent at pH < 4 and pH-independent at the higher pH. This contrasting pH effect could be ascribed to the shift of the primary oxidant of ZVMg from H+ to H2O at the lower and higher pH regimes, respectively.

Overlooked role of aqueous chromate (VI) as a photosensitizer in enhancing the photochemical reactivity of ferrihydrite and production of hydroxyl radical

The reactive oxygen species (ROS) photochemically generated from natural iron minerals have gained significant attention. Amidst the previous studies on the impact of heavy metal ions on ROS generation, our study addresses the role of the anion Cr(VI), with its intrinsic photoactivity, in influencing ROS photochemical generation with the co-presence of minerals. We investigated the transformation of inorganic/organic pollutants (Cr(VI) and benzoic acid) at the ferrihydrite interface, considering sunlight-mediated conversion processes (300–1000 nm). Increased photochemical reactivity of ferrihydrite was observed in the presence of aqueous Cr(VI), acting as a photosensitizer. Meanwhile, a positive correlation between hydroxyl radical (•OH) production and concentrations of aqueous Cr(VI) was observed, with a 650% increase of •OH generation at 50 mg L−1 Cr(VI) compared to systems without Cr(VI). Our photochemical batch experiments elucidated three potential pathways for •OH photochemical production under varying wet chemistry conditions: (1) ferrihydrite hole-mediated pathway, (2) chromium intermediate O-I-mediated pathway, and (3) chromium intermediates CrIV/V-mediated pathway. Notably, even in the visible region (> 425 nm), the promotion of aqueous Cr(VI) on •OH accumulation was observed in the presence of ferrihydrite and TiO2 suspensions, attributed to Cr(VI) photosensitization at the mineral interface. This study sheds light on the overlooked role of aqueous Cr(VI) in the photochemical reactivity of minerals, thereby enhancing our understanding of pollutant fate in acid mining-impacted environments.

Synergistic effects of adsorption and chemical reduction towards the effective Cr(VI) removal in the presence of the sulfur-doped biochar material.

In this study, the anaerobic sludge withdrawn from thickener in a sewage treatment plant served as the precursor for sludge-based biochar fabrication, which was further modified via sulfur (S) heteroatom doping (i.e., S-BC). The S atom doping resulted in the adjustment of the physicochemical properties towards the carbon material, endowment of abundant functional groups on biochar surface, and increasing the binding sites between biochar and Cr(VI). Compared to the primary biochar (i.e., biochar without heteroatomic doping, named BC), S-BC exhibited a rough surface and possessed remarkable advantages in ash content, specific surface area, and pore volume. The existence of graphene carbon crystal structure for S-BC was confirmed through S-BC by XRD and FTIR analysis. The studies of adsorption kinetics and isotherms showed that pseudo-second-order kinetics and the Langmuir model more fitted the Cr(VI) removal behavior in the presence of S-BC. Therefore, the chemisorption and monolayer adsorption were the primary mechanisms involved in the Cr(VI) removal process. Additionally, XPS analysis results illustrated the aqueous Cr(VI) was efficiently eliminated through the synergistic effect of chemisorption and reduction to Cr(III) in the presence of S-BC. Moreover, S-BC could still achieve the Cr(VI) eliminating efficiency of 85.31% undergoing five cycles with unchanged functional group and crystal structure via FTIR and XRD analysis. Thus, the results of this study may shed light on a new approach for simultaneous economical sludge disposal and the sustainable remediation of the Cr(VI)-contaminated wastewater.

Hydrothermal preparation of Nb5+-doped α-Fe2O3 nanorods for efficient visible light-driven photocatalytic reduction of hexavalent chromium

Nb5+ doped α-Fe2O3 (Nb-Fe2O3) nanorods with different Nb5+ contents were prepared using a low-temperature, short-time, one-pot hydrothermal method. All the prepared Nb-Fe2O3 products showed better photocatalytic activities than α-Fe2O3 nanorods in the reduction of aqueous Cr(VI) under visible light (wavelength > 420 nm). 4% Nb-Fe2O3 had the maximum photocatalytic activity, which was 2.24 times that of α-Fe2O3 nanorods and surpassed those of many newly reported visible light photocatalysts. Moreover, the photocatalytic efficiency of 4% Nb-Fe2O3 can be further improved by proper increasing its dosage, decreasing the initial pH and Cr(VI) concentration of K2Cr2O7 solution. Furthermore, 4% Nb-Fe2O3 showed rather good photocatalytic stability and reusability. Besides, 4% Nb-Fe2O3 was efficient in visible light-driven photocatalytic reduction of Cr(VI) in diluted black chrome plating solution. Nb5+-doping can efficaciously enhance the abilities of α-Fe2O3 in absorbing visible light and transferring and separating photogenerated carriers, thereby contributing to the elevated photocatalytic activity of Nb-Fe2O3.

Adsorption of aqueous Cr (VI) onto UiO66NH2 MOF: Isotherm, thermodynamics and mechanism studies

As a member of the UiO-66 family, UiO66NH2 MOF (UNH) possessed enhanced stability, high surface area, increased pore size, and pore volume. Additionally, its amino groups offer spectacular affinity to capture Cr (VI). With these propitious features, UNH was fabricated by a facile solvothermal route in this study. It was then characterized by XRD and FTIR to confirm its successful formation. Its affinity towards efficacious capture of aqueous Cr (VI) was explored with the help of isotherm models and thermodynamic parameters. The Langmuir isotherm model fits best the experimental data, demonstrating monolayer Cr (VI) sorption on a homogenous UNH surface. The maximum adsorption capacity was determined as 51 mg/g. Negative ΔG° values suggested that the Cr (VI) uptake process is spontaneous and favorable, while the endothermic nature of the sorption was evident from positive values ΔH°. Thermodynamic parameters such as standard enthalpy change (ΔH°), standard entropy change (ΔS°), and standard Gibbs free energy change (ΔG°) were determined as 6.651 kJ mol−1, –3.216 kJ mol−1, and 33.76 KJ K−1.mol−1 respectively. FTIR and pH studies evidenced that the sequestration of Cr (VI) onto UNH proceeds through the electrostatic attraction mechanism, supported by the fitted isotherm model and thermodynamic parameters’ results.