Ultrasound-assisted catalytic reduction of Cr(VI) by an acid mine drainage based nZVI coupling with FeS2 system from aqueous solutions: Performance and mechanism

Decoration of dandelion-like manganese-doped iron oxide microflowers on plasma-treated biochar for alleviation of heavy metal pollution in water

Carbon-based biowaste incorporated with inorganic oxides as a composite is an enticing option to mitigate heavy metal pollution in water resources due to its more economical and efficient performance. With this in mind, we constructed manganese-doped iron oxide microflowers resembling the dandelion-like structure on the surface of cold plasma-treated carbonized rice husk (MnFe2O3/PCRH). The prepared composite exhibited 45% and 19% higher removal rates for Cu2+ and Cd2+, respectively than the pristine CRH. The MnFe2O3/PCRH composite was characterized using XRD, FTIR, FESEM, EDX, HR-TEM, XPS, BET, TGA, and zeta potential, while the adsorption capacities were investigated as a function of pH, time, and initial concentration in batch trials. As for the kinetics, the pseudo-second-order was the rate-limiting over the pseudo-first-order and Elovich model, demonstrating that the chemisorption process governed the adsorption of Cu2+ and Cd2+. Additionally, the maximum adsorption capacities of the MnFe2O3/PCRH were found to be 122.8 and 102.5 mg/g for Cu2+ and Cd2+, respectively. Based on thorough examinations by FESEM-EDS, FTIR, and XPS, the possible mechanisms for the adsorption can be ascribed to surface complexation by oxygen-containing groups, a dissolution-precipitation of the ions with -OH groups, electrostatic attraction between metal ions and the adsorbent's partially charged surface, coordination of Cu2+ and Cd2+ with π electrons by aromatic/graphitic carbon in the MnFe2O3/PCRH, and pore filling and diffusion. Lastly, the adsorption efficiencies were maintained at about 70% of its initial adsorption even after five adsorption-desorption cycles, displaying its remarkable stability and reusability.

Degradation of Levofloxacin by a green zero-valent iron-loaded carbon composite activating peroxydisulfate system: Reactivity, products and mechanism

In this study, a green zero-valent iron-loaded carbon composite (ZVI-SCG) was synthesized using coffee grounds and FeCl3 solution through two-steps method, and the synthesized ZVI-SCG was used in the activation of peroxydisulfate (PDS) to degrade Levofloxacin (LEX). Results revealed that ZVI-SCG exhibited a great potential for LEX removal by adsorption and catalytic degradation in the ZVI-SCG/PDS system, and 99% of LEX was removed in the ZVI-SCG/PDS system within 60 min. ZVI-SCG/PDS system showed a high reactivity toward LEX degradation under realistic environmental conditions. Also, the ZVI-SCG/PDS system could effectively degrade several quinolone antibiotics including gatifloxacin, ciprofloxacin and LEX in single and simultaneous removal modes. A potential reaction mechanism of LEX degradation by ZVI-SCG/PDS system was proposed, SO4•-, HO•, O2•- and 1O2 involved in radical and non-radical pathways took part in catalytic degradation of LEX by ZVI-SCG/PDS system, but HO• might be the main reactive species for LEX degradation. The possible degradation pathway of LEX was also proposed based on the identified ten intermediate products, LEX degradation was successfully achieved through decarboxylation, opening ring and hydroxylation processes. The potential toxicity of LEX and its oxidation products decreased significantly after treatment. This study provides a promising strategy of water treatment for the antibiotics-containing wastewater.

A green synthesized medicine residue carbon-based iron composite for the removal of chromium (VI) and cadmium (II): Performance, kinetics and mechanism

Nowadays, clean-up of heavy metals from wastewaters using waste residue carbon-based material has received increasing attention. In this work, a novel Chinese medicine residue carbon-based nano zero-valent iron composite (CM-nZVI) had been successfully prepared using the combined Chinese medicine residue, FeCl3 and green tea extract. Cr(VI) and/ or Cd(II) removal in water by the CM-nZVI were systematacially investigated with a series of batch experiments. The most relevant findings indicated the adsorption efficiecy and capacity of Cr(VI) by CM-nZVI were respecitvely nearly 98% and 26 mg/g under optimized reaction conditions. The negative influences of the cations on the Cr(VI) removal followed the order of Al3+ > Ca2+ > Mg2+ Na+ > K+, but the anions followed the order of HCO3- > PO43- > NO3- > Cl- > SO42-. Humic acid (HA) and ionic strength with high concentrations severely inhibited Cr(VI) removal. The Cr(VI) adsorption on CM-nZVI fitted well by the pseudo-second-order kinetic and Langmuir models. A monolayer endothermic chemisorption occurred on Cr(VI) adsorption over CM-nZVI, and Cr(VI) removal by CM-nZVI primarily involved in the absorption, reduction, precipitation and complexation processes. Both Cr(VI) and Cd(II) removals had been achieved by CM-nZVI at their low concentrations. This CM-nZVI showed a better reusability proprity for Cr(VI) and Cd(II) removal with the regeneration of CM-nZVI through simple pickling. The outcomes of this work show that CM-nZVI could be used an effective material for heavy metals removal from water.

Design of cryogel based CNTs-anchored polyacrylonitrile honeycomb film with ultra-high S-NZVI incorporation for enhanced synergistic reduction of Cr(VI)

An ultra-high NZVI-loaded PAN film (S-CPN) with a unique 3D honeycomb structure was designed based on the cryogel method of green solvent-induced pores and confinement of the spatially free conformation of films by anchoring carbon nanotubes (CNTs), supplemented sulfidation for removing hexavalent chromium (Cr(VI)), and characterized by SEM, AFM, BET, XRD, XPS, and electrochemical corrosion. The doping amounts of the compounds for S-CPN synthesis were optimized to be 0.075 g CNTs, 0.25 g Na₂S, and 0.3 M FeSO₄. S-CPN possessed a 175.247 m²/g specific surface area, -0.365 V reduction potential, and 46.54 mg/g ultra-high NZVI-loading. S-CPN had the strong activity of Cr(VI) removal and tolerance to coexisting ions. The removal efficiency remained at 80 % after age for 30 days or 5 cycles. The pseudo-first-order kinetics and Langmuir model were more favorable to simulate the adsorption of Cr(VI) on S-CPN. The thermodynamics show that S-CPN removing Cr(VI) was a spontaneous exothermic reaction. The reasons for these excellent properties were that CNTs improve the film porosity and ultra-high NZVI-loading, and synergistic the FeS_X layer to chelates-reduces Cr(VI). This was the first time that honeycomb film with 3D structure and potential applications in heavy metal removal was developed via an eco-friendly strategy.

Impacts of coexisting mineral on crystallinity and stability of Fe(II) oxidation products: Implications for neutralization treatment of acid mine drainage

The neutralization treatment of acid mine drainage involves the oxidation of Fe(II), but little is known about the effects of co-existing minerals on the oxidation and hydrolysis of Fe(II) to iron oxides. Here we investigated the transformation of fresh and heated Fe(II) oxidation coprecipitates, which were synthesized in the presence and the absence of five co-existing minerals (montmorillonite, kaolin, quartz (SiO₂)_, aluminium oxide (Al₂O₃) and calcium carbonate (CaCO₃)). In the FeSO₄ system with montmorillonite or kaolin, the formation of lepidocrocite was inhibited with the increase of clay mineral contents. In the same system, heated coprecipitates of montmorillonite were mainly comprised of amorphous ferrihydrite and its transformation was retarded by the excess montmorillonite. In the FeCl₂ system with SiO₂, Al₂O₃ or CaCO₃, akaganeite formation was inhibited with the increase in the corresponding mineral contents. In the same system, goethite formation was blocked by either CaCO₃ or Al₂O₃ and the growth of lepidocrocite was inhibited by CaCO₃ or SiO₂. However, magnetite formation was enhanced by addition of CaCO₃. These findings are important for predicting products of abiotic Fe(II) oxidation during the neutralization of acid mine drainage and for better understanding the transformation of amorphous iron oxides in the complicated environmental matrix.

Facile provision of CuO-Kaolin nanocomposite for boosted sonocatalytic removal of Cr(VI) from hydrous media

Nowadays, nanoscale materials have been widely applied in the removal of contaminants from the water system. Reduction of Cr(VI) (as a poisonous species) to Cr(III) (as a slight toxic species) was performed using CuO-Kaolin with ultrasound (US) irradiation. The CuO-Kaolin nanocomposite was synthesized via a facile co-precipitation method. Then X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscope and Energy dispersive X-ray spectroscopy analyses were performed to identify the structural features of CuO-Kaolin. The role of influential parameters for the reduction of Cr(VI) was investigated in sonocatalytic advanced oxidation system. About 89.35% of Cr(VI) was removed via US/CuO-Kaolin process after 90 min at optimum conditions (pH = 3, sonocatalyst dosage = 1 g L^-1 and [Cr (VI)]₀ = 20 mg L^-1). This outstanding result was due to the synergistic effect of the increased electron delivery to conduction band on CuO-Kaolin nanocomposite and the increased reactive surface region of nanoparticles by sonication. The presence of H₂O₂ as an amplifier improved the removal efficiency of Cr(VI) from 89.35% to 100% after 20 min. Kinetic experimental results were well described by a pseudo-first-order kinetic model. Desorption experiments showed excellent stability of sonocatalyst during the reaction and maintenance of the catalytic activity up to 10 sequential cycles.

Synthesis of ZnO immobilized on recycled polyethylene terephtalate for sonocatalytic removal of Cr(VI) from synthetic, drinking waters and electroplating wastewater

In this study, Cr(VI) was removed sonocatalytically by the zinc oxide nanoparticle coated with polyethylene terephthalate (PET) fabricated through a facile co-precipitation method. The crystal structure, functional groups on the surface, morphology, surface composition and oxidation states of the nanomaterials were investigated by XRD, FTIR, SEM, EDX and XPS techniques. Environmental parameters including solution pH, catalyst dose, hexavalent chromium concentration, H₂O₂ content, purging gases, organic compounds and type and anions strength on the sonotocatalytic removal of Cr(VI) were also investigated. Additionally, the contribution of each process, reusability, Cr(VI) reduction from actual water and electroplating wastewater were evaluated. Under the optimal conditions, [Cr(VI)]₀=20 mg/L, nanocomposite loading=1.6 g/L and pH=5, 99.92% of Cr(VI) was removed within 60 min. By increaing, Cr(VI) concentration (5-50 mg/L), k_obs decreased to values between 0.1498 and 0.0063 min^-1 and the calculated electrical energy per order (E_Eo) increased from 148.68 to 3535.24 kWh.m^-3, respectively. The presence of purging gases, organic compounds and ionic strength negatively affected Cr(VI) reduction. Examination of radical scavengers showed that the most active radicals in Cr(VI) removal were O₂^•- and h⁺. The removal of the Cr(VI) using the US/ZnO-PET method (99.92%) was higher than that of the US/ZnO method (70.78%). The catalyst activity was well maintained up to eight consecutive cycles. In addition, the removal efficiency was approximately 72.23 and 68.55% for drinking water and real electroplating wastewater samples, respectively. The results of toxicity in the sonotocatalytic removal of Cr(VI) by Daphnia magna showed LC₅₀ and toxicity unit (TU) 48 h, which was equal to 81.46 and 1.227 vol percent.

Sulfide-induced repartition of chromium associated with schwertmannite in acid mine drainage: Impacts and mechanisms

The cycling of Fe/S is often related to the formation and fate of schwertmannite which is particularly suitable as a scavenger for heavy metals and metalloids in acid mine drainage (AMD). However, the interactions between reactivity of S(-II) and schwertmannite with structurally incorporated Cr(VI) remain elusive. This work evaluated dissolution experiments in combination with XRD, SEM, FTIR, TEM, and XPS characterization to provide detailed information regarding the transformation of schwertmannite induced by S(-II) following changes in pH, Cr loading, and S(-II) concentration. Our results found that the presence of sulfide significantly decreased the stabilization of schwertmannite under acidic conditions. The reactivity of the three schwertmannite samples depended on the contents of Cr(VI) that were structurally incorporated and followed the order Sch > 0.13Cr-Sch > 0.17Cr-Sch. High S(-II) concentrations and low Cr doping favored the release of Fe and SO₄^2- from schwertmannite. Attenuation of Cr mobility occurred via elevating the S(-II) concentrations and pH values resulting in Cr concentrations spanning ∼1.39 to ∼0.10 mg L^-1 and ∼1.58 to ∼0.12 mg L^-1 for 0.13Cr-Sch and 0.17Cr-Sch, respectively. Combining the results of characterization, goethite was the dominant end product constituted secondary phase together with sulfide minerals (FeS, FeS₂), iron oxides (Fe₃O₄, Fe₂O₃), and CrFe minerals on the bulk mineral surface. The substituted Cr significantly inhibited the reductive transformation of schwertmannite by sulfide and led to the formation of lepidocrocite. Thus, we concluded that a three-stage transformation mechanistic pathway governed partial conversion of schwertmannite to goethite. This finding provides new understanding of the biogeochemical processes of iron minerals affected by reducing substances that control the transport and fate of immobilizing contaminants in an AMD-polluted area.

Role of reactor type on Cr(VI) removal by zero-valent iron in the presence of pyrite: Batch versus sequential batch reactors

This study was conducted to understand the role of application sequence of pyrite and zero-valent iron (Fe⁰) (simultaneous vs. sequential) on chromium (VI) removal by Fe⁰. In batch experiments, pyrite and Fe⁰ were homogeneously mixed in batch reactors maintained at a constant total solids loading of 2 g L^-1. In sequential batch experiments, however, the first reactor containing variable doses of pyrite was operated for 20 min, and the liquid fraction from the first reactor was then subsequently loaded into the second reactor containing a fixed Fe⁰ dose of 1.2 g L^-1. The batch reactors achieved much higher Cr(VI) removal efficiency than sequential batch reactors under similar operating conditions due to discrepancies in Fe redox cycling activities between these two systems. In batch reactors, the Fe⁰ particles deposited on pyrite surface due to electrostatic attraction between negatively charged pyrite and positively charged Fe⁰, thus, rendering the overall solids surface charge neutral at optimum pyrite and Fe⁰ doses. As a result, the whole system behaved like a composite material, with pyrite functioning as a support material for Fe⁰. This stimulated Fe redox cycling more effectively to generate new Fe(II) sites on Fe⁰ for enhanced Cr(VI) removal relative to Fe⁰ only system. In sequential batch reactors, however, the Fe redox cycling activity was limited, but significantly increased with increasing pyrite dose in the first reactor. Overall, our results indicate that the stimulatory effect of pyrite on Cr(VI) removal by Fe⁰ may be much higher if the reactors are operated in batch mode.

Kinetics and Mechanisms of Cr(VI) Removal by nZVI: Influencing Parameters and Modification

In this study, single-spherical nanoscale zero valent iron (nZVI) particles with large specific sur-face area were successfully synthesized by a simple and rapid chemical reduction method. The XRD spectra and SEM–EDS images showed that the synthesized nZVI had excellent crystal struc-ture, but oxidation products, such as γ-Fe2O3 and Fe3O4, were formed on the surface of the parti-cles. The effect of different factors on the removal of Cr(VI) by nZVI were studied, and the opti-mum experimental conditions were found. Kinetic and thermodynamic equations at different temperatures showed that the removal of Cr(VI) by nZVI was a single-layer chemical adsorption, conforming to pseudo-second-order kinetics. By applying the intraparticle diffusion model, the ad-sorption process was composed of three stages, namely rapid diffusion, chemical reduction, and in-ternal saturation. Mechanism analysis demonstrated that the removal of Cr(VI) by nZVI in-volved adsorption, reduction, precipitation and coprecipitation. Meanwhile, Cr(VI) was reduced to Cr(III) by nZVI, while FeCr2O4, CrxFe1−xOOH, and CrxFe1−x(OH)3 were formed as end products. In addition, the study found that ascorbic acid, starch, and Cu modified nZVI can promote the removal efficiency of Cr(VI) in varying degrees due to the enhanced mobility of the particles. These results can provide new insights into the removal mechanisms of Cr(VI) by nZVI.

A porous biochar supported nanoscale zero-valent iron material highly efficient for the simultaneous remediation of cadmium and lead contaminated soil

Risk associated with heavy metals in soil has been received widespread attention. In this study, a porous biochar supported nanoscale zero-valent iron (BC-nZVI) was applied to immobilize cadmium (Cd) and lead (Pb) in clayey soil. Experiment results indicated that the immobilization of Cd or Pb by BC-nZVI process was better than that of BC or nZVI process, and about 80% of heavy metals immobilization was obtained in BC-nZVI process. Addition of BC-nZVI could increase soil pH and organic matter (SOM). Cd or Pb immobilization was inhibited with coexisting organic compound 2,4-dichlorophenol (2,4-DCP), but 2,4-DCP could be removed in a simultaneous manner with Cd or Pb immobilization at low concentration levels. Simultaneous immobilization of Cd and Pb was achieved in BC-nZVI process, and both Cd and Pb availability significantly decreased. Stable Cd species inculding Cd(OH)₂, CdCO₃ and CdO were formed, whereas stable Pb species such as PbCO₃, PbO and Pb(OH)₂ were produced with BC-nZVI treatment. Simultaneous immobilization mechanism of Cd and Pb in soil by BC-nZVI was thereby proposed. This study well demonstrates that BC-nZVI has been emerged as a potential technology for the remediation of multiple heavy metals in soil.

A collaborative strategy for elevated reduction and immobilization of Cr(VI) using nano zero valent iron assisted by schwertmannite: Removal performance and mechanism

A novel collaborative strategy for enhanced removal of Cr(VI) using nano zero valent iron (nZVI) assisted by schwertmannite (Sch) with two synthesis methods was designed. Batch experiments demonstrated that nZVI/Sch-AP (synthesized by abiotic precipitation of Fe³⁺ species) exhibited excellent removal performance for Cr(VI) than nZVI/Sch-CO (synthesized by chemical oxidation of Fe²⁺ species). The results indicated that the removal efficiencies of Cr(VI) by nZVI/Sch-AP and nZVI/Sch-CO were highly pH-dependent and achieved to be 99.99% and 98.01% under the optimal conditions of 10 mg L^-1 Cr(VI) concentration, a pH of 6.3 and a Fe(0)/Cr(VI) molar ratio of 12. But nZVI/Sch-AP emerged greater k of 0.1097 min^-1 than that of nZVI/Sch-CO (0.0485 min^-1). Humic acid exhibited promotion effect on the Cr(VI) removal in low concentration of 1 mg L^-1. Results of XRD and XPS demonstrated that α-FeOOH was the dominant products in both incubations of nZVI/Sch-AP and nZVI/Sch-CO, accompanied with FeCr₂O₄ and CrFe mixed (oxy)hydroxides, and γ-FeOOH was found alone in the incubations of nZVI/Sch-CO. We proposed a consecutive and simultaneous process involving surface absorption-reduction and co-precipitation/immobilization for the removal. This study provides new insights into the elimination of Cr(VI) from wastewater by nZVI/Sch, especially in acid mine drainage.

Biochar for agronomy, animal farming, anaerobic digestion, composting, water treatment, soil remediation, construction, energy storage, and carbon sequestration: a review

In the context of climate change and the circular economy, biochar has recently found many applications in various sectors as a versatile and recycled material. Here, we review application of biochar-based for carbon sink, covering agronomy, animal farming, anaerobic digestion, composting, environmental remediation, construction, and energy storage. The ultimate storage reservoirs for biochar are soils, civil infrastructure, and landfills. Biochar-based fertilisers, which combine traditional fertilisers with biochar as a nutrient carrier, are promising in agronomy. The use of biochar as a feed additive for animals shows benefits in terms of animal growth, gut microbiota, reduced enteric methane production, egg yield, and endo-toxicant mitigation. Biochar enhances anaerobic digestion operations, primarily for biogas generation and upgrading, performance and sustainability, and the mitigation of inhibitory impurities. In composts, biochar controls the release of greenhouse gases and enhances microbial activity. Co-composted biochar improves soil properties and enhances crop productivity. Pristine and engineered biochar can also be employed for water and soil remediation to remove pollutants. In construction, biochar can be added to cement or asphalt, thus conferring structural and functional advantages. Incorporating biochar in biocomposites improves insulation, electromagnetic radiation protection and moisture control. Finally, synthesising biochar-based materials for energy storage applications requires additional functionalisation.

Spatial distribution and morphological transformation of chromium with coexisting substances in tannery landfill

The prosperity and development of tannery industry have brought about rapid economic growth. However, the tannery landfill without anti-seepage measures in the early stage has generated masses of environmental hazards owing to the lack of awareness in environmental protection. Therefore, it is imperative to pay much attention to the understanding of environmental hazards from tannery waste. In this study, solid samples and groundwater samples were collected from a tannery landfill to study the effect of the characteristic pollutants produced by tanning on chromium distribution with other coexisting substances. The results showed that significant correlations were demonstrated between multiple coexisting substances (total organic carbon, total petroleum hydrocarbons, total nitrogen, Cr, F, Ca, Cu and Pb), indicating the possible same source or they coming from the same tannery production stage. The weights of positive effects and negative effects of coexisting substances on total Cr distribution in the profile decreased in the order: total nitrogen > Cu > Ca > Pb > total organic carbon > F > SO₄^2-> Cd, and Ni > Cl > Hg, respectively. Moreover, the simulation of Visual MINTEQ showed that the cations were mainly bound to Cr as CrO₄^2-, while the anions were bound to Cr³⁺. This study provided a new perspective on the selection of remediation strategies for Cr-contaminated sites to avoid secondary environmental pollution caused by the release of coexisting heavy metals.

Synergistic chromium(VI) reduction and phenol oxidative degradation by FeS2/Fe0 and persulfate

It is a challenge to simultaneously treat the combined pollutants of chromium(VI) (Cr(VI)) and organics (such as phenol) in wastewater. Here, a stable and efficient redox system based on FeS₂ sulfidated zero valent iron (FeS₂/Fe⁰) and persulfate (PS) was developed to synchronously remove Cr(VI) and phenol. 100% of phenol (10 mg/L) was oxidized in 10 min and Cr(VI) (20 mg/L) was completely reduced to Cr(III) in 90 min in the FeS₂/Fe⁰+PS system with a pH range of 3.0-9.0, respectively. phenol was selectively oxidized without re-oxidizing Cr(III) in such system. The surface-bound Fe²⁺ was the major reactive species to synchronously reduce Cr(VI) and oxidize phenol. The mechanisms were elucidated that the phenol degradation was accelerated by the generated Cr(III) complexing with its products, and that SO₄^2-, whose production speed was accelerated by the PS activation to oxidize phenol and FeS₂, was conductive to corrode Fe⁰ to regenerate the surface-bound Fe²⁺ for reducing Cr(VI) and oxidizing phenol. It is potential to develop a high-performance and large-scaled FeS₂/Fe⁰-based redox platform to remediate the complex pollution of Cr(VI) and organics.

Simultaneous adsorption of Cr(VI) and phenol by biochar-based iron oxide composites in water: Performance, kinetics and mechanism

The pollution of heavy metals and organic compounds has received increased attention in recent years. In the current study, a novel biochar-based iron oxide composite (FeYBC) was successfully synthesized using pomelo peel and ferric chloride solution through one-step process at moderate temperature. Results clearly demonstrate that FeYBC exhibited more efficient removal of Cr(VI) and/or phenol compared with the pristine biochar, and the maximum adsorption amounts of Cr(VI) and phenol by FeYBC could reach 24.37 and 39.32 mg g^-1, respectively. A series of characterization data suggests that several iron oxides such as Fe₂O₃, Fe⁰, FeOOH and Fe₃O₄ were formed on the FeYBC surface as well as oxygen-containing groups. Thermodynamics study indicates that Cr(VI) and phenol adsorption by FeYBC were endothermic and exothermic processes, respectively. Langmuir adsorption isotherm and pseudo-second order models could better explain the Cr(VI) and phenol adsorption behaviors over FeYBC. The Cr(VI) adsorption might be primarily achieved through the ion exchange and surface complexation and reduction, whereas the π-π interaction and electron donor-acceptor complex mainly contributed to phenol adsorption. The findings indicate that the biochar-based iron oxide composites material was an efficient adsorbent for the remediation of industrial effluents containing Cr(VI) and phenol.