Enhanced removal of Cr(VI) from aqueous solution by stabilized nanoscale zero valent iron and copper bimetal intercalated montmorillonite

3D/1D Fe3O4@TiO2/TC-TiO2/SiO2 Magnetic Inorganic-Framework Molecularly Imprinted Fibers for Targeted Photodegradation

To achieve a selective degradation of pollutants in a water body, 3D/1D magnetic molecularly imprinted fibers Fe3O4@TiO2/TC-TiO2/SiO2 were fabricated by an electrospinning method. The molecularly imprinted layer was successfully prepared by a direct imprinting method using TiO2 as a functional monomer. Fe3O4 facilitates the catalyst recovery and light utilization. The as-prepared fibrous photocatalyst has a large specific surface area of 132.4 m2/g. The successful generation of imprinted sites was proven by various characterizations. The weak interaction between the inorganic functional monomer and tetracycline (TC) was determined to be van der Waals force and hydrogen bonds by the IGMH isosurface theory. The construction of the 3D/1D homojunction of molecularly imprinted materials is beneficial to charge transfer. The as-prepared photocatalyst exhibits a high selectivity coefficient α = 737.38 competing with RhB. The TC removal efficiency reached 100% within only 20 min. In addition, the possible degradation pathway and the degradation mechanism are reasonably proposed. This work not only provides an in-depth mechanism of the weak interaction between the inorganic molecularly imprinted functional monomer and pollutant molecules but also offers new thoughts on the fabrication of photocatalysts for the effective and selective treatment of pollutants in water bodies.

Doped Cu0 and sulfidation induced transition from R-O• to •OH in peracetic acid activation by sulfidated nano zero-valent iron-copper

Peracetic acid (PAA) has emerged as a new effective oxidant for various contaminants degradation through advanced oxidation process (AOP). In this study, sulfidated nano zero-valent iron-copper (S-nZVIC) with low Cu doping and sulfidation was synthesized for PAA activation, resulting in more efficient degradation of sulfamethoxazole (SMX, 20 μM) and other contaminants using a low dose of catalyst (0.05 g/L) and oxidant (100 μM). The characterization results suggested that S-nZVIC presented a more uniform size and distribution with fewer metal oxides, as the agglomeration and oxidation were inhibited. More significantly, doped Cu0 and sulfidation significantly enhanced the generation and contribution of •OH but decreased that of R-O• in S-nZVIC/PAA/SMX system compared with that of nZVIC and S-nZVI, accounting for the relatively high degradation efficiency of 97.7% in S-nZVIC/PAA/SMX system compared with 85.7% and 78.9% in nZVIC/PAA/SMX and S-nZVI/PAA/SMX system, respectively. The mechanisms underlying these changes were that (i) doped Cu° could promote the regeneration of Fe(Ⅱ) for strengthened PAA activation through mediating Fe(Ⅱ)/Fe(Ⅲ) cycle by Cu(Ⅰ)/Cu(Ⅱ) cycle; (ii) S species might consume part of R-O•, resulting in a decreased contribution of R-O• in SMX degradation; (iii) sulfidation increased the electrical conductivity, thus facilitating the electron transfer from S-nZVIC to PAA. Consequently, the dominant reactive oxygen species transited from R-O• to •OH to degrade SMX more efficiently. The degradation pathways, intermediate products and toxicity were further analyzed through density functional theory (DFT) calculations, liquid chromatography-mass spectrometry (LC-MS) and T.E.S.T software analysis, which proved the environmental friendliness of this process. In addition, S-nZVIC exhibited high stability, recyclability and degradation efficiency over a wide pH range (3.0∼9.0). This work provides a new insight into the rational design and modification of nano zero-valent metals for efficient wastewater treatment through adjusting the dominant reactive oxygen species (ROS) into the more active free radicals.

Construction of attapulgite decorated cetylpyridinium bromide/cellulose acetate composite beads for removal of Cr (VI) ions with emphasis on mechanistic insights

Eco-friendly and renewable composite beads were constructed for efficient adsorptive removal of Cr (VI) ions. Attapulgite (ATP) clay decorated with cetylpyridinium bromide (CPBr) was impregnated into cellulose acetate (CA) beads, which were formulated through a simple and cost-effective solvent-exchange approach. FTIR, XRD, SEM, Zeta potential, and XPS characterization tools verified the successful formation of ATP-CPBr@CA beads. The composite beads displayed a spherical and porous shape with a positively charged surface (26.6 mV) at pH 2. In addition, higher adsorption performance was accomplished by ATP-CPBr@CA composite beads with ease of separation compared to their components. Meanwhile, equilibrium isotherms pointed out that the Langmuir model was optimal for describing the adsorption process of Cr (VI) with a maximal adsorption capacity of 302 mg/g. Moreover, the D-R isotherm model verified the physical adsorption process, while adsorption data obeyed the pseudo-second-order kinetic model. Further, XPS results hypothesized that the removal mechanism involves adsorption via electrostatic interactions, redox reaction, and co-precipitation. Interestingly, the ATP-CPBr@CA composite beads reserved tolerable adsorption characteristics with a maximum removal present exceeding 70% after reuse for seven successive cycles, proposing its feasible applicability as a reusable and easy-separable candidate for removing heavy metals from aquatic bodies.

Citric acid-induced photochemical behavior of Cr(III)-substituted ferrihydrite: Fe and Cr release, mineral transformation and reactive oxygen species generation

Cr(III)-substituted ferrihydrite (Fh-Cr(III)) is widespread in the surrounding environment of mining areas. Fh-Cr(III) is unstable and susceptible to the influence of environmental factors, such as dissolved organic matter (DOM) and light, so Cr species embedded in mineral crystal layers are likely to have more profound negative effects on the environment with the photochemical behaviors of minerals. However, the photochemical behaviors of Fh-Cr(III) in the presence of DOM remains poorly understood. For this reason, citric acid (CA) was chosen as the representative DOM to study its combined effects with visible light irradiation on Fh-Cr(III) dissolution and phase transition. The results showed that CA hindered the agglomeration of Fh-Cr(III) particles, thereby slowing the phase transition of Fh-Cr(III). However, CA exacerbated the release of Fe and Cr by maintaining Fh-Cr(III) under unstable crystal structure. Moreover, due to the occurrence of ligand-metal charge transfer (LMCT) in (CA)n-Fh-Cr(III) formed on the Fh-Cr(III) surface, the synergistic effect between CA and light irradiation greatly promoted the dissolution of Fh-Cr(III). In the mixed system of Fh-Cr(III) (3 g/L) and CA (5 mM) at initial pH 3.0, the maximum concentrations of TFe and TCr were 18.17 and 5.68 mg/L after 6 h of light reaction, which were 1.82 and 3.62 times of those in the corresponding system in the darkness. Meanwhile, the Fe(III)/Fe(II) cycling in solution and solid surfaces was affected by various reactive oxygen species (ROS) generated from the LMCT process, in which the photoproduced Fe(II) further accelerated Cr(III) dissolution under acidic solution. The fast release of Cr(III) may pose greater danger to the environment as the more toxic Cr(VI) can be easily formed through the oxidation of dissolved Cr(III). This work offers fresh insights into the migration and transformation of Cr elements in the natural environment.

Removal and reduction mechanism of Cr (VI) in Leersia hexandra Swartz constructed wetland-microbial fuel cell coupling system

Cr (VI) is extremely harmful to both the environment and human health, and it can linger in the environment for a very long period. In this research, the Leersia hexandra Swartz constructed wetland-microbial fuel cell (CW-MFC) system was constructed to purify Cr (VI) wastewater. By comparing with the constructed wetland (CW) system, the system electricity generation, pollutants removal, Cr enrichment, and morphological transformation of the system were discussed. The results demonstrated that the L. hexandra CW-MFC system promoted removal of pollutants and production of electricity of the system. The maximum voltage of the system was 499 mV, the COD and Cr (VI) removal efficiency was 93.73% and 97.00%. At the same time, it enhanced the substrate and L. hexandra ability to absorb Cr and change it morphologically transformation. Additionally, the results of XPS and XANES showed that the majority of the Cr in the L. hexandra and substrate was present as Cr (III). In the L. hexandra CW-MFC system, Geobacter also functioned as the primary metal catabolic reducing and electrogenic bacteria. As a result, L. hexandra CW-MFC system possesses the added benefit of removing Cr (VI) while producing energy compared to the traditional CW system.

Efficient spatial separation of charge carriers over Sv-ZnIn2S4/NH2-MIL-88B(Fe) S-scheme heterojunctions for enhanced photocatalytic H2 evolution and antibiotics removal performance

The exploration of highly efficient sunlight-assisted photocatalyst for photodegradation of organic contaminants or energy conversion is strongly encouraged. In this work, we designed a novel three-dimensional spindle-like Sv-ZIS@NMFe heterojunction made of amino functionalized NH2-MIL-88B(Fe) (NMFe) and ZnIn2S4 nanosheets with abundant sulfur vacancies (Sv-ZIS). The structural properties of NMFe materials, such as a clearly defined system of pores and cavities, were retained by the Sv-ZIS@NMFe composites. Additionally, the incorporation of sulfur vacancies, -NH2 functional groups, and well-matched energy level positions led to various synergistic effects that considerably enhanced internal electron transformation and migration, as well as improved adsorption performance. Consequently, under visible light irradiation, the optimized sample exhibited superior hydrogen production activity and tetracycline hydrochloride photodegradation performance. At last, density functional theory calculations was used to further elucidated the possible photoreactivity mechanism. This study demonstrates that the Sv-ZIS@NMFe heterojunction materials formed by ZnIn2S4 with suitable sulfur vacancies and amino functionalized Fe-MOFs have promising applications in photocatalysis.

Insight into the effect of microplastics on photocatalytic degradation tetracycline by a dissolvable semiconductor-organic framework

The presence of microplastics (MPs) in large quantities in the aqueous environment significantly affects the degradation process of water pollution. Still, the interaction between MPs and pollutants during photocatalytic degradation has not been studied. Here, a soluble BiOCl-OH semiconductor-organic framework (BOCH-SOF) was prepared from xylitol, and the polystyrene (PS) MPs' effect on the photocatalytic degradation of tetracycline (TC) was investigated. It was found that the appropriate number of PS can promote TC degradation and also change degradation products and pathways. At the same time, the presence of TC can effectively enhance PS aging and reduce the molecular weight of PS. This indicates that BOCH-SOF produces a synergistic effect in treating the combined pollution of TC and PS. Through free radical analysis and density functional theory calculations, it was proposed that PS and TC can complement each other. A certain concentration and size of PS can promote the conversion of Bi(III) to Bi and enhance charge separation and radical generation; the TC can change the interfacial charge distribution and free radical depletion, extend the light absorption in the system, and the PS and TC work together to ultimately achieve the synergistic degradation of the TC and the aging PS. This paper provides an in-depth analysis of the mechanism of the effect of MPs on the photocatalytic degradation of antibiotics, which is significant in controlling the combined pollution of MPs and associated pollutants in the water environment.

Emerging iron-based mesoporous materials for adsorptive removal of pollutants: Mechanism, optimization, challenges, and future perspective

Iron-based materials (IBMs) have shown promise as adsorbents due to their unique physicochemical properties. This review provides an overview of the different types of IBMs, their synthesis methods, and their properties. Results found in the adsorption of emerging contaminants to a wide range of IBMs are discussed. The IBMs used were evaluated in terms of their maximum uptake capacity, with special consideration given to environmental conditions such as contact time, solution pH, initial pollutant concentration, etc. The adsorption mechanisms of pollutants are discussed taking into account the results of kinetic, isotherm, thermodynamic studies, surface complexation modelling (SCM), and available spectroscopic data. A current overview of molecular modeling and simulation studies related to density functional theory (DFT), surface response methodology (RSM), and artificial neural network (ANN) is presented. In addition, the reusability and suitability of IBMs in real wastewater treatment is shown. The review concludes with the strengths and weaknesses of current research and suggests ideas for future research that will improve our ability to remove contaminants from real wastewater streams.

Au doped metal organic frameworks as di-functional photocatalysts for clearing organics in wastewater

Along with the development of productive forces, the use of organic compounds including diversified dyes and multiple drugs has become more and more commonly, resulting in the accelerating water contamination. Herein in this paper, Au doped PCN 224 are designed as bi-functional wastewater treatment agents to absorb and decompose organics molecules efficiently under light irradiation. After inserted with Au, the PCN 224 nanoparticles, which is kind of porous, stable and photosensitive metal-organic framework, show enhanced photodegradeability. Because the Au inserted could inhibit the re-combination of electrons and holes by absorbing photo-electrons; decrease the nanoparticles' band gap, and finally produce much more free radicals. In the meanwhile, due to the lower binding energy between S and Au, the Au modified PCN 224 perform better in absorbing organic compounds consisted of S contained heterocyclic ring (such as methylene blue). This work provides new insights into the precious design of materials in clearing organic compounds.

Fabrication and performance of novel multifunctional sodium alginate/polyvinylpyrrolidone hydrogels

In this work, a novel alginate/polyvinylpyrrolidone (SA/PVP-Fe) hydrogel spheres were prepared by cross-linking with Fe2+ ions after blending sodium alginate with polyvinylpyrrolidone. The degradation performance of the hydrogels was assessed through the degradation of phenol, achieving 100% degradation and about 64% total organic carbon (TOC) mineralization within 60 min (initial concentration of phenol = 20 mg/L; H2O2 concentration = 5 mM; initial pH = 6.5; catalyst dosage = 1.0 g/L). Degradation kinetics were monitored using high-performance liquid chromatography (HPLC). The structural and chemical properties of the hydrogels were characterized using scanning electron microscopy (SEM), energy spectroscopy (EDS), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and Inductively coupled plasma mass spectrometry (ICP-MS). Additionally, the hydrogels exhibited multiple reuse cycles, albeit with a gradual decline in degradation performance. Mechanistic investigations revealed that the hydroxyl radical derived from the Fenton reaction was the primary active species responsible for the degradation process. This research provides valuable insights into improving the mechanical properties of sodium alginate hydrogels, opening up avenues for their practical applications.

Laser-cum-KOH activation allows interfacial engineering of cardboard-derived carbon, tunable ionic states, and universal dye adsorption

In this study, we demonstrate the preparation of laser-cum KOH-activated porous carbon with tunable ionic states, unique surface chemistry, and physical texture from renewable and environmentally friendly precursors (waste cardboard boxes). The adsorption performance of the engineered adsorbents is examined on the adsorption of methyl blue (MB, anionic) and methylene blue (MeB, cationic). The adsorption mechanism was determined using detailed batch adsorption, and the MB was adsorbed via the formation of ternary complexes, whereas the MeB was adsorbed through cation-π interaction. Furthermore, the non-activated laser-induced carbon (LIC100) and the KOH-activated carbon (KAC(L)) exhibit superior dye adsorption capacities of 9610 (MB) and 1882 mg g-1 (MeB), respectively. To the best of our knowledge, this is the highest ever reported at dye removal in the field of adsorptive dye removal. Langmuir isotherm model and pseudo-second-order are fitted well for both MB and MeB adsorption. Lastly, the carbons generated through this modern technique can be remodeled into a more secure device to obtain clean and microorganism-free water. This study showed the potential of laser-induced carbonization of KOH-activated substrate and provided unique insights into future development for universal dye adsorption and other possible applications.

Visualized adsorption and enhanced photocatalytic removal of Cr6+ by carbon dots-incorporated fluorescent nanocellulose aerogels

In this study, carbon dots (CDs) and titanate nanofibers (TNs) were mixed with TEMPO-oxidized nanocellulose (TOCNC) to prepare fluorescent nanocellulose aerogels (FNAs) by a Schiff base reaction. The resulting FNA can detect the adsorption of Cr6+ through the fluorescence quenching in CDs and promote the removal of Cr6+ through the synergistic effect of CDs in photocatalysis. The optimized FNA has a maximum adsorption capacity of 543.38 mg/g, higher than most reported Cr6+ adsorbents. This excellent performance is due to the porous structure of the aerogel, which gives it a high specific surface area of 20.53 m2/g and provides abundant adsorption sites. Simultaneously, CDs can enhance the amino-induced Cr6+ adsorption, improve the photocatalytic performance of TNs, and expose more adsorption sites through electrostatic adsorption of amino-induced reduction products (Cr3+). This study explores the preparation of visualized nanosorbents with enhanced photocatalytic removal of Cr6+ and provides a new direction for nanoscale photocatalysts.

Sulfidated nanoscale zero valent iron for in situ immobilization of hexavalent chromium in soil and response of indigenous microbes

This study aimed to investigate the immobilization efficiency of sulfidated nanoscale zero valent iron on Cr(VI) in soil. Reactions between sulfidated nanoscale zero valent iron and Cr(VI) in soil system and effects of sulfidated nanoscale zero valent iron on microbes had been demonstrated. Solid characterization results confirmed the incorporation of sulfur into nanoscale zero valent iron. Furthermore, the main oxidation products of iron after the reactions were magnetite, goethite and lepidocrocite. Fe-Cr complexes indicated that Cr(VI) was reduced to Cr(III). The results of 16 S rRNA gene analysis indicated that the sulfidated nanoscale zero valent iron had a limited bactericidal effect but further stimulated the sulfite reductase gene population, representing its positive effect for the soil remediation. The study showed that some microflora such as Protobacteria were promoted, while others community such as Firmicutes, were depressed. Furthermore, Cr mainly converted from a high toxic state such as exchangeable (EX) to less bioavailable state such as iron-manganese oxides bound (OX) and organic matter-bound (OM), thus reducing the toxicity of Cr when sulfidated nanoscale zero valent iron was added. High immobilization efficiency of the Cr(VI) compared to nanoscale zero valent iron indicated an improvement on selectivity and reactivity after sulfidation. Overall, sulfidated nanoscale zero valent iron was promising for the immobilization of Cr(VI) immobilization soil.

Strategies for improving the stability of perovskite for photocatalysis: A review of recent progress

Photocatalysis is currently a hot research field, which provides promising processes to produce green energy sources and other useful products, thus eventually benefiting carbon emission reduction and leading to a low-carbon future. The development and application of stable and efficient photocatalytic materials is one of the main technical bottlenecks in the field of photocatalysis. Perovskite has excellent performance in the fields of photocatalytic hydrogen evolution reaction (HER), oxygen evolution reaction (OER), carbon dioxide reduction reaction (CO2RR), organic synthesis and pollutant degradation due to its unique structure, flexibility and resulting excellent photoelectric and catalytic properties. The stability problems caused by perovskite's susceptibility to environmental influences hinder its further application in the field of photocatalysis. Therefore, this paper innovatively summarizes and analyzes the existing methods and strategies to improve the stability of perovskite in the field of photocatalysis. Specifically, (i) component engineering, (ii) morphological control, (iii) hybridization and encapsulation are thought to improve the stability of perovskites while improving photocatalytic efficiency. Finally, the challenges and prospects of perovskite photocatalysts are discussed, which provides constructive thinking for the potential application of perovskite photocatalysts.

Effective photocatalytic degradation of amphotericin B and naproxen from aqueous solutions using carbon quantum dots combined in MIL-88B(Fe) under visible light

A photocatalytic adsorbent composed of carbon dots (CD) embedded in a metal-organic framework (MOF) of MIL-88 B(Fe) was prepared by solvothermal technique. The synthesized CD@MIL-88 B(Fe) was characterized by different X-ray-based microscopic and spectroscopic methods, as well as electrochemical impedance spectroscopy, UV-Vis, FT-IR, DRS, TGA, and photoluminescence (PL) analysis. The prepared adsorbent showed a remarkable photocatalytic activity for eliminating amphotericin B (AmB) and naproxen (Nap) from aqueous solutions under visible light, reaching up to 92% and 90% removal, respectively, with an RSD value of around 5%. The parameters affecting the degradation process of pharmaceuticals were investigated. The optimal conditions for the degradation process were determined, including pH values (3 and 4 for AmB and Nap), photocatalyst concentration (0.2 g L-1), and H2O2 concentration (40-50 mM). Reactive oxidative species were also identified (·OH, ·O2) by examination of different scavengers. The adsorption isotherm and kinetic studies reveal that the synthesized photocatalyst exhibits dual functionality as an effective adsorbent (with maximum adsorption capacities of 42.5 and 121.5 mg g-1 for AmB and Nap) and a photocatalytic agent for removal purposes.

Performance and degradation pathway of florfenicol antibiotic by nitrogen-doped biochar supported zero-valent iron and zero-valent copper: A combined experimental and DFT study

Fluorinated compounds are a class of organic substances resistant to degradation. Although zero-valent iron (Fe0) has a promising reducing capability, it still fails to degrade fluorine-containing antibiotics (i.e., florfenicol) efficiently. In this study, we applied a simple one-pot pyrolytic approach to synthesize nitrogen-doped biochar supported Fe0 and zero-valent copper (Cu0) composite (Fe/Cu@NBC) and investigated its performance on florfenicol removal. The results clearly showed that approximately 91.4% of florfenicol in the deionized water was removed by Fe/Cu@NBC within 8 h. As the reaction time was extended to 15 d, the total degradation rate of florfenicol reached 96.6%, in which the defluorination and dechlorination rates were 73.2% and 82.1%, respectively. Both experimental results and density functional theory calculation suggested that ∙OH and ·O2- triggered β-fluorine elimination, resulting in defluorination prior to dechlorination. This new finding was distinct from previous viewpoints that defluorination was more difficult to occur than dechlorination. Fe/Cu@NBC also had a favorable performance for removal of florfenicol in surface water. This study provides a new insight into the degradation mechanism and pathway of florfenicol removal in the Fe/Cu@NBC system, which can be a promising alternative for remediation of fluorinated organic compounds in the environment.

Simple ultrasonic integration of shapeable, rebuildable, and multifunctional MIL-53(Fe)@cellulose composite for remediation of aqueous contaminants

Metal-organic frames (MOFs) have been recognized as one of the best candidates in the remediation of aqueous contaminants, while the fragile powder shape restricts the practical implementation. In this work, a shapeable, rebuildable, and multifunctional MOF composite (MIL-53@CF) was prepared from MIL-53 (Fe) and cellulose fiber (CF) using a simple ultrasonic method for adsorption and photocatalytic degradation of organic pollutants in wastewater. The results showed MIL-53(Fe) crystals were uniformly growth on CF surfaces and bonded with surface nanofibrils of CF through physical crosslinking and hydrogen bonding. Because of the high bonding strength, the MIL-53@CF composite exhibited an excellent compressive strength (3.53 MPa). More importantly, the MIL-53@CF composite was rebuildable through mechanical destruction followed by re-ultrasonication, suggesting the excellent reusability of MIL-53@CF for water remediation. The MIL-53@CF composite also had high adsorption capacities for methyl orange (884.6 mg·g-1), methylene blue (198.3 mg·g-1), and tetracycline (106.4 mg·g-1). MIL-53@CF composite could degrade TC through photocatalysis. The photocatalytic degradation mechanism was attributed to the Fe(II)/Fe(III) transform cycle reaction of MIL-53 crystal located on MIL-53@CF. Furthermore, the mechanical property and remoldability of MIL-53@CF composite increased its practicability. Comprehensively, MIL-53@CF composite provided a possible strategy to practically apply MOF in the remediation of aqueous contaminants.

Study on the removal effect and mechanism of calcined pyrite powder on Cr(VI)

Pyrite exhibits considerable potential as an adsorbent in wastewater treatment. However, few pyrite adsorbents are directly obtained from natural pyrite, as most are composite materials that require a complex preparation process. To develop a pyrite-based adsorbent with a simple preparation process, pyrite was processed by calcination at 400, 600, and 800 °C for 4 h and ball-milled into a fine powder. The adsorption properties of the pyrite powder were systematically explored. The calcined pyrite powder was characterized by SEM-EDS and XRD. The results revealed that the pyrite calcined at 600 °C exhibited excellent adsorption properties and was primarily composed of Fe7S8. The optimum conditions for Cr(VI) removal were a temperature of 45 °C, an adsorbent dosage of 1 g, an equilibration time of 60 min, and an initial pH of 3. Moreover, the calcined pyrite powder exhibited excellent reusability, and the Cr(VI) removal rate exceeded 65% after three cycles. The Cr(VI) adsorption on pyrite can be well described by the Freundlich model and pseudo-second-order kinetic equation. The calcination temperature is the main factor affecting the adsorption performance of pyrite. Therefore, the calcined pyrite powder is expected to be an excellent adsorbent for Cr(VI) in the wastewater treatment industry.

Alginate@ZnCO2O4 for efficient peroxymonosulfate activation towards effective rhodamine B degradation: optimization using response surface methodology

A facile chemical procedure was utilized to produce an effective peroxy-monosulfate (PMS) activator, namely ZnCo₂O₄/alginate. To enhance the degradation efficiency of Rhodamine B (RhB), a novel response surface methodology (RSM) based on the Box-Behnken Design (BBD) method was employed. Physical and chemical properties of each catalyst (ZnCo₂O₄ and ZnCo₂O₄/alginate) were characterized using several techniques, such as FTIR, TGA, XRD, SEM, and TEM. By employing BBD-RSM with a quadratic statistical model and ANOVA analysis, the optimal conditions for RhB decomposition were mathematically determined, based on four parameters including catalyst dose, PMS dose, RhB concentration, and reaction time. The optimal conditions were achieved at a PMS dose of 1 g l^-1, a catalyst dose of 1 g l^-1, a dye concentration of 25 mg l^-1, and a time of 40 min, with a RhB decomposition efficacy of 98%. The ZnCo₂O₄/alginate catalyst displayed remarkable stability and reusability, as demonstrated by recycling tests. Additionally, quenching tests confirmed that SO₄˙^-/OH˙ radicals played a crucial role in the RhB decomposition process.

A correlation of the adsorption capacity of perovskite/biochar composite with the metal ion characteristics

LaFeO₃/biochar composite is prepared by cellulose-modified microwave-assisted method at 450 °C. The structure is identified by Raman spectrum which, consists of characteristics biochar bands and octahedral perovskite chemical shifts. The morphology is examined by scanning electron microscope (SEM); two phases are observed, rough microporous biochar and orthorhombic perovskite particles. The BET surface area of the composite is 57.63 m²/g. The prepared composite is applied as a sorbent for the removal of Pb²⁺, Cd²⁺, and Cu²⁺ ions from aqueous solutions and wastewater. The adsorption ability reaches a maximum at pH > 6 for Cd²⁺, and Cu²⁺ ions, and is pH-independent for Pb²⁺ ions adsorption. The adsorption follows pseudo 2nd order kinetic model, Langmuir isotherm for Pb²⁺ ions, and Temkin isotherms for Cd²⁺, and Cu²⁺ ions. The maximum adsorption capacities, q_m, are 606, 391, and 112 mg/g for Pb²⁺, Cd²⁺, and Cu²⁺ ions, respectively. The electrostatic interaction is responsible for the adsorption of Cd²⁺, and Cu²⁺ ions on LaFeO₃/biochar composite. In case of Pb²⁺ ions form a complex with the surface functional groups of the adsorbate. LaFeO₃/biochar composite shows high selectivity for the studied metal ions and excellent performance in real samples. The proposed sorbent can be easily regenerated and effectively reused.

Efficient degradation of hexabromocyclododecane using montmorillonite supported nano-zero-valent iron and Citrobacter sp. Y3

The coupling of modified nanoscale zero-valent iron (nZVI) with organohalide-degrading bacteria provides a promising solution for the remediation of hexabromocyclododecane (HBCD)-contaminated environments. However, the interactions between modified nZVI and dehalogenase bacteria are intricate, and the mechanisms of synergistic action and electron transfer are not clear, and requires further specific investigation. In this study, HBCD was used as a model pollutant, and stable isotope analysis revealed that organic montmorillonite (OMt)-supported nZVI coupled with the degrading bacterial strain Citrobacter sp. Y3 (nZVI/OMt-Y3) can use [¹³C]HBCD as the sole carbon source and degrade or even mineralise it into ¹³CO₂ with a maximum conversion rate of 100% within approximately 5 days. Analysis of the intermediates showed that the degradation of HBCD mainly involves three different pathways: dehydrobromination, hydroxylation, and debromination. The proteomics results showed that nZVI introduction promoted the transport of electrons and debromination. Combining the results from XPS, FTIR, and Raman spectroscopy with the analysis results of proteinomics and biodegradation products, we verified the process of electron transport and proposed a metabolic mechanism of HBCD degradation by the nZVI/OMt-Y3. Moreover, this study provides insightful avenues and models for the further remediation of HBCD and other similar pollutants in the environment.

Polyethylene glycol-modified mesoporous zerovalent iron nanoparticle as potential catalyst for improved reductive degradation of Congo red from wastewater

In this study, bare zero-valent iron nanoparticles (nZVI) have been modified using polyethylene glycol (PEG) of various molecular weight in a facile technique. The synthesized nZVI modified with PEG, M.W. of 600 and 6000 was denoted by nZVI-PEG₆₀₀ and nZVI-PEG₆₀₀₀, respectively, and compared their catalytic activity towards the reductive degradation of Congo red (CR) using NaBH₄.The existence of PEG layer surrounds the nZVI core was confirmed by several characterization tools, such as XRD, FTIR, FESEM and TEM. Herein, both nZVI-PEG₆₀₀ and nZVI-PEG₆₀₀₀ exhibited remarkable removal efficiencies of 89.6% and 99.2% within 14 min of reaction time. The optimum reaction parameters were found to be as follows: 0.2 g L^-1 catalyst dose and initial dye concentration of 2 × 10^-5 molL^-1 etc. Kinetic studies of dye degradation were investigated which follow pseudo-1^st-order kinetics. The TOC analysis confirmed the complete mineralization of CR dye by nZVI-PEG₆₀₀₀ nanocatalyst. GCMS analysis of plausible degraded products was performed to elucidate a probable mechanistic pathway of CR degradation. Further, we have investigated the degradation of two anionic dyes mixture, i.e., CR and methyl orange (MO) using best catalyst, i.e., nZVI-PEG₆₀₀₀.

Nanoporous hydrogel absorbent based on salep: Swelling behavior and methyl orange adsorption capacity

This study used the gas-blowing method to develop a nanoporous hydrogel using poly (3-sulfopropyl acrylate-co-acrylic acid-co-acrylamide) grafted onto salep. The synthesis of the nanoporous hydrogel was optimized by various parameters for maximum swelling capacity. The nanoporous hydrogel was characterized using FT-IR, TGA, XRD, TEM, and SEM analyses. Images from SEM showed numerous pores and channels in the hydrogel with an average size of about 80 nm, forming a honeycomb-like shape. The change in surface charge was investigated by zeta potential and revealed that the surface charge of the hydrogel ranged from 20 mV at acidic conditions to -25 mV at basic conditions. The swelling behavior of optimum superabsorbent hydrogel was determined under different environmental conditions, such as different pH values, ionic strengths of the environment, and solvents. In addition, the swelling kinetics and the absorbance under loading of the hydrogel sample in different environments were investigated. Moreover, Methyl Orange (MO) dye was removed from aqueous solutions using the nanoporous hydrogel as an adsorbent. The adsorption behavior of the hydrogel was examined under various conditions, and the adsorption capacity of the hydrogel was found tobe 400 mg g^-1. The maximum water uptake was obtained under the following conditions: Salep weight = 0.01 g, AA = 60 μL, MBA = 300 μL, APS = 60 μL, TEMED = 90 μL, AAm = 600 μL, and SPAK = 90 μL. Lastly, the adsorption kinetics was studied by employing pseudo-first-order, pseudo-second-order, and intra-particle diffusion models.

3D porous carbon-embedded nZVI@Fe2O3 nanoarchitectures enable prominent performance and recyclability in antibiotic removal

Overcoming the instability and poor recyclability during the practical applications of contaminant scavengers is a challenging topic. Herein, a three-dimensional (3D) interconnected carbon aerogel (nZVI@Fe₂O₃/PC) embedding a core-shell nanostructure of nZVI@Fe₂O₃ was elaborately designed and fabricated via an in-situ self-assembly process. The porous carbon with 3D network architecture exhibits strong adsorption towards various antibiotic contaminants in water, where the stably embedded nZVI@Fe₂O₃ nanoparticles not only serve as magnetic seeds for recycling, but also avoid the shedding and oxidation of nZVI in the adsorption process. As a result, nZVI@Fe₂O₃/PC efficiently captures sulfamethoxazole (SMX), sulfamethazine (SMZ), ciprofloxacin (CIP), tetracycline (TC) and other antibiotics in water. In particular, an excellent adsorptive removal capacity of 329 mg g^-1 and a rapid capture kinetics (99% of removal efficiency in 10 min) under a wide pH adaptability (2-8) are achieved using nZVI@Fe₂O₃/PC as an SMX scavenger. nZVI@Fe₂O₃/PC displays exceptional long-term stability given that it shows excellent magnetic property after it is stored in water solution for 60 d, making it an ideal stable scavenger for contaminants in an etching-resistant and efficient manner. This work would also provide a general strategy to develop other stable iron-based functional architectures for efficient catalytic degradation, energy conversion and biomedicine.

Preparation of sludge-cyanobacteria composite carbon for synergistically enhanced co-removal of Cu(II) and Cr(VI)

Traditional sludge disposal is currently restricted by the risk of secondary pollution. Sludge carbon material has gained widespread attention because of its low cost and environmentally sustainable properties. However, owing to the high ash content and low-energy density of sludge, sludge pyrolysis alone has certain limitations, and the performance of carbon materials needs to be improved. Herein, a sludge-cyanobacteria composite carbon (SCC) was easily synthesized, and the adsorption process of Cu(II) and Cr(VI) by SCC was examined. SCC-700-2-50% exhibited a high S_BET (1047.54 m²/g) and developed pore structure rich in functional groups (such as -NH, -OH, and C-O). The combination of pore structure and functional groups improved the adsorption performance of SCC. The adsorption processes exhibited a synergistic effect in a binary system: the q_m of Cu(II) and Cr(VI) were 386 mg/g and 341 mg/g, respectively, and the selectivity of Cu(II) adsorption by SCC was greater than Cr(VI). The adsorption process, examined by SEM-EDS, FTIR, and XPS analysis, indicated that Cu(II) as a cationic interface strengthens Cr(VI) adsorption through electrostatic interaction, and the anion Cr(VI) created a valid electrostatic shield against the electrostatic repulsion between H⁺ and Cu(II), facilitating Cu(II) adsorption. SCC had great reusability: Cu(II) and Cr(VI) adsorption capacity were 90% and 84%, of the initial adsorption capacity, respectively, after six cycles. This study demonstrates the prospect of SCC as a valid adsorbent for multiple heavy metal contaminations removal.

Non-toxic and biodegradable κ-carrageenan/ZnO hydrogel for adsorptive removal of norfloxacin: Optimization using response surface methodology

Antibiotic resistance is increasing globally due to increased prescription and easy dispensing of antibiotic drugs universally. Hence, to mitigate this effect, efficient, biodegradable, and non-toxic adsorbents are required to be developed. Carrageenan (CG), a natural polymer, having multiple functional groups, provides a backbone for crosslinking with borax and incorporation of ZnO nanoparticles that formed borax-cross-linked κ-carrageenan (CG/Bx/ZnO) hydrogel which is used for efficient adsorption of norfloxacin from water. Surface morphology of as-synthesized hydrogel revealed the rough surface, which was determined by FESEM. Surface area of CG/Bx/ZnO hydrogel was found to be 22.90 m²/g with 3.41 nm pore radius. Systematic batch adsorption studies revealed that 99.4 % removal efficiency could be achieved at a dosage level of 20 mg/L of norfloxacin with 10 mg of hydrogel at pH of 4 in 8 h at room temperature. Experimentally optimized key parameters affecting the overall efficiency of adsorption matched well with the results assessed from ANOVA using Box-Behnken composite design model. The adsorption process was well fitted with the pseudo-second-order model and Langmuir isotherm with 1282.05 mg/g adsorption capacity. Thermodynamic study results show that adsorption is spontaneous and endothermic. The CG/Bx/ZnO hydrogel demonstrated excellent repeatability with minimal loss in norfloxacin adsorption for seven cycles.

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.

Efficient Biosorption of Hexavalent Chromium from Water with Human Hair

The triphenyl group (trityl radical) possessing three-phenyl rings, self-assembled through aromatic π-π stacking interactions, can form interesting crystalline organic nano-flowers. In this work, we have synthesized a hybrid material of 1,2-bis(tritylthio)ethane and magnetite, which reduces toxic Cr(VI) to non-toxic Cr(III). We validated the efficacy of the hybrid in reducing toxic Cr(VI) along with three other adsorbent systems. Among the five adsorbent systems tested, we observed that human hair has higher Cr removal efficiency, which prompted us to explore further using different mechanical forms of human hair. Pulverized hair (PH), hair powder (HP), and raw hair (RH) were evaluated by employing different reaction factors such as the adsorbent dose, pH, initial Cr(VI) concentration, and contact time. The comparative evaluation showed that PH has greater adsorption capacity (15.14 mg/g), followed by RH (13.27 mg/g) and HP (10.5 mg/g). While investigating the adsorption mechanism, we observed that it follows pseudo-second-order kinetics suggesting chemisorption. The Freundlich isotherm model fitted well for Cr(VI) adsorption by human hair, suggesting a multi-layered adsorption process. Overall, this study promises a cost-effective and eco-friendly bio-adsorbent for Cr(VI), which may be scaled up to design automated industrial waste disposal systems.

Oxygen-Deficient Engineering for Perovskite Oxides in the Application of AOPs: Regulation, Detection, and Reduction Mechanism

A perovskite catalyst combined with various advanced oxidation processes (AOPs) to treat organic wastewater attracted extensive attention. The physical and chemical catalytic properties of perovskite were largely related to oxygen vacancies (OVs). In this paper, the recent advances in the regulation of OVs in perovskite for enhancing the functionality of the catalyst was reviewed, such as substitution, doping, heat treatment, wet-chemical redox reaction, exsolution, and etching. The techniques of detecting the OVs were also reviewed. An insight was provided into the OVs of perovskite and reduction mechanism in AOPs in this review, which is helpful for the reader to better understand the methods of regulating and detecting OVs in various AOPs.

Structure-performance correlation guided cerium-based metal-organic frameworks: Superior adsorbents for fluoride removal in water

Fluoride in the hydrosphere exceeds the standard, which could be critically hazardous to human health and the natural environment. The adsorption method is a mature and effective way to remove pollutants in water, including fluoride. In this study, we synthesized three kinds of cerium-based metal-organic frameworks (Ce-MOFs) with different structures and properties by modulating the organic ligands (i.e., trimesic acid (BTC), 1,2,4,5-benzenetetracarboxylic acid (PMA), and terephthalic acid (BDC)) via the solvothermal method. The adsorption kinetics of Ce-MOFs on fluoride well fit the pseudo second order model, and their adsorption isotherms also conform to Langmuir isothermal model. The thermodynamic study reveals that the adsorption process is a spontaneous endothermic reaction. The maximum saturated adsorption capacities of Ce-BTC, Ce-PMA, and Ce-BDC are 70.7, 159.6, and 139.5 mg g^-1, respectively. Ce-MOFs have stable and excellent adsorption capacity at pH = 3-9. Coexisting anions (Cl^-, SO₄^2-, and NO₃^-) do not affect the performance of Ce-MOFs for fluoride removal. Moreover, Ce-MOFs also show their broad prospect as superior fluoride adsorbents because of their excellent performance and reusability in real water samples. Organic ligands have a remarkable influence on the defluoridation performance of Ce-MOFs. This work will provide a feasible idea for designing MOFs as superiors adsorbents for defluoridation.

Chromium Transport and Fate in Vadose Zone: Effects of Simulated Acid Rain and Colloidal Types

Chromium (Cr) can enter groundwater through rainfall infiltration and significantly affects human health. However, the mechanisms by which soil colloids affect chromium transport are not well investigated. In this study, column experiments were conducted to simulate the chromium (Cr) transport mechanism in two typical soils (humic acid + cinnamon soil and montmorillonite + silt) in the vadose zone of a contaminated site and the effects of acid rain infiltration conditions. The results showed that Mt colloids have less influence on Cr. The fixation of Cr by colloid mainly occurs in the cinnamon soil layer containing HA colloid. The adsorption efficiency of Cr was increased by 12.8% with the addition of HA. In the HA-Cr system, the introduction of SO4^2- inhibited the adsorption of Cr, reducing the adsorption efficiency from 31.4% to 24.4%. The addition of Mt reduced the adsorption efficiency of Cr by 15%. In the Mt-Cr system, the introduction of SO4^2- had a promoting effect on Cr adsorption, with the adsorption efficiency increasing from 4.4% to 5.1%. Cr release was inhibited by 63.88% when HA colloid was present, but the inhibition owing to changes in acidity was only 14.47%. Mt colloid promotes Cr transport and increases the leaching rate by 2.64% compared to the absence of Mt. However, the effect of acidity change was not significant. Intermittent acid rain will pose a higher risk of pollutant release. Among the influencing factors, the type of colloid had the most significant influence on the efficiency of Cr leaching. This study guides the quantitative assessment of groundwater pollution risk caused by Cr in the vadose zone.

Insights into the mechanism underlying remediation of Cr(VI) contaminated aquifer using nanoscale zero-valent iron@reduced graphene oxide

Currently, there is an urgent need to develop functional nanomaterials for highly effective environmental remediation. However, the long-term effect of remedial materials upon their injection into contaminated aquifer has frequently been overlooked. Here, the remediation of Cr(VI) contaminated aquifer by reduced graphene oxide (rGO) supported nanoscale zero-valent iron (nZVI@rGO) was investigated from a long-term perspective. The performances of nZVI@rGO samples with different rGO loadings in the removal of aqueous Cr(VI) were evaluated in batch experiments. The electron transfer properties different nZVI@rGO samples were investigated by measuring their corrosive potentials using the steady-state Tafel polarization curves. The results show that the electron transfer efficiency between Cr(VI) and nZVI@rGO is enhanced owing to the large reactive conjugated structure of rGO. Besides, the surface passivation of nZVI is effectively retarded due to the uniform accommodation of Cr(III) precipitates on rGO. The structure and composition of nZVI@rGO before and after Cr(VI) removal were analyzed by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The characterization results revealed that most Cr(VI) ions (∼90%) will be reduced to Cr(III) precipitates on nZVI@rGO as the passivation product. Accordingly, Cr(VI) ions tend to react more readily at less blocked regions on the surface of rGO, and a layer-by-layer passivation model on nZVI@rGO surface is proposed. Our results provide new insights into the mechanism underlying the long-term remediation of Cr(VI) contaminated aquifer using nZVI@rGO, which helps design new materials and approaches for practical in-situ remediation engineering.

Preparation of Montmorillonite Nanosheets with a High Aspect Ratio through Heating/Rehydrating and Gas-Pushing Exfoliation

Montmorillonite (MMT) is an abundant silicate mineral with ultrahigh stability. The exfoliation of stacked MMT into high-aspect-ratio nanosheets is of crucial importance for various applications such as toxic gas suppression, barrier property enhancement, flame retardancy, and ion conduction. In this work, we develop a new heating/rehydrating and gas-pushing method that can successfully exfoliate MMT into nanosheets with aspect ratios (600-5000) far higher than the currently reported values (1-120). The MMT first goes through a "starvation pretreatment" under different heating temperatures to improve its hydrophilicity and is then rehydrated in a hydrogen peroxide solution. The hydrogen peroxide in the MMT interlayer space can decompose into water and oxygen bubbles, thus finally leading to the exfoliation via gas-pushing while preserving the large lateral size (mainly in the range of 1-6 μm) of the nanosheets. By changing the pretreatment temperature and pH value of the hydrogen peroxide solution, the exfoliation performance can be tuned. This simple and low-cost exfoliation method is promising to achieve the mass production of MMT nanosheets with a high aspect ratio and may promote its application in various fields such as energy conversion, drug delivery, and photocatalysis.

Fluorescent nanocellulose-based hydrogel incorporating titanate nanofibers for sorption and detection of Cr(VI)

Chromium pollution is a major environmental concern; thus, effective and multifunctional adsorbents for removing the Cr(VI) ion are urgently needed. A fluorescent nanocellulose-based hydrogel (FNH) incorporating titanate nanofibers (TNs) was developed for the sorption and detection of Cr(VI) ion. The chemical and physical structures of the hydrogels, as well as their sorption and detection properties, were studied. The predicted maximum adsorption capacity and the lowest detection limit of FNH were 648.4 mg/g and 0.039 μg/L, respectively. Furthermore, the sorption and detection mechanisms of FNH were discussed in detail. These results showed that the excellent sorption and detection might be mainly attributed to the three-dimensional (3D) porous structure constructed by TNs and cellulose nanocrystals modified with carbon dots, which improved the sorption ability and provided the rapid visual response to Cr(VI). Furthermore, cost analysis showed that FNH was cheaper than activated carbon in removing the Cr(VI) ion. This work established a facile technique in developing low-cost and multifunctional adsorbents.