Preparation and characterization of bifunctional Ti–Fe kaolinite composite for Cr(VI) removal

Efficient Cr(VI) removal by pyrite/porous biochar: Critical role of potassium salt and sulphur

The excessive accumulation of hexavalent chromium (Cr(VI)) in the environment poses a risk to environment and human health. In the present study, a potassium bicarbonate-modified pyrite/porous biochar composite (PKBC) was prepared in a one-step process and applied for the efficient removal of Cr(VI) in wastewater. The results showed that PKBC can significantly remove Cr(VI) within 4 h over a wide range of pH (2-11). Meanwhile, the PKBC demonstrated remarkable resistance towards interference from complex ions. The addition of potassium bicarbonate increased the pore structure of the material and promoted the release of Fe2+. The reduction of Cr(VI) in aqueous solution was primarily attributed to the Fe(II)/Fe(III) redox cycle. The sulphur species achieved Fe(II)/Fe(III) cycle through electron transfer with iron, thus ensuring the continuous reduction capacity of PKBC. Besides, the removal rate was also maintained at more than 85% in the actual water samples treatment process. This work provides a new way to remove hexavalent chromium from wastewater and demonstrates the potential critical role of potassium bicarbonate and sulphur.

Biochar-SO prepared from pea peels by dehydration with sulfuric acid improves the adsorption of Cr6+ from water

A new biochar was produced from pea peel residues by the dehydration process. The effect of the obtained new biochar on the ability to remove Cr6+ ions from the aqueous solution was investigated. Biochar-SO was obtained from pea peel by dehydration of biochar with 50% sulfuric acid. The obtained biochars were characterized by Fourier transform infrared (FT-IR); Brunauer, Emmett and Teller (BET); Barrett-Joyner-Halenda (BJH); thermogravimetric analysis (TGA); differential scanning calorimetry (DSC); scanning electron microscope (SEM); and energy-dispersive X-ray (EDAX) analyses. The optimum pH value for Cr6+ ion removal was determined as 1.48. The maximum removal percentage of Cr6+ ions was 90.74% for Biochar-SO of 100 mg·L−1 Cr6+ ions initial concentration and 1.0 g L−1 adsorbent dosage. The maximum adsorption capacity (Qm) of biochar-SO was 158.73 mg·g−1. The data obtained were analyzed with Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich (D-R) isotherm models. In addition, the data obtained from these isotherm models were tested using different error functions (hybrid error function (HYBRID), average percent errors (APE), the sum of the absolute errors (EABS), chi-square error (X2), and Marquardt’s percent standard deviation (MPSD and the root mean square errors (RMS)) equations. It was the Freundlich isotherm model that best fits the experimental data of biochar-SO. Kinetic data were evaluated by pseudo–first-order (PFO), pseudo–second-order (PSO), Elovich, and intraparticle diffusion models. The adsorption rate was primarily controlled by the PSO rate model with a good correlation (R2 = 1). The adsorption mechanism of biochar-SO to remove Cr6+ ions can be based on electrostatic interaction and ion exchange with exchangeable cations in biochar such as aluminum, silicon, and calcium ions for chromium. The results indicate that biochar-SO is a promising adsorbent for the adsorption of Cr6+ ions that can be employed repeatedly without substantial loss of adsorption effectiveness.

Laboratory investigation on the retention performance of a soil-bentonite mixture used as an engineered barrier: insight into the effects of ionic strength and associated heavy metal ions

Soil-bentonite (S-B) materials are promising backfill materials for use as engineered barriers in heavy metal-contaminated sites. The effects of contaminant exposure on the retention performance of the S-B barrier remain unrevealed. In this study, based on the pollution status of an abandoned ferroalloy factory located in southern China, the retention performance of the S-B mixture toward Cr(VI) and Zn(II) was studied through adsorption and diffusion experiments sequentially; the separate effect of ionic strength (binary solution) and the combined effect of ionic strength and associated heavy metal ion (ternary solution) were discussed. In NaCl-Cr(VI)/Zn(II) binary solutions, the adsorption of Zn(II) onto the S-B mixture is larger than that of Cr(VI). K_d, Q_max, and ɛ_acc (accessible porosity) of Cr(VI) increase through increasing ionic strength, while Zn(II) shows the opposite trend; D_e (effective diffusion coefficient) values for both Cr(VI) and Zn(II) increased with increasing ionic strength and follow a sequence of Cr(VI) > Zn(II), indicating a better retention performance of the S-B mixture to Zn(II). For a given ionic strength, the adsorption of Zn(II) was larger than that of Cr(VI), which can be attributed to the retention specificity of the S-B mixture to anion and cation. In Cr(VI)-Zn(II)-NaCl ternary solutions, the adsorptions of Cr(VI) and Zn(II) are enhanced in varying degrees when compared with their binary solution, which probably could be attributed to the ion bridge role of Cr(VI)/Zn(II) to connect each other that relatively increased the adsorption capacity of S-B material. This work will contribute to an in-depth understanding of the retention performance of the S-B mixture in complicated chemical environments and facilitate the selection of future remediation strategies.

Efficient removal of Cr(VI) by a 3D Z-scheme TiO2-ZnxCd1-xS graphene aerogel via synergy of adsorption and photocatalysis under visible light

Efficient and robust photocatalysts for environmental pollutants removal with outstanding stability have great significance. Herein, we report a kind of three dimensional (3D) photocatalyst presented as Z-scheme heterojunction, which combining TiO₂ and Zn_xCd_1-xS with graphene aerogel to contrast TiO₂-Zn_xCd_1-xS graphene aerogel (TSGA, x=0.5) through a moderate hydrothermal process. The as-prepared Z-scheme TSGA was used to remove aqueous Cr(VI) via a synergistic effect of adsorption and visible light photocatalysis. The adsorption equilibrium can be reached about 40 min, then after about 30 min irradiation under visible light (wavelength (λ) > 420 nm) the removal rate of Cr(VI) almost reached 100%, which is much better than the performance of pristine TiO₂ and Zn_0.5Cd_0.5S, as well as TiO₂ graphene aerogel (TGA) and Zn_0.5Cd_0.5S graphene aerogel (SGA). The virulent Cr(VI) was reduced to Cr(III) with hypotoxicity after photocatalysis on TSGA, meanwhile the as-synthesized TSGA presented a good stability and reusability. The reduced graphene oxide (rGO) sheets between TiO₂ and Zn_0.5Cd_0.5S played a role as charge transfer mediator, promoting the photoinduced electrons transfer and photocatalysis ability of TSGA was enhanced significantly. Hence, such photocatalyst exhibits a potential application on removing heavy metals with high efficiency and stability from polluted aqueous environment.

Removal of Cr(VI) from aqueous solutions via simultaneous reduction and adsorption by modified bimetallic MOF-derived carbon material Cu@MIL-53(Fe): Performance, kinetics, and mechanism

Cr(VI) has drawn growing concern because of its acute toxicity and strong carcinogenic properties to most organisms. Metal-organic frameworks (MOFs) have attracted broad interest in removing Cr(VI) as a novel porous adsorbent. In this work, a novel modified Cu@MIL-53(Fe) material and its derivatives have been successfully synthesized via solvothermal and calcination methods and applied for Cr(VI) removal. Experimental parameters, such as the amount of the added Cu, the calcination temperature, the pollutant concentrations, the pH value of solution, etc. were optimized. The Cu@MIL-53(Fe) optimized synthesis parameters were determined as a 0.5 M ratio of Cu/Fe and 800 °C of calcination temperature. The Cr(VI) removal capacities were 20.65 mg/g at 180 min and 13.35 mg/g in 15 min, and 45.55% of total chromium and 99.05% of Cr(VI) were removed at a dose of 0.5 g/L, pH = 3, 25 °C. Batch experiments revealed that the reaction process applied for Langmuir adsorption isotherm and pseudo-second-order models most suitable with q_m = 724.6 mg/g. Additionally, Cr (VI) could be reduced to less toxic Cr(III) by Fe⁰ and Cu⁰ during redox reactions. According to further mechanism analysis, the process was primarily monolayer chemical adsorption, followed by electrostatic interaction, redox reaction co-precipitation and coordination effect, etc. A novel promising method of Cr(VI) removal from acidic water by MOFs adsorption is presented in this study.

Assessment of algal biomass towards removal of Cr(VI) from tannery effluent: a sustainable approach

The current investigation focuses on a systematic study of application of two dried algal biomass (i.e., Nostoc sp. and Turbinaria vulgaris) in removal of Cr(VI) from synthetic solution as well as tannery industrial wastewater. The optimized conditions for Cr(VI) removal are nearly same for the both the biosorbents (i.e., pH 2.8, initial Cr(VI) concentration 100 mg L^-1, biomass dosage of 1.2g L^-1, contact time 120 and 110 min). Nostoc sp. (q_max=23.94mg g^-1) was observed to possess a superior removal capability when compared to Turbinaria vulgaris (q_max=21.8mg g^-1). Desorption studies were performed with four different desorbing agents. Application study was conducted using tannery wastewater with Nostoc sp. and 94.20% removal of Cr(VI) was obtained. Hence, this study revealed that Nostoc sp. and T. vulgaris both have great potential to be an environment friendly and economic biosorbent for removal of Cr(VI) containing industrial effluent.

Rust triggers rapid reduction of Cr6+ by red phosphorus: The importance of electronic transfer medium of Fe3

Red phosphorus (P) is one of the metalloid materials, with five external electrons, it should be an excellent electron donor. However, the activity of red P to reduce Cr⁶⁺ is limited. Due to electrostatic repulsion, it is difficult for the electrons on the red P to transfer to the chromate anion (Cr⁶⁺). Interestingly, we found that Fe³⁺ derived from rust, waste iron or Fe³⁺ reagents can be used as the electron transport medium to solve the electron transport obstacles between red P and Cr⁶⁺. As a result, the reduction of Cr⁶⁺ by red P/rust system takes only 20 min, which is far lower than the 140 min of red P. The reduction rate of Cr⁶⁺ in the red P/rust system is about 12.3 times that of red P. The reaction mechanism is that red P is not easy to access chromate anions but can easily adsorb Fe³⁺. The adsorbed Fe³⁺ will be reduced to Fe²⁺ by red P, and the regenerated Fe²⁺ will diffuse into the solution to rapidly reduce Cr⁶⁺. Therefore, this work provides an alternative waste iron reuse pathway and also sheds light on the important role of electron medium in reduction reaction.

Mineral-Supported Photocatalysts: A Review of Materials, Mechanisms and Environmental Applications

Although they are of significant importance for environmental applications, the industrialization of photocatalytic techniques still faces many difficulties, and the most urgent concern is cost control. Natural minerals possess abundant chemical inertia and cost-efficiency, which is suitable for hybridizing with various effective photocatalysts. The use of natural minerals in photocatalytic systems can not only significantly decrease the pure photocatalyst dosage but can also produce a favorable synergistic effect between photocatalyst and mineral substrate. This review article discusses the current progress regarding the use of various mineral classes in photocatalytic applications. Owing to their unique structures, large surface area, and negatively charged surface, silicate minerals could enhance the adsorption capacity, reduce particle aggregation, and promote photogenerated electron-hole pair separation for hybrid photocatalysts. Moreover, controlling the morphology and structure properties of these materials could have a great influence on their light-harvesting ability and photocatalytic activity. Composed of silica and alumina or magnesia, some silicate minerals possess unique orderly organized porous or layered structures, which are proper templates to modify the photocatalyst framework. The non-silicate minerals (referred to carbonate and carbon-based minerals, sulfate, and sulfide minerals and other special minerals) can function not only as catalyst supports but also as photocatalysts after special modification due to their unique chemical formula and impurities. The dye-sensitized minerals, as another natural mineral application in photocatalysis, are proved to be superior photocatalysts for hydrogen evolution and wastewater treatment. This work aims to provide a complete research overview of the mineral-supported photocatalysts and summarizes the common synergistic effects between different mineral substrates and photocatalysts as well as to inspire more possibilities for natural mineral application in photocatalysis.

Highly Efficient and Simultaneous Removal of Cr(VI) and Imidacloprid through a Ferrocene-Modified MIL-100(Fe) Composite from an Aqueous Solution

A novel nanocomposite [Fc-MIL-100(Fe)] was constructed by combining ferrocene (Fc) with the porous structural metal-organic framework [MIL-100(Fe)]. The proposed composite material could simultaneously and efficiently remove hexavalent chromium [Cr(VI)] and imidacloprid and reduced strongly noxious Cr(VI) to weakly noxious trivalent chromium [Cr(III)]. The removal efficiencies of the composite material for Cr(VI) and imidacloprid could reach 95% after 15 h. The adsorption process was determined by kinetics, isotherms, and thermodynamics. The results demonstrated that the adsorption kinetics of Cr(VI) followed the pseudo-second-order model mainly by chemisorption; meanwhile, the adsorption of imidacloprid by the material conformed to the pseudo-first-order kinetics, which indicated that physical adsorption was the main process. Additionally, the intraparticle diffusion model revealed that the uptake of imidacloprid and Cr(VI) occurred via intraparticle diffusion at the composite material. The adsorption procedure for Cr(VI) was fitted to the Langmuir model (R² = 0.995) via monolayer adsorption, and that for imidacloprid was fitted to the Freundlich model (R² = 0.995) due to multilayer or heterogeneous adsorption. The thermodynamic research confirmed that the adsorption procedure was exothermic and spontaneous. Infrared spectroscopy, X-ray photoelectron spectra, and the pH effect implied that intermolecular hydrogen bonding and electrostatic interaction played a crucial role during the removal process. Fc-MIL-100(Fe) also exhibited long-term stability and satisfactory regeneration and reusability. Therefore, this method may enhance an environmentally friendly and prospective approach for concurrently removing imidacloprid and Cr(VI) from wastewater.

Microscopic understanding about adsorption and transport of different Cr(VI) species at mineral interfaces

Cr(VI) ranks as one of the most toxic heavy metals and herein, microscopic mechanisms for adsorption and transport of different Cr(VI) oxyanions (Cr₂O₇^2- and CrO₄^2-) at kaolinite interfaces are addressed by dispersion-corrected periodic density functional theory calculations. Cr(VI) oxyanions adsorb favorably at both tetrahedral and octahedral surfaces, and K⁺ ions serve as bridge for Cr(VI) oxyanions and tetrahedral surfaces while Cr(VI) oxyanions serve as bridge for K⁺ ions and octahedral surfaces. Adsorption structures are altered significantly by pH variation, and stability trends at different pH ranges are deciphered by the dominant interaction force with clay surfaces: Electrostatic interaction with K⁺ ions at tetrahedral surfaces whereas combined action of electrostatic and H-bonding interactions with Cr(VI) oxyanions at octahedral surfaces. Electron transfers are strongly pH-dependent, and clay surfaces serve as electron reservoirs. CrO₄^2- rather than Cr₂O₇^2- is dominant at clay interfaces, and HCrO₄^- can co-exist under acidic conditions. Cr₂O₇^2- transformation to CrO₄^2- is kinetically blocked at pH ≈ PZC while preferred at pH < PZC. Cr(VI) removal and reclamation should proceed at pH > 7.0 and pH < PZC, respectively. Results greatly promote the understanding about Cr(VI) bioavailability and fate in surficial environments and are also useful for Cr(VI) removal and reclamation.

Mechanisms for Cr(VI) reduction by alcohols over clay edges: Reactive differences between ethanol and ethanediol, and selective conversions to Cr(IV), Cr(III) and Cr(II) species

Catalytic reduction by alcohols over clay minerals works efficiently under a wide range of pH and represents an emerging approach to control Cr(VI) contamination. Herein, mechanisms for Cr(VI) adsorption and reduction at clay edges are addressed by dispersion-corrected periodic DFT calculations, considering different active sites, and types (monohydric and polyhydric) and coverage of alcohols. Cr(VI) adsorbs favorably at clay edges, forming direct bonds and strong H-bonds. Mechanisms for Cr(VI) reduction by alcohols are largely determined by π-conjugation development, and efficient conversion conduces to Cr(VI) removal. Cr(II), Cr(III) and Cr(IV) are useful for different purposes, and high selectivity towards these products is realized through rational catalysts design: 1) Cr(IV) dominates at Al³⁺ site with all ethanol coverage, Al³⁺ site with high-coverage ethanediol, and Mg²⁺ site with low-coverage ethanol; 2) Cr(III) dominates at Al³⁺ and Mg²⁺ sites with low-coverage ethanediol; 3) Cr(II) dominates at Mg²⁺ site with high-coverage ethanol or ethanediol. Results agree finely with experimental observations available, and significant new insights have been provided for Cr management and recycling. Detailed electronic structure and vibrational analyses, which can also guide future experimental studies, manifest that Cr(VI) reduction progresses are effectively monitored by ESR and FT-IR techniques.

Application of pyridine-modified chitosan derivative for simultaneous adsorption of Cu(II) and oxyanions of Cr(VI) from aqueous solution

The bioadsorbent C1, which is a chitosan derivative prepared in a one-step synthesis, was successfully used to adsorb Cr(VI) and Cu(II) simultaneously. Here, for the first time the simultaneous adsorption of a cation and an anion was modeled using the Corsel model for kinetics and the Real Adsorbed Solution Theory model for equilibrium data. Batch studies of the adsorption of Cu(II) and Cr(VI) in single and binary aqueous solutions were performed as a function of initial solute concentration, contact time, and solution pH. The maximum adsorption capacities of C1 in single and binary aqueous solutions were 1.84 and 1.13 mmol g^-1 for Cu(II) and 3.86 and 0.98 mmol g^-1 for Cr(VI), respectively. The reuse of C1 was investigated, with Cu(II) ions being almost completely desorbed and fully re-adsorbed. For Cr(VI), the desorption was incomplete resulting in a lower re-adsorption. Energy-dispersive X-ray spectroscopy was used for mapping the distributions of Cr(VI) and Cu(II) adsorbed on the C1 surface in single and binary adsorption systems. Isothermal titration calorimetry experiments were performed for Cr(VI) and Cu(II) adsorption in single solutions. The thermodynamic parameters of adsorption showed that the adsorption of both metal ions was enthalpically driven, but entropically unfavorable.

Preparation of a New Iron-Carbon-Loaded Constructed Wetland Substrate and Enhanced Phosphorus Removal Performance

Iron-carbon substrates have attracted extensive attention in water treatment due to their excellent processing ability. The traditional iron-carbon substrate suffers from poor removal effects, separation of the cathode and anode, hardening, secondary pollution, etc. In this study, a new type of iron-carbon-loaded substrate (NICLS) was developed to solve the problems of traditional micro-electrolytic substrates. Through experimental research, a preparation method for the NICLS with Fe and C as the core, zeolite as the skeleton, and water-based polyurethane as the binder was proposed. The performance of the NICLS in phosphorus-containing wastewater was analyzed. The results are as follows: The optimal synthesis conditions of the NICLS are 1 g hydroxycellulose, wood activated carbon as the cathode, an activated carbon particle size of 200-60 mesh, and an Fe/C ratio of 1:1. Acidic conditions can promote the degradation of phosphorus by the NICLS. Through the characterization of the NICLS (scanning electron microscope (SEM), X-ray diffractometer (XRD), and energy-dispersive spectrometer (EDS), etc.), it is concluded that the mechanism of the NICLS phosphorus removal is a chemical reaction produced by micro-electrolysis. Using the NICLS to treat phosphorus-containing wastewater has the advantages of high efficiency and durability. Therefore, it can be considered that the NICLS is a promising material to remove phosphorus.

Removal of Cr(VI) from wastewater by artificial zeolite spheres loaded with nano Fe-Al bimetallic oxide in constructed wetland

In order to solve the problems of poor adsorption capacity and low stability in treating heavy metal wastewater with traditional constructed wetland (CW) fillers, a new type of filler, artificial zeolite spheres loaded with nano Fe-Al bimetallic oxide (hereinafter referred to as composite zeolite spheres), was prepared for Cr(VI) removal from wastewater. The results indicated that nano Fe-Al bimetallic oxide was an effective material for Cr(VI) removal with the maximal removal efficiency of 84.9% at initial Cr(VI) concentration of 20 mg/L (pH = 3). The micro-reactor experiment further verified that composite zeolite spheres had better removal performance on Cr(VI) than traditional filler. Fourier-transform infrared (FT-IR), X-ray diffractometry (XRD) and X-Ray photoemission spectroscopy (XPS) results demonstrated that -OH groups reduced Cr(VI) to Cr(III), and then the Cr(III) was removed by forming Cr_xFe_1-x(OH)₃ precipitation with Fe(III) or formed Cr(OH)₃ precipitation with Al-OH through the ion exchange. This study provided an effective approach for treating Cr(VI) wastewater by using a new composite zeolite in constructed wetlands (CWs).

Mixed Redox-Couple-Involved Chalcopyrite Phase CuFeS2 Quantum Dots for Highly Efficient Cr(VI) Removal

Iron-based nanosized ecomaterials for efficient Cr(VI) removal are of great interest to environmental chemists. Herein, inspired by the "mixed redox-couple" cations involved in the crystal structure and the quantum confinement effects resulting from the particle size, a novel type of iron-based ecomaterial, semiconducting chalcopyrite quantum dots (QDs), was developed and used for Cr(VI) removal. A high removal capacity up to 720 mg/g was achieved under optimal pH conditions, which is superior to those of the state-of-the-art nanomaterials for Cr(VI) removal. The mechanism of Cr(VI) removal was elucidated down to an atomic scale by combining comprehensive characterization techniques with adsorption kinetic experiments and DFT calculations. The experimental results revealed that the material was a good electron donor semiconductor attributed to the existence of "mixed redox couple of Cu(I)-S-Fe(III)" in the crystal structure. With the size-dependent quantum confinement effect and the high surface area, the semiconducting chalcopyrite QDs could effectively remove Cr(VI) from aqueous solution through a syngenetic photocatalytic reduction and adsorption mechanism. This study not only reports the design histogram of the iron-based CuFeS₂ QD ecomaterial for efficient Cr(VI) removal but also paves the way for understanding the atomic-scale mechanism behind the syngenetic effects of using the QD semiconducting material for Cr(VI) removal.

Preparation of Reduced Graphene Oxide Aerogel and Its Adsorption for Pb(II)

In this study, reduced graphene oxide (rGO) aerogels were successfully prepared using a facile hydrothermal method and determined with Fourier transform infrared spectroscopy, X-ray diffraction spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller surface area, and scanning electron microscopy. The rGO aerogels were used to remove Pb(II) from aqueous solution, and the adsorption performance of rGO aerogels was investigated. In addition, the adsorption-desorption cycle experiments were carried out to evaluate the recyclability and stability of rGO aerogels. The adsorption data were consistent with the pseudo-second-order kinetic model and Langmuir isotherm model. The experimental results showed that rGO aerogels have good adsorption capacity for Pb(II) and can be utilized for wastewater treatment.

Screening for the action mechanisms of Fe and Ni in the reduction of Cr(VI) by Fe/Ni nanoparticles

Zero-valent iron (ZVI), Fe²⁺ and H₂ are possible electron donors in the reduction of Cr(VI) by nanoscale ZVI (n-ZVI). However, it is often ambiguous about the roles of these electron donors in the reductive removal of Cr(VI) from groundwater and wastewater. This study investigated the action mechanisms of Fe and Ni in Cr(VI) reduction by Fe/Ni nanoparticles (n-Fe/Ni). Among the three possible reduction mechanisms of ZVI, direct electron transfer from ZVI and its corrosion product, Fe²⁺, were confirmed to be responsible for the reduction removal of Cr(VI). H₂, another product of ZVI corrosion, was found incapable of reducing Cr(VI). In addition, the secondary metal Ni in n-Fe/Ni was found to facilitate the direct electron transfer from ZVI owing to its ability to inhibit the passivation of ZVI and to enhance the production of Fe²⁺ due to the formation of FeNi galvanic cells. The results of characterizations on n-Fe/Ni before and after the reaction with Cr(VI) demonstrated that Cr(VI) was reduced to Cr(III), which existed as FeCr₂O₄ precipitates on the surface of n-Fe/Ni, resulting in effective sequestration of Cr(VI). These findings are important for understanding the main mechanisms of bimetallic nanoparticles or nanomaterials for reductive immobilization of Cr(VI), and may guide further ZVI-based technology development for remediation of contaminated water or soil with redox-active contaminants.

Nanoscale zero-valent iron intercalated 2D titanium carbides for removal of Cr(VI) in aqueous solution and the mechanistic aspect

MXenes, as new members of the 2D materials group, are regarded as good candidates for heavy metal removal particularly for radioactive metal element because of their high irradiation stability. However, the small interlayer distance and lack of active sites on the surface limit their further application. In this report, nanoscale zero-valent iron has been introduced into the inter-layer structure of alkaline intercalated Ti₃C₂ (Alk-Ti₃C₂) and investigated to Cr(VI) removal. The XPS spectra, SEM images, TEM images, and N₂ adsorption-desorption isotherms characterizations revealed that the OH groups on the Alk-Ti₃C₂ surface assisted the introducing of nZVI into the inter-layer of Alk-Ti₃C₂ and subsequently stabilized the nZVI. The increased active sites of nZVI and extended interlayer space of Alk-Ti₃C₂ could improve the uptake capacity of Cr(VI) (194.87 mg/g at pH = 2). The highly efficient removal of Cr(VI) was maintained even in the presence of coexisting cations, which showed great potential for real environment remediation. Mechanistic study indicated that the synergistic effects of nZVI and Alk-Ti₃C₂ nanosheet in nZVI-Alk-Ti₃C₂ composites are keys for Cr(VI) removal.

In situ construction efficient visible-light-driven three-dimensional Polypyrrole/Zn3In2S6 nanoflower to systematically explore the photoreduction of Cr(VI): Performance, factors and mechanism

Photoreduction of highly toxic Cr(VI) has been regarded as an efficient and green method to achieve water purification. In this process, better charge carrier separation is vital to achieving excellent performance. Besides, it is vital to systematically explore the influencing factors and reaction mechanism. Herein, a novel 3D PPy/Zn₃In₂S₆ nanoflower composite was successfully fabricated via in-situ polymerization. The remarkable conductivity of PPy provides a good electron transport path to facilitate the separation and migration of charge carriers, which benefits to the activity improvement. The results show that 5% PPy/Zn₃In₂S₆ exhibits superior photocatalytic activity with almost 100% Cr(VI) reduction just within 24 min and 99.4% of Methyl orange (MO) is degraded in 25 min. On this basis, factors of different catalyst dosage, concentration, ions and pH under the reduction system were systematically investigated. Especially, different organic acids were in-depth analyzed and the activity could be significantly enhanced just adding 0.1 mmol organic acids. 5% 3D PPy/Zn₃In₂S₆ nanoflower composites (with tartaric acid) exhibits superior photocatalytic activity, which can achieve 100% photoreduction of Cr(VI) just within 6 min. Finally, a possible reaction mechanism was proposed. Moreover, 3D PPy/Zn₃In₂S₆ nanoflower also presented an efficient photodegradation activity for organic pollution.

Enhanced removal of ethidium bromide (EtBr) from aqueous solution using rectorite

Ethidium bromide (EtBr) is an intercalating agent commonly used as nucleic acid fluorescent tag in various techniques of life science field. It is considered as a serious biohazard due to its mutagenicity and carcinogenicity. As such, developing high efficiency and low cost materials as cleanup kits is in urgent need although many methods have already been developed. In this study we take use of the affinity of organic cations for clay minerals of high cation exchange capacity (CEC) and large specific surface area (SSA) and tested the removal of EtBr using rectorite, a type of clay mineral made of 1:1 regularly mixed layers of illite and montmorillonite. Our results showed that the uptake of Et⁺ on rectorite could be as high as 400 mmol/kg and the removal of Et⁺ was extremely fast. Desorption of inorganic cation Ca²⁺ and sorption of counterion Br^- revealed that cation exchange was the dominating mechanism of Et⁺ removal using rectorite. Thermal analyses revealed that the EtBr could be thermally destructed inside the interlayer of rectorite and the material could be thermally regenerated. Thus, clay minerals could have a great potential to be fabricated into cleanup kits for the removal of EtBr in case of spill.

Hourglass-type polyoxometalate-based crystalline materials as efficient cooperating photocatalysts for the reduction of Cr(vi) and oxidation of dyes

Hourglass-type polyoxometalate-based crystalline materials exhibit efficient photocatalytic activities towards simultaneous photocatalytic Cr(vi) reduction and organic MB oxidation.

Porous NiFe-oxide nanocubes derived from prussian blue analogue as efficient adsorbents for the removal of toxic metal ions and organic dyes

A novel porous NiFe-oxide nanocubes (NiFe NCs) binary material was successfully fabricated via a facile and scalable tactic, which involved a morphology-inherited heat treating of Ni₃[Fe(CN)₆]₂·xH₂O prussian blue analogue nanocubes as self-sacrificial templates. Consequently, it was demonstrated that the NiFe NCs consisted of primary nanostructure units and interconnected pores, with an average size of ˜80 nm. When employed as adsorbents, the as-prepared NiFe NCs displayed remarkable adsorption capacities for heavy metal ions (232.3 mg g^-1 for As(V) and 350.71 mg g^-1 for Cr(VI)) and organic dyes (284.99 mg g^-1 for XO and 31.97 mg g^-1 for CR at 298 K). The resulting NiFe NCs further revealed efficient regeneration and reusability even after five consecutive adsorption/desorption cycles. The microscopic spectrum analysis demonstrated that the interaction between As(V) and NiFe NCs was mainly ascribed to the metal-oxide bonds (MO) and hydroxyl groups (OH), while Cr(VI) adsorption was in conjunction with the reduction reaction of Cr(VI) to Cr(III). Furthermore, the adsorption of organic dyes on NiFe NCs depended on the pore structure and molecule sizes of the organic dye molecules. These findings make cost-efficient NiFe NCs materials a powerful candidate for remediating water contaminated with inorganic and organic contaminants.

Removal of aqueous fluoroquinolones with multi-functional activated carbon (MFAC) derived from recycled long-root Eichhornia crassipes: batch and column studies

Fluoroquinolones (FQs) occur broadly in natural media due to its extensive use, and it has systematic effects on our ecosystem and human immunity. In this study, long-root Eichhornia crassipes was reclaimed as a multi-functional activated carbon (MFAC) to remove fluoroquinolones (FQs) from contaminated water. To get insight into the adsorption mechanism, multiple measurements, including FTIR and XPS analyses, were employed to investigate the adsorption processes of ciprofloxacin and norfloxacin as well as the experiments of effect of exogenous factors on adsorption performances. The results confirmed that the adsorption of FQs by MFAC was mainly attributed to the electrostatic interaction, hydrogen bond interaction, and electronic-donor-acceptor (EDA) interaction. In addition, the kinetics and thermodynamics experiments demonstrated that the MFAC possessed great adsorption performance for FQs. According to the Langmuir model, the saturated adsorption capacities exceeded 145.0 mg/g and 135.1 mg/g for CIP and NOR at 303.15 K, respectively. The column experiments were conducted to explore the application performance of MFAC on the advanced treatment of synthetic water at different flow rates and bed depths. The adsorption capacity of CIP on MFAC was estimated by the Thomas models and the bed-depth service time (BDST) models, reaching 127.56 mg/g and 11,999.52 mg/L, respectively. These results also provide a valid approach for the resource recycling of the redundant long-root Eichhornia crassipes plants. Graphical abstract.

Structural Design of a Cellulose-Based Solid Amine Adsorbent for the Complete Removal and Colorimetric Detection of Cr(VI)

A cellulose-based solid amine adsorbent (MCC/TEPAA) with high amino density for the detection and removal of Cr(VI) was designed and prepared through using epichlorohydrin cross-linking with MCC (microcrystalline cellulose) and tetraethylenepentamine (TEPA). The structure and amino density of the cellulose-based solid amine adsorbents could be tailored by adjusting the structure of the amines (triethylenetetramine or diethylenetriamine). The as-prepared cellulose-based solid amine adsorbents could detect and completely remove Cr(VI) from water, and the concentration of Cr(VI) solution after adsorption met the standard concentration of Cr(VI) solution for drinking water (0.05 mg/L). In particular, the MCC/TEPAA, supported by MCC with porosity as a framework, promoted the adsorption rate (adsorption equilibrium within only 10 min), removal rate (100%) of Cr(VI), and adsorption capacity (327.72 mg/g). In addition, the limit of colorimetric detection of Cr(VI) by MCC/TEPAA was 0.5 mg/L at 20 min when other interfering heavy metal ions exist. The adsorption and colorimetric detection mechanism of Cr(VI) on MCC/TEPAA was proposed to include electrostatic interactions, chelating reactions, and oxidation-reduction reactions, all of which contributed to the excellent adsorption and detection performance.

Facile conversion of kaolinite into clay nanotubes (KNTs) of enhanced adsorption properties for toxic heavy metals (Zn2+, Cd2+, Pb2+, and Cr6+) from water

Kaolinite nanotubes (KNTs) were synthesized from kaolinite by ultrasonic scrolling and characterized using X-ray diffractometer, scanning and transmission electron microscopes; and FTIR-FT Raman spectrometer. The synthetic KNTs appear as multi-walled scrolls of 12 nm average pore diameter and 50-600 nm particle length; and exhibit surface area of 105 m²/g. KNTs were used as adsorbents for Zn²⁺, Cd²⁺, Pb²⁺, and Cr⁶⁺ with uptake capacities of 103 mg/g, 116 mg/g, 89 mg/g, and 91 mg/g, respectively. The equilibration time of Cd²⁺ and Pb²⁺ adsorption is 360 min and for Cr⁶⁺ and Zn²⁺ area 120 min and 240 min, respectively. KNTs adsorption systems can be described mainly by Lagergren-second order and Freundlich models (R²> 0.95) as kinetic and isotherm models. This reflected multilayer adsorption forms with chemical sharing or ion exchange processes. KNTs exhibits high reusability and used for five cycles in the removal of the studied metals (100 mg/L). The removal percentages declined by 20.5%, 15.12%, 22.8% and 23.16% with repeating the reused cycles from cycle 1 to cycle 5 for Zn²⁺, Cd²⁺, Pb²⁺, and Cr⁶⁺, respectively. KNTs were applied successfully in realistic purification of tap water, groundwater, and sewage water from the inspected metals.

Photocatalytic removal of SO2 using natural zeolite modified by TiO2 and polyoxypropylene surfactant

Air pollution due to emission of various hazardous gases such as SO₂ into the atmosphere and its control is an important environmental issue. Application of photocatalysts is considered as a suitable process to control the gaseous pollutants. In this study, the efficiency of clinoptilolite as a natural zeolite (Ze) modified by TiO₂ (Ze-Ti) and a polymeric surfactant polyoxypropylene (Ze-Ti-POP) for removal of SO₂ was investigated. The nanocomposites were characterized by SEM, EDX, and BET analyses. The photocatalytic oxidation experiments of SO₂ by the nanocomposites and natural zeolite were done under UV irradiation with initial SO₂ concentration of 500 ppm in a photoreactor. The effects of different factors including reaction time, catalyst dose, UV irradiation intensity, humidity content, and calcination temperature and dose of TiO₂ were studied. The modification of clinoptilolite by TiO₂ and POP increased considerably the BET specific surface area of the nanocomposites. The results showed that maximum removal efficiencies of SO₂ by Ze-Ti and Ze-Ti-POP under the optimum experimental conditions were 82.1 and 87.4%, respectively. Adsorption kinetics data well fitted with the Langmuir-Hinshelwood model. Moreover, reusing of nanocomposites after three regeneration cycles indicated that application of Ze-Ti and Ze-Ti-POP nanocomposites could be a promising approach for SO₂ removal. Graphical abstract ᅟ.

Biosynthesis and Characterization of Iron Nanoparticles for Effective Adsorption of Cr(VI)

In this study, iron nanoparticles (FeNPs) were synthesized via a green method using loquat (Eriobotrya japonica) leaves aqueous extract as a renewable reducing agent. The synthesized FeNPs were characterized by DLS, XRD, FT-IR, SEM/EDX, and TEM analysis, and then, they were used as an adsorbent for Cr(VI) removal from aqueous solutions. Batch adsorption experiments were carried out to investigate the optimum adsorption parameters such as the initial pH of the solution, temperature, initial Cr(VI) concentration, and adsorbent concentration. The optimum adsorption conditions were determined as initial pH 3.0, temperature 45°C, and adsorbent concentration 1 g/L. Also, a linear increase was observed in adsorbed Cr(VI) amounts with the increasing initial Cr(VI) concentrations. The biosynthesized FeNPs showed the high removal levels higher than 90% for Cr(VI) adsorption at a wide range of initial Cr(VI) concentrations (50–500 mg/L). The experimental equilibrium data were modelled with Langmuir and Freundlich isotherm models, and it was found that experimental equilibrium data could be well described by the Langmuir isotherm model. The maximum monolayer coverage capacity of FeNPs for Cr(VI) adsorption was found to be 312.5 mg/g. The pseudo-first-order and the pseudo-second-order kinetic models were applied to the experimental adsorption data, and it was concluded that the data were defined as the best agreement with the pseudo-second-order kinetic model. Weber–Morris model was used to investigate the effect of mass transfer on the adsorption of Cr(VI) onto FeNPs; it was observed that both the film (boundary layer) and intraparticle diffusion affected the studied adsorption process. The thermodynamic studies suggested that Cr(VI) adsorption onto FeNPs was endothermic and nonspontaneous, and the positive ΔS value indicated increased disorder at the solid-solution interface during the adsorption.

Simultaneous removal of As(V)/Cr(VI) and acid orange 7 (AO7) by nanosized ordered magnetic mesoporous Fe-Ce bimetal oxides: Behavior and mechanism

In this study, nanosized ordered magnetic mesoporous Fe-Ce bimetal oxides (Nanosized-MMIC) with highly well-ordered inner-connected mesostructure were successfully synthesized through the KIT-6 template method. This Nanosized-MMIC displayed excellent adsorption capacities for As(V), Cr(VI) and AO7, and the corresponding calculated maximum adsorption capacities of material were 111.17, 125.28 and 156.52 mg/g, respectively. As(V) and Cr(VI) removal by Nanosized-MMIC were slightly dependent on the ionic strength but highly solution pH-dependent, the coexistent silicate and phosphate ions competed remarkably with both As(V) and Cr(VI) for the adsorption active site. Mechanisms indicated As(V) and Cr(VI) formed inner-sphere complexes on Nanosized-MMIC interface via the electrostatic interaction and surface complexation, while the total organic carbon (TOC) change demonstrated that AO7 could be removed completely and no organic intermediates formed through the adsorption process. In addition, Nanosized-MMIC also possessed superior adsorption performance in As(V)/Cr(VI)-AO7 binary systems, and the reusable and regeneration properties indicated that the obtained nanomaterials could maintain at a comparatively high level after several recycling. Finally, fixed-bed experiments suggested the Nanosized-MMIC was expected to have a promising excellent nano-adsorbent with high application potential for co-existed toxic heavy metals and organic dyes removal in practical wastewater treatment.

Ternary assembly of g-C3N4/graphene oxide sheets /BiFeO3 heterojunction with enhanced photoreduction of Cr(VI) under visible-light irradiation

A novel ternary composite of graphitic carbon nitride (g-C₃N₄)/graphene oxide (GO) sheets/BiFeO₃ (CNGB) with highly enhanced visible-light photocatalytic activity toward Cr(VI) photoreduction is prepared and characterized. The characterization and photocatalysis experiments corroborate its reasonable band gap, efficient charge separation and transfer, widened visible-light adsorption, easy solid-liquid separation, good stability and superior catalytic activity of CNGB. Three CNGB samples with different ratios of g-C₃N₄ and BiFeO₃ (CNGB-1, -2, -3 with 2:4, 3:3, and 4:2, respectively), though possessing different adsorption ability, eventually remove all Cr(VI) ions via photocatalysis within 90 min. The catalytic efficiency of the composite is the highest at pH 2; increases in pH decrease the catalytic ability. The inorganic anions such as SO₄^-, Cl^-, and NO₃^- only slightly affects the photocatalytic process. The matching of the band structure between BiFeO₃ and g-C₃N₄ generates efficient photogenerated electron migration from the conduction band of g-C₃N₄ to that of BiFeO₃, which is also facilitated by the electron bridging and collecting effects of GO, and holes transfer from the valence band of BiFeO₃ to that of g-C₃N₄, yielding the efficient separation of photogenerated electron-hole pairs and the subsequent enhancement of photocatalytic activity. The research provides a theoretical basis and technical support for the development of photocatalytic technologies for effective application in wastewater treatment and Cr-contaminated water restoration.

Enhanced Photocatalytic Simultaneous Removals of Cr(VI) and Bisphenol A over Co(II)-Modified TiO2

To enhance the electron-hole separation and boost the practical performance of commercial titania (Degussa P25) under natural solar light, in this work, P25 was modified with Co(II) species (CoP25) through post-treatment with decomposition of Co-ethylenediaminetetraacetic acid precursors in a wet chemical anchoring process. With appropriate Co(II) loading amount as molecular cocatalyst, the resulted CoP25-4 showed significantly improved photocatalytic performance for Cr(VI) reduction and bisphenol A (BPA) oxidation under UV-light irradiation. The coexistence of Cr(VI) and BPA promoted mutually the degradation of both pollutants. Under simulated solar light (AM 1.5G) illumination, the Cr(VI) reduction rate over CoP25-4 was 8.5 times enhanced compared with that over P25, whereas the simultaneous degradation rate of BPA over CoP25-4 was 8 times higher than that over P25. Further investigations indicated that the covalent atomic Co(II) anchoring on P25 significantly promoted the photogenerated electron-hole separation and facilitated Cr(VI) reduction via the formation of a Co(I) intermediate and simultaneously boosted BPA oxidation. Our results demonstrated a facile strategy to modify P25 with remarkably improved performance for the practical application in environmental pollution management under natural light excitation.

Highly Efficient and Sustainable Spent Mushroom Waste Adsorbent Based on Surfactant Modification for the Removal of Toxic Dyes

The treatment of wastewater always demands eco-friendly and cost-efficient adsorbents. In this paper, spent mushroom waste (SMW) was modified by a cationic surfactant (cetyltrimethylammonium bromide, CTAB) to eliminate toxic dyes. A characterization of adsorbents confirmed that CTAB was successfully embedded into the SMW structure. The spent mushroom waste, modified by CTAB (SMWC), exhibited an excellent adsorption capacity of 249.57 mg·g−1, 338.67 mg·g−1, and 265.01 mg·g−1 for the Direct red 5B (DR5B), Direct blue 71 (DB71), and Reactive black (RB5) dyes, respectively. Batch experiments indicated that the dye adsorption of SMWC depended mainly on pH, dye concentration, temperature, and ionic strength. The adsorption isotherm could be fitted to the Langmuir model and described by the pseudo-second-order kinetic model. The dye adsorption mechanism was dominated mostly by the chemosorption of the dyes and the SMWC surface. Thermodynamic parameters showed that the adsorption was endothermic and spontaneous. SMWC could successfully remove over 90% of dyes from various water samples. This can be considered a feasible waste resource utility, since it meets both the ecological and the economic requirements for auspicious industrial applications.

Removal of rhodamine 6G with different types of clay minerals

With an increased use of color dye in textile industries and elevated fabrics output, more scientific studies and technology developments are needed to effectively treat wastewater containing dyes. However, better understanding of the interactions between dyes and suspended solids is a necessity to advance such developments. In this study the interactions between rhodamine 6G (R6G), a cationic dye, and different types of clays minerals, commonly found in the wastewater sludge, were elucidated. The uptake of R6G on the clay minerals was attributed to cation exchange on the external surfaces of non-swelling clays, and at both external and interlayer spaces for swelling clays. In the interlayer of montmorillonite, the R6G molecules form monolayer and bilayer configurations under low and high uptake levels. The significant amounts of R6G uptake indicate that clay minerals are good sorbents for the removal of cationic dyes from water. And the R6G could be readily removed in wastewater treatment by adding small amount of clays and flocculated the clays out.

Strategies to reduce mass and photons transfer limitations in heterogeneous photocatalytic processes: Hexavalent chromium reduction studies

The current work presents different approaches to overcome mass and photon transfer limitations in heterogeneous photocatalytic processes applied to the reduction of hexavalent chromium to its trivalent form in the presence of a sacrificial agent. Two reactor designs were tested, a monolithic tubular photoreactor (MTP) and a micro-meso-structured photoreactor (NETmix), both presenting a high catalyst surface area per reaction liquid volume. In order to reduce photon transfer limitations, the tubular photoreactor was packed with transparent cellulose acetate monolithic structures (CAM) coated with the catalyst by a dip-coating method. For the NETmix reactor, a thin film of photocatalyst was uniformly deposited on the front glass slab (GS) or on the network of channels and chambers imprinted in the back stainless steel slab (SSS) using a spray system. The reaction rate for the NETmix photoreactor was evaluated for two illumination sources, solar light or UVA-LEDs, using the NETmix with the front glass slab or/and back stainless steel slab coated with TiO₂-P25. The reusability of the photocatalytic films on the NETmix walls was also evaluated for three consecutive cycles using fresh Cr(VI) solutions. The catalyst reactivity in combination with the NETmix-SSS photoreactor is almost 70 times superior to one obtained with the MTP.

Microwave hydrothermal-assisted preparation of novel spinel-NiFe2O4/natural mineral composites as microwave catalysts for degradation of aquatic organic pollutants

In this study, novel spinel-NiFe₂O₄/natural mineral (sepiolite, diatomite and kaolinite) composites were developed using microwave (MW) hydrothermal method, and applied in MW-induced catalytic degradation (NiFe₂O₄/natural mineral/MW) of organic pollutants such as sodium dodecyl benzene sulfonate (SDBS), azo fuchsine (AF), methyl parathion (MP), and crystal violet (CVL) in solution. Catalytic activities of three NiFe₂O₄/natural mineral composites were compared. The effects of material synthesis process parameters such as molar ratios of NiFe₂O₄ and natural mineral, and pH of precursor solutions for synthesizing catalysts, and degradation parameters such as MW irradiation time and catalyst reuse cycles were also investigated. The principle on NiFe₂O₄/natural mineral/MW degradation was provided. The results reveal that organic pollutants in wastewater can be removed completely using NiFe₂O₄/natural mineral/MW within minutes. NiFe₂O₄/sepiolite shows higher catalytic activity than the others. The calculated degradation rate constants are 1.865, 0.672, 0.472, and 0.329 min^-1 for SDBS, AF, MP, and CVL, respectively, using NiFe₂O₄/sepiolite/MW system. The performance of NiFe₂O₄/natural mineral can be maintained for three reuse cycles. Active species OH, O₂^-, and h⁺ play main roles in NiFe₂O₄/sepiolite/MW degradation. Hence, NiFe₂O₄/sepiolite/MW technology with rapid and cost-effective degradation, magnetic separation, and no secondary pollution, demonstrates to be promising in treating organic contaminants in wastewater.

Distinguished Cr(VI) capture with rapid and superior capability using polydopamine microsphere: Behavior and mechanism

Toxic heavy metal containing Cr(VI) species is a serious threat for ecological environment and human beings. In this work, a new mussel-inspired polydopamine microsphere (PDA-sphere) is prepared through in situ oxidative polymerization at air condition with controllable sizes. The adsorption of Cr(VI) ions onto PDA-sphere is highly pH dependent with the optimal pH ranging from 2.5 to 3.8. A rapid Cr(VI) removal can approach in 8min for equilibrium. More importantly, the prepared materials exhibit a remarkable sorption selectivity, coexisting SO₄^2-, NO₃^- and Cl^- ions at high levels; The applicability model further proves its effective performances with treated capacity of 42,000kg/kg sorbent, and the effluent can be reduced from 2000ppb to below 50ppb, which meets the drinking water criterions recommended by WHO. 1kg sorbent can also purify approximately 100t Cr(VI) contaminated wastewaters basing on the wastewater discharges of China. Such capacity for application ranks the top level for Cr(VI) removal. Additionally, the exhausted materials can be well regenerated by binary alkaline and salts mixtures. Such efficient adsorption can be ascribed to the well-dispersed morphology as well as the strong affinity between Cr(VI) and catechol or amine groups by XPS investigation. All the results suggest that polydopamine microspheres may be ideal materials for Cr(VI) treatment in waters.