Preparation, characterization and application of a Ce-Ti oxide adsorbent for enhanced removal of arsenate from water

Synthesis of Mesoporous Tetragonal ZrO2, TiO2 and Solid Solutions and Effect of Colloidal Silica on Porosity

Metal oxides possessing a large surface area, pore volume and desirable pore size provide more varieties and active industrial potentials. Nevertheless, it is very challenging to produce crystal metal oxides while keeping satisfactory porosity features, especially for ternary compositions. High temperature is usually needed to produce crystal metal oxides, which readily leads to the collapse of the pore structure. Herein, by employing a 'soft' dispersant agent and a hard silica template, ZrO2, TiO2 and Zr-Ti solid solutions having a tetragonal crystal structure are produced and the silica-leached materials are characterized from macroscopic to atomistic scales. The micron-sized particulate powders are composed of nanoscale 'building blocks', with crystallite sizes between ~8 and 21 nm. These polycrystalline ceramic powders exhibit a high specific surface area (up to ~200 m2·g-1) and pore volume (up to 0.5 cm3·g-1), with a pore size range of ~5-20 nm. Importantly, the Zr/Ti-O-Si-OH chemical bonds exist on the particle surface, with about two-thirds of the surface covered by silica. The hydroxyl groups can further post-graft organic ligands or directly associate with species. Synthesized mesoporous metal oxides are highly homogenous and could potentially be used in various applications because of their tetragonal structure and porosity features.

Sous Vide-Inspired Impregnation of Amorphous Titanium (Hydr)Oxide into Carbon Block Point-of-Use Filters for Arsenic Removal from Water

Carbon block filters, commonly employed as point-of-use (POU) water treatment components, effectively eliminate pathogens and adsorb undesirable tastes, odors, and organic contaminants, all while producing no water waste. However, they lack the capability to remove arsenic. Enabling the carbon block to remove arsenic could reduce its exposure risks in tap water. Inspired by Sous vide cooking techniques, we developed a low-energy, low-chemical method for impregnating commercially available carbon block with titanium (hydr)oxide (THO) in four integrated steps: (1) vacuum removal of air from the carbon block, (2) impregnation with precursors in a flexible pouch, (3) sealing to prevent oxygen intrusion, and (4) heating in a water bath at 80 °C for 20 h to eliminate exposure and reactions with air. This process achieved a uniform 13 wt % Ti loading in the carbon block. Our modified carbon block POU filter efficiently removed both arsenate and arsenite from tap water matrices containing 10 or 100 μg/L arsenic concentrations in batch experiments or continuous flow operations. Surprisingly, the THO-modified carbon block removed arsenite better than arsenate. This innovative method, using 70% fewer chemicals than traditional autoclave techniques, offers a cost-effective solution to reduce exposure to arsenic and lower its overall risk in tap water.

A review on multi-synergistic transition metal oxide systems towards arsenic treatment: Near molecular analysis of surface-complexation (synchrotron studies/modeling tools)

The science and interface chemistry between the arsenic (As) anions and the different adsorbent systems have been gaining interest in recent years in environmental remediation applications. Metal-oxides and the corresponding hybrid systems have shown promising performance as novel adsorbents in various treatment technologies. The abundance, surface chemistry, high surface area (active-centres), various synthesis and functionalization methodologies, and good recyclability make these metal oxide-based nanomaterials as potential remediating agents for As oxyanions. This work critically reviews eight different platforms focused on the arsenic contamination issue, where the first classification describes the origin of arsenic contamination and presents geographical and demo-graphical considerations. The following section briefs the state-of-the-art remediation techniques for arsenic treatment with a comparative evaluation. An emphasized discussion has been provided regarding the adsorption and classification of various metal oxide adsorbents. In the next classification, various multi-synergism abilities like Redox activity, Surface functional groups, Surface area/morphology, Heterogeneous catalysis, Reactive oxygen species, Photo-catalytic/electro-catalytic reactions, and Electrosorption are detailed. The classification of various characterization tools for accessing the arsenic remediation qualitatively and quantitatively are given in the fifth chapter. The first-of-its-kind dedicated analysis has been given on the surface complexation aspects of the arsenic speciation onto various metal adsorbent systems using synchrotron results, surface-complexation modeling, and molecular simulation (e.g., DFT) in the sixth chapter. The current sensing applications of these novel nano-material systems for arsenic determination using colorimetric and electrochemical-based analytical tools and a note about the economic parameters, i.e., regeneration aspects of various adsorbent systems/the sustainable applications of the treated sludge materials, are provided in the final sections. This work makes a critical analysis of 'Environmental Nanotechnology' towards 'Arsenic Treatment'.

Simultaneous and efficient removal of fluoride and phosphate in phosphogypsum leachate by acid-modified sulfoaluminate cement

The high concentration of fluoride and phosphate in phosphogypsum leachate is harmful to the environment and ecosystem. Thus, there is a need to develop feasible materials or technologies to remove both fluoride and phosphate in acidic phosphogypsum leachate. In this study, sulfoaluminate cement (SC) was used to simultaneously remove fluoride and phosphate in wastewater based on its moderate alkalinity and rich content of metal elements (Ca, Al and Fe, etc). The acidized sulfoaluminate cement (ASC) composite was prepared through modifying SC with hydrochloric acid, which can increase the specific surface areas of the raw SC, as well as the activity of the metal elements in SC. Compared with other coagulants, ASC showed excellent removal performance for fluoride and phosphate, such as higher removal efficiency, better effluent quality, and accelerated settling rate. The fluoride and phosphate removal performances of ASC herein were investigated at different dosages, pH values, coexisting substances, and initial concentrations. As a result, ASC exhibited wide pH adaptability and satisfactory selectivity for fluoride and phosphate. The possible removal mechanisms of fluoride and phosphate by ASC included chemisorption, ion exchange, and precipitation. The main end products associated with fluoride were fluorite (CaF₂), aluminum fluoride (AlF₃), and iron trifluoride (FeF₃). The main final products amid phosphate removal, on the other hand, were brushite (CaHPO₄·2H₂O), aluminophosphate ((H₃O)·AlP₂O₆(OH)₂), silicocarnotite (Ca₂SiO₄·Ca₃(PO₄)₂) and iron phosphate (Fe(H₂PO₄)₃). More importantly, ASC can effectively treat the phosphogypsum leachate at a wide range of concentrations, and the concentrations of phosphate and fluoride in the effluents were lower than 0.5 mg P L^-1 and 4 mg L^-1, respectively. To sum up, ASC is a competitive candidate to treat wastewater with high fluoride and phosphate content, such as phosphogypsum leachate.

A systematic study of arsenic adsorption and removal from aqueous environments using novel graphene oxide functionalized UiO-66-NDC nanocomposites

This study investigates the removal of As(V) from aqueous media using water stable UiO-66-NDC/GO prepared via the solvothermal procedure. The synthesized material was analyzed by Raman spectroscopy, UV-visible, X-ray powder diffraction (XRD), Transmission electron microscopy (TEM), Fourier Transform Infrared spectroscopy (ATR-FTIR), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) support its applicability as a super-adsorbent for the adsorption of As(V) ions from aqueous solutions. The effect of various parameters, including initial ion concentration, temperature, adsorbent dose, and pH on the adsorption of As(V) was studied to recognize the optimum adsorption conditions. The qmax obtained for this study using Langmuir isotherms was found at 147.06 mg/g at room temperature. Thermodynamic parameters ΔH°, ΔG°, and ΔS° were also calculated and negative values of ΔG° represent that the As(V) adsorption process occurred exothermically and spontaneously. Meanwhile, theoretical density functional simulation findings are accommodated to support these experimental results. It is observed that the dynamic nature of graphene oxide and the UiO-66 NDC nanocomposite system becomes superior for adsorption studies due to delocalized surface states. UiO-66-NDC/GO also showed high reusability for up four regeneration performances using 0.01 M HCl as a regenerant.

Attenuation mechanisms of arsenic induced toxicity and its accumulation in plants by engineered nanoparticles: A review

The excessive arsenic (As) accumulation in plant tissues enforced toxic impacts on growth indices. So, the utilization of As-contaminated food leads to risks associated with human health. For the reduction of As concentrations in foods, it is obligatory to fully apprehend the take up, accretion, transportation and toxicity mechanisms of As within plant parts. This metalloid impairs the plant functions by disturbing the metabolic pathways at physio-biochemical, cellular and molecular levels. Though several approaches were utilized to reduce the As-accumulation and toxicity in soil-plant systems. Recently, engineered nanoparticles (ENPs) such a zinc oxide (ZnO), silicon dioxide or silica (SiO₂), iron oxide (FeO) and copper oxide (CuO) have emerged new technology to reduce the As-accumulation or phytotoxicity. But, the mechanistic approaches with systematic explanation are missing. By knowing these facts, our prime focus was to disclose the mechanisms behind the As toxicity and its mitigation by ENPs in higher plants. ENPs relives As toxicity and its oxidative damages by regulating the transporter or defense genes, modifying the cell wall composition, stimulating the antioxidants defense, phytochelatins biosynthesis, nutrients uptake, regulating the metabolic processes, growth improvement, and thus reduction in As-accumulation or toxicity. Yet, As-detoxification by ENPs depends upon the type and dose of ENPs or As, exposure method, plant species and experimental conditions. We have discussed the recent advances and highlight the knowledge or research gaps in earlier studies along with recommendations. This review may help scientific community to develop strategies such as applications of nano-based fertilizers to limit the As-accumulation and toxicity, thus healthy food production. These outcomes may govern sustainable application of ENPs in agriculture.

Efficient removal arsenate from water by biochar-loaded Ce3+-enriched ultra-fine ceria nanoparticles through adsorption-precipitation

Biochar-loaded Ce³⁺-enriched ultra-fine ceria nanoparticles (Ce-BC) were used as a novel nanostructured adsorbent for the removal of arsenate (As(V)) from aqueous solutions. The effect of cerium valence on As(V) adsorption and the mechanism of As(V) adsorption onto Ce-BC were investigated using batch experiments and a series of spectroscopy detection technologies. The adsorption isotherm data fitted with the Langmuir model, with maximum As(V) sorption capacity of 219.8 mg g^-1 at pH 5.0 and 25 °C. The adsorption kinetics fitted well with the pseudo-second-order model. Ce³⁺ on the surface of Ce-BC plays an important role in the adsorption of As(V). The decrease in Ce³⁺ concentration from 60.1% to 48.9% on the Ce-BC surface, significantly decreased the adsorption of As(V) on Ce-BC. Furthermore, a strong affinity between As(V) and Ce³⁺-enriched Ce-BC was revealed, resulting in irreversible adsorption. Most importantly, the adsorbed As(V) could further react with Ce³⁺ of the ultra-fine cerium oxide nanoparticles in Ce-BC to form rod-like CeAsO₄ precipitates. Through the novel adsorption-precipitation process, Ce-BC can be used to remove trace As(V).

Anchoring Al- and/or Mg-oxides to magnetic biochars for Co-uptake of arsenate and fluoride from water

The co-occurrence of arsenic and fluoride in the water environment has led to many health concerns for living beings. Simultaneous removal of such ions is crucial to the safety of water resources, and biochar has been extensively engaged to address this issue. Here four magnetic biochars (mBCs) including pristine magnetic biochar and three aluminum (Al) and/or magnesium (Mg) oxides-anchored magnetic biochar (i.e., Al-mBC, Mg-mBC, and MgAl-mBC) were prepared via a facile pyrolysis method and then comprehensively evaluated as adsorbents for enhanced co-uptake of arsenate (As^V) and fluoride (F^-) from synthetic water. The mBC shows a high specific surface area of 205 m² g^-1, which dropped to 116, 80, and 114 m² g^-1 upon the anchoring of Al, Mg, and Mg + Al, respectively. Our results suggest that the adsorption of either As^V or F^- is highly pH-dependent, and pH 4-6 is the optimal range for maximum adsorption. The adsorption isotherm data indicate that the MgAl-mBC adsorbent outranks all other mBCs for co-uptake of both As^V and F^-. The adsorption capacity maxima of MgAl-mBC are 34.45, and 21.59 mg g^-1 for As^V and F^-, respectively (pH = 5, T = 10 °C), also highly outstripping other biochars reported in the literature. The magnetic feature of these mBCs enables us to fast reclaim and regenerate the exhausted adsorbents by an external magnet and dilute NaOH. The Al- and Mg-anchored mBCs are expected to be used as highly efficient adsorbents for environmental remediation of waters contaminated by both As^V and F^-.

Synthesis of Fe3O4@mZrO2-Re (Re = Y/La/Ce) by Using Uniform Design, Surface Response Methodology, and Orthogonal Design & Its Application for As3+ and As5+ Removal

In this study, iron oxide (Fe₃O₄) was coated with ZrO₂, and doped with three rare earth elements((Y/La/Ce), and a multi-staged rare earth doped zirconia adsorbent was prepared by using uniform design U₁₄, Response Surface methodology, and orthogonal design, to remove As³⁺ and As⁵⁺ from the aqueous solution. Based on the results of TEM, EDS, XRD, FTIR, and N₂-adsorption desorption test, the best molar ratio of Fe₃O₄:TMAOH:Zirconium butoxide:Y:La:Ce was selected as 1:12:11:1:0.02:0.08. The specific surface area and porosity was 263 m²/g, and 0.156 cm³/g, respectively. The isothermal curves and fitting equation parameters show that Langmuir model, and Redlich Peterson model fitted well. As per calculations of the Langmuir model, the highest adsorption capacities for As³⁺ and As⁵⁺ ions were recorded as 68.33 mg/g, 84.23 mg/g, respectively. The fitting curves and equations of the kinetic models favors the quasi second order kinetic model. Material regeneration was very effective, and even in the last cycle the regeneration capacities of both As³⁺ and As⁵⁺ were 75.15%, and 77.59%, respectively. Adsorption and regeneration results suggest that adsorbent has easy synthesis method, and reusable, so it can be used as a potential adsorbent for the removal of arsenic from aqueous solution.

Removal of lead ions by two FeMn oxide substrate adsorbents

Lead pollution has become a global concern due to its ubiquity and persistence. This study describes two FeMn oxide substrate adsorbents, namely, FeMn binary oxides (FMBO) and mesoporous FeMn binary oxide (MFMBO) covered with tannic acid film (FMBO@TA-Fe³⁺ and MFMBO@TA-Fe³⁺), for the treatment of Pb²⁺ in water. The characterization results showed that TA was successfully coated onto the surfaces of FMBO and MFMBO. The maximum capacities of Pb²⁺ on FMBO@TA-Fe³⁺ and MFMBO@TA-Fe³⁺ were 322.08 and 357.14 mg g^-1, respectively, which were twice those of FMBO and MFMBO. The adsorption of Pb²⁺ on the adsorbents was a spontaneous, endothermic process with increasing disorder through thermodynamics studies. An overall mechanism was proposed for Pb²⁺ adsorption, the improved adsorption performance of FMBO@TA-Fe³⁺ and MFMBO@TA-Fe³⁺ is ascribed to the mesoporous characteristics and the introduction of hydroxyl groups. Further investigation indicated the adsorption of Pb²⁺ could be attributed to electrostatic interactions on FMBO@TA-Fe³⁺ and MFMBO@TA-Fe³⁺, and cation exchange existed through the formation of these internal surface complexes. The Pb²⁺-loaded adsorbents could be effectively desorbed in a dilute hydrochloric acid solution, promoting recycling and reuse of the regenerated adsorbents. These results warrant the promising application of FMBO@TA-Fe³⁺ and MFMBO@TA-Fe³⁺ for the removal of Pb²⁺, and this work first proposed TA film-modified FMBO and MFMBO to improve its adsorption capacity in the application of environmental remediation.

Inorganic arsenic toxicity and alleviation strategies in rice

Direct or indirect exposure to inorganic arsenic (iAs) in the forms of AsIII (arsenite) and AsV (arsenate) through consumption of As-contaminated food materials and drinking water leads to arsenic poisoning. Rice (Oryza sativa L.) plant potentially accumulates a high amount of iAs from paddy fields than any other cereal crops. This makes it to be a major source of iAs especially among the population that uses it as their dominant source of diet. The accumulation of As in human bodies poses a serious global health risk to the human population. Various conventional methods have been applied to reduce the arsenic accumulation in rice plant. However, the success rate of these techniques is low. Therefore, the development of efficient and effective methods aimed at lowering iAs toxicity is a very crucial public concern. With the current advancement in technology, new strategies aimed at addressing this concern are being developed and utilized in various parts of the world. In this review, we discuss the recent advances in the management of iAs in rice plants emphasizing the use of nanotechnology and biotechnology approaches. Also, the prospects and challenges facing these approaches are described.

Enhanced As(III) transformation and removal with biochar/SnS2/phosphotungstic acid composites: Synergic effect of overcoming the electronic inertness of biochar and W2O3(AsO4)2 (As(V)-POMs) coprecipitation

The activation of carbon atoms in biochar is an important approach for realizing the reuse of discarded woody biomass resources. In this work, a strategy for the construction of carbon-based catalysts was proposed with Magnoliaceae root biomass as a carbon source, doped by SnS₂ and further decorated with heteropoly acid. The introduction of SnS₂ can activate the carbon atom and destroy the electronic inertness of the disordered biochar with 002 planes. In addition, the synergy between the Keggin unit of phosphotungstic acid and biochar/SnS₂ can suppress recombination of e^--h⁺ carriers. The adsorption and photocatalysis experiments results showed that the efficiency of removing As(III) by biochar/SnS₂/phosphotungstic acid (biochar/SnS₂/PTA) systems was 1.5 times that of biochar/SnS₂ systems, and the concentration of total arsenic in the biochar/SnS₂/PTA composite system gradually decreased during the photocatalysis process. The formation of As-POMs can simultaneously realize As(III) photooxidation and As(V) coprecipitation. The phase transfer of arsenic by As-POMs could significantly increase the As adsorption capacity. Specifically, the composites achieved the conversion of S atoms at the interface of biochar into SO₄^•- radicals to enhance the As(III) photooxidation performance.

A practical approach on reuse of drinking water treatment plant residuals for fluoride removal

The sustainable management of the voluminous waste from drinking water treatment plants has motivated environmental researchers towards several reuse options. Water treatment residues (WTR) are proven adsorbent for remediation of many water- and soil-borne anions (perchlorate, selenium and arsenic), and may be able to remove fluoride from contaminated water. In this study, the sustainable reuse of the freely available waste of the drinking water treatment plants, namely WTR, was explored for their fluoride removal potential to meet drinking water standards. WTR was characterized by specific surface area, Fourier transform infrared (FT-IR), scanning electron microscopy and X-ray powder diffraction (XRD). Batch adsorption experiments were conducted as a function of WTR dose, contact time, agitation speed, initial fluoride concentration, initial temperature and water pH to get best adsorption capacity. About 90% fluoride removal (from initial 5.0 mg/L) was observed within 2 h contact time at WTR dose of 28 g/L. Also, WTR effectively removed fluoride in the pH range of 5-8, whereas removal efficiency decreased at pH 9 or higher. The adsorption equilibrium was established within 120-150 min. Adsorption isotherm data were best fit to Langmuir (R ² = 0.984) and Freundlich models (R ² = 0.983), while adsorption kinetic study exhibited that second-order kinetic model was followed with rate constant of 0.038 g/mg min. The FT-IR and XRD analyses affirmed that the metal hydroxyl and metal oxide groups contributed to the fluoride removal. The experimental results show the promising potential of WTR as an adsorbent in fluoride removal from real contaminated groundwater.

One-step phosphite removal by an electroactive CNT filter functionalized with TiO2/CeOx nanocomposites

Compared with phosphate (+5 valence), phosphite (HPO₃^2-/H₂PO₃^-, +3 valence) possesses higher solubility, and is more resistant to biotransformation. Herein, we designed a one-step electroactive filter technology for rapid and efficient phosphite removal. The filter consists of carbon nanotubes (CNT) and functionalized with nanoscale TiCe binary oxides. The phosphite removal kinetics and capacity increased with electric field (e.g., from 54.5% at 0 V to 75.6% at 2 V) and flow rate (e.g., from 63.1% at 1.5 mL/min to 81.2% at 6 mL/min). This can be attributed to synergistic effects of the filter's electrochemical reactivity, limited pore size, more exposed active sites and flow-through design. Meanwhile, phosphite can be converted to phosphate once adsorbed under electric field. The TiO₂/CeO_x-CNT filter could work effectively across a wide pH range, and the presence of various coexisting anions posed negligible impact on phosphite removal. Electrochemical characterizations verified the essential role of CeO_x and applied electric field, which synergistically accelerated electron transfer rate and increased charge capacity. The TiO₂/CeO_x-CNT filter can be regenerated effectively by chemical washing. The system efficacy was further supported by a comparable phosphite removal efficiency of 72.8% in actual lake water conditions. Therefore, this TiO₂/CeO_x-CNT filter technology is promising for mitigating the challenging issue of phosphite contamination from water bodies.

Influence of dissolved organic matter components on arsenate adsorption/desorption by TiO2

The influences of different dissolved organic matter (DOM) components and ionic matters on As(V) adsorption/desorption behavior on the TiO₂ surface were investigated. The results demonstrated that the characteristics and involving order of DOM significantly affected the As(V) adsorption/desorption behavior. The presence of DOM decreased the As(V) adsorption quantity. Fulvic acid (FA) exhibited the most negative effect, and followed by the order of alginate ≈ BSA > SDBS. The precomplexation DOM prevented more As(V) adsorption. While, the presence of DOM caused more As(V) release when the surrounding changed and FA exhibited the strongest effect. The results indicated that the site competition and electrostatic repulsion were the major mechanisms to resist As(V) adsorption. The presence of Fe³⁺ and Ca²⁺ increased As(V) adsorption by bridge effect, while PO₄^3- and CO₃^2- decreased As(V) adsorption owing to the competition.

Titanium-based nanocomposite materials for arsenic removal from water: A review

Arsenic is highly carcinogenic element and less concentration of this chemical element makes natural water unsafe for human consumption. Versatile techniques including adsorption method have been established to remove the arsenic from water. However, adsorption is found to be one of effective method for the remediation of arsenic from contaminated water. Different types of natural adsorbents i.e. clays, waste materials, carbon based material have been studied widely for the adsorption of arsenic. Recently, nanotechnology is considered to be one of the best technology for waste water treatment. Therefore researchers have synthesized several types of nanoadsorbents and investigated them for the removal of various pollutants including arsenic from water. Now days, attention is paid on development of nanocomposite materials which are proven as competent arsenic adsorbent candidate as compared to other adsorbents due to dominant structural and surface features. Various metal/metal oxide based nanocomposites have been developed and studied for arsenic removal from aqueous media. It has been reported that TiO2 based nanocomposite exhibit stong affinity for both inorganic form of arsenic. Therefore, in this review numerous metal or metal oxide based titania nanocomposites i.e. TiO2-αFe2O3, NHITO, Ce-Ti oxide, Zr-TiO2, RGO-MFT etc. have been discussed in details for the water treatment containing arsenic. This review also presents an overview of low cost adsorbents, titania based nanoadsorbent and hybrid titania nanostructures for the removal of arsenic. In this review paper the particle size, surface area and adsorption efficiency of these titania based materials at different pH are also been presented in tabulated form. It provides the opportunity to choose best titania based nanocomposites for the treatment of arsenic polluted water.

Synthesis and application of Zn/Ce bimetallic oxides for the decontamination of arsenite (As-III) ions from aqueous solutions

Arsenic contamination has threatened water safety due to its high toxicity and carcinogenicity. Therefore, it is urgent and significant to develop simple and effective approach for dearsenification of drinking water. In present study, Zn/Ce bimetallic oxide particles of various atomic ratios were synthesized by sol-gel process and were applied for adsorption of arsenite from aqueous solutions. The Zn/Ce bimetallic oxide of atomic ratio Zn_0.2:Ce_0.05 shows better adsorption proficiency in comparison to their monometallic counterparts as well as synthesized bimetal oxides of other atomic ratios. Sorption behavior of arsenite on Zn/Ce bimetal oxide was investigated through batch experiments and optimum conditions were found to be pH = 7.5, adsorbent dose = 0.36 g/L, and contact time = 30 min. The arsenite adsorption data was explained by Langmuir isotherm model and maximum adsorption capacity found to reaching 88.49 mg/g at 318 K. Adsorption mechanism was interpreted using FTIR and XPS data, the former suggesting formation of bond between As(III)Zn/Ce oxide nanoparticles while, latter reveals presence of both As(III) and As(V) peak which further infer that some fraction of As(III) may be get oxidized to As(V) by O₂ based on Ce³⁺ as electron mediation agent between As(III) and O₂.

Ultrafast removal of arsenic using solid solution of aero-gel based Ce1-XTixO2-Y oxide nanoparticles

An aero-gel based solid solution of titanium and cerium oxide nanoparticles have been used for the first time for ultra fast and trace level removal of arsenic from water. The interconnected long range ordered mesoporous structure was observed from TEM analysis which has been verified as an essential facet for the fast removal of arsenic in this study. The HR-XRD spectra indicated the face centred cubic structure with Fm3¯m space group. Le-Bail refinement and Raman spectroscopy confirmed the formation of single phase solid solution of Ce_1-XTi_xO_2-Y oxide nanoparticles. The HR-XPS and FT-IR study indicated the surface complexation and partial oxidation of As(III) to As(V) via electron transfer mechanism by reduction of Ce(IV) to Ce(III) and Ti(IV) to Ti(III) simultaneously during adsorption process. The kinetics study demonstrated 99% removal of As(III) within 10 min of initiating the adsorption process. The effect of pH and interfering ions confirmed the wide range of applicability for solid solution of titania and cerium oxide nanoparticles over the different environmental conditions for the removal of arsenic. The adsorption capacity for our best adsorbent (Ce_0.8Ti_0.2O_2-y) was found to be 2 × 10⁵ mg kg^-1 while the lowest concentration of water body system was 7 μg L^-1 which is the minimum concentration of arsenic achieved by any metal oxide based adsorbent.

A biomimetic SiO2@chitosan composite as highly-efficient adsorbent for removing heavy metal ions in drinking water

Highly efficient adsorbents for drinking water purification are demanded since the contaminants are generally in a low concentration which makes it difficult for conventional adsorbents. Herein, we present a novel biomimetic SiO₂@chitosan composite as adsorbent with a high adsorption capability towards heavy metal ions including As(V) and Hg(II). The hollow leaf-like SiO₂ scaffold within the adsorbent has a stable chemical property; while on the surface SiO₂, the chitosan nanoparticle provide a large amount of active sites such as amino and hydroxyl groups for adsorbing heavy metal ions. The special SiO₂ structure also prevents the agglomeration and loss of chitosan, which enables the efficient contact between the functional groups of chitosan and heavy metal ions. The SiO₂@chitosan composite exhibits maximum adsorption capacities of 204.1 and 198.6 mg g^-1 towards Hg(II) and As(V), respectively. In addition, the removal efficiency reaches over 60% within 2 min. The adsorption performance enables the presented biomimetic adsorbent suitable for adsorbing low-concentration heavy metal ions, especially possessing a promising potential for drinking water purification.

Electrically enhanced adsorption and green regeneration for fluoride removal using Ti(OH)4-loaded activated carbon electrodes

An electrically enhanced fluoride removal method was developed that blended the merits of electrosorption and adsorbent adsorption. This method has the advantages of high adsorption selectivity and capacity for fluoride. The saturated adsorption capacity of Ti(OH)₄ for fluoride in the electrode of Ti(OH)₄-loaded activated carbon reached 115.2 mg/g when a voltage of +1.2 V was applied to the electrode. The electrode was easily and cleanly regenerated in a short time in aqueous solution with high fluoride concentrations when a voltage of -1.6 V was applied. The adsorption capacity of the Ti(OH)₄-loaded electrode for fluoride did not decrease after multiple cycles of electrically enhanced adsorption and regeneration. Excellent adsorption selectivity for fluoride was achieved. The electrically enhanced adsorption method showed potential for fluoride removal.

Engineering metal (hydr)oxide sorbents for removal of arsenate and similar weak-acid oxyanion contaminants: A critical review with emphasis on factors governing sorption processes

To create an integrative foundation for engineering of the next generation inexpensive sorbent systems, this critical review addresses the existing knowledge gap in factor/performance relationships between weak-acid oxyanion contaminants and metal (hydr)oxide sorbents. In-depth understanding of fundamental thermodynamics and kinetics mechanisms, material fabrication, and analytical and characterization techniques, is necessary to engineer sorbent that exhibit high capacity, selectivity, stability, durability and mass transport of contaminants under a wide range of operating and water matrix conditions requirements. From the perspective of thermodynamics and kinetics, this critical review examines the factors affecting sorbent performances and analyzes the existing research to elucidate future directions aimed at developing novel sorbents for removal of weak-acid oxyanion contaminants from water. Only sorbents that allow construction of simple and inexpensive water treatment systems adapted to overcome fiscal and technological barriers burdening small communities could pave the road for providing inexpensive potable water to millions of people. Novel sorbents, which exhibit (1) poor performances in realistic operating and water matrix conditions and/or (2) do not comply with the purely driven economics factors of production scalability or cost expectations, are predestined to never be commercialized.

Aluminum Substituted Cobalt Ferrite (Co-Al-Fe) Nano Adsorbent for Arsenic Adsorption in Aqueous Systems and Detailed Redox Behavior Study with XPS

Arsenic [As(III) and As(V)] adsorption on aluminum substituted cobalt ferrite (Co-Al-Fe) ternary metal oxide adsorbent is reported by means of qualitative and quantitative spectroscopy tools. IR and Raman active signals were observed around 810-920 cm^-1 band indicate different As-OH_complexed and As-O_uncomplexed stretching vibrations on to the adsorbent. The adsorption behavior of arsenic (III and V) onto these adsorbents is studied as a function of contact time, different concentrations, and pH conditions. The kinetics study on adsorption were performed to understand nature of adsorption which supports the Pseudo Second Order (PSO) model. The adsorption isotherms study indicates Freundlich type of adsorption. The maximum adsorption capacity of Co-Al-Fe adsorbent is observed around 130 and 76 mg g^-1 for As(III) and As(V) systems, respectively. Detailed XPS study of As 3d, Fe 2p, Co 2p, and O 1s spectra has been reported in explaining the redox behavior and ligand exchange reactions in supporting arsenic adsorption mechanism.

Formation of nickel-doped magnetite hollow nanospheres with high specific surface area and superior removal capability for organic molecules

A strategy for the formation of magnetic Ni _x Fe_3-x O₄ hollow nanospheres with very high specific surface areas was designed through a facile solvothermal method in mixed solvents of ethylene glycol and water in this work. The Ni/Fe ratios and the crystal phases of the Ni _x Fe_3-x O₄ hollow nanocrystals can be readily tuned by changing the molar ratios of Ni to Fe in the precursors. An inside-out Ostwald ripening mechanism was proposed for the formation of uniform Ni _x Fe_3-x O₄ hollow nanospheres. Moreover, the obtained Ni _x Fe_3-x O₄ hollow nanospheres exhibited excellent adsorption capacity towards organic molecules such as Congo red in water. The maximum adsorption capacities of Ni _x Fe_3-x O₄ hollow nanospheres for Congo red increase dramatically from 263 to 500 mg g^-1 with the increase of the Ni contents (x) in Ni _x Fe_3-x O₄ hollow nanospheres from 0.2 to 0.85. The synthesized Ni _x Fe_3-x O₄ nanoparticles can be potentially applied for waste water treatment.

Recent progress of arsenic adsorption on TiO2 in the presence of coexisting ions: A review

Arsenic (As)-contaminated wastewater and groundwater pose a pressing environmental issue and worldwide concern. Adsorption of As using TiO₂ materials, in combination with filtration, introduces a promising technology for the treatment of As-contaminated water. This review presents an overview on the recent progress of the application of TiO₂ for removal of As from wastewater and groundwater. The main focus is on the following three pressing issues that limit the field applications of TiO₂ for As removal: coexisting ions, simulation of breakthrough curves, and regeneration and reuse of spent TiO₂ materials. We first examined how the coexisting ions in water, especially high concentrations of cations in industrial wastewater, affect the efficacy of As removal using the TiO₂ materials. We then discussed As breakthrough curves and the effect of compounded ions on the breakthrough curves. We successfully simulated the breakthrough curves by PHREEQC after integrating the CD-MUSIC model. We further discussed challenges facing the regeneration and reuse of TiO₂ media for practical applications. We offer our perspectives on remaining issues and future research needs.

Hydrous TiO2@polypyrrole hybrid nanocomposite as an efficient selective scavenger for the defluoridation of drinking water

An adsorptive process for the defluoridation of drinking water was performed using a hybrid nanocomposite of hydrous titanium oxide@polypyrrole (HTiO₂@PPy), as a scavenger.

Microwave-hydrothermal method for the synthesis of composite materials for removal of arsenic from water

Composite material Zr-doped TiO2, suitable for the removal of arsenic from water, was synthetized with fast and simple microwave-hydrothermal method. Obtained material, Zr-TiO2, had uniform size and composition with zirconium ions incorporated into crystal structure of titanium dioxide. Synthetized composite material had large specific surface area and well-developed micropore and mesopore structure that was responsible for fast adsorption of As(III) and As(V) from water. The influence of pH on the adsorption capacity of arsenic was studied. The kinetics and isotherm experiments were also performed. The treatment of natural water sample containing high concentration of arsenic with composite material Zr-TiO2 was efficient. The concentration of arsenic was reduced to the value recommended by WHO.

Comparative study on arsenate removal mechanism of MgO and MgO/TiO2 composites: FTIR and XPS analysis

The arsenate removal mechanism using MgO and MgO/TiO₂ adsorbents was revealed by Fourier transform infrared and X-ray photoelectron spectroscopy.

Arsenic contamination, consequences and remediation techniques: a review

The exposure to low or high concentrations of arsenic (As), either due to the direct consumption of As contaminated drinking water, or indirectly through daily intake of As contaminated food may be fatal to the human health. Arsenic contamination in drinking water threatens more than 150 millions peoples all over the world. Around 110 millions of those peoples live in 10 countries in South and South-East Asia: Bangladesh, Cambodia, China, India, Laos, Myanmar, Nepal, Pakistan, Taiwan and Vietnam. Therefore, treatment of As contaminated water and soil could be the only effective option to minimize the health hazard. Therefore, keeping in view the above facts, an attempt has been made in this paper to review As contamination, its effect on human health and various conventional and advance technologies which are being used for the removal of As from soil and water.

Titanium dioxide-coated carbon nanotube network filter for rapid and effective arsenic sorption

In this study, a TiO2-coated carbon nanotube (CNT) network filter was prepared via a simple filtration–steam hydrolysis method and evaluated with respect to aqueous arsenic removal. The TiO2 coating was 5.5 ± 2.7 nm thick, completely covered the CNT network surface, and had a specific surface area of 196 m(2) g(–1), which was ∼2-fold greater than that of the CNT network. The TiO2–CNT As sorption kinetics increased with both increasing flow rate and cell potential, with increasing flow rate having a significantly stronger effect. At 6 mL min(–1) in the absence of potential and in recirculation mode, the first-order As sorption rate constants were 4.3 and 4.4 s(–1) for As(III) and As(V), respectively. The TiO2–CNT electro-assisted equilibrium sorption capacities at a cell potential of 2 V for effluent [As] = 10 ppb in single-pass mode were 1.8 and 1.3 mg g(–1) for As(III) and As(V), respectively. The enhanced TiO2–CNT filter As sorption kinetics and capacity result from increased mass transport due to internal convection and pore radius range, improved sorption site accessibility due to porosity and TiO2 dispersion, and reduced TiO2 negative surface charge due to anodic capacitance. Groundwater samples containing 44 ppb As were treated by single-pass filtration, and 12,500 bed volumes (residence time of 4.5 s; 127 L m(–2) h(–1); 5.8 mg m(–2) h(–1)) were filtered prior to the effluent As level reaching >10 ppb. A spent TiO2 filter was successfully regenerated by 5 mM NaOH for both As(III) and As(V).

Enhanced arsenic removal from water by hierarchically porous CeO₂-ZrO₂ nanospheres: role of surface- and structure-dependent properties

Arsenic contaminated natural water is commonly used as drinking water source in some districts of Asia. To meet the increasingly strict drinking water standards, exploration of efficient arsenic removal methods is highly desired. In this study, hierarchically porous CeO₂-ZrO₂ nanospheres were synthesized, and their suitability as arsenic sorbents was examined. The CeO₂-ZrO₂ hollow nanospheres showed an adsorption capacity of 27.1 and 9.2 mg g(-1) for As(V) and As(III), respectively, at an equilibrium arsenic concentration of 0.01 mg L(-1) (the standard for drinking water) under neutral conditions, indicating a high arsenic removal performance of the adsorbent at low arsenic concentrations. Such a great arsenic adsorption capacity was attributed to the high surface hydroxyl density and presence of hierarchically porous network in the hollow nanospheres. The analysis of Fourier transformed infrared spectra and X-ray photoelectron spectroscopy demonstrated that the adsorption of arsenic on the CeO₂-ZrO₂ nanospheres was completed through the formation of a surface complex by substituting hydroxyl with arsenic species. In addition, the CeO₂-ZrO₂ nanospheres were able to remove over 97% arsenic in real underground water with initial arsenic concentration of 0.376 mg L(-1) to meet the guideline limit of arsenic in drinking water regulated by the World Health Organization without any pre-treatment and/or pH adjustment.

Application of titanium dioxide in arsenic removal from water: A review

Natural arsenic pollution is a global phenomenon and various technologies have been developed to remove arsenic from drinking water. The application of TiO(2) and TiO(2)-based materials in removing inorganic and organic arsenic was summarized. TiO(2)-based arsenic removal methods developed to date have been focused on the photocatalytic oxidation (PCO) of arsenite/organic arsenic to arsenate and adsorption of inorganic and organic arsenic. Many efforts have been taken to improve the performance of TiO(2) by either combing TiO(2) with adsorbents with good adsorption property in one system or developing bifunctional adsorbents with both great photocatalytic ability and high adsorption capacity. Attempts have also been made to immobilize fine TiO(2) particles on supporting materials like chitosan beads or granulate it to facilitate its separation from water. Among the anions commonly exist in groundwater, humic acid and bicarbonate have significant influence on TiO(2) photocatalyzed oxidation of As(III)/organic arsenic while phosphate, silicate, fluoride, and humic acid affect arsenic adsorption by TiO(2)-based materials. There has been a controversy over the TiO(2) PCO mechanisms of arsenite for the past 10 years but the adsorption mechanisms of inorganic and organic arsenic onto TiO(2)-based materials are relatively well established. Future needs in TiO(2)-based arsenic removal technology are proposed.

Manganese doping of magnetic iron oxide nanoparticles: tailoring surface reactivity for a regenerable heavy metal sorbent

A method for tuning the analyte affinity of magnetic, inorganic nanostructured sorbents for heavy metal contaminants is described. The manganese-doped iron oxide nanoparticle sorbents have a remarkably high affinity compared to the precursor material. Sorbent affinity can be tuned toward an analyte of interest simply by adjustment of the dopant quantity. The results show that following the Mn doping process there is a large increase in affinity and capacity for heavy metals (i.e., Co, Ni, Zn, As, Ag, Cd, Hg, and Tl). Capacity measurements were carried out for the removal of cadmium from river water and showed significantly higher loading than the relevant commercial sorbents tested for comparison. The reduction in Cd concentration from 100 ppb spiked river water to 1 ppb (less than the EPA drinking water limit of 5 ppb for Cd) was achieved following treatment with the Mn-doped iron oxide nanoparticles. The Mn-doped iron oxide nanoparticles were able to load ~1 ppm of Cd followed by complete stripping and recovery of the Cd with a mild acid wash. The Cd loading and stripping is shown to be consistent through multiple cycles with no loss of sorbent performance.

Preparation, characterization and application of a copper (II)-bound polymeric ligand exchanger for selective removal of arsenate from water

A copper (II)-bound polymeric ligand exchanger named WH-425-Cu was prepared by loading Cu(2+) onto poly (4-vinylpyridine) resin. The performance of WH-425-Cu as the ligand exchanger to remove arsenate [As (V)] from aqueous solution was also investigated by using static equilibrium and dynamic adsorption experiments. Results of static experiments indicated that WH-425-Cu had higher adsorption selectivity for As (V) than other ubiquitous anions in nature water body such as SO(4)(2-), Cl(-), SiO(3)(2-), and PO(4)(3-). The optimal pH for adsorption of As (V) on WH-425-Cu was in the range of 6.0-8.0. The As (V) adsorbed on WH-425-Cu could be easily eluted with 7 BV of 6% NaCl solution (at pH = 9.0) with elution efficiency above 99%. The prepared WH-425-Cu could be used as a highly selective and reusable ligand exchanger for selective removal of As (V) from water.

Mn-Ce oxide as a high-capacity adsorbent for fluoride removal from water

A novel Mn-Ce oxide adsorbent with high sorption capacity for fluoride was prepared via co-precipitation method in this study, and the granular adsorbent was successfully prepared by calcining the mixture of the Mn-Ce powder and pseudo-boehmite. High-resolution transmission electron microscopy (TEM) image showed that the Mn-Ce adsorbent consisted of about 4.5 nm crystals, and X-ray diffraction (XRD) analysis indicated the formation of solid solution by Mn species entering CeO(2) lattices. The surface hydroxyl group density on the Mn-Ce adsorbent was determined to be as high as 15.3 mmol g(-1), mainly responsible for its high sorption capacity for fluoride. Sorption isotherms showed that the sorption capacities of fluoride on the powdered and granular adsorbent were 79.5 and 45.5 mg g(-1) respectively at the equilibrium fluoride concentration of 1 mg L(-1), much higher than all reported adsorbents. Additionally, the adsorption was fast within the initial 1 h. Fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy (XPS) analysis revealed that the hydroxyl groups on the adsorbent surface were involved in the sorption of fluoride. Both anion exchange and electrostatic interaction were involved in the sorption of fluoride on the Mn-Ce oxide adsorbent.

Modified native cellulose fibers--a novel efficient adsorbent for both fluoride and arsenic

Native cellulose fibers were surface modified by poly(N,N-dimethyl aminoethyl methacrylate) (PDMAEMA) to generate an anion adsorbent, which was characterized by scanning electron microscopy, fourier transform infrared spectroscopy and elemental analyzer. This adsorbent had high efficiency in removal of F(-), AsO(2)(-) and AsO(4)(3-) from aqueous solutions, even at low initial concentrations. Adsorption kinetics showed that the adsorption equilibrium could be reached within 1 min. The distribution coefficient did not change with adsorbent dose, indicating the adsorption was a homogenous process. Langmuir, Freundlich and Temkin models were used to fit the adsorption isotherms. Based on the parameters calculated from the models, the adsorption capacity was in the order of AsO(4)(3-)≫AsO(2)(-)>F(-), and the adsorption was a favorable process. Compared with Freundlich and Temkin models, the isotherms followed Langmuir model a little better.