Arsenate adsorption onto Fe-TNTs prepared by a novel water–ethanol hydrothermal method: Mechanism and synergistic effect

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'.

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 q_max 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.

Hydrothermally synthesized titanate nanomaterials for the removal of heavy metals and radionuclides from water: A review

Hazardous heavy metals and radionuclides in water and wastewater are of drastic concern owing to their detrimental impacts on the organisms as well as the circumambient ecosystem. To remove them as much as we can, both technique and materials were studied in the past years. The adsorption technique as superior water remediation method with the simplicity of design, environmental friendliness and high efficiency was well established. Consequently, it is practically important to explore advanced and economically feasible absorbents for removing these poisonous pollutants from aqueous solutions. So far, large numbers of experiments proved hydrothermally synthesized titanate nanomaterials (TNMs) could be a prospectively excellent adsorbent extracting heavy metals and radionuclides from water due to the high specific surface area, tunable pore size, abundant surface active sites, favorable hydrophilic properties. The objective of this work is to give an overview of hydrothermal synthesis, adsorption performance of TNMs for heavy metals and radionuclides, as well as the various influencing factors for water purification. It comprehensively reviews the structural changes and regenerability of TNMs after adsorption, and different modification methods adopted for improving removal capacity. Additionally, it uniquely highlights the efficient decontamination of the pollutants through a synergistic effect of adsorption and photocatalysis by TNMs. This review provides detailed information for the development, application, and research challenges faced by hydrothermally synthesized TNMs for the removal of heavy metals and radionuclides from aqueous solutions, which will serve as a reference guide for scientists in related fields.

Oxygen defective titanate nanotubes induced by iron deposition for enhanced peroxymonosulfate activation and acetaminophen degradation: Mechanisms, water chemistry effects, and theoretical calculation

The large consumption of acetaminophen (APAP) worldwide and unsatisfactory treatment efficiencies by conventional wastewater treatment processes give rise to the seeking of new technology for its effective removal. Herein, we proposed a facile one-step hydrothermal method to synthesize defective iron deposited titanate nanotubes (Fe/TNTs) for peroxymonosulfate (PMS) activation and APAP degradation. The retarded first-order reaction rate of APAP degradation by Fe/TNTs was 5.1 times higher than that of neat TNTs. Characterizations indicated iron deposition effectively induced oxygen vacancies and Ti³⁺, facilitating the electrical conductivity and PMS binding affinity of Fe/TNTs. Besides, oxygen vacancies could act as an electron mediator through PMS activation by iron. Moreover, the formation of Fe-O-Ti bond facilitated the synergistic redox coupling between Fe and Ti, further enhancing the PMS activation. SO₄^•- was the major radical, causing C-N bond cleavage and decreasing the overall toxicity. In contrast, APAP degradation by neat TNTs-PMS system mainly works through nonradical reaction. The Fe/TNTs activated PMS showed desired APAP removal under mild water chemistry conditions and good reusability. This work is expected to expand the potential application of titanate nanomaterials for PMS activation, and shed light on facile synthesis of oxygen defective materials for sulfate-radical-based advanced oxidation processes.

Fabrication, application, optimization and working mechanism of Fe2O3 and its composites for contaminants elimination from wastewater

Fe₂O₃ and its composites have been extensively investigated and employed for the remediation of contaminated water with the characteristics of low cost, outstanding chemical stability, high efficiency of visible light utilization, excellent magnetic ability and abundant active sites for adsorption and degradation. In this review, the potentials of Fe₂O₃ in water remediation were discussed and summarized in detail. Firstly, various synthesis methods of Fe₂O₃ and its composites were reviewed and compared. Based on the structures and characteristics of the obtained materials, their applications and related mechanisms in pollutants removal were surveyed and discussed. Furthermore, several strategies for optimizing the remediation processes, including dispersion, immobilization, nano/micromotor construction and simultaneous decontamination, were also highlighted and discussed. Finally, recommendations for further work in the development of novel Fe₂O₃-related materials and its practical applications were proposed.

Facile integration of FeS and titanate nanotubes for efficient removal of total Cr from aqueous solution: Synergy in simultaneous reduction of Cr(VI) and adsorption of Cr(III)

In this study, a novel composite composed of iron monosulfide nanoparticles (FeS NPs) and titanate nanotubes (TNTs) was hydrothermally synthesized. Characterizations revealed the encapsulation and homogenous dispersion of FeS NPs into the interlayers of TNTs. Significant performance in removal of aqueous total Cr was acquired by efficient conversion of Cr(VI) to Cr(III) on FeS and simultaneous adsorption of Cr(III) on TNTs. Moreover, the high activity of FeS-TNTs in reduction of Cr(VI) can maintain at high oxicity or alkalinity of its solution. The synergistic effect between FeS and TNTs was derived from sheltering of FeS NPs from their self-aggregation, O₂-oxidation and the affinity of Cr(III) to TNTs. The unique properties, e.g. the solid acidity, the hollow and interlayered configuration of TNTs played important roles in high activity, good stability and reusability of FeS-TNTs.

Efficient adsorption of europium (III) and uranium (VI) by titanate nanorings: Insights into radioactive metal species

Radioactive wastewater containing high concentration of radionuclides poses severe threats to ecosystem and human health, so efficient removal of these toxic heavy metals is urgently needed. Titanate nanomaterials have been demonstrated good adsorbents for heavy metals due to ion exchange property. In this study, titanate nanorings (TNRs) were synthesized using the facile hydrothermal-cooling method. The TNRs were composed of sodium trititanate, with a chemical formula of Na_0.66H_1.34Ti₃O₇•0.27H₂O and a Na content of 2.38 mmol/g. The TNRs demonstrated sufficient adsorption performance to radionuclides europium (Eu) and uranium (U) ions. Specifically, even at a high initial concentration of 50 mg/L, 86.5% and 92.6% of the two metal ions can be rapidly adsorbed by the TNRs within 5 min, and equilibrium was reached within 60 min at pH 5. The maximum adsorption capacity (Q _max) obtained by the Langmuir isotherm model was 115.3 mg/g for Eu(III) and 282.5 mg/g for uranium U(VI) at pH 5, respectively. The adsorption capacities of the two metals under various water chemical conditions were highly related to their species. Ion exchange between metal cations and Na⁺ in the TNR interlayers was the dominant adsorption mechanism, and adsorption of U(VI) was more complicated because of the co-existence of various uranyl (UO₂ ²⁺) and uranyl-hydroxyl species. The spent TNRs were effectively regenerated through an acid-base or ethylenediamine tetraacetic acid (EDTA) treatment and reused. Considering the large adsorption capacity and quick kinetic, TNRs are promising materials to remove radionuclides in environmental purification applications, especially emergent treatment of leaked radionuclides.

Simultaneous oxidation and sorption of highly toxic Sb(III) using a dual-functional electroactive filter

One of the topics gaining lots of recent attention is the antimony (Sb) pollution. We have designed a dual-functional electroactive filter consisting of one-dimensional (1-D) titanate nanowires and carbon nanotubes for simultaneous oxidation and sorption of Sb(III). Applying an external limited DC voltage assist the in-situ conversion of highly toxic Sb(III) to less toxic Sb(V). The Sb(III) removal kinetics and efficiency were enhanced with flow rate and applied voltage (e.g., the Sb(III) removal efficiency increased from 87.5% at 0 V to 96.2% at 2 V). This enhancement in kinetics and efficiency are originated from the flow-through design, more exposed sorption sites, electrochemical reactivity, and limited pore size on the filter. The titanate-CNT hybrid filters perform effectively across a wide pH range of 3-11. Only negligible inhibition was observed in the presence of nitrate, chloride, and carbonate at varying concentrations. Our analyses using STEM, XPS, or AFS demonstrate that Sb were mainly adsorbed by Ti. DFT calculations suggest that the Sb(III) oxidation kinetics can be accelerated by the applied electric field. Exhausted titanate-CNT filters can be effectively regenerated by using NaOH solution. Moreover, the Sb(III)-spiked tap water generated ∼2400 bed volumes with a >90% removal efficiency. This study provides new insights for rational design of continuous-flow filters for the decontamination of Sb and other similar heavy metal ions.

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.

Novel synergistic hydrous iron-nickel-manganese (HINM) trimetal oxide for hazardous arsenite removal

A novel hydrous iron-nickel-manganese (HINM) trimetal oxide was successfully fabricated using oxidation and coprecipitation method for metalloid arsenite removal. The atomic ratio of Fe:Ni:Mn for this adsorbent is 3:2:1. HINM adsorbent was identified as an amorphous nanosized adsorbent with particle size ranged from 30 nm to 60 nm meanwhile the total active surface area and pore diameter of HINM area of 195.78 m²/g and 2.43 nm, respectively. Experimental data of arsenite adsorption is best fitted into pseudo-second order and Freundlich isotherm model. The maximum adsorption capacity of arsenite onto HINM was 81.9 mg/g. Thermodynamic study showed that the adsorption of arsenite was a spontaneous and endothermic reaction with enthalpy change of 14.04 kJ/mol and Gibbs energy of -12 to -14 kJ/mol. Zeta potential, thermal gravimetric (TGA) and Fourier transform infrared (FTIR) analysis were applied to elucidate the mechanism of arsenite adsorption by HINM. Mechanism of arsenite adsorption by HINM involved both chemisorption and physisorption based on the electrostatic attraction between arsenite ions and surface charge of HINM. It also involved the hydroxyl substitution by arsenite ions through the formation of inner-sphere complex. Reusability of HINM trimetal oxide was up to 89% after three cycles of testing implied that HINM trimetal oxide is a promising and practical adsorbent for arsenite.

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.

Simultaneous removal of Cr(VI) and 4-chlorophenol through photocatalysis by a novel anatase/titanate nanosheet composite: Synergetic promotion effect and autosynchronous doping

Clean-up of wastewaters with coexisting heavy metals and organic contaminants is a huge issue worldwide. In this study, a novel anatase/titanate nanosheet composite material (labeled as TNS) synthesized through a one-step hydrothermal reaction was demonstrated to achieve the goal of simultaneous removal of Cr(VI) and 4-cholophenol (4-CP) from water. TEM and XRD analyses indicated the TNS was a nano-composite of anatase and titanate, with anatase acting as the primary photocatalysis center and titanate as the main adsorption site. Enhanced photocatalytic removal of co-existent Cr(VI) and 4-CP was observed in binary systems, with apparent rate constants (k1) for photocatalytic reactions of Cr(VI) and 4-CP about 3.1 and 2.6 times of that for single systems. In addition, over 99% of Cr(VI) and 4-CP was removed within 120min through photocatalysis by TNS at pH 7 in the binary system. Mechanisms for enhanced photocatalytic efficiency in the binary system are identified as: (1) a synergetic effect on the photo-reduction of Cr(VI) and photo-oxidation of 4-CP due to efficient separation of electron-hole pairs, and (2) autosynchronous doping because of reduced Cr(III) adsorption onto TNS. Furthermore, TNS could be efficiently reused after a simple acid-base treatment.

Aluminum substituted nickel ferrite (Ni–Al–Fe): a ternary metal oxide adsorbent for arsenic adsorption in aqueous medium

In this article synthesis and characterization of ternary metal oxide of aluminum substituted nickel ferrite and its adsorption capability for arsenic [(As(iii)) and (As(v))] species have been reported.

Dual-Enhanced Photocatalytic Activity of Fe-Deposited Titanate Nanotubes Used for Simultaneous Removal of As(III) and As(V)

Fe-deposited titanate nanotubes (Fe-TNTs) with high photocatalytic activity and adsorptive performance were synthesized through a one-step hydrothermal method. Initial As(III) oxidation followed by As(V) adsorption by Fe-TNTs could simultaneously remove these two toxic pollutants from aqueous solutions. The apparent rate constant value for photo-oxidation of As(III) under UV irradiation by Fe-TNTs was almost 250 times that of unmoidified TNTs. Under visible light, the Fe-TNTs also exhibited enhanced photocatalytic activity after Fe was deposited. Fe3+ located in the interlayers of TNTs acted as temporary electron- or hole-trapping sites, and attached α-Fe2O3 played the role of a charge carrier for electrons transferred from TNTs. These two effects inhibited electron-hole pair recombination thus promoting photocatalysis. Moreover, the As(V) adsorptive performance of Fe-TNTs also improved, owing to the presence of additional adsorption sites, α-Fe2O3, as well as increased pHPZC. Furthermore, Fe-TNTs exhibited good photocatalytic and adsorptive performace even after 5 reuse cycles. The present tests, concerning an initial As(III) photocatalysis and subsequent As(V) adsorption process, highlight the feasibility and importance of Fe used to modify TNTs. This study proposes a feasible method to simultaneously remove As(III) and As(V) from contaminated water using a novel Ti-based nanomaterial.