Simultaneous photooxidation and sorptive removal of As(III) by TiO2 supported layered double hydroxide

Hybridization of layered double hydroxides with functional particles

Layered double hydroxides (LDHs) are a class of materials with useful properties associated with their anion exchange abilities as well as redox and adsorptive properties for a wide range of applications including adsorbents, catalysts and their supports, electrodes, pigments, ceramic precursors, and drug carriers. In order to satisfy the requirements for each application as well as to find alternative applications, the preparation of LDHs with the desired composition and particle morphology and post-synthetic modification by the host-guest interactions have been examined. In addition, the hybridization of LDHs with various functional particles has been reported to design materials of modified, improved, and multiple functions. In the present article, the preparation, the heterostructure and the application of hybrids containing LDHs as the main component are overviewed.

Application of natural minerals in photocatalytic degradation of organic pollutants: A review

Photocatalysis is an effective, inexpensive and environmentally friendly technology for the decomposition of various aqueous organic pollutants and plays an increasingly critical role in the degradation of pollutants. Natural minerals are abundant natural resources on Earth and can be obtained directly from nature. Natural minerals are excellent photocatalyst carriers that are environmentally friendly, low in price, and will not cause secondary pollution to the environment. Natural minerals have the characteristics of a large specific surface area, providing more active centres, and adsorbing pollutants to concentrate catalysis. Natural minerals are also excellent photocatalysts, such as haematite and magnetite, which play a very good role in the degradation of water pollutants. Studies that make full use of natural minerals are of great significance. This review covers the latest research on natural minerals as photocatalytic composite materials to degrade organic pollutants in water, including three parts: the classification of natural minerals, the structural description of natural mineral composites, and the photocatalytic degradation of organic pollutants by natural mineral composites. In addition, the current limitations and opinions of natural mineral composites are discussed to achieve better results in applying natural minerals.

Photoredox catalysis of As(III) by constructed CSnS bonds: Using biomass as templates leads to bio‑carbon/SnS2 nanosheets capable of the efficient photocatalytic conversion of As(III) and calcium arsenate capture

In this paper, a new interface design strategy for bio‑carbon/SnS₂ nanosheets equipped with CSnS bonds was proposed by using biomass as a template for the efficient photocatalytic conversion of As(III). The characterization results illustrated that the CSnS bonds could effectively prevent the agglomeration of SnS₂, expand the photoresponse range and improve the hydrophilicity of the bio‑carbon/SnS₂ composites while reducing their transfer resistance. Therefore, the construction of CSnS bonds could more efficiently promote the photoredox catalysis of As(III) to As(V) compared with pure SnS₂, attributing to the polarization and conjugation effects of the CSn bonds. Meanwhile, CaSO₄·nH₂O (n = 0, 0.5, 2) could rapidly convert AsO₄^3- into Ca₃(AsO₄)₂ precipitates to eliminate arsenic from the aqueous solution in one step. In particular, 7500 μg/L As(III) could not only be photocatalyzed into As(V) but also be converted to Ca₃(AsO₄)₂ to achieve the removal of arsenic within only 55 min in the coexistence of CaSO₄. In addition, the electron transfer path in the photocatalytic oxidation system on arsenite was proposed according to the Mott-Schottky (MS) plots of SnS₂ and graphitic carbon. The electron paramagnetic resonance (EPR) results implied that O₂^- and h⁺ were the main active substances in the photooxidation arsenic system and the effect of OH could be negligible. Thus, the possible mechanism of the photocatalytic conversion of As(III) was discussed.

Simultaneous oxidation and adsorption of arsenic by one-step fabrication of alum sludge and graphitic carbon nitride (g-C3N4)

The oxidation of trivalent arsenic (As(III)) to pentavalent arsenic (As(V)) is a common pretreatment to remove As(III) from the aqueous phase. The graphitic carbon nitride as a photocatalyst can transform As(III) to As(V), but the photocatalyst does not adsorb any species of arsenic. In this study, a new composite material to achieve the simultaneous oxidation and adsorption of arsenic was synthesized by co-pyrolyzing alum sludge and melamine. It was hypothesized that graphitic carbon nitride derived from melamine oxidizes As(III) to As(V) and pyrolyzed alum sludge provides strong adsorption sites for the oxidized As(V). The composites were characterized by X-ray diffraction, Brunauer-Emmett-Teller(BET) surface, scanning electron microscope, and X-ray photoelectron spectrometer. The composite material effectively converted As(III) to As(V) under the light, and the total arsenic concentration decreased in the aqueous phase via the adsorption of As(V). Speciation analysis of arsenic on the composite showed that both As(III) and As(V) species were present on the surface of adsorbent, from which desorption by mixing with deionized water was difficult. This new and green composite material can oxidize As(III) and adsorb arsenic simultaneously under the light, which can be used to treat arsenic-containing water.

Comparision of photocatalysis and photolysis processes for arsenic oxidation in water

The oxidation of As(III) to As(V) in aqueous solution was evaluated using heterogeneous photocatalysis and photolysis. The influence of TiO₂ as catalyst in different crystalline (rutile, anatase) and commercial forms was evaluated in a batch reactor and an insignificant difference was observed between them. The process by photocatalysis reached up to 97% As(III) oxidation and no significant difference was observed comparing to results obtained by photolysis. The photolysis experiments (UV radiation only), also carried out in a batch system, showed a high oxidation rate of As(III) (90% in 20min). The influence of different matrices (well water, river water and public water supply) were evaluated. Additionally, the effect of As(V) concentration, generated during the oxidation process, was studied. Continuous photolysis experiments using only UV radiation were performed, resulting in a high As(III) oxidation rate. Using a flow rate of 5mLmin^-1 and an initial concentration of As(III) 200µgL^-1, gave an oxidation percentage of As(III) of up to 72%, showing a simple and economical alternative to the oxidation step of As(III) to As(V) in the treatment of water contaminated with arsenic.

Efficiently Visible-Light Driven Photoelectrocatalytic Oxidation of As(III) at Low Positive Biasing Using Pt/TiO2 Nanotube Electrode

A constant current deposition method was selected to load highly dispersed Pt nanoparticles on TiO2 nanotubes in this paper, to extend the excited spectrum range of TiO2-based photocatalysts to visible light. The morphology, elemental composition, and light absorption capability of as-obtained Pt/TiO2 nanotubes electrodes were characterized by FE-SEM, energy dispersive spectrometer (EDS), X-ray photoelectron spectrometer (XPS), and UV-vis spectrometer. The photocatalytic and photoelectrocatalytic oxidation of As(III) using a Pt/TiO2 nanotube arrays electrode under visible light (λ > 420 nm) irradiation were investigated in a divided anode/cathode electrolytic tank. Compared with pure TiO2 which had no As(III) oxidation capacity under visible light, Pt/TiO2 nanotubes exhibited excellent visible-light photocatalytic performance toward As(III), even at dark condition. In anodic cell, As(III) could be oxidized with high efficiency by photoelectrochemical process with only 1.2 V positive biasing. Experimental results showed that photoelectrocatalytic oxidation process of As(III) could be well described by pseudo-first-order kinetic model. Rate constants depended on initial concentration of As(III), applied bias potential and solution pH. At the same time, it was interesting to find that in cathode cell, As(III) was also continuously oxidized to As(V). Furthermore, high-arsenic groundwater sample (25 m underground) with 0.32 mg/L As(III) and 0.35 mg/L As(V), which was collected from Daying Village, Datong basin, Northern China, could totally transform to As(V) after 200 min under visible light in this system.

Effective As(III) Removal by A Multi-Charged Hydroacid Complex Draw Solute Facilitated Forward Osmosis-Membrane Distillation (FO-MD) Processes

Effective removal of As(III) from water by an oxalic acid complex with the formula of Na3[Cr(C2O4)3] (Na-Cr-OA) is demonstrated via an forward osmosis-membrane distillation (FO-MD) hybrid system in this study. Na-Cr-OA first proved its superiority as a draw solute with high water fluxes and negligible reverse fluxes in FO, then a systematic investigation of the Na-Cr-OA promoted FO process was conducted to ascertain the factors in As(III) removal. Relatively high water fluxes of 28 LMH under the FO mode and 74 LMH under the pressure retarded osmosis (PRO) mode were achieved when using a 1000 ppm As(III) solution as the feed and 1.0 M Na-Cr-OA as the draw solution at 60 °C. As(III) removal with a water recovery up to 21.6% (FO mode) and 48.3% (PRO mode) were also achieved in 2 h. An outstanding As(III) rejection with 30-3000 μg/L As(III) in the permeate was accomplished when As(III) feed solutions varied from 5 × 10(4) to 1 × 10(6) μg/L, superior to the best FO performance reported for As(III) removal. Incorporating MD into FO not only makes As(III) removal sustainable by reconcentrating the Na-Cr-OA solution simultaneously, but also reduces the As(III) concentration below 10 μg/L in the product water, meeting the WHO standard.

Enhanced Arsenate Removal Performance in Aqueous Solution by Yttrium-Based Adsorbents

Arsenic contamination in drinking water has become an increasingly important issue due to its high toxicity to humans. The present study focuses on the development of the yttrium-based adsorbents, with basic yttrium carbonate (BYC), Ti-loaded basic yttrium carbonate (Ti-loaded BYC) and yttrium hydroxide prepared using a co-precipitation method. The Langmuir isotherm results confirmed the maximum adsorption capacity of Ti-loaded BYC (348.5 mg/g) was 25% higher than either BYC (289.6 mg/g) or yttrium hydroxide (206.5 mg/g) due to its increased specific surface area (82 m²/g) and surface charge (PZC: 8.4). Pseudo first- and second-order kinetic models further confirmed that the arsenate removal rate of Ti-loaded BYC was faster than for BYC and yttrium hydroxide. It was subsequently posited that the dominant removal mechanism of BYC and Ti-loaded BYC was the carbonate-arsenate ion exchange process, whereas yttrium hydroxide was regarded to be a co-precipitation process. The Ti-loaded BYC also displayed the highest adsorption affinity for a wide pH range (3-11) and in the presence of coexisting anionic species such as phosphate, silicate, and bicarbonate. Therefore, it is expected that Ti-loaded BYC can be used as an effective and practical adsorbent for arsenate remediation in drinking water.