Arsenic Photoactive Adsorption Mechanism On Titanium Modified Composites

Arsenic contamination is a global environmental problem, adsorption is one ofthe most effective and common method for the removal of arsenic, and an efficientarsenic adsorbent is also dependent on a deep understanding of the adsorptionmechanism. Recently, nanocrystalline titanium dioxide has been developed foreffective removal of arsenic due to its good photochemical stability, non-toxic, highcatalytic activity for As(III) oxidation. In this study, we make a comprehensive reviewon the application of TiO2and TiO2bifunctional adsorbents for arsenic adsorption.We also make many efforts to understand the the performance of TiO2by eithercombing TiO2adsorbent with good adsorption property in one system or developingbifunctional adsorbents with both great photocatalytic ability and high adsorptioncapacity.Anatase TiO2, two metal mixed oxide Ti-Fe and Ti-Mn binary oxide composites,and a series of titanium and iron pillared montmorillonites (Ti-Fe/MMT) wereprepared by a hydrolysis method and to comprehensively evaluate arsenicadsorption/oxidation on TiO2with and without UV irradiation. The as-preparedsamples were characterized by powder X-ray diffraction (XRD), scanning electronmicroscope (SEM), transmission electron microscope (TEM), and nitrogenadsorption-desorption techniques. After a general characterisation of those adsorbents,the adsorption kinetics and isotherm data were analysed. The effects of pH and theinfluence of coexisting ions on arsenic adsorption were investigated in order tounderstand the mechanism of arsenic adsorption under UV irradiation.XRD, nitrogen adsorption-desorption techniques, electron paramagneticresonance (EPR), SEM, TEM and X-ray fluorescence analyzer (XRF) investigationsrevealed that the TiO2and TiO2bifunctional adsorbents have better surfaceperformance for arsenic adsorption, and the anatase nanoparticles of TiO2in the TiO2bifunctional adsorbents were responsible for As(III) oxidation under UV irradiation.The optimum adsorption capacity on the adsorbent was achieved at pH below7forAs(V) and at neutral pH for As(III). Among all the common coexisting ionsinvestigated, silicate and phosphate ions were the greatest competitor with arsenic foradsorptive sites on the adsorbent. Zeta potential and Fourier transform infrared (FTIR)analysis indicated that the hydroxyl groups on the TiO2and TiO2bifunctionaladsorbents surface were involved in arsenic adsorption, while X-ray photoelectronspectroscopy (XPS) provided further evidence for the involvement of hydroxyl groups in the sorption and the formation of monodentate and bidentate complexes on theadsorbent surface. Furthermore, the XPS and Raman analysis confirmed that the Mninvolved in the oxidation-sorption of As(III). The study suggests that Ti-Fe and Ti-Mn binary oxide composites were effective adsorbent for arsenic removal due to itsphotocatalytic oxidation property and the presence of high affinity surface hydroxylgroups. The good reusability of Ti-Fe/MMT also indicated that the Ti-Fe/MMT was avery promising adsorbent for arsenic removal from arsenic contaminated water.After adsorbed with TiO2and TiO2bifunctional adsorbents, the final arsenicconcentration in solution below10μg/L, and meet the arsenic standard in drinkingwater. The results suggest that the TiO2and TiO2bifunctional adsorbents canfunctions as both photocatalyst and adsorbent in the presence of UV light, but itworks only as adsorbent in the absence of UV light.