Development of heteropolyacid-modified natural ilmenite photocatalysts for the oxidation of As(III) and insights into their photoelectrochemical behavior

Natural ilmenite (FeTiO 3 ) was superficially modified with phosphotungstic acid (H 3 PW 12 O 40 , PTA), aiming to improve its photocatalytic performance in the oxidation of As(III), by the formation of type II heterojunctions. The heterostructure PTA/ilmenite materials were obtained through physical wet impregnation method, in which the volumetric ratio ethanol:water of the impregnation media was varied (2:1, 1:1 and 1:2). The content of W in PTA/ilmenite materials was determined by atomic absorption spectroscopy, and it was established that higher proportion of water in the medium (1:2) leads to improved impregnation of PTA. Interestingly, even though the adsorption of As(III) was hindered by PTA modification, it was found that PTA/ilmenite heterostructures exhibited efficiencies 1.3–2.6 times higher than bare ilmenite in the photo-oxidation of As(III) to As(V) under visible light irradiation. The amount of As(V) produced as a function of As(III) adsorbed suggest the improvement of As(V) desorption, enabling the re-adsorption of As(III), thus increasing the global oxidation process. The photocurrent response from chronoamperometry measurements showed that PTA-surface anions act as storage sites for photogenerated electrons, enhancing the spatial separation of electron-hole pairs. Upon As(III) addition, the increase in current density suggests that the surface-PTA trapping effect also prevents the reduction of formed As(IV) back to As(III), thus, leading to the obtention of As(V) as final product. The joint analysis of the photocatalytic and photoelectrochemical behavior evidenced that addition of PTA led to the enhancement of regenerative capability of surface-active sites, along with the reduction in the recombination of electron-hole pairs. Furthermore, the oxidation mechanism depicting the role of PTA as electron acceptor was proposed.