Catalytic system based on recyclable Fe0 and ZnS semiconductor for UV-promoted degradation of chlorinated organic compounds

Fe-0 (ZVI) is one of the most effective reactants for treatment of water contaminated with chlorinated hydrocarbons, pollutants recalcitrant to degradation. Unfortunately, ZVI use is limited by corrosion and loss of reactivity over time. In order to suppress oxidation and passivation phenomena of iron, in this study ZVI was coupled to a suitable semiconductor (ZnS). Under UV light, electrons in the valence band of ZnS are promoted in the conducting band (E-CB = 1.04 eV vs NHE) and are able to reduce Fe2+ to Fe-0 (E-0 = 0.44 eV vs NHE). The ZnS_ZVI composite was simply prepared by coating commercial ZnS with Fe-0 obtained by reducing Fe (II) salt with sodium borohydride. The achieved ZnS_ZVI catalytic system was characterized by FESEM, WAXD, Raman and N2 adsorption at 196 degrees C, with the aim to analyze morphology and crystalline structure of both components of the ZnS_ZVI system. Catalytic activity tests showed that ZnS_ZVI is effective in degrading probe molecules, eriochrome black T dye (EBT) and chlorobenzene (PhCl), from aqueous solutions under UV light irradiation. The experimental results show that in both cases the reductive action of Fe-0 on the N = N bond of EBT and C-Cl of PhCl is synergistically coupled to the oxidative action of ZnS towards by-products. OH radicals, generated by the reaction between the electron holes in the valence band of ZnS and H2O (OH-), were indeed confirmed by photoluminescence analysis. Recyclability tests indicate that ZnS_ZVI is highly stable since its catalytic activity, on both probe molecules, remains unchanged even after several cycles of reuse under UV irradiation. ZnS_ZVI catalytic behavior towards EBT has been carefully analyzed and a possible mechanism of EBT degradation has been proposed, which involves the formation of a Fe2+-EBT complex whose structure has also been suggested.