As(III) removal of aqueous solutions using zerovalent iron nanoparticles: The role of the UVA-Vis irradiation wavelength

Abstract

Arsenic contamination in drinking water is dramatic in many countries, particularly in Asia and Latin America. Arsenic removal from water can be achieved through some conventional technologies but some of them have drawbacks or are very expensive, especially in developing countries, where the population uses groundwater for drinking purposes. Therefore, the need of searching for new and low-cost methodologies for As removal from water is mandatory. Because As(V) is removed more easily than As(III), oxidation of this species, when present, is needed as the first step. The use of zerovalent iron nanoparticles (nZVI), particularly reactive due to their extremely small particle size and large surface area can be proposed at a household scale and in isolated regions. The effect of UV–Vis irradiation on the removal of arsenical species using nZVI presents scarce reports in the literature. In the present work, experiments of As(III) removal with commercial nZVI under UVA-Vis irradiation (λem = 300–800 nm) are described. The effect of the As(III) concentration, the As:nZVI molar ratio (MR = [As(III)]:[FeTOTAL]), and the irradiation wavelength on the As(III) removal were analyzed. Two initial As(III) concentrations, 1 and 10 mg L−1, and different MR in the range 1:10–1:100 were tested at pH 6–6.8. The increase of the nZVI amount and the use of UV–Vis irradiation enhanced the As removal under all conditions. The optimal MR for the As(III) removal after 60 min of irradiation at both initial concentrations was 1:30, being 85% for [As(III)]0 = 10 mg L−1, and 92% for [As(III)]0 = 1 mg L−1. By the use of chemical optical filters, it was determined that the most efficient wavelengths to improve the As(III) removal were those lower than 450 nm. All experimental data were adjusted to a biexponential decay suggesting the occurrence of two removal processes, one faster than the other. It was proposed that the main mechanisms under UVA-Vis irradiation were the action of oxides and oxyhydroxides already present or formed on the surface of nZVI during the reaction as semiconductors originating oxidative species and a ligand-to-metal charge-transfer reaction on As(III)-Fe(III) complexes formed in the system with simultaneous As(III) oxidation and Fe(III) reduction. It was concluded that the effect of the light depends on the pollutant and not on the iron material.