Fe(II)-mediated activation of oxygen by goethite for the As(III) oxidation and the mechanisms

Abstract

<p indent="0mm">The contamination of arsenic (As) in paddy soils is one of the most critical issues in China, and subsequently threatening food safety and human health. Previous studies suggest that the transformation of As species in paddy soil is a key factor determining its availability, uptake and accumulation in crops, particularly rice grains; arsenite As(III) is more toxic and mobile than arsenate As(V). Being important components in soils, evidence shows that redox-active iron minerals play an important role in the transformation and fate of As in the rhizosphere zone in paddy soils, however, the underlying mechanisms are largely unknown. To address the above mentioned scientific questions, in this work, a simulated rhizosphere reaction system in paddy soil was built to investigate the reaction process of As(III) on the surface of goethite with and without the addition of Fe(II) under different oxic conditions (e.g., the effects of Fe(II) concentration and goethite dosages), and the transformation of As(III) to As(V) was monitored and compared in systems. Furthermore, the possible reactive oxygen species (ROSs) were identified by quenching experiments and electron spin resonance (ESR). Together with X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), the mechanism of As(III) oxidation on the surface of goethite mediated by Fe(II) under oxic conditions were proposed. Our results indicate that both Fe(II) concentration and goethite dosage determine the As(III) oxidation; the presence of goethite significantly enhances the oxidation extent of As(III) from 12% (without goethite) to 18% <sc>(1.5 g/L</sc> goethite). Despite both homogenous and heterogeneous reactions of As(III) oxidation occur, results suggest that the oxidation reaction of As(III) on the surface of goethite predominates due to the rapid adsorption of As(III) onto the goethite within seconds. The increase of goethite dosage leads to a higher oxidation extent of As(III), which is likely caused by the presence of higher oxygen vacancies on goethite. Moreover, the increase of Fe(II) concentration from 5 to <sc>15 mg/L</sc> significantly enhances the As(III) oxidation extents from ~10% to ~25% (the As(III) and Fe(II) initial concentrations are 10 and <sc>15 mg/L),</sc> while the adsorption of total As is insignificantly affected by the concentration of Fe(II). Quenching experiments indicates that the <bold>·</bold>O₂⁻ species are the dominant ROSs determining the As(III) oxidation, which has also been confirmed by the ESR spectra. Besides, the generated As(V) mainly adsorbs on or incorporates in goethite. In conclusion, our findings suggest that the Fe(II) adsorbed on the goethite can directly activate oxygen to generate reactive oxygen species and then promote the oxidation of As(III) under oxic conditions. Importantly, the oxygen vacancies on goethite can further accelerate the oxidation of As(III) by enhancing the iron cycling and the decomposition of H₂O₂ to <bold>·</bold>O₂⁻species. The findings of this study provide new insight into understanding the reaction process of As(III) near the rhizosphere zone and can be useful for developing new techniques for regulating the As contamination in paddy soils.