REDOX REACTIONS DURING ARSENITE ADSORPTION ON FERRIHYDRITE

Redox transformations of arsenic and iron strongly affect their fate and transport in the environment. It is of significance to investigate redox reactions of Fe and As on the surface of major metal oxide in sediments. There are controversies in As(III) oxidation on ferrihydrite and ferrihydrite reduction in previous investigations. In this work, oxidation of As(III) and reduction of ferrihydrite were studied by using HPLC-AFS and X-ray absorption near edge spectroscopy (XANES). Iron isotopic fractionation was also used to evaluate redox reactions.

Results showed that kinetics of As(III) adsorption was better fit to the pseudo-second order adsorption model in comparison with the pseudo-first order adsorption model, regardless of ferrihydrite dosage and pH. Adsorption equilibrium time was longer at the lower adsorbent dosage. Adsorption was independent of solution pH between 4 and 7. Results of As species in solutions and solids showed that no redox reactions occurred in As(III)-ferrihydrite system in the absence of oxygen, but As(III) was partially oxidized in the presence of oxygen. As(III) oxidation slightly affected As removal from solutions. Redox reactions were also confirmed by iron isotope fractionation. Fe isotope may be a good tool showing ferrihydrite as oxidant in the As(III)-ferrihydrite system in the presence of oxygen, instead of catalyst.

Acknowledgement

The study has been financially supported by NSFC (Nos. 41222020 and 41172224), the Chinese Universities Scientific Fund (No. 2652013028), and the Chinese Scholarship Council (CSC).