Oxidation reactivity of As(III)-containing pyrites: Differences between structurally-incorporated and adsorbed As(III)

Abstract

Natural pyrites contain high contents of adsorbed and structurally-incorporated arsenic (As), which may in turn affect the oxidation rates of pyrite as well as the transformation and immobilization behaviors of As contaminants. However, the speciation and distribution of As on the oxidation reactivity of pyrites is still not clear yet. In this study, the oxidation reactivity of three different pyrites, namely, pure pyrite (Py-free), As(III)-adsorbed pyrite [Py*-As(III)ad] and As(III)-structurally-incorporated pyrite [Py-As(III)in, including Py-0.02 and Py-0.04] will be compared. In Py-As(III)in, As(III) replaces S(-I) and changes the relative ratio of Fe: S. The XPS results reveal that Fe(III)-S(-II) site densities on the pyrite surface increase with incorporated As, indicating that Py-As(III)in with more As carries more surface defects. The oxidation rates of pyrites are accelerated at pHs 7.0 and 9.0 by both structurally-incorporated and adsorbed As(III), which is more significant in structurally-incorporated As(III). At pH 2.5, structurally-incorporated As(III) promotes the oxidation of pyrite, whereas adsorbed As(III) inhibits its oxidation. The Tafel plots and the electrochemical impedance spectroscopy (EIS) analysis further support that As in Py-As(III)in decreases the charge transfer resistance on the pyrite surface and facilitates electron transfer between dissolved oxygen (DO) and the pyrite surface, thus enhancing the oxidation rates of pyrites. Variations of H2O2 in Py-As(III)in in the full pH range also indicate that As increases the two-electron transfer from pyrite to DO, resulting in the formation of more H2O2. At pH 2.5, the generated OH from H2O2 is the predominant species in pyrite oxidation, and at pHs 7.0 and 9.0, H2O2, Fe(IV) and OH all contribute to pyrite oxidation. Compared with Py-As(III)in, As in Py*-As(III)ad is more readily to be mobilized, with more As(V) being detected at pH 2.5. Subsequent decreases of As(III) and As(V) suggest re-immobilization of the released As via the adsorption or co-precipitation pathways. Solid characterizations further reveal that dissolved or adsorbed As(III) significantly inhibit the development of lepidocrocite, goethite and magnetite, whereas structurally-incorporated As(III) does not, triggering differences in the speciation of secondary Fe minerals. The present study reveals that the speciation and distribution of As(III) significantly affects the chemical and electronic properties of different pyrites, thus affecting the corresponding oxidation reactivity of different pyrites.