The impact of electron transfer between structural Fe(II) and aqueous Fe(III) on the redox transformation of arsenic

In the high-arsenic and iron groundwater system, the interfacial Fe(II)/Fe(III) redox couples are easily formed due to the seasonal change of redox potential. In general, this Fe(II)/Fe(III) redox couple is more redox-active than the Fe(II) or Fe(III) along. Most of the published works focused on the electron transfer between aqueous Fe(II) and structural Fe(III) in clay minerals, or the opposite pathway, i.e., electron transfer from structural Fe(II) to aqueous Fe(III). However, few research has reported the effect of Fe(II)–Fe(III) electron transfer on the redox transformation of arsenic. Understanding this process is important for assessing iron cycling in subsurface environments, as well as the fate and transport of arsenic in the shallow groundwater system. A series of hydrochemical and spectroscopic tests (i.e., XPS) were applied to analysis the prerequisite and pathway of the Fe(II)str–Fe(III)aq electron transfer, and deciphering the oxidation mechanism of As(III) by this process. The main findings are: in the PRN/Fe(III)aq/As(III) system (PRN refers to partially reduced NAu-1) under anoxic condition, the As(III) was oxidized rapidly and the XPS results show the appearance of Fe(II)–As(V) bond, which suggests that a short-time intermediate product [active Fe(III)str] was formed on the surface of the PRN. This active Fe(III)str can oxidize aqueous As(III) to As(V). In the PRN/Fe(II)aq/As(III) system experiencing anoxic and oxic process, a small portion of As(III) were oxidized by the PRN itself or the ·OH/Fe(IV) produced by Fenton reaction, while large amounts of As(III) were oxidized by the reactive Fe(III)str formed during the Fe(II)str–Fe(III)aq electron transfer. We considered that the aqueous Fe(II) was first oxidized to Fe(III) by H2O2, triggering the Fe(II)str–Fe(III)aq electron transfer which acts on the oxidation of As(III).