Phytic acid inhibits Cr(VI) reduction on Fe(II)-bearing clay minerals: Changing reduction sites and electron transfer pathways

Abstract

The presence of organic phosphorus may influence the characteristics of Cr(VI) reduction and immobilization on Fe(II)-bearing clay minerals under anoxic conditions, as the organic phosphorus tends to bind strongly to clay minerals in soil. Herein, reduced nontronite (rNAu-2) was used to reduction of Cr(VI) in the presence of phytic acid (IHP) at neutral pH. With IHP concentration from 0 to 500 μM, Cr(VI) reduction decreased obviously (17.8%) within first 5 min, and then preferred to stagnate during 4–12 h (≥50 μM). After that, Cr(VI) was reduced continuously at a slightly faster rate. Density functional theory (DFT) calculations revealed that IHP primarily absorbed at the edge sites of rNAu-2 to form Fe-IHP complexes. X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), and Fourier transform infrared spectroscopy (FTIR) results demonstrated that IHP hindered the ingress of CrO₄²⁻ into the interlayer space of rNAu-2 and impeded their reduction by trioctahedral Fe(II) and Al-Fe(II) at basal plane sites in the initial stage. Additionally, Fe(II) extraction results showed that IHP promoted the electron from interior transfer to near-edge, but hindered it further transfer to surface, resulting in the inhibition on Cr(VI) reduction at edge sites during the later stage. Consequently, IHP inhibits the reduction and immobilization of Cr(VI) by rNAu-2. Our study offers novel insights into electron transfer pathways during the Cr(VI) reduction by rNAu-2 with coexisting IHP, thereby improve the understanding of the geochemical processes of chromium within the iron cycle in soil.