The influence of Si(iv) on the reactivity of [Fe(iii)]/[Fe(ii)] couples for 2-nitrophenol reduction in γ-Al2O3 suspensions

Abstract

In a natural environment, Fe(II) adsorbed onto the surfaces of natural particles to form various surface complex species can influence the transformation of contaminants. The reductive reactivity of the [Fe(III)]/[Fe(II)] couples are close correlated with the surrounding conditions. In this study, we investigated the effects of Si(IV) on the reductive reactivity of [Fe(III)]/[Fe(II)] couples adsorbed onto γ-Al₂O₃. Experiments were conducted under different conditions to investigate the effects of Si(IV) on the reactivity of [Fe(III)]/[Fe(II)] couples for 2-nitrophenol (2-NP, selected as the model pollutant) reduction in γ-Al₂O₃ suspensions. Kinetics results revealed that chemical adsorption is the rate limiting step in Fe(II) and Si(IV) adsorption processes and the reduction of 2-NP is an endothermic reaction. The linear correlations between the reduced peak oxidation potential (E_p) (versus SCE) and 2-NP reduction rate (ln k), and between the adsorbed Fe(II) density (ρ_Fe(II)) and ln k, illustrated that E_p and ρ_Fe(II) are two key factors in the inhibiting effects of Si(IV) on the reductive reactivity of Fe(III)/Fe(II) couples on γ-Al₂O₃. The results of Fe K-edge X-ray absorption spectroscopy revealed that the increase of Si(IV) concentration resulted in the gradual change in the composition of the adsorbed Fe species from pure AlOFe⁺ (γ-Al₂O₃ surface-bound Fe(II) species with higher reductive reactivity) to a mixture of AlOFe⁺ and SiOFe⁺ (SiO₂ surface-bound Fe(II) species with lower reductive reactivity), leading to the decrease in ρ_Fe(II), the positive shift in E_p, the increase in activation energy (E_a), and consequently the decrease in the reduction rate (ln k) of 2-NP.