Coupled effects of aging and weak magnetic fields on sequestration of selenite by zero-valent iron

The sequestration of Se(IV) by zero-valent iron (ZVI) is strongly influenced by the coupled effects of aging ZVI and the presence of a weak magnetic field (WMF). ZVI aged at pH 6.0 with MES as buffer between 6 and 60 h gave nearly constant rates of Se(IV) removal with WMF but with rate constants that are 10- to 100-fold greater than without. XANES analysis showed that applying WMF changes the mechanism of Se(IV) removal by ZVI aged for 6-60 h from adsorption followed by reduction to direct reduction. The strong correlation between Se(IV) removal and Fe²⁺ release suggests direct reduction of Se(IV) to Se(0) by Fe⁰, in agreement with the XANES analysis. The numerical simulation of ZVI magnetization revealed that the WMF influence on Se(IV) sequestration is associated mainly with the ferromagnetism of ZVI and the paramagnetism of Fe²⁺. In the presence of the WMF, the Lorentz force gives rise to convection in the solution, which narrows the diffusion layer, and the field gradient force, which tends to move paramagnetic ions (esp. Fe²⁺) along the higher field gradient at the ZVI particle surface, thereby inducing nonuniform depassivation and eventually localized corrosion of the ZVI surface.