Coupled
Effects of Aging and Weak Magnetic Fields
on Sequestration of Selenite by Zero-Valent Iron
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Abstract
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 Fe<sup>2+</sup> release suggests direct reduction of Se(IV)
to Se(0) by Fe<sup>0</sup>, 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 Fe<sup>2+</sup>. 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. Fe<sup>2+</sup>) along the higher
field gradient at the ZVI particle surface, thereby inducing nonuniform
depassivation and eventually localized corrosion of the ZVI surface