Influence
of Riboflavin on Nanoscale Zero-Valent Iron
Reactivity during the Degradation of Carbon Tetrachloride
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Abstract
Experiments were conducted to investigate
the effect of riboflavin
on the reactivity of nanoscale zerovalent iron (NZVI) during three
reaction cycles of carbon tetrachloride (CT) degradation. The degradation
kinetics of CT by NZVI without riboflavin (0.556 ± 0.044 h<sup>–1</sup>) was 1.5 times higher than that with riboflavin (0.370
± 0.012 h<sup>–1</sup>) in the first cycle. Riboflavin
was rapidly reduced (65.0 ± 7.0 h<sup>–1</sup>) by NZVI
during CT degradation, resulting in the slow degradation kinetics
of CT in the first cycle due to competition for electrons from NZVI
between riboflavin and CT. These results indicate that riboflavin
is not effective as an electron shuttle for reduction of CT by NZVI.
On the other hand, the degradation kinetics of CT by NZVI without
riboflavin decreased to 0.122 ± 0.033 h<sup>–1</sup> in
the third cycle, while that with riboflavin was significantly enhanced
(0.663 ± 0.005 h<sup>–1</sup>). The results from X-ray
analyses and transmission electron microscopy suggest that the decline
in reactivity of NZVI without riboflavin in the third cycle resulted
from continuous Fe(0) oxidation to iron oxides on the NZVI surface.
In contrast, riboflavin enhanced the reactivity of NZVI by reductive
dissolution of passive iron oxides on NZVI surface by reduced riboflavin.
The experimental results suggest that riboflavin can play a pivotal
role in the prolongation of NZVI reactivity in long-term in situ and
ex situ applications of NZVI