Influence of Riboflavin on Nanoscale Zero-Valent Iron Reactivity during the Degradation of Carbon Tetrachloride

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(-1)) was 1.5 times higher than that with riboflavin (0.370 +/- 0.012 h(-1)) in the first cycle. Riboflavin was rapidly reduced (65.0 +/- 7.0 h(-1)) 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(-1) in the third cycle, while that with riboflavin was significantly enhanced (0.663 +/- 0.005 h(-1)). 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.