Insight into the high reactivity of commercial Fe-Si-B amorphous zero-valent iron in degrading azo dye solutions

Improving intrinsic reactivity is one of the key requirements in applying zero-valent iron in the field. As a new kind of zero-valent iron, iron based amorphous alloys were recently found to be capable of rapidly remediating wastewater. However, the mechanisms for the rapid degradation have not yet been fully understood. In this study, commercial Fe-Si-B amorphous alloy ribbons (Fe-Si-B-AR) were used to degrade azo dyes (Direct Blue 6 and Orange II) to study the reaction kinetics, pathway and mechanism behind the high reactivity of these iron based amorphous alloys. The results show that, under the same conditions, the surface normalized reaction rate constants for the decomposition of Orange II and Direct Blue 6 by Fe-Si-B-AR could be 1300 and 60 times larger respectively than those obtained by using 300 mesh iron powders. Through UV-vis spectrophotometry and mass spectrometry, it is found that the intermediate products of the azo dyes degraded by Fe-Si-B-AR are similar to those produced in degradation by iron powders. However, the controlling step of the degradation reaction by Fe-Si-B-AR turns out to be the diffusion process rather than the surface chemical reaction found in the reaction by iron powders. Further analysis indicates that the high degradation efficiency of Fe-Si-B-AR results from its amorphous structure and the metalloid additions, which could enhance the catalytic effect and promote the formation of a non-compact and easily detached oxide layer on the surface. The experiments under different environmental conditions show that the factors that influence the degradation efficiency of crystalline iron powders affect that of Fe-Si-B-AR in a similar way, but Fe-Si-B-AR is capable of efficiently degrading wastewater under broader conditions than the crystalline iron powders. The results indicate that Fe-Si-B-AR is a promising environmental catalyst for wastewater treatment.