Oxidative transformation of contaminants using colloidal zero-valent iron

It has recently been demonstrated that nano scale zero-valent iron (nZVI) oxidizes the herbicide molinate when it is used in the presence of oxygen. Further batch and column experiments were conducted to investigate the potential application of nZVI for the treatment of contaminants. Results of batch studies reveal a rapid initial degradation of molinate followed by slower degradation over more than a day. Additional insight into the nZVI-mediated process has been obtained from studies of the formation of para-hydroxybenzoic acid (p-HBA) from the oxidative degradation of benzoic acid with slow ongoing production of p-HBA over 24 h observed after an initial pulse of byproduct production. Addition of EDTA enhances the initial pulse by 50% but does not appear to reduce surface passivation over a longer time frame. Oxygen availability during the initial pulse appears to be a limiting factor. Results of column studies reveal that the arrangement of the nZVI, sand and gravel within the column strongly affect the degradation performance despite each column having the identical nZVI loading. Under the optimal column configuration, >90% removal of 100 ppb molinate was observed over a 3 h period. These promising results suggest that nZVI, despite initial rapid oxidation of the particle surface, has sufficient residual oxidizing power to enable it to be incorporated into a continuous treatment process.