Roles of silica coating on nanosized zero-valent iron in sequential reduction-oxidation process in a system containing persulfate.

A sequential reduction-oxidation process using silica-coated nanosized zero-valent iron (nZVI) particles (nZVI@SiO) and persulfate for mineralizing recalcitrant compounds was developed, and the effects of the process on nitrobenzene were evaluated. This sequential process significantly enhanced contaminant mineralization, which could not be effectively achieved by reduction or oxidation alone. The nZVI@SiO rapidly reduced nitrobenzene to aniline, then the aniline concentration gradually decreased after persulfate had been added and initiated sequential oxidative degradation. The SiO coating on the nZVI@SiO limited outward mass transfer of reaction products and increased the efficiency with which nitrobenzene was converted into aniline. Slow release of Fe(II) caused by the coating caused persulfate activation and subsequent aniline oxidation to be more sustained and efficient than without the coating. The final nitrobenzene-aniline mineralization efficiency was higher for the nZVI@SiO/persulfate system than the nZVI/persulfate system. The SiO coating of the nZVI@SiO particles was an excellent protective layer, protecting the particles from undesirable consumption through reactions with groundwater components. nZVI@SiO particle transformations during the sequential process were investigated, and the operating conditions were optimized to maximize the recalcitrant compound removal efficiency. The results indicated that nZVI@SiO and persulfate could be used to mineralize organic contaminants in groundwater through sequential reduction-oxidation.