Chitosan capped Ag0, Fe0 and Fe doped Ag@Fe as persulfate activators for the oxidative degradation of metronidazole in water.

Chitosan-capped Ag0, Fe0, and Ag@Fe prepared via chemical reduction and seed-growth methods were used to activate persulfate (S2O82-) for oxidative degradation of metronidazole (MTZ) antibiotic drug. This study compared the results of MTZ degradation by monometallic (chitosan-Ag0 and chitosan-Fe0) and bimetallic (chitosan-Ag@Fe) activated S2O82-. The kinetics of the MTZ decay was monitored by the absorbance measurements at 317 nm. The results showed that the Fenton-like system (chitosan-Ag@Fe/S2O82-) leads to 96.7% of concentration removal for MTZ under optimal experimental conditions ([MTZ] = 0.05 mM, [activator] = 5.0 mg, [S2O82-] = 3.0 mM, temperature = 23 °C, and pH = 5.0 after 60 min). The MTZ degradation followed pseudo-first-order kinetics with a rate constant of 0.032 min-1 at 3.0 mM S2O82-. The results showed that chitosan-Ag@Fe could activate S2O82- effectively better than chitosan-Ag0 and/or chitosan-Fe0. The acidic conditions were found to be more favorable for MTZ removal by chitosan-Ag@Fe activated S2O82-. The radical quenching study using methanol and tertiary butanol revealed that both sulfate (SO4- •) and hydroxyl (HO•) radicals were responsible for MTZ removal at a pH of 5.0. Chitosan was stable with S2O82- at 23 °C and prevents the oxidative etching of Ag+ and Fe2+ by the dissolved molecular oxygen from the surface of Ag@Fe in an aqueous solution. Chitosan-Ag@Fe remained an excellent activity after being recycled five times, indicating the redox recycles of Ag2+/Ag+ and Fe3+/Fe2+ processes may be a synergistic strategy for the long-term application for water treatment.