Hydroxyl radical generation by zero-valent iron/Cu (ZVI/Cu) bimetallic catalyst in wastewater treatment: Heterogeneous Fenton/Fenton-like reactions by Fenton reagents formed in-situ under oxic conditions

Zero-valent iron (ZVI) has been recognized as a heterogeneous Fenton/Fenton-like catalyst generating hydroxyl radical (OH radical). ZVI bimetallic catalysts modified by the deposition of transition metals on the ZVI surface have been proposed as alternatives to enhance the reactivity of ZVI catalyst. However, it is not clear from the literature whether OH radical is actively generated via the Fenton/Fenton-like reactions in ZVI bimetallic catalyst systems. To quantify the generation of OH radical by ZVI/Cu bimetallic catalysts, the amount of generated OH radical was measured under the oxic condition. Although the maximum amount of generated OH radical by ZVI/Cu bimetallic catalysts was obtained at pH 3 being an optimal pH for the Fenton reaction, OH radical generation by ZVI/Cu bimetallic catalysts was considerably restricted by the inadequate in-situ generation of H2O2 and the generation of OH radical was inhibited due to Cu deposition on ZVI surface. While the deposition of Cu on the ZVI surface enhanced the removal of Orange II via the facilitation of reductive degradation and the increase in adsorption capability on the surface of ZVI/Cu bimetallic catalysts, it could not stimulate the OH radical generation or the oxidative reactivity of heterogeneous Fenton/Fenton-like catalyst. The present study confirmed that the passivation of ZVI surface accelerated by the deposition of Cu on ZVI surface inhibited the OH radical generation. The kinetic models for OH radical generation by ZVI/Cu bimetallic catalysts were developed by considering the linkage of OH radical generation with eluted Fe and Cu ions. The model predictions could simulate the experimental results reasonably.