Exploring the mechanism and kinetics of Fe-Cu-Ag trimetallic particles for p-nitrophenol reduction

Preparation conditions of Fe-Cu-Ag trimetallic particles were optimized by single-factor and response surface methodology (RSM) batch experiments to obtain high-reactive Fe⁰-based materials for p-nitrophenol (PNP) removal. Under the optimal conditions (i.e., Fe⁰ dosage of 34.86 g L^-1, theoretical Cu mass loading of 81.87 mg Cu/g Fe, theoretical Ag mass loading of 1.15 mg Ag/g Fe, and preparation temperature of 52.1 °C), the actual rate constant (k_obs) of PNP reduction in 5 min was 1.64 min^-1, which shows a good agreement between the model prediction (1.85 min^-1) of RSM and the experimental data. Furthermore, the high reactivity of Fe⁰-based trimetals was mainly attributed to the plating order of transition metals (i.e., Ag and Cu). Furthermore, we propose a new theory that the pyramid trimetallic structure of Fe-Cu-Ag could improve the electron transport and create active sites with high electron density at the surface (Ag layer) that could enhance the generation of surface-bonded atomic hydrogen ([H]_abs) or the direct reduction of pollutant. Moreover, Fe-Cu-Ag trimetallic particles were characterized by SEM, EDS, and XPS, which also could confirm the proposed theory. In addition, the leached Cu²⁺(<10 μg L^-1) and Ag⁺ (below detection limits) in Fe-Cu-Ag system could be neglected completely, which suggests that Fe-Cu-Ag is reliable, safe, and environment friendly. Therefore, Fe-Cu-Ag trimetallic system would be promising for the removal of pollutants from industrial wastewater.