Mechanochemically Sulfidated Zero Valent Iron as an Efficient Fenton-like Catalyst for Degradation of Organic Contaminants

Abstract

Mechanochemically sulfidated zero valent iron (S-ZVI), prepared from ball milling of ZVI and sulfur powder, was used as a catalyst for heterogeneous Fenton oxidation of a variety of persistent organic compounds including phenol, chlorophenols, nitrobenzene, bisphenol A and tetracycline. The 100% removal of phenol was achieved within 1 min in S-ZVI/H2O2 system while it took 10 min in ZVI/H2O2 system. The initial surface area normalized phenol degradation rate by S-ZVI was 5 times of that of ZVI, suggesting the much higher efficiency of S-ZVI in catalyzing the decomposition of H2O2 for oxidative degradation of organic contaminants. In addition, an initial lag period of phenol degradation in ZVI/H2O2 system was absent in S-ZVI/H2O2 system. The removal efficiency of phenol was dependent on the initial H2O2 concentration, S-ZVI dosage, initial phenol concentration, and pH. The optimum pH and H2O2 concentration was 3.0 and 2 mmol·L-1, respectively, when the initial phenol concentration was 0.2 mmol·L-1 and the S-ZVI dosage was 0.12 g·L-1. The phenol degradation was effectively scavenged by a ·OH probe compound, ethanol and the electron paramagnetic resonance (EPR) studies successfully detected DMPO (5, 5-dimethyl-1-pyrroline-N-oxide)-OH signals, which collectively suggests that the reactive species responsible for contaminant degradation in S-ZVI/H2O2 system was ·OH. S-ZVI particles before and after reaction were characterized by scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM-EDS) and X-ray diffraction (XRD). SEM-EDS results showed that the oxidation of S-ZVI by H2O2 resulted in the formation of iron hydroxide nanoparticles on the particle surface while FeS was not significantly consumed. Tafel analysis of S-ZVI and ZVI modified electrodes demonstrated that S-ZVI had a greater overall rate of electron transfer than ZVI. Therefore, FeS as a better electron conductor facilitated the electron transfer from Fe0 to H2O2 resulting in faster Fe2+ releasing and H2O2 activation, which enhanced contaminant degradation.