Overlooked pyrite-mediated heterogeneous Fenton processes: Mechanisms of surface hydroxyl radical generation and associated decontamination performance

Abstract

Pyrite-based Fenton-like processes have been extensively studied for wastewater decontamination; however, most relevant studies placed excessive emphasis on the homogeneous Fenton reaction mediated by aqueous Fe2+, resulting in the proposed technologies facing issues such as additional acid requirements for pH adjustment and excessive iron sludge production. Herein, through in situ shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), custom dual-chamber reactor experiments, and a series of control experiments, significant hydroxyl radical generation was identified during the pyrite/H2O2 process, while the dominant reactive iron species was verified to the structural Fe sites on the pyrite surface, rather than structural Fe(II) in secondary iron minerals and surface adsorbed Fe2+. Consequently, even with significant suppression of the homogeneous Fenton pathway, the pyrite/H2O2 process exhibited significant degradation efficiency for sulfamethoxazole (SMX) at pH 4. Moreover, the pyrite/H2O2 process was found to selectively remove 50 μM of pollutants with high affinity for pyrite (bisphenol A, carbamazepine, nitrobenzene, and SMX), even in the presence of 50–100 mM methanol. Compared to the typical iron-based reductive catalyst (zero-valent iron, ZVI), pyrite mediated a Fenton process with greater potential for practical applications at pH 4, achieving a 43.75-fold reduction in iron sludge production and almost doubling the H2O2 utilization efficiency. Additionally, in contrast to ZVI, minimal iron oxide formed on the pyrite surface during the oxidation process. Thus, after seven cycles of degradation experiments, the decontamination efficiency of the pyrite/H2O2 process remained stable. These findings are crucial for understanding the complex environmental behavior of pyrite in both natural and engineering processes and provide a new perspective for the efficient utilization of pyrite resources as well.