Effects of humic acid on pyrite surface oxidation and interfacial interactions

Pyrite, as a primary component of tailings, can be readily oxidized and result in the generation of acid mine drainage (AMD). Humic acid (HA), the most common organic matter in the environment, tends to be adsorbed on pyrite surfaces. This study aims to clarify how HA influences the oxidation of pyrite surfaces and the related interfacial mechanisms. The passivation of pyrite by HA, which involves both electrostatic interactions and surface complexation, yielded a maximum adsorption capacity of 25.30 mg g^-1. This HA passivation layer effectively isolates the oxidizing medium, and the oxidation inhibition rate on the pyrite is 40%. Furthermore, electrochemical analyses indicated that after pyrite is covered with HA, its dominant oxidation pathway does not fundamentally change, with the current density of the electrode reaction is significantly suppressed (from 14.03 to 0.1713 µA cm^-2) and a remarkable rise in charge transfer resistance (from 1091 to 9.07 × 10⁴ Ω cm^-2), confirming that HA effectively hinders the electrochemical oxidation process. HA's adsorption on pyrite and its passivating effect against oxidation were further affirmed by SEM-EDS and FTIR characterization, which supported the electrochemical findings. At the molecular level, XAFS analysis further reveals that HA, through its hydroxyl groups, forms stable Fe-O coordination with iron atoms on the pyrite surface, thereby constructing a passivating surface structure. This work reveals the HA-pyrite interfacial mechanisms, shedding light on the surface passivation process that inhibits oxidation.