Ball-Milled Natural Pyrite for Enhanced Phosphate Adsorption: Mechanism of Surface Complexation and Precipitation

Phosphate removal in aquatic systems is critical for mitigating eutrophication. In this study, ball-milled natural pyrite (FeS₂) was employed to examine the effects of mechanochemical activation on the phosphate adsorption behavior and mechanism. Notably, the adsorption capacity (4.43 mg/g) of ball-milled pyrite (FeS₂^bm) was 5.8 times that of pristine FeS₂ (0.77 mg/g). Kinetic and isotherm analyses indicated that the phosphate adsorption process aligned well with both the Elovich and Freundlich models, indicating that multimolecular layer chemical adsorption governed the adsorption of phosphate. The Brunauer-Emmett-Teller (BET) specific surface area analysis, attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR), and ⁵⁷Fe Mössbauer spectroscopy demonstrated that moderate ball milling enhanced the BET specific surface area, facilitated the formation of the oxygen functional group, and increased the exposed adsorption sites of FeS₂^bm, attributing to the better adsorption performance of FeS₂^bm compared to pristine FeS₂. ATR-FTIR, X-ray photoelectron spectroscopy, and ⁵⁷Fe Mössbauer spectroscopy indicated that the main removal mechanism of phosphate by FeS₂^bm involved precipitation with surface Fe species as well as inner-sphere complexation through ligand exchange with surface -OH groups. Unfortunately, excessive ball milling leads to the aggregation and caking of FeS₂^bm particles, resulting in a reduction of both the specific surface area and phosphate adsorption capacity. The commonly observed coexisting cations and anions and a pH range of 3-11 did not significantly fluctuate the phosphate removal of FeS₂^bm, while only humic acid exhibited moderate interference at elevated levels. This work established ball-milled pyrite as a sustainable mineral-based adsorbent, simultaneously addressing phosphate pollution control and resource-oriented utilization of sulfide minerals.