Innovative coupling of hydrothermal treatment and ball milling for PFAS-enriched aquatic plants

A novel simultaneous hydrothermal treatment and ball milling (SHTBM) is developed. • SHTBM disrupts plant tissues efficiently and releases PFOA and PFOS completely. • NaOH is confirmed to achieve de-SO 3 H, de-COOH, and defluorination. • Mg 2 Al-LDH is confirmed to achieve adsorption, de-SO 3 H, and de-COOH. Phytoremediation using aquatic plants is a promising and sustainable strategy for in-situ per - and polyfluoroalkyl substances (PFASs) removal from contaminated waters. However, the PFAS-enriched wet biomass is difficult to handle due to the strong embedding of PFAS within plant tissues and the lack of environmentally friendly degradation processes. This study develops a novel process-intensification strategy called simultaneous hydrothermal treatment and ball milling (SHTBM). It can completely disrupt plant structures and ensure the thorough release of embedded perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS). Solid layered double hydroxide (LDH) is employed as a catalyst to replace conventional strong alkalis, yielding a neutral effluent (pH = 6.5–7.5) and simplifying downstream wastewater management. Corrosion of iron-based milling balls in the hydrothermal environment generates reactive species (e − and HO·), which enhance PFAS removal through a two-step radical pathway. The control test of traditional NaOH catalysis confirms that the one-step ionic substitution by OH − is more efficient in all degradation reactions of de-SO 3 H, de-COOH and defluorination. However, defluorination in LDH tests remains uncertain due to its strong adsorption. Under the optimal conditions of 250 °C and cast‑iron balls, all three enriched plants achieve 100% removal of PFOA. PFOS removal exceeds 80% in the LDH-catalyzed system and surpasses 90% in the NaOH-catalyzed system, with defluorination rates ranging from 52.51% to 90.35%. Future work should focus on developing solid catalysts without adsorption capacity and increasing the reaction temperature to achieve complete mineralization of PFAS. Nevertheless, this work provides an innovative post-treatment solution for phytoremediation. Download: Download high-res image (259KB) Download: Download full-size image