Designing biochars for improved sorptive removal of per- and polyfluoroalkyl substances

The widespread occurrence of per- and polyfluoroalkyl substances (PFAS) in aquatic environments, along with their adverse impacts on human health, has been recognized as an emerging issue. Sorption using carbon-based sorbents is the most common approach for PFAS removal. However, conventional biochars typically underperform compared to activated carbon, which is effective but relatively costly. In this study, a designer biochar was developed by using lime sludge, a no-cost byproduct from drinking water treatment plants, as a pretreatment for woody biomass followed by systematic optimization of lime-sludge loading, pyrolysis temperature, and residence time. The results showed that the pretreatment process enhanced pore development and modified the surface chemistry of the designer biochar. Under optimal conditions (1:4 lime sludge-to-biomass, 850°C, 5 h), the designer biochar achieved near-quantitative removal (>99%) of perfluorooctanoic acid and both linear and branched perfluorooctane sulfonate, outperforming unmodified biochar and comparable to a commercial activated carbon. Sorption kinetics and isothermal studies confirmed the superior performance of the designer biochar, exhibiting rapid equilibration (<1 h) and increased sorption capacity. Mechanistic analysis revealed that improved PFAS removal by the designer biochar was driven by multiple adsorption mechanisms, including hydrophobic interactions, electrostatic attraction, and cation bridging facilitated by Ca^{2 +} and Mg^{2 +} species derived from lime sludge. This study highlights lime sludge pretreatment as a sustainable and cost-effective strategy for producing high-performance carbon-based sorbents for PFAS remediation in contaminated water systems.