Microalgal-based carbon encapsulated iron nanoparticles as novel adsorbents for PFAS removal: From dye proxies to target compounds

This study evaluates the performance of microalgal-based carbon-encapsulated iron nanoparticles (ME-nFe) for the adsorption of per- and polyfluoroalkyl substances (PFAS) and synthetic dyes from aqueous solution at laboratory scale. ME-nFe were produced by hydrothermal carbonization (225°C, 3 h) of wastewater-grown microalgae, combining the high reactivity of iron nanoparticles (40% total Fe) with a mixed macro–mesoporous structure supporting them (total pore volume 0.65 cm3·g−1, BET surface area 117 m²·g−1). Batch tests at pH 3 and dosages of 1–2 g·L−1 showed removal above 90% for medium- and long-chain PFAS (PFUnDA, PFDoDA, PFOS, PFDA, PFNA) and more than 60% removal of PFOA at concentrations typically found in industrial wastewater or landfill leachate (≈15 µg·L-¹) after 200 min, whereas short-chain PFBS, PFHxA and PFPeA were only weakly removed. Dye adsorption at equilibrium (30 min) confirmed strong affinity for cationic dyes at neutral pH and for anionic dyes under acidic conditions. Based on their log Kd, Procion Red, 3B Red, Methyl Orange and Sudan Black closely reproduce the adsorption behaviour of medium- and long-chain PFAS on ME-nFe, supporting their use as cost-effective proxies during adsorbent optimisation. Overall, ME-nFe emerge as a promising adsorbent for PFAS remediation.