Polyelectrolyte membranes for selective nutrient recovery from PFAS-contaminated wastewater in forward osmosis

Reimagining nutrient-rich wastewater contaminants (e.g., NH₄⁺, PO₄³⁻) as recoverable resources represents an emerging yet challenging paradigm. This study addresses this challenge by evaluating layer-by-layer (LbL) membranes for simultaneous nutrient recovery and per- and polyfluoroalkyl substance (PFAS) rejection in forward osmosis (FO), enabling precise separation and resource recycling. The membrane fabricated by alternating deposition of polystyrene sulfonate (PSS) and poly(dimethyl diallyl ammonium chloride) (PDDA) on a porous substrate demonstrated superior performance over commercial cellulose triacetate (CTA) FO membranes. In single-solute tests, the LbL membrane achieved a 4- to 38-fold increase in nutrient (NH₄Cl and KH₂PO₄) recovery per unit of produced water and 5- to 95-fold higher selectivity for nutrients over PFAS. When tested with AnMBR effluent containing PFAS, nutrient recovery and selectivity also improved remarkably compared to CTA membrane. Enhanced performance stems from the membrane's high PFAS rejection and enhanced nutrient/salt permeability in FO, driven by an extended electrical double layer (EDL) within polyelectrolyte membrane pores, which impedes PFAS transport despite larger pore sizes than CTA membranes. Narrower pore size distribution amplified steric exclusion, while improved nutrient/salt permeability facilitated recovery. Notably, while the LbL membrane outperformed CTA in FO, it exhibited lower PFAS rejection in reverse osmosis (RO), highlighting process-dependent efficacy. These findings demonstrate the potential of LbL membranes for dual resource recovery and PFAS mitigation, advancing sustainable water treatment strategies through precision separation.