Impacts of Natural Organic Matter and Dissolved Solids on Fluoride Retention of Polyelectrolyte Multilayer-Based Hollow Fiber Nanofiltration Membranes

This study examines the effects of natural organic matter (NOM) and dissolved solids on fluoride (F^-) retention in polyelectrolyte multilayer-based hollow-fiber nanofiltration membranes (dNF40). Lab-scale filtration experiments were conducted under varying operating conditions (initial salt concentration, NOM concentration, permeate flux, crossflow velocity, and recovery rate). dNF40 membranes exhibited F^- retention above 70% ± 1.2 in the absence of NOM and competing ions. However, when filtering synthetic model water (SMW) designed to simulate groundwater contaminated with high total dissolved solids (TDSs) and NOM, F^- retention decreased to approximately 60% ± 0.7, which was generally attributed to ion competition. Furthermore, despite limited declines in normalized permeability, the addition of NOM to SMW notably deceased F^- retention in the steady state to~20% due to fouling effects. The facilitated transport of the divalent cations Ca²⁺ and Mg²⁺ could be observed, as they accumulated in the organic fouling layer. While SO₄^2- retention remained relatively stable, the retention of monovalent anions (NO₃^-, Cl^-, and F^-) decreased substantially due to drag effects. Na⁺ retention improved slightly to maintain electroneutrality. Feed salinity was shown to significantly affect separation efficiency, with PEC layers undergoing swelling and certain structural changes as the ionic strength increased. During batch filtration experiments at varying recovery rates, the retention of monovalent anions further decreased, with F^- retention reducing to just ~10% at a 90% recovery rate. This study provides valuable insights into better understanding and optimizing the performance of PEC-based NF membranes across diverse water treatment scenarios.