Copper(II)-induced alginate fouling in forward osmosis: a novel water-state perspective using low-field nuclear magnetic resonance (NMR)

Membrane fouling limits the dewatering efficiency of forward osmosis (FO) for waste sludge (WS). Industrial waste sludge (IWS), due to prevalent heavy metals with high binding affinity for polysaccharides, favors severe membrane fouling. Understanding this fouling behavior is crucial for broader FO application in sludge dewatering. This study systematically investigated the fouling mechanism in a copper(II)‑sodium alginate (Cu(II)-SA) model system during FO from a novel perspective of water-state distribution. Filtration tests showed a unimodal filtration resistance ( R ) of SA-Cu(II) solutions with increasing Cu(II) (0–1.5 mM), peaking at 0.5 mM Cu(II). Density Functional Theory (DFT) simulation revealed that above 0.5 mM Cu(II), intramolecular coordination arising from excess Cu(II) drove the conformational transition of SA-Cu(II) complexes from linear-porous structures to compact flocs. Low-field nuclear magnetic resonance (NMR) revealed that interstitial water peaked at 0.5 mM Cu(II) due to high surface tension and abundant porosity. Conversely, vicinal and mobile water increased above 0.5 mM Cu(II) because of interstitial water release upon compaction and increased surface polarity. Bound water (the sum of interstitial and vicinal water) was identified as the primary driver of unimodal R , a conclusion supported by (1) an extremely significant Pearson correlation between R and bound water ( p  < 0.001), and (2) filtration thermodynamics quantifying that bound water-induced water potential gap accounted for 91.9% of R . These findings establish a novel framework for understanding heavy metal-induced polysaccharide fouling in FO and provide critical insights for developing effective membrane fouling control strategies for FO applications.