The overlooked role of lignin: comparable enhancement on colloid transport with humic acid in porous media

Abstract

The transport of colloidal particles in subsurface environments is critically influenced by natural organic matter (NOM), with humic acid (HA) being the most studied facilitator. However, the role of other abundant NOM components, particularly lignin, a major plant-derived polymer, remains poorly understood. This study systematically investigated and compared the effects of HA and lignin on the transport of fluorescent polystyrene nano- (0.51 μm) and micro-colloids (1.1 μm) in saturated quartz sand columns under varied ionic conditions (10 mM NaCl and 1 mM CaCl₂). Column experiments demonstrated that both HA and lignin significantly enhanced the mobility of both nano- and micro-colloids. At high concentration of HA or lignin (10 mg L⁻¹), even complete breakthrough was observed. Mechanistic investigations through zeta potential measurements and DLVO interaction energy calculations revealed that HA and lignin, possessing less negative surface charges and lower interaction energy barriers than the colloids, preferentially adsorbed onto sand surfaces via competitive adsorption. This was further directly verified by pre-equilibration experiments, where columns pre-treated with HA or lignin produced colloid breakthrough curves (BTCs) nearly matching those obtained with dissolved HA or lignin present. A slight but consistent reduction in colloid recovery in pre-equilibration columns compared to those with HA or lignin and both FTIR and SEM measurements of colloid suspensions with HA or lignin further indicated a secondary contribution from steric hindrance. The findings of this study demonstrated lignin as an important promoter of colloid transport, underscoring the necessity to broaden the current HA-centric framework and highlighting the importance of NOM heterogeneity in lignin-rich environments.