Lignin particle size influences sodium hypochlorite inactivation efficiency against E. coli in aqueous systems

Lignin, a natural biopolymer in plant cell walls and a common contributor to water turbidity, attenuates the disinfection efficacy of sodium hypochlorite (NaClO). However, the influence of lignin size on disinfection performance remains insufficiently characterized. This study investigated the impact of micro- and nano-sized lignin on Escherichia coli viability under NaClO exposure. Results demonstrate that lignin exhibits a size-dependent attenuation effect on the bactericidal activity of NaClO. Specifically, micro-sized lignin provides greater protection to E. coli than nano-sized lignin. In the presence of micro-scale lignin, E. coli exhibited reduced inactivation rates, decreased membrane permeability, and increased growth activity. Transcriptomic analysis combined with physiological assays revealed that these responses correlated with reduced oxidative stress, enhanced energy metabolism, and increased biofilm formation in surviving bacteria. Mechanistically, scanning electron microscopy and fluorescence staining demonstrated that bacteria aggregate on micro-scale lignin surfaces, while nano-scale lignin forms a protective layer around individual bacterial cells. Adsorption kinetics and isotherm modeling confirmed that micro-scale lignin possesses higher bacterial adsorption capacity than nano-scale lignin, indicating that size-dependent lignin-bacteria interactions contribute to varying degrees of disinfectant resistance. This study elucidates the distinct protective mechanisms of micro- and nano-sized lignin against NaClO, which may compromise bacterial inactivation during water treatment.