Effects of seasonal biofouling on diffusion coefficients through filter membranes with different hydrophilicities in natural waters

Highlights: • The effect of biofilm in a lag phase on the diffusion coefficient can be ignored. • Biofilm had adverse effect on the diffusion coefficients during long-term deployment. • Biofilm effect on the diffusion coefficients varied with seasons and deployment time. • Both the structures of membrane and analyte can affect the diffusion coefficients. Biofouling is a major issue for the diffusive gradients in thin films (DGT) passive samplers during long-term deployment. Although biofilms have a negative effect on DGT samplers, the effects of seasonal biofilms on the flux of targets through different membranes are poorly understood. Herein, we evaluated the relationship between the biofilm growth and diffusion coefficients' decline rate through two membranes with different hydrophilicities during the four seasons in natural waters. Cu²⁺ and tetracycline were selected as the model metal and organic contaminant, respectively. Rapid biofilm growth on the membrane surface was observed, and the fouling growth rate increased in the order: winter < spring < autumn < summer. Biofouling had a negative effect on the diffusion coefficients of Cu²⁺ and tetracycline. Generally, the decreasing tendency of diffusion coefficients agreed with the increasing tendency of the fouling growth rate. Biofilms in a lag phase with little bacterial colonies had insignificant effect on the diffusion coefficients. After 30 days, the decline ratios of diffusion coefficients were in the range of 38.14%–53.05%, 69.63%–83.19%, 51.57–68.42%, and 19.43–35.84%, respectively, during the spring, summer, autumn and winter. The flux through the membrane with higher hydrophilicity was greater. Both the hydrophilicity of membrane and structure of target analytes had important effects on the diffusion coefficients through biofouled membranes. Owing to similar physical and chemical characteristics, there was insignificant difference in diffusion coefficients decline trend between the Yalu River water and Hunhe River water in the summer.