A Biosorption Study of Triclosan and Benzalkonium Chloride: Exploring the Role of Biofilm and Soil Grain Surface Chemistry in Drainfields

Although sorption is crucial for removing contaminants of emerging concern (CECs) in on-site wastewater treatment systems (OWTS), and biofilms develop extensively within drainfields, little is known about the effects of sorption and biosorption (with biofilm) on contaminant fate. To gain insights, the transport of triclosan (TCS) and benzalkonium chloride (BAC) (hydrophobic and amphiphilic antimicrobials, respectively) was studied in saturated sand columns with and without 1-day-old and 3-day-old biofilms. Triclosan S-shaped breakthrough curves (BTCs) indicated cooperative sorption, and BAC two-step BTCs suggested irreversible sorption with a maximum capacity. One-day-old biofilms increased TCS retardation while showing no effect for BAC. The 3-day-old biofilms further increased TCS retardation and decreased BAC retardation. Therefore, early stage biofilms can affect contaminant sorption in as little as 1 day and add new TCS sorption sites, whereas hindering BAC sorption. Quartz crystal microbalance with dissipation (QCM-D) using SiO₂ and Fe₂O₃ sensors showed higher protein deposition than humic acids and polysaccharides at pH 7, suggesting that proteins drive extracellular polymeric substance (EPS) deposition. Higher BAC deposition on the clean SiO₂ sensor than on the EPS-coated sensor revealed that EPS likely impaired BAC electrostatic interactions with the surface. These findings support that biofilms affect contaminant mobility and highlight the need for considering biosorption in optimizing OWTS design.