Enhanced Cd2+ adsorption and toxicity for microbial biofilms in the presence of TiO2 nanoparticles

Titanium dioxide nanoparticles (TiO2 NPs) easily combine with other pollutants such as heavy metals because of their excellent physiochemical properties. However, how such an interaction may affect the binding behavior of metals onto biofilms remains largely unclear. This study, examined the effects of TiO2 NPs on Cd2+ accumulation and toxicity for natural periphytic biofilms were examined. The adsorption kinetics showed that adding 0.1 and 1 mg/L TiO2–NPs increased the Cd2+ adsorption of biofilms at equilibrium by 23.5% and 35.8%, respectively. However, adding 10 mg/L TiO2 NPs increased the Cd2+ adsorption of biofilms at equilibrium by only 1.9%. The adsorption isotherms indicate that the presence of TiO2 NPs considerably increased the Cd2+ adsorption capacity of the biofilms; however, this effect became less prominent at high TiO2 NP concentrations. The optimum pH for Cd2+ adsorption increased with increasing Cd2+ and TiO2 NP contents. At low concentrations, the coexistence of Cd2+ and TiO2 NPs may facilitate their respective accumulation by stimulating the secretion of extracellular polymeric substances and enhancing the microbial activity of the biofilm. The presence of TiO2 NPs increases the surface binding energy between Cd2+ and functional groups such as carboxyl groups, enhancing the Cd2+ accumulation on the biofilm.