Transport and retention of titanium dioxide nanoparticles in a drinking water treatment plant. Effects of coagulation, filtration and raw water properties on the removal efficiency

Increasing the use and release of titanium dioxide nanoparticles (TiO₂ NPs) into aquatic systems, including water compartments used to produce drinking water, represent a risk for human health through direct NPs ingestion. Since drinking water treatment plants (DWTPs) must provide and guarantee the quality of the water for human consumption, in this study, TiO₂ NPs removal efficiency of a DWTP which provides drinking water for half million consumers is investigated. For that purpose, a pilot-scale designed to closely reproduce each treatment process of the main DWTP is considered. Experiments were first conducted without considering the coagulation process to evaluate the efficiency of the filtration processes (sand and granular activated carbon) and the specific impact of the coagulation with Polyaluminum Chloride on NPs removal. Using an original and novel combination of different analytical techniques (turbidity, ζ-potentials, size distribution measurements, electron microscopy, total organic carbon determination) we found that filtration processes through sand and GAC achieve an overall NPs removal of 96.3 % ± 1.0. NPs removal is mainly attributed to straining and adsorption processes during filtration. Then experiments were conducted in presence of PACl to quantify the impact of coagulation on the TiO₂ NPs removal efficiency. It was found that the addition of coagulant significantly improves TiO₂ NPs removal with a global removal efficiency greater than 99.5 % ± 0.5. The higher removal efficiency in presence of coagulant was related to a significant TiO₂ NPs surface charge reduction and subsequent formation of aggregates increasing their retention and attachment in the filter media.