Titanium dioxide nanoparticle removal in primary prefiltration stages of water treatment: Role of coating, natural organic matter, source water, and solution chemistry

Abstract

Since nanoparticles are considered emerging contaminants in water, there is a pressing need to ensure their removal. Specifically, recent studies have found titanium dioxide nanoparticles (TiO2) to potentially cause adverse environmental and health effects. Typical treatment plants have three stages of prefiltration treatment in which particles may be removed: coagulation, flocculation, and sedimentation. Scaled-down jar tests, with constant dosage of aluminum sulfate as the coagulant, were utilized to investigate the efficacy of conventional treatment facility to remove nanoparticles and the fundamental mechanisms involved under ideal and environmentally relevant conditions. Results showed that removal by sedimentation was the most efficient in the presence of divalent cations and in the absence of natural organic matter (NOM) and synthetic organic coating on the nanoparticles, achieving >1 log removal. With the addition of a synthetic organic coating on the nanoparticles and NOM, removal decreased to ∼80%. Despite the high dosage of coagulant used, without pH modification (typical of practical treatment), under realistic water chemistries there is a possibility for significant release, and specifically ∼5 ppm of particles smaller than 450 nm were observed after sedimentation, which raises concern regarding the ability for these engineered treatment facilities to adequately remove these nanoparticles that are increasing in production and use.