The effect of multivalent anions on removal of Titanium dioxide nanoparticles from drinking water sources by coagulation-sedimentation processes: Efficacy and mechanisms

• Anions led to TiO 2 NPs removal with an improved order of NO 3 − = Cl − < SO 4 2− < PO 4 3− . • >89% of TiO 2 NPs was removed in tap water in 45 min in SO 4 2− - and PO 4 3− -spiked C/S systems. • Charge neutralization was primarily responsible for enhanced TiO 2 NPs removal. • Removal pathways of TiO 2 NPs were proposed in SO 4 2− - and PO 4 3− -added C/S processes. Titanium dioxide nanoparticles (TiO 2 NPs) are emerging potential hazard contaminants. There is a pressing need to understand TiO 2 NPs removal from anions-spiked complex waters by coagulation-sedimentation (C/S) processes. In this study, we investigate impacts of Cl − , NO 3 − , SO 4 2− , and PO 4 3− on TiO 2 NPs removal at circumneutral pHs by C/S processes. Little removal of TiO 2 NPs was observed in the absence and presence of Cl − or NO 3 − . However, 93% and 82% of 10 mg/L TiO 2 NPs were removed within 45 min as dosing 0.6 mM PO 4 3− and 0.8 mM SO 4 2− , respectively, at 10 mg/L polyaluminum chloride (PAC) and pH ini. 6.2 in C/S processes, with pseudo-first-order rate constants of 0.098 ± 0.002 and 0.072 ± 0.001 min −1 , respectively. Reaction rates in this work were several orders of magnitude faster than those in previous studies. Removal efficiencies of TiO 2 NPs reached higher than 89% with a spike of multivalent anions in tap water, thereby exhibiting high practical implication potential. Additionally, the temporal evolution of total particle number and fractal dimension demonstrated that the presence of SO 4 2− and PO 4 3− improved the formation of denser spherical flocs and consequently precipitation rates mainly due to charge neutralization. Moreover, appropriate coagulants, optimized levels of PAC, SO 4 2− and PO 4 3− , and acidic pHs favored nanoparticle removal. Together, this work suggests that the presence of background water composition could facilitate nanoparticle removal in a C/S process.