Effect of AlCl sub(3) concentration on nanoparticle removal by coagulation

In recent years, engineered nanoparticles, as a new group of contaminants emerging in natural water, have been given more attention. In order to understand the behavior of nanoparticles in the conventional water treatment process, three kinds of nanoparticle suspensions, namely multi-walled carbon nanotube-humic acid (MWCNT-HA), multi-walled carbon nanotube-N,N-dimethylformamide (MWCNT-DMF) and nanoTiO sub(2)-humic acid (TiO sub(2)-HA) were employed to investigate their coagulation removal efficiencies with varying aluminum chloride (AlCl sub(3)) concentrations. Results showed that nanoparticle removal rate curves had a reverse "U" shape with increasing concentration of aluminum ion (Al super(3 +)). More than 90% of nanoparticles could be effectively removed by an appropriate Al super(3 +) concentration. At higher Al super(3 +) concentration, nanoparticles would be restabilized. The hydrodynamic particle size of nanoparticles was found to be the crucial factor influencing the effective concentration range (ECR) of Al super(3 +) for nanoparticle removal. The ECR of Al super(3 +) followed the order MWCNT-DMF > MWCNT-HA > TiO sub(2)-HA, which is the reverse of the nanoparticle size trend. At a given concentration, smaller nanoparticles carry more surface charges, and thus consume more coagulants for neutralization. Therefore, over-saturation occurred at relatively higher Al super(3 +) concentration and a wider ECR was obtained. The ECR became broader with increasing pH because of the smaller hydrodynamic particle size of nanoparticles at higher pH values. A high ionic strength of NaCl can also widen the ECR due to its strong potential to compress the electric double layer. It was concluded that it is important to adjust the dose of Al super(3 +) in the ECR for nanoparticle removal in water treatment.