Influence of ions on the coagulation and removal of fullerene in aqueous phase

Increased attention has been paid to the transportation and removal efficiencies of nanoparticles during water treatment. Here, C₆₀ was selected as nano-pollutant to investigate its removal efficiency in the process of coagulation by Al³⁺ in regard to different ions. Negatively charged C₆₀ tends to adsorb hydrated Al³⁺ to form positively charged C₆₀-Al(OH)_n^(m-δ)+ particles, leading to the restabilization of C₆₀. Multivalent anions, i.e., CO₃^2-, SO₄^2-, HPO₄^2-,and humic acid (HA), were shown to bridge the C₆₀-Al (OH)_n^(m-δ)+ particles together and induce the formation of flocs, resulting in enhanced C₆₀ removal (>80%). Monovalent anions, such as OH^-, NO₃^- and the surfactant SDS, were not able to bridge the C₆₀-Al (OH)_n^(m-δ)+ particles together; hence, they had no effect on the C₆₀ removal. The addition of Na⁺ and Ca²⁺ increased the ionic strength of the aqueous solution, which compressed the electric double layer and induced the C₆₀-Al(OH)_n^(m-δ)+ particles to aggregate. However, they are not the essential factors for flocs formation. This is the first study to report the bridging phenomena between multivalent anions and C₆₀-Al(OH)_n^(m-δ)+ in water treatments, and it highlights that the presence of multivalent anions and cations in raw water are important for the effective removal of C₆₀.