The feasibility and mechanism of poly-aluminum/titanium silicate composite coagulants for the efficient removal of nano- and micro-sized plastics

Nano- and micro- sized plastics, a sort of emerging persistent pollutant, have attracted extensive attention worldwide. In this work, poly-aluminum silicate sulphate (PSiAS) and poly-titanium silicate sulphate (PSiTS) were simply obtained to remove 100 nm–5.0 μm polystyrene microplastics (PS MPs). The coagulation performance of PSiAS and PSiTS was systematically evaluated by investigating the effects of the different Si/metal molar ratios, various sized and typed MPs, the solution pH and the MPs in different water sources. These two composite coagulants showed much higher efficiency in MPs removal than the combinations of their two precursors, i.e., metal sulphate and polysilicic acid. The PSiAS and PSiTS were effective in removing PS MPs in weak acidic conditions. The interaction energies between MPs and coagulant obtained from the extended Derjaguin-Landau-Verwey-Overbeek theory further confirmed that PSiAS and PSiTS could achieve effective coagulation and thus high efficiency in MPs removal. The feasibility of the composite coagulants with three-dimensional network structure was further confirmed by efficient removal of various MPs in different real water sources due to the synergistic effects, charge neutralization and efficient netting-bridging effects. The characterizations of fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy were analyzed to further explore the interactions between the coagulant and MPs, accordingly, hydrogen bonding and complexation existed between the hydrolysates of coagulants and the oxygen-containing functional groups on MPs. These findings demonstrated that PSiAS and PSiTS have a great advantage on coagulative removal of MPs due to its low cost, easy to operate, and high efficiency, which could potentially become a simple and promising way for MPs removal from water.