Effect of Microplastics on the Coagulation Mechanism of Polyaluminum–Titanium Chloride Composite Coagulant for Organic Matter Removal Revealed by Optical Spectroscopy

Polyaluminum–titanium chloride composite coagulant (PATC) is a good candidate coagulant for the removal of composite pollutants formed by humic acid (HA) and microplastics (MPs). However, to date, little is known about the microinterfacial interaction process between the dominant species of PATC and different types of natural organic matter (NOM) in the coexistence of MPs. Elucidating the interaction process between the predominant species of PATC and complex pollutants is a critical strategy for optimizing coagulation performance. In this study, the effect of aged polystyrene (APS) on the coagulation mechanism of PATC to remove NOM from actual surface water was investigated by optical spectroscopy. It was found that PATC effectively removed UV254 (51.58%) and turbidity (98.72%) from actual surface water, and carboxyl O–H in the protein-like fraction interacted fastest with PATC through complexation. However, the addition of high concentrations of APS led to the fulvic-like fraction with highly aromatic structures preferentially interacting with PATC. The competition between APS and NOM for complexation active sites (i.e., Al–O and Ti–O) of high polymers (e.g., Al2Ti2, Al4Ti, Al2Ti3, and Al13Ti13) and adsorption active sites (−OH) of amorphous hydrolysates reduced the removal efficiency of fulvic-like organics. Moreover, the occupation of the NOM binding sites by APS decelerated the decomposition of Al13Ti13, the most active species in PATC, thus reducing the content of residual Al. This study can provide new insights into the coagulation mechanisms for NOM removal in actual water by the dominant species of PATC in the existence of MPs.