New insights into the fate and interaction mechanisms of hydrolyzed aluminum-titanium species in the removal of aged polystyrene

Polyaluminum-titanium chloride composite coagulant (PATC) has been demonstrated to be a promising coagulant in microplastics (MPs) treatment. However, the interaction process between the dominant species of PATC and MPs remains unclear, which will hinder our understanding of the coagulation mechanisms. Here, the species transformation of PATC during its interaction with aged polystyrene powder (APSp) was studied. The results showed that the rise of O-containing functional groups in APSp increased the possibility of forming C-O-M coordination bonds and hydrogen bonds between APSp and PATC, which improved the removal of PSp. Furthermore, Al(OH)TiO(HO) (AlTi) was considered to be the most effective species of PATC. At pH 4, electrostatic attraction brought AlTi approached APSp first, followed by hydrogen bonding and complexation occurred, respectively. However, the AlTi-APSp complexes were easily converted to monomers and dimers during coagulation, which influenced the coagulation efficiency. With the increase of pH, OH in the solution would further polymerize the depolymerized AlTi into oligomers and mesomers. Under weakly acid conditions, the diversity of PATC hydrolysates and the increase in APSp binding sites correspondingly led to the maximum APSp removal of 75%. When the pH further increased to 10, PATC interacted with APSp mainly by hydrogen bonding and sweeping effect.