Coaggregation assisted by cationic polyelectrolyte and clay minerals as a strategy for the removal of polystyrene microplastic particles from aqueous solutions

This study aims to test the removal of polystyrene microplastics (PSMPs) from aqueous solution by using the cationic polyelectrolyte polydiallyldimethylammonium chloride (polyDADMAC) to stimulate coaggregation of PSMPs and clay minerals with different shape and surface charge properties, namely kaolinite, montmorillonite, and palygorskite. Colloidal stability, aggregation kinetics and surface charge properties were evaluated simultaneously. For this purpose sedimentation experiments in test tubes were run together with measurements on the course of the hydrodynamic diameter over time by dynamic light scattering and linked with particle charge analyses. In the pH range from 4 to 8, negative surface charge of all clay minerals introduced as well as PSMPs can be neutralized by the addition of polyDADMAC. When being introduced into suspensions of PSMPs and clay minerals, the long-chain poly-DADMAC acts as a bridge to structuralize a matrix of PSMPs and clay particles, by which coaggregation is favoured. The efficiency to remove PSMPs from aqueous solution, follows the order: palygorskite > montmorillonite > kaolinite. Although these clay minerals have been used in water purification for decades, our findings with the amendment of an organic polycation illustrate their extended ability, particularly for palygorskite, to efficiently remove emerging pollutants such as microplastics.