Hybrid treatment of polystyrene nanoplastics by coagulation-coupled with adsorption

Nanoplastics (NPs) in aquatic ecosystems pose serious environmental and public health problems due to their inherent toxicity and persistent ability to attract various adsorbable contaminants on their surfaces. This study evaluated a hybrid coagulation-adsorption process in series for the removal of amidine-functionalized polystyrene (PS) NPs in water. Coagulation was first performed using ferric chloride to remove the PS NPs. Apart from this, an adsorption test was conducted at varying PS concentrations, demonstrating that PS NPs undergo chemical adsorption onto granular activated carbon (GAC), as evidenced by pseudo-second-order kinetics (R<sup>2</sup> = 0.991-0.999), while showing that intra-particle diffusion was not the only rate-limiting step. Other rate-limiting steps include boundary layer diffusion and surface adsorption, both of which contribute significantly to the overall adsorption process. The process was better fit by the Langmuir isotherm model (R<sup>2</sup> = 0.985) than by the Freundlich model (R<sup>2</sup> = 0.927), indicating that monolayer adsorption might be predominant on a homogeneous surface. Nanoparticle tracking analysis (NTA) showed that coagulation, adsorption, and the combination of the two removed PS NPs at 30.0%, 98.0%, and 99.4%, respectively. Turbidity and total organic carbon (TOC) revealed that they were removed in a similar manner to those of NTA. The combined treatment process could achieve the highest removal rate. It demonstrates the effectiveness of integrating coagulation and adsorption in series for PS NP removal in water treatment, which highlights properly arraying the unit treatment process to maximize the removal efficiency.