The role of abiotic and biotic aging in the elimination of polyethylene microplastics by coagulation

Coagulation has been extensively examined for the removal of microplastics from water; however, to date, there is a significant lack of knowledge regarding the removal efficiency of aged microplastics. To address this challenge, this study investigates the effects of abiotic and biotic aging on microplastic properties and coagulation efficiency. Mechanical, thermal, biological, and combined mechanical-biological aging of polyethylene (PE) were conducted. The influence of time (7, 14, 28, and 60 days), pH (3, 7, and 10), temperature (5, 25, and 60 °C), and the presence of sand particles and sunlight was analyzed. All aging factors affected removal efficiency, which ranged from 27.93% to 56.79% for mechanical aging, 26.98% to 57.80% for thermal aging, 59.52% to 66.97% for biotic aging, and 44.92% to 65.48% for combined mechanical-biotic aging. The most significant effect was observed following biotic incubation of PE with Pseudomonas aeruginosa, due to biofilm formation on the PE surface. After 7, 14, and 28 days of incubation, the measured biofilm amounts were 70.2, 121.6, and 149.6 mg/g, respectively. Biofilm formation led to an increase in sinking ratio from 0.01 for pristine PE to 0.068, 0.07, and 0.1425 after 7, 14, and 28 days, respectively. FT-IR analysis revealed signals at approximately 2100 cm^-1 (CN group) and 1100 cm^-1 (nucleic acid residues), along with increased carbonyl index values, indicating biofilm formation during incubation with P. aeruginosa. LDIR analysis showed a reduction in PE particle size following thermal and mechanical aging, whereas an increase in particle size was observed after biological aging.