Coagulation-flocculation performance and floc properties for microplastics removal by magnesium hydroxide and PAM

Microplastics (MPs) have attracted much attention worldwide as one kind of emerging pollutant, with the social demand for plastic products surging and microplastic pollution increasingly serious. In this experiment, mag-nesium hydroxide formed under alkaline conditions was combined with anionic polyacrylamide (PAM) as a dual-coagulant to deal with the simulated natural water containing polyethylene (PE). And the whole process was monitored by intelligent Photometric Dispersion Analyzer (iPDA), the Flocculation Index (FI) value of flocs and MPs removal efficiency were used as indicators to evaluate coagulation-flocculation performance. The results suggested that the flocs formed were not sufficient to remove PE particles only in the case of dosing magnesium ion. When PAM was added after magnesium hydroxide formation, the maximum FI value reached up to 7.2 and the PE removal efficiency was greatly improved, reaching 84.9% +/- 3%. Additionally, flocs settling rate with average size of 57.19 mu m reached to 4.8 x 10(-3) m/s. At the same time, the types, dosing time and dosage of anionic PAM also had a great influence on the removal of MPs. Based on investigations of zeta potential and floc properties, adsorption bridging and sweeping were the main coagulation mechanisms. The MPs removal be-haviors exhibited during coagulation processes have potential application in water treatment.

Automated summary

Microplastics have become a global concern due to the increasing demand for plastic products and the worsening microplastic pollution. In this study, magnesium hydroxide and anionic polyacrylamide were used as a dual-coagulant to effectively treat water containing polyethylene, improving PE removal efficiency significantly. Various factors such as floc settling rate, type and dosage of PAM were found to significantly influence the removal of MPs, with adsorption bridging and sweeping identified as the main coagulation mechanisms.