Quantitative analysis of microplastics coagulation-removal process for clean sea salt production

Abstract

Sea salt is produced from the evaporation of seawater containing harmful microplastics, and this process leads to sea salt enrichment with microplastics. To solve this issue, the coagulation method for seawater microplastics removal was studied, and Fourier transform infrared spectrometer spectrum was adopted to analyze the amounts of MPs in coagulated flocs. AlCl3, Al2(SO4)3, FeCl3, CH3(CH2)3SiCl3, and CH3(CH2)7SiCl3 were employed as coagulants. The salinities of simulated seawater were manipulated using commercial sea salt. Micron-size poly(n-butyl methacrylate) spheres were used as representative microplastics after confirming their stabilities in seawater. The results indicated that the high microplastic removal efficiencies (exceeding 60%) were achieved by all coagulants. Further, the removal efficiency of silane-based coagulants was enhanced via NaCl addition, whereas the removal performance of AlCl3 was reduced in the presence of excess NaCl. The best performance was obtained using CH3(CH2)7SiCl3: microplastic removal efficiency > 98% for 3 and 15% salinities. Quantitative analysis of coagulated microplastic by Fourier transform infrared spectrometer was in good agreement with experimental data and confirming that aggregated flocs were mainly composed of MPs. Therefore, this coagulation method can be applied for seawater microplastic coagulation, and employing CH3(CH2)7SiCl3 in 15% saline seawater is considered the most efficient process. Furthermore, the developed Fourier transform infrared spectrometer method would be an effective analytical tool for quantitative analysis of microplastic removals from seawater, and optimization of coagulants dosage for the microplastic coagulation–flocculation process. Graphic abstract: [Figure not available: see fulltext.].