Total-organic-carbon-based quantitative estimation of microplastics in sewage

Although many studies have been conducted to quantify microplastics in various aquatic environments, there is no easy and standardized analytical method or apparatus. Recently, a number of attempts have been made to standardize microplastic-measuring methods using micro-Fourier transform infrared spectroscopy (μ-FTIR), Raman spectroscopy, or pyrolysis–gas chromatography/mass spectrometry (Py-GC/MS). However, they still require time-consuming and labor-intensive sample pretreatment, instrument configuration, and complex data processing. Especially, when sample matrix is as complex as sewage, the quantification of microplastics is even more difficult. Therefore, in this study, we have proposed an innovative method which quantifies total organic carbon (TOC) of plastic particles to estimate the mass of microplastics in sewage. Then, the method was applied to evaluate the fate of microplastics in sewage flowing into and out of a sewage treatment plant (STP). In the proposed method, sewage samples were collected and filtered using a sampling module equipped with stainless-steel filters to harvest particles between 45 μm and 500 μm. Then, the retentates of the filter were digested by Fenton's reagent to remove organic matters other than plastic particles before TOC determination. The method detection limit of the proposed method was 0.003 mg (0.15 μg L⁻¹ for a 20 L sample), and the recovery efficiencies estimated with six different types of plastic particles were ranged from 76% to 98%. Using the proposed method, the performance of a STP in Seoul in excluding microplastics from sewage was evaluated; more than 99% of microplastics could be removed. In fact, the result was also confirmed by μ-FTIR.