Distribution Characteristics and Adsorption Performance of Microplastics in Domestic Sewage: A Case Study of Guilin, China

Microplastics (MPs) resulting from plastic fragmentation with a size less than 5 mm have become one of the main pollutants endangering the water environment. Therefore, it is necessary to know about the abundance and size distribution of MPs in sewage waters and their relationship with water quality. In this study, water samples are collected from 20 sewage outlets in Guilin, China to analyze the abundance and morphology of the MPs and their hydrochemical characteristics. Multivariate statistical analyses are conducted to identify the major factors related to the MP distribution in sewage water samples. Results showed that MPs in sewage water samples are mainly composed of fiber and film, and about 67.8% are sized <0.3 mm. The abundance is in the range of 6 (±1)–47 (±3) items/L. The correlation analysis presents that the abundance of MPs is weakly correlated with hydrochemical parameters and metal ions due to the complexity of the abundance data. The redundancy analysis indicates that the MP morphology distribution is significantly affected by NO3–N, Zn, Ca, and Cu contents, and the MP size distribution is mainly related to Zn, Ca, and Cu contents. Adsorption kinetics are analyzed using pseudo-first-order, pseudo-second-order, and intraparticle diffusion models, revealing that the adsorption process is predominantly governed by chemisorption for smaller MPs (0.3–0.5 mm), while larger MPs (1.0–5.0 mm) are constrained by internal diffusion. Isothermal adsorption experiments are fitted using Langmuir and Freundlich models, indicating that the adsorption of nutrients (NH3–N, TN, TP) and metal ions (Ca, Mg, Cu, Pb, Zn) on MPs follows a monolayer adsorption mechanism, with smaller MPs showing higher adsorption capacities due to their larger specific surface areas. This study highlights the occurrence characteristics and environmental influencing factors of MPs in sewage water, which may be significant for future studies on the pollution control of MPs.