Plastic debris is a pervasive pollutant in marine environment released from terrestrial origin through river transport. Evaluating behavior of plastic debris especially microplastics (MPs) in river environment is vital to assess risks of plastic contamination with respect to their physical and chemical characteristics in river ecosystem. Since microplastics (MPs) have become long-lasting anthropogenic debris, the current study aimed to identify occurrence of natural organic substances on polystyrene microplastics (PS-MPs) and to evaluate behavior of the PS-MPs based on the surface property of PS-MPs in river environment. Samples of PS-MPs were collected from the river shores along the Tuul River in the urban area of Ulaanbaatar, Mongolia. Samples were digested with mixture of hydrogen peroxide (30%) and iron (II) sulfate for further spectroscopic analyses. The spectroscopic analysis was carried out for digested and non-digested samples by micro-Fourier transform infrared spectroscopy to obtain organic functional groups and surface photodegradation status of plastic particles using carbonyl index (CI). A wide spectrum of organic functional groups was identified from the samples, indicating that growth and metabolism of microorganisms promotes formation of biofilms on the surface of PS-MP particles based on the environmental factors. The CIs in the samples revealed surface chemical deterioration of PS-MPs by solar radiation before microbial growth on PS-MPs. The values of CIs could be reflected the weathered surface of PS-MPs in the environment with further potential to aggregate with other plastic particles and contaminants in the environment mediated by microbial biofilms. Owing to variation in residential time of plastic particles in the environment, PS-MPs are prone to interact with contaminants as vectors for pollutants in the aquatic environments during the development of biofilms followed by formation of MP aggregates. The fate and behavior of plastic particles and associated pollutants can be greatly influenced by development of biofilms in aquatic environments.