Investigating microplastic adsorption by natural biofilms: Insights from the metro river ecosystem

Microplastic contamination in freshwater systems is an emerging environmental concern, yet the capacity of naturally occurring river biofilms to act as microplastic sinks remains insufficiently quantified under controlled conditions. This study aims to fill this research gap by quantitatively evaluating microplastic adsorption by biofilms formed in situ in the Metro River, Indonesia. River water and biofilm samples were collected, and microplastic particles were extracted using oxidative digestion, density separation, and optical microscopy. Batch adsorption experiments were performed in triplicate using size-classified high-density polyethylene microplastic fragments (250–400 micrometers) at initial loads of 0.01–0.50 g with contact times of 5–120 min. Adsorption behaviour was examined using kinetic and isotherm models, and adsorption mechanisms were investigated using Fourier transform infrared spectroscopy. Results are presented as mean ± standard deviation. Biofilms harboured significantly higher microplastic concentrations (9.5 ± 1.06 particles per wet gram) than the surrounding river water (0.0011 ± 0.00065 particles per gram). Adsorption followed pseudo-second-order kinetics (R² = 0.981), indicating surface-mediated interactions. Equilibrium data fitted the Langmuir isotherm (R² = 0.84), with a maximum adsorption capacity of 0.112 g per wet gram (≈2124 particles per wet gram) and a Langmuir constant of 0.062 L per wet gram, consistent with monolayer adsorption. Functional groups associated with adsorption included hydroxyl, amine, carbonyl, and carboxyl groups. The results demonstrate that natural river biofilms function as temporary but capacity-rich microplastic sinks, with implications for microplastic retention, residence time, and potential remobilization in fluvial systems.