Beyond Equilibrium Adsorption: Transient Retention and Release of Dyes from Textile Wastewater in Natural Biofilm Systems Reveals a Dynamic Pollutant Buffering Mechanism

Understanding pollutant interactions with natural biofilms is essential for accurately describing contaminant dynamics in aquatic systems. This study investigates the apparent retention behavior of a commercially designated target dye (Yellow HR) in a textile-dye-affected river-water sample using naturally developed biofilm matrices collected from the Konto River in Kediri Regency, East Java, Indonesia. The temporal profile exhibited a non-monotonic trend characterized by a rapid initial increase in apparent retention during the first 60 minutes, followed by an apparent decline in the UV-Vis signal; however, the overall observed removal efficiency remained low. Baseline FTIR analysis suggested that the biofilm matrix is dominated by hydrated extracellular polymeric substances (EPS) with functional groups that potentially provide heterogeneous, pH-responsive charge sites, as apparent retention was mildly enhanced under acidic conditions. Nonlinear kinetic modeling provided exploratory estimates for the initial increasing interval, but equilibrium-based models failed to capture the subsequent non-monotonic profile. Crucially, the observed declining signal cannot be definitively attributed to desorption due to significant analytical limitations, including the absence of experimental controls, potential mixed-dye UV-Vis interference, and reliance on wet-mass normalization. Consequently, rather than demonstrating a confirmed physical mechanism, these findings support the conceptual hypothesis that biofilms may act as dynamic buffering systems governed by weak, transient interactions rather than operating as permanent pollutant sinks. This study highlights the critical necessity for rigorous mechanistic validation to accurately characterize biofilm-mediated sorption in complex environmental systems.