Passive Retention of Plastic Pellets and Microplastics by Coconut Fiber in a Mangrove-Supporting Floating Island in an Urban Estuary

Plastic pollution is a growing concern in urban estuarine environments, where hydrodynamic conditions and structural features of vegetated habitats can promote the interception and accumulation of plastic debris. This study evaluated the coconut-fiber matrix of a mangrove-based floating island as a passive retention substrate for plastic pellets and smaller plastic particles under real field conditions in the Estero Salado, Guayaquil, Ecuador. The system consisted of three connected floating modules containing coconut fiber and 27 mangrove propagules and was originally developed to support mangrove establishment and local phytoremediation. Following approximately one month of estuarine exposure, the floating structure remained stable and plastic particles were visibly retained within the coconut-fiber substrate. A total of 300 visible plastic particles were recovered, of which 40 were randomly selected for morphological and ATR-FTIR characterization. Pellet-like particles represented 72.5% of the analyzed subset, and polymer analysis identified polyethylene (PE) in 87.5% and polypropylene (PP) in 12.5% of the characterized visible particles. Smaller retained particles were additionally recovered from coconut-fiber subsamples using saturated NaCl flotation, membrane filtration, stereomicroscopy, and FTIR microspectroscopy. Spectroscopic analysis provided evidence of synthetic microplastics, including spectra associated with PET and LLDPE, together with a separate fraction of cellulosic fiber-like materials associated with cotton, linen, and regenerated cellulose (rayon). Overall, the results demonstrate that the coconut-fiber matrix performed an additional function beyond its original role as a planting substrate by acting as a retrievable passive interception matrix for plastic particles under urban estuarine exposure. These findings support the further development of coconut-fiber-based floating systems for plastic interception while providing a basis for future quantitative assessments of retention performance.