Assessment of microplastics accumulation and aging in horizontal subsurface flow constructed wetlands

Microplastics (MPs) are emerging pollutants whose fate after removal from wastewater remains uncertain. Constructed wetlands (CWs), as nature-based solutions with high retention of MPs, provide a suitable context to assess MP fate through extended interactions with sediments, vegetation, and microbial communities. This study evaluated MP distribution and aging in two sequential horizontal subsurface flow CWs (secondary wastewater treatment CW1 and sludge treatment CW2) in a residential area (Spain), in operation since 2014. Fibers were identified as the predominant type across depth profiles, with polymer-type abundance linked to urban wastewater inflows. Vertical patterns revealed denser polymers and smaller MPs accumulating in deeper layers, while lighter or later-introduced particles remained near the surface. MP distribution varied with treatment configuration, showing peak concentrations either in the top 5 cm at the CW1 inlet zone (566 MP/g) or in the deepest layer at the CW2 (464 MP/g). An exploratory phenomenological MP balance model reproduced the observed 97% removal efficiency by accounting for substrate retention and estimating accumulation and loss terms. A significant decrease in MP abundance from the inlet to the center confirmed efficient retention, with vegetation roots exhibiting MP levels equivalent to ∼34%-150% of the local substrate concentrations. Evidence of physical deterioration and spectra-detected chemical alterations (oxidation, hydrolysis) reflects long-term environmental aging and suggests potential pathways for polymer degradation. These findings support the hypothesis that CW conditions may favor MP aging, consistent with studies reporting microbiota-driven polymer degradation in similar natural environments. Further research should employ complementary techniques to confirm polymer aging mechanisms and optimize microbiota-based CWs to enhance MP removal.