Wastewater Treatment Performance of Gravel, Graphite, and Biochar as Wetland Substrates in the Presence and Absence of Mixed Microplastics

Traditional constructed wetlands (CWs) are primarily designed for removing conventional pollutants from wastewater; however, the presence of emerging contaminants such as microplastics (MPs) in wastewater poses new challenges. In this study, two experimental CWs with carbonaceous substrates (granular graphite and biochar) were compared against a gravel-based control to evaluate their performance in treating wastewater with and without a mixture of polyethylene terephthalate (PET) and high-density polyethylene (HDPE) MPs. The unplanted substrate-only configuration was intentionally used to isolate the effects of MPs on substrate-mediated treatment processes while minimizing plant-associated confounding effects; however, future studies should incorporate vegetation and microbial-community characterization to better represent planted CWs. The CWs were operated in two phases, with and without MPs, to assess changes in pollutant removal performance and system stability. In the presence of MPs, ammonium (40.78 ± 1.87 mg/L) removal efficiencies decreased by 2.90, 12.03, and 26.96 percentage points, and sulphate (136.64 ± 6.09 mg/L) removal declined by 2.03, 25.65, and 25.61 percentage points in the gravel CW, graphite CW, and biochar CW, respectively. Conversely, COD removal (549 ± 45.93 mg/L) improved by 16.17, 20.34, and 9.27 percentage points across the respective CWs. MPs also disrupted environmental condition, including dissolved oxygen and redox potential, while their hydrophobic nature enhanced volatile suspended solids accumulation. Under the tested PET–HDPE exposure conditions, the observed reductions in ammonium and sulphate removal were more pronounced in the carbonaceous-substrate CWs than in the gravel-based system. This study highlights the need for further research into how MP characteristics, such as functional groups, size, and shape, influence CW performance and resilience.