Distribution and removal of microplastics in a horizontal sub-surface flow laboratory constructed wetland and their effects on the treatment efficiency

[Display omitted] •99.99% of microplastics were retained in a laboratory constructed wetland (LCW).•More microbeads than fibres were detected in the effluent.•Microplastics affected the removal of N-NO2- and P-PO43- in the LCW.•The removal of carbonaceous matter was not affected by the presence of microplastics.•The first order areal rate coefficient was comparable to that of large CWs. Constructed wetlands are commonly used to treat domestic wastewater, but the extent to which microplastics are retained in constructed wetlands and the impact this has on their efficiency are not known. In this context, we investigated retention of microplastics (microbeads and fibres) and their effects on the removal of carbonaceous (expressed as chemical oxygen demand (COD), biochemical oxygen demand (BOD5), and dissolved organic carbon (DOC)), nitrogenous (N-NH4+, N-NO3-, and N-NO2-), and phosphorous (P-PO43-) matters in a horizontal sub-surface flow laboratory constructed wetland (LCW) under continuous flow. The LCW was in operation for 257 days, with the first 78 days without microplastics and microplastics added weekly during the next 179 days. The presence of microplastics had no significant effect on COD, BOD5, DOC, N-NH4+, and N-NO3- removal, while a statistically significant decrease in N-NO2- and an increase in P-PO43- concentrations in the effluent were observed after the addition of microplastics. Microplastics were efficiently retained in the LCW, and only 0.296% and 0.003% of the total microbeads and fibres, respectively, were detected in the effluent. Thus, it is evident that constructed wetlands are an effective barrier to the release of microplastics into the aquatic environment, but the high retention and thus continuous accumulation of microplastics may alter the nutrient cycling in constructed wetlands.