Closing the loop. Repurposed iron-coated granulate improves pollutant removal in nature-based solutions

Optimising nature-based solutions for the removal of dissolved pollutants is essential for their adoption as end-of-pipe solutions in urban drainage systems. This study evaluates the performance of a repurposed iron-coated granulate, sourced from a drinking water treatment plant, as an amendment in bioretention systems. In a column experiment, a control media mixture of pumice and coconut fibres was compared to one amended with 10 % iron-coated granulate. Four configurations were tested: media-only, media with vegetation, media with a zone, saturated with water, and media with both vegetation and a saturated zone. Systems amended with the iron-coated granulate consistently achieved >90 % removal of phosphorus, copper, and zinc from the semi-synthetic stormwater, representing improvements of up to 33.3 %, 4.1 %, and 2.8 %, respectively, compared with the control systems. Copper was primarily removed in the upper media layers, while the presence of a saturated zone was essential for effective phosphorus and zinc removal at the bottom layer. Vegetation had little influence under the tested conditions, likely due to low temperatures and limited plant growth during the study period. While the combination of vegetation and a saturated zone improved nitrate removal in the top layer of amended systems (61.6 %), overall removal was higher (73.9-78 %) in unamended control systems with saturated zones, suggesting possible inhibition of denitrification by elevated iron concentrations. Practical applications should consider that carbon addition, especially in saturated zones, may mobilise previously bound contaminants, and that amendment sourcing requires careful evaluation of granulate quality and arsenic content before broader implementation.