Enhancing emerging pollutant removal mediated by root iron plaques: Integrated abiotic and biotic effects

As a contact interface among plants, microbes, and the liquid phase, root iron plaque (IP) occupies a crucial ecological niche on the root surface in constructed wetlands. However, research on the integrated work mechanisms of the various processes mediated by root IP in removing emerging pollutants is limited. This study analyzed four IP-mediated pathways in plant hydroponic systems, categorizing them into abiotic (adsorption and ·OH oxidation) and biotic (plant uptake and microbial degradation) effects. Employing sulfamethoxazole (SMX) as a target emerging pollutant, the results revealed that root IP improved pollutant removal efficiency by 2.44 times, which could be attributed to the enhanced abiotic effects. Root IP increased the amount of adsorbed SMX from 0.135 to 1.328 µg/g FW. Fe^2 + in root IP could activate O₂ to generate adsorbed hydroxyl radicals (·OH), which subsequently facilitate the oxidative removal of adsorbed SMX. The contribution of ·OH oxidation to SMX removal reached 27.35 %. However, from the biotic perspective, the stable adsorbed SMX was difficult to transport into plant root tissues, and root IP served as a "barrier" that hindered the plant uptake. In addition, IP weakened biotic effects by altering root-associated microbial community. The total relative abundance of SMX degradation-related genera decreased by 12.4 % with root IP, and the amounts of microbial degradation decreased from 8.33 to 3.49 µg/L.