Defluoridation in constructed wetlands: Effect of auxin on performance and kinetics

Constructed wetlands (CWs) were evaluated for fluoride removal and the impact of auxin on system performance and kinetics. Nine horizontal subsurface flow CWs, varying in hydraulic retention times (HRT) (2, 5, 10 days), plants (Eichhornia crassipes), and auxin amendment were exposed to fluoride-enriched water (15 mg L^-1). Levels of fluoride were determined both in effluent and in plant-tissues, and the latter also underwent anatomical assessments. Planted CWs outperformed unplanted CWs, with HRTs of 5 or 10 days achieving > 69 % fluoride removal. Although two-day CWs faced higher loadings, they also showed reasonable performance, sometimes removing more fluoride in terms of mass. Auxin improved removal rates, especially in two-day planted CWs. Fluoride accumulated primarily in the shoot tissue (translocation factors > 1), and anatomical changes included epidermal alterations, exodermis and parenchyma loss in the roots, and exposed aerenchyma. CWs proved effective for defluoridation, and plants were tolerant despite minor adverse effects. First-order kinetics better described the trends in fluoride removal.