Removal mechanisms of benzotriazoles in duckweed Lemna minor wastewater treatment systems.

The fate of five benzotriazoles (1H-benzotriazole, BTR; 4-methyl-1H-benzotriazole, 4TTR; 5-methyl-1H-benzotriazole, 5TTR; xylytriazole, XTR and 5-chlorobenzotriazole, CBTR) was studied in batch and continuous-flow Lemna minor systems and the role of different mechanisms on their removal was evaluated. Single and joint toxicity experiments were initially conducted using the Organization for Economic Co-operation and Development (OECD) protocol 221 and no inhibition on specific growth rate of Lemna minor was observed for concentrations up to 200μgL-1. All tested substances were significantly removed in batch experiments with Lemna minor. Excepting 4TTR, full elimination of CBTR, XTR, 5TTR and BTR was observed up to the end of these experiments (36d), while the half-life values ranged between 1.6±0.3d (CBTR) and 25±3.6d (4-TTR). Calculation of kinetic constants for hydrolysis, photodegradation, and plant uptake revealed that for all BTRs the kinetic constants of plant uptake were by far higher comparing to those of the other mechanisms, reaching 0.394±0.161d-1 for CBTR. The operation of a continuous-flow Lemna minor system consisted of three mini ponds and a total hydraulic residence time of 8.3d showed sufficient removal for most target substances, ranging between 26% (4TTR) and 72% (CBTR). Application of a model for describing micropollutants removal in the examined system showed that plant uptake was the major mechanism governing BTRs removal in Lemna minor systems.