The removal mechanisms of microcystin-LR through electrolysis biofilters

Abstract

Microcystin-leucine arginine (MC-LR) is the most common cyanotoxin produced by harmful cyanobacterial blooms with a cyclic structure which is hard to be degraded. In this study novel electrolysis biofilters (EBFs) with two Ti/RuO2-IrO2 anodes and one iron cathode were developed for intensified MC-LR degradation in contaminated water. Test results showed that biochar-EBF could efficiently remove intracellular and extracellular MC-LR. The MC-LR concentration decreased to 0.18 ± 0.06 ug·L−1 when the mean influent concentration was 2.51 ± 0.08 ug·L−1 with a removal rate of 92.83 ± 1.2%. The degradation rate was 0.47 ± 0.006 ug·(m2·h) −1, and the energy consumption was 0.3 kWh·m−3, which meets the World Health Organization standard for drinking water quality. A removal kinetic study found that electrolysis, adsorption and biodegradation were the main roles in the degradation process. The removal curves were more consistent with the pseudo-second-order model, as the R2 were all 0.99. The biofilm bacteria genera were mainly chemoheterotrophic, aerobic chemoheterotrophic, and methylotrophic. These biofilm bacteria genera could metabolize MC-LR and its intermediate products. According to the liquid chromatography-high resolution mass spectrometry (LC-HRMS) analysis of the MC-LR degradation intermediates by biochar-EBF, the degradation pathway of MC-LR first broke the amide bond between Adda and arginine, and then formed a linear MC-LR, which further degraded into a tetrapeptide compound. That compound was then further broken into phenylacetic acid (PAA) and small molecule amino acids. Thus, biochar-EBF enhanced the MC-LR degradation, and the biological toxicity of MC-LR was also reduced.