Mechanism and kinetics of methylisothiazolinone removal by cultivation of Scenedesmus sp. LX1

Methylisothiazolinone (MIT) is a widely used non-oxidizing biocide for membrane biofouling control in reverse osmosis (RO) systems usually with high dosages. However, few investigations have focused on MIT removal through bio-processes, since it is highly bio-toxic. This study proposed a novel biotreatment approach for efficient MIT degradation by Scenedesmus sp. LX1, a microalga with strong resistance capability against extreme MIT toxicity. Results showed that MIT (3 mg/L) could be completely removed within 4 days' cultivation with a half-life of only 0.79 d. Biodegradation was the primary removal mechanism and this metabolic process did not rely on bacterial consortia, soluble algal products secretion or algal growth. The main pathway was proposed as ring cleavage followed by methylation and carboxylation through the identification of MIT transformation products. MIT biodegradation followed the pseudo-first-order kinetics under growth control. A new kinetic model was presented to depict the MIT removal considering algal growth, and this model could be used for generally describing non-nutritive contaminants biodegradation. The algal biodegradation capability was independent of the initial biocide concentration, and MIT removal could be enhanced by increasing the initial algal density. Our results highlight the potential application of algal cultivation for MIT-containing wastewater biotreatment, such as RO concentrate.