Study on the removal of benzisothiazolinone biocide and its toxicity: The effectiveness of ozonation

• Ozone could remove benzisothiazolinone (BIT) rapidly and effectively.
• Toxicity of BIT solution was eliminated by ozone as soon as no BIT remained.
• An ozonation pathway was proposed using MS detection and energy calculation.
• Municipal ROC matrix showed little negative effect on BIT ozonation process.

Benzisothiazolinone (BIT) is widely used as a biocide in different industries, and, in particular, in wastewater reclamation plants in which reverse osmosis (RO) is used. BIT can cause dermatitis in humans and is toxic to fish. It is therefore significant to control BIT concentrations in water that is being treated, however, appropriate methods for BIT control and removal have not yet been developed. This study investigated the changes in the concentrations and toxicities of aqueous solutions of BIT, caused by ozonizing the solutions. It was found that BIT was rapidly degraded by ozonation effect, and the rate constant kO3,BITkO3,BIT was 4.17 × 106 L/(mol s). Samples were analyzed using time-of-flight mass spectrometry, and a possible ozonation pathway was proposed from the results. Besides, free energy calculations were performed for characterization of possible products of BIT ozonation. One product, saccharin, was identified, which was produced through the sequential oxidation of sulfur atoms in BIT. The specific ultraviolet absorbances of the BIT solutions decreased and sulfate ions were produced during ozonation. Furthermore, BIT was found to be toxic to zebrafish embryos, with a median lethal concentration of 6.8 μmol/L. Nevertheless, ozonation markedly decreased the toxic effects of BIT to zebrafish embryos, making the toxic effects undetectable. This indicated that the oxidized products were ecologically benign. Since the kinetic rate of BIT ozonation is very fast, compared with (bi)carbonate or humic acid, there is no observable negative interference on BIT ozonation process. Even in the RO concentrate matrix, namely existing complex organic/inorganic components, ozone is still able to quickly degrade BIT as in pure water system. Consequently, ozonation can be considered as an effective method for removing BIT from RO concentrates.

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