Formation of THMs and HANs during bromination of Microcystis aeruginosa

Bromine-contained disinfectants and biocides are widely used in swimming pools, recreational waters and cooling towers. The objective of this study was to evaluate the formation of thrihalomethanes (THMs) and haloacetonitriles (HANs) and their cytotoxicity in algae solutions during free bromine disinfection. Disinfection by-products formation potential experiments were conducted using model solutions containing 7 mg/L (as total organic carbon) Microcystis aeruginosa cells. Effects of free bromine dosage, pH and ammonia were investigated. The results showed that brominated disinfection by-products were the major products when free bromine was applied. The total THMs formed during bromination was much as that formed during chlorination, whereas HANs were elevated by using bromination instead of chlorination. Dibromoacetonitrice (C₂H₂NBr₂) and bromoform (CHBr₃) were the only detected species during free bromine disinfection. The production of C₂H₂NBr₂ and CHBr₃ increased with disinfectant dosage but decreased with dosing ammonia. CHBr₃ increased with the pH changing from 5 to 9. However, C₂H₂NBr₂ achieved the highest production at neutral pH, which was due to a joint effect of variation in hydrolysis rate and free bromine reactivity. The hydrolysis of C₂H₂NBr₂ was base-catalytic and nearly unaffected by disinfectant. Finally, estimation of cytotoxicity of the disinfected algae solutions showed that HANs formation was responsible for the majority of toxicity. Considering its highest toxicity among the measured disinfection by-products, the elevated C₂H₂NBr₂ should be considered when using bromine-related algaecide.