In response to increasing concerns in the literature about human toxicity of microcystin-LR (MC-LR) in drinking water at very low concentrations, this study investigated the destruction of MC-LR down to ng L−1 concentrations by chlorine and permanganate. Results showed that decomposition in the low concentration range (1–2 ng L−1) exhibits a pseudo-first-order reaction with respect to both oxidants in pure water solutions. However, when the reaction proceeded down to a very low concentration (<10 ng L−1) a much smaller rate constant dictates the kinetics. In the presence of humic acids, the reaction rates and the final concentration removal were affected. The extent of such influence varied between the two oxidants, likely due to the different reaction mechanisms involved. This creates some profound effects of the governance of an oxidant exposure CT (residual oxidant concentration (C) × contact time (T)) over MC-LR degradation between the two oxidants. This study indicates that chlorine and permanganate may be still effective for MC-LR decomposition down to a concentration of 10 ng L−1 or below. However, the influence of solution chemistry must be carefully examined before a feasible CT is determined for the control of this toxin in a water treatment system.

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