Modelling of chlorophyll-a and Microcystis aeruginosa decay under the effect of different oxidants in culture media

Abstract

Blooms of the cyanobacterium Microcystis aeruginosa are common in many eutrophic freshwater bodies and pose a serious threat to water quality, potentially giving rise to high turbidity, food web alterations, increased production of toxic microcystin (MC-LR) and odorous compounds. The comparative effectiveness of oxidant treatment of M. aeruginosa cells in culture media was evaluated by applying a mathematical model of chlorophyll-a (Chl-a), cells and MC removal. The oxidants were chlorine (1–5 mg∙L^-1), hydrogen peroxide (HP: 50–150 mg∙L^-1), percitric acid (PCA: 10–50 mg∙L^-1), and peracetic acid (PAA: 1.5–7.5 mg∙L^-1). The Weibull distribution model was applied to assess the degree of inactivation of M. aeruginosa viability under different oxidant treatments. First-order kinetics was successfully applied to the experimental data for Chl-a decay. Using the Weibull model, it was possible to predict the required exposure time (T_r) for oxidants to achieve a 99.9% reduction in viable M. aeruginosa cells with respect to the initial value. 5 mg∙L^-1 chlorine produced a 81% degradation of [D-Leu¹] MC-LR after 72 h, with an exposure time (T_r) of 141 h. Among the peroxide treatments (HP, PCA and PAA), PCA (10–50 mg∙L^-1) produced the highest level of [D-Leu¹] MC-LR degradation (39–79%), with low exposure times (T_r = 119–125 h). Chl-a concentration and M. aeruginosa counts for each oxidant treatment were highly correlated and successfully linked by a cubic polynomial. This is the first modelling report of M. aeruginosa decay by oxidant treatments.