Treatment and kinetic study of cyanobacterial toxin by ozone

Abstract

A rapid scan-stopped flow (RS-SF) reactor was used to study reaction between ozone and cyanobacterial toxins [microcystin-LR (MC-LR) and microcystin-RR (MC-RR)] at different pH values and over a temperature range of 20-30 degrees C. The ozonation reaction was very effective for elimination of microcystin; solutions of concentration up to 5 mg/L MC-LR were totally oxidized by an ozone dosage of 2 mg/L. Reactions were dependent on ozone dose, temperatures, and pH. A more effective reaction took place at a higher ozone dose, higher temperatures, and more acidic pH. Spectrophotometer analysis was used to study the ozonation kinetics. Reactions were very fast: with an initial ozone concentration of 2 mg/L the half-life time of the toxins was less than 20 s. Ozonation reaction was successfully modeled to an overall second-order kinetics and with first-order kinetics for both ozone and toxins. Overall rate constants (K) were found to be 6.79 x 10(4) M(-1)s(-1) for MC-LR and 2.45 x 10(5) M(-1)s(-1) for MC-RR at 20 degrees C, with a pH of 2. The main degradation intermediates and the toxicity of the treated solution were also evaluated. The identified by-products were related to ozone dose. The high available ozone concentration degraded the toxins into smaller by-products and led to a ring opening. On the other hand, at a low ozone dose larger intermediates were detected. The treated solution toxicity was also found to be related to the ozone available in the aqueous solution; a high ozone dose led to cleavage of the Adda side chain from the toxin and reduced the toxicity.