Water Journal : Water Journal September 2011
water treatment refereed paper technical features 92 SEPTEMBER 2011 water WTP. Results are shown in Figure 4, along with the corresponding CT values. The purpose of the chlorine decay experiments was to determine the free chlorine residual of chlorine in the waters at predetermined times so that the stoichiometric volume of quenching agent (sodium thiosulphate) could be added during the chlorination of cyanotoxins. In addition, the chlorine decay curves allowed for the calculation of the CT values. Only slight differences were evident between the waters when using chlorine doses of 1.5 and 2.0 mg L-1 with the chlorine consumption following the trend: Plant A ≥ Plant B > Plant C. This is reflected in the first-order decay rate constants shown in Table 2. The differences may be attributed to the water quality, in particular, the natural organic material (NOM) characteristics. Plant C water displayed the lowest specific UV absorbance (SUVA) values compared with the other waters; SUVA has been shown to correlate well with the conjugated and aromatic moieties in NOM and it is these NOM constituents that are more susceptible to chlorine oxidation (Croué et al., 1999; Kitis et al., 2002). Furthermore, the slightly lower pH of Plants A and B waters (compared with Plant C water) may have also influenced the reactivity with chlorine, as the acid-base equilibrium favours hypochlorous acid, a stronger oxidant than the hypochlorite ion. Chlorination of CYN For the chlorination of CYN experiments, a chlorine dose of 1.5 mg L-1 was applied to the waters from Plants B and C, while a chlorine dose of 4.0 mg L-1 was applied in Plant A water; these represent the average chlorine dose applied at the respective WTPs. Two different initial concentrations of CYN (3 and 20 μg L-1) were evaluated in the three waters for the chlorination experiments; these concentrations were selected as they represent the concentration range detected in Australian water sources (McGregor and Fabbro, 2000). CYN has previously been shown to be highly susceptible to oxidation by chlorine (Rodriguez et al., 2007b; Ho et al., 2008) and the results in Figure 5 confirmed this. CYN was readily oxidised in the three waters with minimal difference observed between the two initial CYN concentrations. CT values of approximately 19, 7 and 7 mg.min L-1 were required to oxidise CYN to below 1 μg L-1 in waters from Plants A, B and C, respectively. These values are comparable to those reported in the literature (Ho et al., 2008). Furthermore, these CT values correspond to the first sample taken at 5 mins, so it is possible that efficient oxidation occurred at a time less than 5 mins, potentially translating to lower CT values required for efficient CYN oxidation. The susceptibility of CYN to chlorination can be attributed to its structure, in particular, the uracil moiety, which is also partially responsible for its toxicity (Banker et al., 2001). The reaction of uracil with chlorine is quite rapid, with a documented rate constant of 90 M-1 s-1 (Gould et al., 1984). Chlorination of saxitoxins The chlorination of saxitoxins was conducted using two initial STX-eq concentrations: 8 and 12 μg L-1. These 0 50 100 150 200 250 300 350 400 0.0 0.5 1.0 1.5 2.5 3.0 3.5 4.0 4.5 5.0 Time (min) Free chlorine concentration (mg/L) Plant A 1.5mg/L 2.0mg/L 4.0mg/L Plant B 1.5mg/L 2.0mg/L Plant C 1.5mg/L 2.0mg/L 0 50 100 150 200 250 300 350 400 0 200 400 600 800 1000 CT (mg.min/L) Time (min) Figure 4. Decay of chlorine in waters from Plants A, B and C. Inset: CT plots for the respective chlorine decay curves. Table 2. First order rate constants (k) for the decay of chlorine in waters from Plants A, B and C. Correlation coefficients (R2) presented in parentheses. Chlorine dose (mg L-1) k (s-1) Plant A Plant B Plant C 1.5 4.0 x 10-5 (0.90) 4.1 x 10-5 (0.96) 2.5 x 10-5 (0.93) 2.0 2.6 x 10-5 (0.91) 2.8 x 10-5 (0.92) 1.8 x 10-5 (0.92) 4.0 2.4 x 10-5 (0.98) - - 0 100 200 300 400 500 600 0 20 40 60 80 100 Percent toxin oxidised CT (mg.min/L) Plant A 4.0mg/L chlorine 3μg/L CYN 20μg/L CYN 0 20 40 60 80 100 120 140 160 0 20 40 60 80 100 Percent toxin oxidised CT (mg.min/L) Plant B 1.5mg/L chlorine 3μg/L CYN 20μg/L CYN 0 50 100 150 200 0 20 40 60 80 100 Percent toxin oxidised CT (mg.min/L) Plant C 1.5mg/L chlorine 3μg/L CYN 20μg/L CYN Figure 5. Chlorination of cylindrospermopsin (CYN) in waters from Plants A, B and C.
Water Journal November 2011
Water Journal August 2011