2.1 Site selection and sample collection
Experimental waters were sourced from three South Australian locations that have had a history of cyanobacterial blooms and include Happy Valley Reservoir, Torrens River, and Myponga Reservoir. Happy Valley Reservoir is located in Adelaide and supplies drinking water to over half a million people. The Torrens River is a recreational water course located near Adelaide’s CBD. Myponga Reservoir is located about 60 km south of Adelaide and it is fed by the Myponga River and other smaller catchment streams, and provides about 5% of Adelaide's water supply.
Samples collected from these three sites were filtered through a 90 mm glass microfiber filter Gf/C and kept in a refrigerator under 4°C. All glassware and storage containers were washed three times with distilled water, followed by three washes with ultrapure deionised water (Milli-Q element, 18.2 MΩ) before a final rinse with the reservoir water used in each experiment.
2.2 Materials, reagents and solutions
Sodium percarbonate (SP) stock solution was prepared by dissolving 18.382g granulated SP (IXOM Operations Pty Ltd) in 1 L of Milli-Q water to give a final hydrogen peroxide concentration of 5 g/L. This stock solution was used to simulate the same method of H2O2 dosing in reservoir water. Copper sulphate solution was prepared by dissolving 9.82 g of CuSO4.5H2O (AR, Chem-Supply) in a litre of Milli-Q water. Buffer stock to perform the H2O2 concentration measurement was prepared by mixing 18 mL of 0.5 mol/L Na2HPO4 and 82 mL of 0.5 mol/L NaH2PO4 to achieve pH 6 in tested samples. N,N-diethyl-1,4-phenlenediammonium sulphate (DPD) reagent was prepared by diluting 0.1 g of N,N-diethyl-1,4-phenlenediammonium sulphate, Fluka, in 10 mL of 0.1 N H2SO4. Horseradish peroxidase reagent was prepared by diluting 10 mg of horseradish peroxidase, Type II, 181 purpurogallin units/mg, in 10ml of Milli-Q water (Drábková et al. 2007).
2.3 H2O2 analysis / concentration determination
The method used to measure the H2O2 concentration was adapted from Drábková et al. (2007), which was modified from Bader et al. (1988). 30% H2O2 solution (AR, Chem-Supply) was used to create H2O2 standard solutions. Two H2O2 standard curves were prepared to determine the H2O2at low and high concentration. The standard curve to determine low H2O2 concentration of less than 5 mg/L (0, 0.056, 0.113, 0.226, 0.564, 1.13 and 5.64 mg/L) was prepared by mixing 16.6 μL of H2O2 standard solution to 1 L of Milli-Q water, followed by multiple dilutions. The standard curve to measure high H2O2 concentration of more than 5 mg/L (0, 0.11, 0.57, 1.13, 2.27, 3.4, 6.8 and 13.6 mg/L) was prepared by mixing 1 mL of H2O2 to 1 L of Milli-Q water, then followed by multiple dilutions. Standard solutions were then analysed in Agilent Cary 60 UV-Vis spectrophotometer at 551.0 nm.
For H2O2 concentrations up to 5 mg/L, 100 μL of the buffer solution, 50 μL of DPD reagent, and 50 μL of HRP reagent and 900 μL of the water sample were mixed in a polystyrene cuvette by inverting before immediately placing in UV-VIS spectrophotometer and measuring absorbance. Concentration was then determined from the corresponding standard curve. For H2O2 concentration greater than 5 mg/L, 100 μL of the buffer solution, 850 μL of Milli-Q water, 50 μL of DPD reagent, and 50 μL of HRP reagent was used. All measurements were made in triplicate.
2.4 Investigate the effects of different parameters on H2O2 decomposition
a. The effects of different water types
A 200 mL volume of either Milli-Q water or experimental water samples were dosed with 40, 200, 400 and 800 μL of SP solution to achieve initial concentration of 1, 5, 10 and 20 mg/L H2O2. All samples were incubated (Thermoline Scientific TRIL-1175-2-SD) at 20 °C in the dark over a 24-hour period to ensure the change in H2O2 concentration was not affected by changes in light or temperature. The water samples were collected at the time of initial dosage, and on every hour for a period of 7 hours to determine the H2O2 concentrations. At the same time, pH was measured using Oakton pH Meter (510 series) every hour for each water sample across all the four concentrations. Additionally, the H2O2 level was again measured 24 hours later from the initial dosage.
b. Additional Control Experiments
i.The effect of UV light
The experiment was conducted in Happy Valley reservoir water using the same method, concentrations and conditions as presented in 2.4a except in the presence of UV light. All samples were kept in the incubator under two UV lamps (Dermfix 3000 phototherapy 310-312 nm spectrum). H2O2 concentrations were measured at the time of initial dosage and on every hour for a period of 7 hours, with a final sample taken 24 hours after the initial dosage. pH was also determined over the same period.
ii. The effect of copper sulphate under dark condition
The experiment was carried out in Milli-Q water using same methodology presented in section 2.4a but replacing hydrogen peroxide with 0.5 mg/L Cu2+. pH was determined as described previously.
iii. The effect of DOC under dark condition
The experiment was carried out in Milli-Q water as described in section 2.4a but with inclusion of dissolved organic carbon (DOC) extract as surrogate of natural organic matter (NOM) present in natural water samples. The DOC was added to the samples using the laboratory organic extract adopted from the Spent Brine of Mt Pleasant, South Australia, MIEX based Water Treatment Plant. 200 μL of the organic compound was dosed into 200 mL samples to achieve 10 mg/L DOC in all samples. H2O2 concentrations and pH were also determined using the same procedure as previously described.
2.5 Data analysis and model development
Experimental data was initially processed and analysed using Microsoft Excel. For further data analysis and model assessment, the three-column H2O2 concentration, dosage and time data were further analysed using TableCurve3D, Systat Software, Inc., to obtain the equation of best fit. TableCurve3D used the built-in equation types / equations to fit the experimental data and ranked against the r2 value. The simple equation option was initially selected, if r2 values were good, >0.95, then simple equation will be used to provide the best equation for the decay characteristic of each water sample, which were then used to establish an initial prediction model. By using the coefficients which were determined by the fitting procedure, the H2O2 decay behaviour can be established for each water source, thus, the CT (contact time x concentration) value can be determined.