2.4 Photocatalytic experiments
Experiments were performed using Milli-Q water (in Pyrex glass bottles with a volume of 80 mL) containing 0.3 mg L-1 (0.2 mg L-1 of total iron) of goethite and fluoride (0.15-1.20 mg L-1) and bicarbonate (83.6-596.0 mg L-1) concentrations typically found in natural well waters at initial pH 6.9; this solution was stirred overnight. Then, 10 mg L-1 of H2O2 and 30 mg L-1 of 2,4-D were spiked under constant magnetic stirring on the sample and irradiated by a Hanau Suntest (AM-1) solar simulator (Total Intensity: 300 W m-2). The lamp has a spectral distribution with about 0.5% of emitted photons at wavelengths shorter than 300 nm (UV-C range), and about 4% between 300–400 nm (UV-B and UV-A ranges). The distribution of the photons emitted between 400 and 800 nm follows the solar spectrum. The radiant flux was monitored with a Photo-radiometer (HD 2102.2-Delta Ohm). The temperature of the experiments was never superior to 38 °C. Samples were periodically collected to follow the 2,4-D concentration by HPLC (LC20AT-Shimadzu) using as mobile phase acetonitrile (55 %), an aqueous solution of acetic acid at pH 3.0 (30 %), and Milli-Q water (15 %) and an HPLC column C-18 Nucleosil 100–5. An isocratic flux of 0.8 mL min−1 and a UV detector at 280 nm were used. The final pH was monitored in all experiments reaching a value of 7.5. Experiments were performed in triplicate and their average was reported. Although the initial 2,4-D concentration is not representative of concern emerging contaminants (CECs) in water, it was chosen to permit adequate detection by HPLC equipment available in the laboratory.