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.