2

2

2

2

(1)

(2)

A first series of experiments were devoted to the study of the ability of Fe(II) in combination with hydrogen peroxide to form a DMPO-OH adduct, as well as the variation of the signal attributed to this adduct vs time. The pH value was selected to be five, since it is the highest pH value which can be employed to drive the photo-Fenton process in the presence of HLS with an acceptable efficiency. (5) Figure 1 a shows the signal obtained at pH = 5 in the presence of the HLS (20 mg/L) and Fe(II) (5 mg/L), immediately after the addition of hydrogen peroxide (34 mg/L); this corresponds to a molar ratio Fe/Hof 1:10 that has been reported to optimize the generation of hydroxyl radicals. (33) The high signal/noise ratio shows that there was an efficient generation ofOH under these experimental conditions. To see if the ability of the Fe(II)–HLS is constant or varies with time, the spectra were recorded again at different times after the addition of hydrogen peroxide. The intensity of the signal was calculated as an average of the height of the four characteristic peaks of the spectrum and given in relative values. The obtained relative intensities were plotted vs the delay time between addition of Hand recording the spectrum. Figure 1 b shows that there is a fast decrease in the signal, what is compatible with an inactivation of the Fenton system. This is a well-known phenomenon, which is attributable to the existence of two main reactions in the Fenton process ( eqs 1 and 2 ). The first equation is very fast and generates efficientlyOH; however, Fe(II) is oxidized to Fe(III) and the reduction of Fe(III) is slow. (34) Hence, as the initial Fe(II) is oxidized, its concentration decreases and there is a noticeable loss of efficiency in the generation ofOH.Taking into account those results, it seems interesting to test the behavior of the system when iron is added as Fe(III). Figure 2 a shows that the signal to noise ratio is clearly lower in this case, showing that the generation of hydroxyl radicals under these conditions is less efficient. However, Figure 2 b indicates that the signal of the adduct is nearly constant vs time; hence, the Fe(III) does not suffer a remarkable loss of efficiency in generatingOH. This is in agreement with the above given explanation: eq 2 is the limiting step of the process, and as Fe(III) is reduced to form Fe(II),OH is quickly formed by eq 1 , and iron oxidized again to Fe(III) closing the cycle; in other words, when starting with Fe(III) instead of Fe(II), a steady state for Fe(II)/Fe(III) speciation is quickly reached and hence a stable signal is obtained. Hence, for practical reasons, the use of Fe(III) seems more advisable for the rest of EPR-based experiments.