3.3 Evaluation of the effect of fluoride and bicarbonate concentration on 2,4-D photoinduced degradation under simulated sunlight irradiation by a factorial experimental design.
A factorial design 22 was used to study the effect of natural concentrations of fluoride (0.15-1.2 mg L-1) and bicarbonates (83.6-596.0 mg L-1) typically found in natural groundwater samples on simulated sunlight irradiated-goethite (0.3 mg L-1; 0.2 mg L-1 of total iron) degradation of 2,4-D (30 mg L-1) in presence of 10 mg L-1 of H2O2 at pH 6.9 after 90 min of sunlight irradiation. ANOVA results confirmed a polynomial equation of second order with R2=99.87 and P-value<0.05.
Surface response (Figure 8a) shows that with increasing fluoride and bicarbonate concentrations, the 2,4-D removal rises as well. The Pareto chart (Figure 8b) revealed that the presence of fluoride at its maximum level (1.2 mg L-1) plays a significant role. Moreover, the combined interaction of bicarbonates and fluoride at their maximum levels (596.0 mg L-1 and 1.2 mg L-1respectively) also exerted an important role in herbicide degradation. This was corroborated in Figure 8c where the factor interactions chart was shown, and it was observed that high bicarbonate and fluoride concentrations exhibited a high 2,4-D degradation.
Results suggest that fluoride and bicarbonate ranging at concentrations typically found in natural groundwater samples could have a positive effect on the 2,4-D removal using goethite at natural concentrations by adding H2O2 doses of 10 mg L-1 under simulated sunlight irradiation. This finding has important environmental implications since it should be probable that the simple addition of hydrogen peroxide onto natural groundwater samples upon natural sunlight irradiation may enhance several photo-induced phenomena implying natural fluorinated colloidal iron for the removal of organic pollutants or bacteria inactivation.