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.