Interactive effect of independent variables on the TC degradation
Three-dimensional surfaces and contour plots are graphical representation of regression equation for the optimization of reaction Status. The results of the interactions between four independent variables and dependent variable are indicated in Figs. 6 and 7.
Fig 6 Contour and 3-D plots showing Interactive effect of: (a) TC concentration (mg/L) and PS concentration (mM); (b) TC concentration (mg/L) and sonication time (min)
Figure 6(a) indicates the interaction effect of TC concentration and PS concentration on the TC degradation rate with reaction time of 120 min. with the increasing PS concentration, the TC degradation rate significantly enhanced. With increasing PS concentration from 2 to 4 mM, the TC degradation rate increased from 75.56 % to 94.25 % at TC concentration of 30 mg/L. These results suggest that with increasing PS concentration, more sulfate radicals are produced which leads to more quickly TC degradation [32].
Figure 6(b) indicates the interaction effect of initial TC concentration and reaction time on the TC degradation rate. The TC degradation rate strongly increased with increase of sonication time from 60 to 120 min. with increasing reaction time from 60 to 120 min, TC concentration of 30 and 70 mg/L, the TC degradation rate increased from 70.44 % to 94.25 % at TC concentration of 30 mg/L. With increasing the TC concentration from 30 to 70 mg/L, the TC degradation rate decreased from 94.25 % to 85.05 %. In the constant conditions, with the increasing TC concentration, possibility of reaction between TC molecules and reactive species were declined. Moreover, the higher concentration of TC may lead to the creation of resistant byproducts and consequently decreases the degradation rate of TC [14, 61]. However, the total amount of degraded TC increased with the increasing initial TC concentration. This results are in agreement with the results obtained by other researchers [50].
Figure 7 indicates the interaction influence of pH value and initial TC concentration on the TC degradation rate. With increasing pH from acidic (5) to natural (7.5), the degradation rate slightly decreased, whereas with increasing pH from neutral (7.5) to alkaline (10), the degradation rate significantly enhanced. The TC degradation rate increased from 86.62 % to 94.25 % with increasing pH from 5 to 10, at TC concentrations of 30 mg/L. Under alkaline conditions (pH ≥10), alkaline-activated persulfate is the primary responsible for the production of SO4-•, O2-• and HO• radicals as following equations: [62, 63].10 S2O82−+2H2O→OH−HO2−+2SO42−+3H+11 HO2−+S2O82−→SO4−•+SO42−+H++O2−•12 SO4−•+OH−→SO42−+HO•
Also, at alkaline pH, sulfate radicals can react with hydroxyl anions to generate hydroxyl radicals (HO•) according to Eq. (3). In addition, a theory was introduced by other researchers that with increasing pH, the PS degradation into HO• and SO4-• increased [64].
The SO4-• is the predominant radical responsible for TC degradation at acidic pH, whereas both SO4-•and OH• are contributing in TC degradation at natural pH. Thus, three reactions compete with each other in natural pH: the reaction between SO4-• and HO•, the reaction between SO4-• and TC, and the reaction between HO• and TC, the simultaneous occurrence of these reactions may reduce the TC degradation rate [37, 65].