The effect of temperature on the degradation of tetracycline
To investigate the effect of temperature on the TC degradation rate, experiments were done with various temperature varying from 25 to 65 °C. With increasing temperature from 25 to 65 °C, the degradation rate constant increased from 0.0229 to 0.1042 min −1. Complete TC degradation occurs after 40, 60 and 75 min of reaction at 65, 55 and 45 °C respectively. The activation of S2O82− can be done under heat to form SO4-• radical as following Eq. (15). Therefore, complete removal of TC by high temperature could be as a result of thermally activated S2O82− oxidation. Moreover, the increase of temperature significantly enhanced the cavitation activity and chemical effects, resulting in greater degradation rate of TC by US/S2O82− process [22, 60].15 S2O8−2+→Termal−activation2SO4−•30°C<T<99°C
To investigate the effect of ultrasound on the process kinetics, significant parameters such as activation energy (Ea) play a remarkable role. The effect of temperature on the rate of the reaction and rate constant (k) is obtained by Arrhenius equation according with Eq. (16) [66].16 LnK=Aexp−EaRT
Arrhenius plot can be used to calculate the Activation Energy at various temperatures by graphing ln k (rate constant) versus 1/T (kelvin). The graph between ln k and 1/T is a straight line with an intercept of ln A and the slop of the graph is equal to –Ea/R, where R is a constant equal to 8.314 J/mol-K. According with Arrhenius plot (Fig. 8), the activation energy values of 32.01 (kJ/mol) obtained for degradation of TC by S2O82−/US process. It means that for a successful reaction, the colliding molecules must have a total kinetic energy of 32.01 kJ/mol. The low activation energy indicates that the degradation of TC by S2O82−/US process is thermodynamically feasible.
Fig 7 Contour and 3-D plots showing Interactive effect of pH and TC concentration (mg/L)