Oxidation of Congo red by thermally activated persulfate process: Kinetics and transformation pathway

The degradation of Congo Red (CR) in thermally activated persulfate (PDS) oxidation (TAP) system was investigated in this study. Effects of chloride (Cl-), carbonate/bicarbonate (HCO3-/CO32-), natural organic matter (NOM), and operating parameters including reaction temperature, solution pH, and PDS dosage on the observed pseudo-first-order rate constants for CR degradation (k(obs)) were evaluated. The calculated activation energy of CR in TAP system was approximately 76( +/- 0.8) KJ mol(-1) within the range of 313-353 K, and CR could be degraded effectively. The removal efficiency of CR increased with increasing the activation temperature (20-80 degrees C) and dosage of PDS (0.1-5.0 mM). The change of pH could not affect the removal of CR directly, but the lower scavenging effect of dihydrogen phosphate than hydrogen phosphate resulted in faster degradation of CR in acidic conditions (i.e., pH = 5 and 7) than alkaline condition (i.e., pH = 9). The presence of high-concentration of Cl - and CO32-/HCO3- ( > 1.0 mM) could accelerate the degradation of CR in TAP system, which was attributed to the contribution of those secondary radicals (i.e., chlorine radicals, carbonate radical). The presence of NOM could inhibit the degradation of CR effectively, which could be explained by the sulfate radical (SO4-center dot) scavenge effect of NOM. Further, seven degradation products were detected in the reaction of CR with SO4-center dot A tentative pathway was proposed, where the SO4-center dot attack the -SO3-, -NH2 functional group and -C-C- bond by electron transfer reactions. These results indicated that CR could be degraded effectively in TAP system under various conditions.