Kinetics of the reaction between a lignin model compound and chlorine dioxide

In this experimental study using 5,5'-diallyl-2,2'-dihydroxy-3,3'-dimethoxybiphenyl (DDD, a laboratory-synthesized biphenyl lignin model compound), the activity, kinetics and mechanisms in aqueous solution were investigated for the delignification of ClO2. The influence of the process operating conditions (initial pH and temperature) on the reaction pathway and product distribution were studied as well. The DDD instant concentration was calculated by a special differential ultraviolet-visible (UV-vis) method to exclude the effect caused by newly generated products. Kinetic parameters for the reaction in aqueous solution were determined for DDD and ClO2 with a kinetic model by regression analysis. The reaction with excess ClO2 at a pH of 3.32 was monitored to elucidate the reaction kinetics. The reaction was proven to be first-order with respect to DDD. The overall kinetic order of the reaction obeys the characteristics of a second-order kinetic reaction at lower pH values, but the fit to a second-order rate law is gradually lost with an increase in the pH. Under the experimental conditions first-order rate constants (k), second-order rate constants (K) and reaction stoichiometries were affected by the initial pH and temperature. Gas chromatography-mass spectrometry (GCMS), UV-vis spectroscopy and an AOX analyzer were employed to characterize the newly generated products. The activation energy was calculated to be 141.58 kJ mol(-1) by the Arrhenius equation. The potential reaction pathway of one of the newly generated product ((1Z,3Z,5Z,7Z)-2,7-diallylocta-1,3,5,7-tetraene-1,4,5,8-tetracarboxylate detected by GCMS) is proposed.