Theoretical limiting concentration for mineralization of trichloromethane and dichloromethane in aqueous solutions by AOPs

It is widely stated that most organic contaminants could be completely mineralized by Advanced Oxidation Processes (AOPs). This statement means that the concentration of the organic contaminant at equilibrium (limiting concentration, LC) is low enough to be neglected. However, for environmental safety, especially drinking water safety, this statement needs to be verified from chemical engineering thermodynamic analysis. In this paper, trichloromethane (CHCl₃) and dichloromethane (CH₂Cl ₂) are selected as the model systems, and the equilibrium concentration (theoretical limiting concentration, TLC) for the mineralization of chlorinated methanes in aqueous solutions at the different initial concentrations of chlorinated methanes, pH values and •OH concentrations by AOPs are investigated by thermodynamic analysis. The results in this paper show that the TLC for the mineralization of CHCl₃ and CH ₂Cl₂ with •OH increases with increasing initial concentrations of CHCl₃ and CH₂Cl₂, decreases with increasing concentration of •OH, and the TLC for the mineralization of CHCl₃ decreases with increasing pH values except that the pH value changes from 3.0 to 3.5. For the mineralization of CH₂Cl₂ with •OH, at the concentrations of •OH obtained from the literature, there is no obvious change of the TLC with pH values, while as the concentrations of •OH increase by 10 and 100 times, the TLC decreases with the increasing pH values from 2.0 to 3.0 and from 3.5 to 4.5, and increases with the increasing pH values from 3.0 to 3.5 and from 4.5 to 5.0. The investigations in this paper imply that high concentration of •OH, a bit higher pH values (4.0-5.0) in acid environment and low initial concentrations of the organic contaminants are beneficial for the complete mineralization of chlorinated methanes by AOPs.