A system and method for the treatment of process streams. A catalyst mediates a wet oxidation process at elevated temperatures and pressures for treating at least one undesirable constituent in an aqueous mixture. A catalyst may be selected for its solubility at a detected pH level of the aqueous mixture. Alternatively, a pH level of the aqueous mixture may be adjusted to solubilize a selected catalyst and/or maintain the selected catalyst in a soluble form. A controller in communication with a pH sensor may be configured to generate a control signal to adjust the pH level of the aqueous mixture in response to the pH sensor registering a pH level outside a predetermined pH solubility range for a selected catalyst.