Diuron degradation in irradiated, heterogeneous iron/oxalate systems: The rate-determining step

The purpose of this study was to examine the various factors that control t he kinetics of diuron degradation in irradiated, aerated suspensions contai ning goethite (alpha -FeOOH) and oxalate, in the following denoted as heter ogeneous photo-Fenton systems. In these systems, attack by hydroxyl radical s (HO.) was the only pathway of diuron degradation. Studies were conducted in systems containing initially 80 or 200 mg L-1 goethite (corresponding to 0.9 or 2.25 mM total iron) and 20, 50, 75, 100, 200, and 400 muM oxalate a t 3 less than or equal to 5 pH less than or equal to 6. Both oxalate concen tration and pH greatly affected the rate of light-induced diuron transforma tion. In the presence of initial 200 muM oxalate, the rate of diuron degrad ation was maximal at pH 4, coinciding with the maximal extent of oxalate ad sorption on the surface of goethite. At pH 4,the rate of light-induced diur on degradation increased with increasing oxalate concentration, reaching a plateau at initial 200 muM oxalate, i.e., at the oxalate solution concentra tion at which the extent of oxalate adsorption on the surface of goethite r eached a maximum. These experimental results suggest that the rate of Fe(II )(aq) formation through photochemical reductive dissolution of goethite, wi th oxalate acting as electron donor, determines the kinetics of diuron degr adation in these heterogeneous photo-Fenton systems.