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 the kinetics of diuron degradation in irradiated, aerated suspensions containing goethite (alpha-FeOOH) and oxalate, in the following denoted as heterogeneous photo-Fenton systems. In these systems, attack by hydroxyl radicals (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 microM oxalate at 3 < or = pH < or = 6. Both oxalate concentration and pH greatly affected the rate of light-induced diuron transformation. In the presence of initial 200 microM oxalate, the rate of diuron degradation was maximal at pH 4, coinciding with the maximal extent of oxalate adsorption on the surface of goethite. At pH 4,the rate of light-induced diuron degradation increased with increasing oxalate concentration, reaching a plateau at initial 200 microM oxalate, i.e., at the oxalate solution concentration at which the extent of oxalate adsorption on the surface of goethite reached a maximum. These experimental results suggest that the rate of Fe(II)(aq) formation through photochemical reductive dissolution of goethite, with oxalate acting as electron donor, determines the kinetics of diuron degradation in these heterogeneous photo-Fenton systems.