Atrazine Degradation in Irradiated Iron/Oxalate Systems: Effects of pH and Oxalate

Abstract

The purpose of this study was to examine the various factors that control the kinetics of atrazine degradation in irradiated Fe(III)/oxalate systems, in the following denoted as photo-Fenton systems. In these systems, attack by hydroxyl radicals (HO • ) was the only pathway of atrazine degradation. Transformation of pollutants by HO • that are produced in photo-Fenton systems is of interest in atmospheric waters, in iron-rich surface waters, and possibly on soil surfaces. Studies were conducted in systems containing 6 μM iron and 0, 18, and 180 μM oxalate at 3 ≤ pH ≤ 8, irradiated with simulated sunlight. Both oxalate concentration and pH greatly affected the rate of atrazine transformation. In the presence of initial 18 μM oxalate, the rate increased in the order of pH 7.5 < 5.6 < 3.2 < 4.3, and with 180 μM in the order of pH 7.9 < 3.2 < 4.6 ≈ 5.4. At all pH values, the rates were considerably higher at the higher initial oxalate concentration. In both cases, no degradation occurred at pH > 7. In the absence of oxalate, atrazine transformation was slower and occurred only up to pH 4.1. These experimental results can be explained by various competing effects. First, both pH and oxalate concentration control the iron(III) speciation and thus the rate of photolysis of Fe(III) complexes. Second, also the Fe(II) speciation and hence the rate of the Fenton reaction [oxidation of Fe(II) by H 2 O 2 ] are affected by pH and oxalate concentration. Finally, oxalate acts as a scavenger of hydroxyl radicals that are produced in the Fenton reaction and hence competes with atrazine for HO • . We have identified the individual reactions taking place in these complex systems at pH 3 by combining batch experiments with kinetic modeling.