Reactivity of Effluent Organic Matter (EfOM) with Hydroxyl Radical as a Function of Molecular Weight

The application of advanced oxidation processes (AOPs) for the treatment of wastewater is hindered by scavenging of the hydroxyl radical (HO•) by effluent organic matter (EfOM). This scavenging is directly proportional to the second-order reaction rate constant between EfOM and HO• (kEfOM-HO•). To understand the kinetics of this reaction as a function of the subcomponents of EfOM, four wastewater samples were fractionated by ultrafiltration into distinct apparent molecular weight (AMW) fractions (<1, <3, <5, and <10 kDa), and their kEfOM-HO• values were quantified. In general, the values for kEfOM-HO• decreased as the AMW increased. The values of kEfOM-HO• for the bulk waters varied between 6.32 and 14.1 × 108 MC−1s−1 (units of per molar carbon concentration per second). In the case of the <1 kDa fraction, the values of kEfOM-HO• varied from 14.3 to 35.0 × 108 MC−1s−1, or approximately 2.31(±0.24) times that of the corresponding bulk waters. For the <3 kDa, <5 kDa, and <10 kDa fractions, the kEfOM-HO• values were 1.83(±0.25), 1.32(±0.23), and 1.26(±0.35) times that of the bulk waters, respectively. Based on the obtained results, the variability and general magnitude of the kEfOM-HO• values were attributed to the production and reactivity of soluble microbial products (SMP), a major component of EfOM. Two samples collected at a wastewater treatment facility with different treatment variables had different kEfOM-HO• values, indicating that wastewater treatment processes will impact overall HO• scavenging by EfOM and should be considered during the implementation of AOPs.