Evaluating Interferences in the External Calibration Method for Hydroxyl Radical Scavenging Capacity Measurement

Abstract

In most advanced oxidation processes (AOPs), hydroxyl radical (•OH) is produced to transform micropollutants. However, it can also be consumed by non-target compounds including organic and some inorganic matter, known as •OH scavengers. The pseudo-first order consumption rate of •OH is termed as the hydroxyl radical scavenging capacity (HRSC). HRSC directly influences AOP performance, which is a crucial parameter for AOP design and operation. An external calibration method has been developed to measure HRSC, using methylene blue (MB) as the probe compound. This method assumes that MB only reacts with •OH, but experimental investigation is needed to confirm this. In this study, MB decay was monitored under reaction conditions featuring high concentrations of superoxide radical, organic radical, carbonate radical, and singlet oxygen. The results showed that these reactive species did not contribute to MB decay. Additionally, as chloride, bromide, and chloramines may introduce new reactive species through UV photolysis or reaction with •OH, their impacts on HRSC measurement were investigated. Our results show that chloride has no impact on HRSC, while bromide at low mg/L levels significantly increases it. Chloramines were found to significantly increase HRSC. Our findings confirm that the external calibration method is reliable for most drinking water samples due to the negligible interferences from non-•OH species. For water samples containing chloramines, a protocol was developed to measure background HRSC with chloramines quenched using sodium sulfite. This approach assumes that the contribution of chloramines can be modeled when their concentrations are accurately determined.