Hydroxylamine enhanced degradation of naproxen in Cu²⁺ activated peroxymonosulfate system at acidic condition: Efficiency, mechanisms and pathway

Peroxymonosulfate (PMS) activated by Cu⁺ has been regarded as an effective activation method to generate sulfate radical and hydroxyl radical, however, this process is limited for the poor reduction of Cu²⁺ to Cu⁺ by PMS. In this study, hydroxylamine (HAm) was introduced to improve the oxidation capacity of Cu²⁺/PMS process, meanwhile the degradation efficiency of naproxen (NPX) in Cu²⁺/HAm/PMS system was investigated for the first time. It was found that HAm could accelerate the redox cycle of Cu²⁺/Cu⁺, leading to an enhancement on NPX degradation. The NPX degradation efficiency in Cu²⁺/HAm/PMS system was 10 times and 2 times of that in Cu²⁺/PMS and HAm/PMS in 5 min at pH 4.0. The analysis of steady-state concentration of Cu species showed that HAm could enhance the transformation of Cu²⁺ to Cu⁺, leading to an improvement on NPX degradation in Cu²⁺/HAm/PMS system. Both sulfate radical and hydroxyl radical were identified as the dominant reactive species by indirect and direct methods, and PMS activation was attributed to the formation of Cu⁺ proved by neocuproine experiments. Eight degradation products of NPX were identified by LC-MS/MS, and four oxidation pathways were proposed. This study gives a preliminary interpretation on the enhancement of NPX degradation in Cu²⁺/HAm/PMS system under the acidic condition, and broadens the scope of HAm enhanced Cu-catalyzed PMS system for the treatment of organic contaminants.