Efficient degradation of ethanolamine by UV/chlorine process via organic chloramine photolysis: Kinetics, products, and implications for ethanolamine wastewater treatment

Abstract

The degradation of ethanolamine (ETA), a recalcitrant pollutant containing a primary amine moiety, can be significantly enhanced in a water treatment with chlorination followed by UV (UV/chlorine) via photolysis of the N–Cl bond of monochloro- (N-Cl ETA) and dichloro-ethanolamine (N-Cl2 ETA) which are formed from the reaction of chlorine with ETA. This study investigated the reaction kinetics, products, and mechanisms of ETA degradation in UV254/chlorine and the effects of process variables on ETA’s treatment efficiency. Fluence-based, photolysis rate constant (kUV) was 1.0 × 10−3 cm2/mJ for N-Cl ETA and 1.3 × 10−3 cm2/mJ for N-Cl2 ETA. UV/chlorine was found effective at treating ETA under wide-ranging conditions for the free available chlorine (FAC) dose over ETA concentration (i.e., [FAC]0/[ETA]0 ≥ 1), pre-chlorination duration (i.e., >a few minutes), and solution pH (i.e., 2–11), due to the rapid formation of N-Cl ETA from the reaction of HOCl with the amine of ETA (k of 1.2 × 108 M−1 s−1). Increasing the FAC dose was capable of enhancing the degradation rate of ETA so that it attained a kUV of 1.5 × 10−3 cm2/mJ at [FAC]0/[ETA]0 of 3, mainly due to the formation of N-Cl2 ETA (≥75%) and to a lesser extent from the involved OH reaction. The energy consumption of UV/chlorine to achieve a 90% ETA abatement ranged from 0.22 kWh/m3 ([ETA]0 = 0.025 mM) to 3.59 kWh/m3 ([ETA]0 = 10 mM). Ammonia, formaldehyde, and glycolaldehyde were the major degradation products of the UV/chlorine treatment of ETA, which could have formed through the photolysis of N–Cl bonds forming aminyl radicals and their transformation via a 1,2-hydrogen shift and β-scission. Overall, this study provides useful kinetic and mechanistic information for designing an effective UV/chlorine process for treating ETA wastewater.