Investigation of Ultrasonically Induced Degradation of Tris(2-chloroethyl) Phosphate in Water

Abstract

The widespread use of hazardous organophosphate ester (OPE) flame retardants has led to the contamination of groundwater and drinking water sources. Given the negative impact of OPEs on environmental and human health, there is a critical need to identify effective remediation processes. This study reports that ultrasonic irradiation at 640 kHz leads to effective degradation of tris(2-chloroethyl) phosphate (TCEP), a model organophosphate flame retardant. The concentration of TCEP in an irradiated aqueous solution was monitored by a gas chromatography- nitrogen phosphorus detector (GC-NPD) technique. TCEP has a half-life of less than 1 h under the used experimental conditions. The degradation follows pseudo-first-order kinetics with rate constants varying from 0.09 to 0.02 min−1 depending on initial concentrations ranging from 3.1 to 84 μM. The observed rate constant decreases with the increase in initial TCEP concentration, implying the process likely involves a heterogeneous process controlled by partitioning at the gas–liquid interface during ultrasonic cavitation. The degradation fits the heterogeneous Langmuir-Hinshelwood model, further suggesting the degradation occurs at the gas–liquid interface. Detailed product studies using liquid chromatography orbitrap high-resolution mass spectrometry confirm the primary degradation products are the mono and diester adducts of TCEP, specifically 2-chloroethyl dihydrogen phosphate and bis(2-chloroethyl) hydrogen phosphate. Mineralization of TCEP to chloride and phosphate was monitored by ion chromatography, yielding mass balances of 48% and 32% for chloride and phosphate, respectively, after 6 h of treatment. The results demonstrate that ultrasonic irradiation is effective for the degradation of the halogenated flame retardant TCEP. The results suggest ultrasonic treatment can be used alone or in combination with other methods for the remediation of problematic organophosphorus flame retardants.