Mechanism and products of the photolysis of hexabromocyclododecane in acetonitrile–water solutions under a UV-C lamp

Abstract

The photolysis of hexabromocyclododecane (HBCD), a new persistent organic pollutant (POPs), under a UV-C lamp (λmax 220–260nm) in a mixed solution of acetonitrile–water was investigated to clarify the contribution of direct and indirect photolysis on the decomposition of HBCD and the photolysis products. The results show that the photolysis reaction follows the pseudo-first order kinetics. Isopropanol inhibited the photolysis by scavenging hydroxyl radicals (HO), from which the contributions of direct and indirect photolysis to HBCD decomposition were determined to be 85.7% and 14.3%, respectively. Using a competitive reaction with Rhodamine B in a photo-Fenton system, the second-order rate constant of HBCD with HO was estimated to be 2.48±0.23×108Lmol−1s−1. Moreover, the effect of singlet oxygen (1O2) was investigated through a scavenging experiment (using NaN3 as scavenger) and a 1O2 generated experiment by using indirect photolysis of methylene blue. Results show that 1O2 cannot degrade HBCD directly, but has an indirect influence on photolysis of HBCD through reacting with the final debrominated products of HBCD, cyclododecatriene. Products of sequential debromination (tetrabromocyclododecene, dibromocyclododecadiene, and cyclododecatriene), and 1O2 oxidation products (dibromo-epoxy-cyclododecene and epoxy-cyclododecadiene) during photolysis were identified. Both direct photolysis and indirect photooxidation by HO and 1O2 should be taken into account when HBCD is exposed to UV-C light. The quantum yields for photolysis of HBCD were measured at wavelengths of 224, 240 and 254nm, which were 0.22, 0.15 and 0.033, respectively.