TiO2 photocatalytic degradation of the flame retardant tris (2-chloroethyl) phosphate (TCEP) in aqueous solution: A detailed kinetic and mechanistic study

摘要

The presence of halogenated organophosphate flame retardants in natural water systems is a widespread concern because of their potential threat to human health and the environment. TiO2 photocatalysis at 350 nm leads to rapid degradation of tris (2-chloroethyl) phosphate (TCEP). The observed degradation follows pseudo-first-order kinetics at a specific concentration. The apparent rate constants varied from 0.28 to 0.03 min(-1) depending on the initial concentrations over the range of 18-270 mu M, indicating the heterogeneous degradation process is likely controlled by mass transfer (adsorption <-> desorption) at the surface of TiO2. The degradation kinetics also fit the Langmuir-Hinshelwood model with apparent kinetic parameters of 0.03 mu M-1 and 13.1 mu M min(-1) for the apparent equilibrium constant (K-LH) and the reactivity constant (k(rxn-LH)) respectively. H-1- and P-31-NMR studies indicate sequential oxidation of the alkyl ester chains (alkyl phosphate), initially leading to the diester product, followed by the formation of the monoester and ultimately producing phosphate. Under strongly alkaline conditions the degradation is enhanced, from pH 4 to 9 the degradation is relatively constant, while under highly acidic conditions the degradation is inhibited. Effective mineralization is achieved as demonstrated by excellent chloride (98%) and phosphate (94%) mass balances as well as the loss of total organic carbon (TOC) > 95%. The addition of an equal molar amount of the hydroxyl radical scavenger, coumarin, leads to pronounced reduction in degradation indicating hydroxyl radicals mediate the degradation process. These results demonstrate TiO2 photocatalytic oxidation has promise for the treatment of aqueous solutions contaminated with organophosphate flame retardants.