The photodecay of TCE in surfactant solutions with and without the presence of acetone and additional hydrogen sources

Abstract

Various clean-up technologies have been developed for the removal and/or destruction of trichioroethene (TCE) in subsurface systems. Several studies have shown that surfactant flushing followed by a photodegradation process could be a promising approach for degrading the chlorinated compounds. Therefore, the photodegradation of TCE in surfactant micelles has been investigated. The decay of TCE was studied in the Rayonet RPR-200 merry-go-round photoreactor at 253.7 inn monochromatic ultraviolet (UV) lamps in the presence of surfactants. Surfhctants were used to improve the photodegradation rates of TCE. About three times of rate increment is observed in the presence of Brij 35 surfactant micelles than that in water alone. A lag phase is observed at the commencement of the degradation, but the duration of the lag phase is significantly reduced as the initial pH increases. The increasing concentrations of H+ and Cl- indicate that they are the final products of TCE photodegradation (i.e., photodechiorination is the dominant mechanism in this system). Photodecomposition of TCE in surfactant micelles is also proven to be a clean and effective process. It generates no chlorinated byproducts nor intermediates during the process, and TCE can be filly decomposed within a reasonable time. The effects of photosensitizer (acetone, ACE) and hydrogen sources (tnethylainine, TEA and humic acids, HA) in the TCE photodecay were examined Quantum yields of TCE decay in solution with surfhctant Brij 35 and optimum ACE dosage is about 25 times higher than that in Brij 35 alone. However, with an excess ACE dosage, ACE will act as a light barrier, which attenuates the light intensity available for TCE photodegradation. The investigation of different initial pH levels reveals that the percentage removal and the duration of lag phase are pH dependent. Apart from the direct photodegradation, photosensitization is postulated to be another major pathway contributing to the overall decay. The addition of TEA in solution containing fixed amount of TCE, Brij 35 and ACE could also speed up the reaction, but similar to ACE, overdosing of TEA would quench the reaction. HA causes a slight rate increment at low HA concentration while at high HA dosage, the photodecay of TCE will be quenched. The quenching effect of HA may be due to the quantum yield reduction by the attenuation of the incident light through the reactor when large amount of HA was added. The possible photoreduction mechanisms of TCE in ACE/TEA and HA were proposed. Two models were derived and the reaction at both low and high HA concentrations were described.