On-line measurement of oxidative degradation kinetics for trace gasoline contaminants in aqueous solutions and natural water by membrane introduction tandem mass spectrometry

Membrane introduction mass spectrometry (MIMS) was used to directly monitor the loss of trace gasoline contaminants (benzene, toluene, 2-methylthiophene and methylcyclohexane) in nanomolar (ppb) aqueous solutions under a variety of UV-induced advanced oxidation processes (AOP). The decay kinetics of these contaminants were followed simultaneously in "real-time" via tandem mass spectrometric techniques by re-circulating the reaction mixture in a closed loop over a semi-permeable membrane interface. The photocatalyzed degradations were observed to follow pseudo-first-order kinetics with rate constants ranging from 0.006 to 0.2 min⁻¹ depending on the reaction conditions. We report rate enhancements for several UV-based advanced oxidative processes using physiosorbed titanium dioxide (TiO₂/UV, TiO₂/UV/O₂, TiO₂/UV/H₂O₂) and compare these to the direct photolysis of H₂O₂ under otherwise identical conditions. The relative degradation rates of 4 trace contaminants are reported for reactions carried out in the same solution. The degradation kinetics were also monitored directly in a natural surface water spiked with the same contaminant suite. The observed decay kinetics in the presence of TiO₂ in air-saturated natural water were similar to those carried out in deionized water. However, when the photo-oxidation was enhanced by the addition of H₂O₂, the degradation was markedly slower in natural water relative to deionized water due to competition for photons by dissolved organic matter. This work further demonstrates the use of MIMS as a sensitive on-line measurement technique for "in-situ" reaction monitoring of organic contaminants at environmentally relevant concentrations in complex solutions and reactive media.