Sonochemical Decomposition of Perfluorinated Surfactants: Chain Length Effects

Citation

Campbell, Tammy Y. (2010) Sonochemical Decomposition of Perfluorinated Surfactants: Chain Length Effects. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/HDZ3-X116. https://resolver.caltech.edu/CaltechTHESIS:05232010-212126893

Abstract

The sonochemical degradation kinetics of the aqueous perfluorochemicals (PFCs) perfluorobutanoate (PFBA), perfluorobutanesulfonate (PFBS), perfluorohexanoate (PFHA) and perfluorohexanesulfonate (PFHS) have been investigated. Surface tension measurements were used to evaluate chain-length effects on equilibrium air-water interface partitioning. The PFC air-water interface partitioning coefficients, Κ^(PF)_(eq), and maximum surface concentrations, Γ^(PF)_(max), were determined from the surface pressure equation of state for PFBA, PFBS, PFHA and PFHS. Relative Κ^(PF)_(eq) values were dependent upon chain length Κ^(PFHS)_(eq) ≅ 2.1K^(PFHA)_(eq) ≅ 3.9K^(PFBS)_(eq) ≅ 5.0K^(PFBA)_(eq), whereas relative Γ^(PF)_(max) values had minimal chain length dependence Γ^(PFHS)_(max) ≅ Γ^(PFHA)_(max) ≅ Γ^(PFBS)_(max) ≅ 2.2Γ^(PFBA)_(max). The rates of sonolytic degradation were determined over a range of frequencies from 202 to 1060 kHz at dilute (< 1 μM) initial PFC concentrations. Under all conditions, the time-dependent PFC sonolytic degradation was observed to follow pseudo-first order kinetics suggesting bubble-water interface populations were below saturation. The PFHX (where X = A or S) sonolysis rate constant was observed to peak at an ultrasonic frequency of 358 kHz, similar to PFOX. In contrast, the PFBX degradation rate constants had an apparent maximum at 610 kHz. Degradation rates observed for PFHX are similar to previously determined PFOX rates, k^(PFOX)_(app,358) ≅ k^(PFHX)_(app,358). PFOX is sonolytically pyrolyzed at the transiently cavitating bubble-water interface suggesting that rates should be proportional to equilibrium interfacial partitioning. However, relative equilibrium air-water interfacial partitioning predicts that K^(PFOX)_(eq) ≅ 5K(PFHX)_(eq). This suggests that at dilute PFC concentrations, adsorption to the bubble-water interface is sonochemically mediated. PFC sonochemical kinetics are slower for PFBS and further diminished for PFBA as compared to longer analogs suggesting that PFBX surface films are of lower stability due to their greater water solubility. Furthermore, application of a Langmuir type kinetic model on the basis of a heterogenous solution system is applied in evaluating the sonochemical effects on surface activity and the absolute rates over four orders of magnitude of initial PFYX (Y = H or B, X = A or S). Investigations on the effects of ultrasound power densities at 83.3, 167, 250, and 333 W L^(-1) have been carried out. Rate constants exhibit a linear increase with increasing power density for all species, thus rates can be increased by simply increasing the power density. Degradation rates are compared at single frequency exposures of 202, 358, and 610 kHz and compared to dual frequency exposures at 20 + 202 and 20 + 610 kHz under the same power conditions (250 W L^(-1). A synergistic enhancements in degradation rates for PFOS (~12%) and PFOA (~23%) were observed for 20 + 202 kHz simultaneous exposure. Frequency and power are parameters that can affect the sonochemical efficiency by modifying peak collapse temperature and by controlling the size and population of transiently cavitating bubbles.

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