Heterogeneous Photocatalytic Decomposition of Perfluorocarboxylic Acids (PFCAs): Mechanism and Efficiency Studies
Date Issued
2010
Date
2010
Author(s)
Chandana Panchangam, Sri
Abstract
The increasing worldwide contamination of environmental matrices with numerous industrial chemicals poses a significant threat to both aquatic and human life. Fluorine containing organics like perfluorochemicals (PFCs) are of significant environmental concern due to their persistent and bioaccumulative characteristics. The persistence of PFCs is due to the greater stability offered by the extremely strong carbon-fluorine bonds. PFCs resist several conventional treatment processes in the drinking water and wastewater treatment plants and demands more refined treatment techniques.
Advanced oxidation processes (AOPs) such as sonication, photocatalysis, ozonation, peroxone etc. are promising treatment technologies that have shown to be efficient in decomposing several organic contaminants. Before realizing the full advent of these processes towards the decomposition of PFCs, the complete mechanism of decomposition, treatment parameters like pH, and the energy requirements are need to be critically studied.
This study explores the capabilities of heterogeneous photocatalysis and combination of sonication and photocatalysis towards the decomposition of PFCs. Perfluorooctane sulfonate (PFOS) was found to be non-susceptible to the oxidative decomposition by photocatalysis and therefore further efficiency and mechanism studies were carried out on perfluorocarboxylic acids (PFCAs). Among PFCAs, perfluorooctanoic acid (PFOA) was chosen as a model compound for all the background and mechanism studies. Loss of PFOA and PFOS was not found due volatilization and adsorption onto glass or TiO2 but only a minor loss was observed during direct photolysis. Efficient decomposition of three important PFCAs such as PFOA, perfluorononanoic acid (PFNA) and perfluorodecanoic acid (PFDA) is studied by heterogeneous photocatalysis with TiO2 as a photocatalyst in acidic aqueous solutions. The PFCAs were decomposed into shorter carbon chain length PFCAs and fluoride ions. Photoholes of excited TiO2 generated upon UV-irradiation are found to be the oxidation sites for PFCAs. Therefore, creation and sustenance of these photoholes in the acidic aqueous medium has enhanced the decomposition of PFCAs. Heterogeneous photocatalytic treatment achieved more than 99% decomposition and 38% complete mineralization of PFOA in 7 h. The decomposition of other PFCAs was as high as 99% with a defluorination efficiency of 38% for PFDA and 54% for PFNA. The presence of perchloric acid was found to enhance the decomposition by facilitating the ionization of PFCAs. The oxygen present in the medium served both as an oxidant of perfluoroalkyl radicals and an electron acceptor. The mechanistic details of PFCA decomposition and their corresponding mineralization are elaborated. The PFCAs photocatalytic decomposition involves predominantly four steps: ionization, electron transfer, decarboxylation and oxidation. Initially PFCAs are ionized in the water and the corresponding anions will donate electron to the excited TiO2 forming a perfluoroperoxy radical. This radical undergoes immediate decarboxylation transforming to perfluoroalkyl radical. The perfluoroalkyl radicals thus generated are oxidized by either superoxide or hydroxyl radicals resulting in PFCA with one less carbon releasing two fluorides. This decomposition mechanism follows a cyclic order until all the fluoride ions are stripped off from the perfluorinated carbons. Mean while the oxygen being supplied to the system will act as electron acceptor and keeps the catalytic cycle of TiO2 active.
Further, decomposition of PFOA was studied by a combination of sonication and photocatalysis in two different strategies. At first, photocatalysis was coupled with half-an-hour of sonication which is called as sonication-assisted photocatalysis. Another strategic way of combination was concurrent use of sonication with photocatalysis called sonophotocatalysis. Sonication-assisted photocatalysis with commercial TiO2 (RdH) and home-made TiO2 (sol-gel) at ambient temperature, pressure and near neutral pH was studied for the decomposition of PFOA. The process was efficient to decompose PFOA into fluoride ions and to several perfluorinated carboxylic acids with a shorter carbon chain length such as perfluoroheptanoic acid (PFHpA), perfluorohexanoic acid (PFHxA), perfluoropropanoic acid (PFPA), and trifluoroacetic acid (TFA). The efficiency of sonication-assisted photocatalysis was found to be 64%. In all the cases, higher efficiencies were obtained when sol-gel TiO2 was used as a photocatalyst than the commercial RdH TiO2 catalyst. The specific surface area is three times higher for sol-gel TiO2 than commercial RdH TiO2 and appears to be the probable reason for the observed differences in the corresponding efficiencies. It is also interesting to note that pH plays a determining role in the decomposition of PFOA and correspondingly photocatalyses were carried out under different controlled pH. Perfluoroalkyl radicals are presumably oxidized by superoxide and hydroxyl radicals generated during the TiO2-mediated photocatalysis at pH 4 and 10, respectively. The role of sonication in sonication-assisted photocatalysis was construed to be an aid to photocatalysis than a tool itself. Sonication enhances photocatalysis through physical dispersion of TiO2 and eases mass transfer which keeps on rejuvenating the TiO2 surface.
Sonophotocatalysis was shown to be a promising treatment technology to decompose PFOA and PFDA. Significant synergistic efficiency of about 58% was observed when sonication is combined with photocatalysis. The kinetic rate constant was found to decrease with increase in PFOA initial concentration. The energy required by sonophotocatalysis was also three times less than the sum of energy required by individual processes. From energy requirement comparisons and efficiency wise, heterogeneous photocatalysis and combinatorial process (sonication-assisted and sono- photocatalysis) emerge to be viable treatment techniques as demonstrated in this study.
Subjects
Heterogeneous photocatalysis
perfluorooctanoic acid
perfluorocarboxylic acids
decomposition
mechanism
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