Environ. Sci. Technol. 1997, 31, 2761-2764
Influence of Water Solubility, Side-Chain Degradability, and Side-Chain Structure on the Degradation of Phthalic Acid Esters under Methanogenic Conditions JO ¨ RGEN EJLERTSSON,* MAGNUS ALNERVIK, SUSANNE JONSSON, AND BO H. SVENSSON Department of Water and Environmental Studies, University of Linko¨ping, S-581 83 Linko¨ping, Sweden
Water solubility and the degradability of side chains estrifying phthalic acid are some of the factors that could influence the degradation of phthalic acid esters (PAEs). To assess the importance of these factors, the degradation of butyl 2-ethylhexyl phthalate (BEHP), bis(2-ethylhexyl) phthalate (DEHP), dibutyl phthalate (DBP), dihexyl phthalate (DHP), dioctyl phthalate (DOP), and didecyl phthalate (DDP) as well as the alcohols estrifying these PAEs was examined using a methanogenic butylbenzyl phthalate (BBP)degrading enrichment culture. We also set out to determine whether the degradation of resistant PAEs could be stimulated by the addition of a PAE known to be degradable. The investigation strongly indicates that water solubility is a major factor limiting degradation of hydrophobic PAEs. In the study conducted, PAEs having high water solubility (DBP, BBP, BEHP, and DHP) were degraded, whereas those having low water solubility were not (DEHP, DOP, and DDP). The investigation also showed that all alcohols, commonly used in PAEs, were degraded to methane and carbon dioxide. It also seems possible that anaerobic degradation of persistent PAEs may be stimulated by organisms in cultures degrading less resistant phthalates.
Introduction Esters of 1,2-benzenedicarboxylic acid (phthalic acid esters, PAEs, phthalates) comprise a group of organic compounds used in large quantities by present day society. The worldwide production of PAEs was estimated to be ca. 4.2 × 109 kg during 1994 (A.-L. Rykfors, Neste Oxo AB, personal communication) and has increased by roughly 50% during the last 20 years (cf. ref 1). PAEs are mainly used as plasticizers in PVC plastics and may constitute up to 67% of their total weight. They are also used in a variety of other products such as cosmetics, ammunition, inks, etc. (2). Due to their broad range of applications, PAEs are ubiquitous environmental pollutants. In 1975, the rate of PAEs entering the environment was estimated at approximately 2.3 × 107 kg annually as a result of leaching from plastic wastes and the direct application of various formulations (3). Bis(2-ethylhexyl) phthalate (DEHP) is the most commonly used PAE, accounting for approximately 40-50% of the global annual PAE production (A.-L. Rykfors, Neste Oxo AB, personal communication). Studies on the aerobic degradation of PAEs accelerated after 1972, owing to doubts about their degradability and * Corresponding author.
S0013-936X(96)01055-3 CCC: $14.00
1997 American Chemical Society
concerns regarding their accumulation in the environment. In 1973, Saeger and Tucker (4) reported on the aerobic degradation of PAEs in activated sludge, and since then, numerous studies have shown that PAEs can be transformed by inoculates from various aerobic environments (cf. refs 5-14). Under anaerobic methanogenic conditions, the capacity for PAE transformation appears to vary among the habitats investigated and the PAEs studied. Some PAEs were shown to be degraded by sewage sludge inoculates, whereas others were of more persistent nature (1, 15, 16). Similar observations were made by Ejlertsson et al. (17) with landfilled municipal solid waste (MSW) and MSW treated in a biogas digestor as inoculates. Previous studies on the degradation of PAEs have shown that it starts with a hydrolysis of the ester bond under both oxic and anoxic conditions (cf. refs 6, 16, 18). Kurane et al. (18) showed that the hydrolytic enzymes from an isolate degrading DEHP aerobically had a low specificity, i.e., they were able to hydrolyze several PAEs as well as olive oil. The aim of this study was to assess the influence of various factors on the degradability of PAEs. The following questions were addressed: (1) does the degradability of the alcohol(s) estrifying phthalic acid limit the anaerobic degradation of PAEs, (2) does the use of branched alcohols for estrifying phthalic acid limit the degradation, and (3) is the anaerobic degradation of PAEs regulated by their solubility in water. We also set out to ascertain whether the degradation of a resistant PAE (DEHP) could be stimulated by feeding it to active methanogenic cultures degrading butylbenzyl phthalate (BBP) and butyl 2-ethylhexyl phthalate (BEHP), respectively.
Experimental Section To determine how the suitability of PAE-estrifying alcohol as a substrate influences anaerobic PAE degradation, the degradation of six alcohols commercially used in PAE synthesis was studied. The effects of using branched side chains were followed in degradation experiments with BEHP and DEHP, each having one and two branched alcohols, respectively. The effect of their solubility in water was studied using PAEs with different hydrophobicities [water solubility according to Staples (19) is given in milligrams per liter within parentheses]: di-n-butyl phthalate (DBP; 11.2), BBP (2.7), BEHP (0.11), di-n-hexyl phthalate (DHP; 0.05), DEHP (0.003), di-n-octyl phthalate (DOP; 0.0005), and di-n-decyl phthalate (DDP; no water-solubility data found). To evaluate the possibility of triggering degradation of resistant phthalates, DEHP and BBP were given to a methanogenic culture actively degrading BBP. The triggering effect was also studied with a PAE mixture of BEHP, DEHP, and DBP. PAE and alcohol degradation were measured as the difference in methane production between triplicate experimental bottles and triplicate control bottles. Added substrate, equivalent to 1 mM (except for octanol, which was 1.3 mM), was considered to be completely degraded if more than 80% of the amount of methane expected from a stoichiometric conversion to methane and carbon dioxide was obtained. To assess the degradation of PAEs and formation of intermediates, incubated experimental bottles were frozen periodically for extraction and analysis later on. The monoesters used to confirm formation of possible intermediates during PAE degradation were synthesized by Ejlertsson et al. (17). Chemicals and Gases. 2-Ethyl hexanol (2-EH), benzyl alcohol (BeOH), benzyl 2-ethylhexyl phthalate (BEHP), ndecanol (DeOH), di-n-decyl phthalate (DDP), di-n-hexyl phthalate (DHP), di-n-octyl phthalate (DOP), n-hexanol (HeOH), mono-2-ethylhexyl phthalate (MEHP), and n-octanol (OcOH) were gifts from Neste Oxo AB, Stenungsund (Sweden).
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The purity of the compounds supplied was at least 96% except for BEHP (73%), which, due to the synthesis process, also contained DBP (20%) and DEHP (7%). Yeast extract (Oxoid) and all other chemicals (Merck products) were purchased from Kebo, Stockholm (Sweden). The gases used for medium preparation (N2,
