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a TUHH Technologie GmbH (TuTech) Integrated Management, Schellerdamm 4, D-21079 Hamburg, Germany
b Limnological Institute Dr. Nowak, Mayenbrook 1, D-28870 Ottersberg, Germany
c Univ. of Linköping, Dep. of Water and Environmental Studies, S-58183 Linköping, Sweden
* Corresponding author (mersiowsky{at}tutech.de)
Received for publication July 14, 2000.
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Abbreviations: DBT, dibutyltin • DHT, diheptyltin • DMT, dimethyltin • DOT, dioctyltin • MBT, monobutyltin • MHT, monoheptyltin • MMT, monomethyltin • MOT, monooctyltin • MSW, municipal solid waste • PEC, predicted environmental concentration • PNEC, predicted no-effect concentration • TBT, tributyltin • TePT, tetrapropyltin • TOC, total organic carbon • TPT, tripropyltin • VFA, volatile fatty acids
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Organic metal compounds comprise a variety of species: alkylated metal species may be present in landfill leachate, and volatile methylated metal species may be found in gaseous emissions from waste deposits (Feldmann and Hirner, 1995). In particular, organotin species are scrutinized in risk assessments due to their widespread use as industry chemicals and their possible toxicological relevance (Summer et al., 1996). Trialkyltin compounds are used as biocides in antifouling paints and other preservatives for roof linings, wood, textiles, etc. The predominant application for nonbiocidal mono- and dialkyltin compounds is stabilizers for PVC products (OSPARCOM, 1997).
However, the knowledge about product-specific emissions from sanitary landfill sites is insufficient. In order to assess the potential risk to soil and ground water arising from seepage of contaminated landfill leachate, the monitoring of target substances in original samples is required. These findings could then possibly be attributed to specific waste materials, and predicted environmental concentrations (PEC) could be derived.
The aim of the present study was to perform a screening investigation regarding the occurrence of organotin compounds in the leachates of various European landfill sites. The results are discussed with respect to possible evaluation approaches and need for research. The selected target compounds are monomethyltin (MMT), dimethyltin (DMT), monobutyltin (MBT), dibutyltin (DBT), tributyltin (TBT), monooctyltin (MOT), and dioctyltin (DOT).
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MATERIALS AND METHODS |
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After the field blank had been obtained, one 1000-mL polyethylene bottle was filled with leachate from the sampling point. The temperature was measured directly in this leachate sample by means of a digital field thermometer equipped with a probe. Directly after the temperature measurement, the pH was measured using a pH meter (Model E50; WTW, Hamburg, Germany). The measurement procedure was based upon German Standard Method DIN 38404 C5.
Finally, the samples for the speciation of organotin compounds were obtained. The leachate samples were stored in 1000-mL polycarbonate bottles, filling the containers to nominal capacity, thus leaving some headspace with respect to the intended freezing. Leachate storage tests showed these bottles to be free of any detectable background contamination with organotin compounds. In order to prevent deterioration, all samples were kept in dark and cool storage and immediately delivered to the laboratory, if at all possible (i.e., unless transboundary shipping was necessary). Upon reception at the lab, samples for trace analysis were immediately frozen, while standard leachate analyses were conducted right away.
Selection of Landfill Sites for the Sampling Program
Landfill Sweden B
The landfill Sweden B is situated in central Sweden and receives municipal solid waste from a suburban area. For reasons of ongoing research at the site, separate cells had been constructed and were equipped with their own gas collection systems. The leachate is collected and recirculated.
The capacity of each cell corresponds to the waste generated in the area during one year. Hence it was possible to sample cells from consecutive years: four leachate samples were taken (waste from 1995, 1996, and 1997), including one mixed sample of 1995–1996. The samples Sweden B 1995 and Sweden B 1996 were obtained from the gas drains of the respective cells, the mixed sample Sweden B 1995–1996 from the connected leachate collection of these two cells, and the sample Sweden B 1997 from the leachate collection of that cell.
Landfill Germany A
The landfill Germany A is located in northern Germany and receives household and commercial waste from the surrounding municipality. The landfill consists of an old sector operated from 1958 until 1986, which was then partly excavated and expanded by a new sector currently being filled. A base liner and drainage system were installed, but some cells do not possess a top cover yet due to ongoing filling.
The landfill was sampled twice at different locations, and in both cases the samples were scooped from an open access shaft. The sample Germany A 1 was obtained in winter at the storage lagoon of the treatment plant receiving leachate from the whole landfill; the bulk volume flow originates from the not-yet-closed cells, which are currently being filled. The sample Germany A 2 was obtained in early autumn from a collection well receiving leachate from only one recent landfill cell. The cell was just being filled at that time and contained waste of a maximum age of 2 yr. The first sampling took place at the central leachate collection.
Landfill Germany B
The closed landfill Germany B is located in northern Germany and was operated from 1982 until 1992, having received solid waste from the surrounding municipalities, including household, commercial, and construction–demolition waste as well as sewage sludge. A base lining system is present, and a top cover was constructed during the years 1990–1995. The leachate is collected and about half the volume is recirculated; consequently, the content of moisture in the landfill body was supposed to be rather high and evenly distributed. The sample Germany B was obtained by scooping from a peripheral storage tank receiving leachate from the whole landfill.
Landfill Germany C
The closed landfill Germany C is situated in a hilly rural area in southern Germany and was operated by a PVC manufacturer during 1967 until 1971. The landfill served solely as a deposit for production residues originating from the compounding of PVC foils, wallpapers, and plastisols as well as cans of solvent residues. A simple drainage system is present, and leachate is stored in a collection well. There is no base liner and only cover soil for capping, nowadays being used for agriculture. The sample Germany C was obtained by scooping from the collection well.
Landfill Germany D
The landfill Germany D is situated in a rural area of northern Germany. The sampled sector was commenced in 1982 and is still operated. The landfill receives solid waste from the surrounding municipality, a mixture of household waste, commercial waste, sewage sludge, and construction–demolition waste. The site is equipped with a base liner and an intermediate top cover of gravel and soil, and the leachate is collected in several wells along the outer edge of the landfill. The sample was obtained by drawing from a sampling tap at the pumping station where the raw leachate enters the treatment plant.
Landfill Germany E
The landfill Germany E is located in a rural area of northern Germany. The sampled sector was operated from 1992 until 1996 and had been closed since; it received solid waste from the surrounding municipality; a mixture of household waste and commercial waste as well as a certain amount of sewage sludge. A leachate collection system is present, but there is as yet only an intermediate top cover consisting of a geotextile and bitumen, which currently does not sufficiently block infiltration. The sample was drawn from a sampling tap at the pumping station where the raw leachate enters the treatment plant.
Landfill Italy A
The landfill Italy A is located in a suburban area of northern Italy. Operation was commenced in late 1987, and the site receives municipal solid waste from the area of the nearby city. A base liner is present, and an intermediate top cover has been constructed from compost and geotextiles. The leachate is collected in wells, then pumped into a storage tank above ground. The sample was obtained by tapping from this storage tank, by necessity using a rubber tube attached to the outlet. Both Italian landfills were sampled in early autumn, with ambient temperatures and leachate temperature being correspondingly high.
Landfill Italy B
The landfill Italy B is located on a southern Italian island in a rural region. The operation of this site commenced in 1992, receiving solid waste from the area of the neighboring municipality and a major city as well as sewage sludge. One half of sampled sector was already closed and covered, whereas the other was currently being filled. The leachate is collected in one peripheral well and then recirculated to be irrigated in ponds on the top surface of the landfill, thus preventing any discharge of leachate into municipal treatment facilities. The sample was obtained from this well by scooping.
Chemicals
All solvents were of purity "for organic trace residue analysis" grade by Merck (Darmstadt, Germany). Furthermore, the following reagents were employed: sodium tetraethyl borate (NaBEt4, 98%) by Alfa (Karlsruhe, Germany); all organotin compounds as chlorides: monomethyltin (MMT), monobutyltin (MBT), dibutyltin (DBT), tributyltin (TBT) at purity >95% by Aldrich (Deisenhofen, Germany), monoheptyltin (MHT), diheptyltin (DHT), monooctyltin (MOT), dioctyltin (DOT), tetrapropyltin (TePT) at purity >99% by Witco (Bergkamen, Germany), tripropyltin (TPT) at purity >95% by Alfa; potassium hydroxide (KOH) granules (p.a.) at purity >85%, silica gel 60 (0.063–0.200 mm) and aluminium oxide 90 active neutral for column chromatography, silver nitrate (AgNO3) (p.a.) at purity >99.8%, and sodium sulfate (Na2SO4) water-free/granulated (pro analysi grade) (0.25–2 mm by Merck.
Speciation of Organotin Compounds
The species-selective analysis of organotin compounds were performed in accordance with an emerging German standard method (Draft DIN 38407-13, "Determination of selected organotin compounds by means of gas chromatography (F 13)", as of October 1999), which was adapted to the specific problem at hand by modifying the sample preparation and extraction. Since landfill leachates are rich in suspended solids, colloids, and solute organic matter, the target organotin compounds are probably sequestered, adsorbed, or incorporated. During pre-experiments an optimization was accomplished by destroying the organic leachate matrix, thus enhancing the solubility and availability of the target organotin compounds and performing an additional cleanup step for the removal of sulfur compounds. These modifications in comparison with the draft standard method DIN 38407-13 are described in more detail below.
After storage at -20°C, the frozen samples were allowed to thaw at room temperature during 16 h. A quantity of 100 mL of leachate was then transferred by weighing into a 250-mL bottle (Type NS 19/26, DIN 12039). In order to break up the organic leachate matrix and release the organotin compounds to be available for complete derivatization and extraction, 6 mL of a 25% potassium hydroxide solution in methanol (KOH–MeOH) and 10 mL methanol were added, stirring vigorously for 2 h. Higher extraction efficiencies had been confirmed by the pre-experiments. The pH value was adjusted to 4.5 by the addition of 5 mL glacial acetic acid, followed by the addition of 1.0 mL of the internal standard solution during vigorous stirring for 20 min. The internal standard contained tripropyltin (TPT), mono- and diheptyltin (MHT, DHT), and tetrapropyltin (TePT), dissolved in acetone at concentrations of 0.1 mg/L of each organotin cation.
For in situ derivatization and liquid extraction, 20 mL n-hexane and 3 mL of the derivatization agent sodium tetraethyl borate (NaBEt4, 20%) were added, stirring for 16 h. Then the hexane phase was retrieved, dried above 2 g Na2SO4, and evaporated (50°C, 450 hPa) thus concentrating to 2 mL, in accordance with the draft standard method DIN 38407-13. The standard method calls for a cleanup of extracts on silica gel. A 15-mL glass column was filled with 3 g silica gel and rinsed with 20 mL n-hexane. In order to remove sulfur compounds, a layer of 1.5 g aluminum oxide covered with silver nitrate (Al2O3–AgNO3) was added to the silica. A solution of 90 g Al2O3 and 10 g AgNO3 in 40 mL water had been heated (50–120°C in 5 h) and activated (125°C, 16 h). The extract was applied to the silica column by means of a Pasteur pipette, then eluted by 15 mL n-hexane and subsequently by 10 mL of a 5% acetone and n-hexane mixture. This additional cleanup step was introduced because the presence of sulfur compounds had been found to be impairing the quality of the chromatograms. The cleaned extracts must not be concentrated to dryness due to the volatility of tetraalkyltin species. The sensitive species-selective detection of organotin compounds relied on gas chromatography separation and detection on a flame-photometric detector (GC–FPD) with an interference filter selective for tin (Table 2). From the peak areas, concentrations were calculated by means of a six-point calibration, using the internal standards. With respect to similar substance properties, the internal standard TPT was used to calculate MMT, DMT, MBT, DBT, and TBT; whereas the internal standard DHT was used to calculate MOT and DOT. The other two compounds in the internal standard were used to check the completeness of derivatization (MHT) and extraction (TePT). Quality assurance tests were conducted by including one blank control (demineralized water) and one standard solution as positive control in each series of leachate samples, imposing criteria for error margins.
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For the German and Italian samples, the volatile fatty acids (VFA) were quantified as a sum parameter by vapor distillation and subsequent titration in accordance with German Standard Method DIN 38409 H21. For the Swedish landfill Sweden B, the VFA were calculated as the total of the range from acetate through caproate as analyzed by gas chromatography and flame ionization detection (GC–FID) according to Ejlertsson et al. (unpublished data, 2000).
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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The findings of organotin compounds were generally close to or below the detection limit, with the exception of cell B 96 showing monobutyltin at 1.2 µg/L and di- and tributyltin also being detectable (Fig. 2).
Germany
All of the samples from the landfill sites Germany A to Germany E showed slightly alkaline pH values. Correspondingly, the contents of TOC and VFA were rather low (approximately 1000 mg/L and 200 mg/L or less, respectively), with the exception of the very fresh leachate from the recently filled cell (A 2) that shows the highest organic load at VFA 8600 mg/L and TOC 9400 mg/L (Table 5).
It is noteworthy that the two samples from landfill site Germany A showed the maximum concentrations of monobutyltin of this sample series, at 1.9 and 4.1 µg/L, respectively. Likewise, the sample A 2 had the highest findings of octyltin compounds, with MOT at 1.7 and DOT at 0.8 µg/L, respectively. Of all investigated landfills, the sample from Germany D showed the highest concentration of tributyltin at 0.9 µg/L (Fig. 2).
Italy
The alkalinic pH values of these two leachate samples constitute the upper boundary of this investigation (Table 5). Otherwise, the leachate matrix was rather comparable with the samples obtained in Germany.
The samples Italy A and Italy B showed the maximum findings of methyltin compounds throughout the investigation, with MMT at 0.6 µg/L (A) and DMT at 0.5 µg/L (B), respectively. Octyltin compounds were also present in the sample Italy A at concentrations around 0.5 µg/L. This is remarkable in that otherwise only sample Germany A 2 showed distinct findings of octyltin compounds (Fig. 2).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Of all target compounds, monobutyltin may be considered ubiquitous as it was detected in nearly all of the samples and showed the overall maximum findings of 4 µg/L. Otherwise, the concentrations of mono- and dialkyltin compounds rarely exceed 1 to 2 µg/L. Tributyltin is seldom found at concentrations below 1 µg/L. In a very conservative approach, the predicted environmental concentrations (PEC) for all investigated organotin species, as indicated by the median and mean values of the raw leachate, would range from below the detection limit (0.1 µg/L) up to 1 µg/L, with maxima for MBT and MOT at 4 and 2 µg/L, respectively.
For a preliminary risk assessment, these concentrations in the raw undiluted leachate are compared with the predicted no-effect concentrations (PNEC), calculated for aquatic toxicity (Table 6). None of the above PEC exceed these PNEC values. Since attenuation and leachate treatment would mitigate any contamination, the present findings of organotin compounds in landfill leachate are assessed to constitute no relevant environmental effect.
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Some PVC products contain nonbiocidal organotin compounds as stabilizers. These are mono- and dialkyltin compounds, with the alkyl groups being either methyl, butyl, or octyl. In particular, mono- and dioctyltin can safely be assumed to originate from PVC products since these compounds are employed solely as stabilizers. This does not hold true for methyl- and butyltin compounds that may arise from a number of other sources. Tributyltin, in particular, is used as a biocide in antifouling coatings and varnishes, disinfectants, wood fungicides, and other preservatives, for instance in roof linings. It is common in outdoor products and may even occur in landfill liners. Since tributyltin is subject to biodegradation via successive dealkylation, di- and monobutyltin may arise as transformation products of the landfill microflora (Kuballa et al., 1996; OSPARCOM, 1997).
A biological methylation of inorganic tin is also possible (Kuballa et al., 1996), drawing upon the abundant deposits of inorganic tin that are present in municipal solid waste (e.g., cans and scrap metal). Findings of methylated metal species in landfill gas have been reported elsewhere (Feldmann and Hirner, 1995). The occurrence of mixed alkylated tin species (e.g., methylbutyltin) should therefore be assumed and would corroborate the relevance of these processes. The transformation and relocation of the original substances between solid residues, leachate pollution, and gaseous emissions thus disrupts any linear causality between substance inventory and emission potential.
Another relevant secondary source may be sewage sludge, which is known to exhibit findings of organotin compounds and is often co-disposed with municipal solid waste in sanitary landfills. For a conservative estimate of the contents of organotin compounds in sewage sludge, the order of magnitude may be indicated as 100 µg/kg TS (cf. Kuballa et al., 1998). It is quite uncertain, however, whether the findings are actually attributable to dispersed organotin compounds adsorbed to the organic sludge matrix or rather PVC (micro)particles containing organotin stabilizers.
The characterization of the organic matrix of the leachate is of particular importance since suspended solids and colloid matter enable leaching and transport of hydrophobic compounds beyond their water solubility (Bauer and Herrmann, 1998; Mersiowsky et al., 1999). The standard parameters pH, TOC, and VFA of the investigated leachate samples (cf. Table 5) indicated a wide variety of conditions. The landfill sites showed considerable differences in their progress through the degradation stages, ranging from the strongly acidic early fermentative phase to the weakly alkaline mature phase of stable methanogenesis. The deposit Germany C could even be mineralized, possibly aerobic. During acidogenesis, volatile fatty acids (VFA) and colloids constitute the major share of the TOC, but humic substances become gradually more prevalent in mature landfills. Both may act as carriers for organotin compounds.
The possibility of mass balances is contentious due to the heterogeneity of landfill body. Local conditions in different cells of the landfill or even within one bulk of waste may vary widely in waste composition, moisture contents, and biological activity. The leachate flux may either involve short-cut conduits or a percolation of larger waste volumes. Moreover, sorption and attenuation processes occur: the landfill body itself acts as an anaerobic fixed-bed filter, partially retaining compounds, such as octyltin species, which possess an affinity for solid matter.
The possible fate of organotin compounds may therefore be summarized in the following way. First, the substances may be retained in the solid waste matrix, either being averse to release by leaching (e.g., organotin stabilizers fixed in rigid PVC products) or being adsorbed in the organic matrix due to an affinity for solids (e.g., octyltin species). Second, the target compounds may be transported by the leachate, either as solute or adsorbed to colloids or suspended solids. The respective findings presented here indicate that the relative relevance of this route is comparatively low. Third, the target compounds may be volatilized into the landfill gas (e.g., methyltin species), and the relative relevance of this emission pathway is as yet uncertain.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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These findings were determined to possess no significant environmental relevance. At the present stage it is difficult to allocate the presence of organotin compounds in landfill leachate to any specific waste fraction. There are numerous possible sources, including stabilizers from PVC products and biocides from diverse impregnated materials as well as sewage sludge and other diffuse carriers. Only octyltin species may be attributed to PVC products with any certainty, whereas the origin of butyltin and methyltin species is rather indeterminate. It would be instrumental to compare the release patterns of all respective materials that are subject to leaching and biodegradation processes. The microbial dealkylation and methylation processes are as yet not properly understood. The occurrence of mixed alkylated species might provide pertinent indications. As for emission pathways, the transformation of metals and alkylated metals into volatile methylated species and release by landfill gas should be monitored carefully.
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ACKNOWLEDGMENTS |
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The investigation was supported by grants from CK Witco/Crompton (Bergkamen, Germany) and the Organotin Environmental Programme (ORTEP) Association. The project was co-sponsored by Norsk Hydro ASA (Porsgrunn, Norway), European Council of Vinyl Manufacturers (ECVM), European Council for Plasticisers and Intermediates (ECPI), European Stabiliser Producers Association (ESPA), and the Vinyl Institute (VI).
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