Soil Science Society of America Journal 67:795-797 (2003)
© 2003 Soil Science Society of America
DIVISION S-2—NOTES
Dissolved organic carbon is released from sealings and glues of pore-water samplers
Jan Siemens* and
Martin Kaupenjohann
Institute of Soil Science, University of Hohenheim, D-70593 Stuttgart
* Corresponding author (jan.siemens{at}tu-berlin.de)
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ABSTRACT
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To quantify solute concentrations in soil pore waters, their alteration on contact with sampler components should be negligible compared with naturally occurring concentrations. We hypothezise that elastomer sealings and glues are potential sources of dissolved organic C (DOC) in pore water samples. We immersed different sealing materials and filled assembled samplers with water and determined the release of DOC from glues, elastomers, and other components of suction cups and plates into water samples. Between 0.8 and 63 mg DOC L-1 were released on contact with sampler parts. We conclude that soil solution samplers should be designed without glues or elastomers, if DOC concentrations <10 mg L-1 are the targets of investigations and expected flux rates are low. Glass suction plates entirely made from borosilicate glass avoided the contamination of samples with sealing derived DOC.
Abbreviations: DOC, dissolved organic C • PMMA, polymethylmetacrylate • POM, polyoxymethylene • PTFE, polytetrafluoroethylene
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INTRODUCTION
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POROUS CUPS AND PLATES that are used to sample pore waters of soils and sediments may alter the sample composition by filtration effects, precipitation of solid phases, gas exchange between sample and atmosphere, and sorption or release of chemical species (Grossmann and Udluft, 1991; Krejsl et al., 1994; Wessel-Bothe et al., 2000). The alteration of sample composition should be negligible compared with naturally occurring concentrations of the target solute. Porous cups and plates are commonly sealed with elastomers or glues. Although glues and elastomers contain organic C in the form of polymers, organic solvents and plasticizers, the possible release of such sealing-derived organic C into pore water samples has received only limited attention. We tested whether the release of organic solutes from sealings affects the quantification of DOC concentrations in pore water samples. Furthermore we present a suction plate entirely made from borosilicate glass that did not release organic C.
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Materials and Methods
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In a first experiment, samples of 3 cm2 of neoprene (polychloroprene) and buna-N (nitrile) were immersed in 10 mL of tap water containing 4 mg DOC L-1 in glass beakers for 5 d to compare DOC release. It was tested whether DOC release could be reduced by placing the sealings in 250 mL of tap water for 14 d before the DOC release experiment (Table 1, Test A). In a second step, the alternative sealing materials silicone rubber and polytetrafluoroethylene (PTFE) were compared with neoprene and buna-N. The silicone sealings were cut from 1-mm thick transparent sheets of silicone rubber of different hardness (shore 40 and shore 60) provided by two distributors (Fähnletechnic, Göppingen and Renner, Stuttgart, both Germany). After comparing DOC release of different materials, suction plates of 90-mm diam. were assembled and sealed using silicone O-rings (Table 1, Test B). The void volume of the suction plates was filled with tap water (10 mL, containing 4 mg DOC L-1) for 2 d to get an estimate of the magnitude of DOC release into pore water samples. The contact time of 2 d is equivalent to a flux rate of 0.8 L m-2 d-1. Five silicone O-rings were retrieved from suction plates that were installed into soil from Nov. 1998 till Sept. 1999. Dissolved organic C release from these aged O-rings into tap water was compared with DOC release from new O-rings (Table 1, Test C). In a further set of experiments, we examined the release of DOC from glues (Table 1, Test D) and assembled suction cups and plates that were sealed with glues (Table 1, Test E). Finally, suction plates entirely made from borosilicate glass were checked for DOC release into deionized water (Table 1, Test F). Deionized water was chosen instead of tap water, because the tests were run parallel to the cleaning of suction plates by rinsing them with deionized water.
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Table 1. Change of dissolved organic C (DOC) concentration in water on contact with sealings and components of suction plates and suction cups.
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The change in DOC concentrations on contact with sampler parts was compared with the concentration change in controls. Dissolved organic C concentrations were determined on a DIMATOC 100 TOC analyzer (Dimatec, Essen, Germany). Given the low number of replicates it was not feasible to test the data for normal distribution. Therefore, we tested the significance of differences of DOC concentrations relative to pure water controls by the nonparametric Mann-Whitney U-test that does not rely on a normal distribution of data (SPSS 9.0.1, SPSS Inc., Chicago, IL).
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Results and Discussion
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Buna-N and neoprene induced a large increase in DOC concentrations in 5 d, which reduced after aging these materials in water for 14 d (Table 1, Test A). However there was still a significant increase in DOC concentrations on contact with these materials compared with the control. Release of DOC from silicone was much lower than release from buna-N and neoprene. Yet also silicone increased DOC concentrations significantly. Only PTFE did not change the DOC concentration of tap water significantly. However, attempts to seal suction plates with PTFE sealings were not successful. Because of the hydrophobic surface of PTFE, we were not able to realize bubbling pressures higher than approximately 60 hPa. Therefore, suction plates were assembled and sealed using the Fähnletechnic silicone O-rings. While polymethylmetacrylate (PMMA) and polyoxymethylene (POM) components did not induce a significant change in DOC concentrations (Test B, Table 1), the silicone O-rings increased the DOC concentration by 9.4 mg L-1. Hence, the specific DOC release rate (mg DOC L-1 cm-2 d-1) of O-rings in assembled suction plates was as high as the specific DOC release rate from sole O-rings.
Aging of buna-N and neoprene showed that DOC release from elastomers decreases with time because of the leaching of mobile organic components, indicating that the release of DOC might be reduced to an acceptable magnitude by equilibration in the field. Similarly, Guggenberger and Zech (1992) showed that equilibration of ceramic suction cups mitigated sorption of DOC by ceramic cups. We retrieved silicone O-rings that were installed in the soil for 1 yr and determined their release of organic C. The increase in DOC concentrations was less than half of the increase that was induced by new O-rings (Test C). Nevertheless, the increase in DOC concentrations was still significant.
Dissolved organic C was not only released by elastomers, but also by different glues that are used to assemble samplers (Tests D and E).
To relate the increase in DOC concentrations in the batch experiments to the field situation we have to compare both the residence time of pore water in the sampler, and the magnitude of the increase in DOC concentrations. The residence time of 2 d would be exceeded at a flux rate <0.8 L m-2 d-1, which is frequently observed in the field especially in summer (J. Siemens and Kaupenjohann unpublished data, 1999). The absolute increase in DOC concentrations of 4.7 to 14.5 mg L-1 in assembled samplers because of release of organic C from sealings and glues is of similar magnitude as DOC concentrations in pore waters of subsoils and sediments (Moore, 1998; Michalzik et al., 2001). Hence, the release of organic C from sealings may significantly affect the monitoring of naturally occurring DOC concentrations, and possibly the speciation of other solutes.
To avoid a contamination of pore water samples with organics, we designed a sampler entirely made of borosilicate glass (Fig. 1) . By fusing the glass parts at high temperature the use of sealings could be avoided. After rinsing these samplers with water, no release of organic C was detected (Test E).
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Fig. 1. Section of glass suction plate (hydraulic conductivity: 6 x 10-7 ± 4 x 10-7 m s-1, n = 14; bubbling pressure: >400 hPa).
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We conclude that pore water samplers should be designed without elastomers and glues if expected DOC concentrations are <10 mg L-1 and additionally the contact time of solution samples and samplers is in the range of days because of low flux rates.
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ACKNOWLEDGMENTS
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We thank the water works/agriculture cooperation programs of the City of Münster and the City of Warendorf, the Stadtwerke Münster GmbH, the Wasserversorgung Beckum GmbH, the Westfälisch-Lippischer Landwirtschaftsverband, and the Environmental Department of the City of Münster for financial support. We also thank Alfons Peine for coordinating the financial aspects. Martti Haas determined the release of DOC from suction cups.
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NOTES
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Currently at: Institute of Ecology, Department of Soil Science, Technical University of Berlin, Salzufer 11-12, D-10587 Berlin, Germany.
Received for publication July 10, 2002.
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REFERENCES
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- Grossmann, J., and P. Udluft. 1991. The extraction of soil water by the suction cup method. A review. J. Soil Sci. 42:83–93.
- Guggenberger, G., and W. Zech. 1992. Sorption of dissolved organic carbon by ceramic P80 suction cups. Z. Pflanzenernaehr. Bodenkd. 155:151–155.
- Krejsl, J., R. Harrison, C. Henry, N. Turner, and D. Tone. 1994. Comparison of lysimeter types in collecting microbial constituents from sewage effluent. Soil Sci. Soc. Am. J. 58:131–133.[Abstract/Free Full Text]
- Moore, T.R. 1998. Dissolved organic carbon: Sources, sinks and fluxes and the role in the soil carbon cycle. p. 281–292. In R. Lal (ed.). Advances in soil science, soil processes and the carbon cycle. CRC Press, Boca-Raton, FL.
- Michalzik, B., K. Kalbitz, J.H. Park, S. Solinger, and E. Matzner. 2001. Fluxes and concentrations of dissolved organic carbon and nitrogen- a synthesis for temperate forests. Biogeochemistry 52:173–205.
- Wessel-Bothe, S., S. Pätzold, C. Klein, G. Behre, and G. Welp. 2000. Sorption of pesticides and DOC on glass and ceramic suction cups. (In German, with English Abstract.) J. Plant Nutr. Soil Sci. 163:53–56.
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ABSTRACT
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