chemical analytical procedures which are sensitive to interference from inorganic ..... of the halogenated organic compounds present in soil are found in fractions ...
Origin and occurrence of halogenated organic matter in soil Gunilla Asplund Department of Water and Environmental Studies, Linkoping University, S-581 83 Linkoping, Sweden.
Introduction The past few years of research have shown that organically bound halogens are natural constituents of organic matter, that key processes occur in the terrestrial environment and that a major pool is found in soil (Asplund and Grimvall, 1991; Hjelm et aI., 1994; Gron et aI., 1994). These findings strongly indicate that the turnover of organohalogens in the environment encompass a complex pattern of processes, such as production (halogenation), mineralisation (dehalogenation) and transformation in addition to retention and release from pools and large scale fluxes. In the present paper, existing knowledge of the occurrence and origin of organohalogens in the terrestrial environment is reviewed in the light of recent findings. To begin with, the chemical analytical procedures which are sensitive to interference from inorganic halides are discussed and evaluated. An evaluation of the methods used to determine organically bound halogens in soil Two methods used to determine organically bound chlorine in soil are discussed in the present paper, and these procedures were developed by modifying the sample treatment for determination of organohalogens in water samples (AOX). The AOX method is based on silver-ion microcoulometric titration of the hydrogen halides formed after incineration of the sample. When using this procedure it is crucial that inorganic halides present in the sample are removed prior to incineration.
The AOX method In the standard procedure for AOX determinations, an acidic nitrate solution and activated carbon are added to the water sample. The organic matter is adsorbed onto the activated carA. Grimvall and E. W. B. de Leer (eds.), Naturally-Produced Organohalogens, 35-48. © 1995 Kluwer Academic Publishers.
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bon whereafter the mixture is filtered, and the filter and the filter cake are then incinerated (DIN, 1985). Removal of inorganic halides is accomplished by adding nitrate to induce ion exchange. Several investigators have shown that dissolved inorganic chloride does not interfere with the determination of AOX as long as the chloride concentration does not approach the levels found in brackish waters (DIN, 1985; Wigilius et al., 1988; Jonsson et at., 1991). It is also generally accepted that the concentrations of bromide and iodide normally found in freshwater do not affect the AOX determination. Salkinoja-Salonen (pers. comm.) has suggested that inorganic chloride retained in microbial cells, despite sample pretreatment, might interfere with the analysis of AOX. In freshwater, bacterial abundance varies from 105 cells/ml in oligotrophic waters to 107 cells/ml in eutrophic waters, and the mean volume has been estimated to 0.09 (/lm)3 for planctonic bacteria and 0.5 (/lm)3 for attached bacteria (Kato, 1989). Assuming that microorganisms are isotonic (0.9 % NalKCI), with a density of 1.3, the amount of chloride thus retained can be estimated to less than 400 ng Cill in oligotrophic waters and to the high level of 40 /lg Cill in eutrophic waters. These calculated values indicate that chloride in microorganisms should not cause any analytical artefacts when analysing samples of oligotrophic waters but might do so when the samples are from eutrophic waters. In further consideration of the possible interference from chloride in microorganisms, 19 surface water samples from the county of Osterbotten in Finland and the counties of Norrbotten and Ostergbtland in Sweden were analysed before and after sterile-filtration through a 0.22 /lm membrane. In general, the results obtained for filtered and unfiltered samples were almost identical, although a small but significant decrease could be observed in five of the samples (fig. 1). This shows that the major part of the organohalogens detected as AOX in these samples did not originate from inorganic chloride retained within microorganisms. Evaluating chloride interference on the basis of AOX analysis of the investigated Finnish and Swedish surface waters may provide unreliable results, since samples were collected from December to February, when biological activity is low. In other words, there might be significant interference when analysing water sampled during periods of extensive biological activity. However, a study involving monthly sampling of Swedish rivers showed that organohalogen concentrations were almost exclusively lowest during the summer months; the only exceptions were found in the northernmost part of the country, where four rivers exhibited a reversed concentration pattern of organohalogens (Enell and Wennberg, 1991). Furthermore, in the cited study, fractionation of the organic matter in water samples from five Swedish rivers revealed that a minor part of the AOX was present in the particulate fraction « 0.45 /lm). Together, these results imply that inorganic chloride in microorganisms does not interfere with the AOX analysis.
The TOX and LOX procedures By modifying the sample treatment, the AOX analysis described for water can also be used to determine the presence of organohalogens in solid samples. So far, a standard method has been developed for determination of the total amount of organohalogens in sediments and
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Origin and occurrence of halogenated organic matter in soil
sludges (DIN, 1989) and several procedures have been suggested to be appropriate for analysis of total and leachable amounts of organohalogens in soil (e.g. Muller and Schmitz, 1985; Morton and Pollak, 1987; Martin and Takahashi, 1988; Laschka and Schall, 1989; Asplund et ai., 1994). The study by Asplund et at. (1994) was devoted solely to the problem of determining the total and leachable amounts of organohalogens in soil. It was shown that the investigated procedures for analysis of the total amount of organohalogens (TOX) and the leachable amount of organohalogens (LOX) in soil can be standardized to provide reproducible results with a relative standard deviation of less than 10%. In the LOX procedure, a soil sample is leached with Milli-Q water and filtered through a coarse filter (0.1 mm) and the filtrate is then treated according to the AOX procedure. By comparing the chlorine-tocarbon ratios in coarse (0.1 mm) and sterile (0.22 ~m) filtered soilleachates, Asplund et at. (1994) found that inorganic chloride in organisms does not significantly interfere with the determination of LOX (fig. 2).
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