Environ. Sci. Technol. 2011, 45, 461–467
Dioxins in Primary Kaolin and Secondary Kaolinitic Clays M A R T I N S C H M I T Z , * ,† G E O R G S C H E E D E R , † S A R A H B E R N A U , †,‡ R E I N E R D O H R M A N N , †,§ A N D KLAUS GERMANN‡ Federal Institute for Geosciences and Natural Resources (BGR), Stilleweg 2, D-30655 Hannover, Germany, Technical University of Berlin (TUB)sInstitute of Applied Geosciences, Ackerstraβe 76, D-13355 Berlin, Germany, and State Authority of Mining, Energy and Geology (LBEG), Stilleweg 2, D-30655 Hannover, Germany
Received September 1, 2010. Revised manuscript received November 10, 2010. Accepted November 12, 2010.
Since 1996 dioxins have been repeatedly detected worldwide in Tertiary ball clays used as anticaking agent in the production of animal feed and a variety of other applications. The dioxins of these natural clays are very unlikely of anthropogenic source, but no model of dioxin enrichment has been established. A hypothetical model is presented which explains the highly variable dioxin loadings of the Tertiary kaolinitic clays by natural addition during clay-sedimentation. To prove this hypothesis, Tertiary primary nonsedimentary kaolin and sedimentary kaolinitic clays were collected at three profiles in Europe and analyzed for mineralogy, chemistry, organic carbon, and polychlorinated dibenzo-p-dioxins/-furans (PCDD/F). Primary kaolin, kaolinitic, and lignitic clays contained almost no PCDFs. PCDD concentration differed markedly between primary kaolin (3-91 pg/g) and secondary kaolinitic clay (711-45935 pg/g), respectively, lignitic clays (13513-1191120 pg/g). The dioxin loading of secondary kaolinitic and lignitic clays is approximately 10 to a few thousand times higher than in the primary kaolin or recent environmental settings. The dioxin concentrations decrease from octachlorodibenzo-p-dioxin to the tetrachlorodibenzo-p-dioxins and exhibit the “natural formation pattern”. No correlation between PCDD/F concentration and bulk composition of clays was found. These findings support the hypothesis of the enrichment of dioxin in clays during sedimentation.
Introduction In 1996, polychlorinated dibenzo-p-dioxins/-furans (PCDD/ Fs) were detected in Tertiary ball clays of the Mississippi embayment, used as anticaking agent in the production of animal feed in the U.S. (1, 2). Studies that followed confirmed that numerous Tertiary clays, specified as kaolin or ball clay, in the U.S., Europe, and Asia also contain PCDD/Fs. Ferrario et al. (3-5) reported the distribution and the congener profiles of PCDDs in ball clay samples from the Mississippi embayment and of processed ball clays in the U.S. The congener data of these studies indicate that especially the higher chlorinated PCDDs were thoroughly * Corresponding author phone: +49 511 643 3093; fax: +49 511 643 3661; e-mail:
[email protected]. † Federal Institute for Geosciences and Natural Resources (BGR). ‡ Technical University of Berlin (TUB). § State Authority of Mining, Energy and Geology (LBEG). 10.1021/es103000v
2011 American Chemical Society
Published on Web 12/02/2010
dominant, whereas the PCDF contents were low to undetectable. The dominant homologue was octachlorodibenzo-p-dioxin (OCDD), followed by the groups of heptachlorodibenzo-p-dioxins (HpCDD) and hexachlorodibenzo-p-dioxins (HxCDD). The homologue-groups of pentachlorodibenzo-p-dioxins (PeCDD) and tetrachlorodibenzo-p-dioxins (TCDD) had the lowest concentrations. Among the 2,3,7,8-substituted HxCDD-congeners the 1,2,3,7,8,9-HxCDD dominated, followed by the 1,2,3,6,7,8HxCDD. The average concentration of the most toxic 2,3,7,8-TCDD in the raw ball clays was 711 pg/g and the average WHO-TEQ-value (WHO-TEQ ) World Health Organisation-Toxic Equivalent) of the clays was 1513 pg/g (4). The most important information regarding the question of the possible source of PCDD/F loading is that congener patterns of the samples were different from those of anthropogenic sources. In addition, some of the clays were situated in more than 10 m depth and contained no other anthropogenic contaminants. Based on these findings Ferrario et al. (4) concluded that the PCDD/F loading could have been of natural origin. Rappe and Anderson (6) found strong variations in the PCDD/F content in ball clays from Kentucky, U.S., and in kaolin from Germany, however, all samples had almost the same congener patterns. The U.S. ball clays contained on average 1035 pg/g WHO-TEQ PCDD/F whereas the German kaolin showed a much lower content of 198 pg/g WHO-TEQ. In contrast to these relatively high loadings, kaolin samples from Georgia and North Carolina, contained PCDD/Fs close to the detection limit (0.23-0.5 pg/g WHO-TEQ). Jobst and Aldag (7) analyzed 33 German ball clays. PCDD/F contents of these clays varied from 3.9 to 1132 pg/g I-TEQ (I-TEQ ) International Toxic Equivalent) and showed almost the same isomer congener patterns as described by Ferrario et al. (3, 4). The median of the PCDD contents was 154 pg/g I-TEQ and the 90%-percentile was 578 pg/g I-TEQ. Green et al. (8) found the same “natural PCCD/F pattern” in deep soil samples from the Rothamsted archive, U.K., which were considerably different from those samples close to the surface which were believed to be affected by anthropogenic input. Gaus et al. (9) and Prange et al. (10) determined the PCDD/F contents of marine sediments, river sediments, kaolinitic clays, and topsoils from the coastal environment of Queensland, Australia. They found strong similarities to the PCDD/F congener profiles and isomer patterns reported from German and North American ball clays and kaolins. Gaus et al. (9) concluded that this might be an indication of a similar source or formation process that is unidentified. In 2002 Gaus et al. (11) analyzed the same PCDD/F patterns within estuarine sediment cores in Queensland, Australia. They presented the hypothesis that an anthropogenic precursor, for example, pentachlorophenol, presumably caused the contamination. Abad et al. (12) studied the contamination of animal feed ingredients by dioxins in Spain. They analyzed two kaolins among other types of clay, showing congener patterns almost similar to those of presumably natural origin. The PCDD/F contents of the kaolins were almost identical to those of German ball clays with values of 232 and 461 pg/g WHOTEQ. Spanish Bentonites and sepiolites were considerably less contaminated (0.16-0.47 pg/g WHO-TEQ). To investigate the vertical distribution of PCDDs and to determine the correlation between the mineralogy and the dioxin concentrations, Gadomski et al. (13) analyzed 27 samples from three ball clay cores from locations in Kentucky VOL. 45, NO. 2, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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and Tennessee. All samples showed the assumed “natural formation pattern” with maximum PCDD/F concentrations of the cores of 2500, 440, and 15 000 pg/g WHO-TEQ. The authors reported that along the profiles of the clays no correlation existed between the mineralogical composition and the PCDD concentrations, respectively, the PCDD homologues. Horii et al. (14) determined the specific carbon isotope composition of OCDDs in ball clays from the U.S. and Japan. They were able to distinguish the carbon isotope composition of the OCDD in the ball clays from those of recent anthropogenic sources. This study gives strong evidence to the hypothesis of a natural formation of PCDD/Fs. Biotic and Abiotic Formation of Natural Dioxins. Although several studies indicated occurrence of PCDD/Fs in kaolin and ball clays of possibly natural origin, no accepted model was developed to explain their source or their relation to the formation of the clays. The enzymatic biotic formation of PCDD/Fs was described ¨ berg and Rappe (16). Under by Wagner et al. (15) and O laboratory conditions microorganisms like “Phanerochaete chrysosporium” were able to produce chlorphenol, a precursor of PCDD/Fs, by degradation of lignite, however the strong aerobic atmosphere conditions of the experiments are not realized in natural systems. Bunge (17) proved in his doctoral thesis that special kinds of bacteria are able to dehalogenate and transform PCDD/Fs under anaerobic conditions. The natural abiotic formation of PCDD/Fs was discussed by a number of authors. Martinez et al. (18) examined the dioxin content of vegetation and soil burned in Catalan (Spain) forest fires and compared the results with the unburned material. They concluded that forest fires were not an important natural source of dioxin-like compounds. Gullet and Touati (19) found variable PCDD/F patterns in the smoke of forest fires. Each pattern was dependent on the type of the burned material and the location. In their analysis of estuarine and river sediments, topsoils, and kaolinitic clays in the coastal zone of Queensland, Australia, Gaus et al. (9) and Prange et al. (10) reported congener patterns almost similar to the natural pattern. They supposed that PCDD/Fs possibly were formed by an unknown natural, possibly abiotic process. Those samples taken close to the sea coast contained more dioxin than samples taken further from the sea. The authors pointed out that coastal environments are subject to elevated chlorine levels, however, biotic processes or different geological processes could not be excluded to explain the differences between the areas studied. Gaus et al. (20) detected presumably anthropogenic OCDD precursors in soil samples from Queensland, Australia. The authors concluded that these soils from the Australian National Park could not be considered as undisturbed and a dioxin contamination by agrochemicals seemed to be at least possible. Gu et al. (21) provided the first evidence for clay-catalyzed OCDD formation. They mixed Fe(III)-montmorillonite with pentachlorophenol under ambient temperature in the presence of water. After minutes up to days approximately 5 mg OCDD/kg clay were formed. This study is a remarkable indication of the proposed hypothesis of in situ formation of OCDD in soils, whereas pentachlorophenol has to be regarded as an anthropogenic precursor. In 1989 Nolan et al. (22) produced hydroxy-aluminumtreated clay minerals (kaolinite and smectite) which showed a much higher affinity for chlorinated PCDD/Fs than the untreated clays. The authors concluded that adsorption of PCDD/Fs from the aqueous solution by the hydroxyaluminum polymer of the clays led to a higher PCDD/F loading of the clay minerals. 462
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FIGURE 1. Definition of in situ kaolin and kaolinitic clay with synonyms. PCDD/Fs are lipophilic and hydrophobic, they are persistent in the environment and usually adsorbed to clay particles and/or organic matter. It is very likely that under natural conditions, particularly over geological times, the PCDD/F pattern may have been modified (degraded) by bioor photodegradation. Niu et al. (23) noted that the photolysis rates of PCDFs adsorbed on spruce needles are higher than those of the PCDDs. Additionally the degradation by photolysis of the higher chlorinated PCDD/Fs tends to be slower than the lower chlorinated congeners. Hutzinger and Fiedler (24) described that higher chlorinated PCDDs and PCDFs were more lipophilic. Accordingly these congeners are preferentially bound to particles rather than evaporate to the gaseous phase, where they can undergo photolysis. The fact that PCDD/Fs were detected in several deposits or profiles in remote areas of Tertiary kaolinitic soft rocks accompanied with the typical natural congener pattern led to the hypothesis that PCDD/Fs might have accumulated in the paleoenvironment followed by conservation in clays as natural sinks over geological periods of time. Consequently these PCDD/Fs should then be regarded as naturally fixed and preserved from Tertiary age. On their way from source to sink or in the sink, congener patterns might have changed, probably by photolysis or biodegradation, showing the distribution that can be observed today. If natural PCDD/Fs were introduced as a byproduct during erosion, transportation and finally sedimentation of primary in situ kaolin to form kaolinitic clays, then in situ kaolins that were formed only by kaolinitic weathering without any transportation should be virtually free of PCDD/Fs. The aim of this study is to identify whether such typical Tertiary primary kaolin is PCDD/F-free and whether in contrary typical sedimentary kaolinitic clays of Tertiary age, which can be related to these primary weathering products, are PCDD/F-loaded.
Materials and Methods The term “kaolin” provides no information about the origin and composition of the sampled material. In general, kaolin is a white or colored soft-rock, mainly composed of the clay mineral kaolinite with minor and variable amounts of illite, montmorillonite, quartz, and mica. Weathered relicts of feldspar are very common. Kaolin can be formed by different processes. In situ kaolin, often labeled as primary kaolin or china clay, is mainly formed by weathering of acid-rocks like granite (Figure 1). On the contrary kaolinitic clays, also named ball clays, plastic kaolins, and sometimes sedimentary or secondary kaolins, are sedimentary soft rocks, mainly built by erosion, transportation, and sedimentation of the mineralogical components of the in situ kaolin. Compared with the in situ kaolin, the average grain size of kaolinitic clays is smaller and they are usually enriched in clay minerals. Additionally, they often
FIGURE 2. Schematic sketch of the three profiles studied. PCDD/F concentration is given in pg/g WHO-TEQ. contain variable amounts of sedimentary organic matter such as lignite or peat. Samples. From the geological point of view, the situation for comparing the composition of in situ kaolins and genetically related kaolinitic clays is favorable in Central Europe. A thick kaolinitic weathering crust was formed during Late Mesozoic and Tertiary weathering periods and partly converted into kaolinitic clays by erosion and redeposition. In this study, one profile of mid-European in situ kaolin and two profiles of mid-European kaolinitic clays as well as their recent topsoils were sampled in different locations (Figure 2). In outcrops vertical kaolin and clay profiles were investigated which consist of different horizons. These horizons were sampled separately. To avoid influences of anthropogenic sources only fresh and unweathered material was sampled. Each sample was taken using precleaned tools and kept in an acetone cleaned beaker glass, closed hermetically by a screw cap wrapped with an acetone cleaned tinfoil. During preparation only glass bottles were used, treated with acetone before usage. The samples were not touched by hands or plastics to avoid contamination. A total of three in situ kaolin samples (K), six kaolinitic clay samples (C), two lignitic clay samples (LC), and two samples of recent topsoils (S) were taken (Figure 2). The PCDD/F content and the congener pattern of all samples were determined as well as the mineralogical and chemical composition of the clay samples. The samples were labeled with regard to the profile (P1-P3) and to the depth from which they were taken. Samples taken close to the surface were given the number 1, samples taken from a lower level were given numbers 2 and 3 respectively. Preparation. The samples were dried at 40 °C, homogenized, and split. One sample split was ground in a disk vibratory mill for chemical and mineralogical analysis. A second split was used for dioxin analysis. A third one was suspended in distilled water and sieved to pass a 63 µm sieve. The clay fractions PeCDD. In the samples P1K1 and P1K2 no TCDDs were detected. In general, all values are close to or below the detection limit, therefore a complete PCDD pattern cannot be provided. Kaolinitic Clays (P2 and P3). The clays of profile 2 (P2C1-P2C3) contain 35-60% kaolinite, 15-45% quartz, and 20-25% illite. Accessories are montmorillonite, anatase, goethite, pyrite, and organic carbon (0.14-1.8%). The lignitic clay (P2LC) consists of 45% kaolinite, 15% quartz, and 15% illite, the amount of organic carbon is approximately 12.3%, accessories are montmorillonite, anatase, goethite and pyrite. The kaolinitic clays of profile 3 (P3C1-P3C3) contain more kaolinite (46-57%) and minor amounts of quartz (5-35%) and illite (10-12%) if compared with profile 2. Accessories are the same minerals as in profile 2. The lignitic clay of profile 3 (P3LC) consists of 45% kaolinite, 1% quartz, 5% illite, 20.5% organic carbon and HpCDDs > HxCDDs > PeCDDs > TCDDs. Particularly TCDDs exhibit the lowest concentration, with exception of the recent topsoil samples (P2S and P3S). The PCDD/F concentrations of the samples of the clay profiles are approximately in the same range, with the exception of the lignitic clay in profile 3 (P3LC). The soil samples show significant PCDF concentrations; on the contrary the kaolinitic clays and lignitic clays contain PCDFs close to or below the detection limit (Table 1). In the VOL. 45, NO. 2, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. TOC (wt.%), WHO-TEQ (pg/g) and Concentration of the Homologue-Groups and the 2,3,7,8-Substituted PCDD/F Congeners (pg/g)a Congener
P1K1
P1K2
P1K3
P2S
P2C1
P2C2
P2LC
P2C3
P3S
P3LC
P3C1
P3C2
P3C3
2,3,7,8-TCDD Sum TCDD 1,2,3,7,8-PeCDD Sum PeCDD 1,2,3,4,7,8-HxCDD 1,2,3,6,7,8-HxCDD 1,2,3,7,8,9-HxCDD Sum HxCDD 1,2,3,4,6,7,8-HpCDD Sum HpCDD OCDD 2,3,7,8-TCDF Sum TCDF 1,2,3,7,8-PeCDF 2,3,4,7,8-PeCDF Sum PeCDF 1,2,3,4,7,8-HxCDF 1,2,3,6,7,8-HxCDF 1,2,3,7,8,9-HxCDF 2,3,4,7,8,9-HxCDF Sum HxCDF 1,2,3,4,6,7,8-HpCDF 1,2,3,4,7,8,9-HpCDF Sum HpCDF OCDF Sum PCDD Sum PCDF WHO-TEQ TOC
0.40 1.4 6.0 7.4 0.005 0.04
1.2 0.29 0.31 0.46 7.7 9.0 25 58 91 0.21 0.02
0.44 0.15 2.5 3.0 HpCDD > HxCDD > PeCDD or TCDD. Considering only the kaolinitic clays the proportion of OCDD in the clays increases with depth. Remarkably the highest PCDD concentration was measured in the lignitic clay of profile 3. Both analyzed lignitic
clays exhibit the highest WHO-TEQ values (with P2LC , P3LC) of their profiles (Table 1). This fact, together with the chemical properties of PCDD/Fs, led to the hypothesis, that the PCDD/Fs could be predominantly associated with the organic matter of the clays. In Figure 6 the sum of the PCDD/F concentrations compared to the “total organic carbon” (TOC) content of each sample is presented. TOC however does not correlate with PCDD/F content. Clays with high PCDD/F loadings contain both small and large amounts of TOC (P3C3 and P3LC). Therefore, the comparison of the PCDD/F content and the TOC indicates that the percentage of organic carbon is not a reliable proxy of the PCDD/F concentration in clays. It is assumed that not the content of organic carbon, but the VOL. 45, NO. 2, 2011 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Comparison of TOC and PCDD/F concentration in kaolinitic and lignitic clays and topsoils. type of organic matter could be a very important factor of dioxin enrichment in kaolinitic clays.
Discussion In contrast to the kaolinitic and lignitic clays the in situ kaolin does not contain significant PCDD/F concentrations. This result is consistent with the analysis of U.S. kaolin by Rappe and Anderson (6). Accordingly, the PCDD/Fs in kaolinitic clays must have been added in the course of the sedimentary processes forming clays. The PCDD/F patterns of the clays are very different from those of the recent topsoils. The topsoils contain PCDFs in considerable amounts and the kaolinitic and lignitic clays only show insignificant concentrations of highly chlorinated PCDFs. The PCDD/F content of the clays is 10s to hundreds of times higher than that of the topsoils. The topsoils represent a typical recent anthropogenic contamination pattern, whereas the clays exhibit the natural formation pattern described by Ferrario et al. (3, 4). The toxic equivalent values
(WHO-TEQ) are very similar to those of the kaolin and kaolintic clays analyzed by Jobst and Aldag (7) and Abad et al. (12). In general the average PCDD/F content of the European kaolinitic and lignitic clays of this study seems to be lower than of the U.S. ball clays from Georgia, Kentucky, and Tennessee (3, 4, 6, 14). No significant statistic correlations between the mineralogical and chemical composition of the kaolinitic and lignitic clays and the PCDD/F contents were found, confirming the findings of Gadomski et al. (13) and Ferrario et al. (5). Given that the sedimentary kaolinitic and lignitic clays contain considerable amounts of PCDD/Fs and the nonsedimentary in situ kaolin contains markedly lower and nondetectable amounts of PCDD/Fs, it is very likely that dioxins were added to the clays during transportation and sedimentation of their components. This assumption is the basis of the proposed model of natural dioxin formation and accumulation in clays. Hypothetical Model of Natural Dioxin Formation and Accumulation in Clays. The present data does not give clear evidence for the primary sources of the PCDD/Fs or the processes of chlorination or dechlorination. In the proposed model it is assumed that the dioxin was (1) either generated by biotransformation or by natural abiotic processes in soils or (2) built by abiotic processes like forest fires and released to the atmosphere (Figure 7). The furans and lower chlorinated dioxins were decomposed in the atmosphere by phototransformation and subsequently deposited in the soils. Within the soils, the dioxin patterns were presumably modified by evaporation and altered by biotransformation. Due to their higher refractory nature the higher chlorinated and more lipophilic PCDDs were thereby enriched relatively to the lower chlorinated and less lipophilic PCDD/Fs. This alteration pattern is dominated by the most refractory OCDD congener. Therefore, the natural formation pattern could be regarded as an alteration product of any primary PCDD/F pattern. It is assumed that PCDD/Fs were initially associated with organic matter or fine mineral particles (clay minerals) of the Tertiary soils. They were eroded and transported by rivers together with parts of the deeper weathering crust (in situ kaolin). The components were deposited together in the same
FIGURE 7. Hypothetical geological and paleoenvironmental model of natural PCDD formation and accumulation in clays. The gray arrows indicate dioxin transfer paths. 466
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environment, due to the similar hydrodynamic properties. In zones of low fluid flow in lakes and rivers kaolinitic clays were formed. Swamps developed above the low permeable kaolinitic clay sediments in all sampled areas. This caused anaerobic conditions after sedimentation. The PCDD/F alteration patterns of the soils were locked in the kaolinitic clays and preserved inside the newly generated clay formations. Soil-organic matter and particles of different origin, which were loaded with different amounts of PCDD/Fs, were mixed during transportation and sedimentation. This could explain the different PCDD/F contents and patterns in different clay formations, even within a single clay profile, measured by numerous authors. The PCDD/F concentrations found in the sedimentary Tertiary kaolinitic clays can reach remarkably high levels compared to the present-day contamination found in remote environmental areas. Unknown specific processes excluded, the hydrodynamic sedimentation process, which led to the separation of clay minerals and subsequently to the accumulation of secondary kaolinitic clays with coaccumulation and preservation of PCDD/Fs, is likely a major factor for the development of enhanced PCDD/F concentrations in clay deposits. Nevertheless, a PCDD/F accumulation factor of 100-1000 from environment to clay deposit due to the sedimentation process is unlikely. A temporary higher dioxin contamination of the environment in the Tertiary due to natural processes has to be assumed to bias the PCDD/F accumulation toward higher levels compared to current levels. Hence, the variability and the amount of PCDD/F contaminations in unworked Tertiary sedimentary kaolinitic clay horizons and deposits found today were most likely controlled by geological processes.
Acknowledgments We thank the Hans-Joachim-Martini-Foundation for financing the project.
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