The last 30 years has seen an increased global awareness of toxic organic compounds in the environment, leading to the definition of a set of long-lived compounds that are highly toxic. These persistent organic pollutants (POPs) consist of chlorinated pesticides and industrial chemicals that accumulate in the environment, in the food chain and in tissues. They travel long distances intact, contaminating otherwise pristine regions such as the Arctic.

One set of POPs is the dioxins, a large group of compounds consisting of polychlorinated dibenzo-p-dioxins, with different numbers of chlorine atoms attached to the triple ring system at different positions. Of these, 17 are known to be highly toxic with 2,3,7,8-tetrachlorodibenzo-p-dioxin the most potent. One recent analysis in the US concluded that there is no safe dose of dioxins that does not cause cancer, although the World Health Organisation has set a safe daily exposure level at 1-4 pg (1-4 x 10^-12 g).

It is widely believed that the presence of the POPs is the fault of human activity, due to the release of industrial chemicals from factories and the application of pesticides that are washed into rivers or evaporate into the air. However, for the dioxins, this is not the whole story. Dioxins are also released by burning wood for heating, from forest fires and volcanoes, and are produced during the incineration of waste.

Any additional source is a cause for concern because dioxins are among the most toxic chemicals known. And in the last few years, a new natural source of dioxins was unearthed, literally, in the USA when they were discovered in the mineral known as ball clay. Ball clay consists mainly of kaolinite (20-80%), mica (10-25%) and quartz (6-65%) and is mined largely for the ceramics industry. Since the initial discovery, dioxins have also been found in kaolin clays in Europe.

The clays are several million years old and the congener distribution of the dioxins was noticeably different from that typically found in anthropogenic sources of contamination. In addition, the congener profile was remarkably similar for clays from the different geographic locations. Other types of environmental contaminant were absent and all of this evidence points to a natural source for the dioxins.

Now, scientists from the USA, Sweden, Germany and Japan have provided further proof of the natural origin based on the congener-specific carbon isotopic signatures of the dioxins. They compared the delta-¹³C values of the principal congener in the clays, octachlorodibenzo-p-dioxin (OCDD), with those of OCDD in anthropogenic source materials such as fly ash, soils, river sediments, sediment core samples and the pesticide pentachlorophenol.

Clays from mines in Kentucky and Tennessee, Spanish kaolin and two types of Japanese ball clays were examined. Following accelerated solvent extraction, the extracts were cleaned up by multi-column chromatography then by double column HPLC with activated carbon and pyrenyl silica columns to separate OCDD from the closely related compound octachlorodibenzofuran.

The delta-¹³C values were determined by two-dimensional GC-isotope ratio mass spectrometry. The dual column set up yielded significant improvement in the sensitivity and selectivity of OCDD since it was the only compound selected for transfer to the oxidation furnace, based on its retention time. The extensive clean up removed all neighbouring interferences to give a detection limit of 20 ng.

The concentrations of OCDD in the US ball clays, measured by high-resolution GC/MS, averaged 408 ng/g dry wt., which is far greater than the average in waste incinerator fly ash (3.4-381) and contaminated environmental samples (257 ng/g). The levels in the Spanish and Japanese clays were 24 and 0.76 ng/g. The data "suggest the existence of significant amounts of OCDD in the strata of the Tertiary era", say the researchers.

The total yield of dioxins in mined American (2006) and Japanese (2005) ball clays was estimated to be 490 kg and 290 g, respectively. They are a health hazard for those involved in the mining and processing industries but not for clay modellers, since the dioxins are destroyed by kiln heat treatment before distribution.

The delta-¹³C values of OCDD in all of the ball clays examined were similar, ranging from -27.1 to -23.5 ppt. The average values were depleted by 2 ppt relative to the fly ash, enriched 6 ppt relative to the PCP value and 4% relative to the values for OCDD standards. This suggests that the OCDD in ball clay originated from a different carbon source.

Comparison of the bulk delta-¹³C values with those of OCDD present in them also strongly supported the natural origin of dioxins in ball clays. In comparison, the bulk values for soils and sediments were consistent with dioxin formation originating from PCP.

There are several proposed mechanisms for dioxin formation in ball clay, including the pyrolysis of vegetation by fire or volcano, biotic chlorination, or the percolation of dioxins from the surface. But the team declared that their results were consistent with abiotic formation due to surface-promoted reactions, influenced by the high temperatures and pressures experienced by deep layers.

This conclusion requires further investigation but, regardless of the mechanism, the data unambiguously support the formation of dioxins in ball clays deposited over 2 million years ago by a natural process, ruling out the influence of anthropogenic chlorinated compounds.

Related Links:

• Wadsworth Center, New York State Department of Health
• Department of Environmental Health Science, State University of New York at Albany
• MTM Research Center, Orebro University
• Leibniz Institute of Marine Sciences, University of Kiel
• National Institute of Advanced Industrial Science and Technology (AIST)
• Chemosphere 2007 (Article in Press): "Novel evidence for natural formation of dioxins in ball clay"

Article by Steve Down

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.