Dioxins in clay: Broad geographic study leads to global inventory

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  • Published: Sep 5, 2011
  • Author: Steve Down
  • Channels: Gas Chromatography
thumbnail image: Dioxins in clay: Broad geographic study leads to global inventory

Dangers of dioxins in clay

It is widely known that the highly toxic dioxins are ubiquitous pollutants, contaminating the air, land and water and reaching otherwise pristine parts of the globe. It is also widely believed that they are produced during industrial processes such as paper production, smelting and waste incineration but there are also some significant natural sources such as forest fires and volcanoes.

A much less heralded natural source is kaolin ball clay, which was laid down millions of years ago and is embedded in sedimentary rock layers. The polychlorinated dibenzo-p-dioxins (PCDDs) present are dominated by the octachloro congener (OCDD), with the related polychlorinated dibenzofurans (PCDFs) at low levels or undetectable.

The clay-bound dioxins have been recognised in soils and sediments from their unique congener profile, due to natural leakage from the clays. However, millions of tons of kaolin ball clay are mined each year for use in the ceramics industry and the industrial processes involved release dioxins into the environment.

Other applications have also caused problems with dioxin release. The presence of PCDDs in some animal products, particularly chicken and fish, was blamed on the use of ball clay as an anticaking agent in animal feedstuffs. High levels of PCDDs in milk were attributed to the use of the clay in a potato sorting process before moving up the food chain.

The amount of ball clay mined each year worldwide was estimated at 38.5 million tonnes in 2007. Given the scale, it would be interesting to estimate the dioxin content of the clay and compile a global inventory.

There have been some estimates of the levels of dioxins in kaolin ball clays from the USA, Germany and Spain which revealed a consistency between the PCDD congener compositions that implied a common mechanism of formation

Now, the picture has been made a lot clearer following a new study by scientists across four countries who have examined ball clays from ten locations around the world.

Global dioxin survey

The research was carried out by Yuichi Horii from the Center for Environmental Science in Saitama, the National Institute of Advanced Industrial Science and Technology, Ibaraki, Japan, the City University of Hong Kong and the Wadsworth Center and State University of New York at Albany.

Kaolin ball clays from the USA, Brazil, UK, Japan, China, South Korea, Indonesia, India, New Zealand and Australia were examined. Samples were acquired from retail suppliers or the ceramics industry. The dioxins were Soxhlet extracted with toluene and purified on acidic silica gel and activated carbon columns for analysis by GC/MS.

The congeners were separated on a strongly polar column coated with a highly substituted cyanopropyl phase for some of the compounds and a less polar diphenyl dimethylpolysiloxane phase for the others.

The eluting compounds were detected by electron ionisation mass spectrometry operating under high resolution, with quantitation by selected ion monitoring.

Dioxin data and inventory

Dioxins were found in all 33 clays studied but in widely different concentrations. Those from Brazil, the UK and New Zealand contained low levels of 1.2, 3.9 and 5.4 pg/g, respectively, although only one sample was analysed in each case.

Clays from Indonesia and South Korea contained 35 and 41 ng/g dioxins, respectively, and the mean value for Chinese kaolin was 300 pg/g, with a range of 3.1-1500 pg/g.

The most abundant clays were those from the USA with 520,000 pg/g dioxins. Two types of Japanese clay, Kibushi and Gaerome, also contained relatively high concentrations at 5900 and 1600 pg/g, respectively, the different values reflecting the characteristics of the clays. Kibushi is a dark clay stained with organic matter that resembles US ball clay, whereas Gaerome comprises course quartz grains and some feldspar grains.

In all of the samples, OCDD was dominant, taking up about 85% of total dioxins and confirming previous observations by other research groups. PCDFs were at trace levels or below.

The dioxin profile was notably different from those of anthropogenic sources, with the ratios of particular PCDDs appearing to differentiate between the two. For instance, the ratio of 1,2,3,7,8,9-hexa-CDD to 1,2,3,6,7,8-hexa-CCD generally had a value greater than one in the clays but less than one in anthropogenic sources.

The measured concentrations were used to calculate the global inventory of dioxins from kaolin production, leading to a total of 650 kg/yr. In terms of toxic equivalents (TEQs), this equated to 2400 g-TEQ/yr. The contribution of OCDD was estimated at 480 kg/yr. The US ball clay was by far the most abundant source, contributing 550 kg/yr.

The team calculated that the total TEQs for dioxins from Japanese clay would be 4.8 g-TEQ, assuming that all of the dioxins were released to the environment during high-temperature processing. This accounts for about 2.9% of the total dioxin release in Japan.

In contrast, total dioxin release from the US ball clays would correspond to an additional 130% of the dioxins released in the USA, illustrating the importance of kaolin ball clay as a potential dioxin source.

This is the first published report on dioxin levels in kaolin clays from Asia and, with the other regions, illustrates the widespread occurrence of dioxins. The data suggest that dioxin release from ceramics factories should be examined more closely.

Horii proposed that further research into the presence of dioxins in the mother rocks and sedimentary products in lakes and marshes should be carried out in an attempt to derive their origins and their mechanism of formation.



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


The levels of naturally produced dioxins in kaolin ball clays from around the world have been measured by GC/MS to produce a global inventory, with US sources providing the greatest estimated release into the environment

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