Investigations Into the Presence Of PCDDs in Ball Clay Exhibiting the 'Natural Formation' Profile

Recently, there have been many reports of environmental samples exhibiting a unique PCDD/F profile, which can not be attributed to any of the known anthropogenic sources. Samples exhibiting this PCDD/F 'natural formation' profile include ball clay and kaolin from different regions throughout the world. Although the PCDD/Fs found in ball clay and kaolin are thought to be of natural origin, no definitive evidence has been brought forward to explain their presence in this specific environmental matrix. A series of experiments involving three ball clay cores were conducted, with the goal of determining the origin of the PCDDs. Principal Component Analysis was used to evaluate the data and provide visual representations of it, which aids in determining relationships among a multitude of variables.

All clay core samples analyzed exhibited the PCDD natural formation profile, containing elevated levels of PCDDs, dominated by OCDD and decreasing concentration with decreasing chlorination. Maximum TEQs for the three cores were 2,500, 440, and 15,000 pg WHO-TEQ/g, respectively. Results from the bulk mineralogical analysis demonstrated there is no statistical correlation between the mineralogy of the ball clay samples and the PCDD concentrations and/or homologue profiles.

It was determined from the elemental and oxide analysis that there is a direct correlation between specific oxides (e.g., Al2O3) and total PCDD concentration associated with the samples in two of the cores, which have considerably different mineralogy. Results from a black carbon analysis confirmed that there is no correlation between the carbon content of the ball clay total PCDD concentrations. These BC data support the hypothesis that the PCDDs associated from the ball clay are not the result of an ancient combustion source.

The findings from these experiments advanced the understanding of the possible origin of dioxins associated with the ball clay. From these findings, coupled with what has been reported in the literature, it is hypothesized that the PCDDs associated with the ball clay are the result of a formation process from naturally produced chlorinated organic compounds present during the deposition of the clay. In this hypothesis, the varying chemical composition of the clay (e.g., amount of Al2O3) is the predominant property controlling PCDD concentration as observed throughout the cores.