Tuesday, May 6, 2014

Using Environmental Forensics to Solve a Chemical Mystery in the Passaic River

On April 15, 2014, the Editorial Board of the New York Times wrote an op-ed concerning the proposed plan for the cleanup of the Passaic River in New Jersey – a river that has often been described as “toxic,” “dirty,” and “troubled.” The drivers for the cleanup, and for such descriptions, are the pollutants contained in the sediments of the river – vestiges of decades upon decades of heavy industrial activity. The contaminants typically discussed are PCBs, dioxin, mercury, and heavy metals; but as the New York Times notes, the list is far longer than that. Many of the contaminants are not just the products that were manufactured along the banks (e.g. DDT), but also the byproducts from chemical manufacturing (e.g. dioxin or PCB 11). It is this latter category that is sometimes less understood due to the unintentional nature of production, and the analytical challenges (especially historically) of detecting the presence of these by-products. In September of 2014, however, Matson & Associates environmental chemist and engineer, Dr. Robert Parette, and Wendy Pearson, president of the firm, published an article in the journal Chemosphere, discussing a compound that while not well known to the general public, may provide an important tracer for contaminants in the Passaic River and Newark Bay.

The compound is 2,4,6,8-tetrachlorodibenzothiophene, or 2,4,6,8-TCDT.

For the last twenty years the presence of 2,4,6,8-TCDT in river sediments and aquatic life has been somewhat of a mystery. While a number of sources were previously suggested, none adequately explained the presence of this unique dioxin-like compound.[1]

In “2,4,6,8-Tetrachlorodibenzothiophene in the Newark Bay Estuary: The likely source and reaction pathways” Parette and Pearson (2014) evaluate past hypotheses and then put forth their own: the compound is likely the result of an impurity in the starting materials for the well-known herbicide, 2,4-dichlorophenoxyacetic acid (2,4-D), and chlorophenols that were produced at a former chemical manufacturing facility that historically made Agent Orange located on the Passaic River.

This connection between 2,4-D and 2,4,6,8-TCDT is far from obvious at first glance, which is one of the reasons the relationship possibly evaded researchers for so long. First, to form 2,4,6,8-TCDT, a source of sulfur would be necessary during production (thiophenols are organosulfurs). In modern times the starting material for 2,4-D (phenol) is not produced via any method that would incorporate sulfur. However, during the 1940’s and up until the 1970’s a process called sulfonation was utilized to commercially produce phenol. In this process, benzene reacted with sulfuric acid and in doing so also produced a thiophenol impurity. As Parette and Pearson show, when chlorinated during the 2,4-D production process this impurity likely became 2,4,6,8-TCDT.
 A second impediment to understanding the link between the two compounds is that the facility along the Passaic River would have needed to actually utilize phenol containing this impurity. As Parette and Pearson detail, almost all of the phenol used in the production of 2,4-D at the facility  was synthesized using the sulfonation method. Further, operational practices at the plant resulted in the discharge of wastes from the 2,4-D process to the river.

Employing the techniques of environmental forensics, specifically industrial chemistry reconstruction – utilizing a blend of historical and modern literature, industrial process knowledge and organic chemistry – the paper walks through the likely synthesis and production, discusses soil samples taken at the site, and explains why 2,4,6,8-TCDT has not been found in more modern samples of 2,4-D.  While 2,4,6,8-TCDT is only one of the many contaminants that lie in the sediments that EPA has proposed to remove; the compound is evidence of the importance of understanding not only the end product in manufacturing, but all the products created in the process of synthesis. For not only might they be more toxic than the end product (e.g. dioxin impurities in the manufacture of organochlorides), they might prove to be valuable tracers – as 2,4,6,8-TCDT may prove now that its presence is explained.

[1] It should be noted that 2,4,6,8-TCDT has a relatively low toxicity in comparison to the oft cited 2,3,7,8-TCDD.

Submitted by Kate McMahon, Research Associate