Wei-Haas, M. L., Hageman, K. J., & Chin, Y.-P. (2014). Partitioning of Polybrominated Diphenyl Ethers to Dissolved Organic Matter Isolated from Arctic Surface Waters. Environmental Science & Technology. doi:10.1021/es405453m
Polybrominated diphenyl ethers (PBDEs) are brominated flame retardants that are classified as persistent organic pollutants (POPs) and found ubiquitously in the environment. They are now banned in many regions of the world, however certain mixtures of PBDEs are still widely produced and used in the United States and can be found in all sorts of products from couches to roof shingles to pajamas.
Marine dissolved organic matter (DOM) is a complex mixture of organic matter found in seawater. Largely it consists of (and as such is measured by) a huge pool of organic carbon in the oceans that is of a very small size that can pass through a very small filter (i.e. < 0.5 microns). However it is important to note that in addition to carbon, there are thousands of complex molecules within the DOM pool that often make it difficult to completely characterize.
PBDEs are largely hydrophobic molecules, meaning they hate the water. Thus, they are much more likely to bind or sorb to something they have a greater affinity, or liking, to such as organic matter or fatty tissues in organisms (e.g. fish or humans). There are various different methods we can use to measure how likely organic contaminants, such as PBDEs, are to be bound or sorbed to a certain material – these are referred to as partition coefficients, and are usually denoted with the letter “K.” When you see Kow, it is referring to the partitioning between octanol and water, or the ratio of the compound you will find in octanol versus water. Octanol in many cases is used as a proxy, or substitute, for fatty tissue; so if you see a compound with a high Kow, you might expect that it will really want to be in an organism versus the water. In a similar sense, a compound Koc, is the ratio of a contaminant you see in organic carbon versus water. In most cases, compounds with high Kow values will also have high Koc values. This study aims to find Kdoc values for 6 different PBDEs, which means the focus it only on the dissolved component of organic carbon.
Since we know PBDEs are largely hydrophobic and don’t like to exist in water, we want to know where they might end up. Some options, and often where we find them, are bioaccumulating up the food chain through organisms and binding to sediment. The authors of this paper focus on the Arctic environment and investigate dissolved organic matter (DOM) and the interaction that PBDEs might have with it. Interactions have been proven in the past between DOM and other hydrophobic contaminants that link to altered environmental mobility, bioavailability, and degradation in the environment. If PBDEs, which have typically high Kow values, are likely to bind to DOM, their transport and fate may be greatly affected.
How they did it
DOM is extremely complex and its composition may vary, depending on the region of interest. Structures of DOM can range from high aromaticity and molecular weight, which often comes from land-derived sources, to low aromaticity and molecular weight, which is often indicative of marine sources such as phytoplankton and microbial action. In simpler terms, the level of complexity of DOM can affect the sorption of hydrophobic contaminants, such as PBDEs.
The authors collected and isolated DOM from the North Slope of Alaska using what’s called a ‘solubility enhancement method and PPL solid phase extraction cartridges’ (fancy terminology for the lab work performed), choosing to look at the interactions of 6 PBDE congeners or varying bromination (BDE-28, -47, -77, -99, -153, and -190) with the isolated Arctic DOM as well as laboratory standards. They analyzed their observations and compare them to predicted values using what’s called a poly-parameter linear free energy relationship, or pp-LFER. This is basically just a way of plotting a combination of various parameters of the compounds (such as how many hydrogens it’s willing to donate or accept) in logarithmic form to come up with a KDOC.
What they found
DOM concentrations in the Arctic surface waters are high and high Kdoc values were measured for the PBDEs observed in this study. When you link high amounts of DOM and high Kdoc values, you can get as much as 70% of the dissolved mass of higher brominated PBDEs partitioning to DOM, which may largely affect the fate and bioavailability of the compound.
It is clear that PBDEs are unique and do not always follow the same trends as observed in other hydrophobic compounds. However, certain trends observed were that the Log Kdoc increased with increasing bromine substitution, but when looking at molecules with the same number of bromines, the Log Kdoc may also be affected by the position of the bromine on the molecule (i.e. if it’s in the “ortho” position on the molecule). Values determined for KDOC in this experiment are 1-2 orders of magnitude lower than what has been reported previously in the literature, which illustrates the complexity of aquatic DOM and demonstrates that using laboratory derived standards for organic matter may not always be the best option.