Previous investigators have tended to model trace metal speciation as resulting from a single type of ligand, while generally acknowledging that a range of ligand types are present. In some instances, two ligands are posited, one strong and one weak (L1 and L2), though even this is a simplification of the real world. I argue that: (1) a continuum of ligand types is found in most freshwaters, (2) a systematic relationship describes the strength and abundance distribution of these ligands, and (3) treating ligands as a single type can lead to errors in predicting metal speciation, whereas exploiting knowledge of the distribution relationship can result in better prediction with less analytical effort.
The figure shows Cu binding ligand abundance against strength for fresh waters in studies where multiple detection windows were used, including our research in Linsley Pond. Taken together, these studies suggest that a single relationship may describe the distribution of ligand strength and abundance in many freshwater systems. The long, dotted line has a slope of -0.25 and reflects a regression of all freshwater data.
In this research, I am testing the hypothesized quantitative relationship between Cu binding ligand strength and abundance, which may be generally applicable across fresh water systems, and to begin to explore its causes(s). I am explicitly testing the hypothesized relationship between CL and K’ in individual freshwater systems, while using a broad range of analytical methods with detection windows spanning several orders of magnitude. Specifically, I am examining ligands with strengths ranging from log K’ 5 to 15 or greater, while using a minimum of 5 detection windows. Work of this kind has never before been done on individual natural aquatic systems. Even if the proposed relationship is invalidated, we will provide the first ever data on the distribution of ligand strengths across many orders of magnitude of site abundance and how it varies with time and the critical parameters pH and dissolved organic carbon (DOC).
Results of this research should have the immediate practical benefit of improving and simplifying our ability to predict Cu speciation and its consequences, toxicity and bioavailability. At a more fundamental level, the research should provide new insights into the nature and characteristics of copper binding ligands. We focus on Cu because it is an important biologically active metal and its speciation has received more attention than any other. If successful, we plan to extend the research to other metals, like Co, Zn, and Ni, in the future.
This project is funded by the National Science Foundation