Benoit Lab

Trace metals and their speciation

Evaluation of metal speciation changes in response to diurnal pH variations.

Our past research has shown that the Quinnipiac River has elevated levels of toxic metals, including chromium, copper, mercury, lead, cadmium, and silver.  However, the total amount of metal is only part of the story.  Just as important is whether or not the metal is bioavailable.  Only bioavailable forms of metals can be taken up and be toxic to biota.  Our recent study showed that, although total copper levels might be of concern in some locations, the bioavailable form of this metal was consistently low in the samples we collected.  So far, this is good news from the standpoint of aquatic organisms and the human population of the region.  However, there is a risk that low pH can undermine this otherwise desirable scenario.  We have found that pH (acidity) changes dramatically over the course of the day, presumably because of photosynthesis and respiration, and may be lowered even further by acid rain or other pollution.  Low pH makes metals more bioavailable and toxic and also directly impacts aquatic life.  Our measurements suggest that acidity can vary daily by a factor of 30 or greater, and is even worse after rain storms.  We are documenting pH variations and the resulting changes in copper’s bioavailability and toxicity in a number of tributaries and in the main stem of the Quinnipiac River.  Our results to date include several important findings. 
  • All streams we have tested show diurnal variations in pH (and dissolved oxygen and temperature), which are apparently a consequence of photosynthesis and respiration.
  • Variation is greatest for all chemical parameters at sites with the most developed watersheds.
  • Average dissolved oxygen (DO) levels tend to be greatest at sites with the most developed watersheds and presumably the poorest water quality.  This finding contradicts the common belief that high DO is generally indicative of good water quality.
  • Surprisingly, there is a diurnal variation in turbidity and suspended matter that coincides with pH minima that occur at night.
We are continuing this research to investigate the nature of the daily variation in suspended matter and to evaluate Cu speciation changes that occur as a result of the measured pH variations.
This project is funded by the Quinnipiac River Fund

Examination of the speciation of copper in fresh waters with a view to testing whether there is a common, widely-occurring distribution of ligand strength vs. abundance.

Chemical speciation controls a metal’s environmental behavior and biological availability in natural waters.  Speciation is largely controlled in aquatic systems by metal-binding ligands that decrease the free (aquo) metal ion concentration through the formation of stable metal complexes.  Sources of naturally occurring organic ligands include humic and fulvic acids derived from soil, direct exudates from plankton, and strong ligands based on reduced sulfur.
The bioavailability and toxicity of trace metals to aquatic organisms have been linked to the activity of the free aquo ion rather than their total concentrations both in the Free Ion Activity Model (FIAM) and its extension the Biotic Ligand Model (BLM).  Beyond influencing trace metal bioavailability, strong trace metal complexation can significantly influence the reactivity of trace metals with aquatic colloids, larger particles, and other surfaces, and consequently drive the geochemical behavior of the metals.
trace metal
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
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Release of ANC and metals during leaching of concrete in urban watersheds.

Stormwater pollution is a growing problem in the United States, due to increases in urbanized land, impervious surface area, and road traffic.  Hexavalent chromium is a carcinogenic pollutant found in concrete, and our lab is monitoring its release in stormwater.

Our research also aims to improve hexavalent chromium monitoring in stormwater by developing methods to track the reaction of urban concrete surfaces with stormwater runoff through measurement of acid neutralizing capacity, or ANC.  ANC is released from concrete in a predictable way, which can then be compared with hexavalent chromium concentrations, whose release is less predictable, so ANC provides a point of comparison for this pollutant.

Our lab studies concrete chemistry under controlled laboratory conditions with simulated rainfall and in situ, under field conditions during rainstorms in the Quinnipiac River watershed.  By scaling lab observations to field phenomena, we are modeling the chemistry of concrete leaching to quantify hexavalent chromium in urban stormwater.
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