The importance of dissolved organic matter to the binding of copper and the release of trace elements from coal ash.

摘要

The bioavailability and toxicity of copper to aquatic life depends on its speciation. Dissolved organic matter (DOM) plays an important role in the speciation of copper, but there is still much uncertainty about what controls the strength and formation of the Cu2+-DOM complex. The ratio of copper to DOM is known to affect the strength of Cu2+-DOM binding, but previous methods to determine Cu2+-DOM binding strength have generally not measured binding constants over the same Cu:DOM ratios, making these results difficult to directly compare. A competitive ligand exchange-solid phase extraction (CLE-SPE) method and a copper ion-selective-electrode (Cu-ISE) method were used to determine conditional stability constants for Cu2+-DOM binding at near neutral pH and 0.01 M ionic strength over a range of Cu:DOM ratios that bridge the detection windows of previous measurements reported in the literature. As the Cu:DOM ratio increased from 0.0005 to 0.1 mgCu mgDOM-1, the measured conditional binding constant (cKCuDOM) decreased from 1011.5 to 105.6 M-1. This behavior is consistent with the presence of Cu2+ binding sites of higher affinity and lower abundance that become filled as the total copper concentration increases. A comparison of the binding constants measured using CLE-SPE with those measured by Cu-ISE and voltammetry methods demonstrates that the Cu:DOM ratio is an important factor controlling the Cu2+-DOM binding strength for a variety of DOM isolates and whole water samples.;Using correct Cu2+-DOM binding constants to accurately model copper speciation is important for predicting copper toxicity in cases like the waters draining the Pebble deposit in southwestern Alaska, where small increases in the dissolved copper concentration may be harmful to salmonids and other aquatic biota. Experimentally determined Cu2+-DOM binding constants were used as inputs in Visual MINTEQ to model copper speciation and calculate Cu2+ concentrations. The results were then compared to results from the biotic ligand model (BLM), a speciation and toxicity model recommended by U.S. EPA for calculation of stream copper standards, which uses the Windermere Humic Aqueous Model (WHAM) to model metal interactions with DOM. The BLM was found to over-estimate Cu2+ at low total copper concentrations and under-estimate Cu2+ at high total copper concentrations, which may result in over- or under-estimations of toxicity.;DOM also has also been shown to enhance the dissolution of soils, sediments and minerals, which could result in the release of toxic trace elements into aqueous systems. Coal ash contains high concentrations of toxic trace elements that may have the potential to be released into the water. Releases of coal ash to rivers and streams, such as that which occurred from the Kingston Fossil Plant in Kingston, TN, into the Emory and Clinch Rivers in December 2008, are a concern because of the potential for human health problems, as well as ecological effects. In order to understand the effect that DOM has on the release of trace elements from coal ash, a series of release experiments were performed using coal ash generated from the Kingston Fossil Plant as a function of DOM concentration, DOM aromaticity, and calcium concentration. Various DOM isolates and filtered site-water samples collected near the Kingston Fossil Plant were used to produce coal ash suspensions at a fixed ash:water ratio of 1:1000 and a near-neutral pH. The major and trace elemental composition of the solution, and specific trace metals (mercury, lead, copper, aluminum) and metalloids (arsenic and selenium) were measured. The results indicate that DOM enhances the release of mercury, lead, copper, and aluminum from coal ash. The concentration of mercury, lead, copper and aluminum released from the coal ash was positively correlated with the SUVA254 (ultraviolet absorbance at 254 nm divided by the dissolved organic carbon concentration) of the DOM. The release of arsenic and selenium from the coal ash was not dependent on the DOM concentration or SUVA254. Calcium was shown to inhibit the release of mercury, lead, copper and aluminum from the coal ash, but did not have the same effect on the release of arsenic and selenium.