Measuring Bioavailable Copper in Estuarine Waters using an Ion Exchange Resin

Abstract

Anion exchange methods were used to measure the percent of weakly bound and strongly bound copper in estuarine waters and agricultural drainage. Copper bound to an ion exchange resin has been operationally defined as bioavailable copper, or copper that is available for uptake by aquatic organisms.

Initial laboratory testing indicated that the percent of copper bound to the resin did not differ between a freshwater solution (pH 6.5 buffered with 30 mg/L as calcium carbonate) and a saline solution (25 ppth salinity at pH 8). Adding 20 mg/L natural organic matter (NOM) to each solution lowered the amount of copper sorbed to the resin by the same amount in saline and fresh water. Varying salinity from 16 to 30 ppth did not effect % bioavailability. This indicates that the ions typically found in seawater do not compete with copper for ion exchange onto the resin. Varying initial copper concentration from 195 to 495 ppb for the given NOM content of 12.5 mg/L did not effect % bioavailability, indicating that as the copper concentration of this water is increased over the range studied, the fraction of potentially toxic copper is constant.

Decreasing the pH of the system from 8.5 to 4 and the NOM from 12.5 to 0 mg/L statistically increased % bioavailability in both cases. However, copper retention onto the resin at pH values near 4 was hindered, possibly because resin functional groups become inefficient at low pH values. MINEQL+ predicted that the resin was not retaining approximately 30% Cu⁺² at a pH of 4. Copper retention to the resin was also not as high as expected at NOM contents between 0 and 3 mg/L at pH 8.1. MINEQL+ indicated that the water studied at pH 8.1 was comprised primarily of strongly bound copper-organic ligand species and Cu(OH)₂. Experimental results suggested that the resin retained only 50% Cu(OH)₂, which is a potentially bioavailable copper complex. However, MINEQL+ computer modeling indicated that the resin was accurately retaining potentially bioavailable copper in waters with a pH from 5.5 to 8.5, containing 3 to 12.5 mg/L NOM.

After assessing the characteristics of copper retention by the resin, it was applied to an estuarine system and agricultural drainage to measure bioavailable copper. The results indicated that in copper-containing agricultural runoff and groundwater the percent bioavailable copper was approximately 15% of the dissolved copper. This percentage was relatively constant over the range of copper concentrations observed. In estuarine systems receiving agricultural runoff, the percent bioavailable copper was about 11%.

This research was supported by a grant from the Virginia Pesticide Control Board.