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Copper(II) Activity in Aged Suspensions of Goethite and Organic Matter

Dep. of Soil, Crop, and Atmospheric Sciences, Cornell Univ., Ithaca, NY 14853

*Corresponding author (mbm7{at}cornell.edu).

ABSTRACT

Suspensions of natural organic matter and iron oxide (FeOOH) were equilibrated with a wide range of concentrations of added Cu²⁺ (50–5000 mg kg^-1) for as long as 400 d, in order to observe the time-dependent solubility of the free Cu²⁺ ion. Adsorption was conducted at two pH ranges (5.0–5.5 and 6.5–7.0) and in the presence and absence of phosphate. The Cu²⁺ activity, measured by ion-selective electrode, was one to two orders of magnitude lower for the organic systems at all Cu loadings investigated when the pH was near 5.5, indicating that the organic matter was a more effective adsorbent for Cu²⁺ than goethite. Longer reaction times generally caused the Cu²⁺ activities to decrease due to drifting pH and slow reactions. At higher pH, organic matter again resulted in lower Cu²⁺ activity than the oxide at all Cu loading levels, but the difference was generally less than at low pH. A downward pH trend in the organic system initially at pH 6.5 caused the Cu²⁺ activity to increase with longer reaction times. Most of the long-term changes in Cu²⁺ activity could be attributed to shifting pH, although an aging effect on activity could be discerned. Phosphate added to the oxide or organic matter had little effect on the activity of Cu²⁺. The EDTA extracted a larger fraction of Cu²⁺ adsorbed on organic matter than on the oxide, suggesting that strong chelating agents selectively extract metals in quantities not directly related to activity or potential toxicity.

Received for publication November 18, 1997.

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