Copper(II) activity in aged suspensions of goethite and organic matter

Suspensions of natural organic matter and iron oxide (FeOOH) were equilibra ted with a wide range of concentrations of added Cu2+ (50-5000 mg kg(-1)) f or as long as 400 d, in order to observe the time-dependent solubility of t he 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, mea sured 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 nea r 5.5, indicating that the organic matter was a more effective adsorbent fo r Cu2+ than goethite. Longer reaction times generally caused the Cu2+ activ ities to decrease due to drifting pH and slow reactions. At higher pH, orga nic matter again resulted in lower Cu2+ activity than the oxide at all Cu l oading levels, but the difference was generally less than at low pH. A down ward pH trend in the organic system initially at pH 6.5 caused the Cu2+ act ivity to increase with longer reaction times. Most of the long-term changes in Cu2+ activity could be attributed to shifting pH, although an aging eff ect 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 lar ger fraction of Cu2+ adsorbed on organic matter than on the oxide, suggesti ng that strong chelating agents selectively extract metals in quantities no t directly related to activity or potential toxicity.