The partition of Cu among model soil constituents such as Fe oxides (2-line-Ferrihydrite), silicate clays (K-saturated Montmorillonite), and organic matter (Leaf Compost) was studied using a dialysis equilibration technique.

Specific amounts of individual solid phases in suspension were placed in dialysis bags enabling different solids to be equilibrated together in the same external solution. Copper was added to the multicomponent system and copper activity was determined at increasing periods of time. Dialysis bags were made from 0.01 μm polycarbonate membrane layers by sealing two 3.5x10 cm rectangles and clamping one end with a weighted closure. Five mL of 0.01 M KNO₃ containing 0.5 g of an individual solid phase were placed in the dialysis bags and a second closure was used to seal the bag. Three bags, each containing an individual solid phase, were placed together in a 1 l polypropylene reactor containing 975 ml 0.01 M KNO₃. After 24 h, a load of Cu equivalent to 100 mg Cu Kg^-1 solids was added to the system. pH was kept constant at 6.0 ± 0.2 using a pH-stat device with additions of 0.1 M HNO₃ or 0.1 M KOH.

External solutions were analyzed for copper speciation and dissolved organic carbon. Copper was determined as free copper by ISE, as labile copper by DPASV, and as total dissolved copper in acidified samples by both ISE and DPASV. For easy comparison all the values are shown in pCu units (-log Cu²⁺). Dissolved organic carbon was determined as non-purgable organic carbon in a Simadzu TOC-5000A Total Organic Carbon Analyzer.

Results presented in Table 1 show that the activity of copper is two orders of magnitude lower than the initial Cu load. However, copper added to the multicomponent system remained in solution after 24 h. One tenth of the total Cu in solution was DPASV-labile (free hydrated cupric ions and Cu soluble complexes that dissociate very rapidly to yield the free metal) while nine tenths of the total soluble Cu exist as non-labile species (cupric ions complexed with organic matter in solution). Practically no copper was sorbed onto the solid phases after 24 h, but remained in solution as Cu-DOM complexes (Table 1). Results from cyclic voltammetry and ISE indicate the presence of several metal-organic complexes with different labilities.

Data collected during longer periods of time will be presented together with analyses of the solid phases for total Cu concentration and chemical forms.

Table 1. Copper speciation in the multicomponent system studied over 24 h.

	Cu Load	Free copper	Labile copper	Total dissolved copper
Method	pCu
ISE	5.61	7.63		5.95
DPASV	5.61		6.58	5.63