Partitioning of metals to sediment organic carbon

The partition coefficients and sorption capacities for cadmium, copper, and lead in freshwater sediments are derived from fitting a Langmuir isotherm to data from 14 sediments. The metal activity in the solution phase was measured using specific ion electrodes. The metal sorption due to acid-volatile sulfide was subtracted from the total sorbed metal to produce the metal bound to other sediment phases. Organic carbon normalized isotherms provide a reasonable representation of the metal sorption. Partition coefficients and capacities are calculated for pH = 6, 7, and 8 (for Cd). The organic carbon normalized isotherm parameters for the sediments are compared to those reported for soils and sludges. The comparisons for cadmium exhibit differences for which no ready explanations are available. For copper and lead, however, the differences can be attributed to the use of specific ion electrodes for measuring metal activity in the sediment experiments. The cadmium sediment partition coefficients contribute a small increment to the sediment quality criteria. For copper and lead, however, the additional binding to sediment organic carbon is significant and can amount to 1 to 10 and 10 to 100 mu mol/g dry weight for lead and copper, respectively, depending on the organic carbon concentration of the sediment.