Behaviour of trace cadmium in boom clay reducing sediment I. Complexation with in situ dissolved humic acids

Abstract

Boom clay is a reducing clay layer that contains 1.5% pyrite, 1% CaCO 3, 1.2% organic carbon and about 50% clay. Trace Cd conditional complexation constants were determined from Cd and organic matter distribution measurements at different solid/liquid ratios under in situ reducing conditions of solid- and liquid-phase composition. Cadmium forms 1:1 complexes with boom clay humic acids. The conditional complexation constant of trace Cd with boom clay humic acids under in situ solution conditions (pH 9, E h = −250 mV) is log K 1 = 7.5. Conditional complexation constants decrease by about 0.15 log K 1 units for each 10-fold increase in Cd loading in the range 10 −6 to 10 −10 eq. Cd/g organic matter. Cadmium-organic matter complexes were predicted to dominate the speciation of Cd in the in situ solution phase (pH 9, CO 3 = 0.015 M, 100 ppm DOM).