Colloidal pumping: Evidence for the coagulation process using natural colloids tagged with Hg-203

Colloids isolated from waters of the Trinity River (Texas) were tagged with high specific activity mercury (Hg-203) and re-equilibrated with ultrafiltered water and particles. The transfer of radioisotopic Hg from the colloidal pool to the particulate pool was monitored over a 5-day period. The experiment was repeated with a range of particle concentrations from 10 to 82 mg/L and at salinities of 13.6 and 24.5. Partitioning of Hg-203 to particles and colloids during the radiotracer experiments was comparable or slightly higher than that observed for stable Hg in Galveston Bay. In all cases, Hg-203 was transferred from the colloidal pool (1 kDa-0.4 mu m), consisting mostly of macromolecular organic matter, to the particulate pool (>0.4 mu m), indicating that colloids serve as an intermediary in the transfer and removal of Hg and perhaps other trace metals in natural systems. The rate of transfer was determined by applying an irreversible kinetic model to the data. The transfer appeared to occur by two processes, a fast (<1 h) initial process and a slower process that occurred over days. Both processes were proportional to particle concentration (C-p) expressed as a power function C-p(0.3), suggesting that Brownian coagulation is the mechanism responsible for slow transfer. Measured rates were consistent with coagulation rates calculated from coagulation theory.