The kinetics of aggregation of riverine dissolved Fe colloids upon addition of seawater have been studied using samples from two New Zealand rivers.

Results show that the kinetic data are not well-described by a second-order rate law as found in previous studies. A significant fraction of the dissolved Fe was aggregated immediately upon the addition of seawater, with the fraction aggregated in this way increasing as the salinity was increased. Storage of river water before salt addition for 8–48 hr was found to markedly decrease the fraction of Fe aggregated upon salt addition. The rate and extent of aggregation decreased as the pore size of the filters used to separate dissolved and particulate iron was increased. The rate and extent of aggregation were markedly decreased when pre-existing particles already larger than the nominal pore size of the filters were removed before addition of salt, showing that large particles are essential to the aggregation mechanism.

A statistical model of particle collisions has been developed that explains this dependence of aggregation rate on the presence of large particles and on the pore size of the filter.