AGGREGATION OF IRON COLLOIDS IN ESTUARIES - A HETEROGENEOUS KINETICS STUDY USING CONTINUOUS MIXING OF RIVER AND SEA WATERS

The kinetics of iron colloid aggregation in estuaries have been simula ted with a model estuary in which seawater is continuously pumped into a reservoir initially containing river water. Profiles of colloidal F e concentration versus salinity produced in this apparatus closely res embled field data for actual estuaries. Synthetic Fe colloids prepared by peptising Fe(OH)(3) with humic acid and phosphate showed very simi lar kinetic behaviour. Aggregation rate was found to be almost indepen dent of velocity shear rate, implying that most aggregations are induc ed by brownian interparticle collisions. A heterogeneous kinetic model is proposed to explain the kinetic behaviour of Fe colloids during se awater-induced aggregation, This model describes kinetic behaviour in terms of a log-normal distribution of rate constants characterised by a mean value [k] and a standard deviation gamma. Experiments showed th at [k] is linearly related to the rare constant for salinity increase during mixing. This coupling of salinity changes and aggregation rate leads to Fe-salinity profiles that are nearly independent of the rate of salinity change, but which are dependent on gamma.