PARTICLE-SIZE EFFECTS ON COLLOID DEPOSITION KINETICS - EVIDENCE OF SECONDARY MINIMUM DEPOSITION

The influence of particle size on the deposition kinetics of several s ubmicron carboxyl latex spheres, sulfate latices, and hematite particl es was examined in granular quartz beds as a function of ionic strengt h. Experiments were performed near pH 10 in the presence of sodium dod ecyl sulfate to reduce surface heterogeneity by masking possible hydro phobic regions on the interacting surfaces. The onset of unfavorable f iltration conditions and the sensitivity of the experimental attachmen t efficiencies, alpha(exp), to changes in the ionic strength were both particle size dependent. The influences of pH and ionic strength, how ever, were opposite to those predicted by filtration theory based on t he primary interaction energy barrier height. Correlations of alpha(ex p) with the secondary minimum of the interaction energy showed that, a s attachment efficiencies approached unity, the depth of the secondary well increased. These observations suggest that particles may be reta ined within the secondary minimum even when an energy barrier is suffi cient to inhibit attachment in the primary energy well. This hypothesi s was further supported by particle detachment experiments which revea led that approximately 80% of the particles attached under unfavorable deposition conditions could be released when the secondary minimum wa s eliminated.