Effects of Ionic Strength and Flow Rate on Colloid Release: Relating Kinetics to Intersurface Potential Energy

The effects of ionic strength and flow rate on the release of hematite colloids from quartz surfaces were observed in a packed bed column. The rates of colloid release, prompted by an abrupt increase in solution pH, increased as ionic strength was decreased; however, the corresponding intersurface potential energy barriers, calculated using extended DLVO theory and a Born repulsion collision parameter of σ = 5 Å, increased in size. Furthermore, the observed release rates increased with increasing flow rate. These results suggest that colloid transport "over an energy barrier" was not the rate-limiting step in these experiments. We hypothesized that the increase in solution pH was sufficient to cause the disappearance of the detachment energy barrier in these experiments. When we calculated the double layer potentials with surface potentials (estimated using a surface complexation-double layer model) instead of zeta potentials (which decrease with increasing ionic strength owing to compression of the double layer), the potential energy profiles still contained energy barriers. To force removal of the energy barriers in the calculated potential energy profiles, we increased the short-range repulsive energy by increasing the Born collision parameter from 5 to 20 Å, resulting in effective distances of closest approach of about 7 Å. This change resulted in correspondence of the trends in the observed and predicted detachment behavior. In the absence of an energy barrier, we hypothesized that colloid transport across a diffusion boundary layer would control the rate of colloid release. In this case, the colloid release rate should be related to the flow rate through the influence of the flow rate on the thickness of the diffusion boundary layer. The observed dependence of the release rates on the estimated boundary layer thickness was close to that predicted for the release rate-boundary layer thickness relationship.