SREL Reprint #3211

Mobile Colloid Generation Induced by a Cementitious Plume: Mineral Surface-Charge Controls on Mobilization

Dien Li1, Daniel I. Kaplan1, Kimberly A. Roberts1, and John C. Seaman2

1Savannah River National Laboratory, Aiken, South Carolina 29808, USA
2Savannah River Ecology Laboratory, University of Georgia, Aiken, South Carolina 29802, USA

Abstract: Cementitious materials are increasingly used as engineered barriers and waste forms for radiological waste disposal. Yet their potential effect on mobile colloid generation is not well-known, especially as it may influence colloid-facilitated contaminant transport. Whereas previous papers have studied the introduction of cement colloids into sediments, this study examined the influence of cement leachate chemistry on the mobilization of colloids from a subsurface sediment collected from the Savannah River Site, USA. A sharp mobile colloid plume formed with the introduction of a cement leachate simulant. Colloid concentrations decreased to background concentrations even though the aqueous chemical conditions (pH and ionic strength) remained unchanged. Mobile colloids were mainly goethite and to a lesser extent kaolinite. The released colloids had negative surface charges and the mean particle sizes ranged primarily from 200 to 470 nm. Inherent mineralogical electrostatic forces appeared to be the controlling colloid removal mechanism in this system. In the background pH of ~6.0, goethite had a positive surface charge, whereas quartz (the dominant mineral in the immobile sediment) and kaolinite had negative surface charges. Goethite acted as a cementing agent, holding kaolinite and itself onto the quartz surfaces due to the electrostatic attraction. Once the pH of the system was elevated, as in the cementitious high pH plume front, the goethite reversed to a negative charge, along with quartz and kaolinite, then goethite and kaolinite colloids were mobilized and a sharp spike in turbidity was observed. Simulating conditions away from the cementitious source, essentially no colloids were mobilized at 1:1000 dilution of the cement leachate or when the leachate pH was ≤8. Extreme alkaline pH environments of cementitious leachate may change mineral surface charges, temporarily promoting the formation of mobile colloids.

SREL Reprint #3211

Li, D., D. I. Kaplan, K. A. Roberts, and J. C. Seaman. 2012. Mobile Colloid Generation Induced by a Cementitious Plume: Mineral Surface-Charge Controls on Mobilization. Environmental Science and Technology 46(5): 2755-2763.

This information was provided by the University of Georgia's Savannah River Ecology Laboratory (srel.uga.edu).