Foam as a Delivery Vehicle in Vadose Zone Remediation: Transport; Amendment Distribution; and Tc-99 Immobilization

Foam is a promising low water content advection technology for the distribution of remedial amendments to the deep vadose zone for metals and radionuclides remediation. Foam flow in the vadose zone can be directed by manipulating the pressure field, therefore the foam delivery technology allows more even distribution of remedial amendments under vadose zone conditions than water-based delivery. With minimal water content in foam flow, foam delivery minimizes the potential for contaminant mobilization and spreading. Laboratory scale experiments were conducted to investigate the physical aspects of foam transport in sediments under vadose zone conditions, to study the (poly)phosphate amendment foam-delivery; and to evaluate technetium-99 immobilization by foam-delivered calcium polysulfide. Foam quality (percentage of gas volume in total foam volume) between 95% and 99% was tested to study its influence on foam injection pressure. It was observed that the injection pressure increased inversely as a function of foam quality. The effect of sediment permeability on foam injection pressure was evaluated in sediments with a range of permeabilities. Results suggested that when the foam injection rate was low, the injection pressure increased with sediment permeability; but when the foam injection rate was high, the injection pressure versus permeability relationship was reversed. Foam bubbles rupture at the foam-flow-front in the unsaturated sediments. The front of the gas injected with foam traveled significantly faster (~ 8 times) than the foam front observed in the sediment. The liquid carried by foam was left behind the foam front. In addition a water accumulation front developed in sediment ahead of the foam-flow-front, i.e. sediment not swept by foam flow, as native pore water was displaced by the foam front. The water content in this liquid accumulation front increased with the increasing of initial water content in the sediment. However, the water content in the sediment swept by foam flow was negligibly affected by the initial water content. Very low surfactant concentration was observed in the accumulated liquid ahead of the foam front, indicating the initial moisture in the sediment was pushed forward by foam flow. In the foam-delivery of (poly)phosphate column tests, the delivery of phosphate was retarded slightly compare to the foam front and the distribution of surfactant in the sediment. The profile of pore water electric conductivity was similar to the phosphate distribution profile. Immobilization of Tc-99 by foam-delivered calcium polysulfide (CPS) in sediments under vadose zone conditions was evaluated. Foams generated from 0.5 wt% STEOL CS-330 surfactant solution at quality of 98% was used to deliver CPS sediment in columns for reductive immobilization of Tc-99. The foaming solution contained 5 wt% of calcium polysulfide. Using the foam-delivered calcium polysulfide, 66% of the total Tc-99 in the contaminated sediment was immobilized in the sediment. No technetium remobilization was observed when up to 6 pore volume of synthetic ground water was flushed through the treated sediment column.