The delivery of calcium polysulfide (CPS) to the vadose zone using foam and the immobilization of Cr(VI) via reduction by the foam-delivered CPS was studied in a series of batch and column experiments. Batch tests were conducted to select the foam-generating CPS–surfactant solutions, to determine the solution foamability and the reducing potential of CPS-containing foams, and to study the influence of foam quality, surfactant concentration, and CPS concentration on foam stability. Column experiments were performed to test the foam delivery of CPS to sediments under conditions similar to a field vadose zone, to study the foam transport and interaction with sediments, and to determine the extent of Cr(VI) immobilization using this novel delivery approach. Foams containing CPS with high reducing potential were prepared based on the batch tests. Sediment reduction by foam-delivered CPS was observed in the columns. Massive mobilization of Cr(VI) from sediments occurred when CPS was delivered in aqueous solution. The Cr(VI) mobilization was minimized in column tests when CPS was delivered by foams, resulting in significant Cr(VI) in situ immobilization. These results demonstrated for the first time that foam injection can be successfully used for CPS delivery to the vadose zone and that foam-delivered CPS can be applied for Cr(VI) immobilization in contaminated vadose zones.

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