Adsorptive Behavior of Zeolite-Amended Backfills for Enhanced Metals Containment

Abstract

The containment function of soil-bentonite (SB) cutoff walls can be enhanced when the adsorption capacity of the SB backfill for targeted contaminants is increased. In this regard, the potential use of three commercially available zeolites, i.e., two chabazites and a clinoptilolite, as SB backfill amendments to enhance the adsorption capacity for two metal sorbates, potassium (K) and zinc (Zn), was evaluated via batch equilibrium adsorption testing. The adsorptive behaviors of K and Zn to both the unamended and zeolite-amended backfill sorbents were nonlinear over the concentration ranges of interest. Depending on the specific zeolite, the addition of only 5% zeolite increased the adsorption capacity relative to that for the unamended backfill sorbent by a factor ranging from 6.2 to 7.3 for K and from 2.8 to 3.4 for Zn, whereas 10% zeolite amendment increased the adsorption capacity by a factor ranging from 7.5 to 13.5 for K and 3.1 to 3.7 for Zn. The lower increase in adsorption capacity for Zn relative to K was attributed to preferential selectivity of K relative to Zn. The adsorption behavior of both K and Zn was consistent with cation exchange as the dominant mechanism, provided that the chemical speciation (complexation) of Zn was taken into account. The results show that adding zeolite to SB backfills can enhance the containment of metals via an increase in adsorption capacity, but the enhanced adsorption capacity will be a function of both the metal and the type and amount of zeolite, such that material specific adsorption studies will be required for practical applications.