Long-Term Column Testing of Zeolite-Amended Backfills. I: Testing Methodology and Chemical Compatibility

Eight column tests were performed with specimens comprising unamended and zeolite-amended sand-bentonite backfills for vertical cutoff walls permeated with solutions of either 35 mM KCl (five tests) or 20 mM ZnCl2 (three tests) to evaluate the potential use of zeolite amendments for enhancing the sorption capacity of K+ and Zn2+. The column tests used flow-pump systems to maintain constant flow rates and flexible-wall cells to minimize the potential for side-wall leakage and short circuiting of solutes. However, the use of low flow rates (2.06–8.05 mL/d) to prevent membrane blow-out and initial permeation with deionized water to control soluble salts contributed to extensive test durations ranging from 385 days (1.05 year) to 1,370 days (3.75 year). The column specimens were characterized by soil:solution ratios ranging from 3.3∶1 to 3.7∶1 and hydraulic conductivity, k, based on permeation with a salt solution that were <1.0×10−9 m/s. Thus, the k of the backfill specimens was consistent with that typically required of backfills for vertical cutoff walls used for containment of contaminated groundwater. Also, effluent electrical conductivity (EC) and pH attained the values for the influent in the case of the 35 mM KCl solution, whereas for the low pH (1.83), 20 mM ZnCl2 solution, the buffering capacity of the backfill specimens reduced the contribution of H+ to the EC of the effluent, resulting in a slight increase in the effluent pH (2.80≤pH≤3.74). Finally, delayed breakthrough of K+ and Zn2+ was consistent with preferential cation exchange of K+ and Zn2+ relative to exchangeable Na+ and/or Ca2+ initially bound to the exchange sites of the bentonite and zeolite components of the backfills.