Mixed metals migration through zeolite-amended backfills for vertical cut-off walls

Column tests were performed with two sand–bentonite backfills comprising 5·8% bentonite with 5% of one of two high-cation-exchange-capacity (CEC) zeolite amendments – clinoptilolite and chabazite upper bed (chabazite-UB). The backfill specimens were permeated with a mixed salt solution of 17·5 mM potassium chloride (KCl) and 10 mM zinc chloride (ZnCl₂) to evaluate the extent of attenuation of two metals – potassium (K) and zinc (Zn). The specimens were characterised by low hydraulic conductivity (<1·0 × 10⁻⁹ m/s) and delayed breakthrough of potassium and zinc that was consistent with exchange of potassium (K⁺) and zinc (Zn²⁺) ions for the sodium (Na⁺) and/or calcium (Ca²⁺) ions initially occupying the exchange sites of the bentonite and zeolite components of the backfills. The results were consistent with those previously reported for the same backfills permeated with single-salt solutions of the same metals, indicating that the competition between potassium and zinc ions in the mixed salt solution had little effect on the metals migration. Overall, the metals attenuation was enhanced relative to that for the unamended backfill by a factor ranging from 2·0 for zinc with both zeolite-amended backfills to 3·7 for potassium with the chabazite-UB-amended backfill, further illustrating the benefit resulting from amending a sand–bentonite backfill with a small amount of high-CEC zeolite.