Sorption of heavy-metal cations by Al and Zr-hydroxy-intercalated and pillared bentonite

The sorption of Cd, Cu, Pb, and Zn ions by Na-rich bentonite, Al and Zr-pil lared Na-rich bentonite (Al-MX80, Zr-MX80), the uncalcined hydroxy-intercal ated precursors (HAI, HZr-MX80), and commercial Al-pillared bentonite EXM 5 34 was investigated. Experiments were conducted in ultrapure water and arti ficial leachate with varying pH. The experiments were performed over period s to 30 wk. Sorption characteristics were described with one and two-site L angmuir isotherms. The non-exchangeable quantities of heavy metals were det ermined by fusion of the sorbents after ion exchange with ammonium acetate. The sorption of Cd, Cu, Pb, and Zn by bentonite was dominated by cation ex change. In artificial leachate, the sorption was reduced due to competition with alkali and alkaline-earth cations. The sorption of Cu, Zn, and Pb at pH 4.9 and Cd at pH 6.9 by Al and Zr-hydr oxy-intercalated and pillared MX80 was governed also by cation exchange. In contrast, the sorbed quantities of Zn at pH 6.9 exceeded the cation exchan ge capacity (CEC) of HAl, HZr, Al, Zr-MX80, and EXM 534 and were partially nonexchangeable. The increase of the sorption of Zn with pH and its indepen dence of the ionic strength of the solution at neutral pH suggest a complex ation of Zn ions to surface hydroxyl groups of the intercalated Al and Zr-p olyhydroxo cations and pillars. This complexation is the dominating sorptio n mechanism. Removal of dissolved Zn from solution with time is attributed to surface precipitation. Al-hydroxy and pillared bentonites are considered potential sorbents of Zn ions from neutral pH aqueous solutions, such as w aste waters and leachates.