Abstract: A systematic investigation is presented on chemico-osmotic membrane behaviors and diffusive properties of model sand/hexametaphosphate (SHMP)-amended bentonite cutoff-wall backfill (SHMP-SB) for the containment of zinc-impacted groundwater. A multi-stage chemico-osmotic test is conducted to determine the chemico-osmotic efficiency coefficients, effective diffusion coefficients and retardation factors of the backfill using the zinc nitrate (Zn(NO₃)₂) solution with varied concentrations. The results indicate that the measured chemico-osmotic efficiency coefficients of SHMP-SB gradually decrease and then tend to stabilize with the increasing Zn(NO₃)₂ concentration. A comparison with the previous studies shows that the measured chemico-osmotic efficiency coefficient of SHMP-SB is about 2-3 times that of the parent soil-bentonite backfill in the Zn(NO₃)₂ solutions. The effective diffusion coefficient increases with the increasing source concentration of Zn(NO₃)₂ solution, whereas the retardation factor of zinc decreases with the increasing Zn(NO₃)₂ concentration.