Evaluation of metal stabilization ability of adsorbents for toxic metals in solid waste by sequential extraction

The stabilization of toxic metals (Cd, Zn, Pb, Cu and Mn) in a solid waste from a zinc refinery was studied by the addition of either a synthetic adso rbent (zeolite A) or an organic adsorbent (dried water hyacinth and rice hu ll carbon). The stabilization ability of an adsorbent was evaluated by mean s of sequential extraction, which separated the metals into five fractions. Metals extracted in the first two fractions by relatively weak solvents ar e considered as mobile. These metal contents provide an estimation of the a mount available for plant uptake. Metals extracted in the last three fracti ons are considered to be immobile. Stabilization ability of an adsorbent wh en mixed with solid waste can be considered as the transformation of a meta l from mobile to immobile form. In this work an index, the 'stabilization dose' (SD), is defined for use as a measure of the stabilization ability of an adsorbent for a metal. SD50 v alue is the percentage amount (w/w) of an adsorbent which, when added to so lid waste, can reduce 50% of a metal formerly in the mobile fractions. The lower the SD50 value the greater is the stabilization ability of the adsorb ent. By comparing the SD values for four metals, zeolite and dried water hy acinth are found to be more effective than rice hull carbon. The SD50 value s of zeolite were 6.9, 18.6, 25.5, and 30.0 %w/w for Pb, Cd, Zn and Cu, res pectively. The SD50 values of dried water hyacinth were 23.8 and 46.4%w/w f or Cu and Zn. Only copper could be stabilized by rice hull carbon with SD50 value of 45.1%w/w.