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 adsorbent (zeolite A) or an organic adsorbent (dried water hyacinth and rice hull carbon). The stabilization ability of an adsorbent was evaluated by means of sequential extraction, which separated the metals into five fractions. Metals extracted in the first two fractions by relatively weak solvents are considered as mobile. These metal contents provide an estimation of the amount available for plant uptake. Metals extracted in the last three fractions are considered to be immobile. Stabilization ability of an adsorbent when mixed with solid waste can be considered as the transformation of a metal 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 value is the percentage amount (w/w) of an adsorbent which, when added to solid 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 adsorbent. By comparing the SD values for four metals, zeolite and dried water hyacinth are found to be more effective than rice hull carbon. The SD50 values of zeolite were 6.9, 18.6, 25.5, and 30.0 %w/w for Pb, Cd, Zn and Cu, respectively. The SD50 values of dried water hyacinth were 23.8 and 46.4%w/w for Cu and Zn. Only copper could be stabilized by rice hull carbon with SD50 value of 45.1%w/w.