HEAVY-METAL RELEASE FROM A SERPENTINE SOIL USING A PH-STAT TECHNIQUE

Serpentine soils show high contents of Cr and Ni. Acidification may convert the metals bound in silicate structures into bioavailable forms. The effects of mineral acids on a serpentine-derived Typic Hapludoll were examined in column studies. Cadmium and Pb were also examined. One end of the soil-packed column was placed in 1 mM HCl solution and constant pH was maintained by pumping the acid through a H-saturated cation-exchange resin. The experiment was conducted for 100 d, after which the soil column was vertically sectioned. Chemical and mineralogical analyses showed steep vertical gradients in the sectioned columns. Base saturation decreased by about 60% at the contact of the column with the acid; exchangeable Fe, Cr, Pb, and Al increased while Ni decreased. Cadmium exhibited a differential behavior in that it increased in concentration in the middle of the column. Extracts using ethylenediaminetetraacetic acid (EDTA) gave similar results. X-ray diffraction indicated chlorite dissolution by acid loading. The resin-based pH-stat technique is a useful method in simulating soil weathering.