Solid-Phase Speciation and Surface Association of Metals in Serpentinitic Soils

Serpentinitic soils are of great ecological and environmental interest, yet pedologic studies of such soils are relatively rare, particularly concerning heavy metal solidphase speciation. This study examined the surface association and solid-phase speciation of metals, the mineralogy and the surface chemistry of serpentinitic soils. Surface association and solid-phase speciation were assessed by selective wet chemical methods. Mineralogy was determined by X-ray diffraction and surface chemistry by potentiometric titration. The morphology of clay minerals was studied by transmission electron microscopy. Surface-as-sociated metals were divided into exchangeable and nonexchangeable fractions. Adsorbed Mg and Ca were predominantly in exchangeable form. More than 10% of adsorbed Mn and Ni were present in exchangeable form, but generally <1% of adsorbed Al and Fe were exchangeable. Serpentine and Fe oxides were the major soil mineral components, along with lesser amounts of quartz, whereas gibbsite and smectite were not detected. Iron oxides contained more than 10 mole-% of non-Fe elements (Al, Cr, Ni, and Si). The soils also contained Mn oxides that appeared to incorporate Co and Ni.