SPECIATION AND SORPTION CHARACTERISTICS OF CD AND ZN IN BAUXITIC SOILS OF JAMAICA

Cadmium and Zinc are strongly associated in many environmental materials; in rocks the Zn/Cd ratio is typically 100/1. Soils in Manchester Parish, Jamaica have been shown to contain unusually high contents of Cd (mean=52 mg/kg and maximum=931 mg/kg compared with world mean=roughly 0.5 mg/kg). In addition, the Zn/Cd ratio is unusually small at about 20/1. The soils are bauxitic, deep profiles are formed in commercial bauxite deposits, shallow soils have been described as Terra Rossa. They are generally of near neutral pH as a consequence of unconformably overlying pure Miocene limestones.

Since cadmium is potentially highly toxic to humans, a better understanding of its soil chemistry is needed. The soils are Oxisols rich in hydrous oxides of Fe and Mn which are known to sorb Cd and Zn strongly. The sorption characteristics of the high Cd soils have been investigated to find an explanation for the unusual Zn/Cd ratios and the ability of these soils to retain Cd under the high leaching regime of humid tropical conditions. A chemical fractionation procedure was used in order to identify the chemical pools in which Cd and Zn are held in these soils. The experimental approaches comprise: 1) batch equilibration of soil with solutions of Cd and Zn at low concentration and constant temperature. Metal sorption was modelled using the Langmuir isotherm. This was preferred over other models since the resulting concentration plots provide sorption maxima for each element and each soil. 2) a sequential chemical extraction procedure designed to investigate the relative amounts of Cd and Zn in the freely exchangeable pool, the carbonate-bound pool, the Fe oxide-bound pool, the Mn oxide-bound pool and in the residue.

The paper will report results from these experiments and will show that soils with the highest Cd contents appear to be nearly saturated with respect to Cd. Zinc contents exceed the measured sorption maxima. Much of the soil Cd is easily removed during the fractionation procedure. Both Zn and Cd are relatively easily leached from soil profiles and deposits of bauxite require one or more million years to form. The experimental data are therefore interpreted to suggest the soil Cd is unlikely to have originated from the bauxite parent rock and some active geological process must be sought in which Cd is still being supplied to the soils.