Distribution and Speciation of Cr in Selected Serpentinized/Ultramafic Soils from Zimbabwe.

This study was conducted on soils derived from serpentinized /ultramafic toposequence from Zimbabwe, with the objective of determining the concentration, mineral forms, distribution, solubility and speciation of Cr in the soils. Cr is of particular concern since the Cr(VI) species is highly toxic and mobile in soils. These soils contain elevated levels of Cr, Fe, Mn, and Ni, which introduces both environmental and agronomic challenges. The transition metals (including Cr) can exist in multiple oxidation states that determine their bonding mode, solubility, mobility and bio-availability in soil and aqueous environments. Total Cr and exchangeable Cr (VI) were determined using the instrumental neutron activation analysis and s-diphenylcarbazide methods respectively. A sequential extraction procedure was used to determine the distribution of Cr among Fe- and Mn-oxide phases. Cr was localized and predominant Cr-containing phases were identified by means of X-ray diffraction in combination with SEM and EDS. The results are discussed in terms of mineralogy and geochemistry and their implications to agricultural and environmental management.