Towards the Use of Software Modelling for Determination of Heavy Metal Speciation in Soils in Zambia

Abstract

The toxicity of heavy metals in water is typically related to the concentration of the metals in water. However, it is known that apart from the concentration the speciation of the heavy metals is also a critical factor. In this study, the concentration and type of species (ionic form) of Cu, Co, and Pb from two different soil samples on the Copperbelt Province in Zambia were investigated by modeling using Visual MINTEQ software. The concentration and the conductivity of the sample from ore-dump site were higher than that of the second site further away owing to the higher mineral content in the waste ore-dump site. Modeling by Visual MINTEQ uses defined parameters such as concentration, ionic conductivity and pH of the sample solutions to analyze the free metal ion and bound-metal distribution as a function of pH. The results of the model showed that Cu2+, Pb2+ and Co2+ ions thrive in aqueous acidic solutions up to pH 6.5. At higher pH, formation of the hydroxo and bicarbonate species leads to a decline in the concentration of the divalent ions. Interestingly, the Co2+ ion persists in solution up to pH 8.5 with little formation of the hydroxo or carbonate forms. Finally, the results show that without elaborate analytical tools, speciation modeling can be used to ascertain the availability of metal ions in soils/aqueous solutions when chelating and competing metal ions are taken in consideration.