Trace Metal Speciation in a Soil Profile Irrigated with Waste Waters

Abstract

A chemical equilibrium computer model: GEOCHEM, was used to predict the trace metal behavior in a soil profile if subjected to a long-term irrigation with waste waters. Various categories of interactions such as acid-base equilibria, soluble complex formation of metals with organic and inorganic ligands, dissolution-precipitation reactions and ion-exchange reactions were included in simulation. The metal-soluble organic interactions were accounted for by a mixture model. The computations included a 10 metal - 15 ligand system with 262 soluble complex species and 21 possible solid phases. The results predicted that a major fraction of alkali elements in solution tend to be in free ionic forms, whereas, major fractions of the alkaline earths were predicted to be present in adsorbed and/or precipitated forms. In marked contrast, significant soluble fractions of transition series metals were predicted to be in adsorbed and/or complexed forms. The degree of attenuation of these transition series elements moving through the soil profile seems to be governed mainly by the degree of adsorption on soil surfaces and the tendency of these elements to form soluble organic complexes. The predicted mobilities of these elements in this soil profile was Cu > Ni > Zn > Cd. Application of this equilibrium model appears to provide a first approximation approach to simulate the trace element behavior in soil profiles.