Role of metal-organic complexation in metal sorption by soils

This chapter focuses on the nature of interaction among trace metals in soil solution, dissolved organics in soil solution, and solid surfaces. The interaction between metal cations and dissolved polyfunctional organic compounds of low molecular weight is important because of its role in mineral-weathering and soil-forming processes and its potential role in heavy metal contamination of soil and groundwater. The chapter presents the organics and metals in the soil solution. Dissolved organics that interact with soil constituents and trace metal ions are of two major kinds: a range of low-molecular-weight organic acids—including polyphenols, simple aliphatic acids, amino acids, sugar acids, and hydroxamate siderophores; and a series of soluble humic/fulvic acids. Numerous environmental issues arise in relation to the interaction of metal ions with soluble organics. Some of these include the phytoavailability of metals, plant nutrient availability, toxicological effects of coordinated metal ions on aquatic and marine organisms, and transport of contaminants, particularly in relation to implications for surface and groundwater quality and soil genesis. All of these issues are highly dependent on the nature and concentration of the contaminant in the soil solution phase. Extant research indicates that low-molecular-weight ligands in soil solution may either enhance or retard reactions with solid surfaces—depending on the functional groups on the organic molecule, soil surface properties, and soil solution conditions. It is imperative that increased research efforts be devoted to evaluating the effects of these organics on metal reactions in the soil. © 1995 Academic Press Inc.