Mobilization of Heavy Metals as Affected by pH and Redox Conditions

Water is a major vector of transport of heavy metals in the lithosphere. The solids present in soils, aquifers and surface water bodies (i.e., suspended and deposited sediments in rivers, lakes, oceans) can trap significant quantities of toxic heavy metals and act as reservoirs in the various hydrocycles taking place at the earth’s surface. The geochemical phenomena controlling the retention of heavy metals are adsorption and precipitation, while dissolved complexation influences advective and dispersive transport. For all of these, pH and redox conditions are master variables controlling the potential release of stored pollutants to the aqueous phase and therefore their dispersion in the environment and their availability to biota. While the redox status of natural environments is usually expressed as Eh (potential measured at an electrode), by analogy with pH, it is sometimes useful to express redox potentials in terms of pe (-log of electron activity). In this way electrons can be treated like classical reactants and products so that both chemical and electrochemical reactions can be expressed by a single equilibrium constant. Various solids control the fixation of heavy metals: clay minerals, organic matter, oxides and hydroxides of Fe, Mn and Al for adsorption, and low solubility sulfide, carbonate and phosphate minerals for precipitation. Complexing by organic matter, also very important, is addressed in Chapter 3 (Geiger et al.).