Metals

Metals are found in industrial wastes in a variety of forms. When these metals are introduced into the subsurface environment, they can react with water and soil in several physicochemical processes to produce appreciable concentrations that affect the quality of groundwater. The most important processes that affect the concentration and mobility of metals in groundwater include filtration, precipitation, complexation, and ion exchange.

Filtration occurs when dissolved and solid matter are trapped in the pore spaces clogging the pore spaces and decreasing the permeability of the soil system (Dragun 1988b).

Precipitation occurs when metal ions react with water to form reaction products which precipitate in soil as oxide and oxyhydroxide minerals or form oxyde and oxy-hydroxide coatings on soil minerals. Precipitation of metals as hydroxides, sulfides, and carbonates is common (Dragun 1988b).

Complexation involves the formation of soluble, charged or neutral complexes between metal ions and inorganic or organic anions called ligands. The complexes formed influence the mobility and concentration of the metal in groundwater. For example, the mobility of zinc in groundwater is affected by the formation of complex species between the zinc ion and inorganic anions present in the water, such as HCO-, CO|-, SOf-, Cl-, F-, and NO -3 (Freeze and Cherry 1979). The complexation of cobalt-60 ions by synthetic organic compounds enhances its mobility in groundwater (Killey et al. 1984). Other metal species are reported to be highly mobile in ground-water after soluble complexes are formed with humic substances or organic solvents (Bradbent and Ott 1957; Griffin and Chou 1980).

The predominant complex species in an aqueous solution are influenced by the redox and pH of the soil. The relationship between the redox, pH, and the complex species is commonly expressed in Eh-pH diagrams for each metal; Eh is the electronic potential. Figure 9.12.1 shows an example of an Eh-pH diagram for mercury. Methods for calculating Eh-pH diagrams are discussed by several authors (Brookings 1980; Garrells and Christ 1965; Verink 1979).

Using Eh-pH diagrams, environmental engineers can qualitatively determine the most important complexes formed by the metal in water and estimate the concentration and mobility of the metal in groundwater. The concentration of cations reported in chemical analyses of groundwater normally represents the total concentration of each element in water. However, most cations exist in more than one molecular or ionic form. These forms can have different valences and, therefore, different mobilities due to different affinities to sorption and different solubility controls.

Adsorption is another process affecting the concentration and mobility of metals in groundwater. Positive adsorption involves the attraction of metal cations in water by negatively charged soil particles. Therefore, adsorption can decrease the concentration of dissolved metals in water and retard their movement. The cation exchange ca

FIG. 9.12.1 Stability fields of solid phases and aqueous species of mercury as a function of pH and Eh at 1 bar total pressure. (Reprinted from J.D. Hem, 1967, Equilibrium chemistry of iron in groundwater, In Principles and applications of water chemistry, edited by S.D. Faust and J.V. Hunter, New York: John Wiley and Sons.)

FIG. 9.12.1 Stability fields of solid phases and aqueous species of mercury as a function of pH and Eh at 1 bar total pressure. (Reprinted from J.D. Hem, 1967, Equilibrium chemistry of iron in groundwater, In Principles and applications of water chemistry, edited by S.D. Faust and J.V. Hunter, New York: John Wiley and Sons.)

pacity (CEC) of a soil, defined as the amount of cations adsorbed by the soil's negative charges, is usually expressed as milliequivalents (meq) per 100 grams of soil. In general, clay soils and humus have a higher CEC than other soils.

Some cations are more attracted to a soil surface than others based on the size and charge of their molecule. For example the Cu2+ cation in water can displace and replace a Ca2+ cation present at the soil surface through a process known as ion exchange. Also, trivalent cations are preferentially adsorbed over divalent cations which are preferentially adsorbed over monovalent cations. The release of

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