Strong adsorption on Fe and Mn oxides and hydrous oxides Precipitation
Cr is very mobile in neutral to alkaline conditions.
As is more mobile under anaerobic than aerobic conditions and in alkaline conditions. Pb2+2 is relatively immobile except in highly acidic environments. Cd, Cr(IV), Hg, Ni, Se.
Cr + organic material ^ insoluble (aerobic conditions) precipitates.
Cr(III) hydroxide, carbonate, and sulfide precipitate (pH > 6); Cr(VI) does not precipitate in these conditions.
Pb typically precipitates as Pb(OH)2, PbCO3, Pb5(PO4)3OH. NaCl increases solubility.
Ni carbonates, hydroxides, and sulfides are relatively insoluble; Ni oxides in acidic solution may precipitate with neutralization.
Many selenium compounds can be reduced to produce elemental selenium when exposed to organic matter in subsurface environment.
As(OH)3 to As(CH3)3 (anaerobic); Hg (inorganic) to methyl mercury (anaerobic).
Source: U.S. EPA, Assessing the Geochemical Fate of Deep-Well-Injected Hazardous Waste: A Reference Guide, EPA/625/ 6-89/025a, U.S. EPA, Cincinnati, OH, June 1990.
Solution complexation is of major importance for the fate of metals in the deep-well environment. Soluble metal ions in solution can be divided into three major groups: simple hydrated metal ions,103 metals complexed by inorganic anions, and organometallic complexes.104 The stability of complexes between metals and organic matter is largely independent of ligand, and follows the following general relationships105:
1. Monovalent ions: Ag > Tl > Na > K > Pb > Cs
2. Divalent ions: Pt > Pd > Hg > UO2 > Cu > Ni > Co > Pb > Zn > Cd > Fe > Mn > Sr > Ba
3. Trivalent ions: Fe > Ge> Sc > In > Y > Pl > Ce > La
Hydration reactions between metal ions and water affect mobility and adsorption but not toxicity. Hydrolysis is particularly important in the chemistry of cyanide.
Oxidation-reduction reactions may affect the mobility of metal ions by changing the oxidation state. The environmental factors of pH and Eh (oxidation-reduction potential) strongly affect all the processes discussed above. For example, the type and number of molecular and ionic species of metals change with a change in pH (see Figures 20.5-20.7). A number of metals and nonmetals (As, Be, Cr, Cu, Fe, Ni, Se, V, Zn) are more mobile under anaerobic conditions than aerobic conditions, all other factors being equal.104 Additionally, the high salinity of deep-well injection zones increases the complexity of the equilibrium chemistry of heavy metals.106
Forstner and Wittmann107 reported the following observations about the general mobility of heavy metals in groundwater:
1. Mobility tends to increase with increasing salinity because alkali- and alkaline-earth cations compete for adsorption sites on solids.
2. A change in redox conditions (lower Eh) can partly or completely dissolve Fe and Mn oxides and liberate other coprecipitated metals.
3. When natural or synthetic complexing agents are added, soluble metal complexes may form.
FIGURE 20.5 Distribution of molecular and ionic species of divalent cadmium at different pH values. (From U.S. EPA, Assessing the Geochemical Fate of Deep-Well-Injected Hazardous Waste: A Reference Guide, EPA/625/6-89/025a, U.S. EPA, Cincinnati, OH, June 1990.)
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