Mineral Formation and Weathering

The formation and weathering of the mineral matrix of sediments plays a primary role in the retention and release of ions and compounds. The formation of sediments by authigenic precipitation removes ions from solution and creates fresh surfaces for the adsorption of other inorganic and organic compounds as a secondary retention mechanism. The weathering ofsediments, conversely, releases both adsorbed components and parent material into solution. The overall chemical processes of mineral precipitation and dissolution can be generally described in terms of nucleation, crystal growth, and weathering.

Nucleation is often the rate-limiting step of mineral precipitation and can occur by homogeneous or heterogeneous mechanisms. During homogeneous precipitation, crystal nuclei are formed in a saturated solution by the random collision of ions. Heterogeneous nucleation involves the formation of mineral nuclei on the surfaces of reactive solids already present. Regardless of the nucleation pathway, once stable nuclei are formed, mineral growth proceeds spontaneously until the solution is no longer saturated with respect to that mineral. The rate of nucleation is controlled by the degree of supersaturation, temperature, and the geometry ofinitial nuclei formed or heterogeneous materials available for nuclei seeding. Nucleation rates are also dependent upon the specific interfacial free energy, which is the difference in free energy between an ion bound within the mineral matrix and an equivalent ion bound to the mineral surface.

Crystal growth involves transport of reacting ions to the mineral surface, transport of reaction products away from the mineral surface, and surface interactions including adsorption, surface complexation, dehydration, and cation exchange. Mineral growth can be transport controlled, where growth is limited by the rate at which ions or complexes migrate to the surface via diffusion and advection, or surface controlled, where the rate-limiting step is the surface interaction involved. Most mineral formation reactions are surface-controlled processes.

The nature of sediment weathering products and their subsequent release depends upon the mineralogical composition of the parent material, the chemical composition of the aqueous phase, and the nature of fluid flow. Chemical weathering reactions are often classified by the attacking agent type and the manner in which the mineral is altered. Dissolution and hydrolysis reactions are mediated by potential attacking substances including acids, oxygen-containing ligands, and water. Since many weathering reactions can be treated as acid-base reactions, the pH of sediment porewaters exerts a primary control on sediment dissolution and weathering rates. The source of acidity in sediments can be organic acids exuded by plant roots or from the decay of organic matter; sulfuric acid from sulfide oxidation; nitric, sulfuric, and hydrochloric acids in acid rain; and carbonic acid, with CO2 derived from the atmosphere or the respiration of organic matter.

If simple dissolution is the primary mechanism of weathering, the process is termed congruent dissolution, as illustrated by the dissolution of quartz (SiO2(qtz)):

This reaction is reversible, and dissolved silica in the form of silicic acid, H4SiO4(aq), can precipitate to form quartz. But the kinetics of quartz precipitation are quite slow below 70 °C, and as a result silicic acid concentrations are often measured in excess of its predicted solubility of about 180 p,M in low-temperature soil and sediment porewaters. Therefore, while thermodynamics predict the precipitation of quartz from many soil and sediment porewaters, the slow kinetics involved often result in the supersaturation of dissolved silica.

The dissolution of calcite, CaCO3(calc), is another congruent weathering reaction:

In this process, calcium carbonate and carbonic acid react to form calcium cations, Ca2+, and bicarbonate anions, HCO3~. This reaction yields no solid phase, because the carbonate mineral undergoes complete dissolution. The equilibrium of this reaction, as well as that of the preceding reaction, is pH dependent, with higher acidity resulting in greater dissolution of the product ions into solution.

If a secondary mineral forms during a chemical weathering reaction the process is referred to as incongruent dissolution. A common form this type of process takes is the weathering of aluminosilicates minerals, which are stable at high temperatures and pressures characteristic of the Earth's interior, to clay minerals, which are more stable at the low temperatures and pressures characteristic of the Earth's surface. An example of incon-gruent dissolution is the hydrolysis of potassium feldspar, KAlSi3O8(Kspar), to kaolinite, Al2Si2Os(OH)4(kaol):

! Al2Si2O5(OH)4(kaol)

9 H2O

The incongruent dissolution of potassium feldspar not only forms the solid-phase kaolinite, but also releases silicic acid, H4SiO4(aq), and potassium cations, K+, into solution. The stability of the different mineral phases is, again, dependent upon pH and fluid chemistry, with increased acidity and decreased silicic acid and potassium cation concentrations promoting feldspar weathering. Determination of which mineral phase is most stable under prevailing sediment conditions can be made with thermodynamic models and stability (Eh:pH) diagrams.

Weathering reactions generally neutralize acids and release base cations (Ca2+, Mg2+, Na+, K+) into solution. Weathering of aluminosilicates releases dissolved silica, but not comparable levels of aluminum due to its relatively low solubility around neutral pH (7). But in freshwater lakes with sediments and soils low in carbonates and low acid-neutralizing capacities that are impacted by acid rain, the pH of porewaters eventually drops to levels where the dissolution of aluminum (Al3+) is sufficient to cause asphyxiation in fish and invertebrates.

Nevertheless, aluminum is often treated as a conservative element and its concentration is compared to that of other elements in sediments to estimate levels of contamination. These qualitative estimates are based on the assumption that the concentration of aluminum in contaminated sediments is not enriched relative to its average crustal abundance 8.1 mgg-1), while concentrations of other elements may be enriched in contaminated sediments relative to their average crustal abundance. The resulting enrichment factor (EF) is then derived with a simple normalization of those ratios:

Is t/[Al]s, crustal abundance m crustal abundance where[X] is the concentration of the element of interest and [^1] is the concentration of aluminum.

The presence of ligands and organic matter in sediment porewaters can increase mineral weathering by complexing cations involved in the weathering reaction. This decreases the cation's free ion concentration and causes a thermodynamic shift which favors further mineral dissolution, as predicted by Le Chatelier's principle. Complexation by organic matter also increases the solubility of metals via this mechanism, which acts to facilitate the dissolution of minerals containing metals exhibiting low solubilities at near neutral pHs, such as Fe3+, Al3+, and Hg2+.

Finally, microbes commonly play an important role in the rates of both mineral precipitation and weathering, and thus the sequestering or release of nutrients and contaminants in sediments. Microbes often mediate the underlying chemical reactions involved or take advantage of the energy released by the associated reactions. For example, microbes can facilitate nucleation by their cell membranes when they act as nucleation sites or when they produce an organic molecule that serves as a template for an inorganic crystal, a phenomenon known as biomineralization. Similarly, microbes often effectively control the rate of sulfide mineral oxidation, a thermodynamically favorable but kinetically slow inorganic reaction, by their production of enzymes which catalyze the reaction as a means of harnessing the energy released.

Figure 1 Stylized representation of a clay particle with surface coatings: (a) iron oxyhydroxides, (b) manganese oxyhydroxides, (c) aluminosilicates, (d) other inorganics (e.g., calcite, apatite), (e) organic matter, (f) microorganisms (e.g., bacteria), (g) cross section of the clay particle showing net negative charge on its surface, (h) clay surface with a net negative charge resulting in the subsequent adsorption of cations and other coatings as detailed above. Although each of these coatings is shown as being discrete, they generally overlap and overlay each other.

Figure 1 Stylized representation of a clay particle with surface coatings: (a) iron oxyhydroxides, (b) manganese oxyhydroxides, (c) aluminosilicates, (d) other inorganics (e.g., calcite, apatite), (e) organic matter, (f) microorganisms (e.g., bacteria), (g) cross section of the clay particle showing net negative charge on its surface, (h) clay surface with a net negative charge resulting in the subsequent adsorption of cations and other coatings as detailed above. Although each of these coatings is shown as being discrete, they generally overlap and overlay each other.

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