Weathering of minerals

Contamination of waters with ai senic occurs as a result of a number of industrial activities such as treatment of industrial wastes, fertilizers, pesticides production, mining, metal smelting etc. and natural processes (e.g. weathering of minerals, volcanic and biological activities). 

Groundwater environments can be represented as a simple flow-through system. For the situation where chemical weathering of mineral grains is transport controlled, the weathering rate of a mineral should be directly dependent on the rate of throughput of water. For the situation where rates are controlled by surface... 

As noted earlier, diverse forms of organomercury are released into the environment as a consequence of human activity. Methyl mercury presents a particular case. As a product of the chemical industry, it may be released directly into the environment, or it may be synthesized in the environment from inorganic mercury which, in turn, is released into the environment as a consequence of both natural processes (e.g., weathering of minerals) and human activity (mining, factory effluents, etc.). 

Most of the rocks that make up the upper crust of the earth lie hidden beneath layers of sediments, unconsolidated accumulations of particles derived from the weathering of minerals and rocks (see Fig. 44 and Textbox 45) (Keller 1957). Once formed, the particles are either carried away or moved by the wind, rain, and gravitational forces into the seas and oceans or, before they get there, into depressions in the land. There they accumulate in a wide range of shapes and sizes (see Table 49) (Rocchi 1985 Shackley 1975). 

Vinegar is recommended for cleaning a variety of appliances and other items that may be stained by hard water deposits. Automatic coffee makers, steam irons, dishwashers, teapots, faucet heads, and shower heads — over time, all accumulate calcium deposits from hard water. Groundwater, that is, water that travels through soil and rocks, accumulates dissolved calcium ions as a consequence of the natural weathering of minerals that contain calcium such as limestone and calcite, shells, and coral. At the same time, carbon dioxide in the air dissolves in water to form carbonate ions that combine with calcium ions to form a white solid, calcium... 

Chemical weathering of minerals during pedogenesis can be enhanced by microbial activity by a factor as high as 106 (Kurek 2002). Microorganisms can dissolve minerals by direct and indirect actions under aerobic and anaerobic conditions (Robert and Berthelin 1986 Ehrlich 2002 Kurek 2002). In some cases of attack, the microorganisms may be dispersed in the soil solution in others, they may grow in biofilms on the surface of susceptible minerals. 

In this chapter, we build on applications in the previous chapter (Chapter 26), where we considered the kinetics of mineral dissolution and precipitation. Here, we construct simple reactive transport models of the chemical weathering of minerals, as it might occur in shallow aquifers and soils. 

Small amounts of cadmium enter the environment from the natural weathering of minerals, but most is released as a result of human activities such as mining, smelting, fuel combustion, disposal of metal-containing products, and application of phosphate fertilizers or sewage sludges (USPHS 1993). In 1988, an estimated 306,000 kg of cadmium entered the domestic environment as a result... 

The weathering of minerals forms particles with a size continuum from ions to grains. Mineral dissolution and precipitation occur more or less continuously as a function of ambient conditions. Particles of the clay textural fraction may be suspended in solution as colloids as well as occurring as part of the stationary solids. 

The rate of chemical weathering of minerals in the subsurface depends on a number of factors, including mineralogy, temperature, flow rate, surface area, presence of ligands and CO, and H+ concentrations in the subsurface water (Stumm et al. 1985). Figure 2.3 shows the rate-limiting steps in mineral dissolution consisting of... 

Many technical-chemical processes take maximum benefit of similarities with ongoing processes in Nature, with increased purity or reaction speed as the most important differences. The production of carbonates is a typical example of this, and the process of C02 mineralization for carbon capture and storage (CCS) (see Section 14.4) is in fact the accelerated version of what is known as the natural weathering of minerals. This is a combination of the interacting processes of mechanical and chemical weathering, and relevant to the current discussions are the chemical weathering processes of dissolution and hydrolysis that involve C02 [6, 7]. A dissolution equilibrium reaction that proceeds in Nature with dissolved C02 in water and calcite gives a bicarbonate solution ... 

Metal complexation by low-molecular-weight organic substances released by plants and microbes have been involved in weathering of minerals and release of nutrients (e.g., Fe) in the rhizosphere (Violante et al., 2003). 

In Figure 9.21 all of the carbon eventually used in weathering of minerals by CC>2-charged soil water is shown as entering the atmosphere. The difference between the flux of CO2 owing to precipitation of carbonate minerals in the ocean and the total CO2 released from the ocean is that CO2 used to weather silicate minerals on land, and agrees with the calculations of riverine source materials made earlier in this chapter, in which it was shown that 30% of the HCC>3 in river water comes from weathering of silicate minerals. 

The weathering of minerals can be understood as a continuous dissolution-precipitation process involving them. The process can be very complicated, leading to multiphase equilibria, in which more than one solid phase, solution, and gas phase may be present. For example, primary silicates transform to secondary silicate minerals via such weathering reactions (Stumm and Wollast, 1990), as in the formation of kaolinite (AljSijC OH) from anorthite (CaAl2Si208) ... 

Chemical weathering of minerals results not only in the introduction of solutes to the aqueous phase but often in the formation of new solid phases. Dissolution is described as congruent, where aqueous phase solutes are the only products, or incongruent, where new solid phase(s) in addition to aqueous phase solutes are the products. These reactions... 

The present chapter has described the presence of EMF where soil minerals occur and their formation of intimate contact with soil minerals. It seems clear that EMF can dissolve at least some of these minerals when exposed to them in laboratory conditions. Furthermore, EMF obtain large amounts of carbon from their host trees and convert much of it to organic and carbonic acids, making them potentially important in dissolving minerals in natural ecosystems. The main questions for future research are to quantify the contribution of EMF to total weathering of minerals in the soil and to determine whether this contribution will change as a result of more intensively managed forest ecosystems and increased... 

Strong weathering of minerals in the confinement area. This may be why water trapped within sedimentary generated strata becomes more saline at greater depths. It is interesting to observe that... 

The interplay of acid-base, solubility, and complex ion equilibria is often important in natural processes, such as the weathering of minerals, the uptake of nutrients by plants, and tooth decay. For example, limestone (CaCOj) will dissolve in water made acidic by dissolved carbon dioxide ... 

Because natural weathering of minerals in the upper soil horizons is not sufficient to neutralize that amount of acid, a gradual acidification takes place so that even calcareous soils can be affected, especially at the stem base of beech and red oak, both possessing high stem flow rates. In silicate soils, clay minerals can be destroyed, releasing AP " and Fe, by replacing... 

High arsenic concentrations can also occur in alkaline, closed-basin lakes. Mono Lake, California, USA has dissolved arsenic concentrations of (10-20) X lO pg with pH values in the range 9.5-10 as a result of the combined influences of geothermal activity, weathering of mineralized volcanic rocks, evaporation of water at the lake surface, and a thriving population of arsenate-respiring bacteria (Maest et al., 1992 Oremland et al., 2000). 

As noted above, the principal natural sources of selenium in water are likely to be sulfides or metal oxides containing adsorbed selenium, especially Se(IV). Coal can be an additional primary source of selenium either directly through oxidation or indirectly via atmospheric precipitation following combustion. Selenium is readily oxidized during the weathering of minerals. Seleniferous ground-water areas such as those in the USA and Pakistan... 

Stage 1 corresponds to a steady state where base cation concentrations in runoff are relatively constant, averaged over periods longer than an annual cycle. BS of soils is also relatively constant. The rate of export of base cations is equal to the rate of chemical weathering of minerals... 

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