Weathering, 296 alkalinity chemical

The influence that variations of temperature and levels of atmospheric CO2 and O2 have on chemical weathering are more subtle. Temperature appears to have a direct effect on weathering rate (White and Blum, 1995). The silica concentration of rivers (Meybeck, 1979, 1987) and the alkalinity of ground waters in carbonate terrains (Harmon et al., 1975) are both positively correlated with temperature variations. It is not clear, however, whether temperature-related variations in weathering rates are largely due to variations in vegetational activity that parallel temperature variations. 

As shown in Figure 14.4, each clay mineral exhibits a large range in the type and degree of isomorphic substitution. The central silicon atom in the tetrahedral layers can be replaced by aluminum, alkali, alkaline earth, and trace metal atoms. In the octahedral layers, the central Al and Mg atoms can be similarly replaced. The large range in composition within each mineral type reflects variability in the environmental conditions under which crystallization and chemical weathering occur. Thus, the... 

These alkaline conditions are due in part to the consumption of acid during the low-temperature chemical weathering of plagioclase in a process similar to Eq. 19.4. 

Reverse weathering Chemical reactions that are theorized to occur in the sediments. In these reactions, seawater is thought to react with clay minerals and bicarbonate producing secondary clays and consuming alkalinity and some cations. This process is approximately the reverse of chemical weathering on land that produces clay minerals. 

The weathering of surface rocks has had a critical role in the chemical evolution of the continental crust for most of the Earth s history. In the presence of air and water, mafic minerals tend to rapidly weather into iron (oxy)(hydr)oxides, clays, and other silicate minerals, and at least partially water-soluble salts of alkalis (sodium and potassium) and alkaline earths (calcium and magnesium). In contrast, quartz in felsic and intermediate igneous rocks is very stable in the presence of surface air and water, which explains why the mineral readily accumulates in sands and other sediments. 

Clay minerals and clay colloids are the products of the advanced weathering of primary silicates. They are comprised mainly of silica and alumina, often with appreciable amounts of alkali and alkaline earth metals and iron. Most also have varying amounts of water bound to their surfaces and can take on a variety of different chemical and physical properties depending on the amount of water adsorbed. They have the ability to exchange or bind cations and anions and are capable of complex formation with a wide variety of organic molecules. 

Rainwater is essentially free of mineral solutes. It is usually slightly acidic due to the presence of dissolved carbon dioxide, or more highly acidic because of acid rain-forming constituents. As a result of its slight acidity and lack of alkalinity and dissolved calcium salts, rainwater is chemically aggressive toward some kinds of mineral matter, which it breaks down by a process called chemical weathering. 

In fact, recent work has shown that the export of carbonate alkalinity in the Mississippi River has increased in the past half-century due to significant increases in rainfall and chemical weathering in the Mississippi Basin (Raymond and Cole, 2003). Moreover, it appears that watersheds with forested areas have less alkalinity transport than cropland systems. While the mechanisms of alkalinity export are not well understood, these results have major implications for management of land-use patterns in watersheds as it relates to carbon sequestration issues. 

The pH of a system determines the reactions that define the concentration of many dissolved chemical species in water containing salts and minerals, supplied by weathering reactions, rain, runoff, and lixiviating processes. The pH is a key parameter for biological growth and for the sustainment of life for the different aquatic flora and fauna species. As discussed in Chapter 2 the contribution of the different species will affect the final pH and vice versa (i.e., the pH on its own often determines the form of the species present). That is why the distribution diagrams of chemical species are frequently defined as functions of pH (Section 2.1.2). In summary, the main environmental processes that affect the pH and the alkalinity of natural waters include ... 

The cementitious substrates include concrete, masonry, sand-cement, and gypsum plasters. All these substrates retain moisture and are alkaline in nature. The surface alkalinity can result in a chemical attack or saponification of certain types of binders used in paints, notably oils and alkyds, resulting in a marked diminution in the paints resistance to washing, abrasion, and weathering. Alkyd paints are, therefore, not used on fresh concrete, masonry, and plaster surfaces. 

The most abundant anion delivered by rivers to the oceans is bicarbonate ion (HCO ), and most of the bicarbonate alkalinity in rivers comes from the weathering of carbonate rocks (Meybeck, 1987). The chemical weathering of limestones and dolostones by dissolved CO2 can be represented by the reactions for dissolution of calcite and dolomite ... 

Soils that have been contaminated by mine wastes, tailings, smelter slags, or smelter particulates can contain a complex mixture of minerals present in the soils prior to contamination minerals contributed by the contaminants minerals formed by soil weathering, biological reactions, and chemical reactions with infiltrating waters and soil moisture windblown dust, and other anthropogenic materials (Ruby et al, 1999). For example, reactions of lead oxide with soil moisture in alkaline soils can precipitate lead carbonate, whereas reactions in acidic soils can precipitate lead sulfate. 

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