Rocks: acid-base properties

Nagy, N. M., and J. Kdnya. 2006. Acid-base properties of bentonite rocks with different origin. J. Coll. Interface Sci. 295 173-180. 

In this chapter, the relationship of geological origins and interfacial properties of bentonite clay will be reviewed first. Then we will discuss the migration of water-soluble substances in rocks and soil, and the effect of sorption on the migration. A linear model will be derived by which the quantity of ion sorbed on rocks can be estimated when the mineral composition and sorption parameters of the mineral components are known. Surface acid-base properties of soils will be discussed, and the sorption of an anion (cyanide ion) will be shown on different soils and sediments. 

Bentonite rocks have many uses in the chemical and oil industries and also in agriculture and environmental protection. The usefulness of bentonite for each of these applications is based on its interfacial properties. These properties are determined by geological origin, chemical and mineral composition (especially montmorillonite content), and particle size distribution, and they include the specific surface area (internal and external), cation-exchange capacity (CEC), acid-base properties of the edge sites, viscosity, swelling, water permeability, adsorption of different substances, and migration rate of soluble substances in bentonite clay. 

Some characteristic properties of bentonites (CEC, sorption properties) are mainly governed by the montmorillonite content and the layer charge of montmorillonite. Other properties, however, depend on the circumstances under which the rock is formed. These are particle size distribution, external specific surface area, and surface acid-base properties. The quantity of the edge sites mainly depends on the specific surface area. The protonation and deprotonation reactions take place on the edge sites of other silicates and aluminosilicates present beside montmorillonite, so their effects manifest via surface reactions. Consequently, the origin of bentonite determines all properties that are related to external surfaces. 

An understanding of the acid-base properties of solid surfaces is extremely important in almost all fields of science and engineering. For example, the solid/ gas interface intervenes when solid acids (or bases) are used as heterogeneous catalysts in the oil refining and chemical industries. The solid/liquid interface between rocks, minerals, and soils and natural waters is continuously transformed by weathering processes that result in the geohydrochemical circuit of elements. These are only two examples in which the primary reactivity factor is determined by the acid-base chemistry at solid interfaces, usually confined to molecule-sized spaces. 

Oxides, especially those of Al, Si, Fe, and Mg, are essential components of the earth s minerals their surface sites may have either Bronsted or Lewis acid-base properties. The weathering of rocks and the formation of soils are processes at the solid/liquid interface between minerals and natural waters or solutes from the medium in which solid phases are either formed, altered, or dissolved. Dissolution of solid minerals is controlled by slow surface reactions rather than... 

Acidity and alkalinity titrations determine the total capacity of natural waters to consume strong bases or acids as measured to specified pH values defined by the endpoints of titrations. Of more interest for many purposes is the ability of a water or water-rock system to resist pH change when mixed with a more acid or alkaline water or rock. This system property is called its buffer capacity. Buffer capacity is important in aqueous/environmental studies for reasons that include ... 

Of the substances studied the most abundant in the Paleozoic rocks of the area are furfurals presumably derived from carbohydrates, higher molecular weight hydrocarbons, and in a few samples, amino acids. Acid and base soluble, low boiling substances having some properties of heterocyclic compounds, phenols, and organic acids exist in smaller amounts. 

Alkaline flooding is based on the reaction that occurs between the alkaline water and the organic acids, naturally occurring in some crudes, to produce in-situ surfactants or emulsifying soaps at the oil/water interface. Recent literature (i-J.) summarizes several proposed mechanisms by which alkaline water-flooding will enhance oil recovery. These mechanisms include emulsification and entrapment, emulsification and entrainment, and wettability reversal (oil-wet to water-wet or water-wet to oil-wet). Depending on the initial reservoir and experimental conditions with respect to oil, rock and injection water properties, one or more of these proposed mechanisms may be controlling. 

Silica deserves especial attention as it is distinguished among oxides by its unique properties and special role in the formation of most insoluble minerals and rocks. Other nonmetal oxides usually form acidic oxides, which relatively easily interact with H O and bases, forming acids or soluble salts. Silica is perhaps most insoluble and most common among them. 

Rock wool fibers can be characterized by their acidity modulus, Mj, which describes the ratio of acidic to basic oxides. If Ms< 1.2, the fiber is called slag wool, the base material of which is cinder. Nowadays, such low quality fibers are not produced any more because they are very brittle and show a poor chemical resistance. If Mj= 1.2-1.5, the fiber is considered to be a mineral wool, the base materials of which are basic volcanic rock and cinder. These fibers are brittle, but have acceptable insulation properties, hence their significance in the construction industry is high. If Ms> 1.5, the fiber is called rock wool, and if its base material is basalt then it is named basalt wool (basalt fiber, BF). The base of basalt fiber is basalt, which is a volcanic, over-ground, effusive rock saturated with 45-52 wt.% Si02. Due to the circumstances of its formation, basalt has several excellent properties. In addition to its high elasticity modulus and excellent heat resistance. 

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