Water-soluble substances, rock

Migration of Water-Soluble Substances in Rocks and Soils... 

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. 

Migration of water-soluble substances is a common process occurring in rocks and soils (Chapter 1, Section 1.3.6.2). This process is the net result of different hydrological and sorption processes. Hydrological processes (Chapter 1, Equation 1.127), that is, the movement of water, are governed by the water levels. Sorption processes can decrease the rate of water movement via interfacial processes (Chapter 1, Equations 1.128-1.131). In this chapter, the effect of sorption will be shown on the migration rate of water-soluble substances. 

Efflorescence. The solvent properties of water also causes efflorescence, a phenomenon whereby soluble or slightly soluble substances migrate from the interior of porous solids to the surface, where they precipitate. Efflorescence is an important factor in the decay and disintegration of many rocks, and of human-made porous materials such as ceramics, and even of some types of glass. On archaeological objects, efflorescence generally occurs mostly as a white, powdery, but sometimes consolidated accretion on the surface of the objects. Calcite, a form of calcium carbonate, is one of the most common substances to effloresce on archaeological ceramics. 

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. 

When rain falls over land some drain off the surface directly into surface water courses in surface runoff. A further part of the incoming rainwater percolates into the soil and passes more slowly into either surface waters or underground reservoirs. Water held in rock below the surface is termed groundwater, and a rock formation that stores and transmits water in useful quantities is termed an aquifer. Water that passes through soil or rock on its way to a river is chemically modified during transit, generally by addition of soluble and colloidal substances washed out of the ground. Some substances are removed from the water for example, river water often contains less lead than rainwater one mechanism of removal is uptake by soil. 

In the soil, the FA and low molecular weight HA, which are water-soluble, are of particular interest. These humic substances are important in the process of natural weathering of rock materials via complexataion, dissolution, and transport. 

Figure 2. Conceptual scheme of a small catchment ecosystem. Fluxes of element i between a hydrological basin and its surroundings Wi, total weathering of rocks Pi, wet atmosphere deposition Di, dry deposition Ai, possible anthropogenic inputs (e.g. fertilization) Ri, surface and subsurface runoff of soluble substances Mi, possible water erosion of solid substances Bi, biomass export (harvesting) (Moldan and Cherny, 1994).

Let me list water s notable properties in a somewhat different order and present a variety of specific examples. Water comes closer to being a universal solvent than any other known substance, a basic property familiar to anyone putting sugar into a cup of coffee. In the human body, the digestive process takes place after nourishment has been dissolved into a liquid - water - solution. Even rocks can be subject to water s dissolving powers witness the ocean s salinity. The solubility of... 

The simplest reactions that take place on chemical weathering are the solution of soluble minerals and the addition of water to substances to form hydrates. Solution commonly involves ionization, for example, this takes place when gypsum and carbonate rocks are weathered. Hydration takes place among some substances, a common example being gypsum and anhydrite ... 

At equilibrium, the least soluble substance in a system that can form will precipitate. Much phosphate contained in sea water is precipitated as tricalcium orthophosphate or hydroxyl apatite, Caio(P04)6(OH)2, and fluorapatite, Caio(P04)6(F)2. Oceans floors are covered with these deposits and are referred to as marine pellets. There are many ways in which this problem may be approached, but it is obvious that if phosphates are to be leached from igneous rocks, large boulders will leach very slowly. Smaller particles of rock caused by grinding, weathering, and aging solubilize more rapidly than larger particles. As a first approximation, rates of solubilization are proportional to fresh surfaces of solubilized rocks. 

Abraumsalze ) contain 55 to 65 per cent, of camallite, associated with 20 to 25 per cent, of rock-salt, 10 to 20 per cent, of kieseritc, MgS04,H20, and 2 to 4 per cent, of tachydrite, CaCl2,2MgCl2,12H20. The technical preparation of potassium chloride from these deposits depends on the ready solubility of camallite, and the crystallization of potassium chloride from hot saturated solutions of this substance.1 Kainite is employed as a source of potassium chloride, and the compound is also obtained by fractional crystallization of the salts present in sea-water and in the ash of seaweed. 

There are other possible explanations when a model calculation indicates a water is supersaturated with respect to one or more carbonate minerals. They include (1) the use of inaccurate, inconsistent, or incomplete thermodynamic data for carbonate minerals and aqueous complexes (2) nonstoichiometry (i.e.. solid solution) and/or small (submicron) particle sizes of the carbonates, making them more soluble than the well-crystallized pure phases assumed in the calculation (cf. Busenberg and Plummer 1989) (3) different solution models used to define the mineral and in the calculation of saturation state in a natural water (4) inhibition of carbonate nucieation by adsorbed substances (cf. Inskeep and Bloom 1986) and (5) slow nucieation and precipitation rates that require times exceeding residence times of the water in the water-rock system (cf. Herman and Lorah 1987). The same possible explanations apply to model-computed supersaturations obtained for noncarbonate minerals. 

Stoltman and Mainfort discussed some of the problems with NAA and most chemical composition studies of pottery. Neutron activation does not identify the minerals in the pottery it identifies only the chemical elements, and those elements that can occur in many different types of parent rock. Pottery is a human artifact whose chemicals derive from at least five sources (1) the clay (2) any added material such as temper (3) the water used to moisten the clay, which may contain such soluble elements as sodium, potassium, calcium, magnesium, or iron (4) any substance stored, cooked, or transported in the pot and (5) diagenesis, the absorption of chemicals from the soil in which the sherds have lain buried for millennia. Because it identifies minerals, ceramic petrography can link the sherd to the bedrock geology from which the temper came NAA, by contrast, cannot distinguish among the five sources that contributed the elements recorded. 

Rock disintegration by hydrolysis is usually even more important than by simple solution. In this process, the water reacts with minerals to form new products that differ physically and chemically from the original parent material. The new products may be more soluble and be lost by leaching, or they may remain as colloidal materials and become a very important part of the new soil being formed. Clay is a product of this process. Oxygen of the air may also react with some minerals to form new substances that may be either more or less soluble than the original ones. 

---