4) Precipitation and dissolution
The process of precipitation and dissolution is an important mass transfer mechanism in the subsurface, where in dissolution increases and precipitation decreases the concentration of contaminants in pore water. Water is a good solvent for a variety of solids, liquids and gases. Dissolution is the process of complete solubility of an element in groundwater. Some natural minerals also undergo dissolution. For example,
SiO2 + 2H2O gives H2SiO3 (dissolution of quartz)
Kaolinite + 5H2O gives 2Al(OH)3 + 2H4SiO4 (dissolution of kaolinite)
Precipitation is reverse process of dissolution where in dissolved element comes out of the solution due to the reaction with dissolved species. Due to precipitation, the concentration of the element reduces in pore water. For example, Lead gets precipitated from pore water due to its reaction with sulfides, carbonates or chlorides. Iron, zinc and copper precipitates due to hydrolysis reaction, and chromium, arsenic precipitates due to redox reaction. In some cases, both dissolution and precipitation occurs one after the other as the pore water advances.
pH is important factor governing dissolution and precipitation. An element has a solubility limit in water. Beyond the solubility limit the solution becomes supersaturated and starts precipitating. pH governs the solubility limit and hence when pH changes, there is a possibility of precipitation reaction. It is found that solubility reduces with pH, reaches a minimum value and then again increases. This indicates that there exists a
particular pH where optimum precipitation will occur. Metal hydroxides are amphoteric (increasingly soluble at both low and high pH) and the pH for minimum solubility (optimum precipitation) is different for different metal. For example, cadmium-pH 11, copper-pH 8.1, chromium-pH 7.5, zinc-pH 10.1, nickel-pH 10.8. A small change in pH would therefore result in considerable changes in precipitation reaction.
5) Exsolution and volatilization
This process involves mass transfer between gaseous and liquid or solid phase. Similar to precipitation this process removes mass from pore fluid to gaseous phase. This process is mostly governed by the vapour pressure (pressure of gaseous phase) with respect to liquid or solid at a particular temperature. There are a lot of volatile contaminants disposed into subsurface that finds its way to atmosphere. A thorough knowledge on the exsolution and volatilization is required to understand the mass transfer mechanism of these organic contaminants.
6) Radioactive decay
In this process, unstable isotopes decay to form new ones with release of heat and particles from element nucleus. The process is known as α or β decay depending on whether the element looses α particle (helium) or a β particle (electron). The process of decay is irreversible and daughter isotope increases in quantity. The disposal of radioactive waste generated from nuclear installations, mining etc. to subsurface will considerably increase the heat. Moreover, the radioactive isotope such as uranium, plutonium, cesium etc gets transported to far field and would pollute the groundwater. Preventing such harmful pollution and reducing the ill effect of overheating of subsurface is
a challenging geoenvironmental problem.
7) Sorption and partitioning
When contaminant laden pore water flow past the soil surface, mass transfer of these contaminants takes place on to the solids. The process is referred to as sorption or partitioning. The amount of partitioning depends on the soil surface (sorbent) and the reactivity of contaminant (sorbate). This is one of the predominant mechanisms governing the fate of contaminant once it is released into the geoenvironment. The term sorption refers to the adsorption of dissolved ions, molecules or compounds on to the soil surface. The mechanism of sorption includes physical and chemical sorption as well as precipitation reaction. These reactions are governed by surface properties of soil, chemistry of contaminant and pore water, redox
potential and pH of the environment. Physical adsorption refers to the
attraction of contaminant on to the soil surface mainly due to the surface
charge (electrostatic force of attraction). Physical sorption is weak
bonding and can be reversed easily by washing with extracting solution.
Chemical sorption is strong force of attraction due to the formation of
bonds such as covalent bond. High adsorption energy is associated with
chemical sorption and it can be either exothermic or endothermic reaction. The
details of sorption reaction and mass transport mechanisms will be discussed in detail in module 3 on how to use these information for predicting the fate of contaminants in geoenvironment.
8) Biological transformation
Biological transformation is the degradation or assimilation of contaminants (mostly organic) by microorganisms present in the soil. Transformations from biotic processes occur under aerobic or anaerobic conditions. The transformation products obtained from each will be different. The biotic transformation processes under aerobic conditions are oxidation reaction. The various processes include hydroxylation, epoxidation, and substitution of OH groups on molecules (Yong 2001). Anaerobic biotic transformation processes are mostly reduction reaction, which include hydrogenolysis, H+ substitution for Cl– on molecules, and dihaloelimination (Yong 2001). The major application of biological transformation is in organic contaminant remediation,
which is discussed in detail in module 4.