Rainwater is able to dissolve gas present in the atmosphere (leading to acid rain, for example; see Section 6.2.6). Rainwater is also able to dissolve chemicals present at the road surface (e. g. metals, salts and some organics). Road materials are of course selected for not being soluble but trace elements present in natural and alternative materials can be released by dissolution when leached by seepage. The rise in a water table can also bring about dissolution. Dissolved elements can precipitate downstream where hydrous, pH and/or redox conditions differ from those upstream. Dissolution/precipitation sequences are also part of the circulation of chemicals. Dissolution of CO2, whether from the air or biological activity, is of great importance to the pH of the soil solution, also in the road context.
Dissolution is the process by which a solution is formed when a soluble substance (a solute) is dissolved in a liquid (a solvent). A true solution is a uniform molecular or ionic mixture of one or more solutes in a solvent, as distinguished from a colloidal solution or dispersion in which the dispersed material is in the form of extremely small particles, 1 ^m or less. The solute can be a solid or a gas.
As a polar molecule, water can dissolve ionic substances such as salts but also substances consisting of polar molecules with which the water molecule forms hydrogen bonds. The solubility (i. e. the maximal quantity of a chemical compound that can be dissolved per litre of solvent) is dependent on temperature, pH and activity coefficient.
Whether a substance is present in the dissolved or in the precipitated form is of crucial importance to its mobility and transport in the soil. This is especially true of heavy metals and other micro-pollutants (Ramade, 1998). Depending on variations in the chemical and physical properties of its environment, a given pollutant present in the soil can repeatedly change from being dissolved to being precipitated, and thus from being mobile to being less mobile. Some salts (ionic solids) are very soluble, for instance NaCl and CaCl2 which are used for de-icing and dust-binding, respectively.
The degree of solution of any salt MpXq is governed by the dissolution equilibrium:
MpXq(s) <-> pMq+ + qXP — (6.17)
where the solubility product Ks = (Mq+)p ■ (XP-)q. Thus, the greater the solubility product, the greater the solubility of the salt. This equilibrium can be coupled with other equilibria, e. g. acid-base, redox or complexation equilibria. The solubility of a salt is, e. g., dependent also on the pH and the redox status of the soil.
Carbonates are important to the mobility of heavy metals. Carbonates are dissolved upon the interaction with water and with the carbon dioxide present in air, water and soil. Calcium carbonate (CaCO3) is a major constituent of calcareous rock. Where enough free carbonate ions (CO32-) are present, they will react with heavy metal ions to form immobile precipitates, e. g. lead carbonate. The mobility of many heavy metals is low in calcareous soils. On the contrary, heavy metals are often more mobile in acidic soils where carbonates are largely absent (Selim & Sparks, 2001).
Hydroxides of Fe and Mn also play a major role in natural waters and soils. The solubility of hydroxides depends on the acidity (pH) of the water or the soil solution. The solubility of hydroxides decreases when pH increases, passes through a minimum and then increases at higher pH.
Organic molecules that contain polar groups or create hydrogen bonds are to a great extent soluble in soil solution and water. This is the case for organic molecules with groups such as hydroxyl, amine, carboxylic acid, carbonyl, ester or ether.
In the road situation, adsorption and desorption will happen routinely whereas precipitation will depend on the ion concentration. At low concentrations, many metals are under-saturated with respect to their associated mineral phases so that their mobility/retardation is governed by adsorption/desorption. At higher concentrations, both adsorption and precipitation may be occurring to take ions out of solution, but it is the dissolution/precipitation processes that will determine the aqueous concentration of the metal.
