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Water and pollutants

Water quality is determined by the composition of pollutants. The presence and concentration of pollutants may affect the properties of the water in various ways. The composition of pollutants may also indicate the suitability of the water for specific use.

The polar structure of water molecules allows them to form numerous donor-acceptor bonds (including hydrogen bonds) between themselves and with other molecules and ions, which largel determines good solubility and electrolytic dissociation of substances in the water.
Therefore, absolutely pure water can only be obtained in laboratory conditions by synthesis from atomic hydrogen and oxygen or by multi-stage purification of aqueous solutions from pollutants. Pollutants that may be contained in water can be divided into 4 major groups: mineral substances, organic compounds, microbiological organisms, and gases

Minerals are soluble or insoluble inorganic compounds. Examples of such compounds are acids (HCl, H2SO4, etc.), bases (NaOH, Ca(OH)2, etc.), salts (NaCl, CaSO4, etc.), oxides (Fe3O4, Al2O3, etc.). Acids, alkali, and salts can reversibly decompose into cations and anions, i.e. dissociate in aqueous solutions. Cations and anions form donor-acceptor bonds with water molecules. The degree of solubility, i.e. how much of a substance can be dissolved in a volume of water, depends on how much the bonds between the ions of the substance and the water molecules are stronger than the bonds between the ions themselves. Of course, the mechanism described above is very simplified; for example, solubility also depends on the temperature and the properties of the dissolved substance.

Organic compounds are substances whose composition and structure include a carbon atom (the exception is some inorganic substances that also contain a carbon atom, such as carbonic acid and its derivatives). Organic compounds can be soluble in water, such as sugars (glucose, fructose, sucrose, etc.), organic acids (acetic, formic), surfactants, and poorly soluble or insoluble, such as various hydrocarbons (oils, paraffins, benzene) and their derivatives (chloroform, dichloroethane). The solubility of organic compounds is determined by the formation of donor-acceptor bonds: more bonds per molecule lead to higher solubility. The principle of "like dissolves like" applies, i.e. the more polar is the molecule (the more the charges in it are separated), the easier it dissolves in water and vice versa

Water is a habitat for microorganisms and a substrate that delivers nutrients for them. Also, water is literally a vital part of many of them. Many microorganisms coexist with humans and animals, one of them form symbiotic links, the others are pathogenic, i.e. can cause diseases. Also, microorganisms can produce harmful substances in the process of their life and/or their decomposition.

Gases dissolve in water in proportion to the partial pressure above the water surface. Some gases can form hydrogen bonds with water molecules or react chemically (e.g., carbon dioxide, ammonia). The solubility of gases depends on pressure and temperature, so the higher the pressure is and the lower the temperature is, the higher is the solubility.

Mineralization (salinity), is the total amount of inorganic and organic salts dissolved in water. Water sources, depending on the amount of salt content, can be divided into: fresh, brackish, salty and brine.
Dry residue is a dry weight of a substance after evaporation and drying at 110° C to a constant mass for a known volume of water. Unlike mineralization, it doesn’t include volatile organics and some carbon dioxide compounds. Can produce inflated values due to the hygroscopic nature of some salts.

The hardness of water is a set of properties defined by presence of alkaline earth elements, mainly calcium and magnesium ions (Ca2+, Mg2+).

pH is a measure of the acidity of hydrous solution. It is calculated as the negative decimal logarithm of the activity of hydrogen ions in a solution: pH= – lg a(H+). This means the higher the concentration of H+ ions in a solution, the lower the pH of the solution. The pH values range from 1 to 14, with pH=7 being considered a neutral value. pH value is measured by indicators or by potentiometric method.

Electric conductivity is a measure of flow of electrical current through solution. In the physical sense, conductivity is the inverse of electrical resistance in Ohm. Dissolved salts provide an increase in the conductivity of water, so measuring conductivity allows for indirect determination of the salt content (method is called conductometry). This is why conductivity meters are used to estimate the purity of water. But it should be noted that different ions produce different values for conductivity. Also, some dissolved gases, such as hydrated CO2, can increase the conductivity of the solution, while the organic pollutants pass the current very weakly.

The resistivity of water is the inverse of conductivity. Resistivity, as a measure of water purity, is convenient for almost pure water. Wilhelm Kohlrausch first generated ultrapure water by 45 times vacuum distillation. The measured resistance value of the water was 23.8 Mohm at 18°C and 16.4 Mohm at 25°C. The theoretical maximum of electrical resistance of pure water is 18.18 Mohm*cm at 25°C.

Total Organic Carbon is the total amount of carbon atoms in organic substances. It is clear from the name that this parameter shows the degree of organic contamination of water. During the typical measurement, the difference in conductivity of the water sample before and after the destruction of organic compounds are recorded, followed by recalculation according to the calibration curves. Photo-oxidation is used to destroy organic compounds; in this case the conductivity is increased due to release of carbon oxides and other substances into the water. Current methods allow to measure TOC in continuous mode, which is also useful for indirect estimation of microbiological contamination.