Science of Water : Concepts and Applications

108 The Science of Water: Concepts and Applications
√ Important Point: As mentioned, solutions can become saturated with solute. This is the case
with water and oxygen. As with other solutes, the amount of oxygen that can be dissolved at
saturation depends upon the temperature of the water. In the case of oxygen, the effect is just the
opposite of other solutes. The higher the temperature, the lower the saturation level; the lower
the temperature, the higher the saturation level.
METALS
Metals are elements, present in chemical compounds as positive ions or in the form of cations in
solution. Metals with a density over 5 kg/dm 3 are known as heavy metals. Metals are one of the
constituents or impurities often carried by water. Although most of the metals are not harmful at
normal levels, a few metals can cause taste and odor problems in drinking water. In addition, some
metals may be toxic to humans, animals, and microorganisms. Most metals enter water as part of
compounds that ionize to release the metal as positive ions.
Table 4.1 lists some metals commonly found in water and their potential health hazards.
√ Important Point: Metals may be found in various physical and chemical forms. These forms,
or “species,” can be particles or simple organic compounds, organic complexes, or colloids. The
dominating form is determined largely by the chemical composition of water, the matrix, and,
in particular, the pH.
ORGANIC MATTER
Organic matter or compounds are those that contain the element carbon and that are derived from
material that was once alive (i.e., plants and animals). Organic compounds include fats, dyes, soaps,
rubber products, plastics, wood, fuels, cotton, proteins, and carbohydrates. Organic compounds in
water are usually large, nonpolar molecules that do not dissolve well in water. They often provide
large amounts of energy to animals and microorganisms.
√ Important Point: Natural organic matter (NOM) is used to describe the complex mixture of
organic material, such as humic and hydrophilic acids present in all drinking water sources.
NOM can cause major problems in the treatment of water as it reacts with chlorine to form
disinfection by-products (DBPs). Many of the DBPs formed by the reaction of NOM with
disinfectants are reported to be toxic and carcinogenic to humans if ingested over an extended
period. The removal of NOM and hence reduction in DBPs is a major goal in the treatment of
any water source.
TABLE 4.1
Common Metals Found in Water
Metal Health Hazard
Barium Circulatory system effects and increase in blood pressure
Cadmium Concentration in the liver, kidneys, pancreas, and thyroid
Copper Nervous system damage and kidney effects, toxic to humans
Lead Same as copper
Mercury Central nervous system (CNS) disorders
Nickel CNS disorders
Selenium CNS disorders
Silver Turns skin gray
Zinc Causes taste problems—is not a health hazard