**3. Heavy metals in the environment**

#### **3.1 Soil**

Natural as well as anthropogenic sources of heavy metals including soil erosions, volcanic eruptions, forest fires, mining operations, industrial activities, fertilizers application as well as urban wastes may lead to the contamination of soils [3]. Heavy metals occur naturally in ores in different chemical forms such as sulfides or oxides. Industrial pollution results from various activities such as chemical manufacturing, oil refining, metal processing and plating, tanneries and plastics.

Wind, water and gravity are the principal factors controlling the heavy metals mobility in soils and landscapes. The distribution of metals between solid and solution phases depends on their chemical forms in each phase and on chemical factors such as pH, metal concentration or soil composition. The soil contamination by heavy metals may affect soil fertility by the reduction in populations of soil fauna [3].

#### **3.2 Water**

The outbreak of "Minamata disease" and "itai-itai-byo" or "ouch-ouch disease" in Japan during the 1940s and the 1950s drew the worldwide attention to the environmental hazards caused by aquatic heavy metal pollution. Minamata disease was due to the ingestion of fish and shellfish contaminated with highly toxic methylmercury, while ouch-ouch disease was caused by eating rice polluted with lethal amounts of cadmium [1].

Heavy metals enter the aquatic environment from both natural and anthropogenic sources. Entry may be due to direct discharges into both fresh and marine ecosystems or through indirect routes such as dry and wet deposition. Anthropogenic wastes, geochemical structure and mining effluents create potential sources of heavy metals pollution in the aquatic environment.

Once introduced into the aquatic environment, heavy metals are distributed among four interactive compartments (water, suspended matter, sediment and biota). Metals in the aquatic environment can exist in dissolved or particulate form. Sedimentation, adsorption/desorption, dilution and dispersion are the main processes governing the distribution of heavy metals in aqueous ecosystems. Adsorption could be the first step of metals removal from water. Both in marine environments and in fresh water, the permanent or temporary storage of metals

*Heavy Metals in the Environment and Health Impact DOI: http://dx.doi.org/10.5772/intechopen.97204*

takes place in sediments. Microbial activity and oxidative processes can alter the properties of sediments and influence the composition of pore water. Snooping organisms can also bring sediments to the surface, which will release a significant fraction of the metal [11]. The absorption of heavy metals by both fauna and flora could cause an increase in the concentration of the metals in the living organisms. If the evacuation phase is slow, it may result in a phenomenon of bioaccumulation, which could contaminate the aquatic food chain [11].

Unlike biosorption, bioaccumulation is an active process, whereby metals are integrated in the interior of cells [12]. Certain metals bind preferentially, either transiently or permanently, to the cell membrane and then cause structural and functional modifications, which are often fatal to the cell. A study by Dao and Beardall showed that increasing doses of lead administered to the algae *Chlorella sp.* were associated with a decrease in algal growth, as well as an inhibition of the photosynthetic function by the algae [13].

#### **3.3 Air**

Heavy metals occur in the atmosphere mainly in particulate form. Particulate matter refers to a mixture of solid and liquid particles that are dispersed in the air. It is composed of primary particles that are emitted directly into the atmosphere and secondary particulates formed through chemical transformation of gaseous pollutants [1]. Metals in the atmosphere result from natural processes (volcanic eruption, soil erosion, sand storms, dust re-suspension) and anthropogenic sources (mainly industrial, agricultural and vehicle emissions). Metal oxides constitute a major class of inorganic particles in the atmosphere. They result from the combustion of fuels containing metals.

Generally, the concentrations of atmospheric heavy metals are higher in winter than in summer. This is probably due to the high temperature, the strong diffusion capacity and the rainfall in summer season. Inversely, the atmospheric deposition of heavy metals tend to be higher in summer than in winter. According to Duan and Tan, the average atmospheric concentration of lead in China is 261.0 ± 275.7 ng m−3, which falls below the guideline limit of the World Health Organization (WHO) of 500 ng m−3 [14]. The mean concentration of atmospheric cadmium in the same area is 12.9 ± 19.6 ng m−3, which exceeds the WHO limit of 5 ng m−3. The principal sources of atmospheric heavy metal pollution in China are coal burning, iron and steel industry and vehicle emissions [14].
