The Heavy Metal Equilibrium in the Soil

Abstract

.” Friedrich NietzcheThe presence of heavy metals in soils is an environmental hazard key and one of the most difficult contamination control problems to solve. There are two main causes: firstly, the chemical character of heavy metals (they are not subjected to biodegradation processes), favours accumulate in the environment and, secondly, the complexity of the geochemical soil matrix. Simplification of this matrix increase chances of recognition of basic soil processes [1]. Soil is a major reservoir for xenobiotic compounds as it possesses an ability to bind various chemicals [2]. The soil as an important ecological factor due to several characteristics: close correlation with the climate and region and its structure, configuration and nature; its own quality on which relies formation and protection of surface and underground matter resources; influence on humans nutrition and its role in social and economic development of mankind. The heavy metals have an important influence on plants, animals and humans development. The chemical compositions of these heavy metals don’t exceed 0.01% in the analyzed systems and their physiologic role is essential for normal biological life cycle of organisms [3]. They are absolutely necessary for living organisms but their biological role is still difficult to establish. If taken alone they don’t show deficiency symptom, only in combination with other mineral elements and don’t seem to have a direct function in plants nutrition. Soil has the ability to immobilise introduced chemicals like heavy metal ions [4,5]. From the geological and chemical point of view the heavy metals can express lots of proprieties. The solid phase of soils and environmental factors play an important role in heavy metals solubility, dynamics and balance in the soil.Between solid, liquid and gaseous phases exists a dynamic equilibrium in the soil.When the concentration of the elements in the soil solution exceeds the normal level, the mineral elements start to precipitate in order to maintain the normal balance. When their concentration is low they start to dissolve from the solid phase [6]. The period of time in which every element re-establish his own balance in the soil, depends on the soil nature, structure, texture, chemical composition pH, humus content etc. [7]. The heavy metal equilibrium in the soil can last from several hours to few days or years. The manganese and iron hydroxides represent the basic phase, where the heavy metals found in small quantities are adsorbed, co–precipitated and integrated in the soil.The organic matter in the soil can retain a large amount of heavy metals or can release some of them due to microbiological activities [8,9]. From these reasons we can discuss about a dynamic balance. The elements with metal characteristics, in their inferior oxidation states (one, two, three and in some cases even four states) can form combination with most of bases. For these the most abundant and important, due to their effects on plant development carbon, nitrogen and sulphurous compound and also carboxylic acids salts [10,11]. Metals can be found in the soil as native elements, characterized by a fine crystal structure with metal bond like Van der Walls hetero–polar bonds.The humus, humic acids and the natural chelate agents possess the capacity to make more complex heavy metals. A substantial part of the humic acids is represented by functionally carboxylic acids groups which give them the ability to incorporate positive charged ions (Zn