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Monica Butnariu*
Chemistry and Vegetal Biochemistry, Banat’s University of Agricultural Sciences and Veterinary Medicine from Timisoara, Romania
Received Date: January 19, 2012; Accepted Date: January 23, 2012; Published Date: January 26, 2012
Citation: Butnariu M (2012) The Heavy Metal Equilibrium in the Soil. J Ecosys Ecograph 2:e103. doi:10.4172/2157-7625.1000e103
Copyright: © 2012 Butnariu M. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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“Man and man’s earth are unexhausted and undiscovered. Wake and listen! Verily, the earth shall yet be a source of recovery. Remain faithful to the earth, with the power of your virtue. Let your gift-giving love and your knowledge serves the meaning of the earth.” Friedrich Nietzche
The 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 (Zn2+, Cr3+), in other word most of the valuable elements for plants, and also, other types of ions without positive biological role (Cd2+ and Pb2+) [12]. This chelating capacity ions metals is probably the most important role of humic acids in the living organisms. Another role is attributed to the detoxification capacity. The humic acids are important stopper system for heavy metals present in the soil, because of pollution or other natural factors. The humic and the fulvic acids are also present in the soil like mixtures of iron hydroxides and other hydroxides of heavy metals. The chelating agents are formed with metal ions when two or more coordinated positions of metals ions are occupied by donor groups of donor ligand, which give the ring like internal structure [13,14]. The humic and fulvic acids capacity to form complex structures is mainly due to the oxygen content in the functional groups (COOH, OH and C=O). The organic compound from soils forms either soluble or insoluble complexes with metal ions and in this way playing a double role in the soil. The compounds with low molecular weight (fulvic acids) are responsible for metal ions solubility and alter their transport through plant roots, and the compounds with high molecular weight (humic acids) function as a blocking barrier for polyvalent cations [15,16]. The interaction between organic substances with clay has lots of consequences which are reflected in the physical, chemical and biological proprieties of soils mixture. In soils (like in the all types of soils) the heavy metals can exist in different forms, being more or less accessible to plants [17]. These forms are results of different physical and chemical factors action like heavy metals–organic, inorganic anions interactions and the inter–atomic distances values, or from different environment conditions. The effects of above mentioned factors can justify the behavior of different organics or inorganic compound of heavy metals, for which the chemical bound presents pronounce covalent character. In soil the heavy metals are found as follows: native state; as mineral compound; organic matter; chelating agents; retained by adsorption; poled in the clays; and dissolved in the soil solution [18]. These heavy metals can be adsorbed by plants, altering the soil balance, can be released from crystal minerals and from precipitates, can be retained in the organic matter in the soil and in the same time can be released from the microbiological activities. Due to these phenomena, a stable balance in the soil cannot be reached and so we cannot took about dynamic equilibrium.
If the minerals found in the heavy metals, the thermodynamic proprieties and the way to reach the balance in the soil solution are known, one can predict the heavy metals solubility in the soil taking in to account the environmental factors like: pH, CO2, O2 and temperature [19]. Due to erosion and tectonic movements, helpful mineral accumulations, including the heavy metals can reach the earth surface, were are under the atmosphere actions, from which an important role goes to temperature variations and waters extremely active, because of their oxygen content, carbon dioxide, salts and other compounds. The heavy metals tendency to adapt to the new environmental conditions leads to alteration processes. Some of the elements are taken from waters (in the forms of solutions) and placed in the circulation to the earth underground, were they will form new minerals.
Another part is taken and used by the plants in the metabolic processes [20]. An important role in the alteration is due to the temperatures and water regime. The higher temperatures leads to an accelerate alteration meanwhile the rainfalls lead to dilution, determines transits from FeSO4 a hydrolysis reaction followed by precipitations. If a warm climate follows then the solutions acidity becomes higher, the hydrolysis doesn’t take place and the heavy metals migrate in deepness of the soil. Another important factor is the chemical composition of the underground waters. The waters from the regions with recent volcanic activity have a higher chloride and basis compounds, compared with the others which have a les more salt content [21]. According to their chemical composition waters can be active or inert, accelerating or reducing the oxidative reactions, with different contribution to the undergoing chemical processes from the soils. In the soil the heavy metals suffer a reduction process from a superior oxidative state to an inferior one which means an electron transfer [22].
This process is possible due to low energy charged metallic cations which can interact with 3d orbital of some inorganic anions to give a low valence bands. The electron transfer will be favourable when the cation will have a stabile low electronic configuration. These configurations are: d5 (Mn2+); d6 (Fe2+, Ru2+ and Os2+); d8 (Pd2+) and d10 (Zn2+ and Cd2+). To discuss about the solubility of these compound in the soil is very difficult because in most of the cases the dissolving is followed immediately by a reaction with water and other organic compounds. The very different solubility of heavy metals can be explained by different energetic levels from the network and must be correlated with factors which can influence these levels. From this reason we consider two factors: metal–ions interaction and inter–atomic distances values [23,24].
Considering these two factors they might justify the behaviour of different heavy metals inorganic compounds, for which the chemical bound has a strong covalent character. The inorganic combinations of heavy metals have the lowest solubility [25]. Regarding the ionic potential (ionic target/ crystalline ray which marks the density of the ion target) and the nature of the bond formed by heavy metals ions a dependence to the electronic structure of the metal can be observed.
So, one can say that: in the inferior oxidative states the heavy metals form general combinations with a strong ionic character (weak basic character and low stability); in the superior oxidative states the heavy metals form general combinations with a strong covalent character and only with the most electron negative (flour, oxygen, chloride) elements (acidic character and high stability) and the tetravalent state of the heavy metals forms amphoter combinations. The implications of such a high number of factors points out the fact that possibility of such a variety of organic and inorganic chemical combinations has to be carefully interpreted.
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