Soil Biology 19 Soil Heavy Metals Bearbeitet von Irena Sherameti, Ajit Varma 1. Auflage 2012. Taschenbuch. xviii, 492 S. Paperback ISBN 978 3 642 26157 2 Format (B x L): 15,5 x 23,5 cm Gewicht: 777 g Weitere Fachgebiete > Chemie, Biowissenschaften, Agrarwissenschaften > Agrarwissenschaften > Ackerbaukunde, Pflanzenbau Zu Inhaltsverzeichnis schnell und portofrei erhältlich bei Die Online-Fachbuchhandlung beck-shop.de ist spezialisiert auf Fachbücher, insbesondere Recht, Steuern und Wirtschaft. Im Sortiment finden Sie alle Medien (Bücher, Zeitschriften, CDs, eBooks, etc.) aller Verlage. Ergänzt wird das Programm durch Services wie Neuerscheinungsdienst oder Zusammenstellungen von Büchern zu Sonderpreisen. Der Shop führt mehr als 8 Millionen Produkte. BookID 162110_ChapID 2_Proof# 1 - 15/10/2009 Chapter 2 Definition of “Heavy Metals” and Their Role in Biological Systems Klaus-J. Appenroth 2.1 Introduction At first glance, it would appear to be a rather simple matter to define a “heavy metal” – it is a metal that is “heavy”. Unfortunately, a more in-depth consideration reveals a huge amount of problems with this simple definition. This definition is meant to suggest that the density of a heavy metal is high, but this physical property is quite meaningless in the context of plants and other living organisms. Plants do not deal with metals in their elemental (valence state of 0) forms; they are not accessible to plants. Metals are only available to them in solution, and it is necessary for metals to react with other elements and form compounds before they can be solubilised. Once such a chemical compound is formed (e.g. a salt), the density of the metal does not play any role. We do not know of any correlation between the density of a metal and its physiological or toxicological effects, or even the chemical properties of its compounds. Therefore, let us leave the question of how to define a “heavy metal” until later, and first consider the definition of a “metal”. 2.2 The Definition of Heavy Metals in Plant Science 2.2.1 Metals Metals are often characterised and distinguished from nonmetals by their physical properties – the ability to conduct heat, and an electrical resistance that is directly proportional to temperature, malleability, ductility and even lustre (Housecroft and Sharpe 2008; Müller 2007). These properties, especially that of a temperature- dependent conductivity, at least allow us to define what a metal is in contrast to K.-J. Appenroth (*) Friedrich-Schiller University of Jena, Institute for General Botany and Plant Physiology, Dornburgerstr 159, 07743, Jena, Germany e-mail: [email protected] I. Sherameti and A. Varma (eds.), Soil Heavy Metals, Soil Biology, Vol 19, 19 DOI 10.1007/978-3-642-02436-8_2, © Springer-Verlag Berlin Heidelberg 2010 BookID 162110_ChapID 2_Proof# 1 - 15/10/2009 BookID 162110_ChapID 2_Proof# 1 - 15/10/2009 20 K.-J. Appenroth nonmetals and metalloids. However as mentioned above, all of these physical properties are lost after the metal has been chemically transformed into a chemical compound that can be taken up by plants (Shaw et al. 2004). It is well known that the properties of chemical elements can be determined from their positions in the periodic table of the elements (Fig. 2.1). In general, the chemical elements become more metallic as we move towards the lower left corner of the table and nonmetal- lic towards the upper right corner. In other words, metallic character decreases from left to right and from the bottom to the top of the table. Metalloids (elements with properties intermediate between metals and nonmetals) occur close to the diagonal border between metals and nonmetals in the table. A metal can be categorised according to the last electronic subshell in its atom. There are s-elements, which can be subdivided into alkaline elements (first main group) and alkaline earth ele- ments (second main group). All s-elements are metals except for H (the first ele- ment in the first main group). The first element in the second main group, Be, is also somewhat special (its oxides are amphoteric), but it is still considered to be a metal. Among the other groups of the periodic table, d-group elements (transition elements) are all metals. Many of them form compounds with different valence states, which is an important factor in their toxicity. Some of the oxides of transition elements have slightly amphoteric properties, but they are still all considered to be metals. Then there are the f-group elements, also known as the rare earth elements, which are subdivided into the lanthanide series (including La) and the actinide series (including Ac). All of these rare earth elements are also metals and so are sometimes called rare earth metals. The next group, the p-group, occurs towards the right hand side of the periodic table and thus represents a mixed group of Alkali elements Ia 2 1 Alkali earth elements H IIa IIla IVa Va Vla VIla He 3 4 5 678910 Li Be Lead BCNOF Ne 11 12 Transition elements group 13 14 15 16 17 18 Na Mg Illb IVb Vb VIb VIIb VIIIb Ib IIb Al Si P SCl Ar 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 K Ca Sc Ti Y Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd ln Sn Sb Te l Xe 55 56 57 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 Cs Ba La Hf Ta W Re Os Ir Pt Au Hg T1 Pb Bi Po At Rn 87 88 89 104 105 106 107 108 109 110 111 112 113 114 115 116 Fr Ra Ac Rf Db Sg Bh Hs Mt Ds Rg Lanthanide 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Actinide 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 Ac Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Fig. 2.1 Periodic table of the elements. Metals and some metalloids are indicated. The transition elements, the rare earth elements (lanthanide series, actinide series) and the lead-group elements on the right hand side of the table are relevant to the definition of “heavy metals” provided in this chapter BookID 162110_ChapID 2_Proof# 1 - 15/10/2009 2 Definition of “Heavy Metals” and Their Role in Biological Systems 21 metals, metalloids and nonmetals. This includes the elements of the third to seventh main groups of the periodic table, but excludes the rare gases (the eighth main group). Metallic members of this group include Al, Ga, In, Tl, Sn, Pb, Cb, Bi, Te and Po. All of them (except Bi) form amphoteric oxides. Si, Ge, As and Te are consid- ered to be metalloids; sometimes B and Sb are included too (Fig. 2.1). Since there is no common name for the metal/metalloid members of the p-group, we suggest that these metals and metalloids should be termed “lead-group elements”, as lead is the representative of this group that has been studied in the greatest depth in plant science. As plant scientists, we should stress at this point that we never talk about the elemental forms of these elements. We usually only deal with their salts. There are, of course, special cases where the properties of a compound formed from elements from any of the groups defined above are modified (e.g. by organic ligands or sub- stituents). This should then be treated as a special case and does not necessarily have an impact on the divisions and subdivisions of elements. Classifying metals according to their positions in the periodic table of the elements makes sense because the chemical properties of their compounds are related to it. 2.2.2 Heavy Metals In the fundamental review paper written by Duffus (2002), 13 different works were cited that used lower limits on the density of a “heavy” metal ranging from 3.5 to 7 g cm−3. The author stated that the threshold varied depending on the author, and that “it is impossible to come up with a consensus”. Moreover, he concluded that “any idea of defining “heavy metals” on the basis of density must be abandoned as yielding nothing but confusion”. However, this is beside the point; although half of the works cited suggested similar lower limits of 4.5 or 5 g cm−3, plants do not have the ability to detect the density of a metal. Thus, “heavy metal” remains an obscure term in the life sciences. It should also be noted that the review paper of Duffus (2002) was commissioned by the International Union of Pure and Applied Chemistry, and certainly represents a chemical point of view that is often neglected by biologists. Apart from the specific weight, the atomic weight, the atomic number, specific chemical properties, and the toxicity were all mentioned as a possible basis for classification – and then rejected for good reasons. So what should we base our definition of “heavy metals” upon? Indeed, is it necessary to use the term at all? Let us now consider what defining “heavy metals” according to the chemical properties of compounds can offer us. 2.2.3 Lewis Acid Strength and Ionic Indices Any positively charged ion is able to accept electrons, thus defining it as a Lewis acid.