**2. Heavy metals: An overview**

food chain, thus reducing the risk of toxicity in living organisms posed by the method of

The evaluation of heavy metals in soil can be made in several ways. The basic and most common is to quantify the total concentration [11]. The great advantage of this measure is its robustness, that is, it does not easily change with environmental conditions, but it may be altered by the addition of metals from external sources, such as sludge, which allows its use as an indicator of soil contamination by heavy metals. However, the total concentration not satisfactorily represents the amount of metal that would be available for uptake by plants, that is, the fraction of contaminant which could cause phytotoxicity and enter the food chain, affecting animals and humans. Due to its direct relationship with potential toxic effects, the

The availability of heavy metals to plants has been characterized using chemical extractants, some of them already employed in routine soil analysis, as Mehlich 1, Mehlich 3 and DTPA. In general, extractants are (i) acid, (ii) chelating agents, (iii) acid-chelating or (iv) saline solutions [11]. The chemical nature of the extracting solution interferes with the ability of extracting metals and, ultimately, the efficiency of the extractant to represent the available fraction. Therefore, the extractants should be systematically tested before being used in

Fractionation is also interesting technique to evaluate heavy metals in soils. Its principle is to separate the metals in soil fractions in which they have variable solubility [12]. With this procedure, it is possible to determine the contribution of each fraction in the availability of metals to plants [13, 14]. It also allows selecting the best chemical extractant, based on its relationship with the fractions that most contribute to uptake of metals by plants. More‐ over, the redistribution of metals among fractions in response to changes in soil condi‐ tions can be studied [15]. Thus, the fractionation can indicate whether the metals added to the soil by the sludge are to be redistributed in fractions in which they are either more or less available, that is, whether they have either greater or lesser potential to cause toxici‐

The study of speciation is another interesting strategy to evaluate heavy metals in soils, since it enables to distinguish different chemical species in the soil solution. Each species has a particular chemical behavior in terms of availability and mobility in soil. Free ions are more relevant to the availability of metals, because they are the preferred forms of plant uptake [11]. In contrast, organo-metal complexes are more related to the mobility of metals and, conse‐ quently, to their leaching [17]. Thus, the speciation can indicate if the risk of phytotoxicity and contamination of the food chain is higher or lower than the risk of groundwater contamination, based on the proportion of the chemical species formed in response to application of sewage

Besides the contamination of soil, it is also necessary to evaluate whether the sewage sludge applied to land can contaminate crops with heavy metals. The assessment of crop contamina‐ tion must include studies of differential capacity of uptake, translocation, accumulation and allocation of metals in different plant species. In the case of allocation, it is essential to evaluate

available concentration should be determined in addition to total concentration.

monitoring heavy metals in soils amended with sewage sludge.

disposal of sludge on land.

874 Environmental Risk Assessment of Soil Contamination

ty problems respectively [7, 16].

sludge.

Heavy metals comprise a class of not very well defined chemical elements. They have been commonly characterized as metals and metalloids (semimetals) with density higher than 5 g cm–3 associated with problems of environmental contamination and toxicity, although some of them are required in low concentrations for some organisms such as higher plants, animals and humans [23, 24]. However, this definition (attempt) has been criticized for its chemical and toxicological inadequacy and it has had no institutional support of IUPAC (International Union of Pure and Applied Chemistry) [25]. Despite the criticism, there is no more appropriate term to refer collectively to the elements known as heavy metals. Therefore, we use this terminology in this text and support its use in the definition introduced by Hawkes [26], according to which heavy metals comprise a block in the Periodic Table with all metals and metalloids in Groups 3 to 16 that are in periods 4 and greater. As some authors consider selenium (Se) it as a metalloid and therefore heavy metal [10, 27], we also consider it as metalloid in this chapter, even though it is a non-metal. Thus, we expand the definition of Hawkes [26] to include Se as a heavy metal, keeping in mind which this redefinition is simply operating, just to join in the same class specific chemical elements commonly associated to problems of environmental contamination and toxicity.

Heavy metals can be divided according to their need for different organisms. There are those which are doubtless essential and those which are not recognized as essential (Table 1). Copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn) are essential to plants, animals and humans. Cobalt (Co) and Se are essential only to animals and humans, while chromium (Cr) and nickel (Ni) are essential to humans and plants, respectively. In contrast, arsenic (As), Cd, Pb and mercury (Hg) are not essential to any of these organisms. However, essential or not essential metals may be toxic. For example, manganese (Mn) is an essential element (i.e., micronutrient) to plants, but its excessive uptake can cause toxicity in crops [28, 29]. On the other hand, As, Cd, Pb and mercury (Hg) are not essential to humans but in excess can also cause toxicity. People exposed to Pb or Hg develop neurological disorders, while exposure to Cd is associated with kidney damage and fragile bones, and various forms of cancer can occur due to the ingestion of food or water contaminated with As [30]. A summary of the toxicity of heavy metals commonly associated with environmental contamination is given in Table 1.



(Cu), iron (Fe), manganese (Mn), molybdenum (Mo) and zinc (Zn) are essential to plants, animals and humans. Cobalt (Co) and Se are essential only to animals and humans, while chromium (Cr) and nickel (Ni) are essential to humans and plants, respectively. In contrast, arsenic (As), Cd, Pb and mercury (Hg) are not essential to any of these organisms. However, essential or not essential metals may be toxic. For example, manganese (Mn) is an essential element (i.e., micronutrient) to plants, but its excessive uptake can cause toxicity in crops [28, 29]. On the other hand, As, Cd, Pb and mercury (Hg) are not essential to humans but in excess can also cause toxicity. People exposed to Pb or Hg develop neurological disorders, while exposure to Cd is associated with kidney damage and fragile bones, and various forms of cancer can occur due to the ingestion of food or water contaminated with As [30]. A summary of the toxicity of heavy metals commonly associated with environmental contamination is

**(Symbol) Organism† Essentiality‡ Toxicity§ Reference**

Plants No Increased oxidative stress and reduced plant

Plants No Increased oxidative stress and reduced plant

Plants No Increased oxidative stress and reduced plant

Plants No Chlorosis in younger leaves (restricted Fe

Animals No Blindness and reduced weight gain. [9] Humans No Increased cancer risk. [30]

Animals No Reduced weight gain. [36] Humans No Kidney damage and fragile bones. [30]

Animals No ? [38]

Animals Yes Body weight loss and muscular incoordination. [9, 44] Humans Yes Cardiomyopathy and Increased cancer risk. [45]

Animals Yes Gastroenterits, liver damage and death. [9, 47]

reduced plant growth.

Humans Yes Wilson's disease, hemolysis, hepatic necrosis and

Plants Yes Leaf bronzing, roots with black coating and reduced

Intervenial chlorosis in younger leaves, reduced branching, thickening, darkening of rootlets and

Anorexia, diarrhea, metabolic acidosis, reduced

Humans Yes Allergy and increased cancer risk. Cr(VI) is more toxic

growth. [31-33]

growth. [34, 35]

growth. Cr(VI) is more toxic than Cr(III). [9, 37]

than Cr(III) or Cr(V). [39-41]

translocation) and reduced plant growth. [42, 43]

kidney damage. [9, 48]

plant growth. Common in flooded rice. [9, 49]

body growth rate and death. [9, 44]

[9, 43, 46]

given in Table 1.

876 Environmental Risk Assessment of Soil Contamination

**Heavy metal**

Arsenic (As)

Cadmium (Cd)

Chromium (Cr)

Cobalt (Co)

Copper (Cu)

Iron (Fe)

Plants Yes

Animals Yes


†In case of animals, they are livestock animals.

‡Yes: essentiality recognized. No: essentiality unrecognized.

§Most common manifestations, symptoms and consequences of heavy metal toxic effects.

**Table 1.** Essentiality and toxicity of heavy metals for different organisms.

The relative importance of heavy metals toxicity was addressed by McLaughlin et al. [10] in terms of food chain contamination. According to these authors, Cd is the metal with greatest potential to contaminate plants and subsequently to be transferred to animals and humans that eat these contaminated plants or part of them. This statement is based on the fact that (i) Cd poses animal and human health risks in plant tissue concentrations that are not generally phytotoxic and (ii) Cd concentrations in agricultural soils are increasing in many parts of world due to Cd inadvertent additions through the use of fertilizers, sewage sludge and soil amend‐ ments. Due to the high risk of contaminating the food chain, the risk of Cd to cause toxicity is considered to be high as well. Despite increased concern with Cd, the toxicity risk of other heavy metals should not be neglected.

The toxicity of heavy metals in living organisms is a phenomenon somewhat complex. Toxic effects of a metal depend on a number of factors that often include (i) rate, (ii) exposure time, (iii) tolerance of the organism and (iv) environmental conditions. In recent years, the effect of the interaction between heavy metals on the expression of toxicity has been considered very intensely. As a result of the interaction, a given metal may increase or decrease the negative effects of other metal in the organism [65].

Despite the complexity, the toxicity of heavy metals in plants and in animals and humans that eat contaminated plants is primarily associated with previous environmental contamination. Soils may be contaminated with such hazardous elements by the use of sewage sludge. High concentrations of metals in the sludge increase the risks of contamination and therefore toxicity. Thus, it is important to know the chemical composition of sewage sludge.
