Toxic Elements in Soils

Although ion retention by soils can cause elemental deficiencies, it also prevents excessive or toxic concentrations in most soil solutions. The evolution of life took advantage of the naturally low concentrations in water and low plant availability higher concentrations evolved as toxic. Table 2.4 shows the natural soil contents of ions that are generally harmless. The upper values are conservative estimates of soil contents that might lead to toxicity. All soils contain the toxic elements, even in amounts that are mined as ores, and these concentrations do not necessarily harm plants or groundwater. 

Element Typical Value (mg kg-1) Range (mg kg-1) Soil Solution (mgL-1) Plants Range (mg kg 1) 

Landfill leachate is a potential source of heavy metal contamination and a popular horror story in newspapers. Landfills concentrate wastes, the wastes are not always mixed thoroughly with soil, and soils have limits in their capacity to react. Landfills 

The total amount of a heavy metal or any substance in the soil is a poor indicator of its availability to plants or ability to move in soils. Defining the amounts that are toxic to plants or to animals subsisting on those plants is very difficult. Plant and soil concentrations that are harmful to plants are unknown for most ions and for most plants. In addition, the harmful concentrations vary with the species, age, health, and general nutrition of the organization. The soil solution column of Table 2.5 gives estimates of the concentrations that are found in soils and that are immediately available 

Soil solution concentrations of most trace metals are largely unknown because of difficulties in measuring small concentrations. The values in Table 2.4 marked with b are only rough estimates derived from the composition of seawater. Reported Mn and Cu concentrations in soil solutions are about 30 times greater than their concentrations in seawater. This factor was applied to the remaining ions of the table as well. 

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Such methods are used in more fundamental studies to elucidate the soil chemistry, to determine the structure and composition of soil components and to improve understanding of the processes in the soil that control the mobilisation and retention of nutrient and toxicant elements in soil as well as to illuminate their transport mechanisms. They are, therefore, more important for the soil physical chemist than the functionally defined procedures that are the main concern of the agronomist. Both methods are of major interest to the environmental scientist particularly in the study of the fate of environmental pollutants. Many of the extractants intended to target particular species are also used in a functional speciation role. 

It is possible to summarize the principal forms of mobile and mobilizable toxic elements in soil (Berrow and Burridge, 1980) as follows ... 

Also consider the use of NIST sediments 1646, 2704, and soils 2709-2711 in exploration geochemistry. These samples were certified largely in view of the demand for samples to support monitoring of toxic elements in environmental samples. However, many of the elements certified overlap either the list of primary ore elements or the list of pathfinder elements. Thus, these samples may legitimately be used in a very different application than the one that prompted certification. The sample matrix is ideal for the alternative application, and so is the suite of certified elements. 

Reduction-oxidation is one of the most important processes controlling solubility and speciation of trace elements in soils, especially for those elements with changeable values, such as Cr, As and Se. Within normal ranges of redox potentials and pH commonly found in soils, the two most important oxidation states for Cr are Cr(III) and Cr(VI). Cr(III) is the most stable form of chromium and less soluble and nontoxic, but Cr(VI) is mobile, soluble and toxic. The main aqueous species of Cr(III) are Cr3+, Cr(OH)2+, Cr(OH)3° and Cr(OH)4" and the major aqueous species of Cr(VI)... 

Acid-digestion is often used with composts derived from municipal wastes, sewage and slurry, where toxic amounts of heavy metals may cause problems on the land to which they are applied. It is probably more convenient to determine total elements in soils by a benchtop X-ray fluorescence spectroscopy (XRF) instrument. This only requires the soil to be ground, and several reference standards of a similar soil. A Reference Materials Catalogue, Issue 5, 1999, is available from LGCs Office of Reference Materials, Queens Road, Teddington, Middlesex TW11 OLY, UK. Tel. -i-44 (0)20 8943 7565 Fax h-44 (0)20 8943 7554. 

The total content of the major elements in soil is of little practical significance since only a tiny, soluble fraction is available for absorption by plant roots (West, 1981 Tinker, 1986). To some extent this is less true for trace elements and micronutrients where for example, analyses of total copper or zinc can be used to assess the likelihood of plant deficiencies or toxicities. Nonetheless, for an element to be bioavailable it has to be relatively soluble. 

Uptake of elements by plants is important in understanding essential nutrients for plants, potential health concerns for humans and animals that eat the plants, and use of plants as a remediation agent to remove toxic elements from soils. The uptake of heavy metals by wheat from fertilizers and sewage sludge applied to agricultural lands was investigated by using ICP-MS [263]. 

Aqua regia extraction is a strong partial extraction method that dissolves carbonates, most sulphide minerals, some silicates like olivine and trioctahedral micas, clay minerals and primary and secondary salts and hydroxides (Salminen, 1995). It can be considered a quasi-total extraction method, since actual total concentrations can be higher. On the other hand, this leaching method overestimates the bioavailable amount of toxic elements in a soil since metals trapped in the silicate lattice are released very slowly in the environment and are not easily involved in plant nutrition processes. 

Overview of Selected Soil Pore Water Extraction Methods for the Determination of Potentially Toxic Elements in Contaminated Soils Operational and Technical Aspects... 

Chemical elements that are either present naturally in the soil or introduced by pollution are more usefully estimated in terms of availability of the element, because this property can be related to mobility and uptake by plants. A good estimation of availability can be achieved by measuring the concentration of the element in soil pore water. Recent achievements in analytical techniques allowed to expand the range of interest to trace elements, which play a crucial role both in contaminated and uncontaminated soils and include those defined as potentially toxic elements (PTE) in environmental studies. A complete chemical analysis of soil pore water represents a powerful diagnostic tool for the interpretation of many soil chemical phenomena relating to soil fertility, mineralogy and environmental fate. This chapter describes some of the current methodologies... 

The varieties of rice examined in this study show only modest differences in trace element content between the Italian and Asian samples. The slight variations observed can be ascribed to specific characteristics of the botanical varieties, soil composition, and availability for each element, and local environmental contamination. The concentration of potentially toxic elements in rice ascertained in this study raise no specific concerns. On the other hand, the key role played by rice consumption in Asia diets calls for careful evaluation of the average daily intake for elements such as As, Cd, and Pb, which might exceed the established tolerance level. 

Wine is a widely consumed beverage worldwide, with thousands of years of tradition and a remarkable commercial and social value. The evaluation of the quality of wine is a permanent concern for manufactures, merchants, and consumers. The presence of potentially toxic elements in wines is associated with soil contamination and also related to atmospheric precipitation, pesticides, and materials used in production, transportation, and storage. Although at the end of alcoholic fermentation there is a significant reduction of the mineral content, this may not be enough to prevent some problems related to wine stability, namely,... 

Danika, L. and LeDuc, N.T. 2005. Phytoremediation of toxic trace elements in soil and water. Journal of Industrial Microbiology and Biotechnology, 32 514-20. 

To facilitate fundamental understanding of the linkage of trace elements in soils with plant—animal—human—environment systems and related geomedical problems and to provide practical solutions to their deficiency and toxicity problems, it is essential to promote research on the relationship between soil physicochemical-biological interactions and the impacts on the transformation, transport, bioavailability, toxicity, and fate of trace elements in the terrestrial environment. 

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