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<ul><li><p>525Soil Remediation and Plants. http://dx.doi.org/10.1016/B978-0-12-799937-1.00018-8Copyright 2015 Elsevier Inc. All rights reserved.</p><p>Soil Contamination, Remediation and Plants: Prospects and Challenges</p><p>M.S. AbdullahiDepartment of Chemistry, Federal College of Education, Kontagora, Nigeria</p><p>INTRODUCTION</p><p>Vegetables constitute parts of the human diet since they contain carbohydrates, proteins, vitamins, minerals and trace elements. Their consumption is on the increase, particularly among the urban community, due to increased awareness of their food values as a result of exposure to other cultures and education. However, they contain both essential and toxic elements over a wide range of concentrations. Metals accumulation in vegetables may thus pose threat to human health. Heavy metals are contaminants in the tissues of vegetables. Heavy metals such as cadmium, copper, lead, chromium and mercury are pol-lutants, particularly in areas under irrigation using wastewater. Investigations on water, soil, sediments and vegetables irrigated with wastewater are available in literatures from different researchers of different parts of Africa.</p><p>Agriculture is one of the major sources of environmental pollution (Gucel et al., 2009; Ashraf et al., 2010 a, b, 2012; Ozturk et al., 2010; Hakeem et al., 2013). Efforts were made and are still being made to improve the productiv-ity of the low-nutrient status of soil in tropical Africa so as to enhance food production in order to sustain the projected population growth necessitate irrigated farming system. To ensure the success of this objective, the use of inorganic fertilizers had tripled. Superphosphate fertilizers contain not only the necessary elements for plant nutrients and growth, but impurities such as Cd, Pb or Hg. Thus fertilizer application is a source of soil contamina-tion. As such, the intake of these heavy metals by plants, especially the leafy vegetables, is an avenue of their entry into the human food chain with harm-ful effects on health. The level of heavy metals in soils through phosphate fertilizer application depends on the origin of phosphate rocks from which the fertilizer is manufactured.</p><p>Chapter 18</p></li><li><p>526 Soil Remediation and Plants</p><p>Industrial or municipal wastewaters are used for the irrigation of crops in a particular ecosystem, due to their availability and due to the scarcity of fresh water (Ozturk et al., 2011). Irrigation with wastewater contributes to heavy metal contents in soil. Heavy metals are harmful because of their non-biodegradable nature, long biological half-lives and their potentials to accumulate in different body parts. Heavy metals are toxic because of their solubility in water. Low concentrations of heavy metals have damag-ing effects on humans and animals because there is no mechanism for their elimination from the body. Wastewater contains substantial amounts of toxic metals which create problems. A high accumulation of these metals in agri-cultural soils through wastewater irrigation does not only result in soil con-tamination, but also affects food quality and safety. Food and water are the sources of essential metals: these are also the media through which we are exposed to toxic metals and their accumulation in the edible parts of leafy vegetables.</p><p>Environmental pollution is the undesirable change in the atmosphere, hydro-sphere and lithosphere. Advanced industrialization processes have provided a comfort to man but have also resulted in the indiscriminate release of gases and liquids which pollute the environment. Large amounts of untreated sewage and industrial water have been discharged into surface bodies of rivers for dis-posal and caused their contamination. With water shortage, farmers use waste-water to irrigate their fields. Such irrigation practices give good crop yields as the water contains organic material and some inorganic elements essential for their growth. It may also contain non-essential metals which, when present in large amounts, could be transferred to man and animals through the food chain (Saeed and Oladeji, 2013).</p><p>Long-term use of polluted water for cultivation of vegetables has resulted in the accumulation of trace metals in agricultural soils, affecting microbial activities as well as their transfer to various crops under cultivation with levels of contamination that exceed permissible levels. The uptake and bio-availability of trace metals to plants is controlled by factors associated with soil and climatic conditions, plant genotype, and agronomic management, activities / passive transfer processes, sequestration and speciation, soil PH, soil organic matter content, redox states, type of root system, the response of plants to the elements, seasonal cycles and competitive metal interactions (Tukura et al., 2013).</p><p>Similarly, the uptake and bioavailability of heavy metals in vegetables are influenced by many factors such as climate, atmosphere dispositions, the con-centrations of heavy metals in soil, the nature of soil and the degree of maturity of the plant at harvest. Air pollution may pose a threat to post-harvest vegetables during transportation and marketing, causing elevated levels of heavy metals in vegetables (Sale et al., 2013).</p><p>Rapid urbanization and industrial development during the last decades have provoked some serious concerns for the environment (Ozturk et al., </p></li><li><p>527Chapter | 18 Soil Contamination, Remediation and Plants: Prospects</p><p>2010a, b). Heavy metal contaminations are a very serious health issues in many cities of the world especially the highly concentrated industrial areas. The sources of heavy metals in the environment are natural or anthropogenic. Usually the concentration of heavy metals in a natural environment is very low and is mostly derived from the mineralogy and the weathering material components. Main anthropogenic sources of heavy metal contamination are mining, disposal of untreated and partially treated effluents, indiscriminate use of heavy-metal-containing fertilizer and pesticides in agricultural fields. The natural sources are volcanic eruptions, erosion and weathering. Trace metal concentrations are important due to their potential toxicity to the envi-ronment and humans. Some of the metals like Cu, Fe, Mn, Ni and Zn are essential micronutrients for the life processes in animals and plants while many other metals such as Cd, Cr, Pb Hg and As have no known physiologi-cal activities. Metals are non-degradable and can accumulate in the human body causing damage to the nervous system and internal organs (Ibiam and Ekpe, 2013).</p><p>SOURCES OF HEAVY METALS IN SOIL</p><p>Heavy metals occur naturally in the soil environment from the pedogenetic processes of weathering of parent materials at levels that are regarded as trace (&lt; 1000 mg kg1) and rarely toxic (Pierzynski et al., 2000; Kabata-Pendias and Pendias, 2001). Due to the disturbance and acceleration of natures slowly occurring geochemical cycle of metals by man, most soils of rural and urban environments may accumulate one or more of the heavy metals above at defined background values high enough to cause risks to human health, plants, ani-mals, ecosystems or other media (Raymond and Felix, 2011). The heavy metals essentially become contaminants in the soil environments because (1) their rates of generation via man-made cycles are more rapid relative to natural ones, (2) they become transferred from mines to random environmental locations where higher potentials of direct exposure occur, (3) the concentrations of the metals in discarded products are relatively high compared to those in the receiving environment and (4) the chemical form (species) in which a metal is found in the receiving environmental system may render it more bioavailable.</p><p>Heavy metals in the soil from anthropogenic sources tend to be more mobile, hence bioavailable than pedogenic or lithogenic ones (Kuo et al, 1993; Kaasalainen and Yli-Halla, 2003). Metal-bearing solids at contaminated sites can originate from a wide variety of anthropogenic sources in the form of metal mine tailings, disposal of high metal wastes in improperly protected landfills, leaded gasoline and lead-based paints, land application of fertilizer, animal manures, biosolids (sewage sludge), compost, pesticides, coal combustion resi-dues, petrochemicals and atmospheric deposition are discussed below ( Alloway, 1990; Adriano, 2001; Nicholson et al., 2003; Zhang, 2006; Huang et al., 2007; Raymond and Felix, 2011).</p></li><li><p>528 Soil Remediation and Plants</p><p>Fertilizers</p><p>Historically, agriculture was the first major human influence on the soil. To grow and complete the lifecycle, plants must acquire not only macronutrients (N, P, K, S, Ca and Mg), but also essential micronutrients. Some soils are defi-cient in the heavy metals (such as Co, Cu, Fe, Mn, Mo, Ni and Zn) that are essential for healthy plant growth, and crops may be supplied with these as an addition to the soil or as a foliar spray. Cereal crops grown on Cu-deficient soils are occasionally treated with Cu as an addition to the soil, and Mn may similarly be supplied to cereal and root crops. Large quantities of fertilizers are regularly added to soils in intensive farming systems to provide adequate N, P and K for crop growth. The compounds used to supply these elements contain trace amounts of heavy metals (e.g. Cd and Pb) as impurities, which, after con-tinued fertilizer application, may significantly increase their content in the soil (Raymond and Felix, 2011). Metals, such as Cd and Pb, have no known physi-ological activity. Application of certain phosphatic fertilizers inadvertently adds Cd and other potentially toxic elements to the soil, including F, Hg and Pb.</p><p>Pesticides</p><p>Several common pesticides used fairly extensively in agriculture and horticulture in the past contained substantial concentrations of metals. For instance, in the recent past, about 10% of the chemicals approved for use as insecticides and fun-gicides in United Kingdom were based on compounds which contain Cu, Hg, Mn, Pb or Zn. Examples of such pesticides are copper-containing fungicidal sprays such as Bordeaux mixture (copper sulphate) and copper oxychloride. Lead arse-nate was used in fruit orchards for many years to control some parasitic insects. Arsenic-containing compounds were also used extensively to control cattle ticks and to control pests in banana in New Zealand and Australia, timbers have been preserved with formulations of Cu, Cr and As (CCA), and there are now many derelict sites where soil concentrations of these elements greatly exceed back-ground concentrations. Such contamination has the potential to cause problems, particularly if sites are redeveloped for other agricultural or nonagricultural pur-poses. Compared with fertilizers, the use of such materials has been more local-ized, being restricted to particular sites or crops (McLaughlin et al., 2000).</p><p>Biosolids and Manures</p><p>The application of numerous biosolids (e.g. livestock manures, composts and municipal sewage sludge) to land inadvertently leads to the accumulation of heavy metals such as As, Cd, Cr, Cu, Pb, Hg, Ni, Se, Mo, Zn, Tl, Sb and so forth, in the soil (Raymond and Felix, 2011). Certain animal wastes such as poultry, cattle and pig manures produced in agriculture are commonly applied to crops and pastures either as solids or slurries. Although most manures are seen as valuable fertilizers, in the pig and poultry industry, the Cu and Zn added </p></li><li><p>529Chapter | 18 Soil Contamination, Remediation and Plants: Prospects</p><p>to diets as growth promoters and As contained in poultry health products may also have the potential to cause metal contamination of the soil (Chaney and Oliver, 1996). The manures produced from animals on such diets contain high concentrations of As, Cu and Zn and, if repeatedly applied to restricted areas of land, can cause considerable build-up of these metals in the soil in the long run.</p><p>Biosolids (sewage sludge) are primarily organic solid products, produced by wastewater treatment processes that can be beneficially recycled (Raymond and Felix, 2011). Land application of biosolids materials is a common practice in many countries that allow the reuse of biosolids produced by urban popula-tions (Weggler et al., 2004). The term sewage sludge is used in many refer-ences because of its wide recognition and its regulatory definition. However, the term biosolids is becoming more common as a replacement for sewage sludge because it is thought to reflect more accurately the beneficial characteristics inherent to sewage sludge (Raymond and Felix, 2011). It is estimated that in the United States, more than half of approximately 5.6 million dry tons of sewage sludge used or disposed of annually is land applied, and agricultural utilization of biosolids occurs in every region of the country. In the European community, over 30% of the sewage sludge is used as fertilizer in agriculture (Weggler et al., 2004). In Australia over 175,000 tons of dry biosolids are produced each year by the major metropolitan authorities, and currently most biosolids applied to agricultural land are used in arable cropping situations where they can be incor-porated into the soil (McLaughlin et al., 2000).</p><p>There is also considerable interest in the potential for composting biosolids with other organic materials such as sawdust, straw or garden waste. If this trend continues, there will be implications for metal contamination of soils. The potential of biosolids for contaminating soils with heavy metals has caused great concern about their application in agricultural practices (Raymond and Felix, 2011). Heavy metals most commonly found in biosolids are Pb, Ni, Cd, Cr, Cu and Zn, and the metal concentrations are governed by the nature and the intensity of the industrial activity, as well as the type of process employed dur-ing the biosolids treatment (Mattigod and Page, 1983). Under certain conditions, metals added to soils in applications of biosolids can be leached downwards through the soil profile and can have the potential to contaminate groundwater. Recent studies on some New Zealand soils treated with biosolids have shown increased concentrations of Cd, Ni and Zn in drainage leachates (Raymond and Felix, 2011).</p><p>Wastewater</p><p>The application of municipal and industrial wastewater and related effluents to land dates back 400 years and is now a common practice in many parts of the world. Worldwide, it is estimated that 20 million hectares of arable land are irri-gated with wastewater. In several Asian and African cities, studies suggest that agriculture based on wastewater irrigation accounts for 50% of the vegetable </p></li><li><p>530 Soil Remediation and Plants</p><p>supply to urban areas (Bjuhr, 2007). Farmers generally are not bothered about environmental benefits or hazards and are primarily interested in maximizing their yields and profits. Although the metal concentrations in wastewater efflu-ents are usually relatively l...</p></li></ul>

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