The Earth’s crust is composed of various elements some of which are essential for plant growth while others detrimental to growth and development of plants and animals. A portion of the earth’s crust is soils; which serve as a medium for plant growth. Soils contain a balance of elements arranged as nutrient elements and compounds. These elements can be mis-balanced by the addition of trace elements, some of which are heavy metals which are either not required by the plants/ animals or are detrimental to the biotic life.
The addition of heavy metals to agriculture soils is primarily attributed to the use of waste water irrigation in the urban and peri-urban surroundings around the globe but more specifically in the developing world. The second major attribution is the blatant and atrocious use of chemicals pesticides and insecticides on every crop particularly the vegetables, the main produce of urban agriculture soils in developing countries.
Urban wastewater comes from domestic sewage; commercial establishments and institutions and from industries. The use of polluted water in the immediate urban surroundings for the growing of crops and vegetables is a common practice. The urban wastewater is considered a rich source of nutrients (Kennish, 1993), over a period of time, induces heavy metals accumulation in soil and that may be toxic to the plants and also cause deterioration of soil (Kirkhan, 1983).
The disposed waste waters are contaminated with trace elements like lead (Pb), copper (Cu), zinc (Zn), boron (B), cobalt (Co) chromium (Cr), arsenic (As), molybdenum (Mo), manganese (Mn) etc. many of which are non essential and over time toxic to plants, animals and human beings. Elements like Cd, Cr, Pb and Ni which are industrially used are included in the group-I carcinogens by the International Agency for Research on cancer i.e. they are known to cause cancer in humans. These problem heavy metals are attracting the attention of researchers around the world for developing a safe and effective low-cost in-situ remediation technology; to decrease the mobility of toxic heavy metals in soils, rendering them less mobile and more stable, thereby decreasing their entry into the plant body.
Pre-treatment of wastewater is always recommended before agricultural application but due to the costs involved, in most places around the globe, to be more specific; in the third world countries, wastewater, irrespective of its origin, is applied to the crops untreated. The laws are seldom followed and the implementation is weak. This direct application inflicts much of the unseen hazards into the food chain to which the farmers are oblivious, e.g. heavy metal bio-magnification, pathogenic infections and other toxic elements in the agricultural produce.
The trace elements, more specifically heavy metals, cannot be destroyed on the analogy of organic contaminants but only be relocated from one place to another, e.g. contaminated site to landfills or from industries to agriculturally productive lands via wastewater irrigation. The high cost of traditional soil remediation techniques, the need to produce more food for the ever increasing population necessitates the use of alternate means to grow cops. The introduction of high yielding varieties, hybrid crops and the need to grow crops in the close vicinity of the settlements have necessitated the use of all available means, from blatant use of chemical insecticides, use of all available land and the use of any available water including industrial wastewater for agricultural use and prompted the development of alternative techniques that are cost-effective and less disruptive to the environment such as soil stabilization. For the comparison of soil stabilization with other commonly used soil remediation techniques, refer to Mulligan et al. (2001). In-situ stabilization is the chemical stabilization of heavy metals induced by immobilizing soil additives (amendments). As, and four heavy metals, Cr, Pb and Zn, in soils are of environmental relevance as they are common soil contaminants. As, Cr, and Zn can be found all together in soil contaminated with a wood impregnation chemical, chromated copper arsenate (CCA), while they can vary in composition in industrial wastewater depending on the nature of industry.
The in-situ stabilization of heavy metals in waste water contaminated agriculture soils is a remediation technique wherein the mobility of heavy metal is reduced in soil through addition of a stabilizing/immobilizing agent. The main aims being to slightly change the environmental condition for the element fractions to be stabilized, leached down or transform into unavailable complexes that are not taken up by plants.
The stabilization of heavy metals (Cd, Cr, Pb and Ni) in soil can be achieved by application of amendments able to adsorb, complex or precipitate the heavy metals. The addition of soil amendments has been practiced for soils of mine tailings and under various individual metal conditions, e.g. addition of lime, clays, organic matter, phosphates etc. to improve soil availability of depleted nutrients and reduce soil phytotoxicity of various metals by decreasing the mobility and bioavailability of toxic elements (Bolan et al., 2003). The contaminant immobilization in soil by the action of amendments has been studied by various researchers (Gupta et al., 2000; Mench et al., 2000; Knox et al., 2001; Morgan et al., 2002; Rautaray et al., 2003; Tripathi et al., 2004; Chiu et al., 2006; de Mora et al., 2005).
A combination of various amendments will be applied to improve the treatment efficiency of multi-element contaminated sites such as wastewater irrigated agricultural soils. For example, Humic acid amended soils can be neutralized applying appropriate amounts of lime, and leaching of metals could be avoided (Warren et al., 2003). Soil acidification due to phosphoric acid can also be managed by the following application of phosphate rock and calcium phosphates (Melamed et al., 2003).
• To investigate the extent of carcinogenic heavy metals in wastewater irrigated agriculture soils and devise cost effective in-situ techniques for stabilization of heavymetals in order to ensure safe agricultural crops.
The specific objectives of the study are;
• To investigate the extent of accumulation of heavy metals in wastewater irrigated fields.
• To determine the effects of various organic and inorganic amendments on the stabilization of various heavy metals in soil under incubation studies.
• To study the effect of amendments on heavy metals immobilization under hydroponic conditions.
• To study the effect of organic and inorganic fertilizer or chemicals on phytostabilization /immobilization of heavy metals in soil.