Heavy Metal Toxicity in Soils: Sources, Remediation Technologies and Challenges

Soil pollution with heavy metals draws a serious concern because of their detrimental effects on the living biota. The persistent and non-biodegradable nature of heavy metals eases their accumulation in the environment. Agricultural soils are receiving tremendous amount of pollutants from the various sources. During the last few decades, rapid urbanization, industrialization, injudicious and faulty agricultural practices influence the release of metal contaminants in the agricultural fields. Heavy metal contamination in agricultural soils may impart functional disorders of soils, retarded plant growth and even harm the health of humans through contamination of food chain. The heavy metals bioaccumulate in the living systems and their concentrations increase as they pass from organism of lower trophic level to higher trophic level, a phenomenon known as biological concentration. According to Babula et al. [1] heavy metals are the ill-defined subset of elements having a higher molecular weight that includes transition metals, some metalloids, lanthanides and actinides with specific density greater than water, i.e., 5g cm-3. They are classified as essential and non-essential on the basis of their function in living organisms. Essential heavy metals like Zinc (Zn), Nickel (Ni), Manganese (Mn) and Iron (Fe) etc are required by the plants for performing various physiological and biochemical functions. Non-essential heavy metals like Lead (Pb), Aresenic (As), Cadmium (Cd), Chromium (Cr) and Mercury (Hg) etc have no physiological role and they entered the living systems through food chain. Beyond critical limits, heavy metals impart hazardous impact on human health as they hinder the normal functioning of the living systems. ATSDR [2] has listed arsenic, lead, mercury and cadmium at first, second, third and seventh position in terms of their frequency, toxicity and potency for human exposure. The increasing production of livestock and poultry products has led to generation of large amount of wastes into the soil from the industries. The large quantity of waste produced must be treated properly keeping into consideration the environmental measures involved in land treatment. The elevated heavy metal levels in the agricultural soils depend on the characteristics of the soil and the rate of application by the supplier with its elemental concentration. These heavy metals or metalloids do not undergo microbial and chemical degradation and persist in the soil for longer duration. Due to the increasing awareness among the public and the detrimental effects of these contaminants on human health, scientific communities are focusing on development of some new technologies for removal of these metals from contaminated soils. This review focuses on sources of heavy metals in soil, strategies or the technologies involved in their removal from the contaminated soils and further the challenges involved in use of various amendments for remediation of soils.