Sources of heavy metals

Dr. Rabah Salim Shareef Gumaa

Mining, manufacturing, and the use of synthetic products (e.g. pesticides, paints, batteries, industrial waste, and land application of industrial or domestic sludge) can result in heavy metal contamination of urban and agricultural soils. Heavy metals also occur naturally, but rarely at toxic levels. Potentially contaminated soils may occur at old landfill sites (particularly those that accepted industrial wastes), old orchards that used insecticides containing arsenic as an active ingredient, fields that had past applications of waste water or municipal sludge, areas in or around mining waste piles and tailings, industrial areas where chemicals may have been dumped on the ground, or in areas downwind from industrial sites. Some researchers clarify that the heavy metals are classified among the most dangerous groups of human environmental pollutants due to their toxicity and persistence in the environment. Consequently, the evaluation of the level of metal deposition is of vital importance for the assessment of plant exposure. Heavy metals are commonly considered potential pollutants to the soil and groundwater environment. Major sources of heavy metal contamination including industrial and anthropogenic wastes, mining, and smelting. It is recognized that the fate and transport of heavy metals in soils and geological media is significantly impacted by various interactions in the soil systems.

In the study for a different heavy metal concentrations in plants and soil irrigated with industrial / sewage waste water that the application of waste water for irrigation purposes has increased over the past years confirmed that the waste water contains high amount of trace elements and heavy metals. The results revealed that heavy metals concentration in soil irrigated with waste water was higher of toxicity level at depth of 0-15 cm than the lower layer 16-30 cm while the leaves and fruits of vegetables also showed higher concentration of heavy metals. The maximum concentration of lead, iron and manganese was recorded in soil samples taken from Nawabanwala, Malkanwala and Sheikhanwala respectively. The results of another study showed that the highest and the lowest concentrations of (Pb, Cu, Zn, Co, Cr, Cd and Ni) were found near the road. This indicates that the vehicular traffic has been major source of heavy metal contamination in urban areas. Tree leaves have been widely used as indicator of atmospheric pollutions. the study indicated that the increase in industrial and agricultural activities, the content of  Cd in soils has also increased.

Effect of heavy metals on human and plant

Heavy metal pollution in air, water and agricultural soils is one of major ecological concern due its impact on human health through the food chain and its high persistence in the environment. Excess heavy metal accumulation in soils is toxic to humans and other animals. Exposure to heavy metals is normally chronic (exposure over a longer period of time), due to food chain transfer. Acute (immediate) poisoning from heavy metals is rare through ingestion or dermal contact, but is possible.

Heavy metals could interact with many metabolic functions of plants. For instance; inhibition of growth processes or decreases in the activity of the photosynthetic apparatus often correlated with progression of senescence processes etc. . As well as the exposure of plants to toxic levels of heavy metals triggers a wide range of physiological and metabolic alterations. However, as different heavy metals have different sites of action within the plant, the overall visual toxic response differs between heavy metals. The most widespread visual evidence of heavy metal toxicity is a reduction in plant growth including leaf chlorosis, necrosis, turgor loss, a decrease in the rate of seed germination, and a crippled photosynthetic apparatus, often correlated with progressing senescence processes or with plant death.  Anyone interested in studying heavy metals said in all cases that the amount of metal accumulated in the plant increased by increasing the concentration of the applied metal, and the roots accumulated more metal than did the aerial part of the plant. The results of another study indicated that the number of factors that affect the metals uptake by the plant from the soil such as heavy metal concentration, type of the fertilizer applied, plant growth stage and species. Heavy metals affect a number of soil characteristics, including microbial growth and enzyme activity and crop growth.

Heavy metals in vegetables accumulate through uptake from soil, water and atmosphere. The intake of heavy metals through diet leads to several disorders such as kidney damage, nervous disorder, bone disease. Heavy metals, such as cadmium, copper, lead, chromium and mercury are major environmental pollutants, particularly in areas with high anthropogenic pressure. [Heavy metal accumulation in soils is of great concern in agricultural production due to the adverse effects on food safety and marketability, crop growth due to phytotoxicity, and environmental health of soil organisms]. The influence of plants and their metabolic activities affects the geological and biological redistribution of heavy metals through pollution of the air, water and soil. A common consequence of heavy metal toxicity is the excessive accumulation of reactive oxygen species (ROS) and methyl glyoxal (MG), both of which can cause peroxidation of lipids, oxidation of protein, inactivation of enzymes, DNA damage and/or interact with other vital constituents of plant cells.

Another study indicated that the heavy metals is of special concern in Phosphate fertilizer (PF) industry. These are naturally present in earth’s crust in the amount required for all the living biota. But with the excessive use of fertilizers in an area the extra amount of these are transferred to the soil and then to crops and enter in to food chain. Entrance of these elements in different compartment of environment poses threat not only to soil, air and water but ultimately to living population of the concerned area. Furthermore continuous use of these fertilizers causes these elements to accumulate in the environment which after sometime may exceed their tolerable limits which in turn poses various health problems to human life. Also the deficiency and surplus of these elements may affect the growth and yield of any crop plant.