Literature Review
2.7 Lead and Cadmium in Environment
Heavy metals are defined as metals or metalloids present naturally in the earth, and became accumulated as a result of anthropogenic activities resulting into ecotoxicity (Duf- fus, 2002). Heavy metals are disposed from the outlet of various industries such as mining, smelting, metal plating, and ore processing which causes hazardous effects on humans, animals, and environment (Volesky and Holan, 1995; Duffus, 2002). At very low con- centrations heavy metals are essential for metabolic activities of living organisms but can cause severe toxic effects on humans, plants and animals at higher concentrations (Lo et al., 1999). Mercury, cadmium, and lead called “the big three” have major impact on the envi- ronment among all the toxic heavy metals (Volesky, 1994).
2.7.1 Causes and Effects of Lead (Pb(II)) Contamination
Lead is present in the earth’s crust majorly as lead sulphide at a low level. The widespread prevalence of lead is a result of human activities such as industries, lead acid batteries, sol- dering, mining, paints, ammunition, smelting, pipes, use of leaded petrol (gasoline), jew-
2.7. Lead and Cadmium in Environment
ellery making, electronic waste and plastic stabilizers (WHO, 2011). Consequently, lead is the one of the most prevalent heavy metal contaminant on earth. The exposure modes of lead to the environment include contaminated air, water, soil, food, and consumer products.
Causes
•Industrial processes
The production of lead-acid batteries, plumbing materials and alloys use lead mainly.
Additionally, lead is also used in paint industries, cable making, insecticides and ammuni- tion. Occupational exposure is a common cause of lead poisoning in adults (Staudinger and Roth, 1998). Contact with materials painted with lead-containing paints through weld- ing, cutting, grinding, construction, fabrication, electronic waste recycling, petrol etc. are significant sources of human exposure to lead.
•Food and smoking
Smoking of tobacco causes lead exposure. The people who are non-smokers are ac- customed to lead exposure through either food intake or the materials used in storing and cooking food. The amount of lead present in the plants depends upon the concentration of lead in soil, mostly soils in the region of mining and smelting are contaminated with lead.
Food colouring, contaminated spices, food and beverage cans made with lead soldering can also cause food contamination. Especially for storage of alcoholic drinks and acidic foods, storage in cans should be avoided. Also, usage of pottery having lead glazed ceramic causes intake of lead through food.
•Drinking water
Significant adverse effects are associated with usage of lead containing plumbing ma- terials. Lead is barely present in tap water. However, water present in these pipes comes in contact with lead, especially the overnight or after longer duration storage will have higher concentration of lead. Acidic water dissolves most of the lead.
•Domestic sources
Lead contaminated paint on the household walls and dust are the main sources of exposure for infants and young children. Some toys made of lead containing plastic and paints, and also other traditional makeup (e.g. kohl).
•Exposure routes of lead in environment and its effects
The biogeochemical cycle in the environment influences the chemical and physical prop- erties of lead to move throughout the ecosystem. The metal can enter into the different components of environment and affect them until reaching the equilibrium. Lead gets accu- mulated in different components of environment such as plants, ground, and water surface.
Lead is the most ubiquitously spread heavy metal in the soil. It exerts adverse effects on the germination, physiology, morphology and metabolism of plants (Nagajyoti et al., 2010).
•Effects on humans
Lead is a cumulative general poison, infants and pregnant women being most suscepti- ble to adverse health effects, it deteriorates the central nervous, kidney and the reproductive system (Sheng et al., 2004). Lead attacks the brain and central nervous system leading to coma. Children suffering from lead poisoning can have mental disorder. Other effects of lead exposure include anaemia, hypertension, immune dysfunction, renal failure, toxicity in reproductive organs (Staudinger and Roth, 1998; WHO, 2011).
•Effects on soil
Urban and agricultural soils are contaminated with lead due to mining, manufacturing, paint industries, pesticides, batteries, and industrial waste. It can result in heavy metal contamination of urban and agricultural soils. Lead in the soil can be ingested by children through hand-to-mouth activity, eating vegetables grown in the soil and from inhalation of re-suspended lead in the air (UNEP, 2010).
•Permissible limits of lead
According to world Health Organization (WHO) the permissible limits lead in drinking water is 10µg/L and for air is 0.5µg/m3. The provisional tolerable weekly intake (PTWI) of lead should be 25µg/kg of body weight for infants and children, considering that lead is a cumulative poison there should be no accumulation lead in body. The Indian standard for lead by Bureau of Indian Standard (BIS) in drinking water is 0.01 mg/L (BIS, 2009) and for air is 0.5µg/m3 (CPCB, 2009).
2.7.2 Causes and effects of Cadmium (Cd(II)) Contamination
Cadmium is present in earth’s crust at a concentration of 0.1-0.5 ppm in common as- sociation with zinc, lead, and copper, it is also present naturally in the ocean waters with
2.7. Lead and Cadmium in Environment
average levels of 5-110 ng/L (Kirk, 2007). Common sources of heavy metal contamina- tion are mining, industrial wastes, batteries, plastics, fertilizers, vehicle emissions, paints etc. (Waalkes, 2000; Kirkham, 2006; Oh et al., 2009). Exposure modes of Cadmium to the environment include contaminated air, water, soil, food, and consumer products.
Industrial processes: The production of NiCd batteries also causes cadmium deposition.
Also the consumption of refined cadmium in pigments preparation for the plastics, enamels and ceramics deposits cadmium in household surroundings. Iron and steel production, cement production, alloying with metals such as lead, tin, and copper
Food and Smoking: Food contamination by cadmium can be caused by the cadmium polluted soil, or usage of cadmium-contaminated water for irrigation.Cigarette smoking increases risk of cadmium exposure to both active and passive smokers, an average smoking of twenty cigarettes a day results into intake of 2-4µg of cadmium (WHO, 2011).
Municipal installations: Open burning, landfills or incineration of cadmium containing waste increases risk of cadmium in air particles (UNEP, 2015).
Drinking water: The contamination of drinking water due to the presence of cadmium in cadmium-containing solders, water coolers, taps, and water heaters (WHO, 2011).
•Effects of Cadmium (Cd(II)) in the environment
Effects on humans: Cadmium affects the human health due its high mobility and toxic nature. Liver dysfunction, disruption of hematopoietic and immune system, osteoporosis are reported as consequences of short-term exposure of cadmium. Other effects include the kidney dysfunction, DNA degradation, and cancer. Inhalation of relatively high con- centration of cadmium in atmosphere causes great risk of lung cancer, other targeted sites for cadmium carcinogenesis are stomach, reproductive system, liver, and kidney (Waalkes, 2000; Lin et al., 2012; UNEP, 2013).
Effects on soil: The toxicity and bioavailability of cadmium in soil depends on the charac- teristics of soil. Cadmium enters the soil through composts, sludge or fertilizers (Kirkham, 2006). Cadmium enters the geochemical cycle through plants grown in cadmium loaded soil, consequently it enters into the food chain causing problem of bioaccumulation and biomagnification (Volesky, 2007; Gadd, 2009).
Permissible limits of Cadmium: World health organization (WHO) has recognized that the average weekly intake of cadmium present in food lies in the range of 0.7-2.8µg/kg of body weight. According to EPA maximum contaminant level for cadmium exposure is 0.005 mg/L. WHO recommended in its guidelines that 0.003 mg/L of Cd is its threshold limit in drinking water.