Shishir Reza :
More than 170 million people are living in Bangladesh which is one of the most densely populated countries in the world. Agriculture plays a fundamental role in the revival of Bangladesh economy. Agricultural soils are extremely diversified in appearance and quality, yet they consist of different proportions of the same elements; which are in various states of chemical combination, or mechanical mixtures. The quality of the soil determines the nature of plant ecosystems and the capacity of land to support animal life and society. The contamination of soil, rice and vegetables by heavy metals is currently a global environmental issue. They are universal in the environment through various pathways, due to natural and anthropogenic activities.
As trace elements, some heavy metals are essential to maintain the metabolism of human body. However, Mercury, Cadmium and Lead are highly toxic in nature. On the basis of health importance, heavy metals can be categorized into four major groups.
Cadmium is a heavy metal and it is not essential for daily life. Cadmium is a 67th most abundant element which has a density of 8.64 g/cm3 at 200C, a melting point of 3200C, and a boiling point of 7670C. It is a silver-white, lustrous and ductile metal. The atomic number is 48 and atomic mass is 112.4. The most significant use of Cd is in Ni/Cd batteries, as rechargeable or secondary power sources exhibiting high output, long life, low maintenance, and high tolerance to physical and electrical stress. Cadmium coatings provide good corrosion resistance coating to vessels and other vehicles, particularly in high-stress environments such as marine and aerospace. Other uses of cadmium are as pigments, stabilizers for polyvinyl chloride (PVC), in alloys and electronic compounds. Cadmium is also present as an impurity in several products, including phosphate fertilizers, detergents and refined petroleum products and also metal based pesticides, industrial emissions, transportation, harvesting process. Agricultural soil contamination with heavy metals through the repeated use of untreated or poorly treated wastewater from industrial establishments and application of chemical fertilizers and pesticides is one of the most severe ecological problems in Bangladesh.
Soil Cadmium is derived from weathering of rocks and minerals or from numerous anthropogenic sources. Two primary sources of heavy metals in the soil are (i) the natural background i.e. metals derived from parent rocks; and (ii) the anthropogenic contamination i.e. those originated from human activities. Vegetables are part of daily diets in many households forming an important source of vitamins and minerals required for human health. The daily vegetable consumption by an adult of Bangladesh is 130 g. The concentrations of these trace elements in vegetables may vary depending on the inherent (varieties, maturity, genetics, and age) and environmental (soils, geographical locations, season, water source and use of fertilizers) conditions of plants and animals and on methods of handling and processing. An average man accumulates as about 30 mg cadmium in his body by the age of 50 years. Refined foods, water foods, water pipes, coffee, tea, coal burning and cigarettes are the most important source of cadmium.
It has been observed that cadmium concentration is <0.1 mg/kg in Potato, Red amarantha, Spinach amarantha, Carrot, Cabbage, Tomato, and Brinzal at Pakshi union, Ishwardi, Pabna. The average cadmium concentration in Amaranthus, Radish and cauliflower is 2.545mg/kg at Ruppur, Pabna district. Research explains Cadmium levels in leafy vegetables. Of these lalshak contained the highest Cadmium level (303 µg/kg; mean 100.5. It is reported that Cadmium level is the highest in spinach with a range of 0.559 to 1.400 µg/g followed by tomato with a range of 0.630 to 1.303 µg/g and cauliflower having 0.506 to 0.782 µg/g. Report implies that the range of Cadmium concentration in the in Rupganjupzila (sub-district) under Narayanganj district besides the Shitalakhya river vegetables was from 0.095 to 0.283 mg/kg and the trend of accumulation of Cadmium concentration.
Heavy metal contamination in paddy soils is one of the most serious problems facing rice production and soil management in Asian countries. Among agricultural food products, rice (Oryza sativa) is one of the most widely consumed staple cereal foods in the world constituting about 89% of the diet of people in Asian countries. In many East and South Asian countries including Bangladesh, Japan, Indonesia, and Korea, the accumulation of metal particularly cadmium (Cd), in rice ecosystems and its subsequent transfer to the human food chain is a major environmental issue. In the world, especially in Latin America, the East, and South Asia, the Middle East and the West Indies rice is one of the most important staple foods which are the staff of life for 3 billion people (Stone, 2008). It also provides about 30% of the dietary energy and 20% of the dietary protein in Asia. However, rice may contain significant amounts of contaminants such as arsenic (As), cadmium (Cd) and lead (Pb).
Jahiruddinet (2017) reported that irrigated rice samples collected from Mymensingh exhibited the highest Cd concentration (mean 0.080 mg/kg), which was threefold higher than Saltha (0.024 mg/kg) and between the two rice seasons, the difference of Cd concentration was small. The Cd level of rice samples depending on the varieties ranged from 0.007-0.297 mg/kg; however, the majority varieties had Cd concentration below 0.05 mg/kg. Study of Bangladesh Rice Research Institute (BRRI) finds 8/86 samples of industrial field rice were highly contaminated with cadmium(>0.07 mg/416.01 g). They also found 21/76 market samples of rice contained greater amount than the risk level of daily intake of cadmium.
Cadmium is known mostly for its toxicity to animals and humans. It causes various health-related problems in humans through accumulation in different organs. Cadmium enters through ingestion and inhalation in the human body. The acute effects of this metal can be seen in the respiratory and digestive tracts. The toxic effects of excess Cadmium on humans are hypertension, emphysema, carcinogenic changes (mainly kidney and prostate), skeletal deformation and low reproductive function. Cadmium exposure may increase important manifestations of cardiovascular disease Environmental exposure to Cadmium are associated with significantly increased stroke and heart failure prevalence.
(The Writer is an Environmental Analyst and Associate Member, Bangladesh Economic Association).
