A Review on Removal of Heavy Metals from Water by Adsorption

Abstract

Sources and health effects Arsenic Arsenic is one of the contaminants found in the environment which is notoriously toxic to man and other living organisms. It is generally accepted that the inorganic species, arsenite [As +3 ] and arsenate [As +5 ], are the predominant species in most environments, although the organic ones might also be present. Although As +5 tends to be less toxic compared to of As +3 , it is thermodynamically more stable due to it predominates under normal conditions and becomes the cause of major contaminant in ground water. The occurrence of arsenic in natural waters is usually associated with sedimentary rocks of marine origin, weathered volcanic rock, fossil fuels, geothermal areas, mineral deposits, mining wastes, agricultural use, and irrigation practices. Anthropogenic sources of arsenic include nonferrous metal mining and smelting, pesticide application, coal combustion, wood combustion, and waste incineration. Most anthropogenic releases of arsenic are due to land or soil, primarily in the form of pesticides or solid wastes. Chronic intake of inorganic arsenic being present in concentrations above 50 µg/L in drinking water, different kinds of skin lesions (e.g., hyperpigmentation, hyperkeratosis) and cancers (e.g., skin, lung, kidney, bladder) can develop, which are collectively termed as arsenicosis. Cadmium It is divalent metal and has +2 oxidation states. The average concentration of cadmium in earth's crust is found to be 0.1-0.5 ppm. Cadmium is insoluble in water and non-flammable. Cadmium is a toxic metal which has hazardous effect on environment as well as human beings. Cadmium is widely used in making nickel-cadmium batteries, pigments, phosphate fertilizers, alloy and metal plating. Runoff from waste batteries and paints, corrosion of galvanized pipes, and discharge from metal refineries are sources of cadmium discharge in water. Natural emission of cadmium in water is by volcanic eruptions and in form of ores of zinc, lead and copper, and coal. As per the drinking standards given by Bureau of Indian Standards the limit in drinking water is from 0.003mg/L to 0.005mg/L [3]. High concentration of cadmium in drinking water causes serious poisoning in human beings. Cadmium affected humans has a risk of cancer, cardio vascular diseases, and osteoporosis. Chromium About 60% of the chromium produced goes into chromium-based alloys, 20% into chemical use including electroplating, and most of the remainder into furnace bricks and other refractory products. At many locations, chromium has been released into the environment via leakage, poor storage or improper disposal practices. Sources of chromium waste include metal plating, steel fabrication, paint and pigment production, wood treatment, leather tanning, and chromium mining and milling. Chromate is also an important constituent of waste streams from coal-burning power plants. Chromium and its salts are used in the leather tanning and textile dyeing industries, the manufacture of catalysts, laundry chemicals, pigments and paints, fungicides, the ceramics and glass industry, and in photography, and for chrome alloy and chromium metal production, chrome plating and corrosion control. Chromium has oxidation states from-2 to +6; however mostly trivalent and hexavalent occurs in water. Chromium toxicity is dependent on chemical speciation and thus associated health effects are influenced by chemical forms of exposure. Cr (VI) compounds are much more soluble than Cr (III) and are much more toxic (mutagenic and carcinogenic) to microorganisms, plants, animals and humans. Cr (VI) is toxic, producing liver and kidney damage, internal haemorrhage and respiratory disorders. An oral dose of 2-5 g of a soluble hexavalent chromium compound can be fatal to an adult human. Ingesting less than 2 g of hexavalent chromium compound can result in kidney and liver damage after 1-4 days of exposure. Sub-chronic and chronic effects include dermatitis and skin ulceration. Cr (VI) has been shown to cause cancer in humans and animals through inhalation exposure, but has not been shown to be carcinogenic through ingestion exposure. Cr (VI) compounds have been associated with lung cancer in humans. Copper In nature, copper occurs in a variety of minerals, including native copper, copper sulphides such as chalcopyrite, bornite, digenite, covellite, and chalcocite, copper sulfosalts such as copper (II) oxides such as cuprite and tenorite, respectively. Copper has +1 and +2 oxidation states. Copper enters in the environment through natural and anthropogenic sources. The contamination of air and water by copper is contributed from mining, milling, refining of copper ores, electroplating, petroleum and refinery industries. Copper is discharged in water effluent of various industries like chlor alkali, electroplating, paints, dyes, fertilizer, mining and metallurgy, explosives, pesticides, iron and steel industries etc. Copper is a toxic metal which is hazardous for flora, fauna, and human beings. Ingestion of 15-75 mg of copper causes gastro intestinal diseases. Excessive intake of copper may cause haemolysis, hepatotoxic and nephron toxic. Its toxicity leads to widespread capillary damage, hepatic and renal damage, and also central nervous system irritation followed by depression. Mercury (Hg) Mercury is a naturally occurring metal that is present in several forms. Mercury is a persistent environmental pollutant with bioaccumulation ability in fish, animals, and human beings. Mercury salts and organo mercury compounds are among the most poisonous substances in our environment. The mechanism and extent of toxicity depend strongly on the type of compound and the redox