ADSORPTION OF HEAVY METALS FROM AQUEOUS SOLUTIONS USING MANGROVES
Heavy metal release to the environment has been increasing continuously as a result of industrial activities and technological development, posing a significant threat to the environment, public and soil health (Cerbasi and Yetis; 2001). Among the heavy metals released into environment, chromium is an important pollutant. Chromium is a redox active element, with oxidation states from –2 to +6, but only the +3 and +6 states are prevalent in the aqueous environment. Cr(III) and Cr(VI) are environmentally stable oxidation states and exhibit different types of toxicity mechanism on biota. Chromium is released into the environment by a large number of industries such as mining, iron sheet cleaning, chrome plating, leather tanning and wood preservation (Krishna and Philip; 2005). These industries contains Cr(III) and Cr(VI) at a concentration ranging from 10 to 100 mg/L (Park et al; 2005). Long-term release of such wastewater may result in the accumulation of heavy metals in soil exerting a selection pressure on soil micro-biota. Cr(VI) is a highly soluble and toxic chromate anion, and is a suspected carcinogen and mutagen (Costa; 2003). Also the wide use of chromium (Cr) by modem industries has resulted in a large quantity of this element being discharged into the environment via air emission, waste water or land disposal. One of the important sources of the Cr in the waste water is due to the use of chromates in cooling water for corrosion control. Cr may exist in water in both hexavalent and trivalent states. Trivalent Cr has a lower toxicity than that in the hexavalent state.
Important sources which add chromium (Cr) to the environment are: Ferro-chromium production units, refining of ores, chemical industries and the combustion fossil fuels. In rural areas chromium content in ambient air is usually less than 0.1 ~g/cubic meter, while in industrial areas it may range from 0.01-0.03 pg/cubic meters. Particulate ejected from coal fired power generation plants may contain 2.3-32 ppm of chromium. Waste waters from tanneries may contain 10-50 ppm, textile wastes up to 32 ppm while spent chrome liquors may contain up to 4500 mg/litre of Cr. Only trivalent and hexavalent forms of Cr are of biological significance. Trivalent Cr is the most common form of the metal in nature and it is in this state the metal almost always occurs in a living system. Hexavalent Cr is capable of crossing the cell membrane and is rapidly reduced to trivalent state inside a living cell. Small quantities of trivalent Cr are essential to carbohydrate metabolism in mammals while it is also a co-factor for action of insulin. A little amount of Cr is helpful in improving glucose tolerance in diabetic patients, weak and old individuals. Most of the mammals can tolerate almost a hundred times more Cr in trivalent state than their usual body burden without any toxic effects. Acute Cr toxicity causes serious renal tubular necrosis. Exposure to hexavalent Cr has been found to cause dermatitis, allergic skin reactions, chronic ulceration and injury to nasal septum, gastrointestinal ulcers etc. Chronic Cr toxicity has been associated with incidence of cancers of respiratory track in occupationally involved workers. Both forms of Cr (hexavalent and trivalent) are considered equally
3 potent carcinogenic agent. In view of the paradoxical role of the Cr, as an essential micronutrient in human and animal nutrition at low concentrations, and a known carcinogen at the elevated levels, there is a growing concern about the fate and effects of Cr in the environment. The conventional methods for removing Cr (VI) ions from wastewater are based on the combination of different physical and chemical reduction processes (Kurniawan et al; 2006). However, such processes are becoming undesirable due to the use of expensive and toxic chemicals.
Further these methods are also inefficient in the removal of Cr(VI), particularly with wastewater containing with a low Cr concentration and also associated with the production of secondary effluents (Baral and Engelken, 2002). Prakasham et al., (1999) and Kadimpati et al. (2013) suggested that the removal of heavy metals by adsorption on various biomass surfaces is the best alternative to the physico and chemical methods. In recent years, many researchers used the various bio-materials such as tea factory waste (Cay et al. 2004; Malkoc and Nuhoglu 2005, 2006), sawdust (Acar and Malkoc 2004; Yu et al. 2003), soya cake (Daneshvar et al; 2002), sugarcane bagasse (Gupta and Ali 2004), green algae (Gupta et al. 2001; Malkoc and Nuhoglu 2003), distillery sludge (Selvaraj et al. 2003) for removal of heavy metals from waste waters. Most of the low-cost sorbents have the limitation of poor sportive capacity, and thereby, for the same degree of treatment it generates more solid waste (pollutant laden sorbent after treatment) leading to disposal problems. Therefore, there is a need to explore low cost, economically viable and effective sorbent, having high pollutant sorption capacity (Ho et al. 2005)