Introduction

Chromium is an essential nutrient for plant and animal metabolism. However, the increasing accumulation of chromium in the environment from industrial outputs has caused great concern. Chromium contaminated wastewaters can originate from dyes and pigment manufacturing, wood preserving, electroplating and leather tanning. Chromium exists in +3 or +6 oxidation states, as all other oxidation states are not stable in aqueous solutions. Both valences of chromium are potentially harmful (Dakiky et al., 2002). Hexavalent chromium which is primarily present in the form of chromate (CrO₄^2-) and dichromate (Cr₂O₇^2-) poses significantly higher levels of toxicity than the other valency states (Sharma and Forster, 1995).

Conventional methods for removing Cr (VI) ions from industrial wastewater include reduction (Kim et al., 2002), reduction followed by chemical precipitation (Ozer et al., 1997), adsorption on activated carbon (Lotfi and Adhoum, 2002), solvent extraction (Mauri et al., 2001), freeze separation, reverse osmosis (Padilla and Tavani, 1999), ion exchange (Rengaraj et al., 2003) and electrolytic methods (Namasivayam and Yamuna, 1995). These methods have found limited application because they often involve  high capital and operational costs. Adsorption is an effective and versatile method for removing chromium. Natural materials that are available in large quantities or certain waste products from industrial or agricultural operations, may have potential as inexpensive sorbents. Most of the low cost sorbents have the limitation of low sorptive capacity and thereby for the same degree of treatment, it generates more solid waste (pollutant laden sorbent after treatment), which poses disposal problems. Therefore, there is a need to explore low cost sorbents having high contaminant sorption.