Introduction

Rapid industrialisation has led to increased disposal of heavy metals into the environment and hence effluent treatment is one of the most important targets for industry. Different alternatives for treating effluents are described in literature, including chemical precipitation, carbon adsorption, ion exchange and membrane separation process, among others (Juang and Shiau, 2000; Lacour et al., 2001; Yan and Viraraghavan, 2001). The most common of these alternatives is chemical precipitation However some limitations in the process can be pointed out, such as high cost, low efficiency, labour intensive operation, and lack of selectivity of the precipitation process (Lee et al., 1998). On the other hand, the use of other natural materials such as peanut shells, soybean hulls and corncobs, which are available in large quantities, may present high potential as inexpensive sorbents for effluent treatment (Marshall et al., 1999; Wafwoyo et al., 1999; Vaughan et al., 2001). Bailey et al., (1999) have reviewed the potential of a wide variety of low cost sorbents for heavy metals. A low cost sorbent is the one that requires less processing, is abundant in nature or is either a byproduct or waste material from another industry. These materials could be an alternative for expensive treatment processes.

In India, tamarind (Tamarindus indica L.) is an economically important tree which grows abundantly in the dry tracts of Central and South Indian States. Indian production of tamarind is about 3 lakh (0.3 million) tonnes per year. The hard pod shell is removed (deshelled) when the fruit is ripe and the fruit is the chief acidulant used in the preparation of foods. The shells are discarded as waste and since it is available free of cost, only the transport cost is involved for hauling it from the point of generation for wastewater treatment. Hence, recycling of this waste for wastewater treatment would not only be economical but also help to solve waste disposal problems.

Hexavalent chromium frequently encountered in electroplating effluent, is one of the most toxic heavy metal in the environment and is also a known carcinogen and mutagen in humans and animals (Zhao and Duncan 1998; Salunkhe et al., 1998). The possibility of using Tamarind shells, an agricultural waste for the removal of Cr (VI) from aqueous solutions has been examined in this paper.