The pollution caused because of heavy metals has received widespread attention in the recent years (Bishnoi et al., 2004) due to the toxicological importance in the ecosystem, agriculture and human health. It leads to the development of alternative technologies for the removal of these pollutants from aqueous effluents. The use of low-cost and waste biomaterials as adsorbents of dissolved metal ions has been shown to provide economic solutions to this global problem (Park et al., 2005). In this context, our adsorbent (RCC: Ricinus communis seed shell) could be used as an effective and environment-friendly adsorbent for the removal of Cr(VI) containing water and wastewater.
Chromium is a highly toxic pollutant generated from many industrial processes such as leather tanning processes, electroplating and manufacturing of dye, paint and paper. Chromium exists in the aquatic environment mainly in two states; trivalent chromium and hexavalent chromium. Hexavalent chromium is primarily present in the form of chromate and dichromate ions (Khezami and Capart, 2005). The USEPA has set the permissible level for chromium in drinking water at 0.05 mg/L. These standards are based on the total concentration of the trivalent and hexavalent forms of dissolved chromium. Chromium has the potential to cause the following health effects from long-term exposure to above the minimum cleanliness level (MCL); damage to liver, kidney and nerve tissues (USEPA, 1995).
Conventional methods for removal of dissolved heavy metal ions included the chemical precipitation, chemical oxidation and reduction, ion exchange, filtration, electrochemical treatment and evaporative recovery. However, these high-technology processes have significant disadvantages, including incomplete metal removal, requirements for expensive equipment and monitoring systems, high-cost reagents, energy requirements, generation of toxic sludge and other waste products that require disposal (Aksu et al., 2002).
Adsorption on activated carbon has been found to be an effective process for Cr(VI) removal, but it is too expensive. Natural materials are available in large quantities; certain waste products from industrial and agricultural operations may have potential as inexpensive adsorbents. Due to their low cost, after these materials have been expended, they could be disposed without expensive regeneration. Most of the low-cost adsorbents have the limitation of low sorptive capacity and thereby same degree of treatment, which leads to disposal problems. Therefore, there is need to explore ‘low-cost biosorbent’ having high adsorption capacity (Agarwal et al., 2006). Consequently, numerous low-cost alternatives have been studied including sawdust (Acar and Malkoc, 2004), eucalyptus bark (Sarin and Pant, 2006), green algae (Malkoc and Nuhoglu, 2003), seaweeds (Vijayaraghavan et al., 2005), coir pitch (Kadirvelu et al., 2001), peanut husks carbon (Ricordel et al., 2001), Zeolite tuff (Al-Haj and El-Bishtawi, 1999), activated carbon fabric cloth (Mohan et al., 2005), bagasse fly ash (Gupta et al., 1999), activated slag (Srivastava et al., 1997), green algae Spirogyra species (Gupta et al., 2001), nonviable cyano bacterium Nostoc muscorum biomass (Gupta and Rastogi, 2008), fertiliser industry waste material (Gupta et al., 2010), fertiliser waste material (Srivastava et al., 1996) and bagasse fly ash—a sugar industry waste material (Sharma and Park, 1999). New economical easily available and highly effective adsorbents are to be still needed. Generally, biosorptive processes could reduce capital costs by 20%, operational costs by 36% and total treatment costs by 28%, compared with the conventional systems (Loukidou et al., 2004).
The literature survey indicated that R. communis has not been used as adsorbent for the removal of chromium so far. The common name of R. communis carbon is castor and it belongs to the family ‘Euphorbiaceae’. It is commonly cultivated in dry lands along with groundnuts in India. The Pericarb is removed from the seed before going for the castor oil extraction. It is one of the large quantities of agricultural waste and it is used as a fuel to small extent. RCC-activated carbon as an adsorbent is a low cost material and was used for the removal of Cr(VI) in wastewater. In the present work, values of well-known thermodynamic functions, kinetics and isotherms studies have been performed to elucidate the equilibrium adsorption behaviour of Cr(VI) at different temperatures. Also the effect of agitation time, pH and temperature on the adsorption has been investigated.