Adsorption of malachite green on groundnut shell waste based powdered activated carbon
Introduction
Activated carbons are extensively used as efficient and versatile adsorbents for purification of water, air and many chemical and natural products. Water treatment is, by far, the largest outlet for activated carbon, and powdered activated carbon (PAC) is more commonly used than granular activated carbon (GAC) to control taste, colour and odor in drinking water treatment.
Colour is the first contaminant to be recognized in water and has to be removed from wastewater before discharging it into water bodies. Residual dyes are the major contributors to colour in wastewaters generated from textile and dye manufacturing industries, etc. (Ramakrishna and Viraraghvan, 1997). Colour impedes light penetration, retards photosynthetic activity, inhibits the growth of biota and also has a tendency to chelate metal ions which result in micro-toxicity to fish and other organisms (Garg et al., 2004, Mckay et al., 1980). It should be noted that the contamination of drinking water by dyes at even a concentration of 1.0 mg l−1 could impart significant colour, making it unfit for human consumption.
Most of the used dyes are stable to photodegradation, bio-degradation and oxidizing agents (Garg et al., 2004, Ramakrishna and Viraraghvan, 1997). Currently, several physical or chemical processes are used to treat dye-laden wastewaters. However, these processes are costly and cannot effectively be used to treat the wide range of dye wastewater. The advantages and disadvantages of some methods of dye removal from wastewaters are given in Table 1 (Robinson et al., 2001). The adsorption process is one of the efficient methods to remove dyes from effluent (Nigam et al., 1996, Malik, 2003). The adsorption process has an advantage over the other methods due to the excellent adsorption efficiency of activated carbon (powdered or granular) for organic compounds even from dilute solutions, but commercially available activated carbons are very expensive.
Various carbonaceous materials, such as coal, lignite, coconut shells, wood and peat are used in the production of commercial activated carbons (Bansode et al., 2003, Hayashi et al., 2000). However, the abundance and availability of agricultural by-products make them good sources of raw materials for activated carbons. Agricultural by-products (Ankur et al., 2001) are renewable sources of raw materials for activated carbon production because the development of methods to reuse waste materials is greatly desired. Residues from agriculture and agro-industries are the non-product outputs from the growing and processing of raw agricultural products such as rice, corn, beans and peanuts (Tsai et al., 2001). Disposal of agricultural by-products is currently a major economic and ecological issue, and the conversion of by-products to adsorbents, such as activated carbon, represents a possible outlet. A number of agricultural waste materials are being studied for the removal of different dyes from aqueous solutions at different operating conditions (Table 2).
India is the second largest producer of groundnuts in the world; the country has 8 million ha of cultivation producing 8,004,000 metric tons/yr. Nutshell is a carbonaceous, fibrous solid waste, which creates a disposal problem and is generally used for its fuel value. Therefore, it was of interest to prepare a higher value product, such as activated carbon, from groundnut shell. These by-products (Toles et al., 1998) offer the advantage of having a greater percentage of non-carbon constituents in their composition compared to coal or peat and therefore afford a greater chance of retaining functional groups, especially oxygenated groups, in the carbonized product. However, the lower carbon content of by-products translates to lower activated carbon yields, but the low cost of waste cancel out the lower yields. So far, there have been few published data on carbonization of groundnut shell by chemical activation and its application in wastewater treatment for colour removal (Kannan and Sundaram, 2001).
The high adsorptive capacities of activated carbons are related to properties such as surface area, porosity, and surface functional groups. These unique characteristics are dependant on the type of raw material employed and method of activation. Basically, there are two different processes for the preparation of activated carbon: physical and chemical activation (Ahmadpour and Do, 1996). Physical activation involves carbonization of the carbonaceous precursor followed by activation of the resulting char in the presence of activating agents such as carbon dioxide or steam. Chemical activation, on the other hand, involves the carbonization of the precursor in the presence of chemical agents. In physical activation, the elimination of a large amount of internal carbon mass is necessary to obtain a well developed porous structure, whereas in chemical activation process, chemical agents used are dehydrating agents that influence pyrolytic decomposition and inhibit the formation of tar, thus enhancing yield of carbon (Rodriguez-Reinoso and Molina-Sabio, 1992). Chemical activation has more advantages (Lillo-Rodensas et al., 2003) over physical activation with respect to higher yield, more surface area and better development of porous structure in carbon. It also helps to develop oxygenated surface complexes on the surface of activated carbon.
Consequently, the aim of this work was to study the feasibility of developing an efficient adsorbent from groundnut shell by ZnCl2 activation and to investigate its adsorption capacity by removal of cationic dye from aqueous solution. The influence of activation variables on the adsorption characteristics in terms of iodine adsorption capacity and on % yield has been investigated. To evaluate the suitability of the product for its use in water and wastewater treatment systems, its characterization has been done comparatively with commercial powdered activated carbon for physical, chemical and adsorption properties because these preliminary studies provide good information about the applicability of the product in a treatment system. Malachite green was selected for the adsorption experiment due to its presence in the wastewaters of several industries (such as textile, leather, and jute); influence of dye concentration, adsorbent dose and contact time were investigated. The Langmuir, Freundlich and BET isotherm models were tested for their applicability for both carbons, and results will be useful for further application of groundnut shell based powdered activated carbon in colour removal from wastewater.
Section snippets
Raw material
Agricultural solid waste, i.e., groundnut shell, was used as a precursor for the preparation of powdered activated carbon. Nut shell was properly washed to remove any mud, debris, etc., dried and then ground to a particle size of 2 mm for the further process of carbonization.
Preparation of adsorbent
The waste material was carbonized in a conventional electric furnace by a two stage carbonization process under optimized conditions. The general schematic diagram for preparation of activated carbon has been given in Fig. 1
Effect of operating variables
Operating conditions for lower and higher carbonization stages were obtained by optimizing various operating variables in terms of iodine adsorption capacity mainly (Table 4). During low carbonization, maximum iodine adsorption was found for char prepared at 450 °C but % yield was relatively low, so 400 °C was considered the optimum temperature for charring as it showed comparative iodine adsorption which ensures effective removal of volatile matter present in biomass. During higher
Conclusions
The results of the present investigation show that groundnut shell based powdered activated carbon is an effective adsorbent for the removal of malachite green dye from aqueous solutions, and its adsorption capacity is quite comparable to the commercial powdered activated carbon. Groundnut shell was selected for studying adsorption due to its availability in India, as well as to assess the possibility of utilizing an agricultural waste for dye removal. The results show that initial dye
Acknowledgements
The authors are thankful to Director, NEERI, Nagpur (India) for providing facilities to carry out this research work and kind permission for this publication.
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