Adsorption kinetics and thermodynamics of malachite green dye unto acid

A carbonaceous adsorbent prepared from an indigenous waste by acid treatment was tested for its efficiency in removing Malachite green (MG). The parameters studied include agitation time, initial dye concentration, carbon dose, pH and temperature. The adsorption followed first order reaction equation and the rate is mainly controlled by intra-particle diffusion. Freundlich and Langmuir isotherm models were applied to the equilibrium data. The adsorption capacity (Qm) obtained from the Langmuir isotherm plot were 9.7377, 9.6246, 9.6339, 9.5693 mg/g respectively at an initial pH of 6.0 at 30, 40, 50 and 60C. The temperature variation study showed that the Malachite green adsorption is endothermic and spontaneous with increased randomness at the solid solution interface. Significant effect on adsorption was observed on varying the pH of the Malachite green solutions. Almost 80% removal of Malachite green was observed at 60C. The type I and II isotherm obtained, positive H value, pH dependent results and desorption of dye in mineral acid suggest that the adsorption of Malachite green on PDC involves chemisorption as well as physisorption mechanism. @ JASEM The discharge of highly coloured effluents into natural water bodies is not only aesthetically displeasing, but it also impedes light penetration, thus upsetting biological processes within a stream. In addition, many dyes are toxic to some organisms causing direct destruction of aquatic communities. Some dyes can cause allergic dermatitis, skin irrigation, cancer and mutation in man. Recent estimates indicate that, approximately, 12% of synthetic textile dyes used each year are lost during manufacture and processing operation and 20% of these dyes enter the environment through effluents that result from the treatment of residual industrial waters (Selvarani., 2000) Wastewaters from dyeing industries are released in to nearby land or rivers without any treatment because the conventional treatment methods are not cost effective in the Indian context. Adsorption is one of the most effective methods and activated carbon is the preferred adsorbent widely employed to treat wastewater containing different classes of dyes, recognizing the economic drawback of commercial activated carbon. Many investigators have studied the feasibility of using inexpensive alternative materials like pearl millet husk, date pits, saw dust buffing dust of leather industry, coir pith, crude oil residue tropical grass, olive stone and almond shells, pine bark, wool waste, coconut shell etc., as carbonaceous precursors for the removal of dyes from water and wastewater (Singh and Tiwari.,2003: Sekaran et al.,1995) The present study was undertaken to evaluate the efficiency of a carbon adsorbent prepared from acid activated Pandanus carbon for removal of dye in aqueous solution. In order to design adsorption treatment systems, knowledge of kinetic and mass transfer processes is essential. In this paper, we have reported the applicability of kinetic and masstransfer models for the adsorption of Malachite green (MG) onto activated carbon. MATERIALS AND METHODS Materials Dried leaves of Pandanus were carbonized with concentrated sulphuric acid in the weight ratio of 1:1(W/V). The carbonization and activation was completed by heating for twelve hours in a furnace at 400°C. The resulting was carbon washed with distilled water until a constant pH of the slurry was achieved. Then the carbon was dried for four hours at 100°C in a hot air oven. The dried material was grinded to a fine powder and sieved All Chemicals used were of high purity, commercially available AnalaR grade. Stock solutions of 1000 mg / L of dyes were prepared using doubly distilled water. Methods Malachite green (MG) dye stock solution was made up to a known concentration. From that solution 50 ml of 5,10,15,20,25 and 30mg/L dye solutions were taken and to it was added 100mg of activated carbon adsorbent. These solutions were agitated at 30°, 40°, 50°, and 60°C in a defined time intervals, samples were withdrawn from the shaker and filtered. The solution was analyzed in an UVVisible spectrophotometer. Desorption studies were carried out using the spent carbon. Carbon loaded with dyes was separated and gently washed with distilled water to remove any unasdsorbed dyes. The dye-laden carbons were agitated with 50 ml each of water, 0.2 M sulphuric acid, hydrochloric acid, and nitric acid and sodium chloride solutions separately for 30 minutes and analyzed for any dye content using UV-Visible spectrophotometer. RESULTS AND DISCUSSIONS Effect of contact time and initial dye concentration The experimental results of adsorptions of Malachite green (MG) dye on the activated carbon Adsorption kinetics and thermodynamics of malachite green dye onto acid * Corresponding author: Arivoli S. 44 at various concentrations (5,10,15,20,25 and 30mg/L) with contact time are shown in Figure 1. The equilibrium data were collected in Table 1, reveals that, percent adsorption decreased with increase in initial dye concentration, but the actual amount of dye adsorbed per unit mass of carbon increased with increase in dye concentration. This means that the adsorption is highly dependent on initial concentration of the dye. It is because this that at lower concentration, the ratio of the initial number of dye molecules to the available surface area is low, subsequently the fractional adsorption becomes independent of initial concentration. However, at high concentration the available sites of adsorption becomes fewer and hence the percentage removal of dye is dependent upon initial concentration. Equilibrium was established at 40 minutes for all concentrations. Figure 1 reveals that the curves are single, smooth, and continuous, leading to saturation, suggesting the possible monolayer coverage of the dye on the carbon surface (Senthilkumar et al.,2005). Table 1: Equilibrium parameters for the adsorption of Dyes onto activated Carbon [D]0 Ce (mg / L) Qe (mg/g) Dye removed (%)