MODELING OF ADSORPTION ISOTHERMS AND KINETICS OF 2,4,6- TRICHLOROPHENOL ONTO MICROPOROUS ZnCl2 ACTIVATED COIR PITH CARBON

Activated carbon prepared from coir pith using ZnCl2 was investigated to find the feasibility of its application for removal of 2,4,6-trichlorophenol (TCP) in aqueous solution through adsorption process. Batch mode kinetics and isotherm studies were carried out to evaluate the effects of contact time, initial concentration, adsorbent dose, pH, and temperature. Lagergren, second order, and Banghams models were used to fit the experimental data. Langmuir, Freundlich, D-R and Temkin isotherm models have been employed to analyse the adsorption equilibrium data. The Langmuir adsorption capacity was found to be 172 mg g of the adsorbent. pH effect and desorption studies showed that ion exchange mechanism was involved in the adsorption process. Removal of TCP from synthetic wastewater was also tested. *Corresponding author Email: [email protected] INTRODUCTION Coir pith, an agricultural solid waste indigenously available in coir fibre industries, is a light fluffy material, which is generated in the separation process of the fibre from coconut husk. Annual production of coir pith is around 7.5 million tons in India [1]. Raw coir pith consists of 27.1% of cellulose, 28.3% of lignin and 8.1% of soluble tannin like phenolic compounds [2]. Coir pith carbon showed good adsorption capacity towards anions, heavy metals and dyes when the surface was modified with activating agents (such as ZnCl2) due to high microporosity [3]. Utilization of coir pith for the treatment of ground water would be helpful not only to the economy of the treatments but also for solving the solid waste disposal problems of the coir industries. Since raw coir pith did not show good adsorption capacity for organics, the surface of coir pith was activated with ZnCl2 in order to enhance its capacity towards organics. Chlorophenols are persistent and toxic organic compounds released into water as waste from plastic, pesticide and other organochemical industries and research centers [4]. 2,4,6-trichlorophenol (TCP) is used as a wood and glue preservative, as an antimildew agent for textiles [5], in pesticide formulations (herbicides, fungicides, bactericides) and in leather tanning and fishing industries. It has been detected in the emission from fossil fuel combustion, municipal waste incineration, and chlorination of water containing phenols or certain aromatic acids with hypochlorite or during the disinfection of drinking water sources [6], river water, landfill leachate, chemical plant and sewage treatment plant effluents and in ambient air [7]. TCP causes respiratory effects such as cough, chronic bronchitis, chest wheezing, altered pulmonary function and pulmonary lesions [8]. It displays a pronounced undesirable effect in portable water at concentrations as low as 0.1 mg L [9]. Hence, the removal of these organic pollutants from water stream is critical to ensure the safety of water supplies. A considerable effort has been dedicated in the past concerning the removal of these compounds from wastewaters. Several methods have been proposed and developed (physiochemical, chemical oxidation and biological degradation) and the most extensively used method is adsorption process. Adsorption of organic solutes from aqueous phase is a very important application of activated carbon, which covers a wide spectrum of applications from drinking water to wastewa276 J. Environ. Eng. Manage., 18(4), 275-280 (2008) ter treatments in the food, beverage, pharmaceutical and chemical industries [10]. Recently, many researchers have tried to develop activated carbons for the removal of various pollutants using renewable and cheaper precursors which were mainly industrial and agricultural byproducts such as coconut shell [11], activated clay [12], coconut husk [13], and peat and soil [14]. Increased attention has been paid for carbonized coir pith, an agricultural solid waste for adsorption of pollutants from water [3]. The focus of this research was to explore the feasibility of ZnCl2 activated coir pith carbon (ZnCPC) for the adsorption of TCP. Adsorption kinetics, isotherms and effects of pH and adsorbent dose were studied. MATERIALS AND METHODS Preparation and characterization of ZnCPC has already been reported [3]. Experimental solutions of TCP were prepared using analytical reagent grade. Synthetic waste waters containing TCP of 60 and 140 mg L were prepared using analytical reagent grade salts, NaCl and Na2SO4 and their composition: chloride (49.6 mg L); sulfate (58 mg L) and sodium (59.6 mg L) [15]. TCP, NaCl and Na2SO4 were obtained from Loba chemie, Mumbai. Batch mode adsorption studies were carried out by agitating 50 mg of ZnCPC with 50 mL of TCP solution of desired concentration on a thermostated rotary shaker (ORBITEK, Chennai, India) at 200 rpm, 35 °C and at an initial pH 2.0. At the end of adsorption, the solution and adsorbent were separated by centrifugation at 10,000 rpm for 20 min and the concentration of residual TCP was determined spectrophotometrically using UV-visible spectrophotometer at 290 nm (Specord 200, Analytic Jena, Germany). Effect of contact time was studied by withdrawing samples at predetermined time intervals and then residual concentration was analyzed as before. Effect of pH was studied in the pH range 2.0 to 11.0 by adjusting the pH using 0.1 M HCl and 0.1 M NaOH solutions by means of a pH meter (Elico, Mode LI-107, Hyderabad, India). Effect of adsorbent dose was studied with different adsorbent doses (10-250 mg) for TCP solutions (60-140 mg L). Effect of temperature on adsorption of TCP was studied using 60-140 mg L concentration and 50 mg of the adsorbent at 35, 40, 50, 60 C in a thermostated rotary shaker. The adsorbent (50 mg per 50 mL) that was used for the adsorption of 60-140 mg L of TCP solution was separated from the solution by suctionfiltration using Whatman filter paper and washed gently with water to remove any unadsorbed TCP. Then the spent adsorbent was mixed with 50 mL of distilled water whose pH was adjusted to various values (2.011.0) and agitated at time intervals longer than the equilibrium time. Then the desorbed TCP was estimated as before. RESULTS AND DISCUSSION The physio-chemical characteristics of ZnCPC in comparison with coir pith carbon in the absence of ZnCl2 activation has been already reported [16]. 1. Effect of Reaction Time and Kinetics The amount of TCP adsorbed (mg g) increased with increase in agitation time and reached equilibrium. The equilibrium time was found to be 30, 40, 70, 80 and 100 min for 60, 80, 100, 120, 140 mg L, respectively. The amount of TCP removed at equilibrium increased from 58 to 127 mg g with increase in TCP concentration from 60 to 140 mg L. It shows that the adsorption at different concentrations was rapid in the initial stages and gradually decreased with the progress of adsorption until the equilibrium was reached. 2. Adsorption Kinetics Adsorption kinetic models correlate the adsorbate uptake rate with bulk concentration of the adsorbate. Kinetic data were fit into Lagergren [17], second order [18] and Banghams pore diffusion [19] models. The first order kinetic model is represented as: log (qe-q) = log qe – 2.30 t k1 (1) where qe and q are the amounts of TCP adsorbed (mg g) at equilibrium and at time t, respectively, and k1 is the Lagergren rate constant of first order adsorption (min). The second order kinetic model is expressed as: