International Journal of Environmental Science and Development, Vol. 2, No. 5, October 2011 Thermodynamics of the Adsorption of Cd(II) from Aqueous Solution on NCRH Upendra Kumar, Senior Member, IACSIT was found to be a better indicator of understanding the Abstract— Batch kinetic studies were conducted for the process mechanism adsorption of Cd(II) on NCRH. At the adsorbent dose of 10 g/l and initial Cd(II) concentration of 10 mg/l, 93 to 97 % removal were observed when the temperature increases from 15 o to 40o. II. MATERIALS AND METHODS Equilibrium time was found to be 60 min. This paper attempts to develop simple and easily understandable thermodynamic A. Preparation of NCRH parameters related sorption process at the equilibrium. The overall scenario represented by thermodynamic parameters Fresh rice husk was obtained from a local rice mill and was found to be a better indicator of understanding the process was passed through different sieve size. The fraction of mechanism. Sorption of Cd(II) on NCRH followed pseudo particle between 425 and 600µm (geometric mean size: 505 second-order kinetic equation. The standard Gibbs free energy µm) was selected [3]. Rice husk was washed thoroughly ΔGo value for the adsorption processes of Cd(II) was obtained ◦ o with distilled water and was dried at 60 C. This was treated as – 7.30 kJ/mole. The negative value, ΔG confirms the with 0.1 mol/l sodium carbonate solution at room feasibility of the adsorption process and spontaneous nature of o o temperature for 4 h. Excess of sodium carbonate was adsorption. The values of ΔH and ΔS for Cd(II) were ◦ obtained as 26.4, kJ/mol and 113.2 J/mol respectively. The removed with water and the material was dried at 40 C. positive value of ΔHo indicates endothermic nature of Sodium carbonate treated rice husk was designated as adsorption, while positive ΔSo value confirms the increased NCRH [3]. randomness at the solid-liquid interface during adsorption. The activation energy for the sorption of Cd(II), was found as B. Batch Kinetic Studies 11.37 kJ/mol indicating chemisorption. All chemicals used were of analytical grade (E. Merck, India). Stock solutions of 1000 mg/l was prepared for Cd(II) Index Terms— Activation energy, Gibbs free energy, NCRH, using cadmium nitrate. In order to avoid hydrolysis as well Cadmium, Thermodynamics, Adsorption. as high adsorption of species in the flask wall the stock solution was prepared with HNO3 2% (v/v), which were I. INTRODUCTION diluted with distilled water to prepare working solutions. pH of the working solutions were adjusted to 6.8±0.2 for all Cadmium is a metal widely used in industries such as studies using dilute NaOH solution. The experiments were cadmium plating, alkaline batteries, copper alloys, paints carried out with 10 g/l of sorbent dose for initial metal and plastics. It is a non-essential and non-beneficial element concentration of 10 mg/l. The mixture was agitated using a to plants and animals. Its toxic effects are well documented. thermostatic rotary shaker at 180 rpm in the temperature Diseases such as renal damage, anemia, hypertension and o o range of 15 – 40 C. At the end of the experiment the itai–itai are associated with excess cadmium [1], [2]. Hence sample was allowed to settle for about 1 min and the it is important to eliminate trace of cadmium from drinking supernatant sample was analyzed for its Cd(II) concentration water, or to remove cadmium from wastewaters before they using atomic absorption spectrophotometer (AA-6650, are discharged into receiving bodies. In our continued study Shimadzu, Japan ). on the use of low-cost material for the removal of organic and organic pollutants from water and wastewater we investigated rice husk as a sorbent for the removal of Cd(II). III. RESULT AND DISCUSSION Some simple and low-cost chemical modifications resulted in increasing the sorption capacity of raw rice husk. The A. Effect of Temperature highly efficient low cost and the rapid uptake of Cd(II) by The removal kinetics of Cd(II) by NCRH were obtained NCRH indicated that it could be an excellent alternative for at 15o, 25o and 40o C (Fig. 1). At an equilibrium time of 60 the removal of heavy metal by sorption process [3]. This min for initial Cd(II) concentration of 10 mg/l, the paper attempts to develop simple and easily understandable percentage removal increases form 93 to 97 % with the thermodynamic parameters related sorption process at the increase in temperature from 15o to 40o C (Fig. 2).. The equilibrium, such as as entropy change, enthalpy change, sorption capacity increases with increase in temperature Gibbs free energy and activation energy of sorption. The indicating that the sorption process was endothermic and the overall scenario represented by thermodynamic parameters sorption of metal ions by NCRH may involve not only physical but also chemical sorption [4]. The increase in sorption capacity of NCRH at high temperature may be Manuscript received May 4, 2011, revised October 7, 2011. attributed to enlargement of pore size or increase in the Upendra Kumar is with the Department of Civil Engineering, National Institute of Technology, Silchar, Assam, India (e-mail: active surface for sorption. This could also be due to the [email protected]). enhanced mobility of the metal ions from the bulk solution 334 International Journal of Environmental Science and Development, Vol. 2, No. 5, October 2011 towards the adsorbent surface and extent of penetration 90 within NCRH structure overcoming the rate of intraparticle 288 K 298 K 313 K 80 diffusion [4], [5]. R2 = 0.9999 R2 = 0.9999 R2 = 1 70 10 60 9 50 8 15 40 7 25 30 6 40 5 20 4 10 Cd(II), mg/l 3 0 2 0 10203040506070 1 Time, min 0 Fig. 3. Pseudo second order model plot for Cd(II) at different temperature 0 10203040506070 in K. Time, min Fig. 1. Kinetic profile of Cd(II) uptake at 15, 25 and 40 o C. C. Thermodynamic Parameters of Adsorption The adsorption process of metal ions can be summarized 100 by the following reversible process, which represents a heterogeneous equilibrium. 95 Metal ions in solution ↔ Metal ions adsorbent 90 The apparent equilibrium constant (Kc) of the adsorption 85 is defined as: 80 C A (2) 15 K = c C 75 e 25 o Removal efficiency Removal −=Δ ln KRTG c (3) 70 40 ΔS o ΔH o 65 ln K = − (4) c R RT 60 0 10203040506070 where CA is the solid phase concentration at equilibrium Time, min. (mg/l), Ce is the equilibrium concentration (mg/l), T (K) the Fig. 2 . The rate of removal efficiency of Cd(II) at 15, 25 and 40 o C. absolute temperature, R the gas constant (8.314 J/mol K), ΔSo the entropy change (J/mol) and ΔHo the enthalpy change o o B. Sorption Kinetic (kJ/mol). ΔH and ΔS values were obtained from the slope and intercepts of van’t Hoff plot, ln Kc versus 1/T (Fig. 4). Pseudo second-order kinetic model has been widely used The Kc value thus obtained is also used to determine the for the sorption kinetics [6]-[8]. It was observed that most of Gibbs free energy ΔGo at 25 ºC. The Gibbs free energy the sorption systems followed a pseudo second-order kinetic indicates the degree of spontaneity of the adsorption process model as also reported by Ho and McKay [6], which can be and the higher negative value reflects a more energetically expressed as favorable adsorption [10]-[13]. The standard ΔGo values for t 1 t (1) += 4.5 hq 0 qe The initial sorption rate can be obtained as t approaches 4.0 R?= 0.9998 zero (t → 0): 3.5 2 c ho = Kp2 qe K ln3.0 where ho is the initial sorption rate (mg/g min), t is the 2.5 contact time (min), q and qe are the quantities of sorbate (in mg/g), sorbed at time t and at equilibrium respectively and 2.0 K is the rate constant (g/mg min). The rate constant K p2 p2 1.5 was found out from the slope of the linear test plot between 3.15 3.20 3.25 3.30 3.35 3.40 3.45 3.50 t/q versus time t as shown in Fig. 3. The values of Kp2 for 1000/T, 1/K 15o, 25o and 40o C was obtained as 0.697, 0.763 and 1.011 g/mg min respectively. Fig. 4. van’t Hoff plots for Cd(II 335 International Journal of Environmental Science and Development, Vol. 2, No. 5, October 2011 the adsorption processes of Cd(II), was obtained as – 7.30, process was endothermic and it was further conformed by kJ/mole respectively using equation (3). The negative value the positive value of ΔHo (26.4 kJ/mol) and further positive ΔGo confirms the feasibility of the adsorption process and value of ΔSo (113.2 J/mol) confirms the increased spontaneous nature of adsorption [10]-[13]. The values of randomness at the solid-liquid interface during adsorption. ΔHo and ΔSo for Cd(II), was obtained as 26.4, kJ/mol and The standard Gibbs free energy ΔGo value for the adsorption 113.2 J/mol respectively. The positive value of ΔHo processes of Cd(II) was obtained as – 7.30 kJ/mole. The indicates endothermic nature of adsorption while positive negative value, ΔGo confirms the feasibility of the ΔSo value confirms the increased randomness at the solid- adsorption process and spontaneous nature of adsorption.