DETERMINATION OF EQUILIBRIUM, KINETIC AND THERMODYNAMIC PARAMETERS FOR THE ADSORPTION OF CADMIUM (II) ONTO CASTANEA SATIVA SHELL
Advancing the chemical engineering fundamentals
Transport Phenomena in Porous/Granular Media - III (T2-7c)
Keywords: cadmium adsorption, chestnut shell, kinetics, equilibrium, thermodynamic parameters
Castanea sativa shell is generated as a residue of a food factory during the peeling process in the “marrón glace” production. Apart from its use as fuel, the shell has little application, and although other similar lignocellulosic materials have received considerable attention as adsorbents of cations from wastewaters, there is no much information in the literature on the utilization of chestnut shell as adsorbent. Therefore, in this work, we proposed the study of the adsorption of cadmium ions from aqueous solutions onto the chestnut shell.
There are several parameters which determine sorption rate, like structural properties of the adsorbent, metal ion properties, initial concentration of metal ions, pH and temperature. In general, the characteristics of the adsorption behaviour are inferred in terms of both adsorption kinetics and equilibrium isotherms. They are also important tools to understand the adsorption mechanism for the theoretical evaluation and interpretation of thermodynamic parameters.
Two adsorption models have been applied to evaluate the experimental data: the Lagergren pseudo-first order kinetic model and the Ho’s second-order rate equation. The dynamic behaviour of the adsorption was investigated analysing the effect of concentration (15.3, 50.5 and 87.3 mg/L) at 25ºC and temperature (15, 25 and 35ºC) at an initial cadmium concentration of 20 mg/L. In addition, Langmuir and Freundlich isotherms were used to describe the adsorption equilibrium at the three temperatures essayed.
Chestnut shell was air-dried, ground and sieved to a particle size less than 2 mm, and then it was treated with formaldehyde in acid-medium. Adsorption experiments were conducted in a series of Erlenmeyer flasks covered with a Teflon sheet to prevent contamination. 100 mL of cadmium ion solution were transferred in the flasks together with 1g of pre-treated chestnut shell, and placed in a water bath shaker maintained at the desired temperature. When the adsorption process was completed, the solutions were filtered and analyzed for residual cadmium (II) ion concentrations.
For all cases essayed, the amount of cadmium adsorbed increased rapidly with time at the beginning and became slow towards the end of the process. The adsorption equilibrium was attained within four days. The efficiency of adsorption was increased with increasing contact time and initial cadmium concentration, and with decreasing temperature.
Experimental results have indicated that the pseudo-second order reaction model provided the best description of the data with correlation coefficients higher than 0.99 for all initial cadmium concentrations and temperatures. It was found that the values of the rate constant and the equilibrium sorption capacity obtained from model equations were affected by the two variables analysed. Thus, the rate constant decreased slightly from 5.32•10-3 to 4.53•10-3 g.min-1.mg-1 and the capacity increased from 1.3 to 4.4 mg/g with an increase in initial cadmium concentration from 15.3 to 87.3 mg/L. The data also showed that, when the temperature increased from 15 to 35ºC, the rate constant also increased (from 4.2•10-3 to 7.1•10-3 g•mg-1• min-1), whereas the equilibrium sorption capacity was little affected (from 1.4 to 1.2 mg/g). A linear relationship between the rate constant and the reciprocal absolute temperature was found and therefore, the variation of the pseudo-second order rate constants with temperature could be represented in an Arrhenius form. The rate constant of sorption, k0, took the value of 12.72 g•mg-1•min-1 and the activation energy for sorption, E of 19.2 kJ/mol, which is inside the range (8-22 kJ/mol) of diffusion-controlled processes. These results suggested that the rate-controlling step for the adsorption of cadmium on chestnut shell is likely not chemical in nature.
Freundlich equation represented a better fit to the experimental data than the Langmuir equation. This result predicts the heterogeneity of the adsorption sites on chestnut shell. The values of n, greater than unity, indicated that cadmium (II) ions are favourably adsorbed by Castanea sativa shell at all the temperatures studied.
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Presented Wednesday 19, 11:00 to 11:20, in session Transport Phenomena in Porous/Granular Media - III (T2-7c).
