(529e) Adsorption Equilibrium and Kinetics of Cadmium (II) from Aqueous Solution Onto Activated Carbon Fiber. Hindered Diffusion

The contamination of water by heavy metals represents a hazardous problem for human health and the environment. The ingestion of water contaminated with metals can cause enzymatic alterations and affect the renal, respiratory and digestive systems, among others. One of the most efficient processes for the removal of these pollutants from aqueous solution is adsorption, which is a surface phenomenon that involves the accumulation of substances on a surface or interphase. The primary objective of this work is to study the adsorption equilibrium and kinetics of Cd(II) from aqueous solution on activated carbon fiber (ACF), as well as to model the adsorption rate data using a diffusional model.

The experimental adsorption equilibrium data were obtained in a batch adsorber consisting of 50 mL plastic vial, and the effect of pH, temperature and ionic strength of the solution on the adsorption capacity was studied thoroughly. The adsorption rate data was procured in a differential column batch adsorber composed of a polyethylene column packed with ACF, a feed vessel and a peristaltic pump.

The textural properties of ACF were the BET surface area of 1039 m²/g, the average pore diameter of 1.1 nm, the total pore volume of 0.57 cm³/g and the volume of the micropores of 0.50 cm³/g, the latter was calculated by the Dubinin-Radushkevich’s method. Thus, the ACF is a very microporous material since the micropore volume represents 88 % of the total pore volume.

The experimental adsorption equilibrium data were satisfactorily interpreted with the Prausnitz-Radke isotherm. It was found that the adsorption capacity of FCA was enhanced by raising the solution pH from 5 to 7. This behavior was attributed to the electrostatic interactions between the Cd²⁺ and the negatively charged surface of the ACF. The adsorption capacity of the ACF decreased markedly with increasing ionic strength of the solution, corroborating the importance of electrostatic interactions in the adsorption of Cd(II) on ACF. Adsorption equilibrium experiments at different temperatures revealed that the adsorption capacity of FCA toward Cd(II) does not vary by increasing the temperature when the concentration of Cd(II) is high. The electrostatic interactions and ion exchange play an essential role in the adsorption mechanism.

A diffusional model was proposed to interpret the adsorption rate of Cd(II) on ACF from a water solution. The system was comprised of two phases: a liquid solution of Cd(II) and a solid adsorbent. In deriving this model, mass balances of Cd(II) were performed in each phase, and the following suppositions were assumed: (i) the fibrils of ACF were cylindrical, ii) the external mass transfer was represented by a mass transfer coefficient, iii) the intraparticle mass transfer was only due to pore volume diffusion, and (iv) the adsorption rate at an active site is instantaneous.

The kinetic study revealed that the adsorption equilibrium of Cd(II) on FCA was reached in 45 min. The experimental data of the concentration decay curves were interpreted reasonably well by a numerical solution of the diffusional model, and the overall rate of adsorption was shown to be controlled by intraparticular diffusion. Additionally, it was found that the intraparticular diffusion of Cd(II) in the FCA is drastically affected by restricted diffusion due to exclusion, friction in the walls of the micropores and obstruction caused by the Cd (II) cations adsorbed on the surface of the pores.