External mass transfer and hindered diffusion of organic compounds in the adsorption on activated carbon cloth

Abstract

The overall rate of adsorption of organic compounds on activated carbon cloth (ACC) was interpreted with a diffusional model which takes into account external mass transfer and intrafiber diffusion. It was assumed that the intrafiber diffusion was solely due to pore volume diffusion (Fick diffusion). The effect of molecular size of the organic compounds on the overall rate of adsorption was studied using pyridine (0.49 nm), phenol (0.53 nm), naphthalenesulfonic acid (NAS) (0.85 nm), pentachlorophenol (PCP) (0.87 nm) and methylene blue (MB) (1.03 nm). The experimental concentration decay data were obtained in a differential column batch adsorber. The overall adsorption rate of pyridine and phenol was controlled by the external mass transfer whereas the intrafiber diffusion was the controlling mechanism in the adsorption rate of NSA, PCP and MB. Additionally, the intrafiber diffusion of NSA, PCP and MB was substantially reduced by hindered diffusion, which was due to steric exclusion, wall drag effects and obstruction caused by the molecules adsorbed on the pore walls. The controlling mechanism in the adsorption rate on ACC is a function of the molecular size of the organic compound.

Introduction

Activated carbon (AC) is the most commonly employed adsorbent for industrial applications, especially for eliminating organic compounds from aqueous solution due to its physicochemical and textural properties. The activated carbon is mainly commercialized in the granular (GAC) and powdered (PAC) forms. Recently, a new form of AC has been developed by carbonization and activation of fibers made up of pitch tar and of diverse polymeric materials such as Nylon, Rayon, cellulose, phenolic resins and polyacrylonitrile. This new form of AC is known as activated carbon fiber (ACF), and it can be manufactured in the forms of cloth or felt. The properties of the ACF are highly dependent on the precursor material and preparation method.

The porous structures of GAC and PAC are very complex and are formed by macropores (pore diameter > 50 nm), mesopores (2 nm ≥ pore diameter ≥ 50 nm) and micropores (pore diameter < 2 nm), whereas the pores of ACF are predominantly micropores. The diameter of the fibrils or threads of the ACFs ranges from 0.005 to 0.017 mm, and this diameter is near 100 times smaller than the particle diameters of GAC (1–3 mm). The adsorption rate in ACF is much faster than that in GAC inasmuch as the intrafiber distance is 100 times shorter than the intraparticle distance. This is one of the main advantages of using the ACFs. The adsorption rate in PAC is also faster than that in GAC since the particle diameter varies between 0.015 and 0.025 mm. However, the use of PAC is limited due to handling problems. Besides, a fixed bed adsorber cannot be packed with PAC due to very high pressure drops.

The adsorption equilibrium of phenol, 1-naphthalenesulfonic acid (NSA), pentachlorophenol (PCP) and methylene blue (MB) on ACFs have been previously studied in various works. At pH 7 and T = 30 °C, the adsorption equilibrium data of phenol on an ACF manufactured from polyacrylonitrile (PAN) was interpreted with two-parameter isotherms (Freundlich and Langmuir) and three-parameter isotherms (Redlich–Peterson, Toth and Jossens–Prausnitz–Fritz–Schlunder–Myers, JPFSM) [1]. The regression analysis showed that the JPFSM isotherm best fitted the adsorption data because it presented the lowest average percentage deviation. Diaz-Flores et al. [2] investigated the adsorption of PCP on PAN-based ACFs in the forms of cloth and felt and found that adsorption capacity of the cloth was 1.7 times greater than that of the felt. Furthermore, the adsorption capacity of both types of ACFs diminished by increasing the pH from 6 to 12, and the adsorption capacity of the felt increased by raising the temperature from 15 to 35 °C. Ayranci and Duman [3] investigated the adsorption of NSA on an ACC and found that the adsorption capacity diminished by reducing the pH from 5.8 to 1.9. The adsorption of NSA was attributed to π–π interactions between the aromatic rings of the NSA and the aromatic rings of graphene layer. The adsorption isotherms of Freundlich and Langmuir were fitted to the adsorption equilibrium data of MB on an ACF at pH 7 and T = 25 °C and the Freundlich isotherm better fitted the experimental data [4].

In a previous work, Leyva-Ramos et al. [5] studied the adsorption rate of PCP in an ACF in the form of felt. The overall adsorption rate of PCP was interpreted with a diffusional model considering that the overall adsorption rate was affected by the external mass transport and the Fick diffusion inside the pores of the fibrils of the ACF (Intrafiber diffusion). The results revealed that the overall rate of adsorption of PCP in an ACF was controlled by the intrafiber diffusion, but was not affected by the external mass transfer. However, Leyva-Ramos et al. [5] pointed out that the rate of adsorption in the ACF was not as fast as it was expected because restricted diffusion diminished considerably the intrafiber diffusion of PCP in the ACF.

The restricted diffusion has been found in the diffusion of organics in the pores of AC when the molecular size of the organic compound is close to the mean pore diameter of AC [6]. Restricted diffusion is due to the steric exclusion interactions, and frictional drag between the molecule diffusing in the pore volume and the pore walls. Restricted diffusion is likely to happen in the adsorption of organic compounds on ACCs because the ACCs are mainly composed of micropores [5].

This work is aimed at studying the overall rate of adsorption of pyridine, phenol, PCP, NSA and MB in an ACC. Besides, the effect of restricted diffusion on the intrafiber diffusion was examined in detail.