Elsevier

Chemical Engineering Journal

Volume 145, Issue 2, 15 December 2008, Pages 196-202
Chemical Engineering Journal

Modelling single compound adsorption onto porous and non-porous sorbents using a deformed Weibull exponential isotherm

https://doi.org/10.1016/j.cej.2008.04.001Get rights and content

Abstract

A new isotherm equation, the Brouers–Sotolongo isotherm (BSI), a deformed (Weibull) exponential equation was used to describe adsorption equilibrium of phenol and methylene blue (MB), respectively onto a non-porous adsorbent, Posidonia oceanica fibres and two porous adsorbents, chemically and physically activated carbons (ACs) prepared from vetiver roots. The adsorption characteristic parameters obtained were also calculated using classical adsorption isotherm, i.e. the Langmuir isotherm (LI), Freundlich isotherm (FI) and Redlich–Peterson isotherm (RPI). Using only R2 to determine the best-fitting model was not sufficient, calculation of the error deviation using both the Marquardt's percent standard error (MPSE) and the average relative error (ARE) was also done. For the case of phenol, and among all studied models, the BSI involving adsorption on a heterogeneous surface is definitely the most suitable one to satisfactorily describe biosorption of phenol on posidonia, but as well adsorption of phenol on the ACs giving the highest squared correlation coefficients and the lowest MPSE. For posidonia and as well for the ACs, when MB is the adsorbate, the RPI is the best-fitting model.

Introduction

In the last decade, a large number of papers have been devoted to model adsorption process involved in the removal of pollutants from contaminated waters [1], [2], [3], [4]. It has been found that like activated carbons (ACs), in many instances, natural biological materials; as well biomass wastes can be used for water treatment purposes [5], [6], [7]. The isotherm, describing the retention of a substance on a solid at various concentrations at a constant temperature is a major tool to predict the efficiency of a sorbent to remove a given pollutant from polluted waters. The isotherm data are traditionally fitted with either the Langmuir isotherm (LI) or the Freundlich isotherm (FI) [8], [9]. The Redlich–Peterson isotherm (RPI) tending to the two previous isotherms at low and high value of solute concentration, respectively has been as well used with some success [10], [11]. These formulas are however essentially empirical, indeed, the physical situation is generally very far from the one postulated by Langmuir which derivates its equation supposing that the adsorption takes place on a homogeneous surface. Contrary to the proposed Langmuir picture, the sorption mechanism should be dominated by the most energetically active sites and it is the nature of the non-Gaussian energy distribution of these sites which determines the shape of the isotherm. Following the work of Brouers and Sotolongo on the statistical properties of physical complex systems, it has been recently demonstrated how the exponent of the FI could be related to an exponential distribution of sorption energies (energy landscape). A deformed exponential (Weibull) isotherm, namely the Brouers–Sotolongo isotherm (BSI) has been proposed to analyse sorption processes on highly heterogeneous systems [12]. In the present paper, the validity of the use of the BSI equation for describing isotherms of phenol and methylene blue (MB) adsorption, respectively, onto ACs prepared from vetiver (Vetivera zizanoides) roots and posidonia leaf sheath fibres (Posidonia oceanica) is investigated.

Section snippets

Biomass preparation

P. oceanica leaf sheaths (basal parts of leaves of the Mediterranean seagrass) were collected from Chott–Meriem bay (Sousse, Tunisia). The fibres are manually separated, washed thoroughly with distilled water to remove salt and then dried in an oven at 40 °C for 48 h to a constant weight, without altering the polymeric composition of the fibres. After drying, the biomass was blended in a waring blendor to get homogenous particle size (2 mm sieve) and stored in dessicator.

Activated carbons preparation

The ACs were obtained from

Results and discussion

The equilibrium data for MB and phenol adsorption onto fibrous ACs from vetiver roots and posidonia fibres were analysed by non-linear curve fitting analysis using Langmuir, Freundlich, Redlich–Peterson and the newly established model: the BSI.

For all studied models, the accuracy of the fit with experimental data was determined based on R2, MPSE and ARE. Modelling results were considered suitable to satisfactorily describe the biosorption process if the squared correlation coefficient was

Conclusion

The new BSI equation, as well, the LI, FI and RPI were used to described adsorption equilibrium of phenol and MB, respectively onto a non-porous adsorbent, P. oceanica fibres and two porous adsorbents, ACs prepared from vetiver roots. Using only R2 to determine the best-fitting model was not sufficient, calculation of the error deviation using both the MPSE and the ARE allow us to determine the best-fitting model. Indeed, according to the Freundlich related results, the use of this model

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