Removal of phenol from aqueous solutions by adsorption onto organomodified Tirebolu bentonite: Equilibrium, kinetic and thermodynamic study

Abstract

A natural bentonite modified with a cationic surfactant, cetyl trimethylammonium bromide (CTAB), was used as an adsorbent for removal of phenol from aqueous solutions. The natural and modified bentonites (organobentonite) were characterized with some instrumental techniques (FTIR, XRD and SEM). Adsorption studies were performed in a batch system, and the effects of various experimental parameters such as solution pH, contact time, initial phenol concentration, organobentonite concentration, and temperature, etc. were evaluated upon the phenol adsorption onto organobentonite. Maximum phenol removal was observed at pH 9.0. Equilibrium was attained after contact of 1 h only. The adsorption isotherms were described by Langmuir and Freundlich isotherm models, and both model fitted well. The monolayer adsorption capacity of organobentonite was found to be 333 mg g⁻¹. Desorption of phenol from the loaded adsorbent was achieved by using 20% acetone solution. The kinetic studies indicated that the adsorption process was best described by the pseudo-second-order kinetics (R² > 0.99). Thermodynamic parameters including the Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) were also calculated. These parameters indicated that adsorption of phenol onto organobentonite was feasible, spontaneous and exothermic in the temperature range of 0–40 °C.

Introduction

As a result of rapid development of chemical and petrochemical industries, the surface and ground waters are polluted by various organic and inorganic chemicals such as phenolic compounds, dyes and heavy metals. Phenol and its derivatives are considered as noxious pollutants, because they are toxic and harmful to living organisms even at low concentrations [1]. Phenols are being discharged into the waters from various industrial processes such as oil refineries, petrochemical plants, ceramic plants, coal conversion processes and phenolic resin industries [2]. The utilization of phenol-contaminated waters causes protein degeneration, tissue erosion, paralysis of the central nervous system and also damages the kidney, liver and pancreas in human bodies [3]. According to the recommendation of World Health Organization (WHO), the permissible concentration of phenolic contents in potable waters is 1 μg L⁻¹ [4] and the regulations by the Environmental Protection Agency (EPA), call for lowering phenol content in wastewaters less than 1 mg L⁻¹ [5]. Therefore, removal of phenols from waters and wastewaters is an important issue in order to protect public health and environment.

The traditional methods such as adsorption, chemical oxidation, precipitation, distillation, solvent extraction, ion exchange, membrane processes, and reverse osmosis, etc. have been widely used for removal of phenols from aqueous solutions [6]. Among them, removal of phenols by adsorption is the most powerful separation and purification method because this technique has significant advantages including high efficiency, easy handling, high selectivity, lower operating cost, easy regeneration of adsorbent, and minimized the production of chemical or biological sludge [7]. Adsorption process is strongly affected by the chemistry and surface morphology of the adsorbent. Therefore, new adsorbents, which are economical, easily available, having strong affinity and high loading capacity have been required. A number of adsorbents such as activated carbon, [8], red mud [9] and rubber seed coat [10], etc. have been used for phenol removal. Adsorption of phenol onto activated carbons is a well-known process because activated carbon has a large surface area and high adsorption capacity. However, its high cost and the difficulties in recovering of activated carbon particles from treated water, limit its use as an adsorbent. In recent years, clay minerals have been widely used as adsorbents for the removal of toxic metals and organic pollutants from aqueous solutions due to their low cost, large specific surface area, chemical and mechanical stability, layered structure and high cation exchange capacity [2], [11], [12], [13], [14], [15], [16], [17], [18], [19].

Bentonite is a member of 2:1 clay minerals (meaning that it has two tetrahedral sheets sandwiching a central octahedral sheet) which consists essentially of clay minerals of montmorillonite group. Bentonite is characterized by an Al octahedral sheet between two Si tetrahedral sheets. It has a negative surface charge created by the isomorphous substitution of Al³⁺ for Si⁴⁺ in tetrahedral layer and Mg²⁺ for Al³⁺ in octahedral layer. The bentonite surface is hydrophilic in nature because inorganic cations, such as Na⁺ and Ca²⁺, are strongly hydrated in presence of water. As a result, the adsorption efficiency of natural bentonite for organic molecules is very low [20], [21]. The adsorption properties of bentonite can be improved by the modification of clay mineral surface with a cationic surfactant. The cationic surfactants, known as quaternary amine salts, are in the form of (CH₃)₃NR⁺, where R is an alkyl hydrocarbon chain. Replacement of inorganic exchangeable cations with cationic surfactants, converts the hydrophilic silicate surface of clay minerals to a hydrophobic surface and the obtained complex is referred as organoclay. It is generally accepted that adsorption of hydrophilic long-chain quaternary ammonium cations onto clays occurs according to the ion-exchange mechanism [22]. As a result organoclay complex is an excellent adsorbent for the removal of phenolic compounds, other organic contaminants and also heavy metals from aqueous solutions.

The objective of this study was to investigate the adsorption potential of bentonite for removal of phenol from aqueous solutions. The natural bentonite was obtained from Tirebolu-Giresun region of Turkey, and modified with a cationic surfactant, cetyl trimethylammonium bromide (CTAB), in order to increase the adsorption capacity. The structures of natural and organobentonite were characterized by using a variety of instrumental techniques including Fourier transform infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM). Also the surface area, cation exchange capacity and pH of the bentonite samples were estimated. The effects of experimental parameters such as initial pH of the solution, contact time, initial phenol concentration, organobentonite concentration, etc. were studied. The adsorption mechanisms of phenol onto organobentonite were evaluated in terms of thermodynamics and kinetics. The adsorption isotherms were described by using Langmuir and Freundlich isotherm models.