Abstract
Wastewater from oil refineries is characterized mainly by the presence of hydrocarbons and, in particular, non-ionic volatile organic compounds such as benzene, toluene, ethylbenzene and xylene; known as BTEX. The removal of BTEX compounds is carried out by commercial activated carbon; however, the widespread use of this adsorbent is limited due to its high cost. As such, alternative novel adsorption techniques using non-conventional low-cost adsorbents are promising methods to remove BTEX compounds. The present study’s main focus is to carry out a critical analysis on the removal efficiency of BTEX compounds from aqueous environment in single and multi-component systems, investigate the advantages and limitations of each adsorbent, and evaluate the various adsorption mechanisms. Batch adsorption experiments of BTEX-contaminated waters (5-200 mg/1) in single component systems were carried out by using chitin and chitosan as adsorbent materials, in order to evaluate the removal performance and to obtain the isotherm profiles. The effects of various parameters such as initial concentrations, adsorbent dose and contact time on the removal performance of BTEX were investigated. The next part of this work examines the equilibrium sorption of BTEX compounds; in multi-component system, from aqueous solutions by chemically modified chitosan. The enhancement of chitosan was carried out by crosslinking the chitosan with glutaraldehyde, and then grafting the poly(methacrylic acid) onto its backbone. Then, experimental work was carried out to study the adverse effects of major competing ions such as sulfate, phosphate and ammonium ions on the BTEX sorption isotherms and kinetics using chitosan and modified chitosan. The equilibrium data were analysed using Langmuir, Freundlich, Redlich-Peterson, Temkin and Dubinin-Radushkevich isotherm models. The linear and non-linear regressions were carried out to determine the best fit model for each system. The linear correlation coefficient was found for each system and the Redlich-Peterson provided the best fit, over the concentration range studied. The non-linear regressions were carried out to evaluate the data by five error analysis methods; namely, the sum of the squares of the errors (ERRSQ), the hybrid fractional error function (HYBRID), Marquardt's percent standard deviation (MPSD), the average relative error (ARE), and the sum of the absolute errors (EABS). Overall, the values of error indicated that the Langmuir isotherm model provided the best quality of fit for the isotherm equilibrium data, for the selected adsorbents. The kinetic studies revealed that the adsorption followed the pseudo-second order rate model for the selected adsorbents, and the pore diffusion is not the only rate controlling step in the removal of BTEX compounds. In terms of added competing ions, it was found that the adsorption behaviour of BTEX compounds was insignificantly affected by the presence of ammonium, phosphate and sulphate ions. The major ions reduced the BTEX adsorption in order of HPO2-4 > SO2-4 > NH+4. This study demonstrated that the chemically enhanced chitosan is a potential adsorbent for the removal of BTEX at concentrations as high as 200 mg/1.