Resorcinol - formaldehyde activated carbon gels for adsorption of methylene blue and congo red

Abstract

The issues related to the presence of inorganic and organic pollutants such as dyes in water bodies has become a subject of considerable concern. Chemically-formulated carbon gel is a promising material for continuous adsorption in wastewater treatment because it can be mould into desired shape to decrease hydraulic resistance leading to high pressure drop and operating costs. The effluents need a decisive treatment before been discharged to water resources. This work aims to evaluate the adsorption properties of activated carbons prepared from resorcinol-formaldehyde for the removal of positively charged methylene blue and negatively charged congo red from water. The activated carbons were prepared by chemical activation using potassium hydroxide at activation temperature of 550 °C for 2 h. The physicochemical properties of the activated carbons were characterized using Fourier transform infrared spectroscopy, thermal gravimetric analysis, Brunauer-Emmett-Teller (BET) surface area, pH value at the point of zero charge, Boehm titration, scanning electron microscope and elemental analysis. The BET surface area of the activated carbons are 427 and 433 m2/g for resorcinol to catalyst ratios of 100 and 2000, respectively. The adsorption of methylene blue and congo red were studied at varying concentrations (10 – 300 mg/L) and contact times (1 h – 288 h). The isotherm and kinetics models were employed to describe the adsorption data. The maximum adsorption of methylene blue is 135 mg/g, while the maximum adsorption of congo red is 16 mg/g. The equilibrium data of methylene blue adsorption fitted well with the Freundlich and Redlich-Peterson models, while that of congo red adsorption obeyed Langmuir and Redlich-Peterson models. The kinetics data were best fitted to pseudo-first-order and pseudo-second-order models, indicating that both physisorption and chemisorption may have occurred simultaneously. The intraparticle diffusion model revealed that the intraparticle diffusion may be involved, but it is not the only rate – limiting step, while Boyd model showed that film diffusion may be the controlling mechanism. For adsorption dynamic, the adsorption of methylene blue was studied at varying bed heights (2 and 6 cm), flow rates (5, 10 and 20 mL/min) and inlet concentrations (5, 10 and 20 mg/L). The results show that the performance of activated carbons for methylene blue adsorption is better at lower flow rate, lower inlet concentration and higher activated carbon bed height. The adsorption dynamic data of methylene blue adsorption fitted well with Thomas, Yoon-Nelson and Adams-Bohart, indicating that the kinetics system was controlled by external mass transfer or film diffusion.