Adsorption of ethyl violet dye in aqueous solution by forest wastes, wild carob

Abstract

The adsorption of basic dye (i.e. ethyl violet (EV) or basic violet 4) from aqueous solutions onto the forest waste non-modified wild carob (NMWC) was carried out by varying some process parameters, such as initial concentration, pH, and temperature. The experimental results showed that an increase in the pH from 2 to 7 led to a strong decrease in the adsorption capacity of the dye (EV) on NMWC, showing the predominance of the dispersion forces compared to the electrostatic interactions, owing to the cationic character of the dye and the pH_pzc of the biosorbent (~6). The adsorption process can be well described by means of a pseudo-second-order reaction model showing that boundary layer resistance was not the rate-limiting step, as confirmed by intraparticle diffusion. In addition, experimental data were accurately expressed by the Sips equation if compared with the Langmuir and Freundlich isotherms. The high “m” values of the Sips model characterized a multilayer adsorption and the maximum amount adsorbed given by the Sips model was 100.4 mg/g at 20°C, namely close to the experimental value and increased only weakly with the temperature. The values of ΔG⁰ and ΔH⁰ confirmed that the adsorption of EV on NMWC was spontaneous and endothermic in nature. The positive values of ΔS⁰ suggested an irregular increase in the randomness at the NMWC–solution interface during the adsorption process.

Keywords:

• Adsorption
• Wild carob
• Isotherm models
• Kinetics
• Thermodynamic parameters