engleski

Influence of initial pH value on the adsorption of Reactive Black 5 dye on powdered activated carbon: kinetics, mechanisms and thermodynamics

Textile industry is considered to be one of the major environmental pollutants, primarily due to wastewaters quantity and composition. Wastewaters have a wide range of pH values and temperatures and mainly contain various types of pollutants, most commonly dyes, surfactants, solvents, inorganic salts etc. It is therefore necessary to treat the wastewaters by reducing the amount of harmful substances to legally limited concentrations. One of the most commonly used and effective methods of treating coloured water is adsorption, while one of the most often used adsorbent is activated carbon. In this paper, the effect of initial pH value (pH0 = 2.00, 4.00, 8.00 and 10.00) on the process of isothermal adsorption of Reactive Black 5 (RB5) dye on powdered activated carbon at 45(±1) °C was carried out. Adsorption experiments were performed at different contact times (15, 30, 60, 120 minutes and 16 hours until equilibrium is reached). In order to investigate the mechanism of adsorption kinetic studies have been also performed, using pseudo-first-order and pseudo-second-order kinetic models, as well as intraparticle diffusion model. Additional equilibrium experiments at temperatures of 29(±1), 35(±1) and 55(±1) °C at pH0 = 4.00 were also done in a wish to determine thermodynamic parameters of adsorption (standard Gibbs free energy, standard enthalpy and standard entropy changes). Results of this research showed that initial pH value significantly influences the adsorption of RB5 dye onto activated carbon. The highest adsorption capacities and efficiencies of decolouration (Ed) were observed for initial pH values of pH0 = 2.00 and 10.00 (Ed = 98.4 % and 95.6 %, respectively), while values for pH0 = 4.00 and 8.00 are much lower. High Ed values at pH0 = 2.00 can be explained by the strong electrostatic interactions between positively charged activated carbon surface and coloured anion of RB5 dye, while at pH0 = 10.00 by attractive Pi···Pi interactions. The experimental data also showed that adsorption at all pH0 values is kinetically controlled assuming a pseudo-second order process, and that intraparticle diffusion is not the only process that influences on the adsorption rate. Negative values of standard Gibbs free energy change indicated that the adsorption process is spontaneous, and that adsorption of RB5 on activated carbon at the highest temperature is energetically the most favourable. The positive value of standard enthalpy change revealed the endothermic nature of the adsorption process at pH0 = 4.00 and that hydrogen bonds are driving force for RB5 dye adsorption. The positive value of standard entropy change suggests the increased randomness in the dye-adsorbent interaction at the solid/liquid interface.

Adsorption ; activated carbon ; Reactive Black 5 ; adsorption kinetics ; adsorption thermodynamics

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