Kinetic, equilibrium and thermodynamic phenomenological modeling of reactive dye adsorption onto polymeric adsorbent

Abstract

This work focused in the assessment of the potential of reactive blue dye 5G (RB5G) adsorption by polymeric adsorbent Dowex Optipore SD-2 (DOSD-2) in closed batch system by a phenomenological investigation of the adsorption process by both experimental data and mathematical modeling of kinetics, equilibrium and thermodynamics. The kinetic model of adsorption on the adsorbent sites described experimental data, consequently the adsorption itself is the rate-limiting step of the overall process, instead of diffusional steps. This kinetic model reduces to Langmuir isotherm in equilibrium condition, which in turn adequately described the solid-liquid interface equilibrium. Both kinetic, namely, adsorption kinetic constant (ka), and isotherm parameters: maximum adsorption capacity (qmax) and adsorbent-adsorbate affinity (b) provided data to the thermodynamics investigation, which was evaluated by means of: (i) activation energy (Ea) calculated Arrhenius equation; and (ii) enthalpy of adsorption (ΔH) determined by the Henry constant with the estimated parameters. In this sense, the proposed model presents a theoretical investigation of the entire process by an interdependent approach. Results showed higher adsorption capacity in acidic pH values (i.e. pH=2 – lower than adsorbent’s zero point charge: 4<pHzpc<10) suggesting electrostatic interactions as the main intermolecular forces in adsorption’s mechanism. The positive and low values of ΔH and Ea suggested to a physical nature and endothermic sorption process. Morphological and chemical characteristics of the adsorbent-adsorbate system indicated the possibility of aggregates formation and hence sterical limitations for the dye in the DOSD-2 adsorbent pores, on spite of that high adsorption yield of RB5G dye presented by DOSD-2 (≈700mgg−1) was achieved demonstrating its great potential for textile effluents containing reactive dyes.