Removal of acid turquoise blue 2G from aqueous solution by adsorbent derived from sludge and straw: kinetic, isotherm and thermodynamic study

Abstract

The main objective of this work was to evaluate the adsorption capacity of acid turquoise 2G onto the adsorbent derived from sludge and straw. The sludge–straw adsorbent was prepared by pyrolysis in a dried state. The BET surface area of the adsorbent was 459.77 m²·g⁻¹, micropore volume was 0.164 cm³·g⁻¹, and average pore radius was 3.345 nm. The experimental data were analyzed using four adsorption kinetic models (pseudo-first-order, pseudo-second-order, intra-particle diffusion, and elovich) and related parameters were calculated. The results showed pseudo-second-order kinetic model was better fitted with experimental data than the others; the pseudo-second-order adsorption mechanism was predominant for the adsorption of acid turquoise 2G. Adsorption equilibrium was analyzed by Langmuir and Freundlich models. Langmuir isotherm was found to be more suitable than Freundlich isotherm for correlation of equilibrium data. The maximum amount of acid turquoise blue 2G adsorbed (q_m) was 1.215, 1.413, and 1.574 mmol·g⁻¹ at 288, 298, and 308 K, respectively. Free energy of adsorption (ΔG⁰), enthalpy change (ΔH⁰), and entropy change (ΔS⁰) were calculated, and the results indicated that the adsorption processes of acid turquoise blue 2G on the sludge–straw adsorbent were spontaneous (ΔG⁰ < 0), endothermic in nature (ΔH⁰ > 0), and accompanied by an increase in entropy change (ΔS ⁰ > 0).

Keywords:

• Acid turquoise blue 2G
• Adsorption
• Sludge
• Kinetic
• Thermodynamic

Acknowledgments

This work was financially supported by Shanxi Science and Technology Agency Research Project (No. 20100321086) and Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (No. 20111029).