On the Adsorptive Detoxification of Chrome Tan Liquor: Kinetics, Thermodynamics and Mode of Transport

Aim: The aim of this study is to regenerate spent activated carbon (RGAC) for use as adsorbent and to study the kinetics, thermodynamics and mode of transport of removing chromium from chrome tanning effluent (TEff).

Methodology: The chemical regeneration approach was adopted in the recycling. Regenerated adsorbent was characterized using FTIR, SEM and classical methods. A batch adsorption experiment was carefully followed to de-chrome the chrome tan effluent. Equilibrium phase chromium was quantified with Atomic Absorption Spectrophotometer (AAS). Adsorption phenomena were investigated with Kinetic, thermodynamic and transport (diffusion) models to study the behavior of Cr uptake. Kinetic models viz; First order, pseudo-second order, Elovich and Bhattacharya-Venkobachor kinetic models were subjected to three model applicability tests. Thermodynamic parameters which include changes in free energy (ΔG), entropy (ΔS) and enthalpy (ΔH) were monitored in standard states. Prediction of applicable diffusion model was based on comparing linearity of film diffusion, intra-particle diffusion and intraparticulate diffusivity models.

Results: Highlights from this study unveiled the influential roles of parametric factors (Initial effluent concentration on vol./vol. bases, solution pH, particle size, adsorbent dose, contact time and temperature). R2 values for models considered showed good fit except for the Bhattacharya-Venkobachor kinetic model. The rate law of the adsorption kinetics is best explained using the Pseudo-second order kinetic model. The chromium adsorption efficiency using both Commercial and regenerated GAC are in good agreement at 95% confidence interval. The regenerant-chromium mode of diffusion, as predicted by the “best-fit’’ transport models, was not suitable for use in the intraparticle diffusion mode (with least R2 and high transport rate) as it does for the film diffusion. In addition, thermodynamic parameters of sorption have also been determined in favour of spontaneity and chemisorptions mechanism.

Conclusion: An overall from this study is the recommendation of regenerated adsorbent as a sure economically viable substitute to the commercial activated carbon for chrome tan effluent de-chroming. This generalization was based on statistical test of significance which reports good agreement between the two adsorbents for the investigated adsorption phenomena.