Batch Adsorption Studies on the Removal of Acid Blue 25 from Aqueous Solution Using Azolla pinnata and Soya Bean Waste

Abstract

Azolla pinnata (AP) and soya bean waste (SBW) were studied for their potentials to remove hazardous dye, acid blue 25 (AB25), from aqueous solution in a batch adsorption process. Various parameters such as pH, contact time, concentration and temperature were studied. The optimum pH was found to be at pH 2.0, and short duration of contact time at 180 min was sufficient to attain equilibrium. The experimental data were fitted to three different isotherm models, and the adsorption was best described by the Langmuir isotherm model. The maximum monolayer capacities were estimated to be 38.3 and \({50.5\,{\rm mg}\,{\rm g}^{-1}}\) for SBW and AP, respectively. Kinetics studies showed that the adsorption system for both adsorbents follow pseudo-second-order model. Weber–Morris model showed that intraparticle diffusion is not the rate-limiting step, while Boyd model suggested that film diffusion may be the controlling mechanism for both adsorbent. The adsorption processes were found to be thermodynamically feasible. AP-AB25 system is endothermic in nature, while SBW-AB25 is exothermic. Regeneration experiment showed that NaOH is effective at regenerating the spent adsorbent, where at fifth cycle, the adsorption capacities of AP and SBW were comparable to the unspent adsorbents. All of these discoveries highlighted the potential of both AP and SBW as effective adsorbents for removal of AB25.