Design of a hybrid bio-adsorbent based on Sodium Alginate/Halloysite/Hemp hurd for methylene blue dye removal: kinetic studies and mathematical modeling

This research aims for designing and fabricating green and sustainable systems for an efficient removal of methylene blue from aqueous solutions. The hybrid beads, obtained by using ionotropic gelation technique and calcium chloride as cross-linking agent, are composed of sodium alginate matrix, hemp hurd as agro-waste resource and halloysite nanoclay (HNTs). The produced composite beads were characterized morphologically by using SEM and EDX techniques. The batch adsorption studies were performed up to 24 h using methylene blue (MB) as model hazardous dye in order to investigate the effect of HNTs content (0-35% wt) and pH (2, 7 and 12) on the adsorption capacity. In neutral conditions, a maximum adsorption capacity of 32 mg/g was obtained for sample with 0% of HNTs while it increases up to 41, 43 and 50 mg/g for 5%, 20% and 35%wt, respectively. Moreover, swelling/deswelling behavior and point of zero charge (PZC) analysis were even investigated since they are known to affect the adsorption process. A slight increase in PZC can be observed by adding clay nanotubes from 7.42 (0% wt HNTs) up to 7.98 (35% wt HNTs). The experimental data were fitted through first order model, pseudo-second order model and intraparticle diffusion model, demonstrating that the adsorption phenomenon is well described by second order process as well as that the process is mainly characterized by boundary diffusion and intraparticle diffusion. Finally, diffusion of dye was evaluated as function of HNTs content and H+ concentration appearing to decrease from pH= 2 up to pH= 7 and then to increase in alkaline medium (pH=12). Diffusion coefficient and recovery % trends were modeled through design of experiment methodology.