Methylene blue removal from aqueous solutions using a biochar/gellan gum hydrogel composite: Effect of agitation mode on sorption kinetics

Hydrogel membranes are prepared by casting a mixture of gellan gum (associated with PVA) and biochar produced from a local Egyptian plant. The mesoporous material is characterized by a specific surface area close to 134 m2 g−1, a residue of 28 % (at 800 °C), and a pHPZC close to 6.43. After grinding, the material is tested for Methylene Blue sorption at pH 10.5: sorption capacity reaches 1.70 mmol MB g−1 (synergistic effect of the precursors). The sorption isotherms are fitted by both Langmuir and Sips eqs. MB sorption increases with temperature: the sorption is endothermic (∆H°: 12.9 kJ mol−1), with positive entropy (∆S°: 125 J mol−1 K−1). Uptake kinetics are controlled by agitation speed (optimum ≈200 rpm) and resistance to intraparticle diffusion. The profiles are strongly affected by the mode of agitation: the equilibrium time (≈180 min) is reduced to 20–30 min under sonication (especially at frequency: 80 kHz). The mode of agitation controls the best fitting equation: pseudo-first order rate agitation for mechanical agitation contrary to pseudo-second order rate under sonication. The sorption of MB is poorly affected by ionic strength (loss 97 % with a sorbent dose close to 1 g L−1; a higher dose is required for maximum reduction of the COD (60 % at 3 g L−1). [Display omitted] •Methylene Blue is well sorbed using a bio-based composite (Gella gum/PVA-Biochar).•Langmuir and Sip's equations show the synergistic effects of GG/PVA and biochar.•Optimum agitation speed at 200 rpm (on both equilibrium and kinetic performances).•Ultrasonication strongly enhances kinetics and reverses the kinetic model fitting.•Efficient MB desorption with an acidic solution and remarkable stability at reuse.