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 X200 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 6 % in 45 g L-1 NaCl solution). Desorption (faster than sorption) is completely achieved using 0.7 M HCl solution. At the sixth recycling, the loss in sorption is close to 5 % (??? decrease in desorption efficiency). The process is successfully applied for the treatment of MB-spiked industrial solution: the color index decreases by 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).

Automated summary

Hydrogel membranes were prepared using a mixture of gellan gum and biochar from an Egyptian plant. The material showed good sorption capacity for Methylene Blue, with a specific surface area of 134 m2/g. The sorption process was endothermic and temperature-dependent.