Hydrogel‐biochar composites for removal of methylene blue: Adsorption performance, characterization, and adsorption isotherm, kinetics, thermodynamics analysis

Abstract Low‐concentration organic dye wastewater severely threatens the growth and reproduction of animals and plants, as well as the survival and development of human beings and the environment. Although versatile hydrogel composites have been prepared owing to their excellent adsorption performance, the high‐removal efficiency and fast adsorption rate are still two crucial problems related to practical applications. Herein, xanthan gum‐g‐poly (acrylic acid‐co‐acrylamide)/biochar composite hydrogels with high‐removal efficiency and fast adsorption rate of methylene blue (MB) are synthesized using sodium dodecyl sulfate as a pore‐foaming agent. The composite hydrogels exhibit rough and porous three‐dimensional structures, which can make the internal active adsorption sites to be fully exposed and facilitate the MB diffusion process. MB's removal efficiency can reach 96.64% within 30 minutes, which is almost the first time that a biochar‐contained hydrogel could show superior removal efficiency of more than 95% in such a short adsorption time. The factors affecting the adsorption performance are studied comprehensively. Langmuir and pseudo‐second‐order models fit the adsorption process well. Moreover, the MB removal rate still exceeds 80% after eight cycles, which is also a vital indicator for practical treatment. Adsorption mechanism analysis shows that the synergistic effect among biochar, xanthan gum, and polymer chains and the porous structure improve the adsorption performance of the composite hydrogels. The proposed hydrogel‐biochar composites with high‐removal efficiency and adsorption rate are highly promising as an alternative adsorbent for advanced printing and dyeing wastewater treatment.