High-Strength Apatite/Attapulgite/Alginate Composite Hydrogel for Effective Adsorption of Methylene Blue from Aqueous Solution

This study proposed a new gelation method to prepare apatite/attapulgite/alginate composite hydrogels with high structural stability for effective adsorption of methylene blue (MB) from aqueous solution. Three types of conventional gels, together with the newly proposed apatite/attapulgite/alginate composite hydrogels, were compared for their mechanical strengths and adsorption behaviors and investigated for effects of the gelation process on their adsorption capacities. The isotherms, kinetics, and thermodynamics of the adsorption of MB onto the four types of gels were studied at different initial MB concentrations, temperatures, and contact times. The swelling test results proved that the apatite and attapulgite particles reinforced the structure of hydrogels to maintain its integrity in a saline solution. The adsorption kinetics results showed that all the adsorption followed the Langmuir model and the pseudo-second-order model. Calculations of thermodynamic parameters proved the adsorption processes to be spontaneous. The new prepared attapulgite/apatite/alginate composite gels were demonstrated to have the highest adsorption capacity among the four types of hydrogels. A possible reason could be that the porosity and functional groups of attapulgite and apatite provide enough active sites to improve the affinity of dye molecules to the surface of hydrogels.