Adsorption behavior and mechanism of MB, Pb(II) and Cu(II) on porous geopolymers

In this study, a low-cost porous geopolymers (PGP) were prepared using coal gangue and fly ash as raw materials to remove organic and heavy metal ions from wastewater. Various characterization techniques were employed to investigate the microstructure, morphology, pore size distribution, and surface functional groups of the porous geopolymers. The results indicate that the geopolymer exhibits a porous structure with hydroxyl groups on the surface after foaming. It was found that the porous geopolymers exhibited good adsorption capacity for Methylene Blue (MB), Cu(II), and Pb(II), with optimal adsorption capacities reaching 35.60 mg/g, 98.31 mg/g, and 85.67 mg/g, respectively. Fitting analysis suggests that the adsorption of MB, Cu(II), and Pb(II) is better described by the pseudo-second-order kinetic model and the Freundlich isotherm model. This indicates that the adsorption process is primarily chemical adsorption with multilayer adsorption. Competitive adsorption studies showed that Cu(II) could enhance the adsorption of MB, while Pb(II) had no significant effect on the adsorption of MB. Mechanistic studies revealed that the adsorption of MB by PGP is mainly through pore adsorption, electrostatic attraction, and hydrogen bond formation; whereas the adsorption of Cu(II) and Pb(II) is primarily through pore adsorption, electrostatic attraction, chemical reactions, and ion exchange. Recycling experiments demonstrated that after five repetitions, the adsorption capacity of PGP for the three pollutants remained above 60 %. These findings confirm the potential of porous geopolymers as adsorbents. Preliminary cost analysis shows that the adsorption cost of porous geopolymers is only 2.07 RMB/kg, making the geopolymer adsorbent economically advantageous and promising for application, especially considering its ability to utilize industrial solid waste.