Development of porous and reusable geopolymer adsorbents for dye wastewater treatment

Geopolymer materials exhibit a huge potential for treating wastewater to improve the water quality. Limited specific surface area and subsequent treatment are the major challenges that currently hinder the development of adsorbents. In this paper, a cost-effective hierarchical pore geopolymer based on solid waste was synthesized by dealuminization with acid treatment after geopolymerization to efficiently remove methylene blue (MB) dye from wastewater. The changes of pore structure, morphology and functional groups were systematically analyzed by various characterization techniques, and the dialectical effect of such alterations on adsorption properties was further investigated. The acid-treated geopolymer particles (0.425–1 mm) showed an excellent specific surface area of 292.05 m2/g with a microporosity of 79%, providing abundant sites and broad channels for the adsorption process. The optimal geopolymer adsorbent exhibited a high adsorption efficiency for 600 mg/L of MB dye solution (pH = 8) at room temperature, with the maximum adsorption capacity of 115 mg/g and the removal efficiency of 97.8%. The equilibrium time was shortened from 100 min to 30 min when the pH value was adjusted from 6 to 8 or 10. Furthermore, the removal efficiency of the adsorbent was still above 95% after successful regeneration for 10 cycles, and the selectivity of the adsorbent to cationic dyes was much higher than that of anionic dyes and amphoteric dyes. The superior porosity and adsorption properties of the adsorbents are related to the modulated matrix structure of the geopolymer, which can be easily implemented by acid treatment to mainly release Al and Na of the network.