Development of geopolymer for dye industrial and heavy metal removal

The aim of this work is to study removal of dyes and heavy metal ions from aqueous solutions by using geopolymers synthesized from metakaolon and commercial silica extracted from rice husks and fly ash from the Mae Moh power plant of preparing sodium silicate (Na2SiO3). Non-treated fly ash was used as an adsorbent. Geopolymer was synthesized from metakaolin. The adsorbent materials were characterized by x-ray diffraction (XRD), x-ray fluorescence (XRF) scanning electron microscope (SEM), brunauer-emmett-teller or (BET) and Fourier transform infrared spectroscopy (FT-IR). Based on the adsorption efficiency of the geopolymers from commercial Na2SiO3 (GEO8M5D) for methylene blue and brilliant green dyes, it was found that the adsorption efficiency for both dyes are higher than 90%. The optimum parameters for adsorption including adsorbent dosage and adsorption time, adsorption isotherm, kinetic, thermodynamic and regeneration test parameters were studied at the initial dye concentration of 300 ppm. The optimum dosage is 20 g/L. The contact time is 60 minutes. The adsorption efficiency is 95%. The regeneration of GEO8M5D can be obtained for 9 cycles (qe=296.92 mg/g). The geopolymer prepared from rice husks (RH) and fly ash (GSiRH and GFA) for GSiRH is higher than GFA. Therefore, GSiRH was selected for optimal conditions for the study. The results showed that the optimum dosage is 40 g/L, the contact time is 60 minutes and the adsorption efficiency is 91%. The regeneration of GSiRH can be obtained for 4 cycles (qe=71.92 mg/g). Fly ash’s adsorption efficiency for removing brilliant green was investigated. It was found that the optimum dosage is 4 g/L, the contact time is 30 minutes and the adsorption efficiency is 93%. The regeneration of fly ash can be obtained for 3 cycles (qe=227.10 mg/g).
The adsorption isotherm onto GEO8M5D, GSiRH and fly ash corresponded well to the Langmuir isotherm which described the monolayer adsorption. Kinetic study showed that the adsorption was followed the pseudo-second-order model. The thermodynamic properties of the adsorption, the positive value of DeltaS๐ indicated the increment of revealing the samples, affinity for the dyes showed increasing instability and randomness in the dyes interaction interface which resulted from the excess endothermic heat. Negative DeltaH๐ and DeltaG๐ indicated that dyes adsorption are an endothermic and spontaneous physisorption, respectively.
In addition, application of GEO8M5D was used for heavy metal removal from solution. The adsorption efficiency of Cr(III), Zn(II) and Cd(II) onto GEO8M5D are 90.68, 92.86 and 93.48%, respectively. The result indicated that GEO8M5D can be used to reduce heavy metals from the solutions. Therefore, these results indicated that the synthetic geopolymer and the fly ash can be used as potential adsorbents for dyes and heavy metal removal from industrial wastewater.