Synthesis and optimization of porous metakaolin–rice husk ash geopolymer for efficient safranine dye removal using response surface methodology

Industrial effluents and unsustainable development practices contribute significantly to environmental pollution, leading to the deterioration of water quality and a growing prevalence of non-potable water sources. The porous geopolymer adsorbent was synthesized in this study from metakaolin and rice husk ash as the precursor materials. Activation was done using a hybrid alkaline solution (mixture of NaOH and sodium silicate Na₂SiO₃ with a 1.1 modulus) and NaOH activation concentration of 10 M (with H₂O₂ functioned as pore-forming agent). Fourier-transform infrared spectroscopy (FTIR) was used to investigate the functional groups, while X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to assess crystalline structure and gel phase, surface morphology, and porosity of the synthesized materials. Performance test of different geopolymer formulations, such as MG-A, MRG-A, and MRG-Ap (hybrid solution-activated with and without porosity) and NaOH-activated (with and without porosity) named as MG-N, MRG-N and MRG-Np were conducted through safranine dye batch adsorption tests. Among all, the porous NaOH-activated geopolymer (MRG-Np) showed the highest adsorption capacity - 110 mg/g and dye removal efficiency 98.5% achieved. This data was also evaluated varies isotherm and kinetic models. The models that best suited the adsorption data were pseudo-first-order and Freundlich isotherms, which indicated possible chemisorption with intra-particle diffusion and covers multi-site (heterogeneous) adsorption process. Thermodynamic analysis demonstrated that the adsorption process was spontaneous, endothermic, and entropy-driven. The theoretical adsorption performance to shed light on the mechanisms underlying its superior adsorption behavior. Subsequently, four significant factors were investigated on the effect of the adsorbent. The removal efficiency predicted by the model was 96.13% under optimized conditions, and it was very close to the experimental value of 98.5%. These characteristics establish MK-RHA-based porous geopolymers as potential low-cost, effective adsorbents for dye removal in wastewater treatment applications.