ABSTRACT
A geopolymer foam was synthesized for the evaluation of its capacity for fast retention and slow release of potassium in water. The base matrix of the geopolymer binder was obtained by mixing metakaolin, sodium silicate, and sodium hydroxide solutions. A factorial design of experiments (DOE) was applied to analyze the effect of adding hydrogen peroxide, aluminum powder, soybean oil, and iron ore flotation tailing on the water absorption and compressive strength of the geopolymer samples. The best-performing samples were characterized and evaluated for morphological aspects, chemical stability in different pH ranges, optimal adsorption time, and the ability to release potassium in the water. The inclusion of hydrogen peroxide was not a statistically significant factor at the 95% confidence level. The inclusion of aluminum powder and oil increased the water absorption, whereas the addition of tailing decreased it. The addition of soybean oil and tailing contributed positively to the compressive strength, but the addition of aluminum powder caused its reduction. The tailing addition not only increased the porous samples compressive strength but also prevented large cracks in their structure. The optimized response of 63 wt % of water absorption and 6 MPa of compressive strength was achieved by adding 0.05% of Al-powder, 1.5% of soybean oil, and 20% of tailing on the geopolymer binder. This sample exhibited 72% open porosity, a maximum adsorptive capacity of q = 5.7 mg/g, and chemical stability at pH > 4. Adsorption stabilized after 9 h of contact with water. Kinetics modeling suggested that the concentration gradient and chemical reactions probably drove potassium adsorption. The potassium release step was possibly controlled by the concentration gradient. After 60 days, the water solution released only 28% of absorbed potassium.
