In this paper, raw natural metakaolin (MK, Serbia) clay was used as a starting material for the synthesis of geopolymers for thermal treatment. Metakaolin was obtained by calcination of kaolin at 750 °C for 1 h while geopolymer samples were calcined at 900 °C, which is the key transition temperature. Metakaolin was activated by a solution of NaOH of various concentrations and sodium silicate. During the controlled heat treatment, the geopolymer samples began to melt slightly and coagulate locally. The high-temperature exposure of geopolymer samples (900 °C) caused a significant reduction in oxygen, and even more sodium, which led to the formation of a complex porous structure. As the concentration of NaOH (6 mol dm−3 and 8 mol dm−3) increased, new semi-crystalline phases of nepheline and sanidine were formed. Thermal properties were increasingly used to better understand and improve the properties of geopolymers at high temperatures. Temperature changes were monitored by simultaneous use of thermogravimetric analysis (TGA) and differential thermal analysis (DTA). The loss of mass of the investigated samples at 900 °C was in the range of 8–16%. Thermal treatment of geopolymers at 900 °C did not have much effect on the change in compressive strength of investigated samples. The results of thermal treatment of geopolymers at 900 °C showed that this is approximately the temperature at which the structure of the geopolymer turns into a ceramic-like structure. All investigated properties of the geopolymers are closely connected to the precursors and the constituents of the geopolymers.
The present work was focused on doping of 1% and 5% both of Nd2O3 and Sm2O3 in geopolymer gels. One of the main goals was to determine the influence of the behavior of Nd and Sm as dopants and structural nanoparticles changes of the final geopolymer formed. It is shown that the disorder formed by alkali activation of metakaolin can accommodate the rare earth cations Nd3+ and Sm3+ into their aluminosilicate framework structure. The main geopolymerization product identified in gels is Al-rich (Na)-AS-H gel comprising Al and Si in tetrahedral coordination. Na+ ions were balancing the negative charge resulting from Al3+ in tetrahedral coordination. The changes in the structures of the final product (geopolymer/Nd2O3; Sm2O3), has been characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) analysis with energy dispersive spectrometry (EDS). Nucleation at the seed surfaces leads to the formation of phase-separated gels from rare earth phase early in the reaction process. It is confirmed that Nd and Sm have been shown to form unstable hydroxides Nd(OH)3 and Sm(OH)3 that are in equilibrium with the corresponding oxides.
Water hyacinth biomass was used for the synthesis of activated carbons in the process of chemical activation with H3PO4, followed by controlled carbonization. The study investigates the effect of various impregnation weight ratios of H3PO4 and dry hyacinth (0.5–3.0), as well as different carbonization temperatures (T = 400–800 °C), on the surface characteristics of the produced activated carbons (AC). The activated carbon obtained with an impregnation ratio of 1.5 and a carbonization temperature of 600 °C (1.5 AC/600) showed the highest values of specific surface area of 1421 m2 g−1, representing a selected adsorbent for metformin removal. The chosen sample was characterized by elemental analysis, adsorption–desorption isotherms of nitrogen at −196 °C, via FTIR spectroscopy and the SEM method. The modeling of the experimental adsorption data showed that metformin adsorption: (i) can be best described by the Langmuir isotherm model, with the value of qmax = 122.47 mg g−1; (ii) led the pseudo-second order kinetic model; and (iii) is a spontaneous (ΔG° = −3.44 kJ mol−1) and endothermic (ΔH° = 8.77 kJ mol−1) process. A desorption study has shown that 92% of metformin was successfully desorbed in the presence of a 0.1 MHCl/ethanol mixture (volume ratio 2:1). The recovery of the adsorbent of 84%, after five successive cycles, indicated that the 1.5 AC/600 has potential to be applied in the real systems for water treatment.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.