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 aim of this study was determination of physical-chemical and radiological characteristics of kaolin and products of alkali-activated thermally treated kaolin (geopolymer). Also, the objective of presented research was to investigate the possibility of kaolin application as a pigment or as a raw material for obtaining geopolymer materials as a relatively new ones in a building material industry. Physicochemical characterization of one set of samples was conducted using X-ray diffraction (XRD), Fourier transform infra - red (FTIR) and X-ray photoelectron spectroscopy (XPS). Activity concentration of naturally occurring radionuclides in kaolin, metakaolin and geopolymer were determined. The absorbed dose rate (D) and the annual effective dose rate (EDR), calculated in accordance with the UNSCEAR 2000 report, are also presented in this paper. Kaolin was heat-treated on 750oC and specific activity of natural radionuclide in metakaolin increased up to 1.6, while measured specific activities in geopolymer were the lowest.

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 q_max = 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.

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.

In this paper, the properties of organic-inorganic hybrid polymer materials, which were synthesized from an aluminosilicate inorganic matrix with the addition of brushite and aminosilane grafted on one side and PEI covalently bonded composites on the other side, were examined. The synthesized organic-inorganic hybrid polymers were examined in terms of a structural, morphological, thermo-gravimetric, and adsorption-desorption analysis and also as potential CO₂ capturers. The structural and phase properties as well as the percentage contents of the crystalline and amorphous phase were determined by the X-ray diffraction method. The higher content of the amorphous phase in the structure of hybrid polymers was proven in metakaolin and metakaolin-brushite hybrid samples with the addition of amino silane and with 1,000,000 PEI in a structure. The DRIFT method showed the main band changes with the addition of an organic phase and inorganic matrix. Microstructural studies with the EDS analysis showed a uniform distribution of organic and inorganic phases in the hybrid geopolymers. The thermo-gravimetric analysis showed that organic compounds are successfully bonded to inorganic polymer matrix, while adsorption-desorption analysis confirmed that the organic phase completely covered the surface of the inorganic matrix. The CO₂ adsorption experiments showed that the amine-modified composites have the higher capture capacity, which is 0.685 mmol·g⁻¹ for the GM10 sample and 0.581 mmol·g⁻¹ for the BGM10 sample, with 1,000,000 PEI in the structure.

The study of Cu(II) from aqueous solutions using the adsorption process on synthetically modified geopolymers was performed under static conditions. Three geopolymers (based on metaphase of Serbian clay, metaphase of German clay and metaphase of German clay plus 10% of carbon cloth) were used. The geopolymers were made by condensing a mixture of metaphases and alkali activator solution at a fixed ratio at room temperature and then at a temperature of 60 °C in a dry oven. Then, the geopolymer samples were pre-crashed to a fixed-radius size. Their properties were characterized by X-ray diffractometry (XRD), Diffuse Reflectance Infrared Fourier Transform (DRIFT) analysis and Scanning Electron Microscopy (SEM) with Energy Dispersive Spectroscopy (EDS). Adsorption experiments were carried out under batch process as a function of the dose, concentration of metal, and contact time. The uptake of Cu(II) was rapid, and it increased with increasing metal concentration. The sorption percentage decreased with increasing concentration of Cu(II). The equilibrium adsorption capacity of geopolymers was measured and extrapolated using more isotherms. The data fit very well the linear Langmuir isotherm model. The pseudo-second-order kinetic model can well describe the adsorption behavior of Cu(II) ions with geopolymers samples. These results show that used geopolymers hold great potential to remove Cu(II) from industrial wastewater.