Technical Faculty in Bor
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Change in pH and conductivity during the rinsing and the biosorption of copper ions onto pumpkin peel
(Univerzitet u Zenici, Fakultet inženjerstva i prirodnih nauka, 2025) Marković, Marina; Gorgievski, Milan; Marković, Miljan; Grekulović, Vesna; Štrbac, Nada; Zdravković, Milica; Božinović, Kristina
The changes in pH and conductivity during the rinsing of the pumpkin peel, and the biosorption of Cu2+ ions, were the subject of this work. The obtained data showed that the pH value of the solutions increased during the rinsing of the biosorbent, as a result of the transfer of H+ ions from the aqueous phase into the structure of the pumpkin peel. An increase in the conductivity value was observed in the initial period of rinsing the pumpkin peel, followed by a decrease. The increase in conductivity in the initial phase is contributed to the self-leaching of the alkali and alkaline earth metal ions from the structure of the pumpkin peel, which were transferred into the aqueous phase. The further decrease in conductivity is a result of the dilution of the aqueous phase. The pH value decreased during the biosorption of Cu2+ ions, as hydrogen ions were transferred from the pumpkin peel structure into the solution, and then exchanged with Cu2+ ions. The conductivity value increased during the biosorption process, with a rapid increase in the initial period of 5 minutes, due to the transfer of alkali and alkaline earth metal ions into the solution.
DTA-TGA and thermodynamic studies of walnut shells used for the biosorption of Cu2+ ions from synthetic solutions
(Fakultet Tehničkih nauka, Kosovska Mitrovica, 2025) Marković, Marina; Gorgievski, Milan; Marković, Miljan; Štrbac, Nada; Grekulović, Vesna; Zdravković, Milica; Milošević, Nemanja
Biosorption is a simple, economical and environmentally friendly process of removing pollutants by binding them to biological material [1]. Biological materials that can be used to remove or recover organic and inorganic substances from aqueous solutions include living or dead microorganisms and their components, seaweed, plant material, industrial and agricultural waste and natural residues [2]. In this paper, the thermodynamic analysis of the biosorption of copper ions on walnut shells, as well as the DTA-TGA analysis of walnut shells, are presented. Thermogravimetry was used to study the thermal degradation characteristics of walnut shells. The sample was heated in an inert atmosphere from 20 to 900 oC, using a simultaneous DSC-TGA device SDT Q600 (TA Instruments). The TGA curve showed two stages of weight loss during heating. In the interval from 20 to 150 oC, a weight loss of 6.64% was recorded, which was due to the evaporation of moisture from the sample. Further weight loss of 69.05% was observed in the temperature range from 150 to 900 oC, which was due to the decomposition of hemicellulose, cellulose and lignin present in the biosorbent structure [3]. The DTA curve showed that the process was accompanied by an endothermic peak at a temperature of 279.81 oC. The total weight loss was 75.69%. Thermodynamic studies are conducted to investigate the influence of temperature on the biosorption process and to obtain information about the feasibility, spontaneity and nature of the process [4]. Therefore, thermodynamic parameters such as the change in Gibbs free energy (ΔG0), the change in enthalpy (ΔH0) and the change in entropy (ΔS0) were determined. In order to determine the thermodynamic parameters, 0.5 g of walnut shells were brought into contact with 50 mL of a synthetic solution of copper ions with a concentration of 0.2 g L-1, at different temperatures (25 oC, 35 oC and 40 oC). The obtained results for the change in Gibbs free energy (ΔG0 ) were negative at all three temperatures, indicating the feasibility of the process and spontaneous nature of the biosorption. The obtained positive value of enthalpy (ΔH0 = 1.12 kJ mol-1) indicates that the biosorption reaction is endothermic and consequently consumes energy. The negative value of entropy (ΔS0 = -6.26 J mol-1 K-1) indicates that there is less randomness at the interface between solid and solution.
Al-Sn alloys as composite phase change materials for thermal energy storage: microstructural and thermal characterization
(Fakultet Tehničkih nauka, Kosovska Mitrovica, 2025) Manasijević, Dragan; Balanović, Ljubiša; Marković, Ivana; Gorgievski, Milan; Stamenković, Uroš; Kovacević, Avram
Al–Sn alloys are well known for their excellent resistance to wear and corrosion, as well as their solid mechanical performance, making them popular choices for bearing applications [1,2]. More recently, however, they have attracted attention as potential composite phase change materials (C-PCMs) for thermal energy storage (TES) systems. To determine their suitability for such applications, a thorough investigation of their thermal characteristics is essential. In this study, Al–Sn alloys containing 11.7, 22.4, 32.8, 41.1, and 53.4 atomic percent Sn were prepared by melting high-purity metals. Continuous stirring of the melt was employed to prevent segregation, followed by casting into stainless steel molds. The resulting ingots displayed uniform microstructures, free of cracks and pores. Thermal diffusivity of the solid Al–Sn samples was measured between 25°C and 150°C using the light flash method. Density at room temperature was determined via the Archimedes principle, while specific heat capacities across different temperatures were calculated using the CALPHAD (Calculation of Phase Diagrams) approach. Thermal conductivity values were then derived using an appropriate conversion relationship. Additionally, phase transition temperatures and the associated thermal effects were analyzed by differential scanning calorimetry (DSC). The study mapped how thermal conductivity changes with both alloy composition and temperature, and also assessed how latent heat of fusion varies across the compositions. Microstructural and phase composition analyses were performed using scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS). The findings offer valuable insights into the thermal behavior and microstructural characteristics of Al–Sn alloys, contributing to the development of advanced PCMs for thermal energy storage applications.
Kinetic and DTA-TGA analysis of Cu2+ biosorption on hazelnut shells
(Fakultet Tehničkih nauka, Kosovska Mitrovica, 2025) Gorgievski, Milan; Marković, Miljan; Štrbać, Nada; Manasijević, Dragan; Grekulović, Vesna; Jovanović, Dalibor; Marković, Marina
Biosorption can be defined as the ability of biomaterials to accumulate and concentrate heavy metals from even the most dilute aqueous solutions. The biosorption process involves a solid phase (biological material) and a liquid phase (solvent, usually water) containing the dissolved metal. [1]. In this work, hazelnut shells were used as biosorbents for the biosorption of Cu2+ ions from synthetic solutions. This paper presents the results of kinetic and DTA-TGA analysis of the biosorption of Cu2+ ions on hazelnut shells. A pseudo-first order kinetic model, a pseudo-second order kinetic model, an intraparticle diffusion kinetic model and an Elovich kinetic model were used to describe the kinetics of biosorption of Cu2+ ions with hazelnut shells. The change in the adsorption capacity of copper ions with time was determined by contacting a series of solutions of copper ions with an initial concentration of 0.2 g dm-3 with 0.5 g of the sample for different contact times. Biosorption was terminated after 90 minutes, assuming that this time was long enough to establish equilibrium in the system [2]. It can be seen from Fig. 1a that at the beginning of the process (in the first 10 minutes) the adsorption capacity increases sharply with the contact time. The sudden increase in capacity in the first 10 minutes of the process is due to the large number of active sites available on the surface of the adsorbent. Thereafter, until the end of the experiment, a slight increase in adsorption capacity can be observed, which is due to the decrease in available active sites and the decrease in the concentration of copper ions in the solution [3]. The pseudo-second order kinetic model, with the correlation coefficient R2 = 0.961, showed the best agreement with the experimental data. Based on this model it can be concluded that chemisorption is a possible binding mechanism of copper ions on the surface of hazelnut shells [4].
Microstructural and thermal properties of the Al-Cu eutectic alloy
(Fakultet tehničkih nauka, Kosovska Mitrovici, 2025) Manasijević, Dragan; Balanović, Ljubiša; Cimpoesu, Nicanor; Markovic, Ivana; Gorgievski, Milan; Stamenković, Uroš; Stepanović, Aleksandra
Understanding thermal characteristics such as thermal conductivity, specific heat capacity, and latent heat of fusion is crucial when developing phase change materials (PCMs) for latent heat energy storage (LHES) systems [1]. Among metal-based PCMs, aluminum-based eutectic alloys have emerged as some of the most extensively studied due to their favorable thermal and mechanical properties [2].In this study, the Al–33.6 mass% Cu eutectic alloy was investigated in terms of its microstructure, thermal diffusivity, thermal conductivity, specific heat, and latent heat of melting. Techniques including scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), differential scanning calorimetry (DSC), and the light flash method were employed. Analysis revealed that the alloy’s microstructure contains both fine and coarse (Al)+Al₂Cu eutectic phases. Measurements showed that specific heat, thermal diffusivity, and thermal conductivity all increase with temperature over the range of 25–400 °C. At room temperature, the alloy exhibits a thermal conductivity of 134.3 W·m⁻¹·K⁻¹. The latent heat of fusion was determined to be 319.5 J·g⁻¹. Overall, the findings highlight the strong potential of the Al–Cu eutectic alloy as a candidate material for use in phase change thermal energy storage applications.