Adsorption of gold using magnetic solid phase extraction method and determination by ICP-OES

Gold (Au) has held significant importance throughout history not only as an ornament and a means of economic value but also as a symbol of social power, wealth, and authority. Due to its exceptional physical and chemical properties—particularly its high electrical conductivity, chemical inertness, corrosion resistance, and rarity—this precious metal finds wide application in various industrial fields. Gold plays a strategic role in areas ranging from microelectronics and biomedical devices to aerospace technologies and catalysis, becoming even more critical with advancing technology. However, the gradual depletion of natural resources has brought the recovery of gold into focus as a prominent research topic. In particular, the recovery of gold accumulated in electronic waste and low-concentration aqueous environments through eco-friendly, cost-effective, and efficient methods is of great importance in terms of both environmental sustainability and economic value. The vast majority of traditional gold recovery methods rely on cyanide-based processes. Although cyanide is effective in leaching gold due to its high solubility, it is an extremely toxic and environmentally harmful chemical. For this reason, the development of alternative solvent systems that are environmentally friendly and have low toxicity to replace cyanide has become the focus of scientific research. The new-generation adsorption techniques developed in this context offer sustainable solutions from both environmental and economic perspectives. Adsorption is the process by which target ions or molecules in a solution medium are physically or chemically bound to a solid surface. This method is a separation technique frequently used in the fields of recovery and environmental remediation. Modern methods such as magnetic solid-phase extraction (MSPE) stand out due to their advantages such as high selectivity, short processing time, reusability, and easy magnetic separability. Magnetic nanoparticles used in the MSPE method can be specifically designed for target ions thanks to surface modifications, enabling selective and efficient separation even in complex matrices. In this study, a novel magnetic nanoparticle system (Fe3O4@SiO2@CPTES@DAT) functionalized with 3,5-diamino-1,2,4-triazole (DAT) on its surface was developed for the selective separation and enrichment of gold Au(III) ions from aqueous media. DAT is a triazole derivative that exhibits high affinity toward metal ions due to the nitrogen atoms in its structure and has good solubility in water. In the nanoparticle synthesis, Fe3O4 magnetic cores were first prepared by the co-precipitation method, followed by coating their surfaces with silica (SiO2) via tetraethyl orthosilicate (TEOS). Subsequently, 3-chloropropyltriethoxysilane (CPTES) was applied for surface functionalization, and finally, DAT molecules were chemically bonded to the surface. The Fe3O4@SiO2@CPTES@DAT nanostructure obtained at the end of this synthesis process was evaluated as an effective adsorbent for both analytical and environmental applications, owing to its high surface area, specific binding sites, and superparamagnetic properties. The characterization of synthesized nanoparticles was performed using various advanced analysis techniques to verify the morphological, structural, and chemical properties of the nanomaterial. Field Emission Scanning Electron Microscopy (FESEM) determined the surface morphology and dimensions of the nanoparticles, showing that the particles were spherical in shape and homogeneously distributed. High-resolution transmission electron microscopy (HRTEM) was used to observe the crystal structure and finer structural details, confirming that the particles were in the 50–100 nm size range. The surface chemical composition of nanoparticles was analyzed using Energy Dispersive X-ray Spectroscopy (EDX), and the presence of elements such as Fe, Si, O, C, and N in the structure was confirmed. X-ray Diffraction (XRD) analysis identified the characteristic crystal structures of the Fe3O4 core, and the crystalline structure was preserved after the coating processes. In Fourier Transform Infrared Spectroscopy (FT-IR) analysis, characteristic bands associated with silanol (-Si-OH), amino (-NH2), and triazole rings were detected, demonstrating the successful surface functionalization. Finally, using thermogravimetric analysis (TGA), the amount of coating and functional groups was evaluated along with thermal stability, and the thermal resistance of the obtained nanostructure was found to be high. These characterization data demonstrate that the synthesized Fe3O4@SiO2@CPTES@DAT structure possesses the desired properties and is a suitable candidate for the adsorption of gold ions. Within the scope of experimental studies, parameters affecting gold adsorption have been systematically optimized. In the single-variable analyses conducted, factors such as the pH value of the solution, the amount of adsorbent, the contact time, and the desorption conditions were evaluated separately. The experiments conducted within this scope not only determined the optimal conditions but also provided an in-depth understanding of the potential of the developed system for environmental and analytical applications. The optimum adsorption conditions were determined to be pH 2, 10 mg adsorbent amount, and 20 minutes contact time. The high yield obtained under these conditions supports the strong affinity of the triazole group for Au(III) ions. This strong adsorption capacity, particularly observed at low pH, also demonstrates the system's advantage in terms of kinetic rate. Systems capable of operating at high efficiency in a short time offer a significant advantage, particularly for environmental applications. Additionally, the achievement of high adsorption performance despite the low amount of adsorbent used highlights the method's economic feasibility and operational efficiency. In desorption experiments, it was found that a 1 M HCl solution containing 0.1% (w/v) thiourea provided the highest yield, with over 95% of Au(III) ions successfully recovered under these conditions. This high recovery rate clearly supports the system's potential for reuse and sustainability. In selectivity tests conducted to evaluate the effect of foreign ions, it was observed that the synthesized adsorbent maintained its high selectivity towards Au(III) ions even in the presence of metal ions such as Cu²⁺, Zn²⁺, and Fe²⁺, which are known to cause interference. The absence of a significant decrease in the adsorption efficiency of gold ions demonstrates that the developed nanostructure can function effectively even in multi-component and complex matrices. This finding reveals that the system can perform stably against potential ionic interferences in real samples and increases the analytical reliability of the method. The adsorption properties of the adsorbent were studied in detail using different isotherm models. The experimental data obtained were applied to the Langmuir, Freundlich, and Temkin isotherm models, and the highest correlation coefficient was found to belong to the Freundlich model. This indicates that the adsorption process occurs on heterogeneous surfaces and through a multilayer structure. The high fit to the Freundlich model indicates the presence of active sites with different binding energies on the adsorbent surface and that Au(III) ions can be adsorbed onto these surface regions in a stepwise manner. Thus, it confirms that the synthesized adsorbent offers high adsorption capacity and selectivity due to its surface functionality and morphological diversity. Within the scope of kinetic modeling, pseudo-first-order and pseudo-second-order kinetic models were applied, and it was determined that the experimental data highly conformed to the pseudo-second-order kinetic model. This result indicates that the adsorption mechanism is controlled by chemical interactions and that the rate-determining step is a chemical reaction. In other words, the interaction of Au(III) ions with the adsorbent surface is not limited to physical adsorption but also occurs through strong complexation and coordination bonds. These chemical interactions reveal that the functional groups of the adsorbent form specific coordination bonds with the target ions, thereby enabling the system to exhibit high selectivity and adsorption capacity. The determination of gold was performed using an Inductively Coupled Plasma – Optical Emission Spectroscopy (ICP-OES) device. ICP-OES is widely used in environmental analysis due to its superior analytical properties, such as multi-element determination capacity, low detection limits, wide dynamic range, and fast analysis time. In this study, analyses performed using the ICP-OES device yielded a relative standard deviation (RSD) of 2.51%, a detection limit (LOD) of 0.019 µg/L, and a quantification limit (LOQ) of 0.065 µg/L for the determination of gold ions. The obtained sensitivity parameters clearly demonstrate that the method provides high reliability and suitability for analyses at trace concentrations. Additionally, the low detection limits prove that gold ions present at trace levels can be accurately and precisely determined. Natural sample applications were conducted to test the method using real environmental samples, and tests were performed on tap water, lake water, and wastewater samples. High recovery rates were obtained in all samples, confirming that the method provides reliable results independent of matrix effects. The fact that the method can be applied with high efficiency under both laboratory and natural sample conditions confirms its validity in practical environmental analyses. In conclusion, the triazole-functionalized magnetic adsorbent with the Fe₃O₄@SiO₂@CPTES@DAT structure developed in this thesis has yielded extremely successful results in terms of selective adsorption of trace amounts of Au(III) ions and their accurate determination by ICP-OES. Characterization analyses clearly demonstrated that the obtained nanostructure possesses the desired morphological and chemical properties, while under optimized experimental conditions, significant advantages such as high adsorption capacity, effective recovery with short contact time, high selectivity, and low detection limit were achieved. Additionally, the method's applicability in environmental samples with high recovery rates demonstrates that the system can be reliably and sustainably used in real samples. Based on all these data, it is assessed that the developed adsorbent system could offer an effective alternative not only for gold but also for the monitoring and removal of valuable or toxic metal ions with similar structures.