Eggshell-Derived Fe-Mg Particles and Hydroxyapatite for Removal of Tetracycline and Metronidazole from Aqueous Systems

Abstract

In this study, eggshells were upgraded to two active adsorbents, iron (Fe)-magnesium (Mg) particles and hydroxyapatite (HAP), and their subsequent utilization for the elimination of tetracycline (TET) and metronidazole (MNZ) from aqueous systems was explored. As a first step, the strength of the functionalization solution (0.5 M) and synthesis temperature (600°C) were optimized for the improved performance of the Fe-Mg particles. Through batch experiments, the maximum TET and MNZ adsorption capacities (qm) were estimated as 9.47 and 6.29 mg/g, respectively, for the Fe-Mg particles. In the case of HAP, the qm were 8.70 and 6.07 mg/g for TET and MNZ, respectively. Monolayer and chemisorption phenomena were identified as the mechanisms for TET's removal with both Fe-Mg particles and HAP. By contrast, monolayer and physisorption were responsible for MNZ's removal with both adsorbents. In a binary system, i.e., in the presence of both TET and MNZ, the antagonistic effect was observed, which hindered the efficient removal of one by the other. The van der Waals force was involved in MNZ adsorption, whereas the interaction between the hydrogen (H) atoms of the phenolic diketone group(s) of TET and the anions of Fe-Mg particles/HAP favored TET's removal. In the recyclability study, both adsorbents performed well for the first 20 cycles, but thereafter the removal efficiency reduced drastically. Overall, eggshell waste can be managed effectively by converting it into adsorbents, which can be a viable alternative for adsorption-based water/wastewater treatment. The release of emerging contaminants from conventional wastewater treatment plants has been a major environmental concern in recent years. Consequently, these contaminants find their way into environmental matrices, affecting organisms at various scales by disrupting their metabolic activities. Advanced treatment techniques like ozonation, sonocatalysis, photocatalysis, adsorption, electrochemical oxidation, and Fenton processes are effective in the removal of emerging contaminants. Specifically, the current focus is on adsorption, which is scalable and cost-effective. Moreover, the use of naturally derived adsorbents promotes sustainability of the process. However, the applicability of these adsorbents varies depending on their availability and ease of synthesis. In this context, eggshells from confectionary and poultry processing industries can be a suitable alternative for synthesizing naturally derived adsorbents. This study successfully evaluated eggshell-based adsorbent synthesized utilizing the holistic components of eggshells (Fe-Mg particles derived from the inner parietal membrane and hydroxyapatite synthesized from the hard calcareous shell) for the removal of commonly used pharmaceutical compounds (i.e., tetracycline and metronidazole as single and binary systems). The possible removal mechanism, recyclability, and cost calculations are presented and the major gap of competitive adsorption mimicking real-time wastewater was also explored.