Adsorption of tetracycline using silica nanoparticles: a comparative study of sodium silicate-derived and fly ash-derived silica nanoparticles

• Silica nanoparticles synthesized from fly ash (FA-SiNPs) and sodium silicate (SSSNPs) • FA-SiNPs were modified with iron to produce Fe-SiNPs • Fe-SiNPs showed the highest tetracycline (TC) removal efficiency • TC adsorption capacity of Fe-SiNPs reached 32.31 mg/g • Adsorption on Fe-SiNPs involved electrostatic forces, chemisorption, and H-bonding Fly ash (FA), a fine particulate residue produced from the combustion of coal in thermal power plants, is generated in substantial quantities and presents significant challenges related to its environmental management and disposal. Repurposing FA can mitigate waste accumulation while contributing to sustainable environmental solutions. One promising approach is utilising FA as an adsorbent for removing pollutants from aqueous solutions, particularly emerging contaminants (ECs). In this study, silica nanoparticles were synthesised from FA (FA-SiNPs) and subsequently modified with iron to produce Fe-SiNPs. As a control, silica nanoparticles were synthesised directly from a sodium silicate solution (SSSNPs). The different silica nanoparticles were used as adsorbents to remove tetracycline (TC) from aqueous solutions under identical conditions, to compare their adsorption performance. Under the identical conditions, Fe-SiNPs demonstrated superior performance, achieving 59 % removal of TC, compared to 30 % and 20 % removal by FA-SiNPs and SSSNPs, respectively. The enhanced adsorption capacity of Fe-SiNPs was attributed to the presence of iron, which facilitated TC removal through chelation. Upon optimisation of experimental parameters, a maximum TC removal efficiency using Fe-SiNPs of 86 % was observed at the optimal dosage of 5 g/L within a pH range of 4 to 5. The adsorption kinetics were best described by the Elovich model, whereas the equilibrium data fitted the Langmuir isotherm model with a maximum adsorption capacity of 32.31 mg/g at 30 °C. Thermodynamic analysis revealed that the adsorption process was both spontaneous and exothermic in nature. The adsorption of TC on Fe-SiNPs involved chemisorption, electrostatic attraction and hydrogen bonding. This study highlights the potential of FA-derived silica nanoparticles, particularly Fe-SiNPs, as cost-effective and sustainable adsorbents for TC removal from contaminated water.