Granular tri-metal oxide adsorbent for fluoride uptake: Adsorption kinetic and equilibrium studies

A novel adsorbent embedding Mg-Al-Zr mixed oxides with millimetre-sized calcium alginate beads (SA-CMAZ) was synthesized, characterized, and applied for the secondary removal of fluoride from wastewater. Key factors affecting the fluoride adsorption, including initial fluoride concentration, contact time, initial pH and coexisting anions, were investigated. The results showed that fluoride could be removed by SA-CMAZ over a wide pH range, from 4 to 10. The presence of coexisting anions weakened the adsorption of fluoride, and the decreasing order of the removal towards fluoride was PO₄^3->CO₃^2->SO₄^2->NO₃^-. The adsorption follows a pseudo second order kinetic with theoretical adsorption capacity (Q_e,cal) and experimental adsorption capacity (Q_e,exp) close to each other at the temperatures of 303K, 308K, and 313K. The equilibrium data could be fitted by the Freundlich isotherm model as the SA-CMAZ is a heterogeneous. The value of the thermodynamic parameter indicated an endothermic adsorption process. A negative value shows the feasibility and spontaneity of the material-anion interaction.