Selective removal of Fe and Al by raw and functionalized rice husk silicas as a pre-treatment step of phosphogypsum leachates for subsequent recovery of rare earth elements

Rare earth elements (REE) recovery from phosphogypsum by acid leaching has already proven effective. However, the obtained leachates often contain high concentrations of interfering ions, such as Fe³⁺ and Al³⁺, which hinder subsequent REE recovery steps. This work proposes a novel pre-treatment approach for phosphogypsum leachates, focusing on the removal of Fe³⁺ and Al³⁺ through adsorption using raw rice husk-derived silica (Si) and rice husk-derived silica functionalized with 3-aminopropyl triethoxysilane (Si-APTS) and 2,6-pyridinedicarboxylic acid (Si-APTS-CO). The characterization results demonstrated the success of the silica production and functionalization. The Fe³⁺ and Al³⁺ adsorption onto the raw and functionalized rice huks-derived silicas was highly pH-dependent, reaching 30 % Fe³⁺ removal at pH 5.0 and 90 % Al³⁺ removal at pH 5.5. The adsorption of Fe³⁺ ions was faster, and the pseudo-second-order model demonstrated greater suitability to the kinetic data of both metal ions. The Freundlich model fitted the equilibrium data well. The highest q_e values for Fe³⁺ and Al³⁺ were, respectively, 34.16 mg g⁻¹ (with Si-APTS-CO) and 32.42 mg g⁻¹ (with Si-APTS) at 333 K and an adsorbent dosage of 1.5 g L⁻¹. Functionalization changed the thermal behavior, with Si showing exothermic adsorption, while the functionalized silicas exhibited endothermic adsorption. The adsorbents demonstrated promising potential to remove Fe³⁺ and Al³⁺ from a real-world phosphogypsum leachate. In addition, they did not show affinity for the REE³⁺ ions, meaning that there are no losses of these elements in the proposed pre-treatment step.