Retention of Gallium(III) by Hematite: Batch Sorption, Local Structure Analysis, and Surface Complexation Modeling

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Abstract
Gallium(III) sorption onto hematite was investigated as a function of pH using batch sorption experiments, precipitation controls, zeta potential measurements, extended X-ray absorption fine structure spectroscopy, and thermodynamic surface complexation modeling. Sorption experiments at initial Ga(III) concentrations of 40 and 100 µM showed a steep sorption edge at low pH and nearly complete gallium retention over a broad pH range, from pH 3.5–8.0. Parallel precipitation experiments and thermodynamic calculations demonstrated that bulk precipitation alone cannot account for the observed retention behavior, particularly at low pH and under alkaline conditions where hematite strongly immobilizes gallium. Zeta potential measurements revealed a systematic increase in surface charge in the presence of Ga(III), indicating strong specific interaction with the hematite surface. EXAFS spectroscopy showed a highly consistent local coordination environment over the entire investigated pH range, characterized by octahedral Ga–O coordination and distinct Ga–Fe backscattering paths. The spectroscopic results indicate a structurally robust retention mechanism and are consistent with either strongly bound inner-sphere surface complexes or partial incorporation of Ga(III) into near-surface Fe(III) sites of the hematite structure. Surface complexation modeling reproduced the sorption data for both initial concentrations using a single hydrolyzed Ga surface species and lead to a logK° value of −1.23 ± 0.04 for the surface complexation formation. The combined results demonstrate that Ga(III) is strongly retained by hematite over a broad pH range, leading to substantial stabilization of gallium surface species relative to dissolved species and bulk hydroxide precipitation.

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