Abstract
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•Synergistic quaternization/phosphonation of algal/PEI beads favors W(VI) binding.•Langmuir dual site and Sips models for fitting isotherm for bi-functional sorbent.•Fast kinetics (≈60–90 min) fitted by pseudo-second order rate equation.•High stability in sorption and desorption performance (at 5th recycling).•Selectivity for W against base and alkali-earth metals (in real effluent).
Algal/polyethyleneimine composite beads (ALPEI) have been successfully functionalized by grafting phosphonate (ALPEI-P), quaternary ammonium groups (ALPEI-Q), and by dual moieties (ALPEI-PQ). The materials are characterized by a wide diversity of analytical methods: SEM and SEM-EDX, TGA, BET, FTIR and XPS, elemental analysis and titration (detailed discussion in Supplementary Information). FTIR and XPS analyses show contributions of quaternary ammonium, hydroxyl, and phosphonate groups in tungstate binding (which may involve polynuclear species, depending on the pH). The synergistic effects of phosphonate and quaternary ammonium moieties contribute to strongly increase W(VI) sorption capacity up to 1.8 mmol W g−1 (i.e., 4-times compared with pristine beads), at pH 5. The strong increase of sorption properties with temperature (2.45 mmol W g−1 at T: 50 °C) demonstrates the endothermic nature of metal binding, which is spontaneous (and characterized by positive entropy change). The Langmuir Dual Site equation successfully fits sorption isotherm for functionalized sorbents (bearing phosphonate and phosphonate/quaternary ammonium groups in addition to proper reactive groups of pristine beads) contrary to the isotherms for raw beads (fitted by Langmuir equation). The pseudo-first order rate equation fits kinetics (equilibrium reached within ≈60 min). The bi-functional sorbent shows marked preference for W(VI) against base and alkali-earth metals (especially at pH close to 3.5). These properties are confirmed in the treatment of raffinate solutions (after ore leaching and solvent extraction): despite the complexity of the effluent, ALPEI-PQ reveals highly efficient for W(VI) recovery and separation. The sorbent can be used as a polishing treatment for enhancing the recovery of strategic metals. Sorbed tungstate is readily desorbed from the sorbent using 0.2 M NH4Cl solution. The sorbent is efficiently recycled for at least 5 cycles (complete desorption, loss in sorption performance less than 4% at the fifth cycle).