FeOOH and (Fe,Zn)OOH hybrid anion exchange adsorbents for phosphate recovery
A determination of Fe-phases and adsorption–desorption mechanisms
C. Belloni (Wetsus, European Centre of Excellence for Sustainable Water Technology, TU Delft - RST/Fundamental Aspects of Materials and Energy)
L. Korving (Wetsus, European Centre of Excellence for Sustainable Water Technology)
G. J. Witkamp (King Abdullah University of Science and Technology)
EH Brück (TU Delft - RST/Fundamental Aspects of Materials and Energy)
P. de Jager (Aquacare Europe, 'sHertogenbosch)
AI Dugulan (TU Delft - RID/TS/Instrumenten groep, TU Delft - RST/Fundamental Aspects of Materials and Energy)
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Abstract
Hybrid anion exchange adsorbents (HAIX) seem promising to prevent eutrophication and recover phosphate (P). HAIX consist of an anion exchange resin (AIX) backbone, promoting anion physisorption (outer-sphere complex), impregnated with iron (hydr)oxide nanoparticles (NPs), for selective P chemisorption (inner-sphere complex). In this work, for the first time, as far as we know, Zn-doped iron (hydr)oxide NPs were embedded in AIX, and the performances compared with conventional HAIX, both commercial and synthesized. Zn-doped HAIX displayed improved P adsorption performances. Mössbauer spectroscopy (MS) revealed the goethite nature of the NPs, against the “amorphous hydrous ferric oxide” claimed in literature. The P adsorption comparisons, made in synthetic solution and real wastewater, underlined the crucial role of the NPs for selective P adsorption, while improving the understanding on the competition between physisorption and chemisorption. In pure P synthetic solutions, especially at high P concentrations, physisorption can “hide” chemisorption. This depends also on the anion form of the AIX, due to their higher affinity for multivalent anions, which affects HAIX adsorption selectivity and P desorption. In fact, a mild alkaline regeneration over three adsorption–desorption cycles revealed a complex interaction between the regenerant OH− and the adsorbed P. OH− molecules are consumed to transform phosphate speciation, causing (stronger) P re-adsorption and preventing desorption. Finally, Mössbauer spectroscopy revealed NPs agglomeration/growth after the three cycles plus final regeneration at pH 14. This study provides further understanding on the P adsorption–desorption mechanism in HAIX, drawing attention on the choice of experimental conditions for reliable performance assessment, and questioning HAIX consistent P removal and efficient P recovery in the long-term.
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