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.
Bibliographical noteKAUST Repository Item: Exported on 2023-09-06
Acknowledgements: This work was performed in the cooperation framework of Wetsus, European Centre of Excellence for Sustainable Water Technology (www.wetsus.nl). Wetsus is co-funded by the Dutch Ministry of Economic Affairs and Ministry of Infrastructure and Environment, the European Union Regional, Development Fund, the Province of Fryslân and the Northern Netherlands Provinces. This research received funding from the Netherlands Organization for Scientific Research (NWO) in the framework of the Innovation Fund for Chemistry, and from the Ministry of Economic Affairs and Climate Policy in the framework of the TKI/PPS-Toeslagregeling. The authors thank the participants of the research theme “Phosphate recovery” for the interest, fruitful discussions, and financial support. A special thanks goes to Raimonda Buliauskaitė (Aquacare) for the frequent knowledge exchange and interest in the research, Michel Steenvoorden and Maxim Ariens (TU Delft) for the support with Mössbauer spectroscopy related matters, Antony Cyril Arulrajan, Thomas Prot, Wokke Wijdeveld, Amanda Larasati, Sam Rutten, Daniele Chinello, Qingdian Shu (Wetsus) for the brainstorming sessions and insightful discussions, Gwendolina van der Linden for the work done together.
ASJC Scopus subject areas
- Environmental Chemistry
- Chemical Engineering(all)
- Industrial and Manufacturing Engineering