Molecular-scale determination of facet- and adsorbent-dependent phosphate adsorption by metal-based adsorbents

Phosphate (P) adsorption systems featuring metal-based adsorbents have been claimed to achieve ultralow P removal. However, little is known about micron- and molecular-scale energetic information on P-adsorbent interactions including facet- and adsorbent-dependent adsorption of P. Herein, representative metal-based adsorbents including an iron (Fe)-based hematite material with different facets and a cerium (Ce)-based Ce-bound peptide material are utilized to characterize their facet- and adsorbent-dependencies. By exposure to a simple P solution, the adsorption experiment is performed on surfaces of hematite and Ce-bound peptide followed by the quantification of adsorbed P at the macroscopic scale and the Kelvin potential at the nanoscale. Furthermore, AFM-based force spectroscopy (FS) techniques are used to in situ map and measure the binding strengths of phosphate group-adsorbents through a functionalized AFM tip at the molecular scale by systematically changing the aqueous environment. The molecular-scale determinations of phosphate group-hematite/Ce-bound peptide interactions provide the energetic basis for the adsorption capacity of different adsorbent surfaces, which follows the trend Ce-bound peptide > the (1010) > (1120) > (0001) surfaces of hematite. Moreover, AFM-based DFS measurements further reveal that a higher P adsorption preferentially occurs in a more acidic or lower salt concentration solution condition. Changing the medium from a simple P solution to a complex P solution does not negatively affect the higher adsorption capacity of the Ce-bound peptide compared to hematite, which is dominated by an inner-sphere complexation mechanism through a ligand exchange process. Overall, these direct measurements of the molecular-scale binding free energy provide mechanistic insights into P-adsorbent interactions for achieving high P adsorption, which could potentially be used to characterize P adsorption systems that explicitly include different metal-based adsorbents.