Understanding the impact of MgO crystal facet on phosphorus adsorption and recovery from water: an experimental and theoretical computational study

MgO-based adsorbents have been extensively utilized for phosphorus (P) resource recovery from wastewater. However, the microscopic interfacial adsorption mechanisms of P on different MgO crystal facets remain insufficiently understood. Elucidating interfacial mechanisms is crucial for the directional regulation of optimized-performance MgO-based adsorbent development. Herein, three MgO materials, octahedral MgO (OC-MgO), cube MgO (CU-MgO) and hexagonal MgO (HE-MgO) were synthesized and tested for P adsorption and recovery from water. OC-MgO primarily exposes the (111) facet, CU-MgO exhibits the (100) facet, whereas HE-MgO displays both the (111) and (110) facets, with exposure ratios of about 69.5 % and 30.5 %, respectively. Batch experiment results proved that P adsorption performance correlated with the exposed crystal surface. The saturated P uptake capacities are 403.44, 278.04, and 221.47 mg/g for OC-MgO, CU-MgO, and HE-MgO, respectively, at a solution pH of 7.0, as determined by Sips model fitting. However, CU-MgO shows a faster P capture rate and a stronger resistance to pH interference than the other two MgOs. The combination of model fitting, Zeta potential analysis and X-ray photoelectron spectroscopy (XPS) characterization demonstrated that the P adsorptive mechanism involved synergistic monolayer and multilayer adsorption, driven by surface complexation, ligand exchange, and electrostatic attraction. In addition, the density functional theory (DFT) calculations further provided mechanistic insights into facet-dependent stability and adsorption behaviors. The MgO (100) facet can stably exist in aqueous solutions, and its primary P adsorption mechanism involves chemisorption driven by monodentate-mononuclear, bidentate-mononuclear and bidentate-binuclear complexation and lattice oxygen replacement. In contrast, the MgO (111) and (110) facets required hydroxylation to maintain structural stabilization in water. On these hydroxylated surfaces, their P capture mechanisms are predominantly governed by electrostatic attraction and –OH group replacement. This study elucidates the fundamental structure–property relationships between MgO crystal facets and P adsorption behavior, revealing facet-dependent stabilization mechanisms in aqueous environments and their associated P capture pathways. These findings provide novel perspective of surface design for developing MgO-based adsorbents for P capture and recovery from water.