Face-dependent phosphate speciation on goethite: CD-MUSIC modeling and ATR-FTIR/2D-COS study

Understanding the face-dependent phosphate adsorption mechanisms and their variations with environmental conditions is of great significance for revealing phosphate adsorption mechanisms on various goethites and predicting phosphorus speciation in iron-rich soils. In this study, micro- (MicroGoe) and nano-sized goethite (NanoGoe) were synthesized and used to investigate the face-dependent adsorption behaviors of proton and phosphate on goethite by combining the charge distribution-multisite surface complexation (CD-MUSIC) model and attenuated total reflectance Fourier transform infrared (ATR-FTIR). The results demonstrated that MicroGoe had a higher charge density and phosphate adsorption capacity than NanoGoe, which could be attributed to the higher site density of ≡FeOH^-0.5 and inner-layer capacitance arising from a higher proportion of capping face and rougher surface of MicroGoe. The logK_H of ≡FeOH^-0.5 on the main and capping face was 8.2 and 8.9, respectively. Three types of monodentate mononuclear phosphate complexes in different protonated states were identified, along with the non-protonated bidentate complex. Protonated monodentate complexes were formed at relatively low pH and high surface loadings, whereas non-protonated complexes were the predominant species at intermediate to high pH. MicroGoe had a higher percentage of monodentate complexes than NanoGoe, and both goethites had considerably lower phosphate adsorption on the capping face than on the main face. The results provide valuable insights into the interfacial reactivity of goethite prepared with various methods and facilitate further prediction of phosphorus speciation and availability in iron-rich soils.