TY - GEN A1 - Ahmed, Ashour A. A1 - Gypser, Stella A1 - Freese, Dirk A1 - Leinweber, Peter A1 - Kühn, Oliver T1 - Molecular level picture of the interplay between pH and phosphate binding at the goethite-water interface T2 - Physical Chemistry Chemical Physics N2 - The soil pH plays a substantial role in controlling phosphorus (P) adsorption and mobilization. These processes are strongly affected by the phosphate interaction strength with P-fixing soil minerals such as goethite. The target of the current contribution is to draw a molecular level picture of the interplay between pH and phosphate binding at the goethite–water interface via a joint experimental–theoretical approach. Periodic density functional theory (DFT) calculations were carried out to provide a molecular level understanding of the pH dependence of P adsorption. To validate the modeling approach, adsorption experiments of phosphate at goethite were performed in the pH range of 4–12. There was agreement between experiments and simulations in the description of the adsorption behavior by two pH-dependent successive stages. The adsorption increases along the pH change from 4 to 8. A further increase of pH leads to a decrease of adsorption. By comparing with literature data it is concluded that the first stage will be observed only if there is no significant change of the surface charge at low pH. Moreover, the molecular modeling results point to the abundance of the monodentate (M) binding motif at both extremely low and high pH ranges. Otherwise, the bidentate (B) one is predominant along the intermediate pH range. These observations could resolve the existing debate about the assignment of phosphate–goethite binding motifs. Furthermore, the results point to a decrease of pH upon phosphate sorption due to an induced acidification of soil solution. The present joint experimental–theoretical approach provides a better understanding and description of the existing phosphate sorption experiments and highlights new findings at the atomistic/molecular scale. KW - Phosphate KW - Goethite KW - Periodic density functional theory KW - Adsorption KW - pH Y1 - 2020 U6 - https://doi.org/10.1039/d0cp04698a SN - 1463-9084 VL - 22 IS - 45 SP - 26509 EP - 26524 ER - TY - GEN A1 - Ahmed, Ashour A. A1 - Gypser, Stella A1 - Leinweber, Peter A1 - Freese, Dirk A1 - Kühn, Oliver T1 - Infrared Spectroscopic Characterization of Phosphate Binding at the Goethite-Water Interface T2 - Physical Chemistry Chemical Physics N2 - The interaction between phosphates and soil mineral surfaces, such as Fe- and Al-(oxyhydr)oxides, plays a crucial role in the P immobilization and thus its availability for plants. The reactions of phosphates with Fe-hydroxides and especially goethite have been studied extensively. But a molecular-level picture about the phosphate binding mechanism at the goethite-water interface is still lacking. Therefore, in the current contribution we have explored the molecular binding mechanism for the adsorbed phosphate at the goethite–water interface by performing sorption kinetics experiments for orthophosphate and characterizing the adsorbed species by FT-IR spectroscopy. In parallel, periodic DFT calculations have been performed to explore the interaction mechanism as well as to calculate the IR spectra for monodentate (M) and bidentate (B) orthophosphate complexes at two different goethite surface planes (010 and 100) in the presence of water. In general, our interaction energy results give evidence that the mono-protonated B phosphate complex is more favored to be formed at the goethite–water interface although the M motif could exist as a minor fraction. Moreover, it was found that water plays an important role in controlling the phosphate adsorption process at the goethite surfaces. The interfacial water molecules form H-bonds (HBs) with the phosphate as well as with the goethite surface atoms. Further, some water molecules form covalent bonds with goethite Fe atoms while others dissociate at the surface to protons and hydroxyl groups. The present theoretical assignment of IR spectra introduces a benchmark for characterizing experimental IR data for the adsorbed KH2PO4 species at the goethite–water interface. In particular, IR spectra of the mono-protonated (2O+1Fe) B complex at the 010 goethite surface plane and the M complex at the 100 goethite surface plane were found to be consistent with the experimental data. In order to explore the role of different abundancies of surface planes and binding motifs, IR spectra obtained from weighted averages have been analyzed. Results confirmed the above conclusions drawn from interaction energy calculations. Y1 - 2019 U6 - https://doi.org/10.1039/C8CP07168C SN - 1463-9084 SN - 1463-9076 VL - 21 SP - 4421 EP - 4434 ER - TY - GEN A1 - Ahmed, Ashour A. A1 - Gypser, Stella A1 - Leinweber, Peter A1 - Freese, Dirk A1 - Kühn, Oliver T1 - New insights into IR spectroscopic characterization of phosphate binding at the goethite-water interface T2 - IPW9, ETH Zurich, Switzerland, 8-12 July, 2019 - Abstracts N2 - Facing the projected peak phosphorus (P) scenario, substantial research activities have been triggered to improve our knowledge towards a more efficient and sustainable use of P resources. One major factor playing a role in the P immobilization and thus its availability for plants is the strong interaction between phosphates and soil mineral surfaces, such as Fe- and Al-(oxyhydr)oxides. The reactions of phosphates with Fe-hydroxides and especially goethite have been studied extensively. But a molecular-level picture of the phosphate binding mechanism at the goethite-water interface is still lacking. Therefore, in the current contribution we have explored the molecular binding mechanism for the adsorbed phosphate at the goethite–water interface by performing sorption kinetics experiments for orthophosphate and characterizing the adsorbed species by FT-IR spectroscopy. In parallel, periodic quantum mechanical calculations, at the density functional theory (DFT) level, have been performed to explore the interaction mechanism as well as to calculate the IR spectra for monodentate (M) and bidentate (B) orthophosphate complexes with goethite at two different goethite surface planes (010 and 100) in the presence of water. In general, our interaction energy results give evidence that the mono-protonated B phosphate complex is more favored to be formed at the goethite–water interface although the M motif could exist as a minor fraction. Moreover, it was found that water plays an important role in controlling the phosphate adsorption process at the goethite surfaces. The interfacial water molecules form H-bonds (HBs) with the phosphate as well as with the goethite surface atoms. Further, some water molecules form covalent bonds with goethite Fe atoms while others dissociate at the surface to protons and hydroxyl groups. The present theoretical assignment of IR spectra introduces a benchmark for characterizing experimental IR data for the adsorbed KH2PO4 species at the goethite–water interface. In particular, IR spectra of the mono-protonated (2O+1Fe) B complex at the 010 goethite surface plane and the M complex at the 100 goethite surface plane were found to be consistent with the experimental data. In order to explore the role of different abundancies of surface planes and binding motifs, IR spectra obtained from weighted averages have been analyzed. Results confirmed the above conclusions drawn from interaction energy calculations. Moreover, the soil solution pH effect on the phosphate binding mechanism to goethite surface has been explored as well. Y1 - 2019 UR - https://pure.au.dk/portal/files/159039284/IPW9_AbstractBook.pdf SP - 108 CY - Zürich ER -