The interaction of phenylphosphonic acid with the surface of goethite: Isotherms, kinetics, electrophoretic mobility and ATR-FTIR spectroscopy

The increased industrial use of phosphorus-containing substances, such as phenylphosphonic (PhP) acid, have raised significant environmental concerns. This study investigates the interaction of PhP with the surface of goethite, a ubiquitous mineral in soils and natural waters. Adsorption kinetics and isotherms are employed to examine the dynamic and equilibrium conditions of the adsorption process. DFT calculations, ATR-FTIR spectroscopy in a flow cell, and electrophoretic mobility measurements are used to determine the ionization of PhP and the type of binding involved in the PhP-goethite interaction. PhP adsorption decreased by increasing pH. Batch adsorption kinetics coincide with results from ATR-FTIR, showing that the rate-controlling step is the bond formation between PhP and the surface. ATR-FTIR with the flow cell proves to be very powerful for elucidating the process, because the time evolution of the adsorption extent and identity of the adsorbed species can be simultaneously tracked. Regardless of PhP speciation in solution and adsorption time, the adsorbed species is consistently an inner-sphere surface complex, where the phosphonate group establishes a direct P-O-Fe bond with an Fe(III) cation on the surface. These findings provide a comprehensive understanding of the PhP-goethite interaction, offering valuable insights into the environmental mobility of PhP.