The competitive adsorption of phosphate with natural organic matter on hydrous ferric oxide colloids as investigated by chemical force microscopy

The adsorption interactions of phosphates and carboxylic acids on the surfaces of hydrous ferric oxide (FeO(OH)) colloids were investigated using chemical force microscopy (CFM) force titration experiments. Infrared spectroscopy and zeta potentiometry were employed to characterize the substrates used in the CFM experiments. Alkylthiol self-assembled monolayer (SAM) methods were used to prepare the chemically modified AFM tips and control surfaces. Phosphate modified atomic force microscope tips were prepared by deposition of the compounds bis-(11-thioundecyl) phosphate and 11-thioundecylphosphate, and derivatization with carboxylic acid groups was achieved using 16-thiohexadecanoic acid.; The pK_a values of the phosphate SAM species were determined by force titrations on control surfaces and these were found to be 5.0 for the monoprotic phosphate SAM species, and 4.8 and 8.4 for the diprotic phosphate SAM. The monoprotic phosphate (PO₂H) and carboxylic acid (CO ₂H) probes were both seen to have strong forces of adhesion in their interactions on FeO(OH) and showed single broad force titration features that were centred at about pH 7. The interactions of the CO₂H probe extended over a broader pH range than did those of the PO₂H probe, indicating that it interacted with a wider distribution of surface functional groups. The diprotic phosphate probe was seen to have two main peaks in its force titration on FeO(OH), with maxima at pH values of approximately 5.0 and 8.0. This result is in agreement with models of phosphate specific adsorption from solution to the FeO(OH) surface via reaction with A-type hydroxyl sites. It was also observed that longer range electrostatic interactions make negligible contributions to the CFM results.; Force titrations conducted on colloid surfaces that had been post-precipitated with orthophosphate, dimethylphosphate, and gallic, tannic and humic acids showed that the various surface-adsorbed species strongly reduced the mid-pH interactions of the probes with FeO(OH) and introduced new force titration features which were associated with hydrogen bonding of the probe groups with latent functional groups of the surface-adsorbed species. A PO₂H force titration was performed on tannic acid post-precipitated colloids that were treated with FeCl₃ so that Fe(III) ions were complexed within the organic overlayer. The result of this experiment was consistent with models of bridging humic-Fe3+—phosphate interactions in Podzolic soils.