Notes on the adsorption of octyl hydroxamic acid on pyrochlore and calcite

XPS measurements showed that octyl hydroxamic acid (OHA) adsorbed on pyrochlore by both chemisorption (N 1s binding energy peak at around 399 eV) and physisorption (N 1s binding energy peak at above 400 eV). The ratio of the integrated areas of the physisorption peak and chemisorption peak increased from nearly zero to about 4, and remained at 4 with increasing OHA concentrations. However, adsorption isotherm showed a continuous increase in the adsorption density of OHA. The discrepancy was found to be the result of how the OHA-treated pyrochlore was separated from the suspensions. Maximum adsorption of OHA on pyrochlore in fact remained at the second plateau (about 50 μmol/m2), i.e., N 1s peak ratio of 4, and the rest of the physisorbed OHA could be washed off. On calcite, only a small and symetrical N 1s binding energy peak could be detected, indicating only physisorbed OHA.