The sorption of Pm (1×10-9 M) onto TiO2 and goethite has been studied as a function of pH and ionic strength (0.01, 0.1 M NaClO4) at ambient temperature under N2 atmosphere. For both minerals studied there is no effect of the different ionic strengths and the sorption (log Ka vs. pH) increases with a slope of ∼2. At the same pH sorption is lower on the more positively (or less negatively) charged goethite (pHpzc=8.4) than on the TiO2 (pHpzc=6.2). A comparison to other trivalent cations sorbed on different substrates (made by extracting values from %-sorbed curves and calculating these to Ka) indicate a similar pH-dependence. The results have been fitted using a 1-pK basic Stern model with FITEQL. An a priori restriction was that only one sorption reaction stoichiometry at a time was to be used to keep the number of parameters as small as possible. Outer-sphere and mono-, bi- and tri-dentate inner-sphere complexes were fitted. For each fit the value of the capacitance was varied to find the value that provided the best fit. Two different site densities were used: 1 site/nm2 and 2.31 sites/nm2. For the low site density the reaction which provides the best fit was: ≡SOH0.5-+Pm+3↔≡SOHPm2.5+ for both minerals. At the high site density the same reaction fits for TiO2 while there are several reactions that are equivalent in the fit for goethite. |
