A study was carried out to examine the competition in adsorption of sulfate (SO4) and oxalate (OX) on goethite as affected by pH, concentration of the ligands, order of anion addition, and the presence of phosphate (PO4). The purpose was to obtain useful information about the factors that influence the adsorption of ligands, which have different affinity (PO4 >> OX > SO4) for this variable charge mineral. More OX than SO4 was adsorbed for the pH values between 3.0 and 7.5. When equimolar amounts of SO4 and OX were added, the molar ratio of adsorbed OX to adsorbed SO4 ranged from 2.25 to 3.78 in the pH range between 3.5 and 6.0. The sequence of anion addition influenced the adsorption of SO4 and OX only at pH <5.0. Each ligand prevented the adsorption of the other anion mainly when it was added first to goethite (SO4before OX or OX before SO4 systems). In the SO4before OX system, the greatest inhibition of OX adsorption by SO4 occurred at pH 3.0 (34%). There were many indications that at low pH values, some SO4 ions were so strongly adsorbed by the positively charged surface of goethite that they were able to compete efficaciously with OX. At pH 4.0 and at an initial OX/SO4 molar ratio of 3.33, SO4 adsorption decreased by 72.0%, whereas at an initial SO4/OX molar ratio of 3.33, the OX adsorption decreased by 38.7%.
The addition of PO4 facilitated SO4 and, to a lesser extent, OX desorption. In the pH range of 3.0 to 4.5, 85 to 25% of the OX adsorbed when added first with SO4 remained adsorbed on the goethite after the addition of equimolar amounts of PO4. In this range of pH, even large amounts of PO4 did not desorb OX from the surfaces of goethite completely, indicating that at low pH values, OX ligands form particularly strong complexes on some sites of goethite.
At low pH values, the total amounts of PO4, OX, and SO4 adsorbed on goethite were significantly greater than the quantities of PO4 adsorbed when added alone.