COMPETITIVE ADSORPTION OF SULFATE AND OXALATE ON GOETHITE IN THE ABSENCE OR PRESENCE OF PHOSPHATE

Abstract

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.