Effects of pH, Ca- and SO₄-concentration on surface charge and colloidal stability of goethite and hematite - Consequences for the adsorption of anionic organic substances

authored by

J. Walsch, S. Dultz

Abstract

Soil solution chemistry, especially pH and the presence of multivalent ions, affects the surface charge (SC) of Fe oxides and accordingly colloidal stability and sorption properties. The SC of synthetic goethite and hematite was quantified in the presence of different electrolytes (NaCl, CaCl₂, Na₂SO₄ and CaSO₄) by combining the streaming potential with polyelectrolyte titration. The point of zero charge (PZC) for goethite was observed at pH 8.2 and the stability field around the PZC, where colloids are flocculated, is more extended (±1 pH unit) than that of hematite with a PZC at pH 7.1 (±0.5 pH unit). The SC decreases with increasing SO₄ concentration, indicating adsorption of SO₄ on the oxide, whereas the presence of Ca increases the SC. At pH 4, the addition of 0.1 mmol l^-1 Na₂SO₄ induced a decrease in SC from 1.5 to 0.380 μmol_c m^-2 for goethite and from 0.85 to 0.42 μmol_c m^-2 for hematite. In a suspension with 0.1 mmol l^-1 Na₂SO₄, the number of colloids is already reduced, and both oxides flocculate rapidly and completely at >0.5 mmol l^-1 Na₂SO₄. While the addition of SO₄ did not affect charge titrations with the cationic polyelectrolyte, the anionic polyelectrolyte formed complexes with Ca, resulting in an overestimation of positive SC. The electrolyte CaSO₄ is most efficient at keeping goethite and hematite in the pH range 4-10 in the flocculated state. Besides pH, the presence of multivalent ions should also be considered when predicting colloid mediated transport and adsorption properties of anionic substances by Fe oxides in soil systems.

Organisation(s)

Institute of Soil Science
Section Soil Chemistry

Type

Article

Journal

Clay minerals

Volume

45

Pages

1-13

No. of pages

13

ISSN

0009-8558

Publication date

01.03.2010

Publication status

Published

Peer reviewed

Yes

ASJC Scopus subject areas

Geochemistry and Petrology

Electronic version(s)

https://doi.org/10.1180/claymin.2010.045.1.01 (Access: Unknown)