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 (61 pH unit) than that of hematite with a PZC at pH 7.1 (60.5 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 μmolc m–2 for goethite and from 0.85 to 0.42 μmolc 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.

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