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

Abstract

AbstractSoil 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, Na2SO4and 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 SO4concentration, indicating adsorption of SO4on the oxide, whereas the presence of Ca increases the SC. At pH 4, the addition of 0.1 mmol l–1Na2SO4induced a decrease in SC from 1.5 to 0.380 μmolcm–2for goethite and from 0.85 to 0.42 μmolcm–2for hematite. In a suspension with 0.1 mmol l–1Na2SO4, the number of colloids is already reduced, and both oxides flocculate rapidly and completely at >0.5 mmol l–1Na2SO4. While the addition of SO4did not affect charge titrations with the cationic polyelectrolyte, the anionic polyelectrolyte formed complexes with Ca, resulting in an overestimation of positive SC. The electrolyte CaSO4is 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.