Electrochemical Perspective on Hematite–Malonate Interactions

Abstract

Organic matter (OM) interactions with minerals are essential in OM preservation against decomposition in the environment. Here, by combining potentiometric and electrophoretic measurements, we probed the mode of coordination and the role of pH-dependent electrostatic interactions between organic acids and an iron oxide surface. Specifically, we show that malonate ions adsorbed to a hematite surface in a wide pH window between 3 and 8.7 (point of zero charge). The mode of interactions varied with this pH range and depended on the acid and surface acidity constants. In the acidic environment, hematite surface potential was highly positive (+47 mV, pH 3). At pH < 4 malonate adsorption reduced the surface potential (+30 mV at pH 3) but had a negligible effect on the diffuse layer potential, consistent with the inner-sphere malonate complexation. Here, the specific and electrostatic interactions were responsible for the malonate partial dehydration and surface accumulation. These interactions weakened with an increasing pH and near PZC, the hematite surface charge was neutral on average. Adsorbed malonates started to desorb from the surface with less pronounced accumulation in the diffuse layer, which was reflected in zeta potential values. The transition between specific and non-specific sorption regimes was smooth, suggesting the coexistence of the inner- and outer-sphere complexes with a relative ratio that varied with pH.

Keywords

• ion adsorption
• dicarboxylate ions
• iron oxide α-Fe₂O₃
• hematite
• electrical double layer
• surface potential