Electrochemical Perspective on Hematite–Malonate Interactions

Karolina Kędra,Karolina Lament,Tajana Begović,Wojciech Piasecki,Piotr Zarzycki,Marzena Łazarczyk,Danijel Namjesnik

doi:10.3390/colloids5040047

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.

Highlights

The quality of soil depends primarily on its organic matter content [1]
One of the soil organic matter (SOM) preservation mechanisms relies on SOM adsorption to mineral surfaces, which apparently limits microbial decomposition
We present an insight into hematite–malonate interactions by combining three electrochemical descriptors of the electrical double layer formed at the mineral/electrolyte interface: (a) surface potential, (b) surface charge density, and (c) zetapotential [12,13,14,15,16,17,18,19,20,21,22,23,24]

Summary

Introduction

The quality of soil depends primarily on its organic matter content [1]. The interactions of soil organic matter (SOM) with minerals governs the SOM preservation in soil, but these interactions are still poorly understood [2,3]. It remains a challenge to identify the actual modes of SOM protection by mineral surfaces because many phenomena may contribute, ranging from the SOM aggregates or co-precipitates with mineral phase to structural and physicochemical changes in SOM induced by interactions with minerals that disable some degradation pathways [4,5]. Those mechanisms set apart OM from decomposing biota, as well as limiting oxygen diffusion [6].

Methods

Results

Conclusion