On the Adsorption Mechanism of Humic Substances on Kaolinite and Their Microscopic Structure

Edgar Galicia-Andrés,Martin H Gerzabek,Daniel Tunega,Chris Oostenbrink

doi:10.3390/min11101138

Edgar Galicia-Andrés, Martin H Gerzabek + Show 2 more

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https://doi.org/10.3390/min11101138

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Abstract

Soil organic matter (SOM) and various inorganic minerals represent key components of soils. During pedogenesis and due to biological activity these species interact, having a crucial impact on the formation of an aggregated soil structure with a hierarchical arrangement from nano to macro scale. In this process, the formation of organo–mineral microaggregates represents a dominant factor affecting soil functions and properties. This study focuses on the interactions between humic substances (HSs) and the mineral kaolinite as typical representatives of SOM and soil minerals. By performing classical molecular dynamics (MD) simulations on models of HSs and kaolinite, we demonstrate how two dominant but chemically different kaolinite surfaces affect the stability of HSs microaggregates. By analyzing volumetric, structural, and energetic properties of SOM–kaolinite models, we explain possible mechanisms of the formation of stable SOM–clay aggregates and show how a polarized environment affects the electrostatic interactions, stabilizing the microscopic structure of SOM–mineral aggregates. Our results showed that when stable aggregates of HSs are confined in kaolinite nanopores, their interactions with kaolinite surfaces disintegrate them into smaller subaggregates. These subaggregates are adsorbed more strongly on the polar aluminol surface of kaolinite compared to less the active hydrophobic siloxane surface.

Highlights

Soil represents a heterogeneous complex system formed from a mixture of different phases
We focus on the modeling of soil organic matter (SOM)–clay interactions using models of kaolinite clay mineral that are typical for many and highly weathered soils, as well as the model of the leonardite humic acid (LHA) aggregate developed in our previous work [37,38,39]
The plateau achieved at long distances means that COO ordering is local, i.e., r < 1.15 nm, with a mean value of ~9.5 COO’s oxygen atoms per carboxylate group

Summary

Introduction

Soil represents a heterogeneous complex system formed from a mixture of different phases. The formation of different soil types during the pedogenesis is dictated by numerous factors such as the origin of parent materials, climate, vegetation, topographical relief, presence and activity of soil (micro)organisms, soil management, and time. Owing to external conditions and aging, SOM is under permanent development and transformation, leading to the formation of persistent and stable organic phases. Detailed mechanisms by which SOM persists are not completely known. Several processes such as humification, selective preservation, and progressive decomposition were suggested to be responsible for SOM stabilization [2]. The recalcitrance and persistence of SOM arise as the result of the complex interplay of molecular properties, environmental, and microbial conditions [3]

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