『Abstract
Soil organic matter consists of a highly complex and diversified
blend of organic molecules, ranging from low molecular weight
organic acids (LMWOAs), sugars, amines, alcohols, etc., to high
apparent molecular weight fulvic and humic acids. The presence
of a wide range of functional groups on these molecules makes
them very reactive and influential in soil chemistry, in regards
to acid-base chemistry, metal complexation, precipitation and
dissolution of minerals and microbial reactions. Out of these
functional groups, the carboxylic and phenolic ones are the most
abundant and most influential in regards to metal complexation.
Therefore, chemical equilibrium models have progressively dealt
with organic matter in their calculations.
This paper presents a review of six chemical equilibrium models,
namely NICA-Donnan, EQ3/6,
GEOCHEM, MINTEQA2, PHREEQC and WHAM, in light
of the account they make of natural organic matter (NOM) with
the objective of helping potential users in choosing a modelling
approach. The account has taken various faces, mainly by adding
specific molecules within the existing model databases (EQ3/6, GEOCHEM, and PHREEQC) or by using either a discrete (WHAM) or a continuous (NICA-Donnan
and MINTEQA2) distribution of the deprotonated
carboxylic and phenolic groups.
The different ways in which soil organic matter has been integrated
into these models are discussed in regards to the model-experiment
comparisons that were found in the literature, concerning applications
to either laboratory or natural systems. Much of the attention
has been focused on the two most advanced models, WHAM
and NICA-Donnan, which are able to reasonably
describe most of the experimental results. Nevertheless, a better
knowledge of the humic substances metal-binding properties is
needed to better constrain model inputs with site-specific parameter
values. This represents the main axis of research that needs to
be carried out to improve the models. In addition to humic substances,
more non-humic compounds should also be introduced in model databases,
notably the ones that readily interact with the soil microorganisms.
Thermodynamic data are generally available for most of these compounds,
such as low molecular-weight organic acids. However, the more
complex non-humic substances, exhibiting a ratio of hydrophobic
versus hydrophilic bonds lower than humic substances, need to
be further characterised for a comprehensive implementation in
chemical equilibrium models.
Keywords: Modelling; Metal complexing; Organic acids; Humic acids;
Soils; Aqueous solutions
1. Introduction
2. Main chemical equilibrium processes
2.1. Basic speciation and phase equilibrium
2.2. Surface interaction equilibrium
3. Why should NOM be accounted for in chemical equilibrium speciation
models?
3.1. The nature of NOM
3.2. The properties of NOM
3.3. The functions of NOM
4. How do the existing models cope with NOM?
4.1. Non-humic substances
4.2. Humic substances
4.2.1. Continuous site distribution−the NICA-Donnan
model
4.2.2. Discrete site distribution−the WHAM model
5. Has organic matter been well accounted for? Examples of model
applications
5.1. Laboratory applications
5.2. Model applications to natural systems
5.2.1. Analogue models
5.2.2. Humic ion-binding models
5.2.2.1. On the need and difficulties to constrain field parameters
5.2.2.2. Lack of knowledge of NOM speciation and binding properties
5.2.2.3. Applications supported by a better knowledge of HS-binding
properties
5.2.2.4. Applications with some consideration on non-humic
substances
6. How could organic matter be better accounted for in chemical
equilibrium models? Some perspectives
Acknowledgements
References