A unifying model of cation binding by humic substances

Class: Advanced Environmental Chemistry (II)Presented by: Chun-Pao Su (Robert)

Date: 2/9/1999

• Abstract

Model V describes the binding of ions by humic substances in terms of complexation at discrete sites, modified by electrostatic attraction and repulsion, also takes account of nonspecific binding due to counterion accumulation.

The model operates over wide ranges of pH (3-11) and ionic strength (0.001-1M).

Electrostatic effects on specific binding are described with an empirical relationship involving net humic charge and an electrostatic interaction factor.

Accumulation of counterions is described by Donnan-type expressions.

Binding at the monodentate and bidentate sites is characterized by intrinsic equilibrium constants for cation-proton exchange.

Model parameters are derived from published data for fulvic-type material on proton dissociation and metal binding.

Introduction• About Humic substances

The humic substances are mixtures that are formed from transformations of biogenic organic matter.

Humic substances are recognized to interact extensively with cations in natural waters and soils.

These are polyelectric acids occurring in soils and natural waters.

Acidity of Organic Acids

An idea of the functional groups involved

• Purpose

To formulate a model of ion-binding by humic substances that could be used over a range of conditions, and to obtain parameters by analyzing published data on proton-humic and metal-humic interactions.

The study aims to place available data into a unifying framework in order to rationalize present knowledge and aid the iterative processes of further experiment and consequent model improvement.

Model Description• General

The use is made of intrinsic equilibrium constants.

The electrostatic effect due to the attraction of ions to the humic molecular surface is taken into account.

Also, the model recognizes binding due to the accumulation around the humic molecule of counterions.

• Binding sites

In model V, the only binding sites considered are composed of major proton-dissociating groups.

We ignore the possibility of the presence of small numbers of strong binding sites, perhaps containing nitrogen and sulphur atoms.

—COOH, pKint < 7

phenolic-OH, pKint 9~11

There are assumed to be eight proton-dissociating sites, described by four parameters as follows:

Metal binding can occur at single proton-dissociating sites ( monodentate), or when the single groups are sufficiently close, in bidentate.

• Electrostatics

1. The effect of electrostatic on the binding of ions at the specific sites is related to net charge on the humic molecule (Z).

K: intrinsic proton dissociation constants

w: electrostatic interaction factor

Z: net humic charge (eq/g)

2. Values of w can be derived from surface potential and molecular dimensions using electrostatic theory, e.g., the Debye-Hückel/ Gouy-Chapman theory.

P, Q: constants defining w

I: ionic strength

3. The concentration of counterions in the diffuse layer , averaged with respect to distance from the humic surface, is estimated with Donnan-type expressions.

Nomenclature

• Adjustable Parameters

For fitting proton dissociation data, Model V has seven adjustable parameters, namely, na,

pKa, pKb, pKa, pKb, P and Q.

• Calculation Procedures

Amounts of bound cations were calculated from mass- and charge-balance equations essentially. Ion activity coefficients were calculated with the extended Debye-Hückel equation for ionic strength 0.1M.

Data Sets

Model parameters were obtained by fitting published data. The contents of the data sets are summarized in the following table.

Results

• Proton Dissociation

• Binding of Metal Species

Here we concerned with metal species that have affinities for humic functional groups appreciably greatly than monovalent ions like Na+.

The pH-dependence of binding is accounted for, since this supports the assumptions about proton-metal competition.

Different humic samples studied have similar binding properties.

The values of pKMHA shown in the following table indicate that binding strength increases in the order : Mg2+ < Ca2+ < Mn2+ < Cd2+ <Co2+ < Ni2+ ~ Zn2+ < Pb2+ < Fe2+ < Cu2+ < Vo2+.

• The Distribution of Bound Metal Over the Different Sites of Model V

Conclusion Model V achieves its primary objective of

describing data for the binding of protons and a number of metals within the same framework.

Proton-Metal competition is described satisfactorily within the pH ranges covered by the data considered here, but further data are needed for proper testing of predictions of the effects of competition among metals and of ionic strength.

The essential ingredients of Model V are its discrete binding sites, common to protons and metals, and the description of the effects of electrostatic charge.

The semi-empirical description of electrostatic effects in Model V is designed to be simple, adaptable to the fitting of experimental data, and applicable to field conditions.

The charge dependence of binding at specific sites is treated by application of Debye-Hückel/Gouy-Chapman theory and more general electrical double layer theory.

The strong ionic strength dependence of some metal binding cannot be accounted for in the same way as can ionic strength effects on proton binding.

In principle, Model V could be developed to take into account both a distribution of molecular weights and a variability in the contents of proton-dissociating groups among molecules.

The key is to find the minimum degree of complexity that has to be assumed in order to give acceptable agreement between theory and observation.