A unifying model of cation binding by humic substances

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A unifying model of cation binding by humic
substances

• Class Advanced Environmental Chemistry (II)
• Presented by Chun-Pao Su (Robert)
• Date 2/9/1999

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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).

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• 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.

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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.

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Acidity of Organic Acids
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An idea of the functional groups involved
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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.

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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.

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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 lt 7 phenolic-OH,
pKint 911
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There are assumed to be eight proton-dissociating
sites, described by four parameters as follows
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Metal binding can occur at single
proton-dissociating sites ( monodentate), or when
the single groups are sufficiently close, in
bidentate.
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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)
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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
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3. The concentration of counterions in the
diffuse layer , averaged with respect to distance
from the humic surface, is estimated with
Donnan-type expressions.
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Nomenclature
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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.
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Data Sets
Model parameters were obtained by fitting
published data. The contents of the data sets are
summarized in the following table.
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Results
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Proton Dissociation
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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.

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Different humic samples studied have similar
binding properties.
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The values of pKMHA shown in the following table
indicate that binding strength increases in the
order Mg2 lt Ca2 lt Mn2 lt Cd2 ltCo2 lt Ni2
Zn2 lt Pb2 lt Fe2 lt Cu2 lt Vo2.
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The Distribution of Bound Metal Over the
Different Sites of Model V
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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.

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• 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.

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• 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.

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• 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.