Binding of metals to macromolecular organic acids in natural waters
ABSTRACT Trace metal speciation and bioavailability have become keys to current day toxicity and risk assessments. For many metals, macromolecular organic acids constitute the major ligand in fresh water and soil solution. Therefore, understanding their characteristics and behaviour is necessary for understanding trace metal behaviour. This study comprises investigations of the proton- and copper-binding properties of hydrophobic and hydrophilic dissolved organic matter fractions, and competition effects of iron(III) and aluminium. The solutions studied were a forest floor solution and a municipal solid waste incinerator bottom ash leachate. Two geochemical models (SHM and NICA-Donnan) were tested and calibrated against the experimental data. A structural analysis of the binding mode of iron(III) to fulvic acid in acid aqueous solutions was made using extended X-ray absorption fine structure (EXAFS) spectroscopy. Dissolved organic carbon (DOC) in the bottom ash leachate had fulvic acid-like properties and was dominated by the hydrophilic acid fraction. Three organic fractions (hydrophobic, transphilic and hydrophilic) were isolated from the forest floor solution using an XAD-8/XAD-4 tandem. All fractions were characterised by distinct but differing proton-binding properties, suggesting a more acidic character than 'generic' fulvic acid. The copper-binding isotherms were very similar for all three fractions and suggested strong copper binding to a small number of sites. In general, both models tested could be adjusted to obtain good fits to data on both proton- and copper-binding, but iron(III) and aluminium competition was better predicted by the SHM than the NICA-Donnan model. Only mononuclear iron(III) complexes were included in the model calculations, as the EXAFS study showed that these ¬dominated in the aqueous phase. Studies on untreated soil solution indicated that the three isolated fractions were the only contributors to the observed copper binding and together constitute the 'active' DOC fraction. Thus, combination of Leenheer fractionation data with the model parameters derived in this study is recommended for improved predictions of trace metal speciation in soil solutions. However, further studies along this research line, including other samples and trace metals, are highly recommended.
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ABSTRACT: Humic Ion-Binding Model VI, a discrete site/electrostatic model of the interactions of protons and metals with fulvic and humic acids, is applied to 19 sets of published data for proton binding, and 110 sets for metal binding. Proton binding is described with a site density, two median intrinsic equilibrium constants, two parameters defining the spread of equilibrium constants around the medians, and an electrostatic constant. Intrinsic equilibrium constants for metal binding are defined by two median constants, log KMA and log KMB, which refer to carboxyl and weaker-acid sites respectively, together with a parameter, LK1, defining the spreads of values around the medians. A further parameter, LK2, takes account of small numbers of strong binding sites. By considering results from many data sets, a universal average value of LK1 is obtained, and a correlation established between log KMB and log KMA. In addition, a relation between LK2 and the equilibrium constant for metal-NH3 complexation is tentatively suggested. As a result, metal-binding data can be fitted by the adjustment of a single parameter, log KMA. Values of log KMA are derived for 22 metal species. Model VI accounts for competition and ionic strength effects, and for proton-metal exchange.Aquatic Geochemistry 02/1998; 4(1):3-47. · 1.53 Impact Factor
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ABSTRACT: It is widely accepted that the bioavailability, toxicity, and mobility of trace metals are highly dependent on complexation reactions with functional groups in natural organic matter (NOM). In this study, the coordination chemistry of Cd in NOM was investigated by extended X-ray absorption fine structure spectroscopy. Soil organic matter (SOM) from different types of organic soils and dissolved organic matter (DOM) from an organic and a mineral soil horizon of a Spodosol and aquatic DOM from Suwannee River were investigated. In SOM samples (1000-25000 microg of Cd g(-1), pH 4.6-6.6), Cd was coordinated by 1.0-2.5 S atoms at a distance of 2.49-2.55 A and by 3.0-4.5 O/N atoms at a distance of 2.22-2.25 A. In DOM samples (1750-4250 microg of Cd g(-1), pH 5.4-6.3), Cd was coordinated by 0.3-1.8 S atoms at a distance of 2.51-2.56 A and 3.6-4.5 O/N atoms at a distance of 2.23-2.26 A. In both SOM and DOM samples a second coordination shell of 1.7-6.0 carbon atoms was found at an average distance of 3.12 A. This is direct evidence for inner-sphere complexation of Cd by functional groups in NOM. Furthermore, ion activity measurements showed that less than 1% of total Cd was in the form of free Cd2+ in our samples. Bond distances and coordination numbers suggest that Cd complexed in SOM and DOM is a mixture of a 4-coordination with S (thiols) and 4- and 6-coordinations with O/N ligands. Given that Cd-S associations on average are stronger than Cd-O/N associations, our results strongly indicate that reduced S ligands are involved in the complexation of Cd by NOM also at native concentrations of metal in oxidized organic-rich soils and in humic streams.Environmental Science and Technology 06/2005; 39(9):3048-55. · 5.26 Impact Factor
- Environmental Science and Technology 10/1994; 28(11):1853-8. · 5.26 Impact Factor
Binding of metals to macromolecular
organic acids in natural waters
Does Organic Matter?
Joris W.J. van Schaik
Faculty of Natural Resources and Agricultural Sciences
Department of Soil and Environment
Swedish University of Agricultural Sciences
Acta Universitatis agriculturae Sueciae
© 2008 Joris W.J. van Schaik, Uppsala
Tryck: SLU Service/Repro, Uppsala 2008
Cover: Experimental setup for the isolation of DOM fractions
(photo: J.W.J. van Schaik)
Trace metal speciation and bioavailability have become keys to current day toxicity
and risk assessments. For many metals, macromolecular organic acids constitute the
major ligand in fresh water and soil solution. Therefore, understanding their
characteristics and behaviour is necessary for understanding trace metal behaviour.
This study comprises investigations of the proton- and copper-binding properties of
hydrophobic and hydrophilic dissolved organic matter fractions, and competition
effects of iron(III) and aluminium. The solutions studied were a forest floor solution
and a municipal solid waste incinerator bottom ash leachate. Two geochemical
models (SHM and NICA-Donnan) were tested and calibrated against the
experimental data. A structural analysis of the binding mode of iron(III) to fulvic
acid in acid aqueous solutions was made using extended X-ray absorption fine
structure (EXAFS) spectroscopy. Dissolved organic carbon (DOC) in the bottom
ash leachate had fulvic acid-like properties and was dominated by the hydrophilic
acid fraction. Three organic fractions (hydrophobic, transphilic and hydrophilic)
were isolated from the forest floor solution using an XAD-8/XAD-4 tandem. All
fractions were characterised by distinct but differing proton-binding properties,
suggesting a more acidic character than ‘generic’ fulvic acid. The copper-binding
isotherms were very similar for all three fractions and suggested strong copper
binding to a small number of sites. In general, both models tested could be adjusted
to obtain good fits to data on both proton- and copper-binding, but iron(III) and
aluminium competition was better predicted by the SHM than the NICA-Donnan
model. Only mononuclear iron(III) complexes were included in the model
calculations, as the EXAFS study showed that these dominated in the aqueous
phase. Studies on untreated soil solution indicated that the three isolated fractions
were the only contributors to the observed copper binding and together constitute
the ‘active’ DOC fraction. Thus, combination of Leenheer fractionation data with
the model parameters derived in this study is recommended for improved
predictions of trace metal speciation in soil solutions. However, further studies
along this research line, including other samples and trace metals, are highly
Keywords: trace metal, humic substance, DOC, EXAFS, copper, heavy metal,
hydrophobic, hydrophilic, MSWI, metal toxicity.
Author’s address: Joris van Schaik, Department of Soil and Environment, SLU
Box 7014, 750 07 Uppsala, Sweden
There’s no metal like heavy metal!
List of Publications 7
Trace metals in the environment
2.1.1 How did they get there?
2.1.2 Total or biologically available?
2.2.1 Does it matter?
2.2.2 ‘Natural’ macromolecular acids
2.2.3 Fractionate, isolate, investigate
2.2.4 Practical limitations
2.3.1 General background
2.3.3 NICA-Donnan model
2.3.4 Stockholm Humic Model
EXAFS – A closer look
Objectives of this study 27
4.1.1 Study site & sampling of soil solutions
4.1.2 MSWI Bottom ash
4.1.3 Total versus hydrophilic NOM
4.1.4 Isolation of organic matter fractions
4.1.5 Proton and copper binding
4.1.6 Modelling approaches
Results and Discussion
Iron(III) under the microscope (Paper I)
5.1.1 Binding chemistry of iron(III) to aqueous fulvic acid
5.1.2 Influence of pH on iron redox state
Isolation work (Papers II-IV)
Hydrophilic acids unmasked (Paper II)
5.3.1 Composition of MSWI bottom ash DOC
5.3.2 Copper binding
Isolated fractions – alone but by no means lonely (Paper III)
5.4.2 Proton binding
5.4.3 Copper binding
5.4.4 Competition and model performance
Theory put to the test (Paper IV)
5.5.1 The theory
5.5.2 The test
5.5.3 The verdict
Implications & future research 55
List of Publications
This thesis is based on the work contained in the following papers, referred
to by Roman numerals in the text:
I van Schaik, J.W.J., Persson, I., Kleja, D.B., Gustafsson, J.P. (2008).
EXAFS study on the reactions between iron and fulvic acid in acid
aqueous solutions. Environmental Science & Technology 42(7), 2367-2373.
II Olsson, S., van Schaik, J.W.J., Gustafsson, J.P., Kleja, D.B., van Hees,
P.A.W. (2007). Copper(II) binding to dissolved organic matter fractions
in municipal solid waste incinerator bottom ash leachate. Environmental
Science & Technology 41(12), 4286-4291.
III van Schaik, J.W.J., Kleja, D.B., Gustafsson, J.P. Acid-Base and copper-
binding properties of three organic matter fractions isolated from a forest
floor soil solution (manuscript).
IV van Schaik, J.W.J., Olsson, S., Kleja, D.B., Gustafsson, J.P. Copper-
binding properties of total and hydrophilic organic matter in a forest floor
soil solution (manuscript).
Papers I and II are reproduced with the permission of the publishers.
LMWOA Low molecular weight organic acids
MSWI Municipal solid waste incinerator
MWCO Molecular weight cut-off
NICA Non-ideal competitive adsorption
NOM Natural organic matter
SHM Stockholm humic model
TiA Transphilic acids
XANES X-ray absorption near edge structure
Diffuse double layer
Diffusive gradient thin-film
Dissolved organic carbon
Dissolved organic matter
Extended X-ray absorption fine structure
Free ion activity model
‘Heavy metal’ is a term that may provoke quite different reactions,
depending on the recipient. Some people may stick out a hand with two
fingers spread out wide, start banging their head violently and reach for the
beer can. Others may get an indifferent look in their eyes and mumble
something incoherent along the lines of ‘all metals are heavy’. Still others
may jump to attention and engage in a discussion concerning environment,
pollution and scientific research. For me personally, any of the above could
happen, depending on mood, who is asking, surroundings, etc.
Obviously the term ‘heavy metal’ can be interpreted in several ways. In
fact, it was called ‘meaningless and misleading’ in an IUPAC (International
Union of Pure and Applied Chemistry) technical report, due to its
contradictory definitions and lack of a coherent scientific basis (Duffus,
2002). Hodson (2004) presented several alternatives in a paper dedicated
specifically to the term, referring to heavy metals as ‘bogey men’. In this
thesis, the focus is on heavy metal from a soil chemical and scientific point
of view. Rather than engaging in a continuous struggle with terminology,
the term ‘trace metals’ will be adopted from this point onward:
Trace metals - Chromium, cobalt, copper, iron, manganese, magnesium, molybdenum,
selenium, zinc and other elements that occur in very small amounts (usually less than 1 to
10 parts per million) as constituents of living organisms, and are necessary for their growth,
development and health. Whereas the shortage of trace elements in the body may result in
stunted growth or even death, their presence in larger amounts is also harmful
(Business Dictionary, Retrieved: August 27, 2008).
Trace metals are naturally present in most soil systems, due to weathering
of parent material (Ross, 1994). Whereas small quantities of trace metals are
considered essential for optimal functioning of biological processes and
organs in humans and other living beings, ingestion of excess amounts can
result in a variety of symptoms and diseases (e.g. Fergusson, 1990; Merian &
Clarkson, 1991). Examples can be nausea, vomiting, diarrhoea or memory
and concentration problems (copper), seizures, dizziness or impotence (zinc)
and asthma, anxiety, angina or other cardiac symptoms (nickel and cobalt)
(Roth, 2008). Due to this potential toxicity, trace metal contamination is a
global problem (Siegel, 2002).
Various strategies have been developed for making risk assessments of
contaminated and polluted areas. The common feature of most of these
strategies is that they consider total soil metal contents as criteria. However,
awareness has increased that the relevant variable in risk assessment is not the
total content, but rather the distribution of these contents over various
chemical forms (Souren, 2006). In simple words, there is no need to worry
just because a soil contains elevated amounts of trace metals. It is when those
metals become mobile and/or accessible that problems may arise, as they
find their way into food webs via plant, animal and microorganism uptake,
and into natural waters via leaching to groundwater and subsequent
This is where natural macromolecular acids come into the picture. A
macromolecule is defined as a very large molecule, such as a polymer,
consisting of many smaller structural units linked together. Natural
macromolecular acids are the most active and relevant part of natural organic
matter (NOM), a collective term assigned to broken down organic matter
originating from plants, animals and microorganisms, in terms of proton
buffering and trace metal binding. Since macromolecular acids have the
potential to dissolve under natural conditions, formation of soluble organic
complexes is an important mechanism for the mobilisation of many trace
metals in soils (Bergkvist et al., 1989; Berggren, 1992b; Lundström, 1993).
Dissolution of organic matter and the formation of organic metal
complexes are controlled to a large degree by soil acidity (Sposito et al.,
1978; Cabaniss & Shuman, 1988; McBride, 1994). Changes in soil acidity
will therefore have major effects on the behaviour of trace metals. Acid rain
is a well-known and actual phenomenon that results in soil acidification.
Another cause of soil acidification can be the termination of lime application
and conversion of arable lands to forests, pasture, heathlands or wetlands
(Johnston et al., 1986; Römkens & De Vries, 1995). In addition, plantation
of forest on former agricultural soils (afforestation) results in accumulation of
organic material in the topsoil, which in turn can increase dissolved organic
matter (DOM) concentrations in the soil solution (Andersen et al., 2002).
In short, increasing our understanding of the fate and behaviour of metal
contaminations in soil systems is of major importance for improving risk
assessments of contaminated areas.
2.1 Trace metals in the environment
2.1.1 How did they get there?
The origin of trace metals in our soils and environment is twofold. Firstly,
trace metals can be present in the parent material, in the form of constituent
and replacement elements in rock and soil minerals. In situ chemical
weathering of such rock minerals results in a local accumulation of trace
metals in soils; the ease and speed of weathering, as well as the trace metal
contents of the parent material, are the main factors determining the rate of
trace metal release. Secondly, trace metals are released into the environment
due to anthropogenic (human) activities and sources, such as industry,
atmospheric deposition, agriculture, waste disposal on land and metalliferous
mining and smelting (Ross, 1994; Alloway, 1995).
Due to the large differences in parent material trace metal contents, trace
metal concentrations in soils can vary widely. In contrast to this natural
variation, the input from anthropogenic sources is determined by the
location of a soil or site relative to metal pollution sources and, inherently,
human activity. In general, however, the quantity of trace metals originating
from anthropogenic sources exceeds that from natural sources by several
orders of magnitude (Campbell et al., 1983). Once metals enter the soil
system, the options are limited; i) metals are sorbed onto various solid
components and retained in the soil (e.g. Mighall et al., 2002), or ii) they are
dissolved and consequently susceptible to leaching and plant uptake.
Although anthropogenic inputs are site-specific, the fate of trace metals once
they enter the soil system is soil-specific; actual soil properties determine the
distribution of the trace metals between the solid and solution phase.
In general, the mobility of metals in soil systems is fairly low, which
results in accumulation and consequently elevated trace metal levels,
particularly at sites with high anthropogenic inputs (e.g. Brinkmann, 1994;
Hamon et al., 1998; Cicek & Koparal, 2004). Unfortunately, although major
efforts are – and have been – made to reduce these anthropogenic inputs,
many soils are already contaminated beyond ‘acceptable’ levels.
2.1.2 Total or biologically available?
In order to be able to speak of ‘acceptable’ levels, it is essential that critical
toxicity guidelines and limits for trace metals are established. As a well-
known saying goes, ‘wisdom comes with the years’; during the past decade,
the emphasis in assessing heavy metal toxicity and critical limits has shifted
from total metal contents to soluble and bio-available concentrations (e.g.
Salbu, 1991; Ashmore et al., 2000). Morel (1983) introduced the free ion
activity model (FIAM), which proposes that metal toxicity is related to
uptake of specific metal species at the organism-solution interface. Thus, the
concentration of the free metal ion in solution is supposed to be a better
predictor of toxicity than the total (dissolved) metal content or
An essential concept in defining bio-availability is the chemical speciation
of a trace metal. Metal speciation has been defined as determination of the
individual concentrations of the various physico-chemical forms of a metal
that together make up the total concentration of that metal in a sample
(Florence, 1986). Knowledge of the speciation of a metal is essential in order
to assess i) the bioavailability or toxicity of the metal or ii) the mechanism
controlling the solubility of the metal.
Many researchers have found evidence in support of the ‘free metal ion
hypothesis’, where the toxicity or bioavailability of any metal ion is assumed
to be directly related to the activity of the free, hydrated form of the metal
(Bingham et al., 1984; Brümmer et al., 1986; Hue et al., 1986; Sauve et al.,
1998). However, dissolved organic matter has been found to be an
important factor in the uptake of trace metal ions by plants, although no
consensus has been reached concerning its exact role (e.g. Jones & Darrah,
1992; Parker et al., 2001; Molas & Baran, 2004; Inaba & Takenaka, 2005).
Stumm & Morgan (1996) reported that the bioavailability of metal ions, and
their toxicity to soil organisms, is lowered by complex binding to dissolved
organic matter. Others have reported increased plant metal uptake in soils
with increased DOM levels (Pietz et al., 1989; Clemensson-Lindell, 1992).
These discrepancies might be explained by the dynamic nature of organic
metal complexes. Zhang et al. (2001), for example, suggested that the
presence of (labile) metal-organic complexes might indirectly increase metal
bio-availability by acting as a buffer for metal ions in the soil solution. In a
similar fashion, breakdown of DOM into low molecular weight organic
acids (LMWOA) might further increase metal solubility and availability to
plants (Parker et al., 2001; Inaba & Takenaka, 2005).
Several studies have reported good correlations between ‘effective’ metal
concentrations in soil solution, determined by a technique called DGT
(diffusive gradients in thin films), and plant metal contents (Zhang &
Davison, 2000; Zhang et al., 2001; Tye et al., 2004; e.g. Zhang et al., 2004;
Hough et al., 2005; Nolan et al., 2005; Zhang et al., 2006). The effective
concentrations have been reported to incorporate both the free metal ion
concentrations in solution and a labile complex fraction. Thus, the
correlations found give further weight to the importance of organic metal
2.2 Organic matter
2.2.1 Does it matter?
Natural organic matter is a general collective term for organic molecules
formed by decomposition of plants, animal products and microbial material.
Due to the large variety in original material and decomposition stage, NOM
is far from a ‘simple molecule’, as it lacks an unique structure or
composition, cannot be crystallised and is extremely difficult to characterise.
Instead, NOM consists of a heterogeneous mixture of complex molecules,
with basic structures created from cellulose, tannin, cutin, and lignin, along
with other various proteins, lipids, and sugars. The chemically most
significant fraction consists of the humic substances, which are generally of
an acidic character (Swift, 1989).
Due to their origin, humic substances are abundant in the environment;
due to their character, they interact with a variety of solutes that may be
present. Consequently, humic substances (HS) are factors in a range of
environmental issues (Table 1), and thus give rise to a worldwide interest in
attempting to understand their characteristics and environmental behaviour.
Of particular interest within the scope of this thesis is the role of humic
substances in controlling solubility and mobility of trace metals (Schnitzer &
Skinner, 1963; Schnitzer & Skinner, 1964; Schnitzer & Skinner, 1965;
Kerndorff & Schnitzer, 1980; Perdue & Lytle, 1983; Weber, 1988). For
many metal ions such as aluminium(III), iron(III), copper(II), lead(II) and
mercury(II), macromolecular organic acids constitute the major ligand in
fresh water and the soil solution (Tipping, 2002).
Table 1. Environmental issues involving humic substances (adapted from Tipping, (2002)
Issue Role of humic substances
Light penetration into waters
Soil and water acidification
Properties of fine sediments
Radioactive waste disposal
2.2.2 ‘Natural’ macromolecular acids
Humic substances have traditionally been divided into three main groups; i)
humic acids – soluble in bases, insoluble in acids, ii) fulvic acids – soluble in
both bases and acids, and iii) humin –soluble in neither bases nor acids. In
general, fulvic acids have a lower molecular weight, degree of
polymerisation and carbon/oxygen (C/O) ratio than humic acids, but a
higher exchange acidity (Figure 1). Solubility is related to the size-relative
charge of the humic molecules, which in turn is related to pH – the higher
the pH, the more acidic groups will be deprotonated and carry a negative
charge. Adding one and one, this implies that fulvic acids generally carry a
higher size-relative charge than humic acids and, even more so, humin. As a
Major carbon pool, transformations, transport and
Absorption and attenuation of light by humic
Absorption of solar radiation by soil humic matter
Binding of protons, aluminium and base cations in
soils and waters
Reservoir of carbon, nitrogen, phosphorus and
Binding of iron and phosphate
Substrate for microbes
Enhancement of mineral dissolution rates
Translocation of dissolved humic substances and
associated metals (aluminium, iron)
Adsorption at surfaces and alteration of colloidal
Aggregating effect on soil mineral solids
Mediation of light-driven reactions
Binding, transport, influence on bioavailability, redox
Binding, transport, influence on bioavailability
Binding and transport of radionuclide ions in
Control of proton and metal ion concentrations,
result of their surface charge, part of the humic substances may be present in
the aqueous phase under natural conditions. This dissolved organic matter
(DOM) maintains many of the characteristics of its solid precursor, and
therefore plays an important role in bioavailability and transport of trace
metals (Bergkvist et al., 1989; Berggren, 1992b; Berggren, 1992a).
Figure 1. Classification and observed chemical properties of humic substances (adapted from
Scheffer and Ulrich, (1960).
The majority of macromolecular acids in ecosystems have a natural
origin, being the products of NOM decomposition. However, there are
additional sources which have a more anthropogenic character, such as
municipal solid waste incinerator (MSWI) bottom ash and sewage sludge.
MSWI bottom ash is used for various purposes, such as raw material for
cement (Polettini et al., 2001; Aubert et al., 2006; Pan et al., 2008), road
construction (Schreurs et al., 2000; Åberg et al., 2006; Hjelmar et al., 2007;
Izquierdo et al., 2007), and as a drainage layer in landfills (Palmer et al.,
2000). Sewage sludge is often used as a soil amendment and fertiliser, and
has been used extensively to stimulate re-vegetation of mine tailings
(Bergholm & Steen, 1989; Pichtel et al., 1994; Voeller et al., 1998).
Consequently, organic acids that are present in the ash or sludge have the
Fulvic acidHumic acid
(Decomposition products of organic residues)
increase in degree of polymerization
increase in molecular weight
increase in carbon content
decrease in oxygen content
decrease in exchange acidity