On the nature of organic matter from natural and contaminated materials : isolation methods, characterisation and application to geochemical modelling

A. van Zomeren

Research output: Thesisinternal PhD, WU

Abstract

Natural organic matter (NOM) is the material that is formed after the natural
decomposition and transformation of dead plant and animal matter. The fresh
organic matter (e.g. plant leaves or animal debris) is decomposed and
transformed by microbial activity. As such, NOM is found everywhere in the
natural environment; in soils, surface water and oceans. Due to its abundance
at the earth’s surface, the production and decomposition of NOM plays an
important role in the global carbon cycling. In soil systems, NOM has an
extremely important influence on essential properties like soil structure, water
retention, nutrient availability and binding of contaminants. In water systems
(e.g. surface or river water), NOM is important in many (bio)-chemical
processes and the geochemical cycling of elements/nutrients.
Due to the importance of NOM in soils for agricultural production, research on
its chemical and physical properties and classification dates back many
centuries. Numerous laboratory procedures and classifications have been
developed to relate organic matter properties to plant growth and soil fertility.
However, since the growing public and political awareness of environmental
pollution in the 1960s, the properties of NOM (especially humic substances, see
below) have increasingly been investigated in the context of its interactions
with potentially toxic compounds such as heavy metals and pesticides.
Although the first phenomenological observations that natural organic matter
can bind heavy metals have already been made in the 19th century, scientists
started to gain more extensive data on the strong interaction of heavy metals
with NOM in natural soils and aquatic environments during the past decades. At
present, the important influence of NOM on the mobility of such potentially
toxic compounds in the environment is widely acknowledged by the scientific
community.
Waste materials do often also contain organic matter, and in addition, high
contents of potentially toxic contaminants such as heavy metals. In many
countries, waste materials are increasingly being recycled in construction works
(e.g. in road foundations, embankments and sound barriers). The potential
environmental risk associated with the re-use of waste materials in such
applications depends on the extent to which contaminants can be released as a
result of “leaching”. Leaching is the release of contaminants from the solid
phase (e.g. a waste material) to the water phase with which the material may
be in contact (e.g., percolating rainwater). The leaching of contaminants such
as heavy metals can be strongly enhanced by complexes with (soluble) organic
matter. Therefore, it is important to characterise the properties of organic
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Matter matter in (contaminated) waste materials in the context of long-term risk
assessment of waste applications in construction. In addition, this knowledge
can contribute to the development of waste treatment technologies, e.g., to
improve the leaching properties of waste materials. Knowledge of the
fundamental binding properties of NOM with respect to contaminants can also
be specifically used for the development of models to predict the (long-term)
leaching behaviour of waste materials.
Natural organic matter is known to include a broad spectrum of organic
constituents, many of which have their counterparts in biological tissues, and
each with different chemical and physical properties. Two major types of
compounds can be distinguished:
Humic substances: a series of unidentifiable organic compounds of
relatively high-molecular-weight. They are brown to black in colour and
formed by secondary synthesis reactions. The term “humic substances”
is used as a generic name, but many scientist discriminate between
humic- and fulvic acids (HA and FA, respectively) based on their
dissolution properties in alkaline and acid solutions. Humic acids are
generally dark-brown to black in colour and have a relatively high
molecular weight (several thousand to several hundred thousand
atomic mass units (AMU), depending on the applied analysis
techniques). Fulvic acids are light yellow to golden brown in colour and
have molecular weights ranging from several hundred to about ten
thousand AMU. The basis of this classification is also used throughout
this thesis.
Non-humic substances: all identifiable (biochemical) organic molecules
that can be placed in one of the categories of discrete compounds such
as sugars, amino acids, fatty acids etc.
This thesis focuses on the development of methods for the isolation and
characterisation of natural organic matter in general and organic matter in
(contaminated) waste materials in particular. The overall purpose of this
research is to develop a better understanding of the role of NOM with respect
to the mobility of contaminants such as heavy metals in the environment.
The main difficulty of studying humic substances is that these substances are
very heterogeneous by their nature. Various classifications of humic substances
are used in the scientific literature and these are all based on operational
definitions. There is still no consensus today as to how HA and FA should be
operationally defined. It is important to note that the terminology that is being
used does not represent pure compounds. Each class of the humic substances
consists of highly complex and heterogeneous mixtures of organic molecules.
The study of chemical properties of NOM often requires isolation and
purification of different fractions of the organic matter. The advantage of
purification is that it reduces the heterogeneity in the properties of NOM.
Numerous isolation and purification methods have been developed over the
past decades to enable the further characterisation of organic matter
properties. Although these procedures are well established and widely used by
scientists, they all share the disadvantages that they are very laborious and
primarily aimed on purification rather than quantification of the different
fractions. Therefore, this thesis has a strong focus on developing analytical
methods that enable an improved characterisation and quantification of the
different fractions that are important for metal binding, preferably in a more
time- efficient manner.
In Chapter 2, a Competitive Ligand Exchange-Solvent extraction (CLE-SE)
method was used to measure Cu binding to DOC in leachates from municipal
solid waste incinerator (MSWI) bottom ash. The copper binding properties of
dissolved organic carbon (DOC) were investigated with specific attention for the
identification and quantification of the organic ligands. Evidence was found for
an important role of fulvic acids (FA) in the strongly enhanced leaching of Cu
from MSWI bottom ash. This work implies that the complexation of
contaminants with natural organic matter is an important process in these
relatively inorganic waste materials.
Chapter 3 describes the development of an automated procedure to isolate
and purify HA and FA from various materials. The conventional (manual)
isolation and purification procedures that are widely used by scientists, share
the disadvantage of being very laborious and time consuming. These
disadvantages are largely overcome by automation and enabled to gather an
extensive set of purified HA and FA samples from diverse origin for further
characterisation purposes. The main objective for automation of the
conventional procedure was to save a significant amount of labour and total
throughput time in the performance of HA and FA isolation and purification. The
novelty of the method lies in the automated handling of the multiple liquids and
columns, required in the isolation/purification procedure, in both forward and
back elution mode. By automating the procedure, better standardisation of HA
and FA isolation and purification methods is feasible. The automated procedure
significantly reduces the total throughput time needed, from 6–7 days to 48 h,
and the amount of labour to obtain purified HS for further characterisation.
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In chapter 4, the development is described of a rapid batch method for the
In chapter 4, the development is described of a rapid batch method for the
experimental characterisation and quantification of HS in natural and
contaminated systems. In principle, the quantification of HS in the environment
can be studied with the manual or the developed automated isolation and
purification procedure (chapter 3). However, the established models for
calculation of metal binding to HS require absolute concentrations of HA and/or
FA as input. The conventional isolation and purification procedures are too
elaborate (and mainly focussed on purification) for this type of application.
Because the principles of the new batch method are essentially the same as
those of the well-known conventional isolation and purification procedures, the
HA and FA properties identified in this study are, therefore, of general
importance for the interpretation of the occurrence and behaviour of HS in the
environment. The novelty of this method lies in the fact that it greatly
facilitates the analysis of HA and FA concentrations (e.g. for use in geochemical
modelling, chapter 5). The new method can be performed within 1.5-4 hours
per sample and multiple samples can be processed simultaneously, while the
conventional procedures typically require approximately 40 hours for a single
sample.
Based on results from previous chapters, “multi-surface” geochemical
modelling of heavy metal leaching from MSWI bottom ash was used to develop
a mechanistic insight into the beneficial effects of accelerated aging of MSWI
bottom ash on the leaching of copper and molybdenum (Chapter 5).
Therefore, the rapid batch procedure described in Chapter 4 is used to
characterise DOC quantitatively in terms of humic, fulvic and hydrophilic acids
over a wide pH range. Important processes controlling the solid/liquid
partitioning of humic and fulvic acids and their role in the effects of aging on
contaminant leaching are identified. In addition, a new approach is developed
to model the pH dependent leaching of fulvic acids from MSWI bottom ash
based on adsorption to reactive Fe/Al-(hydr-)oxides. This chapter shows that
accelerated ageing results in enhanced adsorption of FA to reactive Fe/Al-
(hydr-)oxides, leading to a significant decrease in the leaching of both FA and
associated Cu.
In Chapter 6, the carbon speciation (inorganic, organic and elemental carbon)
in MSWI bottom ash samples is studied to identify the amount and properties
of the different carbon species present in MSWI bottom ash. The carbon
speciation was quantitatively measured (partly based on the method described
in Chapter 4), and its relation with the leaching of Cu, in fresh and carbonated
MSWI bottom ash. Results show that up to only 25% of loss on ignition (LOI)
consists of organic carbon (OC), while about 15% of organic carbon (OC) in the
three samples consists of HA and FA. Since only these small reactive carbon
Summary and synthesis
171
fractions contribute to enhanced metal leaching from MSWI bottom ash,
fractions contribute to enhanced metal leaching from MSWI bottom ash, it is
concluded that LOI measurements are insufficiently discriminative for a
quantitative assessment of environmentally relevant organic carbon species in
MSWI bottom ash. The results of this study imply that dedicated methods,
focusing on specific carbon fractions, are more appropriate for assessment of
environmentally relevant organic carbon species than the measurement of LOI.
These methods may greatly improve the assessment of the long-term
environmental properties of bottom ash in utilisation or disposal scenarios.
Chapter 7 describes a comparative study regarding the proton binding
properties of the previously isolated and purified HS (Chapter 3). Binding
models for HS, such as the NICA-Donnan model, have so far been developed
and calibrated against organic matter from natural origin (e.g. soils and surface
waters). In Chapter 5, the NICA-Donnan model was found to perform well
when applied to a contaminated (waste) material, i.e. municipal solid waste
incinerator (MSWI) bottom ash. However, the proton binding properties of HS
originating from waste environments have not been analysed directly and
demonstrated to fall within the observed range for natural materials. The aim
of this study is to analyse the proton binding properties of humic and fulvic acid
samples originating from secondary materials, waste materials and natural
samples in order to assess whether the charge development of these HS can be
described with generic NICA-Donnan parameters. New proton binding
parameters are presented for these HS and are shown to be similar to those of
HS originating from natural environments. These results suggest that the
NICA-Donnan model and generic binding parameters are adequate to describe
proton binding to HS in both natural and contaminated materials. This finding
widens the range of environments to which the NICA-Donnan model can be
applied and justifies its use in geochemical speciation modelling of metal
mobility in contaminated (waste) materials.
Original languageEnglish
QualificationDoctor of Philosophy
Awarding Institution
  • Wageningen University
Supervisors/Advisors
  • Comans, Rob, Promotor
  • van Riemsdijk, Willem, Promotor
Award date10 Oct 2008
Place of Publication[S.l.]
Print ISBNs9789085049937
Publication statusPublished - 2008

Keywords

  • organic matter
  • soil pollution
  • pollution
  • heavy metals
  • geochemistry
  • soil chemistry
  • humus chemistry

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