Aggregation of Colloids Observed by X-Ray Microscopy
J. Thieme1, J. Niemeyer2, G. Machulla3, U. Schulte-Ebbert4
1
Forschungseinrichtung Röntgenphysik, Georg-August-Universität Göttingen,
Geiststraße 11, D-37073 Göttingen, Germany
E-mail: jthieme@gwdg.de
2
Fachbereich VI – Geowissenschaften, Abteilung Bodenkunde, Universität Trier,
D-54286 Trier, Germany
E-mail: niemeyer@uni-trier.de
3
Institut für Bodenkunde und Pflanzenernährung, Martin-Luther-Universität,
Weidenplan 14, D-06108 Halle, Germany
E-Mail: laoec@mlucom2.urz.uni-halle.de
4
Institut für Wasserforschung GmbH Dortmund,
Zum Kellerbach 46, D-58239 Schwerte, Germany
E-Mail: ifw_mail@compuserve.com
Abstract. Many aggregation processes of colloidal particles take place in an
aqueous phase. Thus, to ensure a detailed visualisation of the aggregation
processes it is necessary to image the aggregates within this environment. Due
to the size of the primary particles many of these processes can not be observed
directly in light microscopy, as the resolution is too low. The aim of these
studies is to show that by X-ray microscopy aggregation phenomena in aqueous
phase can be observed directly.
1 Introduction
X-rays within the wavelength range between the K-absorption edges of oxygen at
λ = 2.34 nm and carbon at λ = 4.38 nm are very well suited for X-ray microscopy
studies of aqueous colloidal systems [1]. Here, photoelectric absorption and phase
shift are the two dominating processes of interaction of X-rays with matter. The
radiation is weakly absorbed by water but strongly absorbed by iron oxides, silicates,
organic matter, etc. resulting in a good amplitude contrast of objects in aqueous
environments. These differences are even larger when looking at the phase shift of Xrays penetrating water or other materials [2]. The graph in Fig. 1 shows the linear
absorption coefficient of three substances, i.e. water, the phyllosilicate smectite, and
the organic molecule phenol, leading to amplitude contrast in X-ray images. Thus, it
is possible with an X-ray microscope to image objects in aqueous media directly and
without preparational steps as drying or staining.
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J. Thieme et al.
linear absorption coefficient / µm-1
8
water
smectite
phenol
7
6
5
C
O
4
Ca
3
2
1
0
1
2
3
4
5
6
wavelength / nm
Fig. 1. Linear absorption coefficient of water, smectite and phenol as function of wavelength
2 Aggregation Phenomena
2.1 Aggregation of Hematite Particles
X-ray microscopy allows to visualise directly aggregation phenomena in colloidal
dispersions. This has been demonstrated exemplary with a hematite dispersion as a
model system [3]. Increasing amounts of Na2SO4 were added to a stable hematite
dispersion to induce coagulation of hematite particles to larger structures which are
called aggregates. Figure 2a shows an X-ray image of a stable dispersion comprising
hematite particles with a radius of 80 nm approximately. The hematite particles were
synthesised following the method described in [4]. Figure 2b was made after adding
7.5 µl of a 1% solution of Na2SO4 to a 1 ml aliquot of the dispersion. The critical
coagulation concentration (ccc) [5], i.e. the concentration above which the dispersion
collapses, was determined to be reached adding 8 µl of the Na2SO4 solution. Figure 2c
was taken after adding 10 µl.
The single aggregates were measured with the box counting method. The fractal
dimension showed an increase from DF = 1.36 after the addition of 7.5 µl Na2SO4, i.e.
below the ccc, to DF = 1.77 after the addition of 10 µl Na2SO4, above the ccc, as can
be seen in Fig. 3. This result of increasing fractal dimension with increasing Na2SO4
concentration up to now is not in accordance with the values produced by light
scattering experiments [6] or by numerical approaches [7].
Aggregation of Colloids Observed by X-Ray Microscopy
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Fig. 2. X-ray images of the stable hematite dispersion (a), and of single hematite aggregates
after the addition of 7.5 µl (b) and 10 µl (c) of Na2SO4
log2 (number of occupied boxes)
14
12
dF=1.77
above ccc
10
8
dF=1.36
below ccc
6
4
2
2
3
4
5
6
7
log2 (number of divisions)
Fig. 3. Box-counting plot, derived below the critical coagulation concentration (ccc) from the
aggregate at the bottom of Fig. 2b. and above the ccc from the aggregate in Fig. 2c.
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J. Thieme et al.
2.2 The Micro Pore System of Soils
Soils are penetrated by atmosphere, hydrosphere and geosphere [8]. This penetration
is expressed in the pore system of soils, where the distribution of the pore radii shows
a wide range. This pore system and with it the form of the inner surface of soils
determines to a great extend the transport of substances within the soils. Transport
processes are extremely important, examples are the water movement and diffusive
transport of nutrients and toxicants. In the range down to 10 µm pore radius the inner
structure of soils can be well determined and characterised by porosimetric methods.
These methods fail in the colloidal range where the radii of the micro pores are
< 1 µm. Indirect model supported methods are used, which base on diffusion
measurements. With X-ray microscopy it is possible to image directly the porous
inner structure of soils in the colloidal range and to study it [9,10]. For example,
Figs. 4 and 5 show the microstructure of a dystric cambisol. The very open form of
the structure can be seen clearly.
Fig. 4. X-ray microscopic image of the microstructure formed by colloidal particles within a
dystric cambisol.
Aggregation of Colloids Observed by X-Ray Microscopy
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Fig. 5. X-ray microscopic image of the microstructure formed by colloidal particles within
a dystric cambisol.
2.3 Interaction of Humic Substances with Soil Colloids
In the upper part of soils the influence of biological activities is especially prominent.
Humic substances, humins, are one result of these activities in soils [11]. Humins are
anionic polyelectrolytes. Reactions with cations occur within the aqueous
environment because of their negative charge. These reactions influence the
microstructure of soils and may even alter it. Humins interact with soil particles
among others by the formation of network-like structures. These structures aggregate
and, in addition, entangle existing aggregates of other soil colloids. Important
parameters of soils can be substantially influenced, as for instance the water flow or
the transport of matter by diffusion. The top image in Fig. 6 shows an X-ray image of
colloidal aggregate within a 1% dispersion of a chernozem. The bottom image shows
an aggregate of this chernozem after the addition of 5% humins (weight-to-weight to
chernozem). The network-like structure between the soil particles is clearly visible.
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J. Thieme et al.
Fig. 6. Microstructure formed by colloidal particles within a 1% dispersion of chernozem
before (top image) and after (bottom) the addiion of humins.
Aggregation of Colloids Observed by X-Ray Microscopy
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2.4 Formation of Colloidal Particles Due to the Interaction of Humic Substances
with Detergents
Humic substances can be extracted from soils by alkaline solutions. As mentioned in
2.3. they are able to influence many reactions in soils, e.g. mass transport and water
flow. Detergents are able to reach the ecosystem and hence soils nearly unchanged.
Therefore, it is important to study the interaction of both substances in the soil
solution. The cationic detergent dodecyltrimethylammoniumbromide (DTB) was
added to a 0.05% dispersion of humins to study these interactions. Figure 7a shows
small spheres which resulted after the addition of 1 µl of a 1% DTB solution to a 1 ml
aliquot. The radius of the spheres is 100 nm within a small limit. Figure 7c shows
larger spheres with more different radii after the addition of 7 µl. In addition,
aggregates occur. By adding larger amounts of DTB spheres do not occur anymore.
Instead, a network like structure appears as can be seen in a very extended form in
Fig. 7c, where 50 µl of a 1% DTB solution was added to a 1ml aliquot.
Fig. 7. Spheres an network like structures as a result of the interaction of
humins with a cationic detergent.
3 Particle Formation in Ground Water
Hydrochemical changes caused by the degradation of organic matter and reduction of
electron acceptors or by mixing of different groundwater types may result in a redox
gradient in the aquifer [12]. This gradient can induce the formation of particles by
precipitation or the remobilisation of particles which were fixed in mineral coatings
on the aquifer material. Iron, as an example, is an abundant cation in groundwater. In
anaerobic groundwater aquifers it is present in a reduced form as a bivalent
cation [13]. At the groundwater surface or by mixing of anaerobic bankfiltrate and
aerobic water the groundwater may get in contact with oxygen. The bivalent iron
cation is oxidised to a trivalent state, insoluble compounds with iron are formed in
consequence [14]. This gives rise to the formation of new colloidal particles at the
transition from anaerobic to aerobic groundwater. Figures 8 and 9 show X-ray images
of aggregates of such particles in originally anaerobic ground water after oxidisation.
Figure 8 shows two such structures, the larger one looking like an oak leaf, both
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J. Thieme et al.
attached to a much denser colloidal particle. These gel-like structures may contract to
form dense iron containing particles in the end. The formation of larger aggregates
consisting of these particles is among other things influenced by the microbial activity
in the groundwater. In Fig. 9 single particles can be seen in open and loose
aggregates, revealing micro organisms and fibrous structures, presumably of organic
origin, on which iron containing particles accumulate preferably.
Fig. 8. Iron containing structure with a gel-like appearance found in oxidised, formerly
anaerobic groundwater.
Fig. 9. Microbial influenced aggregation of iron containing colloidal particles in oxidised,
formerly anaerobic groundwater
Aggregation of Colloids Observed by X-Ray Microscopy
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Acknowledgements
This paper represents publication no. 20 of the Priority Program 546 "Geochemical
processes with long-term effects in anthropogenically-affected seepage- and
groundwater". Financial support was provided by Deutsche Forschungsgemeinschaft.
In addition, this work has been supported by the Federal Ministry of Education,
Science, and Technology, BMBF, under contract number 05 644 MAG, and by the
Deutsche Bundesstiftung Umwelt under contract number 03149. We would like to
thank the staff of BESSY for providing excellent working conditions.
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