Download An overview of factors influencing the colloid/particle favoured

Groundwater
Quality: Natural and Enhanced Restoration of Groundwater Pollution (Proceedings o f file
Groundwater Quality 2001 Conference held al Sheffield. U K . June 2001). IAHS Publ. no. 275, 2002.
165
An overview of factors influencing the
colloid/particle favoured relocation of
contaminants
T. H O F M A N N
Applied Geology, Institute of Geoscience, Bechenveg 21, D-55099 Mainz,
e-mail: [email protected]
Germany
Abstract Contaminants partition between the solid and the aqueous phase.
Those sorbed to the solid phase are regarded as retarded or immobile. The
solid phase—the matrix of the soil or aquifer—can be mobile itself as
suspended or colloidal particles. In the unsaturated zone, with fast changing
gradients in hydrochemistry and water content, suspended particles can play a
major role in contaminant relocation.
Key words colloids; contaminants; filter theory; heavy metals; particles; PAH; unsaturated zone
INTRODUCTION
The ability to predict contaminant transport is one of the main requirements for
sustainable management of site remediation or groundwater protection. The major
pathways and transport mechanisms must be defined. Most approaches to describing
and predicting transport phenomena treat aqueous systems as a two-phase system: a
mobile aqueous phase and an immobile solid phase. Contaminants partition between
these two systems and those that have a strong tendency to sorb readily to the solid
phase are considered to be virtually immobile or retarded.
However, colloidal and suspended particles also may be mobile in saturated or
unsaturated environments. The composition of these colloids/particles is expected to
be chemically similar to that of the surface of the aquifer material. Thus, they can also
sorb organic and inorganic contaminants (McCarthy & Zachara, 1989). Predictions on
the transport of contaminants may fail if micro-particle facilitated transport is not taken
into consideration.
Many nonpolar organics, e.g. polycyclic aromatic hydrocarbons (PAH), PCP and
dioxin, have a strong tendency to adsorb to the solid phase. Inorganic trace substances,
like heavy metals and radionuclides can show the same tendency (Backhus &
Geschwend, 1990) forming colloids/particles themselves. Thus long-term risk assess­
ment should take colloid-bound relocation of contaminants into consideration.
ORIGIN OF PARTICLES
In this paper we refer to colloids (or particles) as potentially mobile solids including
macromolecular (or "dissolved") components of organic carbon. They can cover a
broad size range from a few nm up to 10-20 um. Thus, common filtration techniques
in groundwater quality monitoring, i.e. filtration through 0.45 jam filters, may exagge­
rate the concentration of dissolved elements and underestimate the total element
166
T. Hofmann
content. Particles can be colloidal stable or suspended. The colloidal stability and
transport behaviour of a particle is a complex function of the surface charge density,
surface chemistry, water chemistry and flow velocity. These conditions change along
the flow path in the unsaturated or saturated zone. Thus, for practical reasons colloids
and suspended particles are often not differentiated.
Many different particles can be expected in aqueous environments. The soil itself
consists of small particles <20 um. These particles were deposited during sedimentation
and genesis of the soil or aquifer, or formed by post-depositional weathering of thermodynamically unstable primary minerals. The composition of these particles depends on
the sedimentation conditions, the chemistry of the pore water solution, the mineral
content of the original geological material and the climate. The dominating inorganic
particles are alumino silicates, oxides and hydroxides of aluminium, iron, manganese, as
well as calcium and magnesium carbonates and silica (McCarthy & Zachara, 1989).
A second group of particles is formed by precipitation or mobilization due to a
geochemical gradient along the flow path. Particles can form or be mobilized due to
changes of parameters like pH, Eh, electrical conductivity or the dissolved gas content.
Seepage of water with a high content of organic carbon can lead to anoxic conditions
by microbial reduction of O2, NCV, NCV, MnCb, Fe(lII)-oxide, SO4 " and C 0 . Iron
and manganese coatings of the soil material are dissolved during this process and
liberate formerly fixed particles and elements in these coatings. Nevertheless, the total
content of iron and manganese, released by dissolution of these cements, is limited by
the solubility of carbonates and sulphides.
Bio-organic substances are from a third group of micro-particles (Jenkins & Lion,
1993). Organic macromolecules and humic substances can be released or brought in by
the infiltrating water. Bacteria attached to the matrix may become unattached or can be
infiltrated. They may release extra-cellular matter or cell fragments. Another rare but
very important and conceivably dangerous group are spores and viruses. Most of these
organic substances and microorganisms have a strong tendency to adsorb to particles.
2
2
TRANSPORT OF PARTICLES
Suspended or colloidal particles can be mobile if they are not susceptible to filtration
and are stable with respect to agglomeration and dissolution. Filtration effects can be
either straining filtration or physico-chemical collection. While the maximum size of
mobile particles is limited by straining filtration and pore velocity, the concentration
and size distribution of particles in the nanometer size range are controlled by physicochemical filtration. A minimum filtration rate can be observed at a size range close to
1 um for bacteria and polystyrene beads, and around 0.2 um for clay minerals. At
these size ranges straining filtration is less effective in most porous aquifers and
physico-chemical filtration does not reach its maximum (Fig. 1).
The collection of particles on the soil material strongly depends on the surface
charge. If the surface charge of the particles and soil matrix are opposite, collection is
enhanced. Differently charged particles favour coagulation and consequently sedi­
mentation. This effect is even more complex for clay minerals, because the edges and
faces may show opposite charge. A charge reversal of particles can occur due to the
adsoiption processes of polyelectrolytic substances. The adsorption of humic substances
An overview offactors influencing the colloid/particle favoured relocation of contaminants
167
j«2,44 m/d D4,1 m/d A 5,7 m/d
100
0,1
-I
0,01
,
,
,
0,1
,
1
,
I
10
particle size [pm]
Fig. 1 Results of column experiments with polystyrene beads. The column is filled
with natural sand and gravel with a mean grain size of 2.4 mm. Squares, triangles and
dots are experimental results at different pore velocities. Lines are results calculated
following the filter theory of Rajagopalan & Tien (1976) and Yao (1968). The
filtration factor is defined by the equation at the right top of the figure (c/c =
concentration at column outflow/inflow, x = column length, a = filtration factor).
0
(HUS) to positively charged particles like ferrihydrite can cause a charge reversal and
facilitate transport (Fig. 2). Batch experiments were performed with a Malvern Zetasizer
after an equilibration time of 24 h for particle-HUS interaction. Zeta potential was
measured allowing an equilibration time of ~5 min after p H adjustment. Ferrihydrite
particles show a charge reversal at pH 8. At higher pH values these particles are nega­
tively charged, at lower ph, they are positive. Humic substances are charged negative
throughout the whole p H range. If humic substances are added to ferrihydrite particles
the surface charge of the positively charged particles is masked by the sorption of the
negatively charged polyelectrolyte. Thus, sorption of humic substances can significantly
alter the transport behaviour of particles. Besides the surface charge, the chemical
stability of the particles themselves is also very important with respect to their mobility.
Major importance should be attributed to fingering and preferential flow. In macropore systems flow velocities are high and filtration rates are low. Thus, large amounts of
mobile particles could be relocated by this transport mechanism. In addition, in the
unsaturated zone the gas phase plays an important role in particle transport (Wan &
Wilson, 1994).
Particle bound transport reduces the retardation factor and should be taken into
account if:
particle concentrations are high,
particles are stable (no dissolution),
particles are mobile (no filtration),
particles interact strongly and irreversibly with the contaminant,
particles are more mobile than the aqueous phase.
If the contaminant interacts irreversibly with the particles but particle mobility is lower
than in the aqueous phase, particles lead to an enhanced retardation factor.
168
T. Hofmarm
50
•
0.0 mg/l HUS
o
0.1 mg/l HUS
A
1.0 mg/l HUS
40
30
>
E
2 0
r
io
x * * x
A
X
X
A
n
.2
X - - 1 0 . 0 mg/l HUS
Io °
°-
-10
ra
A
+j
X
X
20.0 mg/l HUS
•
80.0 mg/l HUS
+
80 mg/l HUS
without
particles
<D
N
-20
-30
-40
3
4
5
6
7
10
11
PH
-50
Fig. 2 Zeta potential of ferrihydrite particles without humic substances (0.0 mg 1"
HUS), ferrihydrite with HUS (0.1-80 mg 1"' HUS), and a pure HUS solution without
ferrihydrite particles (80 mg l" HUS).
1
CONCLUSIONS
Colloidal and suspended particles are abundant in all natural aquatic environments.
The size range of these particles is from a few nm up to several um. Inorganic particles
consist mainly of precipitates, alumino silicates and quartz. To the group of organic
particles belong bacteria, spores, viruses and humic substances as well as extra-cellular
material.
The mobility of these particles is linked to the chemical and physical boundary
conditions. Because these conditions vary over small distances and time periods in the
unsaturated zone, it is very difficult to predict particle transport.
Hydrophobic contaminants like P A H can have a strong tendency to sorb to solid
phases. Heavy metals and radionuclides can sorb to a solid phase or form particles
themselves. If the particles are more mobile than the aqueous phase, then contaminant
transport can be enhanced, and conversely, if they are less mobile, interaction with
particles will lead to contaminant retardation.
REFERENCES
Backhus, D. A. & Gschwend, P. M. (1990) Fluorescent polycyclic aromatic hydrocarbons as probes for studying the
impact of colloids on pollutant transport in groundwater. Environ. Sci. Technol. 24, 1214-1223.
Jenkins, M. B. & Lion, L. W. (1993) Mobile bacteria and transport of polynuclear aromatic hydrocarbons in porous media.
Appl. Environ. Microbiol. 59,3306-3313.
McCarthy, .1. F. & Zachara, .1. M. (1989) Subsurface transport of contaminants. Environ. Sci. Technol. 23, 496-502.
Rajagopalan, R. & Tien, C. (1976) 'trajectory analysis of deep-bed nitration with the shere-in-cell porous media model.
Am. Inst. Chem. Engngll, 523-533.
Wan, J. & Wilson, J. L. (1994) Colloid transport in unsaturated porous media. Wat. Resour. Res. 30, 857-864.
Yao, K.-M. (1968) Influence of suspended particle size on the transport aspect of water nitration. PhD Thesis, University
of North Carolina, Chapel Hill, North Carolina, USA.