Environ. Sci. Technol. 2009 43, 8860–8866

Transport of Copper
Oxychloride-Based Fungicide
Particles in Saturated Quartz SandP A R A D E L O M . , † Š I M Ů N E K J . , ‡N O V O A - M U Ñ O Z J . C . , †ARIAS-ESTEVEZ M.,† AND
J . E U G E N I O L O P E Z - P E R I A G O * ,†Soil Science Group, Department of Plant Biology and Soil
Science, Faculty of Sciences, University of Vigo, E-32004
Ourense, Spain, and Department of Environmental Sciences,
University of California Riverside, Riverside, California

Received June 5, 2009. Revised manuscript received
September 21, 2009. Accepted September 30, 2009.

Intensive use of copper-based fungicides in agriculturecauses contamination of subsurface environment. While thetransport of dissolved copper in porous media has been widely
studied, transport mechanisms of particles of copper-based
fungicides are poorly understood. This paper reports the resultsof tests involving the transport of colloid-size particles of acopper oxychloride-based fungicide (COF) in water-saturated
quartz sand columns under varying electrochemical andhydrodynamic conditions. The effect of the ionic strength on
colloid attachment and exclusion suggests that interactions of
COF in water depend on the characteristics of the diffuse
layer. Hydrodynamic shear influences the deposition of fungicide
particles, indicating that attachment forces are weak. Particle
deposition dynamics was well-fitted with the two-site kinetic
attachment model, which reflects the heterogeneity of the quartz
surface. Retention also occurs for unfavorable electrochemical
conditions, which was attributed to stagnation zones arisingfrom the physical and chemical heterogeneity of the quartz surface.

Introduction
Copper oxychloride-based fungicides (COFs) are widely used
as foliar sprays against fungal diseases in a number of crops.
Thus, there is a great likelihood that they may be released
into the natural environment, especially soils. The accumulation of Cu in the soil from the use of COFs has been
demonstrated in several studies (1-6). Current commercial
formulations of copper fungicides use copper oxychloride
powders with mean particle diameters of 1 µm. Off-target
deposition and foliar wash-off induced by rainfall canfacilitate the transfer of these fungicides into soils. Sincecopper oxychloride is poorly soluble (between 0.02 and 0.03
mmol Cu L-1 in equilibrium at 25 °C) in the typical pH rangeof rainwater (from 6 to 7), wash-off losses of particulate
copper-oxychloride may be more substantial than thetransport of dissolved forms of Cu, at least during heavy rain
episodes. Laboratory studies have shown that losses ofcopper-based fungicides through wash-off occur mainly inthe form of suspended particles (7).
* Corresponding author phone: +34 988 387 070; fax: +34 988 387
001; e-mail: edelperi@uvigo.es.
†
University of Vigo.
‡
University of California.
8860 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 23, 2009

While mechanisms governing the transport of dissolved
forms of copper in porous media have been widely studied,the movement of copper-based particles is still poorly
understood. Since various environmental factors affectmobility of soluble copper differently than the behavior ofparticulate forms, one can not predict travel distances of
one substance from the knowledge of the other. In order toobtain better understanding of the mechanisms controlling
the transport of particulate copper in saturated porous media,
we examined the transport of colloidal COF in watersaturated quartz sand columns in terms of colloid depositionand colloid filtration theory.

TheoryThe theory of colloid transport has been derived from theclassical convection-dispersion equation (CDE) and thecolloid attachment model. The attachment model assumes
a first-order kinetic attachment of colloids, and an attachment
coefficient that may decrease due to the filling of favorable
attachment sites, or increase due to filter ripening. Straining
represents the trapping of colloid particles when colloids are
retained in pores that are smaller than some critical size. For
an aqueous suspension of monodisperse particles flowing
vertically through a packed column of granular collectors,
the one-dimensional form of the CDE with two site kinetic
attachment may be written (8):θ∂(satt2)∂(satt1)
ωc
∂
∂c
) θD
- Uc - Fb
- Fb
∂t
∂z
∂z
∂t
∂t

(

)

(1)

where c is the colloid concentration in the aqueous phase[N L-3], t [T] is time, z is the vertical spatial coordinate [L],
θ is the volumetric water content [-], D is the hydrodynamic
dispersion coefficient [L2 T-1], U is the average pore-watervelocity [L T-1], Fb is the bulk density of the porous matrix[M L-3], and satt [N M-1] are concentrations of colloids
attached to two different attachment sites, denoted bysubscripts 1 and 2, respectively. The attachment rate to anyof the two sites is given byFb∂(satt)) θkattΨattc - Fbkdetsatt∂t(2)

where katt and kdet are the first-order colloid attachment anddetachment coefficients, respectively [T-1], and Ψatt is adimensionless colloid attachment function [-]. Several
functions examined in this work have been proposed in theliterature to account for colloid deposition: Langmuirian (9),random sequential adsorption (RSA) (10), ripening, and depth
dependent attachment (11).
The attachment coefficient, katt, can be determined using
the filtration theory (12), a quasi-empirical formulation that
expresses the attachment coefficient in terms of the mediangrain diameter of the porous medium (often termed thecollector), the pore-water velocity, and collector and collision(or sticking) efficiencies, accounting for colloid removal due
to diffusion, interception, and gravitational sedimentation(12, 13). Here, we used the HYDRUS-1D program (8) tointerpret the experimental particle breakthrough curves inlight of the colloid attachment models and the colloid
filtration theory.

Material and Methods
Copper Oxychloride. The copper oxychloride-based fungicide used in the tests (COF, manufactured by Industrias
Quı́micas del Vallés S.A., Barcelona, Spain) contains 50% of
10.1021/es901650g CCC: $40.75

 2009 American Chemical Society

Published on Web 10/30/2009

TABLE 1. Experimental Properties of the Fungicide
Cu content

g g-1

0.535

mean particle size
particle size distribution width
particle densityµmµm
g cm-3

0.979
0.272
2.889

TABLE 2. Properties and Parameters of the Packed Quartz
Sand Columns Used in the Fungicide Particle Deposition
Tests
collector diameter (dc)

cm

0.032

fluid density (Ff)fluid viscosity (µ)
temperature (T)Hamaker constant (A)porosity (f)column length (L)column diameter (L)Happel model parameter (As)kg m-3kg m-1 s-1KJ

103
8.9 × 10-4298
1.0 × 10-20
0.41
5
1.5
35.72

cm
cm

copper as a wetting powder (Table 1). The copper content
of the commercial product was determined in quintuplicate
using acid digestion with aqua-regia and hydrofluoric acidto the point of complete dissolution (14). The mineralcomposition as determined by an X-ray diffraction of
crystalline powder on a Siemens D-5000 instrument revealed
that the most abundant mineral phase is atacamite, and that
there are traces of botallackite, wroewolfeite, and quartz;
chemical elemental composition is reported in the Supporting
Information.
A suspension containing 4 g of fungicide per L of distilledwater (as recommended by the manufacturer) was prepared
to determine pH. An aliquot of the suspension was filteredthrough a membrane of 0.45 µm pore size to determine thedissolved Cu. Another aliquot of the suspension (100 mL)
was added, along with 5 mL 13.5 M HNO3, to determine thesoluble Cu in acid. Particulate Cu was calculated as thedifference between soluble and total Cu. The average particle
size diameter and ζ-potential of the fungicide powder in the
aqueous suspensions were measured over the range 0.6 nm
to 6 µm by dynamic light scattering on a Zetasizer Nano
equipment from Malvern Instruments Ltd. (Malvern, UK).
ζ-potentials were estimated from the electrophoretic mobility
using Smoluchowski’s approximation. Particle density wasmeasured by pycnometry (15).
Quartz Sand. Quartz sand (SiO2) was used as column
packing material. Quartz grains were well sorted, with a grain
diameter of 0.32 mm (Aldrich Chemical, Milwaukee, WI).
The sand was thoroughly cleaned prior to use (16). Visualinspection by scanning electron microscopy confirmed thatthe sand grains were prolate spheroidal in shape. ζ-potential
was measured in a near-micrometer size fraction, selectedby sedimentation from a suspension of the ground sandsample.
Transport Tests. Clean quartz sand was wet-packed into
adjustable chromatographic columns (Omnifit type, SigmaAldrich) 50 mm long and 15 mm in diameter, the top andbottom of which were covered with a nylon screen of 0.1 mm
nylon mesh. The water content and the bulk density were
determined by gravimetry in each test. Porosity was estimatedfrom the water content, assuming that the water density atroom temperature (20 °C) equals 0.9982 g cm-3 (17). Table
2 shows the most salient properties of the packed quartz
sand columns used in the fungicide particle deposition tests.
The longitudinal dispersivity of the column was estimatedfrom transport tests using bromide as tracer.

The suspensions used in the column tests were preparedby mixing COF with distilled water or NaNO3 solutions.
Particles larger than 2.5 µm in equivalent diameter were
removed by sedimentation. Copper concentration in thesuspensions (measured after adding 13.5 M HNO3, as
indicated above) ranged from 93 to 130 mg L-1. Particle
density measurements were used in conjunction withnominal particle diameters to calculate the number ofsuspended particles. Fungicide suspensions were vigorously
stirred while applied at the top end of the column via a Gilson
minipuls III peristaltic pump.
Particle concentrations were measured by light absorption
in a 80 µL flow-through cell from Hellma GmbH (Müllheim,
Germany); measurements were made at a wavelength of 800
nm at 80 s intervals on a Jenway 6310 spectrophotometer.
Effluent samples were also collected with a Gilson FC-203B
fraction collector. The correlation between the light absorption of effluent and the total copper content in collected
samples was used to calibrate the photometric readings. The
total copper content in outflow samples was determined forfungicide suspensions as described above. Differences inmass balances of the eluted COF between replicate runs wereless than 4%.
After the transport experiments have been finished, thesand was cut into slices 1 cm thick that were then digested
with acid and dissolved Cu concentrations were thendetermined. The copper concentration in effluent fractions
and sand extracts was measured on a Solaar M5 atomic
absorption spectrophotometer from Thermofisher Scientific,
Inc. (Waltham, MA), using an acetylene-air flame.
Experimental Plan. Column transport experiments of thefungicide in suspension (using NaNO3 of 10-1, 10-3, 10-5 M
and distilled water) were performed in order to obtainbreakthrough curves, and to examine the influence of ionicstrength on the attachment efficiency. The low range ofelectrolyte concentrations was chosen to represent heavy
rainfall episodes when the soil solution is diluted. Ionic
strength, pH, and ζ-potential of the fungicide particles were
measured for each of the four suspensions. In addition, atest similar to that described in (18) was conducted in thefollowing three steps, namely (1) deposition, similar as in
the transport tests described above; (2) elution, by washingthe quartz sand column containing the deposited fungicidewith a fungicide-free solution with the same pH and ionicstrength, and finally (3) further elution, using a solution withthe substantially reduced ionic strength.
A third type of transport test, which involved suspending
the fungicide in ionized water, and incorporated two different
Darcy’s velocities (0.57 cm min-1 and 2.83 cm min-1), wasperformed in order to assess the influence of hydrodynamics
on the fungicide deposition on sand grains.

Results and Discussion
The measured ζ-potential of the COF particles in thesuspensions used in the deposition experiments (Table 3)
became more negative as the ionic strength increased.
Additional measurements in the range of electrolyte from
10-5 to 0.1 M (Figure 1) showed a minimum Zeta ζ-potential
for electrolyte concentrations near 0.01 M. This behavior
may be related to the so-called atypical behavior also reported
in electrophoretic mobility experiments made with latex (19)and polystyrene spheres (20). This minimum could be aconsequence of the relaxation (polarization of the electric
double layer around the colloid during the ζ-potential
measurement) that can cause mobility to increase as thesurface true ζ-potential becomes more negative (21). Smoluchowski’s approximation does not consider this relaxation.
However, the maximum does not occur until the ζ-potential
reaches an absolute value of 100 mV (22). Other researchersreported that anomalous behavior may also occur for highVOL. 43, NO. 23, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 8861

TABLE 3. Properties of the Colloid Suspensions in Deionized (DW) Water Containing Variable Electrolyte Concentrations
NaNO3 concentration
(mM)

pH

ζ-potential(mV)copper in suspension(mg L-1)

fungicide concentration(mg L-1)

particle concentration,c0 (particles L-1)

DW
0.01
1.0
100.0

6.1
6.4
6.3
6.3

-31
-35
-40
-47

93
130
123
93

175
243
230
174

1.23 × 1011
1.71 × 1011
1.62 × 1011
1.23 × 1011

electrolyte concentrations and surface charge densities when
the counterion concentration in the immediate neighborhoodof the surface is so large that the charge of colloids isovercompensated. This phenomenon is commonly referred
to as overcharging (23). Multivalent counterions make asignificant contribution to overcharging, and hence on theelectrophoretic mobility of colloidal particles (24). In view ofthese studies, it is likely that Cu2+ dissolved from the copper
oxychlorhide (from 0.062 to 0.075 mM, see SI Figure S1) canconcentrate near the surfaces of fungicide particles, producing the so-called overcharging, the anomalous behavior
observed here. A qualitative consequence of this phenomenon is that, due to small additions of NaNO3, the presenceof divalent (Cu2+) rather than monovalent (Na+) counterions
intensifies counterion-particle attractions, and consequently,
particle-particle attractions (25).
Given the electrochemical conditions imposed in thetransport experiments, both the fungicide colloids and sandgrains were negatively charged. Consequently, electrostatic
repulsion between both types of particles may occur, at least
for the lowest ionic strengths used in the deposition
experiments in sand columns. However, the theoretical valuesof the Dejarguin-Landau-Verwey-Overbeek (DLVO) interaction energy profiles for colloids upon approach tocollector surface predict the presence of an energy barrier(about 900 times greater than the thermal energy of colloids)
against attachment at primary minima at the ionic strengths
used here.
Influence of the Ionic Strength on the Fungicide
Transport. The results of the colloid transport tests conducted
at different ionic strengths are shown as breakthrough curves(BTC) in Figure 2. The relative colloid concentration in theeffluent, C/C0, increased rapidly at the start of each test.
However, colloid attachment prevented the outflow concentrations from reaching the inflow concentration during
each experiment. That means that colloid filtration still
occurred despite of unfavorable electrochemical conditions
for attachment predicted by the DLVO theory for ionic

FIGURE 1. Dependence of ζ-potential for copper-based
fungicide particles on the solution ionic strength. Note a
minimum of zeta potential at the ionic strength near 10-2 M.
8862 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 23, 2009

strengths smaller than 1 mM. The mass of particles retainedin the columns for each of the experiments is shown in Table
4.
The experimental colloid BTCs (Figure 2a-d) were used
to analyze particle deposition using the dispersivity length(0.01 cm) determined for the transport of Br-. Several
combinations of deposition models were tested. While thetransport model with one site attachment was unable toreproduce the results, the two-site particle deposition modelwith blocking attachment functions offered good predictions
for the experimental BTCs. Convergence and uniqueness ofthe parameters were tested by rerunning the program withdifferent initial parameter estimates. Repeated runs resulted
in the same final parameter set in all cases. This likely
occurred because of the high data point density and low
scattering of the experimental BTCs’ data. The experimental
concentration profiles were not exponential, indicating thatthe deposition of the COF could not be explained by the
first-order deposition model without further modifications
by various blocking functions (see SI Figure S2). Consideringexperimental particle concentration profiles together with
the BTCs during parameter estimation did not significantly
affect the final values of the fitted parameters. While thetransport models that assumed either the ripening blocking
function or the depth dependent colloid straining gave poor
predictions of the BTCs, the transport model with an RSA
attachment model for both attachment sites provided thebest results. The RSA model was used to estimate themaximum attachment capacity, smax and the attachment rate
katt. The RSA model defines an attachment function as follows:ψatt ) 1 - 4a + 3.308a2 + 1.4069a3 for satt < 0.8smax(3)ψatt )(1 - bsatt)3
2 3
2d50
b

for satt g 0.8 smax

(4)where b ) smax-1 and a ) 0.546 satt/smax, in which smax is the
maximum solid phase concentration [M M-1]. Fitted parameters of the two-site RSA model are presented in Table
4. Whereas one attachment type had a large capacity (from1454 to 5760 mg Cu kg-1 sand) and slow kinetics (from 0.65
to 0.919 min-1), the other had low capacity (from 60.2 to 112
mg Cu kg-1 sand) and was between 3 and 10 times faster(from 3.2 to 6.5 min-1). The BTC obtained with distilled water(Figure 2a) shows that the deposition rate at later times was
greater than predicted by the RSA, indicating a slow
enhancement in deposition.
The maximum attachment capacity on the faster sites isclose to the theoretical hard sphere jamming limit value of
0.546 (26). However, for the second type of the attachment
site smax1 exceeds several times this value, suggesting that thesurface area is larger than a sphere, or that multilayer
deposition occurs. At ionic strengths smaller than 10-3 M,
there is a increase in katt2 because the long-range of the double
layer interactions favors increases in the attachment rate(22). Given the electrostatic repulsive forces, the long-range
attractive interactions should occur at the secondary minimum for ionic strengths from 100 to 1 mM. Aggregation

FIGURE 2. Breakthrough curves for copper-based fungicide particles at different solution ionic strengths. Experimental conditions:
Darcy fluid velocity U ) 2.83 cm min-1; collector diameter 0.032 cm; bed diameter 1.5 cm; column length 5 cm; temperature 25 °C.
Symbols represent the experimental breakthrough curves and lines the two-site random sequential attachment model for (a)deionized water, and three different NaNO3 concentrations of (b) 10-5 M; (c) 10-3 M; and (d) 10-1 M.

TABLE 4. Mass of Retained COF Particles and Fitting Parameters of the Two-Site Random Sequential Adsorption Model, As
Estimated from the Particle Breakthrough Data Obtained in Saturated Packed Quartz Sand at Different Ionic Strengths (Standard
Error of Fitting Parameters between Parentheses)
a

I

b

0.1
0.001
0.0001
D.W.

21
18
14
12

m

c

m/m0
41
38
27
23

d

smax2

112 (2.2)
91.8 (2.0)
102 (2.0)
60.2 (1.9)

f

katt2

3.17 (0.106)
2.87 (0.118)
6.11 (0.356)
6.89 (0.567)

e

smax1

5060 (90.0)
5760 (83.8)
4750 (159)1454 (49.2)

g

katt1

0.91 (0.007)
0.83 (0.005)
0.53 (0.005)
0.61 (0.008)

h

SSQ

i 2

0.005
0.006
0.015
0.012

0.995
0.994
0.987
0.985

r

Ionic stregth (mol L-1). b Mass of particles retained in the column (mg). c Percentage retained. d Maximum capacity of
attachment (mg Cu kg-1 sand). e Constant of attachment (min-1). f Maximum capacity of attachment (mg Cu kg-1 sand).
gConstant of attachment (min-1). h Sum of squares of the residuals. i Squared Pearson’s correlation coefficient.
abetween the COF particles prior to deposition may contributeto multi layer deposition, and therefore to the high Smax1.
Also, the surface heterogeneity of the quartz grains may playa significant role. The low values of katt2, in comparison withkatt1, indicate that the interactions in a multi layer deposition(i.e., aggregation between COF particles) may be of shorter
range than between the COF and the clean quartz surface.
Tests involving variable NaNO3 concentrations revealed
that the attachment of the fungicide increased with increasing
electrolyte concentrations, which we ascribed to an increasing
collision efficiency. This behavior is in accordance with thestandard Poisson-Boltzmann theory; as the ionic strength
increased, diffuse double layers were compressed, andrepulsive electrostatic double-layer forces between colloid
particles diminished as a result. The discrepancy between
ζ-potential measurements and colloid deposition experiments may reside in factors influencing electrophoretic
mobility, such as polarization of the ion layer surrounding
the colloid particles, and not from the attachment. Underchemically unfavorable attachment conditions (electrostatic
repulsion), colloids would interact with the solid phase by
virtue of the presence of secondary energy minima at
separation distances greater than the location of the energy

barrier (27). However, attachment in the chemically unfavorable secondary minimum can significantly contribute to
the retention of colloids in filtration experiments (28, 29).
Both heterogeneity and the influence of the electrolyte
concentration suggest that the attachment exhibited in our
experiments may occur in the secondary minimum. But, thedepth of the secondary minimum is critical for its ability tocreate deposition. Figure 3 shows the representation of thetotal energy of interaction versus surface-to-surface separation distance profiles for quartz grains and COF particle sandcalculated using the DVLO theory. It can be seen that usinga typical value of the Hamaker constant for aqueous mineraldispersions of 10-20 J, the repulsive barrier lies in the range
from 275 to 1564 KB T, whereas the depth of the secondary
minimum from -73 to -15 KB T could explain colloid
attachment for ionic strengths between 100 and 10 mM. Other
factors that may have influenced attachment include theshape of the colloidal particles and non-DVLO forces.
Heterogeneity of the surface of quartz sand, the presence of
impurities, and heterogeneity in the shape and size of thefungicide particles can increase the retention well above what
can be expected based on the DLVO theory (30).
VOL. 43, NO. 23, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 8863

FIGURE 3. Representation of the total energy of interaction versus surface-to-surface separation distance profiles for quartz grain
and colloid with sizes used in this work, suspended in aqueous electrolyte concentrations ranging from 0.1 to 100 mM. (a) Shows
the repulsive maximum barrier that prevents attachment in the primary minimum; (b) The same data in enlarged beginning of they-axis show the secondary minimum for each of the electrolyte concentration 1, 10, and 100 mM NaNO3. The Hamaker constant for
the quarz-water-COF system was 1.0 10-20 J at 298 K.

FIGURE 4. Scanning electron microphotographies of quartz sand grains covered with Fe obtained using the back scattering electron
detector. The light areas correspond with a higher amount of sorbed Fe, whereas the darker areas are clean (less reactive) quartz
surfaces (confirmed using the X-ray fluorescence microprobe). Note that the dark areas are flat, indicating a lower reactivity of thequartz crystal sides and minor local surface area, whereas light areas have a rougher surface and a greater local surface area.
The chemical and physical heterogeneity of the quartz
sand surface was studied on a sample of coated iron-oxide
grains (31) from the same sand batch. The surface analysis
was carried out by scanning electron microphotography usingbackscattered electron detection and X-ray fluorescenceprobe. Since iron adsorbs preferentially to the negative
8864 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 23, 2009

surfaces, the amount of Fe adsorbed reveals the sites withmore negative surface charge.
Figure 4 shows the chemically adsorbed iron patches (lightpatches) in the negatively charged sites, where dissociatedsilanol groups (dSisOH, pKa ∼3.5-6.9) occur at the pH∼6.3 used in the column experiments. Silanol groups are

FIGURE 5. The three-phase breakthrough curve for fungicide
particles. A suspension of fungicide particles was deposited ats ) 0.1 M NaNO3 in phase 1 (from 0 to 80 min), eluted with 0.1
M NaNO3 in phase 2 (80 to 120 min), and further eluted with
deionized water in phase 3 (from 120 to 160 min). Experimental
conditions: water velocity U ) 2.83 cm min-1; collector
diameter 0.032 cm; column diameter 1.5 cm; column length 5
cm; and temperature 25 °C.
located at imperfections in the crystal lattice; cracks andbroken ends with more ruptured sSi2sOsSis bonds per
surface area. These patches also have higher roughness, which
increases the local surface area available for attachment. The
smooth crystal sides have less specific surface, which was
revealed as dark patches with minor electrostatic repulsion(less negative charge). These surface properties agree withthe two-site deposition model suggested by the colloidtransport experiments. Therefore, the faster attachment sites
with a low smax correspond to low repulsion and smaller
surface area, whereas the slower sites with higher repulsion
have a high smax due to larger surface area. The surface
potential on the crystal sides would decrease below -30 mV,
which is about the threshold potential of the repulsive barrierthat the thermal Brownian energy can overcome, and thisprevents particles from attaching in the primary minimum.
Mass balance shows that retention of 23% of particles
still occurs for electrolyte concentrations smaller than 1 mM
(i.e., without the secondary minimum), indicating thatphysical factors may also contribute to retention. A recent
work shows that surface roughness of the collector may createzones of relative stagnation, in which pore space geometry
and solution chemistry’s coupled effects have a significant
influence in colloid retention during transport (32).
Three-Phase Test. Based on the previous results, we
assumed that the attachment of COF depends on theelectrolyte concentration. In order to confirm this hypothesis,
we ran a second test, during which we changed the electrolyte
concentration within a single breakthrough (18).
The BTC for the three-phase experiment is shown in Figure
5. A suspension containing 1.4 1011 particles L-1 in 0.1 M
NaNO3 was applied during phase 1 to a column of packed
quartz sand and about 25% of the particles were depositedin the column. Rinsing the column with 0.1 M NaNO3 duringphase 2 failed to elute a measurable quantity of deposited

FIGURE 6. Breakthrough curves for copper-based fungicide
particles showing the influence of the water velocity on
particle attachment. Experimental conditions: collector diameter
0.032 cm; bed diameter 1.5 cm; bed depth 5 cm; temperature 25°C. Symbols denote the results obtained at different watervelocities U ) 2.83 cm min-1 (circles) and 0.566 cm min-1(triangles). Solid lines denote 2-sites RSA model.
fungicide. Rinsing of the column with deionized water duringphase 3 released only 8.5% of the particles deposited duringphase 1 and retained during phase 2. This result showed theinfluence of electrochemical conditions on the particle
attachment. However, retention of particles in phase 3
supports the hypothesis that physical heterogeneity andporous space geometry may contribute to retention (32).
Hydrodynamic Effects. As can be seen from breakthroughcurves obtained for different flow velocities (Figure 6), themaximum outflow concentrations decreased by a factor of
2 when flow velocity decreased five times. The fittedparameters of the two-site kinetic attachment model shownin Table 5 indicate that slower flow results in an increase insmax1. A decrease in attachment rates at both sites occurred
when the inflow rate of the particles became limiting toretention.
The effect of hydrodynamics implies that the forces thatkeep the colloids attached in the smax2 are weak. The
assumption of hydrodynamic influence on attachment iscompatible with the weak forces at the secondary minimum,
because increased shear stress forces would tend to pull
attached colloids away (27). In addition, this behavior alsosuggests that increasing the flow rate tends to reduce
stagnation zones, in which colloids can be retained.
To sum up, the results of depth-bed deposition tests witha copper oxychloride-based fungicide in water saturated
quartz sand columns suggest the influence of the ionicstrength and the hydrodynamic shear on the attachment
rate. Attachment can occur in the weak secondary minimum
for ionic strengths from 100 to 1 mM NaNO3. However,retention in the absence of the secondary minimum (i.e.,with ionic strength less than 1 mM) indicates that physicalfactors may also contribute to retention. Surface roughnessof collector may create zones of relative stagnation, in whichpore space geometry and solution chemistry’s coupled effects
have a significant influence on colloid retention duringtransport (32). This shows the importance of pore structure
on colloid retention. Although attachment forces are weak,

TABLE 5. Fitting Parameters of the Two-Site Random Sequential Adsorption Model at Two Different Water Flow Velocities inPacked-Sand Columns
a

U

2.83
0.57

b

Smax2

60.2 (1.9)
55.3 (0.86)

d

katt2

6.89 (0.57)
1.56 (0.07)

c

e

Smax1

kattS11454 (49.2)
9815 (1862)

0.61 (0.01)
0.26 (0.00)

f

g 2

SSQ

r

-2

1.18 × 10
4.36 × 10-3

0.985
0.999

a
Water flow velocity (cm min-1). b Maximum capacity of attachment (mg Cu kg-1 sand). c Constant of attachment(min-1). d Maximum capacity of attachment (mg Cu kg-1 sand). e Constant of attachment (min-1). f Sum of squares of theresiduals. g Squared Pearson’s correlation coefficient.

VOL. 43, NO. 23, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 8865

the attachment of copper as oxychloride particles in quartz
sand beds is larger than the maximum sorption capacity fordissolved copper in some acid soils (33). The environmental
implications of these results lie in the fact that mobility ofcolloidal copper oxychloride-based fungicides is determinedby characteristics of the diffuse layer, hydrodynamics andphysicochemical heterogeneity of the porous media.

Acknowledgments
This work was funded by Spain’s Ministry of Science andTechnology (Project AGL2006-04231/AGR) and Xunta de
Galicia (Project INCITE 08PXIB 383190PR). J.E.L.-P. wasadditionally funded under a Ramón y Cajal contract (European Social Fund RYC-2003-004996, Spain’s MCyT), andso was J.C.N.-M. under a Parga Pondal contract (Galicia’sCouncil of Education), we also thank Erasmus student Mr.
Adrian Letzner from the University of Duisburg-Essen for
helping us with the iron coatings.

Supporting Information Available
Influence of electrolyte added (NaNO3) in suspensions ofcopper-oxychloride fungicide on the activity of Cu2+aq,
observed and predited profiles of concentration at the endof the breakthrough experiment using deionized water. This
material is available free of charge via the Internet at http://
pubs.acs.org.

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