Environmental Pollution 255 (2019) 113124

Contents lists available at ScienceDirect

Environmental Pollution
journal homepage: www.elsevier.com/locate/envpol

Transport and retention of engineered silver nanoparticles incarbonate-rich sediments in the presence and absence of soil organic
matter* 
Yorck F. Adrian a, Uwe Schneidewind a, b, Scott A. Bradford c, *, Jirka Sim
unek d,eaErwin Klumpp , Raﬁg AzzamaDepartment of Engineering Geology and Hydrogeology, RWTH Aachen University, Lochnerstr. 4-20, 52064 Aachen, Germany
Department of Civil and Environmental Engineering, Western University, London, ON N6A3K7, Canada
US Salinity Laboratory, USDA, ARS, Riverside, CA 92507, USAd
Department of Environmental Sciences, University of California, Riverside, CA 92521, USAe
Agrosphere (IBG-3), Institute of Bio- and Geosciences, Forschungszentrum Jülich GmbH, Jülich, Germanybca r t i c l e i n f o

a b s t r a c t

Article history:
Received 26 March 2019
Received in revised form
28 June 2019
Accepted 26 August 2019
Available online 28 August 2019

The transport and retention behavior of polymer- (PVP-AgNP) and surfactant-stabilized (AgPURE) silver
nanoparticles in carbonate-dominated saturated and unconsolidated porous media was studied at thelaboratory scale. Initial column experiments were conducted to investigate the inﬂuence of chemical
heterogeneity (CH) and nano-scale surface roughness (NR) arising from mixtures of clean, positively
charged calcium carbonate sand (CCS), and negatively charged quartz sands. Additional column experiments were performed to elucidate the impact of CH and NR arising from the presence and absence ofsoil organic matter (SOM) on a natural carbonate-dominated aquifer material. The role of the nanoparticle capping agent was examined under all conditions tested in the column experiments. Nanoparticle transport was well described using a numerical model that facilitated blocking on one or two
retention sites. Results demonstrate that an increase in CCS content in the artiﬁcially mixed porous
medium leads to delayed breakthrough of the AgNPs, although AgPURE was much less affected by the
CCS content than PVP-AgNPs. Interestingly, only a small portion of the solid surface area contributed to
AgNP retention, even on positively charged CCS, due to the presence of NR which weakened the adhesiveinteraction. The presence of SOM enhanced the retention of AgPURE on the natural carbonate-dominatedaquifer material, which can be a result of hydrophobic or hydrophilic interactions or due to cation
bridging. Surprisingly, SOM had no signiﬁcant impact on PVP-AgNP retention, which suggests that a
reduction in electrostatic repulsion due to the presence of SOM outweighs the relative importance ofother binding mechanisms. Our ﬁndings are important for future studies related to AgNP transport inshallow unconsolidated calcareous and siliceous sands.
© 2019 Elsevier Ltd. All rights reserved.

Keywords:
Calcium carbonate
Engineered silver nanoparticles
Stabilizing agent
Blocking
Soil organic matter

1. Introduction
Engineered silver nanoparticles (AgNPs) are frequently appliedin textiles and personal care products due to their antimicrobial andantifungal properties (Maillard and Hartemann, 2013; Panacek
et al., 2009). Their release into the environment (Farkas et al.,*This paper has been recommended for acceptance by Baoshan Xing.
* Corresponding author. USDA-ARS, U.S. Salinity Laboratory, 450 W. Big Springs
Road, Riverside, CA, 92507, USA.
E-mail address: scott.bradford@ars.usda.gov (S.A. Bradford).

https://doi.org/10.1016/j.envpol.2019.113124
0269-7491/© 2019 Elsevier Ltd. All rights reserved.

2011; Kaegi et al., 2010), bioavailability (Navarro et al., 2008) andpotentially toxic or detrimental effects on a multitude of organisms(Juganson et al., 2017; Makama et al., 2016; Mallevre et al., 2014;
Siller et al., 2013) have been well documented in the scientiﬁcliterature. Recent studies have demonstrated and discussed theiroccurrence in surface waters (Peters et al., 2018), wastewater
treatment plant inﬂuents/efﬂuents (Kaegi et al., 2013; Kaegi et al.,€uerlein et al., 2017).
2011) and sewage sludge (BaTransport studies for AgNPs have been carried out to improve
our understanding of factors that control their environmental fate
and to better assess potential risks. Many previous transport

2

Y.F. Adrian et al. / Environmental Pollution 255 (2019) 113124

studies have been conducted in pure quartz sands (Liang et al.,2013b; Tian et al., 2010), topsoils (Braun et al., 2015; Cornelis
et al., 2013; Liang et al., 2013a; Makselon et al., 2018; Makselon
et al., 2017; Sagee et al., 2012), sand ﬁlters (Degenkolb et al.,2018), or consolidated rock such as sandstone (Neukum et al.,2014a,b). Calcium-based carbonate minerals such as calcite(CaCO3) or dolomite (CaMg[CO3]2) are major components of carbonate aquifers around the world that are vital for drinking-waterproduction (Bayat et al., 2014). However, only few studies on
engineered nanoparticle transport through carbonate-dominatedmaterial exist. Rahmatpour et al. (2018) compared polyvinylpyrrolidone (PVP)-coated AgNP transport through pure quartz
sand and two calcareous topsoils with considerable clay content(sandy loam and loam). They found that their two topsoils retained
around 99% of the injected PVP-AgNPs, whereas pure quartz sandretained about 11%. Experiments with graphene oxide nanoparticles in limestone porous media show an increase in mobility
associated with increasing pH and humic acid concentration (Donget al., 2017). Bayat et al. (2014) and Bayat et al. (2015) studied thetransport of Al2O3 and TiO2 nanoparticles through limestone byvarying the background solution. Laumann et al. (2013) showedthat the mobility of polydisperse nano zero-valent iron strongly
decreases with increasing carbonate content in crushed limestone.
These authors attributed retention of negatively charged nanoparticles in limestone to cation bridging and elimination of theenergy barrier to attachment by charge heterogeneity and/or near
neutral zeta potentials (Laumann et al., 2013; Dong et al., 2017;
Rahmatpour et al., 2018), as well as to the morphology of thelimestone (Bayat et al., 2014, 2015). Systematic studies considering
predeﬁned unconsolidated mixtures of carbonate- and quartzbased sands as well as natural carbonate-rich aquifer material do
not exist in the literature to the knowledge of the authors.
The charge of calcite surfaces is determined by the presence ofCa2þ and CO2in the diffuse layer (Al Mahrouqi et al., 2017;3
Heberling et al., 2011). The zero point of charge for pure calcite isaround 8 to 9.5 (Somasundaran and Agar, 1967), so it is expected topossess a net positive charge under neutral pH conditions. However, a negative surface charge on calcite may arise from impurities
with lower zero points of charge such as silicate minerals (Chenet al., 2014) or when soil organic matter (SOM) is adsorbed onto
its surface (Al Mahrouqi et al., 2017; Vdovi
c, 2001), and thus, charge
heterogeneity on the calcite surfaces is expected.
Soil (or sediment) charge heterogeneity has frequently beenreported to strongly inﬂuence nanoparticle transport and retention
by locally reducing or eliminating the energy barrier for primary
minimum interactions (Bendersky and Davis, 2011; Bradford et al.,2017). However, no research has systematically examined the inﬂuence of calcite with impurities from silicate minerals and/or SOM
coatings on AgNP transport. Chemical heterogeneities (CH) in suchmedia will produce variations in charge, Hamaker constant(Bradford et al., 2018), and/or hydrophobicity (Tschapek, 1984).
Humic acid (HA), an amphiphilic component of SOM, has beenfound to stabilize and enhance the transport of AgNPs (Park et al.,2016; Yang et al., 2014), as well as other nanoparticles (Wanget al., 2012), in porous media. Enhanced nanoparticle transport in
the presence of HA, surfactants, and SOM has been attributed todecreases in the electrostatic attraction, increases in steric repulsion, and competitive blocking of retention sites (Ryan et al., 1999).
However, the stabilizing effect of SOM has been found less distinct
for PVP-coated AgNP compared to citrate-coated AgNP due to differences in the bonding energy between the capping agent and thesilver, which is higher in the case of PVP (Gunsolus et al., 2015).
Consequently, interactions of SOM with AgNPs are expected todepend on the physicochemical properties of the SOM and the

AgNP capping agent, although limited research has examined thisspeciﬁc issue to date.
Differences in soil mineral weathering (e.g., quartz and calcite)
and SOM coatings are anticipated to inﬂuence the nanoscaleroughness (NR) properties of porous media. Furthermore, theconformation of SOM coatings can change depending on the watercontent as well as the solution chemistry (Sposito, 2008). Various
capping agents may also alter the roughness properties of AgNPs.
The collector surface roughness has been found to play an important role in controlling the mobility and fate of nanoparticles due toits strong inﬂuence on interaction energies, hydrodynamic forces,
and lever arms associated with hydrodynamic and adhesive torques (Morales et al., 2009; Torkzaban et al., 2010). When small
amounts of NR occur on both porous medium and colloid surfaces
the magnitudes of the secondary and primary minima, and theenergy barriers are all strongly reduced in comparison to a smoothsurface (Bendersky and Davis, 2011; Bhattacharjee et al., 1998;
Bradford et al., 2018; Bradford and Torkzaban, 2013; Hoek andAgarwal, 2006), and the colloid may be susceptible to diffusive
and/or hydrodynamic removal (Bradford and Torkzaban, 2015;
Rasmuson et al., 2017). These changes in the interaction energy
proﬁle may even occur in the presence of CH, such that electrostatically favorable locations are unfavorable for colloid retention(Zhang et al., 2016). However, the coupled inﬂuence of CH and NR
from calcite minerals, SOM, and capping agents has not yet beenstudied.
Here we conduct laboratory-scale column experiments andnumerical modeling to systematically study the transport andretention behavior of polymer- and surfactant-stabilized AgNPs incarbonate-dominated saturated and unconsolidated porous media.
Our research provides novel experimental data that provide new
insight into AgNP mobility related to the inﬂuence of soil/sediment
CH and NR. First, we examine the inﬂuence of CH and NR arisingfrom mixtures of clean calcium carbonate and quartz sands. Next,
we study the inﬂuence of CH and NR arising from the presence andabsence of SOM on a natural carbonate-dominated aquifer material. Finally, the role of the AgNP capping agent is inspected underboth of these soil CH and NR conditions. These results are interpreted using mathematical modeling of AgNP transport andblocking behavior on one or two kinetic retention sites, calculations
to determine the fraction of the porous media surface that
contributed to retention, and interaction energy calculations thatconsidered the inﬂuence of various fractions of nanoscaleroughness.
2. Material and methods
2.1. Silver nanoparticles and electrolyte solution
For the laboratory experiments, we used two commerciallyavailable types of AgNPs. The ﬁrst type was sterically stabilized
silver nanoparticles AgPURE™ W-10 (ras materials, Regensburg,
Germany) from a stock suspension with an AgNP concentration of
10.16% (weight per weight, w/w) and identical to the OECD reference material NM300K. According to Klein et al. (2011), AgPURE™
stabilization is achieved using 4% w/w of polyoxyethylene glycerol
trioleate and 4% w/w of polyoxyethylene (20) sorbitan monolaurate (Tween 20). Additionally, AgPURE™ W-10 contains 7.5%(w/w) ammonium nitrate (Menzel et al., 2013). About 99% of thespherically-shaped particles are in a size range of 15e20 nm. Theirhydrodynamic diameter was found to be stable over 24 h in 1 mM
KNO3 at 45.1 ± 4.5 nm (Liang et al., 2013b), in 1 mM Naþ at
51.0 ± 1.3 nm and in 1 mM Ca2þ at 56.4 ± 1.5 nm (Adrian et al.,2018). The original suspension also contains 5% of free/unboundsurfactant and less than 1% of the total silver mass was released as

Y.F. Adrian et al. / Environmental Pollution 255 (2019) 113124

Agþ within three days (Liang et al., 2013b).
The second type of AgNP used in this work was coated with PVP(Getnanomaterials GNM, OOCAP SAS, France, stock number:
7023HZ, powder form). Data provided by the manufacturer showsthat the PVP-AgNP powder consists of 10% silver (purity of 99.99%),89.8% PVP and 0.2% moisture. The PVP-AgNP diameter is 20 nm andthe density 2.07 g cm3. Adrian et al. (2018) showed that the PVPAgNP is stable in 1 mM Naþ and 0.1 mM Ca2þ with hydrodynamic
diameters of 66.9 ± 4.4 nm and 61.8 ± 0.9 nm, respectively.
Electrolyte solutions were prepared using ultrapure water(Merck Millipore Milli-Q, 18.2 MU cm, TOC: 1e2 ppb, 0.22 mm
membrane ﬁlter) and 1 mM NaHCO3 (Merck KGaA, Darmstadt,
Germany) with a pH of 8.3. To better compare our results with otherstudies using column experiments (Liang et al., 2013a; Wang et al.,2014), the total silver concentration in the inﬂuent solution (C0) for
transport experiments was adjusted to 10 mg L1, which is above
reported concentrations found in soil, surface or groundwater(Klitzke et al., 2015; Peters et al., 2018; Troester et al., 2016;
Yecheskel et al., 2018).
Transmission electron microscopy (TEM) was performed toprovide further information on the size and shape of the AgNPs.
Samples were dispersed in an ultra-sonic bath for 15 min. Copper
specimen grids coated with an ultrathin layer of carbon (Plano
GmbH, Germany) were immersed in the sample solution and thenair-dried. Micrographs were obtained using a CM 20 FEG (Philips,
Netherlands) operating at 200 keV.
2.2. Porous media
To study the effect of varying calcium carbonate sand (CCS)
content on AgNP transport, ﬁve mixtures of natural quartz sand(Schlingmeier Quarzsand GmbH & Co. KG, Schwülper, Germany)and CCS were prepared, with CCS contents (w/w) of 0, 20, 50, 80
and 100%, respectively. According to the distributors, the natural
quartz sand (average particle density of 2.65 g cm3) consisted of
99.5% SiO2, 0.02% K2O, 0.036% Fe2O3, 0.01% TiO2, and other minor
fractions, while the CCS (average particle density of 2.70 g cm3,
Ulmer Weiss from Eduard Merkle GmbH & Co. KG, Blaubeuren,
Germany) contained 99.2% (weight) CaCO3, 0.4% MgCO3, about
0.135% iron oxides and aluminum oxides, and 0.25% SiO2. Bothmaterials were wet sieved into size fractions of 0.71e1 mm and1e1.25 mm, and then cleaned to remove organic impurities using
the methods of Chu et al. (2001) for the quartz sand (i.e. soaked in
30% H2O2 for 24 h) and Mikutta et al. (2005) for the CCS (described
below). The sand was subsequently oven dried before use. Eachcolumn experiment employed a selected CCS content with a similar
grain size distribution. The median grain size diameter for CCS
mixtures was determined to be 0.92 mm with a coefﬁcient of uniformity of 1.3 and coefﬁcient of curvature of 0.9.
The natural carbonate-dominated aquifer material was obtained
from an open pit mine near Eching, Germany (Münchner Kiesunion
GmbH & Co. Sand-und Kieswerke KG, Unterschleißheim, Germany).

3

It was sieved following DIN 18123 (2011) to obtain a size range from
0.063 to 2 mm and a median grain diameter (d50) of 0.7 mm. The
average particle density was determined according to DIN 18124(2011) using pycnometers (n ¼ 3). The mineralogy of the aquifer
material was determined by means of X-ray powder diffraction(XRD) using a Siemens D5000. To evaluate the effect of SOM onnatural aquifer material, experiments with cleaned and uncleaned
material were conducted. Cleaning of the material was performed
using the method suggested by Mikutta et al. (2005). In brief, SOM
from the grain surfaces was removed by soaking each grain size
fraction three times over 24 h in NaClO (Hedinger, Stuttgart, Germany) that was diluted to 6%, adjusted to pH ¼ 8.5, and at room
temperature. Note that this NaClO solution causes no dissolution ofmineral components (Gaffey and Bronnimann, 1993). Therefore it isexpected that roundness and sphericity of the porous medium will
not signiﬁcantly change. After three cleaning cycles, the material
was rinsed with ultra-pure water over a 21 mm PES membrane(Franz Eckert GmbH, Waldkirch, Germany) and oven-dried at 50  C.
The organic matter content was determined by weight loss. The pHof all porous media used was determined according to DIN EN
15933 (2012) in 10 mM CaCl2 in the absence and presence of SOM.
A CamSizer P4 (Retsch Technology, Germany) was employed todetermine roundness and sphericity of the CCS, quartz sand, andthe aquifer material (cleaned and uncleaned), respectively. The
surface area of all materials was determined by N2-BET measurements (n ¼ 3). The zeta potential of all materials was measured
using a Malvern ZetaSizer Nano ZS (Malvern GmbH) by the method
presented in Sondi et al. (2009). In short, the respective material
mixtures were crushed, and 40 mg of each was dispersed in 100 mLof the background solution at 25  C before measurements.
2.3. Transport experiments
Transport experiments were carried out in borosilicate glasscolumns with an inner diameter of 2.6 cm, a length of 15 cm, and a
volume of 79.6 mL. The columns were wet-packed in 1 mM
NaHCO3. The porosity of the material packed into each column isprovided in Table 2. A 2 mm-layer of quartz wool was used at thecolumn inlet and outlet to retain ﬁne particles in the material
matrix. All experiments were conducted in an up-ﬂow mode using
a peristaltic pump (ICP High Precision Multichannel Dispenser,
Ismatec). The average pore water velocity in the experiments was
8.64 m d1. Before AgNP transport experiments, tracer tests with
NaBr (Merck KGaA, Darmstadt, Germany) were run. For that, thecolumns were ﬁrst ﬂushed with at least 20 pore volumes (PV) of
1 mM NaHCO3. Each tracer test consisted of ﬂushing 3 PV of NaBr in
1 mM NaHCO3 solution through the column, followed by continued
elution with 3 PV of 1 mM NaHCO3 to prevent dissolution of the
carbonate material. After each tracer test, the respective column
was again ﬂushed with 20 PV of 1 mM NaHCO3 before at least 3 PV
of AgNP suspension was injected, followed again by ﬂushing with atleast 3 PV of AgNP-free 1 mM NaHCO3. Column efﬂuent fractions

Table 1
Zeta potential (n ¼ 3) of the porous media and silver nanoparticles in 1 mM NaHCO3 at pH ¼ 8.3 and temperature of 25  C, hydrodynamic diameter (dH, n ¼ 6) of PVP-coated
AgNP and AgPURE in 1 mM NaHCO3 at pH ¼ 8.3, roundness and sphericity information obtained from CamSizer measurements and BET surface area of the porous medium.
SampleQuartz sand
CCS
Eching (uncleaned)
Eching (cleaned)
PVP-AgNP
AgPURE

Zeta potential

dH

Roundness

Sphericity

BET

[mV]

[nm]

[-]

[-]

[m2 g1]

0.65
0.33
0.44

0.75
0.64
0.72

0.13
0.21
1.09
0.94

60.7
þ1.1
26.1
30.0
9.8
17.1

60.9 ± 1.1
51.1 ± 0.2

e
e
e
e
1.17
e
1.55
e
e
e
e
5.29
6.13
8.44

]  10
[s
]

e
e
e
e
e
e
e
e
e
e
e
0.52
0.40
0.48

[cm g

0.98
3.66
6.11
2.42
14.50
5.38
151.00
6.36
36.00
28.90
8.05
18.80
92.80
125.00]  10
[s
]

e
0.17
0.14
0.38
0.29
0.10
0.05
0.42
1.25
1.49
0.12
0.06
0.35
0.24

[cm g

0.39
0.32
0.28
0.29
0.78
0.19
0.38
0.19
0.18
0.29
0.41
0.86
1.15
0.56

[m]  10]  10

2.9
4.0
4.1
3.6
3.8
3.8
4.2
4.4
4.5
4.4
3.8
3.9
4.0
4.0
0
20
50
80
100
0
20
50
80
100
EchingEchingEchingEching
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.9
0.7
0.7
0.7
0.7
AgPURE
AgPURE
AgPURE
AgPURE
AgPURE
PVP-AgNP
PVP-AgNP
PVP-AgNP
PVP-AgNP
PVP-AgNP
AgPURE
AgPURE
PVP-AgNP
PVP-AgNP
2a
S1a
2a
S1a
2a
2b
S1b
2b
S1b
2b
3a
3a
3b
3b

0.38
0.38
0.38
0.44
0.46
0.37
0.41
0.43
0.44
0.45
0.39
0.39
0.39
0.39

[m s
[%]
[mm]
[-]

4: porosity, determined gravimetrically; d50: mean grain size diameter; CCS: Calcium carbonate sand content in the artiﬁcially mixed porous media; q: Darcy velocity (discharge per unit area); lL: dispersivity obtained fromtracer experiments; Smax1: maximum solid phase concentration on the ﬁrst sorption site; ksw1: ﬁrst-order retention coefﬁcient for the ﬁrst sorption site; Smax2: maximum solid phase concentration on the second sorption site;
ksw2: ﬁrst-order retention coefﬁcient for the second sorption site; R2: coefﬁcient of determination; AgNP input: volume of AgNP suspension injected into the column; Meff: mass recovery in the efﬂuent; Msolid: mass recovery inthe soil; Mtotal: total mass recovery; SOM: soil organic matter; Sf: solid surface area available for retention.

e
16.63
11.64
27.47
19.29
39.27
17.32
123.69320.11351.04
1.70
7.08
37.60
31.12
No
No
No
No
No
No
No
No
No
No
No
Yes
No
Yes
90.30
79.37
89.13
101.26
94.31
96.32
99.95
90.00
94.23
100.99
99.50
75.87
66.33
79.91
9.20
4.07
4.67
25.17
23.41
10.5
23.07
21.00
61.83
72.63
12.1
10.98
14.83
27.33
90.12
108.57897.84
114.58193.6090.01
134.60122.56214.57237.35181.18186.53189.59189.30
0.98
0.95
0.93
0.98
0.95
0.98
0.99
0.97
0.85
0.96
0.95
0.96
0.99
0.99

81.10
75.31
84.46
76.09
70.9
85.82
76.88
69.00
32.40
28.36
87.40
64.90
51.50
52.58

[%]
[%]
[%]
[mL]
[-]

MSolidMeff
AgNP input
R2

4

ksw2

1
1
3

Smax2/C0

4

ksw1

1
1
3

Smax1/C0

2

lL

5
1

Q
CCS
d50
4
AgNP
Fig.

Table 2
Summary of experimental conditions and simulated parameter values of surfactant- and polymer-stabilized AgNP in calcium carbonate sand and carbonate-dominated aquifer material.

MtotalSOM[-]  104

Y.F. Adrian et al. / Environmental Pollution 255 (2019) 113124

Sf4

were collected in polypropylene centrifuge tubes (VWR International GmbH. Germany) using a fraction collector (Pharmacia FRAC100 Fraction Collector, Amersham Biosciences). For each pore volume three to four samples were analyzed. Inﬂow and outﬂow pH
were monitored using a SenTix 41 pH electrode (WTW, Weilheim,
Germany).
The total Ag concentration in efﬂuent samples was determinedusing ICP-MS (Agilent 7900 ICP-MS, Agilent Technologies). The
electrophoretic mobility of the AgNPs in the stock solution (n ¼ 3
for each background solution) was determined with a Malvern
Zetasizer Nano ZS at 25  C and then converted to a zeta potential[mV] using the Smoluchowski equation.
At the end of the transport experiments, the column material of
selected experiments was carefully extracted in 1 cm layers, ovendried, and homogenized to generate retention proﬁles. For this,
0.5 g of the homogenized porous medium was transferred into a
50 mL centrifuge tube and acidiﬁed using 5 mL HNO3 (w/w 65%).
The total silver concentration was determined using ICP-MS. Most
AgNP transport experiments were replicated, and results are shownin the supporting information (SI).
Section S1 of the SI provides detailed information aboutmodeling the tracer and AgNP transport experiments, which was done with the numerical ﬁnite element code HYDRUS 1D (Simunek
et al., 2008; Simunek et al., 2016). For the AgNP experiments,
Langmuirian blocking with two kinetic retention sites was
assumed. Inversely determined model parameters included theretention rate coefﬁcients (ksw1 and ksw2) and the maximum solid
phase AgNP concentrations (Smax1 and Smax2), where the subscripts
1 and 2 on parameters denote the site. Fitted values of Smax1 andSmax2 were used to determine the fraction of the solid surface areathat contributed to AgNP retention (Sf) using Eq. (S5). The Akaike
information criterion (AIC) (Akaike, 1974) was calculated to quantitatively compare the quality of different models for AgNP retention. It was applied to justify the use of one or two retention sites inthe various models.
Interaction energy calculations that considered various surface
roughness fractions (frs ¼ 0, 0.01, 0.025, 0.05, and 0.1) and aroughness height (hrs) of 35 nm were performed to better understand mechanisms contributing to AgNP retention on the various
sands (quartz, carbonate, the Eching aquifer material (denoted
below as Eching), and Eching without SOM) in 1 mM NaHCO3 at
pH ¼ 8.3 solution. Section S2 of the SI provides details about thesecalculations for AgNP-sand interactions. The dimensionless depthsof the primary (F1min) and secondary (F2min) minima, and the energy barrier height (Fmax) were obtained by analyzing the interaction energy proﬁles.

3. Results and discussion
3.1. Characterization of AgNPs and porous media
TEM images (Fig. 1) show the near-spherical nature of the AgNPs
with diameters of around 16e20 nm for AgPURE and 11e25 nm for
PVP-AgNP, respectively. Hydrodynamic diameters (dH) were foundto be around 51.1 nm ± 0.2 nm for AgPURE and 60.9 nm ± 1.1 nm for
PVP-AgNP in inﬂuent samples determined via DLS (Table 1), whichis in good agreement with previous studies (Liang et al., 2013b;
Adrian et al., 2018). The zeta potential was slightly more negative
for AgPURE (17.1 mV) than PVP-AgNP (9.8 mV) in 1 mM NaHCO3,
which suggests a potential for greater suspension stability.
CamSizer measurements (Table 1, on a scale from 0 to 1)
revealed that the quartz sand had the highest roundness (0.65) andsphericity (0.75), while the CCS was less rounded (0.33) and lessspherical (0.64). The total organic carbon content of the quartz sand

Y.F. Adrian et al. / Environmental Pollution 255 (2019) 113124

5

Fig. 1. TEM images of a) PVP-AgNPs and b) AgPURE at 20 nm range.

following H2O2 treatment was determined by ignition loss as
0.091%. The BET measurements showed a higher surface area for
the CCS (0.21 m2 g1) as compared to the quartz sand (0.13 m2 g1).
This implies that the CCS had more roughness and/or internal
porosity than the quartz sand. At pH 8.3, the zeta potential of thequartz sand was more negative (60.7 mV) compared to the AgNPs(Table 1). In contrast, the CCS showed a slightly positive zeta potential (þ1.1 mV). A positive zeta potential was also encountered by
Vdovi
c (2001) for synthetic calcium carbonate at pH ¼ 8.4 ± 0.1 inthe absence of organic matter. The AgNPs are therefore expected to
exhibit electrostatic attraction for the CCS and repulsion for thequartz sand, especially for AgPURE.
Results from the X-ray diffraction analysis (Table S1) showedthat the natural aquifer (Eching) material was dominated by carbonate minerals (56.1% calcite and dolomite), while 37.6% arequartz minerals. Chlorite clay minerals, smectite clay minerals, andkaolinite are present only in small amounts (<1.4%). The rest of thematerial is formed by microcline and albite. The average particle
density for the Eching material was determined from pycnometer
experiments to be 2.733 g cm3, which is close to that of purecalcite with 2.7 g cm3. The uncleaned Eching material had a BET
surface area of 1.09 m2 g1, whereas the surface area decreased to
0.94 m2 g1 (Table 1) after the removal of SOM, which also produced a weight loss of 0.24 ± 0.07% on the natural carbonatedominated material. SOM consists of a complex mixture of fulvic
acid, humic acid, and humin fractions (Huang et al., 2003) that
includes carboxylic and phenolic groups (MacCarthy and Rice,
1985), biopolymers such as lignin and polysaccharides(MacCarthy and Rice, 1985), mineral-bound lipids and humic acidlike materials (Rice and MacCarthy, 1990), and kerogen and black
carbon (Song et al., 2002). The presence of calcium carbonate in a
porous medium can be accompanied by free excess Ca2þ which caninhibit the degradation of SOM by forming Ca-SOM bridges (Clough
and Skjemstad, 2000). Although the SOM fraction of the Eching soil
is very small, it can coat large portions of the exposed mineralsurface (Doerr et al., 2000). DLS measurements show that the
Eching material exhibited a negative zeta potential in the presenceof SOM (26.1 mV) due to a fairly large fraction of quartz minerals(Table 1). After cleaning, the zeta potential further decreasedto 30.0 mV. Consequently, a net repulsive electrostatic interaction
is expected between the AgNPs and the Eching aquifer material
with and without SOM.
The pH of the artiﬁcial quartz-carbonate sand mixtures in
10 mM CaCl2 increased with increasing CCS content from 5.8 (100%quartz sand) to 8.4 (100% CCS). During the transport experiments,
the pH of the inﬂuent remained stable at 8.3 ± 0.1 in 1 mM NaHCO3.
For experiments with pure quartz sand, the efﬂuent pH remained at
8.3 ± 0.1 while the efﬂuent pH in experiments with CCS only
slightly increased to 8.4 ± 0.1. This small increase in the efﬂuent pH
can be attributed to the buffering capacity of calcium carbonate(Dong et al., 2017). The pH of the natural aquifer material was
determined at 8.4 ± 0.1 (n ¼ 3) before and 8.5 ± 0.1 (n ¼ 3) after
cleaning of SOM. Here, the efﬂuent pH was 8.4 ± 0.1.
Dissolution of calcium carbonate during our transport experiments and its effect on the AgNPs was considered negligible for two
reasons. First, Ca2þ concentrations in selected efﬂuent samples
were never higher than 0.01 mM in our experiments, as determined
by ICP-MS (data not shown). This concentration is much lower thanthe 2.5 mM, for which Topuz et al. (2014) and Topuz et al. (2015) did
not observe any signiﬁcant effect on the agglomeration behavior of
PVP-AgNPs. Second, Topuz et al. (2014) demonstrated that prolonged exposure of PVP-AgNPs to carbonate (over a week) had little
effect on agglomeration behavior. Our experiments did not takelonger than 4.5 h, and the hydrodynamic diameters of AgNPs in thepresence of calcium carbonate did not signiﬁcantly change during
this time interval.

3.2. Effect of calcium carbonate content and stabilizing agent on
AgNP transport
Fig. 2a and b present observed and simulated breakthroughcurves (BTCs) for AgPURE (Fig. 2a) and PVP-AgNPs (Fig. 2b) inporous media containing 0, 50 and 100% CCS content. Figs. S1a andS1b also contain information on 20 and 80% mass of CCS. Retention
proﬁles (RPs) for 0% and 100% CCS are presented in the SI (Fig. S2).
Fig. 2a and b also show example BTCs for the conservative bromide
tracer. All tracer tests showed nearly complete recovery of bromide(97.6e102.6%) and breakthrough close to 1 PV. The bromide tracer
breakthrough occurs in almost all cases before AgNP breakthrough,which indicates that pore-size exclusion of AgNPs was not signiﬁcant. Table 2 provides a summary of the considered physicochemical conditions, measured and ﬁtted model parameters, andmass balance information for all AgNP transport experiments. The
total mass recovery of AgNPs in the experiments shown in Table 2
varied between 79.4 and 101.3%. Standard errors and conﬁdence
intervals of the ﬁtted parameters as well as values of the AIC areprovided in the SI (Tables S2 and S3). Replicate experiments conducted for the CSS/quartz sand mixtures are listed in Table S2. In

6

Y.F. Adrian et al. / Environmental Pollution 255 (2019) 113124

Fig. 2. Breakthrough curves of AgPURE (a) and PVP-AgNP (b) transport experiments inquartz sand and calcium carbonate sand (CCS) with various mass percentages of CCS in
the artiﬁcially mixed porous medium. Pore water velocity ¼ 8.64 m d1 andd50 ¼ 0.9 m.

general, replicate experiments agreed well with the results of theexperiments shown in Table 2 as can be assessed by comparison ofﬁtted model parameters and efﬂuent mass balance information for
AgNPs that are presented in Table S3.
AgPURE was highly mobile in the various CCS mixtures, with the
efﬂuent mass ranging from 70.9 to 84.5% (Table 2). Breakthrough of
AgPURE (Fig. 2a and S1a) occurred at around 1 PV, rapidly increased
and reached a plateau, and exhibited little blocking behavior when
CCS  80%. Conversely, the breakthrough of AgPURE in pure CCS(100% CaCO3) was retarded, and then slowly increased to a similarplateau as the other CCS mixtures. The RPs for AgPURE (Fig. S2a)
showed nearly uniform distribution with depth.
The efﬂuent mass of PVP-AgNP rapidly and systematicallydecreased with increasing CCS percentage from 85.8% to 28.4%(Table 2). This reﬂects an increase in the amount of PVP-AgNP
retention with the CCS percentage. The PVP-AgNP BTCs clearly
showed blocking behavior when CCS 50% (Fig. 2b and S1b) andexhibited greater delay in breakthrough with increasing CCS percentage. Decreasing breakthrough with increasing carbonate content (however no signiﬁcant retardation) was similarly observed by
Laumann et al. (2013) for PAA-nZVI nanoparticles in crushedlimestone. At CCS above 20%, much greater retention of PVP-AgNP

occurred than of AgPURE (comparison of Figs. S1a and S1b).
However, both PVP-AgNP and AgPURE exhibit an increased delay inbreakthrough and blocking behavior with increasing CCS
percentages.
The shape of the PVP-AgNP RP in pure quartz sand was uniform,
whereas in 100% CCS the RP shape was nonmonotonic (Fig. S2b).
Other researchers have observed nonmonotonic RPs for AgNPs andother nanoparticles (e.g. Liang et al., 2013b) and have attributed
their ﬁndings to a number of factors, including: competitive
blocking from free surfactants and/or polymers (Becker et al., 2015;
Liang et al., 2013b), solid phase migration (Bradford et al., 2011;
Johnson et al., 2018), and reversible blocking (Leij et al., 2015). All ofthese factors may contribute to the development of nonmonotonic
RPs in our experiments, but the relative importance of these factors
cannot be deduced from the collected information and is beyond
the scope of the present study.
Observed BTCs in Fig. 2a and b were reasonably well describedusing the mathematical model outlined in the SI, with coefﬁcientsof determination (R2) ranging from 0.85 to 0.99. The AIC values
indicated that blocking was not needed to describe the BTC for
AgPURE in pure quartz sand, otherwise blocking was always
justiﬁed on site 1 (see Table S2 in the SI for AIC values). A second
retention site was sometimes needed to describe the BTC data (e.g.,
CCS ¼ 100% for AgPURE and CCS ¼ 20% for the PVP-AgNPs, see e.g.
Fig. S4), but the AIC indicated that blocking was not warranted on
this site. Collectively, the modeling results indicate that blocking
played an important role in AgNP fate. Other researchers have
similarly reported the importance of blocking on colloid andnanoparticle transport through porous media (Adrian et al., 2018;
Liang et al., 2013a; Wang et al., 2015), and noted the need toconsider two retention sites (Bradford et al., 2014; Sasidharan et al.,2014). The mathematical model did not provide a good descriptionof the nonmonotonic PVP-AgNPs RPs. Simulation of nonmonotonic
RPs requires the use of more complex mathematical formulations
which increase the uncertainty in parameter estimates (e.g.,Bradford et al., 2011; Johnson et al., 2018), and was beyond thescope of the current study.
Table 2 indicates that ksw1 and Smax1 tended to increase with the
CCS percentage. This trend is expected because attachment iselectrostatically favorable and rapid between negatively charged
AgNPs and the slightly positively charged calcium carbonate material (Table 1). Values of ksw1 and Smax1 are generally greater for
PVP-AgNPs than AgPURE at a given CCS percentage. This behavior
cannot be explained by electrostatic considerations which predict agreater electrostatic attraction between the calcium carbonate andthe more negatively charged AgPURE than the PVP-AgNPs (Tables 1
and 3). Furthermore, the values of Sf that were determined fromﬁtted values of Smax1 using Eq. (S5) from the SI were very small (e.g.,<0.04 in Table 2). This indicates that only a small fraction of thesurface contributed to AgNP retention, even under the best-casescenario of positive electrostatic interactions (e.g., carbonate inTable 3, CCS ¼ 100% and AgPURE). Consequently, consideration ofchemical heterogeneity alone cannot explain the observed AgNP
retention behavior. Potential explanations for these deviations will
be considered below.
The presence of surfactants and/or polymers on the surface of
AgNPs has been reported to decrease colloid retention due to steric
repulsion (Yang et al., 2014). Steric repulsion is predicted to produce
a large energy barrier to attachment that should hinder any AgNP
retention on the collector surface (Israelachvili, 2011). This result isclearly not consistent with the experimental observations shown in
Figs. 2 and 3. An alternative explanation of the observed AgNP
retention behavior is due to nanoscale roughness which is ubiquitous on natural surfaces (Suresh and Walz, 1996) and AgNPs due

Y.F. Adrian et al. / Environmental Pollution 255 (2019) 113124

7

Table 3The predicted dimensionless energy barrier (Fmax) and primary minima (F1min) for
AgPURE and PVP-AgNPs with the various sands in 1 mM NaHCO3 solution at
pH ¼ 8.3 when the sand roughness height (hrs) is 35 nm and the roughness fraction(frs) is varied from 0 to 0.1.
AgNP

Sand

frs
[-]

AgPURE
AgPURE
AgPURE
AgPURE
AgPURE
AgPURE
AgPURE
AgPURE
AgPURE
AgPURE
AgPURE
AgPURE
AgPURE
AgPURE
AgPURE
AgPURE
AgPURE
AgPURE
AgPURE
AgPURE
PVP-AgNP
PVP-AgNP
PVP-AgNP
PVP-AgNP
PVP-AgNP
PVP-AgNP
PVP-AgNP
PVP-AgNP
PVP-AgNP
PVP-AgNP
PVP-AgNP
PVP-AgNP
PVP-AgNP
PVP-AgNP
PVP-AgNP
PVP-AgNP
PVP-AgNP
PVP-AgNP
PVP-AgNP
PVP-AgNP

QuartzQuartzQuartzQuartzQuartz
CCS
CCS
CCS
CCS
CCS
EchingEchingEchingEchingEchingEching w/o SOM
Eching w/o SOM
Eching w/o SOM
Eching w/o SOM
Eching w/o SOM
QuartzQuartzQuartzQuartzQuartz
CCS
CCS
CCS
CCS
CCS
EchingEchingEchingEchingEchingEching w/o SOM
Eching w/o SOM
Eching w/o SOM
Eching w/o SOM
Eching w/o SOM

0
0.01
0.025
0.05
0.1
0
0.01
0.025
0.05
0.1
0
0.01
0.025
0.05
0.1
0
0.01
0.025
0.05
0.1
0
0.01
0.025
0.05
0.1
0
0.01
0.025
0.05
0.1
0
0.01
0.025
0.05
0.1
0
0.01
0.025
0.05
0.1

Fmax

F1min

[nm]

[-]

[-]

35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35
35

15.3
1.2
1.3
1.6
2.3
0.0
0.0
0.0
0.0
0.0
12.3
0.7
0.8
1.1
1.7
13.2
0.8
0.9
1.2
1.8
6.0
0.6
0.6
0.6
0.9
0.0
0.0
0.0
0.0
0.0
5.5
0.4
0.4
0.5
0.8
5.7
0.4
0.5
0.6
0.8

72.8
0.7
0.4
2.3
6.0
29.4
0.3
0.8
1.5
3.0
1.2
0.6
0.6
0.6
0.7
0.7
0.7
0.7
0.6
0.6
148.9
0.6
2.8
6.6
14.0
21.1
0.2
0.6
1.1
2.1
18.5
0.2
0.1
0.5
1.5
25.7
0.2
0.2
0.8
2.1

hrs

CCS: calcium carbonate sand; SOM: soil organic matter.

to adsorbed surfactants and/or polymers. Table 3 provides calculated interaction energy parameters for AgPURE and PVP-AgNPs on
the various sands when hrs ¼ 35 nm and frs ¼ 0, 0.01, 0.025, 0.05,
and 0.1. Consistent with these results, NR pillars are predicted to
locally reduce or eliminate the energy barrier (Bhattacharjee et al.,
1998; Hoek and Agarwal, 2006), and decrease the magnitudes ofthe primary and secondary minima (Bradford et al., 2017;
Torkzaban and Bradford, 2016). These effects on the interactionenergy proﬁle are most pronounced when small fractions of NR
pillars occur on both the surfaces of the colloid and collector(Bradford et al., 2017), and produce conditions that are susceptible
to diffusive or hydrodynamic removal, even when the surfaces areelectrostatically favorable for attachment (Torkzaban and Bradford,2016). Consequently, only a small fraction of the collector surface
can contribute to irreversible attachment due to spatial variabilityof roughness properties (Bradford et al., 2017). Note that the AgNP
mass transfer rates to roughness pillar tops (fast) and valleys (slow)
are expected to be different and this provides a potential explanation for the need to consider two retention sites. In addition, theenergy barrier can be reduced and/or eliminated and the depth ofthe primary minimum can be increased when roughness and pore
structure create valleys where AgNPs are wedged (Li et al., 2017).

Fig. 3. Breakthrough curves of AgPURE (a) and PVP-AgNP (b) transport experiments inthe presence and absence of soil organic matter (SOM) in the natural carbonatedominated aquifer (Eching) material, respectively. Pore water velocity ¼ 8.64 m d-1
and d50 ¼ 0.7 mm.

Increasing amounts of chemical heterogeneity with the CCS percentage can increase the depths of the primary minimum andenhance limited attachment and wedging on these rough surfaces.
The secondary minimum was always negligible (>-0.002) and wastherefore not further considered. Note that the BET surface area for
CCS was much larger than for quartz (Table 1), which indicates thepresence of greater amounts of surface roughness and/or internal
porosity.
3.3. AgNP mobility in presence and absence of SOM on carbonatedominated aquifer material
Fig. 3 presents observed and simulated BTCs for AgPURE (Fig. 3a)and PVP-AgNP (Fig. 3b) in natural porous media in the presence andabsence of SOM. Retention proﬁles for AgPURE and PVP-AgNP in
the presence and absence of SOM are presented in the SI (Fig. S3).
Table 2 summarizes the considered physicochemical conditions,
measured and ﬁtted model parameters, and mass balance information for these four transport experiments. Standard errors andconﬁdence intervals on the ﬁtted parameters as well as information
regarding the AIC are shown in the SI (Tables S2 and S3). A summary of the replicate experiments is listed in Table S2.
The total mass recovery of AgNPs in the experiments listed inTable 2 varied between 66.3 and 99.5%. Observed AgNP BTCs werereasonably well described with the mathematical model, with R2
ranging from 0.95 to 0.99. The AIC values indicated that blocking ona single retention site was needed to describe the BTC for AgPURE

8

Y.F. Adrian et al. / Environmental Pollution 255 (2019) 113124

in the absence of SOM, whereas blocking on two retention sites wasneeded to describe BTCs for other conditions (e.g., PVP-AgNPs in
the presence and absence of SOM, and AgPURE in the presence of
SOM). An example of a modeled BTC with Langmuirian blocking on
one and two kinetic retention sites is presented in Fig. S5 andfurther information is provided in Table S2 in the SI. Bromide
tracers showed a near complete recovery of 98.2e101.0%, andbreakthrough close to 1 PV.
Fig. 3a indicates that the BTC for AgPURE in the absence of SOM
reached unity after about 4 PV, whereas blocking was more pronounced and produced a slowly ascending plateau when SOM waspresent. Blocking occurs more rapidly for smaller values of Smax
and/or Sf. The calculated value of Sf (Eq. S(5)) was very low, butmore than four times higher in the presence than the absence of
SOM (Table 2). Furthermore, most of the retention sites in thepresence of SOM were more slowly ﬁlled than in the absence ofSOM, although a smaller number ﬁlled more rapidly (Table 2).
Further evidence for blocking was obtained from the RPs whichshowed a near uniform distribution with depth in the presence andabsence of SOM (Fig. S3a). The value of the normalized solid phase
concentration (S/C0) was higher in the presence of SOM due todifferences in Smax1þSmax2.
AgPURE exhibited higher mobility when SOM was removed
from the sediments, with efﬂuent mass recoveries of 64.9% in thepresence of SOM and 87.4% in the absence of SOM (Table 2). This
reﬂects a decrease in the adhesive interaction of AgPURE with the
Eching aquifer material when SOM was removed. This can partiallybe explained by greater electrostatic repulsion between the
AgPURE (17.1 mV) and the sediment after the removal of SOM;
e.g., the zeta potential of the soil decreased from 26.1 mV
to 30.0 mV. In addition, nonionic surfactants (e.g., Tween 20) thatare used to stabilize AgPURE are known to interact with SOM via
various functional groups (Chiou et al., 1983; Hu et al., 2010; Uranoet al., 1984). Interactions discussed in the literature that could favor
increased AgNP retention on SOM-coated particles include hydrophobic interactions, hydrogen bonding or cation bridging (Baker,
1991; Grillo et al., 2015; Hoppe et al., 2014; Zhang et al., 2012;
Zhang et al., 2009). Experiments with viruses showed that thepresence of humic acid on silica surfaces as well as divalent cations
could reduce virus mobility. It was found that calcium is capable offorming cation bridges by binding to carboxylate groups on boththe humic acid and the protein shell of the virus (Pham et al., 2009).
Fig. 3b shows that the BTCs for PVP-AgNPs were similar in thepresence and absence of SOM. In contrast to AgPURE (Fig. 3a), SOM
only had a minor inﬂuence on the mobility of PVP-AgNPs (Fig. 3b);e.g., a breakthrough in the presence of SOM occurred slightly before
that in the absence of SOM, and then reached similar plateau
values. Table 2 indicates that the efﬂuent recoveries of PVP-AgNPs
were similar in the presence and absence of SOM (52.6% and 51.5%,
respectively). BTCs for PVP-AgNPs in both porous media had
Smax1<Smax2 and ksw1[ksw2 (Table 2). The total value of Smax1þSmax2 was similar in the presence and absence of SOM, but thecalculated value of Sf was still very small (Table 2). RPs of PVPAgNPs (Fig. S3b) show no clear trend, although higher Ag concentrations were determined in the presence of SOM.
The efﬂuent mass recovery in Table 2 indicates that PVP-AgNPs
exhibited less mobility than AgPURE in the Eching aquifer material
without and with SOM. Consistent with these observations, theparameters ksw1, ksw2, Smax1, Smax2, and Sf were also larger for the
PVP-AgNPs than AgPURE. These observations can be partiallyexplained by the more negative charge for AgPURE (17.1 mV)
compared to PVP-AgNPs (9.8 mV) in 1 mM NaHCO3 at pH 8.3(Table 1). The electrostatic repulsion between AgPURE and theaquifer material is, therefore, higher than between PVP-AgNPs andthe aquifer material leading to more favorable attachment conditions for PVP-coated AgNPs. Several other factors can also
contribute to differences in interactions of AgPURE and PVP-NPs
with SOM. It is possible that hydrogen bonding or hydrophobic
interactions that are relevant for Tween 20-binding to SOM might
not be signiﬁcant in the case of the polymer-SOM interactions. SOM
has been found to mask charge heterogeneities and cation bridging
with clay particles (Cabal et al., 2010; Han et al., 2008). The interaction of SOM with AgNPs is also expected to be sensitive to theroughness density and height of the AgNP capping agent (Bradford
et al., 2017, 2018) and the bonding energy between the capping
agent and the AgNP (Gunsolus et al., 2015). Additional research iswarranted to determine the underlying cause for differences in
SOM-AgNP interactions, but a systematic evaluation of these factors is beyond the scope of this work.

4. Conclusions
We systematically examined the fate of AgNPs in the presence ofcarbonate minerals. Increasing the CCS content in the artiﬁciallymixed porous medium tended to lead to greater retardation of the
breakthrough curve and increasing retention of AgNPs, although
AgPURE was much less affected by the CCS content than PVPAgNPs. Interestingly, only a small portion of the solid surface area
contributed to AgNP retention, even on positively charged CCS.
Interaction energy calculations demonstrated that the presence ofsmall amounts of nanoscale roughness on the CCS will dramaticallyweaken the adhesive interaction and can explain why only limited
AgNP retention occurred on these surfaces. Consequently, theexperimental and modeling results, and interaction energy calculations demonstrate that chemical heterogeneity played a secondary role in AgNP retention than nanoscale roughness. Furthermore,these experiments demonstrate that AgNP breakthrough in CCScontaining porous media mixtures depends on the AgNP stabilizing agent, the solid surface roughness, and the amount of favorable
attachment places available on the porous medium. The latter isdirectly dependent on the CCS content that in a natural material
would be deﬁned by geological and geochemical processes.
Experiments with the natural aquifer material (Eching) showedthat AgPURE is retarded in its breakthrough in the presence of SOM,
whereas SOM removal had no signiﬁcant impact on the retention
behavior of PVP-AgNP. AgPURE interactions with SOM have been
attributed to hydrophobic interactions, hydrogen bonding or cation
bridging. The insensitivity of PVP-AgNP to the presence of SOM waspartially attributed to a decrease in the electrostatic repulsion of
PVP-AgNPs, which diminished the relative importance of hydrophobic interactions or cation bridging.
This study demonstrates the complex interplay between stabilizing agent, surface roughness and soil organic matter regarding
the transport and fate of surfactant- and polymer-stabilized AgNPs
in carbonate-rich porous media. Based on the information presented in this work we can predict that the transport of AgNPs in
porous media with increasing carbonate content will be reduced
more strongly in the case of polymer-coated than surfactantstabilized AgNPs. In contrast, the retention of surfactantstabilized AgNPs will be larger than that of polymer-stabilized
ones in the presence of natural organic matter.
Calcium carbonate-rich sand is part of many vadose zones andaquifers used for drinking water production around the world. As
such, our ﬁndings could become important for future ﬁeld-scalestudies related to silver nanoparticle transport in shallow unconsolidated calcareous and siliceous porous media, especially at locations where these are exposed to signiﬁcant input of silver
nanoparticles.

Y.F. Adrian et al. / Environmental Pollution 255 (2019) 113124

Conﬂict of intrest
The authors declare no competing interests.
Acknowledgments
The contributions of Yorck F. Adrian and Uwe Schneidewind
were funded in part by the Federal Ministry of Education andResearch (BMBF), Germany, Project Nanomobil (03X0151A). The€hler
authors would like to thank Yvonne Münchow and Bruno Ko
from RWTH Aachen University, Department of Engineering Geology and Hydrogeology for their help with some of the laboratory
work and Claudia Walraf from Research Center Jülich for the BET
measurements. CamSizer measurements were carried out withhelp of the Department of Engineering Geology at TU Berlin. TEM
measurements were carried out by Nina Siebers (IBG-3) at the Ernst
Ruska-Centre (ER-C), Research Center Jülich, Germany.
Appendix A. Supplementary data
Supplementary data to this article can be found online athttps://doi.org/10.1016/j.envpol.2019.113124.
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