Transport of graphene oxide in saturated quartz sand containing iron oxides

Science of the Total Environment 657 (2019) 1450–1459

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Science of the Total Environment journal homepage: www.elsevier.com/locate/scitotenv

Transport of graphene oxide in saturated quartz sand containing iron oxides Zhichong Qi a,b,1, Tingting Du b,1, Pengkun Ma b, Fangfei Liu b, Wei Chen b,⁎ a

College of Chemistry and Chemical Engineering, Henan Joint International Research Laboratory of Environmental Pollution Control Materials, Henan University, Kaifeng, Henan 475004, China College of Environmental Science and Engineering, Ministry of Education Key Laboratory of Pollution Processes and Environmental Criteria, Tianjin Key Laboratory of Environmental Remediation and Pollution Control, Nankai University, Tianjin 300350, China

b

H I G H L I G H T S

G R A P H I C

A B S T R A C T

• Iron oxides heterogeneity in saturated porous media markedly inhibits GO transport. • Extent of transport inhibition depends on morphology of iron oxides in porous media. • Presence of iron oxide coatings magnifies inhibition effects of cations on transport. • Iron oxides provide additional surface hydroxyl groups that attract cations.

a r t i c l e

i n f o

Article history: Received 10 October 2018 Received in revised form 7 December 2018 Accepted 10 December 2018 Available online 11 December 2018 Editor: Jay Gan Keywords: Graphene oxide Transport Porous media Iron oxides Cation bridging

a b s t r a c t The environmental implications of graphene oxide (GO) have received much attention. Transport of GO in subsurface environment is a critical process affecting the migration and potential risks of this important class of carbonaceous nanomaterials. To date, the effects of heterogeneity in porous media, in particular, iron oxides, on GO transport are not well studied. In this study, we investigated the transport properties of GO in saturate quartz sand as affected by the presence of iron oxides, using goethite, hematite and ferrihydrite as the model iron oxide species, and applied a two-site transport model (which accounts for both attachment and straining) to fit the transport data. We found that iron oxide coating on sand surfaces markedly inhibited GO transport, mainly due to increased electrostatic attraction between particles and collectors, as the positively charged iron oxides provided favorable deposition sites for the negatively charged GO nanosheets. Additionally, increased surface roughness was likely an additional mechanism leading to the enhanced GO deposition. The extent of transport inhibition by iron oxides also depended on the morphology iron oxides, in that at the same Fe loading a larger effect was observed when iron oxides existed as the coating on sand surface than as discreet particles. The presence of iron oxide coatings (tested using goethite) could magnify the effects of cations on GO transport. Specifically, the presence of goethite facilitated the accumulation of cations on the surface of sand, and in the case of Ca2 + , the binding of GO via the cation-bridging mechanism was enhanced, as goethite contained abundant surface hydroxyl groups that are strong metal-complexing moieties. © 2018 Elsevier B.V. All rights reserved.

1. Introduction ⁎ Corresponding author. E-mail address: [email protected] (W. Chen). 1 Zhichong Qi and Tingting Du contributed equally to this work.

https://doi.org/10.1016/j.scitotenv.2018.12.143 0048-9697/© 2018 Elsevier B.V. All rights reserved.

The production and application of graphene oxide (GO) materials have been increasing rapidly, owing to their superior electromagnetic,

Z. Qi et al. / Science of the Total Environment 657 (2019) 1450–1459

optical, mechanical, and thermal properties (De Marchi et al., 2018). Meanwhile, the potential environmental implications of GO, upon its accidental and incidental release, have also received much attention. Understanding the fate and transport of GO in the environment is critical for the risk assessment and management of this new class of nanomaterials. Transport properties of GO in saturated porous media, as well as the effects of solution chemistry parameters (e.g., ionic strength, ionic species, pH, and natural organic matter), have been examined in a number of studies (Feriancikova and Xu, 2012; Lanphere et al., 2013; Liu et al., 2013; Lanphere et al., 2014; Qi et al., 2014a; Qi et al., 2014b; Qi et al., 2014c; Xia et al., 2015; Fan et al., 2015a; Fan et al., 2015b; He et al., 2015; Sun et al., 2015; Dong et al., 2016; Zhou et al., 2016; Dong et al., 2017; He et al., 2017; Lu et al., 2017; M. Wang et al., 2017; Xia et al., 2017; Zhang et al., 2018; Chen et al., 2018; Jiang et al., 2018). These studies show that GO exhibits relatively high mobility in saturated porous media under conditions relevant to typical subsurface environments. Ionic strength can markedly influence the transport of GO by affecting both the surface charges and particle sizes of GO (Lanphere et al., 2013; Liu et al., 2013; Qi et al., 2014b; Qi et al., 2014c). In systems dominated with monovalent cations transport of GO is not very responsive to pH (5 to 9), because the surface of GO contains large amounts of hydroxyl and carboxyl functional groups varying in dissociation constant, Ka (Lanphere et al., 2013; Liu et al., 2013; Qi et al., 2014b; Qi et al., 2014c; Konkena and Vasudevan, 2012). The presence of natural organic matter or surfactants can enhance the transport

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of GO, via steric hindrance or by competing with GO for deposition sites on grain surfaces (Qi et al., 2014b; Qi et al., 2014c; Xia et al., 2015; Fan et al., 2015a). Notably, divalent cations (e.g., Ca2+) have significant effects on the transport of GO, not only because they are effective in compressing the electrical double layer, but also because they may act as bridging agents between the O-functional groups of GO and hydroxyl groups of sand grains, resulting in enhanced binding of GO nanosheets to porous media (Qi et al., 2014b; Ren et al., 2014; Xia et al., 2015; Fan et al., 2015b; Xia et al., 2017; Gao et al., 2017; Ren et al., 2018). Thus far, most of the studies on GO transport have been conducted using purified quartz sand. Nonetheless, surface charge heterogeneity is known to influence the transport and retention properties of colloids and nanoparticles in granular media (Ryan and Gschwend, 1992; Song et al., 1994; Fang et al., 2009). In particular, the iron and aluminum oxides represent common source of surface charge heterogeneity in natural subsurface environments (Ryan and Gschwend, 1992; Johnson et al., 1996). To date, only two studies have been conducted to elucidate the transport of GO in iron oxide (FeOx)-coated sand (Duster et al., 2016; D. Wang et al., 2017). Duster et al. (2016) reported that GO deposition onto iron oxide-coated sands was largely controlled by electrostatic forces, however, deposition rate at high ionic strength could not be explained with electrostatic interactions alone and may be influenced by nanoparticle aggregation. D. Wang et al. (2017) reported that the positively charged iron oxide coating on sand surfaces immobilized the negatively charged GO in the primary minimum. Interestingly, they

Table 1 Experimental protocols of column tests. Column no.

Column properties Fe content (mg-Fe/g-sand)a

Influent properties msand/mcoated sand

b

Length (cm)

Bulk density (g/cm3)

Porosity (−)

Background electrolyte

pH

GO conc. (mg/L)

ζ-Potentialc (mV)

Zaved (nm)

dp/dce

−10.1 ± 0.6 −10.1 ± 0.6 −10.1 ± 0.6 −10.1 ± 0.6 −10.1 ± 0.6 −10.1 ± 0.6 −10.1 ± 0.6 −10.1 ± 0.6 −12.6 ± 0.8 −11.3 ± 0.3 −10.1 ± 0.6 −21.3 ± 0.4 −17.6 ± 1.1 −15.7 ± 1.0 −14.0 ± 0.5 −20.6 ± 1.2 −14.9 ± 0.3

103.2

0.0004

103.2

0.0004

103.2

0.0004

103.2

0.0004

103.2

0.0004

103.2

0.0004

103.2

0.0004

103.2

0.0004

98.7

0.0004

96.8

0.0004

125.6

0.0005

276.7

0.0011

109.0

0.0004

187.3

0.0007

291.1

0.0011

100.8

0.0004

272.5

0.0010

1

0

/

6.9

1.55

0.42

10 mM NaCl

4.9

9.5

2

0.64 (G, coated)

0.48/0.52

6.9

1.53

0.42

10 mM NaCl

5.1

10.5

3

1.24 (G, coated)

/

6.9

1.55

0.42

10 mM NaCl

4.9

10.0

4

0.64 (H, coated)

0.47/0.53

6.9

1.56

0.41

10 mM NaCl

5.0

10.0

5

0.64 (F, coated)

0.87/0.13

6.9

1.55

0.42

10 mM NaCl

5.0

9.8

6

0.61 (G, mixed)

/

6.9

1.55

0.42

10 mM NaCl

5.0

9.5

7

0.61 (G, coated)

/

6.9

1.55

0.42

10 mM NaCl

5.0

9.8

8

1.24 (G, mixed)

/

6.9

1.55

0.42

10 mM NaCl

5.0

9.9

9

0.64 (G, coated)

0.48/0.52

6.7

1.55

0.42

0.5 mM NaCl

5.0

10.7

10

0.64 (G, coated)

0.48/0.52

7.0

1.49

0.44

5 mM NaCl

5.0

10.2

11

0.64 (G, coated)

0.48/0.52

7.1

1.34

0.45

0.5 mM KCl

5.0

10.8

12

0.64 (G, coated)

0.48/0.52

7.2

1.44

0.42

10 mM KCl

5.0

10.4

13

0.64 (G, coated)

0.48/0.52

6.9

1.52

0.42

0.1 mM CaCl2

5.0

10.8

14

0.64 (G, coated)

0.48/0.52

7.1

1.53

0.42

0.3 mM CaCl2

5.0

10.3

15

0.64 (G, coated)

0.48/0.52

7.0

1.52

0.42

0.5 mM CaCl2

5.1

10.1

16

0.64 (G, coated)

0.48/0.52

7.1

1.34

0.44

5.1

9.9

17

0.64 (G, coated)

0.48/0.52

7.1

1.44

0.42

0.1 mM MgCl2 0.5 mM MgCl2

5.0

10.1

a G, H, and F represent goethite, hematite and ferrihydrite, respectively; the term coated indicates that iron oxide was coated on the surface of quartz sand, while the term mixed indicates iron oxide particles were mixed with the quartz sand. b The ratio of mass of quartz sand to mass of FeOx-coated sand in a column. c Zeta potential of GO; values after ± sign represent standard deviation of three replicates. d Hydrodynamic diameter of GO based on DLS analysis. e dp/dc represent ratio of Zave of GO aggregates to average diameter of sand grains.

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reported that the contribution of straining, as affected by elevated degree of particle aggregation with increased concentration of Ca2+, to GO retention was more pronounced for the iron-coated sand than for clean quartz sand. This was likely because surface charge heterogeneity caused more significant retention of GO, which narrowed the pore throats and further induced GO retention via straining. This interesting observation indicates that iron oxide-bearing porous media may respond to the changes of solution chemistry differently than purified quartz sand, and calls for further understanding of the cation-specific effects on surface properties of FeOx heterogeneities in porous media and consequently, on FeOx-inhibited GO transport. The findings of the previous studies also underscore the need to understand the effects of iron oxides on GO transport as the functions of iron oxide species and morphology. The overall objective of this study was to further understand the effects of iron oxides on the transport of GO in saturated porous media. In recognition that there are many different types of iron oxides—which vary significantly in size, morphology, surface chemistry and reactivity (Vodyanitskii, 2010)—in natural environments, we chose goethite, hematite and ferrihydrite (the most common forms of crystalline iron oxides in natural environments) (Jerez and Flury,

2006; Wang et al., 2016) as the model iron oxides in this study to gain insight on the role of iron oxide species in GO transport. Since iron oxides not only can present as coatings on silicate mineral grains (Song et al., 1994; Coston et al., 1995; Ryan and Elimelech, 1996), but may exist as discrete particles (Asamoa, 1973), we compared the effects of goethite on GO transport in goethite-coated sand and goethite-mixed sand to understand the role of iron oxide morphology. Moreover, we examined GO transport in iron oxide-coated sand as affected by cation species, i.e., Na+ vs. K+ and Mg2+ vs. Ca2+, to reveal the cationspecific effects on surface properties and on GO–surface heterogeneities interactions. The mechanisms controlling the transport and retention of GO in the FeOx-bearing sand were discussed. A two-site transport model was applied to analyze the transport data to gain further insights on retention mechanisms. 2. Materials and methods 2.1. Materials A GO material was purchased from Plan Nano Materials Tech Co. (Tianjin, China). The product was synthesized using a modified

(a) 1.2 quartz sand goethite-coated sand (0.64 mg-Fe/g-sand) goethite-coated sand (1.24 mg-Fe/g-sand)

1.0

C/C0

.8 .6 .4 .2 0.0 0

20

40

60

80

100

120

140

160

PV (b) 1.2 quartz sand goethite-coated sand hematite-coated sand ferrihydrite-coated sand

1.0

C/C0

.8 .6 .4 .2 0.0 0

20

40

60

80

100

120

140

160

PV Fig. 1. Breakthrough curves of GO as affected by the presence of iron oxide coatings: (a) effects of goethite loading (Columns 1–3) and (b) effects of iron oxide species (Columns1, 2, 4 and 5). Dash-dotted lines (−∙−) were plotted by fitting the breakthrough curves with the two-site transport model.

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Hummers method and contained 62.1% C (wt:wt), measured using an automatic elemental analyzer (Elementar Vario EL Cube, Germany). The measured BET surface area (ASAP 2020, Micromeritics, GA, U.S.A.) was 207.1 m2/g (Qi et al., 2014a, 2014b, 2014c). Quartz sand (0.21–0.30 mm: average grain size was 0.26 mm) was obtained from Sigma–Aldrich (St. Louis, MO). The sand was cleaned, following the methods of Mattison et al. (2011), by washing with HCl (0.1 M) followed by H2O2 (5%) to remove impurities on the grain surface. Goethite-, hematite- and ferrihydrite-coated sands, as well as discreet goethite particles, were synthesized based on the methods reported in the literature (Stahl and James, 1991; Grafe et al., 2002; Ona-Nguema et al., 2005). Details of the coating procedures are described in Supplementary Material. The coating procedures did not change the average grain size of quartz sand. The ζpotential of the sand, goethite-coated sand and discrete iron oxides was measured by electrophoretic mobility, using a ZetaPALS (Brookhaven Instruments, Holtsville, NY). The surface functional groups were identified using a Fourier transform infrared (FTIR) spectrophotometer (Nicolet 6700, Thermo Scientific, Waltham, MA).

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2.2. Preparation and characterization of GO suspensions To prepare a GO stock suspension, 30 mg GO material was added to 300 mL deionized (DI) water. Next, the mixture was ultra-sonicated with a Vibracell Ultrasonic Disintegrator VCX 800 at 100 W (Sonics and Material, Newtown, CT) for 30 min to disperse the GO powder, and finally filtered through 0.45-μm membrane filters. The concentrations of GO in the filtrates, i.e., the stock suspensions were quantified by measuring the total organic carbon (TOC) content (Shimadzu Scientific Instruments, Columbia, MD) (X.W. Wang et al., 2012). Before each experiment, an aliquot of the GO stock suspension was transferred to electrolyte solutions of varied solution chemistry. The resulting concentration of GO in the influents were approximately 10 mg/L. Hydrodynamic diameters (Zave) and ζpotential of GO suspensions were measured by dynamic light scattering (DLS) and electrophoretic mobility, respectively. 2.3. Column experiments Quartz sand or FeOx-coated sand was dry-packed into Omnifit borosilicate glass columns (10 cm in length and 0.66 cm in inner diameter).

(a) 0.61 mg-Fe/g-sand 1.2 goethite-mixed sand goethite-coated sand

1.0

C/C0

.8 .6 .4 .2 0.0 0

20

40

60

80

100

120

PV (b) 1.24 mg-Fe/g-sand 1.2 goethite-mixed sand goethite-coated sand

1.0

C/C0

.8 .6 .4 .2 0.0 0

20

40

60

80

100

120

PV Fig. 2. Effects of goethite morphology on transport of GO. Goethite heterogeneity in the columns were introduced either by mixing discreet goethite particles with quartz sand (referred to as “goethite-mixed sand”) or by using goethite-coated sand. Two different Fe loadings were tested: (a) 0.61 mg-Fe/g-sand (Columns 6 and 7); and (b) 1.24 mg-Fe/g-sand (Columns 3 and 8). Dash-dotted lines (−∙−) were plotted by fitting the breakthrough curves with the two-site transport model.

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2003) was applied to analyze the transport data (the detailed equations are given in Supplementary Material).

The Fe content (either 0.64 or 1.24 mg-Fe/g-sand) in the columns was controlled by mixing purified sand with goethite-coated sand or discreet iron oxide particles. The columns were operated in an upward direction using syringe pumps (KD Scientific). Before initiating column experiments the columns were first flushed with 100 mL DI water to establish a steady-state saturated flow condition, and then equilibrated with 180 mL background electrolyte solution. Column properties are summarized in Table 1. The influents were prepared immediately before the column experiments, by diluting a stock suspension of GO with an electrolyte solution. The mixtures were then equilibrated for 2 h. To obtain the breakthrough curves (BTCs), the effluents were collected every 2–3 pore volumes (PV), and the concentrations of GO were determined by measuring the absorbance at 230 nm with a Shimadzu (UV-2401-PC) UV–vis spectrophotometer (Wang et al., 2009), based on a predetermined calibration curve. The retention profiles of GO were obtained by dissecting the sand columns after transport experiments (see Supplementary Material for detailed procedures). The mass balance of the column experiments was within the range of 96.2 to 104% (Table S1). A two-site transport model that accounts for both attachment and straining (Bradford et al.,

3. Results and discussion 3.1. FeOx-inhibited transport of GO The presence of iron oxides can markedly affect the transport of GO in saturated quartz sand. For example, coating quartz with goethite, at two different goethite loadings of 0.64 and 1.24 mg-Fe/g-sand, significantly inhibited GO transport (at 10 mM NaCl) (Fig. 1a). When the porous media was quartz sand, nearly 100% breakthrough was quickly reached within 10 PV; GO breakthrough occurred at later times and the maximum C/C0 value was lower in the presence of goethite coating, especially for the sand with higher goethite loading. For instance, when the Fe content was 1.24 mg-Fe/g-sand, the C/C0 value did not reach the maximum value (89%) until 45 PV. Transport modeling results also indicate that the fitted parameters (Katt and Smax) increased almost linearly with the increase of goethite loading (Fig. S1). These observations are consistent with the literature, in that the positively charged iron oxides

(a) 1.2 0.5 mM NaCl 5 mM NaCl 10 mM NaCl

1.0

C/C0

.8 .6 .4 .2 0.0 0

20

40

60

80

100

PV (b) 120 0.5 mM NaCl 5 mM NaCl 10 mM NaCl

µg-GO/g-Sand

100 80 60 40 20 0 0

1

2

3

4

5

6

7

Distance from Inlet (cm) Fig. 3. Effects of Na+ on transport of GO in goethite-coated sand at an Fe content of ~0.64 mg-Fe/g-sand (Columns 2, 9 and 10): (a) breakthrough curves; and (b) retained particle profiles. Dash-dotted lines (−∙−) were plotted by fitting the breakthrough curves with the two-site transport model.

Z. Qi et al. / Science of the Total Environment 657 (2019) 1450–1459

provide favorable deposition sites for GO (Wang et al., 2013). Furthermore, increased surface roughness might also have contributed to the enhanced retention of GO in FeOx-coated sand. Previous studies showed that the high roughness of collector surfaces can decrease the repulsive interactions between the collector and colloids, causing colloids to deposit more easily on the surface of FeOx-coated sand (Shellenberger and Logan, 2002; Morales et al., 2009; Shen et al., 2011). The sizes of the sand grains used in this study ranged from tens to hundreds of microns. Considering that the average hydrodynamic diameters of GO is generally less than 300 nm (Table 1), the rough patches on sand surface likely can create local regions that are hydrodynamically favorable for GO retention (Saiers and Ryan, 2005; Morales et al., 2009; Shen et al., 2011). In Fig. S2 the SEM images of goethite-coated sand grains and those of purified quartz sands are compared. The uncoated sand grains are irregularly shaped grains from tens to hundreds

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of microns, with smooth surfaces. Increased surface roughness from the coating of goethite was evident, and Energy dispersive X-ray Spectroscopy (EDS) chemical analysis of Fe element (Fig. S2c and d) confirmed that the increased roughness was due to the formation of iron oxides on the surface of quartz. The BTCs of GO from columns containing different FeOx-coated sands (i.e. goethite, hematite or ferrihydrite coating at ~0.64 mg-Fe/gsand) at 10 mM NaCl are compared in Fig. 1b. Overall, goethite and hematite exerted greater transport-inhibition effects than did ferrihydrite. Specifically, the BTCs from columns containing goethite- and hematitecoated sands nearly overlap, and after 19 PV the maximum breakthrough reached ~90%. However, coating quartz sand with ferrihydrite resulted in little transport inhibition of GO, as the BTC of GO from ferrihydrite-coated sand and that from clean quartz sand nearly overlap, with the maximum breakthrough reaching ~98%. The ζ-potential values

(a) 1.2 0.5 mM NaCl 0.5 mM KCl

1.0

C/C0

.8 .6 .4 .2 0.0 0

20

40

60

80

100

120

PV (b) 1.2

10 mM NaCl 10 mM KCl

1.0

C/C0

.8 .6 .4 .2 0.0 0

20

40

60

80

100

120

PV Fig. 4. Effects of different monovalent cations on transport of GO in goethite-coated sand at an Fe loading of ~0.64 mg-Fe/g-sand: (a) 0.5 mM Na+ or K+ (Columns 9 and 11) and (b) 10 mM Na+ or K+ (Columns 2 and 12). Dash-dotted lines (−∙−) were plotted by fitting the breakthrough curves with the two-site transport model.

Z. Qi et al. / Science of the Total Environment 657 (2019) 1450–1459

of the three iron oxides were found to be very similar (33.2, 35.3 and 34.1 mV for goethite, hematite and ferrihydrite, respectively). Intuitively, at the same Fe loading the three types of FeOx heterogeneities should have exhibited similar extents of inhibition effects on GO transport. One possible explanation for the considerably weaker effect of ferrihydrite was that the as-prepared ferrihydrite-coated sand had a higher Fe content than did goethite-coated sand and hematite-coated sand. Therefore, to achieve the similar Fe loading (~0.64 mg/g) for the columns designated to examine the effects of different FeOx coatings (i.e., Columns 2, 4 and 5), different sand to FeOx-coated sand mixing ratios (msand/mcoated sand) were used: the ratios for the columns containing goethite-, hematite- and ferrihydrite-coated sands are 0.48/0.52, 0.47/ 0.53, 0.87/0.13, respectively (Table 1). Accordingly, for the column containing ferrihydrite-coated sand, the chance of GO nanosheets to make contact with FeOx was likely smaller than those for the columns containing goethite- or hematitie-coated sand, and consequently, a smaller transport-inhibition effect was observed. 3.2. Effects of iron oxide morphology on GO transport In the previous studies addressing the effects of iron oxides on transport and retention of nanoparticles in saturated sand, surface charge heterogeneity was typically created by coating quartz sand with iron oxides (D. Wang et al., 2012; Wang et al., 2013; Duster et al., 2016; D. Wang et al., 2017). In natural environments, however, iron oxides often exist as discreet particles (Ogunsanwo, 1986; Zhang et al., 2015; Locat et al., 1984), which may affect the transport of nanoparticles differently than those coated on quartz surfaces. Thus, we compared the effects of goethite coating versus discreet goethite particles, at two different Fe contents (i.e. 0.61 and 1.24 mg-Fe/g-grain). In general, goethite in the form of coating inhibited the transport of GO more significantly compared with its particle counterpart, and the difference between the two forms of goethite was considerably larger at the lower Fe loading (Fig. 2). The larger transport-inhibiting effect of goethite coating at low Fe loading was understandable, in that goethite particles (which had a rod shape with average size of ~50 nm (diameter) × 500 nm (length); see the SEM images in Fig. S3) are markedly smaller than sand grains (~260 μm) and likely resided in the interstices of sand grains, which limited their interactions with GO nanosheets, whereas in the case of coating the available contact area of goethite with GO is considerably greater. It is noteworthy that as the mass of goethite particles increased, the physical effects of the smaller heterogeneous component of porous media became increasingly more important, i.e., the accumulation of small goethite particles at the interstices would narrow the pore throats and consequently, increase the GO retention via straining. This could partially compensate the abovementioned smaller chemical effects on GO transport exerted by goethite as discrete particles.

the dp/dc values (where dp and dc are the diameters of the particle and the collector, respectively) were well below 0.002 (Table 1), the generally assumed threshold for straining (Bradford et al., 2003; Lanphere et al., 2013). The facts that the fitted Katt values are approximately one order of magnitude greater than the Kstr values under any given Na+ concentrations (Table S2) and that when Na+ was the background cation negligible size fractionation occurred during transport (Fig. S4) both corroborate the negligible contribution of straining. Note that the effects of cations on electrostatic repulsion between nanoparticles and porous media are likely more significant for geothite-coated sand than for purified quartz sand, because it was suggested that coating with goethite can increase surface area, introduce small pores, and change the surface charge distribution of silica, which in combination would facilitate the accumulation of cations on the surface of silica (Xu and Axe, 2005; Hsu et al., 2008; Wu et al., 2011). Additionally, we compared the effects between Na+ and K+ on the transport of GO in porous media containing goethite-coated sand (Fig. 4). The difference in the transport-inhibition effects between Na+ and K+ was insignificant at the lower cation concentration (0.5 mM). However, at the higher cation concentration tested (10 mM), K+ resulted in more significant transport inhibition of GO (also see the fitted two-site model parameters in Table S2). The result was likely related to the weaker hydration of K+, as demonstrated in our previous study (Xia et al., 2017), as the more weakly hydrated K+ can result in more effective charge neutralization for both sand grains and GO nanosheets, and thus greater extent of GO deposition. 3.4. Inhibition effects of FeOx on GO transport as affected by divalent cations The BTCs and retention profiles of GO in the columns containing goethite-coated quartz sand (Fe content was ~0.64 mg-Fe/g-sand) at different CaCl2 concentrations are shown in Fig. 5. With the increase of (a) 1.2 0.1 mM Ca2+ 0.3 mM Ca2+ 0.5 mM Ca2+

1.0 .8 C/C0

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.6 .4 .2 0.0 0

20

40

60

80

100

PV (b)

3.3. Inhibition effects of FeOx on GO transport as affected by monovalent cations

120 0.1 mM Ca2+ 0.3 mM Ca2+ 0.5 mM Ca2+

The BTCs and retention profiles of GO in the columns containing goethite-coated quartz sand (Fe content was ~0.64 mg-Fe/g-sand) at different NaCl concentrations (Fig. 3) show that increasing Na+ concentration had significant effect on GO transport. With the increase of NaCl concentration, the maximum C/C0 value reached more slowly and to gradually smaller values, as increasing amount of GO was retained in the column with the increase of Na+ concentration. The total mass of GO retained in column were 21.7, 24.4 and 34.2% when the influent contained 0.5, 5 and 10 mM NaCl, respectively (Fig. 3b and Table S1). The observations were generally consistent with the mechanisms governing the transport of negatively charged nanoparticles, in that, increasing concentration of cations compresses the double layer thickness and reduces the electrostatic repulsion between nanoparticles and grain surfaces (Ryan and Elimelech, 1996; Jiang et al., 2012), whereas straining due to particle aggregation likely played insignificant role, as

μg-GO/g-Sand

100 80 60 40 20 0 0

1

2

3

4

5

6

7

Distance from Inlet (cm) Fig. 5. Effects of Ca2+ on transport of GO in goethite-coated sand at an Fe content of ~0.64 mg-Fe/g-sand (Columns 13–15): (a) breakthrough curves; and (b) retained particle profiles. Dash-dotted lines (−∙−) were plotted by fitting the breakthrough curves with the two-site transport model.

Z. Qi et al. / Science of the Total Environment 657 (2019) 1450–1459

Ca2+ concentration in the influent the maximum C/C0 value of GO decreased markedly, from 95% at 0.1 mM Ca2+, to 88% and 71% at 0.3 and 0.5 mM Ca2+, respectively; accordingly, the mass of GO retained in column increased from 11.7% to 15.6% and to 38.3% (Fig. 5b). With the increase of Ca2+ concentration both GO nanosheets and goethitecoated sands became less negatively charged (Fig. S5). The strong charge screening effects of Ca2+ can reduce the electrostatic repulsion between GO nanosheets and the porous media, as discussed earlier for Na+ and K+. Strikingly, significant size fractionation of GO aggregates occurred in the presence of Ca2+, as opposed to the case when the background cation was Na+ (Fig. S4), indicating that different retention mechanisms were involved in the presence of Ca2+ vs. Na+. It has been proposed that Ca2+ could inhibit the transport of negatively charged nanoparticles by inducing particle−collector bridging (Chowdhury et al., 2013; Qi et al., 2014a, 2014b, 2014c; Fan et al.,

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2015b; Xia et al., 2015). In this study, Ca2+ could serve as the bridging agent between the surface O-functional groups of GO and the hydroxyl groups of porous media. The cation-bridging effects were likely significant in the presence of goethite coating on sand surface, as goethite contains abundant surface hydroxyl groups (Parfitt et al., 1977; Wu et al., 2008; El-Badawy et al., 2013). For example, goethite (α-FeOOH) contains mainly two types of reactive surface groups, including singly (`FeOH(H)) and triply (`Fe3O(H)) coordinated oxygen, with apparent site density as high as 3.45 and 2.7 nm−2 (Hiemstra et al., 1996). The FTIR spectrum of goethite-coated sand shows a sharp peak at 780 cm−1 (Fig. S6), which can be assigned to Fe–OH (Huang et al., 2014; Yoon et al., 2014). This is consistent with the stronger adsorption of Ca2+ to goethite-coated sand compared with purified quartz sand (Fig. S7). An interesting observation was that even though the average hydrodynamic diameter of GO increased substantially with the increase

(a) 1.2 0.1 mM Mg2+ 0.1 mM Ca2+

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.8 .6 .4 .2 0.0 0

20

40

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PV (b) 1.2 0.5 mM Mg2+ 0.5 mM Ca2+

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C/C0

.8 .6 .4 .2 0.0 0

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60

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PV Fig. 6. Effects of different divalent cations on transport of GO in goethite-coated sand at an Fe content of ~0.64 mg-Fe/g-sand: (a) 0.1 mM Mg2+ or Ca2+ (Columns 13 and 16) and (b) 0.5 mM Mg2+ or Ca2+ (Columns 15 and 17). Dash-dotted lines (−∙−) were plotted by fitting the breakthrough curves with the two-site transport model.

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of Ca2+ concentration (from 109.0 to 291.1 nm, as CaCl2 concentration increased from 0.1 to 0.5 mM), the relatively small dp/dc and Kstr values (Tables 1 and S2) do not indicate significant straining. This further corroborates that the significant effect of Ca2+ on transport inhibition of GO was exerted mainly through cation-bridging. To further verify the role of Ca2+ as the bridging agent, additional column transport experiments were conducted using Mg2+ as the background cation in the influent. The effects of Mg2+ versus Ca2+ on GO transport are compared in Fig. 6. At the lowest divalent cation concentration tested (i.e., 0.1 mM), Ca2+ inhibited GO transport slightly more than did Mg2+ (Fig. 6a); however, at the higher cation concentration (0.5 mM) Ca2+ exerted much larger effects (Fig. 6b). The different effects on GO transport between Mg2+ and Ca2+ were consistent with the cation-bridging mechanism, in that, Ca2+ can form inner-sphere complexes with GO nanosheets whereas Mg2+ mainly forms outersphere complexes (Chowdhury et al., 2013; Wu et al., 2013; Yang et al., 2016; Gao et al., 2017), consequently, in the presence of Ca2+, GO could bind chemically to the surfaces of the porous media, in particular, the highly hydroxylated surfaces of goethite. 4. Conclusions The presence of iron oxides can significantly affect the transport of GO in saturated porous media. The transport inhibition effects are exerted mainly by increasing electrostatic attraction between particles and collectors, in which the positively charged iron oxides render favorable deposition sites on the collector surfaces to attract negatively charged GO nanosheet. Additionally, the formation of iron oxide coating on sand surface can increase surface roughness, causing the deposition of GO nanosheets on collector surfaces to be hydrodynamically more favorable. The morphology of iron oxides in porous media is an important factor controlling the significance of iron oxide-inhibited transport of GO, in that, iron oxides present as coating on sand surfaces can exert markedly stronger effect on GO transport than iron oxides existing as discreet particles. The presence of iron oxide coatings can magnify the effects of cations on GO transport via two main mechanisms. First, coating with iron oxides can increase surface area, introduce small pores, and change the surface charge distribution of silica, which in combination would facilitate the accumulation of cations on the surface of silica. Second, iron oxides contain abundant surface hydroxyl groups that are metal-complexing moieties; thus, for cations that are able to complex with surface O-functional groups (e.g., Ca2+), the binding of GO via the cation-bridging mechanism can be significantly enhanced. Acknowledgments This project was supported by the National Natural Science Foundation of China (Grants 21425729 and 21707081), the Fundamental Research Funds for the Central Universities, and the 111 Program of the Ministry of Education of China (T2017002). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.scitotenv.2018.12.143. References Asamoa, G.K., 1973. Particle size and free iron oxide distribution in some latosols and groundwater laterites of Ghana. Geoderma 10 (4), 285–297. Bradford, S.A., Simunek, J., Bettahar, M., van Genuchten, M.T., Yates, S.R., 2003. Modeling colloid attachment, straining, and exclusion in saturated porous media. Environ. Sci. Technol. 37, 2242–2250. Chen, C., Shang, J., Zheng, X., Zhao, K., Yan, C., Sharma, P., Liu, K., 2018. Effect of physicochemical factors on transport and retention of graphene oxide in saturated media. Environ. Pollut. 236, 168–176.

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