Nanobubble Retention in Saturated Porous Media under Repulsive

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Nano-Bubble Retention in Saturated Porous Media under Repulsive van der Waals and Electrostatic Conditions Shoichiro Hamamoto, Takuya Sugimoto, Takato Takemura, Taku Nishimura, and Scott A. Bradford Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00507 • Publication Date (Web): 06 May 2019 Downloaded from http://pubs.acs.org on May 14, 2019

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Nano-Bubble Retention in Saturated Porous Media under Repulsive van der Waals and Electrostatic Conditions Shoichiro Hamamoto,1,* Takuya Sugimoto,1

Takato Takemura,2 Taku Nishimura,1

Scott A. Bradford3

1Graduate

School of Agricultural and Life Sciences, The University of Tokyo, 1-1-1,

Yayoi, Bunkyoku, Tokyo, 113-8657, Japan.

2College

of Humanities and Sciences, Nihon University, 3-25-40, Sakurajousui,

Setagaya, Tokyo, 156-8550, Japan

3US

Salinity Laboratory, USDA, ARS, 450 W. Big Springs Road, Riverside, CA

92507-4617, USA

KEYWORDS : Nano-Bubble; Retention; Hydrophobic Interaction; Ion Species; Surface Roughness

1 ACS Paragon Plus Environment

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ABSTRACT - An understanding of nano-bubble (NB) migration in porous media is needed for potential environmental applications. The solution chemistry is well-known to be a critical factor in determining interactions of other colloids and nanoparticles with surfaces. However, little quantitative research has examined the influence of solution chemistry on NB transport. One-dimensional column experiments were therefore conducted to investigate the transport, retention, and release of NBs in glass beads under different solution chemistry conditions. NB concentrations in the effluent were reduced with an increase in ionic strength (IS) or a decrease in pH due to a reduction in the repulsive force between the glass surface and NBs, especially when the solution contained Ca2+ as compared to Na+ and for larger NBs. This result was somewhat surprising because electrostatic and van der Waals interactions for NBs were both repulsive on a homogeneous glass bead surface. NB retention on the surface was explained by ubiquitous nanoscale roughness on the glass beads that significantly lowered the energy barrier, and localized attractive charge heterogeneity and/or hydrophobic interactions. In contrast to Na+, adsorbed Ca2+ ions produced charge heterogeneity that enhanced NB retention and inhibited release with IS reduction.


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ABSTRACT ART


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INTRODUCTION Nano-scale bubbles (NBs) have recently gained increasing attention due to their unique physicochemical properties (large surface area, charged surface, stability, and high gas internal pressure) and many potential applications such as a use in water treatment by flotation　 1,2,

and in sterilization applying ozone gas 3, and for accelerating the metabolism in shellfishes

and vegetables 4. In addition, NBs show considerable potential for soil remediation of organic chemicals in combination with bioremediation

5–8.

An understanding of NB transport

characteristics in soils is therefore required for successful soil remediation and agricultural applications using NBs. However, fundamental studies on NB behavior in porous media are very limited 9–11. The transport of solid colloids (e.g., microorganisms, clays, and nanoparticles) in porous media is well-known to depend on the solution pH, ionic strength (IS), and ion composition 12–14. The interaction of solid colloids with porous medium is usually explained by considering the sum of van der Waals and electric double layer interactions

15.

The van der

Waals interaction between solid surfaces is attractive 16, whereas it is repulsive for NBs and a porous media surface

17.

Most porous medium surfaces, including glass beads, exhibit a net

negative charge at neutral or alkaline pH conditions 18. NBs also exhibit a negative charge under these pH conditions due to the preferential adsorption of OH– ions


19,20,21.

Consequently, a

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repulsive electrostatic interaction is expected between glass beads and NBs, although compression of the double layer thickness and alteration of the zeta potentials with increasing IS are expected to decrease this repulsion

22.

This indicates that consideration of only van der

Waals and double layer interactions cannot explain the observed retention of NBs in porous media. Furthermore, no systematic studies have examined the influence of solution IS and ionic composition on NB retention to date. Alternative explanations for observed NB retention in porous media therefore requires consideration of other interactions, factors, and/or processes. The air-water interface has been reported to be extremely hydrophobic 23. However, previous research examining the transport of NBs has not yet considered the potential influence of hydrophobic interactions 9. Ubiquitous nanoscale roughness and chemical heterogeneity on porous medium surface have also been demonstrated to locally reduce and/or eliminate the energy barrier to attachment of solid colloids on porous media surfaces

24.

Multivalent cations are reported to produce greater

amounts of solid colloid retention at the same IS than monovalent cations because of charge neutralization or reversal, and/or cation bridging

25–27.

However, no research to date has

considered the potential influence of nanoscale heterogeneities on NB retention. Straining of NB has previously been reported as one of major contributors to NB retention 10. However, they have not considered the potentially confounding effects of hydrophobic interactions and


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nanoscale heterogeneities on retention. Consequently, there are still many unanswered questions about factors that influence NB retention. NB release has also previously been observed with a decreasing in solution IS

10,11.

The electric double layer interaction is typically more repulsive under lower IS conditions because of expansion of the double layer thickness and larger magnitudes in the zeta potentials　 14,28.

Consequently, NB release with IS reduction cannot be explained by traditional interaction

energy calculations due to the increased height of energy barrier on smooth surfaces

29.

In

addition, colloid release with IS reduction is reported to be much greater when solid colloids are deposited in the presence of monovalent cations than divalent cations such as Ca2+

25–27.

Nanoscale roughness has been reported to theoretically explain the release of solid and hollow colloids with IS reduction 30, but experimental information for NBs is still lacking, especially in the presence of divalent cations. This research investigates factors that control the transport, retention, and release of air NBs in glass beads. One-dimensional column transport studies with air NBs were conducted under different solution IS, ionic composition, and pH conditions. Collected data were simulated using a mathematical model that considered advective and dispersive transport, first-order kinetic retention, and transient release. Results were interpreted with the aid of interaction energy calculations that considered electric double layer, van der Waals, and Lewis


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acid-base interactions on surfaces with different amounts of nanoscale roughness. Findings from this study help to explain the critical roles of hydrophobic interactions, chemical heterogeneity, and nanoscale roughness in controlling the retention and release of NBs in porous media under different solution chemistry conditions.

MATERIALS AND METHODS Materials NBs were produced by a pressurized dissolution method (FZIN-10-I, IDEC Co., Ltd., Japan). In this method, air was allowed to dissolve in water by applying a pressure of approximately 300 kPa and water was flushed through a nozzle inside the water pool. The supersaturated condition was released due to the reduction in the pressure into the expelled water in the form of bubbles. DI (deionized) water, where distilled water was ion-exchanged and filtered (Autostill WA53, Yamamoto Scientific Co., Ltd., Japan), was used for producing the NBs. The NB generator was operated for around 60 min in 10 L of water, resulting in NB water with peak bubble size of around 170 nm and bubble concentration ranging from 1.0-2.0 x 105 numbers ml-1. The bubble-size (diameter) distributions and the concentrations of NB water were measured using a resonant mass measurement (Archimedes, Malvern Instruments Ltd.) 31. We studied the effect of freezing and thawing on NB suspension by keeping the samples at a


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temperature of – 18 oC for 24 hr and then leaving to thaw at room temperature. As shown in Fig. S1, the majority of NBs disappeared after thawing without any significant agglomeration, suggesting the rupture of the gas filled bubbles 32,33. However, the small amount of nano-entities which have not disappeared after thawing likely suggests the existence of impurities whose densities are lower than water; e.g., oil contaminants. Some recent theoretical and experimental works showed that organic (hydrophobic) substances partially cover the NBs, resulting in enhancement of NB stability in water

34,35.

The total organic carbon (TOC-VCHP, Shimadzu,

Co., Ltd., Japan) of a NB suspension was measured to quantify any residual oil contamination. The measured TOC values for the NB suspension were below the lower limit of quantification. The density of NBs in this study was therefore assumed to have the same density as air for calculations of bubble diameter. Glass beads (Toshin Riko Co., Ltd., Japan) composed of silicon dioxide (70–73%), sodium oxide and potassium oxide (13–16%), calcium oxide (7–12%), magnesium oxide (1– 4%), aluminum oxide (0.5–1.5%), ferric oxide (0.1–0.15%), and residuals ( 0.85). Values of 𝑘𝑑𝑒𝑝 increased with the IS at the same pH and electrolyte solution (i.e., NaCl or CaCl2), again representing the reduced repulsive force between NBs and glass beads. In addition, the higher value of 𝑆𝑚𝑎𝑥 in the presence of Ca2+ at IS=10 mM and pH=8 or at IS= 1 mM and pH= 6 as compared to the ones in the presence of Na+ with the same IS and pH indicate more locations that were favorable for NB retention, likely produced by cation bridging or charge neutralization or reversal 42–44.


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Table 1. Fitted parameters for NBs deposition and release models. pH

Electrolyte in NB water

IS

NB deposition Smax/C0

kdep

mM

m3 kg-1

NB release R2

fnr

ksw

R2

min-1

　

　

min-1

　

8

NaCl (Case 1)

10

1.50E-03

0.03

0.99

0.85

0.17

0.98

8

NaCl

50

3.45E-03

0.13

0.98

　

　

　

8

NaCl

100

3.53E-02

0.41

0.85

　

　

　

8

CaCl2

10

9.93E+01

0.48

0.87

　

　

　

8

NaCl (Case 2)

10

2.82E-03

0.15

0.97

n.d.

n.d.

　

8

CaCl2 (Case 3)

10

7.31E-01

0.69

0.92

0.92

0.08

0.90

6

NaCl

1

8.05E-03

0.09

0.98

　

　

　

6

NaCl

5

1.32E+01

0.23

0.91

　

　

　

6

NaCl

10

4.60E+01

0.42

0.85

　

　

　

6

CaCl2

1

4.00E-02

0.26

0.92

　

　

　

6

CaCl2

10

n.d.

n.d.

n.d.

　

　

　

C0 – input bubble concentration


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Figure 4. Total interaction energies between NBs and glass surface at different electrolyte solutions with IS = 10 or 100 mM (a) without considering hydrophobic interaction and (b) with hydrophobic interaction. Blue lines in (b) show the total interaction energies when considering surface roughness with a roughness height of 38 nm and different roughness fraction (fr). kB is the Boltzmann constant and Tk is the absolute temperature taken as 293 K.

The calculated interaction energy for NBs in NaCl and CaCl2 electrolyte solutions of different IS (10 and 100 mM) at pH 8 conditions is shown in Fig. 4. When the hydrophobic interaction was not considered (Fig. 4a) a repulsive force was prominent, especially in the presence of 100 mM NaCl and CaCl2 (IS=10 and 100 mM). This is because the electrostatic and van der Waals interactions were both repulsive. The van der Waals interaction was repulsive due to the negative effective Hamaker constant in the system of water-NBs (i.e., air)-glass beads. These interaction energy calculations are clearly not consistent with the experimental observations of significant NB retention in Figure 3. In contrast to Figure 4a, the interaction energy profiles in Figure 4b exhibit a distinct primary minimum for all conditions when van der


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Waals, double layer, and hydrophobic interaction are considered. The primary minimum for IS=10 mM NaCl was deepest in all conditions but a significant energy barrier (~ 18 kBTK) was observed. Shen et al.

45

reported that few colloids can transport over the energy barrier and be

deposited in the primary minimum when the energy barrier is larger than 10 kBTK. Therefore, NBs are not expected to be attached in a primary minimum on the glass surface under this condition. In contrast, our experiments showed attachment of NBs under this condition (i.e., peak relative turbidity was around 0.8, Fig. 3a). Discrepancies between theoretical interaction energy predictions and experimental observations have also been found in numerous previous studies 46. One possible explanation is due to the influence of nanoscale roughness and/or chemical heterogeneity on glass bead surfaces. Glass has been demonstrated to exhibit significant amounts of nanoscale roughness. For example, Rasmuson et al.47 reported a root mean square roughness height of 38 nm for a glass microscope slide that was cleaned with sodium hydroxide. Nanoscale roughness can have a profound influence on the interaction energy profile. The energy barrier height and the depth of the primary minimum can be reduced or eliminated depending on the local scale roughness parameters

24,30,48.

For example, Table S1 shows the predicted influence of different nanoscale roughness fractions with a height of 38 nm on interaction energy parameters when considering van der Waals, double layer, and hydrophobic interactions for the various solution chemistry conditions at pH=8. Figure 4b also


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showed reduced energy barrier height at IS=10 mM NaCl when considering surface roughness. Note that nanoscale roughness can create locations that are favorable and unfavorable for NB retention, and that the influence of roughness changes with the solution chemistry30. This can partially explain the observed NB retention in Figure 3. In addition to nanoscale roughness, glass beads that were mainly composed of SiO2 also

contain various metal oxides on their surface that produces charge heterogeneity. At the solution pH maintained during experimentation (pH=8), the metal oxides will be near-neutrally charged or carry a slightly positive charge in comparison to the bulk SiO2 surface which will be highly negatively charged

49.

Hence, these surface charge heterogeneities are expected to provide

favorable deposition sites on what is otherwise an unfavorable surface for deposition

13,24,50.

Adsorbed Ca2+ can also produce cation bridging, and charge neutralization and/or reversal 42–44. This can likely explain the enhanced retention of NBs in the presence of Ca2+ in comparison to Na+ for the same IS and pH conditions. However, it should be mentioned that nanoscale roughness has been demonstrated to control the interaction energy profile when considering both nanoscale roughness and charge heterogeneity simultaneously 24. Figure 5 shows bubble size distributions of applied NBs and effluent NBs at the time corresponding to peak effluent turbidity for each experiment, except for conditions with negligible amounts of NBs in the effluent. At both pH conditions, peak bubble size in the


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effluents was similar to the one in the applied NB suspension. However, there was a tendency for larger NB sizes to be reduced in the effluent in comparison to the influent with increasing IS of NaCl at each pH condition (i.e., NBs larger than 250 nm for IS=50 mM at pH 8, and NBs larger than 210 nm for IS=5 mM at pH 6). Similar bubble size distribution at 1 mM of CaCl2 electrolyte solution was observed with one at 5 mM of NaCl electrolyte solution. These results suggest that NB attachment to the glass beads are dependent on the bubble size and larger NBs are more favorable for attachment, especially at higher IS and CaCl2 electrolyte solution. However, filtration theory predicts that smaller colloids will be more efficiently removed from the liquid phase because of greater rates of mass transfer to the solid phase 51. Therefore, this discrepancy might be attributed to physical trapping caused by surface straining at locations with macroscopic roughness or grain-grain contacts retention has been also reported in Hamamoto et al.

52.

11

Bubble size dependency on the NBs

where higher retention of oxygen NBs

with larger size was obtained especially at lower pH.


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Figure 5. Bubble size distributions of applied and effluent NB water at (a) pH 8 and (b) pH 6. The values in the legends represent ionic strength.

Figure 6a shows the breakthrough curves of NBs at IS=10 mM (pH 8) with different electrolytes types for the NB suspension and the control (resident and eluting) solution as follows: NB suspension and control solution used NaCl (Case1); NB suspension used NaCl and control solution used CaCl2 (Case 2); and NB suspension used CaCl2 and control solution used NaCl (Case 3). Figure 6b shows ion concentrations of Na+ and Ca2+ in the effluents of Cases 2 and 3. In Case 2, the peak relative turbidity was highly reduced as compared to Case 1, suggesting introduction of CaCl2 influenced retention of the subsequently applied NB suspension with NaCl. As shown in Fig. 6b, relative Ca2+ and Na+ concentrations during NB injection were not completely 0 or 1, respectively, indicating the Ca2+ in both bulk and exchangeable phases was not fully replaced by Na+. Therefore, residual Ca2+ in the phases contributed to enhanced NB retention. The Ca2+ also worked as a cation bridge (ionic bonds)


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between NBs and glass surface

37,53.

In Case 3, almost negligible NB effluents were obtained.

The repulsive force between NBs and glass beads was reduced when Na+ in bulk and exchangeable phases was replaced by Ca2+ during NB water injection (Fig. 6b) and this enhanced NB retention. Even after switching to the control solution with NaCl, detachment of NBs was not observed even though the interaction between NBs and glass surface was more unfavorable. The NB transport model well captured the breakthrough curves in Cases 2 and 3 (Fig. 6a), and higher values of 𝑘𝑑𝑒𝑝 and 𝑆𝑚𝑎𝑥 were obtained in the presence of 10 mM Ca2+ (Case 3) (Table 1). The value of 𝑆𝑚𝑎𝑥 was larger in Case 2 in comparison to Case 1. This indicates an increase in the number of favorable sites for NB retention on the glass beads due to introduction

of CaCl2 prior to NB injection.

Figure 6. (a) Breakthrough curves for NBs at experiments with different combinations of control solutions and NB water at IS = 10 mM, (b) Ion concentrations of Na+ and Ca2+ in the effluents. The dotted vertical line represents pore volume when starting injection of control solution. Solid lines in Fig. 6a represent model simulations with Eqs. (1)-(3).


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Figure 7. Bubble concentrations and relative electrical conductivity after DI water injection. Solid lines represent model simulations with Eqs. (5)-(7).

Figure 7 shows the bubble concentration and relative EC after the DI water injection following the application of the NB water at IS of 10 mM (i.e., Cases 1, 2, 3 in Fig. 6a). The EC dramatically decreased after approximately one pore volume, indicating that the fluid throughout the whole column was replaced by DI water from the control solution. A decrease of EC in the effluent was accompanied by a rapid increase in the NB concentration for Cases 1 and 3, while no increase in NB concentration was observed for Case 2. The NB concentrations for Cases 1 and 3 subsequently decreased with increasing pore volumes. Thus, a decrease of IS in the aqueous phase enhanced the detachment of NBs that were trapped inside the column during the injection of NB water for Cases 1 and 3. Diffusive release of attached NBs with an IS decrease is expected to occur when the energy barrier to detachment decreases below a critical threshold of about 5.7 to 7 54. On smooth surfaces and in the presence of chemical heterogeneity the energy barrier to detachment is actually expected to increase with IS


55.

Conversely, the

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energy barrier to detachment has been demonstrated to decrease with decreasing IS for certain roughness conditions

55.

Case 3 showed lower peak bubble concentration and more gradual

decrease after the peak as compared to Case 1. Similarly, no NB effluents were observed for the DI water injection experiments using NB water at IS of 10 mM CaCl2 (Fig. 3a). These observations likely reflect an increasing role of charge heterogeneity and/or cation bridging in the presence of adsorbed Ca2+, which would increase the energy barrier to detachment. Thus, existence of Ca2+ highly reduced the mobility of NBs in the porous media. The release model (Eqs. (5)-(7)) during DI water injection provided a good description of NB release for Cases 1 and 3. The values of 𝑓𝑛𝑟 were similar for Case 1 and 3, while a lower value of 𝑘𝑠𝑤 was obtained for Cased 3. This also reflects the influence of adsorbed Ca2+ on the reduction of NB release due to

the presences of localized favorable sites.

CONCLUSIONS In this study, effects of solution chemistry on the transport, retention, and release of NBs in glass beads were investigated. Overall, the results from this study revealed that with increasing IS and decreasing pH the mobilities of NBs were decreased due to reduced repulsive force between NBs and glass surface. The NB retention was explained by attractive hydrophobic interaction and reduced energy barrier caused by nanoscale surface roughness and surface charge heterogeneities. Divalent ion, Ca2+ in the electrolyte solution and adsorbed Ca2+ 26 ACS Paragon Plus Environment

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onto glass beads, highly reduced the mobility of NBs in the porous media because of induced charge neutralization or reversal, and/or cation bridging.

ASSOCIATED CONTENT

Supporting Information

Bubble size distribution before freezing and after thawing of NB suspensions, schematic illustration on the experimental setup, and details of DLVO interaction energy calculations are provided as supporting information.

AUTHOR INFORMATION

Corresponding Author *[email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGEMENTS This work was supported by Grant-in-Aid for Scientific Research of Japan Society for the


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Promotion of Science (JSPS) (Leading Initiative for Excellent Young Researchers, and 16H04411), and Grant-in-Aid for Exploratory Research for Research Institute for Sustainable Humanosphere, Kyoto University. We would like to especially thank Prof. Ohshita and Assoc. Prof. Nihei from the Graduate School of Agricultural and Life Sciences, The University of Tokyo for their assistance to zeta potential measurements and NBs water production.

ABBREVIATIONS NB, Nano-Bubble; IS, Ionic Strength; DI, Deionized Water

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