Adsorption of Cerium Salts and Cerium Oxide Nanoparticles on

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Adsorption of Cerium Salts and Cerium Oxide Nanoparticles on Microbubbles Can Be Induced by a Fluorocarbon Gas Camille Justeau, Andrea Victoria Vela Gonzalez, Alex Jourdan, Jean G. Riess, and Marie Pierre Krafft ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01471 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 14, 2018

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ACS Sustainable Chemistry & Engineering

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Adsorption of Cerium Salts and Cerium Oxide Nanoparticles on Microbubbles Can Be Induced by a Fluorocarbon Gas

Camille Justeau,a Andrea V. Vela-Gonzalez,a Alex Jourdan,b Jean G. Riess,c and Marie Pierre Kraffta* a

University of Strasbourg, Institut Charles Sadron (CNRS). 23 rue du Loess. 67034

Strasbourg Cedex (France) b

c

AREVA NC, Pierrelatte (France)

Harangoutte Institute, 68160 Sainte-Croix-aux-Mines (France)

*Corresponding author [email protected] Tel: +33388414060 Fax: +33388414099 ID: orcid.org/0000-0002-3379-2783

ACS Paragon Plus Environment


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Abstract Retrieving heavy metals from wastewaters has become an important environmental challenge. We report that exposing dilute aqueous solutions or dispersions of cerium compounds (CeO2, Ce(SO4)2, CeF4) to perfluorohexane-saturated air results in substantial adsorption of these salts at the air/water interface, as consistently reflected by a marked decrease in interfacial tension, as assessed by bubble shape profile analysis tensiometry. No detectable adsorption is observed in the absence of the fluorocarbon. Adsorption to the interface is also achieved when, and only when, CeO2 nanoparticle dispersions are exposed to the fluorocarbon vapor. We also found that microbubbles could be generated in cerium salt solutions and CeO2 nanoparticle dispersions when they are formed in the presence of perfluorohexane-saturated air, without need for any surfactant or chelating agent. Optical microscopy, static light scattering and zeta potential measurements were used to establish the ability for the fluorocarbon to induce the formation and stabilize these microbubbles. These findings could provide the basis for a new approach to heavy metal (including radioactive element) recovery and recycling from industrial and other effluents that would combine ionic flotation and fluorocarbon gas-driven adsorption on microbubbles. Extraction of critical raw materials from dilute solutions could also be considered.

Keywords:

Heavy

metal;

Perfluorohexane;

Fluid

interface;

Ionic

flotation;

Tensiometry; Radioactive element; Critical resource; Depollution; Environmental remediation


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Introduction Averting the release of heavy metal-containing industrial effluents to the environment has become mandatory. This is the case, in particular, for wastewaters released from mining, recycling or nuclear energy production sites, not mentioning nuclear power site decommissioning. Unrelenting efforts are being devoted to assessing the ecological impact and health risks and setting authorized limits for heavy metal and radioactive element discharges, reducing the environmental impact of such effluents, and remediation.1 On the other hand, there is concern that supplies will not meet demand for many strategic metals in the future,2-3 thereby founding another powerful incentive for developing new procedures for metal recovery from dilute aqueous media and for recycling critical metallic materials. Such procedures could therefore have substantial economic and environmental value and be part of effective sustainability strategies.4 The recycling rates of metals are presently far below their potential and need boosting, which calls for innovative retrieval technologies.5-7 Recycling also contributes to climate change mitigation by saving energy and reducing greenhouse gas emissions.8 Cost- and energy-effective technologies could moreover open access to numerous heavy metals from seawater, a virtually inexhaustible potential source for strategic raw materials, including over four billion tons of uranium.9-10 Several techniques and combinations of techniques, including chemical precipitation, ionexchange, ultrafiltration, coagulation-flocculation, liquid/liquid or liquid/solid extraction, flotation, gravitational, electrochemical, electromagnetic and bioremediation methods are currently being used to separate and collect heavy metals, including radioactive metals, from wastewater.11-12 Commonly used liquid-liquid processes for decontamination require use of – not so environmentally friendly – solvents and surfactants.13 Another effective process for heavy metal ion recovery, particularly from dilute solutions, is ionic flotation.14-18 Separation by ionic flotation involves the generation and stabilization by a surfactant or chelating agent of air bubbles on which metal ions adsorb. The bubbles will thus collect the metal ions, and the metal ion-loaded bubbles will then float toward the surface and form foams that are easily



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retrieved. However, subsequent separation of the surfactant adds an extra step that is not always simple to achieve on a large scale. Microbubbles stabilized by fluorocarbons have been extensively investigated, in particular for biomedical applications, including as contrast agents for ultrasound imaging and for drug delivery.19-21 It was recently reported that the fluorocarbon present in the bubbles’ gas phase can promote and help control the self-assembly at a gas/water interface of a variety of compounds, including phospholipids,22 proteins,23 polymers,24 fluorinated therapeutics and biomarkers.25 A fluorocarbon gas was found to strongly accelerate the diffusion of watersoluble (or dispersible) compounds towards an air/water interface, and to promote their adsorption at this interface. In certain cases, the recruited compounds could be immobilized in the interfacial phospholipid monolayer.25 The present paper reports a new fluorocarbon-driven ion and nanoparticle adsorption phenomenon that could provide the basis for an innovative approach to heavy metal recovery from aqueous media. This approach involves ionic flotation using fluorocarbon gasloaded microbubbles without need for a surfactant. The fluorocarbon serves both to stabilize the microbubbles and to promote the adsorption of the metal compound on their surface. Cerium, a rare earth element, the oxidation states, ionic radii and properties of which are close to those of uranium,26 was selected for this study. CeO2 itself has found wide applications, under various forms, in polishing powders, catalytic converters, fuel cells, fuel additives, optical coatings, sunscreens and as antibacterial agent, eventually resulting in exposure of aquatic and terrestrial organisms to CeO2 nanoparticles, with still unclear impact on human and ecosystem health.27-28 We first investigated the effect of a volatile fluorocarbon, perfluorohexane (F-hexane) on the adsorption of various cerium(IV) compounds on tethered millimeter-size bubbles using bubble shape profile analysis tensiometry. Second, we investigated the ability for this fluorocarbon to generate and stabilize microbubbles, and the capacity for the latter to adsorb cerium compounds from variously concentrated aqueous solutions and dispersions. The cerium compounds investigated included both soluble species, such as Ce(SO4)2 (solubility


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~23 g per 100 mL at 20°C29) as well as two dispersions of quasi-insoluble27 CeO2 nanoparticles (CeO2NPs) of two different mean diameters, formally featuring anions of diverse charges and sizes. The solubility of CeF4 is very low (~0.2 mg per 100 mL30) but it is still partially soluble at the lowest concentrations investigated. The CeO2NP dispersions consisted of an aqueous dispersion of bulk commercial CeO2 having a mean diameter of 85 ± 10 nm and of a commercial CeO2NP dispersion that had a mean diameter of 35 ± 5 nm (Supporting Information, Figure S1). The former and latter dispersions will henceforth be designated as large (L-CeO2NPs) and small (S-CeO2NPs) nanoparticles, respectively. Fhexane was selected as the fluorocarbon because it uniquely combines extremely low water solubility with high vapor pressure (218 mm Hg at 25°C20) relative to molecular weight, low surface tension and outstanding thermal, chemical and biological stability. Also, this fluorocarbon has proved highly effective in stabilizing microbubble suspensions due to both osmotic19, 31 and co-surfactant contributions.20, 32

Materials and Methods Materials. Bulk CeO2 (99.95%), Ce(SO4)2 (>98.5%) and CeF4 (99%) and the aqueous, acetic acid-stabilized CeO2 nanoparticle (CeO2NP) dispersion (30-50 nm in diameter, 20 wt.%), were purchased from Sigma Aldrich. Perfluorohexane (98%) was from Alfa Aesar. No additional purification was made. Water was from a MilliQ (Millipore) system (γ = 71.5 ± 0.5 mN m-1 at 25°C; resistivity 18.2 MΩ cm). DLS measurements confirmed the presence of nanoparticles in both the S-CeO2NP dispersion (35 ± 5 nm; Figure S1a) and L-CeO2NP dispersions (85 ± 10 nm; Figure S1b) and their absence in the Ce(SO4)2 test solutions (Supporting Information, Experimental Details). Preparation of aqueous cerium salt solutions and cerium oxide nanoparticle dispersions. See Supporting Information. Preparation of microbubble dispersions. The Ce salt solutions or dispersions at 1 mg L-1 were placed in a 50 mL round-bottomed flask and submitted to rotor/stator homogenization (Ultra-Turrax T25, Ika) under N2 or under N2 saturated with F-hexane. In the latter case, N2



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was bubbled through three vials containing liquid F-hexane at room temperature, and the atmosphere above the Ce salt solution or dispersion was flushed with F-hexane-saturated N2 for 20-30 min. Pure N2 was used for the controls. The homogenizer probe of the Ultra-Turrax was plunged about 2 cm deep in the solution and operated for 2 min (power 4) at 25°C. The pH of the microbubble dispersions was 7.3-7.5. Bubble profile analysis tensiometry (see also Supporting Information). Axisymmetric bubble shape analysis33 was applied to a millimetric rising gas bubble formed in aqueous solutions or dispersions of the Ce species. A kinetic adsorption profile was obtained by measuring the interfacial tension γ over time (using a Tracker® tensiometer, Teclis Instruments, Lyon, France),24, 34 reflecting the adsorption of the Ce salts or CeO2NPs on the bubble’s surface. A syringe filled with F-hexane-saturated air was mounted on the injection cell of the tensiometer. A bubble (5 µL) was formed at the end of a steel capillary with a tip diameter of 1 mm. Since the experiments lasted for up to 2 h, a lid was fitted on the measuring glass cell (10 mL) to prevent evaporation of water. Temperature was 25 ± 0.5°C. Each solution or dispersion was tested at least three times for each concentration in order to assess the experimental error (± 0.5 mN m-1). Optical microscopy. Three to four droplets of bubble dispersion were placed in a concave slide, covered with a glass slide, and observed using a Nikon Eclipse 90i microscope. Rapid image acquisition was achieved with an Infinity 2 CCD camera (Lumenera, Ottawa, Canada). Mean bubble diameter was measured on 5 to 10 slides using the ImageJ software. Static light scattering. The microbubble size distributions were further characterized by light scattering using a laser diffraction particle size analyzer (LS 13320 MW, Beckman Coulter, Inc. Brea, CA, USA) equipped with a universal liquid module. Dynamic light scattering and zeta potential. The measurements were achieved with a Zetasizer Nano ZS (Malvern Pananalytical).


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Results and Discussion Impact of F-hexane on the adsorption of cerium salts and cerium oxide nanoparticles at an air/water interface The adsorption of cerium compounds at the air/water and F-hexane-saturated air/water interfaces was first investigated using bubble shape profile analysis tensiometry. The concentrations of the cerium species ranged from 0.1 to 10 mg L-1. The variations of the air/water interfacial tension γ over time are collected in Figure 1a-d. We first confirmed that Fhexane alone, when introduced in the gas phase of the tensiometer bubble, adsorbs rapidly at the interface, as shown by an immediate decrease of γ by about 4 mN m-1 (from 71.6 to 67.9 ± 0.5 mN m-1) (Figure S2), in agreement with our earlier findings.35

Figure 1. Adsorption kinetics of cerium species in aqueous media, as reflected by the interfacial air/water tension decrease of solutions or dispersions of a) Ce(SO4)2, b) CeF4, c) LCeO2NPs, and d) of S-CeO2NPs, at 25°C. Concentrations in Ce compound (mg L-1): 0.1 (dark cyan), 0.3 (orange), 0.5 (red), 0.7 (olive green), 1 (blue), 3 (purple), 5 (dark grey), 7.5 (green) and 10 (magenta); no F-hexane: black.



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The foremost finding of our study is that, for all the cerium salt solutions or dispersions and concentrations investigated, introduction of F-hexane gas above the aqueous phase causes a marked adsorption of the Ce compounds on the air/water interface, as consistently supported by a decrease in interfacial tension γ (Figure 1). By contrast, no significant decrease of the tension was measured in the absence of F-hexane. F-hexane reduces γ by about 12 mN m-1 after a few hours for the 10 mg L-1 Ce compound concentration. It is notable that cerium oxide nanoparticles dispersed in water are also driven to the interface when, and only when, the dispersion is exposed to F-hexane (Figure 1cd). In the absence of F-hexane, there is no detectable variation of γ, which remains constant over time at 70-71 ± 0.5 mN m-1 for all cerium salt solutions or dispersions and for all the concentrations investigated (Figure 1, Figure S3), indicating an absence of adsorption. The situation changes drastically when F-hexane is introduced in the gas phase: a consistent marked decrease of γ is then observed over time that exceeds significantly the γ lowering due to F-hexane adsorption. These observations demonstrate that the cerium species do adsorb at the air/water interface when the fluorocarbon gas is present, while they do not in its absence. The case of CeO2 is particularly interesting. The promotion of nanoparticle adsorption to an air/water interface by introducing an additional component in the gas phase (here a fluorocarbon gas) above a suspension of nanoparticles has, to our knowledge, never been reported. Also notable is that the shape of the corresponding interfacial tension decay curves (Figure 1cd) is very similar to those measured for the dissolved cerium salt Ce(SO4)2 (Figure 1a). Again, no γ lowering, and hence, no adsorption of the nanoparticles is seen in the absence of the fluorocarbon. In order to quantify these adsorption phenomena, both for ionic cerium solutions and CeO2 dispersions, we have investigated the adsorption kinetics and thermodynamics of these species at the interface. As for the kinetics, the characteristic adsorption time τ for each cerium species and concentration was calculated by fitting the adsorption curves


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(Figures 1a-d) with an exponential decay function. Figure 2 shows the variation of τ as a function of concentration for the dissolved cerium species, as well as for the CeO2 dispersions.

Figure 2. Variation of the characteristic adsorption time τ as a function of Ce compound concentration under F-hexane-saturated air for Ce(SO4)2 (black), CeF4 (green), L-CeO2NPs (blue) and S-CeO2NPs (red) at 25°C. The diffusion rate of the species to the interface τ does not vary significantly for CeSO4, CeF4, and S-CeO2NPs within the range of concentrations investigated. By contrast, for LCeO2NPs, τ is larger and decreases exponentially with concentration. The slower diffusion of the latter to the interface may be explained by the larger size of the nanoparticles. In order to assess the adsorption thermodynamics of the cerium species at the interface, the value of γ at equilibrium was extrapolated (γextr) by fitting the interfacial tension profiles with an exponential decay function. Adsorption profiles, usually of surfactants, at the air/water interface of a raising bubble are indeed commonly described by an exponential decay model.36 This model takes into account the rearrangement of the amphiphiles at the interface, which determines the dynamics of surfactant adsorption. Figure 3 shows the variation of γextr as a function of Ce compound concentration. In all cases, γextr first decreases steeply and then reaches a plateau when concentration increases. This shows first that, whatever the compound, saturation of the tethered tensiometric bubbles with F-hexane is effective in inducing the adsorption of the Ce species at the interface; second, that the extent



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of the adsorption does depend somewhat on the anion. For concentrations higher than 3 mg L-1, the interfacial tension lowering effect follows the sequence: CeF4 > L-CeO2NPs > Ce(SO4)2 > S-CeO2NPs. The variation of γ taken at the arbitrary time of 2 h shows a similar behaviour (Figure S4).

Figure 3. Variation of the interfacial tension γextr as a function of Ce concentration under Fhexane-saturated air for Ce(SO4)2 (black), CeF4 (green), L-CeO2NPs (blue) and S-CeO2NPs (red) at 25°C. Altogether, these data demonstrate that introduction of F-hexane in the millimetric tensiometric bubble has a critical impact on interfacial tension, revealing the adsorption on the air/water interface of all the Ce salts and CeO2 nanoparticles present in aqueous solutions or dispersions. By contrast, no measurable adsorption is seen when the fluorocarbon is absent. F-hexane gas allows generation of microbubbles coated with cerium salts or CeO2 nanoparticles without need for a surfactant The above section demonstrates that both soluble and quasi-insoluble cerium compounds, in the presence of F-hexane, adsorb on tethered millimetric air bubbles. Consequently, we investigated whether a shell of adsorbed cerium species could stabilize floating microbubbles. To our knowledge there is no previous report of bubbles of a few micrometers in diameter solely stabilized by ions. Since CeO2 nanoparticles were also found capable of adsorbing at the gas/water interface when exposed to the fluorocarbon gas, we


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wanted to determine whether these nanoparticles could stabilize microbubbles as well. Microbubbles shelled with nano- or microparticles alone (e.g. silica, polystyrene, gold-silica) have been obtained, provided the contact angle was appropriate.37-41 We therefore assessed whether fluorocarbon-induced adsorption could allow microbubble formation from aqueous Ce salt solutions and dispersions. In the absence of F-hexane it proved impossible, in spite of sustained efforts, to obtain microbubbles stable enough for investigation from any of the cerium solutions or CeO2 dispersions investigated. Visual inspection and optical microscopic examination provided no evidence for microbubble formation. By contrast, when F-hexane is introduced in the gas phase, formation of microbubbles with half-lives of at least 30 min could be consistently achieved in all Ce salt solutions and dispersions. When subjected to homogenization using an Ultra-Turrax device these solutions and dispersions become turbid as a result of light diffusion (Figure S5). The liquid becomes clear again after flotation, creaming or collapse of the bubbles. Figure 4a shows a typical optical micrograph of microbubbles generated in a L-CeO2NP dispersion in the presence of F-hexane. The highest densities of bubbles are observed for Ce(SO4)2 and S-CeO2NPs, whereas CeF4 provided smaller bubbles. Monitoring of the microbubble dispersions over time indicated bubble half-lives of ∼1-2 h, but for CeF4 (~30 min). Bubble size histograms and mean bubble diameters were determined by image analysis of micrographs (Figure 4, Table 1) and static light scattering (Figure 5, Table 1).



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Figure 4. a) Typical optical micrograph (10x) of microbubbles formed in the presence of Fhexane in a 1 mg L-1 L-CeO2NP dispersion. Bubble size histograms, obtained by image analysis of micrographs are presented for a) L-CeO2NP, b) Ce(SO4)2, c) CeF4 and d) SCeO2NP solutions or dispersions at a 1 mg L-1 concentration. Table 1. Mean diameters of microbubbles stabilized by F-hexane and shelled by cerium compounds. Mean diameter

Mean diameter

Zeta potential

(image analysis,

(SLS, bubble

bubble dispersions

bubble count)

volume, µm)

(mV)

Ce(SO4)2

7.5 ± 0.6

7.5 ± 0.5

-36.2 ± 4.6

CeF4

3.5 ± 0.2

3.2 ± 0.3

-31.7 ± 2.8

L-CeO2NPs

4.7 ± 0.3

4.3 ± 0.4

-35.7 ± 3.1

S-CeO2NPs

3.8 ± 0.3

3.5 ± 0.3

Sample

-34.7 ± 5.3 9.3 ± 0.7

7.4 ± 0.4


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Figure 5. Diameter distributions of F-hexane-stabilized microbubbles with a shell of a) LCeO2NPs b) Ce(SO4)2, c) CeF4, and d) S-CeO2NPs (1 mg L-1,, 25°C), as determined by static light scattering. The results obtained by SLS (Figure 5, Table 1) corroborate those obtained by optical microscopy, including for the bimodal population character observed for S-CeO2NPs (Figures 4d and 5d). When calculated in Vol%, the surface ratio of the two peaks (5.5) is on the same order than that obtained for the optical microscopy (2.5, Figure S6). Additionally, we determined the net charge on the microbubbles through zeta potential measurements. Zeta potentials are expected to provide an assessment of the electrostatic colloidal stability of the bubble dispersions.42 The combination of both attractive and repulsive forces between particles results in an energy barrier sufficient for preventing particle aggregation. The zeta potential was measured on microbubbles produced from 1 mg L-1 ionic solutions or nanoparticle dispersions in the presence of F-hexane (pH 7.3-7.5). Table 1 shows that all the microbubbles have negative zeta potentials with absolute values in the 3035 mV range. These values are in close agreement with earlier investigations on microbubbles aiming at determining the electrical properties of the gas/water interface,43 at characterizing bubble contrast agents for ultrasound imaging (e.g. SonoVue),44 or assessing the performance of bubbles in a coagulation flotation process for coke waste-water.45 These values are also above the +/- 30 mV threshold usually considered as ensuring electrical colloidal stability of the bubble dispersions.



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To sum up, the main novel, clear cut finding of this study is that exposing cerium salt solutions or dispersions to F-hexane gas induces the adsorption of these compounds at the air/water interface of tethered millimetric bubbles, as evidenced by a sizeable decrease in interfacial tension, while no such adsorption is seen in the absence of the fluorocarbon. This notable fluorocarbon-induced adsorption phenomenon is further supported, and put to use, by our demonstration that free floating microbubbles, with half-lives on the order of 0.5-2 h, can be produced by blowing F-hexane vapor through the cerium salt solutions or dispersions, while no bubbles could be obtained without F-hexane present in the gas phase. This effect was observed for all the cerium compounds investigated. Also significant and essentially unpredictable is that adsorption of differently sized nanoparticles of CeO2 at interfaces of millimetric bubbles can be achieved by introducing F-hexane in the gas phase. The CeO2 nanoparticles-shelled microbubbles could only be generated in the presence of F-hexane. These findings may open new prospects in terms of decontamination and retrieval of heavy metals and radioactive elements from industrial effluents and wastewaters through the development of an ionic flotation process that would involve fluorocarbon gas-driven metal salt or nanoparticle adsorption on fluorocarbon-stabilized microbubbles. This process would not require use of any surfactant or chelating collecting agent. It is noteworthy that the bubbles generated in the presence of the fluorocarbon are smaller than those usually produced for ionic flotation, thus providing larger surface area for metal ion or nanoparticle adsorption. F-hexane should be more easily recoverable and recyclable than the surfactants used for standard liquid-liquid extraction or flotation processes. Use of lighter fluorocarbons, including F-pentane and F-butane could also be considered. The environmental issues related to some heavier fluorinated materials have been discussed.46-47 Our approach should also be adaptable to retrieval and recycling of other critical resources from dilute solutions or suspensions without need for metal-specific surfactants or ligands.


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Acknowledgements The authors thank A. Cagna (Teclis Instruments, Lyon) for discussion. C.J. thanks AREVA NC for a 6-month master grant. A.V. thanks CONACYT (Mexico) for a Ph.D. fellowship.

Supporting Information Experimental Details. DLS sizing of CeO2 nanoparticle dispersions. Effect of F-hexane on interfacial tension of pure water. Absence of evolution of the interfacial tension of cerium salt solutions/dispersions in the absence of F-hexane. Evolution of the surface tension measured after 2 h. Effect of F-hexane on turbidity of CeO2 solution. Diameter distribution of microbubbles stabilized by CeO2NPs as determined by optical microscopy (Vol%).

References (1) IAEA. Setting authorized limits for radioactive discharges: practical issues to consider; IAEA: Vienna, 2010. (2) Graedel, T. E.; Harper, E. M.; Nassar, N. T.; Reck, B. K. On the materials basis of modern

society.

Proc.

Nat.

Acad.

Sci.

USA

2015,

112,

6295–6300,

DOI

10.1073/pnas.1312752110. (3) Elshkaki, A.; Graedel, T. E.; Ciacci, L.; Reck, B. K. Resource demand scenarios for the major metals. Environ. Sci. Technol. 2018, 52, 2491−2497, DOI 10.1021/acs.est.7b05154. (4) European Commission. Report on critical raw materials for the EU European Commission: Bruxelles, 2014. (5) Graedel, T. E.; Allwood, J.; Birat, J.-P.; Reck, B. K.; Sibley, S. F.; Sonnemann, G.; Buchert, G.; Hagelüken, C. Recycling rates of metals. A status report; United Nations Environnement Programme (UNEP) 2011. (6) Graedel, T. E.; Allwood, J.; Birat, J.-P.; Buchert, M.; Hagelüken, C.; Reck, B. K.; Sibley, S. F.; Sonnemann, G. What do we know about metal recycling rates? J. Ind. Ecol. 2011, 15, 355-366, DOI 10.1111/j.1530-9290.2011.00342.x.



Page 16 of 21

16 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(7) Reck, B. K.; Graedel, T. E. Challenges in metal recycling. Science 2012, 337, 690-695, DOI 10.1126/science.1217501. (8) Ciacci, L.; Harper, E. M.; Nassar, N. T.; Reck, B. K.; Graedel, T. E. Metal dissipation and inefficient recycling intensify climate forcing. Environ. Sci. Technol. 2016, 50, 11394−11402, DOI 10.1021/acs.est.6b02714. (9) Bardi, U. Extracting minerals from seawater: An energy analysis. Sustainability 2010, 2, 980-992, DOI 10.3390/su2040980. (10) Tsouris, C. Uranium extraction: fuel from seawater. Nat. Energy 2017, 2, 17047, DOI 10.1038/nenergy.2017.22. (11) International Atomic Energy Agency (IAEA). Combined methods for liquid radioactive waste treatment: Vienna, 2003. (12) Fu, F.; Wang, Q. Removal of heavy metal ions from wastewaters: A review. J. Environ. Manage. 2011, 92, 407-418, DOI 10.1016/j.jenvman.2010.11.011. (13) Zemb, T.; Bauer, C.; Bauduin, P.; Belloni, L.; Déjugnat, C.; Diat, O.; Dubois, V.; Dufrêche, J.-F.; Dourdain, S.; Duvail, M.; Larpent, C.; Testard, F.; Pellet-Rostaing, S. Recycling metals by controlled transfer of ionic species between complex fluids: en route to “ienaics”. Colloid Polym. Sci. 2015, 293, 1-22, DOI 10.1007/s00396-014-3447-x. (14) Rubio, J.; Souza, M. L.; Smith, R. W. Overview of flotation as a wastewater treatment technique. Minerals Engineer. 2002, 139-155. (15) Doyle, F. M. Ion flotation—its potential for hydrometallurgical operations. Int. J. Miner. Process. 2003, 72, 387–399, DOI 10.1016/S0301-7516(03)00113-3. (16) Polat, H.; Erdogan, D. Heavy metal removal from waste waters by ion flotation. J. Hazard. Mater. 2007, 148, 267–273, DOI 10.1016/j.jhazmat.2007.02.013. (17) Hoseinian, F. S.; Irannajad, M.; Nooshabadi, A. J. Ion flotation for removal of Ni(II) and Zn(II) ions from wastewaters. Int. J. Miner. Process. 2015, 143, 131-137, DOI 10.1016/j.minpro.2015.07.006.


Page 17 of 21


17 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(18) Taseidifar, M.; Makavipour, F.; Pashley, R. M.; Rahman, A. F. M. M. Removal of heavy metal ions from water using ion flotation. Environ. Technol. Innovation 2017, 8, 182–190, DOI 10.1016/j.eti.2017.07.002. (19) Schutt, E. S.; Klein, D. H.; Mattrey, R. M.; Riess, J. G. Injectable microbubbles as contrast agents for diagnostic ultrasound imaging: The key role of perfluorochemicals. Angew. Chem. Int. Ed. 2003, 42, 3218-3235, DOI 10.1002/anie.200200550. (20) Szijjarto, C.; Rossi, S.; Waton, G.; Krafft, M. P. Effects of perfluorocarbon gases on the size and stability characteristics of phospholipid-coated microbubbles - Osmotic effect versus interfacial film stabilization. Langmuir 2012, 28, 1182-1189, DOI 10.1021/la2043944. (21) Krafft, M. P. Fluorine in medical microbubbles – Methodologies implemented for engineering and investigating fluorocarbon-based microbubbles. J. Fluorine Chem. 2015, 177, 19-28, DOI 10.1016/j.jfluchem.2015.02.013. (22) Nguyen, P. N.; Trinh Dang, T. T.; Waton, G.; Vandamme, T.; Krafft, M. P. A nonpolar, nonamphiphilic molecule can accelerate adsorption of phospholipids and lower their surface tension

at

the

air/water

interface.

ChemPhysChem

2011,

12,

2646-2652,

DOI

10.1002/cphc.201100425.. (23) Gazzera, L.; Milani, R.; Pirrie, L.; Schmutz, M.; Blanck, C.; Resnati, G.; Metrangolo, P.; Krafft, M. P. Design of highly stable echogenic microbubbles through controlled assembly of their

hydrophobin

shell.

Angew.

Chem.

Int.

Ed.

2016,

55,

10263-10267,

DOI

10.1002/anie.201603706. (24) Ando, Y.; H. Tabata; Sanchez, M.; Cagna, A.; Koyama, D.; Krafft, M. P. Microbubbles with a self-assembled poloxamer shell and a fluorocarbon inner gas. Langmuir 2016, 32, 12461-12467, DOI 10.1021/acs.langmuir.6b01883. (25) Yang, G.; O'Duill, M.; Gouverneur, V.; Krafft, M. P. Recruitment and immobilisation of a fluorinated biomarker across an interfacial phospholipid film using a fluorocarbon gas. Angew. Chem. Int. Ed. 2015, 54, 8402-8406, DOI 10.1002/anie.201502677. (26) Gregson, M.; Lu, E.; Tuna, F.; McInnes, E. J. L.; Hennig, C.; Scheinost, A. C.; McMaster, J.; Lewis, W.; Blake, A. J.; Kerridge, A.; Liddle, S. T. Emergence of comparable



Page 18 of 21

18 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

covalency in isostructural cerium(IV)– and uranium(IV)–carbon multiple bonds. Chem. Sci. 2016, 7, 3286–3297, DOI 10.1039/c6sc00278a. (27) Dahle, J. T.; Arai, Y. Environmental geochemistry of cerium: applications and toxicology of cerium oxide nanoparticles. Int. J. Environ. Res. Public Health 2015, 12, 1253-1278, DOI 10.3390/ijerph120201253. (28) Ma, X.; Wang, Q.; Rossi, L.; Zhang, W. Cerium oxide nanoparticles and bulk cerium oxide leading to different physiological and biochemical responses in Brassica rapa. Environ. Sci. Technol. 2016, 50, 6793−6802, DOI 10.1021/acs.est.5b04111. (29) Paulenova, A.; Creager, S. E.; Navratil, J. D.; Wei, Y. Redox potentials and kinetics of the Ce3+/Ce4+ redox reaction and solubility of cerium sulfates in sulfuric acid solutions. J. Power Sources 2002, 109, 431–438, . (30) Mioduski, T.; Guminski, C.; Zeng, D. IUPAC-NIST Solubility Data Series. 100. Rare earth metal fluorides in water and aqueous systems. Part 2. Light lanthanides (Ce–Eu). J. Phys. Chem. Ref. Data 2015, 44, 013102-1-55, DOI 10.1063/1.4903362. (31) Kabalnov, A.; Bradley, J.; Flaim, S.; Klein, D.; Pelura, T.; Peters, B.; Otto, S.; Reynolds, J.; Schutt, E.; Weers, J. Dissolution of multicomponent microbubbles in the blood stream: 2. Experiment. Ultrasound Med. Biol. 1998, 24, 751-760, DOI 10.1016/S0301-5629(98)000337. (32) Rossi, S.; Waton, G.; Krafft, M. P. Phospholipid-coated gas bubble engineering - Key parameters for size and stability control as determined by an acoustic method. Langmuir 2010, 26, 1649-1655, DOI 10.1021/la9025987. (33) Hoorfar, M.; Neumann, A. W. Recent progress in Axisymmetric Drop Shape Analysis (ADSA). Adv. Coll. Interface Sci. 2006, 121, 25-49, DOI 10.1016/j.cis.2006.06.001. (34) Benjamins, J.; Cagna, A.; Lucassen Reynders, E. H. Viscoelastic properties of triacylglycerol/water interfaces covered by proteins. Colloids Surf. A 1996, 114, 245-254, DOI 10.1016/0927-7757(96)03533-9. (35) Nguyen, P. N.; Veschgini, M.; Tanaka, M.; Waton, G.; Vandamme, T.; Krafft, M. P. Counteracting the inhibitory effect of proteins towards lung surfactant substitutes: a


Page 19 of 21


19 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

fluorocarbon gas helps displace albumin at the air/water interface. Chem. Commun. 2014, 50, 11576-11579, DOI 10.1039/c3cc47840h. (36) Serrien, G.; Joos, P. Dynamic surface properties of aqueous sodium dioctyl sulfosuccinate solutions. J. Colloid Interface Sci. 1990, 139, 149-150, DOI 10.1016/00219797(90)90452-T. (37) Binks, B. P. Particles as surfactants - Similarities and differences. Curr. Opin. Colloid Interf. Sci. 2002, 7, 21-41. (38) Du, Z.; Bilbao-Montoya, M. P.; Binks, B. P.; Dickinson, E.; Ettelaie, R.; Murray, B. S. Outstanding stability of particle-stabilized bubbles. Langmuir 2003, 19, 3106-3108, DOI 10.1021/la034042n. (39) Subramaniam, A. B.; Abkarian, M.; Stone, H. A. Controlled assembly of jammed colloidal shells on fluid droplets. Nature Mat. 2005, 4, 553–556, DOI 10.1038/nmat1412. (40) Binks, B. P.; Murakami, R. Phase inversion of particle-stabilized materials from foams to dry water. Nature Mat. 2006, 4, 865–869, DOI 10.1038/nmat1757. (41) Fujii, S.; Yokoyama, Y.; Nakayama, S.; Ito, M.; Yusa, S.-I.; Nakamura, Y. Gas bubbles stabilized by Janus particles with varying hydrophilic−hydrophobic surface characteristics. Langmuir 2018, 34, 933−942, DOI 10.1021/acs.langmuir.7b02670. (42) Hunter, R. J. Zeta Potential in Colloid Science; Academic Press: London, 1981. (43) Takahashi, M. Zeta potential of microbubbles in aqueous solutions: electrical properties of

the

gas-water

interface.

J.

Phys.

Chem.

B

2005,

109,

21858-21864,

DOI

10.1021/jp0445270. (44) Cai, W. B.; Yang, H. L.; Zhang, J.; Yin, J. K.; Yang, Y. L.; Yuan, L. J.; Zhang, L.; Duan, Y. Y. The optimized fabrication of nanobubbles as ultrasound contrast agents for tumor imaging. Sci. Rep. 2015, 5, 13725, DOI 10.1038/srep13725. (45) Liu, S.; Wang, Q.; Sun, T.; Wu, C.; Shi, Y. The effect of different types of micro-bubbles on the performance of the coagulation flotation process for coke waste-water. J. Chem. Technol. Biotechnol. 2012, 87, 206–215, DOI 0.1002/jctb.2698.



Page 20 of 21

20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(46) Krafft, M. P.; Riess, J. G. Selected physicochemical aspects of poly- and perfluoroalkylated substances relevant to performance, environment and sustainability-Part one. Chemosphere 2015, 129, 4-19, DOI 10.1016/j.chemosphere.2014.08.039. (47) Krafft, M. P.; Riess, J. G. Per- and polyfluorinated substances (PFASs): Environmental challenges.

Curr.

Opin.

Colloid

Interface

Sci.

10.1016/j.cocis.2015.07.004.


2015,

20,

192-212,

DOI

Page 21 of 21


21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

TOC Graphical Abstract

Synopsis Fluorocarbon gases cause adsorption of cerium salts and CeO2 nanoparticles at fluid interfaces, with implications for heavy metal recovery from effluents.


Adsorption of Cerium Salts and Cerium Oxide Nanoparticles on

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