Extraction of Cobalt Ion from Textile Using a Complexing

Research Note pubs.acs.org/IECR

Extraction of Cobalt Ion from Textile Using a Complexing Macromolecular Surfactant in Supercritical Carbon Dioxide Mathieu Chirat,†,§ Tiphaine Ribaut,†,§,⊥ Sébastien Clerc,† Frédéric Charton,§ Bruno Fournel,‡ and Patrick Lacroix-Desmazes*,† †

Ingénierie et Architectures Macromoléculaires (IAM), Institut Charles Gerhardt − UMR 5253 CNRS/UM2/ENSCM/UM1, ENSCM, 8 rue de l’Ecole Normale, 34296 Montpellier Cedex 5, France ‡ DEN/DTCD/DIR and §DEN/DTCD/SPDE/LPSD, CEA Marcoule BP17171, 30207 Bagnols Sur Cèze, France ⊥ Institut de Chimie Séparative de Marcoule, UMR 5257, ICSM Site de Marcoule, BP 17171, 30207 Bagnols Sur Cèze Cedex, France S Supporting Information *

ABSTRACT: Cobalt ion under the form of cobalt nitrate is removed from a textile lab coat using supercritical carbon dioxide extraction. The process involves a macromolecular additive of well-defined architecture, acting both as a surfactant and a complexing agent. The extraction efficiency of cobalt reaches 66% when using a poly(1,1,2,2-tetrahydroperfluorodecylacrylate-covinylbenzylphosphonic diacid) gradient copolymer in the presence of water at 160 bar and 40 °C. The synergy of the two additives, namely the copolymer and water which are useless if used separately, is pointed out. The potential of the supercritical carbon dioxide process using complexing macromolecular surfactant lies in the ability to modulate the complexing unit as a function of the metal as well as the architecture of the surface-active agent for applications ranging for instance from nuclear decontamination to the recovery of strategic metals.

1. INTRODUCTION Extraction processes in supercritical carbon dioxide (scCO2) have been investigated by researchers1−5 as an alternative to solvent-based processes performed in aqueous medium or organic solvents. A lot of industrial sectors can take advantage of scCO2 metal extraction. For instance, the nuclear industry is one of them, and more particularly the decontamination activities are facing effluent management issues.6 Indeed, the laundry of contaminated textiles in the nuclear industry is performed in aqueous medium. This process generates contaminated aqueous effluents which have to be further treated. To reduce the amount of wastewater, supercritical carbon dioxide is increasingly considered as an alternative solvent because it is environmentally benign, inexpensive, and readily available; it also has easily accessible critical parameters (T = 31 °C and P = 73.8 bar) and offers tunable solvent properties. Moreover, some options are available for the separation and recovery of the contamination from CO2, leading to solid and isolated radioactive wastes.7 Since, supercritical CO2 is an apolar solvent, some complexing agents, surfactants, and/or cosolvents are commonly used as additives in the scCO2 process to enhance the metal extraction yield.7−9 In our approach, we propose to use a macromolecular monocomponent system which can play the role of both surfactant and complexing agent. In addition to this, the macromolecular architecture (block, gradient, statistical, grafted copolymer) and the nature of the complexing unit can be modulated as a function of the applications.10 Such compounds can be obtained thanks to their route of synthesis which allows the incorporation of a specific complexing unit in the copolymer architecture through a functional styrenic monomer (Scheme 1-A). Thus, starting from chloromethyl© 2012 American Chemical Society

styrene, a chemical modification is carried out to incorporate the complexing function G. The new G-styrenic monomer is then copolymerized with 1,1,2,2-tetrahydroperfluorodecylacrylate (FDA) to form a CO2-soluble macromolecular additive. The surface-active properties of the additive in scCO2 (CO2philic/CO2-phobic balance) are achieved thanks to the controlled free-radical copolymerization technique, leading to a controlled and well-defined macromolecular architecture. The CO2-philic part of the copolymer (FDA unit) has been selected due to its exceptional solubility in scCO2. It is now wellestablished that only a few polymers are soluble in scCO2.11,12 Among them, siloxane-based polymers, 1 3 − 1 9 poly(vinylacetate),20,21 and poly(propylene oxide)20−22 are reported but the best type of polymer in terms of solubility in scCO2 are the poly(fluoroalkyl (meth)acrylate)s.23,24 We therefore report herein the use of a fluorinated macromolecular surfactant bearing a complexing group for the extraction of cobalt from a lab coat in scCO2 medium. The G functional unit is a phosphonic diacid25,26 which is hydrophilic and could allow the formation of water-in-CO2 microemulsion that might be beneficial for the extraction as shown later (Scheme 1-B).

2. EXPERIMENTAL SECTION 2.1. Materials. The surfactant, poly(1,1,2,2-tetrahydroperfluorodecylacrylate-co-vinylbenzylphosphonic diacid) (poly(FDA-co-VBPDA)), was synthesized by reversible addition− Received: Revised: Accepted: Published: 538

July 2, 2012 November 21, 2012 December 9, 2012 December 19, 2012 dx.doi.org/10.1021/ie301754v | Ind. Eng. Chem. Res. 2013, 52, 538−542

Industrial & Engineering Chemistry Research

Research Note

Scheme 1a

a

(A) Route for the synthesis of CO2-soluble surface-active macromolecular complexing additives (G: complexing group or its precursor); (B) structure of poly(FDA-co-VBPDA) gradient copolymer synthesized by RAFT copolymerization.

and the mixture was stirred with a magnetic stirring bar for 1 h 30 min at 1100 rpm. Then, 100 mL of CO2 were passed through the cell at constant pressure (160 bar) at a flow rate of 5 mL/min. Finally, the cell was depressurized. The textile was analyzed by inductively coupled plasma mass spectrometry (ICP−MS), after a complete dissolution in an aqueous sulfuric acid solution, to determine the quantity of residual cobalt. A control analysis was performed on the contaminated sample to ascertain that 10 μg of Co was initially present. The absence of copolymer on the sample at the end of the procedure was also checked by phosphorus analysis performed by inductively coupled plasma atomic emission spectroscopy (ICP−AES).

fragmentation chain transfer (RAFT) polymerization to obtain a copolymer with a gradient architecture, predetermined molecular weight and narrow polydispersity index, as already described.27 The surfactant characteristics are the following: Mn = 19500 g/mol and VBPDA molar fraction = 19.0%. The gradient architecture has been selected for several reasons. First, gradient copolymers require only one synthetic step (in contrast to at least two steps for the synthesis of block copolymers). Second, the solubility properties in scCO2 are better for copolymers with gradient architectures than with block architectures.28 Finally, gradient copolymers can have surfactant properties (which random copolymers do not have specifically). Furthermore, it must be noticed that, unlike gradient copolymers synthesized by controlled free-radical copolymerization, random copolymers prepared by conventional free-radical copolymerization are heterogeneous in terms of molecular weight and composition. Therefore, the solubility and self-assembly of random copolymers in scCO2 is somewhat less controlled than for gradient copolymers.27 In addition, the surfactant contains phosphonic diacid groups in order to interact with the metal ion (cobalt(II)) as well as to provide hydrophilic properties. The lab coat (textile) was provided by Sas Vanneuville et Fils, composition: 50/50 polyester/cotton. Cobalt(II) nitrate hexahydrate (Co(NO3)2·6H2O) (Aldrich, 99.999%), CO2 (99.99992%, SFE 5.2, Linde Gas SA, France) and all other chemicals were used as received unless otherwise stated. A high pressure, variable volume view cell (maximal volume of 15 mL) equipped with a sapphire window for visual observations is used for pressurized experiments. The cell is equipped with a pressure transducer, a rupture disk, and an internal thermocouple. The cell is thermostatted by a water/ glycol mixture delivered by a Lauda RE206 circulating pump. An ISCO 260D automatic syringe pump is used to deliver CO2. 2.2. Extraction Procedure in Supercritical Carbon Dioxide. On a piece of textile (20 mm diameter), 10 μL of an aqueous solution of cobalt nitrate (1 g/L of Co) was deposited with a micropipet to provide a sample with 10 μg of Co. The sample was dried overnight at 70 °C under vacuum. The sample was introduced in the high pressure cell (V = 8.69 mL) with the copolymer additive or/and deionized water. The cell was pressurized with carbon dioxide at 160 bar and 40 °C,

3. RESULTS AND DISCUSSION 3.1. Solubility of the Copolymer in Dense CO2. The solubility of the gradient copolymer poly(FDA-co-VBPDA) in dense CO2 was determined by cloud point measurements (Figure 1) using a procedure reported elsewhere.29 The copolymer exhibits a lower critical solution temperature (LCST) behavior. The data confirmed that this kind of fluorinated copolymer has a high solubility in dense CO228

Figure 1. Cloud points in dense CO2 of poly(FDA-co-VBPDA), Mn = 19 500 g/mol, VBPDA molar fraction y = 19% (■) and poly(FDA), Mn = 22,400 g/mol (▲) at a polymer weight fraction of 4.0%. 539

dx.doi.org/10.1021/ie301754v | Ind. Eng. Chem. Res. 2013, 52, 538−542


Research Note

compositions of the extraction medium. The error bar for the extraction yield was estimated from the 10% uncertainty of the cobalt content obtained by ICP−MS and the 2% uncertainty of mass deposited on the sample. When using scCO2 alone (Table 1, run 1), the extraction yield is 11% ± 9%. This experiment shows that only a small amount of the contamination is carried away by scCO2 or removed from the sample during the rinsing step and/or the depressurization step. An additive is required to improve the extraction efficiency. The addition of some water to the process decreases the yield of extraction to 0% ± 12% (Table 1, run 2). Approximately 6 × 10−3 in molar fraction of water is soluble in scCO2 at 40 °C and 160 bar (determined by King et al.30 and confirmed by Sabirzyanov et al.31). For our experiments, it means that roughly half of the 33 mg of water is soluble in scCO2. The other half of the water is certainly absorbed on the textile matrix which contains hydrophilic cotton and/or dispersed roughly in the medium. The absorbed water could have enhanced the affinity of the cobalt salt for the textile and thus retained the contamination on the matrix which leads to a lower extraction yield. When the copolymer is used as the sole additive in scCO2 (Table 1, run 3), the extraction yield is 23% ± 11%. Compared to the use of scCO2 alone, the improvement is not very high. However, some interactions between the copolymer and the cobalt probably took place since more cobalt was extracted in the presence of the copolymer. When the two additives, namely the poly(FDA-co-VBPDA) gradient copolymer and water, are introduced in scCO2 (Table 1, run 4), the extraction performance rises up to 66% ± 6%. Therefore, the use of either water or copolymer, separately, gives low yields whereas a synergetic effect is clearly noticed when these two additives are used together. The role of water can be triple. First, water can interact with cobalt nitrate (hydration), making the cobalt more prone to be extracted by the CO2-soluble complexing agent.1,32,33 Second, water allows the formation of phosphonate species which are likely to be more efficient for complexation of metal ions (it must be noticed that, the pH of water in scCO2 being around 2.834 and the pKa values of alkylphosphonic diacids being around 2.3 and 7.55,35 the extraction efficiency could be probably enhanced by a better adjustment of the pH in order to favor the formation of the phosphonate form of the ligand groups while keeping the cobalt in its Co(II) form36). Third, the formation of a water-inCO2 (micro)emulsion is likely to occur,9,37−40 making the water-swollen micelle able to dissolve and/or complex the cobalt contaminant. Indeed, we have previously reported that in the presence of a small amount of water, a microemulsion-like system was evidenced by small-angle neutron scattering with this type of poly(FDA-co-VBPDA) gradient copolymer in scCO2.41 The key role of the hydrophilic group (VBPDA group) borne by the CO2-soluble polymer was confirmed with the poor extraction yields (9% ± 9%) obtained when using a copolymer bearing vinylbenzylphosphonic diethyl ester (VBPDE) groups. These VBPDE groups are hydrophobic, therefore the formation of water-in-CO2 microemulsion is unlikely to occur. Moreover, complexing properties are also required in the copolymer to reach high extraction yields. Indeed, a low extraction yield was obtained (1% ± 11%) when the dispersion of water in CO2 was performed by a

which is a process key-point to work at relatively low pressure. The solubility properties are brought by the well-known FDA unit.24 The effect of the incorporation of a complexing unit (VBPDA) is very low in terms of solubility: Figure 1 shows only a difference of roughly 10 bar between the cloud point curve of the homopolymer (poly(FDA), Mn = 22 400 g/mol) and the cloud point curve of the copolymer. The complexation of cobalt by the VBPDA units of the copolymer increases the copolymer cloud point pressure by around 15 bar (Supporting Information). 3.2. Extraction Experiments in Supercritical Carbon Dioxide. Extraction trials were performed according to different experimental conditions (Table 1). Extraction experiTable 1. Quantities of Additives and Experimental Conditions for Extraction Experiments run

copolymer (mg)

water (mg)

pressure (Bar)

temp (°C)

1 2 3 4

0 0 42.5 38.5

0 33 0 33

160 160 160 160

40 40 40 40

ments were first carried out with only scCO2 as a reference to check if scCO2 has any extractive properties. Besides this, the gradient copolymer poly(FDA-co-VBPDA) was added to the extraction mixture in the presence or absence of water to investigate its extraction capacity. Water was also used as sole additive to distinguish the effect of the different additives on the extraction yields. When the copolymer was used, the VBPDA complexing unit was in a molar excess of approximately 100 compared to cobalt. When water was introduced with the copolymer, the molar ratio water/copolymer was about 400. The pressure and temperature have been selected to perform the extraction in the supercritical domain of the mixture and slightly above the cloud point pressure of the copolymer to manage a process at the lowest possible pressure. The extraction yield (E) of cobalt was calculated by E = (m0Co − mrCo)/m0Co, where m0Co is the initial mass of Co in the sample before extraction, and mrCo is the residual mass of Co in the sample determined by ICP−MS after extraction. Figure 2 presents the extraction yield of the experiments for different

Figure 2. Extraction yields with error bars of cobalt nitrate from textile lab coat in supercritical carbon dioxide (T = 40 °C, P = 160 bar). 540



Research Note

Figure 3. Schematic representation of the combined action of the complexing macromolecular surfactant and water in the supercritical extraction process.

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poly(ethylene oxide)-block-poly(FDA) copolymer (details are provided in Supporting Information). To sum up, the extraction process required both dispersed water (microemulsion) and a complexing group within the micelle to interact with the cobalt. The extraction process is represented in Figure 3. Herein, the extraction yield of 66% without optimization is quite promising. Better results are found in the literature8,9 but less water per Co (4.3 times less) and less complexing unit per Co (40 times less) are used in the present process. The effect of operating parameters such as the modification of pressure and temperature, the application of ultrasound, the modification of the copolymer/water ratio, the application of numerous extraction cycles, and the use of other contaminated matrixes will be detailed elsewhere in view to further improve the extraction efficiency while preserving the above-mentioned advantages of our system.

ASSOCIATED CONTENT

S Supporting Information *

Additional extraction experiments, complexation experiment. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +33-4-67-14-72-05. Fax: +33-4-67-14-72-20. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors of this paper would like to thank the Laboratoire de métallographie et d’analyses chimiques DEN/DTEC/ SGCS/LMAC at CEA Marcoule for ICP−MS and ICP−AES analyses, CNRS and CEA for the joint Grant No. 042832, CEA (Contract No. 045079) and Région Languedoc-Roussillon (Chercheur d’Avenir, Grant No. 2010-0-018, SUPERPOL project) for financial support of this research.

4. CONCLUSIONS A 66% extraction yield was obtained for cobalt-contaminated textile treated in supercritical carbon dioxide in the presence of a tiny amount of water using a macromolecular monocomponent system, namely a poly(1,1,2,2-tetrahydroperfluorodecylacrylate-co-vinylbenzylphosphonic diacid) gradient copolymer, acting as both a surfactant and a complexing agent. Our results pointed out the synergy of the two additives, the copolymer and water, which are useless if used separately. One can think to optimize the quantity of each additive, the thermodynamic conditions (P and T), or to apply ultrasound to further improve the efficiency of the extraction process. Beyond this, the presented strategy opens the route toward the development of a platform composed of a wide range of specific macromolecular additives by modulating the nature of the complexing unit as a function of the metal as well as by varying the architecture of the surface-active copolymer. Lastly, the use of such copolymers in other applications such as recovery of critical metals which are nowadays more and more strategic could also be considered.

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REFERENCES

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Extraction of Cobalt Ion from Textile Using a Complexing

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