Microfluidic solvent extraction of calcium: Modeling and optimization of the process variables

https://doi.org/10.1016/j.seppur.2019.115875Get rights and content

Highlights

  • A method was proposed for separation of species with very low separation factor.

  • Extraction efficiency of microfluidic device was modeled and optimized (=62.28%).

  • Relatively high overall volumetric mass transfer coefficients were achieved.

  • Full phase separation was achieved for potential numbering-up of the system.

Abstract

In this study, a new systematic method was proposed for the highly efficient separation of species with very low separation factor. Here, solvent extraction and microfluidic technology were integrated for the extraction and separation of Ca2+ ion. A cyclic compound (DC18C6) was used as the selective extractant. Full separation of organic and aqueous phases at the end of the microchannel was achieved by exploiting a flow regime map. N-butyl acetate was chosen as organic solvent among from other potential candidates with different characteristics. The Box-Behnken experimental design was used to provide data for modeling, while the variables of the model were DC18C6 concentration, flow rate and pH of the aqueous phase. The results showed that Ca2+ extraction efficiency was 62.28% under the optimum conditions: DC18C6 concentration was 0.014 M, the aqueous phase flow rate was 20 µl/min and pH of the aqueous phase was 5.1. Multi-stage microfluidic solvent extraction in series was performed and the possibility of numbering-up with maintaining full phase separation was demonstrated. The overall volumetric mass transfer coefficients were determined quantitatively in a single microchannel, and their values were in the ranges of 0.19–0.41 s−1, which are two orders of magnitude higher than those of conventional liquid-liquid contactors.

Introduction

Among various isotopes of calcium, 48Ca is widely used in clinical research, mainly in nutritional studies and absorption of calcium in the body. Other applications of this isotope include tracking the migration of elements in soil and plants as well as bombarding lead and bismuth to create super-heavy elements. In spite of having a very high neutron-to-proton ratio (=1.4), 48Ca is a stable isotope of calcium and double beta decay is the only route of its radioactive decay. This decay is one of the extremely scarce types of radioactive decays that has various applications in the nuclear industry [1], [2]. The natural abundance of 48Ca is about 0.187%, so the enrichment process of this isotope has been a challenge so far [3]. Generally, the enrichment of isotopes is carried out through specific processes such as centrifugation, thermal diffusion, chemical exchange, gaseous diffusion, molecular laser and electromagnetic separators. Since calcium-to-gas conversion is virtually impossible, centrifugation, thermal diffusion and molecular laser methods, that use gaseous compounds as working materials, are not the proper options for enrichment [4], [5], [6], [7], [8], [9], [52]. Currently, the separation of stable isotopes of calcium on a commercial scale is carried out solely by electromagnetic separators, which provides a very expensive product (200000$ per gram). Chemical exchange methods such as liquid-liquid and resin extraction are advantageous to electromagnetic methods both in terms of technology and cost.

Among various separation methods, solvent extraction is an efficient technique that uses differences in solubility of a substance in two immiscible fluids, which are usually an aqueous phase and an organic phase, for the purpose of separation [10], [11], [12], [13], [14], [15], [16], [17]. Jepson et al. [18] carried out the enrichment of 44Ca isotope by Dicyclohexyl-18-crown-6 (DC18C6). They showed that 040Ca isotope is transferred into the organic phase by DC18C6 and the heavier isotope (044Ca) of calcium is concentrated in the aqueous phase. Recently, Umehara et al. [4] fractionated various isotopes of calcium using Benzo 18-crown-6-ether resin and a feed solution containing 0.1 M calcium chloride. The maximum isotopic ratio 048Ca/040Ca in the effluent solution was equal to 2.032×10-3, versus 1.956×10-3 of the feed solution. The liquid-liquid extraction approach of chemical exchange method was first used by Hazama et al. [19] for the enrichment of 048Ca using DC18C6. They showed that the size and shape of the nucleus have less effect on the enrichment of 048Ca than the isotope nucleus mass. Okumara et al. [20] reported 048Ca isotope separation using crown ether chromatography. They detected that the resulting isotope separation coefficient between 048Ca and 040Ca is 3.8×10-3. Umehara et al. has recently conducted a research on 048Ca isotope enrichment by band chromatography using crown ether resin, and reported separation coefficient ranging between 2×10-3 and 3×10-3 [21].

However, the maximum amount of enrichment (separation factor) obtained by chemical exchange methods in one step is in the order of magnitude of 0.001. In order to achieve higher values, the number of required steps is very high, which increases the cost, space, and time required for the process. Besides, conventional solvent extraction has some intrinsic drawbacks such as higher response time, high energy consumption and so on [22], [23]. Moreover, in this technique, an additional step called “settling stage” is required to separate the phases [24]. To overcome these obstacles, the integration of microfluidic technology as relatively new technology and liquid-liquid extraction can be used.

Microfluidic devices have been extensively used for the solvent extraction of various molecules [23], [25], [26], [27], [28], [29]. These devices have some advantageous over conventional methods like short diffusion length and high surface-to-volume ratio which leads to a high mass transfer rate across the interface [30], [31]. Thus this technology is well suited for intensifying chemical processes, and low throughput problem can easily be solved by numbering-up [32]. This features along with high safety, compact structure, and well-defined hydrodynamic conditions have made µSX a promising technology in the separation processes [33].

Also, microfluidic devices have the potential to eliminate the settling stage through the in-situ separation of phases [34]. These could be obtained by a parallel flow regime. Type of flow regime in a microdevice highly depends on characteristics like geometry, viscosity and density of phases, interfacial tension between phases and walls [35]. Parallel flow with a stable interface between the two immiscible fluids allows separation of two phases. In other words, it is a phase separation method used in microfluidic devices which enables easy numbering up of extraction stages [36]. This method allows to simply scale-up the process (in this case by numbering up) and achieve commercialized products. However, parallel flow forms in a very narrow range of flow ratios and requires specific sets of liquids and geometries. In order to overcome these difficulties, various approaches such as chemical modification and internal structures like guides and pillars have been proposed [37], [38]. Chemical modification approach is not a good option for continuous processes due to the gradual reduction of hydrophobicity or hydrophilicity of the affected regions. Internal structures, also, decrease the interfacial area and therefore mass transport. Adding a surfactant allows for a reduction in the interfacial tension; this leads to a parallel flow in a wide range of flow ratios as well as a stable interface [39]. In a study by Pohar et al. [40], a correlation has been proposed for reaching an interface in the middle of the microchannel for different operational conditions.

Crown ethers are cyclic compounds that consist of hydrophilic interiors and hydrophobic exteriors. These compounds have been widely accepted as complexing agents for alkaline earth metal ions. Complexation of a crown ether and a metal ion is strongly influenced by crown ether ring size and radius of a cation. On the other hand, a complexed crown ether loses its hydrophilic part and transfers to the organic phase. Owing to these characteristics, crown ethers are ideal candidates for the selective separation of different species. Crystal ionic radius of Ca2+ is 0.99 A°, which becomes slightly larger when dissolves in an aqueous phase. Thus, DC18C6 with a ring size of 1.3–1.6 A° is a good candidate for extraction of Ca2+ion [41].

Reynolds number of a typical microfluidic device is low enough to consider the flow within them to be laminar. Hence, the controlling mechanism of mass transfer in these devices is diffusion. Besides, Wilke-Chang equation indicates that the diffusion coefficient is inversely proportional to the molal volume of solute [42]. Thus, since the molal volume of heavier isotopes of Calcium is higher than lighter ones, the diffusion coefficient of 48Ca is lower than other isotopes. Consequently, mass transfer rate of this isotope from one phase to another phase is lower than other isotopes, and 48Ca is enriched in the initial phase. Along these lines, introducing a method for optimizing the separation process of Ca2+ consequentially leads to 48Ca enrichment.

The present paper aims to introduce a comprehensive approach for optimum single stage micro-solvent extraction of Ca2+ion, which has the potential of numbering-up. Particular emphasis is placed on the obtainment of parallel flow regime that leads to full phase separation. The extraction performance of a microfluidic device is investigated at different operational conditions and optimized via Box-Behnken experimental design. Moreover, traditional solvent extraction experiments are carried out to evaluate the efficiency of the microfluidic device.

Section snippets

Reagents

In order to perform experimental examinations, calcium chloride (anhydrous, beads, −10 mesh, ≥99.9%), dicyclohexano-18-crown-6 (purity: 98.0%), picric acid (moistened with water, ≥98.0%), and sodium hydroxide were purchased from Sigma Aldrich. The organic phase was obtained by dissolving three different concentrations of DC18C6 (0.005, 0.01 and 0.015 M) in the selected organic solvent. The aqueous solution was prepared by dissolving 0.01 M picric acid and 0.005 M CaCl2 into the deionized water

Solvent selection

A suitable organic solvent has to be chosen based on a compromise between the amount of extraction efficiency, the potential of producing parallel flow regime in a wide range of flow rates, being less hazardous and not being expensive. For this purpose, six organic solvents with different characteristics were used to investigate the abovementioned criteria (Table 2). The investigation was performed by lab-scale mixer-settler method (separating funnel and magnetic stirrer). Type of organic

Conclusions

In this research, a systematic method is proposed for the extraction of calcium ions. This method can be used for the extraction of other elements of alkaline and alkaline earth metals. Using this method, at a residence time of 4.05 s, the extraction efficiency was equal to 22.59%, while for obtaining the same amount of extraction by the traditional liquid–liquid method, at least 3 min is required. Therefore, these systems substantially reduce the process time and simultaneously eliminate the

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