Effect of temperature on the heat treatment to recover green solvent from emulsion liquid membranes used in the extraction of Cr(VI)
Graphical abstract
Introduction
Traditionally, a special attention has been paid to the removal of Cr(VI) ions from waste effluents due to its high toxicity and numerous industrial applications. Cr (VI) is classified as a carcinogenic, mutagenic and teratogenic compound. Therefore, its recovery and concentration from industrial effluents become a necessary task for environmental safety [1,2].
One of the main problems that present Cr (VI) recovery is its usual low concentrations in wastewater effluents. In this way, liquid-liquid extraction using liquid membranes (ELMs) has been reported as a promising technique since ELMs showed good performance recovering metal ions such as cadmium, chromium and nickel and pollutants of other nature as dyes and acetic acid from industrial wastewaters [[3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]]. ELMs presents high metal extraction efficiency, easy installation and process economy. However, frequently toxic organic solvents are used [14,15] even some recent works use alternative solvents such as ionic liquids [16].
Extraction with ELMs consists of four steps. (i) firstly, a stable water in oil (W1/O) emulsion is prepared [[17], [18], [19], [20], [21], [22]], (ii) W1/O emulsion is mixed with the wastewater which will act as an external aqueous phase of a water-in-oil-in-water (W1/O/W2) emulsion. The mixture is continuously stirred to enhance the metal extraction. (iii) The low stability of the external interface oil-water makes that, once the stirring is stopped, a phase separation takes place immediately, a W1/O emulsion in the upper part and a purified aqueous phase at the bottom. (iv) The W1/O emulsion should be destabilized in order to obtain the new highly concentrated waste effluent and, if possible, to reuse the oily phase. Fig. 1 schematically describes the processes mentioned.
For economic and environmental considerations, the reuse of the organic membrane solution is one of the most critical factors for developing a commercially viable ELM process, since extractant and stabilizers are more expensive than other chemicals used herein. Moreover, solvents need specific and frequently expensive treatments to ensure environmental requirements before its disposal. In that sense, solvent reuse will perform a clear economic and environmental advantage on process operation.
W/O demulsification from ELMs after pollutant extraction had been investigated [23,24]. However, the destabilization of W/O emulsion from petroleum industry had been widely studied by several techniques such as coalescence by surfactants or coagulation agents, gravity settling, heating, microwave, ultrasounds, freeze/thaw, magnetic field application, membrane processes or the combination of several of them [15,[25], [26], [27], [28], [29], [30], [31], [32], [33], [34]]. Normally, this destabilization process requires a variety of chemicals and the recovered water phase needs a secondary purification, involving additional energy and equipment requirements.
Heating treatment demulsification is a simple process to be industrialized and scaled up, but the process is mainly influenced by some physical characteristics of internal droplet size and the oil properties such as their nature and viscosity, which make their application not always possible. In this case, heat capacity of sunflower oil is slightly higher than other solvents frequently used in extraction process, such as kerosene or toluene, which could lead to an increase of around 10 % of the energy required to warm up the system and makes necessary to optimize the temperature and time required to ensure a satisfactory demulsification. However, this type of solvent does not produce potential toxic vapours from other aforementioned solvents. The main economic advantage of heating demulsification is that the separation can be easily made by settling, no needing expensive equipment and lowering investment and operation costs. Moreover, is also important to point out that other demulsification techniques require also high temperatures in order to achieve significant efficiency [24,34].
One of the most important factors affecting emulsion stability and instability is drop diameter. Large droplet diameters result in poor stability and low extraction efficiencies in ELMs [35], because of a low surface/volume ratio and, consequently, reduced interfacial surface area [36]. On contrary, small droplet diameters provide stable emulsions, larger mass transfer area, and hence higher extraction efficiencies. However, if the droplet diameters are too small, the emulsion destabilization step by any mechanical process will be more difficult, once the extraction had been done (fourth step) [35].Therefore, in order to ensure high ELM extraction and easy W1/O emulsion destabilization, formulation and preparation method should be optimized [37,38].
The use of sunflower oil in the formulation of ELM is a good alternative to conventional organic solvents and their high efficiency on metal extraction has been already tested [5,39,40]. Furthermore, the use of sunflower oil as a extractant solvent presents a clear advantage compared to other frequently used toxic solvents, since it can be heated in safe conditions in order to get the demulsification of the used W1/O [41].
Additionally, the surfactant selection is also a key factor on ELMs formulation in order to have a stable emulsion and high demulsification efficiency. ELMs demulsification did not show high efficiency when common surfactants, such as Span-80, were used to stabilize W1/O emulsion [24]. On contrary, surfactants with ethoxylates groups, such as Tween 80, can form hydrogen bonds between oxygen atoms in the chain and water. However, this hydrogen bounds can easily be broken at high temperature, being heat treatment a suitable technique for emulsions destabilization where this type of surfactants are present on their formulations [25]. Polyglycerol polyricinoleate (PGPR) is a synthetic emulsifier widely used to stabilize food W1/O emulsions [42] and their use has been recently tested in ELMs stabilization with high extraction efficiency [39,40]. However, no studies about the demulsification of emulsions with this compound as stabilizer have been reported.
In the present work, the extraction and recover of Cr(VI) at different concentrations by ELMs processes was studied. Sunflower oil was used as environmental-friendly solvent and as an alternative to synthetic organic solvents and PGPR and Tween 80 were used as stabilizers.
The main goal of this work is to study the feasibility of heat treatment as a demulsification technique of ELMs used for Cr (VI) extraction in a range of temperatures from 30 to 90 °C. ELMs were formulated with sunflower oil as solvent and PGPR and Tween 80 as stabilizers. Moreover, the feasibility of the use of recovered oil for ELMs formulated will be studied. W1/O and ELMs were characterized in terms of droplet size distribution, stability and Cr(VI) extraction capacity. Recovered oil quality was evaluated by dynamic scanning calorimetry (DSC) and Fourier transform transform infrared (FTIR).
Section snippets
Materials
The liquid membrane solution was formulated using two different stabilizers: Tween® 80 (polyoxyethylene sorbitan monooleate), a hydrophilic surfactant from Sigma-Aldrich (USA) with hydrophilic-lipophilic balance (HLB) of 15.0, and the lipophilic surfactant PGPR (polyglycerol polyricinoleate), supplied by Brenntag AG (Germany), with HLB of 3.0. The mobile carrier or extractant used was TOPO (tri-n-octylphosphine oxide) supplied by Alfa Aesar, Germany. Food grade commercial sunflower oil was used
Effect of the temperature on the destabilization kinetics of ELMs
The destabilization kinetics at different temperatures were studied by comparing the TSI values as a function of time. Results are shown in Fig. 2. It can be seen that the heating has a positive effect on emulsion destabilization, since the TSI increases with temperature. When the temperature increases from 30 °C to 50 °C, no noticeable destabilization was observed during the 10 h monitored, but from the 60 °C–90 °C the TSI increase sharply during the first 2 h, indicating a strong
Conclusions
In this study, the application of a heating demulsification treatment in an ELM process using sunflower oil as vegetable solvent and PGPR and Tween 80 as surfactants was evaluated for the extraction and recover of Cr(VI) at different concentrations from aqueous solution.
An extraction efficiency higher than 99 % was observed in the extraction of Cr(VI) from aqueous solutions in heavy metal concentrations ranging from 0.043 ppm to 50 ppm. This so formulated green ELMs were able to concentrate
CRediT authorship contribution statement
Katia Anarakdim: Investigation, Data curation, Writing - original draft, Funding acquisition. María Matos: Data curation, Conceptualization, Supervision, Funding acquisition. Angel Cambiella: Conceptualization, Visualization, Writing - review & editing. Ounissa Senhadji-Kebiche: Conceptualization, Supervision. Gemma Gutiérrez: Data curation, Supervision, Writing - review & editing, Funding acquisition, Project administration.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
Katia Anarakdim gratefully acknowledges scholarship under the National Exceptional Program (P.N.E 2016/2017) provided by the Ministry of Higher Education and Scientific Research Algeria for her research stay at the Department of Chemical and Environmental Engineering, University of Oviedo (Oviedo, Spain).
This study was also financed by the Consejería de Economía y Empleo del Principado de Asturias (Plan de Ciencia, Tecnología e Innovación, 2013e2017) through the Grants Refs. GRUPIN14-022 and
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2022, Colloids and Surfaces A: Physicochemical and Engineering AspectsCitation Excerpt :These are inflammable, biodegradable, non-volatile, non-toxic, and renewable. Owing to these qualities, these are introduced by the researchers as a substitute for volatile organic solvents known as green solvents [17,19,31,32]. Green-ELM technology is a new technology introduced and continuously attracted the scientific community and researchers for its ability in the field of separation technology.