Essay: CPE methodology

The CPE methodology was introduced by Watanabe and co-workers (Goto K, Fukue Y and Watanabe H., 1977; Watanabe H and Tanaka H.,1978), and its analytical potential was reported by Watanabe in 1982 (Watanabe H and Mitta K L.,1982) and its application for organic pollutants have been extensively reviewed(Tani.H, Kamidate.T and Watanabe.T.,1997; Quina.F.H, Hinze.W.L.,1999; Sanz-Medel.A.,et.al.1999; Huddleston.J.G.,et.al.1999; Constantine D. Stalikas,2002; Evangelos K.et.al. 2005). Most recently, cloud point extraction (CPE) has been extended to the extraction/preconcentration and analysis of environmental organic pollutants. Few reviews on CPE for analysis of metal ions, organic compounds, drugs and other bioactive compounds have also appeared in literatures (Paleogos.E.K., 2005; Bhairi S M, 2001; Shinoda K.,1963; Manzoori J L and Bavili-Tabrizi A.; http://en.wikipedia.org/wiki/Micelle; Bosch Ojeda .C and F. S??nchez Rojas,2009; Catalina Bosch Ojeda and Fuensanta S??nchez Rojas,2012).
1.2.1 Compliance with Green chemistry principles:
CPE has an added advantage fits into the principles of ‘green chemistry’. Green chemistry can be defined as ‘those procedures for decreasing or eliminating the use or generation of toxic substances for human health and for the environment during analysis. (Anastas, P.T,1999)’. CPE is identified as a green method due to the following reasons: (a) it uses dilute solutions of the surfactants which are inexpensive, resulting in the economy of chemicals and generation of few residues; and (b) surfactants are not easily flammable, non toxic in nature, nonvolatile, unlike organic solvents used in liquid’liquid extraction.
According to Quina and Hinz (Quina.F.H, Hinze.W.L., 1999), Pramauro and Prevot(Pramauro, E. and Prevot, A.B.,1995), ‘CPE approach permits the design of extraction strategies that are simple, inexpensive, and highly efficient when compared to those extractions that use organic solvents. The main limitation of CPE is the relatively low partition coefficients of several metal species with determinate chelates. However, it can be circumvented with the use of highly hydrophobic ligands’. Another advantage of CPE over traditional procedures like solid-phase extraction (SPE), co precipitation and liquid’ liquid extraction (LLE) is due to the high preconcentration factor which can be obtained from the small initial volumes of the sample. Traditional techniques often require an additional metal re-extraction stage which generates a larger final volume. (Constantine D. Stalikas, 2002).
1.2.2 Fundamentals of cloud point extraction (CPE):
1.2.2.1 Micelle:
Surfactants are amphiphilic organic molecules consisting of hydrophilic head (polar) and hydrophobic hydrocarbon tail (nonpolar). With these opposing properties, they are soluble in both organic solvents and water. In contrast to purely non-polar orpolar molecules, surfactant molecules exhibit typical properties in water. The polar group forms hydrogen bonds with water molecules, and the hydrocarbon chains (nonpolar) forms an aggregate due to hydrophobic interactions. In aqueous solutions, they tend to form spherical organized structures called micelles (Figure 1.1). Surfactants are capable of solubilizing nonpolar hydrophobic compounds in water due to their amphiphilic nature (Bhairi S M, 2001).
Figure 1.1: Surfactant micelles in Aqueous medium
Normal micelle Reverse micelle
The formation of micelles can be understood using thermodynamics: Micelles can form spontaneously because of a balance between entropy and enthalpy. In water, the hydrophobic effect is the driving force for micelle formation, despite the fact that assembling surfactant molecules together reduces their entropy (http://en.wikipedia.org/wiki/Micelle). Explicitly, above the CMC, the entropic penalty of assembling the surfactant molecules is less than the entropic penalty of caging water molecules. Also important are enthalpic considerations, such as the electrostatic interactions that occur between the charged parts surfactants. This analysis of the thermodynamic parameters of the micelles formation has been done for a nonionic surfactant. The enthalpy of micelle formation in aqueous solutions is usually small and can be negative. The main driving force for the micelle formation is the aggregation of the alkyl tails of the surfactant (‘hydrophobic interactions’) (http://en.wikipedia.org/wiki/Micelle).
Micelles with ellipsoid, cylinder, and bilayer shapes are also possible. The shape and size of a micelle depends on the molecular geometry of its surfactant molecules, [surfactant], temperature, pH, and ionic strength. Micelles formed in nonpolar solvents like benzene known as a reverse micelle (Figure. 1.1`) where the polar ends on the inside of the micelle, non-polar ends on the outside where they can come into contact with the nonpolar liquid.
In general, a polar solvent (hydrophilic) is able to dissolve polar substances (solutes), and a non-polar (hydrophobic) solvent is able to dissolve non-polar substances. However, a micellar solution, is unique because it helps non-polar solutes to dissolve in a polar solvent. Owing to the orientation of the surfactant molecules in a micellar media, a hydrophobic region exists within the micelle. This hydrophobic part will entrap non-polar solutes. This leads to the use of surfactants as detergents since most soils are non-polar (Zuhair A-A. Khammas., 2009).
1.2.2.2 Phase separation in micelles/ Principle of cloud point extraction (CPE):
An aqueous solution of a surfactant at and above a certain temperature becomes turbid. This temperature is called cloud point. At cloud point, the surfactant solution separates into two phases. The first one is a surfactant-rich phase with small volume, and in the second phase the concentration of the surfactant is small and approximately equal to the critical micelle concentration (CMC). Any organic or inorganic species interacts with the aggregates of micelles formed , and at and above cloud point it will be pre concentrated in the small volume of the surfactant-rich phase. This phenomenon is reversible and the re-establishment of the initial solution conditions drives the micelles to merge with the aqueous to form a single isotropic phase again. The micellar phase also called coacervate phase, rich in the surfactant and entrap the analyte (organic or inorganic species) (Hinze, W.L. and Pramauro, E., 1993; Pereira, M.G. and Arruda, M.A.Z.).This is due to the decrease of solubility of the surfactant in water. Cloud point varies from one surfactant to another. Hinze and Pramauro (Hinze, W.L. and Pramauro, E.,1993) have summarized the CP temperatures of various non-ionic and zwitterionic surfactants.
There are two important micelle mediated techniques that are attracting the workers world over for extraction/pre concentration/and remediation. They are: Cloud point extraction (CPE) and other Adsorptive micellar flocculation (AMF). Both of them offer simple and inexpensive methods for removing organic/ inorganic pollutants. These techniques are nearly green methods due to: (a) use dilute solutions of the surfactants which are inexpensive, resulting in the economy of chemicals and generation of few residues; and (b) non toxic in nature, nonvolatile, unlike organic solvents used in liquid’liquid extraction.
CPE mainly depends on the solubilization of surfactant solution and phase separation for the extraction and preconcentration of analytes. The whole process similar to traditional liquid’liquid extraction (LLE), the only difference being the “organic’ phase is generated within the aqueous phase, converting a previously homogeneous solution to heterogeneous one by simply gathering its previously scattered hydrophobic suspension.
Adsorptive micellar flocculation (AMF) is a hybrid of Micellar Enhanced Ultra Filtration (MEUF) and Coagulation-Flocculation with flocculants. In AMF, micelles coagulate/flocculate by adsorption of the flocculant cation, which suppresses electrostatic repulsion between micelles. At the same time, pollutants are captured by the flocculate and can be removed by decanting it. The remaining chemicals are unbound pollutant, unbound flocculant and monomeric surfactant. AMF is aimed to reductions of pollutant loads in the order of magnitude of gram per litre.
Among the organic pollutants, pollution by dyes is of great importance in India in which there is sprawling textile industry. The present author therefore taken up a detailed investigation of applicability of these two techniques to address this nagging problem. The author has chosen Five dyes with different structural properties i.e Bromothymolblue(BTB), Bromophenolblue(BPB), Methylene blue(MB), Erythrosine(ER) and Brillent Blue FCF(BBF). Among these, Methylene Blue is basic and remaining are dyes are acidic in nature. Erythrosine is a Xanthene dye, Methylene Blue is Thaizine dye and BTB, BPB, BBF are triphenyl methane type of dyes. BTB, BPB and MB are textile dyes and BB FCF, Erythrosine are food dyes.
The scope encompasses the following aspects:
‘ Study the solubilization capacity of selected dyes by CPE by using the nonionic surfactant Triton X-100
‘ Examine the effect of added electrolytes on the cloud points of the micellar solutions of these nonionic surfactants and optimization of the preconcentration factor.
‘ Develop a simple and effiicient cloud point extraction technique to extract dyes from aqueous samples.
‘ Predicting the design parameters from the isothermal studies andexamining their validity by comparing with experiment data.
‘ To study the feasibility of micelle flocculants as adsorbent for the removal of dyes.
‘ Studying the effect of operating parameters such as dye concentration, adsorbent
Dosage, temperature on AMF
‘ Interpretation of the adsorption mechanism, through the application of kinetic models such as pseudo-first order, pseudo-second order.
‘ Equilibrium studies for adsorption by the application of equilibrium isotherm models such as Langmuir and Freundlich to find the most suitable model that can be used for design purpose.
‘ To study the effect of temperature and determine the values of the thermodynamic parameters.
4.2 Organization of thesis
The fundamental concepts involving in these techniques and literature survey presented in Chapter one. It includes the brief introduction on removal of dyes, the principle and methodology adopted in CPE as well as AMF.
Chapter two describes the application ofcloud point extraction method for the removal of selected dyes. This chapter is divided into five sections. Results of the investigations related to the extraction of BTB, BPB, MB, ET and BBBF are presented in Section-1, section-2, section-3, section-4 and section-5 respectively. Each section also outlines the experimental procedures, materials and methods, factors influencing the efficiency which include effect of pH, initial [Dye], [TX-100] temperature. The investigation of solubilization isotherm, thermodynamic parameters and experimental design are also described in this. Each section also focuses clouding phenomena and effect of electrolytes on cloud point temperature as well as the pre concentration factor, phase volume ratio, partition coefficient which govern the recovery efficiency.
The optimum operating pH is 6.5-7.0 in all extractions of dyes using CPE.. The addition of dyes has no impact on the cloud point of TX-100 aqueous solution. The cloud point extraction of dyes can remove the color from wastewater using the nonionic surfactant TX-100. The extraction efficiency of these dyes increased with increased temperature, TX-100 concentration. The addition of electrolytes enhances the extraction of dyes. An approach to design a cloud point extractor has been proposed to estimate the surfactant required for a known temperature, initial and desired dye concentration. This approach involves calculation of solubilization of dyes in surfactant and variation of fractional coacervate phase volume with operating conditions. A Langmuir type isotherm is found to adequately describe the solubilization isotherms of all dyes in TX-100. Correlations are developed to account for the variation of the isotherm parameters with temperature. The cloud point extraction procedure adopted is straightforward while some of the previously reported methods require pretreatments with chemical compounds for long times. Moreover, cloud point extraction strategy can easily be adopted for large-scale samples.
Chapter three incorporates on the extraction of these dyes using adsorptive micellar flocculation. This chapter is also divided into five sections. These include the AMF studies on BTB, BPB, MB, ET and BBF which are presented in Section-1, section-2, section-3, section-4 and section-5 respectively. Each section outlines the experimental procedures, materials and methods, factors influencing the efficiency of AMF (initial [Dye], ratio of [aluminiumsulphate] and [SDS], temperature and mechanism. Each section also describes stern potential, equilibrium constant and partition coefficient, sorption capacity and adsorption isotherms(Langmuir and Freundlich), adsorption kinetics and thermodynamic parameters. The adsorption processes were mostly found to be of second order with respect to the dye. In the present study, monolayer adsorption of dye on surface of adsorbent is found to be maximum and KL values were found to be very low closest to zero thereby RL values are equal to one. Thus there exists a linear relationship between adsorbate and adsorbent which are the property of amorphous materials. Both the linearized form of Langmuir and Freundlich equations are compared with correlation coefficients and yields a better fit for the experimental data. In case of Freundlich, 1/n shows nearly equals to one indicating homogeneous nature of adsorbent as discussed in Langmuir isotherm. Binding of dyes onto micellar flocs can be described as an electrostatic attraction of a negatively charged species by the positively charged portions of an adsorption substrate which is non homogeneously charged. The proportion between available dye and available surface defines the apparent average Stern potential of the ‘binding area’. The results suggest that direct adsorption into the micellear palisade or adsorption of complexes are the more likely mechanisms for the capture of dyes by AMF.
An integrated picture of application of the CPE and AMF for the preconcentration, extraction and removal of these dyes from aqueous solutions is presented in chapter four. The range of concentration of dyes employed in these techniques clearly shows that these techniques can be effective for the remediation of industrial effluents.
4.3 Future Scope of the work
Although further work is required in order to fully understand and design cloud point extraction and AMF systems for environmental work, the results of the present study indicate that the technique is viable and can be applied for the extraction and/or preconcentration of organic compounds from aqueous solutions. Specifically, the optimized cloud point extraction system and adsorptive micellar flocculation developed in this work was demonstrated to function very well for the extraction of selected dyes from a bulk aqueous solution. The general extraction approach should be applicable to other organic species.The present author is committed to further research aimed at comparing and contrasting the proposed techniques to that of conventional liquid – liquid extractions and consider use of such surfactant-mediated phase separation technique in lieu of the traditional liquid – liquid extraction procedure for the extraction and preconcentration of organic species.