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Statistical Modeling for Stability of Emulsion Liquid Membrane for the Removal of Anionic Dyes from Textile Wastewater

The importance of statistical modeling is obvious because it imparts an imperative role in predicting the effects of significant factors on any experimental process. In recent days, ELMs (Emulsion Liquid Membranes) are considered as an effective technique for the treatment of industrial wastewater. Despite of many advantages over the other treatment methods, ELM technology encounters one serious drawback of the stability of emulsion. The subject of this research is to identify the factors which are important to study the stability of ELM and also to evaluate the response of these parameters on stability. The membrane used in this study consisted of Span-80 and Hexane as surfactant and diluent respectively. The internal aqueous phase was H 2 SO 4 . The experimental setup was designed by using a well-known statistical approach of DoE (Design of Experiment) and the data was analyzed by Taguchi Method using a fractional factorial design. All the parameters including aqueous phase concentration, surfactant concentration, volume ratio of organic to aqueous phase, emulsification speed and emulsification time were selected as key factors to study their effect on the stability of ELM. Using different statistical techniques, it was found that emulsification speed and volume ratio of organic to aqueous phase are two most significant parameters. The significance level of these factors i.e. emulsification speed and volume ratio of organic to aqueous phase was statistically found as 99.7 and 99.9% respectively. A statistical model was also developed and the experimental results were compared with estimated results. The value of correlation coefficient, R was calculated as 0.997 indicating that the developed model fits the data very well.

Processing sector of textile industry consumes a huge quantity of water in many of its operations including dyeing, printing and finishing. Fresh water after being utilized in different processes is finally drained directly into the water bodies without any proper cleaning process. This textile wastewater usually contains a rich quantity of different contaminants in the form of hazardous chemicals, dyes and pigments [3]. Different classes of dyes are the major pollutant usually observed in the effluent released by any textile wet processing industry. It was reported [4] that 99% of the dyes used now-a-days in textile industry are of synthetic origin which contains different toxic and hazardous compounds in their basic structure. Toxic nature of these dyes imposes a carcinogenic and mutagenic effect on almost all the living species including humans, animals and aquatic fauna and flora [5]. Therefore, the treatment of textile wastewater contaminated with hazardous dyes is now-a-days one of the most serious environmental matter of thought for the industries as well as the researchers. Several methods for dye removal have been developed which includes biological treatments [6], coagulation/flocculation [7], chemical oxidation [8], adsorption [9] and photo catalytic processes [10]. However, with the ever upgrading of dye compounds in terms of their stability and solubility, these conventional methods tend to be rather ineffective or less efficient. In comparison to other treatment technologies, ELMs are considered as much efficient tool for the treatment of industrial wastewater containing various hazardous contaminants including dyes. ELMs provide high surface area which promotes better extraction of the target from source [11]. Another advantage of this technology is the possibility of extraction from very dilute solutions like dye solutions in many cases where dye presents in wastewater in a very small amount [11]. Inspite of having many advantages over the other treatment methods, the ELM technology encounters one serious drawback of the stability of emulsion. Stability in ELM system is generally considered as the resistance of the individual globules against coalescence [12]. In other words, it is the ability of emulsions to resist changes in time [5]. The stability of emulsion liquid membranes is very important and it must be optimized before their use [13].
Most often, in order to determine the effect of different factors on the required response, conventional single factorial design method is used. In this method, one parameter is altered at different levels while the rest of the parameters are fixed at constant value. This parameter is then optimized and its value is set as constant while another parameter is varied. This method continues till all the operating parameters in the specific study are optimized. However, this method is not considered effective when the number of factors and their interactions are highly significant in the specific study [14]. In this case, the use of DoE is essential. In Using this experimental design, the coefficients for each parameter can be calculated along with its importance in the specific study. In this study, an approach of DoE was applied to determine the effect and significance of different parameters influencing the stability of an ELM.
This ELM can subsequently be used for the extraction of anionic class of dyes from the industrial wastewaters.
Anionic dyes are the most extensively utilized class of dyes in textile industry particularly in cotton textiles [15].

Reagents and Material
The internal phase of the required ELM was prepared with H 2 SO 4 (97% purity) manufactured by Merck

Procedure
In order to make required stable membrane, both the aqueous and organic phases were separately prepared.
Where Vt is the total volume of the emulsion in the test tube, and Vw is the separated volume of the aqueous phase in the test tube after 24 hours.

RESULTS AND DISCUSSION
The Pareto chart of effects is a useful field to identify the most important factors [19]. It shows the estimated main plot against the horizontal effect. Form Fig. 1   On the other hand, the effect of aqueous phase concentration is negative and very low. Table 3 shows the coefficients and P-values of the parameters which affect the stability. The negative sign before the coefficient value identifies the negative effect of the corresponding parameter on the stability of the emulsion liquid membrane. P-value is a statistical parameter, used to determine the significance of the operating factors on any experimental system. If any effect is significant then there will be high probability that the effect is "real".
From Table 3, it can be seen that volume ratio of organic to Aqueous Phase has P-Value of 0.001 that means this parameter has 99.9% significance over the stability yield for this required ELM. Emulsification speed is the second most significant factor with significance level of 99.7%.

STATISTICAL MODEL FOR STABILITY OF ELM
In order to facilitate the prediction of response and system optimization, the method of DoE also assists in developing a mathematical representation of the response according to all factors. Equation (2)

CONCLUSION
In this study an approach of DoE was used to investigate the effect of operating parameters on the stability of ELM.