ТЕОРЕТИЧЕСКИЕ ОСНОВЫ ХИМИЧЕСКОЙ ТЕХНОЛОГИИ, 2013, том 47, № 3, с. 341-347
УДК 66.021.2.081.3
REMOVAL OF 4-NITROPHENOL FROM BINARY AQUEOUS SOLUTION WITH ANILINE BY GRANULAR ACTIVATED CARBON USING TAGUCHI'S DESIGN OF EXPERIMENTAL METHODOLOGY © 2013 г. S. Surest, V. C. Srivastava4, I. M. Mishra4
aDepartment of Chemical Engineering, Maulana Azad National Institute of Technology, Bhopal, Bhopal-462 051, Madhya Pradesh, India bDepartment of Chemical Engineering, Indian Institute of Technology, Roorkee, Roorkee-247667,
Uttarakhand, India sureshpecchem@gmail.com Received 10.01.2012
This paper reports the studies on the simultaneous removal of aniline and 4-nitrophenol from aqueous solutions of the binaries using granular activated carbon as an adsorbent. The effect of operating parameters such as initial component concentrations, temperature, adsorbent dosage and contact time on the adsorption of the above said mixtures of solutes in the binary aqueous solution was studied. Taguchi's method of design of experiments was used to study the effect of each parameter at three levels on the selected response characteristic (total amount of solutes adsorbed on granular activated carbon (qtot, mmol/g)).The analysis of variance shows that the adsorbent dosage is the most significant parameter affecting the adsorption process with 22.42 and 37.62% contribution to qtot and signal-to-noise ratio data. The contribution of interactions between initial concentration of binary components is also significant. Confirmation experiments were carried out to check the effectiveness of the Taguchi's methodology at the optimum levels of process parameters.
DOI: 10.7868/S0040357113030135
INTRODUCTION
Phenols and anilines are ionizable organic compounds which cause environmentally relevant contaminants widely found in the effluents from pesticides, dyestuffs, pharmaceuticals, petrochemicals, and other industries [1]. Phenols and anilines are relatively high solubility in water, they can transport favourably in the natural environment. Phenol and its substituted compounds like 4-nitrophenol (NP) is toxic and hazardous in nature (contact and exposure through all routes — oral, dermal, ingestion and inhalation). Aniline (AN) comes to light as a toxic compound for human and aquatic plants [2—4]. Aniline and phenols are found simultaneously in wastewaters. Phenols and aniline are mutagenic and also carcinogenic [5]. Phenol and its derivatives are toxics and are considered as priority-pollutants (11th in the list of 126 chemicals) by the US Environmental Protection Agency (USEPA) [6]. The USEPA [6], Central Pollution Control Board [7] as well as the Bureau of Indian Standards [8] have set a discharge standard of1.0 mg/L concentration of phenols in the industrial effluents for their safe discharge into the surface wasters. These compounds impart objectionable taste and odour to drinking water at concentrations as low as 0.005 mg/L. Although several treatment methods have been recommended for the treatment of wastewaters containing phenols and aniline, adsorption has been used by
many investigators. For the removal of phenols and anilines from water and understanding their environmental behaviours, the literature reports many studies concerning their adsorption by adsorbents [1, 9, 10].
However, these adsorption studies focused on the uptake of a single adsorbate. Since industrial effluents generally contain several phenolic compounds and aniline simultaneously, therefore, it is necessary to study the simultaneous adsorption of two or more ad-sorbates and also to quantify the interference and interaction of one adsorbate with the adsorption of the other. No information is, however, available in literature for the simultaneous removal of AN—NP from their binary mixtures in water by granular activated carbon (GAC). GAC is the most frequently employed adsorbent. Its outstanding performance stems from a unique combination of geometrical and chemical properties. Generally 'one parameter at a time' experiments have been conducted in most of the previous studies to determine the operating conditions for optimum solute removal from the solution. 'One parameter at a time' designs often overlook interactive effects of various parameters on the adsorption process. Statistical design of experiments uses fractional factorial design. Response surface methodology and Taguchi's orthogonal array have been used by several investigators to study the effects of multiple parameters as well as interactions between these parameters [4, 11—13].
Table 1. Process parameters for multicomponent adsorption study of AN—NP onto GAC using Taguchi's orthogonal array
Factors Parameters Units Levels
0 1 2
A C0, NP mmol/L 0 1.8 3.59
B C0 , AN mmol/L 0 2.68 5.37
C T °C 15 30 45
D m g/L 2 10 18
E t min 60 360 660
The present paper aims at using Taguchi's fractional factorial design of experiments to carry out experiments on the simultaneous adsorptive removal of AN and NP from synthetic wastewaters using GAC. The objective of the present study is to maximize the selected response characteristic (total amount of adsor-bates adsorbed per unit mass of GAC (qtot, mg per g of GAC)) by optimizing the various parameters such as the initial concentration of adsorbate (C0), temperature (T), adsorbent dosage (m) and contact time (t) affecting the simultaneous removal of AN—NP from aqueous solutions. It is also aimed to study the effect of each parameter and its interaction with the other on qtot. The analysis of variance (ANOVA) has been performed for the raw and signal-to-noise ratio (S) data to identify the significant parameters affecting the adsorption and their effect on the response characteristics have been quantified. The optimal values of the process parameters have been established in terms of mean response of characteristics by analyzing the response curves and the ANOVA tables.
MATERIALS AND METHODS
Materials. Analytical reagent (AR) grade chemicals were used in the experimental runs. AN (C6H5NH2; CAS No: 62-53-3) was procured from Qualigens Fine Chemicals, Mumbai, India., NP (4-NO2C6H5OH; CAS No. 10-02-7), NaOH, and HCl were obtained from S.D. Fine Chemicals, Mum-bai, India. Stock solutions ofAN and NP was prepared by dissolving a weighed amount of AN and NP in double-distilled water, respectively. The experimental test solutions were prepared by diluting the respective stock solution ofAN and NP with double-distilled water and mixing them in the desired proportion.
Methods. The synthetic pollutant (AN and NP) stock solutions were prepared as a 500 mg/L and 1000 mg/L concentration of AN or NP. Then these solutions were mixed with equal proportion (for example, 50 mL each). The obtained concentrations were 250 mg/L and 500 mg/L respectively. Similarly, all other concentration of solutions was prepared by diluting with double distilled water. The concentration
of AN and NP in the aqueous solution were determined using high performance liquid chromatograph (HPLC). Detailed methods of the HPLC have already been presented elsewhere [4].
Taguchi's methodology of optimal design of experiments for multicomponent adsorption has been described by Suresh et al. [4], Srivastava et al. [11] and Lataye et al. [13]. The method has been used in multi-component adsorption of metal ions and pyridine and its derivatives by rice husk ash [12, 13] and bagasse fly ash [11]. Therefore, this methodology is not being detailed here. The critical parameters affecting the adsorption are the initial concentration of the adsorbates (C0j), temperature (T), adsorbent dosage (m) and contact time (t). The range of the process parameters and their levels studied during the experimental runs are given in Table 1. The interactive impact of the two solutes on the adsorption of these components was studied by having the interactions of the initial concentrations of the solutes in the binary systems, i.e. C0 AN x C0P and Co,NP x C0p in the experimental design. Taguchi's L27 (313) orthogonal array matrix was used, which incorporates five parameters: each parameter at three levels (0, 1, 2) and one second-order interaction of initial concentrations, as shown in Table 2. The total degree of freedom f) required is 14 (5 x (3—1) + 1 x 4 = 14). This is because a three-level parameter has a f of 2 (number of levels —1) and each two-parameter interaction term has a f of 4 (2 x 2). Thus a total of 27 experimental runs were taken in duplicate, under the same conditions. Table 2 gives the column assignment for the various parameters and three interactions in the Taguchi's L27 (313) orthogonal array matrix.
Batch experimental adsorption studies. Adsorption batch runs were carried out for the simultaneous adsorption of AN and NP from their binary solutions by GAC. The results obtained from each set as qtot are given in Table 3. The results presented in the table are the average of two individual determinations.
In each experimental run, 25 mL each of the aqueous solutions having a known (desired) concentration of the respective solute (AN or NP) were mixed, and 50 mL of this mixed solution was taken in a 250 mL conical glass flask with a glass stopper containing a specific amount of GAC (m, g/L). In all the experimented, the pH of the solution was maintained constant at their natural pH using buffer system of potassium dihydrogen orthophosphate (KH2PO4) and sodium hydroxide (NaOH). The flasks were agitated at a constant shaking rate of150 ± 5 rpm in a temperature-controlled orbital shaker (Metrex Scientific Instruments, New Delhi) at the desired temperature (i.e. 15, 30 or 45°C). The samples were withdrawn after appropriate contact time and were centrifuged using a research centrifuge (Remi Instruments, Mumbai) at 10000 rpm for 5 min. The supernatant solution was
Table 2. Column assignment for the various factors and three interactions in Taguchi's L27 (313) orthogonal array and experimental qtot values f
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