Removal of Carmine Indigo Dye with Moringa oleifera Seed Extract

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Ind. Eng. Chem. Res. 2009, 48, 6512–6520

APPLIED CHEMISTRY Removal of Carmine Indigo Dye with Moringa oleifera Seed Extract J. Beltrań-Heredia,† J. Sańchez-Martıń,* and A. Delgado-Regalado UniVersidad de Extremadura, Department of Chemical Engineering and Physical Chemistry, AVda. de ElVas, s/n, 06071, Badajoz, Spain

Moringa oleifera has been tested as an active agent in removing various types of anionic dyes. Specially, removal of an indigoid dye, Carmine Indigo, has been tested. The fast kinetics of coagulant action and the high potential of this coagulant agent to treat wastewater from dyestuff has been revealed. Moringa oleifera is fully working in coagulation and flocculation process, and it achieves an average level of removal up to 80%. The pH does not affect the coagulant process, and temperature has a negative influence. By increasing initial dye concentration, lower dye percentage removal is achieved and higher q is presented. Coagulation and flocculation processes can be estimated by the Langmuir and Freundlich models, but the first one gives a better explanation of the coagulation and adsorption behavior (r2 equal to 0.97). Carmine Indigo removal presents an optimum q capacity at 343 mg · L-1 and 11.2 °C. Pilot plant installation gives a similar efficiency for dye removal. 1. Introduction One of the main concerns in industrial wastewater treatment is dye removal. Wastewater effluents from industries such as textile, paper, or plastic contain a large pollutant quantity of synthetic dyestuff. There is a quite large variety of dye substances that might be highly pollutant if dumped, and even small amount of dye in water can affect aquatic animal life and, consequently, get into the alimentary chain and reach human beings. Despite their carcinogenic and mutagenic effects,1 over 50 000 tons of dye are discharged into environmental effluent anually.2 Some useful removal techniques are already developed: adsorption onto materials such activated carbon,3,4 physical and chemical degradation,5,6 Fenton’s oxidation, electrochemical degradation, and ozonization.7,8 However, many more processes should be investigated in order to make the removal of dyes an affordable and sustainable practice that leads to universalization of this kind of wastewater treatment. The classification of dyes is done on the basis of their usage in dyestuff. Regarding this fact, there are acid, basic, disperse, and direct dyes (family names that have to do with when and how dyes are used). Regarding, on the other hand, their chemical structure, lots of compounds are included as dyes. In the present work, we have considered five kinds of dyes: azo-dyes (Chicago Sky Blue 6B, Acid Red 88, and Palatine Fast Black WAN), anthraquinonic (Alizarin Violet 3R), triphenylmethane (Eriochromecyanine), indigoid (Carmine Indigo), and thiazinic (Methylene Blue). Apart from this last one, they are anionic dyes. A preliminary screening of Moringa oleifera ability in removing dyes was done on these five types of dye. Then, we centered the investigation on Carmine Indigo. Carmine Indigo structure includes four benzene rings with a double link inside them (see the Supporting Information) and two sulfonated negative-charged groups. This gives it an anionic and aromatic character that allows the Moringa active principle, * To whom correspondence should be addressed. E-mail: jsanmar@ unex.es. † Telephone number: +34 924289 300 ext. 9033. Fax number: +34 924289 385.

which is presumably proteinic, cationic character, to link dye molecules and provoke their destabilization and settlement in a coagulation and flocculation process. The remediation of several pollution problems is a target of many researchers nowadays. Technical ways of solving environmental concerns and menaces such as the dumping of surfactants, dyes, pharmaceuticals, and other hazards have been available for a long time, but making them cheaper and sustainable is still a challenge. A possible source of low-cost materials that could provide a successful solution are natural matters.9,10 In this sense, we have been researching Moringa oleifera as a water treatment agent for several years. As a tropical multipurpose tree, Moringa oleifera is very interesting from the point of view of developing cooperation, as it is a widespread, easy-available water treatment method. The use of Moringa oleifera for water treating has been thoroughly reported previously.11-13 Many researchers have focused their work in studying and optimizing the coagulant/flocculant ability of Moringa oleifera seed extract because it is not technologically difficult to operate by nonqualified personal, it is easy to maintain, and it does not present an external dependency on reagents, as other products do (Al2(SO4)3, FeCl3...). These reasons support this way of treating water in developing countries.14 There are rather interesting previous studies about the coagulant capacity or turbidity removal ability of natural-based products.15,16 We have also reported that Moringa oleifera has as a very significative ability in removing surfactants17 or even heavy metals.18 One of our previous works19 pointed out the adequacy of Moringa oleifera in dye wastewater treatment. There we checked the ability of Moringa seed extract in removing azo dyes, such as Chicago Sky Blue 6B, which was thoroughly studied. Up to 99% of dye removal was reported with a dosage of 250 mg · L-1, so this coagulant agent is a quite effective wastewater treatment product. This paper aims to study the effect of this natural coagulant on Carmine Indigo dye removal. First of all, we have tested different natural agents to remove Carmine Indigo from aqueous

10.1021/ie9004833 CCC: $40.75  2009 American Chemical Society Published on Web 06/05/2009

Ind. Eng. Chem. Res., Vol. 48, No. 14, 2009

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Table 1. Dye Main Characteristics product

chemical formula

Carmine Indigo Chicago Sky Blue 6B Acid Red 88 Palatine Fast Black WAN Alizarin Violet 3R Eriochromecyanine R Methylene Blue

C16H8N2Na2O8S4 C34H24N6Na4O16S4 C20H13N2NaO4S C60H36N9Na3O21S3 · Cr2 C28H20N2Na2O8S2 C23H15Na3O9S C16H18ClN3S · 3H2O

molecular weight (g/mol) wave length analysis (nm) color index number dye content supplier 466.36 992.8 400.4 1488 622.6 536.4 373.9

solutions. Then, we have studied the specific characterization of Moringa oleifera by evaluating the influence of several parameters such as pH, temperature, and initial dye concentration. In order to evaluate the interaction between them, we have developed a design of experiments between the most influential variables: initial dye concentration and temperature. Finally, implementation on a pilot plant has been carried out, so the efficiency of this wastewater treatment has been evaluated in association with another procedure: slow sand filtration. 2. Materials and Methods 2.1. Buffered Solution. All assays were done in a pH-stable medium. A pH 7-buffered solution was prepared by mixing 1.2 g of NaH2PO4 and 0.885 g of Na2HPO4 in 1-L flask. Assays with different pH values were carried out by adjusting this buffered solution to the specific pH by using HCl 0.5 M and NaOH 0.5 M. All reagents were supplied by Panreac in analytical purity grade. 2.2. Natural Coagulant Product Preparation. Apart from Moringa oleifera, another six natural coagulant products were tested in a preliminary screening. They were prepared in the following way: • Cationic starch was supplied by Cargill (USA). It is used as an authorized alimentary supplement. It is presented as powder. • Modified tannin was suplied by Tanac, S.A. (Brazil). Its name is Tanfloc, and it consists of tannins from Acacia mearnsii that have been modified chemically in order to introduce a quaternary nitrogen that gives Tanfloc its cationic character. The other three products with the same nature were suplied by Silvateam, S.A. (Italy), in the case of Silvafloc, and Aquachimica Seta, S.A. (Brazil) in the case of Acquapol C1 and Acquapol S5T. Differences between Silvafloc, Acquapol C1 and S5T, and Tanfloc were in the tannin nature (Acacia mearnsii for Acquapol and Tanfloc and Quebracho for Silvafloc) and in the chemical modification, which is under copyright law. Tanfloc and Acquapol C1 are presented as powders, while Silvafloc and Acquapol S5T are presented as dense, sticky solutions. • Xanthan gum was supplied by Fluka. • Aluminum sulfate Al2(SO4)3 · 18H2O was supplied by Panreac. 2.3. Moringa oleifera Seed Extraction. Dry seeds were obtained from Setropa, Holland. The extraction process were carried out in the following way: shelled seeds were reduced into powder by a domestic mill (Braun). A 1 M NaCl (Panreac) solution was prepared, and 5 g of Moringa seed powder was put into 100 mL of it (the stock solution was so considered to be 5% w/w). The NaCl solution with powder was vigorously stirred at pH 7 and room temperature for 30 min with magnetic agitation. Then, the extract was filtered twice: once through commercial filter paper on Bu¨chner funnel and once again through a fine filtering Millipore system (0.45 µm glass fiber). The result is a clear, milky liquid.

612 618 505 565 549 437 665

73015 24410 15620 15711 61710 43820 52015

99% 85% 75% 90% 90% 90% 99%

Sigma Sigma Aldrich Aldrich Aldrich Panreac Panreac

Table 2. Operation Conditions in Pilot Plant Assays parameter

value

Moringa oleifera concentration in extract NaCl concentration in extract extraction agitation time extraction agitation manner extraction temperature extraction pH extract dose in raw water residence time in slow mixer residence time in sedimentator raw water flow Moringa oleifera extract flow

5% 1M 30 min magnetic 20 °C 7 315 mg · L-1 20 min 60 min 80 mL · min-1 7.7 mL · min-1

Moringa stock solution prepared in this way was used the same day it was producted, although there are references that point the stability of the extract.20 2.4. General Dye Removal Assay. A Carmine Indigo (Sigma) 1000 mg · L-1 solution was prepared by adding 0.250 g in 250 mL. Different volumes of this initial solution was put into 100 mL-flask, and a certain quantity of coagulant was added. The final volume was reached with distilled water. Thirty reps per minute stirring was applied for 1 h (Nahita 686/1 motor stirrer), until equilibrium (that is, the moment when no more dye is removed either by coagulation or adsorption process) was achieved. Then, a sample was taken, and it was centrifuged. Photometric analysis was carried out in a 1-cm glass cell. The maximum absorbance wavelength was 612 nm and a linear relationship of absorbance versus dye concentration was deduced at this wavelength. A Helios UV/vis spectrophotometer was used for photometric measures. 2.5. Removal of Other Dyes. Stock solutions of 1000 mg · L-1 for each six dyes were prepared, regarding to percentual dye content. A screening of Moringa oleifera extract interaction with these products was done by carrying out different assays: 5 mL of Moringa oleifera seed extract and 10 mL of each dye solution into a 100 mL-flask, and then it was filled to the mark. Then, a similar treatment as described before (see section 2.4 above) was carried out. Photometric analysis was developed at appropriate wavelength for each compound. These data are shown in Table 1. 2.6. Pilot Plant Installation. Installation consists of three sections: coagulation/flocculation, sedimentation, and slow sand filtration. Table 2 shows the designing parameters, and a scheme is shown in Figure 1 as well. Incoming water flow was 80 mL · min-1. Twenty minutes was fixed for coagulating and mixing; then, 1 h of sedimentation drives flocs to settle. Two peristaltic pumps complete the installation: one for raw water (Dinko-25-V, Dinter, S.A.) and one for Moringa oleifera seed extract (Masterflex DV, ColeParmer). Both of them have the corresponding flow regulators. Operation conditions are showed in Table 2, and specific data for filters are reported in Table 3. Turbidity was determined by a Hanna HI93703 turbidimeter. 2.7. Mathematical and Statistical Procedures. Linear data adjustment was carried out by using Origin v. 7.0 for Windows. Design of experiments section was statistically analyzed by

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Figure 1. Pilot Plant Installation. Table 3. Characteristic Bed Data dimension

value

units

average particle diameter sphericity factor φ corrector factor FRe bed porosity particle specific surface

0.75 0.95 45 0.4 8421

mm

m-1

using StatGraphics Plus for Windows 5.1. A factorial central composite orthogonal and rotatable design was used with 8 replicates of central point, so the total number of experiments was 16. 3. Results and Discussion 3.1. Preliminary Screening of Dye Removal. Several assays of Carmine Indigo removal were carried out with different natural agents, as well as with alum. Most of these were based on polysaccharides (starch) or proteins (vegetal extracts such as Moringa oleifera), and others were tannin-based flocculant agents (Tanfloc, Silvafloc, and both Acquapol samples). Some previous research papers were found referring the ability of gums and vegetal proteins to remove dyes.21,22 Dye removal by tanninbased coagulants is mentioned just in one previous work.19 A preliminary screening was needed to search for an efficient and operative dye removal mechanism which would be comparable with alum coagulation efficiency.23 Figure 2 shows dye removal percentages that have been carried out using different agents. A standard dosage of 100 mg · L-1 dye and 100 mg · L-1 coagulant agent was fixed, and experiments were caried out at pH 7 at 20 °C. As Figure 2 shows, every product exhibits a slight removal activity, with cationic starch and Moringa oleifera seed extract showing the highest dye removal efficiency and alum and Acquapol S5T the lowest one. This result differs from the bibliographic data, especially those data referred to by Blackburn,24 but it is clear that the dosages are rather different (100 vs 10 000 mg · L-1). With regard to tannin-based flocculants, it is observed that Acquapol C1, Tanfloc, and Silvafloc present a significative dye removal activity. In spite of its polysaccharide nature, xanthan gum presents a significantly low dye removal activity (ca. 5%). Further studies must be done with this product in order to improve this property. Clearly, Moringa oleifera is the coagulant agent that presents a higher efficiency as a dye removal product; therefore, this

Figure 2. Preliminary screening of dye removal.

study is focused on its activity. Aluminum sulfate was used to compare results from nature and synthetic coagulant agents. At pH level equal to 7 and with the mentioned dosage, alum does not present a significant dye removal ability. Other drawbacks and risks linked to aluminum usage as a primary coagulant agent (environmental bioaccumulation, implications with Alzheimer’s disease)25 make this product not recommended for this scope. 3.2. Removal of Other Dyes. In order to test the ability of Moringa oleifera to remove other dyes, several assays were done with five other dyes with different natures, structures, and usages. The result of these experiments are shown in Figure 3. Bearing in mind chemical structures (Supporting Information), it is possible to discuss some aspects of dye removal due to the Moringa oleifera seed extract coagulation process. First, it is obvious that Moringa oleifera presents a very high capability of removing azo dyes. For the three studied azo dyes (Chicago Sky Blue, Acid Red 88, and Palatine Fast Black) the percentage dye removal was higher than 98.8%. It is shown that dye removal is lower in the case of Acid Red 88 than in the other two cases of Chicago Sky Blue and Palatine Fast Black. The anionic charge is lower as well in Acid Red 88, and its chain is smaller, so the coagulation process may be a bit less effective. This fact has been previously reported in the work of Beltrań-Heredia and Sańchez-Martıń.19 Regarding Alizarin Violet 3R, its anthraquinonic nature implies a different chemical structure in which three negative electric charges are involved. There is a very little difference


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Figure 3. Removal of dyes by means of Moringa oleifera seed extract.

in dye removal efficiency between azo dyes and this anthraquinonic dye. A significant difference in dye removal is shown in the case of Eriochromecyanine R. Although almost 40% of dye removal is achieved, this lack of effectiveness may be due to nonlinear structure that may cause steric difficulties and coagulation process may prefer linear molecules rather than other space distributions. Carmine Indigo is not the most easy to remove, but the initial dye concentration is reduced by more than 50%. The structure of this dye is rather different from the others (anthraquinonic and azo dyes), and this reduction in effectiveness of Moringa oleifera seed extract coagulation ability may be due to the presence aromatic double-linked rings in a not so linear molecule. In the case of Methylene Blue, no removal was achieved by Moringa oleifera seed extract. Its cationic nature implies no cationic polyelectrolytes,26 including proteinic flocculants as Moringa oleifera extract,27 which may cause its destabilization. 3.3. Kinetics of Dye Removal. As the first researched data, a kinetic study was done. Three different dosages of coagulant were added in order to treat three dye samples of 150 mg · L-1. Reference dosages were 10, 15, and 25 mL of Moringa oleifera seed extract, which correspond to 315, 470, and 785 mg · L-1. As it can be appreciated in Figure 4, the coagulation process is highly fast; in the first 10 min, the equilibrium dye concentration (that is, what is going to remain because of Moringa depletion) is achieved. This is probably due to complex coagulation mechanisms that may involve netlike structure formation,28 so a very long contact time is not needed. This is a great advantage versus sorption processes, in which contact times seem to be longer.29 According to this kinetic data, further experiments were carried out for 1 h to guarantee that chemical equilibrium was achieved. 3.4. Coagulant Dosage. Experimental data series were made in order to determine extract dosage influence on dye removal. Three fixed initial doses of 100, 250, and 450 mg · L-1 of dye were evaluated to be removed with different doses of Moringa extract: from 2 to 350 mg · L-1. As it can be appreciated in Figure 5, the final dye concentration tends to decrease as the Moringa seed extract dose goes higher. Final dye removal is

Figure 4. Kinetic evolution during Carmine Indigo removal.

Figure 5. Influence of coagulant dosage on Carmine Indigo removal.

easily achieved between 150 and 250 mg · L-1 of Moringa oleifera, depending on the initial dye concentration. It is showed that no flocculant additive or pH adjustment (apart from buffered solution matrix, in order to avoid any pH interaction) was used in these assays, so another advantage is

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Figure 6. Influence of temperature on Carmine Indigo removal.

offered by Moringa oleifera seed extract versus inorganic traditional coagulant agents such as alum, which needs pH adjustment.30 No health or pollutant risks are observed because of the natural origin of this coagulant agent. In addition, the fact that only 1% Moringa oleifera extract has coagulant ability has to be considered,27 so the active principle of this treatment is significatively more effective than other treatments. It is also important to point out the fact that most of the proposed processes for dyestuff wastewater treatment reccommend a two-step procedure: adsorption and coagulation, as just one step is not so effective in removing enough dye concentration or a large amount of coagulant agent is needed.31 3.5. pH Influence. From previous papers,32,33 it is observed that pH has an important role in coagulation processes. Because of this fact, several assays with different pH values have been carried out, varying pH between 4 and 10, with different doses of Moringa oleifera seed extract and a fixed initial dye concentration equal to both cases. No significative affection of pH was found in coagulation and flocculation processes, as it is reported in the Supporting Information. 3.6. Temperature Influence. In spite of previously reported data,17-19 where temperature seems not to be a very important factor in coagulant processes, a series of assays was done in order to carry out this information. The temperature was varied between 10 and 40 °C, pH 7, and a dye initial concentration of 100 mg · L-1. The Moringa oleifera seed extract dosage was equal to 315 mg · L-1. Results are shown in Figure 6. As it can be appreciated, significative variations in treatment efficiency are observed: although a high dye removal is achieved in all cases, both q capacity (see below) and dye removal undergo a decrease as temperature rises. This should be taken into account in order to treat several kinds of industrial effluents and may be due to adsorption phenomena that arises and enhances coagulation-induced dye removal processes. Desorption mechanisms and temperature effects on the active protein may be considered.34 3.7. Initial dye Concentration Influence. A high efficiency in dye removal has been confirmed by all the experiments done before. However, it was considered an important task to research how important initial dye concentration was referring to percentage dye removal. Experimental series were done varying just dye initial concentration between 40 and 400 mg · L-1, and a fixed dosage of 315 mg · L-1 of Moringa oleifera was added. The percentage dye removal is showed in Figure 7. Increasing initial dye concentration leads to a loss of percentual removal, as it is obvious as Moringa coagulant extract tends to be

Figure 7. Influence of initial dye concentration on Carmine Indigo removal.

exhausted, but the q capacity tends to be higher, which means coagulation and flocculation processes are rather more efficient. 3.8. Theoretical Adsorption Modeling. In order to characterize the coagulation phenomenon even more, we will pretended to propose a theoretical model which explains the dye removal by the action of this product. Coagulation and flocculation processes are rather difficult to model mathematically, due to two main reasons: (a) the complex nature of the phenomenom, which implies physicochemical interaction molecule-molecule (van der Waals and hydrogen bridges forces)35 and (b) the fact that the intrinsic composition of the organic matters that forms the flocculant active principle in both products is not completely known. We have worked on the hypothesis that dye removal by coagulation and flocculation processes may involve two phases: The first is a destabilization of colloids, that may be ruled by chemical interactions between coagulant molecules (cationic, positive charged) and dye molecules (anionic, negative charged). Once the complex coagulant dye is formed, flocs begin to grow by sorption mechanisms. This should be the controllant stage, so the whole process can be simulated as an adsorption phenomenon. The possible coagulation mechanisms36 and the fact that Moringa oleifera is known to form bridges and netlike structures28 during adsorption and coagulation processes may be considered in this theoretical model. Other similar conjectures are made and applied previously with similar processes.37 First, adsorption capacity q has been determined, defined as q)

(C0 - Cl)V W

(1)

where C0 is initial dye concentration, (mg · L-1), Cl is equilibrium dye concentration in bulk solution, (mg · L-1), V is the volume of solution (L), and W is coagulant mass (mg). Two main adsorption models have been considered in the present work: the Langmuir and Freundlich models. The first of them assumes that the molecules striking the surface have a given probability of adsorbing. Molecules already adsorbed similary have a given probability of desorbing. At equilibrium, equal numbers of molecules desorb and adsorb at any time. The probabilities are related to the strenght of the interaction between the adsorbent surface and the adsorbate.38 That is the physical meaning of eq 2:


q ) kl1

Cl 1 + kl2Cl

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(2)

where kl1 is the first Langmuir adsorption constant (L · [mg of coagulant]-1) and kl2 is the second Langmuir adsorption constant (L · [mg of dye]-1). The Freundlich model was derived from empirical data39 and assumes that q capacity is an exponential function of the equilibrium dye concentration (Cl). That is what eq 3 expresses: q ) kfCnl f

(3)

where nf is the Freundlich adsorption order (dimensionless) and kf is the Freundlich adsorption constant ([Lnf] · [mg of coagulant]-1 · [mg of dye1-nf]). By combining data series of sections 3.4, 3.5, and 3.7, it is possible to look for a theoretical model that fits rather well to experimental data. Figure 8 shows experimental data. As it can be appreciated, the curve fit is reasonably good with the two proposed equations. However, linear expressions of Freundlich and Langmuir models give a criterion to discriminate the goodness of these data adjustments. A linear fitting graphic for the Langmuir hypothesis is included in Figure 8. Dye removal caused by Moringa oleifera seed extract coagulant seems to work according to the Langmuir hypothesis. Through the linearization method, it is shown that the Langmuir equation fits better to the experimental data than Freundlich’s proposal, according to r2 values. These are equal to 0.97 in the case of Langmuir and 0.78 in the case of Freundlich. The values of the different parameters involved in these two models in each case are kl1 ) 2.24 × 10-2 L · [mg of coagulant]-1 and kl2 )3.29 × 10-2 L · [mg of dye]-1 in the case of Langmuir adjustment and kf )1.38 × 10-1 Lnf · [mg of coagulant]-1 · [mg of dye1-nf] and nf )2.76 × 10-1 in the case of Freundlich. 3.9. Design of Experiments. The traditional experimental method, the one factor at a time approach, can hardly be used to stablish relationships among all the experimental input factors and the output responses. Event through the traditional approach can be useful in finding predominant factors in this situation, it is difficult to observe an optimum value of the working parameters as no interaction among them is considered. To solve this problem and to obtain a probable optimum, design of experiment (DOE) offers a better alternative to study the effect of variables and their response with minimum number of experiments.40 Using design of experiments based on response surface methodology (RSM), the aggregate mix proportions can be arrived with a minimum number of experiments without the need for studying all possible combination experiments. StatGraphics software provides a useful and powerful mathematical and statistical tool in order to develop the experimental planning (in a random order for avoiding hidden effects) and to analyze the results, searching for conclusions. We have selected a central composite design (CCD). Details of this election, as well as the codification of variables are presented in the Supporting Information. Table 4 shows the experiments we have carried out. The specific study involves a range of 100-400 mg · L-1 in initial dye concentration (IDC) and 12-40 °C in temperature. Consequently, a step is equal to 150 mg · L-1 and 14 °C; the central value is equal to 26 °C and 250 mg · L-1. 3.9.1. Numerical Analyses: ANOVA Report. In a first approach, we should refer to the ANOVA analysis that shows us the significance of the different parameters. Table 5 gives

Figure 8. Equilibrium isotherm of Carmine Indigo with Moringa oleifera seed extract. Table 4. Experimental Planning in the Design of Experiments coded coded real temperature IDCa temperature -1.41 -1 -1 0 0 0 0 0 0 0 0 0 0 1 1 1.41 a

0 1 -1 0 -1.41 1.41 0 0 0 0 0 0 0 1 -1 0

6.2 12 12 26 26 26 26 26 26 26 26 26 26 40 40 45.8

real IDC

dye q (mg dye/mg removal (%) Moringa)

250 400 100 250 37.87 462.13 250 250 250 250 250 250 250 400 100 250

57.50 37.76 75.63 50.16 17.59 31.20 48.30 54.65 55.86 53.67 56.19 52.02 51.80 28.18 9.05 41.06

0.46 0.48 0.24 0.40 0.02 0.46 0.38 0.44 0.44 0.43 0.45 0.41 0.41 0.36 0.03 0.33

Initial dye concentration.

Table 5. ANOVA Report variable

sum of squares

freedom degrees

F-ratio

p-value

temperature IDCa temperature2 IDC2 temperature · IDC total error total

0.03 0.17 0.004 0.002 0.07 0.009 0.30

1 1 1 1 1 10 15

35.43 185.97 4.78 2.12 83.96

0 0 0.05 0.17 0

a

Initial dye concentration.

all of the data of this statistical analysis. Three of the five studied factors have a p-value below 0.05 (significance limit), so they are statistically significant. As we are working on q and not on the dye removal, the fact that q capacity may be improved as IDC increases is not obvious. Nonlinear polynomic regression is carried out taking into account the corresponding equation (see the Supporting Information). In this sense, this regression is the following expression (eq 4): q ) 0.42 - 0.06T + 0.14C + 0.02TC - 0.02T 2 - 0.09C2 (4) where the values of C (initial dye dosage) and T (temperature) should be coded according to the model. The q values are given in milligrams of removed dye per milligram of flocculant. The

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Figure 9. Pareto graphic.

Figure 11. Pilot plant assay.

Figure 10. Response surface graphic and contour plot.

adjusted correlation factor r2 is equal to 0.95. This regression leads to an optimum q (0.50 mg · mg-1 at temperatures equal to 11.2 °C and an initial dye concentration of 343 mg · L-1. This optimum is quite near to the lower limit (-1, in temperature coded level), but it is realistic data according to the significant p-value. The ANOVA test also gives us the value of the Durbin-Watson statistic, which has a value equal to 1.97, with a p-value of 0.43. As this p-value is higher than 0.05, there is no evidence of correlation in the residuals series. This means that the random order of experiments has been effective in order to avoid any systematic error. 3.9.2. Graphical Analyses. A new model is made on the basis of five factors which correspond to eq 4. A graphical expression of the ANOVA test results may be the Pareto graphic (Figure 9). Bars represent the standarized effects of each involved factor, considering them as the temperature, the IDC, and combinations of both. Filled bars are a graphical representation of negative-affecting factors, such as temperature. That means that these factors appear in eq 4 behind a negative sign. On the other hand, open bars represent positive-affecting factors, such as IDC. The vertical rule stands near to 2 and has to do with the significance level of the ANOVA test, which is equal to 95% confidence. Bars trespassing the vertical rule are inside the significance region, while bars behind it are not statistically significant. The Pareto graphic also gives us an idea of how factors influence the final response q. Positive bars indicate that by varying those terms the variable q increases. Negative bars indicate the contrary. As can be shown, as the temperature level rises, q decreases and, as IDC rises, q increases. 3.9.3. Response Surface. The most important graphical representation in the RSM is the surface graphic (Figure 10). It plots eq 4 and allows evaluation from a qualitative point of view of the behavior of the whole system studied. As it can be

appreciated, the response is quite a convex surface inside the studied region. Both variables have a similar effect on the target variable q, and an optimum is intuited around near the lower limit of temperature. The contour plot, which is drawn in the same figure, is almost more clear in order to identify the maximum point. It appears in the negative part of the coded temperature values and around the center of the IDC. 3.10. Pilot Plant Installation. In order to evaluate the scaleup of this new coagulation and flocculation wastewater process, pilot plant assays were carried out in the referrenced installation (see section 2.6). Simulated raw wastewater with around 70 mg · L-1 of Carmine Indigo was treated with Moringa oleifera seed extract. The coagulation dosage was ca. 100 mg · L-1. No pH adjustment was set up. The coagulation process was carried out by the generation of flocs that act as the active surface. Because of this, it has been considered interesting to observe the turbidity increase in three points of the installation: inside the mixer, in the exit of the sedimentator, and in the exit of the filter. Turbidity in the incoming flow was consequently 0. Figure 11 shows the average results of these assays. As it can be appreciated, the pseudo-equilibrium regime is achieved very soon, as either turbidity levels and final dye removal are kept constant during the assay period. Turbidity in the mixer is kept constant and close to 2300 NTU, while this measurement decreases rapidly in the sedimentator and is kept constant around 400 NTU (6 times lower). Slow sand filtration removes every nonsettled floc, so the exit turbidity is 0. Regarding dye removal, an important reduction of around 77% just due to the coagulation process is shown. The filtration process just increased this level of dye removal by 5%. With a similar dosage, similar removal levels are obtained in batch assays (see section 3.4), which is a promising result. If q capacity is considered, for pilot plant assays, eq 1 may be modificated in the following form: q)

Q0C0 - QCf Q c Cc

(5)

where Q0 is the dye incoming flow (L · min-1); C0 is the initial dye concentration (mg · L-1); Q is the total incoming flow (L · min-1); Cf is the final dye concentration (mg · L-1); Qc is

Ind. Eng. Chem. Res., Vol. 48, No. 14, 2009 -1

the incoming flow of coagulant (L · min ); and Cc is the concentration of the coagulant (mg · L-1). Equation 5 yields an average value of q equal to 0.16 mg dye/mg Moringa oleifera seed extract. This data is consistent with predicted data in Figure 8, and it is very close to the one predicted by the Langmuir model (eq 2). 4. Conclusions This investigation drives to the following conclusions: • Among several natural products, Moringa oleifera has been found to be a highly effective dye-removal agent through the coagulation process. In the studied case of Carmine Indigo removal, up to 80% dye removal is easily achieved, so this stage may be included into the normal depuration procedure. • Moringa oleifera is fully working also with other kinds of dyes, such as azo dyes or anthraquinonic dyes. • pH does not affect significatively to dye removal process. • As temperature increases, the efficiency of the process decreases, due surely to proteinic character of the Moringa seed extract. • Initial dye concentration affects negatively the percentage dye removal, although q capacity increases as initial dye concentration becomes higher. • Theoretical modeling is possible according to the Langmuir or Freundlich hypotheses. The first of them fits the coagulation process better. • Bearing in mind the influence of initial dye concentration and temperature at the same time, it is possible to evaluate their interaction through a design of experiments. An optimum (maximum) q is found at 11.2 °C and 343 mg · L-1. • Moringa seed extract presents fully ability also in a scaleup installation. Slow sand filtration enhances slightly the dye removal. Acknowledgment This investigation has been supported by the Programa de Iniciacioń a la Investigacioń, Universidad de Extremadura, oriented modality, Banco Santander subprogram. The authors thank also the Comisioń Interministerial de Ciencia y Tecnologıá (CICYT) CTQ 2007-60255/PPQ project as well as Junta de Extremadura under the PRI-07A031 project. Supporting Information Available: Dye chemical structures, pH influence, linearization of Langmuir and Freundlich models, Freundlich linearization, design of experiments, and graphical analysis for the design of experiments. This material is available free of charge via the Internet at http://pubs.acs.org. Literature Cited (1) Crini, G. Non-conventional low-cost adsorbents for dye removal: a review. Bioresour. Technol. 2006, 97 (9), 1061–1085. (2) Brown, D. Effects of colorants in the aquatic environment. Ecotoxicol. EnViron. Safe. 1987, 13 (2), 139–147. (3) Tan, I. A. W.; Ahmad, A. L.; Hameed, B. H. Adsorption of basic dye on high-surface-area activated carbon prepared from coconut husk: Equilibrium, kinetic and thermodynamic studies. J. Hazard. Mater. 2008, 154 (1-3), 337–346. (4) Sanghi, R.; Bhattacharya, B. Review on decolorisation of aqueous dye solutions by low cost adsorbents. Color. Technol. 2002, 118 (5), 256– 269. ¨ stu¨n, G. E.; Yonar, T. Colour and (5) Solmaz, S. K. A.; Birgu¨l, A.; U COD removal from textile effluent by coagulation and advanced oxidation processes. Color. Technol. 2006, 122 (2), 102–109. (6) Yonar, T.; Yonar, G. K.; Kestioglu, K.; Azbar, N. Decolorisation of textile effluent using homogeneous photochemical oxidation processes. Color. Technol. 2005, 121 (5), 258–264.

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ReceiVed for reView March 24, 2009 ReVised manuscript receiVed May 6, 2009 Accepted May 8, 2009 IE9004833

Removal of Carmine Indigo Dye with Moringa oleifera Seed Extract

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