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Nov 19, 2014 - Abstract A 24 factorial design was used to evaluate the quantitative adsorption of Hg(II) ions in an aqueous solu- tion onto radiation crosslinked ...
Arab J Sci Eng (2015) 40:109–116 DOI 10.1007/s13369-014-1484-x

RESEARCH ARTICLE - CHEMISTRY

Full Factorial Design Approach to Hg(II) Adsorption onto Hydrogels Deniz Bingöl · Dursun Saraydin · Dilek Solpan ¸ Özbay

Received: 2 December 2013 / Accepted: 30 May 2014 / Published online: 19 November 2014 © King Fahd University of Petroleum and Minerals 2014

Abstract A 24 factorial design was used to evaluate the quantitative adsorption of Hg(II) ions in an aqueous solution onto radiation crosslinked poly(acrylic acid/acrylamide) hydrogels. The influence on the binding ratio (r ) of variables such as pH, temperature, initial concentration of solution, and material was analyzed statistically using a suggested regression equation. The results demonstrate that the initial concentration of Hg(II) is the most significant parameter. A maximum Hg(II) ion uptake of 15.50 mg/g (with standard deviation, 0.20) was achieved at a high initial Hg(II) concentration (100 mg/L), low pH (2.5), and low temperature (15 ◦ C) for both of radiation crosslinked poly(AAm–co–AAc) hydrogel samples in a significance level of 5 %. Keywords Hydrogel · Mercury · Response surface method · Wastewater

1 Introduction Mercury enters the environment through industrial wastewater (e.g., electrochemical industries, Hg batteries, and electrical switches), pesticides, and fungicides. The permissible D. Bingöl (B) Department of Chemistry, Faculty of Science and Art, Kocaeli University, Kocaeli, Turkey e-mail: [email protected] e-mail: [email protected] D. Saraydin Department of Chemistry, Faculty of Science, Cumhuriyet University, Sivas, Turkey D. S. ¸ Özbay Department of Chemistry, Faculty of Science, Hacettepe University, Ankara, Turkey

limit of Hg(II) is 1 ppb in drinking water. Elemental Hg is toxic, but organomercury is highly toxic due to its lipid solubility. Therefore, to meet quality standards, cleanup is essential prior to discharging the wastewater into the environment. The critical concentration (1 ppb) directly influenced the design and evaluation of numerous Hg(II) removal processes. Thus, several methods have been used to remove Hg(II) from aqueous media, namely precipitation, ionexchange, solvent extraction, sorption, ultrafiltration, and complex formation by polymer. Removal of Hg(II) using polymeric sorbents deserves particular attention because the materials used have proven to be highly efficient and easy to handle, and in several cases, they can be regenerated [1]. Metal ion adsorption onto a solid polymer surface is a simple and attractive method for the removal of metal ions from the effluents due to its high efficiency; nontoxicity, low cost, and speed as compared to precipitation; complex formation; solvent extraction; and ultrafiltration techniques. Design of experiments is a method for simultaneously investigating the effects of multiple experimental variables on a response. This method is used, with minimal effort, for the evaluation of the statistical significance of individual experiment parameters as well as the interaction between factors that are not possible in a classical experiment. Thus, much information can be extracted with a minimal number of experimental trials [2]. In recent years, the most commonly used designs are twolevel full/fractional factorial designs in which every factor is experimentally studied at only two levels. Full factorial design techniques that investigate all possible combinations of the factor levels are very useful for preliminary studies or in the initial steps of an optimization, while fractional factorial designs are almost mandatory when the problem involves a large number of factors. Full factorial designs of the 2k type (k factors, each at two levels) have several advan-

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tages, such as their simplicity, relatively few runs required, data analysis performed using graphical methods, and greater cost-effectiveness than traditional univariate (one at a time) approaches. In addition, another major advantage is that the mathematical models can be utilized to find the predicted optimum response within the studied experimental conditions. The codified model employed for 24 factorial designs is expressed as follows:

air at ambient temperature in a Gammacell-220 type γ irradiator. A dose of 8.0 kGy was applied at a fixed-dose rate of 0.16 kGy/h. Hydrogels obtained in long cylindrical shapes were cut, washed with distilled water, and dried both in the air and in a vacuum and stored for later evaluations. The hydrogels were named hydrogel-1 and hydrogel-2 for AAm/AAc mole ratios, 30/70 and 15/85, respectively. 2.3 Experimental methods

y = β0 +

k 

βi xi +

i=1

k 

βi j xi x j + ε

(1)

1≤i≤ j

where y is the response, xi and x j are coded variables, β’s are regression coefficients, and ε is a random error [3–8]. The uncoded variables are converted to coded variables using the following equation [9]: x=

X − [X max + X min ]/2 [X max − X min ]/2

(2)

where x is the coded variable, X is natural variable, and X max and X min are the maximum and minimum values of the natural variable. The main goal of this paper was to show a new approach using statistically designed experiments for a maximum binding ratio of Hg(II) ions onto radiation cross-linked poly(acrylic acid/acrylamide) hydrogels. The corresponding interactions among the variables were studied and optimized using full factorial design and response surface plots.

Stock solution of 1,000 mg/L of Hg(II) was prepared and suitably diluted to required initial concentrations (10–100 mg/L). For each experiment, 25 mL Hg(II) solution, taken in capped conical flasks containing approximately 0.2 g of the hydrogel, was allowed to stand for seven hours, sufficient time to reach equilibrium concentration specified by pretest, in a water bath. The temperature was maintained at desired values by using a cold-storage unit where the experiments were conducted. The pH of the solutions was held constant at desired values by using buffer solutions of sodium acetate. Aliquots of 1 mL were taken from the solution, transferred to a centrifuge tube, and mixed with 1 mL of distilled water and 1 mL 0.1 % of 1.5-diphenylcarbazide solution. The final concentration of Hg(II) ions was analyzed immediately using a Shimadzu UV-2450 UV–VIS at 530 nm. The correlation coefficient was 0.9991, and the absorbance was linear up to 100 mg/L Hg(II) concentration. Spectroscopic studies were conducted with a Bruker Tensor 27 model FT–IR spectrophotometer. The binding ratio or the amount of adsorption per unit mass of the poly(AAc/AAm) hydrogels (r ) was evaluated using the following equation:

2 Experimental methods r = (C0 − Ce ) V/m

(3)

2.1 Materials All chemicals were used as received. The acrylamide (AAm) and acrylic acid (AAc) monomers used in this study were obtained from BDH (British Drug Houses), and the sodium acetate and acetic acid used to prepare acetate buffer solutions were obtained from Sigma-Aldrich. The deionized water was used in the preparation of buffer solutions, and mercury acetate was supplied from Merck. All compounds were used without any further purification. 2.2 Preparation of hydrogels Cross-linked AAm/AAc hydrogels were synthesized by gamma radiation; this procedure was detailed in our previous study [10]. The solutions of monomers of AAm and AAc were prepared in two different compositions (AAm/AAc mole ratios, 30/70, 15/85). These solutions were placed in 3 mm-diameter polyvinylchloride tubes and irradiated in the

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where r is the binding ratio of Hg(II) ions adsorbed onto the unit dry mass of the poly(AAm/AAc) hydrogels (mg/g); C0 and Ce are the concentrations of the Hg(II) ion in the initial solution and in the aqueous phase after treatment for a certain period of time, respectively (mg/L); V is the volume of the aqueous phase (L); and m is the mass of dry poly(AAm/AAc) hydrogels (g). 2.4 Factorial design of experiments The effect of the experimental parameters chosen as independent variables, including pH of the solution ( A), temperature (B), initial concentration of solution (C), and material (D), was investigated using a 24 full factorial design. The high and low levels defined for them are listed in Table 1. The level selection for each factor was implemented based on the preliminary trials and previous results for radiation cross-linked poly(acrylamide/crotonic acid) hydrogels [11]. Experiments

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Table 1 Design matrix and the results of the 24 full factorial design Factors

Low level (−1)

High level (+1)

(A) pH of solution

2.5

5.0

(B) Temperature (◦ C)

15

35

(C) Initial concentration of solution (mg/L)

10

100

(D) Material

Hydrogel-1

Hydrogel-2

Run

A (pH)

B (temperature)

C (initial concentration)

D (material)

Binding ratio, r (mg/g)

1

−1 (2.5)

−1 (15)

−1 (10)

−1 (hydrogel-1)

0.324

2

+1 (5.0)

−1 (15)

−1 (10)

−1 (hydrogel-1)

0.312

3

−1 (2.5)

+1 (35)

−1 (10)

−1 (hydrogel-1)

0.324

4

+1 (5.0)

+1 (35)

−1 (10)

−1 (hydrogel-1)

0.348

5

−1 (2.5)

−1 (15)

+1 (100)

−1 (hydrogel-1)

15.46

6

+1 (5.0)

−1 (15)

+1 (100)

−1 (hydrogel-1)

10.77

7

−1 (2.5)

+1 (35)

+1 (100)

−1 (hydrogel-1)

11.06

8

+1 (5.0)

+1 (35)

+1 (100)

−1 (hydrogel-1)

10.14

9

−1 (2.5)

−1 (15)

−1 (10)

+1 (hydrogel-2)

0.308

10

+1 (5.0)

−1 (15)

−1 (10)

+1 (hydrogel-2)

0.349

11

−1 (2.5)

+1 (35)

−1 (10)

+1 (hydrogel-2)

0.351

12

+1 (5.0)

+1 (35)

−1 (10)

+1 (hydrogel-2)

0.382

13

−1 (2.5)

−1 (15)

+1 (100)

+1 (hydrogel-2)

15.50

14

+1 (5.0)

−1 (15)

+1 (100)

+1 (hydrogel-2)

10.62

15

−1 (2.5)

+1 (35)

+1 (100)

+1 (hydrogel-2)

10.68

16

+1 (5.0)

+1 (35)

+1 (100)

+1 (hydrogel-2)

9.88

were performed according to the full factorial design matrix given in Table 1. The binding ratio was determined as the average of two parallel experiments. The order in which the experiments were made was randomized to avoid systematic errors. The results were analyzed with the Minitab 16 software to obtain the effects, coefficients, standard deviation of coefficients, and other statistical plots (Pareto, main effects, interactions, surface).

3 Results and discussion 3.1 Statistical analysis The 24 full factorial design used to determine main effects, two-way interactions, and three-way interactions. Significant effects and interactions onto r were evaluated with the student’s t test. Table 2 shows estimated effects and coefficients for r . The null hypothesis that the effects are equal to zero was rejected for pH of the solution ( A), temperature (B), initial concentration of solution (C), pH-initial concentration of solution (AC), pH–initial concentration of solution (BC), and pH of the solution–temperature–initial concentration of solution (ABC) interactions having P values smaller than 0.05.

That is, the binding ratio of Hg(II) onto radiation crosslinked poly(AAm–co–AAc) hydrogel basically depends on these factors. The null hypothesis, stating that the main effects and the two-way and three-way interactions are equal to zero, was also tested in Table 2 by using the F test. From the P value defined as the lowest level of significance leading to rejection of the null hypothesis [5], it appears that the small P values ( 0.05) at the 5 % significance level. Based on the above findings, the regression equation ignoring the insignificant terms is as follows: r = 6.0505 − 0.7017.A − 0.6543.B + 5.7132.C + 0.4915.AB − 0.7121.AC − 0.6682.BC + 0.488.ABC

(4)

A positive sign of a given parameter indicates that the response increases with the increase in the value of that parameter [2]. That is, (+1) level of the included factors increases the binding ratio. The magnitude and sign of C indicate that the contribution of this factor to the binding ratio is 5.7132 for the high level (+1). Similar explanations can be done for

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Table 2 Estimated effects and coefficients for r (mg/g) (coded units) Term

Effects

Coefficients

Standard errors 6.0505

0.03555

170.18

0.000

pH (A)

−1.4033

−0.7017

0.03555

−19.73

0.000

Temperature (B)

−1.3086

−0.6543

0.03555

−18.40

0.000

Constant

T

P

Initial concentration (C)

11.4264

5.7132

0.03555

160.69

0.000

Material (D)

−0.0848

−0.0424

0.03555

−1.19

0.250

A∗B

0.9831

0.4915

0.03555

13.82

0.000

A∗C

−1.4242

−0.7121

0.03555

−20.03

0.000

A∗D

−0.0027

−0.0013

0.03555

−0.04

0.970

B ∗C

−1.3364

−0.6682

0.03555

−18.79

0.000

B∗D

−0.0614

−0.0307

0.03555

−0.86

0.400

C∗D

−0.1052

−0.0526

0.03555

−1.48

0.158

A∗ B∗C

0.9769

0.4885

0.03555

13.74

0.000

A∗B∗D

0.0332

0.0166

0.03555

0.47

0.647

A∗C ∗ D

−0.0173

−0.0087

0.03555

−0.24

0.811

B ∗C ∗ D

−0.0711

−0.0355

0.03555

−1.00

0.332

0.0443

0.0222

0.03555

0.62

0.542

A∗ B∗C ∗ D

S = 0.201, R-Sq = 99.94 %, R-Sq(pred) = 99.77 %, R-Sq(adj) = 99.89 % Analysis of variance for r (mg/g) Source

DF

Seq SS

Adj SS

Adj M S

F

P

Main effects

4

1074.02

1074.02

268.505

6637.75

0.000

2-Way interactions

6

38.37

38.37

6.394

158.07

0.000

3-Way interactions

4

7.69

7.69

1.922

47.51

0.000

4-Way interactions

1

0.02

0.02

0.016

0.39

0.542

0.65

0.040

Residual error

16

0.65

Total

31

1120.73

DF degrees of freedom, Seq SS sequential sum of squares, Adj SS adjusted sum of squares, Adj MS adjusted mean of squares, F factor F, P probability

other terms. A negative sign of a given parameter (i.e., pH of the solution ( A) and temperature (B)) indicates that the binding ratio decreases with the increase in the value of that parameter. According to Eq. (4), the maximum binding ratio of Hg(II) will be 15.48 mg/g. Comparative data from some recent studies for determination of Hg(II) were given in Table 3. From Table 3, there are adsorbents with low and high adsorption abilities for Hg(II) in literature. Analysis of the variance (ANOVA) and P value were used to evaluate the significance of the effects on the binding ratio. The relative importance of main effects and their interactions can be seen the Pareto chart, as shown in Fig. 1. The vertical line in the chart indicates the minimum statistically significant effect magnitude for the 5 % significance level. Any effect that extends beyond this reference line is potentially important. The interpretation of the factorial design by the Pareto chart demonstrates that the main effects of initial concentration of solution (C), pH of the solution (A), temperature (B),

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and the interactions of pH of the solution–initial concentration of solution (AC), temperature–initial concentration of solution (BC), pH of the solution–temperature (AB), and pH of the solution–temperature–initial concentration of solution (ABC) were statistically significant at the 5 % significance level on r . The occurrence of pH of the solution-initial concentration of solution (AC), temperature–initial concentration of solution (BC), pH of the solution–temperature (AB), and pH of the solution–temperature–initial concentration of solution (ABC) interactions imposes the necessity of analysis of the interactions. The initial concentration of solution (C) appears to have the most significant effect, as it lies furthest from the line. The main effect plots in Fig. 2 were used to visualize which factors most affect the response (r ). Each level of the factors affects the binding ratio differently. A lower pH, a lower temperature, and a higher initial Hg(II) concentration of solution should be used to minimize the uncertainty in the binding ratio. Since the slope of material (D) between hydrogel-1 and hydrogel-2 is close to zero, the effect of that

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Table 3 The comparison of the results found in the presented study and some works in literature for Hg(II) pH

Adsorbent

Adsorption capacity (qe , mg/g)

References

5–9

Polyaniline/attapulgite composite

800

[15]

2

A thiocarbamoyl derivative of chitosan

450

[16]

6

Rhizopus oligosporus

33.33

[17]

2–9

Poly(m-phenylenediamine) microparticles

955

[18]

5

A thiourea-modified chitosan derivative

6.29a

[19]

5

P(Sulti/hydroxyethyl methacrylate/acrylic acid)

13.46

[20]

4.4

Polyacrylamide/attapulgite

192.5

[21]

2.5

Radiation crosslinked poly(acrylic acid/acrylamide)

15.50

This study

a mmol/g

Fig. 1 Pareto chart of the standardized effects for r

factor is insignificant. That is, hydrogel-1 or hydrogel-2 has almost the same effect on r . In addition, the initial concentration of solution (C) appears to have a greater effect on the binding ratio, as indicated by a sharp slope. These results are also supported by the ANOVA statistical calculation in Table 2. The two-way interactions are shown in Fig. 3, and it is possible to confirm the interaction between factors. An interaction plot shows the effect that changing the settings of one factor has on another factor. As the lines of two factors (pH of the solution (A) and temperature (B), pH of the solution (A) and initial concentration of solution (C), and temperature (B) and initial concentration of solution (C)) are far from being parallel, and the two factors interact. That is, the effect of a high level of initial concentration of solution (i.e., 100 mg/L) on r will be higher if the pH and temperature are kept at a low level (i.e., 2.5 and 15 ◦ C, respectively). A regression Eq. (4) was used in constructing the surface plots (Fig. 4) for the binding ratio of Hg(II) ions onto radiation cross-linked poly(acrylic acid/acrylamide) hydrogels. From Fig. 4, with the initial Hg(II) concentration increasing

at low pH and temperature, the uptake capacity of Hg(II) ions per unit mass of hydrogel (r , mg/g) was increased. The binding ratio of Hg(II) ions onto radiation cross-linked poly(acrylic acid/acrylamide) hydrogels at very low pH values can be considered quite good, while a consistent lowering of the binding ratio can be noted at pH 5. At very low pH values, the mercury hydrolysis begins with the formation of very stable Hg(OH)+ and Hg(OH)02 aqueous species. There is positively charged species [Hg2+ and Hg(OH)+ ] of mercury in solution at pH ∼3. At pH ∼5, the neutral species Hg(OH)02 occurs [12]. Thus, the uptake of Hg(II) was less at a high pH level, probably due to presence of Hg(OH)02 . Binding ratio decreases as temperature increases. Because the physical attachments between hydrogel and Hg(II) ions were decreased with the temperature increased. As the initial concentration of solution increases, r also increases. This is because the interactions between hydrogel and Hg(II) ions increase along with the Hg(II) ion concentration. The reaction of Hg(II) ions with amides can be depicted as follows: a covalent bond is formed between the Hg(II) and amide nitrogen atoms in aqueous solution. Generally, mercurated amides are insoluble in water, and the reaction is common to all primary and secondary amide compounds. The reaction also forms the basis of one of the most common reactions of proteins, in which the addition of a few drops of Hg(II) solution to an aqueous solution of protein causes instantaneous precipitation of the biomacromolecule. Apparently, this reaction involves crosslinking by mercuric ions via the amide groups. In an aqueous solution, anion exchange can take place, and the counter anion may be hydroxyl as well [13]. On the other hand, a possible interaction between negative charges of radiation crosslinked poly(AAm–co–AAc) hydrogel and positive charges of Hg(II) ions is shown in Fig. 5. The interaction between the cationic Hg(II) ions and carboxyl groups of the hydrogel is ion–ion interaction [9]. These may occur between the negative charge of carboxyl group on the hydrogel and the positive charge on the Hg(II) ions.

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Fig. 2 Main effect plots for r

Fig. 3 Interaction plots for r

Fig. 4 Surface plots for r

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Fig. 5 A possible interaction between the hydrogel and Hg(II) ions

Fig. 6 FT–IR spectrum of a radiation crosslinked poly(AAm–co–AAc) hydrogel b radiation crosslinked poly(AAm–co–AAc) hydrogel bound Hg(II)

As shown in the surface plots, the higher binding ratio (>14 mg/g Hg(II)) onto radiation crosslinked poly(AAm– co–AAc) hydrogel could be obtained in a region of higher initial Hg(II) ion concentration and lower pH and temperature (+, −, –). It is noted that the three contour plots (pH of the solution–temperature (AB), pH of the solution–initial concentration of solution (AC), and temperature–initial concentration of solution (BC)) form a curved line because the model (Eq. 4) contains these two-way interactions. These interactions are significant, as can be seen with the exception of pH of the solution–material (AD), temperature–material (BD), and initial concentration of solution–material (CD) interactions.

C=O group connected to the carboxyl group gives absorption peak at 1,703 cm−1 . C=O group connected to the amide group gives absorption peak at 1,653 cm−1 in poly(AAm-coAAc)–Hg. In Fig. 6, these peaks are sharper than in the hydrogel at the same frequencies. A common feature for all of the complexes is the position of νasym (OCO) (ca. 1,600 cm−1 ) because of the dissociation and coordination to the metal ion of the carboxylic group [14]. Therefore, a possible interaction between radiation crosslinked poly(AAm–co–AAc) hydrogel and Hg(II) ions may occur on the amide and carboxyl groups of hydrogels with the Hg ions, as suggested earlier.

4 Conclusion 3.2 FT–IR studies To examine the nature of the interaction between Hg(II) ions and poly(AAm-co-AAc) hydrogel, the FT-IR spectrums of both radiation crosslinked poly(AAm–co–AAc) hydrogel and radiation crosslinked poly(AAm–co–AAc) hydrogel bound Hg(II) were given in Fig. 6.

By the use of the mathematical model and the surface plots, variation of the binding ratio onto radiation crosslinked poly(AAm–co–AAc) hydrogel of Hg(II), depending on the changes in experimental factors, can easily be investigated and maximized within the limits of the experimental design. The initial Hg(II) concentration was found to have the most statistically significant effect upon the binding ratio. More-

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over, the interactions between factors AC, BC, AB, and ABC also significantly influence the binding ratio. The value of the predicted determination coefficient (R 2 (pred) = 99.77 %) is in reasonable agreement with the value of the adjusted determination coefficient (R 2 (adj) = 99.89 %). The full factorial design technique could be used as the most efficient way to derive the required information with the least experiments, that is, for predicting and understanding the interaction effects between experimental factors.

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