^{1}

^{1}

^{1}

^{1}

Synthetic zeolite Na-A was prepared from Egyptian kaolinite by hydrothermal treatment to be used as an adsorbent for removal of phosphate from aqueous solutions. The present work deals with the application of response surface methodology (RSM) and central composite rotatable design (CCRD) for modeling and optimization of the effect of four operating variables on the removal of phosphate from aqueous solution using zeolite Na-A. The parameters were contact time (0.5 - 6 h), phosphate anion concentrations (10 - 30 mg/L), adsorbent dosage (0.05 - 0.1 g), and solution pH (2 - 7). A total of 26 tests were conducted using the synthetic zeolite Na-A according to the conditions predicted by the statistical design. In order to optimize removal of phosphate by synthetic zeolite Na-A, mathematical equations of quadratic polynomial model were derived from Design Expert Software (Version 6.0.5). Such equations are second-order response functions which represent the amount of phosphate adsorbed (mg/g) and the removal efficiency (%) and are expressed as functions of the selected operating parameters. Predicted values were found to be in good agreement and correlation with experimental results (R2 values of 0.918 and 0.905 for amount of phosphate adsorbed and removal efficiency of it, respectively). To understand the effect of the four variables for optimal removal of phosphate using zeolite Na-A, the models were presented as cube and 3-D response surface graphs. RSM and CCRD could efficiently be applied for the modeling of removing of phosphate from aqueous solution using zeolite Na-A and it is efficient way for obtaining information in a short time and with the fewer number of experiments.

Wastewater is one of the biggest environmental problems all over the world [

Increasing the phosphorous concentration resulted in impairing the water quality and causing many problems as cloudy lakes, depletion of oxygen in deep water in lakes, high purification costs, decreased recreational and conservation value of water bodies, loss of livestock and the possible lethal effect of algal toxins on drinking water [

There are several methods which have been applied to eliminate phosphorous in waste water including chemical precipitation, biological removal, crystallization, adsorption and ion exchange [

Zeolite molecular sieves are crystalline microporousaluminosilicates with an indefinitely extending three- dimensional network of AlO_{4} and SiO_{4} tetrahedrons linked by sharing of oxygen atoms [

Zeolites are usually synthesized under hydrothermal conditions from basic aluminosilicate precursor gels at elevated temperatures [

The aim of this paper focuses on applying, response surface methodology (RSM) and statistical Central Composite Rotatable Design (CCRD) to evaluate the interactive effect of the selected factors and to obtain the optimum conditions for maximum removal of phosphate from aqueous solution by synthetic zeolite Na-A from Egyptian kaolinite.

Response Surface Methodology (RSM) is a collection of mathematical and statistical techniques used in significance of several affecting factors in an optimum manner, even in the presence of complex interactions [

where y is the answer of the system, and x_{i} the variables of action called factors. The goal is to optimize the response variable (y). The task then is to find a suitable approximation for the true functional relationship between independent variables and the response surface [

where Y is the predicted response; X_{1}, X_{2}, ∙∙∙, X_{k} are input factors which influence the response y; β_{0} is constant; β_{i} is linear term coefficients; β_{ii} is quadratic term coefficients; β_{ij} is cross product term coefficients and K is the number of factors.

The experimental design techniques commonly used for process analysis and modeling are the full factorial, partial factorial and central composite rotatable designs. Central composite rotatable design originally developed by Box and Wilson (1951) [^{k} + 2K + cp where (k) is the variable number and (cp) is the number of central points [

The starting kaolinite clay sample used in the present study was supplied by Central Metallurgical Research and Development Institute, CMRDI, Cairo, Egypt. Its chemical composition is 45% SiO_{2}, 35% Al_{2}O_{3}, 3.5% TiO_{2}, 1.5% Fe_{2}O_{3}, 0.1% MgO, 0.6% CaO, 0.3% Na_{2}O, 0.05% K_{2}O and ignition loss nearly 14%. The alkali hydroxide used in the synthesis of zeolite A is NaOH (Alfa Aesar, purity: 97%).

Metakolinization is the process in which kaolinite was subjected to calcination at high temperature of about 650˚ C for 5 h where by dehydroxylation occurs forming amorphous more reactive product (metakaolinite). ZeoliteNa-A was synthesized using Egyptian kaolin and sodium hydroxide by treatment of metakaolinite with sodium hydroxide solution with 1:2 weight ratio with a continuous vigorous stirring of the mixture overnight at room temperature. After stirring, the mixture was transferred to 125 ml stainless steel Parr reactors and heated in an oven at 100˚C for 8 h. After hydrothermal treatment, the vessels were cooled down to room temperature. The contents from Teflon vessels were centrifuged to separate solids and solutions. Then, the solid products were washed with distilled water to remove the excess alkalinity and dried at 65˚C.

X-ray powder diffraction patterns were obtained using a Philips APD-3720 diffractometer with Cu Kα radiation, operated at 20 mA and 40 kV in the 2θ range of 5 - 70 at a scanning speed of 5˚/min. Scanning electron microscopy using a field emission scanning electron microscope (JSM-6510, JEOL, Tokyo, Japan). Particle size analyses and surface area of the syntheticzeolite Na-A were determined by BT-2001 (wet) laser particle size analyzer device.

Response surface design and central composite rotatable design of quadratic model has been designed according to fours elected variables (Contact time, initial concentration of phosphate, Dosage and pH) to modeling and detect the optimum conditions under which, maximum removal of phosphate from aqueous solution can be occurred. The upper and lower limits for the selected variables in the actual and coded values set in (

Factor | Name | Low actual | High actual | Low coded | High coded |
---|---|---|---|---|---|

A | Contact time | 0.5 h | 6 h | −1 | 1 |

B | Phosphate concentrations | 50 ml∙gm | 250 ml∙gm | −1 | 1 |

C | Dosage | 0.05 gm | 0.1 gm | −1 | 1 |

D | pH | 2 | 7 | −1 | 1 |

Run | Contact time (A) | Initial concentration (mg/L) (B) | Dosage (C) | pH (D) | Uptake (mg/g) | Removal efficiency (%) |
---|---|---|---|---|---|---|

1 | 6.00 | 50.00 | 0.05 | 2.00 | 10 | 20 |

2 | 6.00 | 50.00 | 0.10 | 7.00 | 15 | 30 |

3 | 3.00 | 150.00 | 0.08 | 7.00 | 12 | 8 |

4 | 0.50 | 50.00 | 0.10 | 7.00 | 9 | 18 |

5 | 3.00 | 50.00 | 0.08 | 4.00 | 30 | 60 |

6 | 0.50 | 250.00 | 0.05 | 7.00 | 10 | 4 |

7 | 0.50 | 50.00 | 0.05 | 2.00 | 6 | 12 |

8 | 6.00 | 50.00 | 0.05 | 7.00 | 7 | 12 |

9 | 0.50 | 250.00 | 0.10 | 7.00 | 19 | 7.6 |

10 | 6.00 | 250.00 | 0.05 | 2.00 | 12 | 4.8 |

11 | 0.50 | 50.00 | 0.10 | 2.00 | 13 | 26 |

12 | 6.00 | 250.00 | 0.05 | 7.00 | 8 | 3.2 |

13 | 6.00 | 250.00 | 0.10 | 7.00 | 18 | 6.8 |

14 | 3.00 | 150.00 | 0.05 | 4.00 | 29 | 19.3 |

15 | 0.50 | 150.00 | 0.08 | 4.00 | 23 | 15.3 |

16 | 3.00 | 150.00 | 0.08 | 4.00 | 48 | 32 |

17 | 0.50 | 250.00 | 0.05 | 2.00 | 15 | 6 |

18 | 3.00 | 150.00 | 0.10 | 4.00 | 53 | 35.33 |

19 | 0.50 | 50.00 | 0.05 | 7.00 | 4 | 8 |

20 | 6.00 | 250.00 | 0.10 | 2.00 | 34 | 13.6 |

21 | 3.00 | 250.00 | 0.08 | 4.00 | 47 | 18.8 |

22 | 6.00 | 150.00 | 0.08 | 4.00 | 49 | 32.6 |

23 | 0.50 | 250.00 | 0.10 | 2.00 | 26 | 10.4 |

24 | 6.00 | 50.00 | 0.10 | 2.00 | 25 | 50 |

25 | 3.00 | 150.00 | 0.08 | 4.00 | 48 | 32 |

26 | 3.00 | 150.00 | 0.08 | 2.00 | 18 | 12 |

The experiments were performed according to tests suggested by the statistical design where 25 ml from the prepared solutions shake with the suggested dosage for various contact times then the solutions filtered using 45 µm pore size What man filter paper. The results were evaluated by following phosphate concentration in the filtrate solutions using a spectrophotometric method (DIN EN 1189, 1996) at a wavelength of 700 nm with a Lamp 2 UV/VIS Spectrophotometer. The amount of phosphate ions adsorption onto the zeolite (q (mg/g)) and the removal efficiency were calculated as follows:

where C_{0} and C_{e} are the ion concentrations in the initial solution and the solution after equilibration of phosphate ions, respectively. V is the volume of solution in (ml) and m is the mass of sorbent (gm).

The data obtained were fitted to a second-order polynomial equation:

where Y is amount of phosphate uptake; β_{0}, β_{i}, β_{ii}, β_{ij} are constant coefficients X_{i} are the uncoded independent variables. Subsequent regression analyses, analyses of variance (ANOVA) and response surfaces were performed using the Design Expert Software (Version 6.0.5). Optimal reaction parameters for maximum removal were generated using the software’s numerical optimization function.

The XRD patterns of the used natural kaolin and metakaolin are shown in

main crystalline phases of untreated kaolin sample (

SEM images of zeolite Na-A synthesized by hydrothermal treatment of metakaolinite with sodium hydroxide show the transformation of kaolinite (^{2}/g.

The results of analysis of variance (ANOVA) show that, the Model F-values for amount of phosphate uptaking and the removal efficiency are 8.88 and 8.16 respectively which imply that the model is significant; and there is only a 0.04% and 0.06% chances that a “Model F-Value” resulted from noise for the previous stated responses. Pure errors are zero which indicates very good reproducibility of the obtained data. The mathematical equations of the quadratic polynomial model, which represent the relations between the required responses (q(Y_{1}) and removal efficiency of phosphate (Y_{2})) and the selected variables, were obtained from Design Expert Software (Version 6.0.5) for coded units as follows:

The observed (experimental) values and the predicted values obtained using the model equations are given in

Amount of phosphate uptaking (mg/g) | Removal efficiency (%) | ||
---|---|---|---|

Observed values | Predicted values | Observed values | Predicted values |

12 | 14.41 | 12 | 11.8 |

13 | 12.63 | 20 | 23.93 |

25 | 26.52 | 6 | 1.89 |

26 | 27.15 | 4.8 | 2.69 |

34 | 34.86 | 26 | 29.89 |

4 | 2.68 | 50 | 49 |

7 | 6.41 | 10.4 | 7.08 |

10 | 9.31 | 13.6 | 14.85 |

8 | 6.86 | 8 | 7.05 |

9 | 7.25 | 12 | 13.76 |

15 | 16.16 | 4 | 5.08 |

19 | 17.87 | 3.2 | 0.45 |

17 | 20.61 | 18 | 19.36 |

23 | 34.86 | 30 | 33.04 |

49 | 41.08 | 7.6 | 4.49 |

31 | 36.9 | 6.8 | 6.84 |

47 | 45.05 | 15.3 | 20.66 |

29 | 38 | 32.6 | 28.44 |

56 | 50.94 | 62 | 50.17 |

18 | 20.09 | 18.8 | 31.83 |

12 | 13.85 | 19.3 | 22.65 |

48 | 42.08 | 37.3 | 35.15 |

48 | 42.08 | 12 | 13.67 |

12 | 14.41 | 12 | 11.8 |

13 | 12.63 | 20 | 23.93 |

25 | 26.52 | 6 | 1.89 |

in the high determination coefficient (R^{2}). The closer the R^{2} is to 1, the better the model fits the experimental data, the less the difference between the predicted and the observed values. R^{2} values are above 0.9 for all the required responses as appear in

Effect of the selected variables and the interaction between them during removal of phosphate from aqueous solutions using synthetic zeolite Na-A is expressed in 3D and cube response surface diagrams. Response surface plots in terms of two selected factors at any one time maintaining all other factors at fixed levels are suitable in understanding either the main or the interaction effects of these two factors and represented by 3D diagrams. The elliptical shape of the curve indicates good interaction of the two variables and circular shape indicates no interaction between the variables [

The interaction between all the selected variables at fixed pH 4 and their effect on the amount of adsorbed phosphate (mg) and the removal efficiency is represented by cube graphs in

1) Interaction between contact time and pH

Effect of pH appears in increasing the adsorption capacity with pH values from pH 2 to pH 4.5 and then reduced from 4.5 to 7. The optimum pH value for maximum capacity is detected to be from 4 to 4.5. Acidic pH is preferred in removal phosphate anions, due to a greater number of surface sites with a PZC = 5.5 at such conditions [

2) Interaction between initial phosphate concentration and pH

The interaction between initial phosphate concentration and pH values was plotted in

Increasing the initial phosphate concentrations has reversible effect on the removal efficiency (%) (

3) Interaction between solid dosage and pH

4) Interaction between adsorbent dosage and the contact time

The relation between amounts of zeolite Na-A and contact times during the adsorption process represented in

5) Interaction between contact time and initial concentrations

The interaction between the contact time and the initial phosphate concentration was investigated at pH 4.5 and dosage 0.08 gm and represented in

6) Interaction between dosage and the initial concentration

The relation between zeolite dosage and initial phosphate concentrations; and their effect on the required responses is represented in

Observed experimental results in _{4} are the best conditions with respect amount of phosphate adsorbed by the synthetic zeolite Na-A (53 mg/g), but the maximum removal efficiency (60%) occur after 3 h as contact time, 50 mg/L initial concentration for phosphate in the aqueous solutions, 0.08 gm dose and pH_{4}.

The predicted optimum conditions for maximum phosphate anions removal using synthetic zeolite Na-A have been predicted using Design Expert’s optimization function in terms of the upper and lower limits for the selected variables (contact time, initial concentrations, dose and pH). The optimum conditions are listed in

Highly crystalline synthetic Na-A was synthesized from Egyptian kaolinite by hydrothermal treatment of metakaolinite at 100˚C for 8 h. In this study, response surface methodology in conjunction with central composite rotatable design was employed for modeling and optimizing four operations parameters of phosphate removal using synthetic zeolite Na-A. Four variables of the model investigated in this study were: contact time, initial phosphate concentration, amount of zeolite and pH values. The mathematical model equations were derived for both amount of phosphate adsorbed (mg/g) and the removal efficiency (%) from Design Expert Software (Version 6.0.5). Predicted values obtained using the model equations were in very good agreement with the observed values (R^{2} value of 0.918 for the amount of the adsorbed phosphate, R^{2} value of 0.905 for the removal efficiency (%)).

In order to gain a better understanding of the effects of these operational variables and their interactions on the amount of adsorbed phosphate (mg/g) and the removal efficiency (%), the predicted models values were

Contact time (h) | Initial concentration (mg) | Dose (gm) | pH | Removal efficiency (%) | Desirability |
---|---|---|---|---|---|

4.5 | 50 | 0.1 | 4.04 | 61.77 | 0.996 |

4.32 | 50 | 0.1 | 3.98 | 61.38 | 0.989 |

4.5 | 54.11 | 0.1 | 4.02 | 60.33 | 0.972 |

Contact time (h) | Initial concentration (mg) | Dose (gm) | pH | Amount of phosphate adsorbed (mg/g) | Desirability |
---|---|---|---|---|---|

4.55 | 250 | 0.1 | 4.19 | 55.82 | 0.997 |

4.43 | 250 | 0.1 | 4.21 | 55.81 | 0.996 |

4.4 | 250 | 0.1 | 4.24 | 55.71 | 0.994 |

presented as cube and 3D response surface graphs. Taking advantage of the quadratic programming, contact time of 4.5 h, initial concentration of 50 ppm, zeolite dose of 0.1 gm and pH of 4.04 have been determined as optimum levels to achieve the maximum removal of phosphate of 61.77%, whereas it is 60% in the tests conducted, i.e. 1.77 improvements in the removal efficiency could be obtained by the statistical design. In the same way, contact time of 4.52 h, initial concentration of 250 ppm, zeolite dose of 0.1 gm and pH of 4.19 have been determined as optimum levels to achieve the maximum amount of phosphate anions adsorbed by synthetic zeolite Na-A as it reaches (55.82 mg/g); however, the best result from the experimental tests is (53 mg/g).

E. A.Mohamed,A. Q.Selim,M. K.Seliem,Mostafa R.Abukhadra, (2015) Modeling and Optimizations of Phosphate Removal from Aqueous Solutions Using Synthetic Zeolite Na-A. Journal of Materials Science and Chemical Engineering,03,15-29. doi: 10.4236/msce.2015.39003