Factorial Design Analysis of Zn ( II ) Ions Adsorption on Thermally Treated Rice Husk

Adsorption of Zn(II) ions from aqueous solutions by thermally treated rice husk was investigated using factorial experimental design to study effects of heating temperature and period of rice husk, pH, initial Zn(II) ion concentration, adsorption temperature and contact time, and adsorbent dosage. Main and interaction effects of these factors were analyzed using statistical techniques and the results were analyzed statistically using the Student’s t-analysis and analysis of variance which were used to determine significant factors that affect the percentage removal of Zn(II). These significant factors were heating temperature of rice husk, pH, initial metal concentration, contact time, and adsorbent dosage. The interaction between two different effects also affects the percentage removal of Zn(II) ions. These include the interactions between heating temperature of rice husk and initial metal concentration, pH and initial metal concentration, and pH and the adsorbent dosage.


Introduction
Rapid development of industrial and agricultural sectors coupled with the growth of population increase hazardous heavy metals released into our environment.Heavy metals such as lead, chromium, copper, cadmium, nickel, silver, mercury, and zinc are harmful but, in many instances, they are discharged without proper treatment by manufacturers and/or users, therefore, become a major cause of water pollutions (Kilic et al., 2013).Without proper disposal methods, these metals endanger public and environment health (Sheela et al., 2012) as they are stable ions which can be accumulated by living organisms (Demirbas, 2008).complexation, chemical oxidation or reduction, chemical precipitation, reverse osmosis, ion exchange, solvent extraction, membrane filtration, coagulation, phytoextraction and evaporation (Lim et al., 2012).However, the methods such us chemical precipitation, ion exchange and membrane filtration are expensive, ineffective at low concentration (i.e.< 100 mg/L), and produce toxic sludge/ waste materials that also need to be disposed (Kilic et al., 2013).
Recently, several studies have been carried out to explore the usage of agricultural wastes as heavy metal ion adsorbents.Hydroxyl groups of cellulose, hemi-cellulose and lignin, which are major components of these wastes, have the abilities to bind the heavy metals (Nguyen et al., 2013).The adsorption capacity of these wastes to adsorb heavy metals could be increased with the pre-treatment (Bhattacharya et al., 2006).Agricultural wastes such as hazelnut shell (Jamali et al., 2009), pomegranate husk (El Nemr, 2009), sugarcane bagasse (Garg et al., 2009), banana peel (Anwar et al., 2010), and rice husk (Bansal et al., 2009) have been studied for these purposes.
Rice husk is a waste material from rice milling process.Based on the statistical data from Malaysian Ministry of Agriculture, there is 408 000 metric tons of rice husk generated in Malaysia annually (Syuhadah and Rohasliniey, 2013).The presence of abundant floristic fiber, protein and some functional groups such as carboxyl, hydroxyl and amidogen in rice husk increase the possibility of adsorption process to occur.Modification on the textures of rice husk, either chemical and/or thermal treatment/s, could enhance its adsorption capacity (Ye et al., 2010).Rice husk has the potential to be used as raw material to produce activated carbon.Hence, it will be a cheaper alternative compared to existing commercial activated carbons.Activated carbon produced from rice husk has highly porous structure and high specific area (Taha et al., 2011).Rice husk ash which is produced from thermal treated rice husk, was previously used to absorb oil on hard surfaces, filter arsenic from water, adsorption of gold thiourea, and adsorption of metal ions (Mane et al., 2007).
In this study, a 2 7-1 fractional factorial design was used to evaluate the effects of heating temperature and period of rice husk, pH of metal ion solution, initial metal concentration, adsorption temperature and contact time, and an adsorbent dosage on the removal of Zn(II) ions from aqueous solutions by thermally treated rice husk as running full factorial design is too expensive due to large number of experiments have to be performed.Moreover, higher order interactions are negligible.

Method
Rice husk was collected from a local rice mill in Selangor, Malaysia.All chemicals and reagents used were of analytical grade and purchased from Merck Malaysia.The ultrapure water was obtained from a Milli-Q water purification system (Millipore, Germany) for preparation of solutions and rinsing purposes throughout the experiments.

Preparation of Thermal Treated Rice Husk
The rice husk obtained was ground and washed with ultrapure water several times until constant pH was obtained.The powder was then dried in an oven at 105 O C for 24 h, sieved to obtain particles of less than 2 mm, heated in a furnace at selected temperature and period (Table 1) and then cooled in a desiccator and stored in a polyethylene bottle.

Preparation of Metal Solution
Solutions of Zn(II) ion were prepared by diluting appropriate amounts of 1000 mg/L of Zn(NO 3 ) 2 with ultrapure water (resistivity of 18.2 MΩ.cm).The pH was adjusted to the desired pH using 1.0 M of NaOH and HCl.

Adsorption Study
The mixture of 100 mL of Zn(NO 3 ) 2 solution and 1 g of thermally treated rice husk (RH) sample was agitated at 150 rpm for a particular period (Table 2).The mixture was filtered using a 0.45 µm cellulose nitrate membrane filter and the solution was then analyzed using an inductively coupled plasma optical emission spectrometry (ICP-OES) (Perkin Elmer, OPTIMA 5300 DV).All experiments were conducted in duplicates.

Results and Discussion
Table 2 shows the experimental results of the adsorption study of Zn(II) ions using factors in coded unit.The percentage removal of Zn(II) was calculated as shown below: (1) where Co is initial concentration and Ce is final concentration of Zn(II) solutions.Analysis of results was performed using MINITAB 17 for windows.
The determination of main effects and interaction between factors were employed.Percentage removal of Zn(II), which act as the response in this experiment was produced by a change in the level of a factor, heating temperature, heating period, pH, initial concentration of Zn(II), temperature of reaction, contact time, or adsorbent dosage varied from lower to higher level.The 2 7-1 factorial design was employed as codified model stated below: where A 0 is the global mean and A i is the regression coefficient related to the main factor effects and interactions.Table 3 displays the effects, regression coefficients, standard errors, t statistics and p-value, by which it may be concluded that, the removal efficiency increased as the effect of the factor is positive which changed from low to high.Otherwise, the removal efficiency decreased when the effect of the factor is negative.The results revealed that the effects of all main effects is positive except initial concentration as indicated by the effect of its is -10.09,showing that increasing initial concentration from low to high level (30 mg/L to 150 mg/L) cause decreased in the removal efficiency of Zn(II) ions.Interaction between x 1 x 7 shows that effect of heating temperature (x 1 ) depends of the level of adsorbent dosage (x 7 ).When the heating temperature increases from low to high, changing the adsorbent dosage from low level to high level will decrease the percentage of removal by about 7 units.The equation model for Zn(II) removal efficiency is stated below, by substituting the regression coefficients in Eq. ( 2): η = 69.75+ 10.95x 1 + 1.79x 2 + 16.66x 3 -5.04x 4 + 0.07x 5 + 4.10x 6 + 22.19x 7 + 0.48x 1 x 2 -0.73x 1 x 3 + 5.86x 1 x 4 -1.43x 1 x 5 -2.22x 1 x 6 -3.52x x 7 -1.78x 2 x 3 + 0.15x 2 x 4 -0.93x 2 x 5 + 1.25x 2 x 6 -0.81x 2 x 7 -3.87x 3 x 4 -0.13x 3 x 5 -1.56x 3 x 6 -10.96x 3 x 7 + 0.50x 4 x 5 + 1.46x 4 x 6 + 2.17x 4 x 7 -0.11x 5 x 6 + 1.00x 5 x 7 + 1.13x 6 x 7 (3) The relative importance of the individual and interaction effects is shown in Pareto chart, Figure 1.The chart shows the difference between the calculated effects from zero and it was determined based on student's t -test.
For a 95% confidence level, the corresponding critical t-value at 29 degrees of freedom is equal to 2.03 as indicated by the vertical line in the chart which shows the minimum statistically significant effect magnitude for that particular confidence level.Figure 1 shows Pareto chart of standardized effects on removal efficiency for Zn(II) of full model.The effects of adsorbent dosage, pH, heating temperature, initial concentration, contact time, interaction between pH and adsorbent dosage, heating temperature and initial concentration, and pH and initial concentration are significant as the standardized effects exceed the t-value, while heating time and temperature are insignificant.
Figure 1.Pareto chart of standardized effects on removal efficiency for Zn(II) -full model

Analysis of Variance (ANOVA)
The sum of squares used to estimate the each term up to second order polynomial model is shown in

2.00
The reduced model equation of Zn(II) removal efficiency is expressed as below: η = 69.51+ 10.71x 1 + 16.90x 3 -4.81x 4 + 3.87x 6 + 22.43x 7 + 6.09x 1 x 4 -4.11x 3 x 4 -11.19x 3 x (4) The results of ANOVA for the reduced model are presented in Table 6, only shows the significant factors that affecting the removal efficiency of Zn(II) with p-value less than 0.05.The significant factors were heating temperature of rice husk, pH, initial metal concentration, contact time, adsorbent dosage, and two ways interaction between heating temperature of rice husk and initial concentration, pH and initial concentration, and pH and adsorbent dosage.Various previous works have been reported regarding factorial design analysis to determine significant factors that affect the removal of heavy metals ions using different adsorbents.For example, the effects of pH, adsorbent dosage and initial silver ions concentration on adsorption silver ions from water onto montmorillonite was investigated by Geyikçi and Büyükgüngör (2013) using factorial design analysis at two levels.The results were analyzed statistically using the Student's t-test, analysis of variance, F-test and lack of fit to define most important process variables affecting the percentage silver ions adsorption.The most significant effect was found to be initial concentration.
An experimental design technique has been used to study the biosorption of Cr 3+ and Cr 6+ ions from water solutions, simulating typical tanning effluents using the factorial design 2 3 .The effects of three factors pH, temperature, and metal concentration were evaluated at two levels.The results were analyzed statistically and found that the most significant effect affecting Cr 3+ sorption was attributed to interaction between metal concentration and pH, while the most significant effect of Cr 6+ uptake was ascribed to metal concentration (Carmona et al., 2005).
Meski et al. ( 2011) used a factorial design to study the quantitative removal of zinc from aqueous solutions on synthesized hydroxyapatite.The factors investigated were initial zinc concentration in solution, adsorbent dosage, Ca/P molar ratio and calcination temperature of hydroxyapatite.Based on the analysis of variance and the factorial design of experiments, adsorbent dosage has a positive effect on the removal of zinc, whereas zinc concentration, Ca/P molar ratio and calcination temperature have a negative effect on the removal process.
The main effects plot is shown in Figure 3 which clearly indicated that there was huge difference in Zn(II) percentage removal between low and high levels for all significant factors (heating temperature of rice husk, pH, initial metal concentration, contact time, and adsorbent dosage) as indicated by the steep plots in the figure, which shows that the factors are highly significant.Figure 4 shows the interaction plot of effects and revealed that there were interactions between some factors affecting the percentage removal of Zn(II).The interactions occur between two different factors consist of heating temperature of rice husk and initial metal concentration, pH and initial metal concentration as well as pH and adsorbent dosage.The results showed that increasing each factor would increase percentage removal of Zn(II).
Figure 5 shows normal probability plot of residuals.The plot shows that the experimental points were reasonably distributed randomly around the straight line which suggests the data is normal distributed.Meanwhile, plot of fitted response variable versus standardized residuals is displayed in Figure 6.The plot is important not only for checking constant variances assumption but also to identify possible outlier.All points are in the range +2 to -2, except for a point run number 35 which slightly out of the range.

Effect of PH
Removal of heavy metals from aqueous solution was greatly influenced by the pH of the solution.In Table 3, the p-value for pH is less than 0.05 indicated that pH has a significant effect on removal of Zn(II) ions.The removal efficiency of Zn(II) ions increased by 33.79% when pH increased from 2 to 6, similar trend was reported by Elham et al. (2010) and Wahi et al. (2010).This may due to that at lower pH, more hydronium ions form and compete with Zn(II) ions for the bonding sites, hence, the removal of Zn(II) was lower at lower pH.As the pH increased, the concentration of hydronium ions decreased resulting in more availability of binding sites, therefore increased the removal efficiency of Zn(II) ions.

Effect of Heating Temperature of Rice Husk
In this study, thermal treated rice husk was prepared at two different heating temperatures i.e. 500 and 800 O C. The removal efficiency of Zn(II) increased by 21.42 % with increasing heating temperature.According to Taha et al., (2011), the increase in removal efficiency was influenced by higher porosity in the rice husk which obtained at higher temperature.

Effect of Contact Time
Removal efficiency of Zn(II) ions was enhanced when the contact time increased from 30 to 180 minutes.At 30 minutes, 65.65% of Zn(II) ions was removed while 73.38% was removed at 180 minutes.During the early stage of adsorption, there are a large number of vacant binding sites available to bind with Zn(II) ions.As the contact

Fitted Value
Standardized Residual time increased, the binding sites are occupied by Zn(II) ions and become more saturated until it reached equilibrium.

Effects of Adsorbent Dosage and Initial Zn(II) Concentration
Removal efficiency of Zn(II) increased with increasing adsorbent dosage.The removal of Zn(II) was 47.09% when the adsorbent dosage was 2 g, whilst, 91.94% of Zn(II) ions was removed when the adsorbent dosage increased to 10 g, with an increase of 44.85%.The removal efficiency of Zn(II) was greater when higher amount of adsorbent dosage used was due to the presence of large amount of binding sites available for adsorption.On the contrary, the removal efficiency of Zn(II) declined as the initial Zn(II) concentration increased.Increase in initial Zn(II) concentration from 30 to 150 mg/L resulted in decrease of Zn(II) removal efficiency by 9.62%.The decreasing removal efficiency of Zn(II) was due to saturation of all binding sites for adsorption at higher Zn(II) concentration.

Interaction Effects of Heating Temperature of Rice Husk and Initial Metal Concentration, PH and Initial Metal Concentration, PH and Adsorbent Dosage
Heating temperature of rice husk and initial metal concentration, pH and initial metal concentration as well as pH and adsorbent dosage were found to be the significant interaction effects that affect removal of Zn(II) ions.When the heating temperature of rice husk increased from 500 to 800 O C, removal efficiency of Zn(II) increased from 69.71 to 78.94% at 30 mg/L initial concentration and 47.90 to 81.51% at 150 mg/L.Similar trend was observed for the interaction between pH and initial concentration, where removal efficiency of Zn(II) increased from 53.32 to 95.33% from pH 2 to 6 at 30 mg/L initial concentration, while 51.92 to 77.49% was recorded at 150 mg/L, respectively.For the interaction between pH and adsorbent dosage, the increase in pH from 2 to 6 resulted in increase of Zn(II) removal efficiency from 19.00 to 75.18% for 2 g adsorbent dosage, and 86.24 to 97.65% for 10 g adsorbent dosage, respectively.

Conclusion
The results from this study indicated that thermal treated rice husk is effective to be used for removing Zn(II) ions from aqueous solutions.Out of seven factors that have been investigated on the percentage removal of Zn(II) ions, t only five factors were found to be significant.They are heating temperature of rice husk, pH, initial metal concentration, contact time, and adsorbent dosage.The removal efficiency of Zn(II) increases with increasing heating temperature, pH, contact time and adsorbent dosage, while increasing initial metal concentration results in decreasing removal efficiency of Zn(II) ions.The interaction between two different effects also significantly affect the percentage removal of Zn(II).They are interactions of heating temperature of rice husk and initial metal concentration, pH and initial metal concentration, and pH and adsorbent dosage.

Figure 3 .
Figure 3. Main effects plot for Zn(II) percentage removal

Table 1 .
Experimental range and levels of the factors used for the optimization of adsorption of Zn(II) ions

Table 2 .
Experimental results of the adsorption of Zn(II) ions using factors in coded

Table 4
. Fisher's F-ratios and p-values are also shown in the table.A particular effect in the model is statistically significant when p-value is less than 0.05, where p-value is the smallest level of significance leading to rejection of null hypothesis.

Table 4
shows that heating time (x 2 ) and temperature (x 5 ) are not significant as the p-value which are larger than 0.05, with the values of 0.335 and 0.969, respectively.Hence, these insignificant effects are neglected and removed.The effects, regression coefficients, standard error, t statistics and p-value were recalculated for the significant effects to refit the model and the results are presented in Table5.
Figure2shows the Pareto chart for reduced model and displays the student's t-test results.The results also indicated that heating temperature, pH, initial concentration, contact time, and adsorbent dosage are significantly affects the removal efficiency of Zn(II) ions. Figure 2. Pareto chart of standardized effects on the removal efficiency for Zn(II) -reduced model

Table 6 .
Analysis of variance -reduced model