Statistical analysis of the removal of Chromium ( VI ) by Iron Oxide Nanoparticle ( Fe 3 O 4 )

Investigation of the adsorption of Chromium (VI) on Fe3O4 is carried out using batch scale experiments according to statistical design using a software program minitab17 (Box-Behnken design). Experiments were carried out as per Box-Behnken design with four input parameters such as pH (2-8), initial concentration (50–150mg/L), adsorbent dosage (0.05–0.3 g) and time of adsorption (10–60min). The better conditions were showed at pH: 2; contact time: 60 min; chromium concentration: 50 mg/L and magnetite dosage: 0.3 g for maximum Chromium (VI) removal of (98.95%) with an error of 1.08%. The three models (Freundlich, Langmuir, and Temkin) were fitted to experimental data, Langmuir isotherm has better described the adsorption process. Whereas, the kinetics results of the adsorption process obeyed the pseudo-second-order. Keyword: Chromium (VI), Adsorption, Design of experiments, Iron Oxide Nanoparticle (Fe3O4). مورك ةلازلإ يئاصحلإا ليلحتلا يساذسلا ةطساوب ةدام ونانلا ةزاملا ) ذيسكوأ )ذيذحلا رباص ىوجن ذيجم ءاعد يجان ذمحم سْخكد ذعاسه راخسا تبلاط شيخسجاه يلك ت تسذٌِلا عهاج/ ت داذغب يلك ت تسذٌِلا عهاج/ ت داذغب ةصلاخلا مّشكلا صاضخها يف قيقحخلا نخي يساذسلا ذيذحلا ذيسكّأ ىلع ( Fe3O4 ( يعفذلا ظوٌلا بساجح ماذخخساب اِب ) batch experiment ) نيوصخلل اقفّ يئاصحلإا ماذخخساب (يبْساح جهاًشب Minitab17 ) (Box-Behnken design) . جيشجا نيوصخلل اقفّ بساجخلا box-Behnken تجسد لثه ثلاخذوك لهاْع تعبسا عه ( تضْوحلا 2-8 ) , ( يئاذخبلاا ضيكشح 50-150 mg/l ) , ( ٍصاولا ةداولا تعشج 0.05-0.3 g ) , ( صاضخهلاا يه جقْلاّ (10-60 min . ىْكخل ىلثولا فّشظلا ىا ذيفاّ 2 تجسد تضْوحلا 0.3 g َعشج عه ٍصاولا ةداولا يه 50 mg/l يهصّ يئاذخبلاا ضيكشح 1 hr فّشظلا ٍزُ يف تلاصلإا جًاكّ ، 98.95% تبسً عه اطخ ليلق ت .1.08% ثلايدْه ماذخخساب جئاخٌلا ليلحح نح (Freundlich, Langmuir and Temkin) , ليدْه شياوكًلا لثوي لضفأ فصّ تيلوعل صاضخهلاا . ييح يف تيًاثلا تجسذلا ليدْه ىلع داوخعلااب صاضخهلاا تيكشح تساسد نح لهاعه ىلعا جلثه اًِلأ . طابحسا تاملكلا ةيسيئرلا : لا مّشك يساذسلا ، لاا صاضخه ، سكّا ًْاٌلا ةداه ،بساجخلا نيوصح ي ذيذحلا ذ . Journal of Engineering Volume 24 February 2018 Number 2 63 1.INTRODUCTION As human needs increase and civilization changes, more and more finished products of different types are required. A large number of industries are born and grown in every country and industry plays an important role in economic development of the world. It improves the economic welfare of citizens and supplies the material goods they consume. The way in which society will develop in the future is largely dependent on how the growth which industry generates is distributed. The industry is also a major consumer of natural resources and a major contributor to the overall pollution load. Process waste streams contain heavy metals at concentrations exceeding the local discharge limits, Belachew, 2015. Heavy metals are very important that are used in several manufacturing processes such as electroplating industry, petrochemical industries, paint, pigments, sugar mills, distilleries, leather processing industries, paper mill, agrochemicals, mining, plumbing, ammunition, fuel additives, use in pesticides manufacturing industries, pharmaceutical industries, X-ray shielding, crystal glass production and PVC plastics. All these industries effluent have a lot of heavy metals that have to be treated before being discharged to the environment, Mohammed, 2014. Metallic elements have been excessively released into the environment due to rapid industrialization and have created a major global concern, Ngah, and Hanafiah, 2008. Hexavalent chromium, Cr(VI) is highly mobile and is considered acutely toxic, carcinogenic and mutagenic to living organisms, and hence more hazardous than other heavy metals, Gupta, and Babu, 2010. Contact with chromium can result in severe health problems ranging from simple skin irritation to lung carcinoma, Shen, et al., 2009, therefore it should not be present in the leather fabrics. Hexavalent chromium usually exists in wastewater as oxyanions such as chromate (CrO42ˉ) and dichromate (Cr2O72ˉ) and does not precipitate easily using conventional precipitation methods, Amin, et al., 2010. Different technologies have been used for removal of chromium compound from an aquatic system which includes chemical precipitation, ion exchange, and adsorption, use of membrane technology, evaporation recovery and reverse osmosis. However, the conventional treatment technologies require expensive equipment, high energy and generate enormous quantity of sludge. Adsorption processes are one of the most important methods for metals’ removal and attract much attention because of inexpensive, efficient and simple methods, Aftabtalab, and Sadabadi, 2015. The use of magnetic nanoparticles for separation and treatment of waste water is new methodology that is faster and simpler. Magnetite nanoparticles (Fe3O4) have been widely studied because of structural and functional elements have various novel applications, Yeary, et al., 2005. Magnetism is a unique physical property that independently helps in water purification by influencing the physical properties of contaminants in water. Adsorption process combined with magnetic separation has therefore been used extensively in environmental cleanup and water treatment. Iron oxide NMs are promising for industrial scale wastewater treatment, due to their little cost, strong adsorption capacity, easy separation and enhanced stability, Xu, et al., 2012. Magnetite nanoparticles received considerable attention not only in the fields of medical applications, including radiofrequency hyperthermia, photo magnetics, magnetic resonance imaging (MRI), medical diagnostics, cancer therapy, but also in the field of waste water treatment, El Ghandoor, et al., 2012, and Aftabtalab, et al., 2013. Investigating the process parameters and modeling its response is of vital or essential part of a process analysis. For controlling the adsorption independent variables, the factorial experimental design method can be used because it reduces the number of experiments, material Journal of Engineering Volume 24 February 2018 Number 2 64 resources and time by simple statistical design of experiments, Chilab, 2016. In this research Chromium concentration, time, adsorbent loading and pH are selected as most important process factors to be study. Models based on regressions, are used in a constrained optimization to get the best fit of experimental data and optimum conditions for highest removal efficiency. Using classical method to optimize the parameters could be made by changing one variable but keeping all factors constant in the same time. This may be effective in some cases, but it needs extra time, material and large number of trials. Also, the combined effect of factors cannot be fixed and due to all these limitations, analysis and investigation can be efficiently made using statistical experimental design. This paper describes the use of magnetite nanoparticles Fe3O4 for removal of Chromium (VI) from aqueous solutions. The adsorption of Chromium (VI) was investigated as a function of pH, contact time, concentration of chromium (VI) and Fe3O4 dose by using Response Surface Methodology (RSM) using Minitab software. Isotherms and kinetics studies had been performed to describe the process 2. EXPERIMENTAL WORK 2.1 Materials and Methods 2.1.1 Adsorbate Potassium dichromate supplied by SIGMA Aldrich, was chosen in this study. Chemical and physical properties of potassium dichromate are shown in Table 1. 2.1.2 Adsorbent Prepared Nano-Magnetite (Fe3O4) was used as adsorbent. Magnetite nanoparticles were synthesized according to, Parsons, et al., 2014. One liter of a 30 mM solution of FeCl2 was prepared from deionized water and FeCl2.4H2O. The solution was then titrated with NaOH solution at a rate about 1mL/min. The solution kept on constant mixing to attain a well-mixed blend. Then the Fe(OH)x was put in the Teflon container and heated in a programmable electrical furnace. The particles were heated at constant temperature of 100oC for 60 min and consequently cooled to room temperature and then filtered using Buckner funnel with the aid of a vacuum pump and washed twice with deionized water and then dried in oven for 24 hours at 100 °C. Fig.1 shows XRD pattern of prepared Fe3O4. Table 2 shows the characterization of nano Fe3O4. 2.1.3 Procedure of adsorption The ability of iron oxide magnetite to remove chromium (VI) solution was determined under batch mode conditions. 50 ml of chromium (VI) solutions with different initial concentrations (50,100,150 mg/l) were mixed with Fe3O4 at adsorbent dose (0.05, 0.175, 0.3 g). The mixture was added to 50 ml Erlenmeyer flasks, and the flask were shaken at 120 rpm and 25±5 C. After that, the samples were separated by using centrifuge for 10 min at 2000 rpm and the concentration of chromium(VI) were measured using Atomic Absorption Spectrophotometer, (AAS) at wave length (375.9 nm) for chromium(VI). The uptake of chromium (VI) at equilibrium, qe (mg/g) was calculated by the following expression. ( ) (1) Journal of Engineering Volume 24 February 2018 Number 2 65 3. EXPERIMENTAL DESIGNUSING BOX-BEHNKEN DESIGN A standard response surface methodology (RSM) design known as Box-Behnken experimental design was used to study the parameter for adsorption of chromium(VI) on Fe3O4, and was used to create a set of designed experiments by MINITAB software (version 17). In this work, four independent variables were study for the chromium (VI) adsorption; x1= pH, and x2=concentration of chromium (VI) (ppm), x3= Fe3O4 dosage (g), x4= time (min). Box-Behnken experimental design is applied to investigate and validate adsorption process parameters, which affect the removal of Cr (VI) ions onto magnetic nano-adsorbents. Table 3 represents a 27-trial experimental design, where each variable was tested in three different coded levels: low (-1), middle (0) and high (+1). The coded values correspond toX1: -1(2), 0(5), +1(8), X2: -1(50 mgl-1), 0(100 mgl-1), +1(150

resources and time by simple statistical design of experiments, Chilab, 2016.In this research Chromium concentration, time, adsorbent loading and pH are selected as most important process factors to be study.Models based on regressions, are used in a constrained optimization to get the best fit of experimental data and optimum conditions for highest removal efficiency.Using classical method to optimize the parameters could be made by changing one variable but keeping all factors constant in the same time.This may be effective in some cases, but it needs extra time, material and large number of trials.Also, the combined effect of factors cannot be fixed and due to all these limitations, analysis and investigation can be efficiently made using statistical experimental design.
This paper describes the use of magnetite nanoparticles Fe 3 O 4 for removal of Chromium (VI) from aqueous solutions.The adsorption of Chromium (VI) was investigated as a function of pH, contact time, concentration of chromium (VI) and Fe 3 O 4 dose by using Response Surface Methodology (RSM) using Minitab software.Isotherms and kinetics studies had been performed to describe the process

Adsorbate
Potassium dichromate supplied by SIGMA Aldrich, was chosen in this study.Chemical and physical properties of potassium dichromate are shown in Table 1.

Adsorbent
Prepared Nano-Magnetite (Fe 3 O 4 ) was used as adsorbent.Magnetite nanoparticles were synthesized according to, Parsons, et al., 2014.One liter of a 30 mM solution of FeCl 2 was prepared from deionized water and FeCl 2 .4H 2 O.The solution was then titrated with NaOH solution at a rate about 1mL/min.The solution kept on constant mixing to attain a well-mixed blend.Then the Fe(OH)x was put in the Teflon container and heated in a programmable electrical furnace.The particles were heated at constant temperature of 100ºC for 60 min and consequently cooled to room temperature and then filtered using Buckner funnel with the aid of a vacuum pump and washed twice with deionized water and then dried in oven for 24 hours at 100 °C.Fig. 1 shows XRD pattern of prepared Fe 3 O 4 .Table 2 shows the characterization of nano Fe 3 O 4 .

Procedure of adsorption
The ability of iron oxide magnetite to remove chromium (VI) solution was determined under batch mode conditions.50 ml of chromium (VI) solutions with different initial concentrations (50,100,150 mg/l) were mixed with Fe 3 O 4 at adsorbent dose (0.05, 0.175, 0.3 g).The mixture was added to 50 ml Erlenmeyer flasks, and the flask were shaken at 120 rpm and 25±5 C.After that, the samples were separated by using centrifuge for 10 min at 2000 rpm and the concentration of chromium(VI) were measured using Atomic Absorption Spectrophotometer, (AAS) at wave length (375.9 nm) for chromium(VI).The uptake of chromium (VI) at equilibrium, qe (mg/g) was calculated by the following expression.(1)

EXPERIMENTAL DESIGNUSING BOX-BEHNKEN DESIGN
A standard response surface methodology (RSM) design known as Box-Behnken experimental design was used to study the parameter for adsorption of chromium(VI) on Fe 3 O 4 , and was used to create a set of designed experiments by MINITAB software (version 17).In this work, four independent variables were study for the chromium (VI) adsorption; x 1 = pH, and x 2 =concentration of chromium (VI) (ppm), x 3 = Fe 3 O 4 dosage (g), x 4 = time (min).Box-Behnken experimental design is applied to investigate and validate adsorption process parameters, which affect the removal of Cr (VI) ions onto magnetic nano-adsorbents.Table 3 represents a 27-trial experimental design, where each variable was tested in three different coded levels: low (-1), middle (0) and high (+1).The coded values correspond toX1: -1(2), 0(5), +1(8), X2: -1(50 mgl-1), 0(100 mgl-1), +1(150 mgl-1) for X3: -1(0.05g),0(0.175g), +1(0.3g), and for X4: -1(10), 0(35), +1(60).The all set of experiments was performed in triplicate and mean response was used for analysis.A second order polynomial equation was then fitted to the data using the Minitab for the regression and graphical analysis of the data obtained.The reliability of the fitted model was justified through and the coefficient of R², The p-value of less than 0.05 indicates that the model is significant for 95% probability, Ahmad, and Hameed, 2009.

4.1Adsorption isotherms
The adsorption isotherm is the most important information, where the equilibrium amount adsorbed qe (mg/g) is related to the equilibrium concentration Ce (mg/L) indicating how adsorbate molecules are distributed between the liquid phase and solid phase as the process reach equilibrium, Huo, et al., 1996.The adsorption process was performed at initial concentrations (50, 100, 150 ppm).Three most common isotherm equations namely, Langmuir, Freundlich and Temkin were tested in this work to describe the equilibrium adsorption.Table 4 lists the four isotherms along with the constants of the linear plots studied in this work.
By examining Table 4, the results gave a different linearity calculated by the correlation coefficient (R 2 ) which ranges between 0.994 and 0.997, so, based on R 2 , the adsorption of Chromium(VI) is best fitted in the Langmuir isotherm indicating the homogeneous nature, Yuan, et al., 2010, and Bhaumik, et al., 2011.
The values of (1/n) ranging between 0 and 1 in Freundlich isotherm is an indication of surface heterogeneity, and the closer to zero, the more heterogeneous is the surface.It is clear from Table3 that value of 1/n was less than 1 confirming the heterogeneity of the surface of adsorbent.

Kinetics Analysis
In this section, the adsorption rate is investigated.The pseudo first-order and pseudo second order models were adapted to the test experiment data.The results, as shown in

Modeling Using Minitab Software
In this study, the relationship between the response and variable is examined by regression.The predicted removal efficiency of Chromium (VI) or the response of surface methodology (Y) is given in Eq. ( 2).Y = 68.88+ 2.760 X 1 + 0.0723 X 2 + 34.4 X 3 + 0.5115 X4-0.3778X 1 2 -0.000151X 2 2 -43.6 X 3 2 -0.002169X 4 2 -0.00982X 1 X 2 -1.780X 1 X 3 -0.01607X 1 X 4 -0.0276X 2 X 3 -0.000550X 2 X 4 + 0.070 X 3 X 4 ( 2) The predicted values versus the experimental values for Chromium (VI) removal can be seen in Fig. 7 Results reveal that the developed model successfully captured the relation between the variables to the responses within the range of the studied variables.

Effects of Variables and Their Interactions
A hypothesis was assumed that this relationship is statistically significant, for a pvalue coefficient.This probability was set as 95 % of the observation is significant or 5% is rejected.The Final equation is: It can be deduced from

Main Effects and Interaction Effects Plot for Chromium (VI) Removal Efficiency
The main effects plot Fig. 8 was developed based on Table 5 in which the un-coded values of the test variables and the Chromium (VI) removal efficiencies obtained experimentally are shown.The main effects of all parameters on percentage chromium (VI) removal were determined and shown in Fig. 8.The effect of pH on Chromium (VI) removal was studied in the range of pH 2-8.It is apparent from the Fig. 8 that efficiency removal was increased with decreased pH.At high pH values electrostatic repulsion between Chromium (VI) and Fe 3 O 4 might result lower removal efficiencies whereas at lower pH conditions Fe 3 O 4 electrostatic attraction between Fe 3 O 4 and acidic Chromium (VI) may be resulted in higher efficiencies, Yuan, et al., 2010.The percentage removal of Chromium (VI) decreased slightly up to the concentration of 150ppm as shown in figure.The reason for decreasing the removal in higher concentration media is by increasing the initial concentration from 50 ppm to 150 ppm, the removal decreased.On this basis, it can be explored that, for the given adsorbent dose, the available sites for adsorption become less at higher concentrations, and hence, the percentage adsorption of Cr (VI) decreases, Lasheen, et al., 2013.Percentage removal of chromium (VI) was increased with time up to 60 min.This might be due to the fact that increased time allowed the particles to reach equilibrium hence, removal percentage was increased, Parsons, et al., 2014.It is noticed from the Fig. 8 that efficiency removal of Chromium (VI) has increased significantly with increased adsorbent dosage.This means that the toxic ions can be removed effectively from the contaminated water with the proper amount of adsorbent, due to the fact that intensely activated surface of Fe 3 O 4 nanoparticles substantially possess more adsorption sites available for metal ions uptake from the solution.This is in accordance with the reports available on other Chromium (VI) removal processes, Shen, et al., 2009 Fig. 9 is the surface plot showing the combined effect of both pH and concentration using (0.3g) of Fe 3 O 4 after 1hour.From Fig. 9 it can be seen an increase in the percentage Chromium (VI) removal can be observed with decrease in pH from 8 to 2 as well as there is an decrease in concentration from about 50 to 150 ppm.The dramatic increase of the Chromium (VI) uptake with the decrease of pH values was mainly due to that higher pH values made the surface of magnetite more negatively charged, which greatly enhanced the electrostatic repulsion between magnetite and Chromium (VI) anions, leading to a release of the adsorbed Chromium (VI) species from the magnetite surface, Bhaumik, et al., 2011.The reason for decreasing the removal in higher concentration media is by increasing the initial concentration from 50 ppm to 150 ppm, the removal decreased.On this basis, it can be shown that, for the given adsorbent dose, the available sites for adsorption become less at higher concentrations, and hence, the percentage adsorption of Cr (VI) decreases, Ilankoon, 2014.Fig. 10 shows the combined effect of pH and time for Chromium (VI) removal on Fe 3 O 4 .From these figures it can be seen an increase in the percentage Chromium (VI) removal can be observed with decrease in pH from 8 to 2 as well as there is an increase in time from about 10 to 60 min, contact time and removal rate are important factors for selection a design of economical adsorbent for wastewater treatment.The removal efficiency increased with the contact time increased, the contact time was reach to saturation within 60 min for adsorbents that mean the saturation time was independent of adsorbent nature.This relation is found significant in similar work of, Srivastava, and Sharma, 2014.

Optimization of Operating Parameters
So, the optimum conditions were one of the objectives of the experimental design so that the high Chromium (VI) removal can be achieved.The optimum conditions of chromium (VI) removal by Fe 3 O 4 was achieved at pH, weight of Fe 3 O 4 , initial Chromium (VI) concentration and time of 2, 0.3g, 50 ppm and 1hr, respectively.At these conditions, chromium (VI) removal was 98.95%.Table 6 shows the model validation were the predicted and experimental values of the responses for chromium (VI) are presented.The predicted and experimental values obtained at optimum conditions was 100% and 98.95%, showing good agreement between the experimental values and predicted from the model, with relatively small error which was only 1.08%.Model desirability approaching unity and with low error value displays the applicability of the model towards the responses.Relatively small errors were obtained for the predicted and the actual values indicate that the models are suitable in predicting the responses efficiently.

CONCLUSIONS
Chromium (VI) was successfully removed from aqueous solutions onto Fe 3 O 4 nanoparticles.The obtained data for the adsorption is best fitted with the Langmuir isotherm.The adsorption complies with pseudo second order reaction.The statistical approach, response surface Box-Behnken experimental design was successfully employed for experimental design and analysis of results, to study the linear, quadratic and interaction effects between the variables and also to optimize those variables for maximum removal of Chromium (VI).
The experimental results showed, percentage Chromium(VI) removal in permeate increased with decreasing in pH from 8 to 2, increasing time from 10 to 60 min, decreasing initial concentration from 150 to 50 mg/L , increasing adsorbent dosage from 0.05 to 0.3 g.
Appropriate regression model was developed for predicting the removal for Chromium (VI) and satisfactorily predicted the experimental values.Graphical surface response plots were used to obtain the optimum points.The best conditions for maximum Chromium (VI) removal (98.95%) were obtained at pH: 2; Chromium (VI) concentration: 50mg/L; contact time: 1hr and Fe 3 O 4 dosage: 0.3g.Optimum values were confirmed by validation experiments.

Figure 7 .
Figure 7. Relationship between predicted and experimental data for Chromium (VI) removal.

Figure 10 .
Figure 10.The effect of pH and Time on the removal of Chromium (VI).

Table 4 ,
indicate that the adsorption of Chromium (VI) perfectly complies with pseudo 2 nd order reaction based on the correlation coefficient.Similar results were reported Bhaumik

Table 2 .
The characterization of nano Fe 3 O 4 .

Table 3 .
Design of Experiments.

Table 6 .
Model validation for AFD removal by Fe 3 O 4 .