Novel Au – Fe 3 O 4 NPs Loaded on Activated Carbon as a Green and High Efficient Adsorbent for Removal of Dyes from Aqueous Solutions : Application of Ultrasound Wave and Optimization

Present study is devoted on the development of the effective methodology for removal ultrasonic to simultaneous removal of Bismarck Brown (BB) and Thymol Blue (TB) onto Au–Fe3O4 nanoparticles loaded on activated carbon (Au–Fe3O4-NPs-AC) in aqueous solution. The Au–Fe3O4-NPs-AC were synthesized and characterization by different techniques such as XRD, FE-SEM and FT-IR. The process efficiency was confirmed through examination of variables like initial BB and TB concentration (X1, X2, respectively), pH (X3), adsorbent mass (X4) and sonication time (X5). The optimum operating parameters (OOP) were evaluated by Response Surface Methodology (RSM) based on central composite design (CCD) for prediction and simulation of removal of BB and TB dye, while analysis of variances (ANOVA) give the estimate of significance of experimental variables. The experimental equilibrium data were fitted to the conventional isotherm models and accordingly Langmuir isotherm has good applicability for the explanation of experimental data with maximum monolayer capacity (Qmax) of 80 and 76.38 mg g-1 in binary system for BB and TB, respectively. Kinetic evaluation of experimental data showed that the BB and TB adsorption processes followed well pseudo-second-order and intraparticle diffusion models. The results of this study will be useful for further development of magnetic nanostructures for environmental applications.


INTRODUCTION
Industries, such as leather, textile, dye stuffs, tanning, plastics, rubber and cosmetics generate huge amounts of synthetic dyes and pigments [1] and accordingly approximately 15% of total dyes release in to the wastewater.Disposal of colored textile wastewater in to the environment, without efficient treatment, imposes serious damages to aquatic life.Furthermore, some dyes (especially reactive dyes) are recalcitrant, non-biodegradable [2], stable to oxidizing agents [3], and toxic and mutagenic [4,5].AC despite of its porous structure and reasonable surface area supply low adsorption capacity and high cost which simply overcome these and process were accelerated loading, nanostructure material simply simply on its surface [6][7][8][9].This modification can significantly increase the removal percentage and its adsorption capacity due to the increase in surface area and number of reactive sites.Ultrasound irradiation is unique accelerator of chemical process due to the phenomenon of acoustic cavitation, formation, growth and collapse of micrometrical bubbles as consequence of propagation of a pressure wave through liquid [10,11].Secondary effect of ultrasound namely cavitation (nucleation, growth and transient collapse of tiny gas bubbles) improve the mass transfer through convection pathway that is emerged from physical phenomena is consequence of micro-streaming, micro-turbulence, acoustic (or shock) waves and micro jets without significant change in equilibrium characteristics of the adsorption/desorption system [12].Shock waves have the potential of creating microscopic turbulence within interfacial films surrounding nearby solid particles [13].Acoustic streaming induced by the sonication is the movement of the liquid is conversion of sound to the kinetic energy which is known as useful tool in intensifying the mass transfer process and breaking the affinity between adsorbate and adsorbent [14,15].Ghaedi et al. [16-19, 20, 21] studied dyes removal using nanomaterial-loaded activated carbon whith more efficiency than the AC due to improvement in the interface area and number of interface reactive atoms.Metallic nanostructures such as NiSe and ZnSe nanoparticles, [17] Pt nanoparticles, [18] ZnS:Cu nanoparticles [20] and Cd(OH)2 nanowires [21] easily loaded on AC via their binding to OH and COOH present at the interface of AC which causes high improvement in the interface area and adsorbate-adsorbent interaction.The main objectives of present work is development of novel approach based on simultaneous application of ultrasound and Au -Fe3O4-NPs-AC for safe and clean removal of dyes through optimization of various operating parameters using CCD.The adsorbent was characterized by field emission scanning electron microscopy (FESEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) and finally experimental equilibrium and kinetic of adsorption process were studied.

Materials and Methods
The chemicals including Bismarck Brown and Thymol Blue, activated carbon, FeCl3.6H2O,FeCl2.4H2O,NH3, sodium citrate, HAuCl4, NaOH and HCl with the highest purity available were purchased from Merck Co. (Darmstadt, Germany).An accurately weighted amount of dyes (BB and TB) were dissolved in deionized water to prepare 100 mg L -1 as to stock solution, while the working solutions were prepared by diluting this solution.A pH meter (Metrohm model-728) was adjusted for the pH measurement.Ultrasonic Homogenizer (UHP-400) (made in Ultrasonic Technology Development company-Iran) was used for the ultrasound-assisted.Dye concentrations were analyzed by measuring the absorbance using a Jasco UV-Vis spectrophotometer.All the instruments used in this work for the characterization purpose were fully described in our previous publications [22].

Ultrasound Assisted Adsorption Method
A batch process was applied to evaluate the adsorption performance of dyes from aqueous solutions in the presence of ultrasound.Adsorption experiments were carry out in a cylindrical glass vessel by addition of adsorbent (25 mg) into 50 mL of dyes solution at known concentrations (12 mg L -1 for both dye) and pH 6 with a known amount of adsorbent (0.025 g) and sonication time (5 min) at the room temperature.At the end of the adsorption experiments, the sample was immediately centrifuged and analyzed.The efficiency of dyes removal was determined at different experimental condition according to CCD method.The BB and TB removal (R%) and adsorbed dye amount (qe (mg g -1 )) in the aqueous solution by Au-Fe3O4 -NPs-AC were computed according to well-known [22].

Statistical Analysis
The experimental results of the central composite design (CCD) were analyzed using STATISTICA 10.0 software (State-Ease Inc., Minneapolis, USA) and the optimal while RSM was applied for modeling and the optimization of effects of concentration of initial BB and TB concentration (X1 , X2 respectively), PH (X3), amount of adsorbent (X4) and contact time (X5) on the ultrasonic-assisted adsorption of BB and TB by Au-Fe3O4NPs-AC.Five independent variables were set at five levels at which the R% of BB and TB the response was determined and shown in Table 1.Analysis of variance (ANOVA) was carried out to evaluate the important and effective terms for modeling the response based on F-test and p-values [23,24].

Preparation of Au-Fe3O4 -NPs-AC
At first step, The FeCl3.6H2O and FeCl2.4H2Osalts (molar ratio of Fe 3+ : Fe 2+ = 2:1) were dissolved in 200 mL pure water.Temperature of solution adjusted to 80⁰C.Then while stirring and under nitrogen gas atmosphere and NH3 (aq) (25%) were drupelet added to the solution of metallic ions.the solution was stirred for 2 h at 80⁰C under N2.Following this, the color of the bulk solution changed from orange to black.The black Fe3O4 NPs were separated by an external permanent magnet.The precipitates were washed several times with deionized water and ethanol.The synthesized Fe3O4 nanoparticles were dried in 70⁰C.At two-step, in order to loaded Fe3O4 nanoparticles on carbon active, 2g carbon active was added 200 ml ethanol and sonicated for 1 h.then 0.4g Fe3O4 nanoparticles was add to mixture, while mixture was sonicated.Subsequence, prepared mixture was stirred for 20 h.The obtained products were filtrated, washed and collected, then dried in a hot air oven at in 70 ⁰C for 12 h.Finally the dried precipitate was grinded into powder form.Three-step, The Au NPs were synthesized by reducing HAuCl4 with sodium citrate as reducing and stabilizer.Briefly, the 0.0625 mmol HAuCl4 was heated with vigorous stirring until boiling.Addition 10 mL solution of 40 g/L sodium citrate resulted in a continuous color change from colorless to red and stirring continued for an additional 10 min after the color change ceased.The heating source was then removed and stirring continued for 10 min.After the solution cooled to room temperature.Then decorated carbon active with Fe3O4 nanoparticles was add to solution of Au nanoparticles and was sonicated for 30 min.Subsequence, prepared mixture was stirred for 20h.The obtained products were filtrated, washed and collected, then dried in 85 ⁰C for 16 h.and finally used as an absorbent for adsorption experiments.

Characterization of Au-Fe3O4 -NPs-AC
The morphological studied by SEM are shown in (Figure 1a and b) denote uniform size, which in addition to its porous structure with high surface area making it suitable for the adsorption of target compounds.The structural analysis of the Au-Fe3O4NPs loaded on activated carbon by X-ray diffractometer (XRD) (Figure 2a).show peaks at 2θ = 35.2,41.1, 52.8, 57, 62.4 ° and 67.1 belong to the lattice planes of (311), (400), (422), (511), (440), and (222) and confirm the cubic structure of Au-Fe3O4NPs loaded on activated carbon, respectively.The observed XRD peaks (Figure 2a), indicate well-crystalized structure of the Au-Fe3O4NPs -AC characteristic peaks correspond to impurities such as Fe, Au, Fe(OH)2 and Fe(OH)3 and/or other compounds.Fourier transform infrared spectroscopic analysis of Au-Fe3O4 -AC (Figure 2b), shows absorption peak at 1715 cm -1 assign to the stretching vibration of carbonyl groups.The broad peaks at 1180 cm _1 originated from C-O stretching and to C-C bonds.The broad absorption band at 3456 indicates the presence of surface hydroxyl groups (O-H stretching) and the bands at low wave numbers (≤700) are related to vibrations of the Fe-O and Au-O bonds in adsorbent.These functional groups may act as anchoring sites for dye molecules as reported in the literature [25].

Analysis of Central Composite Design
Central composite design (CCD) under RSM was applied to design a systematic series of experiments (50 runs) in five level.RSM makes it possible to nonlinearly model the experimental data [26][27][28].The CCD avoids running unnecessary experiments while is useful for to understanding the synergies amongst the variables while supply value about interaction between the parameters.The analysis of variance (ANOVA) was performed to determine the level of significance of each term (Table 2).RBB = 99.018-6.7290X1+ 3.2493X2 -4.2080X3 + 5.3355X4 + 3.8140X5 + 4.5872X1X2 -5.1675X1X3 + 2.5679X1X4 -0.95248X1X5 + 3.6788X2X3 -0.50336X2X4 + 1.5375X2X5 + 4.7161X3X4 -0.13503X3X5 -1.0939X4X5 -6.7661X1 2 In this equation, the positive values correspond to each term indicate their positive effect on response and their negative values show a decrease in the response following raising their value.Analysis of variance (ANOVA) is a statistical method that partitions the total variation into its parts while each term has the different source of variation [29].The interaction effects are easily estimated and by usual ANOVA, based on results presented in (Table 2), while the calculation is based on a sum of the squares which applied for estimation of factors effect and Fisher's F-ratios and P-values.The model F-value of 18.880 and 18.987 implied it's statistically significant and reveal that only 0.01% chance that ''model F-value" could happen due to noise.The non-significant value of lack of fit (more than 0.05) represents validity of the quadratic model for explanation of experimental data of present study [30].

Response Surface Plots (3D Surfaces)
RSM was then applied for improving the optimization and evaluation of the relative significance and interaction of variables on adsorption processes.The three-dimensional surface response plots of this interaction are shown in Figure 3 and 4, which demonstrate the interaction of TB concentration and pH with sonication time.Maximum of dyes adsorption is achieved at high sonication times, which confirms the strong association between ultrasound and mass transfer.The results show that the initial adsorption rate is very rapid which emerged from the high available surface area and vacant sites s, while (Figure 3c-d) and (Figure 4a) increase in the adsorbent dose leads to a significant decrease in sonication time.On the other hand, the under study percentage dyes removal increases at higher adsorbent mass for particular sonication time.

Optimization of CCD by DF for Extraction Procedure
The profile for the desirable option with predicted values in the STATISTICA 10.0 software was used for the optimization of the process (Figure 5).Additional experiments at the derived optimal conditions were conducted in three replicates to validate the optimum point of the factors.The level of each process parameters, optimal response values and experimental results are shown in Figure 5 which optimum conditions were pH 6.0, sonication (5 min), adsorbent mass (0.025 g) and suggest that 12 mg L -1 of both dyes which supply maximum removal percentage.The optimum conditions were checked experimentally by running eleven experiments under the same conditions at 25 0 C. The results showed an average dye removal efficiency of 98.20%.This is a high degree of agreement between the experimental and prediction indicating that the central composite design could be used effectively for the evaluation and optimization of the effects of the adsorption independent variables on the removal efficiency of dyes from aqueous solution using Au-Fe3O4 -NPs-AC.

Adsorption Equilibrium Study
The experimental adsorption equilibrium data were evaluated for studying the mechanism of BB and TB dyes adsorption onto Au-Fe3O4 -NPs-AC using different models such as Langmuir, Freundlich, Temkin, Dubinin-Radushkevich isotherms [31,32,33] in their conventional linear form.Subsequently, their corresponding constants were evaluated from the slopes and intercepts of respective lines (Table 3).These models were applied at two dosages of adsorbent while other variables were kept in optimal condition (Table 3).Fitting the experimental data to these isotherm models and considering the higher values of correlation coefficients (R2 = 0.999) for both dye, it was concluded that the Langmuir isotherm is the best model to explain the BB and TB dyes adsorption onto Au-Fe3O4 -NPs-AC, which quantitatively describes the formation of a monolayer of adsorbate on the outer surface of the Au-Fe3O4 -NPs-AC.It also shows the equilibrium distribution of metal ions between the solid and liquid phase.

Kinetic Study
The kinetic of reactions is strongly influenced by several parameters related to the state of the solid and to the physicochemical conditions under which sorption occurs.To investigate the BB and TB adsorption onto the adsorbent was checked using different kinetics models such as pseudo-first order [34], pseudo-second-order [35], Elovich [36] and intraparticle diffusion [37] models were studied [38][39][40] and accordingly constant and information corresponding to each model is given in Table 4.The regression coefficient (R 2 ) from pseudo-second order rate equation for adsorbent is higher when compared to the pseudo-first order equation.These results revealed that the kinetics of both dyes onto adsorbent exhibited best fit to the pseudo-second order equation for BB and TB dye by Au-Fe3O4 -NPs-AC.

Comparison of Various Adsorbent
The performance of the proposed method has been compared with other adsorbents (Table 5).The adsorption capacity and contact time for present work are superior for other adsorbents BB and TB removal [41][42][43].The results indicated that the ultrasound-assisted removal method has good ability to improve the efficiency of dyes removal.The ultrasonic-assisted enhancement of dye removal could be attributed to the high-pressure shock wave sand high speed microjets during the violent collapse of cavitations bubbles.

CONCLUSION
In the present study, Au -Fe3O4 -NP-AC were produced and tested as adsorbents for the removal of BB and TB dyes from aqueous samples.CCD was applied to evaluate the interactive effects of adsorption variables and optimize the adsorption process.The RSM combined with CCD suggest best operational conditions as follow: 5 min sonication, 0.025 g Au-Fe3O4 -NPs-AC, 12 mgL -1 for both dye and PH of 6. Langmuir isotherm gave a better fit to adsorption isotherms than Freundlich, Temkin and D-R isotherms using linear and nonlinear methods for prediction removal of dyes.The Langmuir is the best model for fitting experimental data, maximum adsorption capacity due to its higher R 2 value with 80.0, 76.3 mg g _ 1 for BB and TB dyes, respectively.The data indicate that the adsorption kinetics follow the pseudo-second-order rate in addition to inter particle diffusion for all analytes in binary system.

Figure 1 .Figure 2 .
Figure 1.SEM image of (a) Fe3O4 -NPs-AC and (b) Au-Fe3O4 -NPs-AC Figure 4a-b indicate the effect of pH with initial BB concentration, which show that pH has the negative correlation with removal percentage and higher pH led to the achievement of lower removal percentage.At low initial pH, protonation of the adsorbent functional groups led to a generation of positive charge and appearance of the strong attractive forces between the anionic dye molecule and adsorbent surface (increase in removal percentage).

Figure 5 .
Figure 5. Profiles for predicated values and esirability function for removal percentage of BB and TB.Dashed line indicates current values after optimization

Table 1 .
Experimental factors, levels and matrix of CCD

Table 1 (
cont).Experimental factors, levels and matrix of CCD

Table 2 .
Analysis of variance (ANOVA) for R% of TB and BB

Table 3 .
Various isotherm constants and their correlation coefficients calculated for the adsorption of BB and TB onto Au-Fe3O4 -NPs-AC

Table 4 .
Various Kinetic constants and their correlation coefficients calculated for the adsorption of BB and TB onto Au-Fe3O4 -NPs-AC

Table 5 .
Comparison for the removal of dyes by different methods and adsorbents