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Background
Every day, many harmful organic pollutants such as dyes are discharged into the environment by industries (1). Azo dyes are carcinogenic and mutagenic compounds and constitute the largest class of dyes. Colored wastewater discharged into receiving waters leads to environmental contamination such as reduced sunlight penetration and toxicity to the aquatic life (2,3). The orange acid 7 (AO7) is an acidic dye, soluble in water and has an azo group. Because azo dyes are not easily degraded, they must be removed prior to discharged into the water (4,5).
So far, different methods have been used to remove these pollutants from industrial wastewater. Today, advanced oxidation processes (AOPs) based on nanoparticles, due to high efficiency and no secondary contamination, to remove a variety of contaminants such as dyes have been used (6). AOPs are based on generation of reactive radical species such as sulfate, superoxide or hydroxyl. Hydroxyl radicals (OH˙) with high oxidizing and non-selective oxidizing properties are the key oxidizing agent of advanced oxidation processes (7). Sulfate radical-based AOPs are considered as one of the most advanced oxidation processes due to their high oxidizing properties and are seriously considered for the removal of resistant and biodegradable organic pollutants (8,9). Besides the high oxidation–reduction potential (2.6 V), sulfate radicals can also be attractive for application in chemical oxidation due to their high solubility in water, high stability and relative low cost (10,11).
Proxymonosulfate (HSO5-, PMS) is a non-selective anion, soluble and the strongest oxidant in the Peroxygen family. It has unique properties including higher standard reduction potential than hydroxyl radicals and less dependence on natural materials. Under atmospheric conditions, the oxidation of proxymonosulphate does not have much effect on organic pollutants. The activation of PMS can be done by heat, light, or certain metal ions (Fe₂⁺, Co₂⁺,…) (12). According to previous studies, hydrogen peroxide (H₂O₂) is an oxidizing agent that acts as a source for reactive oxygen species and can lead to oxidizing of various organic pollutants. H₂O₂ decomposes slowly at room temperature, so H₂O₂ activation is necessary to produce oxidizing hydroxyl radicals (13-15).
So far, the effectiveness of various adsorbents including active carbon, bentonite, lignin, quartz, kaolinite and other adsorbents has been evaluated as a catalyst bed for the removal of pollutants (7). Powdered activated carbon (PAC) as the most common and most effective adsorbent is known for effective removal of dyes due to high absorption capacity, high surface area and low cost (16). The main problem in the use of nanocomposites or nanoparticles is their separation from the solution due to their small size. The separation of nanocomposite carbon from aqueous solutions using methods such as filtration and centrifugation is difficult, time-consuming and costly. Therefore, the dispersion, and production of secondary pollution are the main problems of these systems (17). Magnetic separation is a method to facilitate the separation of these nanocomposites (18).
Magnetic nanoparticles coated on activated carbon were used for the removal of different pollutants (19,20). Among the magnetic nanoparticles, spherical ferrite structures (AFe₂O₄; A is a divalent  metal cation such as Fe, Cu, Ni, Co, Mg, Mn) has been paid much attention to synthesize magnetic nanocomposites in order to improve the catalyst regeneration (21,22). Cobalt ferrite (CoFe₂O₄) nanoparticles had properties such as excellent chemical stability, rapid separation and moderate saturation magnetization (18,22-23).
The catalyst used in this study is iron oxide and cobalt nanoparticles (CoFe₂O₄) stabilized on powdered activated carbon that in the presence of peroxymonosulfate and hydrogen peroxide, it can produce sulfate and possibly hydroxyl radicals, and thus can lead to the degradation and removal of organic compounds from the wastewater sample.
Aims of the study:
The aim of this study is the investigation of CoFe₂O₄@PAC nanocomposite efficiency in the presence of peroxymonosulfate (PMS) and hydrogen peroxide (H₂O₂) for the removal of acid orange 7 dye from aqueous solutions.
 

Materials & Methods

Materials
The acid orange 7 (AO7) was purchased from Alvan sabet company, Iran. The characteristics of AO7 dye is shown in Table 1 (24, 25). All the other reagents were of analytical grade that were used without further purification. FeCl₂.4H₂O, FeCl₃.6H₂O, Co(NO₃)₂.6H₂O, ammonia hydroxide, and nitric acid were purchased from Merck and used to synthesis the CoFe₂O₄@PAC nanocomposite.
Table 1) Characterization of Acid Orange 7 dye

Chemical structure	Molecular formula	Molar mass	lmax
	C16H11N2NaO4S	350.32 (g/mol)	485 (nm)

Synthesis of nanocomposite
The catalyst used in this study (CoFe₂O₄) that was supported on the modified powdered activated carbon (PAC), was synthesized by co-precipitation method. First, certain amount of activated carbon powder (98% purity, Merck) was dissolved in nitric acid 65%. Then, the solution was stirred at 80 ˚C for 3 hours. The sample was washed by ultrapure water and then was dried in an oven at 80 ˚C overnight.
5.652 g FeCl₂.4H₂O, 3.841 g FeCl₃.6H₂O and 4.133 g Co(NO₃)₂.6H₂O (Fe(III):Fe(II):Co(II)=1:2:1) were added to 80 mL of ultrapure water. 5 g modified activated carbon powder (prepared in the previous step) was added to the solution. With stirring, the solution was mixed for 0.5 h with nitrogen bubbling at 90 ˚C. Then, 40 mL ammonia hydroxide (28%) was added drop wisely at 30 min to make solution pH 10-11. Nitrogen gas flow stopped and to ensure the completion of the reaction, the mixture was stirred for one hour. The sample was then washed by ethanol and ultrapure water for several times (pH=7). Then was dried at 100 ˚C for 8h. It was calcinated in the furnace without oxygen (Nabertherm High-Temperature Tube Furnace) at temperature rate of 5 ˚C/min at 500 ˚C for 4 h (26). Finally, the black solid was formed (CoFe₂O₄@PAC, Figure 1). For the use in advanced oxidation process, it was stored inside the desiccator. As shown in the inset of Figure 2d, the magnetic separation performance of the composite was tested by placing a magnet (~4000 Gauss) near the glass bottle. The product was attracted toward the magnet in a short period, demonstrating its high magnetic sensitivity. These results indicate that the composite can be potentially used as a magnetic adsorbent to remove dye contaminants from aqueous solution in order to avoid the secondary pollution.
  Figure 1) The synthesized catalyst and its magnetic behavior in the vicinity of a strong magnet
Characterization of nanocomposite
To study the surface morphologies of nanocomposite, scanning electron microscopy (SEM) was done. In order to the crystal structural characterization of nanoparticles, X-ray diffraction analysis (XRD) was performed by X-ray diffractometer. Energy dispersive X-ray spectroscopy (EDS) analysis, using an EDX chemical microanalysis module, was used to determine the elemental composition of the samples. The BET (Brunauer–Emmett–Teller) surface area was determined by Micromeritics analyzer under liquid nitrogen environment.
Experiments procedure
Stock solution was prepared by dissolving a known quantity of the dye in distilled water; this solution was diluted to the required initial concentrations (10-250 mg/l). In each stage, the certain amount of nanocomposite added to 50 mL of AO7 solution in a 100 ml Erlenmeyer flask. Then, H₂O₂ and PMS in certain values were added. The pH of the suspension was adjusted by 0.1 M HCl or NaOH solutions. Batch experiments were carried out using an orbital shaker at 250 rpm. At different time intervals, the solution was sampled and a powerful magnet was used for the magnetic separation of the absorbent from the solution. The residual AO7 dye solution concentration was determined using UV–Vis spectrophotometer at wavelength of 485 nm (25). The dye removal efficiency (%R) using nanocomposite was calculated using equation (1): 
                          (1)
C_t and C_o are the equilibrium and initial dye concentrations (mg/L), respectively (3).
The effect of the various parameters, contact time (0–120 min), initial dye concentration (10-250 mg/L), catalyst dosage (0.1-1.5 g/L), solution pH (3-11), PMS concentration (2.5-50 µM) and H₂O₂ concentration (1-15 mM) were investigated. All experiments were repeated three times.
Data analysis:
Data analysis has been done using Excel software.
 

Results

The characterization of nanocomposite
Figure 2a shows the SEM image of the CoFe₂O₄@PAC composite. It can be seen that nanocomposite is spherical (sphere shaped). The approximate crystallite size of nanoparticles (D) was calculated using Scherer’s equation (Eq. 2).
                                        (2)
Where, λ is the X-ray wavelength (1.540598 Å), β is the full-width at half-maximum, and Ɵ is the diffraction angle (27).  The approximate crystallite size of CoFe₂O₄@PAC was found in the range of 33.97-39.60 nm.
The XRD patterns of CoFe₂O₄@PAC, CoFe₂O₄ and PAC are presented in Figure 2b. The XRD patterns show the sharp peaks at 2θ values of 19.98, 30, 36.95, 38.21, 43.25, 56.65, 58.30, and 62 that correspond to 111, 220, 311, 222, 400, 422, 511 and 440 planes. Also the XRD pattern indicated that the CoFe₂O₄@PAC structure obtained by the process is crystalline. The XRD pattern in Figure 2b shows a peak at around 26◦ that correspond to the graphite (002) plane of the PAC (18).
Figure 2c shows energy dispersive X-ray spectroscopy (EDX) analysis. The EDX spectra confirmed the presence of Fe, Co, Ca, Cl, C, and O elements.
Physical and structural properties of Co-Fe₂O₄@PAC nanocomposite is presented in Table 2. The BET surface area of Co-Fe₂O₄@PAC was 595.6 m²/g. The pore volume of Co-Fe₂O₄@PAC nanocomposite was 0.316 cm³/g due to the presence of carbon that leads to the surface roughness (18).
 

(a)	(b)
(c)

Figure 2) Characterization of Co-Fe₂O₄@PAC nanocomposite
a) SEM image; b) XRD spectra pattern; c) EDX spectrum
Table 2) Physical and structural properties of Co-Fe₂O₄@PAC nanocamposite compared to Co-Fe₂O₄ and PAC

Sample	Color	Pore structure	Pore diameter (nm)	Pore volume (cm3/g)	Specific surface (m2/g)
Co-Fe2O4@PAC	black	micropore	3.31	0.316	595.6

 Effect of pH on dye degradation
pH is one of the effective parameters on the dye degradation by nanocomposite. The results of the pH effect on dye removal are presented in Figure 3a. As can be seen, with increasing of pH from 3 to 11, removal efficiency of AO7 dye by H₂O₂/Co-Fe₂O₄@PAC was decreased from 81.5% to 24.5%. According to the results, the removal efficiency of AO7 dye by PMS/Co-Fe₂O₄@PAC increased from 58.5% to 85.4% by increasing pH from 3 to 4; but the increasing of pH up to 4 led to a decrease in the removal efficiency.
The impact of pH also depends on the zeta potential of the catalyst (15,23). For determining pHzpc, 50 mL NaCl 0.01M was added to 10 beakers and pH was adjusted between 2-11 using HCl and NaOH. Then, 0.2 g nanocomposite was added to each beaker and stirred for 48h. The final pH was measured and the initial pH was drawn against the difference of the final pH and the initial pH. pHzpc was obtained from the confluence of the curve with pHzpc zero line that was equal to 6 (Figure 3b).

(a)

(b)
Figure 3) a) Effect of pH on catalytic degradation of AO7 dye; b) pHzpc
(Initial dye concentration: 50 mg/L, H₂O₂ concentration: 10 mM, PMS concentration: 25 μM, Temperature: 298 ºk, Time: 120 min, Catalyst dose: 1 g/L)
 
Effect of catalyst dosage on dye degradation
One of the effective parameters in the advanced oxidation processes is the catalyst dosage (Figure 4). The removal efficiency of AO7 dye in the presence of H₂O₂ increased from 3% to 78.5 % with increasing the catalyst dose from 0.1 to 1.5 g/L. When PMS was used, the removal efficiency increased from 38.3 % to 86.8 %.

Figure 4) Effect of catalyst dosage on catalytic degradation of AO7 dye
(Initial dye concentration: 50 mg/L, H₂O₂ concentration: 10 mM, PMS concentration: 25 μM, Temperature: 298 ºk, Time: 120 min, pH: 4)
Effect of oxidant concentration on dye degradation
The oxidant concentration (in the range of 1-15 mM for H₂O₂ and 2.5-50 µM for PMS) was also investigated. According to results, in the absence of H₂O₂ and PMS, the removal efficiency was 41% and 39.7%, respectively. With increasing the oxidant concentration, the removal efficiency increased. So that, by increasing of H₂O₂ concentration from 1 to 10 mM, the removal efficiency increased from 46.5% to 78.6% (Fig.5a). The dye degradation increased from 44.3% to 86.5% as PMS concentration increased from 2.5 to 25 µM (Fig.5b). With increasing the concentration of oxidants over the optimal amount, dye removal decreased. So that the removal efficiency was 78.5% in the concentration of 15 mM hydrogen peroxide. In the concentration of 50 µM PMS, dye removal was equal to 84.3%.

(a)

Figure 5) Effect of oxidant concentration [(a)H₂O₂, (b)PMS] on catalytic degradation of AO7 dye (Initial dye concentration: 50 mg/L, Catalyst dose: 1 g/L, Temperature: 298 ºk, Time: 120 min, pH: 4)
 
Effect of initial dye concentration on dye degradation
The other parameter examined in this study was the effect of initial dye concentration on the rate of decomposition. To survey this parameter, initial concentration between 10-250 mg/L was examined (Figure 6). As can be seen, the maximum degradation rate was achieved at initial concentration of 10 mg/L. The removal efficiency was 99.5% and 99.9% in the presence of H₂O₂ and PMS, respectively. By increasing initial dye concentration to 250 mg/L, the degradation rate decreased (39.7% for H₂O₂ and 43.7% for PMS).

Figure 6) Effect of initial dye concentration on catalytic degradation of AO7 dye
(H₂O₂ concentration: 10 mM, PMS concentration: 25 μM, Catalyst dose: 1 g/L, Temperature: 298 K, Time: 120 min, pH: 4)
Effect of reaction time on dye degradation
Effect of reaction time on catalytic degradation of AO7 dye is presented in Figure 7. As the time has increased, the removal efficiency has also increased. With a rise in contact time from 5 to 120 minutes, the removal efficiency in the presence of H₂O₂ and PMS increased from 8.4% to 78.5% and from 9.7% to 87%, respectively. By increasing the reaction time from 60 min to 120 min, no significant change was found in the dye removal efficiency. So that dye removal was 75% and 80.6% in the presence of H₂O₂ and PMS at 60 min, respectively. Thus, optimum reaction time was considered 60 min for next steps of the experiments.

 
Figure 7) Effect of reaction time on catalytic degradation of AO7 dye
(H₂O₂ concentration: 10 mM, PMS concentration: 25 μM, Catalyst dose: 1 g/L, Temperature: 298 ºk, Initial dye concentration: 50 mg/L, pH: 4)
Kinetic studies
To investigate the AO7 dye absorption behavior by the Co-Fe₂O₄@PAC nanocomposite, kinetic studies were done in the presence of both types of oxidants under optimum conditions. For analyzing experimental data, the first-second-order kinetic model can be used (Eq.3). Figure 8 shows pseudo-first-order kinetic for AO7 dye absorption by nanocomposite.
                                       (3)
Where C0 and C are dye concentration at t=0 and t=t (mg/L), respectively. k is the pseudo-first-order rate constant (min⁻¹) and t is reaction time (min) (14).
The kinetic parameters of AO7 dye absorption by the Co-Fe₂O₄@PAC nanocomposite are listed in Table 3. DT90 and t 1/2 are the required time for 90% removal of AO7 dye and half-life, respectively. The results show that the data follow a pseudo-first-order model with the maximum correlation coefficient (R²>0.99) for both of catalysts.

Figure 8) Degradation kinetic of AO7 dye by H₂O₂/Co-Fe₂O₄@PAC and PMS/Co-Fe₂O₄@PAC

 
Table 3) Kinetic parameters of AO7 dye absorption by the Co-Fe₂O₄@PAC nanocomposite

Nanocomposite	K (min -1)	R2	half-life (t1/2 or DT50) (min)	DT90 (min)
H2O2/CoFe2O4@PAC	0.0223	0.9914	31	103
PMS/CoFe2O4@PAC	0.0266	0.9937	26	87