Nano-Catalytic Ozonation of 4-Nitrochlorobenzene in Aqueous Solutions

In this paper, efficiency of nano-ZnO particles on catalytic ozonation of 4-nitrochlorobenzene (4NCB) using semi-batch reactor has been studied at various pHs. During the catalytic ozonation, TOC and concentration of nitrate ions was monitored. Results indicate that degradation of 4NCB was improved by combination of nano-ZnO with ozone. The effect of ZnO particle size and pH are also examined. According to the results, concentration of 4NCB decreased with increasing of particle size from nanosized to microsized and pH from 3.0 to 9.0. Based on the results, it suggests radical hydroxyl does not affect on the degradation of 4NCB in catalytic ozonation, but the surface of catalyst plays main role. Kinetic studies showed degradation of 4NCB followed pseudo-first-order kinetic and maximum degradation rate was observed at pH=3.


Introduction
4NCB is an important chemical intermediate that used as basic building blocks for the manufacture of numerous industrial chemicals.For example, it is utilized via chemical reaction to make industrial chemicals that are ultimately used in the preparation of dyes, pigments, pesticides and animal feed ingredients 1 .PCNB is known to produce methemoglobinemia in human and animals [2][3] and is considered hazardous 4-5.Advanced oxidation of organic pollutants present in water is a strong method to degradation of aromatics.On the other hand, advanced oxidation processes exhibit the most effective way of oxidizing organic compounds [6][7][8] .Shen et al., studied the kinetics and mechanism of degradation pCNB in water by ozonation.It has been deduced pCNB completely removed after 20 min of ozonation, while the degree of mineralization was slow 1 .Heterogeneous catalytic oxidation of 4NCB with H 2 O 2 showed that the type of catalyst and oxidation agents affected on the degradation rate 9 .A comparative study of degradation of pCNB by H 2 O 2 /UV and O 3 /UV oxidation processes was carried out 10 .Reduction of concentrations of chloronitrobenzene from 1900 μg/L to less than 20 μg/L could be reached by the application of 8 mg O 3 /L and 3 mg H 2 O 2 /L with a 20 minute contact time 11 .Guittonneau et al., studied the degradation of pCNB by H 2 O 2 photolysis and gave a kinetic model 12 .Yang et al. studied the ozonation of nitrobenzene catalyzed by nano-TiO 2 particles and deduced that degradation efficiency of catalytic ozonation is much higher than ozonation alone 13 .
Objective of this work is to compare the performance of ozonation and catalytic ozonation on degradation of 4-nitrochlorobenzene and determine the kinetic of reaction at various pHs.The ZnO was applied as catalyst in two different sizes, i.e., nano and microsized.

Experimental
1-Chloro-4-nitrobenzen (4NCB) [C 6 H 4 ClNO 2 , M=157.6] (Fluka Co.) was selected as a model compound.A stock solution of 4NCB was prepared at a concentration of approximately 95 µM (15 mg/L).Nano TiO 2 was purchased from Degussa (Germany).Its size was <21 nm and had a surface area in the range of 15-50 m 2 /g.Nano and microsized ZnO were supplied from Aldrich and Sigma-Aldrich (USA), respectively.Nanosized ZnO was <100 nm and its surface area (15-25 m 2 /g) was determined by a BET surface area analyzer.All solutions were prepared by using deionized water.All other chemicals such as acetonitrile, sodium thiosulphate, potassium iodide and etc., were reagent grades and were supplied from Merck, Germany.

Catalytic Ozonation Experiments
Catalytic activity measurements in degradation of 4NCB were carried out in a semi-batch stirred reactor.A pyrex glass reactor of 150 mm diameter and 250 mm length was equipped with a ceramic diffuser.O 2 /O 3 gas mixture (5.64 mg/L) was bubbled continuously through a fine-bubble ceramic diffuser into a reactor (2000 mL) containing 1 L of aqueous solution of 4NCB (15 mg/L) and 300 mg of catalyst that was completely mixed with a magnetic stirring bar.A schematic diagram of the ozonation system is shown in Figure 1.The ozone, generated by an X200 Laboratory Ozone Generator (Baku), was produced from pure oxygen and was fed into the reactor through a porous diffuser located at the bottom of the reactor to produce fine bubbles.Gas flow was kept constant at 1 L/ min, having an ozone concentration of 5.64 mg/L.The gaseous ozone concentration was measured by the iodometric method using 2% potassium iodide for ozone trapping and sodium thiosulfate as a titrant 14 .As soon as the ozone gas contacted the 4NCB solution, ZnO particles were dispersed in the solution.The contact time was 60 min and samples were withdrawn at different intervals (2, 5, 15, 30, 45, and 60 minutes) to determine the residual concentration of 4NCB.The change of concentration was measured by both the photometric and the HPLC methods during the reaction.The two methods gave similar results and the differences ranged from 3.8% to 5.9%.The oxidation reaction was quenched by the addition of a small amount of sodium thiosulphate 15 .During the reaction, the reactor was sealed to avoid the evaporation of aqueous ozone.Triplicate experiments were conducted at 25±2 °C for verification of all results.HCl or NaOH solution was used to control the pH level during the reaction.

Analysis
The concentration of 4NCB was monitored by using high performance liquid chromatography (HPLC, Shimadzu, SCL-6A, Japan) with a UV absorbance detector (Shimadzu, SPD-6AV, Japan) and using a Spherisorb ODS-3 (5 μm, 150×4.6 mm i.d.) column.Elution was carried out by pumping acetonitrile and water (70:30 %v/v) and 0.2 cc acetic acid isocratically at a flow rate of 1.0 ml/min.Absorbance was measured at a wavelength of 279 nm using a Shimadzu UV-2501 recording spectrophotometer.The concentration of nitrate ions formed as a result of organically bounded nitrogen mineralization was measured with a spectrophotometer (HACH, DR/4000 U) at 275 nm.The pH of solution was measured with a 744 pH-meter (Ω Metrohm).TOC was measured with Shimadzu TOC-VCSH equipped with an autosampler (ASI-V) and platinum based catalyst.The carrier gas was synthetic air at the rate of 100 mL/min.Before measurement in all of the experiments, suspensions were centrifuged to collect the filtrate.

Results and Discussion
The experiments were carried out at room temperature during ozonation and catalytic ozonation.

Ozonation of 4NCB
Ozonation of 4NCB was carried out in a semi-batch reactor at a concentration of 15 mg/L using 5.64 mg/L of O 3 .Calculation of consumed ozone per each mole of 4NCB was obtained by the Eq. 1 [16][17] .
Where, [O 3 ] 0 and [M] 0 are initial concentration of ozone and 4NCB, respectively and [M] is a final concentration of 4NCB.From Table 1, maximum of ozone per mole of 4NCB is consumed at basic condition.As shown, after 60 min, 4NCB degradation was 17.91, 19.94, and 29.1% at pH 3,7 and 9, respectively.Ozone is a dipolar molecule, i.e., it had both electrophilic and nucleophilic property.On the base of results, it can be deduced the initial attack of ozone was occurred via nucleophilic nature of ozone, as the nitro and chloro electron withdrawing groups decrease the electron density of the ring and deactivate it against the electrophilic attack of O 3 18 .
Results show that ozonation alone is not significantly affect on degradation of 4NCB, so combination of catalyst with ozone was tested.For this, two different kind of catalysts was used, i.e., TiO 2 and ZnO.The performance of both is examined and results showed nano-ZnO has the most effect on the removal of 4NCB compare to TiO 2 (Figure 3).To investigate the effects of particle size of catalyst, two different size of ZnO were applied, i.e., micro and nano sized.As shown in Figure 4, nano ZnO has the higher performance, due to its high surface area.
Finally, nano sized ZnO selected as an appropriate catalyst during catalytic ozonation of 4NCB.

Catalytic ozonation of 4NCB
Degradation of 4NCB by nano-ZnO catalytic ozonation at different pHs showed the maximum decomposition took place at acidic medium (Figure 5).It is clear pH affects the aggregation of nano-ZnO particles.Nano-ZnO particles significantly enlarged above pH >6.5.At pH 7.5, the size of the ZnO reaches up to 350 nm 19 .It is well known nano particles of metal oxides when suspended in water, formed aggregate 20 .Result proved that though formation of free radicals at high pH, OH° dose not affect on catalytic ozonation process and the main factor is the surface of catalyst.So, due to increase of particle surface area at pH =3, degradation f 4NCB is reached to 42.3%, while at pH 7 and 9 was 40.10 and 32.13%, respectively.
During catalytic ozonation of 4NCB, nitrogen was principally mineralized into nitrate.Concentration of nitrate formed during reaction was monitored (Figure 6) and results showed much formation of nitrate ions is obtained at pH 3 (4.15mg/L).The remaining nitrogen atoms could be in nitro form or may be converted to nitro-intermediates that more resistant to ozone attack 21 .
TOC is total organic carbon that represents the total amount of organically bound carbon present in dissolved and particulate matter in water 16 .In ozonation processes, TOC slowly decrease with time in many cases [15][16] , but ozonation processes catalyzed by transition Time, min Adsorbed 4NCB, mg/L Time, min metals have found to improve the TOC removal as compared to ozonation alone 22 .The effect of pH on mineralization of 4NCB in aqueous solution was shown during catalytic ozonation (Figure 7).The highest mineralization extents was achieved at pH=3(8.26%), while during ozonation it drops maximum to 2.1% (not shown here).It means that only part of 4NCB was mineralized and some by product formed during reaction that can not mineralized 1 .So, this system dose not follow a free radical pathway 23 .

Catalytic Ozonation Kinetic
The rate constant of reaction was obtained following 4NCB degradation at its maximum wavelength (279 nm) at pHs 3, 7, and 9.The reaction form of 4NCB with nano-ZnO/O 3 can be written as: According to Goi et al., direct reaction of molecular ozone with aromatic was predominant during the ozonation at pH values lower than 12 24 .Therefore, the rate equation can be written in the form Here, k' is the overall pseudo-first-order rate constant.
Figure 8 shows the pseudo first-order plot of 4NCB removal versus time at different pHs.The decomposition rate of 4NCB was found to decrease with increasing pH values.This result shows adsorption trend of 4NCB on the nano ZnO surface, which was pH-dependent and was occurred at pH<6.Also, the decay rate of 4NCB improved by increase of process time.

Conclusion
Semi-batch experiments were carried out to investigate the effect of pH on nano-ZnO Time, min catalyzed ozonation of 4NCB.Unlike ozonation, the higher degradation of 4NCB was obtained at acidic condition, i.e., OH° has no main role.However the 4NCB is a recalcitrant compound and even in catalytic ozonation, its degradation reached to maximum 42.3% after 60 min.Nitrate ions detected during the reaction is confirmed conversion of nitrogen to nitrate.TOC was partly changes during process, that means only part of 4NCB was mineralized.Pseudo first rate constant of reaction was obtained at different pHs.The rate of 4NCB degradation was pH dependent and was faster at pH=3 (1.98×10 -4 S -1 )due to high surface area of nano ZnO.

Figure 1 .
Figure 1.Schematic diagram of catalytic oxidation system.(1) Pure oxygen, (2) Flow meter, (3) Ozone generator, (4) Magnetic stirrer, (5) Ozonation reactor, (6) Water sample point, (7) KI trap.The ozone, generated by an X200 Laboratory Ozone Generator (Baku), was produced from pure oxygen and was fed into the reactor through a porous diffuser located at the bottom of the reactor to produce fine bubbles.Gas flow was kept constant at 1 L/ min, having an ozone concentration of 5.64 mg/L.The gaseous ozone concentration was measured by the iodometric method using 2% potassium iodide for ozone trapping and sodium thiosulfate as a titrant14 .As soon as the ozone gas contacted the 4NCB solution, ZnO particles were dispersed in the solution.The contact time was 60 min and samples were withdrawn at different intervals(2, 5, 15, 30, 45, and 60 minutes) to determine the residual concentration of 4NCB.The change of concentration was measured by both the photometric and the HPLC methods during the reaction.The two methods gave similar results and the differences

Figure 2 Figure 2 .
Figure2shows the effect of pH on the ozonation of 4NCB at different pHs.Concentration of 4NCB is plotted as a function of ozonation time.
], [ZnO] and [O 3 ] are the concentrations of 4CNB, zinc oxide, ozone and k o3 and o OH k are the corresponding kinetic rate constant of 4NCB with O 3 and OH°, respectively.
In addition, ozone and zinc oxide concentration in the solution can be assumed to be O ][ZnO] is constant and the Eg. 3 can be simplified to, reaction can be regarded as pseudo first order 25 .The integration Eg. 4 Leads to

Table 1
Consumed mole of ozone per mole of each 4NCB.

Table 2 .
As shown in Table2, k' is higher at acidic condition.
k´ = Pseudo rate constant.