Effect of Various Additives on Photocatalytic Degradation of 4-Nitrophenol

The photocatalytic degradation of 4-nitrophenol (4-NP) assisted by titanium dioxide (TiO2) was investigated in aqueous suspension under irradiation by UV light. The effect of different supporting materials mixed physically with TiO2 on the photocatalytic degradation of 4-NP has been studied. TiO2 with all supports exhibits good degradation efficiency of 4-NP and was better than TiO2 alone. The addition of SiO2 and ZSM-5 only caused a little change in 4-NP degradation. However, degradation of 4-NP was improved from 34.89% to 60.53% within 120 min photocatalysis in the presence of optimal amount of AC. The degradation was also fairly enhanced in the presence of cheaper rice husk and the activity was closed to AC.


Introduction
Nitrophenols are some of the most refractory pollutants, which can be present in industrial wastewater.Among them, 4-nitrophenol (4-NP) is environmentally important for severa1 reasons.Owing to high toxicity and carcinogenic character, 4-NP is characterized as environmentally hazardous material.This toxic pollutant is used in the production of pesticides, insecticides and herbicides 1 and many synthetic dyes 2 .Therefore, 4-NP and its derivatives are common pollutants in many natural water and wastewater systems.It is reported as potential toxic pollutant by United States Environmental Protection Agency (USEPA) and its maximum allowable concentrations in water ranged from 1 to 20 ppb 2 .
The removal of pollutants from wastewater is of great concern, because their complete biodegradation requires several days or weeks.Advanced oxidation processes (AOPs) are efficient treatment methods owing to their ability of complete degradation of wide range of organic pollutants.Titanium dioxide (TiO 2 ) assisted photocatalysis is a well known emerging AOP for the removal of organic pollutants in water and air [3][4][5] .TiO 2 is of great interest due to its non-toxic nature, photochemical stability and low cost, particularly when sunlight is used as the source of irradiation 6,7 .However, shortcoming of using TiO 2 in photocatalytic processes is its rapid aggregation in a suspension resulting in decrease of effective surface area in addition to recombination of generated electron-hole pairs.This disadvantage of TiO 2 results in low catalytic efficiency.
Various methods were documented to improve photocatalytic efficiency of TiO 2 [8][9][10][11] .Another effective method that can increase the photocatalytic efficiency of TiO 2 is to add an additive or support material such as silica (SiO 2 ), alumina (Al 2 O 3 ), zeolite (ZSM-5) or clay and activated carbon (AC) [12][13][14][15][16][17][18][19][20] .TiO 2 with supports or additives offers high specific surface area which helps in more effective adsorption than TiO 2 alone 12,17,19,21,22 .The synergy between TiO 2 particle and the support enhances the degradation which is attributed to reduction in the electron-hole recombination reaction on the surface 23 .Over the years, the opportunity of producing activated carbon from cheaper and readily available source like rice husk (RH) for water purification is interesting.Rice is one of the main crops, which is widely cultivated in Asian countries.The RH is an agricultural waste produced in the milling process when the grain is separated from the outer covering (husk).
Earlier, we have reported the photocatalytic degradation of phenol under the influence of TiO 2 24 and iron-doped TiO 2 nanoparticles 25 .In the present work, the influence of the supports such as activated carbon, silica and zeolite (ZSM-5) on the photoactivity of pure TiO 2 has been examined for the degradation of 4-NP.Thus the prime objective of the present work was to improve the efficiency of photocatalytic process using supports.Results were also compared utilizing cheap material RH as an alternative source of AC.

Experimental
TiO 2 powder of P25 was the product of Degussa Co., Germany.It contains about 80% anatase and 20% rutile form with an average particle size of 21 nm and BET surface area of 50±15 m 2 g -1 .Analytical reagent grade phenol and hydrochloric acid were obtained from Sinopharm Chemical Reagent Co., Ltd.(Shanghai, China).4-nitrophenol (4-NP) was purchased from Aladdin Chemical Reagent Co., China.AC (SSA 950 ±10 m 2 /g) was obtained from Ningxia Coal Co., Ltd.(China).ZSM-5 (Si/Al = 50, 5-6 µm, 320 m 2 /g) was obtained from Nankai University (Tianjin) and SiO 2 (125-425 µm, 460 m 2 /g) was purchased from Qingdao Haiyang Chemical Co., Ltd.All chemicals were used as such without further purification.The rice husk obtained from a farmer near Chang Chun (China) was washed with distilled deionized water to remove all dirt and then oven-dried at 80 °C till constant weight.The dried RH was sieved through 80 mesh.The dried RH was stored in a polythene bag kept in desiccator and was used as such without any physical or chemical treatment.Water used for chemical solutions was purified using a Milli-Q system (Millipore Corporation).

Photocatalytic measurement
Photocatalytic degradation of 4-NP in TiO 2 suspension was performed in an open Pyrexglass cell with 500 mL capacity.0.05 g of TiO 2 was suspended in 200 mL of 4-NP aqueous solution (1 × 10 -4 M) using appropriate amount of support or RH at pH 5. Air was continuously bubbled through the suspension.The suspension was magnetically stirred in the dark for at least 15 min to ensure the establishment of an adsorption/desorption equilibrium.Then light was turned on and it was treated as the starting point (t = 0) of the DE, %
reaction.UV irradiation was carried out using a 125 W bulb from Leijian Special Light Source Co., Ltd.(Shenzhen, China) with a wavelength of 365 nm.The pH of suspension was measured using PH-035 digital pH meter from Changlilai Technology Co., Ltd.
(Shenzhen, China) by adding HCl (0.1 M).After a regular interval of time, the samples of 7mL volume were collected and immediately centrifuged at 3000 rpm for 10 min, and then filtered through a 0.45 µm Millipore filter to remove the TiO 2 particles.The filtered samples were stored at 4 °C prior to analysis.The quantitative determination of 4-NP was performed by measuring its absorbance at 315 nm with a Helios Gamma UV-vis spectrophotometer.
The degradation efficiency (DE) of each sample was computed using following equation: where, A o and A t is the absorbance at time zero and time t, respectively.

Photocatalytic degradation of 4-NP
Aqueous solution of 4-NP was irradiation in the presence or absence of TiO 2 -P25 by 125 W UV-lamp was carried out.The role of photocatalytic degradation and the effect of direct photolysis on the degradation of 4-NP were studied.
The • OH is exclusively strong, non-selective oxidant, which conducts the partial or complete mineralization of organic pollutants 26 .Electrons (e cb -) in the conduction bond are also responsible for the generation of • OH, which have been indicated as the primary source of pollutant degradation 27 .
OH pollutant degradation products The whole mechanism of photoactivity of TiO 2 particle is depicted in scheme 1.

Effect of support concentration
In order to examine the effect of support on the degradation of 4-NP, a series of experimentation was accomplished by the different types of supports, namely AC, SiO 2 and ZSM-5.Figures 2-4 show results for the degradation of 4-NP from aqueous solution employing different supports loading varying from 25 to 100 mg.It can be seen that photocatalytic degradation of 4-NP was found to increase then decrease with the increase in support concentration.This trend is probable for the reason that as the number of support particles surrounding the 4-NP increases, more 4-NP is adsorbed by these particles 28 .It was found that for all supports, a concentration of 50 mg with TiO 2 produced the best photocatalytic degradation of 4-NP.
From Figures 2-4, it can be concluded that all the supports when mingle with TiO 2 enhanced the photocatalytic degradation of 4-NP, with the order AC > ZSM-5 ≅ SiO 2 .TiO 2 aided with AC has the highest degradation efficiency for 4-NP under the experimental conditions.The results indicate that the effective surface area and adsorption capacity of the supported TiO 2 were much higher than that of TiO 2 alone, which favor rapid degradation of 4-NP.

Comparison with rice husk
Figure 5 shows the effect of RH on the absorption intensity of 4-NP versus irradiation time.It is easily seen from Figure 5 that relative to 4-NP degradation over TiO 2 alone (Figure 1), the degradation of 4-NP is fairly increased in the presence of RH.The inset of Figure 5 shows that the synergistic result of TiO 2 and RH is in comparison with TiO 2 and AC system.This suggests that RH is an alternative and cheap material and can be used as support material.

Kinetics of photocatalytic degradation of 4-NP
From engineering point of view, it is useful to find out a simple and user-friendly rate equation that fits the experimental rate data.It can be seen from Figures 2-4 that the dependence of 4-NP concentration on the irradiation time was fitted to an exponential function suggesting that the photocatalytic degradation of 4-NP established pseudo firstorder kinetics with respect to the 4-NP concentration: The integration of equation ( 8 where app k is the apparent first order rate constant (min -1 ).The computed k app values from equation ( 9) at optimal support concentration are listed in

Conclusions
The photocatalytic degradation of 4-nitrophenol (4-NP) was investigated in aqueous suspension of TiO 2 irradiated by UV light.Various supporting material were shown to enhance the photocatalytic degradation of 4-NP.All supports show improved degradation of 4-NP than TiO 2 alone within 120 min of photocatalysis.A comparable result was obtained when an optimal amount of rice husk was used as a supporting material.Therefore, rice husk could be used as an alternative to AC.

Figure 1 Figure 1 .
Figure 1.Effect of UV light and TiO 2 particles on photocatalytic degradation of 4-NP.Irradiation time, min

Figure 5 .
Figure 5. Change in absorption intensity of 4-NP with TiO 2 /RH.The inset shows the DE of 4-NP assisted by TiO 2 with RH and AC.

Table 1 .
It can be seen that k app values for TiO 2 /AC and TiO 2 /RH are in good agreement demonstrating that RH can be used as alternative supporting material to AC.