The effects of initial concentrations of PNP, doses of TiO2, cations and anions have been investigated to find out the conditions for the maximum degradation of PNP in presence of 254 nm UV light. The rate of photocatalytic degradation of PNP was increased with increasing TiO2 dose until the dose concentration reached at a value 0.4 g/100 mL. Further increase of TiO2 decreased the degradation. The maximum degradation of PNP was found with the catalyst dose 0.4 g/100 mL at pH 3. The degradation of PNP was decreased with increasing of PNP concentration. About 90% degradation of PNP was observed when 1.0 × 10﹣4 M PNP was irradiated for 2 hours in 0.4 g/100 mL of TiO2 suspension. The effect of Cu(II) and Fe(II) ions on the degradation was also investigated. Addition of Cu(II) ions enhances the percent degradation but excess of Cu(II) ions decreases the degradation. Under the same experimental conditions, the presence of
Phenolic compounds discharged in effluents from petrochemicals, agrochemicals, plastic industries, preservative industries, coal distillation plants, pharmaceutical industries etc. are carcinogenic and harmful for the environment. PNP is one of these harmful chemicals and its presence in the environment is a threat for living beings. Many investigations have been carried out regarding the removal of PNP from the aqueous media [
Chen and Ray studied the photo catalytic degradation of phenol, PNP and 4-nitrophenol (4-NP) in aqueous suspension and over immobilized Degussa P25 TiO2 in the laboratory [
Commercial PNP was used for experiments without further purification. Titania P-25 (anatase) 99.0% purity was obtained from Fluka. Electrolytes used in the experiments were obtained from Merck.
Experiments were carried out using 100 mL of 0.5 × 10−4 M PNP solution in suspension of TiO2, the concentration of which was varied from 2 g/L to 8 g/L. pH was recorded before irradiation. The suspension was stirred magnetically and irradiated. The irradiated samples were collected after definite intervals of time and were analyzed. The experimental procedure is given elsewhere [
The percent degradation (%) has been calculated as follows:
where Ao is the initial absorbance and At is the absorbance of the sample irradiated for t minutes.
Photodegradation experiments were repeated to investigate the effect of concentrations of TiO2 (
The photodegradation was investigated by varying the initial concentration of PNP over the range of 0.5 × 10−4 M to 2.5 × 10−4 M.
Effect of concentration of TiO2 in the suspension on photodegradation of PNP at concentration of 1.0 × 10−4 M and pH 3.0
Absorbance change of PNP with times of irradiation at different initial concentrations of PNP in suspension[s] containing 0.4 g TiO2/100mL and solution pH 3.0
bance at different intervals of time.
Effect of initial concentration of PNP on photodegradation. [TiO2] = 0.4 g/100 mL. Solution pH = 5.0
Effect of solution pH on photodegradation of PNP. [TiO2] = 0.4 g/100 mL, [PNP] = 1.0 × 10−4 M
With increasing of PNP concentration more PNP molecules accumulate on the surface by replacing water molecules [
Solution pH is an important factor that regulates aqueous-phase semiconductor mediated photocatalytic reaction. It affects dissociation of substrate, surface charge, conduction band potential of semiconductor [
The effect of cations on the photodegradation was studied by monitoring the hydrogen ion concentration of the solution during irradiation. At the beginning, two experiments were carried out, one using only the TiO2 suspension (blank) and the other one using PNP in the suspension. The irradiation was carried out by adding different concentrations of Cu(II) and Fe(II) ions in the suspension of PNP and in the blank, followed by monitoring of H+ ion concentrations at different time intervals.
Any metal ion will be adsorbed on the TiO2 surface if the reduction potential of the metal ion is higher than the conduction band potential of TiO2. The conduction band potential of TiO2 is −0.45 V for pH 5.95 and the redox potentials of Fe(II) (aq)/Fe(s) and Cu(II) (aq)/Cu(I) (aq) are −0.41 and 0.16 V respectively.
The ions adsorbed on the TiO2 particle can trap an electron to form Ti-O-M.
This might be oxidized by oxygen at the interface producing superoxide anion.
The highly reactive superoxide radical anions attach itself to the peroxyl radicals formed from the organic compounds as mentioned earlier in reaction 3 producing an unstable product which contains at least four oxygen and this can decompose into carbon dioxide molecule. Thus the superoxide anions play an important role in en- hancing the rate of mineralization. However, addition of excess ions decreased H+ ion concentration. Excess ions occupy the adsorption sites on the surface of TiO2 and thus decreased the adsorption of PNP on the TiO2
Effect of cations on the photodegradation of PNP. 1 = no PNP (blank), 2 = PNP only, 3 = PNP with 1 × 10−9 M Fe(II), 4 = PNP with 1 × 10−5 M Cu(II), 5 = PNP with 1 × 10−5 M Fe(II), 6 = PNP with 1 × 10−3 M Fe(II), 7 = PNP with 1 × 10−3 M Cu(II)
surface. As a result, photodegradation and mineralization decreased. This observation is in good agreement with the previous findings [
Hua et al. [
To investigate the effect of anions on photodegradation of PNP, the experiments were carried out using sodium salts of
TiO2 mediated photodegradation at 254 nm light for 2 hours showed that about 90% of PNP was degraded in 0.4 g/100 mL of TiO2 suspension containing 1.0 × 10−4 M of PNP. Addition of Cu(II) and Fe(II) ions in TiO2-sus- pension enhances the degradation of PNP suggesting that the presence of metal ions has higher redox potential than the conduction band potential of TiO2 and accelerates the mineralization, but the presence of excess ions decreases the mineralization process. The complete mineralization of 1.0 × 10−4 M PNP may be obtained by using 0.4 g/100 mL of TiO2 suspension with either 10−9 M Fe(II) or 10−5 M Cu(II) as impurities and illuminating for 2 to 3 hours or until steady increase of H+ ion concentration is observed.
Authors would like to acknowledge to the Ministry of Science and Technology of the People’s Republic of Bangladesh for the partial financial support to carry out this work.