Figure 1 shows how the degradation of PNP changes with increasing concentrations of TiO₂ suspension_. The concentration which corresponds to the maximum degradation of PNP was 0.4 g/100 mL. As the concentration of TiO₂ in the suspension was increased, the adsorption site on the surface was increased. As a result the extent of adsorption of PNP on the TiO₂ surface was increased and the percent degradation was also increased. With the continuous increase of concentration of TiO₂ in the suspension, the adsorption increased until the surface was likely to be saturated but percent degradation did not increase. The reason is UV light would face difficulties to penetrate and reach the surface under these conditions. As a result, percent degradation decreased. The present observations are in agreement with earlier observations [8] -[10] .

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. Figure 2 shows five different initial concentrations of PNP solutions and values of absor-

bance at different intervals of time. Figure 3 shows the change of percent degradation of PNP solution with increasing PNP concentration at definite interval of time. It is evident that percent degradation of PNP decreases with increasing concentration. This can be explained by the possible photocatalytic reactions taking place on the surface of TiO₂. An approximately single layer of adsorbed water molecule exists on the surface of TiO₂. Of the two types of carriers formed after light is adsorbed on the surface of TiO₂, h⁺ oxidizes water molecules producing ^.OH radicals which have strong oxidizing power. These radicals can react with PNP producing free radicals. These free radicals can react with molecular oxygen producing organic peroxy radicals which can propagate a chain reaction. PNP molecules are completely degraded within a short time.

With increasing of PNP concentration more PNP molecules accumulate on the surface by replacing water molecules [11] . As a result, reaction 1 slows down resulting in low production of radicals. Neppolian et al. [12] found similar observations with some commercial textile dyes under solar irradiation. The present observations are also in good agreement with the results obtained by a group of researchers, who have investigated photocatalytic degradation of dyes in aqueous solution using TiO₂ suspension [13] -[20] .

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 [7] and other physicochemical properties of the system.

Figure 4 shows the percent degradation of PNP with increasing pH of the suspension. The maximum photodegradation was observed in highly acidic solution. As the pH increases from 3 to 5, degradation decreases. But surprisingly, the rapid increase of degradation was observed as pH was increased from 5 to 7. Beyond 7.5, the degradation was found to decrease. The pH_ZPC of TiO₂ is 6.25 and in acidic condition, the surface pH is lower than pH_ZPC of TiO₂ [11] . As a result in acidic condition surface of TiO₂ becomes positively charged. PNP is strongly adsorbed on TiO₂ surface due to the presence of lone pair of electron in phenolic OH group. This might be the reason for maximum degradation in acidic solution. As the pH increases, surface loses its positive character and degradation decreases. When the pH exceeds 7.5 meaning that the solution is alkaline, the surface becomes negative and PNP remains as anion under these conditions. Obviously PNP molecules have difficulties to approach the surface to be adsorbed. This might be the reason for decreasing photodegradation in alkaline solution. However, neutral region favoured the photodegradation. Under these conditions, adsorption of either PNP or PNP^- would decrease. It is expected that the rate of production of ^.OH radical by the reaction would increase. Increase of production of ^.OH radical would increase the photo- oxidation. Islam et al. and her co-workers investigated the effect of pH, ions and ionic strength on TiO₂-me- diated photodegradation of Brilliant Orange which is an anionic dye, and they found that the degradation rate was high in acidic media and low in the region of pH 6 to 7 [7] .

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. Figure 5 shows the change of H⁺ ion concentrations with time under different experimental conditions.

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

surface. As a result, photodegradation and mineralization decreased. This observation is in good agreement with the previous findings [7] [21] .

Hua et al. [21] studied the effect of presence of Cu(II) ions on the photodegradation of monocrotophos and found that at low concentration of Cu(II) (<10⁻⁵ mol∙dm⁻³) slight enhancement of photodegradation was observed whereas at higher concentration photodegradation was decreased. Zang et al. [22] investigated the effect of Cu(II) ion on 2,4-DCP and showed that the photodegradation of 2,4-DCP was increased for the addition of Cu(II) ion at low concentrations but in presence of higher concentrations of Cu(II) it decreased.

To investigate the effect of anions on photodegradation of PNP, the experiments were carried out using sodium salts of, , and. All these anions decreased the percent degradation of PNP. These observations are in good agreement with the previous results [7] .