The initial pH is regarded as a vital parameter to study the photoreaction for decolorization any dye from suspension solution of photocatalyst. The effect of pH from 4 to 10 on the photocatalytic decolorization of Nigrosin dye are appeared in (Figure 2(a) and Figure 2(b)) and Figure 3.

The results demonstrated that the best initial pH for decolorization of Nigrosin dye is 8.177. This value is near from the zero point charge of ZnO that equal to 9 [12] [17] . This effect depended upon the characteristics of studied dye, the surface charge of catalyst and generation of hydroxyl radicals via photocatalysis process [18] [19] . In acid medium, the surface of photocatalyst is positive chargly and caused photocorrosion via self oxidation, therebye the efficiency of

decolorization depresses. Moreover, In basic medium, the surface of photocatalyst is negative charge, that will lead to dissolution of the photocatalyst according to the following Equations (4)-(7) [19] [20] .

ZnO + 2 h + → Z n 2 + + 1 2 O 2                           ( a c i d i c   p H ) (4)

o r           ZnO + 2 H + → Z n 2 + + H 2 O                       ( a c i d i c   p H ) (5)

Z n − OH + O H − → Z n − O − + H 2 O                     ( b a s i c   p H ) (6)

o r       ZnO + H 2 O + 2 O H − → [ Z n ( O H ) 4 ] 2 −                 ( b a s i c   p H ) (7)

Figure 3 was proved, at initial pH equal to 8.170, the maximum value of the efficiency (E%) of removal Nigrosin dye in presence of ZnO is reached to 99.014 at 25 min and 288.15 K.

The effect of temperature on photodecolorization of Nigresone dye from suspension solution of ZnO was estimated in the range (283.15 - 303.15) K. This effect plays an important role to determine the apparent activation energy (E_a) that used Arrhenius equation (Equation (8)), and some thermodynamics parameters such as (ΔH^#, ΔS^# and ΔG^#) that employed Eyring-Polanyi equation (Equation (9)) and Gibbs equation (Equation (10)). These parameters were calculated by the following Equations [21] [22] [23] [24] .

ln k a p p = − E a R T + ln A (8)

Whereas: k_app is apparent rate constant, E_a is apparent activation energy, A is a frequency constant, R is gas constant and T is temperature of photoreaction.

ln ( k a p p T ) = − Δ H # R T + ( ln k B h + Δ S # R ) (9)

Whereas: k_B is a Boltzmann’s constant, h is a Plank’s constant, R is a gas

constant and T is the temperature of reaction.

Δ G # = Δ H # − T Δ S # (10)

From (Figure 4(a) and Figure 4(b)), it was observed the decolorization of the studied dye declines with increasing temperature that due to the exothermic nature in reaction (ΔH^#). This result is in agreement with information that reported in references [21] [25] .

On basis on calculated data from Figure 4(a) and Figure 4(b), the values of activation energy and the essential thermodynamics parameters that are listed in Table 1.

The negative value of ΔG^# for this photoreaction indicates the reaction spontaneous in nature. The negative value of ΔS^# proved the depress randomness.

The moderate value of activation energy refer to the rate for decolorization of this dye is low and reach to final at 25 min.

Figure 5 displays the effect of addition of varies types of oxidant reagent such as H₂O₂ and Fe(II) on the decolorization of Nigrosein dye in suspension solutuion of ZnO.

In case of used ZnO alone as photocatalyst, The E% for decolorization of above-mentioned dye is equal to 64.9122 at 30˚C (Lab temperature) and 15 min. The hydroxyl radical acts as a power for starting the photoreaction with presence of photocatalyst. This progress of photoreaction was monitored by the following equations [26] [27] .

ZnO + h v → ZnO ( e CB − + h VB + ) (11)

H 2 O + h VB + → H + + HO · (12)

O 2 + e CB − → O 2 · − (13)

O 2 · − + H + → HO 2 . (14)

HO 2 · + H + → H 2 O 2 (15)

H 2 O 2 + h v → 2 HO · (16)

The decolorization of Nigresine dye will suggest in the following equation

Dye + HO · → n CO 2 + n H 2 O + Inorganicions ( NH 4 + , SO 4 2 − + Na + ) (17)

When addition 1% of H₂O₂ to suspension solution of dye and ZnO, that depress the E% to 58.969, that attributable to the high concentration of H₂O₂ that acts as scavenger for •OH radical to form Perhydroxyl radical HO 2 · [12] [26] .

H 2 O 2 + HO · → H 2 O + HO 2 · (18)

HO 2 · + HO · → H 2 O + O 2 (19)

In this work, the process for addition of Fe(II) in concentration 1 × 10⁻⁴ M to the dye solution with presence ZnO has a negative effect. So, E% of this reaction declines to be 43.750. That belongs on the reacted of Fe²⁺ with Perhydroxyl radical, according to the following equation [28] [29] .

HO 2 · + Fe 2 + → HO 2 − + Fe 3 + (20)

HO 2 · + Fe 2 + → O 2 + Fe 3 + + H + (21)

From the other hand, the used of fenton’s reaction in the conditions of the photoreaction, that leads to depress the E% to 36.363. The certain concentration of H₂O₂ and Fe²⁺ causes to produce H₂O and O₂ [30] [31] .

Scheme 1. Suggested mechanism for decolorization of Negrisone in suspension solution of ZnO.

Fe 2 + + H 2 O 2 → Fe 3 + + HO · + HO − (22)

H 2 O 2 + HO · → H 2 O + HO 2 · (23)

HO 2 · + HO · → H 2 O + O 2 (24)

The suggested mechanism for decolorization of Nigreson in presence ZnO under UV-A light was summarized in (Scheme 1).

The essential conclusions for this work were proven the following results:

1) The photoreaction of decolorization of Nigresoin dye in suspension solution of ZnO obeys the pseudo first order kinetics.

2) The Rate of photocatalytic decolorization of the above dye is followed the next sequences:

Rate of decolorization in presence ZnO alone > Rate of decolorization in presence ZnO with H₂O₂ > Rate of decolorization in presence ZnO with Fe(II) > Rate of decolorization in presence ZnO with H₂O₂ and Fe(II).

3) The decolorization reaction is fast, exothermic and spontaneously at initial pH 8.177 of dye solution.

4) At 25 min and 288.15 K, the efficiency (E%) of removal Nigrosin dye in presence of ZnO was reached to maximum value and to be 99.014.