In this photocatalytic experiments, all chemicals were used without any further purification. Commercial Zinc sulfide and Thiourea were supplied by Fluka. Other chemicals such as Chromium (III) chloride, Zinc sulphate hepta hydrate and poly ethylene glycol (PEG-4000) were purchased from Merck. NaOH, NH₄OH and HCl were supplied by BDH. The textile dye (reactive black 5 dye) was supplied by the Hilla textile factory in Babylon city in Iraq, and the characteristics of it are summarized in Table 1.

All prepared solutions were dissolved in distill water then shaking by ultrasonic

(FALC) for 10 min to ensure getting for homogenous solution. The Bare ZnS and metal loaded ZnS procedure were modified from procedure in reference [22] . 0.5 M from ZnSO₄ was used as starting material. The equal volume from 0.5 M thiourea was taken in a burette and added drop by drop into ZnSO₄ solution with vigorous stirring (Labtech-magnetic stirrer) for 25 minutes to get ZnS solution, and simultaneously 0.05 M from CrCl₃ was added to this solution as droping agent using burette for 25 min to create the Cr loaded ZnS. This final formed solution was followed by drop wise addition of 1% of poly ethylene glycol-4000 (PEG-4000) as neutral capping agent solution under vigorous stirring for 1 hour. Afterwards, 0.1 M from ammonia solution was added slowly to solution of metal until the pH reached to range between 10 and 12, that necessary to generate the metal complex. Finally the metal complex solution was filtrated under vacuum using Büchner’s funnel. The precipitate was twice times by using distill water then used absolute ethanol to remove the impurities. The produced precipitate for ZnS and Cr/ZnS were white and grey color as seen as Figure 1, both catalysts were dried overnight by desiccator then dried in oven at less 100˚C for 2 h to obtain powder sample.

The suggested chemical equations were investigated from the following equations.

Z n S O 4 ⋅ 7 H 2 O → Z n 2 + + S O 4 2 − + 7 H 2 O (1)

Z n 2 + + S C ( N H 2 ) 2 → [ Z n ( S C N 2 H 4 ) 2 ] 2 + (2)

[ Z n ( S C N 2 H 4 ) 2 ] 2 + + 8 O 2 → PEG-4 00 Z n S ↓ +   H 2 S O 4 + 2 H N O 3 + 2 N H 2 C O 3 (3)

and

[ Z n ( S C N 2 H 4 ) 2 ] 2 + + C r C l 3 + 8 O 2 → PEG-400 C r / Z n S ↓ +   H 2 S O 4 + 2 H N O 3 + 2 N H 2 C O 3 +   3 C l − (4)

H 2 S O 4 + 2 H N O 3 + 3 C l − + 6 N H 4 O H → ( N H 4 ) 2 S O 4 + 2 N H 4 N O 3 + 2 N H 4 C l + H O C l + 5 H 2 O (5)

Appropriate grams of photocatalysts (ZnS or Cr-ZnS) were added to 100 mL of 25 ppm from reactive black 5 dye solution. The produced suspension solutions were subjected to UV-A light flux at intensity equal to 1.45 × 10⁻⁷ Einstein s⁻¹, which calculated by chemical actinometer [23] . At different time intervals, 3.5 mL was taken out with using syringe and then carried out double separated processes by using centrifuge for 10 min and 4000 rpm. The produced filters were studied by reading the absorption at 595 nm to find the residue concentration of dye after irradiation.

Figure 5 illustrates, that the metalized process of photo-semiconductor (ZnS) by Cr leads to increase the rate constant for decolorization of reactive black 5 dye. The rate constant (kapp.) can be calculated by depended on the Langmuir-Hinshelwood kinetic expression to determine Equations (7) and (8) [34] . This reaction is found to be obeyed the pseudo first order.

C t = C o exp ( − k a p p ⋅ t ) (7)

where: C_t is a concentration of the reactive black 5 dye at t time of irradiation. C = is an initial concentration of same studied dye at (dark reaction) in 0 min as the time of irradiation. The apparent first order equation can be investigated from Equation (8) [34] [35] :

ln ( C o C t ) = k a p p ⋅ t (8)

The % Efficiency (% E) of decolourization for reactive blue 5 dye was calculated using the following relationship that reported in reference [35] .

%   Effeciency = ( C o − C t C o ) × 100 (9)

This result assigns to rise the charge carrier space distance that leads to increase the hole life time and inhibits the recombination process. Moreover, the metal acts as collector of electron by forming a Schottky’s barrier [36] [37] [38] .

The probability for generating this barrier is depended upon the value of work functions fcc (111) for both Cr and Zn in ZnS. Hence, the large value for work function of Cr at fcc (111) 4.5 enhances the interaction between Cr and Zn fcc (111) 4.22 in ZnS photocatalyst [39] .

The proposed mechanism for photodecolorization of reactive blue 5 dye (RB5) is occurred in when UV-A light focused on suspension solution of bare ZnS nanopowder or Cr loaded on ZnS nanopowder, that perform as system (Dye/sem-iconductor/UV light). The more accepted mechanism is outlined in Scheme 1. In outset, azo bond is oxidative cleavage either C-N or N-N cleavage by attaching the hydroxyl radical that generated via photocatalysis process [40] [41] in presence the prepared photocatalysts. The intermediate I and intermediate II were formed with liberated N₂. After that the C-S bond is cleavage in series steps to generate CO₂ and H₂O [41] .