The study focused on the effect of parameters (catalyst concentration, initial diclofenac concentration, ratio r= [H2O2]/[ZnO] and volume flow) on the photo catalytic degradation of diclofenac.
The analyses performed are pH, electrical conductivity, turbidity, surface tension, chemical oxygen demand, nitrite and diclofenac concentration.
III.1 Influence of the ZnO catalyst concentration
The study of heterogeneous photo catalytic degradation was carried out using the ZnO catalyst whose concentrations are (0.5; 1 and 1.5 g/L). The concentration of diclofenac is 100 mg/L. The volume flow rate is 20.5 mL/s.
III.1.1 Hydrogen potential (pH)
The pH variation during heterogeneous photo catalytic treatment as a function of the three ZnO concentrations is shown in Figure.3.
For the 0.5 g/L ZnO concentration; the pH decreases from 7.22 to 4.1 after 2h of treatment and then increases to 6.79 at t=5h.
For the two concentrations of (1 and 1.5 g/L) ZnO; the pH varies very little during treatment (it is between 6.61 and 7.89).
The pH influences the charge on the ZnO particles, the size of the aggregates and the positions of the conductance and valence bands. Thus, the ionization state of ZnO varies with pH (Malato. S et al., 2009).
At pH values more acidic than PZC (Im. J.K et al., 2012), the ZnO particle will be positively charged and attraction will be enhanced for anionic molecules.
III.1.2 Electrical conductivity (EC)
The variation of EC during heterogeneous photo catalytic treatment as a function of the three ZnO concentrations is shown in Figure.4.
For the 0.5 g/L ZnO concentration; the EC increases from 28.9 to 213 µS/cm after 2h of treatment, then decreases to 87.1 µS/cm at t=5h.
For both concentrations of (1 and 1.5 g/L) ZnO; EC increases from 28.9 to (42.9 and 77.8 µS/cm) respectively at t=5h.
The EC of the aqueous solution is mainly related to the presence of ions in the medium.
The higher the conductivity, the greater the degradation of diclofenac.
III.1.3 Turbidity (Tu)
The variation of Tu during the heterogeneous photo-catalytic treatment as a function of the three ZnO concentrations is shown in Figure.5.
For a ZnO concentration of 0.5 g/L; turbidity increases from 2.9 to 27.3 NTU at t=4h and then decreases to 10.1 NTU at t=5h.
For both concentrations of (1 and 1.5 g/L) ZnO; turbidity increases from 2.9 to (15.1 and 15.7 NTU) respectively at t=5h.
Turbidity is an important parameter and is likely to affect the photocatalytic efficiency. Indeed, high turbidity can affect optical properties and hinder light penetration.
Some authors have shown that a turbidity lower than 5 NTU would not affect light penetration.
Above this value, turbidity would interfere with the optimal use of light [15, 16]. This explains the decrease in the efficiency of photo catalytic treatment when turbidity increases.
III.1.4 Surface tension (Ts)
The variation of Ts during heterogeneous photo catalytic treatment as a function of the three ZnO concentrations is shown in Figure.6.
For a ZnO concentration of 0.5 g/L; the Ts increases from 56.9 to 70.26 mN/m after 2 hours of treatment. Beyond 2 hours the Ts is constant until the end of the treatment.
For ZnO concentrations (1 and 1.5 g/L); Ts increases from 56.9 to (57 and 59.76 mN/m) respectively during treatment.
The variations in Ts are explained by the degradation of diclofenac, which leads to the appearance of other molecules, leading to physico-chemical modifications of the solution and the catalyst.
This reflects different types of interactions between the solid (ZnO) and the liquid :
- Interactions due to London forces, also called dispersive forces. They always take place regardless of the molecules present.
- Interactions due to all other types of forces (polar, acid/base, hydrogen bonds, ...).
III.1.5 Chemical Oxygen Demand (COD)
The variation of COD during heterogeneous photo catalytic treatment as a function of the three ZnO concentrations is shown in Figure.7.
For the three concentrations (0.5; 1 and 1.5 g/L) of ZnO; the COD decreases from 360 to (130, 162 and 116 mg O2/L) respectively.
The chemical oxygen demand represents the amount of oxygen consumed by oxidizable materials in the water. The decrease in COD in the presence of the ZnO catalyst is explained by the absorption of light by the catalyst and the production of hydroxyl radicals responsible for the degradation of organic matter.
III.1.6 Nitrites
The variation of nitrites during heterogeneous photo catalytic treatment as a function of the three ZnO concentrations is shown in Figure.8.
For the ZnO concentration of 0.5 g/L; the nitrite concentration increases from 0.0086 up to 0.0162 mg/L after 4h of treatment and then decreases to 0.0130 mg/L at t=5h.
For the concentration of 1 g/L of ZnO; the nitrite concentration increases from 0.0086 to 0.0125 mg/L after 3h of treatment and then decreases to 0.0113 mg/L at t=5h.
For 1.5 g/L ZnO concentration; nitrite concentration increases during treatment from 0.0086 up to 0.0195 mg/L.
Nitrites can affect the photo catalytic process in several ways :
- Radical and hole scavengers which influence the quantum efficiency and thus the oxidation efficiency of the pollutant,
- Compete with the drug during adsorption to ZnO.
- Capture photons. - Corrosion or alteration of the photocatalyst.
- React directly with the photo catalyst (Chong, 2010) :
However, it is interesting to recall that the relative effects of these ions on the photo catalytic process can be variable depending on the pH which influences the zwitterionic nature of ZnO.
III.1.7 Concentration of diclofenac
The variation in concentration during heterogeneous photo catalytic treatment as a function of the three ZnO concentrations is shown in Figure.9.
For the three studied ZnO concentrations (0.5; 1 and 1.5 g/L), the diclofenac concentration decreases to (50.06; 58.65 and 48.11 mg/L) respectively.
The performance of the heterogeneous photo-catalytic degradation of diclofenac as a function of the three ZnO concentrations is shown in Figure.10.
The concentration of 1.5 g/L of ZnO gives a high percentage removal in diclofenac concentration which is 52%.
The presence of more ZnO can lead to the formation of zinc oxide aggregates which finally do not develop a larger ZnO-diclofenac contact surface than in the case of the 1g/L concentration, thus decreasing the percentage of degradation.
Increasing the ZnO concentration to 1.5 g/L also leads to the formation of zinc oxide aggregates, but the catalytic surface area increases, leading to an increase in the percentage of degradation.
The decrease in efficiency of one ZnO concentration compared to another can be explained by many characteristics.
However, characteristics such as specific surface area, charge recombination, interfacial charge transfer dynamics, particle size, crystallinity, pH etc. can have an influence.
On the other hand, the activity of a photocatalyst is also influenced by the type of degradation intermediates generated. A study carried out by (Ryu and Choi 2007) has shown that the photo catalytic activity varies according to the pollutant tested.
Indeed, by applying a high concentration of ZnO, more photons can be absorbed and therefore more radicals can be formed. In addition, more diclofenac can be adsorbed on the surface of the photo-catalyst. This promotes the degradation of diclofenac.
III.2 Influence of diclofenac concentration
The study of heterogeneous photo catalytic degradation was carried out with three concentrations of diclofenac (50; 75 and 100 mg/L). The concentration of the ZnO catalyst is 1.5 g/L. The volume flow rate is 20.5 mL/s.
III.2.1 Hydrogen potential (pH)
The pH variation during heterogeneous photo catalytic treatment as a function of the three concentrations of diclofenac is shown in Figure.11.
The pH increases from (7.38; 7.26 and 7.22) to (8.37; 8.45 and 7.89) respectively for the three concentrations studied and after 1 hour of treatment and then decreases until the end of treatment. These results can be explained by the competition reactions for the active sites on the surface of ZnO which involves the displacement of OH- ions, thus decreasing the trapping of holes by these hydroxyl ions and reducing the charge separation.
Moreover, the occupation of these active sites can lead to the exclusion of organic compounds from the active sites on the surface of the photo-catalyst (Burns 1999). .
III.2.2 Electrical conductivity (EC)
The EC variation during heterogeneous photo catalytic treatment as a function of the three concentrations of diclofenac is shown in Figure.12.
For both concentrations (50 and 100 mg/L) of diclofenac; conductivity increases from (22.6 and 28.9 µS/cm) to (99 and 77.8 µS/cm) respectively during treatment.
For the concentration of 75 mg/L diclofenac; conductivity increases from 24 to 91.4 µS/cm after 1h of treatment and then decreases to 34.5 µS/cm at t=2h and remains constant.
The EC of the aqueous solution is mainly related to the presence of ions in the medium. The results show that the EC value obviously takes a significant part in the degradation of the organic compound. Indeed, the higher the conductivity, the greater the degradation of diclofenac.
III.2.3 Turbidity (Tu)
The variation of Tu during the heterogeneous photo catalytic treatment as a function of the three concentrations of diclofenac is shown in Figure.13.
For the concentration of 50 mg/L diclofenac; turbidity increases from 4.6 up to 32 NTU after 2h above t=2h the Tu is unstable (values are between 13.2 and 18 NTU).
For the concentration of 75 mg/L of diclofenac; turbidity increases from 4.1 to 19.8 NTU after 1 hour of treatment and then it varies little (between 8.6 and 10.7 NTU).
For 100 mg/L diclofenac concentration; turbidity increases from 2.9 to 15.7 NTU at t=5h.
The variation in turbidity during treatment may be due to light-deflecting degradation products. Thus, photo-dismutation of the catalyst may be possible.
III.2.4 Surface tension (Ts)
The variation of Ts during heterogeneous photo catalytic treatment as a function of the three concentrations of diclofenac is shown in Figure.14.
For the concentrations studied (50; 75 and 100 mg/L) of diclofenac; fluctuations are observed during treatment.
The observed fluctuations in surface tension are explained by the fact that during treatment, the diclofenac molecule is degraded leading to degradation by-products that compete with the parent molecule. This calls for a concentration gradient in the liquid which influences the Solid/Liquid contact surface.
One can distinguish the angle at the advance θa which is at the front of the drop or liquid front and the angle at the retreat θr which is at the back and generally, θa>θe>θr (De Gennes 1985, Dussan 1976).
The difference between these different angles comes in particular from the roughness and heterogeneity (chemical, morphological ...) of the surface, or the degradation by-products.
III.2.5 Chemical oxygen demand (COD)
The variation in COD during heterogeneous photo catalytic treatment as a function of the three concentrations of diclofenac is shown in Figure.15.
For all three concentrations (50; 75 and 100 mg/L) of diclofenac; COD decreases from (130; 250 and 360 mg O2/L) to (32; 90 and 116 mg O2/L) respectively.
Chemical Oxygen Demand (COD) represents the amount of oxygen consumed by oxidizable materials in the water.
The decrease in COD in the presence of the ZnO catalyst is explained by the absorption of light by the catalyst and the production of the hydroxyl radicals responsible for degradation.
Degradation products can also induce different interactions between ZnO and diclofenac, especially in the case of negatively charged compounds at basic pH such as carboxylic acids, which appear in the mechanisms of photo-catalytic degradation.
Different diclofenac/ZnO surface interactions could lead to a modified degradation mechanism and thus to different kinetics.
III.2.6 Nitrites
The variation of nitrites during heterogeneous photocatalytic treatment as a function of the three concentrations of diclofenac is shown in Figure.16.
For all three concentrations (50; 75 and 100 mg/L) of diclofenac; the nitrite concentration increases during treatment from (0.0086; 0.0087 and 0.0086 mg/L) to (0.0188; 0.0224 and 0.0195 mg/L) respectively.
The increase in nitrite concentration indicates that degradation begins with the breakdown of the nitrogen bond and the cyclic nucleus of diclofenac.
III.2.7 Concentration of diclofenac
The variation in concentration during heterogeneous photo catalytic treatment is shown in Figure.17.
For all three concentrations (50; 75 and 100 mg/L) the concentration of diclofenac decreases during treatment to (29.15; 42.47 and 48.11 mg/L) respectively.
The yield of heterogeneous photo-catalytic degradation to diclofenac is shown in Figure.18.
The 100 mg/L concentration of diclofenac gives a high percentage of elimination which is 52%.
The higher the initial concentration of diclofenac, the greater the efficiency of the photo-catalytic degradation of diclofenac.
III.3 Influence of the hydrogen peroxide (H2O2) concentration
The study of heterogeneous photo catalytic degradation was carried out using hydrogen peroxide H2O2 whose ratio r = [H2O2]/[ZnO] is (1 ; 5 and 10%).
The concentration of diclofenac is 100 mg/L. The catalyst concentration is 1.5 g/L. The volume flow rate is 20.5 mL/s.
The results showed that degradation is complete after 1h of treatment for all three r-ratios. These results are confirmed by the absence of the diclofenac absorption peak (274 nm). The absorption spectrum of diclofenac is shown in Figure.19.
These results are explained by the effect of hydrogen peroxide on the optical properties of ZnO. A study (Ozgur. U et al., 2005) has shown that in the presence of H2O2, ZnO has an emission band with a maximum of 3.24 eV and a wide composite band composed of three sub-bands at 2.2 eV; 2.06 eV and 1.8 eV. This promotes the formation of a large amount of OH.
Thus, the concentration of hydrogen peroxide causes an increase in the amount of OH. radicals in the medium and therefore the degradation yield increases.
It should also be noted that the quantity of the organic pollutant drops abruptly after one hour of treatment. This high efficiency is mainly due to the rapid formation, in large quantities, of OH radicals. which facilitates the attack between a large quantity of the oxidizing agent and diclofenac.
III.3.1 Hydrogen potential (pH)
The variation of pH during heterogeneous photo catalytic treatment as a function of the three ratios r is shown in Figure.20.
For the 1% ratio; the pH decreases from 7.22 to 6.14 after 1 hour of treatment.
For both ratios (5 and 10%); the pH increases from 7.22 to (8.23 and 7.41) respectively after 1 hour of treatment.
For pH values more acidic than PZC (Im 2012), the ZnO particle will be positively charged and attraction will be favoured for anionic molecules.
III.3.2 Electrical conductivity (EC)
The variation of EC during heterogeneous photo catalytic processing as a function of the three ratios r is shown in Figure.21.
For all three ratios (1; 5 and 10%); EC increases from 28.9 to (43.6; 38.1 and 34.7 µS/cm) respectively after 1 hour of treatment.
The EC of the aqueous solution is mainly related to the presence of ions in the medium. The results show that the EC value obviously takes a significant part in the degradation of the organic compound. The increase in electrical conductivity is due to the presence of the degradation compounds of diclofenac. Indeed, the higher the conductivity, the greater the degradation of diclofenac.
III.3.3 Turbidity (Tu)
The variation of Tu during photo-catalytic treatment as a function of the three ratios r is shown in Figure 22.
For all three ratios (1%; 5% and 10%); Tu increases from 2.9 to (9.6; 7.4 and 8.3 NTU) respectively after 1 hour of treatment.
Turbidity is an important parameter and is likely to affect the photocatalytic efficiency. Indeed, high turbidity can affect the optical properties and hinder light penetration. This will decrease the absorption of photons by the photocatalyst.
III.3.4 Surface tension (Ts)
The variation of Ts during heterogeneous photo-catalytic treatment as a function of the three ratios r is shown in Figure.23.
For all three ratios (1; 5 and 10%); Ts increases from 56.9 to (58.81; 61.35 and 69.74 mN/m) respectively after 1 hour of treatment.
As the surface tension increases, the catalyst is well wetted, which may increase the transfer of liquid/solid material. This increases the photocatalytic efficiency.
III.3.5 Nitrites
The variation of nitrites during heterogeneous photocatalytic treatment as a function of the three ratios r is shown in Figure.24.
For all three ratios (1; 5 and 10%); nitrite increases from 0.0086 to (0.0227; 0.0097 and 0.0267 mg/L) respectively after 1 hour of treatment.
The increase in nitrite concentration indicates that degradation begins with the breakdown of the nitrogen bond and the cyclic nucleus of diclofenac.
III.4 Influence of volume flow rate
The study of heterogeneous photo catalytic degradation was performed at three volume flow rates (20.5; 22 and 25 mL/s). The concentration of diclofenac is 100 mg/L. The catalyst concentration is 1.5 g/L.
The results showed that the degradation is complete for the volume flow rate 25 mL/s after 1h of treatment.
This result is confirmed by the absence of the diclofenac absorption peak (274 nm).
The absorption spectrum of diclofenac for the 25 mL/s volume flow rate is shown in Figure.25.
III.4.1 Hydrogen potential (pH)
The variation of pH during heterogeneous photo catalytic treatment as a function of the three volume flows is shown in Figure.26.
For both volume flows (20.5 and 22 mL/s); the pH increases from 7.22 to (7.89 and 8.73) respectively at t=1h and then varies little.
For the volume flow rate of 25 mL/s, the pH increases from 7.22 to 8.25 after 1h of treatment.
The increase in pH is due to competition reactions for the active sites on the surface, which involves the displacement of OH- ions, thus reducing the trapping of holes by these hydroxyl ions and reducing charge separation.
Moreover, the occupation of these active sites can lead to the exclusion of organic compounds from the active sites on the surface of the photo-catalyst [18].
III.4.2 Electrical conductivity (EC)
The variation of EC during heterogeneous photocatalytic treatment as a function of the three volume flows is shown in Figure 27.
For the volume flow rate of 20.5 mL/s; the EC increases from 28.9 to 74.5 µS/cm after 3 hours of treatment; beyond 1 hour the EC varies little.
For a volume flow rate of 22 mL/s; EC increases from 28.9 to 51.6 µS/cm after 1h of treatment, then decreases to 38.2 at t=2h, after 2h it varies little.
For a volume flow rate of 25 mL/s; EC increases from 28.9 to 40.1 µS/cm after 1 hour of treatment.
The electrical conductivity of the aqueous solution is mainly related to the presence of ions in the medium. The results show that the EC value obviously takes a significant part in the degradation of the organic compound. Indeed, the higher the conductivity, the greater the degradation of diclofenac.
III.4.3 Turbidity (Tu)
The variation of Tu during the heterogeneous photo catalytic treatment as a function of the three volume flows is shown in Figure.28.
For volume flow rate of 20.5 mL/s; Tu increases from 2.9 to 15.7 NTU at t=5h.
For volume flow rate of 22 mL/s; Tu increases from 2.9 to 37 NTU at t=5h.
For volume flow rate of 25 mL/s; Tu increases by 2.9 NTU to 33.1 NTU after 1h of treatment.
High turbidity can affect the optical properties and hinder light penetration. This will decrease the absorption of photons by the photocatalyst.
Some authors have shown that a turbidity lower than 5 NTU would not affect light penetration.
Above this value, turbidity would interfere with optimal use of light (Fernandez-Ibanez 2009, Gelover 2006, Chong 2010).
This explains the decrease in the efficiency of photo catalytic treatment when turbidity increases.
III.4.4 Surface tension (Ts)
The variation of Ts during heterogeneous photo catalytic treatment as a function of the three volume flows is shown in Figure.29.
For a volume flow rate of 20.5 mL/s; Ts varies little (between 56.9 and 59.76 mN/m).
For a volume flow rate of 22 mL/s; Ts decreases from 56.9 mN/m to 40.97 mN/m at t=4h and then increases to 64.96 mN/m at t=5h.
For the volume flow rate of 25 mL/s; Ts increases from 56.9 to 62.31 mN/m after 1h of treatment.
The surface tension of the solutions is lower than that of pure water (72 mN/m). The interaction between the phases in a reactor depends on the velocity and physico-chemical properties of the fluid phases, as well as the geometric and physico-chemical properties of the solid particles. It is important to note that the contact surface between phases, which itself depends on the surface tension, varies according to the flow regime.
III.4.5 Chemical Oxygen Demand (COD)
The variation of COD during heterogeneous photo catalytic treatment as a function of the three volume flows is shown in Figure.30.
For both volume flow rates (20.5; 22 mL/s); COD decreases from 360 to (116 and 15 mg O2/L) respectively during treatment.
For the 25 mL/s volume flow rate, COD decreases from 360 to 15 mg O2/L after 1 hour of treatment.
Chemical Oxygen Demand (COD) represents the amount of oxygen consumed by oxidizable materials in the water. The decrease in COD in the presence of the ZnO catalyst is explained by the absorption of light by the catalyst and the production of hydroxyl radicals responsible for degradation. It can be seen that the rate of mineralization is rapid at the beginning of the treatment and then it decreases. Indeed, at the beginning of the treatment, the mother molecule and its degradation products are more easily oxidized by hydroxyl radicals leading to the formation of aliphatic compounds like carboxylic acids... (Oturan, 2000, Brillas 2003, Boye 2002, Diagne, 2007) that resist well to mineralization by these same radicals.
III.4.6 Nitrites
The variation of nitrites during heterogeneous photo catalytic treatment as a function of the three volume flows is shown in Figure.31.
For both volume flow rates (20.5; 22 mL/s); the nitrites increase from 0.0086 to (0.0195; 0.0178 mg/L) respectively after 5h of treatment.
For the volume flow rate 25 mL/s, nitrite increases from 0.0086 to 0.0109 mg/L at t=1h.
The increase in nitrite concentration indicates that degradation begins with the breakdown of the nitrogen bond and the cyclic nucleus of diclofenac.
Nitrites can affect the photocatalytic process in several ways:
- Radical and hole traps which influence the quantum efficiency and thus the oxidation efficiency of the pollutant,
- Compete with the drug during adsorption to ZnO,
- Capturing photons - Corrosion or alteration of the photocatalyst
- React directly with the photo catalyst (Chong 2010).
However, it is interesting to recall that the relative effects of these ions on the photo catalytic process can be variable as a function of the pH, which influences the zwitterionic nature of ZnO.
III.4.7 Concentration of diclofenac
The variation in concentration during the heterogeneous photo catalytic treatment as a function of the three volume flows is shown in Figure.32.
For both volume flow rates (20.5; 22 mL/s) the diclofenac concentration decreases to (48.11; 24.56 mg/L) respectively during treatment.
For the volume flow rate 25 mL/s, the diclofenac concentration decreases from 100 to 12.35 mg/L after 1 hour of treatment.
The photo-catalytic degradation performance of diclofenac as a function of the three flow rates is shown in Figure.33.
The volume flow rate of 25 mL/s gives the highest removal percentage (100%).
The increase in degradation yield with increasing flow rate is explained by :
The thickness of the absorbent layer of the treated fluid varies along the reactor. Indeed, at the reactor inlet, the conversion is null x = 0, and the absorbent layer consists only of ZnO. In this case, the majority of the photons are absorbed by the ZnO.
Then along the reactor, the conversion progresses and the layer becomes depleted in diclofenac and, conversely, it becomes enriched in degradation compounds. The photons are then absorbed by both ZnO and degradation compounds.
The distribution of the absorbed photons between ZnO and the degradation compounds will depend on the hydrodynamic conditions of the reactor.
The hydrodynamic regime is defined by the Reynolds number.
Table 1 regroups the Reynolds number for the three volume flows studied.
Table 1. The Reynolds number for the different volume flows.
|
ρ (Kg/m3)
|
103
|
|
µ (Pa.s)
|
10-3
|
|
V (mL/s)
|
20.5
|
22
|
25
|
|
Re
|
2611
|
2802
|
3184
|
For both volume flows (20.5 and 22 mL/s) the regime is transient.
For the volume flow rate of 25 mL/s the regime is turbulent.
For a given Reynolds number, the efficiency of photo-catalysis is influenced by :
- The amount of material passing through the reactor.
- The residence time 𝜏𝑠 of the molecules in the reactor.
- The light power.
The flux density of incident photons will play on the local speed of the photochemical reaction, which can impact the local and average characteristic times in the zone of illuminated fluid.