Effect of Nickel-Aluminum Trioxide/Titanium Dioxide Nanocomposites on Photo-oxidation of Olive Mill Wastewater

Raw Wastewater

The characterization of raw OMW taken from the influent of a olive oil production industry in Izmir, Turkey is given in Table 3. This plant is operated with a three phase olive oil extraction process.

Preparation of Ni / Al2O3 / TiO2 Photocatalyst

An Al2O3 (d50 = 210 nm) powder was mixed with a TiO2 (d50 = 220 nm) powder by ball milling in deionized H2O for 24 h. NH3was added drop by drop into the slurry to reach a pH=9.2. A separate solution of the nickel nitrate [Ni(NO3)2•6H2O] was also prepared. The pH value of Ni(NO3)2•6H2O solution was also adjusted to 9.2. The Al2O3 and TiO2 slurry was poured into Ni(NO3)2•6H2O solution and then stirred for 30 min. The Ni+2 ion could then be absorbed onto the surface of Al2O3 particles. The starting amounts of TiO2 and Ni added into the slurry were adjusted to result in 5 vol% each to that of Al2O3. The resulting powder mixtures after coating were filtered, washed and dried. The powder mixtures were reduced in pure hydrogen at 550°C for 1 h, followed by ball milling in ethyl alcohol for 24 h with Al2O3 grinding media. The Al2O3 powder, Al2O3–TiO2 and Al2O3– Ni powder mixtures were also prepared with the same technique.

5 g of TiO2, 5 g of Al2O3 and 2g of Ni(NO3)2•6H2O were weighed, where the mass ratios of (1%/5%/10%; 10%/1%/5% and 1%/10%/5%), respectively. These three mixed raw materials with the designed proportion were grounded with an agate mortar for 30 min, and then sintered in a crucible under nitrogen gas [N2(g)] atmosphere at 400°C for 2 h, with the temperature rise rate of 20°C.1/min. These samples were obtained after annealing and cooling down to the room temperature (at 25°C). The as-prepared composite was named according to the preparation conditions as illustrated by “Ni/Al2O3/TiO2”. The mass ratios of Ni/Al2O3/ TiO2 were adjusted as 1%/5%/10%; 10%/1%/5% and 1%/10%/5%, respectively.

Photocatalytic Degradation Reactor

A two liter cylinder kuvars glass reactor was used for the photodegradation experiments in the OMW under 500 W UV-vis and 50 W sun light irradiations, at different operational conditions. 1000 ml the OMW was filled for experimental studies and the photocatalyst were added to the cylinder glass reactor. The photocatalytic reaction was operated with constant stirring during the photocatalytic degradation process 500 W under UV-vis light and 50 W sun light irradiations. 10 ml of the reacting solution were sampled and centrifugated (at 10000 rpm) at different time intervals.

Experimental Chemicals

Nano-Al2O3, nano-Ni(Ac)2, nano-TiO2 and Ni(NO3)2•6H2O were purchased from Merck, (Germany). Helium, He(g) [gas chromotograpy (GC) grade, 99.98%) and N2(g) (GC grade, 99.98%) was purchased from Linde, (Germany). Catechol (99%), 3-hydroxybenzoic acid (99%), tyrosol (99%), 4-hydroxybenzoic acid (99%), 4-hydroxyphenylacetic acid (99)%, 3-hydroxyphenylpropionic acid (99%), 4-hydroxyphenylpropionic acid (99%), 3,4-dihydroxyphenylethanol, 3,4-dihydroxyphenylacetic acid (99%), aniline (99%), 4–nitroaniline (99%), o–toluidine (99%), anisidine (99%), benzene (99%), nitrobenzene (99%), ethylbenzene (99%), 3,6-bis(dimethylamino)durene (99%), benzidine (99%), dimethylaniline (99%) and 3,3– dichlorobenzidine (99%) were purchased from Aldrich, (Germany).

Microtox Acute Toxicity Test

Toxicity to the bioluminescent organism Aliivibrio fischeri (also called Vibrio fischeri or V. fischeri) was assayed using the Microtox measuring system according to DIN 38412L34, L341, (EPS 1/ RM/24 1992). Microtox testing was performed according to the standard procedure recommended by the manufacturer [33]. A specific strain of the marine bacterium, V. fischeri-Microtox LCK 491 kit was used for the Microtox acute toxicity assay. Dr. LANGE LUMIX- mini type luminometer was used for the microtox toxicity assay [34].

Daphnia Magna Acute Toxicity Test

To test toxicity, 24-h born Daphnia magna were used as described in Standard Methods sections 8711A, 8711B, 8711C, 8711D and 8711E, respectively [35]. After preparing the test solution, experiments were carried out using 5 or 10 Daphnia magna introduced into the test vessels. These vessels had 100 ml of effective volume at 7.0– 8.0 pH, providing a minimum dissolved oxygen (DO) concentration of 6 mg/l at an ambient temperature of 20–25°C. Young Daphnia magna were used in the test (≤24 h old); 24–48 h exposure is generally accepted as standard for a Daphnia magna acute toxicity test. The results were expressed as mortality percentage of the Daphnia magna. Immobile animals were reported as dead Daphnia magna.

Statistical Analysis

ANOVA analysis of variance between experimental data was performed to detect F and P values. The ANOVA test was used to test the differences between dependent and independent groups, [36]. Comparison between the actual variation of the experimental data averages and standard deviation is expressed in terms of F ratio. F is equal (found variation of the date averages/expected variation of the date averages). P reports the significance level, and d.f indicates the number of degrees of freedom. Regression analysis was applied to the experimental data in order to determine the regression coefficient R2, [37]. The aforementioned test was performed using Microsoft Excel Program. All experiments were carried out three times and the results are given as the means of triplicate samplings. The data relevant to the individual pollutant parameters are given as the mean with standard deviation (SD) values.

Characterization Values of the OMW

The characterization values of raw OMW at pH=4.5 taken from the influent of a olive oil production industry in Izmir, Turkey is given in Table 3. This plant is operated with a three phase olive oil extraction process.

Effect of Mass Ratios of Ni/Al2O3/TiO2 on the OMW Pollutant Parameters

The photocatalytic performance of the synthesized catalyst sample which combined with three different Ni/ Al2O3/TiO2 mass ratios of (1%/5%/10%; 10%/1%/5% and 1%/10%/5%) (Table 4). The maximum photodegradation efficiency was obtained at 1%/10%/5% mass ratio of Ni/Al2O3/TiO2 which is the maximum Ni can be act as photosensitizer, absorbing the UV-vis and sun lights (Table 4), which can inject the photogenerated electrons into TiO2 conduction band as mentioned in the study performed by Tuan WH, et al. [38].

In other words, the interfacial behavior between Ni and TiO2 may increase the photo-generated electron mobility in Al2O3. Meanwhile, the synergetic effect of the intrinsic properties of Ni and component in the present NCs is also beneficial for the electron transfer in the conduction band to reduce the pollutants (COD components, polyphenols and polyaromatics) in the OMW. Approximately similar photooxidation yields were obtained for Ni/Al2O3/TiO2 mass ratios of 1%/5%/10% (the yields are very slightly lower than the removal efficiencies at 1%/10%/5% Ni/Al2O3/TiO2

mass ratio), since with high TiO2 mass ratio more electrons were activated by production of high OH● resulting in high pollutant photo-degradation in the OMW (Table 4). The effect of high Ni mass ratios in the Ni/Al2O3/TiO2 NCs formation (10%/1%/5%) was found to be not so significant. However, higher pollutant photodegradation yields were obtained for this Ni mass ratio (Table 4). The slightly lower photodegradation yields can be discussed as follows: The weakly bounded Ni on the stoichiometric TiO2 surface tend to migrate and aggregate to form larger clusters on Al2O3 [19]. TiO2 is softer than Al2O3, so the hardness of samples always decreased with increases of TiO2 contents [39]. In this study, demonstrates that nanometer-sized Ni particles can be distributed uniformly onto the surface of Al2O3 particles by employing a coating technique. As long as both TiO2 and Ni particles are incorporated simultaneously into Al2O3 matrix, the coarsening of matrix grains is constrained.

Establishment of the Optimum Retention Time for Photocatalytic Oxidations of the Pollutants in the OMW

The effects of increasing photooxidation retention times (10, 30, 60, 100 and 120 min) on the photocatalytic oxidation of pollutant parameters in the OMW, under 500 W UV-vis and 50 W sun lights, at 500 mg/l Ni/Al2O3/TiO2 NCs, at pH=9.0, at 50°C are shown in Table 4. The maximum CODtotal , CODdis, polyphenols and polyaromatics yields in the OMW were 99%, 98%, 88% and 94%, respectively, under 500 W UV- vis light, at 500 mg/l Ni/Al2O3/TiO2 NCs, at 1%/10%/5% mass ratio of Ni/Al2O3/TiO2, at pH=9.0, at 50°C, after 100 min, respectively Table 4. The maximum CODtotal , CODdis, total phenols and TAAs yields in the OMW were 96%, 95%, 86% and 92%, respectively, under 50 W sun light, at 500 mg/l Ni/Al2O3/TiO2 NCs, at 1%/10%/5% mass ratio of Ni/Al2O3/ TiO2, at pH=9.0, at 50oC, after 100 min, respectively Table 4. As the photo-oxidation times were increased from 10 min up to 60 min in the presence of 500 mg/l Ni/Al2O3/TiO2 NCs, the photooxidation yields were increased in all pollutant parameters in the OMW under 500 W UV-vis light (Table 4).

Similarly, the removal efficiencies increased as the contact time between pollutants and 500 mg/l Ni/Al2O3/TiO2 NCs posite increased from 10 min to 60 min in the OMW, under 50 W sun light. On the other hand, the removal efficiencies of pollutant parameters in the OMW slightly decreased in the same UV-vis and sun ligths as the increasing retention times from 100 to 120 min, respectively (Table 4). Low contact times cannot be enough for OH● production throughout photooxidation process while high contact times can be decompose the structure and the pores of Ni/Al2O3/TiO2 NCs, and the photocatalysts may to covered completely with the particles of pollutant parameters (CODtotal, CODdis, total phenols and TAAs. Therefore, it was that the maximum removal efficiencies were observed at 100 min retention time during experimental studies Table 4.

Effect of Different Ni/Al2O3/TiO2 NCs Concentrations on the Photocatalytic Oxidation of the Pollutants in the OMW

Since the maximum photooxidation removals of Ni/Al2O3/TiO2 NCs was obtained with mass ratiof of 1%/10%/5% the studies performed with this form of the nanocomposite synthesized under laboratory conditions. The rate of photocatalytic reaction and the removals of pollutants in the OMW are strongly influenced by the amount of the photocatalyst. Heterogeneous photocatalytic reactions are known to show proportional increase in photooxidation with catalyst loading. Generally, in any given photocatalytic application, the optimum catalyst concentration must be determined in order to avoid excess usage of catalyst and to ensure the total absorption of efficient photons.

As shown in Table 5, four different Ni/Al2O3/TiO2 NCs concentrations (50, 250, 500 and 1000 mg/l) were used to determine the maximum yields of pollutant parameters {COD components [CODtotal, CODdis, CODinert, respectively], polyphenols [catechol, 3-hydroxybenzoic acid, tyrosol, 4-hydroxybenzoic acid, 4-hydroxyphenylacetic acid, 3-hydroxyphenylpropionic acid, 4-hydroxyphenylpropionic acid, 3,4-dihydroxyphenylethanol, 3,4-dihydroxyphenylacetic acid, respectively], TAAs metabolites [aniline, 4–nitroaniline, o–toluidine, o-anisidine, benzene, nitrobenzene, ethylbenzene, 3,6-bis(dimethylamino)durene, benzidine, dimethylaniline, 3,3–dichlorobenzidine, respectively]} in the OMW throughout photocatalytic oxidation, under UV-vis and sun lights. The maximum CODtotal, CODdis and CODinert removal effficiencies in the OMW were 99%, 98% and 73%, respectively, under 500 W UV-vis light, at 500 mg/l Ni/Al2O3/TiO2 NCs, at 1%/10%/5% mass ratio of Ni/ Al2O3/TiO2, at pH=9.0, at 50oC, after 100 min, respectively Table 5. The maximum removal efficiencies of total phenols and polyphenols; such as, catechol, 3-hydroxybenzoic acid, tyrosol, 4-hydroxybenzoic acid, 4-hydroxyphenylacetic acid, 3-hydroxyphenylpropionic acid, 4-hydroxyphenylpropionic acid, 3,4-dihydroxyphenylethanol, 3,4-dihydroxyphenylacetic acid in the OMW were 88%, 87%, 86%, 88%, 83%, 86%, 85%, 88%, 88% and 87%, respectively, under 500 W UV-vis light, at 500 mg/l Ni/Al2O3/TiO2 NCs, at 1%/10%/5% mass ratio of Ni/Al2O3/TiO2, at pH=9.0, at 50oC, after 100 min, respectively Table 5.

The maximum removal efficiencies of TAAs and TAAs metabolites; such as, aniline, 4–nitroaniline, o–toluidine, o-anisidine, benzene, nitrobenzene, ethylbenzene, 3,6-bis(dimethylamino)durene, benzidine, dimethylaniline, 3,3–dichlorobenzidine in the OMW were 94%, 92%, 90%, 91%, 89%, 88%, 93%, 92%, 85%, 93%, 89% and 90%, respectively, under 500 W UV-vis light, at 500 mg/l Ni/Al2O3/ TiO2 NCs, at 1%/10%/5% mass ratio of Ni/Al2O3/TiO2, at pH=9.0, at 50oC, after 100 min, respectively Table 5. On the other hand, the pollutant parameters (COD components, polyphenols and TAAs metabolites) removal efficiencies were increased from 50 to 250 mg/l Ni/Al2O3/TiO2 NCs concentrations under 500 W UV-vis light. Therefore, the removal efficiencies of the pollutant parameters (COD components, polyphenols and TAAs metabolites) were slightly decreased from 500 to 1000 mg/l Ni/Al2O3/TiO2 NCs concentrations under 500 W UV-vis light Table 5. The observation was that removal efficiency increased as the quantity of the nanocomposite was increased. However, in this study low photooxidation removals was observed with low (50 mg/l) and high (1000 mg/l) Ni/Al2O3/ TiO2 concentrations. The inhibition effect of over loaded photocatalysts concentrations for the pollutant parameter removals in the OMW was detected. Probably the structural decomposition of photocatalysts, the pore surfaces of photocatalysts may to cover completely with the particles of pollutant parameters (CODtotal, CODdis, total phenols and TAAs) or other radical species [carbon based radicals such as carboxyl radicals (CO2●)]. The effect of photocatalyst quantity can be explained by the fact that decreasing the amount of photocatalyst decreases the number of activities on the Ni–TiO2 surface, which in turn in decreases the numbers of hy OH● and hydroperoxyl (OH2●) radicals.

The maximum CODtotal, CODdis and CODinert removal effciencies in the OMW were 96%, 95% and 70%, respectively, under 50 W sun light, at 500 mg/l Ni/Al2O3/ ZrO2 NCs, at 1%/10%/5% mass ratio of Ni/Al2O3/TiO2, at pH=9.0, at 50°C, after 100 min, respectively (Table 5). The maximum removal efficiencies of total phenols and polyphenols; such as, catechol, 3-hydroxybenzoic acid, tyrosol, 4-hydroxybenzoic acid, 4-hydroxyphenylacetic acid, 3-hydroxyphenylpropionic acid, 4-hydroxyphenylpropionic acid, 3,4-dihydroxyphenylethanol, 3,4-dihydroxyphenylacetic acid in the OMW were 86%, 85%, 86%, 82%, 84%, 83%, 86%, 86% and 85%, respectively, under 50 W sun light, at 500 mg/l Ni/Al2O3/TiO2 NCs,, at 1%/10%/5% mass ratio of Ni/Al2O3/TiO2, at pH=9.0, at 50oC, after 100 min, respectively (Table 5). The maximum removal effciencies of TAAs and TAAs metabolites; such as, aniline, 4–nitroaniline, o– toluidine, o-anisidine, benzene, nitrobenzene, ethylbenzene, 3,6-bis(dimethylamino)durene, benzidine, dimethylaniline, 3,3–dichlorobenzidine in the OMW were 90%, 88%, 90%, 87%, 87%, 91%, 91%, 84%, 92%, 87% and 88%, respectively, under 50 W sun light, at 500 mg/l Ni/Al2O3/TiO2 NCs, at 1%/10%/5% mass ratio of Ni/Al2O3/TiO2, at pH=9.0, at 50°C, after 100 min, respectively (Table 5). The result of the photocatalytic oxidation yields of the pollutant parameters in the OMW with Ni/Al2O3/TiO2 under sun light showed that the yields were slightly lower than the photocatalytic oxidation yields of the pollutant parameters in the OMW under UV-vis light. In addition to, the pollutant parameters (COD components, polyphenols and TAAs metabolites) removal efficiencies were increased from 50 to 250 mg/l Ni/Al2O3/TiO2 NCs concentrations under 50 W sunlight. However, the removal efficiencies of the pollutant parameters (COD components, polyphenols and TAAs metabolites) were slightly decreased from 500 to 1000 mg/l Ni/Al2O3/TiO2 NCs concentrations under 50 W sun light Table 5.

Tuan WH, et al. [38] reported to the dense Al2O3/(TiO2 + Ni) NCs are prepared by pulse electric current sintering (PECS) at 1350°C for 5 min or by pressureless sintering (PLS) at 1600°C for 60 min. The sub-micrometer-sized TiO2 particle acts as microstructural stabilizer that slows down the coarsening of matrix grains in the composites prepared by both processes. Due to microstructural refinement, the strength of the Al2O3/(5%TiO2 + 1%Ni) nanocomposite is ≈ 40% higher than that of Al2O3 alone [38].

Establishment of Optimum Temperature Value for Photocatalytic Oxidation of the OMW Pollutants with Ni/Al2O3/TiO2

The effects of increasing temperature values (15, 25, 50 and 80°C) on the photocatalytic oxidation of the OMW pollutants was investigated, under 500 W UV-vis and 50 W sun lights, at optimum photocatalyst concentration (500 mg/l Ni/Al2O3/TiO2 NCs), at optimum pH value (pH=9.0), at optimum retention time (100 min). The increasing of temperature values from 15 to 25°C showed a raise in the removal efficiences of OMW pollutants in both UV-vis conditions. The maximum CODtotal, CODdis, total phenols and TAAs removal effciencies in the OMW were 99%, 98%, 88% and 94%, respectively, under 500 W UV-vis light, at pH=9.0, at 50°C, after 100 min, respectively Table 6. The removal efficiencies of CODtotal , CODdis, total phenols and TAAs in the OMW were 96%, 95%, 86% and 92%, respectively, under 50 W sun light, at 500 mg/l Ni/Al2O3/TiO2 NCs, at 1%/10%/5% mass ratio of Ni/Al2O3/TiO2, at pH=9.0, at 50°C, after 100 min, respectively Table 6. The photocatalytic oxidation yields of the pollutant parameters slightly decreased as the temperature increased from 50 to 80°C in both types of irradiation ligths Table 6. Therefore, the optimum operational temperature was selected as 50°C for the maximum removals of pollutant parameters in the OMW during photocatalytic oxidation.

Establishment of the Optimum pH Value for Photocatalytic Oxidation in the OMW

Table 7 shows the effect of increasing pH values (4.0- 7.0-9.0-10.0) throughout photocatalytic oxidation on the yields of the OMW, under 500 W UV-vis and 50 W sun lights, at optimum retention time (100 min), at room temperature (25°C), at optimum photocatalyst concentration (500 mg/l Ni/Al2O3/TiO2 NCs). The maximum CODtotal , CODdis , total phenols and TAAs removal effciencies in the OMW were 99%, 98%, 88% and 94%, respectively, under 500 W UV- vis, at 500 mg/l Ni/Al2O3/TiO2 NCs, at pH=9.0, at 50°C, after 100 min, respectively Table 7. In addition to, CODtotal , CODdis , total phenols and TAAs removal efficiencies in the OMW were 96%, 95%, 86% and 92%, respectively, under 50 W sun light, at 500 mg/l Ni/Al2O3/TiO2 NCs, at pH=9.0, at 50°C, after 100 min, respectively Table 7. The photooxidation removal efficiencies decreased as the pH was decreased from 7.0 to 4.0 and increased from 7.0 up to 9.0 and up to 10.0 under both UV-vis and sun lights, at 500 mg/l Ni/Al2O3/TiO2 NCs, at 1%/10%/5% mass ratio of Ni/Al2O3/TiO2, at 50oC, after

100 min, respectively Table 7. This phenomenon may be attributed to the fact that as pH is neutral, the concentration of OH ions also increases, thus causing Ni–TiO2 to generate OH - and O - 2 more efficiently. However, it is also seen that the rate of removal diminished for the values beyond pH=7.0. This is possibly because at higher pH values, the negatively charged photocatalyst surface repulses the pollutant anions, thereby reducing the all pollutant efficiencies in the OMW. The lower photooxidation rate of the phenolic and aromatic compounds at pH=10.0 is likely a result of the low adsorption onto the surfaces of Ni/Al2O3/TiO2 NCs. Another explanation regarding the languor of the process at high pH levels is the presence of carbonate (CO3-2) ions, which could scavenge the OH●, or holes produced on the activated TiO2 surface, comprising a less reactive carbonate (CO3●) radical, slowing the degradation and mineralization process. Samples prepared at neutral pH exhibit more surface area and higher reactivity than those prepared at lower and higher pH. Photocatalytic oxidation process for optimum operational conditions were explained.

Photooxidation Mechanisms of Ni/Al2O3/TiO2 NCs

Overall, the mechanism of photocatalysis can be divided into five steps: (1) transfer of reactants in the fluid phase to the surface; (2) adsorption of the reactants; (3) reaction in the adsorbed phase; (4) desorption of the products; and (5) removal of products from the interface region [41, 42]. A photocatalyst is a substance that, after being irradiated by light, can induce a chemical reaction in such a way that the actual substance of the catalyst will not be consumed [43]. It is well known that the photocatalytic activity could be controlled by varieties of factors such as surface area, phase structure, interfacial charge transfer, and separation efficiency of photo-induced electrons and holes. In this work, Ni molecule which acted as an electron shuttle was mostly in contact with the surface of Al2O3/TiO2 composite so that it could effectively transfer the photoelectrons from conduction band of Al2O3/TiO2 composite after being illuminated under UV light irradiation. Therefore, the photo- generated electrons in the Ni/Al2O3/TiO2 photocatalyst could easily migrate from the inner region to the surface to take part in the surface reaction. Such series this photocatalysts can be easily recycled from the aqueous solution because of the soft magnetism feature of combined Ni particles. The photocatalytic performance and the enhanced photocatalytic mechanism occurred on the interface of Ni-TiO2 [34]. The mechanism of photooxidation of polyphenols, COD and TAAs on Ni/Al2O3/TiO2 NCs surface was as follows: The excitation of Ni/Al2O3/TiO2 NCs by solar energy leads to the formation of an electron–hole pair. The hole combines with H2O to form OH

• while electron converts O2(g) to superoxide radical (O2
• ¯ ), a strong oxidizing species as shown below (Equations 1, 2, 3, 4, 5, 6, 7 and 8):

2 3 2 2 3 2 Ni / / nanocomposite Ni / / nanocomposite VB CB Al O TiO hv Al O TiO h e + − + + +  (1)

2 3 2 2 3 2 Ni / / Ti nanocomposite Ni / / nanocomposite ( ) VB CB Al O O hv Al O TiO h e + − + +  (2)

2 3 2 2 3 2 Ni / / nanocomposite OH Ni / / nanocomposite OH VB Al O TiO h Al O TiO − + + → ● (3)

2 3 2 2 2 3 2 Ni / / nanocomposite OH Ni / / nanocomposite OH H VB Al O TiO h Al O TiO + + + → + ● (4)

2 3 2 org org Ni / / nanocomposite | OH Red Ox photooxidation of OMW pollutants Al O TiO + → → ● (5) org org Red Ox photooxidation of OMW pollutants VB h+ + → → (6)

2 2 OH OH H O + →

( ) 2 2 2 H O or 2OH 2H O + → + ● (8)

COD, polyphenols and TAAs are degraded via photo- oxidation process by reacting with both OH

• and h+VB [according to Eqs (1) → (2), (3) → (4) and (1) → (5)]. The OH
• shows electrophilic character and prefers to attack electron rich ortho or para carbon atoms of COD, polyphenols and TAAs. This results in the formation of polyphenol metabolites (catechol, 3-hydroxybenzoic acid, tyrosol, 4-hydroxybenzoic acid, 4-hydroxyphenylacetic acid, 3-hydroxyphenylpropionic acid, 4-hydroxyphenylpropionic acid, 3,4-dihydroxyphenylethanol, 3,4-dihydroxyphenylacetic acid) from total phenol and TAAs metabolites [aniline, 4– nitroaniline, o–toluidine, o-anisidine, benzene, nitrobenzene, ethylbenzene, 3,6-bis(dimethylamino)durene benzidine, dimethylaniline, 3,3–dichlorobenzidine] from TAAs are formed with photooxidation process in the OMW under UV-vis and sun light irradiation, respectively. Radicals that undergo further reaction with DO in the OMW to yield polyphenols by-products (catechol, 3-hydroxybenzoic acid, tyrosol, 4-hydroxybenzoic acid, 4-hydroxyphenylacetic acid, 3-hydroxyphenylpropionic acid, 4-hydroxyphenylpropionic acid, 3,4-dihydroxyphenylethanol, 3,4-dihydroxyphenylacetic acid), and polyaromatics metabolites [aniline, 4– nitroaniline, o–toluidine, o-anisidine, benzene, nitrobenzene, ethylbenzene, 3,6-bis(dimethylamino)durene, benzidine, dimethylaniline, 3,3–dichlorobenzidine], respectively, with simultaneous generation of hydrogen peroxide (H2O2) and (oxygen radicals (O2
• ). The O2
• ¯ produces H2O2 and O2 again by disproportion, and generation of OH
• accompanied with the production and consumption of H2O2.

Tuan WH et al. [38] also demonstrates that nanometer- sized Ni particles can be distributed uniformly onto the surface of Al2O3 particles by employing a coating technique. As long as both TiO2 and Ni particles are incorporated simultaneously into Al2O3 matrix, the coarsening of matrix grains is constrained. Due to the coarsening during sintering is limited by the addition of the inclusions; the nanocomposite can also be prepared by pressureless sintering.

The applications of ceramics as structural components are restricted because of their poor mechanical performance. To improve the mechanical properties of ceramics has thus attracted much attention. One of the most promising approaches is incorporating second-phase reinforcement into ceramic matrix [44]. The second-phase reinforcement can be either a ceramic or a metallic phase. The presence of the second-phase inclusions can prohibit the propagation of crack and thus enhance the toughness of ceramics.

Yang et al. [41] investigated by the influences of TiO2 NPs on the mechanical properties and microstructure of hot-pressing cerrium-tetragonal zirconia polycrystal (TZP) based aluminium trioxide [Ce-TZP/Al2O3] ceramics were investigated. Meanwhile, t-TiO2 to m-TiO2 transformation toughening mechanism was investigated by X-ray diffractometry (XRD) method, the results show that when the percentage of TiO2 was 20%, the mechanical properties and microstructures of materials are optimum. Moreover, transmission electron microscopy (TEM) observation show dislocation structures formation both in the Al2O3 and on the grain boundary [41].

Mechanisms of polyphenols by products: Pyrolysis differs from other high-temperature processes like combustion and hydrolysis in that it does not involve reactions with O2, H2O or any other reagents. Pyrolysis of organic substances produces gas and liquid products and leaves a solid residue richer in carbon content. Pyrolytic destruction of phenol in the gas phase is negligible; the degradation occurs mainly in the bulk solution. A possible explanation for this is that a considerable increase in the concentration results in the formation of a complex H-bonding network between the phenolic compounds. It is well known that molecules containing carboxyl or carbohydrates (COOH or CHO) groups exist as dimers in solution due to the formation of H-bonds between two neighbouring molecules. This results in a more robust and stable configuration, thus leading to reduced degradation [45]. At this study, the major phenolic compounds in OMW after photocatalytic oxidation process reported by [46, 47] and [48] as shown in Table 3. Benzene, tyrosol, caffeic acid…etc. (major aromatic amines) in OMW [49] during the photooxidation process of polyphenols.

Mechanisms of Polyaromatics Metabolites

Hydrolysis and pyrolysis are main degradation mechanisms for aromatic amines with photooxidation. The attack of non-volatile compounds in the ‘‘bulk’’ water by OHl that destroy the chromophoric system through azo bond cleavage. OHl attack leads to hydroxyl amines followed by subsequent oxidation forming aromatic nitroso and nitro compounds. The attack at the carbon atom adjacent to the azo bond, leading to phenyl derivative radicals. Further degradation pathways are difficult to predict since the fate of the fragments depends on their physical and chemical properties. Further reactions may occur inside the cavity (pyrolysis), in the hypercritical water layer or in the 1616bulk water [49]. In the present study, the major aromatic amines in the OMW after photooxidation process reported by [49] as seen in Table 3. Benzene (major aromatic amines) in the OMW [49] during the photooxidation process of polyaromatics.

Direct Effects of Ni/Al2O3/TiO2 NCs Concentrations on the Acute Toxicity of Microtox and Daphnia Magna in OMW

The acute toxicity test was performed in the samples containing 50 mg/l, 250 mg/l, 500 and 1000 mg/l Ni/ Al2O3/TiO2 NCs concentrations. In order to detect the direct responses of Microtox (with Aliivibrio fischeri) and Daphnia magna to the increasing Ni/Al2O3/TiO2 NCs concentrations the toxicity test was performed without OMW. The initial EC values and the EC50 values were measured in the samples containing increasing Ni/Al2O3/TiO2 NCs concentrations after 150 min photocatalytic oxidation time. Table 10 showed the responses of Microtox and Daphnia magna to increasing Ni/Al2O3/TiO2 NCs concentrations.

The acute toxicity originating only from 50, 250, 500 and 1000 mg/l Ni/Al2O3/TiO2 NCs were found to be low Table 10. 50 mg/l Ni/Al2O3/TiO2 NCs did not exhibited toxicity to Aliivibrio fischeri and Daphnia magna before and after 150 min photocatalytic oxidation time. The toxicity attributed to the 50, 500 and 1000 mg/l Ni/Al2O3/TiO2 NCs were found to be low in the samples without OMW for the test organisms mentioned above. The acute toxicity originated from the Ni/ Al2O3/TiO2 NCs decreased significantly to EC1, EC3 and EC7 after 150 min photocatalytic oxidation time. Therefore, it can be concluded that the toxicity originating from the Ni/ Al2O3/TiO2 NCs is not significant and the real acute toxicity throughout photocatalytic oxidation was attributed to the OMW, to their metabolites and to the photocatalytic oxidation by-products Table 10.