Wastewater problems and treatments

C H A P T E R

8 Wastewater problems and treatments Sahar Mansour, Sarra Knani, Rahma Bensouilah and Zouhaier Ksibi Laboratory of Materials Chemistry and Catalysis, Department of Chemistry, Faculty of Sciences of Tunis, Tunis El Manar University-Tunis, Tunisia

8.1 Introduction Water is a natural resource essential to all life on earth. Its availability, as well as its abundance, plays a fundamental role in the development of living beings. After oxygen, water is the second vital need of our body. Indeed, a man can survive 5 weeks without eating but only 3 4 days without drinking. However on land 97% of water reserves are salty, therefore unsuitable for consumption, and only 3% are potable [1]. This is why it has become known as “Blue Gold.” Apart from the scarcity of freshwater, the industrial revolution has created a serious problem of water contamination and caused alarming harm to humans and living organisms [2]. Water pollutants discharged into natural aquatic environments derived mainly from industrial effluents, agricultural runoff, and domestic sewage [3]. Wastewater management has become a necessity, and the subject of water purification and recycling has become more and more important in recent years. In this context, various approaches are designed to remove hazardous pollutants from wastewater effluents before being spilled directly into the environment without further treatment, including biological treatment, adsorption, sedimentation, flocculation, coagulation, ion exchange, and membrane filtration [4]. Nevertheless, some recalcitrant pollutants, such as pharmaceuticals, pesticides, dyes, heavy metals, and microplastics, are nonbiodegradable and resistant to destruction by conventional treatment methods [5]. Therefore there is an urgent need to find efficient and cost-effective systems for treating effluents before being released into the aquatic environment. Advanced oxidation processes (AOPs), by their nonpolluting character, constitute a powerful and clean solution for the treatment of aqueous effluents [6]. The AOPs are based on the production of highly reactive radicals hydroxyl (OH•) and superoxide anion (O2 2 ), which possess a strong oxidizing power for the degradation of organic pollutants. Heterogeneous photocatalysis is one of the most developed AOPs that had already shown its potential in the treatment of toxic and

Current Trends and Future Developments on (Bio-) Membranes DOI: https://doi.org/10.1016/B978-0-12-816778-6.00008-4

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recalcitrant contaminants [7]. Several semiconductors (SCs) have been investigated as photocatalysts, but titanium dioxide (TiO2) has attracted much attention since the discovery of its photocatalytic effect by Fujishima and Honda in 1972 [8]. The choice fell on TiO2 because of its unique properties, such photostability, nontoxicity, chemical inertness, and strong capability for the removal of hazardous compounds [9]. However, the large bandgap (3.2 eV) of TiO2 and the fast recombination of the photogenerated charges limit its applications under the visible light range [10]. In order to avoid these limitations, tremendous efforts have been devoted to the improvement of the photocatalytic efficiency of TiO2 thanks to the modification of the SC surface with different strategies such as doping [11] or codoping with heteroatoms [12], construction of heterostructure [13], and using of molecular photosensitizers [14]. In the following sections the main wastewater problems and their effects on the environment are reviewed. The performance of the AOPs over titanium dioxide based materials to eliminated water pollutants is also reported.

8.2 Wastewater problems Water contamination has become a serious concern in the present scenario. Therefore chemical pollution is one of the major causes of water pollution due to the intensification and diversification of the industrial world. A wide range of recalcitrant chemicals has been released into the environmental matrices, such as pharmaceuticals, phenolic compounds, microplastics, dyes, heavy metals, and pesticides. Fig. 8.1 presents the percentage of scientific publications of different organic pollutants found in water. Some of these pollutants can still remain several thousand years in the water without degradation, which leads to the impoverishment of freshwater, the destruction of ecosystems and causes potential threats to human life. So far, it has been reported that about 4 million children are shed every day because of diseases transmitted by contaminated water and for 1.2 billion people worldwide have no access to clean water [15].

80

% of publications

70

60 50 40

30 20 10

0 Dyes

Phenolic compounds

Pharmaceuticals

Pesticides

Others

77%

9%

3%

3%

8%

FIGURE 8.1 Percentage of scientific publications of organic pollutant.

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Indonesia 3%

Tanzania 3%

Uganda 3%

Pakistan 3%

Kenya 2% Rest of the world 40%

FIGURE 8.2 Water drinking shortage in the world based on the water gap state of water report (2018).

DRC 6% China 7% Nigeria 7%

Ethiopia 7%

India 19%

Moreover, it has been stated that just 10 countries account for three-fifths of population without access to clean water supply (Fig. 8.2).

8.2.1 Effect of pharmaceuticals Although pharmaceuticals are a fundamental human need, they have been a major concern for aquatic environments. Various studies have reported that low levels of pharmaceuticals could have ecotoxicological effects on water bodies so it causes a real threat for environments and human life [16]. Hignite and Azarnoff [17] are the first to show the presence of pharmaceuticals in water. Indeed, they detected the presence of several types of drugs and drug metabolites, such as chlorophenoxyisobutyrate, salicylic acid, and the metabolites of clofibtate and aspirin into the Missouri river. Kidd et al. [18] conducted experiments on male fishes (Pimephales promelas) exposed to 5 ng/L levels of the potent 17α-ethynylestradiol. The results showed that the presence of this type of molecules in water leads to the feminization of males by the production of vitellogenin, protein, which has effects on the gonad development. Diclofenac is another pharmaceutical compound, which is classified as a nonsteroidal antiinflammatory drug [19]. It is frequently detected in surface water and could some ecotoxic effects on the living organisms [20]. A previous study has shown that the release of pharmaceutical effluents containing diclofenac into freshwater leads to phototransformation of the latter by producing intermediate compounds, which are very stable and could have a more toxic effect than the starting pollutant [21]. Naidoo et al. [22] studied the effect of diclofenac on Gyps vultures. They found that a dose of 98 225 mg/kg is enough to kill this species. It is interesting to highlight in this case that the observed collapse of vultures is caused by the presence of sufficient diclofenac in the vulture food reserves. A further analysis [23] showed that tetracycline, an antibiotic, can induce harmful impacts on the organisms of the aquatic ecosystems. Thereby, the highly hydrophilic and low volatility properties distinguish the tetracycline drug from other antibiotics allowing it to persist in aquatic systems. Borghi et al. [24]

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evaluated the toxicity of tetracycline to the species Daphnia magna by an aquatic toxicity test. The authors found that tetracycline reduced the reproduction and limit the growth of D. magna.

8.2.2 Effect of heavy metals Drinking water containing heavy metals causes dangerous impacts on the environment and health of human beings. Indeed, exposure to heavy metals is caused primarily by water consumption [25]. Several researches have been conducted on various heavy metals, which have many effects in drinking water [26]. Among them, mercury (Hg) is a worldwide environmental pollutant. It is known due to its high toxicity and serious effects that pose a real threat to the aquatic ecosystem and human life [27]. Hg derives from different sources such as industrial processes, volcanic activities, domestic uses, and combustion of fossil fuels [28]. Milioni et al. [29] have mentioned that exposure to mercury, even in small quantities of the order of 1 μg/L, can cause severe health problems on the nervous system. A very recent study has shown that the inhalation of gaseous elemental mercury may also cause harmful effects on the brain and kidneys and may be fatal [30]. Kaleshkumar et al. [31] have shown the relationship between lead and many marine edible puffer fishes Takifugu oblongus. The results showed that all organs of the species studied are very rich in heavy metals, and even the accumulated content has exceeded the limit allowed by the World Health Organization and Food and Agriculture Organization of the United Nations. Cheng et al. [32] studied the effect of arsenic on the progression of chronic kidney disease, and they found that a high concentration of arsenic in drinking water is the main factor of appearance of kidney impairments. Chabicovsky et al. [33] reported the effect of cadmium on hepatopancreatic cells of the terrestrial pulmonary snail Helix pomatia. They showed that exposure to cadmium content greater than 5 μmol/g caused the death of more than 40% of snails. This could be due probably to the establishment of unspecific interactions between excess Cd21 species with other proteins, such as enzymes, resulting in harmful effects on enzyme activities.

8.2.3 Effect of dyes The significant development of the textile industry is the result of the direct relation between both population growth and chemical industrialization evolution. In fact, until the mid-19th century, people have used natural dyes in the dyeing process. After this, William Henry Perkin discovered the “mauve,” which is the first synthetic dye [34]. Synthetic dyes have been widely produced and utilized by various industries, such as cosmetic, paper, printing, plastics, and textile; but the largest consumer of these dyes is the textile industry, accounting for approximately three-fourths of its market [35]. However, these dyes are very dangerous due to their bad effects on environment [36]. Thereby, Chakrabarti and Dutta [37] reported that 15% of dye effluents are unloaded into the receiving water bodies, which endanger the health of consumers. Among the most wellknown dyes, the methylene blue (MB), an organic dye, it is widely used for coloring paper, temporary hair colorant, and dyeing cottons and wools [38]. MB can have some

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harmful effects on human beings. It can cause eye burns in humans and animals. It may stimulate the gastrointestinal tract and cause nausea, vomiting, and diarrhea if ingested [39]. Abe et al. [40] used zebra fish as a model animal to evaluate the toxic effects of the synthetic dye Basic Red 51, on these species. They have proven that these dyes cause developmental malformations on embryos and behavioral impairment on larvae. Furthermore, one of the most used classes in dye chemistry is the class of azo dyes. Chung [41] investigates the effect of azo dyes on human health. Indeed, main results acquired in these studies revealed that these dyes have many hazardous effects on human and animal health, such as allergies, genotoxicity, and mutagenicity. Besides, Puvaneswari et al. [42] demonstrated that some azo dyes such as benzidine could cause cancers. In fact, this carcinogenicity could be due to the presence of cleaved groups and aromatic amines.

8.2.4 Effect of pesticides From the last past decades, the consumption of pesticides witnessed a remarkable rise due to the agricultural development and product quality improvement, which contributes to expand the water pollution concern [43]. In fact, the worldwide pesticide consumption reached 75.74 kg/ha, and this becomes a real threat for the aqua life system [44]. Toxicological tests were carried out in seven wastewater treatment plants in rural and suburban areas of Central Germany to detect pesticide pollution and study its impact on the water environment. As a result, Mu¨nze et al. [45] proved that pesticide residues potentially affect the structure and function of invertebrates. Another study carried out by Yadav et al. [46] showed that long-term and low-dose exposure to certain pesticides acting as endocrine disruptors can cause serious diseases such as cancer, hormonal disturbances, decreased intelligence, and infertility problems. This is related essentially to the fact that these pesticides can mimic or antagonize the natural hormones in the body leading to disruption in their normal functioning. Furthermore, Lai [47] demonstrated that pesticides have serious effects on humans and could even lead to death. Indeed, these contaminants can affect the nervous system by creating neurological abnormalities as well as sperm DNA damage [48], diabetes [49], cognitive impairment [50], and hypospadias [51].

8.2.5 Effect of phenolic compounds According to the first law of thermodynamics which says, “nothing is lost, everything is transformed,” we agree that the secret of a competent industry lies in the total consumption of its products without waste [52]. Unfortunately, this is not the case for the olive oil producing industries. Indeed, Pulido et al. [53] reported that 98% of the phenolic content of the olive is discharged in the natural streams while only the rest is transferred to the oil. These huge amounts of olive mill wastewaters containing phenolic compounds exhibit a severe environmental concern due to their toxicity and high stability in aquatic systems [54]. Siddiqui and Ahmad [55] showed that phenolic compound is one of the highly toxic pollutants present in water ground. Wasi et al. [56] quantified phenol pollution and its toxicological impact on surface water. They found that the perturbation of energy transduction, unrest of membrane barrier, and subsequent cell death are the main

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hazardous effects of phenol. This could be due essentially to the integrity loss of the cytoplasmic membrane, which is directly related to the toxic activity of phenolic compounds.

8.2.6 Effect of microplastics Since the industrial revolution, worldwide production and use of plastic have considerably increased. In fact, plastic production has doubled 276 times during 64 years and was estimated to be 1.7 million tonnes in 1950 and 99 million tonnes in 2014 [57]. According to the most recent statistics, more than 330 million tonnes of plastics across the globe were produced annually until 2016 [58]. Thereby, it is important to know how widespread plastic pollution is. Among diverse plastic contaminants, microplastics have generated a lot of interest because of their frequent occurrence in the environmental ecosystems [59]. Indeed, microplastics are defined as particles in the size range from 1 μm to 5 mm. They are the result of large elements degradation into smaller fragments by the action of sun and natural mechanical forces [60]. Jambeck et al. [61] reported that the potential sources of microplastics are wastes that are thrown into the wild, accidentally lost or wind-driven, and garbage dumps and certain industrial activities. A recent study conducted by Ma et al. [62] proves that microplastics, which recently have been considered a new kind of pollutant, are emerging contaminants of concern due to their widespread presence in marine environments around the world. Moreover, microplastic pollution is not limited to the oceans, it is also present in freshwater ecosystems. It is even more widespread than previously thought. Indeed, some of these pollutants move into sewers and water treatment plants and are released into rivers, lakes, and oceans where they enter the aquatic food chain. Human, fauna, and flora can be exposed to microplastics in a variety of ways presenting a serious hazard that threatens their life on earth [59]. Several studies have been reported on the undesirable effect of microplastics on the environment [63 65]. The first study conducted on the presence of microplastics in living insects was published by Windsor et al. [66]. The authors reported the effects of these contaminants on the aquatic insects living in rivers in South Wales. The researchers demonstrated that plastic microparticles were found in at least half of the aquatic insects. This report warns of plastic pollution that seriously threatens freshwater. At all sampled sites, it was found that pieces of plastic debris less than 5 mm long had been ingested by half of the insects. At all study sites, microplastics were found in insects of all species, whether they live in the water column or in the rivers. A recent study conducted by Wilcox et al. [67] demonstrates that even a single piece of normal size plastic ingested by a sea turtle increases its risk of dying by 20% and that a turtle that consumes 14 pieces of plastic increases its risk of dying by 52%. Furthermore, this investigation showed that three-fourths of sea turtles had plastic in the intestines. This causes a huge number of problems about the survival of some turtle species.

8.3 Wastewater treatment This overview allowed us to imagine the huge amount of sewage with different sources and effects. These pollutants were spilled without further treatment into natural streams Current Trends and Future Developments on (Bio-) Membranes

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and consequently infiltrated into the groundwater. It is clear that effluents containing recalcitrant compounds pose a serious worldwide environmental concern, which threatens ecosystem. Therefore this problem must be solved by treating aqueous effluents before being discharged in nature. Several remediation strategies have been adopted for the treatment of wastewater including conventional treatment processes. Among these techniques, we can cite biological treatments, coagulation/filtration, ion exchange, sedimentation, activated carbon filtration, and adsorption. Hayat et al. [68] compared dye removal from textile effluents using an anaerobic internal circulation reactor as a biological treatment and the Fenton process as chemical treatment. Results showed that the performance of color removal by the Fenton process is much more efficient than that of biological treatment. A study on the biodegradation of clofibric acid (CFA) was conducted by Kimura et al. [69]. They demonstrated that this technique is not reliable for the degradation of CFA. Moreover, they suggested that this could be due essentially to the complex aromatic structure of CFA. Ma et al. [62] applied coagulation method for the removal of microplastic pollutants from wastewater. They found that the obtained yield is less than 40% even when they increased the aluminum-based coagulant dose to 15 mM. Adams et al. [70] investigated the degradation of antibiotics using ion exchange, sedimentation, flocculation, and coagulation with iron and aluminum salts. Experiments showed that conventional strategies, such as coagulation, flocculation, sedimentation, and ion exchange, are not recommended for the treatment of wastewater containing pharmaceuticals. Another study focused on the degradation of sulfamethoxazole and trimethoprim, bacteriostatic antibiotic drugs [71]. This investigation shows that sewage treatment, from pharmaceutical effluents, by conventional strategies leads to transfer drug compounds from one phase to another without degrading them, which could create a new source of contamination that could be more dangerous than the original one [72]. Alexandratos [73] used a strong-base ion-exchange resin (quaternary ammonium) to decompose phenol and chlorinated phenols. They found that the resin used is incapable of exchanging phenolic compounds since it would not release ions. Ortega et al. [74] evaluated the phenol recovery from olive mill wastewaters (OMWs) by the ion-exchange method. In addition, they used both strong- and weak-base anion (Amberlyst A26 and Amberlite IRA-67, respectively) exchange resins. The authors found that phenol removal efficiency of Amberlite IRA-67 reached 57%, while that of Amberlyst A26 was 100%. In this case, it can be said that the adsorption process is directly related to phenol removal efficiency. Indeed, the adsorption capacity of weak and strong resins is 76.3 and 148.6, respectively, which explains the results found. Lakshmanan et al. [75] investigated the degradation of As(III) and As(V) through electrocoagulation. The following study was proven that this strategy is ineffective with respect to the degradation of As(III). Indeed, the results showed that the adsorption capacities of As(III) were limited with a yield of 5% 30% than those reached by As(V). The main problem of these technologies lies in the production of wastes and organic by-products, which are more toxic and stable than the starting products. Furthermore, the decomposition of these contaminants with conventional water treatment processes seems to be very difficult, which requires another treatment steps [76].

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Other treatment strategies were evaluated by various research teams and focused on the recovery of hazardous compounds from freshwater. Among these techniques, we can cite membranes, activated carbon adsorption, and electrodialysis. In this context, Mehta et al. [77] used reverse osmosis membrane for the removal of two phenyl urea pesticides (diuron and isoproturon) from agricultural field water. Interestingly, it was found that isoproturon elimination is satisfactory and reached 95%. Agarwal et al. [78] studied the adsorption of sunset yellow eosin B and MB on copper oxide nanoparticle loaded on activated carbon. Authors reported that the higher adsorption performance is attributed to the application of ultrasound treatment, which accelerates the removal dye reaction, and the presence of copper oxide nanoparticle that improved the adsorption capacities of activated carbon. Another study focused on the ultrasound-assisted adsorption of rhodamine 123 and disulfine blue dyes on Au Fe3O4 nanoparticles loaded on activated carbon [79]. Authors have found that the adsorption process is very fast. This could be due mainly to the high adsorption capacity of modified activated carbon, which reached 71.46 and 76.38 mg/g for rhodamine 123 and disulfine blue dyes, respectively. Polyethyleneimine-loaded activated carbon was elaborated via the hydrothermal method. The obtained material has been evaluated for mercury (Hg) adsorption in order to reduce its concentration in wastewater. As a result, modified activate carbon exhibits a potential adsorption performance with respect to the heavy metal recovery as well as it is effectively regenerated. Moreover, the high specific surface area and increased adsorption capacity are at the origin of the strong Hg(II) removal effectiveness [80]. Recently, promising results were obtained by Babilas and Dydo [81] when studying heavy metals recovery from simulated electroplating industrial wastes containing iron and zinc by novel electrodialysis improved with complexing technique. It is worth to note that this method allows the selective elimination of heavy metals. Thereby, 86.6% of zinc was removed from the studied solution and 92.36% of iron was retained. Hybrid ion exchange electrodialysis was developed by Mahmoud and Hoadley [82] with the final objective of combining the advantages of the two methods. Thereby, this technology was applied to remove copper sulfate from aqueous solutions. Results point out that copper removal performance did not exceed 50%. Several studies confirmed that most recalcitrant contaminants are nonbiodegradable, nonvolatile, water-soluble, and resistant to destruction by physicochemical treatment methods [83]. Besides, conventional techniques are expensive and they are not practical to rural regions. Moreover, these processes convert recalcitrant compounds solely from one phase to another without decomposing them. In this field the search for alternative for wastewater treatment has led in recent years to the emergence of new technologies. Among these strategies, AOPs have attracted substantial attention of several researchers due to their lower cost and less maintenance requirements. AOPs are considered promising methods for the degradation of recalcitrant pollutants, such as pharmaceuticals, pesticides, dyes, and phenols compounds During water treatment using AOPs, oxidative decomposition of organic pollutants is achieved by hydroxyl (OH•) and superoxide (O2 2 ) radicals [84]. Therefore these species, produced in situ, have high reactivity and strong oxidizing ability for removing organic compounds with one or many double bonds into inorganic materials, such as water, carbon dioxide, and organic chain acids [85]. Furthermore, it is recognized that wastewater treatment using AOPs does

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not require secondary treatment because OH• radicals disappear after production by the fact that they cannot live for long time under ordinary medium [86]. Glaze and Kang [87] are the first to publish work on AOPs. They evaluate the effect of OH• scavengers in the oxidation reaction of organic compounds with ozone and hydrogen peroxide in wastewater. Through this investigation, authors prove that hydroxyl radicals are potentially capable of decomposing organic compounds. The range of AOPs is broad and includes various technologies. Among these techniques, we can distinguish (1) homogeneous phase photochemical processes such as photolysis [88], Fenton process [89], and photo-peroxonation [ultraviolet (UV)/H2O2/O3] [90] and (2) heterogeneous phase photochemical processes such as radiolysis [91] and catalytic wet oxidation [92]. Despite the performance of these processes and their classification as clean techniques, they require a long treatment period, energy consumption, and a significant investment and scavenging of OH• radical by carbonate ions [93]. Thus Thokchom et al. [94] have stated that the utilization of individual AOPs procedure seems to be expensive and presents lower energy efficiencies for wastewater remediation. A very recent study conducted by Lu et al. [95] focused on the degradation of nitrobenzene using AOPs based on peroxymonosulfate. The authors showed that the use of this type of procedure in the treatment of wastewater remediation could have a negative effect on the reactivity of OH• radicals. In fact, bicarbonate in natural water acts as OH• radical scavenger, which generate secondary radicals, such as carbonate radicals CO2 3 . Thereby, the nitrobenzene decomposition is inhibited. Therefore looking for a low cost and effective strategy for the degradation of recalcitrant compounds is an urgent challenge.

8.4 Heterogeneous photocatalysis Photocatalysis is a powerful and clean solution that fits in with the objectives of green chemistry and sustainable development. It is widely used in industry because it is more efficient than other techniques in use allowing partial or total degradation of pollutants while respecting the integrity of the environment [96]. Serpone et al. [97] reported that the first work on the photocatalysis concept dates from 1918 when Baur demonstrated that zinc oxide (ZnO) exhibited a photochemical response when it was exposed to sunlight for 3 7 h. According to Teichner [98], the term “heterogeneous photocatalysis” was used for the first time in 1970s to explain photoinduced reactions of carbon monoxide on the surface of titanium dioxide. In 1972 Fujishima and Honda [99] made a major breakthrough in this field. Indeed, they report the production of H2 from water on a photoanode in TiO2 irradiated under UV. During the last decades, many studies have focused on heterogeneous photocatalysis due to its high potential for wastewater purification for the recovery of persistent pollutants [100 102].

8.4.1 The basic on photocatalysis When an SC is irradiated with photons, emitted either by the sun or by a UV lamp, of energy equal to or greater than its bandgap energy, an excited electron (e2) is promoted

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e–

O2• –

Reduction

Conduction band

FIGURE 8.3 Schematic illustration of the photoactivation process of a semiconductor.

O2 ads

hυ υ

Valence band

OH• + H+

Charge recombinaison

Excitation

h+

H2Oads Oxidation

CO2 + H2O

from the valence band (VB) to the conduction band (CB), while a hole (h1) is created in the VB as shown in Fig. 8.3 [103]. SC 1 hυ-e2 1 h1

(8.1)

Then, the photogenerated (e ) reacts with electron acceptors such as O2 to form O2 2 superoxide radicals. 2

O2 1 e2 -O2 2 1

(8.2) 2

As well as (h ) reacts with electron donors such as H2O and OH anions adsorbed on the surface of the SC forming OH•.

(O2 2

H2 Oads 1 h1 -H1 1 OHads

(8.3)

1  OH2 ads 1 h -OHads

(8.4)

Rads 1 h1 -R1 ads

(8.5)

•

and OH ) are highly oxidizing and can decompose the The generated radicals adsorbed contaminants on the surface of the SC until their mineralization to water and carbon dioxide. OH 1 pollutants-H2 O 1 CO2

(8.6)

In the absence of electron hole trappers, the charge carriers tend to recombine both in the bulk and the surface of the SC, and they return to their fundamental energy state with releasing energy as heat, and no chemical reaction takes place [104]. e2 1 h1 -energy

(8.7)

8.4.2 Wastewater remediation by heterogeneous photocatalysis over titanium dioxide A huge variety of potential photocatalytic SCs has been studied in the literature such as oxides (such as WO3, BiVO4, La2O3, Nb2O5, ZnO, and TiO2) [105], nitrides (such as Current Trends and Future Developments on (Bio-) Membranes

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Redox potentials +

H 2O

3.2 eV

3.2 eV

2.6 eV

2.25 eV

3.6 eV

H•/H2 +

h+ h+ h+

C3N4

h+

CdS

Nitrides

ZnS

Sulfides

e–

O2• –

e–

e– 2.7 eV

Potential vs NHE

e–

CO2 + Organic pollutants

+

OH•

H2O/•OH

WO3 ZnO TiO2

Oxides

FIGURE 8.4 Illustration of energy diagram and photocatalytic process of several semiconductors.

C3N4) [106], and sulfides (such as CdS, ZnS, and MOSx) (Fig. 8.4) [107]. Among the already SCs, TiO2 is well known as emperor of photocatalysts due to its environmental friendliness, thermal stability, biological and chemical inertness, low cost, mechanical strength, strong oxidizing ability, and long-term stability against photocorrosion [108]. These characteristics make TiO2 a promising material compared to other photocatalyst such as WO3, ZnO, and CdS. Jones et al. [109] stated that WO3 is an interesting SC for photocatalysis but is limited by its toxicity and instability. Besides, it has been reported that ZnO could be a promising photocatalyst because it has a strong visible activity, but it suffers from photocorrosion problems [110]. CdS has the advantage of absorbing a large part of the solar spectrum, but this SC is unstable during photocatalytic oxidation [111]. It seems that TiO2 is the most popular material used in heterogeneous photocatalysis. However, its low quantum efficiency (due to the recombination of electron hole pairs) hinders its widespread application in the photodegradation of pollutants. To meet the challenge of developing active photocatalysts under visible light, various strategies have been proposed such as doping, metal deposition, surface sensitizers, and heterojunctions [112,113]. In this context, several research teams studied the effectiveness of different photocatalytic systems for the degradation of recalcitrant organic pollutants, such as dyes, pharmaceuticals, pesticides, and heavy metals using materials based on TiO2. Table 8.1 depicts recent works conducted on heterogeneous photocatalytic degradation of toxic contaminants. In fact, Zhang et al. [133] used the simulated solar light/TiO2 microstructure with exposed (001) facets system for the degradation of MB. Results proved that this material exhibits a strong photocatalytic activity and even greater than that of P25. The decomposition of MB reached 92% after 20 min, and total degradation was performed after 30 min. Furthermore, the effective photoactivity is directly related to the presence of highly active (001) facets, which have a strong capability to form hydrogen peroxide and peroxide radicals from absorbed water. Moreover, these facets have a high density of subcoordinated Ti

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TABLE 8.1 Recent studies related to photocatalytic degradation of pollutants. Pollutant

Photocatalytic system

Elaboration method

Arsenic(V) and arsenic(III)

UV/Fe3O4@TiO2 sheets

Mixing method (sol gel approach and [114] facile hydrothermal assisted method)

Cr(VI)

Visible light/N TiO2/gC3N4@diatomite hybrid

Impregnation method

[115]

Pb(II)

UV/GO TiO2

Hydrothermal method

[116]

Amoxicillin

Visible light/Pt and Bi codoped TiO2

Hydrothermal method

[117]

Acetaminophen

UV/TiO2/activated carbon heterostructures

Solvothermal treatment

[118]

Acetaminophen

UV/Ag, Au, Pt/TiO2

Photodeposition

[119]

Micropollutants

UV/TiO2 nanotube

Electrochemical anodization method

[120]

Perfluorooctanoic acid, chloroacetic acid, and benzoic acid

Simulated sunlight/BiOCl/ TiO2

Hydrothermal synthesis

[121]

Microcystins

UV/TiO2 sheet

Titanium dioxide membrane

[122]

Methyl orange

UV/CuO TiO2/RGO

Wet impregnation method

[123]

Methylene blue

Visible light/Al-doped TiO2

Impregnation method

[124]

Methylene blue

Visible light/bismuth TiO2 nanotube

Anodization method

[125]

Orange 7 azo dye

UV/active carbon titania composites

Mixing method (hydrothermal treatment and milling)

[126]

Industrial olive oil mill wastewater

UV/H2O2/TiO2

Commercial TiO2

[127]

Phenol

Simulated sunlight/MgOcoated Ag/TiO2 nanocomposite

Hydrothermal process

[128]

Bisphenol A

Visible light/D35 organic dye-TiO2 nanocrystalline film

Deposition method

[129]

Pesticides

Visible light/TiO2/ graphene/Au nanocomposite

Simple electrophoretic deposition method

[130]

Imidacloprid

Visible light/GO/Fe3O4/ TiO2 NiO

Sol gel method

[131]

Insecticides acetamiprid, imidacloprid, and thiamethoxam

UV, visible, and solar irradiations/Ru, Ni/TiO2

Wet impregnation method

[132]

GO, graphene oxide; RGO, reduced graphene oxide.

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References

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atoms. Also, the important angles of the Ti O Ti bond at the surface make these facets potentially active in comparison with {100} and {101} facets [134]. In a previous study conducted by Cao et al. [135], authors investigated the addition effect of hydrofluoric acid (HF) on the {001} facets evolution. Results showed that adding 0.6 mL of HF could form 77% of {001} facets. Furthermore, the photocatalytic test showed that catalyst with a large amount of exposed {001} facets exhibited the highest photoactivity toward the degradation of methyl orange (MO), MB, and rhodamine B. Indeed, the authors agree that photocatalytic activity improvement is due to the formation of surface heterojunction by {001} facets, which induces more efficient separation of excess charge carriers and reduces the electron hole recombination rate. Carbon allotropes have attracted substantial attention in order to improve the catalytic performance of titanium dioxide. This has been explained by the effect of carbon on increasing the adsorption capacity of TiO2, shifting its adsorption potential toward the visible and increasing the lifetime of the photogenerated electron hole pairs during the photocatalytic process. Graphene and reduced graphene oxide (RGO) are part of the family of carbon allotropes. They have created more attraction because of their excellent mobility of charge transfer, high surface area, and chemical stability since their discovery in 2004 by Geim and Novoselov from Manchester University [136], who managed to manufacture a single layer of graphene to from graphite using micromechanical cleavage. Mohammadi et al. [137] synthesized hybrid RGO TiO2 nanocomposites using a simple hydrothermal route. The evaluation of the catalytic properties of the materials prepared in the photodegradation of MB under UV irradiation has shown that the activity of these nanocomposites is seven times higher than that of pure TiO2. Moreover, the MB degradation reached 90% after 15 min with a large linear kinetic rate constant (k) (153.1023 min21). The enhanced photoactivity is attributed to the RGO, which allows the storage and the electron transport through a stepwise transfer process. Therefore RGO could be considered an effective tool preventing the recombination of electron hole, which leads to a better adsorption ability for MB. The visible light/RGO/TiO2 nanocomposite system was used by Nasr et al. [138] for the removal of MO from aqueous solution (10 mg/L) (Fig. 8.5). A 150 W lamp was used as the visible light source and emits wavelength more than 400 nm. Results show high photocatalytic degradation of MO (90%) with a low RGO amount. However, a large amount of RGO is responsible for the decrease in photoactivity (26%). Therefore results showed that RGO incorporation leads to a decrease in the bandgap energy, which enhances the optical properties of TiO2 in the visible light range. Obviously RGO TiO2 system seems to be a promising candidate for dyes removal. However, the photocatalytic visible light/RGO TiO2 system is not efficient for the photodegradation of phenol, according to Kim et al. [139]. In this case, RGO/TiO2 catalysts were elaborated using anodization method. The experimental tests revealed that the photocatalytic degradation of phenol did not exceed 15%. While the maximal photoelectrocatalytic rate of phenol removal was 28%. Al-Kandari et al. [140] investigated for the first time the effect of the GO reduction on the UV/H2O2/RGO TiO2 and UV/RGO TiO2 photocatalytic systems, toward phenol degradation. It was found that the best phenol removal percentage was attributed to the hydrothermally reduced (RGO) method with 60% conversion in the absence of H2O2 and 80% in the presence of H2O2. These promising results could be explained by the high

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CB

e– e– e– e–

e–

e–

e–

Visible light

e–

e–

e– + O2 ads O2• –

+ 3.2 eV

Degradation h+

VB

h+

h+

TiO2

+

h+

CO2

RGO Methyl orange

FIGURE 8.5 The photocatalytic degradation mechanism of MO over visible light/RGO/TiO2 system. MO, Methyl orange; RGO, reduced graphene oxide.

specific surface area, which improved the adsorption ability of pollutant, as well as the presence of H2O2 on the catalyst surface, which could generate more active species enhancing therefore the photocatalytic performance. In order to better improve the photocatalytic activity, Shen et al. [141] decided to modify the RGO TiO2 system by the fixation of β-cyclodextrin (β-CD) on the surface of RGO. β-CD, cyclic oligosaccharide, acts as a linker between RGO and TiO2. It is characterized by a cylindrical cavity wherein the exterior is hydrophilic and an interior is hydrophobic [142]. Thus enrichment with hydroxyl groups inside its cavity enhances the adsorption capacity of molecules. This investigation shows that RGO β-CD TiO2 composites exhibit high photocatalytic efficiency toward the degradation of phenol. Moreover, results revealed that phenol was completely degraded after 60 min. This powerful photocatalytic behavior could be due to the high surface area, strong connection between RGO, TiO2, and β-CD, and the potential ability of RGO to reduce electron hole recombination. It is very important to note that the optimal value of catalytic performance depends on several parameters: the material synthesis procedure, the light source, and the target molecules. OMW, rich in phenolic compounds, is another source of contamination. They are liquid effluents from the olive oil extraction process and their uses. Nevertheless, OMW decomposition seems to be very difficult because they contain an elevated organic load, phenol, and fatty acids, that is why their removal from wastewater is of great requirement. A pilot-plant treatment was used by Gernjak et al. [143] in order to decompose OMW using commercial titanium dioxide (P25) under solar light illumination. The authors did not find satisfactory results in terms of chemical oxygen demand (COD) removal and phenolic compounds. Even the modification of the TiO2 by peroxydisulfate, which acts as an electron acceptor, and the addition of hydrogen peroxide known for its high oxidizing power cannot improve the degradation rate, but they lead to increasing salt concentrations in the solution. This could be due probably to the use of pure

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contaminants without any dilution or pretreatment and the use of solar radiation as a UV source (5% of the solar spectrum is actually used). Sponza and Oztekin [144] have developed novel photocatalysts based on ZrO2/TiO2 using sol gel method. The authors investigated the effect of ZrO2 content on the photocatalytic properties of TiO2 for the degradation of major phenolic compounds and polyaromatic amines present in the OMW under UV irradiation. Indeed, they prepared a series of catalysts: 5, 10, 14, 25, and 50 wt.% of ZrO2 in the TiO2/ZrO2 nanocomposite. Experimental results showed that the best catalytic activity was obtained in the presence of the 14 wt.% ZrO2/TiO2 catalyst. Thereby, the optimum OMW removal reached 89% after 60 min of irradiation time under UV illumination. Moreover, the maximum COD, total phenol, and total aromatic amines photooxidation were 99%, 89%, and 95%, respectively. This developed activity can be related to the formation of a heterojunction between ZrO2 and TiO2, which makes it possible to increase the photocatalytic activity thanks to high separation efficiency of electron hole pairs. As well as, a developed surface area could influence the catalytic performance by improving its adsorption capacity. Besides heterojunction photocatalysts formation, codoping is considered one of the most promising strategies for improving the photocatalytic efficiency of TiO2. This could shift the absorption edge to the visible light range by introducing new energy levels close to the VB or CB and spatially reduce the photogenerated electron hole pairs. Different strategies have addressed this purpose. Table 8.2 summarizes the recent investigations conducted in the degradation of pharmaceuticals by metals codoped TiO2 systems. Tbessi et al. [157] reported the application of Ce and Mn codoped titanium dioxide for the degradation of diclofenac under UV illumination. The study emphasized the TABLE 8.2 Recent pharmaceutical compounds degraded via codoped TiO2 photocatalysts. Therapeutic class

Target pharmaceuticals

Photocatalyst

References

Nonsteroidal, antiinflammatory

Ibuprofen

N S/TiO2

[145]

Nonsteroidal, antiinflammatory

Naproxen

N S/TiO2

[145]

Antibiotic

Ofloxacin

Bi Ni/TiO2

[146]

Anticancer

5-Fluorouracil

N S/TiO2

[147]

Anticancer

Ifosfamide

Bi B/TiO2

[148]

Antiinflammatory

Diclofenac

N S C/TiO2

[149]

Nonsteroidal, antiinflammatory

Ibuprofen

CNT COCl/TiO2

[150]

Antibiotic

Ofloxacin

Co

21

31

31

and Fe /TiO2

[151]

N/TiO2/diatomite

[152]

Antibiotic

Tetracycline

Ti

Antibiotic

Tetracycline

Sm N/TiO2/diatomite

[153]

Antipyretic

Acetaminophen

Gd La/TiO2

[154]

Antibiotic

Cephalexin

N, Ag/TiO2

[155]

Nonsteroidal

Ibuprofen

C N/TiO2

[156]

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significant improvement of the visible light adsorption of TiO2. This result could be explained by the charge transfer between the d electrons of the Mn and Ce cations and the CB of TiO2. Thereby, this charge transfer could create intermediary energy levels between CB and VB, which are responsible for reducing the band-gap energy. Furthermore, the high harmonious effect between Mn, Ce, and TiO2 enhanced significantly the total organic carbon (TOC) reduction. In fact, the highest percentage of TOC conversion of Ce Mn/ TiO2 was 89% while that of pure TiO2 is 32%. These results lead us to conclude that codoping improves the catalytic activity through Mn and Ce, which act as an electron hole trap, which promotes the lifetime extension of charge carriers. Guevara et al. [158] compared the photocatalytic performances of Fe and N codoped TiO2 solids using two synthesis routes: microwave and sol gel approaches for the decomposition of diclofenac, amoxicillin, and streptomycin in aqueous solution. The authors reported that materials prepared via sol gel method exhibit the highest photocatalytic activity. This is related to the advantage of the sol gel process, which promotes high dispersion of N and Fe dopants into the TiO2 matrix in comparison with microwave method. These results are in good agreement with the work of Zhang et al. [159] who stated that a good dispersion of particles provides more reactive sites than agglomerated particles. Thus a high specific surface could improve oxygen adsorption at the surface of TiO2, which reacts with electrons to form OH• radicals that are endowed with their capability to decompose hazardous pollutants. A few studies have been carried out on photocatalytic degradation of microplastics especially via TiO2 as a catalyst. Tarazona et al. [160] were the first to report a publication on the photodegradation of high-density polyethylene microplastics’ real samples extracted from commercial facial scrub using photocatalysis. Thereby, they used two materials based on N-doped TiO2 (N TiO2). The first one was prepared via conventional sol gel while the second was accomplished by a green process in which the mussel proteins are used as a nitrogen source. In this regard, authors evaluated the photocatalytic performances of SCs in the degradation reaction of polyethylene microplastics under visible light illumination in both solid and aqueous medium. The experimental investigation concluded that catalyst derived from bioprocess exhibits an excellent photocatalytic activity toward the degradation of microplastics, especially in an aqueous environment. This could be associated with the high concentration of OH• hydroxyl radicals generated by adsorbed water in the N TiO2 surface. According to the studies carried out by Goza´lez et al. [161], humid environment favors the generation of reactive species OH•, which enhances the photodegradation process. So far, photocatalytic decomposition efficiency is driven by a strong hydration state of the photocatalyst. In addition, this performance could be due also to the good interaction between pollutants and catalysts, which enhances the adsorption capabilities for contaminants on the surface of the catalyst. Ali et al. [162] developed a blue-green dye photosensitizer of titanium dioxide nanotubes in order to decompose low-density polyethylene (LDPE) under visible light irradiation. In fact, the choice was falling on TiO2 nanotubes because of its unique properties such as chemical stability, high surface area, and easy and efficient synthesis method. Thereby, authors quantified the photooxidation of microplastic contaminants using the carbonyl index method. They found that the carbonyl index for LDPE using titanium nanotubes as support is potentially higher than titanium nanoparticles with a maximum of 2.

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167

Furthermore, they proposed a photodegradation mechanism of LDPE and mentioned that blue-green dye is crucial for the degradation of this pollutant because it acts as generator holes. The photogenerated holes will react with water on the SC surface to form reactive species, which will participate in turn in the photodecomposition process. Another study performed by Shang et al. [163] was conducted on the degradation of polystyrene (PS) over copper phthalocyanine (CuPc) sensitized TiO2 photocatalyst under fluorescent light exposure. Indeed, this inquiry focused on the effect of CuPc addition on the photocatalytic performances of the SC. The photocatalytic test has proved evidence that TiO2/CuPc exhibits potential oxidation properties toward the degradation of PS in comparison with pure TiO2. In fact, PS was completely mineralized. The carbon dioxide detection test revealed that the total amount of CO2 increased significantly during the photocatalytic process, which results in weight loss of PS. These encouraging results are associated with the constructed energetic interface between TiO2 and CuPc. This interface contributes to the charge separation of the photogenerated electron hole pairs, which reduces the recombination rate. Obviously, efficient charge separation results in more oxygen reactive species, which plays a crucial role in PS chain split. Asghar et al. [164] have described liquid impregnation method to prepare TiO2 nanoparticles doped with Fe, Ag and codoped with mix Fe/Ag for the degradation of polythene films under UV radiation, artificial light, and darkness. The aim of this investigation is to compare the degradation behavior of pure and doped TiO2. Results revealed that the maximum weight reduction are 14.34% and 14.28%, respectively, and they are attributed to Fe/Ag mix doped TiO2 under UV irradiation and to Ag-doped TiO2 under artificial light, respectively. The authors reported that the enhanced photocatalytic activity is due to the powerful loading effect of Ag and Fe. Besides, they mentioned that the synergetic effect between mix Fe/Ag and TiO2 plays an important role in the successful hole production in the ground state of Fe/Ag. Zhang et al. [165] evidenced that codoping is a promising strategy to separate the charge carriers and enhance the visible light absorption by reducing the bandgap energy, which in turn promotes the photocatalytic activity of TiO2 catalyst. Another study [166] indicated that Ag and Fe act as a deep electron trap, which prevents the recombination of photogenerated electron hole pairs and hence improves the photodegradation of polythene. The organophosphorous pesticides (OP) have gained an immense interest owing to their high efficiency, especially as insecticides, and their rapid degradation in sunlight. A former work carried out by Harada et al. [167] investigated the photocatalytic degradation of dichlorvos and DEP (dimethyl-2,2,2-trichloro-l-hydroxyethyl phosphonate) in the presence of TiO2 suspensions. The study of their activities has been based on the rates of insecticides disappearance with the use of superhigh pressure mercury lamp or sunlight. The results showed that Pt-loading to TiO2 or the addition of H2O2 significantly enhances the degradation of dichlorvos and DEP and favors the total mineralization of the OP insecti1 cides forming Cl2, PO32 4 , H , and CO2 as final product with formaldehyde as predominant intermediate. Similar study was carried out by Mengyue et al. [168] in the presence of other OP (dichlorvos, monocrotophos, parathion, and phorate) using TiO2 thin films. The kinetic of the disappearance of pesticides was evaluated as function of various parameters; illumination time, initial concentration of OP pesticides, amount of air flow, TiO2, and H2O2 or Fe31 concentration. The authors reported that the photodegradation

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8. Wastewater problems and treatments

efficiency increased with the illumination time because of the release with great amounts of radicals (OH• and O2 2 ). Importantly, these latter promote the breaking of P S and P O bond of the organophosphorous insecticides favoring formation of PO32 4 , CO2, H2O, and mineral acids. Furthermore, the photocatalytic degradation efficiency was found to increase with the addition of a small amount of H2O2 (6 3 1023 mol/dm3) or Fe31 (1024 mol/dm3) and by using suspension of TiO2 with a maximum weight of 0.4 g. However, the increasing of the initial concentration of OP pesticides, at constant illumination time and light intensity reduces the number of OH•, O22 and the amount of formed 2 phosphate. Consequently, photodegradation efficiency decreases. Recently, TiO2 immobilized on silica gel was used to eliminate organophosphate and phosphonoglycine using UV light [169]. The feasibility and the performance of the photocatalytic degradation of acephate, dimethoate, and glyphosatein water were investigated by TOC, ion chromatography as well as high-performance liquid chromatography. It was found that under UV light, acephate and dimethoate degrade naturally with a decrease of 69% and 86% of the TOC content after 120 min, respectively. On the contrary, glyphosate degradation seems to be governed by both adsorption and photocatalytic reaction leading to the release of 74% and 52% of the TOC under dark and irradiated conditions, respectively. The kinetic study performed on pesticides showed that the degradation of both acephate and dimethoate follows the apparent first-order kinetic law. As shown in Table 8.3, it is obvious from the kinetic parameters (kapp) values that the photocatalytic process is faster with dimethoate than acephate. It is worth noting that kapp of glyphosate was not determined since its kinetic degradation does not follow the first order. Moreover, it is found that the photocatalysis of the studied pesticides is concomitant with the formation of intermediates product preventing thereby their complete mineralization. On this basis the authors proposed a mechanism pathway to elucidate the formation of the intermediates and end products. For acephate the researchers reported that its decomposition followed two steps. The first one is an oxidative attacks leading to the cleavage of P N bond with formation of acetamide and PO32 4 that could be adsorbed onto TiO2 surface. The second step consists of a hydroxylation of the methyl groups borne by sulfur bonded to P, which will be further mineralized as photocatalysis proceeds. For dimethoate the photodegradation seems to occur via rapid hydroxylation of the methyl groups borne by oxygen bonded to P and through C N cleavage of the amide bond involving the formation of many TABLE 8.3 Kinetic parameters and intermediates and end products obtained during photocatalysis of acephate, dimethoate, and glyphosate. Pesticides

kapp (min21) (photocatalytic degradability)

kapp (min21) (disappearance)

Acephate

0.011

Dimethoate

0.019

Glyphosate

Intermediate products

By-product

0.031

Formic acid, acetic acid, and glycolic acid

CO2, H2O, H1, 2 SO22 4 , and NO3

0.052

Formic acid and NO2 2

CO2, H2O, H1, 2 SO22 4 , and NO3

Formic acid and glycolic acid

CO2, H2O, H1, 32 NO2 3 , and PO4

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intermediates products such as formic acid, nitrite. These latter can be transformed into CO2 and mineral ions due to the presence of hydroxyl and/or superoxide anion radicals. Concerning glyphosate, the authors admit that P N or C N bond undergoes cleavage leading to formation of PO32 4 and sarcosine or aminomethylphosponic and glycolic acids, respectively. But in both cases the organic intermediates will be transformed into CO2 as photocatalysis progresses. Interestingly, modified titanium dioxide could further enhance the photocatalaytic degradation and decomposition of OP contaminants. WO3/TiO2 photocatalytic activity and degradation of malathion pesticide, under solar irradiation, were evaluated by RamosDelgado et al. [170]. The effect of dopant concentration has been highlighted. Indeed, modified TiO2, with 2 and 3 wt.% of WO3, showed better photocatalytic degradation of malathion compared to bare TiO2. On the contrary, the mineralization rate and TOC release depend strongly on the WO3 concentration. The results demonstrated that the optimal mineralization of malathion is obtained with 2 wt.% of WO3; in this condition, 76% of TOC is removed. The formation of small WO3 clusters and large specific surface area reduces the recombination process and boosts the photocatalytic reactivity between materials and pollutant. However, an excess of dopant (i.e., 5 wt.% WO3) inhibits the formation of hydroxyl radical ions impeding the total mineralization of the intermediate products to CO2. In fact, the TOC removal on 5% WO3/TiO2 is about 28%, a very lower value even than that of bare TiO2 (47%). Furthermore, the authors emphasized that 2% WO3/TiO2 is a very stable photocatalyst with high corrosion resistance so it can be reused without loss of its activity. The photocatalysis is also an effective method to reduce emission of heavy metals. The photodegradation of chromium [Cr(VI)], under visible light, using algae decorated TiO2/Ag hybrid nanofiber membrane was attempted by Wang et al. [171]. According to the authors, the presence of algae promotes the reduction of Cr(VI) to Cr(III). It seems that this latter photogenerate reactive oxygen species serve as scavenger hole, inhibiting thereby the recombination of hole/electron. Indeed, the presence of algal cells was found to enhance both the adsorption of Cr(IV) on TiO2/Ag mat and the removal of more than 50% of Cr(VI) under dark and visible light irradiation conditions, respectively. The highly dependance of the photodegradation efficiency to Cr(VI) concentration is also demonstrated in this work. On the other hand, Diao et al. [172], in a study evaluating the photocatalytic removal of Cr(VI) on TiO2/Fe0 composite, achieved the total reduction of Cr(VI) owing to the capture of photoexcited electron from Fe reduction. They even added that the release of hydroxyl radicals decreases the reduction rate of Cr(VI). Chowdhury et al. [173] studied the photoreduction of cadmium ion [Cd(II)] with Eosin Y-sensitized TiO2 photocatalyst. The authors emphasized the use of the major role of triethanolamine as a sacrificial electron donor and a bandgap reducer. Indeed, its presence accelerates the complete photoreduction of Cd(II) within 3 h at a pH of 7 and under visible solar light. However, the work carried out by Alizadeh et al. [174] concluded that the maximum removal of Cd(II) is reached at pH of 5 on cross-linked magnetic EDTA/chitosan/TiO2 nanocomposite (MECT). They even showed that MECT exhibit a long-term stability, and their adsorption capacity for Cd(II) is very high and reached 209.205 mg/g.

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8.5 Conclusion and future trends In advances oxidation process, the photocatalysis is one of the most efficient approaches to eliminate organic or inorganic pollutants from wastewaters. Whatever the radiation sources, UV light or sunlight, many studies showed the importance of photodegradation conditions (e.g., materials, pH, pollutant concentration, and oxidizing species) to ensure total removal with high degradation rates of all types of contaminants with their different concentrations. In this chapter the state of the art of the wastewater problems is presented within the effect of the released sewage, by domestic, industrial, agricultural, and hospital sources; on both health and groundwater. Moreover, we have presented the results of several studies carried out on the photodegradation of various recalcitrant pollutants using TiO2 photocatalysts. The experimental results showed that the titanium oxide is an efficient material to eliminate the maximum of toxic contaminants. Meanwhile, its recombination with other metals or compounds seems to be interesting to enhance the photocatalytic rates and to insure its long-term stability. For concluding, it is obvious that wastewater treatments attract attentions of many researchers over the world. However, it is important to raise awareness about the scarcity of water and to take measures for protecting it other than reducing its consumption. Even if the water is contaminated, it is inevitable to adopt new approaches that disinfects and decontaminates efficiently such as combination between AOP process and biological treatments with low cost.

List of acronyms AOPs Cd Cr CFA CB COD CuPc DEP HF LDPE MECT MB MO OMW OP PS RGO SC TOC UV UV vis VB β-CD e2 h1

advanced oxidation processes cadmium ion chromium ion clofibric acid conduction band chemical oxygen demand copper phthalocyanine dimethyl-2,2,2-trichloro-l-hydroxyethyl phosphonate hydrofluoric acid low-density polyethylene magnetic EDTA/chitosan/TiO2 nanocomposite methylene blue methyl orange olive mill wastewater organophosphorous pesticides polystyrene reduced graphene oxide semiconductor total organic carbon ultraviolet ultraviolet visible valence band β-cyclodextrin electron hole

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171

List of symbols kapp kinetic constant

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