**3. Water treatment techniques**

In the past years, several biological, chemical, and physical techniques have been established to eliminate poisonous pollutants from water resources [33]. Despite the availability of several techniques to eliminate pollutants from water, each of these techniques has their intrinsic limitations.

#### **3.1. The use of metal oxide polymer nanocomposite in chemical treatment of wastewater**

Chemical treatment is a wastewater treatment technique where chemicals such as hydroxides, carbonates, and sulfides combine with the pollutants in the wastewater to form insoluble precipitates. Chemical treatment is used during wastewater treatment to slow down disinfection. Chemical treatment techniques such as ozonation and oxidation have been used for the removal of contaminants [34].

Oxidation is an essential technique which uses strong oxidising agents to remove pollutants from the wastewater. The two main classes of oxidation technique that have been reported are the chemical and the UV-assisted oxidation that uses hydrogen peroxide, chlorine, potassium permanganate, ozone, and Fenton's reagent [35]. Manganese chloride and magnesium chloride has been used to eliminate Levafix Brilliant Blue EBRA and the results demonstrated a significant increase in the chemical dosage required to remove the dyeing auxiliaries [36]. The discharging solutions of white mud have been used to eradicate Reactive Light Yellow K-6G, Direct Yellow R, Acid Orange II, and Reactive Bright Red K-2BP in wastewater [37]. The results showed almost 90% dye elimination within 90 sec.

Ozonation is also one of the most effective and attractive chemical methods owing to the strong oxidative action of O<sup>3</sup> . In this process, O<sup>3</sup> is decomposed into free radicals follow by instantaneous reaction of the free radicals with the pollutants.

$$\rm O\_3 \rightarrow \rm HO^\* + O\_2^{\left(\rightarrow\right)} + \rm HO\_3^\* + \rm HO\_4^\* \tag{1}$$

$$\rm O^{\bullet} + O\_2^{\bullet \text{-}} + HO\_3^{\bullet \text{-}} + HO\_4^{\bullet \text{-}} \text{-pollutants} \rightarrow \text{universalised by -- products} \tag{2}$$

The O<sup>3</sup> is generally created by passing oxygen *via* the gap between two discharging electrodes. During the oxidation process, agents such as high pH, UV, and peroxides are used along with ozone. The optimal pH may be alkaline or acidic based on the nature of pollutants presents in the wastewater. High dose ozonation (~60 mgL−1) was used for dye removal [38]. They found that the color removal indicates the toxic potential of ozonation by-products.

However, highly toxic and unstable metabolites induce by the oxidation and ozonation technique may cause adverse effects on human health and aquatic life [39]. Chlorination process, which provides protection against regrowth of pathogens and bacteria, results in undesirable odors and tastes as well as in the formation of disinfection by-products [40]. Although ozonation has shown to be very effective for pollutants removal, sometimes ozonation generate by-products, which are more dangerous than the pollutants itself. In addition, ozonation process has been less attractive due to short lifetime and high operating costs. The combination of processes with polymers and metal oxides aids to cut cost while at the same time achieving effective degradation of pollutants. For example, polyaniline/hexanoic acid/ TiO<sup>2</sup> /Fe<sup>3</sup> O4 nanocomposite with different ratios of Fe<sup>3</sup> O4 and TiO<sup>2</sup> was fabricated using *in situ* chemical polymerization *via* template-free approach [41]. The nanocomposite with chemical treatment displays a narrow sharp reflection loss peak with strong absorption at a lower frequency owing to the Fe<sup>3</sup> O4 nanoparticles outside the nanorods while the nanocomposite without chemical treatment displays a broad reflection loss peak with weak absorption at a higher frequency.

a mixture of materials at a high temperature to decompose them in the gas phase. The TGA results are generally obtained as a curve with a percent weight against temperature under

In the past years, several biological, chemical, and physical techniques have been established to eliminate poisonous pollutants from water resources [33]. Despite the availability of several techniques to eliminate pollutants from water, each of these techniques has their intrinsic

**3.1. The use of metal oxide polymer nanocomposite in chemical treatment of wastewater**

Chemical treatment is a wastewater treatment technique where chemicals such as hydroxides, carbonates, and sulfides combine with the pollutants in the wastewater to form insoluble precipitates. Chemical treatment is used during wastewater treatment to slow down disinfection. Chemical treatment techniques such as ozonation and oxidation have been used

Oxidation is an essential technique which uses strong oxidising agents to remove pollutants from the wastewater. The two main classes of oxidation technique that have been reported are the chemical and the UV-assisted oxidation that uses hydrogen peroxide, chlorine, potassium permanganate, ozone, and Fenton's reagent [35]. Manganese chloride and magnesium chloride has been used to eliminate Levafix Brilliant Blue EBRA and the results demonstrated a significant increase in the chemical dosage required to remove the dyeing auxiliaries [36]. The discharging solutions of white mud have been used to eradicate Reactive Light Yellow K-6G, Direct Yellow R, Acid Orange II, and Reactive Bright Red K-2BP in wastewater [37]. The

Ozonation is also one of the most effective and attractive chemical methods owing to the

(−•) + HO<sup>3</sup>

However, highly toxic and unstable metabolites induce by the oxidation and ozonation technique may cause adverse effects on human health and aquatic life [39]. Chlorination

 is generally created by passing oxygen *via* the gap between two discharging electrodes. During the oxidation process, agents such as high pH, UV, and peroxides are used along with ozone. The optimal pH may be alkaline or acidic based on the nature of pollutants presents in the wastewater. High dose ozonation (~60 mgL−1) was used for dye removal [38]. They found that the color removal indicates the toxic potential of ozonation by-products.

• + HO4

• + pollutants → mineralised by − products (2)

is decomposed into free radicals follow by

• (1)

. In this process, O<sup>3</sup>

controlled atmosphere.

limitations.

**3. Water treatment techniques**

178 Descriptive Inorganic Chemistry Researches of Metal Compounds

for the removal of contaminants [34].

strong oxidative action of O<sup>3</sup>

O• + O<sup>2</sup>

The O<sup>3</sup>

results showed almost 90% dye elimination within 90 sec.

instantaneous reaction of the free radicals with the pollutants.

• + HO4

O<sup>3</sup> → HO• + O<sup>2</sup>

<sup>−</sup>• + HO<sup>3</sup>

#### **3.2. Metal oxide polymer nanocomposite in disinfection of biological contaminants**

Biological contaminants from pathogens, as well as free-living microbes, such as protozoans, viruses, and bacteria, are possible in water [42]. Biological contaminants are classified as microorganisms, biological toxins, and natural organic matter [42]. The biological treatment techniques for the elimination of pollutants from water are regarded as extremely useful techniques owing to the less chemical usage, low cost, and eco-friendly. The general process of biological treatment techniques involves the transformation of biodegradable wastes into less toxic and simpler forms *via a* biological process using microorganisms such as fungi, bacteria, or algae. The treatment techniques are grouped into the aerobic or anaerobic process. The resultant products after anaerobic treatment are methane, biomass, and carbon dioxide, whereas water, biomass, and carbon dioxide are the resultant products after aerobic treatment. Biological processes such as microbiological, biodegradation, and enzymatic decomposition have also been used for dye removal from wastewaters. The effect of applying irradiation before *Pseudomonas sp*. to Reactive Red 120 dye was studied [43]. Decoloration and mineralization at a lower dose of irradiation were enhanced significantly. Moreover, 90% TOC removal and 98% decoloration were observed after 96 hours of microbial treatment. The potential of *Pseudomonas putida* toward the elimination of Orange II dye was investigated [44]. The results showed 92.8% dye elimination within 96 hours at pH and temperature of 8 and 30<sup>o</sup> C, respectively.

However, most of these biological treatment techniques are unable to eradicate a wide range of pollutants and most of these pollutants remain soluble in the effluent [45]. Recently, there have been reports on nanomaterials such as Zn, Ag, and Ti as a disinfectant to several waterborne disease-causing microbes owing to their charge capacity. The efficiency of polymer-metal oxide nanocomposites in water disinfections have been emphasized [46]. In particular, a polyaniline/TiO<sup>2</sup> /graphene nanocomposite was fabricated *via in situ* oxidative polymerization of aniline with TiO<sup>2</sup> and graphene nanoparticles [47]. The as-fabricated nanocomposite demonstrated high antibacterial activity toward *Enterobacter ludwigii* and *Escherichia coli*, indicating its potential as a photocatalyst with antibacterial properties. The enhanced photocatalytic activity and antibacterial activity was as a result of the low recombination of the graphene electron scavenging property and the sensitising effect of polyaniline. An ultrafiltration membrane of poly(1-vinylpyrrolidone-*co*-acrylonitrile)-*g*-ZnO and poly(ether sulfone)-*g*-ZnO with high antibacterial performances and water flux was synthesized [48]. The results revealed that both membranes possess improved water flux, high antibacterial activities, and antifouling characteristic. The hybrid reverse osmosis membranes with aromatic polyamide thin films and TiO<sup>2</sup> particles was prepared through a self-assembly route [49]. The nanocomposite under irradiated UV light showed enhanced photocatalytic bactericidal efficiency compared to those under darkness and light condition. The nonskinned anatase/poly(vinylidene fluoride) microporous membrane was synthesized without any loss in anatase using the dry cast method [50]. The as-synthesized nanocomposite membrane showed a strong bactericide effect compared to the membrane with only UV light.

#### **3.3. Metal oxide polymer nanocomposite in adsorptive technologies**

Water can be purified in several ways, such as filtration, desalination, adsorption, osmosis, sedimentation, and disinfection [4], however, adsorption holds several benefits over the other techniques [51]. Adsorption is a surface occurrence where adsorbate are concentrated on the adsorbent surface and the process can be chemisorption or physisorption base on how the adsorbate get adsorb onto the surface of the adsorbent [52]. In this process, pollutants may adsorb on the adsorbent surface via various forces such as electrostatic, hydrogen bonding, and van der Wall interaction [53]. Normally, the adsorbents possess porous structures to permit fluid to pass through faster and increase the surface area. The adsorption process is an economical and simple technique for pollutants elimination from water since it does not require additional chemicals, large amounts of water and high amounts of energy [54]. In developing countries where there is a limited access to huge amounts of financial resources and power, this cheap and simple method might be a feasible alternative. The mechanistic process of adsorption permits flexibility in the development and usage of adsorbent. An isotherm is used to identify and describe the mechanism of adsorption between the adsorbate and adsorbent [55]. There are several adsorption isotherms, but the most commonly used isotherms are Langmuir and Freundlich. Adsorption relies on several factors such as temperature, contact time, pH, particle size, concentration of pollutants as well as the nature of the adsorbent, and adsorbate [56]. The elimination of pollutants from wastewater and water through adsorption are normally carried out using activated carbon, low-cost adsorbents, nanoparticulate adsorbents, and others. As a result of the large specific surface area, nanoadsorbent shows an extensively higher rate of adsorption for pollutants compared to the powdered activated carbon [57]. Recently, several inorganic and organic adsorbents, such as activated carbon, zeolites, clay minerals, biosorbents, montmorillonite, polymer-based adsorbent, trivalent, and tetravalent metal phosphates have been employed as an adsorbent in the adsorption process [58]. Among them, polymer/metal oxide nanocomposite containing polymers, such as polypyrrole, polythiophene, polyfuran, and polyethyleneimine have a strong affinity toward cations owing to the electrostatic interaction between the positively charged ions of the metal oxide and the lone pair of the polymers [59]. In addition, the presence of positively charged nitrogen atoms in polypyrrole offers a potential application in adsorption process as adsorbent [60]. A polyaniline-modified TiO<sup>2</sup> nanocomposite was synthesized through the *in situ* chemical polymerization of aniline in TiO<sup>2</sup> solution [61]. The as-synthesized nanocomposite showed an excellent regeneration and adsorption performances with maximum adsorption capacity (454.55 mg/g) with adsorption equilibrium time of 5 min. The acetate/polypyrrole/TiO<sup>2</sup> , succinic-polypyrrole/TiO<sup>2</sup> , tartaric/polypyrrole/TiO<sup>2</sup> and citric/ polypyrrole/TiO<sup>2</sup> composites were fabricated [62] and the results indicate that the hydroxyl group significantly influenced the adsorption capacity and the surface physicochemical properties of the nanocomposite. Moreover, tartaric/polypyrrole/TiO<sup>2</sup> , tartaric/polypyrrole/TiO<sup>2</sup> and citric/tartaric/TiO<sup>2</sup> showed an improved adsorption capacity of 3–4 times toward ARG and MB compared to that of acetate/polypyrrole/TiO<sup>2</sup> and succinic/polypyrrole/TiO<sup>2</sup> . In addition, all the nanocomposites displayed an excellent adsorption capacity within 30 min and can be reused without any reduction in capacity for at least 4 times. The hybrid ternary reduced graphene oxide(rGO)/ZnFe<sup>2</sup> O4 /polyaniline nanocomposite was fabricated through the *in situ* polymerization of aniline on the ZnFe<sup>2</sup> O4 and rGO surface [63]. The as prepared composite exhibited a superior adsorption performance in the sewage purification process. Moreover, the thermodynamic date confirmed that the adsorption behavior of the nanocomposites conforms to the Langmuir isotherm with a second-order kinetic model.

and antibacterial activity was as a result of the low recombination of the graphene electron scavenging property and the sensitising effect of polyaniline. An ultrafiltration membrane of poly(1-vinylpyrrolidone-*co*-acrylonitrile)-*g*-ZnO and poly(ether sulfone)-*g*-ZnO with high antibacterial performances and water flux was synthesized [48]. The results revealed that both membranes possess improved water flux, high antibacterial activities, and antifouling characteristic. The hybrid reverse osmosis membranes with aromatic polyamide thin films and TiO<sup>2</sup> particles was prepared through a self-assembly route [49]. The nanocomposite under irradiated UV light showed enhanced photocatalytic bactericidal efficiency compared to those under darkness and light condition. The nonskinned anatase/poly(vinylidene fluoride) microporous membrane was synthesized without any loss in anatase using the dry cast method [50]. The as-synthesized nanocomposite membrane showed a strong bactericide effect compared to the

Water can be purified in several ways, such as filtration, desalination, adsorption, osmosis, sedimentation, and disinfection [4], however, adsorption holds several benefits over the other techniques [51]. Adsorption is a surface occurrence where adsorbate are concentrated on the adsorbent surface and the process can be chemisorption or physisorption base on how the adsorbate get adsorb onto the surface of the adsorbent [52]. In this process, pollutants may adsorb on the adsorbent surface via various forces such as electrostatic, hydrogen bonding, and van der Wall interaction [53]. Normally, the adsorbents possess porous structures to permit fluid to pass through faster and increase the surface area. The adsorption process is an economical and simple technique for pollutants elimination from water since it does not require additional chemicals, large amounts of water and high amounts of energy [54]. In developing countries where there is a limited access to huge amounts of financial resources and power, this cheap and simple method might be a feasible alternative. The mechanistic process of adsorption permits flexibility in the development and usage of adsorbent. An isotherm is used to identify and describe the mechanism of adsorption between the adsorbate and adsorbent [55]. There are several adsorption isotherms, but the most commonly used isotherms are Langmuir and Freundlich. Adsorption relies on several factors such as temperature, contact time, pH, particle size, concentration of pollutants as well as the nature of the adsorbent, and adsorbate [56]. The elimination of pollutants from wastewater and water through adsorption are normally carried out using activated carbon, low-cost adsorbents, nanoparticulate adsorbents, and others. As a result of the large specific surface area, nanoadsorbent shows an extensively higher rate of adsorption for pollutants compared to the powdered activated carbon [57]. Recently, several inorganic and organic adsorbents, such as activated carbon, zeolites, clay minerals, biosorbents, montmorillonite, polymer-based adsorbent, trivalent, and tetravalent metal phosphates have been employed as an adsorbent in the adsorption process [58]. Among them, polymer/metal oxide nanocomposite containing polymers, such as polypyrrole, polythiophene, polyfuran, and polyethyleneimine have a strong affinity toward cations owing to the electrostatic interaction between the positively charged ions of the metal oxide and the lone pair of the polymers [59]. In addition, the presence of positively charged nitrogen atoms in polypyrrole offers a potential

**3.3. Metal oxide polymer nanocomposite in adsorptive technologies**

application in adsorption process as adsorbent [60]. A polyaniline-modified TiO<sup>2</sup>

nanocomposite

membrane with only UV light.

180 Descriptive Inorganic Chemistry Researches of Metal Compounds

#### **3.4. Application of metal oxide polymer nanocomposite in membrane technologies**

Currently, membrane technologies have been more efficient in water and wastewater treatment owing to their effective removal of pollutants without producing any harmful by-products. Generally, the basic principle of membrane technology is to apply semipermeable membranes to eliminate particles, gases, fluids, and solutes. For the effective separation of pollutants from water reservoirs, the membrane must be water permeable as well as less permeable to particles or solutes. Water treatment by membrane technologies can be the effective removal of pollutants due to their feasibility, environmentally friendly and cost-effective [64]. Even though inorganic membranes are gaining more consideration, the majority of membranes are made of polymeric materials. Polymer materials such as polysulfone, cellulose nitrates and acetate, polytetrafluoroethylene, polypropylene, polyethersulfone, polyacrylonitrile, polyimide, polyvinylidene fluoride and polyvinyl alcohol are the most extensively used organic membrane materials. These materials are well-known as alternative approaches to pollutants removal owing to their mechanical stability, excellent permeability, chemical resistance, and selectivity of permeate [65]. The immobilization of metal oxide nanoparticles in polymer membrane has been effective for the photodegradation of contaminants in water treatment [66]. For example, membranes containing nano-TiO<sup>2</sup> effectively degrade contaminants (mostly chlorinated compounds) in the water system [67]. The use of TiO<sup>2</sup> immobilized on polyethene membranes has also proved to be very effective in degrading 1,2-dichlorobenzene [68]. A complete degradation of 4-nitrophenol was observed when a nanocomposite membrane consisted of polymers and alumina *via* layer-by-layer adsorption of citrate stabilised Au nanoparticles and polyelectrolytes [69]. A nanocomposite membrane coated with CoFe<sup>2</sup> O4 (–rGO) and polyvinylidene fluoride (PVDF) on a carbon fiber cloth was fabricated in a PVDF casting solution [70]. The as-fabricated nanocomposite functioned as the cathode membrane to efficiently decompose the contaminants in the water system compared to the microbial fuel cell membrane bioreactor system. The commercial TiO<sup>2</sup> nanoparticles modified with polyaniline was synthesized using the *in situ*

polymerization to enhance the property of membrane antifouling and avoid particle agglomeration [71]. The as-prepared nanocomposite membranes showed higher porosity, fewer microvoids, larger surface and finger-like pore size compared to the control polysulfone membrane. In addition, the results revealed that 1.0 wt% of the nanocomposite membrane exhibited water permeability, excellent hydrophilicity, and antifouling property with high rejection rate. The polysulfone ultrafiltration membranes with polyethylene Glycol 1000 as additives and polyaniline/titania nanocomposites were synthesized through the phase inversion technique [72]. The as-synthesized composites membranes displayed enhanced permeability, improved porosity, better hydrophilicity, excellent antifouling ability, and water uptake compared to the polysulfone membranes. Through the phase-inversion route, self-synthesized Cu<sup>2</sup> O nanoparticle was introduced into the poly(ether sulfone) mixed-matrix membrane [73]. About 1.5 wt% of the nanocomposite showed an improved water permeability of 66.72 × 10−9 m s/k/Pa. Poly(l-lactide)/TiO<sup>2</sup> nanocomposite membrane was prepared by immersion precipitation method [74]. Moreover, the porosity and pore size on the nanocomposite membrane surface become denser with increasing the TiO<sup>2</sup> loading. The as-prepared nanocomposite membranes displayed enhance recycling and antifouling activity compared to the bulk poly(l-lactide) membrane. The hybrid polyacrylic acid/TiO<sup>2</sup> ultrafiltration membranes with enhanced antifouling performance were fabricated by incorporating TiO<sup>2</sup> particles grafted with polyacrylic acid [75]. The results showed that the nanocomposite membrane exhibited improved hydrophilicity, dispersed well in the membrane medium, excellent antifouling performance, and water flux compared to the pure polyacrylic acid membranes. A polyvinylidene fluoride ultrafiltration membrane was improved through the phase inversion route by incorporating TiO<sup>2</sup> particles in a polyvinylidene fluoride solution [76]. The results showed that the nanocomposite membranes display a larger and longer macrovoid which resulted in an enhanced water flux activity due to the increased surface hydrophilicity. A hybrid SiO<sup>2</sup> /polyvinylchloride membrane with different loading of SiO<sup>2</sup> nanoparticles was prepared using the phase-inversion technique [77]. The membrane nanocomposite with 1.5 wt% SiO<sup>2</sup> nanoparticles displayed better performance toward bacterial attachment and protein absorption, higher flux recovery ratio, and better antifouling performance compared to the bare polyvinylchloride membrane. Hence, membrane nanocomposites exhibited applicable potential in water and wastewater treatment due to their excellent antifouling performance, cost-effectiveness, and better elimination efficacies of total bacteria (>93.6%), chemical oxygen demand (>82.9%), and suspended solids (>97.2%). The nanoporous poly(ether sulfone)/TiO<sup>2</sup> ultrafiltration membranes were fabricated through a nonsolvent-induced phase separation route [78]. The modified poly(ether sulfone) membrane revealed increased wettability, reduced pore size, and surface-free energy. In addition, the modified membrane with 0.5 wt% TiO<sup>2</sup> nanoparticle loading demonstrated enhanced antifouling activity with ~80% water flux recovery compared to the control membrane.

#### **3.5. Metal oxide polymer nanocomposite in ion exchange technologies**

Hard water often leaves a grey residue when used in cleaning and washing. An ion exchange technique which is similar to the reverse osmosis technique can be used to soften the water. Ion exchange technique is a water treatment technique, which successfully eliminates pollutants from aqueous solutions *via* a strong interaction between the functional group on the charged contaminants and the ion exchange resins [79]. Thus, this technique comprises of exchange of ions to form strong bond between the resins and solutes to achieve efficient separation. Generally, ion exchange membrane is categorized into cation and anion exchangers based on the form of ionic group attached to the membrane medium. The most common used anion exchange are weak base-type, which are type I ion exchange resins (-N(CH<sup>3</sup> ) 3 ), and type II ion exchange resins (-N(CH<sup>3</sup> )2 C2 H4 OH), while cation exchangers are the weak acidic carboxylate groups (-COO-) and strong acid-type groups (-SO<sup>3</sup> ) [80]. The modification of commercially existing ion exchangers and the design of appropriate organic polymeric and inorganic metal oxide membranes with biocide and catalytic performance have been of great interest [81]. Beside polymeric membranes, ion exchange membranes can also be fabricated from other materials such as phosphate salts, bentonite, and zeolites [27]. However, these membranes are ineffective compared to the polymeric membranes owing to their high cost, too large pores and relative bad electrochemical properties [82]. In addition, ion exchange membranes fabricated from polymeric materials exhibits excellent conductivity and high chemical stability [83]. Thus, the combination of polymer materials with inorganic metal oxides offer a new form of ion exchangers with high stability, excellent reproducibility, high ion exchange capacity, and mechanical stability as well as good selectivity toward metal ions [84, 85]. Recently, several excellent metal oxide/polymer ion exchange membranes have been fabricated and effectively used in water remediation process. For instance, nanocomposite material formed by the immobilization of multivalent metal acid salts into conducting polymers, such as polypyrrole, polyaniline, or polythiophene, offers a hybrid ion exchange membrane with high reproducibility, stability, granulometric, and mechanical properties as well as excellent selectivity for heavy metals and ion-exchange capacity [86]. These results indicated that the hybrid organicinorganic ion exchangers were highly selective toward Cd(II). A hybrid polypyrrole polyantimonic acid nanocomposite with good reproducibility, selectivity toward certain heavy metals, excellent ion-exchange capacity, thermal, and chemical stability was fabricated by mixing hydrated antimony oxide with polypyrrole [83]. The as-fabricated nanocomposite was extremely selective toward Hg(II). A crystalline acrylamide stannic silicomolybdate nanocomposite ion exchanger was prepared at pH 0.63 [87]. The authors revealed that the nanocomposite demonstrated a superior exchange capacity of 1.64 meq/g compared to the pure stannic silicomolybdate (0.40 meq/g). Based on the distribution studies results, several significant binary separations such as Cd(II)-Pb(II), Cd(II)-Cu(II), Al(III)-Pb(II), Al(III)-Cu(II), Zn(II)- Pb(II), and Zn(II)-Cu(II) were observed on the acrylamide stannic silicomolybdate column. A hybrid poly-o-toluidine/Ce<sup>3</sup> (PO4 ) 4 and poly-o-toluidine/Sn(WO4 ) 2 nanocomposite with high stability, good reproducibility, high ion exchangeability, and good selectivity for heavy metals was fabricated by incorporating orthotoluidine (poly-*o*-toluidine) into Ce<sup>3</sup> (PO4 )4 precipitate [88, 89]. The distribution studies demonstrated that the nanocomposites were extremely selective toward Cd(II). Using a fibrous-type polypyrrole/Th<sup>3</sup> (PO4 )4 cation-exchanger nanocomposite, the separation of Pb(II) from aqueous solution was explored [90]. The nanocomposite was fabricated by immobilizing polypyrrole into Th<sup>3</sup> (PO4 ) 4 precipitate. Based on the distribution studies, the nanocomposite exhibited excellent selectivity for Pb(II) on the Th<sup>3</sup> (PO4 )4 column. A hybrid poly(methyl methacrylate)/Zr<sup>3</sup> (PO4 )4 cation-exchanger nanocomposites were synthesized by immobilizing poly(methyl methacrylate) into Zr<sup>3</sup> (PO4 ) 4 precipitate [91]. Based on the sorption studies, the nanocomposite was highly selective to Pb(II). A novel cellulose

polymerization to enhance the property of membrane antifouling and avoid particle agglomeration [71]. The as-prepared nanocomposite membranes showed higher porosity, fewer microvoids, larger surface and finger-like pore size compared to the control polysulfone membrane. In addition, the results revealed that 1.0 wt% of the nanocomposite membrane exhibited water permeability, excellent hydrophilicity, and antifouling property with high rejection rate. The polysulfone ultrafiltration membranes with polyethylene Glycol 1000 as additives and polyaniline/titania nanocomposites were synthesized through the phase inversion technique [72]. The as-synthesized composites membranes displayed enhanced permeability, improved porosity, better hydrophilicity, excellent antifouling ability, and water uptake compared to the polysul-

introduced into the poly(ether sulfone) mixed-matrix membrane [73]. About 1.5 wt% of the nanocomposite showed an improved water permeability of 66.72 × 10−9 m s/k/Pa. Poly(l-lac-

Moreover, the porosity and pore size on the nanocomposite membrane surface become denser

enhance recycling and antifouling activity compared to the bulk poly(l-lactide) membrane. The

showed that the nanocomposite membrane exhibited improved hydrophilicity, dispersed well in the membrane medium, excellent antifouling performance, and water flux compared to the pure polyacrylic acid membranes. A polyvinylidene fluoride ultrafiltration membrane was

fluoride solution [76]. The results showed that the nanocomposite membranes display a larger and longer macrovoid which resulted in an enhanced water flux activity due to the increased

attachment and protein absorption, higher flux recovery ratio, and better antifouling performance compared to the bare polyvinylchloride membrane. Hence, membrane nanocomposites exhibited applicable potential in water and wastewater treatment due to their excellent antifouling performance, cost-effectiveness, and better elimination efficacies of total bacteria (>93.6%), chemical oxygen demand (>82.9%), and suspended solids (>97.2%). The nanoporous poly(ether

separation route [78]. The modified poly(ether sulfone) membrane revealed increased wettability, reduced pore size, and surface-free energy. In addition, the modified membrane with 0.5

Hard water often leaves a grey residue when used in cleaning and washing. An ion exchange technique which is similar to the reverse osmosis technique can be used to soften the water. Ion exchange technique is a water treatment technique, which successfully eliminates pollutants from aqueous solutions *via* a strong interaction between the functional group on the charged

nanoparticles was prepared using the phase-inversion technique [77]. The membrane

ultrafiltration membranes were fabricated through a nonsolvent-induced phase

nanoparticle loading demonstrated enhanced antifouling activity with ~80% water

nanocomposite membrane was prepared by immersion precipitation method [74].

loading. The as-prepared nanocomposite membranes displayed

ultrafiltration membranes with enhanced antifouling performance

particles grafted with polyacrylic acid [75]. The results

/polyvinylchloride membrane with different loading of

nanoparticles displayed better performance toward bacterial

O nanoparticle was

particles in a polyvinylidene

fone membranes. Through the phase-inversion route, self-synthesized Cu<sup>2</sup>

improved through the phase inversion route by incorporating TiO<sup>2</sup>

tide)/TiO<sup>2</sup>

SiO<sup>2</sup>

sulfone)/TiO<sup>2</sup>

wt% TiO<sup>2</sup>

with increasing the TiO<sup>2</sup>

hybrid polyacrylic acid/TiO<sup>2</sup>

were fabricated by incorporating TiO<sup>2</sup>

182 Descriptive Inorganic Chemistry Researches of Metal Compounds

surface hydrophilicity. A hybrid SiO<sup>2</sup>

flux recovery compared to the control membrane.

**3.5. Metal oxide polymer nanocomposite in ion exchange technologies**

nanocomposite with 1.5 wt% SiO<sup>2</sup>

acetate/Zr(IV) molybdophosphate (ZMP) cation exchanger was fabricated by incorporating organic polymers into multivalent metal acid salts [92]. The as-fabricated nanocomposites displayed a superior selectivity toward Cr(III) on the ZMP column. A novel polymeric-inorganic cation-exchanger nanocomposite was fabricated through a sol-gel route by immobilizing polyaniline into zirconium titanium phosphate [93]. The nanocomposite showed high ionexchange capacity (4.52 meq/g), good thermal and chemical stability compared to the bulk polyaniline and zirconium titanium phosphate. Moreover, the distribution studies of the nanocomposite revealed highly selectivity to Pb(II) and Hg(II).

#### **3.6. Metal oxide polymer nanocomposite in photocatalytic degradation of pollutants**

Recently, personal care and pharmaceutical products used in drugs, cosmetics, agricultural practice, and veterinary medicine have been considered as emerging contaminants [94]. Moreover, organic dyes normally used in printing, photographic, and textile industries are often wasted during the dying process and discharge into the wastewater effluents. The existence of even low concentrations of pollutants in the wastewater streams extremely affects the nature of water, which makes hard to be oxidised or biodegraded. The photodegradation of pollutants in wastewater and water systems using photocatalysis process has been an effective approach compared to the conventional approaches without high energy consumption as well as the formation of highly toxic and poor biodegradable products [95]. Photocatalysis is an AOP employed in wastewater and water treatment process, such as degradation of highly toxic pollutants as well as the oxidative elimination of microbial pathogens and micropollutants [96]. Photocatalysis process uses semiconductors such as metal oxides, nitrides, and sulfides. Normally, a metal oxide is photoactivated by the incoming photon from the light source. As a renewable and safe energy source as well as abundant and clean, natural sunlight has been the ideal source of energy for the activation process [97]. The sun distributes almost four orders of magnitude of its energy to the earth surface annually larger than the energy consumed by humans. When the metal oxide is irradiated by sunlight, electrons and holes are generated according to Eq. (3) only if the energy of the incident photons is equal to or greater than (≥) the metal oxide band gap energy (**Figure 1**).

$$\text{Metal oxide} + hv(\text{UV}) \rightarrow \text{Metal oxide(e}^{\cdot}(\text{CB}) + h^{\cdot}(\text{VB})) \tag{3}$$

The photocatalytic reaction is initiated when photogenerated electrons are transferred from the filled valence band (VB) of the metal oxide to the empty conduction band (CB), leaving positive holes in the VB (hVB + ). The photogenerated electron then migrates to the metal oxide surface where separation and redox reaction occur. Both reduction and oxidation processes can occur on the surface of the photoexcited metal oxide only if the process is thermodynamically feasible. The photoinduced holes at the VB then react with the adsorbed water molecules to produce OH• radical (Eq. (4)).

$$\text{H}\_2\text{O(ads)} \,\text{+h}^\text{\textasciicircum} \,\text{H}^\text{\textasciicircum} \,\text{+} \,\text{HO}^\text{\textasciicircum} \,\text{+} \,\text{HO}^\text{\textquotedbl{}} \,\text{(ads)} \,\text{+H}^\text{\textquotedbl{}} \,\text{(ads)} \tag{4}$$

Instantaneously, electrons in the CB (eCB − ) also react with the adsorbed oxygen molecule to produce O<sup>2</sup> −• radical as shown in Eq. (5).

$$\rm O\_2 + e^-(CB) \rightarrow \quad O\_2^{\cdot \cdot}(ads \text{ )}\tag{5}$$

**Figure 1.** Schematic of photocatalysis process toward the photodegradation of pollutants.

acetate/Zr(IV) molybdophosphate (ZMP) cation exchanger was fabricated by incorporating organic polymers into multivalent metal acid salts [92]. The as-fabricated nanocomposites displayed a superior selectivity toward Cr(III) on the ZMP column. A novel polymeric-inorganic cation-exchanger nanocomposite was fabricated through a sol-gel route by immobilizing polyaniline into zirconium titanium phosphate [93]. The nanocomposite showed high ionexchange capacity (4.52 meq/g), good thermal and chemical stability compared to the bulk polyaniline and zirconium titanium phosphate. Moreover, the distribution studies of the

**3.6. Metal oxide polymer nanocomposite in photocatalytic degradation of pollutants**

Recently, personal care and pharmaceutical products used in drugs, cosmetics, agricultural practice, and veterinary medicine have been considered as emerging contaminants [94]. Moreover, organic dyes normally used in printing, photographic, and textile industries are often wasted during the dying process and discharge into the wastewater effluents. The existence of even low concentrations of pollutants in the wastewater streams extremely affects the nature of water, which makes hard to be oxidised or biodegraded. The photodegradation of pollutants in wastewater and water systems using photocatalysis process has been an effective approach compared to the conventional approaches without high energy consumption as well as the formation of highly toxic and poor biodegradable products [95]. Photocatalysis is an AOP employed in wastewater and water treatment process, such as degradation of highly toxic pollutants as well as the oxidative elimination of microbial pathogens and micropollutants [96]. Photocatalysis process uses semiconductors such as metal oxides, nitrides, and sulfides. Normally, a metal oxide is photoactivated by the incoming photon from the light source. As a renewable and safe energy source as well as abundant and clean, natural sunlight has been the ideal source of energy for the activation process [97]. The sun distributes almost four orders of magnitude of its energy to the earth surface annually larger than the energy consumed by humans. When the metal oxide is irradiated by sunlight, electrons and holes are generated according to Eq. (3) only if the energy of the incident photons is equal to or greater than (≥) the metal oxide band gap

 Metal oxide + *hν*(UV ) → Metal oxide(e −(CB) + h +(VB)) (3) The photocatalytic reaction is initiated when photogenerated electrons are transferred from the filled valence band (VB) of the metal oxide to the empty conduction band (CB), leaving

surface where separation and redox reaction occur. Both reduction and oxidation processes can occur on the surface of the photoexcited metal oxide only if the process is thermodynamically feasible. The photoinduced holes at the VB then react with the adsorbed water molecules

−

+ e <sup>−</sup>(CB) → O<sup>2</sup>

). The photogenerated electron then migrates to the metal oxide

 O(ads ) + h +(VB) → HO•(ads ) + H+(ads ) (4)

) also react with the adsorbed oxygen molecule to

<sup>−</sup>•(ads ) (5)

nanocomposite revealed highly selectivity to Pb(II) and Hg(II).

184 Descriptive Inorganic Chemistry Researches of Metal Compounds

energy (**Figure 1**).

positive holes in the VB (hVB

to produce OH• radical (Eq. (4)).

Instantaneously, electrons in the CB (eCB

O<sup>2</sup>

−• radical as shown in Eq. (5).

H<sup>2</sup>

produce O<sup>2</sup>

+

The HO• and O<sup>2</sup> −• radicals formed are extremely powerful oxidizing and reducing agent to attack the adsorbed pollutants (Eq. (6)), causing them to mineralize depending on their stability level and structure.

$$\text{Polutants} \star (\text{HO}^\*, \text{O}\_2^{\bullet \cdot}) \rightarrow \text{Degradiation products} \tag{6}$$

Sometimes, the O<sup>2</sup> −• radical may not take part in further oxidation process and gets protonated to generate a hydroperoxyl radicals (HO<sup>2</sup> •) and subsequently into H<sup>2</sup> O2 , which further decomposes into extremely reactive HO• radicals (see Eqs. (7)–(9)).

$$\rm O\_2^{\bullet} \text{(ads)} \,\rm \text{+H}^\* \to \rm HCO^\bullet \text{(ads)}\tag{7}$$

$$\mathrm{HCO^{\*}(ads)} \star \mathrm{H^{\*}} \rightarrow \mathrm{H\_{2}O\_{2}(ads)}\tag{8}$$

$$\text{H}\_2\text{O}\_2\text{(ads)} \rightarrow 2\text{ HO}^\*\text{(ads)}\tag{9}$$

Due to the ability of some of the metal oxides to easily absorb some of the visible light, another mechanism of photodegradation of pollutants was considered. This mechanism consist of the pollutants excitation under simulated visible light irradiation from the ground state (pollutant) to the triplet excited state (pollutant\*) [98]. As a result of the migration of electron into the CB of the metal oxide, these excited state pollutants species are transformed into a semioxidized cation radical (pollutants+•). The trapped electrons combine with the dissolved oxygen molecules to generate O<sup>2</sup> −• radical anions, which can further undergo a series of reactions to generate an HO• radicals which can then oxidize the adsorbed pollutants.

Since the report by Fujishima [99], numerous metal oxide–based photocatalysts has attracted substantial consideration in the degradation of highly toxic and nonbiodegradable compounds [100] owing to its exceptional optical properties, nontoxicity, low cost, and high stability toward photo and chemical corrosion [101]. However, the major limitation of some of the metal oxide– based photocatalyst materials for potential applications includes the fast recombination of the charge carriers due to the low quantum yield and wide band gap limitation in harvesting a wider portion of the solar energy [102]. Hence, the design of metal oxide–based photocatalyst materials for the degradation of contaminants, which are of high photostability, visible light active, and photoactivity are of considerable interest to provide a fundamental insight into the underlying mechanism of photocatalysis as well as designing a more efficient and easily tunable metal oxide–based photocatalyst materials. To address these restrictions, metal oxides are normally immobilized with polymers. The incorporation of metal oxide into a polymeric material with appropriate energy levels enhances the charge migration between the inorganic metal oxide and polymer to reduce the recombination of the charge carriers [103]. Recently, nanocomposites of several conductive polymers and metal oxide nanoparticles have been fabricated with improved photocatalytic responses in the visible region. Polymeric materials are currently been applied in water treatment due to their pore size distribution, perfect mechanical rigidity, tunable, and large surface area [104]. For example, some organic nanofiber membranes, such as cellulose, polyacrylonitrile, polyvinylpyrrolidone, polytetrafluoroethylene, and polyvinyl acetate are considered as an excellent catalyst supports owing to their high porosity, high permeability, large surface area and good flexibility [105]. The improvements in the fabrication approach employed for designing materials, the characterization and computational techniques have facilitated the fine-tuning of compositional and structural characteristics of materials. The novel porous polytetrafluoroethylene nanofiber membrane with Fe<sup>2</sup> O3 was prepared through a three-step method [105]. Under simulated UV light irradiation, the porous nanocomposite displayed superior photocatalytic degradation toward Acid Red dye with a degradation efficiency of >99% after 80 min. TiO<sup>2</sup> /polypropylene nanocomposite has been reported to show superb photocatalytic performance toward pollutants degradation [106]. The photocatalytic performance of a transition metal coordination polymer(TMCP)/polyoxometalate nanocomposite was enhanced by immobilizing polypyrrole into the TMCP surface *via* a superficial *in situ* chemical oxidation polymerization method [107]. Under simulated visible light irradiation, the as-prepared nanocomposite displayed better photocatalytic performance toward the degradation of Rhodamine B (RhB) dye. Transition metal ions were incorporated into TiO<sup>2</sup> / fly-ash cenospheres with poly(o-phenylenediamine) through ion imprinting technology [108]. The results demonstrated that the as-prepared nanocomposite effectively photodegrades the tetracycline with a photodegradation rate of 71.7%. A silica nanohybrid with different Ru(II) polypyridyl nanocomposites exhibited improved photocatalytic degradation with respect to Rhodamine 6G dye compared to the functionalized silica nanohybrid [109]. Hydrothermal and electrospinning route was used to prepare polyvinylidene fluoride/titanium dioxide nanocomposite with different compositions of anatase, brookite, and rutile and the nanocomposite displayed a relatively high photocatalytic degradation toward phenol [110]. The two types of polysulfone/carbon-covered alumina/TiO<sup>2</sup> membranes were successfully prepared and their activity toward RhB dye under visible light irradiation was tested [111]. The results displayed that the nanocomposite without a fabric membrane degraded 78.7% of RhB, while the nanocomposite with fabric membrane degraded 82.4% of RhB after 300 min. Polysulfone-based polyaniline-TiO<sup>2</sup> with a hollow fiber membrane was synthesized through a dry-wet spinning process [112]. The results showed that the polysulfone hollow fibres containing 1.0 wt% of the as-synthesized nanocomposite exhibited a maximum rejection rate of 96.5 and 81.5% for Reactive Orange 16 and Reactive Black 5, respectively. A conductive polypyrrole-polyaniline/ TiO<sup>2</sup> nanocomposite was fabricated *via* an *in situ* oxidative copolymerization [113]. The as-fabricated nanocomposite showed a superior photocatalytic degradation of 4-nitrophenol and this improvement was attributed to the conjugated structure, conductivity and the synergy effect among the polymers and TiO<sup>2</sup> . A polysulfone-sulfated/TiO<sup>2</sup> nanofiltration membrane showed good efficacy for methylene blue (MB) dye removal with a maximum rejection of 90.4% [114]. A novel and highly efficient α-Fe<sup>2</sup> O3 /polypyrrole nanocomposite was successful designed and fabricated *via* a simple and mild one-step chemical route [115]. The as-fabricated nanocomposite showed a significant photocatalytic degradation toward MB dye under simulated UV irradiation and ambient temperature compared to the bulk Fe<sup>2</sup> O3 . The improved photodegradation performance was due to the crystalline nature and mesoporous structure of the nanocomposites as well as the synergetic effect between α-Fe<sup>2</sup> O3 and polypyrrole, which improve the charge separation and recombination rate. The method of preparation and photodegradation activity of polymer/metal oxide nanocomposites are presented in **Table 1**.


**Table 1.** Method of preparation and photocatalytic degradation of metal oxide modified with polymers.
