Keyword

1 Introduction

Water is the most important component for various activities on this planet. The distribution of water on the earth is shown in Fig. 1. 97.5% water is in sea and only 2.5% is available in other areas for various activities.

Fig. 1
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Distribution of water on earth

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Water is the most important component for various activities on this planet and therefore, clean and pure water is required for development as well as for the survival of living organisms. However, the quality of water is continually deteriorating due to rapid urbanization and industrialization [1,2,3,4,5]. The harmful chemicals going into water bodies are heavy metal ions, inorganic cations, dyes and organic compounds. Pollutants mainly come from different sources (Fig. 2) [6].

Fig. 2
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Different sources for pollutants (Reproduced with permission from Elsevier [6])

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Toxic metal ions, from different sources go into water body and make them injurious (Fig. 3) [1].

Fig. 3
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Major sources for generation of toxic heavy ions (Reproduced with permission from Elsevier [1])

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When pollutants are present in water, they are very injurious. Different type of pollutants and their effects are shown in Fig. 4 [7].

Fig. 4
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Pollutants and their impact (Reproduced with permission from Elsevier [7])

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These pollutants if increase beyond certain limit becomes harmful to living system and environment. Therefore, these pollutants are to be removed in an environmentally friendly and economical ways [8,9,10]. There are number of methods used for the removal of pollutants from water and are given in Fig. 5. Out of all, adsorption technique is one of the most convenient techniques for removal of pollutants from water. For this purpose, a suitable adsorbent is needed. Amongst different adsorbents, nanoadsorbents have been reported to be the most effective adsorbents. Some of the adsorbents are given in Fig. 6 [11].

Fig. 5
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Water purification methods

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Fig. 6
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Nanoadsorbents (Reproduced with permission from Elsevier [11])

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In recent years, it is reported that metal oxide NP and many other NMs are found to be as an effective adsorbent and photocatalyst for the removal of pollutants from water (Fig. 7)[12].

Fig. 7
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Adsorption of pollutants by different type of NMs (Reproduced with permission from Elsevier [12])

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In this chapter removal of pollutants from water using different nano metal oxides as adsorbents a photocatalyst have been discussed.

2 Decontamination by Adsorption

Out of different purification techniques, adsorption is one of the most useful technique, considering the ease of operational aspects, low cost, scalability, high efficacy, and regenerability of adsorbents. Number of adsorbents have been used to remove different pollutants from water [12, 13]. Adsorption is a mass transfer process, which includes accumulating a substance between interfaces of two phases. The adsorption processes are classified into three major categories, i.e., physisorption, chemisorption, and ion exchange (Fig. 8) [3].

Fig. 8
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Different type of adsorption (Reproduced with permission from Elsevier [3])

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The qualitative and quantitative aspects of the adsorption process are evaluated by using different adsorption isotherm models, kinetic models, and thermodynamic parameters. There are number of parameters which affect the process of adsorption (Fig. 9).

Fig. 9
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Parameters affecting adsorption

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3 Photocatalytic Degradation of Pollutants

Prof. Fujishima of Tokyo University, Japan, accidently in 1967, discovered that in presence of TiO2, water splitted evolving oxygen. This phenomenon was named as photocatalysis. After that various effects of photocalysis was studied with industrial applications. Number of semiconductors with nanodimensions are being used as photocatalysts. Out of all, transition metal oxides are found to be most effective photocatalysts. Many nano metal oxides have been used as photo catalysts for degradation of dyes and organic compounds contaminated with water. Metal oxide NMs are semiconductors with valence band and conduction band separated by band energy of about 3.0 eV and act as photocatalyst (PC). When light of appropriate wavelength and energy hv falls, electron from the valence band jump to conduction band leaving a positive hole (hVB+) and trapped electron in the conduction band, as given by Eq. (1) [14]

$${\text{PC}} + {\text{hv}} \to {\text{e}}_{{\varvec{CB}}}^- + {\text{h}}_{{\varvec{VB}}}^+$$
(1)

The electrons liberated through irradiation could be trapped by O2 absorbed on the surface of the catalyst and give superoxide radicals (\({\varvec{O}}_2^-\)):

$${\text{e}}_{\text{CB}}^- + {\text{O}}_{2} \to {\text{O}}_2^-$$
(2)

O2·− obtained reacts with H2O to form hydroperoxy (HO2·) and hydroxyl radicals (OH·). These radicals are strong oxidizing agents and decompose the organic molecule, dyes and other type of organic contaminants in water as given by Eq. (3).

$${\text{O}}_2^- + {\text{H}}_{2} {\text{O}} \to {\text{HO}}_2 + {\text{OH}}$$
(3)

Simultaneously, the photoinduced holes could be trapped by surface hydroxyl groups (or H2O) on the photocatalyst surface to give hydroxyl radicals (OH·):

$$\begin{gathered} {\text{h}}_{{\text{VB}}}^+ + {\text{OH}}^- \to {\text{OH}}_{{\text{ad}}} \hfill \\ {\text{h}}_{{\text{VB}}}^+ + {\text{H}}_{2} {\text{O}} \to {\text{OH}} + {\text{H}}^+ \hfill \\ \end{gathered}$$
(4)

Finally the organic molecules will be oxidized to yield carbon dioxide and water as follows:

$${\text{OH}} + {\text{organic molecules}} + {\text{O}}_{2} \to {\text{products}}\,\,({\text{CO}}_{2} {\text{and H}}_{2} {\text{O}})$$
(5)

Meanwhile, recombination of positive hole and electron could take place which could reduce the photocatalytic activity of prepared nanocatalyst:

$${\text{e}}_{{\varvec{CB}}}^- + {\text{h}}_{{\varvec{VB}}}^+ \to {\text{PC}}$$
(6)

A wide range of metal oxides such as tungsten oxides, copper oxides titanium oxides, zinc oxides, iron oxides, magnesium oxide, metal oxides composites, and graphene-metal oxides composites have been studied for photocatalytic degradation and adsorptive removal of organic pollutants viz. phenolic compounds, dyes, pesticides, and so on. Figure 10 represents the photocatalytic degradation of organic pollutants, along with the role of the photogenerated hole and electron pairs [12].

Fig. 10
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General mechanism of photodegradation of pollutants by semiconducting metal oxide NM (Reproduced with permission from Elsevier [12])

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Photocatalytic degradation of some organic pollutants in water by titanium dioxides-based NMs and their composites are given in Table 1 [12].

Table 1 Titanium dioxide NM and their composites as photocatalyst for degradation of organic pollutants from wastewater (Reproduced with permission from Elsevier [12])
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In general photocatalytic degradation is fast and ecofriendly as compared to adsorption technique.

4 Synthesis of Metal Oxide NP

The purpose of synthesizing metal oxide NP is to change the properties of corresponding metal NP. For example oxidizing iron NP are converted into iron oxide NP, which increase the reactivity. In the past ten years, effective synthesis methods to obtain metal/metal oxide NMs with controllable shape, high stability and monodispersion have been extensively studied [15]. In general metal oxide NPs can be synthesized by using Chemical Precipitation, Sol –Gel, Hydrothermal, Chemical Vapour Deposition methods. However, synthesis is divided into three major categories to understand the difference in methodology, advantages/disadvantages: (i) solution-based methods (ii) gas phase methods and (iii) biological methods (Fig. 11). This classification is based on the type of medium in which the oxidation reaction occurs. The physical and chemical properties of NMs like size, shape, dispersibility, morphology, internal/external defects and crystal structure are generally influenced by the choice of synthesis method which ultimately affects their applications. For example, nano Mg doped ZnO (ZnMgO) fabricated via three different synthesis methods were found different in geometry. Where regular cubic structure was obtained by CVS method, mixture of cubes and tetrapods for metal combustion method and irregular nano rods by sol-gel method [16, 17]. To understand the difference in various methods they are briefly discussed in Table 2 [18].

Fig. 11
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Synthetic methods of metal oxide NMs

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Table 2 Summary of most common synthetic methods with their advantages/disadvantages
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Due to the vast and varying applications of these nanostructures, various synthetic methods have been utilized to synthesize them as discussed in the Table 2. All the described methods provide high quality metal oxide nanocrystals with definite size and shape except the biological method. It is very difficult to control all the required features of NMs like size, shape yield, purity, cost etc. in most of the methods. This problem is more common in the case of multi-metallic oxide NPs. The most effective method with respect to high crystal purity is chemical vapour deposition method [19]. This method is also very useful to give stability to otherwise unstable crystal phase. For example, Zinc oxide NPs in cubic crystal form can only be achieved at very high pressure but chemical vapour synthesis method allows c-ZnO NPs to be dispersed on MgO surface [20].

On the other hand, sonochemical method has been successfully applied to get enhanced photocatalytic performance of TiO2 NPs [21] and varying magnetic properties of iron oxide NPs [18, 21]. The sol-gel method has been widely used for synthesizing almost all kind of metal oxide NPs. This method is also very useful in doping group 5 oxides, which is often a challenge, for example Co doped Hf-oxide NPs [22]. This method has been utilized by researchers with certain modifications, for example Corr et al. have reported an improved one-step sol-gel aqueous method for the synthesis of iron oxide-silica NPs [23]. To avoid the oxidation of the products at very high temperature, use of ultrasonic conditions is also reported. Some solution-based manufacturing techniques use surfactants [24], which, in addition to affecting particle size, also tend to reduce the degree of aggregation between particles. In addition, solution-based technology combats pollution problems in the resulting metal oxide products. Most of the solution-based methods suffer with the problem of the contamination in the products specially contamination of anions of precursor salt [25]. Biological method is suitable for biomedical applications due to its biocompatibility but face the problem of contamination and composition of NPs also cannot be defined completely [18].

5 Metal Oxide NP Used for Water Purification

Metal oxide NPs are used in different sectors including water remediation as shown in Fig. 12 [26].

Fig. 12
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Applications of nano metal oxides/mixed metal oxides in different areas (Reproduced with permission from Elsevier [26])

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Numbers of nano metal oxides discussed below were used for water remediation.

5.1 TiO2

TiO2 NPs have become the most widely used NMs for water remediation due to their high photosensitivity, availability, non-toxicity, cost-effectiveness and environmental friendliness [27]. These NMs have been widely used in the oxidation and reduction conversion of organic and inorganic pollutants in air and water, such as phenolic compounds, metal ethylene diamine tetraacetate complexes, microorganisms in the air and odorous chemicals, halogenated compounds degradation, dye removal, metal and metal removal, etc. [28]. Photodegradation leads to complete oxidation and reduction of organic and inorganic pollutants and converts them into carbon dioxide, water and inorganic acids [29]. Its large band gap energy (3.2 eV) requires ultraviolet excitation to induce charge separation within the particles [30]. TiO2 and TiO2 films have been successfully used to degrade atrazine and organochlorine pesticides in water, respectively [27]. Photocatalytic degradation of methyl orange using ZnO/TiO2 composites has been studied [31]. Non metal elements like N, F, C and S can improve the photocatalytic activity of TiO2 NMs by narrowing its band gap. This is achieved by the substitution of oxygen by these nonmetals in the TiO2 lattice [32]. Doping with transition metals like Fe, Co and Cu has also been proved to improve photocatalytic activity of TiO2 NPs under UV irradiation However, noble metals like silver, have received much attention for this purpose [33].

5.2 Iron Oxides

In recent years, the synthesis of iron oxide NMs with modified properties and their applications have gained widespread attention due to their high porosity and surface-to-volume ratio, low cost, strong adsorption capacity, easy magnetic separation response. Iron oxide NMs can act as immobilized carrier to remove contaminants or can also act as photocatalyst/catalyst to degrade the contaminants. Magnetic separation is a unique property of iron oxide NMs which is a challenge due to small size of nanoadsorbents [34]. Therefore, the combination of adsorption process and magnetic separation has been widely used in water treatment and environmental purification. Strong paramagnetic characters of Fe2O3 NMs make them effective for the removal of toxic heavy metals like Cd(II), Pb(II) etc. Super magnetic Fe3O4 NPs have shown excellent catalytic activity for dye degradation in waste water to convert them in less toxic form. According to reports, the preparation method and surface coating medium play a key role in determining the size distribution, morphology, magnetic and surface chemistry of NMs in the form of NP, nanoellipses nanobelts and nanorings or other nanostructures [35].

Green synthesized iron oxide (Fe3O4) NP using an extract of Excoecaria cochinchinensis leaves were found much effective for the removal of a contaminant antibiotic (rifampicin) from aqueous media. This was found much more effective as compared to commercially available Fe3O4 (Fig. 13) [36].

Fig. 13
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Green synthesis of Fe3O4 and removal of rifampicin (Reproduced with permission from Elsevier [36])

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5.3 Zinc Oxide

Zinc oxide is another metal oxide NMs based photo catalyst which shows most promising water treatment due to its high chemical stability and excellent photocatalytic activity. Large number of research groups across the globe have already reported potential applications of ZnO NMs along with their variable morphology and structural characteristics including Nano sheets, nanowires, Nano rods, nanoribbons and complex hybrid structures [37, 38]. ZnO has a wide band gap (3.37 eV), and the excitation binding energy (60 meV) is also large at room temperature which makes it an excellent photo catalyst. In addition, easy availability, low toxicity and antibacterial efficiency of ZnO NMs make them ideal for water treatment. Hollow spheres in these nanostructures are of great interest due to their light trapping efficiency and highly enhanced photocatalytic activity, as well as their high surface area, low density, and good surface permeability [37].

Nano Zno acts as an antibacterial agent and the mechanism of its action is given in Fig. 14 [38].

Fig. 14
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ZnO disinfection mechanisms (Reproduced with permission from Elsevier [38])

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5.4 Copper Oxide

Due to its inherent compatibility, low-cost manufacturing and excellent electrochemical performance, research on copper oxide NMs has grown significantly in the recent years. It is reported that copper oxide NMs show a little photocatalytic activity which can be significantly enhanced by activating it with H2O2 [39]. More than a photo catalyst, CuO NMs are used as a good adsorbent of water pollutants such as congo reed, malachite green, methylene blue, ciprofloxacin, methyl orange dyes and many heavy metals like Pb (II), Hg(II) As (III). The main application of CuO NM in water treatment is its antimicrobial efficiency. Bacterial disinfections, one of the main applications of copper-based NM, which is an essential requirement of potable water. Many biotechnologists have investigated the potential of CuO NMs to disinfect water with respect to microorganisms [9]. Scientific reports suggest few mechanisms behind it (i) Cu is released from copper oxide NPs which damages the bacterial cell membrane and lead to bacterial cell death (ii) interaction with DNA molecule and disorder its helical structure (iii) and by inducing oxidative stress [18].

In many cases metal oxide composites were found more effective in removal of pollutants. Extract of pine needle was used for the synthesis of nano composite of iron and copper oxides (Fe/Cu oxides) and was found an efficient adsorbent for ofloxacin and norfloxacin removal from aqueous media. Mechanism of synthesis and removal of organic pollutant is given in Fig. 15 [36].

Fig. 15
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Synthesis of Fe/Cu oxide (Reproduced with permission from Elsevier [36])

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5.5 Silver Oxide

Silver oxide exists in many nanostructural forms which includes, NPs, nanohorns, nanorods, and nanopyramids. Silver oxide NM exhibits excellent antibacterial properties which has been already used in many commercial products [40]. A few research groups have reported the photocatalytic activity of these NMs for the degradation of dyes like methylene blue and methyl orange present in water along with their antimicrobial activity [22]. Silver oxide NMs are syntheisezd be various synthetic routes which includes direct precipitation, sol gel, hydrothermal and biological route [41, 42]. These are mostly spherical particles (20–80 nm) with high surface area (10–50 m2/g) and good magnetic properties.

5.6 Manganese Oxide

Different forms of manganese oxide NP such as MnO, MnO2, Mn2O3, Mn3O4 are tested for removel of heavy metals in water decontamination process. They are structurally flexible and display novel physical and chemical properties. The primary benefits of managanese oxide NMs come from their low cost, high activity and non-toxic nature. A large number of heavy metals including Cu(II), Cd(II), Pb(II), As(III), As(V), U(VI) and organic contaminants are successfully removed by MnO2 and its NPs. The mutual interference of Zn(II), Cd(II) and Pb(II) ions with various nanostructures of MnO2 e.g. nanoparticle, nanotube and nanobowl are investigated by Zhang et al. [43]. Nanoflakes of MnO2 are reported for the detection and removal of Cr(III) ion [44]. Differential pulse voltammetric method was used for the detection of Hg(II) ion by MnO2 nanotubes [45].

5.7 Cerium Oxide

Cerium oxide (CeO2), a non-toxic rare earth metal oxide is gaining attention for application as UV-blocking agent, sensing agent and in water remediation. Recently nanoscale CeO2 is investigated for their applications in removal of heavy metals from water [46]. The properties of nanocrystalline CeO2 are found to be effective for removal of inorganic heavy metals. Low ionic potential and high basicity leads to dissociation of hydroxy group into hydroxyl ions. The size, porosity, surface area, bulk density etc. are in favor of their selectivity, stability and activity during adsorption process. Recillas et al. reported removal of Cr(VI) metal ions using 12 nm average sized CeO2 NPs [47]. Their results indicate that low concentration of Cr(VI) (80 mg/L) can be effectively removed by CeO2 NPs with maximum adsorption capacity of 121.95 mg/g. Arsenic metal in the form of As(III) and As(V) has successfully been removed from water by CeO2 NPs by Mishra et al. [48]. In their work, the BET surface area of 3–5 nm sized CeO2 NPs was 257 m2/g and the adsorption capacity of As(III) and As(V) ions were 71.9 and 36.8 mg/g−1 respectively. It is observed that the adsorption capacity of CeO2 NPs reduced in the presence of some anions such as sulpahte, bicarbonate, dihydrogen phosphate etc. Further, CeO2 NPs are found to be compatible with other metal oxides for treatment of heavy metals from water [49, 50]. Recently, Meepho et al. have synthesized samaria doped cerium nanopowder (SDC) by doping samaria with different morphologies of cerium nanopowder [51]. The samaria doped cerium nanopowder (SDC) was used for the removal of Cu(II), Zn(II) and Pb(II) ions. The outcome of the investigation indicates that spherically shaped SDC nanopowder was more effective than the plate like SDC nanopowder. The surface modifications of CeO2 NPs enhance the adsorption of heavy metals in terms of material stability and selectivity. The hydrous CeO2 NPs with adequate hydoxyl group help in the adsorption of arsenic through inner sphere mechanism. Composite of CeO2 NPs with graphene oxide has the capability of removal of arsenate and arsenite almost completely (99.99%). A cost-effective adsorbent is developed by supporting CeO2 NP over carbon nanotube (CNT) for removing AS(V) ions [52]. The only drawback of ceria in water remediation is its high cost which can be taken care by the surface modification or composite formation of ceria.

5.8 Magnesium Oxide

Magnesium oxide NP have high potential in removing pollutants from water. MgO NP are associated with exceptionally high absorption ability, abundantly available, non toxic and inexpensive [53]. These unique properties make it one of the sought-after metal oxides NMs for removing heavy metals from water. MgO NP also displays superb antibacterial activity for both gram-positive bacteria, gram-negative bacteria and spore cells [54]. Reported literature also indicated the effect of size of MgO NP in its bactericidal properties. Cai et al. reported simultaneous removal of heavy metals Cd(II) and Pb(II) and Escherichia coli bacteria by MgO NP [53]. Three different nano metal oxide e.g. TiO2, MgO and Al2O3 was investigated for elimination of heavy metals Cd(II), Cu(II), Ni(II) and Pb(II) ions form water [55]. It was observed that the efficiency of MgO NP was better than the other two metal oxide NP. MgO NP follows adsorption and precipitation mechanism for the removal of heavy metals while TiO2 and Al2O3 were via adsorption mechanism only. Interesting work by Madzokere et al. revealed that MgO NP are capable of removing 96% Cu(II) ion compared to the 15% removal ability of commercial MgO [56]. A batch adsorption experiment performed by Xiong et al. indicated excellent adsorption capacity of MgO NP [57]. Langmuir model was used by Jing et al. to establish the remarkably high adsorption of Ni(II) ion over mesoporous MgO nanosheets (Fig. 16) [58]. All these works suggest MgO NP as a very promising material for the removal of heavy metals from water.

Fig. 16
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Schematic illustration for the formation of mesoporous MgO nanosheets. (Reproduced with permission from Elsevier [58])

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5.9 Zirconium Oxide

Among metal oxide NMs, zironia or zirconium oxides also exhibited remarkable potential in removing water pollutants specially the heavy metal ions [59, 60]. They have high thermal and chemical stability, less toxicity and biocompatibility. Zirconia display high resistivity against acids and alkalis. Presence of large number of -OH groups over the surface leads to high surface area which in turn makes zirconia a good adsorbent. Both nanoscale zirconia and hydrous zirconia are excellent for removing heavy metals like Cd(II), Zn(II), Pb(II), arsenate and arsenite ions. Silicate ions adsorb strongly over zirconia surface, thus hampering the adsorption of arsenic ions using zirconia adsorbent. The presence of alkaline earth metals e.g. Ca(II) and Mg(II) ions promote the adsorption of arsenic pollutants by reacting with the silicate ions [61]. It is reported that simultaneous adsorption of arsenate and arsenite is possible over nanocomposite of hydrated zirconia-graphene oxide sheet [62]. The adsorption capacity of this nanocomposite was higher compared to the pristine nano zirconia. In addition to that it is recyclable up to five times. Removal of Cd(II) was investigated by a composite of polystyrene supported nanosized hydrous zirconia [63]. The removal efficiency of this material lies within wide pH range. Further promising result of removal of Pb(II) and Cd(II) ions are observed by nanocomposite based on hydrous zirconium (IV) oxide [64].

Another composite of zirconia with γ-Fe2O3 is investigated for the removal of arsenic from leach out water of gold cyanidation industry [65]. The iron oxide core helps in improving the recyclability of the adsorbent by easy separation. To improve the adsorption capacity towards arsenate ions at strong acidic environment, zirconia is encapsulated in D201 (polystyrene anion exchanger [66]. The electrostatic interaction between arsenate ions and D201 and inner sphere complexation explain the mechanism of the adsorption. Presence of sulphate ions restricts the electrostatic interaction and in turn reduces the adsorption capacity.

An interesting report on removal of Cr(VI) by a series of mesoporous transition metal oxides suggests that ZrO2 as the most attractive adsorbent among other nano metal oxides e.g. TiO2, HfO2 and NbO2 [67]. A hybrid nanocomposite made from ZrO2/B2O3 displayed satisfactory results in removal of Cu(II), Cd(II) and Cu(II) ion [68].

5.9.1 Aluminium Oxide

Aluminium oxide NMs are inexpensive and can be prepared easily. Alumina adsorbents have high efficiency in removing heavy metal ions [69]. Many research works is reported on the application of alumina for the adsorption of several heavy metals. Among several crystalline forms of aluminium oxide, γ-alumina is the most effective for decontamination purpose because of its high surface area [70]. In addition to that γ-alumina has high mechanical strength, excellent thermal stability and high adsorption capacity. Tabesh et al. has reported 97% and 87% removal of Pb(II) and Cd(II) ion respectively by γ-alumina NP [71]. It is observed that adsorption of Zn(II) and Cd(II) ions by alumina become more enhanced in presence of phosphate ions and humic acid while presence of citrate ion reduces the adsorption of Zn(II) ion [72]. Moreover heavy metals ions such as As(III), Hg(II), Ni(II), Cu(II), Cr(VI) are also reported to be removed by alumina NP [73,74,75].

Applications of some selected nano metal oxides in removal of pathogens, dyes and heavy metals are listed in Tables 3, 4 and 5 respectively.

Table 3 Nanometal oxide in pathogen removal for water purification
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Table 4 Nanometal oxide in dye removal for water purification
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Table 5 Nanometal oxide in heavy metal removal for water purification
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6 Challenges

The metal oxide NPs are extensively studied for their application in water purification technology. But the validation and development of nanotechnology for purification of water at mass scale is full of challenges. The toxicity of the nanometal oxides is of primary concern. When nano metal oxides are used for water purification, consumers are exposed to the toxicity of these nano materials. Numerous research works have been performed on toxicity analysis of these materials both in vitro and in vivo [115, 116]. Various factors control the level of toxicity of these engineered NMs. Size of the NMs, dose, administration mode and exposure duration are important factors that controls the toxicity levels. It is reported that large TiO2 NP with size more than 100 nm are non toxic in nature. Concentration of TiO2 nanoparticle in the range of 1000–2000 μg/g is found to be toxic [116]. The health issues from TiO2 NP primarily come from inhalation not from ingestion with water. Thus toxicity of TiO2 is not a serious concern. Oral administration of high dose (2.5 mg/g body weight) of ZnO NP is known to be accumulated in different body parts e.g. lung, kidney, liver and spleen. A detailed in vitro toxicity study on ZnO NM is reported by Vandebriel and Jong [117]. The toxicity of silver oxide nanoparticle is found to be more compared to other nano metal oxides. Silver can interact with most of the biomolecules and impart toxicity which in turn leads to cellular apoptosis [115, 118]. Magnetic iron oxide nano particles used for purification of water has insignificant toxic effect and are not serious issue [119]. Thus, technological advancement on nanometal oxide purification system is possible after addressing the toxicity issues convincingly.

Next the economic viability is another challenge that needs to be sorted out. To make the nano metal oxide based water purification technology acceptable it must be affordable. In this regard development of highly effective filtration membrane with multifunctional capabilities is extremely necessary to reduce the cost of the membrane-based purification technology.

In addition to the above issues, the aggregation and dispersion properties of nanometal oxides make the operational conditions critical. Mixing of nano metal oxides in water gets accumulated and forms aggregate. The surface immobilization of the nano metal oxides is used for killing various water borne microbes and pathogens. However, leaching of NMs beyond their acceptable limit is a serious threat for human and other living beings. Report of aggregation of TiO2 NPs as waste from industry and consumer products in water is well documented [120]. One important strategy to reduce the leaching of nano metal oxide in water is to sediment or coagulates the NPs before supply to the consumers. This method has been successfully applied for TiO2 and silver oxide NPs [121, 122]. More technological innovations are needed in these directions to make the nano metal oxide-based water purification in large scale.

Thus, to assure the safety of the consumers for the use of nano metal oxides-based purification technology, regulatory board must be formed [123]. In China, the use of NM and its issues are taken care by NSCNN (National Steering Committee for Nanoscience and Nanotechnology) which work closely with National Nanotechnology Standardization Technical Committee [124]. Similarly in Europe REACH63 (Registration, Evaluation, Authorization and Restriction of Chemicals) controls the use of NMs and their impact on health and environment is monitored [125]. Few other developed countries are in process of bringing regulatory law to control the usage of NM based technology products. Till now in India there is no such organization for governing the usage and legal constraints of NMs [126].

7 Conclusions

Water is the most important element on this planet for living things and plants. However, the water is contaminated with different type of toxic materials. The major cause of this pollution is industrial waste going into water bodies. Numbers of techniques have been used for remediation but the adsorption technique is found to be the most effective. Nanomaterials have been considered to be the most important adsorbent. Because of various specific properties, numbers of nano metal oxides and their composites have been found to be a suitable adsorbent for removal of pollutants. Synthesis of nano metal oxides and their applications for water remediation have been discussed. These metal oxides have also been used as photocatalysts. Considering the advantages and disadvantages, further research is needed.