1.                  Introduction 

As new compounds are being produced, new ways to prevent toxicity in the environment has to grow as well. Novel pollutants such as dyes, pharmacons, and activated sludge are produced by industries and are being disposed into water bodies. Xenobiotic compounds such as organ chlorinated matter [1].Hydrocarbons[2],among others do not occur naturally, affecting flora and fauna in aquatic ecosystems. In addition, some of these contaminants are recalcitrant and bioaccumulate in aquatic animals, plants and microorganisms. The harm produced by these novel pollutants to wild-lifeis not fully knownand because these are strange molecules, microorganisms do not metabolize some of them. Moreover, conventional methods for wastewater treatment are not prepared to remove these emergentcontaminants [3]. 

As primary and secondary treatment for wastewater, like separation and coagulation [4],are unable to remove these compounds,alternatives must be taken into consideration. Advanced Oxidation Processes (AOPs) serves as an efficient process to remove compounds with low biodegradability in wastewater or improves its degradability by forming new by-products easy to biodegradation[5,6].These processes have demonstrated to oxidize a wide selection of organic matter and also can inactivate microorganisms when desired[7].

 2.                  Advanced Oxidation Processes 

AOPs has grown recently, due to its green approach and being one of the most effective methods towards wastewater treatment or inclusive to obtain drinking water[8].AOPs refer to the oxidation process of pollutants in wastewater in order to get solid minerals or other easy-treatment compounds. The advantages of using AOPs for wastewater treatment include rapid degradation rate, reactions can occur at room temperature, complete mineralization of organic matter, elimination of effects from disinfectants and other residual oxidants, degradation by-products of previous treatments, it does not require external energy, it does not generate sludge, contaminants can be treated at low concentrations, and can be used before biological treatments for pollutants resistant to biodegradation[8,9]. 

In the oxidation process, the system need reactive oxygen species (ROS), in which AOPs produce one of the most potent in solution, hydroxyl radicals OH^•.Oxidation potentials of some oxidants are listed on (Table 1)Compared with other oxidant species, hydroxyl radicals have a very high oxidation potential, which is helpful for AOPs.Hydroxyl radicals are not selective, which make them great oxidants of pollutants as they attacking directly and modify their molecular structure into less dangerous by-products[10]. 

AOPs are classified into degradation of pollutants by UV irradiation,son chemical reactions, ozone based AOPand photo catalysis[11-13]. The main mechanism in any AOP refers to the interaction of hydroxyl radicals with organic matter H-R-H, producing the organic radical HR^• by removing an H^• to form H₂O. This radical reacts rapidly with dissolved oxygen by forming organic peroxoradical RHO₂^•, which is highly unstable. The peroxoradical degrades by turning into R^•, which will react with other free radicals or oxygen and producing a degraded organic compound R [14].

OH^• + H-R-H à RH^•                                          (Eq. 1)

RH^• + O₂à RHO₂^•à R^• + H₂O                          (Eq. 2)

R^• + H₂O + O₂ + OH^•àoxidation                      (Eq. 3) 

2.1.              Photocatalysis 

The process of catalysis that involves light as source to produce ROS is called photocatalysis. Photo-catalysis involves a photochemical reaction at the surface of thephotocatalyst (a metal oxide semiconductor), where electron transfer takes place andspacemanthe electron-hole pair is produced by incoming light [15].This process is capable of modifying inorganic matter, degrading organic matter and inactivate microorganism. UV radiation isthe most used energy source due to its adequate energy, viability and being cost effective. Solar energy is the most abundant and clean energy source available today, and it produces the UV radiation that AOPs need. It is known that the amount of solar energy that strikes the earth in an hour is higher than the energy that humanity consumes in a year [16].This brings the rise in development of materials that can utilize solar irradiation in different areas, such as wastewater treatment and pollution management. However, the solar energy is not used totally, because the common photocatalyst use mainly the UV wave length, which represents only 5%-8% of the solar spectrum at sea level[17],limiting its use. 

Photocatalytic mechanisms are distinguished into homogeneous and heterogeneous. This is linked to the type of catalyst used for the process, being the catalyst that are in the same phase where the reaction occurs (homogeneous) and those which are in different phase (heterogeneous). By taking into account their physical nature, the difference between homogeneous and heterogeneous catalysts differ on its uses, stability, and selectivity.(Table 2) shows a comparison of the main advantages and disadvantages of both types of catalysts[18].

The electronic structure of semiconductors consists of two energetic levels, the Valence Band (VB) and the Conduction Band (CB), with an energetic difference known as theBand Gap (BG).Electrons (e^-) found in the VB have low energy. If energy is transferred to an electron in VB, they jump to the CB and leave a “Positive-Charged” hole (h⁺). Promoted electrons in the CB can later react with another compound and result in an oxidation process. The energy needed for an electron to travel from VB to CB is specific for each semiconductor, and can be obtained from absorbed photons in light. In AOPs, they react with O₂ and H₂O, and produce highly oxidant agents, including hydroxyl radicals [13]. (Figure 1) presents a scheme of the mechanism of the promotion of an electron from the VB to the CB.

Semiconductor + hvà Semiconductor (e^- + h⁺)                            (Eq. 4)

Semiconductor (e^–) + O₂à Semiconductor + O₂^•-                          (Eq. 5)

Semiconductor (h⁺) + H₂O à Semiconductor + HO^•    (Eq. 6)

Even though this process is very effective, the e^-/h⁺pair generation can recombine. Recombination can occur before the electron is transferred to another chemical species, which result in energy dissipation and reduce effectiveness of the process. The recombination is dependent on the properties of the semiconductor. However, the particular properties can be modified to have desirable properties. Specific particle size, doping, and synthesis processes can modify the properties of the semiconductors in order to benefit their performance in AOPs. Modifying the semiconductors can change the BG, and thus the required energy they need to transfer electrons from VB to CB. 

Many metal oxides semiconductors have been studied with promising results, such as titanium dioxide (TiO₂), iron (III) oxide (Fe₂O₃), tungsten trioxide (WO₃), zirconia (ZrO₂), zinc oxide (ZnO), among others. All of these oxides are actively used in environmental wastewater treatment [19,20].The production of hydroxyl radicals from these metal oxides is high enough to degrade pollutants in a short time and with high yield. 

Out of all semiconductors, ZnO arises as a candidate as an efficient photocatalyst for its use in green pollutant treatment because of its unique properties. Some appealing characteristics are its wide band gap in the near-UV spectral region, good photocatalytic property, strong oxidation capability,insoluble in water, and low costproduction [21,22]. 

3.                  ZnOas a Photocatalyst 

ZnO is one of the most used semiconductors worldwide. In aqueous solutions, it produces H₂O₂ if exposed to O₂ and UV radiation, which turn into hydroxyl radicals. ZnO has a BG energy of3.37 eV, large excitation binding energy (60 meV) and UV absorption at room temperature, which make this oxide an adequate photocatalysts[23]and used as antibacterial, antifungal [24],and antifouling agent[25]. 

ZnO has advantages over TiO₂, another well-known semiconductor used in photocatalysis. While both semiconductors possess a very similar BG energy, ZnO exhibits a higher absorption efficiency across a wide fraction of the solar spectrum[26,27].In presence of organic matter, electron transfer degrades the organic molecule up to CO₂ as a final product.To explain this, a transferred electron from the ZnO momentarily produces Zn⁺¹, which reduces the O₂ near the photocatalyst in order to form H₂O₂. This transfer produces a positively chargedZnO. If organic matter is present, an electron is donated to the semiconductor, which results on the oxidation of the organic matter [28].

ZnOà Zn⁺¹O^–2                                                        (Eq. 7)

Zn⁺¹+ O₂à Zn⁺²+ O₂^–                                       (Eq. 8)

(ZnO)⁺ + RH àZnO + RH⁺                       (Eq. 9)

RH⁺à RO₂                                                                 (Eq. 10)

Even if ZnO has great properties for oxidation processes,ZnO faces challenges on its own nature. First, it can only absorb UV radiation with wavelengths near 387 nm, which can be associated with the BG energy of the ZnO. Secondly, and as mentioned beforerecombination of the e^-/h⁺ pair that hinders the performance of the ZnO. This is why research on ZnO modifications is needed in order to expand its absorption range and maximize its performance as a photocatalyst. Several novel studies are aimed to improve the optical properties of ZnO, focusing on minimizing BG energy and inhibiting recombination of the photo-generated e^-/h⁺ pair, and new synthetic methodologies and the effect of various modification methods. 

4.                  Synthesis of ZnONanostructures 

Research on ZnO structures has emerged as a potential candidate for many pronounced applications. Other than wastewater treatment, ZnO is subject for energy harvesting, biomedicine, and electronic devices. The versatility of ZnO permits various synthesis methods, with the ability to adopt many morphologies and sizes. Physical and chemical parameters, such as solvent type, pH, temperature, and Zn precursor, can be modified to adapt the properties of the ZnO structures for a desired application. Each nanostructure has specific structural, optical, electrical, and physicochemical properties [24]. 

Standard methodologies that enable better control of both size and morphologies include sol-gel technique, hydrothermal synthesis, self-combustion, thermal evaporation, double-jet precipitation, solution synthesis and polymerized complex method [29]. 

Some synthesis routes utilize a huge amount of energy or solvents. Approaches for a green route synthesis of ZnONanoparticles (NPs)have been studied by using a biological method. A study used Aloe babadensismiller leaf extract to synthesize highly stable and spherical ZnO NPs. These NPs show poly dispersity and particle ranges from 25-45 nm [30].Another synthesis consisted on the use of the fruit trifoliate orange (Poncirustrifoliata) extract as a way to avoid using toxic reagents. The biosynthesizedZnO NPs showed catalytic properties as it was tested on a reaction procedure of Claisen-Schmidt condensation of 3, 4-dimethyl benzaldehyde with acetophenone, with a yield of 80%[22]. 

ZnO can also adopt different morphologies. ZnOnanorods (NRs) are efficient to degrade methyl orange shown byDanwittayakulet al. Methyl orange was degraded up to 95% with UV radiation for 3 h [31].In a similar matter, Vega-Vaudrit&CorzoLucioni in 2012 published the kinetic study of rhodamine B (RhB). The author found out that a high initial concentration of both target dye and photocatalyst increase the degradation rate, with optimum concentrations of 20 mg/L of RhB and 200 mg of ZnO for each 100 mL of solution [32]. 

A methodology to produce ZnO nanostructures with higher surface area for photocatalysis was published by Khoa et al. ZnO nanoparticle-assembled hollow spheres (nPAHS) can be achieved using a self-assembly route [33].The growth mechanism of the ZnO hollow structures is assisted by trisodium citrate, where the hydroxyl and carboxyl groups in citrate ions cap Zn²⁺ precursors and direct the growth of the ZnO structure. After annealing at 500^oC, the citrate ions are removed and improve the crystalline structure of the remaining ZnO in order to form the nPAHS. 

Bijanzad et al. usedZn(anic)₂, a coordination compound obtained from 2-aminonicotinic acid (Hanic) and Zn(NO₃)₂, as a precursor to obtain hollow microbricks of ZnO after the calcination (550^oC, 4 h). These novel nanostructures possess large surface area of 8.13 m²/g and a pore size of 22.6 nm, ideal for photocatalysis. The micro bricks were applied in the photodegradation of Congo red (CR) dye from aqueous solution. Even with a natural 3% of degradation of CR under the light source, CR removal efficiency after 3 h using the microbricks was 96% [34].

Mesoporous ZnO nanofibers were synthesized by electro spinning method. Synthesis consisted on adding polyacrylonitrileand Zn precursor into DMF solvent. After calcinations, the polymer decomposes and produces porous ZnO nanofibers. The fibers were used for the degradation of anthracene and naphthalene dyes. These nanofiber mats may find varied applications, as they resulted to be reusable and effective in photocatalysis[2].Bercoff et al. demonstrated that ZnO decomposes the dye orange II in a first-order kinetic and a half-life t_1/2 = 58 min. ZnO was used as a doping agent for ferromagnetic particles, acquiring the additional property of being reusable photocatalyst[35]. 

4.1.              ZnOComposites and Doping with other Elements

Just as TiO₂, ZnOneeds improvement due to rapid charge recombination from the generated e^-/h⁺pair. A novel structure of TiO₂-ZnO composite sphere was achieved, with ZnO decoration by using solvothermal method [36].The isolated ZnO synergizes with the TiO₂-ZnO composite, as the composite (type II heterojunction) delay the charge recombination. In contrast with TiO₂-ZnO, TiO₂, or ZnO alone, the novel composite showed better photo-activity in degrading RhB and faster photo-assisted isomeration of norbornadiene, which supported the proposed electron transfer mechanism. 

Priyanka and Srivastava modified the ZnO structure by doping it with Fe. By using solution combustion synthesis method, iron-doped ZnO can be synthesized. By using this synthetic route, a mixture of microporous and mesoporous ZnO-Fe can be achieved, with high crystallinity. The study also tested these catalysts for the photocatalytic oxidation of acid red 1 dye, with variable parameters in order to achieve optimization. Optimum conditions for both synthesis (Fe 2.5 wt%, calcination temperature = 400^oC, calcination time = 4h) and photocatalysis tests (pH = 2, catalyst dose = 1.25 g L^-1, oxidant dose = 6 mmol L^-1, temperature = 50^oC) were studied, having the best results of 94% of dye degradation and 41% of total organic carbon removal [37]. 

Cho et al. synthesized sulfur-doped star-shaped hexagonal ZnO nanowires (NWs). These structures were obtained after doping ZnO NWs with thioureaSC(NH₂)₂. This transformation only occurred with thiourea, but not with urea. The cross-sections of the NWs transformed from hexagon to a hexagram. In order to understand the role of sulfur doping in this transformation, ab initio simulations were conducted. Results on the simulation suggest that sulfur causes a local change in charge distribution that is stronger on the vertices of the NWs than in the edges, leading to the change in the morphology [38]. 

On another study, Emin et al. doped ZnO films with CdX (X = S, Se) in order to modify the usable electromagnetic spectrum range. On a first step, plate films of simonkelloeite(Zn₅(OH)₈Cl₂^•H₂O) grew electrochemically, followed by annealing at 450^oC. Once the porous ZnO electrodes were ready, they were used for photoelectrochemical studies. The authors sensitized the ZnO electrodes with CdX quantum dots, and as expected photocurrent density increased with the doping. The results demonstrate that these new materials could be potentially useful in light-harvesting applications [39]. 

NPs decorated with various metals, such as Ag, Au, and Pd, modify the charge carrier dynamics. Lin et al. presented these novel materials and their correlation between the metallic charge carrier dynamics and their photocatalytic properties. The metal content varied from 0.6 to 2.3 at%, showing that an Au content of 1.3 at% can effectuate the largest electron transfer rate constant for ZnO-Au nanocrystals. The three kinds of decorated nanocrystals were employed on the degradation of methylene blue (MB), showing that ZnO-Pd had the best photocatalytic performance [40]. 

Saoud et al., reported highly crystalline Ag spheres on ZnO NRs which presented photocatalytic activity and antibacterial properties under visible light. Photocatalytic tests were performed on the degradation of MB with visible and UV light sources. Results could demonstrate that the Au NPs lead to enhancement of optical and photocatalytic properties of ZnO NRs, aside from being able to inactivate Escherichia coli. These materials could serve as great photocatalysts for AOPs as they showed to have a wide range of photon absorption of the solar spectrum and the ability to inactivate bacteria [41]. 

Goodall et al., showed the photo catalytic performance of ZnO doped with a wide variety of elements, including transition metals, obtaining 56 different samples. All samples were used on the degradation of MB with low doping percentage. Transition metals (Fe, Ag, Mn, V, Ni, Cr, Co, Ti, and Sr) decreased ZnOphotoactivity, but an increase was shown by rare earth metals (La, Y, Nd, Er, Gd, and Pr) doping [42]. 

A series of co-doped ZnOnanostructure was synthesized by Alam et al. using Y and V. The synthetic approach used V₂O₅, Y(NO₃)₃ and Zn(CH₃COOH)₂ as precursors for the ZnO-V-Y NPs by using a surfactant assisted sol-gel methodology. The doping content of Y varied from 1 to 3%, but for V it remained constant at 1%. Photocatalytic performance was tested by using RhB, MB, and 4-nitrophenol (4-N) under visible light. From all the ZnO-V-Y NPs, the best doping content was found to be 3% Y and 1% V, which notably improved the photocatalytic activity of ZnO. Authors suggest that the introduction of V into ZnO creates impurity level above the VB of ZnO which results on a shift in the spectral response from UV to visible region, but also that the inclusion of Y into ZnO-V retards the recombination of charge carriers since Y is known of being a good electron trapper [43]. 

4.2.              Core-Shell Type Structures 

Core-shell structures are an attractive way to exploit the properties of ZnO while coupling benefits from other materials, complementing each other. Also, adding a shell-like structure to a main core can be used to protect it from the environmental solution. 

It is a fact that ZnO is unstable in acid and alkaline mediums, and a way to protect it against corrosive environments is the use SiO₂as a coat to ZnO in order regain photocatalytic activity in both acid and alkaline solutions. These ZnO/SiO₂ core-shell NPs were successfully synthesized in a two-step methodology [44].As shown with TEM images, the thickness of the SiO₂ shell is dependent on the molar ratio of both oxides, but considerably thinner than the ZnO core. Photocatalytic performance was also studied usingRhB aqueous solution at pH 4.5, 7 and 10. Uncoated ZnO worked best at neutral solution.  In contrast, ZnO/SiO₂ NPs showed a better photocatalytic performance in both acidic and alkaline solutions of RhB. This could be explained as the SiO₂ shell could protect the ZnO core from corrosive environments, improving stability and maintaining photocatalytic capability, unlike unprotected ZnO. These results expand the potential application in specific wastewater treatments because the pH variation in some water samples can be extreme. 

ZnO/In₂S₃ core/shell nanostructures where synthesized to overcome the e^-/h⁺pair recombination and the narrow range of photon absorption of the solar spectrum. ZnO NRs where used as substrate for the In₂S₃ NPs, which successfully covered the NRs. The core/shell photocatalyst exhibited enhanced photocatalytic activity in the degradation of RhB, which was 5 times higher than In₂S₃and 56 times higher than ZnO NRs. This effect can be attributable to the separation of the e^-/h⁺pair due to the formation of heterojunctions between both materials [45]. 

Biocompatible coatings are also being studied, particularly inulin [46].A layer of inulin, a biocompatible and biodegradable polysaccharide, was used to cover hematite (α-Fe₂O₃) and brushite (CaHPO₄). The interaction of inulin with these materials were weak, but coating the surfaces beforehand with ZnO increased the affinity toward inulin. The new α-Fe₂O₃/ZnO/inulin and CaHPO₄/ZnO/inulin core-shell structures were water soluble and stable. This studyshows the wide potential in specific tasks in environmental management and the particular physical and chemical advantage of ZnO to bind other materials as polysaccharides. 

Another interesting core-shell nanostructure V₂O₅/ZnOwas synthesized via a combined hydrothermal growth and magnetron sputtering and was supported on a glass substrate [47].Curiously, after 500^oC annealing, the ZnO shell contracted to the tips of each V₂O₅ NRs, giving a lollipop-like structure. This was attributed to the temperature sensitive crystallization and surface diffusion of V₂O₅. This nanomaterial was capable to decompose 2,6-dichlorophenol under visible light, showing promising potential for wastewater treatment and using white light wavelength as light source make this material a cheaper photocatalyst overall. 

ZnO has also been covered with zeoliticimidazolate framework-8 (ZIF-8) produces a naturally porous structure, making this material biocompatible with potential use in animal or human applications[48].These core-shell structures had uniform thickness of around 30 nm composed by ZIF-8 crystalline polyhedrons. An appealing property of these heterostructures is the selective adsorption and permeation effect of the ZIF-8 shell. The ZnO/ZIF-8 also exhibited enhanced selective reduction of Cr (VI) between Cr(VI) and MB, which make this study a niche with its selective photoactivity in mixture aqueous solutions or in Cr(VI) poisoning. 

ZnO can also be used as shell for other materials. The synergy of Mn₃O₄/ZnO and Mn₃O₄ were used as photocatalysts in the reduction of Cr (VI) to Cr (III) under sunlight. The optimal ratio of Mn₃O₄/ZnO and Mn₃O₄ was set at 2:1, having removal rates of total Cr and Cr(VI) with 90.5% and 95.4% within 160 min. Authors tested Cr removal at different pH levels, and noting the optimum level at pH = 6. The novel type-II heterojunction inhibited the recombination of photo-excited carriers and had synergistic effect of photocatalysis and adsorption of total Cr removal. This composite exhibited excellent performance for Cr removal, which make a significant contribution to remediating Cr-polluted water bodies [49]. 

MetallicNWscan also be fabricated for core-shell structures. Worm-like Ag/ZnO core-shell heterostructural composites where fabricated in a two-step polyol method [50].The Ag NWs were formed by a single-crystal Ag NW and then covered with ZnO particles. The ratio of Ag and ZnO was optimized to be 2.8 atom% of Ag, which had the best photocatalytic performance towards RhB, compared with 0.8 or 4.9 atom% of Ag. Authors calculated the reaction rate constants k, demonstrating the better photocatalytic activity of Ag/ZnO NWs with k = 0.0000668 min^-1, compared to pure ZnO with k = 0.0034 min^-1. 

4.3.              Other Structural Modifications 

An innovative approach of ZnO nanostructures is a textile modification by growing one-dimensional ZnO NRs onto the textile surface[21].The modified textile showed to be multifunctional, showing antibacterial activity against Escherichia coli, and as a flexible working electrode for the detection of aldicarb pesticide, and finally as a photocatalyst for the degradation of organic dyes such asCRand MB. Result regarding the photocatalytic degradation of the dyes, the Ag-based textile with ZnO NRs showed activity upon UV irradiation and was successfully applied for the degradation of both CR and MB (degradation of 60% and 65% respectively under 200 min).  

Another type of hetero-structure of gold-nanoparticle-decorated reduced-graphene-oxide (rGO)-wrapped ZnO hollow spheres have been synthesized [33].ZnOnPAHS is decorated with Au/rGO, which decreases the charge-transfer time up to 4.1x10^-12s. The Au/rGO/ZnO hybrid shows a higher charge-transfer efficiency of 68.0%, compared with rGO/ZnO of 40.3% and other reported ZnO hybrids. Additionally, the Au/rGO/ZnO exhibit great photocatalytic efficiency due to the reduced recombination of the e^-/h⁺ pair, the fast electron-transfer rate and the high charge-transfer efficiency. 

All different nanostructured ZnO designed so far have high potential to be used in different wastewater treatment and applied mainly to eliminate xenobiotic or recalcitrant organic compounds and even heavy metal reduction as depicted on this review. It is important to note the potential use to reduce the toxicity or pre-degradation of recalcitrant compounds and improve their biodegradation. A summary of the different nanostructured ZnO materials is presented on (Table 3). 

5.                  Toxicity Concerns of ZnO

Since ZnO has not shown anytoxicity to human cells, it has been used in cosmetics and sun blocker formulations as NPs [51].Nan scaled particles improve skin retention, UV attenuation properties and especially public acceptance in comparison with non-nanosized materials [52].It has been demonstrated that ZnO can undergo photocatalytic reactions while exposed to UV light and generating several ROS (O₂^•, H₂O₂ and HO^•) [53], which can cause cellular damage and environmental toxicity. For many species, including fish[54]and algae[55], nanosizedZnO is extremely toxic [51].ZnO has a similar toxicity as TiO₂, which is also used as an ingredient for sunscreen lotions. It is ascribed that the toxicity is due to two effects: the oxidative stress due the ROS generated and the release of Zn²⁺ ions [24,51].However, nanosizedZnOis known for its antibacterial activity over many bacterial species [7,24,51,56].ZnO toxic properties are being studied in both bulk and nanoscale formulations. Literature reports that antimicrobial properties are improved as size is decreased up to nanometer range, as nanosizedZnO can interact with both outside and inside of cells [57]. 

This particular property however, needs to be considered because its release to the environment can affect microbial population in water and soil systems. Microbial colonies play an important role to the environment by biodegrading different types of organic matter [58].and act as nitrogen fixers for plants. In this case, it is a priority to control its release to the environment or reduce the toxicity of the nanostructures in some way. 

6.                  Future Challenges 

It is a fact that conventional methods are not enough to eliminate all pollutants and need improved with other processes such as photocatalysisin order to degrade recalcitrant compounds. While it has attractive advantages over other types of water treatment methods, photocatalysis faces several challenges that need to overcome. First of all, photocatalysis degradation must be performed using polluted water and using contaminants of interests rather than scavenger agents or proxies [59].Then, a better understanding of degradation mechanisms is required, along with the mechanism of the interaction between ZnO and pollutants. 

Second, the development of ZnO nanostructures must be improved for specific cases. Therefore, the efficiency is dependent on the photocatalysts structure; studies must focus on techniques to synthesize adequate ZnO nanostructures in terms of particle size, surface area and lifespan of charge carrier [59].A good opportunity is the research on ZnO with both metallic and non-metallic doping, fixingZnO in rigid surfaces and other NPs, and exploration on adequate morphologies. 

Performance of ZnO needs to be optimized, as this process alone has technical issues. The method is limited by the clarity of the water being treated, as UV light must be able to reach the ZnO. Also, in order to be implemented on water treatment plants, ZnOphotocatalysis must be paired with other treatment processes, limiting its usage. Photocatalysis can be ineffective in the long-term as its limited by the water chemistry and the presence of co-contaminants. There is also a need of developing mathematical models for photocatalytic systems, which may help on predicting kinetics and will help to optimize conditions of the process.

Research on the potentialtoxicity of nanoscaledZnO should be enhanced. While ZnO NPs have good biocompatibility to human cells, wild-life suffers from the poor management of these materials. Antibacterial mechanisms are not well understood,and investigations should be focus on the comprehension of the phenomenon. Favoring its reuse or recovering and avoid its inclusion to the environment and water bodies. 

7.                  Acknowledgement 

All authors declare no conflicts of interests in this paper.

Figure 1: Scheme of the photocatalysis mechanism in the generation of hydroxyl radicals.

Table 1: Oxidation Potentials of Some Oxidants.

Table 2: Comparison of main advantages/disadvantages of homogeneous and heterogeneous catalysts.

Table 3:Structure type, synthesis and photocatalytic performance details for ZnO nanostructures.

1. Wong F, Alegria HA, Bidleman TF, Alvarado V, Angeles F, et al. (2009) Passive Air Sampling of Organochlorine Pesticides in Mexico.Environ Sci Technol 43: 704-710.
2. Singh P, Mondal K, Sharma A (2013) Reusable electrospun mesoporous ZnO nanofiber mats for photocatalytic degradation of polycyclic aromatic hydroarbon dyes in wastewater.J Colloid Interface Sci 394: 208-21S5.
3. AdeleyeAS, Conway JR, Garner K, Huang Y, Su Y, Keller AA(2016) Engineered nanomaterials for water treatment and remediation: Costs, benefits, and applicability.ChemEngin J286: 640-662.
4. Plumlee MH, Larabee J, Reinhard M (2008) Perfluorochemicals in water reuse.Chemosphere 72: 1541-1547.
5. Ramírez Sánchez I, Bandala E (2015) Drinking Water and Sanitation in Central America: Challenges, Perspectives and Alternative Water Treatment. Am ChemSoc: 53-70.
6. Oturan MA, Aaron JJ (2014) Advanced Oxidation Processes in water/wastewater treatment: principles and applications. A review. Crit Rev Env Sci Tech 44: 2577-2641.
7. Sánchez-Salas JL, Aguilar Ubeda A, Flores Gómez B, Máynez-Navarro OD, Méndez-RojasMA, et al. (2017) Inactivation of bacterialspores and vegetativebacterialcells by interaction with ZnO-Fe₂O₃ nanoparticles and UV radiation.AIMS Geosciences 3: 498-513.
8. Rodríguez-Narváez OM, Peralta-Hernández JM, Goonetilleke A,Bandala ER (2017) Treatment technologies for emerging contaminants in water: A review. ChemEngin J 323: 361-380.
9. Kommineni S, Zoeckler J, Liang S, Stocking A, Flores A,et al. (2017) Advanced Oxidation Process. de Treatment technologies for removal of methyl tertiary butyl eter (MTBE) from drinking water, G. Melin, Ed, California, Gina Melin: 109-133.
10. Legrini O, Oliveros E, Braun AM (1993) Photochemical process for water treatment.Chem Rev 93: 671-698.
11. Bandala E, Torres L (2008) Pesticide removal from water using advanced oxidation technologies: Challenges and perspectives.Pestic Res Trends 282: 49-90.
12. Belgiorno V, Naddeo V,RizzoL(2011) Water, wastewater and soil treatment by advanced oxidation process (AOPs), Fisciano: ASTER onlus Editore 270.
13. Bekbolet M (2005) Fundamentals of Advanced Oxidation Processes, Fisciano: SEED (Sanitary Environmental Engineering Division).
14. Gil Pavas E, Rojas V, Casas D (2011) Procesos avanzados de oxidación para el tratamiento de residuos líquidos peligrosos procedentes de los laboratorios de Ingeniería de Procesos, Universidad EAFIT.
15. Lam SM, Sin JC,AbdullahAZ, Mohamed AR (2012) Degradation of wastewaters containing organic dyes photocatalyzed by zinc oxide: A review. Desalin. Water Treat 41: 131-169.
16. Lewis NS (2007) towardcost-effective solar energy use. Science 315: 798-801.
17. Ibhadon AO, Fitzpatrick P (2013) Heterogeneous Photocatalysis: Recent advances and applications.Catalysts 3: 189-218.
18. Farnetti E, Di Monte R, Kaspar J (2009) Inorganic and Bio-inorganic chemistry, vol. II, I. Bertini, Ed, Oxford: Encyclopedia of Life Support Systems (EOLSS) 50-58.
19. KudoA, Miseki Y (2009) Heterogeneous photocatalyst materials for water splitting. Chem Soc Rev 38: 253-278.
20. VinuR, Madras G (2010) Environmental remediation by photocatalysis. J Indian Inst Sci 90: 189-229.
21. Hatamie A, Kham A, Mohsen G, Turner A, Beni V(2015) ZnO nanostructure: modified textile and its application to biosensing, photocatalysis, and as antibacterial material. Langmuir 31: 10913-10921.
22. Nagayothi PC, Minh An TN, Sreekanth TVM, Jae-il L, Joo Lee D, et al. (2013) Green route biosynthesis: characterization and catalytic activity of ZnO nanoparticles.Mater Lett 108: 160-163.
23. Choi K, Kang T, OhS-G (2012) Preparation of disk shaped ZnO particles using surfactant and their PL properties.Mater Lett 75.
24. Sirelkhatim A, Mahmud S, Seeni A, Mohamad NHM, Ann LC, et al. (2015) Review on zinc oxide nanoparticles: antibacterial activity and toxicity mechanism. Nano-Micro Lett 7: 219-242.
25. Al-Fori M, Dobrestov S, MyintMTZ, Dutta J (2014) Antifouling properties of zinc oxide nanorod coatings. Biofouling 30: 871-882.
26. Qiu R, Zhang D, Mo Y, Song L, Brewer E, et al.(2008) Photocatalytic activity of polymer-modified ZnO under visible light irradiation.J Hazard Mater 156: 80-85.
27. Yogendra K, Naik S, MahadevanKM, Madhusudhana N (2011) A comparative study of photocatalytic activities of two different synthesized ZnO composites againstCoralene red F3BS dye in presence of natural solar light.J Environ Sci Res 1: 11-15.
28. BohleS,Spina C (2009) Cationic and anionic surface binding cites on nanocrystallineZnO: Surface influence on photoluminescence and photocatalysis. J AmChemSoc 131: 4397-4404.
29. Xu Z, Hwang JY, Li B, Huang X, Wang H (2008) The characterization of various ZnO nanostructures using field-emission SEM.JOM 60:29-32.
30. Sangeetha G, Rajeshwari S, Venckatesh R, (2011) Green synthesis of zinc oxide nanoparticles by Aloe barbadensis miller leaf extract: Structure and optical properties. Mat Res Bulletin 46: 2560-2566.
31. Danwittayakul S, Jaisai M, KoottatepT, Dutta J (2013) Enhanced of photocatalyticdegradation of methylorange by supported zinc oxides nanorods/zinc stannate (ZnO/ZTO) onporoussubstrates.IndEngChem Res 52: 13629-13636.
32. Vega-Vaudrit J, Corzo LucioniA (2012) Estudio cinético de la degradación fotocatalítica oxidativa de colorantes empleados en la industria textilera.Revista Iberoamericanade Polímeros 13: 60-68.
33. KhoaNT, Kim SW, Yoo DH, Cho S, KimEJ, et al.(2015) Fabrication of Au/Graphenewrapped ZnO Nanoparticle-assembled hollow spheres with effective photo induced charge transfer for photocatalysis. ACS Appl Mater Interfaces 7: 3524-3531.
34. BijanzadK, Tadjarodi A, AkhavanO (2015) Photocatalyticactivity of mesoporousmicrobricks of ZnO nanoparticles prepared by the thermal decomposition of bis (2-aminonicotinato) zinc (II).ChinJ Catal 36: 742-749.
35. BercoffP, HermeC, Jacobo S (2012)Degradación fotocatalizada del colorante textil Orange II mediante ZnO soportado en hexaferrita de estroncio.Mar de la Plata.
36. Pan L, Shen GQ, Zhang JW, Wei XC, WangL, et al.(2015) TiO₂-ZnO compositespheredecorated with ZnOclusters for effective charge isolation in  photocatalysis. Ind Eng Chem Res 54: 7226-7232.
37. SrivastavaP, Srivastava VC (2013) Photocatalyticoxidation of dyebearingwastewater by iron-dopedZnO.IndEngChemRes 52: 17790-17799.
38. ChoJ, LinQ, YangS, Simmons JrJG, ChengY, et al.(2012) Sulfur-doped zinc oxide (ZnO) nanostars: Synthesis and simulation of growthmechanism. Nano Res 5: 20-26.
39. EminS, FanettiM, AbdiF, LisjakD, ValantM, et al. (2013) Photoelectrochemicalproperties of cadmiumchalcogenide-sensitizedtexturedporous zinc oxide plateelectrodes.Appl Mater Interfaces5: 1113-1121.
40. LinWH,Chiu YH, ShaoPW, HsuYJ (2016) Metal-particle-decoratedZnOnanocrystals: Photocatalysis and chargedynamics.ACS Appl MaterInteraces8: 32754-32763.
41. SaoudK, SoubaihiRA, SaeedS, BensalahN, Al-FandietM,et al. (2018) Heterogeneous Ag and ZnObasedphotocatalytic for wastewatertreatmentunderdifferentirradiationconditions.J Mater Environ Sci 9: 400-413.
42. Goodall J, IllsleyD, Lines R, MakwanaN,DarrJ (2015) Structure-Property-Compositionrelationship in doped zinc oxides: enhancedphotocatalyticactivity with rareearthdopants.ACS Comb Sci 100-112.  
43. AlamU, KhanA, RazaW, Khan A,Bahnemann D (2017) Highly efficient Y and V co-dopedZnOphotocatalyst with enhanceddyesensitized visible light photocatalyticactivity.CatalToday284: 169-178.
44. ZhaiJ, TaoX, Pu Y, Zeng XF,Chen JF (2010) Core-Shell StructuredZnO/SiO₂ Nanoparticles: Preparation, Characterization and Photocatalyticproperty.Appl SurfSci257: 393-397.
45.  Khanchandani S, KunduS, PatraA, GanguliAK (2013) Band gap tuning of ZnO/In2S3 core/shellnanorodarrays for enhanced visible-light-drivenphotocatalyst.JPhys Chem C 117: 5558-5567.
46. Santillán-Urquiza E, Arteaga-Cardona F, Hernandez-Herman E, Pacheco-García PF, González-Rodríguez R, et al. (2015)  Inulin as a novel biocompatible coating: evaluation of surface affinities toward CaHPO4, α-Fe2O3, ZnO, CaHPO4@ZnO and α-Fe2O3@ZnO nanoparticles. Journal of Colloid and Interface Science 460:  339-348.
47. Zou CW, Rao YF, Alyamani A, Chu W, Chen MJ (2010) Heterogeneous lollipol-like V2O5/ZnO Array: A promising composite nanostructure for visible light photocatalysis.Langmuir 26: 11615-11620.
48. Wang X, Liu J, Leong S, Lin X, Wei J, et al. (2016) Rapid construction of ZnO@ZIF-8 heterostructures with size-selective photocatalysis properties. ACS Appl Mater Interfaces 8: 9080-9087.
49. Li N, Tian Y, Zhao J, Zhang J, Zhang J, et al. (2017) Effective removal of chromium from water by Mn₃O₄@ZnO/Mn₃O₄ composite under simulated sunlight irradiation: Synergy of photocatalytic reduction and adsorption.Appl Catal B: Environ 214:126-136.
50. Liu HR, Shao GX, Zhao JF, ZhangZX, Zhang Y,et al. (2012) Worm-like Ag/ZnO core-shell heterostructural composites: fabrication, characterization, and photocatalysis. J Phys Chem 116: 16182-16190.
51. Juliano C, Magrini GA (2017) Comsetic ingredients as emerging pollutants of environmental and health Concern. A mini-review.Cosmetics 4: 11.
52. Serpone N, Dondi D, Albini A (2007) Inorganic and organic UV filters: their role and efficacy in sunscreens and suncare products.Inorg Chim. Acta  360: 794-802.
53. Lewicka ZA, YuWW, OlivaBL, ContrerasEQ, Colvin VL (2013) Photochemical behavior of nanoscale TiO₂ and ZnO sunscreen ingredients.J PhotochPhotobio A 263: 24-33.
54. Chen TH, Lin CC, Meng PJ (2014) Zinc oxide nanoparticles alter hatching and larval locomotor activity in zebrafish (Danio rerio).J Hazard Mater 277: 134-140.
55. Ma H, Wallis LK, Diamond S, Li S, Canas-Carrell J, et al. (2014)  Impact of solar UV radiation on toxicity of ZnO nanoparticles through photocatalytic reactive oxygen species (ROS) generation and photo-induced dissolution. Environ Pollut 193: 165-172.
56. Jones N, Ray B, Ranjit KT, Manna AC (2008) Antibacterial activity of ZnO nanoparticle suspensions on a broad spectrum of microorganisms.FEMS Microbiol Lett 279: 71-76.
57. Seil JT, Webster TJ  (2012) Antimicrobial applications of nanotechnology: methods and literature.Int. J. Nanomed 7: 2767-2781.
58. Barra CaraccioloA,Topp E,  Grenni P (2018) Pharmaceuticals in the environment: biodegradation and effects on natural microbial communities. A review.J Pharm Biomed Anal 106: 25-36.
59. OngCB, Ng LY, Mohammad AW (2018) A review of ZnO nanoparticles as solar photocatalysts: Synthesis, mechanisms and applications.Renew Sust Energ Rev 81: 536-551.