Influencing Parameters in the Photocatalytic Degradation of Organic Effluent via Nanometal Oxide Catalyst: A Review
This paper aims to review the recent works on the photocatalytic degradation of organic pollutants in the presence of nanophotocatalyst. In this regard the effects of operation parameters which could influence the photocatalytic degradation of organic pollutants (such as catalyst preparation method, initial concentration of organic pollutants, presence of doping, catalyst loading, calcinations temperature, pH, presence of oxidants, UV intensity, temperature, and presence of supports) are discussed. Recent research suggests that the parameters mentioned above have great influence on the photocatalytic activity of prepared nanocatalyst. Also, the general mechanism of photocatalytic degradation and some recent synthesis methods are discussed here.
In recent years development of industries like textile, leather, paint, food, plastics, and cosmetics is enlarged and these industries are connected with the discarding of a vast number of organic pollutants which are harmful to microbes, aquatic system, and human health by influencing the following parameters [1–4]. In addition to that the effect of these organic effluents in the wastewater can be sensed by change in the vital parameters of the discharged water such as chemical oxygen demand, biochemical oxygen demand, toxicity, unpleasant odour, and colour . Even the presence of a trace amount of colored organic compounds in the aquatic system can result in coloration of wastewater. The consequence of colored water is detrimental to environment since the color obstructs the sunlight access to aquatic organisms and plants, and it diminishes the photo synthesis and affects the ecosystem [6, 7]. Therefore, the removal of color and sanitization have become an ecological concern and they are vital for the environmental sustainability. Industrial development and its association with discharge of organic matter into the aquatic systems demands the technological development to solve the environmental problems related to organic effluents . Many practices have been widely applied in the treatment of organic effluent such as biological treatment, reverse osmosis, ozonation, filtration, adsorption on solid phases, incineration, and coagulation . However, each of the methodologies has its own advantages and limitations; as such incineration can result in deadly toxic volatiles products ; biological treatment needs prolonged treatment time and results in ghastly smell; ozonation can be effective way for the treatment of organic effluent but the stability of ozone is the biggest concern which was well influenced by the existence of salts, pH, and temperature [9, 11]. The traditional physical methods (adsorption, filtration, reverse osmosis and coagulation) are quite expensive and also these techniques do not eliminate the organic molecules utterly but just transform one phase to another [12, 13]. In recent era, advanced oxidation processes (AOPs) have been found as an effective and alternative way for the treatment of organic effluent in aqueous system [14–16]. The recent research demonstrates that AOPs based on photocatalysts are valuable and this method benefits complete mineralization of organic molecules into nontoxic CO2 and H2O at the atmospheric conditions [17–20]. Further, AOPs result in the generation of hydroxyl radicals (•OH) as main oxidizing agents which can remove even nonbiodegradable organic compounds from wastewater stream [21, 22]. Photocatalytic degradation of organic effluent emerged as one of the best methodologies for the treatment of toxic organic effluents which uses the semiconductor as catalyst such TiO2, ZnO, ZnS, WO3, CdS and Fe2O3, and SrTiO3 [23, 24, 24–27]. Among them TiO2 and ZnO have been widely applied as photocatalyst due to their high activity, nontoxicity, chemical stability, lower costs, optical and electrical properties, and environment friendly characteristics [28–32]. Since the first revolutionary papers in the 1970s, the attention of researchers in heterogeneous photocatalyst has developed greatly [33, 34] and showed the advantages of photocatalytic practices over the conventional techniques, such as rapid oxidation, no formation of polycyclic compounds, complete oxidation of pollutants, and high efficiency [35–37]. In this manner, the heterogeneous photocatalyst turns out to be an elegant alternative for organic effluent treatment and the catalyst reduced to the nanoscale can demonstrate different properties compared to properties at macroscale size, facilitating unique applications in photocatalyst degradation of organic waste . Nanophotocatalysts have been intensively examined because of their increased surface area which sturdily influences their physiochemical properties [39, 40]. Efficiency of the prepared photocatalyst for the degradation of organic effluent is greatly influenced by various parameters, starting from synthesis of nanocatalyst to operation condition of degradation of effluent. In recent year nanocatalyst synthesis and its application in the degradation of organic molecules were studied extensively . This paper discusses the various factors which could affect the photocatalytic activity of the nanocatalyst staring from synthesis of operating condition.
2. Reaction Mechanism
In general, the photocatalytic degradation involves several steps such as adsorption-desorption, electron-hole pair production, recombination of electron pair, and chemical reaction [42, 43]. The general mechanism of photocatalytic degradation of organic molecules is explained as follows (Figure 1): [39, 44–46]. When the photocatalyst (PC) is irradiated with photons of energy equal to or more than band gap energy of PC, the electrons () are excited from the valence band (VB) to the conduction band (CB) with the simultaneous creation of holes () in the VB:where is the energy essential to transfer the electron from valence band to conduction band. The electrons generated through irradiation could be readily trapped by O2 absorbed on the photocatalyst surface or the dissolved O2 to give superoxide radicals ():
Consequently, could react with H2O to produce hydroperoxy radical () and hydroxyl radical (OH•), which are strong oxidizing agents to decompose the organic molecule:
Simultaneously, the photoinduced holes could be trapped by surface hydroxyl groups (or H2O) on the photocatalyst surface to give hydroxyl radicals (OH•):
Finally the organic molecules will be oxidized to yield carbon dioxide and water as follows:
Meanwhile, recombination of positive hole and electron could take place which could reduce the photocatalytic activity of prepared nanocatalyst:
3. Nanocatalyst Preparation Methods
Here some of the recently used methods for the preparation of nanocatalyst in the photocatalytic application are discussed briefly.
3.1. Sol-Gel Method
Synthesis of photo nanocatalyst by sol-gel method is shown in Figure 2.
Catalyst starting material is dissolved in the suitable solvent and then precipitating agent is added dropwise with vigorous stirring and solvent and precipitate (gel) are separated. To eliminate even the trace amount of the solvents in the obtained gel, the gel has to be dried at temperature between slightly near and above the boiling point of solvent; then product is pulverized and calcined to get the desired nanocatalyst [63, 64].
3.2. Hydrothermal Method
Synthesis of photo nanocatalyst by hydrothermal treatment method is shown in Figure 3.
In hydrothermal treatment method, precursor is dissolved in the solvent. Then, the precipitating agent is added dropwise followed by hydrothermal treatment of prepared solution at 100–200°C for 1–12 hours in autoclave under autogenous pressure. The solid particles are cooled and washed with washing agent and then dried for 24 h at 80°C to give nanoparticles [65, 66].
3.3. Coprecipitation Method
Synthesis of photo nanocatalyst by coprecipitation method is shown in Figure 4.
Precursor of catalyst and doping agent are dissolved separately in deionized water and mixed under vigorous stirring to get the uniform distribution of dissolved materials. Then, precipitating agent (NaOH) was added dropwise into precursor solution with the continuous stirring of the particular period of time. The precipitate is filtered and washed with deionised water until the washed solution reaches neutral pH. The obtained precipitate is dried at 80–100°C in an oven for 24 h and calcined at 400–500°C for 5 h to get the nanocatalyst [65, 67].
3.4. Reverse Microemulsion Method
Synthesis of photonanocatalyst particle by reverse microemulsion method is shown in Figure 5.
Desired mole ratios of precursor salts are dissolved in the deionized water. The surfactant and cosurfactant are mixed in solvent with vigorous stirring. As shown in Figure 5, two types of microemulsions are prepared. In part A, mixture of aqueous solution of precursor salts is added into one of the mixtures of surfactant and cosurfactant to result in transparent microemulsion A. In the same manner, the adequate ammonia is added into the other microemulsion mixture to yield a transparent microemulsion B (part B). Then parts A and B are mixed continuously until clear solution appeared and solution was allowed 1-2-day aging. Subsequently, after centrifugation, washing, drying, and calcinations of separated particles could be done to yield the effective photocatalyst .
4. Factor Affecting the Photodegradation
4.1. Nanocatalyst Preparation Method/Condition
In general, changes in process conditions or preparation methods result in catalyst with different catalytic activity. In recent decades, there are many researchers who has been involved in the modification of catalyst preparation method to improve the photocatalytic activity of prepared nanocatalyst [69, 70]. For example, in the case of TiO2 catalyst, it was found that lab-made rutile exhibited a good photocatalytic activity due to presence of vast number of hydroxyl groups (Table 4). But contradictory to that the rutile obtained at a higher temperature is inactive due to limited surface hydroxyl content . Further, many researchers have observed that size of the prepared catalyst is very well influenced by condition of catalyst preparation . It is desired to have photocatalyst with smaller size to result in a higher photocatalytic activity. But reduction in the size of photocatalyst leads to increase in surface energy of the photocatalyst. This causes the agglomeration of particles which could reduce the porosity of the catalyst. Thereby access of organic molecules could be prevented into interparticle porous surface and reduce the surface area available. It obliterates photocatalytic activity of prepared photocatalyst . But the above could be eliminated by doing slight change in sol-gel method by using stearic acid as capping agent to prevent agglomeration of small size nanoparticles due to surface repulsion [73, 74]. By using surfactant as electrostatic stabilization agent, calcination temperature required for the formation of anatase phase TiO2 could be reduced . And, it was found that deposition of dopant ions on the surface of photocatalyst is well affected by the pH of precursor solution .
4.2. Initial Concentration of Organic Compound
Increasing initial organic compound concentration reduces the photocatalyst degradation efficiency. It might be due to the increase of the initial concentration of organic compound in which more molecules were adsorbed on the surface of the photocatalyst. This results in unavailability of catalyst surface for the building of hydroxyl radicals which reduces the photocatalytic activity of the catalyst . Also, escalating initial concentration of organic compound decreases the number of photons or path length of photon that is arrived on the surface of photocatalysis (the Beer-Lambert law) which reduces the excitation of electron from valance band to conduction band. It results in the decrease in the photocatalytic activity of prepared nanocatalyst [18, 30, 50, 77]. Comparison of removal percentage of organic pollutants at different initial pollutant concentration is presented in Table 1.
Doping of photocatalyst could improve the photocatalyst efficiency in the following way: (1) band gap narrowing ; (2) formation impurity energy levels ; (3) oxygen vacancies; (4) unique surface area for the adsorption of organic molecules ; and (5) electron trapping . In general catalyst with a smaller band gap energy is preferred to be an effective photocatalyst to generate more electron-hole pairs . Band gap narrowing, introduction of impurity energy level, and oxygen-deficient sites can result in a catalyst with higher photocatalytic activity even under visible light [63, 78, 80–83]. The photocatalytic activity of prepared catalyst depend well on how the recombination of photoinduced hole-electron pairs is prevented. Doping prevents recombination of electrons and holes and gets better the photocatalytic activity by trapping the photoinduced electrons [28, 30, 46, 84, 85]. Incorporation of dopant ions in the catalyst is either interstitial or substitutional. In the case of interstitial mode, the radius of doped ions will be lesser than the radius of lattice ions and the lattice space. This allows the doped ions to pierce the crystal cell of the metal oxide. In the case of substitutional mode, doping ions replace the lattice oxide or lattice ions . Further, it has been found that the dopant ions incorporated into catalyst crystal lattice can alter the electronic property of prepared nanocatalyst and improve its light absorption ability in the visible light region [1, 18, 78]. It has been found that the red-shift of absorption edge with respect to pure material is resulted from d electron transfer. But beyond optimum level of doping concentration decrease in photocatalytic activity results from (i) decreasing surface area of catalyst and (ii) narrowing space charge region, and the penetration depth of radiation into photocatalyst may exceed the space charge layer, so that the recombination of the photogenerated hole-electron pairs turns to be easier , and some case doping particles can act as electron-hole recombination centres [28, 50, 87], because the average distance between trap sites is reduced with increasing the dopant concentration . Thus the photocatalytic activity of nanocatalyst is decreased. Tian et al. found that, in the case of TiO2, the composition of rutile is increased to some extent with increasing doping concentration by reducing the thermal stability of anatase and favours the phase transformation from anatase to rutile . In addition to that, the increase in the doping content results in clusters of doping molecule to form overlying agglomerates which can shadow the photocatalytic activity by surface plasmon absorption phenomenon . In the case of mesoporous nanoparticles, doping material might be ascribed to lower photocatalytic activities due to the fact that surface site might be blocked by doping material [88, 89]. In contrast to that, Tong et al. reported that doping increases the surface area by increasing mesoporous structure in the nanocatalyst .
4.3.1. Noble Metal Doping
Noble metal may enhance photocatalytic activity through the following route: (i) enhanced adsorption of organic molecules on the photocatalyst surface , (ii) the surface plasmon resonance of noble nanoparticles on photocatalyst which is excited by visible light, increasing electron excitation and electron-hole separation , (iii) noble nanoparticles acting as electron traps to hinder electron/hole recombination , and (iv) Fermi level equilibration between noble nanoparticles and photocatalyst (semiconductors) which may lessen the band gap of the photocatalyst and in turn weaken the fast electron-hole pairs recombination , and also it is reported that the photo corrosion of ZnO catalyst is prevented by adding pt doping to the photocatalyst. Thereby life time of catalyst is increased .
Comparison of the photocatalytic degradation rate of different organic pollutants for different noble metal doping is given in Table 2.
4.3.2. Metal Doping
Metal doping could alter the morphology, particle size , crystal structure , and specific surface area of nanocatalyst which are significant factors that decide the photocatalytic activity of prepared photocatalyst.
Comparison of photocatalytic activity of different metal doped catalyst is presented in Table 3.
Anionic dopants are reported to be superior than metal dopants regarding stability of the doped photocatalyst, photocatalytic activity, and simplicity of doping process . The quantum yield of doped photocatalyst under visible light irradiation is far lower than that of photocatalyst under UV irradiation. Photocatalyst with anion-doping to obtain the visible light-activated photocatalyst has been reported in last decades [42, 62, 79, 93, 94]. Various anionic species such as nitrogen, carbon , phosphorus, fluorine , and sulphur were recognized to potentially form a new impurity energy level neighboring to the valence band while maintaining the largest band gap for maximum efficiency. Nitrogen can be incorporated into the nanophotocatalyst structure as substitutional and as interstitials . In the case of activated carbon hybridized nanophotocatalyst, the surface area of photocatalyst increases with the increase in activated carbon content but activity decreases due to increase of ZnO/TiO2 concentration, because the pore entrances of the activated carbon could be blocked by excess ZnO/TiO2 composition [62, 96]. Also Yu et al. found that the crystalline nature of prepared catalyst was suppressed by the increase in the amount of doping with cerium and nitrogen, and this trend was strengthened with the increase in doping amount. Meanwhile, the growth of crystal size of catalyst was suppressed to different extent of doping .
4.3.4. Earth Metals Doping
In case of TiO2 nanocatalyst preparation, phase transition from anatase to rutile was inhibited by earth ions during the thermal treatment. Similar result is observed in nonmetal dopant and combination of nonearth and nonmetal dopants [79, 89]. In the case of TiO2, the inhibition of the phase transition was ascribed to the stabilization of the anatase phase by rare earth ions through the formation of TiO2 rare earth element bonds . Rare earth metal doping significantly inhibits the growth of nanocrystalline size [64, 97] due to earth metal-O-Ti bonds formation on the surface of nanocatalyst and in the calcinations [56, 64, 98]. Shi et al.  observed that unit cell volumes and lattice distortion of doped nanocatalyst particles are larger than that of undoped photocatalyst, because small part of Ti ions are replaced by earth metal ions in the TiO2 lattice, and this causes the distortion and expansion of lattice crystal structure. Similar result has been reported for Fe3+ doping . It results in easy absorption of oxygen molecules as electron trap on the defects which originated from the lattice distortion and expansion. Therefore the recombination rate of electron-hole pairs could be decreased and increased photocatalytic activity of the nanocatalyst is observed. Photocatalytic activity of earth metal doped photocatalyst is given in Table 5.
4.4. Catalyst Loading
In general due to increasing in active sites, the rate of photocatalytic degradation organic pollutants increases with photocatalyst dosage [6, 32, 60]. This is mainly due to increase of hydroxyl radical produced from irradiated photocatalyst [46, 99]. At lower catalyst loading, degradation of organic molecule is low, because more light is transmitted through the reactor and lesser transmitted radiation only will be utilised in the photocatalytic reaction [49, 61]. But beyond the optimum amount of catalyst loading, the degradation rate might be reduced due to increase in the opacity of the suspension, and thus increasing the light scattering and also the infiltration depth of the photons is diminished and less photocatalysts could be activated [18, 84, 100]. Additional to what is mentioned above, agglomeration of nanoparticles at high concentrations reasoning a decline in the number of surface active sites available for the photocatalytic degradation [77, 99] and deactivation of activated molecules could result from the collision of activated molecules with ground state molecules [61, 101]. Dependence of photocatalytic activity on catalyst loading is given in Table 6.
4.5. Calcination Temperature
Calcination temperature has a vital influence on the optical property, crystal size, and crystal structure of the prepared photocatalyst . Moreover, the common synthesis of nanophotocatalyst usually involves high-temperature calcinations to translate amorphous into crystal structure, which usually results in particle growth and leads to decrease in surface area and decrease in photocatalytic degradation efficiency [64, 75, 101]. Shi et al. reported  that the absorption profile shifted slightly to longer wavelengths (red-shift) with the increase of calcined temperature. In the case of TiO2, anatase phase has higher thermal stability than rutile phase which results in higher concentration of anatase phase at higher temperature , and it has been reported that anatase possesses higher photocatalytic activity than rutile [64, 102]. Because the adsorptive affinity of anatase for organic molecules is higher than that of rutile, anatase structure also displays low rate of recombination of hole-electron pair in contrast to rutile , and also higher calcination temperature helps the formation of larger crystals and the rutile phase by agglomeration , Sun et al. found that doping of nitrogen with TiO2 would drop increasingly with rise in temperature and the crystal particle sizes of doped photocatalyst became bigger with increasing calcinations temperature . Comparison of photocatalytic degradation of different organic pollutants for different calcinations temperature is shown in Table 7.
pH of aqueous dye solution plays a vital role in the photocatalytic activity of prepared catalyst. pH influences the adsorption and dissociation of the organic molecule [41, 104], surface charge of photocatalyst, and oxidation potential of the valence band . Photocatalyst surface is predominantly negatively charged when the pH is increased beyond isoelectric point of nanophotocatalyst. As the pH reduced, the functional groups are protonated, thus raising the positive charge of photocatalyst surface. The surface of photocatalyst will be charged negatively and results in the increased adsorption cationic molecules at higher pH value while in the reverse situation it would adsorb anionic molecules very easily [27, 49, 50, 99]. In the meantime, the increased pH value increases the hydroxyl radicals generation . But the degradation of organic molecules is repressed when the pH of solution is too high (pH > 12), because hydroxyl ions compete with organic molecules for the adsorption on the surface of the catalysts . On the contrary, at low pH, the adsorption of cationic organic molecule on the photocatalyst surface is reduced, because the surface of photocatalyst is positively charged which results in the decrease in adsorption of cationic organic molecules. Thus, the degradation efficiency is declined in lower pH or acidic solution [18, 77, 106]. That is, the optimum pH of solution has to be fixed for the different organic solution.
On the other hand, Prado and Costa  noticed that the degradation of malachite green was lesser in basic medium (higher pH); the degradation efficiency of photocatalyst increases with the decrease in pH value. Similar effects were observed by other researchers [32, 52]. Optimum pH for the different photocatalytic system is presented in Table 8.
4.7. Effect of Oxidants
The electron-hole recombination in photocatalyst can be diminished by adding some irreversible electron acceptors (H2O2, (NH4)2S2O8, KBrO3, and K2S208) to the reaction mixture [14, 50]. In most cases, H2O2 is used to increase the photocatalytic activity of prepared photocatalyst.
The mechanism in which H2O2 alters the photocatalytic degradation is given as follows:
The increase of H2O2 concentration resulted in a quicker degradation of the organic molecules . Because photolysis of H2O2 is to generate OH radicals and H2O2 helps for trapping electrons and thus prevents the recombination of hole-electron pairs, in consequence, the chance of the formation of OH radical and O2 on the surface of the photocatalyst is increased. Beyond the optimum concentration of H2O2, increase in H2O2 level decreases the degradation rate of dye due to quenching of OH radical by H2O2 . Sobana and Swaminathan reported that addition of H2O2 beyond the optimum level results in generation of peroxide radical, which acts as a hole scavenger and thus results in the decrease of photocatalytic degradation efficiency .
4.8. Light Intensity
Semiconductor catalyst absorbs the light with an energy equal to or more than band gap energy  which causes the movement of electrons from valence band to conduction band by leaving holes in the valance band [36, 77]. The photocatalytic degradation rate increased with increasing intensity of radiation . The chance of excitation of catalyst can be improved by raising the intensity of incident radiation . But recombination of hole-electron pair is a normally encountered difficulty in photocatalysis. At lower light intensity, hole-electron pair separation struggles with recombination which reduces the formation of free radicals and, thus, causes reduction in the degradation of the organic molecules  which could be eliminated by using light with higher intensity. Therefore, enhancement of the photodegradation rate could be achieved with increase in the intensity of incident radiation [28, 101]. Dependence of photocatalytic activity on light intensity is presented in Table 9.
4.9. Effect of Temperature
The photocatalytic degradation efficiency of organic molecules steadily increased as the temperature is raised. When the temperature is increased, it causes the bubbles formation in the solution which results in the generation of free radicals. Additionally, the increase in temperature helps the degradation reaction to overcome electron-hole recombination. Besides that, the increasing temperature may enhance the oxidation rate of organic molecules at the interface . Influence of reaction temperature on photocatalytic degradation is presented in Table 10.
4.10. Effect of the Support
Application of support for the nanophotocatalyst maintains the dispersion of the nanoparticle and prevents agglomeration and sintering [104, 110]. Also, supports increase the photocatalytic activity by enhancing charge separation (high electric conductivity supports) or the adsorption of organic molecules . Generally support/magnetic coating is used to immobilize the nanocatalyst so that the reusability of nanocatalyst can be achieved [111, 112], and also supports help the adsorption of organic molecules to increase the efficiency of photocatalytic degradation. The organic pollutants, which are adsorbed on the mesoporous (support) materials, have an opportunity to be degraded due to the appearance of hydroxyl radical on the support surface [77, 113].
The technology of nanocatalyst as a photocatalyst has undergone an explosive growth during the past decade, for environmental remediation by using both artificial and natural UV-visible irradiation. Hence it is possible to obtain visible light-active photocatalyst achieved by altering the synthesis and operating condition, so that industrial implementation of photocatalytic degradation of organic effluent can be achieved over conventional treatment methods.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
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