Recent Progress in Doped Tio 2 Photocatalysis and Hybrid Advanced Oxidation Processes for Organic Pollutant Removal from Wastewater

Hybrid advanced oxidation processes (HAPOs) for the removal of non-biodegradable organics from wastewater have been studied in recent literature. With the increase in industrial development, the quantity of wastewater generated from these industries also organic wastewater produced by industrial manufacturing has posed threats to the environment. AOP’s are one of the promising advanced technologies for mineralization of organics present in wastewater. Hybrid advanced oxidation process based on the ozonation, sonolysis, Photo-Fenton reagents and electro chemical method, has greater potential for complete mineralization of recalcitrant organics. This review article includes recent progress in the research and application of TiO 2 photocatalysis for the removal of non biodegradable organic pollutants present in water. It will provide a quick reference for various hybrid AOPs systems and their effectiveness. This review article provides quick insights into (1) hybrid AOP for treatment of various industrial effluents or model effluents, (2) work done on doped/ co-doped photo catalyst as heterogeneous catalysts (3) study of parameters affecting the photocatalysis to enhance complete oxidation of organics present in wastewater. A mechanistic investigation of hybrid advanced oxidation processes with combinations of sonolysis and Fenton process coupled with UV, adsorption and addition of biochar has been discussed.


Introduction
Innovations and productions of new medicines increased number of pharmaceutical industries with accumulation of waste in rivers and on land. Environmental management part always found non-focused and lead to degradation of nature.
Researchers are working on these issues to resolve these problems. This situation enforced research towards zero effluent discharge, green technology and cleaner development mechanism. Semiconductor photocatalysis has been extensively studied by many researchers for the complete oxidation of refractory organics present in effluent, [1][2][3] water splitting for hydrogen production 4 and solar cells. 5 The application of TiO 2 as a photocatalyst is limited by UV radiations and recombination of the hole and electron pairs. 6,7 Rapid industrialization has vastly increased water and air pollution problems as the current generation are interested more in profit and less concerned about waste generation. This situation demands fruitful research be done on waste minimization to avoid such situations and to achieve sustainable development. Objective of this review is to search for efficient and cost-effective AOP for wastewater treatment. Solar light-driven effluent treatment methods have been focused and developed for research. 8 Titanium dioxide is an N-type semiconductor having an oxygen deficit in its structure. TiO 2 is a superior, nontoxic stable and economical photocatalyst that provides a nonselective and efficient oxidizing agent, Hydroxyl radical (OH*). 9,10. TiO 2 has shown certain limitations as a photocatalyst: 1) it has a large band gap and works only under UV radiations; 2) its low quantum yield of OH* due to recombination of holes. 11 Metal doping in TiO 2 1) improves its absorbance in the visible region, e.g. a Ag: 300-800 nm, Co: 400-650 nm and Fe: 300-800 nm, 12-14 and allow it to work under solar radiation to make cost-effective treatment.; 2) provides the excellent trap of electrons prevents recombination of e-and holes results in superior photoactivity; 15 3) the Bandgap reduces from pure TiO 2 (3.1 eV) to doped TiO 2 (2.8 eV), 16,17 Silver and iron are extensively investigated as a dopant for TiO 2 and proved superior photocatalysts for mineralization of active pharmaceutical ingredients(API). 18-20 Co-doping of TiO 2 using metal dopants is a promising technology for solar mineralization of refractory organics in wastewater. Doping of TiO 2 with Fe and Ag metals enhances the photocatalytic activity due to large reactive sites for photocatalysis. [21][22][23][24][25][26] Nanomaterials have magical physical and ocular characteristics due to their size and in carceration e to initiate quantum properties. Nanopowder absorbs much more solar radiation compared to nanofilms. Size, morphology and optical properties can be controlled during solar photocatalysis and photovoltaics results in better absorption of solar irradiations. 27,28 Several studies on the photo activity of Ag-doped TiO 2 and Ag-Fe co-doped TiO 2 (Ag-Fe CT) catalyst proved co-doped catalyst superior over undoped TiO 2 . 25,29,30 Anisotropic structure of Ag dopant improved solar radiation absorbance. 31 In this review, we have described recent progress in advanced oxidation processes with metal dopants, co-doped photocatalysts with their properties and bandgap. Synthesis of nano-doped TiO 2 , mechanism of degradation by photocatalysis, operating variables and their effects on degradation and different techniques to modify optical properties of TiO 2 such as the use of metal and non-metal dopants, nanofilms, nanotubes and nanowires are discussed. The feasibility and the effectiveness of recycled photocatalyst have been studied. Hybrid AOPs is proved efficient compared to conventional AOP for complete mineralization of complex organics. Hybrid AOP using Fe doped TiO 2 has shown dual characteristics of photocatalysis and Fenton reaction, which has improved decolorization of wastewater. 32 Photocatalytic treatment work under normal ambient conditions. 33 Efficient methylene blue degradation using combining AOP with Fenton reagents, results in production of more OH radicals. 34 Diclofenac and ibuprofen were converted efficiently in to biodegradable intermediates using planar falling film reactor andCoated TiO 2 on a Pilkington Active glass under UV radiations. 35,36 This review will be useful to select efficient hybrid AOP for specific industrial wastewater treatment.

Advanced Oxidation Processes
AOPs are effluent treatment technology that produces a hydroxyl radical (OH) with highest oxidation potential and performs oxidation of organics to produce carbon dioxide and water as end products. These processes use ozone, photo Fenton reagents, hydrogen peroxide, or semiconductor photocatalysis to generate OH. TiO 2 was focused on photocatalysis by many researchers. It is available in three forms anatase, brookite and rutile. Amongst all these, the tetragonal anatase structure performs efficient photocatalysis. 37,38 Various advanced oxidation processes consist of pollutant removal technologies in which hydrogen radicals serve as an active medium. The methods are separated according to the source of the formation of hydroxyl radicals as shown in Fig. 1 Table 1 shows the oxidation potentials of various oxidizing agents. OH. Radical is nontoxic, nonselective and has the highest oxidation potential hence it is capable to mineralize a major category To set up the highest standards for effluent treatment; 3.
To develop an advanced mode of operation and competitiveness.  Major merits of AOP includes the faster rate of mineralization, non biodegradable organics are completely oxidized into CO 2 and H 2 O, treated effluent can be directly reused without further purification, avoid sludge generation and its handling problems, it can be easily clubbed with existing ETP with little modification, and economic operation and maintenance compared to incineration. Demerits of AOPs are higher capital costs, complex and unknown reaction chemistry may sometimes lead to more hazardous intermediates formation and photochemical reactor design and operation are difficult. Challenges of AOPs arePhotocatalyst deactivation and unknown routes for different reactions, 46 development of proper doped catalysts to enhance the absorption of solar radiation, the selectivity of photocatalyst may sometimes pose a problem in treatment when a mixture of different organics is present, electron and hole recombine to result in lower net generation of OH radicals, scale-up and commercialization of process 47 and UV radiations may sometimes degrade ozone, chlorine and hydrogen peroxide which are useful oxidizing agents in the process. 39

Titanium Dioxide Photocatalysis
Semiconductor oxides have a greater number of surface atoms ona surface which enables photon absorption and performs various oxidation and reduction reactions for complete removal of a variety of organics from aqueous solutions. Titanium dioxide is widely preferred for photocatalysis due to its stability, reusability, nontoxicity, anti-corrosiveness and low cost. Different other oxides that can also be used for photocatalys is are zinc, tin, zirconium, cadmium and iron.Hydroxyl radicals react with organics to produce carbon dioxide and water. 6, 48 The main reactions involved in photocatalys is are shown below (equation (1) to equation (8) (1) to (9) on the surface along with complete oxidation of organics to produce CO 2 and H 2 O.
with improved trapping of charge carriers. Thus Doping increases organics degradation efficiency. 52 Dopant will create oxygen defects and shifts light absorption from UV to the visible region by improving absorption bandwidth. The efficiency of photocatalysis may differ based on the position of the dopant on the TiO 2 structure. Based on synthesis methods, the dopant can take a position on the surface or it can be included in lattice structure or as core and thus these positions may lead to different photocatalytic activity and degradation efficiency. Metals and non-metals both can work as dopants but major research concludes that metal dopants possess strong surface plasmon resonance (SPR), work efficiently under solar radiations during photocatalysis. 53 For efficient photocatalysis, the bandgap should be lower which promotes the transfer of e-and holes. This will also influence the redox potential of photogenerated electrons and the oxidation potential of holes. 53 The handling of TiO 2 powder form is difficult and the cost of UV radiation makes the treatment energy-intensive and uneconomical. These issues limit the commercialization of AOPs for industrial effluent treatment. These limitations can be overcome by surface modification of TiO 2 with transition metal doping which reduces the bandgap and greater absorption of visible light is possible, also the dopant metals trape e-and prevent its recombination with holes, hence, the photocatalysis can be performed under solar radiation to make system economical for removal of refractory organics compared to incineration treatment. Various metal dopants are Chromium, manganese, cobalt, copper, iron Nickle, Zinc, cerium, Neodymium, Eurotium, Lanthanum, etc. and various non-mental dopants are Palladium chloride, carbon, nitrogen, and Flouride.

Recyclability of Photocatalyst
TiO 2 doped with 33% Fe 2 O 3 core-shell photocatalyst has enhanced paracetamol removal by photocatalysis from water and the photocatalyst could be easily separated and reused for four recycle runs. 28 Ag decorated Fe 3 O 4 /TiO 2 coated cenosphere prepared via Modified sol-gel and wet impregnation can be recycled for 8 cycles with a slight reduction in Methylene blue degradation efficiency. 26 The novel engineered photocomposite core-shell structure Fe 3 O 4 @SiO 2 @TiO 2 showed greater photoactivity When semiconductors such as TiO 2 absorb light e-jumps from the vb to the cb. Nanoparticles have a large surface to volume ratio and also contain more atoms on their surface which substantially absorb photons. Nanoparticles can perform photocatalys is rapidly before e-and hole recombine. 17

Doping in Nano-Structured TiO 2 for enhanced photocatalytic activity
Doping is one of the methods to improve optical properties, reduce bandgap and overcome e-/hole recombination as metals trap e-result in enhanced photocatalytic activity of semi conductor oxides. Doping will provide efficient and economical photocatalysis as it can replace UV photocatalysis with solar or visible irradiations. Loading of TiO 2 surface with dopant will engineer the photocatalyst compared to commercial TiO 2 . The catalyst provided easy separability using a magnet and was recycled for 10 numbers of recycling runs without a decrease in efficiency. 22 When the Ag-Fe CT with Ti/Ag mole ratio 30 photocatalysts were reused for six numbers of runs, 63.25% COD was removed in 5 hr solar light irradiation, indicating more deactivation of the catalyst during photocatalysis; which represented that the Ag-Fe CT 30 could be recyclable effectively for 4 cycles. The reduction in % COD removal was only less than 5% after three runs of recycling for Ag-Fe CT 30. Ag-Fe CT 30 catalyst has proved its stability even after 4 recycle runs and it can perform photocatalysis under solar radiation effectively for the photocatalysis of drug intermediates. 16 Dye degradation efficiency by Fe 3+ doped TiO 2 has been found to decrease by 9% at the end of six recycle runs. 55 Ag-Fe CT and Fe 2 O 3 /SiO 2 codoped TiO 2 and Ag-Fe CT supported on graphene oxide has shown good stability for 5 recycle runs. 58 Table 4 summarizes the literature review done for the recyclability of photocatalysts. The photocatalysts can be recovered after treatment and efficiently used for several runs without loss in efficiency of treatment or component degradation. The result showed a decrease in photocatalytic activity with an increase in the number of recycling runs as the poisoning of the catalyst increases due to surface blockage, less adsorption and low rate of oxidation reaction. 7  NH 3 -N removal 88660 mg/L, NH 3 -N:3287 mg/L *NA: data not available Hybrid Advanced Oxidation processes COD removal using three methods, combining electrochemical process with AOP, Fenton reagent and flotation HAOP technology has been proved effective in the treatment of pharmaceutical wastewater for COD removal 69 . An ultrasound when used in combination with photocatalysis, Fenton Reagent and the Photolysis process, proved efficient for non-biodegradable toxic organics removal. This combination of AOP will overcome problems of repelling photocatalyst and pollutants due to similar charges. A sonophotocatalysis has been found effective for the removal of variety of organics present in wastewater. 70 Hybrid AOPs with sonolysis, Fenton and photo-ferrioxalate system with sonolysis has been studied for degradation of two dyes: Acid Red B and Methylene Blue. Sonolysis alone has shown the lowest efficiency. Coupling of sonolysis with either Fenton or photo-ferrioxalate system has shown the greater ability of decolorization. Ternary coupling of all these three systems has shown a negative effect of dyes degradation due to the interaction of individual mechanisms 71 . Table 5 summaries research done on hybrid advanced oxidation processes.