*3.1. Low Reaction Rates*

Due to the low reaction rates, for practical reasons, the photocatalytic technologies should be combined with other techniques such as ozonation [3,196], filtration technology [7], sonication [197–199], thermal activation [200–202]. For instance, Preda et al. [3] investigated the aqueous ammonia oxidation over iron-modified titanate nanorods by using combined treatment with ozone and simulated solar light irradiation. Increasing ammonia conversion was registered relative to the photocatalytic process carried out without ozone, but also the NO3 − formation was significantly reduced by comparison with the dark ozonation assays. The main achievement of this combined procedure was the increased selectivity of ammonia degradation to gaseous nitrogen-containing end products.

Denny et al. [7] reported the advantages of coupling the photocatalytic and filtration technologies in terms of particulate pollutants elimination together with VOC removal from the air stream. In order to enhance the photodegradation of gas phase ethanol to CO2 and to reduce the production of intermediates such as acetaldehyde, a fluidized bed aerosol generator (FBAG) was adapted to prepare TiO2-loaded ventilation filters as an irradiation source used a UV-light-emitting diode (UV-LED).

According to Adewui [197], photocatalytic processes assisted by ultrasound can be significantly improved and used for the treatment of pollutants in water, sonophotocatalysis (SPC) implying either sequential photocatalytic reaction and ultrasonic irradiation or simultaneous light and ultrasonic irradiation of the investigated system. The main advantages of sonophotocatalysis are clearly emphasized by this review [197]: the better transfer

of organic compounds from the bulk solution to catalyst surface, increase the dispersion of chemicals, an extra generation of hydroxyl radicals that are very efficient oxidizing agents as well as the photogenerated holes. The photocatalytic oxidation targeting the mineralization of intermediates such as carboxylic acids can be enhanced by complementary use of ultrasounds [197], many pharmaceuticals and dyes from wastewaters being subjected to sonophotocatalytic studies [198,199].

The "thermo-photo-catalysis" concept, which addresses both environmental and energy fields, was detailed by Nair et al. [200], revealing the main advantages of this dual approach: thermal acceleration of the photocatalytic reactions (known for the limited reaction rates), the occurrence of some photocatalytic processes in the visible-infrared domain (scarcely available in terms of hole/electron photoexcitation), the same efficiency for the pollutant removal as in simple thermocatalytic process but using lower temperatures.

A coupling of advanced oxidation/reduction processes and biological processes for water depollution have been investigated, including ozonation-, Fenton-, electrochemicalbiological processes, and also sequential chemical-biological processes [203]. The ozonation in the presence of UV irradiation has become one of the most used advanced oxidation processes for the degradation of organic compounds in general as acids, alcohols, and organochlorines of low molecular weight. Unfortunately, both UV and ozone are quite expensive to generate and need the consumption of large amounts of electric energy. Within the development and application of wastewater treatment technologies, should be taken into account efficiency, cost, and reliability. If the intermediate products obtained require additional removal, then the purification process becomes expensive and complicated. The combination with other technologies, for example, nanotechnology [204], can also be considered.

Andronic et al. [205] tested three different composites in a pilot plant for solar treatment of wastewater using phenol, imidacloprid, and dichloroacetic acid as model pollutants. The investigated photocatalysts were sol-gel TiO2 (as the reference), Vis-active CuxS prepared by photochemical precipitation, and highly filterable TiO2-fly ash mixture/composites. The experiments were conducted at a laboratory scale in two solar simulators and under natural solar irradiation at the pilot plant scale, at the Plataforma Solar de Almería, in a Compound Parabolic Collector (CPC) solar radiation system. The research group showed that the high phenol removal efficiency under simulated solar irradiation was attained by the reference TiO2. This behavior was due to titania's large surface area and its anatase/rutile phase composition. Contrarywise, under solar radiation in the CPC reactor pilot, all three pollutants were partially mineralized during the first 40–90 min, but by-products clog the surface, and removal continues without fully oxidizing the organic substrates. After 150 min of solar irradiation at the pilot–plant scale, the difference between the apparent phenol removal (55%) and mineralization (33%) confirms the presence of intermediates at the end of the reaction.

Combined photocatalytic processes proved to be also efficient for the abatement of larger molecule contaminants. Efforts were paid to search for the optimal technology for the removal of wastewater microcontaminants by coupling four different electrochemical processes with a solar CPC reactor at a pilot plant scale [206]. Thus, the anodic oxidation (AO), solar-assisted AO, electro-Fenton (EF), and solar photoelectro-Fenton (SPEF) processes were employed for monitoring the treatment of microcontaminats. This research group selected two different water matrices: one coming from a synthetic retentate with medium content of chlorides (in the range of 550 mg L−1, from natural water), while the other one was actual urban wastewater with a higher concentration of chlorides (in the range of 1200–2000 mg L−1), spiked with a mix of four microcontaminants (pentachlorophenol, terbutryn, chlorphenvinfos, and diclofenac). These combined technologies successfully removed contaminants of an actual urban wastewater treatment plant secondary effluent up to 80% of the total amount. However, the main disadvantage was that this process was not efficient for dissolved organic carbon (DOC) removal.

Coupling photocatalysis, catalytic, and photocatalytic ozonation, proved to be excellent processes for the abatement of a mixture of seven insecticides at a pilot plant scale [207]. The authors studied a complex mixture of pesticides (e.g., simazine, terbutryn, buprofezin, procymidone, azoxystrobin, imidacloprid, and thiamethoxam), and three sources of TiO2 (N-TiO2, pure TiO2, P25 Degussa) at bench and pilot plant scale to finally combine with ozonation looking for an increase in the degradation efficiency. Ozonation was demonstrated to be the most effective process for the abatement of the targeted mixture of microcontaminants.
