Abstract
This literature research, although not exhaustive, gives perspective to solar-driven photocatalysis, such as solar photo-Fenton and solar photocatalysis, reported in the literature for the degradation of aqueous organic pollutants. Parameters that influence the degradation and mineralization of organics like catalyst preparation, type and load of catalyst, catalyst phase, pH, applied potential, and type of organic pollutant are addressed. Such parameters may also affect the photoactivity of the catalysts used in the studied solar processes. Solar irradiation is a renewable, abundant, and pollution-free energy source for low-cost commercial applications. Therefore, these solar processes represent an environmentally friendly alternative mainly because the use of electricity can be decreased/avoided.
1. Introduction
The use of solar irradiation to detoxify contaminated effluents has recently increased and become of utmost importance for saving energy and to improve the performance of several degradation aqueous processes. Solar photocatalysis and solar photo-Fenton technologies are classified into advanced oxidation processes (AOP). The AOP involve the in situ generation of highly oxidizing radical species such as by using different energy sources like chemical, natural (solar), and artificial light energy. The hydroxyl radicals, , are capable of transforming toxic and persistent organic compounds into harmless end products ( and mineral acids) [1]. The interaction of solar irradiation with catalysts such as iron ions (), in photo-Fenton, and , in photocatalysis, increases the rate of pollutant degradation by the photochemical reactions produced within these processes.
These two AOP have proved to be very effective for degrading a vast aqueous organic contaminants such as emerging [2, 3], persistent [4–9], textiles [10–13], and bacteria [14–16] pollutants.
This literature research, although not exhaustive, gives perspective to solar photo-Fenton, solar photoelectro-Fenton, and solar photocatalytic processes reported in the literature for the degradation of aqueous organic pollutants. Solar irradiation is a renewable, abundant, and pollution-free energy source for low-cost commercial applications. Parameters that influence the degradation and mineralization of organics that may also affect the photoactivity of the catalysts used in the studied solar processes are addressed.
2. Fenton Processes
Fenton oxidation process consists of the reaction between and (1) in acidic solution to produce hydroxyl radicals , highly oxidant species responsible for pollutant degradation [17–19]:
Following a chain reaction [20–26]:
However, according to Pignatello and collaborators [19], reaction (8) is very slow compared to those involving species and can be neglected.
The reaction between and (2), referred to as Fenton-like reaction, produces less oxidant radical species (); nevertheless, both ferric and ferrous ion species are present simultaneously in the chain reaction (reactions (1)–(7) regardless of which is used to initiate the reaction. Thus, the can be catalytically decomposed by into oxidant radical species [19]. The homogeneous catalytic decomposition of by ferric ions may be also represented by [27–29]
The photo-Fenton process increases the amount of radicals with regeneration (11) by the photoreduction of , produced in (reaction (10)) which maxima concentration is at pH ~3.1 [28, 30]:
Thus, solar light irradiation (UV/visible irradiation) promotes photochemical reactions with light active intermediate species, such as ; reaction (11) produces hydroxyl radicals and regenerates which close the catalytic cycle of generation via reaction (1). This light enhancement has been explained mainly by photolysis of hydroxide complexes of (reaction (11)) and photochemical reactions of ligand complexes formed between and organic acids (ferric carboxylic complexes) such as oxalic citric acids [31, 32]. Nevertheless, the rate of photolysis of ferric carboxylic complexes can be several orders higher than that of hydrated ferric ions, (reaction (11)) [33, 34]. Enhancement of the photo-Fenton reaction with the use of these ligand complexes was attributed to their higher solubility and higher stability constant than that of iron-organic pollutant complexes and high photoactivity under UV-visible light by increasing the quantum yield for production [35].
Table 1 summarizes the experimental degradation of several organic contaminants by solar photo-Fenton. It is shown that the oxidation degree, or mineralization, depends on several parameters like the type of reactor used, pH, initial concentration of the organic contaminant, degradation time, among some others. The oxidation of the organic matter with time was followed by parameters like DOC, TOC, COD, and organic concentration decrease.
The main disadvantages of the Fenton process are (i) the pH of the solution (its optimum conditions is obtained at pH 2.8), (ii) sludge formation which depends on the amount and type of iron used, and (iii) H2O2 storage and handling and its associated cost.
2.1. The Electrochemical Fenton-Based (EF) Processes
The electrochemical Fenton-based (EF) processes, included in the electrochemical advanced oxidation processes, were developed to overcome the drawbacks of the classical Fenton (CF) process. These processes in situ generate the Fenton reagents ( and/or ) to yield radicals [36, 37]. The EF processes such as Fered-Fenton (EF-FeRe) [38, 39], electrochemical peroxidation/anodic Fenton (EF-FeOx) [40–42], electro-Fenton (EF--FeRe) [38, 43], and peroxi-coagulation (EF--FeOx) [44, 45] increase the efficiency of pollutant degradation. The efficiency of pollutant degradation is enhanced when UV light or solar irradiation is used in combination with CF (photo-Fenton or solar photo-Fenton) and EF (photoelectro-Fenton or solar photoelectro-Fenton) processes. Figure 1 schematically shows CF and EF processes: (1) CF: addition of both reagents to the solution, (2) EF-FeRe: addition of both reagents to the solution with the regeneration of from reaction (13), (3) EF-FeOx: generation of iron anions using a sacrificial iron anode with the addition of hydrogen peroxide, (4) EF--FeRe (the most common electrochemical Fenton-based process, known as electro-Fenton): generation of hydrogen peroxide and regeneration of initially added catalyst, (5) EF--FeOx: in situ generation of both hydrogen peroxide and iron anions, and finally, (6) CF and EF combined with UV light or solar irradiation.
The hydrogen peroxide is generated in situ from the two-electron oxygen reduction on cathodes such as reticulated vitreous carbon [46], gas diffusion electrodes [47], carbon felt [48, 49], pristine graphene [50], and boron-doped diamond [51] from reaction (12), and can be electrochemically regenerated, if it was initially added to the solution (reaction (13)) or electrochemically generated from a sacrificial iron anode (reaction (14)).
2.2. Solar Photoelectro-Fenton
The conventional photo-Fenton reaction showed a limited efficiency in the mineralization of real textile wastewater mainly associated to three factors [10, 35, 52–55]: (i) formation of iron-organic pollutant complexes with low solubility at acidic pH values (2.8), leading to iron precipitation and abatement of dissolved organic carbon, cannot be associated with mineralization; (ii) low photoactivity of the iron-organic pollutant complexes under UV-visible light; and (iii) photoreduction via a ligand-to-metal charge mechanism, which occurs on the surface of the iron precipitates, is very slow.
The photo-Fenton reaction can be enhanced by the following: (1)The use of different iron(III)-organic ligand complexes such as Fe(III)-oxalate, Fe(III)-citrate and Fe(III)-EDDS (EDDS: S,S′-ethylenediamine-N,N′-disuccinic acid tri-sodium salt). These ligand complexes have higher solubility and higher stability constant than iron-organic pollutant complexes which prevent iron precipitation that allows to achieve proper mineralization. The superiority of Fe(III)-oxalate over Fe(III)-citrate and Fe(III)-EDDS complexes can be mainly associated to the different quantum yield values for Fe(II) production and the half-life of the Fe(III)-organic ligand complexes under UVA radiation. The photo-Fenton reaction mediated by ferricarboxylates was characterized by an initial fast reaction rate, mainly associated to the fast regeneration of to , enhanced by the photodecarboxylation of ferricarboxylate complexes under UV-visible light, followed by a very slow reaction rate with a low consumption of due to the disappearance of oxalic acid and free iron species complex with other organic oxidation by-products which reduced substantially the dissolved iron concentration and consequently the reaction rate [35].(2)The use of electricity to drive the photo-Fenton process electrochemically (photoelectro-Fenton (PEF)) under sunlight [56]. The PEF process accelerates the degradation of pollutants because of (a) the enhancement of regeneration and production by the photolytic reaction (11) and (b) the photolysis of Fe-complexes with generated intermediates like carboxylic acids by reaction (15) [57–59]:
An interesting comparison of the performance of solar photo-Fenton and solar photoelectro-Fenton experiments, at laboratory scale, was carried out by Serra and collaborators [60]. These authors studied the degradation of the amino acid a-methylphenylglycine (a soluble and nonbiodegradable target pollutant) and observed that the solar-driven photo-Fenton is the most environmentally friendly alternative, mainly because of the use of electricity in the solar photoelectro-Fenton experiments [60]. Indeed, the solar photoelectro-Fenton (SPEF) process can be driven by a photovoltaic system when scale up to a pilot plant or solar plant to decrease the costs associated with the consumption of electricity, which may make sustainable the whole process. Having this in mind, a self-sustainable SPEF, driven by a photovoltaic cell, was designed to study the oxidation of the antibiotic trimethoprim (TMP), producing a high degree of mineralization. Tenaromatic intermediates generated from hydroxylation, carbonylation, and demethylation reactions were identified together with three carboxylic acids (oxamic, oxalic, and formic acids) and two inorganic ions ( and ). This study is a cost-effective approach for TMP degradation and demonstrated that the sunlight can provide the electric power (for mechanical systems) and the UV radiation for SPEF process [61].
Almeida and coworkers [62] studied the degradation of paracetamol by solar photoelectro-Fenton using a flow plant with a Pt/carbon-PTFE air-diffusion electrochemical cell (one-compartment filter-press reactor) coupled with a compound parabolic collector. The was electrogenerated from the cathodic reduction at the carbon-PTFE air-diffusion electrode (ADE). These authors reported 75% of total organic carbon (TOC) decrease with an energy cost of 93 kWh kg−1 TOC (7.38 kWh m−3) and 71% removal efficiency. 80% TOC removal was the maximum value acquired at 150 min, with conversion of 79% of initial N into ion. Thus, according to these authors, total mineralization of paracetamol was not attained because of the formation of derivatives and other undetected products that cannot be destroyed by radical and UV light. The same solar flow plant used by Almeida et al. was employed to oxidize azo dyes [63] and the antibiotic metronidazole [64] by electro-Fenton (EF) and solar photolectro-Fenton (SPEF) processes. Ruiz and coworkers observed that the EF process yielded rapid decolorization with poor TOC removal, and Pérez et al. also reported very weak mineralization of the antibiotic. These results were attributed to the formation of persistent carboxylic acids like oxalic acid as the major component of final electrolyzed solutions; conversely, quick photolysis of Fe(III)-oxalate complexes by UV light of solar irradiation explained the higher oxidation power of SPEF that gave almost total mineralization of the organic contaminants under study. It was described that the incident UV light played an important role in the photodecomposition of several N-derivatives that favored the release of ammonium ions and the loss of volatile N-compounds. Espinoza and coworkers [65] reported that the elimination of an diazo dye (acid yellow 42 dye) and its organic intermediates was due to hydroxyl radicals formed both at the anode surface from water oxidation and in bulk solution from Fenton reaction between electrogeneratd and added . The application of solar radiation in the photolectro-Fenton process yielded higher current efficiencies and lower energy consumptions than electro-Fenton (EF) and electro-oxidation, with electrogenerated by the additional production of hydroxyl radicals from the photolysis of Fe(III)-hydrated species (reaction (11)), and the photodecomposition of Fe(III)-complexes with organic intermediates (ferric carboxylic complexes, like ferric oxalate) that pass through an excitation state under the influence of photons of UV/vis wavelengths. This reaction is called the ligand-metal charge transfer reaction [66–68]:
Table 2 summarizes the experimental degradation of several organic contaminants by solar photoelectro-Fenton. It is shown that the oxidation degree, or mineralization, depends on several parameters like the type of reactor used, pH, initial concentration of the organic contaminant, degradation time, among some others. The oxidation of the organic matter with time was followed by parameters like DOC, TOC, COD, and organic concentration decrease.
2.3. Influencing Parameters on the Degradation of Organic Compounds by Solar Photoelectro-Fenton Process
Complete transformation of organic compounds to by the solar photoelectro-Fenton process depends directly on the applied current density, the concentration of catalyst, organic concentration, and the solar radiation intensity [65, 69, 70]. There are other parameters like pH of the solution, temperature, and electrode materials that are important too in the photoelectro-Fenton process; however, Fenton reaction is efficient at the pH interval of 2.5 < pH < 3 [7, 71–73], oxygen is needed to produce , and its solubility is affected by the temperature [74]; thus, it is better to perform the experiment at room temperature, and the electrode materials like carbonaceous cathodes [46, 48–50] and BDD anodes [56, 75] have been proved to be the best materials (to reduce oxygen and to produce radicals, resp.) so far.
2.3.1. Current Density Influence
The current density (or voltage cell) applied to the electrochemical cell for production in the photoelectro-Fenton process influences the electrode surface reaction kinetics and the extension of electrode reactions. Hydroxyl radicals are in situ generated in the photoelectro-Fenton process. Such oxidant species can be produced simultaneously via oxygen cathodic reduction (reaction (12)) in the presence of catalyst (reactions (1) and (8)) and on the anode boron-doped diamond (BDD) [75]:
enhances the destruction of aromatic pollutants and their aliphatic intermediates such as short-linear carboxylic acids [56]. Thus, the oxidation power of the solar photoelectro-Fenton is additionally enhanced by reactions (11) and (15) under the solar radiation.
Garcia-Segura and collaborators [76] claimed that at current densities > 33.3 mA cm−2, the removal rate of oxalic and oxamic acids was not improved because of the occurrence of parasitic reactions, such as nonorganic oxidizing reactions with hydroxyl radicals, which led to an efficiency, decrease at higher current densities. According to these authors, the parasitic reactions limited the mass transport of acids to the electrodes and increased largely the electric charge consumption. A drop-in TOC mineralization of the antibiotic sulfamethoxazole by photoelectro-Fenton which was observed when the current density reached 25 mA cm−2 was also reported [61, 77]. This drop was attributed to (i) an increase in the degree of cathode polarization as the current density increased, resulting in decomposition to on the anode [59]; (ii) reduction of to [78]; and (iii) present in the solution is anodically oxidized to , and is dimerized to (reaction (18)) [59]. It should be noted that the dimerization generally occurs at higher concentration of hydroxyl radicals, and such higher concentration is possible with the higher voltage (or current) applied [79].
Additionally, to these parasitic reactions, there is the oxidation of to (reaction (19)) together with reactions (3), (4), and (18) [65].
The quantity of may also decrease because of the increase in the formation rate of other weaker oxidants at the BDD anode, such as the peroxodisulfate () ion by oxidation of from the electrolyte (reaction (20)) and ozone generation (reaction (21)) [75]:
Thus, using an appropriated current density can prevent such side reactions and also avoid using high energy consumption.
2.3.2. Effect of the Concentration of Ferrous Ions
The catalyst concentration is an important parameter that limits the rate of Fenton reaction (1) to produce hydroxyl radicals and, therefore, affects the degree of mineralization of the organic contaminants. The decrease in the oxidation power of Fenton reaction is related to the competitive reaction between and (4) which can also be photolyzed [80], reaction (11).
There are experimental evidences that the decolorization rate of azo dyes dropped gradually with increasing the initial ferrous ion content [65, 70]. Espinoza et al. [65] observed that concentration of ≥ 2 mmol L−1 decreased the decolorization which was attributed to reaction (4); also, there were produced colored compounds that decreased the effect of solar radiation, and therefore, the removal of the dye was disfavored. Furthermore, there might have been the generation of unwanted reactions like the formation of complexes ((22) and (23)) that would have prevented the regeneration of or slowed the process [65, 70]:
Feng and collaborators [81] reported that the degradation rate of the anti-inflammatory drug ketoprofen decreased with increasing concentration up to 1.0 mmol L−1. Fast degradation rate of ketoprofen by electro-Fenton was achieved at concentration of 0.1 mmol L−1. Further increase in catalyst concentration resulted in the decrease of oxidation rate due to the competitive reaction between and (4).
2.3.3. Influence of Initial Concentration of the Organic Compound
Generally, an increase in contaminant concentration produces a decrease in the extent of the organic compound mineralization. The oxidation of high amounts of contaminant molecules by the hydroxyl radical species becomes slower since a number of intermediate species are also produced that demand more [61, 82–89], but a constant amount of hydroxyl radicals is produced during fixed experimental conditions used. Fryda and coworkers [90] claimed that the chemical oxygen demand (COD) reduction is a highly suitable parameter for wastewater treatment, in most cases independent of the class or organic pollutants. These authors stated that a COD content between 100 mg L−1 to 25 g L−1 is a cost-effective and efficient range for advanced oxidation processes.
Ammar and collaborators reported that slower abatement of the target compound was observed as its content increased [83]. This was attributed to the following: (i) the amount of generated continuously into the reactor was constant, and thus, it limited the disappearance of the organic pollutant, and (ii) when the organic matter in the solution is sufficiently high, a minor quantity of hydroxyl radicals attacks the pollutant molecules because more amounts of them destroy its intermediates. Similar results were reported by Samet et al., in which the removal of COD decreased as the initial COD content increased in the treated solution [89]; such results were observed using different molar ratios under solar photo-Fenton process.
2.3.4. Influence of Solar Radiation
The naturally occurring wavelengths from the solar radiation, mainly those from the UV and visible radiation wavelengths, produce different photochemical reactions. The solar light is expected to improve the performance of artificial UVA lamps since it provides photons in the UV range of 300–400 nm, as well as in the visible range of 400–650 nm which can also be absorbed by reactants in reactions (11) and (15), respectively [43, 65, 91–94]. There are several designs of solar reactors to concentrate and use efficiently the solar irradiation in the solar photo-Fenton and solar photoelectro-Fenton processes.
2.4. Combination of Solar Photo-Fenton and Solar Photoelectro-Fenton with Other Treatment Processes
The advanced oxidation processes (AOP) are environmentally friendly technologies and have shown the capability of mineralizing organic pollutants [9, 19, 35, 95–98]. Inside the AOP, the electrochemical advanced oxidation processes (EAOP) are versatile processes, like solar photoelectro-Fenton, that can produce hydroxyl radical species under controlled optimal conditions with the application of an electric current that enables high hydroxyl radical production [43, 61, 64, 65]. Hence, the hydroxyl radicals, produced by the solar photoelectro-Fenton, can be used initially to oxidize recalcitrant organics and transform them into nontoxic biodegradable species; then, such intermediate compounds can be treated by the biological treatment used as posttreatment process. An anaerobic biological treatment followed by solar photoelectro-Fenton process to oxidize organic matter present in slaughterhouse wastewater from a Chilean meat company was investigated [91]. It was observed that the combination of both processes produced a totally clarified, odorless effluent, without solids in suspension and higher chemical oxygen demand removal than the one from the separate processes.
It has also been shown that the solar photo-Fenton can be used as a pretreatment for the secondary treatment process. Ballesteros and coworkers [88] proposed a strategy for treating water with high pesticide concentrations to overcome the low biodegradability of solar photo-Fenton intermediates; such strategy consisted of mixing the contaminated water with a biodegradable carbon source before biological oxidation. Hence, this combination of photo-Fenton and acclimatized activated sludge in several SBR cycles led to complete biodegradation of a concentrated pesticide solution of 500 mg L−1 dissolved organic carbon (DOC) in ∼5 h with a carbon removal efficiency of 90%. According to García-Montaño and coworkers [99], the Fenton process at pilot plant can be used as a biological pretreatment. Since after certain time and under experimental conditions, treated dye solutions became enough biocompatible to be fully biodegraded in the IBR reactor, attaining residual DOC values close to the 20 mg L−1 corresponding to biomass metabolites.
There are several papers devoted to oxidize many organic contaminants (like microcontaminants, herbicides, textile dyes, landfill leachates, and emergent contaminants) by solar photo-Fenton and solar photoelectro-Fenton (as depicted in Tables 1 and 2) using solar compound parabolic collectors and solar pilot reactors [2, 52, 53, 82, 88, 100–108]. Most of these studies claimed that photo-Fenton enhanced the degradation of the organic compounds achieving over 50% TOC removal. Also, bacteria inactivation by solar photo-Fenton at near-neutral pH using a CPC solar reactor led to a simultaneous decrease of TOC (55%) and total E. coli inactivation with the absence of bacterial regrowth during 24 h in the dark [103].
The solar photo-Fenton degradation of the 2,4-dichlorophenoxyacetic acid (2,4-D) herbicide in solar pilot-plant reactors was carried out by Conte et al. [100]. These authors proposed a kinetic model to predict the reactant concentrations during the degradation and reported that the solar reactor was able to reach complete degradation of both the 2,4-D and the main intermediate (2,4-dichlorophenol) at 60 min with 98.9% of total organic carbon (TOC) conversion at 210 min.
Durán and collaborators [52] observed that the solar photo-Fenton in the presence of oxalic acid (solar photo-Fenton-ferrioxalate) increased the degradation rate of reactive blue 4 since ferrioxalates strongly absorb a higher portion of the solar spectrum. The addition of oxalic acid increased the operational costs but reduced the pH of the solution. Thus, under optimum conditions, a 66% of TOC elimination with total discoloration and total chemical oxygen demand (COD) removal was achieved. High enhancement of the biodegradability of textile synthetic wastewater with the use of oxalic acid in the solar photo-Fenton process (ferrioxalate) was also reported [53]; in such study, the iron precipitated when over 70% of the initially added oxalic acid was photodecarboxylated. Clearly, the ferric-organic ligand complexes prevent iron precipitation, produce a higher amount of hydroxyl radicals (allowing to use efficiently the solar irradiation), and permit to achieve proper mineralization [35].
Table 3 summarizes the degradation of several organic contaminants at pilot scale using sand filter and microfiltration as pretreatment for the solar photo-Fenton process. Also, it reports the solar photo-Fenton process as pretreatment of the secondary treatment process. It is interesting to note that the volume treated was relatively high and that the mineralization achieved depended on the initial experimental conditions, degradation time, type of contaminants, and their initial concentration. The oxidation of the organic matter with time was followed by parameters like DOC, TOC, and organic concentration decrease.
3. Solar Photocatalysis
has proven to be one of the most promising photocatalysts amongst several metal oxide semiconductors because of its high reactivity, chemical stability, low cost, and nontoxicity [109]. photocatalyst has been mainly used widely in environmental remediation (i.e., wastewater detoxification) and solar energy conversion [96, 110]. (band gap energy 3.2 eV) upon illumination with UV light (λ < 380 nm) produces excited high-energy states of electron (in the conduction band, ) and hole (in the valence band, ) pairs, reaction (24), capable of initiating chemical reactions [111], as shown in the photocatalysis mechanism (Scheme 1).
The holes are responsible for the degradation of organic compounds and can react with water (reaction (25)) or hydroxyl ions (reaction (26)) to produce hydroxyl radicals; though, the recombination of the / pair (reaction (27)) and the reduction of radicals (reaction (28)) produce large inefficiencies in the photocatalytic process [112].
The oxidation of organic pollutants (R-H) can take place by a direct reaction with holes (reaction (29)) or indirectly via hydroxyl radicals (reaction (30)) or free organic radicals (reaction (31)) [109, 111]:
The hydroxyl radicals, generated under ambient conditions in this process, are nonselective powerful oxidant species capable of converting organic pollutants (toxic and nonbiodegradable) into relatively innocuous end products such as , and mineral acids [1].
3.1. Photocatalytic Activity of TiO2 under Sunlight
semiconductor is mainly photoactive with artificial UV lamps that increase the electrical cost of the photocatalytic process. The sunlight is expected to improve the performance of photocatalysis since it provides photons in the UV range (300–400 nm); also, doping extends its photoresponse capacity towards the sunlight visible range (400–700 nm). The chemical surface alteration of (when doped or photodoped with transition metals, noble metals, nonmetals, organic dye sensitization, and coupled semiconductors) modifies its electronic structure producing changes in the electron densities in the conduction band [113] which enhances the solar efficiency of under solar irradiation and, hence, influences the mechanism of the photocatalytic process. Dopants can act as electron (or hole) traps preventing charge recombination, create an intermediate energy level introduced between the conduction band and the valence band, lead to band bend gap narrowing or expansion, and induce band bending or conduction band edge shifts which occur due to disruption of the lattice structure of the dopants and formation of surface oxygen vacancies [113–116]. Introduction of such energy levels in the band gap induces the red shift in the band gap transition and the visible light absorption through a charge transfer between a dopant and conduction band (or valence band) [112]. Nevertheless, these changes depend on the nature of dopants, synthesis methods, annealing process, dopant concentration, and dopant distribution [11, 112, 113, 117–120].
3.2. Influencing Parameters on the Degradation of Organic Compounds by TiO2 Solar Photocatalysis
Mineralization of organic compounds (conversion to and mineral acids) by the solar photocatalytic process depends directly on the synthesis method of the catalyst [121–126], type of doping materials [11, 112, 113, 117–120], annealing temperature [127–131], catalyst load [132–135], pollutant concentration [136–139], and pH of the aqueous solution [140–142]. There are other parameters like reaction temperature, reactor configuration (geometry), reactor material, air sparging in the reactor solution, and the solar radiation intensity (solar concentration ratio) [143] that are important too in the solar photocatalytic process. Nevertheless, these last factors contribute in less extension to the degradation of the pollutants compared to the most commonly studied such as catalyst load, pollutant concentration, and pH of the aqueous solution.
3.2.1. Effect of TiO2 Annealing Temperature
The influence of annealing temperature on the microstructure, surface morphology, and optical property of particles, nanoparticles, and thin films strongly depends on the synthesis methods [144–149]. Three different phases of have been reported which depend on the heat treatment like anatase, rutile, and brookite, anatase and rutile phases being commonly prepared [8, 9, 11, 145, 149]. It has been reported that anatase is more photoactive than rutile [1, 109, 150, 151]; however, it has also been reported that synergistic effects of mixed phases of anatase and rutile enhances the photoactivity of the photocatalyst [152–155].
3.2.2. Influence of Catalyst Concentration
Catalyst load is an important factor that can affect the degradation rate of the organic pollutants; the optimum amount depends on the nature of the organic compound and on the geometry of the photoreactor [156–158]. When concentration increases from 0 to certain values, a significant enhancement of efficiency is recorded for the elimination and mineralization of organics. However, there is a loading range in which either no significant changes are observed or even a decrease in the elimination of organic compounds happens. The number of active surface sites increases with concentration until certain loading since light penetration decreases and aggregation of particles increases at relative high catalyst loading [159]. It has been reported that photocatalysis combined with ultrasound can be benefited [72, 97, 159, 160] due to different factors like (a) an increase in the catalyst surface area due to the aggregation action of ultrasound, which enhances the performance of the photocatalytic system [161]; (b) an improvement of mass transfer of organic compounds between the liquid phase and the surface [162, 163]; and (c) a reduction of charge recombination and promotion production of additional due to the residual generated [162, 163]. Also, the continuous cleaning of the surface by acoustic cavitation [164] might also have some role in modifying the photocatalytic rate. Tables 4–6 show that different amounts of catalyst ranging from 10 mg L−1 to 4 g L−1 yielded different degrees of organic degradation during the photocatalytic oxidation process under the experimental conditions stated.
3.2.3. Influence of Initial Pollutant Concentration and pH of the Solution
The initial concentration of the organic compounds at constant pH and different initial pH of the solution are also important factors that affect the degradation rate of the organic pollutants and the efficiency of the photocatalytic oxidation process.
As the initial concentration of organics increases, the photocatalytic efficiency decreases due to a decrease of the reaction rate, which is attributed to a decrease in the number of photons and particles available, since the number of organic molecules adsorbed onto the catalyst may block catalyst activation. Also, the initial pH of the solution influences the rate of the degradation process. It has been reported that the adsorption of the organic compounds onto the surface is affected by the pH of the solution [97, 160]. The pollutant and thus the rates of degradation will be maximum near the point of zero charge (pzc) of the catalyst [164]. The pzc of surface is at pHpzc between 6.25 and 7.1 [1, 165, 166], depending on the ionic strength [166]. Tables 4, 5, and 6 show that different initial pollutant concentration and solution pH were used to degrade different organics during the photocatalytic oxidation process under the experimental conditions stated. Most of the experiments were carried out in the pH range of 2.0–14, and some were with no pH control, as depicted on Tables 4–6.
3.3. Organic Matter Degradation by TiO2 Solar Photocatalysis
The sunlight can be used efficiently in the photocatalytic process with the use of solar reactors that concentrate the solar irradiation. Thereby, solar photocatalysis is cost-effective and environmentally friendly alternative.
The use of CPC solar photoreactors was shown to be effective for solar photocatalytic degradation of several organic contaminants such as antibiotics [167], azo dyes [168], water disinfection [169, 170], pesticides [8, 171], humic acids [172], agro-food industry effluents [173], and ferricyanide [174].
Pereira and coworkers [167] studied the solar photocatalytic degradation of the antibiotic amoxicillin in aqueous solutions at neutral conditions (pH 7.5). These authors reported that the solar UV radiation alone was unable to attack the antibiotic molecules during the same treatment period as in the solar photocatalysis. They observed that the antibiotic concentration was reduced from 40 to 3.1 mg L−1 (at 4.6 kJUV L−1 of UV accumulated energy in the CPC) by solar photocatalysis which led to a considerable reduction of the antibacterial activity with 71% mineralization at the end of the treatment. They attributed such findings mainly to hydroxyl radicals, although singlet oxygen also played an important role in the antibiotic amoxicillin self-photosensitization under UV/visible solar light.
The combination of solar photocatalysis with and persulfate ions , as electron acceptors, for the degradation of azo dyes using CPC reactors was studied [168]. It was reported that the presence of hydrogen peroxide did not affect appreciably the photodegradation rate of methyl-orange but the presence of was strongly beneficial. It was also reported that the presence of both and was always beneficial for orange-II degradation. Complete decolourization was achieved in a few hours for both dyes but mineralization occurred after longer times with the formation of , and ions. Jiménez-Tototzintle and coworkers [173] added to the solar photocatalytic CPC reactor to treat pesticides (biorecalcitrant compounds) for improving supported photocatalysis efficiency. It was reported that nearly complete removal of pesticides was acquired avoiding expensive separation of slurries from treated wastewater.
Water disinfection by solar photocatalysis in a solar reactor was also studied in simulated wastewater containing E. coli K-12 [169] and urban effluent contaminated with E. coli and F. solani spores [170]. Fernández et al. [169] observed that bactericidal deactivation by sunlight in a CPC solar collector occurred with and without catalyst. The total photocatalytic deactivation of pure E. coli suspensions was a consequence of the combined effect of sunlight and the oxidant species generated in the in suspensions and or by supported . The slurry was more efficient for bacteria deactivation than the supported in the solar photocatalytic process. García-Fernández and collaborators [170] reported that the temperature and the dissolved oxygen, the most commonly used electron acceptor, were key factors for water disinfection. Thus, changes in temperature of photocatalytic reactors for water disinfection should be considered to evaluate the efficiency of the process. Also, air sparging caused improvements in the bacterial and spore photocatalytic inactivation, which was specifically manifested in F. solani spores. This was in line with the important role of dissolved oxygen in photocatalytic water disinfection.
Arellano and Martínez [174] investigated the effects of pH on the degradation of aqueous ferricyanide by photolysis and photocatalysis in the solar CPC reactor. These authors revealed that upon sunlight exposure, the conversion of ferricyanide to ferrocyanide and the reverse reaction depended strongly on the pH of the solution whether or not the catalyst was present. Thus, the pH of the solution dictated the type of redox reaction that would proceed under illumination. Additionally, the extent of the heterogeneous photocatalytic degradation of ferricyanide was influenced by pH, but the initial concentration of ferricyanide did not affect its degradation.
Tables 4 and 5 summarize the degradation of several organics by solar photocatalysis in which suspended or immobilized was used, respectively. As can be seen, the degradation and mineralization were taken place in several solar reactors under different experimental conditions in which the oxidation of the organic matter with time was followed by parameters like DOC, TOC, COD, and organic concentration decrease. Also, the efficiency of the solar photocatalysis depended on the suspended or immobilized catalyst phase, nature of organic contaminant, solution pH, reaction time, and the initial concentration of the organics. Thus, these studies have shown that sunlight was used efficiently in the solar photocatalytic process acquiring high oxidation yields.
In order to take advantage of the solar photons in the visible range (400–700 nm) and make the degradation process more efficient under the sunlight, some studies were conducted using doped catalyst, as shown in Table 6. Solar photocatalysis using nitrogen-doped , carbon-doped , polypyrrole- nanocomposites, and with persulfate ion addition were conducted to oxidize diverse organics. These studies have shown better performance of the solar photocatalytic process using doped catalyst and higher organic mineralization. The efficiency of the solar photocatalysis using carbon-doped was ascribed to the presence of oxygen vacancy state between the valence and the conduction bands because of the formation of species in the as-synthesized carbon-doped titania [175]. Herrmann and coworkers [176] degraded the 4-chlorophenol compound (4-CP) by solar photocatalysis using carbon-doped . These authors reported that the addition of a commercial activated carbon to titania under UV irradiation could have induced a substantial synergy effect in the photoefficiency of the photocatalyst. It had been explained by an important adsorption of 4-CP on activated carbon (AC) followed by a mass transfer to photoactive titania. According to these authors, such transfer occurred mainly via a spillover of 4-CP through the contact surface between AC and . This interface was spontaneously created by a mere mixture of both phases in suspension. The simple mixture of the two solids had permitted to avoid a preparation procedure which would have modified the surface of both constituents and prevented a direct comparison from their initial surface state. These authors stated that the synergy effect of the AC could be extrapolated to a large solar pilot plant scale, working in the near UV fringe of sunlight.
4. Scaling-Up of the Solar Processes
The application of solar-powered photoreactors for wastewater treatment represent an environmental alternative specially for regions receiving strong sunlight throughout the year. The solar photoreactors decrease the operating, capital, and maintenance costs associated to photocatalytic reaction systems that are powered with UV lamps. Thus, an appropriate design of solar photoreactors is important to ensure efficient conversion of incident photons to oxidant reactive species such as and holes, among some others [177]. According to Sagawe et al., several parameters need to be addressed for the design of field-scale photoreactors such as pollutant concentration (COD initial load), volumetric flow rate, light intensity, and solar irradiation area; also, the concentration of dissolved oxygen in water, in equilibrium with atmospheric air, defines the limits of all practical oxidizing processes for removing pollutants in photocatalytic reactors [178].
The variability in both the composition and concentration of real wastewater effluents in addition to the solar photon flux variation during the degradation process makes more difficult the field-scale reactors and also increases the cost. Soares et al. made an economic assessment using UVA-vis radiation (solar light) or UVC radiation (lamps) based on the operation variables obtained for the treatment of 30,240 m3 per day of a textile wastewater after a biological preoxidation process (to achieve the values required by German regulations for discharge into water bodies); from this study, the following conclusions were withdrawn: (a) at acidic pH values, the addition of iron to the UVC/ reaction enhanced the decolourisation rates, resulting in a lower treatment cost when compared to neutral pH conditions; (b) the UVC/ system at natural wastewater pH is cheaper than at acid pH, mainly due to the need for acids and bases; (c) the use of solar radiation (UVA-vis) increases the treatment costs, mainly due to an increase in the capital spending associated with CPC; (d) the addition of oxalic acid to the photo-Fenton reaction decreases the capital spending (high reaction rates), but the cost of consumables increases significantly, making the treatment more costly; and (e) the photo-Fenton system mediated by ferrioxalate at near neutral pH using solar radiation is the most costly treatment process, due to the low decolourization rates and high consumption of reactants, especially oxalic acid [179].
Silva et al. proposed a methodology for the treatment of landfill leachates, after aerobic lagooning and adjusted at pilot scale. Such methodology involved an aerobic activated sludge biological preoxidation, a coagulation/sedimentation step and a photooxidation through a photo-Fenton reaction combining solar and artificial light [108]. These authors reported that the was the reactant that most contributed to the final treatment cost (~42%), while the sulphuric acid and the ferrous iron were the chemicals that contributed less (~1%). Finally, the combination of solar and artificial radiation, taking into account the energetic needs throughout the year, showed to be the best alternative to treat 100 m3/day of leachate targeting a COD value lesser than 150 ppm and 1000 mg O2/L.
Jordá et al. performed an economic evaluation of the photo-Fenton process to degrade paracetamol in water to form biodegradable reaction intermediates which can be finally removed with a downstream biological treatment [180]. These authors observed that the key feature was to find the shortest chemical treatment time necessary to reach a sufficiently high biodegradability of the treated water which minimized the cost of the overall process.
A comparison of solar photocatalysis and solar photo-Fenton for the treatment of pesticides from industrial wastewater was carried out to assess the viability of solar heterogeneous photocatalysis (UV/) and photo-Fenton (UV//) for the treatment of agrochemical industrial wastewater at lab scale using CPC reactors. Also, the operational conditions, kinetics, and treatment costs were estimated [181]. Their results showed that the cost of photo-Fenton process was more economical than (UV/) photocatalysis and (UV//) processes. In addition, the maximum COD removal (90.7%) by photo-Fenton process was higher than that obtained by photocatalysis (79.6%) and the cost was 34.8% more economical, which was considered the best solution concerning the process performance. Finally, these authors observed that the variation of operating costs mainly depends on the type and dosage of chemicals.
Zapata et al. developed a strategy targeting the design of an industrial scale combined solar photo-Fenton/aerobic biological system for the decontamination of commercial pesticides [182] using CPC reactors. These authors recommended that for adequate design of this type of treatment, a detailed study should be done for each particular case using different analytical tools and bioassays, mainly dissolved organic carbon, chemical oxygen demand, toxicity, and biodegradability. It was demonstrated that the dose was a critical parameter for the design and control of operation of the combined system. It also remarked the importance of the wide difference in efficiency found in the biological treatment stage when the integrated system was scaled up. Based on their results, these authors highlighted how decisive and restrictive (for the treatment of very specific and variable wastewater) start-up and growth could be in implementing a new biological reactor, especially when the biotreatment is performed in an immobilized biomass reactor which is much more resistant than a conventional sequencing batch reactor. Their observations highlighted the limitations of the results obtained in small laboratory devices and working with model wastewaters to design a proper treatment [182].
Chong et al. [183] published an interesting review on the research and development progresses of engineered photocatalysts, photoreactor systems, and the process optimizations and modellings of the photooxidation processes for water treatment. These authors mentioned several key technical constraints ranging from catalyst development to reactor design and process optimization that have to be addressed, such as (a) catalyst improvement for a high photoefficiency that can utilize wider solar spectra, (b) catalyst immobilization strategy to provide a cost-effective solid/liquid separation, (c) effective design of solar photoreactors because of the low efficacy design of current solar collecting technology (0.04% capture of original solar photons), and (d) the need of a large scale, solar-driven, photocatalytic treatment process with high efficacy and low site area requirements with different possible pilot plant configurations.
5. Concluding Remarks
Solar-driven photocatalysis, as in the processes of solar photo-Fenton and solar photocatalysis, is an environmentally friendly alternative mainly because the use of electricity can be decreased/avoided. Indeed, the solar-driven processes can be driven by a photovoltaic system when scale up to a pilot plant or solar plant to decrease the costs associated with electricity consumption. These two solar photocatalytic processes represent powerful tools for removing organic pollutants from aqueous solution. Total mineralization of diverse organics has been achieved under different experimental conditions. Mineralization of organics with these solar processes depends on the nature of the contaminants and on other parameters such as solution pH, catalyst amount, and initial concentration of organics. Thus, to yield high mineralization, each process requires optimal experimental conditions of such parameters. However, in order to take advantage of the solar light in these solar photocatalytic technologies, photoreactors need to be specifically designed for these purposes. The literature reports the use of different types of reactors to treat wastewater by these solar-driven processes, but some are difficult to scale up.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
The authors are grateful to the Ministry of Science and Technology (CONACyT), Mexico, for the grant given to A. G. Gutierrez-Mata and S. Velazquez-Martínez.