solar photocatalytic degradation of water and air pollutants: challenges and perspectives

Solar Energy Vol. 66, No. 2, pp. 169–182, 19991999 Elsevier Science Ltd

Pergamon PII: S0038 – 092X( 98 )00120 – 0 All rights reserved. Printed in Great Britain0038-092X/99/$ - see front matter

www.elsevier.com/ locate / solener

SOLAR PHOTOCATALYTIC DEGRADATION OF WATER AND AIRPOLLUTANTS: CHALLENGES AND PERSPECTIVES

´ ´†MANUEL ROMERO , JULIAN BLANCO, BENIGNO SANCHEZ, ALFONSO VIDAL, SIXTOMALATO, ANA I. CARDONA and ELISA GARCIA

CIEMAT, Avda. Complutense 22, E-28040, Madrid, Spain

Revised version accepted 25 September 1998

Abstract—Solar photocatalytic oxidation processes (PCO) for degradation of water and air pollutants haverecently received increasing attention. Some field-scale experiments have demonstrated the feasibility of usinga semiconductor (TiO ) in solar collectors and concentrators to completely mineralize organic contaminants in2

water and air. Although successful pre-industrial solar tests have been carried out, there are still discrepanciesand doubt concerning process fundamentals such as the roles of active components, appropriate modelling ofreaction kinetics or quantification of photoefficiency. Challenges to development are catalyst deactivation, slowkinetics, low photoefficiency and unpredictable mechanisms. The development of specific non-concentratingcollectors for detoxification and the use of additives such as peroxydisulfate have made competitive use ofsolar PCO possible. The challenges and perspectives of solar driven PCO as illustrated in the literature and ourown results in large solar field loops at the Plataforma Solar de Almeria and CIEMAT laboratories aredescribed. 1999 Elsevier Science Ltd. All rights reserved.

1. INTRODUCTION out that low hydroxyl radical production ef-ficiency and slow kinetics are important barriers

Detoxification has recently become the mostto marketing the technology, since they may limit

successful solar photochemical application, witheconomic feasibility of the process. Unless the

some relevant field installations in operation.cost of solar components and/or catalyst efficien-

Since Carey et al. (1976) published their resultscies change drastically, applications should be

on PCB decomposition by illuminating aqueousrestricted to certain specific processes like the

TiO suspensions more than 20 years ago, and2 removal of low-ppm concentrations of such high-Ibusuki and Takeuchi (1986) reported photo-

ly toxic compounds as pesticides in water (mi-degradation of toluene in air by irradiating TiO2 cropollutants) (Vidal et al., 1997), or the elimina-at ambient temperature 10 years ago, references

tion of specific organometallic compounds likeand related patents on heterogeneous photo-

phenylmercury or EDTA 1 Ag in which the ratecatalytic removal of toxic and hazardous com-

of destruction is improved by the organic 1pounds from water and air have multiplied by

metal combination (Prairie et al., 1993). Althoughthousands, on a wide variety of applications and

development is difficult, application to industrialtarget compounds (Blake, 1997). Reports on solar

wastewater is feasible, with the addition of aphotocatalytic detoxification processes (Goswami, 5sacrificial reagent like S O . Organic concen-2 81995) and related subjects, such as fundamental

trations of several hundred ppm have been elimi-chemistry (Bahnemann et al., 1994), photoef-

nated from water with this method (Malato et al.,ficiencies (Serpone, 1997), target contaminants

1996). Economics are far from definitive, but may(Legrini et al., 1993) and catalysts (Hermann,

be considered at least pre-competitive (Blanco et1995) are found in the literature.

al., 1998). Gas-phase degradation is even moreBut even with this dramatic increase in interest

unclear, since there are many conventionaland successful photocatalytic oxidation (PCO)

catalysts available on the market that can work atdemonstration projects, the truth is that pre-in-

temperatures slightly over 1008C. Consideringdustrial applications still involve unsolved fun-

that solar technologies can easily achieve suchdamental questions concerning their chemistry. A

temperatures, solar thermal catalysis might be arecent publication by Parent et al. (1996) points

more efficient degradation process than pure PCOat ambient temperatures. Finding a niche for gas-phase solar PCO for air purification is therefore

†Author to whom correspondence should be addressed. not a trivial matter. Significant photoeffect has

169

170 M. Romero et al.

been demonstrated in a few chlorinated com- benzoic acid, with 2,5-furandione and 1,3-iso-pounds, however results with most typical VOCs benzofurandione intermediates (Blanco et al.,are not encouraging. Although, as in water, 1996). Benzaldehyde from oxidation of tolueneresidence times are high, some air purification by UV-irradiation of TiO had already been2

applications are under development (Watanabe et reported by Ibusuki and Takeuchi (1986) and ital., 1993). has recently been suggested that benzoic acid

Some of the critical challenges to eventual promotes deactivation of the catalyst with thisdeployment of solar PCO are described here in contaminant (Luo and Ollis, 1996). This mecha-detail with reference to own results as related to nism, based on proposals by tropospheric airthe literature. pollution researchers, accounts for the high yields

of benzaldehyde formed during the photocatalyticreaction. First benzyl radicals are produced by

2. MECHANISMS AND KINETICS: A abstraction of H atoms from the methyl group by?PERMANENT SUBJECT OF STUDY OH radicals. The final transformation of benzylradicals into benzaldehyde strongly suggests theMuch of the literature analyzing mechanismspresence of benzyloxyl radicals (Akimoto et al.,contains hypotheses on the involvement of photo-

1 1978), as depicted in Fig. 1. This transformationproduced holes (h ) or assumptions on surface-? must originate with oxygen and an additionaltrapped hydroxyl radicals ( OH) (Hoffmann et al.,

redox reaction. Sephson et al. (1984); Killus and1995). The complex primary events affectingWhitten (1982) report that in smog chamber tests,band-gap irradiation of TiO particles have been2substituted furans are produced by the in-studied with detailed laser-flash photolysis mea-tramolecular H-atom transfer reaction in the initialsurements (Bahnemann et al., 1997). These

? radicals, leading to ring cleavage.events, which yield OH radicals, are usuallyWe have reported the same kind of hydroxy-summarized in three steps in which oxygen is

2 lated intermediates for another aromatic com-often the electron acceptor and available OH andpound (ethylbenzene) in water (Vidal et al.,H O are electron donors.21994a). Ethybenzene photo-oxidation leads to

2 1 intermediates like 4-ethylphenol, acetophenone,TiO 1 hn → e 1 h (1)2 cb vb

2-methylbenzyl alcohol, 2-ethylphenol and 3-?1 2 ? ethylphenol. Again H-atom abstraction by OHh 1 OH → OH (2)vb

radical attack on the ethyl group is presumed to2 2? be the first step in the formation of hydroxylatede 1 O → O (3)cb 2 2 species and further ring cleavage (Fig. 2).

Chemical analysis shows the formation of hy- We were also able to demonstrate the interven-?droxylated intermediates which, in many cases, tion of OH radicals in more complex structures,

coincide with those found in other reactions including heteroatoms. During the degradation ofproduced by the attack of hydroxyl radicals in compounds containing sulfur-like thiocarbamateshomogeneous phase, as in photo-Fenton reactions (EPTC), the formation of -OH, -CHO and a C=Oin water or the hydroxyl radical attack on organic group containing thiocarbamate derivatives, wascontaminants photocatalyzed by smog, as de- observed in water. One of these formyl EPTCscribed in tropospheric studies (Killus and Whit- derivatives detected, S-ethyl-N-formyl-N-propylten, 1982). thiocarbamate, (Vidal et al., 1994b), has been

During our tests with the degradation of toluene reported in the literature on atmospheric OHin air we have detected only benzaldehyde and radical studies.

Fig. 1. Mechanism proposed for benzaldehyde and benzoic acid formation during gas-phase photocatalytic oxidation of toluenewith a monolithic catalyst based on sepiolite /TiO . First benzyl radicals are produced by abstraction of H atoms from the methyl2

?group by OH radicals. The final transformation of benzyl radicals into benzaldehyde strongly suggests the presence ofbenzyloxyl radicals.

Solar photocatalytic degradation of water and air pollutants: challenges and perspectives 171

Fig. 2. Hydroxylated cyclic intermediates detected during photocatalytic degradation of ethylbenzene in water with TiO Degussa2?P-25. H-atom abstraction by OH radical attack on the ethyl group leads to 4-ethylphenol, acetophenone, 2-methylbenzyl alcohol,

2-ethylphenol and 3-ethylphenol.

Even with the general evidence of the forma- oxidation of air contaminants by chlorine radicals,tion of hydroxylated by-products, the fundamen- hydroxyl radicals and holes conducted by d’Hen-tals of the roles of the oxidizing holes, hydroxyl nezel and Ollis (1996) concludes that in theradicals or spectators such as chlorine, continue to presence of chlorine atoms, it is the chlorinebe controversial. Some authors claim that in their radical that appears to be active on the surface.experiments, the substrates were directly oxidized Their results reveal that in the absence of chlor-by holes. Goldstein et al. (1994) suggest that at ine, hydroxyl radicals are not the reaction site, buthigh concentrations of phenol (.0.5 M) photo- most probably, either a hole and oxygen vacancyoxidation is directly conducted by holes. This or a dioxygen anion.dependence on substrate concentration is sup- However, other authors think that both the

? ?ported by others (Sun and Bolton, 1996). How- mechanistical Cl and OH approaches are essen-ever, it is in gas-phase where there is greater tially correct and that the eventual pathway iscontroversy on the role of the oxidants due to the dependent on the characteristics of the catalyst,contradictory results obtained by different authors, e.g., internal surface (Yamazaki–Nishida et al.,depending on the operating parameters, type of 1996). According to this second explanation,contaminant and their concentrations. Involved in when the inner catalyst surface is large enough,

?this controversy are the typical VOC chlorine the process continues with OH, since the porosityradicals, as for example, the contradictory results of the catalyst is sufficient to trap chlorine radi-in the degradation of the relatively well-known cals and avoid the formation of chlorinated by-

?TCE (trichloroethylene). Nimlos et al. (1993) products ( Cl-initiated reactions seldom occur onargue that the high quantum efficiency of TCE the catalyst surface), but when the inner surface isdegradation in air is promoted by chlorine radi- small, the chlorine radicals can escape and reactcals. Tests with physisorbed chloromethane and with the organics in the bulk gas.oxygen suggest the dioxygen anion as another key Our experience with TCE in air streams hasactive species (Lu et al., 1995). A study of the shown chloroform and pentachloroethane to be

´mechanistical correlations in the photocatalytic the main by-products (Sanchez et al., 1997). In


some situations (very low residence times and have become a useful tool for engineers and solarhighly concentrated analytical samples), traces of designers, provided the same kind of collector andphosgene and dichloroacetyl chloride could also photoreactor are used.be identified. The mechanism might be as de- Normal laboratory procedure for obtaining ratescribed in Fig. 3, where chlorine radicals take the constants is to fit the organic substrate concen-lead. The by-products may be explained by the tration plotted over time. Our experience withproposal of Sanhueza and Heicklen (1975), in large solar fields has demonstrated the need to usewhich homogeneous-phase attack by chlorine the photon flux (l,387 nm) incident on theradicals follows a chain reaction. The mechanism receiver as the independent variable instead ofand by-products in Fig. 3 are similar to those time (Malato et al., 1996). With this procedure,

˜presented by Hung and Marinas (1997). conclusions are possible which aid in extrapola-All of the above gives an impression of the tion for scaling up results and obviate the problem

complexity of the process and how, even com- stemming from the use of an intermittent radiationpounds studied in depth such as TCE, are a source source. Figs. 4–6 show examples of solar trans-of controversy and contradictory results. ients and their effect on the estimation of rate

Regarding kinetics, a rigorous treatment by constants. Fig. 4 shows the results of conversionTurchi and Ollis (1990) and further commented for two different tests (A and B). Changing UVby Pelizzetti and Minero (1993), usually leads to radiation during the day and the appearance ofa saturation-type reaction rate expression similar clouds produce different illumination conditions.to the Langmuir–Hinshelwood model (L–H). Fig. 5 shows PCP (pentachlorophenol) degrada-Other authors suggest the use of Freundlich tion for both tests and the estimate of the initial

¨isotherms at moderate concentrations (Bekbolet et reaction rate (r ). r differs substantially in theseo o

al., 1996). The L–H isotherm has been rather two tests, but surprisingly, the k constant, assum-o

useful for process modeling, and it is generally ing first order, is quite similar. This may beagreed that, although rate constants and orders are explained by the differences between residenceonly ‘‘apparent’’, they describe the rate of degra- time and real time and the effect of solar radiationdation and may be used for a particular case of transients during the day. The same experimentsreactor optimization, but they have no physical are depicted in Fig. 6, but this time versus themeaning, and may not be used to identify the accumulated photons per volume of photoreactor

21surface process (Serpone et al., 1993; Minero et (Einstein.l ). The initial reaction rate is more21al., 1996b). The use of the unmodified L–H congruent when expressed in mg.Einstein than

21 21model, through common understanding, seems to in the usual mg.l .min . In this way acceptable

Fig. 3. Scheme of gas-phase TCE degradation mechanism obtained by illuminating a monolithic catalyst based on sepiolite /? 2TiO . Chlorine radicals are more likely adding to C1. Afterwards a second Cl is added to C forming penthachloro ethane (n88).2

With the intervention of O it is possible to produce a peroxy radical (n83) which dimerizes and decomposes into two oxyradicals2

(n84) Then the cleavage of C–C bond leads to either dichloroacetyl chloride (n85) or phosgene (n86) and a CHCl radical (the2

base for chloroform production n87).


Fig. 4. Degradation of PCP (pentachlorophenol) in water at different hours the same day, using a large solar field at Plataforma21Solar de Almeria and 200 mg.l of TiO Degussa P-25.2

reproducibility was obtained in solar field tests (Parent et al., 1996). Any detoxification technolo-using the same solar system. gy involving consumption of energy must use it

efficiently, whatever the source. Solar energyshould be used efficiently both for economic and

3. THROUGHPUTS ANDenvironmental reasons. The few studies on PCO

PHOTOEFFICIENCIEScosts demonstrate its sensitivity to photoefficiency

Even considering the important cost reductions (Turchi and Miller, 1994). However, by mimick-from simplifying the solar technology, and the ing conventional treatment, authors have tradition-

3success in the complete mineralization of many ally reported throughputs of m of water treated2substrates, the physical–chemical limitations of per m of solar collector and time. Table 1

PCO are of great concern. The slow kinetics and summarizes a comparison of our results withassociated mediocre photoefficiencies require a micropollutants in water versus other referenceslarge amount of energy from the original source in the literature. We were able to obtain through-

Fig. 5. Decomposition of PCP in the experiments of Fig. 4, versus residence time (t ).R


Fig. 6. Same case as Fig. 5 but represented versus accumulated number of photons inside the reactor (E ).E

22 21puts of 0.7 l.m .min , which might be compar- hour in 1000 l of water when the incident solar2able to other results. This shows the production irradiance is 1000 W/m (Bolton et al., 1996b).

capacity of the collectors, but is restricted to water The CAO tries to rationalize the previous value ofwith the same characteristics used in the experi- throughput, including the contribution of solarment. The value depends strongly on initial and light and the initial and final concentration offinal concentrations of the contaminant and pro- contaminant. But, in any case the CAO parametervides no information on the energy efficiency of continues to be dependent on the specific con-the process. The same is true for gas-phase, where taminant and the initial concentration. The re-gas results are usually given in spatial velocities, action rate constant changes depending on C , asO

as for instance, in conventional thermal-catalysis, illustrated in Fig. 7. According to this perform-which refers to mol /g.s (mols of contaminant ance, the reaction rate constant is faster at lowertreated per second and grams of catalyst) (Fu et concentrations. Therefore, an order of degradational., 1995), or area velocities in the case of with C makes sense as a reference only underO

monoliths (volume of gas treated per unit of time similar conditions. In addition, CAO requires aand surface area of catalyst channels) (Blanco et detailed description of the solar system, sinceal., 1996). Again spatial velocity merely describes solar collector and reaction efficiencies are com-treatment capacity without any information on bined. It will surely have to be complementedenergy consumption. with information on catalyst performance with

It has recently been suggested that a figure of catalytic and photonic efficiencies.merit be used, like the ‘‘Electrical Energy per Other parameters, like Turnover Frequency,Order (EEO)’’ defined as the kWh of electrical have hardly been used in PCO tests, even thoughenergy required to reduce the concentration of a they are very important for forthcoming studiespollutant by an order of magnitude in 1000 l of on catalyst deactivation, since the number of realcontaminated water with UV lamps (Bolton et al., active sites and true illuminated surface area are1996a) or in solar collectors, the ‘‘Collector Area unknown. Some authors approximate the surface

14 22per Order (CAO)’’ defined as the collector area site density as 5–15.10 sites.cm for their2(m ) required to reduce the concentration of a estimations (Peral and Ollis, 1997).

given pollutant by one order of magnitude in 1 However, use of the quantum yield (f) as an

Table 1. System throughputs of solar field experiences of water detoxification

Solar Reference Concentration Conditions Throughput Reference2collector compound C (pbb) C (pbb) [TiO ] g/ l (l /min m )initial final 2

CPC EPTC 20–500 0.1 0.5 0.7 Vidal et al., 19974 3CPC TCE 10 10 1 1.7 Pacheco et al., 1993

PTC TCE 200 5 1 0.4 Mehos and Turchi, 1992


Fig. 7. Degradation of ethylbenzene in water by PCO with TiO Degussa P-25 for three different initial concentrations.2

expression of catalyst efficiency referred to ab- such as TCE have been reported to achievesorbed photons is more common in the literature. photoefficiencies above 100% in air due to theWith the exception of a few cases, in most chlorine chain mechanisms initiated (Jacoby etexperiments, the researchers do not know the flux al., 1995). Fig. 8 shows the results from a lababsorbed in their system, and moreover, the experiment we have performed with TCE usingreaction rate depends on substrate concentration. TiO on a solgel membrane. Reactor photonic2

Therefore, f is complicated to obtain as a param- efficiencies of up to 90% for TCE disappearanceeter. For TiO slurries, light scattering in the were confirmed, even at low residence times.2

particles is very difficult to evaluate. More com- In contrast, for typical VOCs in air pollutionmon in the literature is to find ‘‘apparent quantum such as BTEX, isooctane or hexane photoefficien-yields’’, or more properly, ‘‘photonic efficiencies of about 10% are reported (Turchi andcies’’ (z ), defined as the number of reactant Miller, 1994). Others like trioxane, carbon tetra-molecules transformed or product molecules chloride, methylene chloride, chloroform, methylformed divided by the number of photons incident chloroform and vinyl chloride are below 1%inside the front window of the photoreactor (Berman and Dong, 1993). Conversions below(Serpone, 1997). Photonic efficiencies for organic 4% at initial concentrations of 116 ppm have beensubstrates converted to CO are low in real published (Jacoby et al., 1996) for aromatics like2

photoreactors. The maximum z seems to be on benzene with complete conversion possible byTOC

the order of 1%: phenol 1% (Terzian et al., 1990), only decreasing C to less than 5 ppm (indooro

4-chlorophenol 0.36% (Lindsebigler, 1995) and air). We found similar conversions with toluene1.1% (Mills and Morris, 1993), 2,4-dichlorophen- for concentrations of about 400 ppm with severalol 0.4% (Serra et al., 1994) and 2-ethoxyethanol TiO /sepiolite-based monoliths (Avila et al.,2

´1.2% (Brezova et al., 1991) Our experience with 1998). By comparing photo1thermal and purelarge solar plants and P-25 suspensions shows thermal tests, an estimated increase in conversionexamples like z 50.063% for atrazine (Minero of 3–5% could be estimated. This increase, whichTOC

et al., 1996a), 1% for PCP (Malato, 1997), 2.7% is negligible considering the much higher contri-21for 88 mg.l of initial TOC in pharmaceutical butions obtained with small additions of thermal

plant wastewater containing amino acids, alcohols energy, is produced by the addition of UV photonsand phenols (Malato et al., 1996) and lindane in the reaction.

21degradation tests with 13 mg.l initial TOC had These efficiencies are one order of magnitudea z 50.5%. An assumption of 1% average higher than for water detoxification but are lowTOC

photoefficiency (TOC) seems to be realistic for compared to thermal gas catalysis. Only for somemost potential applications of solar PCO in water, high-quantum chlorinated olefins like TCE, perch-and points out the poor yields to be expected. loroethylene and trichloropropene might it be-Catalyst modification or addition of electron come competitive versus existing control tech-scavengers are undoubtedly required to increase nologies. As a rough estimate, the available UVthose percentages. solar radiation for a standard ASTM (AM51.5) is

25 22 21Some chlorinated high-quantum compounds 14310 Einstein.m .s . Assuming typical


Fig. 8. Conversion of TCE in an air stream (C 5400 ppm) and Reactor Photonic efficiency at different photonic fluxes by using a024membrane coated with TiO by solgel techniques. Flow rate51.1 l /min. Residence time55310 s.2

solar collector efficiency of 75% (from radiation practical applications of industrial and environ-available to the photoreactor window), an average mental concern without extra processes and this

25 22 21of 10.5310 Einstein.m .s or 0.38 may cause solar PCO to lose its charm as a mild22 21Einstein.m .h incident photon flux may be operation.

estimated. The photonic efficiencies mentioned According to Eqs. (1)–(3), the redox process isbefore for PCO in water (about 1%) lead to a based on electron and hole migration to therough estimate of 0.0038 mols of contaminant semiconductor surface, and two further oxidation

22 21degraded.m .h in water phase and ten times and reduction steps. It is widely argued that themore in gas phase. reduction step may be rate-limiting since electrons

react much slower than holes (Sun and Bolton,1996). Two basic lines of R&D have been

4. CATALYST MODIFICATIONS ANDworking on balancing both half-reaction rates, one

ADDITIVESmodifying catalyst structure and composition and

Only 5% of the solar spectrum is of use for the the other adding electron acceptors. Both try toTiO band-gap. A realistic assumption of 75% promote competition for electrons and avoid2

2 1solar collector efficiency and 1% catalyst ef- recombination of e /h pairs. A third approachficiency means that only 0.04% of the original has focused not only on increasing quantum yield,solar photons are efficiently used in the detoxifi- but finding new catalysts able to work with band-cation process. From the standpoint of the solar gaps that coincide with the solar spectrum better.collector technology, this is a rather inefficient Many attempts have been made within the firstprocess even for a high-added-value application. line of research, improving specific surface, dop-In contrast to other Advanced Oxidation Tech- ing with metal ions and by oxide deposition.nologies, PCO has the advantage of being solariz- Exhaustive testing conducted by Magrini et al.able and the main characteristic of solar PCO is (1995), leads to the conclusion that an increase inthat it is a mild technology. The TiO catalyst is rate does necessarily mean an increase in cost,2

photostable and cheap, and the process may be durability is uncertain, and performance is quiterun at ambient temperature and pressure. Further- variable depending on the organic substrate. It ismore, the oxidant, molecular oxygen, (O ) is the postulated that metal depositions may diminish2

2 1mildest of all. Therefore, in principle, we have a e /h recombination, and there is much workmild catalyst working at mild conditions with reported in the literature on depositing Pt overmild oxidants. However, as the number and TiO (Hermann et al., 1986). Surprisingly, a2

concentration of contaminants increase, the pro- recent note by Sclafani et al. (1997), puts intocess becomes more complicated, and such chal- evidence contradictory behavior in the presence oflenging problems as catalyst deactivation, slow metals in dehydrogenation of 2-propanol: forkinetics, low photoefficiencies and unpredictable rutile, Ag and Pt deposits were beneficial, but formechanisms need to be solved (Parent et al., anatase they were detrimental. According to the1996). It is clear that TiO alone cannot undertake authors, electron transfer to the metal, which was2


supposed to avoid recombinations and sub- The experience in large solar facility tests withsequently benefit photoefficiency, is detrimental different contaminants has shown the use ofin this case, because fewer electrons are available electron scavengers to be the most versatile andfor the reduction in Eq. (3) and furthermore, some useful way of improving reaction rates, providingholes might also be attracted to the surface of the an opportunity to extend application of heteroge-metal, generating recombinations there. neous photocatalysis to hundreds-of-ppm concen-

This is a good example of how complex trations. H O has been used as an additive in2 2

catalyst performance can become. Therefore, an lab-scale suspensions in which it has been able to?innovative catalyst composition has not been increment quantum yield of OH production from

employed in solar plants where P-25 suspensions 4% to 22% (Sun and Bolton, 1996). H O2 2

continue to be the more reliable solution. competes with O for conduction-band electrons:2

Catalysts immobilized by different techniques2 ? 2e 1 H O → OH 1 OH (4)cb 2 2perform two and three times less efficiently than

suspensions. Confusion has also arisen from the But again, these results are not directly applicableattempt to modify the naked titania catalyst, again to any situation, and the contribution of H O is2 2without conclusive results. In comparisons be- reportedly more complex. Both H O concen-2 2tween P-25 and Hombikat UV-100, two titania tration (Pichat et al., 1995) and the procedure bycatalysts with different surface areas and per- which it was added (Enzweiler et al., 1994) maycentages of anatase, leading to degradation of make its role beneficial or detrimental. A recentdichloroacetic acid (Lindner et al., 1995; Goslich paper (Dillert et al., 1996) describes the detrimen-et al., 1997), demonstrate that the degradation rate tal role of H O in the degradation of TNT at2 2is 2–3 times faster with Hombikat, but in contrast, different pH, due to competition between theother tests with phenol as the substrate showed compounds for conduction-band electrons. Al-four times more efficiency for P-25 (Tahiri et al., though at solar field scale we have been able to1996). This kind of behavior is symptomatic of demonstrate a two-fold increase in the reactionhighly specific catalyst–substrate performance, rate in PCP degradation tests with H O , the2 2due to internal events within the semiconductor oxidant had to be added every 15 minutes tothat influence the kinetics and subsequent external maintain a stable concentration since H O de-2 2

21events are influenced by the kind of substrate. composed at a rate of 7mM.h (Malato, 1997).Experiments in the Plataforma Solar de Almeria By contrast, peroxydisulfate has been shown to be(PSA) large solar plant produced very similar a more reliable and cheaper electron acceptor forresults with both catalysts. Operation under real a wide variety of contaminants tested in the PSAsolar conditions, using real multicomponent solar loops (Blanco et al., 1997; Malato et al.,wastewaters and electron oxidant additives seems 1996).to dampen the effect of catalyst properties (Fig. 9)

22 2 2? 22(Malato et al., 1996). S O 1 e → SO 1 SO (5)2 8 CB 4 4

22Fig. 9. Influence of peroxydisulphate (S O ) addition on the degradation rate of residual waste water (distillation effluent from a2 8

pharmaceutical company containing basically amino acids, alcohols and phenols). Two types of TiO : Degussa P-25 and221Hombikat. Collector Helioman type (see Fig. 11). Catalyst slurry (200 mg.l ).


2? ? 22 1SO 1 H O → OH 1 SO 1 H (6) during the process. Recently some work in con-4 2 4

tinuous gas-phase flow has verified significantComplex multicomponent water containing inter- deactivation (Peral and Ollis, 1997; Sauer andmediate loads of pesticides and industrial /phar- Ollis, 1996), after 1–10 equivalent monolayers ofmaceutical effluents have been successfully contaminant have been converted. This work

5treated with TiO 1S O . Table 2 shows the reports irreversible deactivation in the presence of2 2 8

reaction rates for different contaminants using such heteroatoms as N and Si, but not sulfur. Ournaked TiO and TiO with peroxydisulfate. results in water under batch regimes with sulfur-2 2

In gas-phase PCO, much less has been done containing compounds such as thiocarbamateswith regard to additives. The use of TCE to (e.g., EPTC and molinate) did not show anypromote photocatalytic oxidation of air pollutants deactivation of the catalyst (Vidal et al., 1994b),by taking advantage of chlorine radical formation however a dithiocarbamate (metham sodium) didhas been proposed by d’Hennezel and Ollis interrupt TOC degradation. A careful analysis of(1997). The oxidation of mixtures of TCE1 intermediates confirms that the action of methamorganics in some cases has shown photoenhance- is similar to thiocarbamates (Fig. 10), but ament, but only at high TCE/Organic ratios, refractory intermediate (methyl isothiocyanate)otherwise they act as chlorine scavengers and may caused incomplete degradation. XPS analysesblock the process. With acetone or methylene reveal much larger sulfur deposits on the catalystchloride, however, TCE contribution was detri- surface in tests with metham and methyl iso-mental. An exhaustive work with organic binary thiocyanate than EPTC and other thiocarbamates.mixtures has revealed dissimilar performancesunder photocatalytic conditions (Lichtin et al.,

5. SOLAR TECHNOLOGY: THE USE OF1996). Anyhow, it seems that addition of chlori-

DIFFUSE RADIATIONnated compounds such as TCE to a gas-phasePCO process would be not practical in real-life Field tests conducted since 1990 have con-situations, but only in those cases where chlorine tributed to clarify what kind of solar technology isis already present. best for detoxification. The parabolic troughs

Deactivation of catalysts has also been insuffi- initially used for water treatment and the dishes orciently treated. In water phase, the extensive use furnaces used for gas phase have since evolvedof batch reactors has not been very helpful in into lower-flux systems. One-sun systems fordiscriminating possible deactivation, since that water treatment are firmly based on two factors:effect might be masked by changes in adsorption first, the high percentage of UV photons in the

22Table 2. Solar photocatalytic degradation of contaminants in the CPC system at PSA comparing TiO only and TiO 1S O2 2 2 82(0.01 M), (250 l total treated volume, 200 mg/ l TiO , UV solar radiation about 30 W/m in all the cases)2

9r r t0,TOC 0,TOC 95%,TOC21 22 21Substrate TOC (mg.l ) (mg.m .l ) (min)0

21(mg.l ) TiO TiO 1 TiO TiO 1 TiO TiO 12 2 2 2 2 222 22 22only S O only S O only S O2 8 2 8 2 8

PesticidesaImidacloprid 132 0.25 0.75 6.9 20.6 617 277

aAcrinathrin 40 0.25 0.77 6.9 21.1 698 103a bOxamyl 90 0.08 0.41 2.7 12.4 ( ) 183

2,4 dichlorofenoxiacetic acid 13 0.16 – 4.4 – 70 –

PhenolsbPhenol 38 0.19 – 5.2 – ( ) –

4-chlorophenol 72 0.12 0.90 3.3 24.7 600 602,4 dichlorophenol 88 0.09 0.40 2.6 11.0 450 120

Other contaminantsBenzofurane 8 0.14 – 3.8 – 60 –Dichloroacetic acid 120 0.51 0.71 14.0 19.5 224 160

b b bOlive oil mills 250 ( ) 0.83 ( ) 22.8 ( ) 3002TOC : initial concentration of contaminant; r : TOC initial degradation rate; r9 : initial degradation rate of TOC per m0 o,TOC o,TOC

of solar field.t : residence time required to degrade 95% of the existing total initial organic.95%,TOCa Commercial product.b Very long testing time required. Values not determined.(–) Tests not performed.


Fig. 10. Degradation of metham and the intermediate methyl isothiocyanate compared to molinate. Water-phase tests with TiO2

Degussa P-25 (slurry).

diffuse component of solar radiation and, second, parabolic troughs with around 103 concentra-the low order rate-dependence on light intensity. tions, but testing of a second generation of non-Experimental measurements show that above a concentrating thin films, flat plates and CPCcertain UV-photon flux, reaction rate dependence collectors started immediately thereafter (Gos-on intensity changes from one to half an order ¨wami, 1995; Vidal et al., 1997; Bekbolet et al.,(Bahnemann et al., 1994; Sun and Bolton, 1996). 1996; Goslich et al., 1997; Pacheco et al., 1993).This change does not seem to occur at a specific Fig. 11 compares degradation endurance tests inradiation intensity, as different researchers obtain CPCs and parabolic troughs. CPC was five timesdifferent results. This is presumably affected superior for dichloroacetic acid degradation undersignificantly by experimental conditions. Some analogous conditions (Richter et al., 1997) andauthors (Kormann et al., 1991) attribute the this ratio would be even higher if cloudy periods

1.0 0.5transition of r5f (I ) to r5f (I ) to the excess were considered. A similar ratio has been reported2 1 ?of photogenerated species (e , h and OH). At by Pacheco et al. (1993).

higher radiation intensities, another transition Of the one-sun collectors today, CPCs are the0.5 0from r5f (I ) to r5f (I ) is produced. At this most reliable and have the best performance for

moment, the photocatalytic reaction becomes catalyst suspensions, since it is easy to transferindependent of the radiation received, to depend annular studies from the laboratory to the solaronly on the mass transfer within the reaction. So, field. Almost the same yields as CPCs may bealthough the radiation increases, the rate is con- achieved with flat-plate designs with a betterstant. This could be for several reasons, such as potential for cost reduction, since cheaper materi-the lack of electron scavengers (i.e. O ), or als can be used, but in contrast, preventing mass2

organic molecules in the proximity of the TiO transfer limitations in a flat plate is not a trivial2

surface and/or excess of products occupying task. CPCs require a high-UV reflective surfaceactive centres of the catalyst, etc. These phenom- and the number of pyrex or teflon tubes becomesena really appear more frequently when working costly, whereas flat plates may use cheap materi-with supported catalysts, and/or with slow agita- als, but a high-UV transmittance glazing is neces-tion, which implies that catalyst surface in contact sary and the challenge of how to operate underwith the liquid is small and there is less turbu- pressure must be solved. CPCs are a good optionlence. This does not favor reactant contact with for suspensions and flat plates are better adaptedthe catalyst or diffusion of products from the for fixed catalysts.proximity of the catalyst to the liquid. Fig. 12 shows the schematic drawing of a water

More important than dependence on intensity is treatment facility in batch mode operation. En-the fact that 50% of UV photons available at gineering-scale tests at the PSA have shown

25 22 21AM51.5 (I 514310 einstein.m .s ) are initial and final concentrations of 100 and 10 ppmglobal

diffuse. This implies that non-concentrating tech- of TOC respectively to be a suitably practicalnology could double the number of UV photons order of magnitude for a real solar detoxificationincident in the photoreactor. Early engineering- treatment plant. After pre-treatment (neutraliza-scale tests with solar systems were based on tion, filtration, etc.), the contaminated water is


Fig. 11. Overall dichloroacetic acid degradation rate (mg of Total Organic Carbon mineralized per collector square meter andhour) for the concentrating (Helioman, concentration ratio¯10) and nonconcentrating (Compound Parabolic Collector) systems.The ratio of performance (———) and the mean value of global UV (? ? ?) during the initial reaction periods (zero order kinetic)

are represented on the right axis.

transferred to the closed-circuit solar detoxifica- 6. CONCLUSIONStion loop. The drawing includes optional reuse of

After almost ten years of field testing, it is nowwater within the specific process which generates

possible to summarize the challenges remaining tothe waste water and also proposes removal of the

be solved for deployment of the solar detoxifica-last fraction of contaminants by carbon active

tion technology:filter, which is practical and economical.

Fig. 12. Conceptual diagram of a solar photocatalytic detoxification plant.


1. Destruction should lead to complete miner- yearly production has only been extrapolatedalization. Levels are not universal for all from limited-time tests.compounds since it is demonstrated that mech-anisms may vary. Some compounds also have

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solar photocatalytic degradation of water and air pollutants: challenges and perspectives

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