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Applied studies in solar photocatalytic detoxification: an overview

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Applied studies in solar photocatalytic detoxification: an overview
  Applied studies in solar photocatalytic detoxification:an overview Sixto Malato  * , Juli  aan Blanco, Alfonso Vidal, Diego Alarc  oon,Manuel I. Maldonado, Julia C  aaceres, Wolfgang Gernjak CIEMAT––Plataforma Solar de Almer  ııa, Ctra. Sen  ee s Km. 4, Tabernas, 04200 Almer  ııa, Spain Received 7 July 2002; accepted 24 July 2003 Abstract The technical feasibility and performance of photocatalytic degradation of four water-soluble pesticides (diuron,imidacloprid, formetanate and methomyl) have been studied at pilot scale in two well-defined systems which are of special interest because natural-solar UV light can be used for them: heterogeneous photocatalysis with titanium di-oxide and homogeneous photocatalysis by photo-Fenton. The pilot plant is made up of compound parabolic collectorsspecially designed for solar photocatalytic applications. The initial concentration tested with imidacloprid, formetanateand methomyl was 50 and 30 mg/l with diuron, and the catalyst concentrations were 200 mg/l and 0.05 mM with TiO 2 and iron, respectively. Total disappearance of the parent compounds, 90% mineralisation and toxicity reduction belowthe threshold (EC 50 ) have been attained with all pesticides tested. All these results have contributed to an evaluation of photocatalytic treatment capacity and comments on the main parameters of TiO 2  and Fe separation from the treatedwater.   2003 Elsevier Ltd. All rights reserved. 1. Introduction Water, pre-requisite for life and key resource of hu-manity, is abundant on earth. However, 97.5% is saltwater. Of the remaining 2.5% that is fresh water, 70% isfrozen in the polar icecaps; the rest is mainly present assoil moisture or in inaccessible subterranean aquifers.Only less than 1% of the world’s fresh water resourcesare readily available for human use; and even this re-source is very unevenly distributed (WHO, 2002).On the ‘‘blue planet’’ nearly 1000 million people stillhave no access to adequate water sources, and about2400 million have no access to adequate sanitation. As aconsequence, 2.2 million people in developing countries,most of them children, die every year from diseases as-sociated with a lack of safe drinking water, inadequatesanitation and poor hygiene (WHO and UNICEF,2000).Compared to regions that are not as rich, watersupply and sanitation in the EU are fairly well devel-oped. Nevertheless, contamination and other regionalproblems still exist. Furthermore, adverse health andother potential effects of many substances present inwater are still uncertain and lacking in investigation. EUlegislation takes into account increasing knowledge andadapts current law to protect and improve the quality of Europe’s fresh water resources. The most recent updatewas the European Water Framework Directive (Direc-tive 2000/60/EC of the European Parliament and of theCouncil of 23 October 2000 establishing a frameworkfor Community action in the field of water policy).Community water policy aims to achieve sustainabilityand integration into other policies in accordance withthe principles set out, particularly in Article 130r of theTreaty: a high level of protection, the precautionaryprinciples, preventive action, rectification of damage at * Corresponding author. Tel.: +34-950387940; fax: +34-950365015. E-mail address: (S. Malato).0038-092X/$ - see front matter    2003 Elsevier Ltd. All rights reserved.doi:10.1016/j.solener.2003.07.017Solar Energy 75 (2003) 329–  source, the polluter pays principle. The policy is basedon the use of available scientific and technical data,taking into account variability of environmental condi-tions in the regions of the Community and the cost/benefits of action or inaction. Water policy is set out inthe following main Directives in addition to the above-mentioned most recent Water Framework Directive: theDrinking Water Directive 80/778/EEC safeguardinghuman health by establishing strict standards for thequality of water intended for human consumption; theDangerous Substances Directive 76/464/EEC and itsdaughter Directives controlling pollution of surfacewaters with dangerous substances from industrial in-stallations; the Integrated Pollution Prevention andControl Directive 96/61/EEC controlling pollution of surface water with dangerous substances from large in-dustrial installations. To meet these goals and diminishcontamination and environmental risks throughout theEU, pollutant sources have to be identified and appro-priate treatment strategies applied. Compliance withstrict quality standards is especially called for in the caseof toxic substances which affect the biological sphereand prevent activation of biological degradation pro-cesses; the Urban Waste Water Treatment Directive 91/271/EEC controlling pollution, in particular, eutrophi-cation of surface water with nutrients (particularly ni-trogen and phosphorus) from urban waste water; theNitrates Directive 91/676/EEC controlling nitrate pol-lution from agricultural sources, complementing theUrban Waste Water Treatment Directive and the Bath-ing Water Directive 76/160/EEC safeguarding the healthof bathers and maintaining the quality of bathingwaters.Advanced oxidation processes (AOPs) are chemicaloxidative processes, which can be applied to wastewatertreatment to oxidise pollutants. A part of them generateshydroxyl radicals. After fluorine, the hydroxyl radical isthe second strongest known oxidant (2.8 V vs. standardhydrogen electrode). It is able to oxidise and mineralisealmost every organic molecule, yielding CO 2  and inor-ganic ions. Rate constants ( k  OH , Eq. (6)) for most reac-tions involving hydroxyl radicals in aqueous solution areusually on the order of 10 6  –10 9 M  1 s  1 . The reactionsby which hydroxyl radicals attack organic molecules arehydrogen abstraction, electrophilic addition, electrontransfer and also radical–radical reactions (Legrini et al.,1993). The most common methods of hydroxyl genera-tion in AOPs are presented in Table 1.Although the strong potential of AOPs for waste-water treatment is widely recognized, it is also wellknown that the operating costs for total oxidation of hazardous organic compounds remain relatively highcompared to those of biological treatment. Further-more, the total destruction of contaminants is not al-ways necessary. While AOP operating costs are alwayshigher than those of a biological treatment, their use asa pre-treatment for the enhancement of the biode-gradability of wastewater containing recalcitrant ortreatment-inhibiting compounds might be justified. Theintermediate reaction products could then be degradedby microorganisms in a biological post-treatment (Parraet al., 2000; Sarri  aa et al., 2002). By determining thetoxicity of the water at different stages of AOP treatmentwith toxicity bioassays using different microorganisms(Fern  aandez-Alba et al., 2001) AOP operating costs canbe further decreased. In this case, biodegradability can Table 1Hydroxyl generation in different AOPsMethod Key reaction Light necessaryUV/H 2 O 2  H 2 O 2 þ h m ! 2OH  k  <  310 nmUV/O 3  O 3 þ h m ! O 2 þ O ð 1 D Þ  k  <  310 nmO( 1 D)+H 2 O fi 2OH  UV/H 2 O 2 /O 3  O 3 þ H 2 O 2 þ h m ! O 2 þ OH  þ OH  2  k  <  310 nmUV/TiO 2  TiO 2 þ h m ! TiO 2 ð e  þ h þ Þ  k  <  380 nmTiO 2 h þ þ OH  ad  ! TiO 2 þ OH  ad UV/H 2 O 2 /TiO 2  TiO 2 þ h m ! TiO 2 ð e  þ h þ Þ  k  <  380 nmTiO 2 h þ þ OH  ad  ! TiO 2 þ OH  ad H 2 O 2 þ e  ! OH  þ OH  UV/S 2 O 2  8  /TiO 2  TiO 2 þ h m ! TiO 2 ð e  þ h þ Þ  k  <  380 nmTiO 2 h þ þ OH  ad  ! TiO 2 þ OH  ad S 2 O 2  8  þ e  ! SO   4  þ SO 2  4 H 2 O 2 /Fe 2 þ (Fenton-reaction) H 2 O 2 þ Fe 2 þ ! Fe 3 þ þ OH  þ OH  UV/H 2 O 2 /Fe (photo-Fenton reaction) H 2 O 2 þ Fe 2 þ ! Fe 3 þ þ OH  þ OH  k  <  580 nmFe 3 þ þ H 2 O þ h m ! Fe 2 þ þ H þ þ OH  330  S. Malato et al. / Solar Energy 75 (2003) 329–336   very often be predicted and, moreover, biocompatibilitywith the environment can be affirmed. Numerous bio-assay procedures are now available (Tothill and Turner,1996), however, as toxicity is a biological response, auniversal monitoring device is unlikely to be availableand therefore, to increase confidence in the evaluation of the toxicity, it is necessary to use a battery of differentorganisms from different taxonomic groups.The use of AOPs for wastewater treatment has beenstudied extensively (Andreozzi et al., 1999; Chiron et al.,2000; Esplugas et al., 2002) and it has been stated thatUV radiation by lamps is expensive. Therefore, researchis focusing more and more on the two AOPs, which canbe powered by solar irradiation, i.e. light with a wave-length longer than 300 nm, homogeneous catalysis withphoto-Fenton and heterogeneous catalysis with UV/TiO 2 , with and without addition of oxidants (Alfanoet al., 2000; Goswami, 1997; Konstantinou and Albanis,2003; Malato et al., 1999, 2002). Although the interest of research, just as of business is growing, there are still fewknown industrial-scale applications. The first Europeanindustrial-scale solar photocatalytic plant has recentlybeen erected with commercially available compoundparabolic collectors (CPCs) (see Fig. 1) and it hasdemonstrated that the solar photocatalytic technologyis sufficiently developed for industrial use (Solar det-oxification technology to the treatment of industrialnon-biodegradable persistent chlorinated water con-taminants. BRITE/EURAM III project, BRPR-CT97-0424, 4th FWP EC).This paper describes how solar photocatalysis couldbecome a significant sector of the wastewater treatmenttechnologies for persistent toxic compounds. It evaluatestwo well-defined AOP systems that make use of naturalUV light in a large pilot plant, heterogeneous photo-catalysis with TiO 2  and homogeneous photocatalysiswith photo-Fenton. The processes were applied to thedegradation of four pesticides: diuron [3-(3,4-dichlor-ophenyl)-1,1-dimethylurea], imidacloprid [1-(6-chloro-3-pyridylmethyl)-  N  -nitroimidazolidin-2-ylideneamine],formetanate [3-dimethylaminomethyleneaminophenylmethylcarbamate] and methomyl [ S  -methyl  N  -(meth-ylcarbamoyloxy) thioacetamidate]. These four differentpesticides were selected for the following two mainreasons: they have different structures that are repre-sentative of a wide range of modern pesticides (con-taining different heteroatoms and bonding structures)and they are highly soluble in water (of special interestbecause of their extremely easy transport in the envi-ronment, seriously threatening all surface and ground-water). Evaluation of equivalent pilot-scale experimentswas comparable and different bioassays determined theirtoxicity at different stages of mineralisation. 2. Experimental  2.1. Chemicals Technical-grade imidacloprid (97.9%) was suppliedby Bayer Hispania S.A. (Barcelona, Spain). Technical-grade methomyl (98%) and Diuron (98.5%) were sup-plied by Aragonesas Agro S.A. (Madrid, Spain). Tech-nical-grade formetanate (90%) was supplied by ArgosShering AgrEvo, S.A. (Barcelona, Spain). Analytical-standard imidacloprid, methomyl and formetanatehydrochloride were purchased from Riedel-deHa € een(Seelze, Germany). Analytical-standard Diuron waspurchased from Dr. Ehrenstorfer GmbH (Augsburg,Germany). The heterogeneous photocatalytic degrada-tion tests were carried out using a slurry solution (200mg/l of TiO 2 ) of Degussa (Frankfurt, Germany) P-25titanium dioxide (surface area 51–55 m 2 g  1 ). For thephoto-Fenton experiments (0.05 mM iron), the follow-ing chemicals were used: iron sulphate (FeSO 4  Æ 7H 2 O),hydrogen peroxide reagent grade (30% w/v) and sul-phuric acid for pH adjustment (around 2.7–2.8). Theconcentration of peroxide in the reactor was determinedby frequent analyses (iodometric titration) and main-tained constant (around 15 mM) by adding smallamounts as consumed.  2.2. Analytical determinations Pesticides were analysed using reverse-phase liquidchromatography (at 0.5 ml/min) with UV detection in anHPLC-UV (Hewlett-Packard, series 1100) with C-18column (LUNA 5micron-C18, 3 · 150 mm from Phe-nomenex). The mobile-phase composition and wave-length was, in each case: H 2 O pH  ¼  3/acetonitrile(ACN) at 80/20 ratio at 270 nm (imidacloprid), H 2 O/ACN at 90/10 ratio at 234 nm (methomyl), H 2 O/meth-anol at 40/60 ratio at 254 nm (diuron) and K 2 HPO 4 Fig. 1. CPCs developed for photocatalytic applications. S. Malato et al. / Solar Energy 75 (2003) 329–336   331  (18.4 mM in water, pH  ¼  8.9)/ACN at 80/20 ratio at 252nm (formetanate). Total organic carbon (TOC) wasanalysed by direct injection of the filtered samples into aShimadzu-5050A TOC analyser.  2.3. Toxicity studies For  Daphnia magna  and  Selenastrum capricornotum bioassays, stock solutions were prepared by dilution inspecific culturing media. For the  Vibrio fischeri   bioassay,the osmolality of solutions was adjusted to 2% NaCl foroptimum performance. The effect of the test compoundon the luminescent bacterium  Vibrio fischeri   was evalu-ated using the Biotox test for 5, 15 and 30 min EC 50 (Bio-Orbit Oy, Turku, Finland). The bacteria werepurchased as freeze-dried reagents. They were stored at ) 20   C and hydrated prior to testing. Light emitted fromthe bacterium is a result of the interaction of the enzymeluciferase, reduced flavin, and a long-chain aldehyde inthe presence of oxygen. Through the electron transportsystem, the metabolic (chemical) energy generated alongthis pathway is converted into visible light. This meta-bolic pathway is intrinsically linked to cellular respira-tion, so disruption of normal cellular metabolism causesa decrease in light production. The toxicity end-point(EC 50 ) was determined as the concentration of a testsample that caused a 50% reduction in light output. Theacute bioluminescence assay was carried out accordingto ISO 11348 (1994). Light production from luminescentbacteria was measured with a photomultiplier in a lu-minometer equipped with a constant temperature waterbath (15   C).The toxicity of methomyl and its photoproductsfor crustaceans  Daphnia magna  was assessed usingcommercially available Toxkit Daphtoxkit (Creasel,Belgium). The toxicity studies were performed in ac-cordance with testing conditions prescribed by OECDGuideline 202 (1995) and ISO 6341 (1989). Acute tox-icity was assessed by noting the effects of the test com-pounds on the mobility of   Daphnia magna . All the testswere performed in the dark at a constant temperature of 20±1   C. The neonates are considered immobilised af-ter 24 and 48 h of incubation, if they lie on the bottomand do not resume swimming within 15 s of observation.The toxicity end-point (EC 50 ) was determined as theconcentration estimated to immobilise 50% of thedaphnids after 24 and 48 h exposure.The alga growth-inhibition test was performed usingthe freshwater green microalga  Selenastrum capricorno-tum  according to OECD Guideline 201 (1995) and usingthe commercially available Toxkit Algaltoxkit (Creasel,Belgium). Algaltoxkit makes use of de-immobilised mi-croalga beads in an inert matrix. After de-immobilisa-tion and transfer into an adequate culturing medium,the microalgae resume their growth immediately. Theinitial number of algae cells was adjusted to 106 cells/mland the test tubes were then kept at 25   C in an incu-bator under continuous illumination. Each test tubecontaining test compound and algae in the medium wasincubated at 25   C for 3 days. Inhibition of alga growthrelative to control was determined by measurement of the optical density in spectrophotometer with a filter at670 nm. The 72-h EC 50  for  Selenastrum capricornotum was calculated as the test concentration resulting in a50% reduction in growth relative to the control.  2.4. Photoreactor All the experiments were carried out under sunlightin CPCs at the Plataforma Solar de Almer  ııa (PSA, lat-itude 37  N, longitude 2.4  W). The pilot plant (Malatoet al., 2001) is made up of twin systems, each havingthree collectors, one tank and one pump. Each collector(1.03 m 2 each) consists of eight Pyrex tubes connected inseries and mounted on a fixed platform tilted 37   (locallatitude). The water flows at 20 l/min directly from onemodule to another and finally into a tank. The totalvolume ( V   T ) of the reactor (40 l) is separated into twoparts: 22 l total irradiated volume (in Pyrex tubes) ( V   i )and the dead reactor volume (tank+connecting tubes).At the beginning of the experiments, with collectorscovered, all the chemicals are added to the tank andmixed until constant concentration is achievedthroughout the system. Then the cover is removed andsamples are collected at predetermined times ( t  ). Solarultraviolet radiation (UV) was measured by a global UVradiometer (KIPP & ZONEN, model CUV3), mountedon a platform tilted 37   (the same angle as the CPCs),which provides data in terms of incident  W   UV  m  2 . Thisgives an idea of the energy reaching any surface in thesame position with regard to the sun. With Eq. (1),combination of the data from several days’ experimentsand their comparison with other photocatalytic experi-ments is possible. t  30W ; n  ¼ t  30W ; n  1 þ D t  n UV30 V   i V   T ;  D t  n  ¼ t  n  t  n  1  ð 1 Þ where  t  n  is the experimental time for each sample, UV isthe average solar ultraviolet radiation measured during D t  n , and  t  30W  is a ‘‘normalized illumination time’’. In thiscase, time refers to a constant solar UV power of 30Wm  2 (typical solar UV power on a perfectly sunny dayaround noon). As the CPCs do not concentrate lightinside the photoreactor, the system is outdoors and isnot thermally insulated, the maximum temperatureachieved inside the reactor during the experiments is25   C. 332  S. Malato et al. / Solar Energy 75 (2003) 329–336   3. Results 3.1. Mineralisation Fig. 2 shows the experiments (mineralisation results)performed with all pesticides at 50 mg/l except diuron,which was tested at a lower concentration (30 mg/l) dueto its lower solubility. But in any case, this does notaffect the comparison because all of them have first-order kinetics. The four pesticides were successfullydegraded (see Table 2 for kinetic constant) and min-eralised, as shown in Fig. 2. Nevertheless, total miner-alisation (i.e., complete disappearance of TOC) can beattained only after very long irradiation. In any case, theby-products detected as the last steps prior to minerali-sation are always very simple (usually carboxylic acids)and the complete release of heteroatoms as inorganicacids has been confirmed by anion analyses (ionicchromatography) according to the stoichiometry pro-posed in reactions (2)–(5) corresponding to methomyl,diuron, imidacloprid and formetanate, respectively(Fern  aandez-Alba et al., 2002; Malato et al., 2001, 2003;Marinas et al., 2001).C 5 H 10 O 2 N 2 S þ 212 O 2 ! 2HNO 3 þ H 2 SO 4 þ 5CO 2 þ 3H 2 O  ð 2 Þ C 9 H 10 Cl 2 N 2 O þ 13O 2 ! 2HNO 3 þ 2HCl þ 9CO 2 þ 3H 2 O  ð 3 Þ C 9 H 10 ClN 5 O 2 þ 332 O 2 ! 5HNO 3 þ HCl þ 9CO 2 þ 2H 2 O  ð 4 Þ C 11 H 15 N 3 O 2 þ 352 O 2  ! 3HNO 3 þ 11CO 2 þ 6H 2 O  ð 5 Þ Assuming that the reaction between the   OH radicalsand the pesticide is the rate-determining step, the rateequation (6) is written as r  ¼ k  OH ½  OH  C   ¼ k  ap C   ð 6 Þ where  C   is pesticide concentration,  k  OH  is the reactionrate constant and  k  ap  is a pseudo first order constant(Malato et al., 2001). This was confirmed by the linearbehaviour of Ln ð C  0 = C  Þ  as a function of   t  30W , for all thetests performed (see Table 2).Table 2 also shows the initial disappearance rate foreach pesticide and the time necessary for a given amountof mineralisation ( t  30W ; 90%TOC ). As mineralisation doesnot follow simple models like first or zero order kinetics,overall reaction rate constants cannot be calculated. Thecomplexity of the results is caused by the fact that theTOC is a sum parameter often including several hundredproducts that undergo manifold reactions. This stan-dard parameter in wastewater technology is easy tohandle and was therefore chosen as an additional targetparameter. From the data in Table 2 photo-Fenton isseen to be more effective than TiO 2  for treating all thepesticides tested, except for diuron mineralisation, inwhich case both treatments are quite similar. 0102030Photo-FentonTiO 2  Imidacloprid Methomyl Diuron Formetanate     T   O   C ,  m  g   /   L 0 200 4000102030  Imidacloprid Methomyl Diuron Formetanate  t 30W  , min Fig. 2. Mineralisation of pesticides as a function of   t  30W  (illu-mination time). TiO 2  (200 mg/l). Photo-Fenton (0.05 mM Fe).Table 2First order rate constants, initial rate and time necessary for mineralising 90% of the initial TOC for the four pesticides tested with TiO 2 and photo-FentonPesticide  k  ap  (min  1 )  r  0  (mgL  1 min  1 )  t  30W ; 90%TOC  (min)TiO 2  Photo-Fenton TiO 2  Photo-Fenton TiO 2  Photo-FentonImidacl. 0.035 0.22 0.61 4.73 421 187Methomyl 0.046 0.08 1.34 1.44 635 368Diuron 0.092 0.20 0.69 1.06 124 159Formet. 0.026 0.85 0.87 10.79 399 105 S. Malato et al. / Solar Energy 75 (2003) 329–336   333
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