PHOTOLYTIC AND PHOTOCATALYTIC ALTERATIONS OF HUMIC SUBSTANCES IN UV (254nm) AND SOLAR COCENTRIC PARABOLIC CONCENTRATOR (CPC) REACTORS.

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1 PHOTOLYTIC AND PHOTOCATALYTIC ALTERATIONS OF HUMIC SUBSTANCES IN UV (254nm) AND SOLAR COCENTRIC PARABOLIC CONCENTRATOR (CPC) REACTORS. Emmanouela Remoundaki, Roza Vidali, Pavlina Kousi, Artin Hatzikioseyian and Marios Tsezos National Technical University of Athens (NTUA), School of Mining and Metallurgical Engineering, Laboratory of Environmental Science and Engineering, Heroon Polytechniou 9, Zografou 15780, Athens, Greece. ABSTRACT Humic substances (HS) are formed during the degradation of plant and animal material, and both microbiological and abiotic processes contribute to their production. They impart a brown/yellow color to the water, they can bind with metals and organic pollutants, affecting their mobility and bioavailability, and are precursors of mutagenic halogenated compounds in water formed after chlorination. Treatment of drinking water by UV irradiation is a successful disinfection technique. On the other hand, passive systems for drinking water disinfection, based on minimal energy and materials consumption and exploiting only the available solar energy can be an alternative, mainly for remote and dispersed communities. This work presents results on the photolytic and photocatalytic (with TiO 2 ) alterations of humic substances monitored in a UV 254 nm reactor and a Solar CPC reactor for drinking water treatment. Keywords: Humic substances, photolysis, photocatalysis, UV irradiation, Solar light irradiation, passive systems 1. INTRODUCTION The largest pool of organic material in the water column is Dissolved Organic Material (DOM). Aquatic humic substances account for 40-60% of the DOM. Humic substances are formed during the degradation of plant and animal material, and both biotic and abiotic processes contribute to their production [1, 2]. Humic substances are heterogeneous polymeric organic acids of both aliphatic and aromatic character. They impart a brown yellow color to the water, complex with metals and organic pollutants, affecting their mobility and bioavailability, and are precursors of mutagenic halogenated compounds in water formed after chlorination [3]. Humic substances also play an important role as photosensitizers in the aquatic photochemical processes producing OH free radicals which contribute to further oxidation of the molecules [4-7]. The availability of drinking water is a critical problem for a major part of the world. In third world countries, over 1 billion of people suffer diseases related to waterborne microorganisms [8]. In small and/or remote communities, for example in southern European countries such as the small and isolated Greek islands, drinking water availability and treatment problems are also addressed. The development of almost passive systems for drinking water treatment based on minimal energy and materials consumption, including minimum treatment steps and exploiting only the available solar

2 energy, can be an alternative, mainly, for remote and dispersed communities. Solar photocatalysis uses sunlight for the photonic activation, allowing solar energy to activate or accelerate chemical reactions directly. The design of solar reactors, based on heterogeneous photocatalysis, relies on solar thermal collectors adapted to carry out a photochemical process [9, 10]. Most photocatalytic processes designed for water treatment are heterogeneous, meaning that they take place in heterogeneous phases e.g., solid/liquid. Heterogeneous photocatalytic processes can be decomposed into five independent steps [11]: 1. transfer of the reactants in the fluid phase to the (catalyst) surface, 2. adsorption of at least one of the reactants, 3. reaction in the adsorbed phase, 4. desorption of the product(s), and, 5. removal of the products from the interface region. The photocatalytic reaction occurs in the adsorbed phase (step 3) and can be initiated in the presence of a semiconductor or sensitizer. In heterogeneous photocatalysis, the catalyst can be used either in slurry or supported. Water disinfection by these photochemical processes is the first aim. The problem of the fate of the naturally present organic matter in water during treatment by these photochemical processes should also be addressed. Cocentric Parabolic Collectors (CPC) were developed and tested in the frame of a European research project (SOLWATER, INCO) both for drinking water disinfection and organic matter oxidation and final mineralization [10, 12, 13]. As already mentioned above, humic substances are always present in natural waters and may be photolytically altered as they act as self-producing OH free radicals photosensitizers. In water treatment, OH radicals are the major source of oxidation reactions with organic molecules [14]. In the present study, the photolytic and photocatalytic alterations of humic substances are comparatively presented in order to conclude about their relative importance. Data sets on the photolytic (without TiO 2 ) and photocatalytic (with TiO 2 ) alterations of humic substances were obtained during the monitoring of humic substances alterations in a UV 254 nm reactor and a Solar CPC reactor. A UV 254 nm reactor has been chosen because drinking water by UV irradiation at 254 nm is a successful technique for drinking water disinfection [15]. Titanium dioxide was used as a catalyst as it is one of the most suitable semiconducting materials which is stable, non toxic and has a low energy band gap. 2. MATERIALS AND METHODS 2.1 Sample preparation The photolytic and photocatalytic alterations of humic substances were studied for three different substances: a) purified peat humic acid (PPHA), b) aquatic fulvic acid (FA), both isolated by the International Humic Substances Society (IHSS), and c) humic acid purchased from Aldrich as sodium salt (HAA). The isolation and purification method used by IHSS, has been described elsewhere [16, 17]. The origins and methods of extraction and purification of the humic substances obtained from IHSS, together with their physical and chemical properties are reported in the IHSS website [18]. Solutions of humic substances of initial concentrations ranging from 5 to 25 mg L -1, were prepared by dissolving an appropriate amount of dry humic sample in deionized water. For complete dissolution of HS, the addition of some drops of 0.1N NaOH solution was necessary. After the dissolution of the HS, the ph of the solution was adjusted at ph 6.50 by addition of 0.1N HNO 3 solution.

3 For the study of the photocatalytic alterations of humic substances, TiO 2 in suspension and immobilized form was used as a photocatalyst. Titanium dioxide suspensions were prepared using Degussa P25 TiO 2 powder at concentrations of 5 mg L -1. The experiments carried out by TiO 2 in immobilized form, were performed using 1049 AHLSTROM paper containing 20 g m -2 of TiO 2 Degussa P25. All solutions were prepared using analytical reagent grade chemicals (MERCK). Deionized water was used in all the experiments. 2.2 Photoreactors Figure 1 shows the UV reactor used. The UV photoreactor consisted of a cylindrical stainless steel reactor vessel with an effective volume of 250 ml, and a high pressure mercury lamp positioned axially at the center of the reactor vessel emitting monochromatic light at 254 nm. The UV lamp was protected from the solution by a cylindrical quartz tube. Figure 1 Schematic representation of the UV photoreactor. Figure 2 Schematic representation of the CPC photoreactor Figure 2 shows the Solar Cocentric Parabolic Concentrator (CPC) reactor which was developed in the frame of a European research project (SOLWATER, INCO) [12-14]. The results reported in the

4 present study concern the fin-type Solar CPC. The working volume was 50 L. The volume of water circulating in the CPC was 16.2 L and the surface exposed to solar light was 1 m Photolysis Photocatalysis experiments Humic substances solutions were circulated continuously under aerobic conditions in both reactors. In the case of UV reactor, the flow rate was adjusted to ml min -1. In the case of CPC reactor the flow rate was adjusted to 2 L min -1. In the UV 254 nm reactor, photolytic alteration experiments were carried out without catalyst, and included three different humic substances: Purified Peat Humic Acid (PPHA) from IHSS, Fulvic Acid (FA) from IHSS and Humic Acid purchased from ALDRICH (HAA), in order to determine the rate of photoalterations for the three different humic substances. Photocatalytic alterations experiments were carried out in the UV reactor with TiO 2 (5 mg L -1 ) as a catalyst in suspension. Aliquots of about 10 ml were sampled during irradiation for the determination of the remaining unaltered humic substance and TOC. In the CPC reactor, three data series were produced. The first correspond to the use of the CPC reactor without catalyst exposed to solar light. The second correspond to the use of the CPC reactor with immobilized catalyst in the form of AHLSTROM paper containing 20 g m -2 of TiO 2 Degussa P25 but in the dark in order to assess the amount of HS adsorbed on the catalyst (paper) surface. The third corresponds to runs of CPC reactor with AHLSTROM paper containing 20g m -2 of TiO 2 Degussa P25 exposed to solar light. 2.4 Humic substances monitoring Gel permeation chromatography (GPC) was performed using a high-performance liquid chromatography system of Metrohm for the monitoring of HS. The method was calibrated for the remaining unaltered HS. The system injection valve is fitted to a 100 μl sample loop made of stainless steel. The separation system includes a guard column (TSKgel G3000PW, TOSOH BIOSEP, 7.5 mm 7.5 cm) and a size exclusion column (TSKgel G3000PW, TOSOH BIOSEP, 7.5 mm 30 cm). As a mobile phase ultra-pure water obtained by a Barnstead compact ultrapure water system was used. A UV detector (Lambda 1010 Bischoff) with a 10 μl cell, emitting at 254 nm was used. Synthetic standard PPHA, FA and HAA solutions were used for calibration. The data were processed by a personal computer using the Metrohm 714 IC-Metrodata software. Total organic carbon (TOC) was monitored by a HACH Odyssey DR/2500 Spectrophotometer. 3. RESULTS AND DISCUSSION 3.1 Photolysis and photocatalysis of HS in the UV 254 nm reactor. Figure 3 presents the results obtained for PPHA photolysis in the UV 254 nm reactor for initial concentrations of 10 and 20 mg L -1 respectively. From this figure, it is apparent that HS are photolysed by about 90% for both initial concentrations after about 10 hours of irradiation. The photoalterations follow a first order law (Figure 3b). Figure 3c shows that TOC is also reduced by about 70-80% after 10 hours of irradiation. Similar results were obtained for FA and HAA.

5 [HA] mg/l [HA] mg/l (Initial con. 20 mg/l) [HA] mg/l (Initial con. 10 mg/l) -lnc/co a lnc/co (Initial con. 20 mg/l) lnc/co (Initial con. 10 mg/l) b. TOC (mg/l) (Initial con. 20 mg/l) 1 TOC (mg/l) (Initial con. 10 mg/l) 8.00 TOC mg/l c. Figure 3 (a) PPHA concentration (mg L -1 ) as a function of the irradiation time (min), (b) -lnc/co as a function of irradiation time (min), and (c) TOC (mg L -1 ). (Initial PPHA concentration = 20 and 10 mg L -1 ). Figure 4a presents the plots of C/Co versus irradiation time and Figure 4b the plots of lnc/co versus irradiation time obtained for three different initial concentrations of HAA (5, 10 and 25 mg L -1 ). From these figures, it is apparent that the influence of the initial concentration on the photolytic alteration of HAA is not significant and a pseudo first order kinetic constant value could be calculated: k=38 min -1. C/Co mg/l 10 mg/l 5 mg/l Irradiation Time (h) a. -lnc/co lnc/co 5mg/L lnc/co 10mg/L lnc/co 25mg/L Figure 4 Plots of (a) C/Co and (b) -lnc/co as a function of the irradiation time (min). (Initial HAA concentration = 25, 10 and 5 mg L -1 ). Figure 5 presents the photocatalytic alterations of PPHA at initial concentration of 10 mg L -1 (suspension of 5 mg L -1 of TiO 2 ). No residual humic acid was determined and TOC was totally b.

6 removed at the end of the irradiation period (3 hours). In the presence of TiO 2 as a catalyst, a more rapid kinetic was also observed. [HA] / TOC (mg/l) 12,00 10,00 8,00 6,00 4,00 Initial PPHA = 10 mg/l TiO2 = 5 mg/l [HA] mg/l TOC (mg/l) -lnc/co 2 1,5 1 -lnc/co 0,5 2,00 0 0,00-0, Figure 5. PPHA concentration (mg L -1 ), TOC (mg L -1 ) and plot of -lnc/co as a function of the irradiation time (min) (pseudo first order, k= 8 min -1 ), (Initial PPHA concentration =10 mg L -1, TiO 2 =5 mg L -1 ). Table 1. Photolysis and photocatalysis results obtained for different HS in the UV 254 nm reactor. Substance Initial HS (mg L -1 ) Photolysis k (min -1 ) Photolysis % Conversion % TOC Removal k (min -1 ) Photocatalysis TiO 2 (5 mg L -1 ) % Conversion % TOC Removal PPHA PPHA FA HAA HAA HAA Summarizing the results obtained on the photolysis and photocatalysis of humic substances in the UV 254 nm reactor (Table 1), we can conclude that humic substances were efficiently photolysed by 254 nm irradiation and these results are in agreement with data reported in literature [6, 19]. The reduction of TOC was also significant underlying the mineralization of humic substances at significant extent. However, TOC was not totally removed and this is also in agreement to other findings in literature [6, 19]. When a suspension of TiO 2 was used, even at a low concentration of 5 mg L -1, both removal of humic substances and TOC was observed. 3.2 Photolysis and photocatalysis of HS in CPC reactor. In the case of CPC runs exposed to solar light, the data are reported as a function of the energy accumulated (MJ m -2 L -1 ) by appropriate calculation using the available meteorological data corresponding to the periods of exposure. Figure 6a shows the HAA photoalterations in the CPC reactor without catalyst as a function of the energy accumulated (MJ m -2 L -1 ), for three different initial concentrations (5, 10 and 25 mg L -1 ). From this figure, it can be seen that about 80% of HAA are phototransformed at MJ m -2 L -1. This behavior is due to the fact that humic substances act as photosensitizers capable of self-producing free radicals of OH which then contribute to further oxidation and degradation of the humic macromolecule [4-7].

7 Effect of HAA Con. (without catalyst) 25 mg/l 10 mg/l mg/l In the dark 10 mg/l Without Catalyst 10 mg/l With Catalyst C/Co 0.50 C/Co Energy accumulated (MJ/m2*L) Energy accumulated (MJ/m2*L) a. b. Figure 6 a.haa (mg L -1 ) photoalterations in the CPC reactor without catalyst as a function of energy accumulated (MJ m -2 L -1 ) (Co = 5, 10 and 25 mg L -1 ). b. Plots of C/Co as a function of the energy accumulated (MJ m -2 L -1 ), without catalyst and with catalyst in the dark and exposed to solar light (Co = 10 mg L -1 ). Figure 6b shows the results obtained by the CPC reactor and initial HAA concentration of 10 mg L -1 as a function of energy accumulated. Three different data sets are presented. The first data set on Figure 6b represents the data obtained by the CPC reactor when immobilized catalyst was installed in the form of AHLSTROM paper containing 20 g m -2 of TiO 2 Degussa P25 and the reactor was running in the dark. The second data set represents the data obtained by the CPC reactor without catalyst. The results obtained show that about 60% of the initial HAA was adsorbed on the surface of the catalyst. This adsorption was necessary for further interaction between solar light-catalyst and HAA as already explained in the introduction [11]. These results are comparable to those obtained for photolysis. Finally, the third data set corresponds to the data obtained by the CPC reactor with the immobilized catalyst exposed to solar light. In this case, the result of photocatalysis was a 90% phototransformation of HAA at 2 MJ m -2 L -1. However, TOC measurements indicated that residual TOC is significant for both cases of photolysis and photocatalysis (about 50% of the initial). The factors influencing the residual TOC need further experimental investigation. 4. CONCLUSIONS Series of experimental data of humic substances photolysis and photocatalysis by two different reactors were presented, a laboratory scale UV 254 nm reactor and a pilot scale CPC reactor, in order to assess the relative importance of photolysis and photocatalysis to the phototransformations of HS. In the UV 254 nm reactor, humic substances were photolysed by about 80-90% and a significant reduction of TOC by about 80% was also observed, due to their ability to generate OH free radicals initiating photooxidation reactions and degradation of the macromolecules. When a small amount of catalyst was added in suspension, complete mineralization of HS was observed. CPC reactor runs were performed without and with immobilized catalyst. Humic substances photolysis resulted to a 60-70% reduction of their initial concentration. When the immobilized catalyst was installed in the CPC reactor and the reactor was run in the dark, humic substances were adsorbed on its surface by about 60%. The CPC reactor with the immobilized catalyst runs exposed to sunlight resulted to a reduction of humic substance initial concentration by 80-90% for values of energy accumulated of 2 MJ m -2 L -1. However, in the CPC reactor, the problem of the residual TOC should be addressed and investigated by further experimental work.

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