© 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Waste Management and the Environment, D Almorza, CA Brebbia, D Sales & V Popov (Editors). ISBN 1-85312-907-0 Photocatalysis of methomyl using concentrating solar collector and ultraviolet light G. Pefiuelal, G. M. Mejial, G. Hincapi&2, N Cuervo2, J. Marin2 & G. M. Restrepo2 lDepartment of Environmental Engineering& 2Department ojChemical Engineering, Universip ofAntioquia, Medellin, Colombia. Abstract The photocatalytic oxidation of methomyl, using TiOz suspensions under solar radiation and ultraviolet light, has been studied at pilot scale at photodegradation laboratory of University of Antioquia (Medellin, Colombia). Two different reactor designs were tested: a low-concentrating radiation system, Solar- Parabolic-Concentrator Reactor (SPCR), facing south and inclined 6 degrees (geographic position of Medellin: 6 degrees north latitude), and an ultraviolet light system (ULS). Both systems were built in University of Antioquia. In Colombia, methomyl is used as a foliar spray and controls many insects on field crops, fkuit crops, and vegetables. Methomyl was dissolved in water to required mg/L levels in a reservoir tank. In both cases, 1 g/L of Lannate (formulation with an active compound: methomyl) was used. Ti02, in concentrations between 50 mg/L and 250 mg/L, and HzOZ,between 1 mL (300/0)/Land 3 mL (300/0)/L,were used. Aliquots of the water samples were taken at different times of irradiation and total organic carbon (TOC) was analyzed. In some experiments, air was used, without HzOZ,which permitted to compare oxidative efficiency of two oxidant agents (02 and HZOJ; both are very used in photocatalytic processes. 79°/0 of mineralization was obtained using SPCR and 52’%ousing ULS. 1 Introduction Degradation of contaminants in water is an area of research interest and, in this sense, sunlight photoalteration processes are known to play an important role [1]. © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Waste Management and the Environment, D Almorza, CA Brebbia, D Sales & V Popov (Editors). ISBN 1-85312-907-0 322 Waste Management and the En~irownent Another way to study photodegradation of organic compounds in water samples is by means of sensitizers that accelerate the process of degradation which can be used for decontamination purposes [2]. In this sense, photodegradation, using sensitizers such as TiOz, is an area of environmental interest for the treatment of contaminated natural waters. Photosensitized degradation reaction is faster than photolysis reaction because photosensitizer accelerates the formation of reactive species, such as hydroxyl free radicals, which, in turn, attack organic contaminants. Several kinds of Ti02 can be used, but Degussa P-25 is most used to remove organic contaminants due to its great superficial area, 55+15 m2/g; it is very photoactive, and it is a very definite mixture of anatase and rutilo (70:30) [3,4]. Photocatalysis with Ti02 has been used to degrade several organic compounds, such as chlorinated herbicides [5-7], insecticides [8,9], algaecides [10], phenols [11,12], PCBS [13] and polyethoxylated surfactants [14]. The rates and efficiencies of photoassisted degradation with Ti02 are significantly improved in the presence of oxygen or by the addition of several inorganic oxidizing species, such as peroxydisulfate, periodate and peroxides [2]. The objective of this study was (1) to determine the mineralization of methomyl in water by using natural photolysis and ultraviolet radiation, and (2) to determine the photodegradation kinetic of methomyl by photocatalysis with TiOz. 2 Experimental section 2.1 Chemicals Lannate 40 sp (40% of methomyl) was from Du Pent, titanium dioxide P25 was from Degussa (Japan) and hydrogen peroxide (30’Yo)was from Carlo Erba (Milan-Italy). 2.2 Equipment and materials Pump, Little Giant, 2E-38N, Oklahoma City (USA). Membrane filters 0.45 pm and 47 mm id. from Schleicher & Schuell (Dassel, Germany). Optima air pump fi-omRalf Hagen (Mansfield, USA) and TOC-5000 Shimadzu analyzer. ULS (30 liters) and SPCR (80 liters) were built in University of Antioquia. 2.3 Photolysis experiments Irradiation was carried out using a ULS equipped with a mercury lamp. Other experiments were carried out using a SPCR, which was built with a reflecting material as aluminum and Pyrex glass tubes. Since Medellin is situated at 6 degrees, north latitude, SPCR was inclined 6 degrees. With both systems, drinking water was spiked with lannate at 1000 mg/L, and with the help of a pump was passed through ULS or SPCR. The solution with © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Waste Management and the Environment, D Almorza, CA Brebbia, D Sales & V Popov (Editors). ISBN 1-85312-907-0 Waste Management and the En~’irownent 323 ULS was stirred with a shaker. At different periods of time, water samples were removed fkom reactors and filtrated (with membrane filters of 0.45 ~m), zmdthen organic carbon analysis was carried out. All the experiments were done by triplicate. 2.4 Photocatalysis experiments Irradiation was carried out using ULS and SPCR. With both systems drinking, water was spiked with lannate at 1000 mg/L, Ti02 and H202. Table 1: Experiments carried out with ULS and SPCR systems Ti02 (mg/L) H202(mL) Air ULS SPCR 50 1 No Yes No 50 2 No Yes No 50 3 No Yes No 100 1 No Yes Yes 100 2 No No Yes 150 1 No Yes No 200 1 No No Yes 200 2 No No Yes 250 1 No No Yes 250 2 No No Yes 50 0 Yes Yes No 100 0 Yes Yes Yes 200 0 Yes No Yes 250 0 Yes No Yes Air used in photocatalysis was injected in the solution, but H202 was not used. At different periods of time, water samples were removed fi-omreactors and filtrated (with membrane filters of 0.45 pm), and then organic carbon analysis was carried out. In the experiments of photocatalysis using SPCR, two caudals were used, All the experiments were done by triplicate. 2.5 Calculation of half-life time Eqn (1) shows first-order rate equation that was used in the calculation of half-life time [1]: c,= COe”k’ (1) Where C, represents the concentration at time t; C. represents the initial concentration; and k is the rate constant. When the concentration is reduced to 50’ZOof its initial amount, half-life (tI/z)can be determined by: t,/2 = 0.693/k (2) Where k is the degradation constant, © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Waste Management and the Environment, D Almorza, CA Brebbia, D Sales & V Popov (Editors). ISBN 1-85312-907-0 324 Waste Management and the En~irownent 3 Results and discussion 3.1 Photolysis The study of photodegradation kinetic using a ULS was carried out with and without TiOz, which permitted to know the enhancement of methomyl photodegradation rate by photocatalysis. After 60 hours of irradiation with radiation of 254 nm (mercury lamp of ULS), 12.40/o of methomyl was mineralized. Figure 1 shows photolysis of methomyl using a ULS, With SPCR, but with only 40 hours of irradiation, 109’owas mineralized. With both systems, the experiments were carried out in similar experimental conditions; however, the standard deviation of three replicates with SPCR was bigger than three replicates of ULS, which can be explained because it is difficult to control atmospheric conditions. ‘=-+-+-- 32s ! o 1023394059 69 Tme (hews) Figure 1: Average residual TOC of methomyl photolysis using a ULS. Three replicates were carried out. Each replicate of the experiments using a SPCR was carried out during five days, 8 hours of irradiation/day. Moreover, intensity of radiation is not constant in a day. Figure 2 shows photolysis of methomyl using a SPCR. 3.2 Photocatalysis Table 2 shows the results of photocatalysis with ULS. In the experiments with H20Z, the increase of TiOz concentration reduced the percentage of methomyl mineralization, which is due to the fact that high amounts of TiOz attenuate radiation which passes through the transversal section of the photoreactor. Mercury lamp is placed in concentric position at the photoreactor. This same © 2002 WIT Press, Ashurst Lodge, Southampton, SO40 7AA, UK. All rights reserved. Web: www.witpress.com Email [email protected] Paper from: Waste Management and the Environment, D Almorza, CA Brebbia, D Sales & V Popov (Editors). ISBN 1-85312-907-0 Waste Management and the En~’irownent 325 conclusion has been obtained in photocatalysis experiments with air, and so with 50 mg/L of TiOZ,26.84?40of methomyl were mineralized, and with 150 mg/L of TiOz, 23.23’%.ofmethomyl were mineralized. The amount of hydrogen peroxide increased the percentage of methomyl mineralization; in this way, more hydroxyl fkee radicals (HO) were produced and, consequently, more methomyl was oxidized. Two caudals of air injected in the solution were used, but the increase of the mineralization percentage was minimum. The mineralization percentage with hydrogen peroxide was higher than with the air, that is due to that a hydroxyl free radical is an oxidant agent stronger than a superoxide radical anion (02-.), that is produced Ilom oxygen. Zeu , 220 ; IILI 1 1 202 ; I 0 $0 20 30 4C Xm (lb”,,) Figure 2: Average residual TOC of methomyl photolysis using a SPCR. Three replicates were carried out. Table 2: Percentage of methomyl mineralization in photocatalysis using ULS. HZOJ(mL/L) TiOz (mg/L) Air 0/0of mineralization STD 1 50 No 28.61 1.56 1 100 No 24.62 1.33 1 150 No 23.75 0.87 2 50 No 38.62 1.68 3 50 No 51.61 1.52 0 50 Yes 26.84* 0.31 0 50 Yes 29.57** 1.84 0 100 Yes 24.32* 1.45 0 150 Yes 23.23* 1,66 * 5 L/rein.