Effectiveness of Advanced Oxidation Processes with O3 and O3+H2O2
Total Page：16
File Type：pdf, Size：1020Kb
IOA Conference and Exhibition Valencia, Spain - October 29 – 31, 2007 Effectiveness of Advanced Oxidation Processes with O 3 and O 3+H 2O2 in Pesticides Degradation Natividad Miguel , María P. Ormad, Munia Lanao, Cristina Ibarz, José L. Ovelleiro Department of Chemical Engineering and Environmental Technologies. University of Zaragoza. Pedro Cerbuna, 12. 50009 Zaragoza (Spain). Tel: +34 976761156 Fax: +34 976762142 email: [email protected] Abstract The aim of this research work is to study the degradation of 44 organic pesticides, which are systematically detected in the Ebro River Basin (Spain), by advanced oxidation processes with O3 and H2O2. The pesticides studied are: alachlor, aldrin, ametryn, atrazine, chlorfenvinfos, chlorpyrifos, pp’ - DDD, op’ - DDE, op’ - DDT. pp’ - DDT, desethylatrazine, 3,4-dichloroaniline, 4,4’- dichlorobenzophenone, dicofol, dieldrin, dimethoate, diuron, a-endosulphan, endosulphan- sulphate, endrin, α-HCH, β-HCH, γ-HCH, δ-HCH, heptachlor, heptachlor epoxide A, heptachlor epoxide B, hexachlorobenzene, isodrin, 4-isopropylaniline, isoproturon, metholachlor, methoxychlor, molinate, parathion methyl, parathion ethyl, prometon, prometryn, propazine, simazine, terbuthylazine, terbutryn, tetradifon and trifluralin. The techniques applied are combinations of ozone and hydrogen peroxide and dosages used are -1 -1 3 mg O 3 L and weigh ratios H 2O2/O 3 (gg ) of 0.1, 0.5 y 2. The treatment with ozone removes 72% of the studied pesticides, whereas applying O 3+H 2O2 treatment, average degradation yields achieved are less. The maximum average degradation of the studied pesticides by O3+H 2O2 treatment is 51%, and it is achieved with the weight ratio -1 H2O2/O 3 (gg ) of.5. This indicates combined treatment don’t improve the degradation of studied pesticides. Key-words : Ozone, hydrogen peroxide, pesticides. Introduction The industrial activity increase and economic and social develop resulting have generate the growth of bid areas. This entails and complicates the supply of one the most essential element to live, water. Moreover, components give for environment and their human use have produced water pollution. Therefore, some parameters of water must to be altered to use it. In the case of human consumption, drinking process is totally necessary, process carried out in drinking water plants. Surface water and groundwater have a natural chemical composition. This composition results of the dissolution of soluble minerals and organic compounds. This natural composition can be modified by four contamination points: domestic water, industrial processes water, uncontrolled wastewaters and diffuse contamination. The last point can be the origin of the presence of pesticides in natural water, substances considered Hazardous Contaminants in accordance with current legislation about water [10,11]. Pesticides are a group of artificially synthesized substances used to fight pests and improve agricultural production. They are, however, generally toxic for living organisms and are difficult to degrade, being toxic agents with persistent bioaccumulative effects [9]. The use of pesticides also constitutes a risk for water quality in agricultural areas due to the fact that these components may 2.4 - 1 pass through the soil and subsoil and pollute surface waters and groundwater. In the Ebro River Basin (Spain), these substances are controlled via a Pesticides Control Network, which systematically analyzes 44 organic pesticides in surface waters. These pesticides were selected in accordance with their appearance in lists of hazardous substances and/or their high level of use in Spanish agriculture. Although the concentration of these substances detected in natural waters is generally very low, the maximum permissible concentration in human drinking waters in Spain is often exceeded [29], which establishes a limit of 0.5 µg/l as the total amount of pesticides and 0.1 µg/l for any single pesticide. Consequently, the treatment used to produce drinking water must guarantee the removal of these types of substances or at least reduce their concentration below the limits established in current legislation. A drinking water process consists of a group of operations more or less complex in accordance with natural water quality. In general, these operations are: sieve and bar system, preoxidation, activated carbon adsorption, coagulation-flocculation, intermediate oxidation, filtration through sand and final disinfection. Pesticides can be removed from water by oxidation and adsorption onto activated carbon steps. However, these techniques have some disadvantages. Activated carbon adsorption doesn’t destroy pesticides, but it a process that transfers contaminants from water to carbon, which generates a new problem of pollution. With respect to oxidation steps, they use to carry out with chlorine or sodium hypochlorite as oxidant agent. The fundamental problem associated with the use of these agents lies in the generation of by-products such as trihalomethanes, substances with proven carcinogenic power [2,4,7,21,23,25]. Due to this problem, some large plants now apply ozone in oxidation steps instead of chlorine or sodium hypochlorite due to the numerous advantages that this presents, in spite of its higher economic cost. Ozone has a high oxidant power and, in principle, does not generate hazardous organohalogenated by-products, such as trihalomethanes (THMs) [30,32]. Moreover, colour, smell, and dissolved iron and manganese can be removed via ozonation and coagulation may be improved [14,22]. The reactivity of organic compounds with ozone is a function of the functional groups present in each molecule. However, in presence of bromides, the ozonation of natural water produces brominated disinfection by-products, which are potentially carcinogenic [20,27]. Ozone may react with the organic matter present in the water via two distinct mechanisms: a direct or molecular reaction (low pH), by which cycleaddiction reactions and eletrophylic and nucleophylic attacks can be produced, and an indirect or radicalary reaction (basic pH). The indirect reaction takes place via chain mechanism by radicals generated in the decomposition of ozone (hydroxyl, superoxide, ozonide and hydroperoxide radicals). It’s proven that a large number of chlorinated pesticides react via the radical pathway [3,15,16,17,18,19]. The chain mechanism is describing next [24]: - ·- · Iniciation: O 3 + OH O 2 + HO 2 · ·- + HO 2 ↔ O 2 + H ·- + · Propagation: O 2 + O 3 + H 2O 2 + OH · + ·- OH + O 3 H + O 2 + O 2 - · · Termination: combinations of O 2 , HO 2 y OH . The ozone decomposition increases with the presence of OH -, hydrogen peroxide, photolysis by ultraviolet radiation and metallic catalysts. These techniques are called advanced oxidation processes, which are based generation of hydroxyl radicals, which are highly reacting, few selective and capable to mineralize contaminants without generate any by-product [5,6]. 2.4 - 2 The direct comparison of the efficiency of these processes is really complicated by their various of factors, such as pH, temperature, auxiliary oxidants or catalysts concentration, substrate nature, etc. [26]. The most of studied pesticides are organic chlorinated compounds, which reactivity with · hydroxyl radicals (OH ) is low, due to C-Cl bounds, on the contrary that C-H bounds, are inert to · ·- these radicals attack. To degrade these compounds, reductive radicals ( HO 2 /O2 ) are needed. Intermediated compounds with more hydrogen are formed and these compounds react faster with hydroxyl radicals, achieving their total degradation [26]. In this particular study, the aim is compare and study the effectiveness of treatments with O 3 (ozonation) and O3/H 2O2 (peroxone) to degrade 44 studied pesticides, which are systematically detected in the Ebro River Basin (Spain). Studied pesticides are shown in table 1. Material and methods Sample The natural water under study comes from the River Ebro, upstream from Zaragoza (Spain). Sampling took place during February, a month in which large amounts of pesticides are not historically registered due to the fact that the periods in which pesticides are applied and the first rains occur is between May and September. Accordingly, the initial concentration of pesticides obtained is very low and uniform for each of these. The Total Organic Carbon (TOC) of water is 2.5 mg C L -1. A single sample of 10 L is divided among several one litre amber glass bottles which are kept under refrigeration at 4 ºC until their subsequent preparation and analysis. Each 1 L sample is fortified with 500 ng L -1 of each of the pesticides under study so as to ensure its presence and to study its possible removal. Thus, the concentration of each pesticide in each sample is the sum of what was artificially added and what the natural water actually contained. These concentrations are shown in Table 1. The pH of the water is 8.2 and the TOC is 37 mg C L -1. Table 1. Studied pesticides and concentration in the studied sample Concentration Concentration Pesticide Pesticide (ng L -1) (ng L -1) Alachlor 500 γ-HCH 521 Aldrin 500 δ-HCH 500 Ametryn 500 Heptachlor 500 Atrazine 551 Heptachlor epoxide A 500 Chlorfenvinfos 500 Heptachlor epoxide B 500 Chlorpyrifos 520 Hexachlorobenzene 500 pp’-DDD 500 Isodrin 516 op’-DDE 500 4-Isopropylaniline 500 op’-DDT 500 Isoproturon 500 pp’-DDT 500 Metholachlor 524 Desethylatrazine 593 Methoxychlor 519 3,4-Dichloroaniline 658 Molinate 551 4,4’-Dichlorobenzophenone 519 Parathion ethyl 500 Dicofol 568 Parathion