K.-D. Zoh, T.-S. Kim, J.-G. Kim, K. Choi and S.-M. Yi Institute of Health & Environment, Seoul National University, Seoul, 110-799, South Korea (E-mail:
[email protected];
[email protected];
[email protected];
[email protected];
[email protected]) Abstract The solar photocatalytic degradation of methyl parathion was investigated using a circulating TiO2/solar light reactor. Under solar photocatalysis condition, parathion was more effectively degraded than solar photolysis and TiO2-only conditions. With solar photocatalysis, 20 mg/L of parathion was completely degraded within 60 min with a TOC decrease of 63% after 150 min. The main ionic byproducts during 2 þ photocatalysis recovered from parathion degradation were mainly as NO2 3 , NO2 and NH4 , 80% of the 22 32 sulphur as SO4 , and 5% of phosphorus as PO4 . The organic intermediates 4-nitrophenol and methyl paraoxon were also identified, and these were further degraded in solar photocatalytic condition. Two different bioassays (Vibrio fischeri and Daphnia magna) were used to test the acute toxicity of solutions treated by solar photocatalysis and photolysis. The Microtox test using V. fischeri showed that the toxicity expressed as EC50 (%) value increased from 5.5% to . 82% in solar photocatalysis, indicating that the treated solution is non-toxic, but only increased from 4.9 to 20.5% after 150 min in solar photolysis. The acute toxicity test using D. magna showed that EC50 (%) increased from 0.05 to 1.08% under solar photocatalysis, but only increased to 0.12% after 150 min with solar photolysis, indicating the solution is still toxic. The pattern of toxicity reduction parallels the decrease in TOC and the parathion concentrations. Keywords Parathion; TiO2; solar light; 4-nitrophenol; paraoxon; Microtox; EC50; V. fischeri; D. magna
Water Science & Technology Vol 53 No 3 pp 1–8 Q IWA Publishing 2006
Parathion degradation and toxicity reduction in solar photocatalysis and photolysis
Introduction
Organophosphorus pesticides have been widely used in agriculture and industry. Methyl parathion is one of the acutely toxic pesticides registered by the US EPA (US EPA, 2003), and can adversely affect the ecosystem. Parathion is readily absorbed through the skin and mucosal membranes. Parathion represents the number one cause of occupational and accidental intoxication and fatalities among the pesticides in developing countries (FAO, 2003). In the treatment of parathion, biological and physical processes, such as wetland and thermal decomposition, have some limitations in terms of time and energy consumption (Schulz et al., 2003; Germain et al., 2000). Advanced oxidation processes (AOPs) have been proposed as an alternative for the treatment of wastewater. Especially, many researches using TiO2 as heterogeneous semiconductor catalyst and solar light as energy source have been reported (Pignatello and Sun, 1995; Lee et al., 2002; Malato et al., 2003). One of the advantages in this technology is that photocatalysis may completely mineralize a variety of aliphatic and aromatic compounds under suitable conditions. The photocatalytic degradations of parathion with artificial UV were recently investigated (Doong and Chang, 1998; Konstantinou and Albanis, 2003). These studies focused on the kinetics, the identification of some intermediates, however, the degradation mechanism and the toxicity evaluation of the treated water were not investigated. Also, the application of solar light instead of artificial UV on the photocatalysis of parathion has not been researched. doi: 10.2166/wst.2006.069
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This paper focuses on the kinetics, mechanisms and end-product toxicity of parathion degradation in water using TiO2 and solar light. Parathion degradation rates were compared for TiO2-alone, solar photolysis (UV-only) and solar photocatalysis (solar light with TiO2). The mechanism of the solar photocatalytic degradation of parathion is discussed by referring to the analysis of intermediates, byproducts and TOC. Two different bioassays were performed to evaluate the toxicity of the treated water after solar photocatalysis and photolysis, and these results were compared with measurements of the byproducts and TOC. Toxicity was measured at different stages of parathion degradation using a modified Microtox 82% test using V. fischeri and a 48-h acute toxicity test using D. magna. Materials and methods
Methyl parathion (99.4%), methyl paraoxon (98.3%) and 4-nitrophenol (99%) were purchased from Chem Service. TiO2 (Degussa P-25) with a BET surface of 50 ^ 15 m2/g was used with no pre-treatment. All other reagents were of analytical grade and were used without further treatment. The photocatalytic experiments were performed in a circulating solar system consisting of a reservoir, a metering pump (Cole-Palmer Instrument Co.) and photoreaction chamber consisting of six quartz columns (10 mm diameter, 650 mm length), all of which were connected with flexible Teflon tubing, as shown in Figure 1. The reservoir was a stirred 2 L glass bottle. The solutions were circulated with a metering pump at a flow rate of 1 L/min. The light intensity was measured using a VLX-3W radiometer (Cole Parmer Instrument Co.) at 365 nm (UV-A). The experiments were carried out on a sunny day between September and October in 2004 between noon and 4 p.m. in Seoul, Korea (latitude 38 8N). Figure 2 shows the typical patterns of solar light intensities at 365 nm during the experiments. The average solar intensity on sunny days varied from 1.93 to 0.64 mW/cm2, as shown in Figure 2. The initial pH of the reaction solution was adjusted to 7.0 (^0.4). All the liquid samples were filtered through 0.2-mm MCE membrane filters (Advantec MFS) to remove the TiO2 particles before analysis. Parathion and its intermediates were determined by a Summite HPLC system (Dionex), equipped with an UVD340S detector, and RP C-18 silica column (25 cm £ 4.6 mm i.d., 5 mm particles, Supelco Park). The detection wavelength was 270 nm for parathion and paraoxon, and 320 nm for 4-nitrophenol. The ionic byproducts were analysed using a DX-120 ion chromatograph 2 32 (Dionex). The column was an Ionpac AS14 (Dionex) for NO2 and SO22 2 , NO3 , PO4 4 þ analyses, and an Ionpac CS 12-A column (Dionex) for the NH4 analysis. TOC analysis was performed on a Shimadzu TOC analyzer.
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Figure 1 Schematic diagram of the circular solar photocatalytic reactor
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Figure 2 Typical patterns of solar light intensities at 365 nm during the experiments
The acute toxicity assay was carried out using the reaction solutions during solar photocatalytic and photolysis. The toxicity was measured at different mineralization time by a Microtoxe test and a 48-h acute toxicity test. The Microtox test (modified 82% test) using V. fischeri was conducted according to Microbics Corp. (1992). The bacteria were exposed to a series of sample concentrations for 5 and 15 min at 15 8C. A Microtox Model 500 toxicity analyzer was used to measure light production as endpoint. The EC50 value (%) was calculated using MicrotoxOmni software (Azur Corp., ver. 1.16). The 48 h static acute toxicity test using D. magna was conducted in accordance with US EPA guidelines (2002). The endpoint of observation was mortality, which was recovered after 24 and 48-h of exposure. The EC50 value (%) was calculated using the Toxstat program (West Inc., ver. 3.4). Results and discusssion Degradation of parathion and TOC
In order to confirm the role of TiO2 in the solar photocatalytic reaction, three experiments were carried out. One set was performed with parathion (20 mg/L) exposed to TiO2 (1 g/L) but no UV (TiO2-only condition). The second set was performed by exposing parathion (20 mg/L) to solar light without TiO2 (solar photolysis). Then, the third set was performed by exposing parathion to TiO2 (1 g/L) in the presence of solar light (solar photocatalysis). Figure 3 shows the results. The experiment with TiO2-only showed a rapid decrease of less than 10% of parathion from the solution and the concentration remained constant for the remainder of the test. The disappearance of parathion is mostly likely due to adsorption to the TiO2, and similar results with TiO2-only have been observed with TNT (Son et al., 2004). Then, the solar photolysis reaction degraded 78% of the parathion concentration after 150 min, whereas the parathion was completely removed after 60 min in solar photocatalytic conditions. Formation of ionic byproducts and mineralization
Since parathion is a phosphorothioate insecticide that contains a nitro group, sulphur and 2 þ 32 22 phosphorus atom, the formations of NO2 2 , NO3 , NH4 , PO4 and SO4 are expected as the ionic degradation byproducts (Konstantinou and Albanis, 2003). In this study, we measured the ionic byproducts produced during solar photocatalytic and photolytic reactions. Figure 4 shows the change of the concentration of the ionic byproducts during
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K.-D. Zoh et al. Figure 3 Degradation of parathion and TOC reduction under the control conditions ([parathion]: 20 mg/L, TiO2: 0 g/L in the solar light only condition, TiO2: 1.0 g/L in the TiO2 only and solar þ TiO2 conditions)
solar photocatalysis and photolysis reactions, respectively. Figure 4(a) shows that approximately 80% of the total nitrogen initially present in parathion was converted to þ NO2 3 , while 15% of the NH4 was produced at the end of photocatalysis. The concen2 tration of NO2 initially increased to 20% of the initial nitrogen present in parathion, and then disappeared after 90 min in photocatalytic reaction. This result indicates that oxidation is the principal mode of photocatalytic degradation of parathion, and NO2 2 is first cleaved from parathion molecules during the photocatalytic reaction, and is then rapidly 2 oxidized to NO2 3 by a hydroxyl radical. Others have observed the conversion of NO2 to 2 NO3 by a hydroxyl radical (Low et al., 1991; Zoh and Stenstrom, 2002). The formation of NHþ 4 indicates that there is a concurrent reduction during the photocatalytic degradation of parathion. Piccinini et al. (1997) proposed that an electron (e2 CB) can reduce a nitro group to an amine. The resulting amine group can be detached from the benzene ring by a hydroxyl radical. Sulphate was also produced and the total sulphur recovery was 80%. This result indicates that an oxidation attack of hydroxyl radical on the P¼S bond first occurred, resulting in the formation of paraoxon and sulphate (Konstantinou et al., 2001). The formation was only observed after 90 min in the photocatalytic reaction, indicating that of PO32 4 PO32 4 is the last ionic byproduct of methyl parathion produced. (a) 100
(b) 100 80
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(C0-Ct) / C0 * 100 (%)
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Figure 4 Degradation of parathion and the formation of ionic byproducts under (a) solar photocatalysis and (b) solar photolysis ([parathion]: 10 mg/L, TiO2: (a) 1.0, (b) 0 g/L)
In contrast to solar photocatalysis, the total amount of nitrogen byproducts produced in the photolysis of parathion was much lower than photocatalysis. As shown in Figure 4(b), NO2 3 produced by the photolytic reaction, while still the major byproduct, represented only 15% of the total nitrogen initially present in parathion, and only 13% of sulphur was recovered as SO22 4 . This phenomenon can be explained by the much lower hydroxyl radical concentration in photolytic condition than photocatalytic condition.
TOC and ionic byproducts measurements implicate the evidence of mineralization during photolytic and photocatalytic of parathion. Sixty-three percent of TOC was mineralized by solar photocatalytic reaction, and only 43% was mineralized by photolysis. The low conversion of TOC by photolysis is believed to be due to the presence of organic byproducts of parathion, containing nitrogen, sulphur and phosphorus molecules since these were not completely recovered from ionic byproducts. Therefore, in order to confirm the mineralization during photocatalysis, the carbon-based organic byproducts were measured. Since paraoxon and 4-nitrophenol were previously found to be two major intermediates in the photocatalysis of parathion, these two byproducts were measured during photocatalysis and photolysis (Konstantinou and Albanis, 2003). Figure 5 shows the formation of these intermediates during the degradation of parathion. The production patterns of intermediates indicate that the concentrations of these intermediates increased with decreasing parathion concentration. Figure 5(a) shows that under photocatalysis, approximately 2.3 mM of paraoxon and 7.8 mM of 4-nitrophenol, as the highest concentration, were produced. The highest peak of paraoxon and 4-nitrophenol was observed within the first 30 min, and these intermediates were further degraded during solar photocatalysis. Under photolysis, however, only trace amounts of the intermediates were produced, as shown in Figure 5(b). These intermediates did not decrease but steadily increased to approximately 0.3 mM of paraoxon and 1.2 mM of 4-nitrophenol after 150 min.
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Formation of organic intermediates
Comparison of acute toxicity and TOC reduction
Numerous studies have reported on the photocatalytic degradation of organic contaminants in water, while few works have investigated the toxicity profile during the degradation (Fernandez-Alba et al., 2002). A wide range of oxidation intermediates, including the more toxic compounds than the parent compound, may be generated. In this study, less than 30% of carbon was recovered as paraoxon and 4-nitrophenol as the highest (b) 10 M-parathion (Solar+TiO2) M-paraoxon 4-nitrophenol
Methyl parathion (mM)
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Figure 5 Degradation of parathion and the formation of organic intermediates with (a) solar photocatalysis and (b) solar photolysis ([parathion]: 10 mg/L, TiO2: (a) 1.0, (b) 0 g/L)
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concentration in solar photocatalysis of parathion, and even a small amount of carbon was recovered in the photolytic degradation. This indicates the possibility of unknown byproducts production. Therefore, it is necessary to investigate the toxicity of the reaction solution at different reaction times. In order to test the reduction of toxicity during solar photocatalytic and photolytic degradation, acute toxicity assays such as Microtoxe modified 82% test using V. fischeri and 48-hr acute toxicity test using D. magna were conducted. Table 1 shows the results of the Microtox test using V. fischeri in the photocatalytic and photolytic reactions. The EC50 (%) rapidly increased to more than 82% within 90 min during photocatalysis, implying that the acute toxicity completely disappeared. In photolysis condition, the EC50 (%) only increased to less than 30% after 150 min, indicating that the acute toxicity for V. fischeri still remained. The results of the 48-h acute toxicity test using D. magna in the solar photocatalytic and photolytic reactions are summarized in Table 2. Under photocatalysis, the EC50 (%) increased from 0.05 to 1.08% after 150 min. However, in photolysis, the EC50 (%) only increased to 0.12% after 150 min, indicating that acute toxicity for D. magna was rarely reduced. Next, the decrease of parathion and TOC concentrations during photocatalysis and photolysis were compared with the relative toxicity expressed as follows: relative toxicity ¼
initial EC50 ð%Þ £ 100: observed EC50 ð%Þ
Figure 6 shows that the complete removal of parathion and 63% reduction in TOC during photocatalysis was responsible for the almost complete disappearance of relative toxicity for V. fischeri and D. magna. Under photolysis conditions, while 78% of the parathion and 43% of the TOC were removed, the relative toxicity decreased to 58% for D. magna and more than 76% for V. fischeri under photolysis. Two different toxicity assays indicate that parathion is more toxic for D. magna than V. fischeri. These results also indicate that the acute toxicity reduction closely correlated with the decrease of parathion and TOC concentrations. It is also clear that the intermediates such as paraoxon and 4-nitrophenol produced from parathion degradation are less toxic for V. fischeri and D. magna than the parent parathion compound. These are in good agreement with the results of Dzyadevych and Chovelon (2002) and Galli et al. (1994). Table 1 EC50 (%) values for Microtox test using the V. fischeri acute toxicity test Reaction time
EC50 (%) in solar þ TiO2 EC50 (%) in solar only
0 min
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10 min
30 min
90 min
150 min
5.5 (4.9–6.0) 4.9 (4.2–5.8)
11.8 (9.0–15.5) 5.4 (4.9–5.9)
13.8 (12.8 –14.8) 6.0 (5.3 –6.9)
55.4 (45.0 –68.2) 8.9 (8.5 –9.4)
.82
.82
16.4 (15.5 –17.4)
20.5 (19.3 –21.8)
Values in parentheses indicate the 95% confidence interval Table 2 The results of the 48-h acute D. magna toxicity test on the reaction samples Reaction time
EC50 (%) in solar þ TiO2 EC50 (%) in solar only 6
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0.05 (0.04 –0.06) 0.05 (0.04 –0.06)
0.07 (0.07 –0.08) 0.07 (0.05 –0.08)
0.34 (0.28 –0.41) 0.11 (0.10 –0.12)
1.08 (0.93–1.26) 0.12 (0.10–0.15)
Values in parentheses indicate the 95% confidence interval
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Figure 6 Comparison between acute toxicities reduction (V. Fischeri and D. magna) and parathion/TOC degradation in (a) solar photocatalysis and (b) solar photolysis ([parathion]: 10 mg/L, TiO2: (a) 1.0, (b) 0 g/L)
Conclusions
This study examined the efficacy of the degradation of parathion by TiO2 solar photocatalysis and photolysis. The conclusions were as follows: (1) Solar photocatalysis was more effective than solar photolysis in parathion degradation. Under the TiO2 only condition, less than 10% of parathion was adsorbed onto the TiO2 surface. The percent degradation of parathion under solar photocatalysis and photolysis for 150 min was 100 and 75%, respectively. In the same period, the % decrease in TOC was up to 63 and 43%, respectively. (2) The patterns of intermediate production showed that paraoxon and 4-nitrophenol increased with the decrease of parathion concentration, and disappeared completely under solar photocatalysis, while they increased steadily until 150 min under photolysis. 22 (3) NO2 3 and SO4 were the dominant ionic byproducts in the solar photocayalysis and photolysis, and the formation of PO32 4 was observed under photocatalysis. (4) The Microtox test using V. fischeri showed that the toxicity as expressed as EC50 (%) value decreased from 5.5 to . 82% with solar photocatalysis, but only reduced from 4.9 to 20.5% after 150 min in solar photolysis. The acute toxicity test using D. magna also showed that EC50 (%) decreased from 0.05 to 1.08% under solar photocatalysis, but only decreased from 0.05 to 0.12% after 150 min with solar light alone, relative to unreacted parathion. The pattern of toxicity reduction parallels the decrease in TOC and the parathion concentrations. These results show that the parathion degradation using solar photocatalysis is feasible, and not only removed the parathion and its intermediates, but also decreased their toxicity.
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