- Email: [email protected]
Degradation of the (R)- and (S)-enantiomers of the herbicides MCPP and dichlorprop in a continuous field-injection experiment
Water Research 36 (2002) 4160–4164
Research note
Degradation of the (R)- and (S)-enantiomers of the herbicides MCPP and dichlorprop in a continuous field-injection experiment a,1 Kirsten Rugge . , Rene! K. Juhlerb, Mette M. Broholma, Poul L. Bjerga,* a
Environment & Ressources DTU, Technical University of Denmark, Bygningstorvet, Building 115, DK-2800 Lyngby, Denmark b GEUS, Geological Survey of Denmark and Greenland, Thoravej 4, DK-2400 Copenhagen NV, Denmark Received 30 October 2001; received in revised form 21 February 2002; accepted 15 March 2002
Abstract An aerobic field-injection experiment was performed to study the degradation and migration of different herbicides at trace levels in an aerobic aquifer at Vejen, Denmark. Mecoprop (MCPP) and dichlorprop monitored in a dense network of multilevel samplers were both degraded within a distance of 1 m after a period of 120 days. The study showed that no preferential degradation of the (R)- and (S)-enantiomers of MCPP and of dichlorprop took place as the enantiomeric forms of the phenoxy acids were degraded simultaneously in the aquifer. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Degradation; Dichlorprop; Enantiomers; Herbicides; MCPP; Mecoprop
1. Introduction Phenoxy acids are widely used as herbicides. Their frequent occurrence in groundwater indicates that they may be critical contaminants that deteriorate drinkingwater resources. This contamination may be a result of leaching from agricultural areas, but recent investigations show that point sources (landfills, machine pools, or market gardens, etc.) may also contribute. For point sources remedial actions may be a possibility. Phenoxy acids are regarded as degradable under aerobic aquifer conditions [1] and thus one promising approach is monitored natural attenuation (MNA). In MNA, the key issue is to provide evidence for mass removal at a field scale [2]. This does not only include disappearance of the compound in question, but also requires data to support that degradation is actually taking place. A range of different tools (footprints, lines *Corresponding author. E-mail address: [email protected] (P.L. Bjerg). 1 Present address: NIRAS, Consulting Engineers and Planners. Sortemosevej 2, DK-3450 Aller d, Denmark.
of evidence) have been suggested such as depletion of electron acceptors, generation of metabolic by-products, formation of degradation products, isotopic techniques, and enantiomeric ratios [2,3]. The applicability of the different tools will depend on the actual compound (molecular structure, isotopic forms, etc.) concentration levels, and degradation pathways. For herbicides, such an experience is limited and there is a need for investigating the applicability of different methods under well-controlled conditions. The phenoxy acids mecoprop (MCPP) and dichlorprop exist in two enantiomeric forms, the (R)- and (S)forms, but only the (R)-forms are active herbicides. Racemic mixtures of the herbicides have been applied to fields for many years. However, since 1980s mostly the pure active enantiomers of MCPP and dichlorprop (MCPP-P and dichlorprop-P) have been applied. An aerobic field-injection experiment was performed to study the degradation and migration of different herbicides at trace levels [1,4]. The experimental conditions provided a unique possibility to study the possible enantioselective degradation of MCPP and dichlorprop under well-controlled field conditions. Measurements of
0043-1354/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 3 - 1 3 5 4 ( 0 2 ) 0 0 1 3 1 - 8
K. Rugge et al. / Water Research 36 (2002) 4160–4164 .
MCPP and dichlorprop (R)- and (S)-forms were conducted to evaluate whether enantioselective degradation of these herbicides occurred in groundwater under natural conditions. This allowed for evaluation of the use of enantiomeric ratios as a tool for MNA.
2. Experimental section 2.1. Chemicals For the injection experiment, racemic mixtures of MCPP (analytical grade >99% pure, CAS RN 7085-190) and dichlorprop (analytical grade >99% pure, CAS RN 7547-66-2), were purchased from Sigma. For HPLC analysis, standards of reference quality were used for calibration. Dichlorprop (99.8% racemic, CAS RN 7547-66-2), dichlorprop-P (99% (R)-form, CAS RN 15165-67-0), MCPP (99% racemic, CAS RN 7085-19-0), and MCPP-P (99% (R)-form, CAS RN 16484-77-8) were purchased from Dr. Ehrenstorfer (Augsburg, Germany). All solvents were HPLC grade (Romil, Cambridge, UK).
4161
2.2. Injection experiment A continuous, natural-gradient field injection (duration 7 months) of herbicides was carried out in a shallow aerobic sandy aquifer at Vejen, Denmark (see [1] for details). The injection consisted of a stock solution (B1000 mg/L of each herbicide) of the phenoxy acids MCPP and dichlorprop, as well as of bentazone, DNOC, isoproturon, and 2,6-dichlorobenzamid (BAM, degradation product of dichlobenil) with bromide as a non-reactive tracer. The resulting individual herbicide concentration in the groundwater at the injection wells was B40 mg/L. The migration of the herbicides and bromide was monitored in a dense network of 98 multilevel samplers (up to a distance of 25 m from the injection wells) over a period of 14 months. The collected water samples were analyzed for bromide and the herbicides. The data were evaluated based on breakthrough curves, cross-sections, and moment analysis. Presence and disappearance of the chiral forms of MCPP and dichlorprop were followed over time in two centrally placed sampling points at distances of 1 and 5 m from the injection wells (see Fig. 1).
Fig. 1. Monitoring network for the field-injection experiment at the Vejen site, Denmark. Position of the bromide tracer plume from the field injection and locations of groundwater sampling points at 1 and 5 m distance for this study.
4162
K. Rugge et al. / Water Research 36 (2002) 4160–4164 .
2.3. Analytical methods The phenoxy acids were measured by HPLC as described by Broholm et al. [1]. This method only quantified the sum of the (R)- and (S)-forms of the respective compounds. Extraction and clean up for the measurements of the enantiomeric forms of the phenoxy acids were done using SPE as described by Juhler et al. [5]. For the enantiomer analysis, an HPLC system (1050 HP, Hewlett-Packard, Waldbronn, Germany) consisting of a quaternary high-pressure pump, a quaternary lowpressure degasser, an autosampler and a UV/VIS detector was used. For calibration of (S) enantiomers, the extinction coefficients of the (R) form were used. The column was a 200 4 mm2 i.d. Nucleodex a-PM 5 mm column (Phenomenex, Cheshire, UK). The system was operated isocratically using an eluent consisting of 70 vol% methanol and 30 vol% NaH2PO4 buffer (50 mM, pH 3.0 using H3PO4), the flow rate was 0.7 mL/min, and the column temperature was 451C. Total analysis time was 8 min and detection was made at 230 nm. The detection limit for both methods was approximately 1 mg/L.
Fig. 2. Break-through curves for MCPP, at a sampling point at 1 m distance from the injection wells at the Vejen Site, Denmark.
2.4. Enantiomeric fraction The data were interpreted by use of breakthrough curves (concentration versus time) and ratio plots. The enantiomeric fraction (EF) was applied as defined by Harner et al. [15]: EF ¼ ½R=ð½R þ ½SÞ; where [R] and [S] are the concentrations of (R)- and (S)MCPP.
3. Results 3.1. Degradation of MCPP and dichlorprop Degradation of MCPP was observed at 1 m from the injection wells after a lag period of approximately 60–80 days. Degradation was fast, with a decrease in concentration from approximately 40 mg/L to below the detection limit at 106–132 days, depending on the analytical method considered (Fig. 2). The results shown are normalized to bentazone, which were measured in the same analytical round as the phenoxy acids. Bentazone was not degraded, and neither the phenoxy acids nor bentazone were retarded in the injection experiment [1]. Therefore, bentazone could be used as a conservative tracer. In general, degradation of dichlorprop followed the same pattern as observed for MCPP for distances of both 1 and 5 m. A lag period of approximately 60 days
Fig. 3. Break-through curves for dichlorprop, at a sampling point at 1 m distance from the injection wells at the Vejen Site, Denmark. ‘‘Dichlorprop—(S)-dichlorprop’’ indicates the measurements of the total dichlorprop from the HPLC analysis subtracted by the specific (S)-dichlorprop from the enantiomer analysis.
was observed at 1 m distance followed by complete degradation of dichlorprop after 120 days (Fig. 3). 3.2. Degradation of enantiomeric forms MCPP and dichlorprop were measured using both sum and enantiomer-selective HPLC methods. In general, the measurements of the racemic mixture corresponded well to the sum of the two enantiomeric forms for MCPP. The EF for MCPP in the stock solution was 0.50, which confirms the initial racemic composition. The
K. Rugge et al. / Water Research 36 (2002) 4160–4164 .
measurements of (R)- and (S)-MCPP revealed that degradation occurred almost simultaneously for both enantiomers during the experiment (Fig. 2). The same pattern was observed at 5 m from the injection wells with a lag phase of 120 days followed by a complete removal of MCPP (below the detection limit) after 55 more days. Here simultaneous degradation of the two enantiomeric forms was also observed. (R)-MCPP versus (R+S)-MCPP concentrations are plotted (Fig. 4) and a good linear correlation indicates a constant EF value of 0.51 (indicated by the slope) for 1 m distance, while EF for 5 m distance was 0.49. Some scatter at low concentrations (below 2 mg/L) may be ascribed to analytical variations. The overall EFs were constant, strongly supporting that concomitant degradation of enantiomers occurred in the field-injection experiment. The measurements of the enantiomeric forms (R)- and (S)-dichlorprop also indicated that degradation occurred almost simultaneously for the enantiomers (Fig. 3). However, the analysis for the (R)-form was affected by matrix interference. The concentrations of the (R)-dichlorprop were, therefore, calculated from the difference between the HPLC analysis for both enantiomers and the (S)-dichlorprop. The analytical problems made calculation of EF quite uncertain, and no solid conclusions can be drawn from the data for dichlorprop. The degradation of (S)-dichlorprop was identical to the degradation of (R)-dichlorprop taking the variations in a field experiment into account (Fig. 3). At 5 m distance, a lag period of 120 days was observed followed by complete degradation after 146–189 days dependent on which analytical method was considered. Here again, an almost simultaneous degradation of (S)-dichlorprop and (R)-dichlorprop was observed.
4163
4. Discussion 4.1. Preferential degradation Our results suggest almost identical degradation patterns for both the (R)- and (S)-enantiomers (nonenantioselective degradation) of MCPP and probably also of dichlorprop. In contrast, enantioselective degradation has been proposed by other authors. Most of these studies describe degradation in soil, and only a few are concerned with groundwater. Heron and Christensen [16] studied the degradation of MCPP in groundwater and sediment-sampled downgradient in the Vejen Landfill. They showed aerobic degradation of a racemic mixture of MCPP, after a lag phase, to half of the applied concentration followed by a second lag phase before the degradation proceeded. They suggested that this pattern could indicate enantiomer selective degradation. Zipper et al. [3] measured equal concentrations of (R)- and (S)-MCPP in leachate from a disposal site in Switzerland. Samples downgradient from the landfill showed a significant excess of (R)-MCPP, indicating preferential degradation of (S)-MCPP during groundwater passage. Williams et al. [6] observed faster degradation of (S)-MCPP than of (R)-MCPP under aerobic conditions downgradient from landfills in the Lincolnshire Limestone. (S)-MCPP was also preferentially degraded in soil [7,14], by a pure soil bacteria strain [8], and with activated sludge under aerobic conditions [9]. Tett et al. [10], however, showed preferential degradation of (R)MCPP in soil. Preferential degradation of (S)-dichlorprop was observed in soil by Garrison et al. [11], and with activated sludge under aerobic conditions by Zipper et al. [9], whereas marine organisms preferentially degraded (R)-dichlorprop [12]. 4.2. Enantiomerization
Fig. 4. (R)-MCPP versus (R+S)-MCPP for two sampling points (1 and 5 m) at all sampling dates.
Enantiomerization, i.e. conversion of (R)- to (S)forms and vice versa, has been suggested to be of importance in groundwater, soil, lake, and river sediments. Williams et al. [6] observed lower concentrations of (R)-MCPP to (S)-MCPP in the iron- and nitratereducing zones of landfill leachate plumes and noted that one possible process involved might be inversion. Muller . and Buser [7] reported the formation of (R)MCPP and (R)-dichlorprop from the (S)-forms and vice versa in soil, however, equilibrium constants favored the (R) enantiomers. In lake and river sediments Buser and Muller . [13] observed conversion of (R)-MCPP and (R)dichlorprop to the (S)-forms, in contrast to little or no enantiomerization of the (S)- to (R)-forms. This indicates, that the presence of residues of either enantiomers does not necessarily reflect the composition of the compound used.
4164
K. Rugge et al. / Water Research 36 (2002) 4160–4164 .
It is unknown in our study whether the degradation involved enantiomerization of one chiral form of the phenoxy acids to the other followed by degradation of the latter, as discussed by Buser and Muller . [13]. We do not find this phenomenon likely at our field site, since the EF of approximately 0.5 was almost constant with time (Fig. 4), however, the phenoxy acids were injected in a racemic mixture which makes conclusions impossible.
[5]
[6]
[7]
5. Conclusions This study showed that both chiral forms of MCPP and dichlorprop, respectively, could be degraded almost simultaneously under field conditions in the aerobic aquifer at Vejen, Denmark. The absence of preferential degradation weakens the use of enantiomeric forms as an analytical tool for documentation of MNA. However, the discrepancy between our field experiment and the field observations at the . Kolliken landfill site [3], and the data reported by Williams et al. [6] suggest that preferential degradation of enantiomers of phenoxy herbicides is not fully understood.
[8]
[9]
[10]
Acknowledgements Nina Tuxen took active part in groundwater sampling and interpretation of the herbicide analysis. Jens S: S rensen performed sampling and pesticide analyses. Pernille Stockmar asssisted with the chiral analysis. Birte Brejl provided the figures. All contributions are gratefully acknowledged.
[11]
[12]
References [13] [1] Broholm MM, Rugge . K, Tuxen N, H jberg AL, Mosbaek H, Bjerg PL. Fate of herbicides in a shallow aerobic aquifer: a continuous field injection experiment (Vejen, Denmark). Water Resour Res 2001;37:3163–76. [2] NRC (National Research Council). Natural attenuation for groundwater remediation. Washington, DC: National Academy Press, 2000. [3] Zipper C, Suter MJ-F, Haderlein SB, Gruhl M, Kohler H-PE. Changes in the enantiomeric ratio of (R)- to (S)mecoprop indicate in situ biodegradation of this chiral herbicide in a polluted aquifer. Environ Sci Technol 1998;32:2070–6. [4] Broholm MM, Tuxen N, Rugge . K, Bjerg PL. Sorption and degradation of the herbicide 2-methyl-4,6-dinitrophenol (DNOC) under aerobic conditions in a sandy
[14]
[15]
[16]
aquifer in Vejen, Denmark. Environ Sci Technol 2001;35: 4789–97. Juhler RK, S rensen SR, Larsen L. Analysing transformation products of herbicide residues in environmental samples. Water Res 2001;35:1371–8. Williams GM, Harrison I, Noy DJ, Crowley O, Kalin R. The use of enantiomeric ratios to identify natural attenuation of mecoprop in the Lincolnshire Limestone. Proceedings of the Third International Conference on Groundwater Quality, University of Sheffield, June 2001. p. 376–8. Muller . MD, Buser H-R. Conversion reactions of various phenoxyalcanoic acid herbicides in soil. 1. Enantiomerization and enantioselective degradation of the chiral 2phenoxypropanoic acid herbicides. Environ Sci Technol 1997;31:1953–9. Zipper C, Nickel K, Angst W, Kohler H-PE. Complete microbial degradation of both enantiomers of the chiral herbicide mecoprop [(R/S-(2-(4-chloro 2-methylphenoxy) propionic acid] in an enantioselective manner by Sphingoni herbicidovorans sp. nov. Appl Environ Microbiol 1996; 62:4318–22. Zipper C, Bolliger C, Fleischmann T, Suter MJ-F, Angst W, Muller . MD, Kohler H-PE. Fate of the herbicide mecoprop, dichlorprop, and 2,4-D in aerobic and anaerobic sewage sludge as determined by laboratory batch studies and enantiomer-specific analysis. Biodegradation 1999;10:271–8. Tett VA, Willetts AJ, Lappin-Scott HM. Enantioselective degradation of the herbicide mecoprop [2-(2-methyl-4chlorphenoxy) propionic acid] by mixed and pure bacterial cultures. FEMS Microbiol Ecol 1994;14:191–200. Garrison AW, Schmitt P, Martens D, Kettrup A. Enantiomeric selectivity in the environmental degradation of dichlorprop as determined by high-performance capillary electrophoresis. Environ Sci Technol 1996;30: 2449–55. Ludwig P, Gunkel W, Huhnerfuss . H. Chromatographic separation of the enantiomers of marine pollutants. Part 5: enantioselective degradation of phenoxycarboxylic acid herbicides by marine microorganisms. Chemosphere 1992; 24(10):1423–9. Buser H-R, Muller . MD. Occurrence and transformation reactions of chiral and achiral phenoxyalcanoic acid herbicides in lakes and rivers in Switzerland. Environ Sci Technol 1998;32:626–33. Buser H-R, Muller . MD. Conversion reactions of various phenoxyalcanoic acid herbicides in soil. 2. Elucidation of the enantiomerization process of chiral phenoxy acids from incubation in D2O/soil system. Environ Sci Technol 1997;31:1960–7. Harner T, Wiberg K, Norstrom R. Enantiomeric fractions are preferred to enantiomeric ratios for describing chiral signatures in environmental analysis. Environ Sci Technol 2000;34. Heron G, Christensen TH. Degradation of the herbicide Mecoprop in an aerobic aquifer determined by laboratory batch studies. Chemosphere 1992;24:547–57.
Research note
Degradation of the (R)- and (S)-enantiomers of the herbicides MCPP and dichlorprop in a continuous field-injection experiment a,1 Kirsten Rugge . , Rene! K. Juhlerb, Mette M. Broholma, Poul L. Bjerga,* a
Environment & Ressources DTU, Technical University of Denmark, Bygningstorvet, Building 115, DK-2800 Lyngby, Denmark b GEUS, Geological Survey of Denmark and Greenland, Thoravej 4, DK-2400 Copenhagen NV, Denmark Received 30 October 2001; received in revised form 21 February 2002; accepted 15 March 2002
Abstract An aerobic field-injection experiment was performed to study the degradation and migration of different herbicides at trace levels in an aerobic aquifer at Vejen, Denmark. Mecoprop (MCPP) and dichlorprop monitored in a dense network of multilevel samplers were both degraded within a distance of 1 m after a period of 120 days. The study showed that no preferential degradation of the (R)- and (S)-enantiomers of MCPP and of dichlorprop took place as the enantiomeric forms of the phenoxy acids were degraded simultaneously in the aquifer. r 2002 Elsevier Science Ltd. All rights reserved. Keywords: Degradation; Dichlorprop; Enantiomers; Herbicides; MCPP; Mecoprop
1. Introduction Phenoxy acids are widely used as herbicides. Their frequent occurrence in groundwater indicates that they may be critical contaminants that deteriorate drinkingwater resources. This contamination may be a result of leaching from agricultural areas, but recent investigations show that point sources (landfills, machine pools, or market gardens, etc.) may also contribute. For point sources remedial actions may be a possibility. Phenoxy acids are regarded as degradable under aerobic aquifer conditions [1] and thus one promising approach is monitored natural attenuation (MNA). In MNA, the key issue is to provide evidence for mass removal at a field scale [2]. This does not only include disappearance of the compound in question, but also requires data to support that degradation is actually taking place. A range of different tools (footprints, lines *Corresponding author. E-mail address: [email protected] (P.L. Bjerg). 1 Present address: NIRAS, Consulting Engineers and Planners. Sortemosevej 2, DK-3450 Aller d, Denmark.
of evidence) have been suggested such as depletion of electron acceptors, generation of metabolic by-products, formation of degradation products, isotopic techniques, and enantiomeric ratios [2,3]. The applicability of the different tools will depend on the actual compound (molecular structure, isotopic forms, etc.) concentration levels, and degradation pathways. For herbicides, such an experience is limited and there is a need for investigating the applicability of different methods under well-controlled conditions. The phenoxy acids mecoprop (MCPP) and dichlorprop exist in two enantiomeric forms, the (R)- and (S)forms, but only the (R)-forms are active herbicides. Racemic mixtures of the herbicides have been applied to fields for many years. However, since 1980s mostly the pure active enantiomers of MCPP and dichlorprop (MCPP-P and dichlorprop-P) have been applied. An aerobic field-injection experiment was performed to study the degradation and migration of different herbicides at trace levels [1,4]. The experimental conditions provided a unique possibility to study the possible enantioselective degradation of MCPP and dichlorprop under well-controlled field conditions. Measurements of
0043-1354/02/$ - see front matter r 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 4 3 - 1 3 5 4 ( 0 2 ) 0 0 1 3 1 - 8
K. Rugge et al. / Water Research 36 (2002) 4160–4164 .
MCPP and dichlorprop (R)- and (S)-forms were conducted to evaluate whether enantioselective degradation of these herbicides occurred in groundwater under natural conditions. This allowed for evaluation of the use of enantiomeric ratios as a tool for MNA.
2. Experimental section 2.1. Chemicals For the injection experiment, racemic mixtures of MCPP (analytical grade >99% pure, CAS RN 7085-190) and dichlorprop (analytical grade >99% pure, CAS RN 7547-66-2), were purchased from Sigma. For HPLC analysis, standards of reference quality were used for calibration. Dichlorprop (99.8% racemic, CAS RN 7547-66-2), dichlorprop-P (99% (R)-form, CAS RN 15165-67-0), MCPP (99% racemic, CAS RN 7085-19-0), and MCPP-P (99% (R)-form, CAS RN 16484-77-8) were purchased from Dr. Ehrenstorfer (Augsburg, Germany). All solvents were HPLC grade (Romil, Cambridge, UK).
4161
2.2. Injection experiment A continuous, natural-gradient field injection (duration 7 months) of herbicides was carried out in a shallow aerobic sandy aquifer at Vejen, Denmark (see [1] for details). The injection consisted of a stock solution (B1000 mg/L of each herbicide) of the phenoxy acids MCPP and dichlorprop, as well as of bentazone, DNOC, isoproturon, and 2,6-dichlorobenzamid (BAM, degradation product of dichlobenil) with bromide as a non-reactive tracer. The resulting individual herbicide concentration in the groundwater at the injection wells was B40 mg/L. The migration of the herbicides and bromide was monitored in a dense network of 98 multilevel samplers (up to a distance of 25 m from the injection wells) over a period of 14 months. The collected water samples were analyzed for bromide and the herbicides. The data were evaluated based on breakthrough curves, cross-sections, and moment analysis. Presence and disappearance of the chiral forms of MCPP and dichlorprop were followed over time in two centrally placed sampling points at distances of 1 and 5 m from the injection wells (see Fig. 1).
Fig. 1. Monitoring network for the field-injection experiment at the Vejen site, Denmark. Position of the bromide tracer plume from the field injection and locations of groundwater sampling points at 1 and 5 m distance for this study.
4162
K. Rugge et al. / Water Research 36 (2002) 4160–4164 .
2.3. Analytical methods The phenoxy acids were measured by HPLC as described by Broholm et al. [1]. This method only quantified the sum of the (R)- and (S)-forms of the respective compounds. Extraction and clean up for the measurements of the enantiomeric forms of the phenoxy acids were done using SPE as described by Juhler et al. [5]. For the enantiomer analysis, an HPLC system (1050 HP, Hewlett-Packard, Waldbronn, Germany) consisting of a quaternary high-pressure pump, a quaternary lowpressure degasser, an autosampler and a UV/VIS detector was used. For calibration of (S) enantiomers, the extinction coefficients of the (R) form were used. The column was a 200 4 mm2 i.d. Nucleodex a-PM 5 mm column (Phenomenex, Cheshire, UK). The system was operated isocratically using an eluent consisting of 70 vol% methanol and 30 vol% NaH2PO4 buffer (50 mM, pH 3.0 using H3PO4), the flow rate was 0.7 mL/min, and the column temperature was 451C. Total analysis time was 8 min and detection was made at 230 nm. The detection limit for both methods was approximately 1 mg/L.
Fig. 2. Break-through curves for MCPP, at a sampling point at 1 m distance from the injection wells at the Vejen Site, Denmark.
2.4. Enantiomeric fraction The data were interpreted by use of breakthrough curves (concentration versus time) and ratio plots. The enantiomeric fraction (EF) was applied as defined by Harner et al. [15]: EF ¼ ½R=ð½R þ ½SÞ; where [R] and [S] are the concentrations of (R)- and (S)MCPP.
3. Results 3.1. Degradation of MCPP and dichlorprop Degradation of MCPP was observed at 1 m from the injection wells after a lag period of approximately 60–80 days. Degradation was fast, with a decrease in concentration from approximately 40 mg/L to below the detection limit at 106–132 days, depending on the analytical method considered (Fig. 2). The results shown are normalized to bentazone, which were measured in the same analytical round as the phenoxy acids. Bentazone was not degraded, and neither the phenoxy acids nor bentazone were retarded in the injection experiment [1]. Therefore, bentazone could be used as a conservative tracer. In general, degradation of dichlorprop followed the same pattern as observed for MCPP for distances of both 1 and 5 m. A lag period of approximately 60 days
Fig. 3. Break-through curves for dichlorprop, at a sampling point at 1 m distance from the injection wells at the Vejen Site, Denmark. ‘‘Dichlorprop—(S)-dichlorprop’’ indicates the measurements of the total dichlorprop from the HPLC analysis subtracted by the specific (S)-dichlorprop from the enantiomer analysis.
was observed at 1 m distance followed by complete degradation of dichlorprop after 120 days (Fig. 3). 3.2. Degradation of enantiomeric forms MCPP and dichlorprop were measured using both sum and enantiomer-selective HPLC methods. In general, the measurements of the racemic mixture corresponded well to the sum of the two enantiomeric forms for MCPP. The EF for MCPP in the stock solution was 0.50, which confirms the initial racemic composition. The
K. Rugge et al. / Water Research 36 (2002) 4160–4164 .
measurements of (R)- and (S)-MCPP revealed that degradation occurred almost simultaneously for both enantiomers during the experiment (Fig. 2). The same pattern was observed at 5 m from the injection wells with a lag phase of 120 days followed by a complete removal of MCPP (below the detection limit) after 55 more days. Here simultaneous degradation of the two enantiomeric forms was also observed. (R)-MCPP versus (R+S)-MCPP concentrations are plotted (Fig. 4) and a good linear correlation indicates a constant EF value of 0.51 (indicated by the slope) for 1 m distance, while EF for 5 m distance was 0.49. Some scatter at low concentrations (below 2 mg/L) may be ascribed to analytical variations. The overall EFs were constant, strongly supporting that concomitant degradation of enantiomers occurred in the field-injection experiment. The measurements of the enantiomeric forms (R)- and (S)-dichlorprop also indicated that degradation occurred almost simultaneously for the enantiomers (Fig. 3). However, the analysis for the (R)-form was affected by matrix interference. The concentrations of the (R)-dichlorprop were, therefore, calculated from the difference between the HPLC analysis for both enantiomers and the (S)-dichlorprop. The analytical problems made calculation of EF quite uncertain, and no solid conclusions can be drawn from the data for dichlorprop. The degradation of (S)-dichlorprop was identical to the degradation of (R)-dichlorprop taking the variations in a field experiment into account (Fig. 3). At 5 m distance, a lag period of 120 days was observed followed by complete degradation after 146–189 days dependent on which analytical method was considered. Here again, an almost simultaneous degradation of (S)-dichlorprop and (R)-dichlorprop was observed.
4163
4. Discussion 4.1. Preferential degradation Our results suggest almost identical degradation patterns for both the (R)- and (S)-enantiomers (nonenantioselective degradation) of MCPP and probably also of dichlorprop. In contrast, enantioselective degradation has been proposed by other authors. Most of these studies describe degradation in soil, and only a few are concerned with groundwater. Heron and Christensen [16] studied the degradation of MCPP in groundwater and sediment-sampled downgradient in the Vejen Landfill. They showed aerobic degradation of a racemic mixture of MCPP, after a lag phase, to half of the applied concentration followed by a second lag phase before the degradation proceeded. They suggested that this pattern could indicate enantiomer selective degradation. Zipper et al. [3] measured equal concentrations of (R)- and (S)-MCPP in leachate from a disposal site in Switzerland. Samples downgradient from the landfill showed a significant excess of (R)-MCPP, indicating preferential degradation of (S)-MCPP during groundwater passage. Williams et al. [6] observed faster degradation of (S)-MCPP than of (R)-MCPP under aerobic conditions downgradient from landfills in the Lincolnshire Limestone. (S)-MCPP was also preferentially degraded in soil [7,14], by a pure soil bacteria strain [8], and with activated sludge under aerobic conditions [9]. Tett et al. [10], however, showed preferential degradation of (R)MCPP in soil. Preferential degradation of (S)-dichlorprop was observed in soil by Garrison et al. [11], and with activated sludge under aerobic conditions by Zipper et al. [9], whereas marine organisms preferentially degraded (R)-dichlorprop [12]. 4.2. Enantiomerization
Fig. 4. (R)-MCPP versus (R+S)-MCPP for two sampling points (1 and 5 m) at all sampling dates.
Enantiomerization, i.e. conversion of (R)- to (S)forms and vice versa, has been suggested to be of importance in groundwater, soil, lake, and river sediments. Williams et al. [6] observed lower concentrations of (R)-MCPP to (S)-MCPP in the iron- and nitratereducing zones of landfill leachate plumes and noted that one possible process involved might be inversion. Muller . and Buser [7] reported the formation of (R)MCPP and (R)-dichlorprop from the (S)-forms and vice versa in soil, however, equilibrium constants favored the (R) enantiomers. In lake and river sediments Buser and Muller . [13] observed conversion of (R)-MCPP and (R)dichlorprop to the (S)-forms, in contrast to little or no enantiomerization of the (S)- to (R)-forms. This indicates, that the presence of residues of either enantiomers does not necessarily reflect the composition of the compound used.
4164
K. Rugge et al. / Water Research 36 (2002) 4160–4164 .
It is unknown in our study whether the degradation involved enantiomerization of one chiral form of the phenoxy acids to the other followed by degradation of the latter, as discussed by Buser and Muller . [13]. We do not find this phenomenon likely at our field site, since the EF of approximately 0.5 was almost constant with time (Fig. 4), however, the phenoxy acids were injected in a racemic mixture which makes conclusions impossible.
[5]
[6]
[7]
5. Conclusions This study showed that both chiral forms of MCPP and dichlorprop, respectively, could be degraded almost simultaneously under field conditions in the aerobic aquifer at Vejen, Denmark. The absence of preferential degradation weakens the use of enantiomeric forms as an analytical tool for documentation of MNA. However, the discrepancy between our field experiment and the field observations at the . Kolliken landfill site [3], and the data reported by Williams et al. [6] suggest that preferential degradation of enantiomers of phenoxy herbicides is not fully understood.
[8]
[9]
[10]
Acknowledgements Nina Tuxen took active part in groundwater sampling and interpretation of the herbicide analysis. Jens S: S rensen performed sampling and pesticide analyses. Pernille Stockmar asssisted with the chiral analysis. Birte Brejl provided the figures. All contributions are gratefully acknowledged.
[11]
[12]
References [13] [1] Broholm MM, Rugge . K, Tuxen N, H jberg AL, Mosbaek H, Bjerg PL. Fate of herbicides in a shallow aerobic aquifer: a continuous field injection experiment (Vejen, Denmark). Water Resour Res 2001;37:3163–76. [2] NRC (National Research Council). Natural attenuation for groundwater remediation. Washington, DC: National Academy Press, 2000. [3] Zipper C, Suter MJ-F, Haderlein SB, Gruhl M, Kohler H-PE. Changes in the enantiomeric ratio of (R)- to (S)mecoprop indicate in situ biodegradation of this chiral herbicide in a polluted aquifer. Environ Sci Technol 1998;32:2070–6. [4] Broholm MM, Tuxen N, Rugge . K, Bjerg PL. Sorption and degradation of the herbicide 2-methyl-4,6-dinitrophenol (DNOC) under aerobic conditions in a sandy
[14]
[15]
[16]
aquifer in Vejen, Denmark. Environ Sci Technol 2001;35: 4789–97. Juhler RK, S rensen SR, Larsen L. Analysing transformation products of herbicide residues in environmental samples. Water Res 2001;35:1371–8. Williams GM, Harrison I, Noy DJ, Crowley O, Kalin R. The use of enantiomeric ratios to identify natural attenuation of mecoprop in the Lincolnshire Limestone. Proceedings of the Third International Conference on Groundwater Quality, University of Sheffield, June 2001. p. 376–8. Muller . MD, Buser H-R. Conversion reactions of various phenoxyalcanoic acid herbicides in soil. 1. Enantiomerization and enantioselective degradation of the chiral 2phenoxypropanoic acid herbicides. Environ Sci Technol 1997;31:1953–9. Zipper C, Nickel K, Angst W, Kohler H-PE. Complete microbial degradation of both enantiomers of the chiral herbicide mecoprop [(R/S-(2-(4-chloro 2-methylphenoxy) propionic acid] in an enantioselective manner by Sphingoni herbicidovorans sp. nov. Appl Environ Microbiol 1996; 62:4318–22. Zipper C, Bolliger C, Fleischmann T, Suter MJ-F, Angst W, Muller . MD, Kohler H-PE. Fate of the herbicide mecoprop, dichlorprop, and 2,4-D in aerobic and anaerobic sewage sludge as determined by laboratory batch studies and enantiomer-specific analysis. Biodegradation 1999;10:271–8. Tett VA, Willetts AJ, Lappin-Scott HM. Enantioselective degradation of the herbicide mecoprop [2-(2-methyl-4chlorphenoxy) propionic acid] by mixed and pure bacterial cultures. FEMS Microbiol Ecol 1994;14:191–200. Garrison AW, Schmitt P, Martens D, Kettrup A. Enantiomeric selectivity in the environmental degradation of dichlorprop as determined by high-performance capillary electrophoresis. Environ Sci Technol 1996;30: 2449–55. Ludwig P, Gunkel W, Huhnerfuss . H. Chromatographic separation of the enantiomers of marine pollutants. Part 5: enantioselective degradation of phenoxycarboxylic acid herbicides by marine microorganisms. Chemosphere 1992; 24(10):1423–9. Buser H-R, Muller . MD. Occurrence and transformation reactions of chiral and achiral phenoxyalcanoic acid herbicides in lakes and rivers in Switzerland. Environ Sci Technol 1998;32:626–33. Buser H-R, Muller . MD. Conversion reactions of various phenoxyalcanoic acid herbicides in soil. 2. Elucidation of the enantiomerization process of chiral phenoxy acids from incubation in D2O/soil system. Environ Sci Technol 1997;31:1960–7. Harner T, Wiberg K, Norstrom R. Enantiomeric fractions are preferred to enantiomeric ratios for describing chiral signatures in environmental analysis. Environ Sci Technol 2000;34. Heron G, Christensen TH. Degradation of the herbicide Mecoprop in an aerobic aquifer determined by laboratory batch studies. Chemosphere 1992;24:547–57.