Environ. Sci. Technol. 2004, 38, 1537-1544
Test System To Establish Mass Balances for 14C-Labeled Substances in Soil-Plant-Atmosphere Systems under Field Conditions REINER SCHROLL* AND SABINE KU ¨ HN GSF-National Research Center for Environment and Health, Institute of Soil Ecology, Ingolstaedter Landstrasse 1, D-85764 Neuherberg, Germany
A test system has been developed that allows the formation of mass balances for 14C-labeled organic compounds in soil-plant-atmosphere systems under field conditions. The main focus was the quantification of all different 14C-labeled gaseous losses from soil and plant surfaces after the application of 14C-labeled chemicals; therefore, a two-chamber system with specially designed trapping facilities was designed. Volatile 14C-labeled compounds and 14CO2 resulting from the total degradation of the applied 14C-labeled chemical could be trapped separately, and 14C gaseous losses from plant and soil surfaces could be determined separately as well. With this test system, it was feasible for the first time to distinguish between 14C volatile and 14CO2 losses from soil surfaces and from plant surfaces in soil monolith systems under real environmental conditions. The system itself was established on surfaces of soil monoliths (lysimeters) to study the abovementioned processes along with the transport and leaching behavior of the chemicals in soil cores. With the new system, the behavior of organic chemicals was followed up for a whole vegetation period, and a mass balance for the applied chemical was established. Therefore, a better prediction of the long-term behavior of organic chemicals under real environmental climatic conditions was achieved. The first results for the fate of the model herbicide isoproturon in two different types of agricultural soils are presented and compared with results from an extensive laboratory study.
Introduction For many decades, soil monolith (lysimeter) experiments were conducted to estimate the environmental behavior of chemicals, heavy metals, and/or nutrients in different soils, mainly in European countries. The main target of these experiments was to determine the leaching of chemicals (14), heavy metals (5-7), and/or nutrients (8-14) into deeper soil layers. Most of the studies focused on the problem of groundwater contamination by chemicals. The use of radiolabeled compounds has increased the ease, sensitivity, and accuracy with which mass balance measurements can be conducted to determine the fate, behavior, and treatment efficiency of organic chemicals in * Corresponding author telephone: +49 3187 3319; fax: +49 3187 3376; e-mail:
[email protected]. 10.1021/es030088r CCC: $27.50 Published on Web 01/20/2004
2004 American Chemical Society
laboratory soil and water systems (15). This technique can also be used in limited test systems under outdoor conditions. Thus, a large number of outdoor lysimeter experiments were carried out with 14C-labeled chemicals to investigate their fate in agroecosystems for about 30 yr, mainly in Europe. Detailed information on the long-term behavior of chemicals in soils under practice-like conditions can be achieved with this type of experimental facility. These outdoor experiments permit a comprehensive study of the fate of chemicals in soil, plant, and leachate compartments by determining the remaining 14C-labeled residues in the compartments. It has been demonstrated that lysimeters with undisturbed soil monoliths reflect the field situation, referring to topsoil processes and the formation of plant residues with sufficient accuracy (16, 17). But it was shown that, due to 14C-labeled gaseous losses, it was not possible to achieve a complete 14C mass balance for several pesticides in these studies. In some cases (18), up to 45% of the applied 14C-labeled radioactivity was not found in the soil monolith at the end of the lysimeter experiment because processes such as mineralization and photolysis of the applied 14C-labeled chemical to 14CO2 could not be recorded. In laboratory microcosm degradation studies, the use of radiolabeled compounds with alkali or quaternary amine carbon dioxide trapping solutions has become very common in the past. Very low concentrations of radioactive carbon can be measured in each compartment of an environmental system, and liquid scintillation analysis of carbon dioxide trapping solutions is quick and easy (15). The formation of 14CO2 resulting from the mineralization of 14C-labeled pesticides was identified as the main reason for gaseous 14C losses in several laboratory experiments (1820). With newly developed highly sophisticated trapping systems, 14CO2 can now also be trapped in outdoor lysimeter experiments in our days (21). The process of the volatility of pesticides from surfaces of agricultural fields is the second reason for gaseous 14Clabeled losses. Some decades ago, pesticides were generally regarded as involatile. Today we are aware that, for most pesticides, volatilization is just as important as chemical and microbiological degradation in causing the dissipation of pesticides from soils. The atmospheric transport of pesticides after the application onto agricultural fields has been known for many years (22-27). The reason for these contaminations: beside the drift of pesticides during the application in the agricultural field, the process of the chemical volatilization from plant (28) and soil surfaces (29-33) plays an important role. Pesticide volatilization causes not only an initial but also a permanent migration from agricultural fields. The volatilization rates of chemicals are usually highest on the day of application, and then they continuously decrease (34, 35). The effect of chemical volatilization from soil surfaces is influenced by the physicochemical properties of the used pesticide; the amount and quality of soil organic matter; the amount of sand, silt, and clay of the soil; and the climatic conditions such as wind speed, temperature, precipitation, and air humidity (28). In 1994, a wind tunnel for measuring the gaseous losses of environmental chemicals from soil/plant systems under field-like conditions (16) was presented. But with this system it was not feasible to distinguish between 14C-labeled gaseous losses from plants and soil surfaces separately. Therefore, several reasons exist for the development of a new test system: (i) Many pesticides are applied to the agricultural field as so-called “post-emergence pesticides”; therefore, the pesticide is to some extent also applied directly on crop surfaces. VOL. 38, NO. 5, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 1. Two-chamber system to quantify the mineralization and volatilization of 14C-labeled chemicals from plant and soil surfaces separately. It is known that the amount of volatilized pesticide from plant surfaces is much higher than from soil surfaces (29). (ii) It was shown in a simple plant bioreactor (36, 37), where the soil and the root zone was compartmentalized separately from the plant shoots, that depending on the experimental conditions up to 33% of the applied 14C-labeled chemical (dioxan) was evapotranspired by plant leaves to the atmosphere. (iii) In cell culture experiments, it was shown (38) that the 14C-labeled chemical trichloroethylene was mineralized to some extent by hybrid poplars. This points out that plants might be able to some extent to mineralize chemicals and/ or their degradation products.
Materials and Methods A two-chamber system (Figure 1) was constructed (39), which consists of a so-called soil chamber to collect gaseous 14Clabeled compounds from soil surfaces and of a plant chamber to collect gaseous 14C-labeled losses from plant surfaces. Soil Chamber. The volatilization chamber is similar to the chamber that was presented previously (34). The chambersnow made from glass at the top and from stainless steel at the sidessis 28 cm long, 2 cm high, and 8 cm wide. Four metal sieves are placed in the inlet funnel of the soil chamber to obtain a laminar airstream in the chamber. The lower part of the chamber consists of a 10 cm long stainless steel metal frame that is pressed completely into the soil; 1538
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this metal frame limits the sampling area (20 cm length, 8 cm wide). Special designed sensors for determining soil humidity (40) and small thermometers were placed 5 mm below the soil surface in the middle of the volatilization chamber as well as outside of the chamber to compare the soil microclimates inside and outside of the chamber. To be able to simulate rainfall events in the chamber, the amount of precipitation was determined, and aliquots of the rainwater were collected in special reservoirs. Special equipment was installed in the volatilization chamber to simulate the precipitation on the soil surface. This equipment consists of two stainless steel metal tubesseach with a length of 19.5 cm and each with 18 0.7 mm wide holesswhich were placed in the chamber 1 cm above the soil surface. According to the frequency and the amount of the natural rainfall, the collected precipitation water was applied onto the soil in the chamber. The soil chamber was placed 55 d before the pesticide application on the lysimeter surface. Via a Teflon tube, air taken at a distance of 10 m from the lysimeter and not containing any 14C-labeled components was sucked through the chamber with a speed of up to 1.0 m/s according to the German BBA guideline for volatilization experiments (41). Subsequently, the air was washed out in a specially designed trapping system. To minimize wall adsorption effects, the sampling system was mounted shortly after the soil chamber, and all components in contact with the sampled air were made of stainless steel, glass, or Teflon.
The actual sampling head to trap volatile pesticide residues consists of two polyurethane foam plugs (T 3546, polyether type, 35 kg/m3, Seybold, 100 mm o.d.) each 50 mm thick. If the first foam is overloaded, the second foam indicates a breakthrough of the 14C-labeled chemical. The porosity of the foam is 60 ppi (pores per inch), which results in a sampling efficiency of more than 95% of the studied herbicide isoproturon; the efficiency was determined by feeding a known quantity of the 14C-labeled substance to a heated glass pipe, where it was vaporized and flushed via an airstream to the sampling unit. Recoveries exceeded 98% using an automatic press-and-wash apparatus. Air volumes of more than 1000 L/h do not allow an effective trapping of the 14CO2 resulting from the total degradation of the 14C-labeled chemical with the used trapping system. Therefore, behind the trapping system for volatile compounds, a split airstream (between 12.5 and 25.0% of the total airstream) was washed out in two cooled “intensive washing bottles” (Figure 1) for the quantitative absorption of CO2 and 14CO2. These washing bottles were filled with a mixture of ethanolamine and diethylene glycol monobuthyl ether (1:1 v/v). If overloaded, the second washing bottle indicates a breakthrough of the 14CO2. The maximum radioactivity measured in the second washing bottle during the whole lysimeter experiment was 98%. Pesticide Application. The 14C-labeled pesticide mixture was applied with a semiautomatic sprayer (Figure 2) driven by a single-axle linear unit similar to the technique used by refs 16 and 47. This equipment permits the homogeneous application of small volumes of pesticide solutions (10-100 mL/m2). The droplet spectrum was according to good agricultural practice. The distribution of the pesticide mixture on the sprayed surface was tested in preliminary experiments by applying a nonlabeled pesticide mixture containing blue ink on white paper. The variation of the pesticide distribution was approximately less than 10% in this pretest. The pressure at the nozzle (Teejet 8004E, Spraying Systems) was 2.3 bar. To avoid any uncontrolled application of the 14C-labeled herbicide onto the agricultural field around the lysimeters, the soil surface around the lysimeter was covered with a specially designed dashboard-box made of stainless steel. VOL. 38, NO. 5, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Semiautomatic sprayer placed on the lysimeter field during the pesticide application. The box and the sprayer were covered at the top with a plastic foil during the application to avoid any 14C-labeled herbicide drift. After the application, aliquots of the plastic foil were combusted in an automatic sample oxidizer to quantify potential amounts of 14C. No radioactivity was detected on the plastic foil. The dashboard-box was washed completely with organic solvents, and the radioactivity in the solvent was measured in aliquots by scintillation counting. The amount of radioactivity that was applied onto the lysimeter surfaces was calculated by subtracting all application losses () 14C-labeled radioactivity determined from the dashboardbox) from the initial amount of radioactivity. The application losses were between 32.95 and 39.59% of the initial 14C-labeled amount. Isoproturon was applied onto the lysimeters when the barley plants had a height of about 15 cm.
Analytical Procedures Trapping Systems. The polyurethane foams for trapping the volatile organic 14C-labeled compounds were extracted three times in an automatic wash-and-press system with acetone. The total volume of acetone was determined, and aliquots of the solvent were mixed with a scintillation cocktail (Ultima Gold XR, Packard, Dreieich, Germany). The total volume of the mixture for trapping 14CO2 (ethanolamine:diethylene glycol monobuthyl ether, 1:1 v/v) was measured as well, and aliquots were mixed with a scintillation cocktail (Hionic fluor, Packard, Dreieich, Germany). All samples were measured in a liquid szintillation counter (Tricarb 1900 TR, Packard). All samples were taken three times a week (Monday, Wednesday, and Friday). Analysis of Soil Extracts. Soil samples taken during the test periods and/or at the end of the experiments from all test systems were extracted with methanol in an ASE 200 (accelerated solvent extractor Dionex, Idstein, Germany) at 90 °C and with a pressure of 10 MPa. Preliminary experiments ensured that isoproturon could be extracted from soils 1540
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without the formation of any artifacts under these conditions. Aliquots of the extracts were measured by scintillation counting followed by a concentration of the extracts with a rotation evaporator to a volume of 1-2 mL. For the cleanup procedure, this volume was diluted with 300 mL of distilled water and extracted by SPE (ENV 200 mg, Varian, Darmstadt, Germany). After drying the SPE columns in a soft nitrogen stream, they were eluted with methanol (10 mL); the extract was concentrated to a volume of 100 µL and injected to a HPLC system that was equipped with a UV/VIS detector (240 nm, Merck, Darmstadt, Germany) and a radioactive detector LB 506 C1 (Berthold, Wildbad, Germany); column: LiChrospher 100 RP-18, 5 µm, 250 × 4 mm (Merck, Darmstadt, Germany); flow: 1.0 mL/min. Mobile phase (HPLC grade): A ) water, B ) acetonitrile. Gradient: T0, A ) 95%; T20, A ) 70%; T30, A ) 40%; T35, A ) 40%; T40, A ) 95%; T50, A ) 95%. The parent compound and the metabolites in the soil extracts were identified by comparison of their retention times with reference substances. Quantification of Nonextractable 14C-Labeled Residues. After the extraction of the soils, the extracted soil material was dried and homogenized intensively; at least 5 aliquots (around 300 µg each) of all soil samples were filled into combustion cups and mixed with 3-4 drops of saturated aqueous sugar solution to guarantee a complete oxidation of the total 14C in the soil samples. The combustion was conducted with an automatic sample-oxidizer 306 (Packard, Dreieich, Germany). 14CO2 was trapped in Carbo-Sorb E (Packard, Dreieich, Germany) and mixed with Permafluor E (Packard, Dreieich, Germany) prior to scintillation counting. Plant Analysis. The dried plant material was intensively homogenized, and aliquots of this material were combusted in the automatic sample-oxidizer to determine the total amount of 14C radioactivity. Leaching Water Analysis. Samples of the leaching water were taken once a week. The water was extracted with SPE
TABLE 1. Distribution and Mass Balance of 14C in the Lysimeter Soils Feldkirchen and Scheyern after the Harvest of the Barley Plantsa soil soils
leachate
residues
14CO
Feldkirchen Scheyern
0.28 0.68
35.20 72.29
57.95 17.51
a
2
plants volatility
residues
14CO
0.37 0.20
0.08 0.22
2.85 6.80
2
volatility
mass balance
0.12 0.90
96.85 98.60
% of applied radioactivity.
FIGURE 3. Mineralization of isoproturon in soils Scheyern and Feldkirchen, both under laboratory and lysimeter conditions.
columns (see above), and the residues were analyzed by HPLC as described above. Detailed results about the leaching water will be presented elsewhere at the end of the complete lysimeter experiment. Additional Experiments. The filling of a lysimeter with an undisturbed soil column was resource limited. Thus, it was not possible to run several parallels for each lysimeter experiment. As the mineralization of isoproturon was the main target process in this experiment, additional experiments concerning the degradation/mineralization of isoproturon in upper soil material from both lysimeters were conducted in numerous biodegradation tests in a special laboratory biodegradation system (19). The laboratory experiments were run at a temperature of 20 °C and at a soil humidity of 30% for both soils. A total of 46.5 g of soil (
