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A NEW TOOL FOR LABORATORY STUDIES ON VOLATILIZATION: EXTENSION OF APPLICABILITY OF THE PHOTOVOLATILITY CHAMBER

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A NEW TOOL FOR LABORATORY STUDIES ON VOLATILIZATION: EXTENSION OF APPLICABILITY OF THE PHOTOVOLATILITY CHAMBER
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  791 Environmental Toxicology and Chemistry, Vol. 22, No. 4, pp. 791–797, 2003   2003 SETACPrinted in the USA0730-7268/03 $12.00    .00 A NEW TOOL FOR LABORATORY STUDIES ON VOLATILIZATION: EXTENSION OFAPPLICABILITY OF THE PHOTOVOLATILITY CHAMBER A NDRE´  W OLTERS ,*† T HOMAS  K ROMER ,† V OLKER  L INNEMANN ,† A NDREAS  S CHA¨ FFER ,‡ and H ARRY  V EREECKEN † †Forschungszentrum Ju¨lich, Institute of Chemistry and Dynamics of the Geosphere IV: Agrosphere, 52425 Ju¨lich, Germany‡University of Technology Aachen, Biology V: Environmental Chemistry, 52074 Aachen, Germany(  Received   5  November   2001;  Accepted   24  May  2002) Abstract —Volatilization from soil and plant surfaces after application is an important source of pesticide residues to the atmosphere.The laboratory photovolatility chamber allows the simultaneous measurement of volatilization and photodegradation of   14 C-labeledpesticides under controlled climatic conditions. Both continuous air sampling, which quantifies volatile organic compounds and 14 CO 2  separately, and the detection of surface-located residues allow for a mass balance of radioactivity. The setup of the photo-volatility chamber was optimized, and additional sensors were installed to characterize the influence ofsoilmoisture,soiltemperature,and evaporation on volatilization. The modified flow profile in the glass dome of the chamber arising from the use of a high-performance metal bellows pump was measured. Diminished air velocity near the soil surface and a wind velocity of 0.2 m/s in 3cm height allowed the requirements of the German guideline on assessing pesticide volatilization for registration purposes to befulfilled. Determination of soil moisture profiles of the upper soil layer illustrated that defined water content in the soil up to adepth of 4 cm could be achieved by water saturation of air. Cumulative volatilization of [phenyl-UL- 14 C]parathion-methyl rangedfrom 2.4% under dry conditions to 32.9% under moist conditions and revealed the clear dependence of volatilization on the watercontent in the top layer. Keywords —Volatilization Photovolatility chamber Parathion-methyl Soil surface application INTRODUCTION After the application of pesticides, their environmental fateis affected by decisive processes, such as uptake by plants,degradation, fixation, metabolization, and translocation in thesoil. Volatilization from soil and plant surfaces is a majorsource of residues of many pesticides in the atmosphere andthus may lead to pollution of ecosystems and organisms byair or rainwater transport [1–4].Early studies to elucidate the volatilization of soil surface–applied compounds were performed primarily in small labo-ratory volatilization chambers where transferability to outdoorconditions was limited by the experimental design, for ex-ample, low wind speed/air exchange rates [5,6]. Attempts tomeasure volatilization under field-like conditions include thedevelopment of wind tunnel systems, which combine the ad-vantages of laboratory facilities (e.g., use of radioisotopes)and field studies [7–10]. Direct determination of pesticide vol-atilization in the field can be achieved by using micrometeo-rological methods, which are very sophisticated and thereforenot suitable for routine investigations [3,11]. In addition, nocomplete mass balances can be obtained in the field forobviousreasons.Despite intense research, reliable models for predicting thefate of surface-applied pesticides that describe all the relevantprocesses adequately and that have been tested and validatedagainst field measurements are not yet available. The largenumber of parameters and processes to be taken into consid-eration is a major problem in modeling volatilization. Detailedinvestigations varying critical parameters will enhance the un- * To whom correspondence may be addressed(a.wolters@fz-juelich.de).Presented at the Organic Soil Contaminants Meeting, SETAC Eu-rope, Copenhagen, Denmark, September 2–5, 2001. derstanding of key processes and allow theoreticaldescriptionsof volatilization.The laboratory photovolatility chamber was developed inaccordance with the requirements of the U.S. EnvironmentalProtection Agency and the Society of Environmental Toxi-cology and Chemistry [12,13]. It allows the simultaneous mea-surement of volatilization and photodegradation of   14 C-labeledpesticides under controlled constant but variable climatic con-ditions [14,15]. The apparatus is intended to be used for adetailed study of the direct and indirect photodegradation pro-cesses of surface-located pesticides while simultaneously re-cording volatile organic compounds and the CO 2  released by(photo)mineralization.This paper presents the improvement and extension of thephotovolatility chamber to characterize the influence of soilmoisture, soil temperature, and evaporation on volatilizationbehavior from soil surfaces. The setup of the chamber (e.g.,instruments, data logging system, air-conditioning, and waterreplenishing system) and the type of soil application wereoptimized and allow the adjustment of scenarios similar tofield conditions. Because of increased flow rates passingthrough the chamber, the air sampling system had to be val-idated to ensure quantitative adsorption of pesticides. Mea-surements include the compilation of soil moisture profiles of the upper soil layer and the determination of the wind profilein the glass dome of the chamber. After validation of the newdevice, volatilization rates of [phenyl-UL- 14 C]parathion-meth-yl applied on gleyic cambisol were measured using differentenvironmental scenarios, and the results obtained were com-pared with previous studies. METHODS Concept of the improved photovolatility chamber  The photovolatility chamber (Fig. 1) and the air-condition-ing unit are installed in an environmental chamber to obtain  792  Environ. Toxicol. Chem.  22, 2003 A. Wolters et al.Fig. 1. Scheme of the photovolatility chamber (air-conditioning unit and sun test apparatus are installed in a climate chamber). AF  activatedcharcoal filter; AI    air inlet; AO    air outlet; CIW    cooled intensive wash bottle; CP    ceramic plate; D1    first drying stage (silica gel);D2    second drying stage (phosphorus pentoxide); DF    dust filter; FM    flow meter; DQ    double-walled quartz vessel with water jacket;GD    glass dome; GM    gas meter; IR    infrared sensor; KR  cryostat; ML  mercury lamp for ozone generation; MLU  ozone analyzer;P    metal bellows pump;   P    moisture tension; PUF    polyurethane foam plugs; RH/T    control of relative humidity and air temperature;S/P    soil/plant container; V    control valve; WB    wash bottle; WR    water reservoir.Fig. 2. Scheme of the water replenishing system. AI   air inlet; AO   air outlet; CP    ceramic plate; NV    needle valve; SP   suctionpump; TDR  time domain reflectometry; VG  vacuum gauge; WB   Woulff bottle; WR    water reservoir; WS    water surface. constant preconditioned climate parameters. The air passingthrough the chamber is purified via a filter system consistingof activated charcoal and dust filters to ensure defined andreproducible conditions while reducing photochemical effectsfrom trace gases and particulate contaminations in the sampleair. To achieve water-saturated air and to prevent drying of thesoil surface, the airstream passes through two wash bottles. Inorder to study the influence on photodegradation of environ-mental chemicals at the interface to the atmosphere, ozone canbe selectively added using a bypass system containing a low-pressure mercury lamp (Penray) [18]. During the experiments,a metal bellows pump (Ansyco, Karlsruhe, Germany) providesconstant flow rates up to 12 L/min passing through the cham-ber.The center of the chamber consists of two major modulesto be used optionally. Apparatus 1 consists of a water-cooled,ultraviolet-permeable quartz dome mounted inside an irradi-ation device (Sun Test apparatus CPS  , Atlas Material TestingSolutions, Hanau, Germany) on a specially integrated baseplate. The setup guarantees adjustable air temperatures in thechamber [15,17]. The irradiation device is composed of anadjustable xenon burner and a 290-nm cutoff filter in order tosimulate natural sunlight as closely as possible. Precise mea-surements with a spectroradiometer scanning with a doublemonochromator revealed that 0.1% of the spectral photon ir-radiance in the ultraviolet range was below 293 nm. Conse-quently, the irradiation inside the photovolatility chamber wasmore intense when compared to a reference day at the For-schungszentrum Ju¨lich (Germany) site at noon (June 16, 1997;50.6  N, 6.2  E; solar zenith angle of 30  ). Natural variations of the spectral photon intensity due to time of day, latitude, andvariation with season can be simulated approximately by astepwise programming of the irradiation device. Air temper-ature as well as relative humidity of the air fed into the pho-tovolatility chamber can be adjusted to the required conditionsdepending on irradiated and nonirradiated scenarios.Apparatus 2 was developed for process studies on volatil-ization in the dark without using an irradiation device. Anadditional infrared sensor for measuring the temperature of thesoil surface is installed at the center of the glass dome. Anal-ogous to apparatus 1, the glass dome (Fig. 3A) is mounted ona base plate to allow the use of containers with different sur-faces (e.g., glass, Teflon  , soil dust, soil layers, and soil/plantsystems). Containers of adjustable height can be used for soilbodies of different thickness and provide a surface area of approximately 0.01 m 2 (18.0    5.6 cm).An air analysis system is connected to record gaseous loss-es. This system is based on the setup srcinally designed forwind-tunnel experiments [9,10]. The  14 C-labeled organic com-pounds in the sample air are collected in the total volumesampler consisting of a glass cartridge filled with three pre-cleaned polyurethane foam plugs ([PUF], 30 mm o.d., 3  50mm). In addition,  14 CO 2  arising from the complete minerali-zation of the test compound is measured by a medium-volumesampler to gain complete radioactivity and mass balances[10,19].During the experiments, significant climatic parameters, in-cluding ozone concentration, air humidity, air temperature,andsoil temperature, are monitored continuously using varioussensors (Fig. 1).  Photovolatility chamber for studies on volatilization  Environ. Toxicol. Chem.  22, 2003 793Table 1. Physicochemical active ingredient data for parathion-methyl [20,21]CAS RN a Chemical nameEmpirical formula298-00-0 O,O -dimethyl- O -(4-nitro-phenyl)-phosphorus thiorateC 8 H 10 NO 5 PSMolecular weight (g/mole)Water solubility (mg/L)Vapor pressure (20  C) (hPa) [22]Henry’s law constant ( K   H  ) [23]263.2551.1 E-52.5 E-6Structural formula  n -octanol-H 2 O coefficient (log  P ow )Soil adsorption coefficient ( K  oc ) (L/kg)3.09,800.0 a Chemical Abstracts Service registry number.*   14 C-label (phenyl uniformly labeled).Table 2. Application parameters and experimental conditions of photovolatility chamber studies with [phenyl-UL- 14 C]parathion-methyl a on gleyic cambisolSoil application typeFirst soilsurfaceexperimentSecond soilsurfaceexperimentFormulation EC b (40% a.i.)Duration (d)Applied radioactivity ( k  Bq)Applied a.i. (  g/0.01 m 2 )4167.186.16140.872.6Climatic parameters (average values during experimental periods)Air humidity (%) c Air temperature (  C)Soil surface temperature (  C)Air exchange rate (L/min)Evaporation (mm/d) e Soil moisture 2.5 cm (% vol )43.1    5.119.4    0.219.7    0.212.1    0,11.1    0.315.7    0.289.6    1.0 d 19.3    0.119.4    0.110.9    0.71.0    0.120.3    0.2 a UL    uniformly labeled. b EC    emulsion concentrate, specific radioactivity; 1.94 MBq/mga.i., radiochemical purity:   99.0%. c Before passing through the glass dome. d Use of wash bottles to achieve water-saturated air. e Calculated from differences between relative humidity of airstreambefore and after passing through the glass dome.Fig. 3. Determination of air velocity. ( A ) Scheme of the glass domeof the photovolatility chamber. ( B ) Flow profile in the glass dome of the photovolatility chamber. Measurements were performed using athermal anemometer at an air exchange rate of 10 L/min and an airtemperature of 20  C above the center of the soil surface. All data arepresented as means  standard deviation, each after 10-min recordingof air velocity. Validation of air sampling system and determination of  flow profile Prior to use in photovolatility chamber experiments, pre-liminary tests were carried out in order to verify the adsorptioncapacity of the PUF plugs and to prevent pesticide losses be-cause of increased air exchange rates up to 12 L/min afterextension of the chamber. Validation studies were performedusing a special vaporization apparatus [10]. Defined amountsof [phenyl-UL- 14 C]parathion-methyl (  100 mg) were heated,completely vaporized, and passed through the air samplingsystem at an exchange rate up to 16 L/min. Vaporization tem-peratures of up to 65  C were used. After 24 h, the plugs weremanually squeezed (four times in 35–45 ml methanol), andall contaminated components were washed with acetone. Pes-ticide concentrations were determined by liquid scintillationcounting (TRI-CARB 2500, Canberra Packard, Frankfurt, Ger-many).Furthermore, the air velocity at the soil surface was mea-sured using a thermal anemometer (Airflow Developments,High Wycombe, UK). The velocity-sensitive thermistor wasfixed in the glass dome (Fig. 3A) at several heights (intervalsof 3–5 mm). A number of readings of each height were usedto calculate the average velocity. Pressure within the chamberwas measured using a pressure transducer (Leybold Vakuum,Cologne, Germany).  Measurement of soil moisture During the studies on bare soil, the soil moisture was ad- justed by a water replenishing system shown in Figure 2 [14].The volumetric water content was measured by time domainreflectometry equipment at a depth of 2.5 cm. After finishingexperiments, the soil was removed in layers (5 mm thick), andthe soil moisture of the layers was determined by gravimetricanalysis.  Experiments with [phenyl-UL- 14 C]parathion-methyl The  14 C-labeled radioactive parathion-methyl (Table 1) wasobtained from Sigma-Aldrich, Deisenhofen, Germany. Ac-cording to the Food and Agriculture Organization of the UnitedNations [16], the experimental soil was classified as a gleyiccambisol; the A p  horizon (plow layer) contained 0.99% C org ,75.2% sand, and 3.2% clay. For the experiments, the soil wasair-dried and sieved with mesh size 2 mm. For the applicationof emulsion concentrate-formulated  14 C-parathion-methyl, anairbrush with a mean droplet diameter of 200   m and anapplication volume of 400 to 500   l/0.01 m 2 , correspondingto 400 to 500 L/ha, was used. Subsequently, the soil container(depth 5 cm) was mounted onto the base plate of the photo-volatility chamber, and studies were performed using apparatus2 (Fig. 1). The application parameters and experimental con-ditions are compiled in Table 2.Air samples were taken at intervals of up to 24 h. The PUFplugs were manually squeezed (four times in 35–45 ml meth-anol). At the end of the experiments, all contaminated partsof the setup were rinsed with acetone to obtain a completeradioactivity balance. Radioactivity of extracts wasdeterminedby liquid scintillation counting (Canberra Packard). Aliquots  794  Environ. Toxicol. Chem.  22, 2003 A. Wolters et al.Table 3.  14 C recovery in the total volume sampler after vaporization of [phenyl-UL- 14 C]parathion-methyl a Duration (h)Vaporization temperature (  C)Average airflow rate (L/min)Applied radioactivity ( k  Bq)Applied substance (mg)246513.4    0.811.37103.3   PUF b plugs ( k  Bq)PUF sampling efficiency (%)System losses (contamination) (%)Average  14 C recovery (%)11.54    0.08101.5    0.71.8    0.3103.3    0.4 a UL    uniformly labeled. b PUF    polyurethane foam.Fig. 4. Soil moisture profiles for gleyic cambisol in the soil container.( A ) First soil surface experiment. ( B ) Second soil surface experiment.Water content of soil layers was determined gravimetrically at the endof the experiments. of the upper soil layers were extracted with methanol in aSoxhlet apparatus for 16 h. Nonextractable radioactivity in soilmaterial was measured by combustion (Canberra Packard,TRI-CARB Sample Oxidizer 306). The active ingredient of air and soil samples was characterized for parathion-methyland its metabolites by radio-high-performance liquid chro-matography and radio-thin-layer chromatography in combi-nation with a bioimaging analyzer (Fujix BAS 100, Fuji, To-kyo, Japan). RESULTS AND DISCUSSION  Air velocity in the glass dome of the photovolatilitychamber  During previous experiments on photodegradation,constantflow rates of up to 3.5 L/min corresponding to a wind velocityof 0.02 m/s passed through the chamber [15]. With respect tovolatilization studies, under field-like conditions the air ex-change rate had to be enhanced to fulfill the requirements of the German guideline on assessing pesticide volatilization. Forregistration purposes, this guideline implies a wind velocityabove the surface of 1 m/s [24]. With respect to the volumeof the chamber, air exchange rates are to be adjusted to realisticscenarios described by logarithmic flow profiles. Field-likeexperiments using a wind tunnel [10] performed with a definedwind velocity of 1 m/s at 20 cm height require a wind velocityof 0.15 to 0.20 m/s at 1 to 2 cm above the soil surfaceaccordingto Prandtl’s law [25]. Using the photovolatility chamber, thiscorresponds to an air exchange rate of 10 to 12 L/min.Exchange rates were increased using a powerful suctionpump generating the flow profile shown in Figure 3B. Theflow rate of 10 L/min changed the atmosphere in the glassdome of the chamber 30 times per minute and minimized thepotential buildup of volatilized contaminants, as evidenced bylow contaminant concentrations on the surface of the glassdome after experiments with parathion-methyl (Table 4). Thewind velocity rose with increasing distance from the soil sur-face, reaching a maximum at 3.3 cm (0.3 m/s) and decreasedhigher up because of frictional resistance caused by the glassdome. These experimental results, including diminished airvelocity near the soil surface, are in good accordance with thepreviously deviated values and confirm that suitable aerody-namic conditions to perform process studies on volatilizationand to meet the requirements for studies for registration pur-poses can be obtained in the photovolatility chamber.In addition, measurements of the pressure within the cham-ber revealed that the system is under slight negative pressurewhen the flow rate exceeds 9 L/min; for example, a flow rateof 9.5 L/min corresponds to an air pressure of 987 mbar (at-mospheric pressure: 1018 mbar; air temperature: 20  C). Validation of the total volume sampler  In earlier vaporization studies, it was shown that the airsampling system of the photovolatility chamber (total volumesampler) ensures quantitative adsorption of   14 C-parathion-methyl and its metabolites on the PUF plugs using flow ratesof about 3 L/min [14]. The results of preliminary experimentsto verify the effectiveness of the total volume sampler usingincreased air exchange rates are summarized in Table 3. Itcould be shown that the vaporized compound had been com-pletely adsorbed onto the first plug, the following plugs re-maining pesticide free. Dimensions of sampling efficiency andrecovery rate agree with validation studies regarding wind-tunnel experiments with parathion-methyl [9]. Taking into ac-count that the applied concentrations and flow rates in thevalidation experiments were much higher than those used inprocess studies (photovolatility chamber), it is obvious thatthe sampling unit is suitable even for high flow rates.  Determination of soil moisture: Time domain reflectometryequipment and gravimetric analysis A basic requirement for process studies under defined en-vironmental conditions is to ensure constant soil moisture. Thewater replenishing system (Fig. 2) allows the adjustment of adefined water tension, which correlates with the volumetricwater content measured by time domain reflectometry equip-ment. Although the soil moisture remains constant at a defineddepth, a gradient is built up within the soil container; forexample, the airflow causes a fast drying of the surface layer.This gradient is essential for adsorption and volatilization, es-pecially in the top layer [26]. To measure soil moisture profilesafter finishing the experiments, soil layers were removed andwater contents determined by gravimetric analysis. Withoutremoistening of the soil surface (air humidity: 40–50%), analmost complete drying of the top layer was observed, and themoisture profile reached a maximum near the ceramic plate(Fig. 4A). When the surface layer (a few millimeters) driesout totally, adsorption of chemicals to the soil surfaceincreasessignificantly, and volatilization rates decrease [2]. To char-acterize the influence of soil moisture on volatilization, it is  Photovolatility chamber for studies on volatilization  Environ. Toxicol. Chem.  22, 2003 795Table 4.  14 C recoveries from experiments with [phenyl-UL- 14 C]parathion-methyl (data in % of net applied radioactivity)First-soilsurfaceexperimentSecond-soilsurfaceexperimentContamination a Soil (0–0.7 cm)Soil (0.7–1.2 cm)Soil (1.2–5.2 cm)0.291.70.40.10.361.30.10.1LeachateVolatilizationMineralization to  14 CO 2   14 CND b 2.4ND94.8ND32.95.299.9 a Contamination of the glass dome and the air sampler by volatilization. b ND    not detectable.Fig. 5. Cumulative volatilized radioactivity after soil surface appli-cation of [phenyl-UL- 14 C]parathion-methyl to gleyic cambisol deter-mined in polyurethane foam plugs. Net applied radioactivity (AR)  100%. Soil moisture (depth of 2.5 cm): 15.7% vol  (first soil surfaceexperiment) and 20.3% vol  (second soil surface experiment).Table 5. Characterization of   14 C-labeled compounds in the methanol extracts of soil and polyurethanefoam plugs (data in % of extracted radioactivity)Soil layer (0–7 mm)FirstexperimentSecondexperimentPUF plugs a FirstexperimentSecondexperimentParathion-methyl4-NitrophenolParaoxon-methylUnknown polar products82.25.6ND b 12.272.712.0ND15.397.11.10.11.896.90.8ND2.4 a PUF    polyurethane foam; values averaged within the experimental duration. b ND    not detectable. therefore essential to prevent drying and to ensure defined soilmoisture in the top layer.The use of wash bottles to achieve water-saturated air (airhumidity about 95%) increased soil moisture at the surface,and values remained nearly constant down to a depth of 4 cm(Fig. 4B). This effect of relative humidity on surface moistureis in good accordance with previous studies [2]. Experimentson volatilization of parathion-methyl from glass surfacesshowed that increased air humidity did not affect volatilizationrates [22]. Water-saturated air influences volatilization mainlythrough its effect on soil moisture and does not directly influ-ence volatilization of pesticides. Thus, water saturation bywash bottles is appropriate for use in experiments on the in-fluence of soil moisture on volatilization.This improved experimental facility establishes an essentiallink to lysimeter and field studies, in particular to studies onthe volatilization of pesticides. The modifications and im-provements of the photovolatility chamber illustrated in thisarticle allow the performance of studies under well-definedfield-like conditions. This offers the opportunity to selectivelyvary environmental parameters to identify crucial aspects of these processes and enables various processes such as pho-todegradation and volatilization to be studied in detail. As-yet-unanswered questions (e.g., photochemical degradation),as frequently occur in lysimeter, field, and wind tunnel ex-periments, can thus be studied under controlled conditions,making an essential contribution to understanding the pro-cesses. Volatilization after soil surface application of [phenyl-UL- 14 C]parathion-methyl on gleyic cambisol The functionality of the photovolatility chamber and the airsampling unit is documented by total  14 C recoveries of 94.8and 99.9% applied radioactivity (Table 4), which is consideredto be in agreement with acceptable error given the complexityof the experiment. System contamination was very low(  0.3% applied radioactivity), which can be attributed to theuse of glass and polytetrafluoroethylene as the main construc-tion materials and high air exchange rates [27].Because of the constant environmental conditions duringthe experiments (Table 2), with the exception of soil moisture,an unambiguous dependence of volatilization on the watercontent in the top layer of the soil can be established (Fig. 5).A complete drying of the top layer of the soil usually occursunder field and field-like conditions and is caused mainly bysolar irradiation. This results in reduced volatilization rates of pesticides adsorbed on the soil surface [2,10]. As a result of the strong adsorption of parathion-methyl (Table 1; soil ad-sorption coefficient  K  oc  9800.0) and the associated reductionof the effective vapor pressure [28], only a slight volatilizationof 2.4% applied radioactivity in the first experiment was de-tected after 4 d. Cumulative volatilization of the same mag-nitude was observed in wind-tunnel experiments after appli-cation of parathion-methyl on fallow soil under dry conditions[29]. Within the scope of this wind-tunnel study, increasedvolatilization rates were noticed when irrigation was provided,illustrating the general tendency of pesticides toward enhancedvolatilization under moist conditions [30]. In full accordance
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