Experimental study on stable isotopic fractionation of evaporating water under varying temperature Experimental study on stable isotopic fractionation of evaporating water under varying temperature

Experimental study on stable isotopic fractionation of evaporating water under varying temperature

  • 期刊名字:水科学与水工程
  • 文件大小:450kb
  • 论文作者:Hai-ying HU,Wei-min BAO,Tao WA
  • 作者单位:School of Civil Engineering and Transportation,College of Hydrology and Water Resources
  • 更新时间:2020-07-08
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Water Science and Engineering, 2009, 2(2): 11-18dol:10388/jissn.1674-2370.2009.02.002htp://kb.hhu.edu.cne-mail: wse@hhu.edu.cnExperimental study on stable isotopic fractionation ofevaporating water under varying temperatureHai-ying HU*l,2, Wei-min BAO', Tao WANG', Si-min QU2I. School ofCivil Engineering and Transportation, South China University of Technology,Guangzhou 510641, P. R. China2. College of Hydrology and Water Resources, Hohai Universiny, Nanjing 210098, P. R. ChinaAbstract: The variation of stable isotope ratios in natural waters provides valuable information thatcan be used to trace water movement. Evaporation plays a crucial role in determining the variationof stable isotopes. In this paper, several evaporation experiments were conducted in order to studythe stable isotopic fractionation mechanism of water and analyze the influence of differenttemperatures on evaporation fractionation. Three group experiments of water evaporation underdifferent temperatures and initial isotopic values were carried out. The results show thatfractionation factors of hydrogen and oxygen may increase with temperature, and the averageenrichment degree of hydrogen isotope D is 3.432 times that of oxygen isotope 8O . The resultsalso show that the isotopic composition of the initial water has lttle influence on water evaporationfractionation, which is mainly affected by the state variables in the evaporation process, such astemperature. This research provides experimental data for further understanding the evaporationfractionation mechanism.Key words: water evaporation; hydrogen and oxygen isotopes; fractionation mechanism;temperature; experimental study1 IntroductionStable isotopes of oxygen and hydrogen are commonly used as hydrological tracers due totheir properties of mass conservation and measurable variability in water budget components(Wen 2002). As fingerprints of water, the stable isotopic species D and 80 respond greatly toenvironmental changes (Gat 1996; Gibson et al. 2005; Lee et al. 2007). Evaporation plays acrucial role in determining isotopic ratios of the subsequent processes of precipitation andrunoff. The isotopic composition of water in rivers, lakes, and soil may change in the course ofthe water's transformation into different states, owing to evaporation fractionation. Thevariation of hydrogen and oxygen isotopes in surface water and atmospheric vapor is caused bystable isotopic fractionation during phase changes (Zhang et al. 2005; Gremillion andWanielista 2000). The light isotopic species of water, H, 180 , has a higher vapor pressure and中国煤化工This work was supported by the National Natural Science FoulMHC N M H G679024) and theInnovation Program for College Graduate of Jiangsu Province of 2007 (Grant No. CX07B_ 130Z).*Corresponding author (e-mail: 108huhaiying@I 63.com)Received Feb.15, 2009; accepted May 13, 2009diffusivity than the heavy species, H2 180 and HDO, and is removed from the water surfacein greater proportion during evaporation (Tian et al. 2000; Cappa et al. 2003). The remainingwater thereby becomes enriched with the heavy isotopic species.With the improvements in mass spectrometric techniques, it is possible to accuratelymeasure stable isotopic components of water samples. The value of stable isotopic content canbe expressed by isotope abundance (8) ,which represents the relative deviation from theadopted standard representing mean isotopic composition of the global ocean (Qu et al.2006):δ=、 Rvsmow;-1 )|x 1000%0(1)where R and Rvsmow are the isotope ratios of the general water sample and the standardwater sample, respectively.The original model describing stable isotopic fractionation was developed by LordRayleigh for the case of fractional distillation of mixed liquids. The Rayleigh model is used forsystems in which the isotopic species removed at each moment are thermodynamic andisotopic equilibrium with those remaining in the system (Gat et al. 2001). Under Rayleighequilibrium conditions, stable isotopic ratios of remaining water increase exponentially as theremaining water proportion decreases. Craig and Gordon (1965) argued that after the primaryequilibrium fractionation, further fractionation should occur, and they suggested a model forisotope fractionation during non-equilibrium evaporation. Gonfiantini (1986) summarized thearguments of Graig and Gordon, ascribing the further depletion to kinetic effects duringmolecular diffusion within the boundary layer between the water-air interface and the fullyturbulent region. In that boundary layer, diffusion predominates because of slow atmospherictransport (Merlivat 1978).In this study, three group water evaporation experiments under different temperatures andinitial isotope ratios of liquid were conducted, and the stable isotopic fractionation mechanismsof waters were further investigated. The experiments generated some basic data which wouldbe helpful in understanding the basic law of evaporation fractionation and could provide a basisfor better use of isotope technology in hydrology.2 Water evaporation experiments under different temperatures2.1 Experimental introductionThree group water evaporation experiments with different initial isotopic values wereconducted at the State Key Laboratory of Hydrology-Water Resources and HydraulicEngineering, Hohai University, China. There were five evaporation pans in each group, and theevaporation pans were cylindrical, with heights and diameters of 35 cm. Each groupevaporation experiment lasted two days. The five evaporation pans in each group ofexperiments were illuminated by an incandescen中国煤化工ctive surfacetemperatures were controlled at 20C, 30'C, 40C, 5YHC N M H G° temperature12Hai-ying HU et al. Water Science and Engineeing, Jun. 2009, Vol. 2, No. 2, 11-18could be adjusted according to the power of the incandescent lamp, as shown in Fig. 1. Theisotopic values of initial water in each group of experiments are listed in Table 1. The firstgroup experiment was followed by two days of evaporation, after which the isotopicfractionation degrees of these five evaporation pans were different, owing to the difference intheir surface temperatures. The second and the third group evaporation experiments wereconducted in turn.IncandescentEvaporation4olamppanThermometer35 cm.Fig. 1 Schematic diagram of evaporation pan deviceTable 1 Isotopic values of initial water in each group experimentGroup8(*0) (%)8(D) (%o)-7.8-57.52-2.2-31.33.4-14.7Evaporation pans were placed a certain distance from each other in order to prevent themfrom influencing each other during the evaporation process. Observation of the evaporationcapacity was conducted at intervals. Since each group of experiments lasted only two days, thetemperature and relative humidity in the laboratory were nearly constant during thexperimental process, therefore, the influence of external circumstances were eliminated. Inaddition, the temperature and relative humidity of each evaporation pan were recorded.2.2 Sampling methodThe isotopic samplings are supposed to be taken along a vertical line in the water in orderto consider the isotopic stratification effects. However, it is difficult to acquire average samplesof the evaporated water along a vertical line. We designed a new method. As shown in Fig. 2, asmall insulated glass pipe with an open top is vertically inserted into the evaporation pan. Thus,the pipe is flldl with water including all layers. At that time, we envelop the top of the glasspipe that appears over the water surface. The water remains in the pipe due to the air pressure.Then, the glass pipe is taken out carefully, and the pipe water is poured into the sampling bottle.Repeat this process two or three times until the sampling bottles are full,The method described above is very simple and1sed to collect中国煤化工average water samples along a vertical line, avoiding CTYHCNMHGmarginal wallHai-ying HU et al. Water Science and Engineering, Jun. 2009, Vol.2, No. 2, 11-1813of the evaporation pans. Considering the spatial representation of the samples, the manipulationof sampling should be conducted both in the marginal and at the center of the evaporated watereach time. Then, the collected samples are sealed and kept away from direct sunlight and heatsources in order to prevent evaporation loss. All samples are taken to the Stable IsotopeLaboratory at the Institute of Geology and Geophysics of the Chinese Academy of Sciences foranalysis of oxygen and hydrogen stable isotopes. Oxygen samples are prepared using theCO2 H,O equilibration method.Deuterium samplesare prepared using theH2 H28O equilibration method with a Zn catalyst. The samples are measured with a massspectrograph and the analytical uncertainties are approximately +0.1%o for 8(*O) and +1%ofor 8(D).MarginalCentralsamplngsamplingFig. 2 Schematic diagram of sampling method3 Results and discussion3.1 Relationship between variation of stable isotopic valuesand temperatureIn the experiment process, the main factor in evaporation is temperature, since the ambientrelative humidity is comparatively stable. Fig. 3 presents the relationship between the variationof 8(*O) and 8(D) in the remaining water and the temperature. The variations wererepresented by A8(*O) and S8(D), respectively. It shows that S8(*O) and s8(D) areproportional to the temperature. The higher the temperature is, the greater the variation.Moreover, when the temperature is constant, the value of O8(D) is larger than that of△8(*O) , which indicates D is more sensitive to the change of temperature than 8O.8「◆First group◆First group▲Second gro25▲Second gyroup6 t口Third group食20 Third group▲马。写10gA680Temperature (C)(a) Relationship between 08190) and temperaure(b) Relationship between 08(D) and temperatureFig. 3 Variation of 8(*0) and 8(D) in remaining water under dfferent temperaturesThe difference in the vapor pressures of I中国煤化工lisproportionalenrichment in the water phase during evaporation. Thi|Y片CNMHGenrichmentin14Hai- ying HU et al. Water Science and Engineering, Jun. 2009, Vol. 2, No. 2, 11-18water, which is roughly eight times greater than '80 enrichment under equilibrium conditions.In the actual evaporation process, the enrichment times are different owing to the influence ofthe kinetic factor under non-equilibrium conditions. Fig. 4 shows that the variation of 8(D) isgreater than that of 8(*O) in the remaining water in the same evaporation pan under the sametemperature. The regression equation of s8(D) and 08(*O) in the three group evaporationexperiments is A8(D) = 3.43208(*O)+ 1.151 with the correlation coefficient (R2) of 0.845;therefore the average enrichment degree of D is about 3.432 times that of 180.0「李200&(80)(%)Fig. 4 Relationship of variations of 8(*0) and 8(D) in remaining water3.2 Relationship between the isotopic content in remaining water andevaporation capacityThree group experiments were carried out. Fig. 5 shows that the isotopic abundances inremaining water are proportional to the evaporation capacity in the evaporation experiment. Ineach group experiment, the higher the temperature is, the greater the evaporation capacity, andthus the larger the isotopic enrichment degree in remaining water. Moreover, it can be seenfrom Fig. 5 that the enrichment degree of isotope D is greater than that of 8O under the sameconditions. In addition, at a specific evaporation capacity, the variation of isotopic compositionin the remaining water will change if the isotopic composition of the initial water changes.However, the difference is very small, which can be seen from the slopes of the lines. Therefore,this shows that the isotopic composition of the initial water has little influence on waterevaporation fractionation, which is mainly affected by the state variables in the evaporationprocess, such as temperature.15「●Fist group0r●Seconderou▲Second group10 OThird group10口Third group官-30-50上-10-708 12i2Evaporation capacity (cm)(a) Relationship between 81*0) and evaporation capacity(b) Relationship between 8(D) and evaporation capacityFig. 5 Relationship between isotopic abundances in remaining water and evaporation capacity3.3 Analysis of relationship between temperature andfractionation factors中国煤化工YHCNMHG(1) Definition of fractionation factor:Hai-ying HU et al. Water Science and Engineering, Jun. 2009, Vol. 2, No. 2, 11-1815The fractionation factor ( a ) is an important index that reflects the evaporationfractionation degree between the liquid and gaseous phase. It is defined as the ratio of theisotope ratios of evaporating water and remaining water in the calculation interval, which canbe described asRg(2)Rwwhere Rp is the isotope ratios of evaporating water, and Rw is the isotope ratios ofremaining water. The isotope ratios are expressed asR(D)=[HDO][H2O](3)R(*0)=[H, 180][H2°叮]where R(D) is the isotope ratio of hydrogen, R(8O) is the isotope ratio of oxygen, and[HDO],[H,O], [H2 *0], and [H2 °0] are the molecular concentrations of HDO, H2O,H2 180 , and H2 "O, respectively.(2) Relationship between temperature and fractionation factors:Fractionation factors of different isotopic species reflect the change of circumstances(Horita and Wesolowski 1994; Hu et al. 2007). In the experiments, the isotopic fractionationfactors were mainly affected by the temperature. The stable isotopic fractionation factors werecalculated, and the relationships between the temperature and the fractionation factors of thestable isotopes in the three group evaporation experiments are described in Fig. 6. Thefractionation factors increase proportionally with temperature.0.9950.960 r0.9900.950e 0.9400.9800.930-0.9750 2040 60 800.920LTemperature(t)(旧) First group experimcat0.985.955 t0.935 ,0.965204060800.91580(b) Seond group expermeat10000.980 t0.960 |/◆0.9700.9202040608Temperature (C)(C) Third group experinentFig. 6 Relationship between temperature and fractiona中国煤化工rimentYHCNMHG6Hai-ying HU et al. Water Science and Engineering, Jun. 2009, Vol. 2, No. 2.11-184 ConclusionsEvaporation is a water cycle process with particular bearing on the variation of stableisotopes in natural waters. In this study, the relationships between evaporation fractionationfactors of stable isotopes and meteorological factors were analyzed with indoor waterevaporation experiments under different temperatures. The results indicate that the isotopicfractionation factors at the water-vapor interface are directly proportional to the temperature.Moreover, the average enrichment degree of the hydrogen isotope (D) in the evaporationfractionation process is 3.432 times that of the oxygen isotope (8O ), which indicates that D ismore sensitive to the change in the environment. The isotopic composition of the initial wateris also shown to have lttle influence on water evaporation fractionation, which is mainlyaffected by the state variables in the evaporation process, such as temperature. This researchprovides credible experimental data for further understanding the evaporation fractionationmechanism.ReferencesCappa, C. D.. Hendricks, M. B.. DePaolo, D. J.. and Cohen, R. C.2003. Isotopic fractionation of water duringevaporation. Journal of Geophysical Research, 108(D16), 1-10. [doi:10. 1029/2003JD003597]Craig, H, and Gordon, A.1965. Deuterium and oxygen 18 variations in the ocean and the marine atmosphere.Tongiorgi, E, ed. Stable Isotopes in Oceanic Studies and Paleotemperatures. Spoleto: ConsiglioNazionale delle Ricerche.Gat, J. R. 1996. Oxygen and hydrogen isotopes in the hydrologic cycle. Annual Review of Earth andPlanetary Sciences, 24, 225-262. [doi: 10.1 146/annurev.earth.24.1.225]Gat, J. R., Mook, W. G, and Meijer, H. A. J. 2001. Atmospheric Water (Volume I). Mook, W. G, ed,Environmental lsotopes in the Hydrological Cycle: Principles and Applications. Paris: UNESCO/IAEA.Gibson, J. J, Edwards, T. W. D.. Birks, S. J., St Amour, N. A.,. Buhay, W. M.. McEachem, P., Wolfe, B. B., andPeters, D. L. 2005. Progress in isotope tracer hydrology in Canada. Hydrological Progress, 19, 303-327.[doi:10.1002/hyp.5766]Gonfiantini, R. 1986. Environmental isotopes in lake studies. Fritz, P, and Frontes, J. C., eds., Handbook ofEnvironmental Isotope Geochemistry, Volume 2. New York: Elsevier Scientific Pub. Co.Gremillion, P., and Wanielista, M.2000. Effects of evaporative enrichment on the stable isotope hydrology ofa central Florida (USA) River. Hydrological Processes, 14(8), 1465-1484. [doi:10.1002/1099-1085(20000615)14:8]Horita, J.. and Wesolowski, D. J. 1994. Liquid-vapor fractionation of oxygen and hydrogen isotopes of waterfrom the freezing to the critical temperature. Geochimica et Cosmochimica Acta, 58(16), 3425-3437. [doi:10.1016/0016-7037(94)90096-5]Hu, H. Y, Bao, w. M., Qu, s. M, and Wang, T. 2007. Fractionation mechanism of stable hydrogen andoxygen isotope in water body evaporating. Jourmal of China Hydrology, 27(3), 1-5. (in Chinese)Lee, K. s, Kim, J. M., Lee, D. R., Kim, Y, and Lee, D.2007. Analysis of water movement through anunsaturated soil zone in Jeju Island, Korea using stable oxygen and hydrogen isotopes. Jourmal ofHydrology, 345, 199- 211. [doi:10.1016/jjhydrol.2007.08.006]Merlivat, L. 1978. Molecular difusivitis of H2!°O, HD'*o and H2l*O in gases. Jourmal of Chemical Physics,69(6), 2864-2871.Qu, s. M.. Bao, W. M.. Shi, P, and Hu, H. Y. 2006. Review on中国煤化工methods. WaterResources and Power, 24(1), 80-83. (in Chinese)TYHCNMHGHai-ying HU et al. Water Science and Engineering, Jun. 2009, Vol., 2, No. 2, 11-1817Tian, L. D., Yao, T. D., Sun, W. Z., and Numaguti, A.2000. Study on stable isotope fractionation during waterevaporation in the middle of the Tibetan Plateau. Joumal of Glaciology and Geocryology, 22(2), 159-164.(in Chinese)Wen, D. G 2002. Application of the environmental isotopes to study on the attribution of regionalgroundwater resources. Earth Sciences, 27(3), 141-147. (in Chinese)Zhang, X. P.. Tian, L. D., and Liu J. M.2005. Fractionation mechanism of stable isotope in evaporating waterbody. Journal of Geographical Science, 15(3), 375-384. [doi:10.1360/gs050312]中国煤化工MYHCNMHG18Hai-ying HU et al. Water Science and Engineering, Jun. 2009, Vol. 2, No. 2, 11-18

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