Analysis of Volatile Organic Compounds (VOCs) in Soil via Passive Sampling: Measuring Partition and Diffusion Coefficients by Hanqing Liu B.S Chemistry Renmin University of China, 2014 SUBMITTED TO THE DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING IN PARTIAL FULLFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF ENGINEERING IN CIVIL AND ENVIRONMENTAL ENGINEERING AT THE -MASSACHUSETTS INSTMTUTE MASSACHUSETTS INSTITUTE OF TECHNOLOGY ARCHNME OF TECHNOLOLGY JUNE 2015 JUL 02 2015 @2015 Hanqing Liu. All rights reserved. LIBRARIES The author hereby grants to MIT permission to reproduce and to distribute publicly paper and electronic copies of this thesis document in whole or in part in any medium now known or hereafter created. Signature of Author: Signature redacted Department of Civil an&Environmental Engineering Certified by: 21, 2015 Signature redacted Philip M. Gschwend Professor of Cix'il and Environmental Engineering Signature red acted 5,s 2 , - Accepted by: Thesis Supervisor Heidi Nepf Donald and Martha Harleman Professor of Civil and Environmental Engineering Chair, Departmental Committee for Graduate Students 1 Analysis of Volatile Organic Compounds (VOCs) in Soil via Passive Sampling: Measuring Partition and Diffusion Coefficients by Hanqing Liu Submitted to the Department of Civil and Environmental Engineering on May 21, 2015 in Partial fulfillment of the requirements for the Degree of Master of Engineering in Civil and Environmental Engineering ABSTRACT Passive sampling has been used as a qualitative and semi-quantitative method in detecting volatile organic compound (VOCs) concentrations in soil vapors or water. Passive sampling for soil vapor takes an absorptive material and places it underground for a period of time to allow the VOCs to diffuse into the absorptive materials. In this report, I use low density polyethylene (PE) as the absorptive material and determine two key parameters for passive sampling: the PE-water partition coefficient (Kpew) and diffusion coefficient in PE (Dpe). These two parameters help passive sampling to transition from a qualitative method to a quantitative method. The report describes the steps used to carry out the experiments, gives the results for several specific VOCs, and makes an attempt to draw more general conclusions on how to estimate these two parameters according to some other well-known properties. Thesis Supervisor: Philip M. Gschwend Title: Professor of Civil and Environmental Engineering 2 Table of Contents C hap ter 1 Introduction ........................................................................................................................... C hapter 2 Partition Coefficient T est ......................................................................................... Chapter 3 D iffu sion Coefficient T est....................................................................................... C hapter 4 C onclusion ........................................................................................................................... R eferences .................................................................................................................................................. A p p en dix ..................................................................................................................................................... 3 4 . 14 . 34 54 55 57 Chapter 1 Introduction Background. This chapter had been jointly written along with Yu Xiang Jaren Soo and David G. Jensen, both of whom were working on separate aspects of the project. Soo focused on bench where controlled settings can be achieved so that he can change different variables of the experiment. Jensen focused on developing the mass transfer model for soil gas and also " developing the probe prototype to be used for the eventual field testing.2 Every year, there is a large amount of chemicals released into the environment either intentionally or by accident. In 1986 the United States issued a program trying to regulate the underground storage tanks (USTs). There are approximately 571,000 underground storage tanks (USTs) nationwide that store petroleum or hazardous substances. The greatest potential threat from leaking USTs, which mostly contain petroleum products for service stations, is contamination of groundwater, the source of drinking water for nearly half of all Americans.1 In fact, the EPA has reported between 6000 and 7000 confirmed releases of contaminants by registered USTs every year since 2009.2 Statistics shows that just the state of Massachusetts has about 10,000 active USTs, which is almost equivalent to 1 potential release site every square mile.3 These UST tanks are widely spread and may cause a great threat for public health via different exposure path ways, intrusion. 4 such as vapor Vapor intrusion is an important exposure pathway in risk assessment and often related to leaking petroleum situations. This is because when these volatile organic compounds are leaking into the ground, they could volatilize from the contaminant source and be inhaled by people living in the area. People spend a large amount of time inside buildings and they breathe a large volume of air, so this can result in a significant risk of chronic health effects. 4 Because of the large number of USTs and the possible threat that they might bring, EPA has promulgated 40 CFR Part 280. One EPA requirement of 40 CFR Part 280 is "the UST must have a leak detection method that provides monitoring for leaks at least once every 30 days... and that ... leak detection can consist of monitoring vapors in the soil provided that the device is protected from moisture such that the results will not be rendered useless."5 And USTs are just an example of all kinds of chemical release today. Since we cannot stop factories and manufacturers from using chemicals in their production, we cannot completely avoid chemicals leaking into the environmental. Based on this, it is important to find a way to detect and evaluate a leak as soon as it occurs. Active and Passive Sampling. Soil vapor sampling and analysis is a valuable tool for assessing the nature and extent of contamination. Soil gas samples are typically collected by applying a vacuum to a probe inserted below ground in order to collect a whole-gas sample, or by drawing the gas through a tube filled with an adsorbent. These approaches are called active sampling. But there are challenges associated with flow and vacuum levels in low permeability materials, and leak prevention and detection during active sample collection can be cumbersome. 5 Passive sampling has been available as an alternative to such conventional gas sample collection. Passive sampling involves of several steps. First, one must choose an absorptive material, for example polyethylene (PE). When PE is cleaned with dichloromethane, methanol and pure water, the PE does not have organic contaminants in it before placing underground. Then, the absorptive material is attached to a frame made of a non-absorptive material such as aluminum. The frame, usually a pipe shape, has many small holes on its surface, which enables the absorptive material to contact soil vapors and at the same time stay away from soil particles. The passive sampling is commonly performed by drilling a hole into the ground, removing the soil from the hole, and putting the passive sampler frame into the hole. A commonly seen frame is shown in figure 1-1 below 6. After that, the excavated soil is back filled to the mouth of the hole and the absorptive material is left there for a period of time to achieve equilibrium. Finally, the frame is retrieved from underground and the equilibrated absorptive material is passed to laboratory for concentration analysis using methods like gas chromatography. 6 Holder Prnmovable Sorbent Tube Sample Chamber 0-Ring O-Ring Buna-n Washer Gas Entry Holes Diftuslonal Tube Membrane Shield Stainless Steel Point 6 Figure 1-1. Schematic representation of sampling chamber and sorbent tube Much of the historic use of passive sampling has been indoor and outdoor air quality 7 monitoring and industrial hygiene applications. -11 Previous passive soil-gas sampling techniques include a method that uses a thin ferromagnetic wire coated with activated charcoal to collect organic compounds. The sample is analyzed in the laboratory using methods like gas spectrometry. chromatography-mass However, the accumulated contaminant masses are not simply related to soil-gas concentrations. Several passive soil 7 gas sampling methods have been developed over the past quarter century since the earliest efforts1 2, including Petrex tubes,13 1' 4 EMFLUX~cartridges,1 5 Beacon B-Sure Sample Collection KitsTM16 and GoreTMModules (formerly known as the Gore-Sorber@).1 7 Each of these methods provides results in units of the mass adsorbed over the duration of the sample; however, the correlation between the mass adsorbed and the soil vapor concentration has not been quantitatively established.15,16,1 7 Concentration values are needed for comparison to risk-based screening levels when assessing human health risks via vapor intrusion, so many regulatory guidance documents caution that passive soil gas 8 sampling should only be used as a qualitative or semi-quantitative screening tool.1 ' 19 Absorptive Material. In the passive sampling design, we need to choose an absorptive material in order to sorb the contaminant in soil gases. Some of the commonly seen materials are activated carbon or charcoal. However, these have some disadvantages like they are hard to handle and it is not straight forward to relate their sorbed loads to vapor concentrations. Hence, I chose polyethylene as my absorbent for the following reasons. Several polymers have had success in the application as a passive sampling material for uptake of organic environmental pollutants. Materials tested in passive sampling devices from earlier studies, including semi-permeable membrane devices (SPMDs), solid-phase microextraction (SPME) fibers, had simple polymeric materials such as polyoxymethylene polydimethylsiloxane (PDMS) and low-density polyethylene (PE). 20 (POM), Each of these polymers exhibits different properties, such as the free volume within the polymer and the segmental mobility of the polymer chains. Transport of contaminants inside polymers depends on these factors. The glass transition temperature (Tg) of a polymer defines these properties, 8 21 and polymers with lower Tg have higher chemical diffusivities within the polymer. While in search of the most suitable material for the setup, we wanted the polymer to have a low Tg as it facilitates the diffusion process. Amongst these materials, PE has several advantages over the rest. It is readily available, inexpensive, robust, has a modest Tg, and is easy to deploy. Therefore, PE was chosen to be the passive sampler material for our passive samplers. Key Parameters. Based on what we had discussed above, we need to know some key parameters in order to change passive sampling from a qualitative tool to a quantitative one. Two key parameters for a contaminant of interest are its polyethylene-water partition coefficient (Kpew) and its diffusion coefficient in polyethylene (Dpe). In order to translate the concentration of contaminant in PE into the concentration in soil vapor, we need the compound's polyethylene-air partition coefficient, Kpea, which is defined by: Kpea = Cpe a. Cair where Cpe is the concentration of contaminant in the PE and Cair is the concentration in soil air. However, it is somewhat difficult to directly measure Kpea values due to the difficulty of controlling the concentration in vapor phase. As a result, we measured a related parameter, the polyethylene-water partition coefficient, Kpew, defined by: 9 -Cpe Cw Kpew because the Kpew can be linked to Kpea by the following equation: Kpea _Kpew K w - Kaw where Kaw is the air-water partition coefficient of the same chemical. number of reported Kaw values, so as long as we can measure the There is a large values, we will be able Kpew to estimate the Kpea accordingly. Through a literature review, we can find some reported data on Kpew for some large compounds like polycyclic aromatic hydrocarbons (PAHs) and polychlorinated biphenyls (PCBs), however, the contaminants that we are more interested in are fuels and include chemicals like benzene-toluene-ethyl benzene-and xylenes (BTEX) and alkanes. The Kpew values for these chemicals has not been reported, although values for many larger chemicals have been measured (Figure 1-2). a) b) 7 A Met lA.2 o Adarns @ * I a * A x 7 .2007 22 A 8-1 at al., 2003 (70 -)m(28) o Adams t al. 2007 (51 un)(10) 7 urn) (1) ConmeIase eta*I.2008(100un)(29( 2009( 25 um)(21) F...anL. Smnedebl a.. 2009(70 urn) (30 Perronat al.,2009(lIOGun) (31) Hale etal. 2010 (26 uM) (32) Haleta. 2010 (51 um) (32) 6 A A 5 0 4 (70 8 8 5 t al.. um) (10) 2007 (29) * COrneissen at al. 2006 (100 urn) Q Fernandeaa..2009 (25 urn)(11) a S dttal, 20 (7 um) (30 Peron etal 2009(100Li)(31) AHAle t 1A2010 (26 urn) (32) x HMO.t al. 2010 (5Lurn) (32) -Pmalcted line 0 Adam (7 umt0 4 4 A 30 4 -5 -4 -2 -3 0 Iog C."(L) log K. Figure 1-2. Log Kpew for PAHs versus (a) log Kow or (b) log Cwsat (L). 22 Notice that the way we obtain the concentration in gas from the concentration in PE is 10 based on the condition that the contaminant has enough time to diffuse into the PE and get equilibrium. However, it is not easy to determine whether phase equilibrium has been reached or not. One key factor influencing the rate of approach to equilibrium is a compound's Dpe value, and so knowing such values will be useful for supporting development of passive sampling. Choose Performance Reference Standards. In the design for our passive sampling, we use performance reference standards (PRCs) impregnated into PE before deployment to allow evaluation of a given sampler's approach to equilibrium. 23 In its simplest form, the PRCs have similar chemical and physical properties to the target compounds diffusing into the sampler. After deployment, a measurement of the remaining PRC mass permits the calculation of the extent to equilibrium reached. Using this information and the deployment time, one can correct measured concentrations of the target compound in the PE to be what . they would have been at equilibrium with the environment 23 Using a PRC for every target compound would be expensive, so Fernandez et al. proposed using a method to extrapolate PRC properties to different compounds. 23 Using a 1D diffusion mass transport model, they were able to generate a linear regression from a small number of PRCs with which to infer necessary mass transfer properties for other target compounds. A major assumption for this model is that the PRCs experience the same factors limiting their diffusion rates as affect the target compounds. In a later study, Apell and Gschwend validated the PRC diffusion assumption of the quantitative passive sampling method for sediment samping.24 They showed that they could 11 accurately determine equilibrium concentrations in sediment porewater using quantitative passive samplers removed at different times prior to equilibrium (figure 1-3). Deduced quilibrium Concentration Target Accumulation U l I PRC Loss t i i t i i I I Time Figure 1-3. Relationship between target accumulation and PRC loss 2 4 Furthermore, they confirmed the ability to use a linear regression generated from the mass transfer model to infer transport properties. Using these inferred properties they could calculate PE-deduced concentrations that reasonably matched measured equilibrium concentrations (Figure 1-4). 12 I a .1 CL 0. 11 10 0 0.0 0.0 0. Measured Porewater (ng/L) Figure 1-4. Porewater concentrations of PCB congeners 52 (black), 101 (red), 153 (blue), and 180 (green) in seven different sediments from Lake Cochituate measured in laboratorytumbled polyethylene pieces and in extracted porewater. 2 4 It is important that we could use PRCs that are similar to the target chemicals so that we can use the calculation method mentioned above. However, the reported data for focused at large compounds like PAHs and PCBs (Figure 1-5).22 are As a result, we need to do the experiments to measure the Dpe values for the compounds we are interested in. 13 Dpe Ibweilo0 taL, 1985 (66) Siko t al., 1999 (65) x Rsina et al., 2007 (64) Hab et al.. 2010 (51 um) (32) 1 * R ina at al. 2010 (63) Rsina et al, 2010- best fit --- best f ft 100 A -12 2X -13 -14 0 -15 x -16. _ 50 0 100 200 150 Vm (SPARC) 250 _ 300 350 Figure 1-5. Measured log Dpe for selected organic compounds versus their molar volume, Vm (estimated from SPARC). 22 Besides this, Dpe values also play an important role in determining how long do we need to deploy the passive sampler. If the Dpe value is quite large, the diffusion process might be accomplished within one day or a few hours. On the other hand, if the value is quite small, a different deployment time may be needed to reach equilibrium. In chapter 2, I will report the detail process of measuring PE-water partition coefficients for several volative organic compounds or VOCs (Kpew), results and calculation details. including the experiment steps and And in chapter 2, I will report how did I measured the diffusion coefficient for VOCs in PE (Dpe) and the results I obtained. 14 Chapter 2 PE-Water Partition Coefficients Measures for Volatile Organic Compounds (VOCs) Introduction The goal of this work was to find polyethylene-water partition coefficients for several VOCs expected to be important contaminants in soils from leaking fuels. Test Materials. Additive-free low-density polyethylene (PE) with 102 um (4mil) and 2Sum (1mil) thicknesses (density: 0.92g/cm3) was obtained from Ambicat" The PE was put into dichloromethane (J.T.Baker) for 1 day, and then taken out for soaking in another container with clean dichloromethane for 1 more day. After that, the PE was taken out from dichloromethane and put into methanol (J.T.Baker) for 1 day and changed into a clean methanol solution for another day. After cleaning with these solvents, the PE was put into pure water (Vaponics, model: Aries, 11OV) to leach out any residual organic solvent and then it was ready for use. The chemicals used in the experiment included: toluene (J.K.Baker), ethylbenzene (J.K.Baker), o-xylene (Aldrich Chemical Company), pentane (J.K.Baker), hexane (J.K.Baker, 95% n-hexane), and hexadecane (Aldrich Chemical Company, 99%). The instruments used for analysis included a Carlo Erba gas chromatograph (HRGC 5300) - and Tekmar purge and trap connected to a Perkin Elmer gas chromatograph. GC Column J&W Scientific, DB-624 capillary column, 60m, 1.40mm film thickness. 15 Toluene Test. I chose toluene as the chemical with which to start partition coefficient measurements for the following reasons. First, there are some results from some other's previous work 22, so I can support my findings by comparing with those results. Second, toluene has the Henry's Law constant of 6.61 X 1O-3 atm m3 /mol (US Air Force 1989) which is not too big for a volatile organic compound. This means that conducting the experiment with toluene would be a little easier compared with chemicals that more easily to escape into the air, such as pentane and hexane. As a result, the data for toluene would not be affected significantly by any little air bubbles that may appear in the experiment. When I began to do the experiment for toluene partition coefficient measurement, I first prepared several biological A1,A2,A3,B1,B2,B3,C1,C2,C3. oxidation demand (BOD) bottles, numbered with Then, I filled sets of bottles, A1-A3, B1-B3, C1-C3, with 0.4%, 1.2% and 2% of saturated toluene solution, respectively. The saturated toluene solution was made by mixing toluene with pure water in a separatory funnel and held without mixing to keep the aqueous solution clear of toluene droplets in the lower layer. After filling the BOD bottles with known toluene concentration solutions, I then cut 6 pieces of precleaned PE (each about 120 mg) and put them into Al, A2, B1, B2, C1, C2 and sealed each bottle. After 72 hours absorption time, I used gas chromatography with flame ionization detection to measure the toluene concentrations in the water of these 9 bottles. In a given series, for example A1,A2,A3, all bottles has the same initial toluene added, the difference in the concentration between Al and A3, A2 and A3 should represent the toluene that absorbed into the PE. Hence, these results allowed us to calculate the toluene concentration in the PE. And the reason that I made two comparisons is that I want to make sure that the results are replicable. 16 After the impregnation period, the PE was taken out from BOD bottles and the aqueous samples were sent to gas chromatography (GC) for analysis. I injected 1 uL of each solution into the GC to measure toluene in the water. The temperature for GC oven was initially set at 103 'C, then it was increased at 10 degrees per minute to 200*C. After that, the temperature increased at 25 degrees per minute to 225'C and stayed at 225 C for 1 minute. The peak height of GC analysis was recorded and translated to concentrations by fitting with a series of toluene standards (figure 2-1). 120 100 y =15.96 (0.48) x +0.67 (0.006) 80 . 60 40 20 0.000 1.000 2.000 3.000 4.000 5.000 6.000 concentration (ng/uL) Figure 2-1. Concentration of toluene standards versus peak height in GC. Black thinner line shows the linear regression line of standard points with regression equation on left side. And the peak height was translated into concentrations of samples Al to C3 (Table 2-1). 17 Table 2-1. Tare weight, water filled weight, and volume of BOD bottles of Al to C3. PE dry weight before the test. Peak height from GC analysis and calculated concentrations. Sample Number Jar Tare wgt. (g) PE dry Jar H20 filled Jar Volume (g) (mL) Al A2 89.78 117.17 A3 116.29 BI 118.07 B2 114.72 B3 118.94 Cl 116.46 C2 118. 12 154. 176. 176. 177. 175. 177. 175. 177. 18 01 73 67 35 72 C3 118.42 178. 52 75 49 64.4 58.84 60.44 59.6 60.63 58.78 59.29 59. 37 60. 1 wgt. Peak Height Concentrat ion (mg) (mm) (ng/uL) 330.8 329.68 n. d. 340.5 20.5 22 338.56 n. d. 67 352.22 330.8 n. d. 1. 243 1. 337 2. 4. 4. 6. 34 72 110 089 470 156 851 120 7. 352 7. 477 184 11. 488 118 Take Al and A3 as an example to calculate the Kpew for toluene, the calculation is shown here: Concentrationdifference = Concentration(A3)- Concentration(A1) Jar Volume = Cpe = JarH20 filled weight - JarTare weight density of H20 concentrationdifference * Jar Volume PE dry weight Cpe = Cpe Cw Concentration(A1) Using this approach, we found the partition coefficient (Kpew) of toluene is about 105 g(PE)/ml(water), which gives log (Kpew) of about 2.02 18 0.06 (Table 2-2) 14.1 Table 2-2. Kpew and log (Kpew) values of toluene calculated from 6 data sets. Sample Set Kpew log (Kpew) (ml (H20) /g (PE)) (ml/g) Al, A3 A2,A3 Bl,B3 B2,B3 C1,C3 C2,C3 Mean Standard Deviation 132 100 93.2 116 94.7 2.12 2.00 1.97 2.06 1.98 96.3 1.98 105 2.02 14. 1 0.06 If we take the equation from Lohmann (log Kpew= 0.99 log Kow - 0.07) to estimate the Kpew values for toluene, we estimate the number to be about 2.59. Given that the equation was drawn from data of PCBs, perhaps it is not surprising that the extrapolated result was lower than my measurements. So I decided to move to the next compound. o-xylene and ethylbenzene Tests. Having the experience with toluene, I then decided to test o-xylene and ethylbenzene together. For one reason, these two chemicals have some similar properties with toluene (Table 2-3) Table 2-3. Comparison of log (Cwsat (H20)) and log (Kow) of toluene, o-xylene and ethylbenzene (all values are taken from textbook Environmental Organic Chemistry, Schwarzenbach et al. 2003). Toluene o-xylene ethylbenzene Formula C7H8 C8H10 C8H10 -log (Cosat(H20)) (L) 2.22 2.75 2.8 -log (Kow) 2.69 3.16 3.2 Chemical property 19 So it is reasonable to assume that the Kpew values for these compounds should not differ too much from each other. The method for testing o-xylene and ethylbenzene was the same as toluene. I first prepared biological several oxidation A1,A2,A3,B1,B2,B3,C1,C2,C3. demand (BOD) bottles, numbered with Then, I filled sets of bottles, A1-A3, B1-B3, C1-C3, with 0.4%, 1.2% and 2% of saturated o-xylene and ethyl-benzene mixed solution, respectively. After filling the BOD bottles with known o-xylene, ethylbenzene mixed concentration solutions, I then cut 6 pieces of pre-cleaned PE (each about 320 mg) and put them into Al, A2, B1, B2, C1, C2 and sealed each bottle. After 72 hours absorption time, I used gas chromatography with flame ionization detection to measure the o-xylene and ethyl-benzene concentration in these 9 bottles. The temperature program of GC was same too. The peak height of GC analysis was recorded and translated to concentrations by fitting with a series of o-xylene and ethylbenzene standards (figure 2-2). (a) 20 - 160 y = 24.8 140 (1.1)x + 3.0 R2 = 0.987 (3.7) 120 100 80 Cu 60 40 20 0 0.000 1.000 2.000 3.000 4.000 5.000 6.000 5.000 6.000 concentration (ng/uL) (b) 180 160 y=29.8 140 (0.61) x+4.6 R2 = 0.98515 (2.3) 3.000 4.000 120 -i 100 -i 80 Cu 60 40 20 0 0.00 0 1.000 2.000 concentration(ng/uL) Figure 2-2. Concentration of (a) o-xylene (b) ethylbenzene standards versus peak height in GC. Black thinner line shows the linear regression line of standard points with regression equation on left side. And the peak height was translated into concentrations of samples Al to C3 (Table 21 2-4). Table 2-4. Tare weight, water filled weight, and volume of BOD bottles of Al to C3. PE dry weight before the test. Peak height from GC analysis and calculated concentrations for (a) oxylene (b) ethylbenzene (a) H20 Tare filled Jar PE sample weight weight Volume dry weight number (g) (g) (mL) (mg) Al A2 89. 78 117.17 154. 18 64.4 176.01 58.84 A3 Bl B2 B3 C1 C2 C3 116.29 118.07 114.72 118.94 176.73 177.67 116.46 118.12 175.75 118.42 178.52 175. 35 177.72 177.49 Peak Height Concentration o-xylene o-xylene (mm) (ng/uL) 60.44 59.6 60.63 58.78 330.8 329.68 n. d. 340.5 338.56 n. d. 5. 5 5. 5 15. 5 21 59.29 59.37 60. 1 352.22 330.8 n. d. 34 19.5 51.5 34.5 90 0.102 0.102 0.506 0.728 0.667 1.959 1.253 1.273 3. 514 (b) Tare H20 filled sample weight weight number (g) (g) Jar PE Volume dry weight Peak Height ethylbenzene (mg) (mm) Al 89.78 154. 18 (mL) 64.4 A2 A3 BI B2 B3 117. 17 116.29 176.01 176.73 58.84 60.44 329.68 n.d. 118.07 177.67 175.35 59.6 60.63 C1 116.46 177.72 175. 75 58.78 59.29 C2 C3 118. 12 177.49 178.52 59.37 60.1 340.5 338.56 n. d. 352.22 330.8 n. d. 114.72 118.94 118.42 330.8 22 5 5 13.5 Concentration ethylbenzene (ng/uL) 0.014 0.014 0.300 18 0.451 0.451 45 33 1.358 0.955 32.5 0.938 2.768 18 87 Using the same approach as toluene, we found the partition coefficient (Kpew) of o-xylene is about 480 185 g(PE)/ml(water), which gives log (Kpew) 0.17; and the 1735 g(PE)/ml(water), which partition coefficient (Kpew) of ethylbenzene is about 1453 gives log (Kpew) of about 2.87 of about 2.65 0.54 (Table 2-5). However, as we can see from table 2-4, the A serie samples for ethylbenzene is higher than the other two series. The might due to the regression error when fitting into a small concentration. If we ignore the A series in ethylbenzene test, the partition coefficient (Kpew) of ethylbenzene is about 335 25 0.02. g(PE)/ml(water), which gives log (Kpew) of about 2.52 Table 2-5. Kpew and log(Kpew) for o-xylene and ethyl-benzene calculated from 6 sets. Sample Set A1,A3 A2,A3 B1,B3 B2,B3 C1,C3 C2,C3 o-xylene log (Kpew) Kpew (ml (H20) /g(PE)) 769 705 296 347 381 381 Mean Standard Deviation 2.89 2.85 2.47 2.54 2.58 2.58 2.65 0.17 480 185 ethyl-benzene log (Kpew) Kpew (ml(H20)/g(PE)) 3.59 3850 3.55 3529 2.55 351 2.56 360 2.48 305 2.51 325 1453 1735 2.87 0.54 Pentane and Hexane Test. After obtaining the values for these three aromatic compounds above, I then assessed another two aliphatic chemicals, pentane and hexane. These two compounds are more difficult to test because they have large Henry's Law constants, which means pentane and hexane tend to partition into any bubbles in the test system rather just stay in the PE and the water. Since my testing relied on the analysis of aqueous samples, a few air bubbles existing in the BOD bottles may cause a big error in the measurement of concentration in aqueous phase. 23 We can take pentane as an example, the log (Kaw) value for pentane is 1.69 (Schwarzenbach et al. 2003), which gives the Kaw to be 48.9. If we have a 60 mL BOD bottle full of 1% saturated pentane solution and a 1 mL air bubble appears in it, the fraction of pentane that go into the air bubble will be: fw ai 0.54 1 mass in water total mass 1 + Kaw * . Vair Vwater This means just a 1 mL air bubble can cause more than half mass loss in aqueous phase and gives an error in our analysis. First, I tried to follow the same procedure as the other three compounds. However, there were two problems appeared in the experiment. One is that some air went into the BOD bottles and created air bubbles during the 72 absorption time. The other is that the concentrations of the aqueous samples were about the same as the minimum detection limit of GC. As a result the analysis was not very reproducible (Table 2-6). Table 2-6. Kpewl and log (Kpew)J for pentane and hexane calculated from 6 sets. Sample Set Al,A3 A2,A3 B1,B3 B2,B3 pentane log (Kpew) Kpew (ml (H20) /g (PE)) 2.83 676 2.53 336 2.65 2.70 442 496 Kpew hexane log (Kpew) (ml (H20) /g(PE)) 498 189 3.55 1.LE+03 1.8E+03 2.55 2.56 2.7E+03 2.92 825 C1,C3 3.2E+03 2.97 924 C2,C3 1.6E+03 2.76 617 Mean 1.2E+03 0.17 230 Standard Deviation samples are around the for the the concentration Kpew means subscript J for (the 24 3.59 2.48 2.51 2.87 0.54 detection limit of the instrument) Based on the problems stated above, I made two changes in the experiment. One is that I placed the BOD bottles filled with PE and aqueous solutions into a big plastic jar that was filled with clean water. I hope that in this way, the water that surrounded BOD bottles would keep air from going into the samples during the desorption time. Another change was that I switched to a purge and trap instrument for sample analysis. The purge and trap method captures the interested chemicals from a solution by blowing air through the sample. And the good part is that we can inject the VOC content of up to 5 mL of sample instead of 1 uL into GC, which can give us a much more lower detection limit. To optimize the purge and trap method, I made some changes to the temperature program. The temperature first stayed at 35-C, then climbed to 165'C with a speed of 10 degrees per minute. And then temperature stayed at 165 *C for 1 minute. The reason that the upper temperature of GC program was decreased from 225 "C to 165*C is that both pentane and hexane are rather volatile and elute at around 100 degree. As a result, we don't need to wait the temperature to reach 225 *C. The purge and trap program was: purge time was set to 4 minutes, followed by desorption heating of 2 minutes, and then bake for 8 minutes. The method for preparing testing samples are the same with above. I first prepared several biological oxidation demand (BOD) bottles, numbered with A1,A2,A3,B1,B2,B3,C1,C2,C3. Then, I filled sets of bottles, A1-A3, B1-B3, C1-C3, with 0.4%, 1.2% and 2% of saturated hexane and pentane mixed solution, respectively. After filling the BOD bottles with known hexane, pentane mixed concentration solutions, I then cut 6 pieces of pre-cleaned PE (each about 80 mg) and put them into Al, A2, B1, B2, C1, C2 and sealed each bottle. After 72 25 hours absorption time, I used gas chromatography purge and trap method to measure the hexane and pentane concentration in these 9 bottles. The peak area of GC analysis was recorded and translated to concentrations by fitting with a series of hexane and pentane standards (figure 2-3). (a) 0.45 0.4 y = 0.2881x R 2 = 0.99093 0.35 f0- 0.3 0.25 0.2 0.15 0.1 0.05 0 1.5 1 0.5 0 2 Area(uV*sec) (b) 0.14 y = 0.0445x 0.12 R = 0.99703 0.10 0.08 S0.06 8 0.04 0.02 0.00 0 0.5 1.5 1 2 Peak Area(uV*sec) 26 2.5 3 Figure 2-3. Concentration of (a) pentane (b) hexane standards versus peak height in GC. Black thinner line shows the linear regression line of standard points with regression equation on left side. And the peak area was translated into concentrations of samples Al to C3.(Table 2-7) Table 2-7. Tare weight, water filled weight, and volume of BOD bottles of Al to C3. PE dry weight before the test. Peak area from GC analysis and calculated concentrations for (a) pentane (b) hexane (a) Tare H20 filled Jar PE sample weight weight Volume dry weight hexane number Al A2 A3 BI B2 (g) (g) (mg) (uV*sec) B3 C1 89. 78 117. 17 116. 29 118. 07 114. 72 118. 94 116. 46 C2 C3 118. 12 118. 42 154.35 176.04 176.93 177.7 175.29 177. 76 175.48 177.8 178.44 (mL) 64.57 Peak Area 92.3 81.6 n. d. 77.5 81.9 58.87 60.64 59.63 60. 57 58.82 59.02 n. d. 86.5 81.9 n. d. 59.68 60.02 1. 48E+04 2. 43E+04 3. 35E+05 3. 3. 2. 1. 75E+05 40E+05 92E+06 02E+06 1. 56E+06 7. 12E+05 Concentration hexane (ng/uL) 4. 27E-03 7. 9. 1. 9. 8. 2. 00E-03 66E-02 08E-01 79E-02 42E-01 94E-01 4. 50E-01 1. 03E+00 (b) Tare sample number Al A2 A3 BI B2 B3 C1 C2 weight H20 filled weight Jar Volume Peak Area PE (g) 89.78 (g) 154.35 (mL) 64.57 117. 17 116.29 118.07 114.72 118.94 116.46 118. 12 176.04 176.93 Concentration dry weight hexane hexane (mg) (uV*sec) (ng/uL) 92.3 4.94E+04 2.20E-06 58.87 60.64 81.6 4.35E+04 8.32E+05 1.94E-06 3.70E-05 177.7 175.29 177. 76 175.48 59.63 60. 57 58.82 59. 02 77.5 81.9 5.68E+05 2.23E+05 2.53E-05 9.92E-06 n. d. 2.01E+06 8.93E-05 86.5 1.41E+06 6.27E-05 177.8 59.68 81.9 1.39E+06 6.20E-05 27 n. d. C3 n.d. 60.02 178.44 118.42 1.15E+06 2.56E-04 Using the same calculation steps as toluene, we found the partition coefficient (Kpew) for pentane is about 6.3 * 103 5.4 * 103 g(PE)/ml(water), which gives log (Kpew) of about 3.64 0.45; and the partition coefficient (Kpew) of hexane is about 6.06 * 103 g(PE)/ml(water), which gives log (Kpew) of about 3.65 4.9 * 103 0.38 (Table 2-8) Table 2-8. Kpew and log(Kpew) for pentane and hexane calculated from 6 sets. hexane pentane Sample Set log (Kpew) Kpew log (Kpew) Kpew (ml (H20) /g (PE)) (ml(H20)/g(PE)) A1,A3 1.51E+04 4.18 1.11E+04 4.04 A2,A3 B1,B3 9.23E+03 5.23E+03 3.97 3.72 1.31E+04 1.95E+03 4.12 3.29 B2,B3 C1,C3 5.62E+03 1.70E+03 C2,C3 Mean Standard Deviation 9.33E+02 6.30E+03 5.44E+03 3.75 3.23 2.97 3.64 5.92E+03 2.10E+03 2.28E+03 6.06E+03 4.92E+03 3.77 3.32 3.36 3.65 0.45 0.38 We can see that there is still a relatively large deviation between different experiment sets, which may decrease our confidence in the consistency of the data. The possible reason for the deviation may be: 1. There were air bubbles in the BOD bottles during desorption that cause a large fraction of chemicals going into the air. Although I didn't see air bubbles when did the analysis, there might be numbers of tiny little bubbles in there. 2. Because we put BOD bottles in a large jar filled with water, it is possible that the cap for BOD bottles were not tight enough so that chemicals can go outside the samples. 3. There might be some pentane and hexane droplet sticking onto the volume pipettes, thus causing a larger concentration than what I would expect. If this is true, I would suspect the high number in Table 2-8 because the droplet effect. 28 Also notice that the calculated results for A series are obviously higher than the other. This might due to 1. a little droplet in low concentration solutions will cause a larger impact on results than in a higher concentration solution. 2. When we are fitting a linear regression line for the standards of each chemical, there might be a larger error for the low concentration fittings. Discussion Desorption Time Optimization. Before discussing the partition coefficients, I would like to first explain why I choose 72 hours as the desorption time in the experiment. I did an experiment that set the desorption time as independent variable and tested how the concentration of samples changed as the desorption time changed. I chose chlorobenzene and toluene as my test compounds. First, I prepared a bottle with a mixed solution of 1% toluene and 2% of chlorobenzene in hexadecane. Then I sealed the bottle and let it sit on the bench for half an hour in order to make the chemicals vaporize into the air space. Then I put a large piece (18 cm * 25 cm) of pre-cleaned PE into the bottle without directly contacting the liquid phase solution. After 24 hours absorption, toluene and chlorobenzene in the vapor phase has reached equilibrium with the PE According to John K MacFarlane's test results (shown in Appendix A), 8 hours is long enough to reach the air-PE equilibrium; here I choose 24 hours just because it is convenient to continue the experiment after one day. Then, I cut the large piece of PE into 9 small pieces, each with a size about 2cm*4cm, and placed these pieces in BOD bottles numbered from D1 to D9 for desorption. If toluene and chlorobenzene diffused into the PE evenly, the concentration in these small pieces should be identical. However, considering there are some errors in the real practice, we assume that the original concentrations in these pieces of PE are about the same. These small pieces of PE were put into BOD bottles at the same time, and the PE was taken out of the bottle with 29 an increasing time. For example, the PE in bottle D1 was immediately taken out of the bottle after putting in, and the PE in D2 was taken out after 1 hour and so on. The result showed toluene in the water increased with time (Figure 2-4). 4.OOE+07 y = $E+061n(x) + 2E+07 R 2 = 0.91708 3.50E+07 3.OOE+07 2.50E+07 -- 1.50E+07... 1.OOE+07 -y = 5.OOE+06 - - - Z 2.OOE+07 2E+061n(x) + 9E+06 R2 = 0.80593 O.OOE+00 40 20 -5.OOE+06 60 80 desorption time(hour) *Toluene *Cholorobenzene Figure 2-4. Peak Area (uV *sec) measured by Perkin Elmer gas chromatograph versus desorption time (hours). The blue points represent toluene and the red points represent chlorobenzene. The solid and dashed black curves are exponential fitting of the toluene and chlorobenzene respectively, with the equation of each line on its side. For the reason that the data in desorption test was somewhat similar to first order reaction curve, so I also try to fit the data into a first order like formula: Ct = Ceq ( 1 - e-kt) assuming that the desorption has achieved equilibrium after 72 hours. And the fitted k for toluene, chlorobenzene is 0.0968[uV*sec/hour], 0.0914[uV*sec/hour], respectively. (figure 2-5) 30 0 40 20 60 80 -1 -2 0--% -3 -4 -5 -6 -7 y = -0.0968x = -8 0.86346 y = -0.0914x R2 = 0.79377 desorption time (hour) 4 toluene U chlorobenzene Figure 2-5. Desorption data exponential fitting. The x-axis is desorption time. The red square points are chlorobenzene and the blue diamond points are toluene. The solid line is the linear fitting line for chlorobenzene while the dashed one is for toluene. As we can see from Figure 2-4, desorption of toluene and chlorobenzene from PE into water almost got equilibrium after the first 24 hours. The reason I chose 72 hours as a desorption time was simply to guarantee that compounds with larger volume than toluene were able to achieve equilibrium. However, it appears reasonable to reduce the time to 48 hours instead of 72 to save time. Kpew results discussion. We can now proceed to the discussion of partition coefficients for these five VOCs. Polyethylene is a thermoplastic polymer consisting of long hydrocarbon 31 chains. 25 So compared with water, which has polar structure, these organic compounds should be more likely to go into a nonpolar structure of PE. So it is reasonable to guess that (Kow) values or Csat values because these two Kpew values may have a relationship with parameters both represent some property relating to compatibility with water. Also, some previous work, Lohmann found a correlation between log(K0 w) and log(Kpew) values 22 . So here I also made a graph trying to figure out the potential relationship between log (Kpew) and log (Kow)(figure 2-6). 6.00 y = 1.30x - 1.32 R2 = 0.6136 5.00 4.00 3.00 2.00 1.00 0.00 0 -1.00 1 3 2 4 5 6 log(Kow) *toluene U o-xylene A ethyl-benzene X pentane )Khexane Figure 2-6. log(Kpew) values versus log(K 0 .) values for toluene, o-xylene, ethyl-benzene, pentane and hexane. Red line is the linear regression line with its equation on left side. for the linear regression and the equation for the linear regression is log(Kpew) = 1.30( 0.19) log(K 0 w) -1.31 ( 0.65) ( r2 = 0.61, se = 0.45, n= 30 ). Notice that the points of toluene and oxylene are all within the 95% confidential area. However the results for other 3 compounds 32 all have some outliers, especially for hexane and pentane. As stated above, pentane and hexane tend to escape into the air, so a tiny air bubble in the sample may contribute to a large loss of concentration. So it seems likely that some error in conducting the experiment may contribute to the outliers in the results. If we compare the results we got with those reported in Lohmann's review 22, the new data reported here appear consistent, albeit variable, with the rest (Figure 2-7). dm C) Adaf:~maa Z0 -W o 27: Mi.30717')wiw 261, 1 101 a) wna*hdgzuia 7 2O9l26,.ni'1i 0AO*@ 5die1.6 'CI7I"U UMJC. 6 PW1Sqt M 2009 1OCw'K" 4 a kdwiIM.1~ O77 ' 10IM09 0 II 46 3 7 44 4 7 Figure 2-6. Comparison of data from Lohnman's review and our results. Red points are the results obtained by our experiment. The equation from Lohmann's review in Figure 2-7 is log (Kpew) = 1.22 1.22 ( 0.24) ( 0.046)log (KOW)- (r 2= 0.94, SE= 0.27, n= 65). We can see that the equation of our result is very close to the equation from the summary of previous work and given that the summary equation contains large compounds like PCBs and PAHs, we can have confidence that our 33 results give some of the facts about partition coefficients of these chemicals. Chapter 3 Diffusion Coefficients in Polyethylene Introduction. In the previous chapter, we focus on the measurement of partition coefficients (Kpew) for several VOCs. However, knowing the Kpew values alone is not enough to guide passive sampling work. Here are the reasons. First, we need to choose a PRC that is similar in property with a target compound so that we can use the method mentioned in Chapter 1 to calculate equilibrium status. Second, the diffusion coefficient (Dpe) can help us to decide how long to put the passive samplers in the testing field. It is easy to understand that chemicals with larger Dpe, which means they move faster, will need less deployment time. On the other hand, chemicals with smaller Dpe values will need longer diffusion time in order to achieve equilibrium. Test Materials. Additive-free low-density polyethylene (PE) with 102 um (4 mil) thickness and density of 0.92g/cm 3 was obtained from Ambicat" Here I used 4 mil PE instead of 1 mil PE in the partition experiment because I am testing relatively small compounds compared to PCBs and PAHs. By pre-calculation, we can estimate the time for toluene to diffuse through 1 mm thickness of PE. 34 diffusion time = distance2 Dpe given that Dpe of toluene is around 10-11 (m 2 /s) according to Lohnman 22, the time for toluene to diffuse through 1 mm thickness of PE should be around: time = (1 * 10-3)2 10-11 =10)10s s ~ 28 hour In other words, if we choose 4 mil (about 100 um) PE to do the diffusion test, it will take about 1000s to diffuse through the PE, which is less than 1 hour. This means if we take 1 mil PE like we did in partition experiment, the diffusion time will decrease 16 times. For the experiment, I impregnated a piece of PE with a target chemical, and then inserted the impregnated piece between sheets of other pre-cleaned PE. In this way, the chemicals in the middle layer will diffuse out because of the concentration gradient and finally all diffuse out through all the sheets of PE. For the convenience of the experiment, I collected samples for a time span of hour scale. It is not hard to image that if the diffusion time decreased to a few minutes, it would be difficult to do the experiment. The PE was cleaned using the same method as it was for partition coefficient experiments. And the chemicals used in the experiment included: toluene (J.K.Baker), ethylbenzene (J.K.Baker), o-xylene (Aldrich Chemical Company), pentane (J.K.Baker), hexane (J.K.Baker, 95% n-hexane) and hexadecane (Aldrich Chemical Company, 99%). The instruments used for analysis includes Tekmar purge and trap system, connected to a 35 Perkin Elmer gas chromatograph. Toluene Test. Again, I chose toluene as my first compound to do the diffusion test. In order to calculate diffusion coefficient (Dpe), we need to know the diffusion velocity of toluene within the PE. So I designed the experiment as follows. First, I cut 7 pieces of PE (about 30 cm * 20 cm) and chose one of them to impregnate with toluene. Here I prepared jar, which at the bottom of it is 10 mL mix solution of 1% toluene in hexadecane. The air in the jar was expected to build up to about 1% of toluene's vapor pressure (about 1 mg/Lair). Then I placed the PE inside the jar without contacting the liquid and capped it. In this way, the PE equilibrated with toluene in gas phase. Since the PE-air partition coefficient for toluene can be estimated from the ratio of this compound's Kpew/Kaw = 105/0.25 = 420, then we can expect the toluene will build up to about 400 mg/kgpe at equilibrium After the impregnation, I put two lead bricks on a flat bench, and then put a rectangle of aluminum frame above the lead bricks (Figure 3-1). I used a vacuum sealer to seal the edge of the toluene-loaded PE together with other 6 pieces, placing the PE with toluene at the middle of all 7 layers. The sealed layers of PE are then place on the aluminum frame and placed under another rectangle of Al frame. Finally, another two lead bricks are put onto these so that two layers of aluminum frames will be pushed tightly together. The reason for doing this is to keep air out of the space between each layer of PE. We are trying to measure the Dpe for PE, so air existing between the PE would cause error in the measurement for the reason that toluene need to diffuse through air between the PE layers. 36 Step 2 Step 1 Step 3 Figure 3-1 Photos showing experimental setup. One hour after the experiment began, a small area of PE (1.5 cm * 3 cm) was cut at the same location from each of 7 PE sheets and put into a 60 mL BOD bottle completely filled with clean water. Another small area was also cut out after 2 hours. In this way, we got 14 BOD bottles numbered from D1-D7, E1-E7 containing a small piece of PE from each of 7 layers. And after 72 hours of desorption, the samples were taken for analysis by gas chromatography. The temperature program for GC first stayed at 35'C, then climbed to 225'C with a speed of 10 degrees per minute. And then temperature stayed at 225 'C for 1 minute. The purge and trap program was: purge time was set to 4 minutes, followed by desorption heating of 2 37 minutes, and then bake for 8 minutes. Because the PE originally impregnated with toluene was put at the middle of 7 layers and the time for diffusion was not long enough for all the toluene in middle to diffuse out, we would expect the concentration in the middle one should be the largest and decrease as the diffusion distance increases. Moreover, we expected the peak concentration in the middle PE sheet would be greater at 1 hour than what was seen at 2 hour. This pattern is consistent with what we found (Table 3-1). Table 3-1. Toluene diffusion test results from GC analysis. Time (hour) represents the diffusion time, and Distance (m) represents the distances from the middle layer (diffusion distance). Creal is the real concentration measured by GC, and Cest is the estimate concentration by fitting an optimized Dpe values into the concentration equation. Time Distance Creal in H20 Cest in H20 (hour) (m) (ng/uL) (ng/uL) 1 1 0 0.0003048 0.0002032 0.0001016 0 0.0001016 1 1 0.0002032 0.0003048 2 2 0.0003048 0.0002032 2 2 0.0001016 0 2 2 2 0.0001016 0.0002032 0.0003048 0 1 1 1 2.98 0.17 0.35 0.58 0.78 0.57 0.33 0.15 0.10 0.27 0.39 0.43 0.39 0.24 0.12 n.d. 0.16 0.35 0.58 0.68 0.58 0.35 0.16 0.24 0.30 0.34 0.36 0.34 0.30 0.24 Here I will give an example of how the Cest in H20 in table 3-1 is calculated. We know from 38 Schwarzenbach et al. (2003) that when the boundary condition for diffusion process is set to 0, the concentration at a specific location can be calculated by: M * C(x, t) = 2e(xDt)1/2 2 ep(- 2(w~)'/ x2 -) 4Dt This is a normal distribution with standard deviation ox = (2Dt)1/ 2 26 M* is the total mass per unit area which can be calculated by : * C(x)dx = M We know the concentration of toluene in PE from t=0 at the beginning of the test by measuring the aqueous sample equilibrated with PE, which can be denoted as Cw(0,0). Then, in order to calculated the mass per unit area M*, we assume the concentrations in the impregnated PE at t=0 are the same for every tiny part. In this way, M* can be calculated by: M * = Cw(0,0) * Kpew * density of PE * thickness of PE And given a values for M* and Dpe, we can calculate an estimated concentration of toluene in a piece of PE (Cpe) by: M2 Cpe(x, x2 t ) = 2 (7D pet)1/2 exp~ 4D pet 39 Then using the Kpew value that we obtained from previous chapter, we can calculate the toluene concentration in H20 (Conc in H20) by: Cest in H20(x, t) = fW * Cpe(X, t) * Mpe Vw and the fraction of toluene in the water at equilibrium could be calculated by: 1 fw 1 + Mpe Kpe w Cest in H20 is the theoretical number given a particular Dpe, and we would like to find a Dpe that can best fit our real concentration profile into the theoretical distribution. So that we first calculate the concentration difference between measurement and theoretical values by: ConcDiff = Creal in H20 - Cest in H20 Then, we add the ConcDiff for the whole 7 sets of data to give the sum square errors. SSE = ~ConcDiff Conc in H20 In the excel spreadsheet, I tried several Dp, values in order to minimize SSE value, and the Dpe that gives the minimum SSE values is considered to be the optimized Dpe. By doing the 40 ( optimization for 1 and 2 hour test separately, I found the Dpe of the toluene to be: 2.23 0.01) * 10-12 [m 2 /s] and log(Dpe) = - 11.65 ( 0.004) [m 2 /s]. And by plotting the theoretical curve with the optimized Dpe value, we can see how well is the real data fit into theory. (figure 3-2). 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.0006 -0.0004 -0.0002 0 0.0002 0.0004 0.0006 Diffusion Distance(m) 4 Creal_1 Creal_2 Cest_1 Cest_2 Figure 3-2. Toluene concentration profile of diffusion test for 1 and 2 hour tests. The diamond the points represent the data collected from the 1 hour test, and the triangle points represent data collected from the 2 hour test. The x-axis is diffusion distance in unit of m, and the y-axis is the concentration in H20 with PE equilibrated. The blue solid line is the theoretical distribution of toluene concentration given the Dpe value of 3.51 * 10-12, and the dashed green line is the theoretical distribution of toluene concentration given the Dpe value of 2.22 41 * 10-12. Notice that all the diamond points are above the triangle points, which indicates that the concentration in PE after the first hour is higher than that of the second hour. This is reasonable because the pieces of PE at the end of both sides were directly contacting the air, which means toluene will eventually diffuse into the air and cause concentration in PE to decrease. Also, the distribution of the concentration is similar to what we would expect: the middle layer has the highest concentration and concentration decrease as the diffusion distance increases. O-xylene Test. I then moved forward to do the diffusion test on another compound: oxylene. The experiment steps were very similar to the experiment with toluene. First, I cut two pieces of 4 mil pre-cleaned PE (25cm * 30cm) and impregnated them by equilibrium with the gas phase of a mix solution. The solution at the bottom of the jar was prepared with mixing 1% o-xylene into hexadecane. Since the vapor pressure for hexadecane is much lower than the other two volatile organic compounds, again we assume that the vapor phase had about 1% of the vapor pressures of o-xylene . After the impregnation, I then stacked the PE with another 6 pieces of clean PE and wrap them as flat as possible in order to keep away the air between different layers. The stack of PE were then put between two layers of aluminum frame and lead bricks (Figure 3-1). I cut a small area from the PE for each of 7 pieces after 1 and 2 hours and put them into BOD bottles filled with clean airless water. After 72 hours of desorption time, I then tested the sample on gas chromatography using the same temperature program with toluene. The results of concentration distribution are shown as below (Table 3-2). Table 3-2. o-xylene diffusion test results from GC analysis. Time (hour) represents the 42 diffusion time, and Distance(m) represents the distances from the middle layer(diffusion distance). Creal is the real concentration measured by GC, and Cest is the estimate concentration by fitting an optimized Dpe values into the concentration equation. Time Distance Creal in H20 Cest in H20 (hour) (i) (ng/uL) (ng/uL) 0 0.0003048 0.0002032 0.0001016 0 0.0001016 0.0002032 0.0003048 0.0003048 0.0002032 3.22 0.04 n. d. 0.03 0.26 0.60 0.97 0.64 0.24 0. 77 1.13 0.77 0.24 0.03 0.13 0. 36 0.19 0.03 0.09 0. 37 0.47 0.54 0.46 0. 33 0.13 0.0001016 0 0.0001016 0.0002032 0.0003048 0.68 0.84 0.68 0.36 0.13 Using the same approach with toluene, by doing the optimization for 1 and 2 hour test 11.7 ( 0.08) * 10-12 [m 2/s] and log(Dpe) = - separately, I found the Dpe of the o-xylene to be: 1.78 ( 0.01) [m2 /s]. And by plotting the theoretical curve with the optimized Dpe value, we can see how well is the real data fit into theory (figure 3-3). 43 1.2 1 0 -0.0006 * 0.0002 0 -0.0004 -0.0002 Diffusion Distance(m) Creal_1 Creal_2 " 0.0004 Cest_1 0.0006 Cest_2 Figure 3-3. o-xylene concentration profile of diffusion test for 1 and 2 hour tests. The diamond points represent the data collected from the 1 hour test, and the triangle points represent the data collected from the 2 hour test. The x-axis is diffusion distance in unit of m, and the y-axis is the concentration in H20 with PE equilibrated. The blue solid line is the theoretical distribution of o-xylene concentration given the Dpe value of 1.85 * 10-12, and the dashed green line is the theoretical distribution of toluene concentration given the Dpe value of 1.70 * 10-12. Notice that if we separately investigate the data for each hour's test, they both apply to normal distribution. However, the concentration of the 2 hour test is higher than the 1 hour in the position away from the middle layer, which includes diffusion distance more than 0.2 mm. It is interesting to guess what happened here. My hypothesis is that the diffusion coefficient (Dpe) of o-xylene is lower than that of toluene, so the chemicals moving velocity in the PE will be slower. This means some o-xylene could accumulate in the outer layers and gave the results like this. Knowing that the o-xylene has one more methyl group than toluene so it has a larger molar volume, it is reasonable to guess the Dpe of o-xylene is lower 44 than toluene. And the theoretical curve has confirmed my guess (figure 3-3). When the diffusion distance is larger than 0.2 mm, the analytical concentration of the second hour test is bigger than that of the first hour, which is indicted by a higher position in the figure. Chlorobenzene Test. For the chlorobenzene diffusion coefficient measurement, I first used * the same method as toluene and o-xylene. I cut two pieces of 4 mil pre-cleaned PE (25cm 30cm) and impregnated them by equilibrium with the gas phase of a mix solution of 1% chlorobenzene in hexadecane. After the impregnation, I then stacked the PE with another 6 pieces of clean PE and wrap them as flat as possible in order to keep away the air between different layers. The stack of PE were then put between two layers of aluminum frame and lead bricks (Figure 3-1). I cut a small area from the PE for each of 7 pieces after 1 and 2 hours and put them into BOD bottles filled with clean airless water. After 72 hours of desorption time, I then tested the sample on gas chromatography using the same temperature program with toluene and o-xylene. The results of concentration distribution are shown as below (Table 3-3). Table 3-3. Chlorobenzene diffusion test results of 1 hour and 2 hour tests. Time (hour) represents the diffusion time, and Distance(m) represents the distances from the middle layer(diffusion distance). Creal is the real concentration measured by GC, and Cest is the estimate concentration by fitting an optimized Dpe values into the concentration equation. Time Distance Creal in H20 Cest in H20 (hour) (M) (ng/uL) (ng/uL) 0 1 1 0 3.22 -0.0003048 -0.0002032 0.0006 0.0026 0.000558 0.003874 1 1 -0.0001016 0 0.0060 0.0116 0.012388 0.018251 45 1 1 0.0001016 0. 0002032 1 2 2 2 2 2 2 2 0.0003048 -0. 0003048 -0. 0002032 -0.0001016 0.0053 0. 0014 0.0006 0. 0004' 0. 0008' 0. 0016' 0. 0023' 0. 0017' 0. 0007' 0. 0005' 0 0. 0001016 0. 0002032 0. 0003048 Notice that all the results from the second hour test has 0.012388 0. 003874 0.000558 0. 003458 0. 006284 0.008993 0.010134 0. 008993 0. 006284 0. 003458 J label. This is because the peak height was about as large as background noise and does not show a clear peak of a reasonable scale. As a result, although the data still have a normal distribution, I didn't include them for fitting Dpe. Using the same approach with toluene, by doing the optimization for 1 hour test, I found the Dpe of the o-xylene to be: 2.46 * 10-12 [m 2 /s] and log(Dpe) = - 11.6 [m 2 /s]. And by plotting the theoretical curve with the optimized Dpe value, we can see how well is the real data fit into theory. (figure 3-4). 46 (a) 0.02 0.018 0.016 1.014 4.012 00.01 1.008 ).006 ).004 ).002 0 -0.0006 -0.0004 -0.0002 0 0.0002 0.0004 0.0006 Diffusion Distance(m) "nCest 1 4 Creal_1 (b) 0.02 0.018 0.016 )O.014 7).012 S0.01 )..008 .2.006 bO.004 r0.002 0 -0.0006 -0.0004 -0.0002 0 0.0002 0.0004 0.0006 Diffusion Distance(m) " + Creal_1 Cest_1 47 (c) 0.02 0.018 0.016 ZO.014 o.012 0.01 EC.008 ).006 -b.004 '0.002 0 -0.0006 -0.0004 -0.0002 0 0.0002 0.0004 0.0006 Diffusion Distance(m) -Cest_1 * Creal_1 Figure 3-4. Chlorobenzene concentration profile of diffusion test for 1 hour test. The x-axis is diffusion distance in unit of m, and the y-axis is the concentration in H20 with PE equilibrated. The blue solid line is the theoretical distribution of chlorobenzene concentration given the Dpe value of (a) 2.46 * 10- 12[mZ/s] (b) 3 * 10- 12 [m2 /s] (c) 2 * 10-1 2 [m2 /s] For the reason that the data for the second hour test was not reliable enough and the fitted Dpe for chlorobenzene seems smaller than what I would expect, I decided to do another test for chlorobenzene. For the second diffusion test with chlorobenzene, I increased the number of PE layers to 19 instead of 7 in order to make a better fit of normal distribution. The reason that this could work is that by increasing the number of layers, we can investigate in a narrow area of the 48 concentration profile, which can fit a more sooth curve than before. The steps I took were not a big difference from previous tests. First, I cut 19 pieces of PE (about 30 cm * 20 cm) and chose one of them to impregnate with chlorobenzene. Here I prepared a jar with 10 mL of 2% chlorobenzene in hexadecane. Then I placed the PE inside the jar without contacting the liquid and capped it. In this way, the PE equilibrated with chlorobenzene in vapor phase. After the impregnation, I put two lead bricks on a flat bench, and then put a circle of aluminum frame above the lead bricks. I used the vacuum sealer to seal the edge of the chlorobenzene loaded PE together with other 18 pieces, placing the PE with toluene at the middle of all 19 layers. The sealed layers of PE are then place on the aluminum frame and placed another circle of frame on them. Finally, another two lead bricks are put onto these so that two layers of aluminum frames were pushed tightly together. After 6 hours diffusion time, I cut a small piece ( 3 cm * 6 cm) from the same location of all 19 pieces of PE and put them into BOD bottles filled with clean water. Because I increased the number of layers of PE, which is diffusion distance, I increased the diffusion time correspondingly in order to make sure that chlorobenzene have enough time to diffuse out a certain distance. Otherwise we may only be able to detect chlorobenzene in a few layers near the middle one. Because I only have one chemical (chlorobenzene) in my testing samples, I did some changes in the GC temperature program. I set the temperature to be isothermal at 140 'C and recorded peak area every time I injected a sample. The purge and trap program was set to purge for 5 minutes, then followed by 2 minutes heating for desorption, and baked for 8 49 minutes to clean up the residuals. The concentration measured by GC is below.(table 3-4). Table 3-4. Chlorobenzene diffusion test results of 6 hour test. Time (hour) represents the diffusion time, and Distance(m) represents the distances from the middle layer(diffusion distance). Creal is the real concentration measured by GC, and Cest is the estimate concentration by fitting an optimized Dpe values into the concentration equation. Time Distance Creal in H20 Cest in H20 (hour) () (ng/uL) (ng/uL) 0 6 6 6 6 6 6 6 0 -0.0009144 -0.0006096 -0.0003048 27.97420734 0.08 0.62 1.67 2.58 1.51 0.76 0.03 0 0.0003048 0.0006096 0.0009144 n. d. 0.057032863 0.567480281 2.25240319 3.566245637 2.25240319 0.567480281 0.057032863 Using the same approach with toluene, by doing the optimization for 6 hour test, I found the Dpe of the o-xylene to be: 2.34 * 10-12 [m 2 /s] and log(Dpe) = - 11.63 [m 2 /s]. And by plotting the theoretical curve with the optimized Dpe value, we can see how well is the real data fit into theory (figure 3-5). 50 (a) 4 3.5 3 0 1.5 1 10.5 -0 -0.0012 -0.0007 -0.0002 0.0003 0.0008 Diffusion Distance(m) Creal_6 Cest_6 (b) 4 3.5 3 25 1.5 0.5 -0.0012 -0.0007 -0.0002 0.0003 Diffusion Distance(m) Cest_6 Creal_6 51 0.0008 Figure 3-5. Chlorobenzene concentration profile of diffusion test for 6 hours' test. The x-axis is diffusion distance in unit of m, and the y-axis is the concentration in H20 with PE equilibrated. The blue solid line is the theoretical distribution of chlorobenzene concentration given the Dpe value of (a) 2.34 * 10-12 [m 2 /s] (b) 2.0 * 10-12 Results Visualization and Discussion. [m 2 /s] Diffusion coefficients (Dpe) quantify how fast a compound can move in a particular medium. When a molecule moves through PE, it needs to pass through the spaces between all kinds of straight and branched structures, so it is reasonable to guess that increasing molecular weight/ volume would decrease the diffusion coefficient. And results from previous work have confirmed this conclusion already. So I decided to plot Dpe we got in my experiments with their molar volume, Vm, which is calculated by a chemical calculation online software called SPARC27. As expected, increasing Vm for the three VOCs tested correlated (figure 3-6). -11.6 1 0 -11.62 I 105 110 115 120 125 -11.64 -11.66 -11.68 i -11.7 l -11.72 y = -0.007x - 10.903 R 2 = 0.95111 I -11.74 -11.76 U -11.78 Vm(SPARC)[cmA3/mol] *toluene Uo-xylene 52 chlorobenzene Figure 3-6. Measured log (Dpe) for toluene, o-xylene and chlorobenzene versus their molar volume, Vm (predicted from SPARC). Fit the data with linear regression and the equation is shown on left side. Notice that we had a clear trend that the Dpe decreases as the Vm increases, which is - exactly what we would expect. The equation of the linear regression line is log (Dpe) = 0.007( 0.09) Vm - 10.903( 0.08) (r 2 = 0.95, se = 0.17, n= 6). To assess our measures, we incorporated our results into previous work summarized by Lohmann 22 (figure 3-7). The results from our experiment fit pretty well on the regression line from previous work, so that we can have some confidence that our results are within the benchmark. so 100 0s o 35 o Z ast sa 90 661ZOD A x * SMetsa 1999(65) ofee at 200 (641 m P4M aet.h 2012i3 *tat, 2010 bwst fit ..-...--------E I Q I 434 - A6 x 0 50 100 150 20 V. SPAP 250 X0 350 Figure 3-7. Incorporate measured Dpe data into Lohnman's review data 22. The equation for 2 Lohnman's regression line is: log (Dpe) = -0.00145( 0.001) Vm - 10.1( 0.2) ( r = 0.76, se = 0.24, n= 74). 53 Chapter 4 Conclusion The fundamental motivation for carrying out my part of the project is to support the design and implement of passive sampling in a gasoline leaking site. The parameters that I have been measuring are critical to the sampler design and concentration interpretation. For the partition coefficient part, I have measured the Kpew values for toluene, o-xylene, ethylbenzene, pentane and hexane. The results for aromatic compounds seem to be consistent and reasonable. However, there was a quite large variation between different concentration sets for pentane and hexane experiments. Since these alkanes are harder to test in aqueous samples, future work future work may consider to get more data on alkanes. For the diffusion coefficient part, the measured Dpe values have a clear trend with molar volume. However, more data will be helpful to draw a more general relationship with relatively small volatile organic compounds. So that we can estimate a new compound's diffusion coefficient given its molar volume data, which will be save a significant time to measure each compound separately. 54 References 1. US.EPA: http://www.epa.gov/oust/ 2. USEPA, Semiannual Report Of UST PerformanceMeasures. End Of Fiscal Year 2014, 2014 3. USEPA, Conceptual Model Scenarios for the Vapor Intrusion Pathway,2012 4. McAlary, T., Wang, X., Unger, A., Groenevelt, H., & G6recki, T. (2014). Quantitative & passive soil vapor sampling for VOCs-part 1: theory. EnvironmentalScience: Processes Impacts, 16(3), 482-490. 5. USEPA, 40 CFR Part 280 6. Karp, Kenneth E. "A diffusive sampler for passive monitoring of underground storage tanks." Groundwater Monitoring & Remediation 13.1 (1993): 101-106. J. Environ. 7. R. H. Brown, 8. J. Namie'snik, Monit., 2000, 2, 1-9. B. Zabiegala, A. Kot-Wasik, M. Partyka and A. Wasik, Anal. Bioanal. Chem., 2005, 381, 279-301. 9. S. Seethapathy, T. G'orecki and X. Li, 10. T. G'orecki and J. J. Chromatogr. A, 2008, 1184, 234-253. Namie'snik, TrAC, Trends Anal. Chem., 2002, 21(4), 276-291. 11. D. P. Adley and D. W. Underhill, Anal. Chem., 1989, 61(8), 843-847. 12. Kerfoot, Henry B., and C. L. Mayer. Use of industrial-hygiene samplers for soil-gas measurement. No. PB-89-166359/XAB. Lockheed Engineering and Management Services Co., Inc., Las Vegas, NV (USA), 1989. 13. Gomes, D. C., Alarsa, M., Salvador, M. C., & Kupferschmid, C. (1994). Environmental Soil and Ground Water Assessment Using High Resolution Passive Soil-Gas Samplers-Petrex & Method: Methodology and Results of a Case Study Performed in Brazil. Water Science Technology, 29(8), 16 1-172. 55 14. M. Anderson and G. Church, J. Environ. Eng., 1998, 124, 555. 15. Environmental Technology Veriocation Report, Soil Gas Sampling Technology, Quadrel Services, Inc., EMFLUX Soil Gas System, U.S. EPA Office of Research and Development. EPA Report no. 600/R-98/096,1998. 16. Beacon, 2013. http:://www.beacon-usa.com/services/passive-soil-gas-surveys/ 17. Environmental Technology Veriocation Report, Soil Gas Sampling Technology, W. L. Gore & Associates, Inc. GORE-SORBER Screening Survey, U.S. EPA Office of Research and Development. EPA Report no. 600/R-98/095, 1998. 18. ASTM Standard D7758, New Practice for Passive Soil Gas Sampling in the Vadose Zone for Source Identication, Intrusion Evaluations, Spatial Variability Assessment, ASTM International, West Monitoring Conshohocken, and Vapor PA, 2011, http://www.astm.org. 19. Final Guidance for the Evaluation and Mitigation of Subsurface Vapor Intrusion to Indoor Air (Vapor Intrusion Guidance), California Environmental Protection Agency/Department of Toxic Substances Control (EPA/DTSC), October 2011. http:// www.dtsc.ca.gov/AssessingRisk/upload/Final VIG Oct 2011. 20. T.P.Rusina,F.Smedes,and.Klanova,J.Appl.Polym.Sci.,2010,1803-1810 21. M.Saleem,A.DF.A.Asfour,D.DeKee,andB.Harrison,J.Appl.Polym.Sci.1989,37,617-625 22. Lohmann, Rainer. "Critical review of low-density polyethylene's partitioning and diffusion coefficients for trace organic contaminants and implications for its use as a passive sampler." Environmentalscience & technology 46.2 (2011): 606-618. 23. L.A.Fernandez, C.F.Harvey, and P.M.Gschwend, Environ. Sc. Technol.,2009, 43, 8888-94. 24. J.N.Apell and P.M.Gschwend, Environ. Sci. Technol., 2014, 48, 10301-7. 25. Wikipedia: http://en.wikipedia.org/wiki/Polyethylene 26. Schwarzenbach, Rene P., Philip M. Gschwend, and Dieter M. Imboden. Environmental 56 OrganicChemistry.John Wiley & Sons, 2003. 27. SPARC: ARChem's physicochemical calculator 57 Appendix Appendix A. Toluene Desorption Time course 10.0 9.0 o 8.0 7.0 6.0 o _ 5.0 E 4.0 3.0 2.0 0~1 1.0 0 0.00 10 20 30 40 50 60 Vapor Phase Sorption Time (hours) Figure A-1. Desorbed water concentration versus vapor phase sorption time testing on toluene. Method: 1 mL of 100:1 hexadecane:toluene was added to a tared, -64 mL volume, glassstoppered jar. The 4 mil PE was 'ribboned' onto aluminum wire supports for suspension above the hexadecane:toluene solution. At t=0, 24 and 50h PE was removed and placed into a water-filled, glass-stoppered jar. The toluene-exposed, 4 mil PE (removed from the aluminum wire support) was added to a tared, glass-stoppered jar filled with Aries H 2 0 and after 72 hours test the concentration in GC. 58