Materials and Methods

A total of 58 PVC pipes from six manufacturers were purchased from local hardware stores. The pipe diameters ranged from ¾ inch to 2 inch. All the pipes were ASTM 1785D- certified, Schedule 40 and marked NSF-PW. Tests were not conducted on old, used, or compromised pipes since the age, exposure history, and conditions are typically not known. Five organic solvents, benzene, toluene, ethylbenzene, xylene (o-xylene, m-xylene and p- xyene) and trichloroethene (TCE), were selected on the basis of their occurrence in contaminated soil and purchased from Fisher Scientific (Chicago, IL).

To dilute a target organic solvent to a certain strength or activity level, inert solvents that do not interact with the PVC pipe material were used. Polyethylene glycol (PEG), an inert chemical that does not interact with the PVC pipe material was used by Berens (1985) to determine the chemical activity threshold at which significant sorption would occur. The National Institute of Standards & Technology (NIST) specifies a “reference fuel” to simulate free product gasoline, consisting of 91% (v/v) (91.09% w/w) 2,2,4-trimethylpentane (iso- octane) and 9% (v/v)(8.91 w/w) n-heptane. The reference fuel has been used to calibrate instruments for the determination of lead (NIST, 1988) and alcohols (National Bureau of Standards, 1986) in gasoline. This reference fuel may be a good surrogate for the alkanes in gasoline which are inert to pipe materials. Both PEG and the reference fuel were used in determining the chemical activity threshold before permeation occurs. PEG (Carbowax®

PEG400), 2,2,4,-trimehtylpentane, and n-heptane were purchased from Fisher Scientific (Chicago, IL).

3.3.2 Pipe-bottle test

The pipe-bottle test was used to evaluate the usefulness of the microscopic visualization technique in predicting the permeation of organic contaminants through PVC pipes. Experiments were conducted in a pipe-bottle apparatus consisting of a 1-L glass bottle with one PVC pipe mounted horizontally through holes drilled in the glass bottle (Figure 3.2). A chemically resistant sealant, Loctite epoxy putty (Henkel Technologies, Rocky Hill, CT) was used to seal the gap between the glass bottle and the pipe. The ends of the pipes were sealed with Teflon plugs. One of the Teflon plugs had a small hole that was plugged with a removable threaded brass plug to allow filling and draining of the water inside the pipe with a glass syringe. The bottle was capped with a Teflon-lined cap.

Three “mini” pipe-bottle apparatuses were set up with a 1-inch diameter PVC pipe directly exposed to pure benzene, toluene and TCE, respectively. Note that only one 1-inch pipe from a manufacturer was used in the experiments. In this test, the pipe was first filled with deionized water (pipe water) and the solvent was then added into the bottle until a visible liquid level of solvent appeared above the pipe. To minimize the amount of solvents added and thus reduce the amount of hazardous waste produced, the bottle space below the pipes was filled with glass beads. The pipe-water was sampled each day and analyzed for the presence of the target compounds. After each sampling event, the pipe was immediately flushed three times with deionized water before fresh deionized water was added and the pipe sealed with the threaded brass plug.

Benzene, toluene and TCE in water samples were determined using a gas chromatograph (Tracor 540, Tracor Instruments Austin, Inc., Austin TX) equipped with a packed column (1.8 m × 2 mm; 1% SP1000 on 60/80 mesh Carbopack B), a photoionization detector, and an automated purge and trap concentrator (Tekmar LSC2/ALS). The detection limits for benzene, toluene and TCE were 0.24 µg/L, 0.24 µg/L, and 1.2 µg/L, respectively. 3.3.3 Microscopic visualization test

Three types of experiments were conducted: (i) exposing 1-inch pipe specimens to three pure solvents (benzene, toluene and TCE) to correlate the moving front data to the permeation data obtained from the pipe-bottle tests; (ii) exposing 1-inch pipe specimens to toluene at various activity levels (pure solvent, 20-80% of solvent by volume in PEG/NIST reference fuel) to determine the threshold activity level that would result in the formation of the moving front; and (iii) exposing 58 pipes to pure toluene solvent to compare the relative susceptibility to permeation of different size pipes and pipe materials obtained from different pipe manufacturers. For test (i) and test (ii), only one-inch PVC pipe from a manufacturer was used.

In the first experiment, 1-inch diameter pipe specimens were cut to approximately 3.5 cm in length. One end of the pipe was sealed with a glass slide using Loctite epoxy putty (Henkel Technologies, Rocky Hill, CT). The pipe was then filled with deionized water from the unsealed end. This end was then sealed with a glass slide using the same epoxy material. The pipe specimens were then immersed in 50 mL of test solvents (benzene, toluene and TCE) in a 120-mL glass jar with a Teflon-lined lid. At different times, a pipe specimen was removed from the solvent, wiped dry, cut, and the progress of the moving front visualized.

As shown in Figure 3.1, three parameters can be measured: (i) the original pipe thickness (L0), (ii) the thickness of the swollen layer at time t (Ls,t), and (iii) the thickness of the

remaining glass core (or the distance of the sharp boundary to the inner wall, Lg,t). The

penetration distance at time t was then calculated by:

Penetration distance (PDt ) = L0 - Lg, (Eq. 3.1)

In this test, the distance of the moving front from the outside to the inside of the pipe (both of Ls,t and Lg,t) was measured.

In the other two tests (ii and iii), pipes to be tested were cut across the longitudinal axis of the pipe, so as to form ring-like specimens of 1.5 cm in length. The pipe specimens were placed directly into the target solvents with both faces (outside and inside) exposed to the solvent. The specimens were removed from the solvent at specific times and dried using a paper towel of any external solvent. Each dried specimen was first cut parallel to the longitudinal axis and then cut across the longitudinal axis of the pipe specimen, so as to form an approximately 1/5 ring-like sample with a length of 0.5 cm. The sample cross section was observed directly under a reflected light microscope (Olympus BHM) and its image captured by a camera mounted on the microscope. Since in a typical contaminated site a solvent penetrates pipes from the outer wall to the inner wall, only the outer swollen layer (Ls,t) was

measured for these experiments. With PC-Image (version 5.0) software package, the penetration distance of the moving front was precisely measured. All experiments were conducted at room temperature (23 ± 1.5oC) for reasons of operational convenience and economy.

3.4 Results and Discussion