ADSORPTION OF SULFUR HEXAFLUORIDE ONTO 

CRUSHED TUFFS FROM THE YUCCA MOUNTAIN 

AREA, NYE COUNTY, NEVADA

by Gordon W. Rattray, Robert G. Striegl, and In C. Yang

U.S. GEOLOGICAL SURVEY

Water-Resources Investigations Report 95-4057

Prepared in cooperation with the 

NEVADA OPERATIONS OFFICE, 

U.S. DEPARTMENT OF ENERGY under 

Interagency Agreement DE-AI08-92NV10874

Denver, Colorado 

1995
U.S. DEPARTMENT OF THE INTERIOR 

BRUCE BABBITT, Secretary

U.S. GEOLOGICAL SURVEY 

Gordon P. Eaton, Director

The use of trade, product, industry, or firm names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

For additional information write to: Copies of this report can be purchased from:

Chief, Earth Science Investigations U.S. Geological Survey

Program Earth Science Information Center

Yucca Mountain Project Branch Open-File Reports Section

U.S. Geological Survey Box 25286, MS 517

Box 25046, MS 421 Denver Federal Center

Denver Federal Center Denver, CO 80225 

Denver, CO 80225
CONTENTS

Abstract ................................................................................................................................................................................ 1

Introduction .........................................................................................................................................................^^ 1

Purpose and scope ...................................................................................................................................................... 1

Location and geologic setting .................................................................................................................................... 1

Experiment procedure .......................................................................................................................................................... 3

Samples ...................................................................................................................................................................... 3

Sample preparation .......................................................................................................................................... 3

Sample characterization ................................................................................................................................... 3

Density and porosity .............................................................................................................................. 3

Cation-exchange capacity ...................................................................................................................... 5

Surface area ............................................................................................................................................ 5

Mineralogy ............................................................................................................................................. 8

Adsorption test method .............................................................................................................................................. 8

Test apparatus ................................................................................................................................................... 8

Sample conditioning ........................................................................................................................................ 8

Experiment components ................................................................................................................................... 12

Fixed components .................................................................................................................................. 12

Variable components .............................................................................................................................. 12

Analytical instrument calibration and limits .............................................................................................................. 18

Results of sulfur hexafluoride adsorption experiments ........................................................................................................ 18

Adsorption modeling.................................................................................................................................................. 18

Freundlich isotherm equation ........................................................................................................................... 18

Retention equation ........................................................................................................................................... 24

Conclusions .......................................................................................................................................................................... 28

References ............................................................................................................................................................................ 28

FIGURES

1. Location of drill holes UE-25 UZ #4 and UE-25 UZ #5, and nearby geographic features in

southern Nevada.................................................................................................................................................... 2

2. Diagram of the depth and extent of geologic units at boreholes UE-25 UZ #4 and UE-25 UZ #5 ...................... 4

3. Schematic of adsorption test apparatus................................................................................................................. 10

4. Detailed diagram of sample flask.......................................................................................................................... 11

5-12. Graphs showing:

5. Adsorption of sulfur hexafluoride (SF6) onto dry and wet bedded tuff (BT), as a function of time,

with an initial concentration of sulfur hexafluoride in the flask of 1.94 ppmv........................................... 13

6. Adsorption of sulfur hexafluoride (SF6) onto Pah Canyon Tuff (PCT) and Yucca Mountain Tuff 

(YMT), as a function of time, with an initial concentration of sulfur hexafluoride in the flask 

of 1.94 ppmv............................................................................................................................................... 14

7. Adsorption isotherms for bedded tuff (BT) and Topopah Spring Tuff, UE-25 UZ #4 (TST4).................. 19

8. Adsorption isotherms for Yucca Mountain Tuff (YMT), Topopah Spring Tuff, UE-25 UZ #5

(TST5), and clinoptilolite........................................................................................................................... 21

9. Adsoprtion isotherms for Tiva Canyon Tuff (TCT), Pah Canyon Tuff (PCT), and gypsum

cement (GC)................................................................................................................................................ 22

10. Moles of sulfur hexafluoride adsorbed, per square meter of tuff, as a function of the partial pressure

of sulfur hexafluoride.................................................................................................................................. 25

11. Comparison of uncorrected and corrected combined retention equations.................................................. 26

12. Correlation of equilibrium constant (Kj) and surface area with the percent of zeolite in the tuffs............ 27

CONTENTS III
TABLES

Sample collection depth and degree of welding of tuff samples .............................................................................. 5

Densities, porosities, and cation-exchange capacities for crushed tuffs, gypsum cement, and clinoptilolite........... 6

Surface-area analyses (m2/g) of crushed tuffs, gypsum cement, clinoptilolite, and kaolinite.................................. 7

Mineralogical analyses (weight percent) of crushed tuffs, gypsum cement, and clinoptilolite................................ 9

Adsorption test data.................................................................................................................................................. 15

6. Values for constants (N, Kj, K), squared correlation coefficients (r2), and standard error of estimate

(SEE) for the Freundlich isotherm equations and the retention equations ............................................................... 23

7. Values for variables in the retardation equation........................................................................................................ 24

8. Values of variables in the retardation equation using bulk and grain densities (for porosity calculations)

of core samples................................................................................................................^^ 24

9. Predicted adsorption of sulfur hexafluoride (pmolspg/gtuff) onto tuffs using the combined and individual

retention equations.................................................................................................................................................... 28

CONVERSION FACTORS

Multiply By To obtain

cubic centimeter per minute (cm3/min)

cubic meter (m3)

gram per cubic centimeter (g/cm3)

kilometer (km)

meter (m)

meter squared per gram (m2/g)

micrometer (Jim)

millimeter (mm)

millipascal (mPa)

pascal (Pa)

picogram per cubic centimeter (pg/cm3)

picomole per gram (pmol/g)

3.661

3.531 X 101

5.781 X 10' 1

6.214 X 10' 1

3.281

3.050 XlO2

3.937 X 10'5

3.937 x 10'2

1.450X10'7

1.450X1Q-4

5.781 X 10' 13

2.835 X 101

cubic inch per second

cubic foot

ounce per cubic inch

mile

foot

foot squared per ounce

inch

inch

pound per square inch

pound per square inch

ounce per cubic inch

picomole per ounce

Degree Celsius (°C) may be converted to degree Fahrenheit (°F) by using the following equation:

°F = 9/5(°C) + 32.

Kelvin (K) may be converted to degree Celsius (°C) by using the following equation:

°C = K-273.15.

IV CONTENTS
Adsorption of Sulfur Hexafluoride Onto Crushed Tuffs 

from the Yucca Mountain Area, Nye County, Nevada

^/Gordon W. Rattray, Robert G. Striegl, and\t\ C. Yang

Abstract

A laboratory batch-type testing procedure 

was developed that provides a simple, rapid, and 

reproducible method to investigate the adsorptive 

capabilities of crushed materials for gaseous compounds. Several batch tests were conducted to test 

crushed samples of tuff, clinoptilolite, and gypsum 

cement for their retention of sulfur hexafluoride. 

For each sample tested, the surface area, distribution coefficient, and retention equation were determined. The surface areas of the samples decreased 

in the following order: Topopah Spring Tuff, 

UE-25 UZ #5; bedded tuff; clinoptilolite; Yucca 

Mountain Tuff; Topopah Spring Tuff, UE-25 

UZ #4; Pah Canyon Tuff; gypsum cement; and 

Tiva Canyon Tuff. The distribution coefficients 

show that sulfur hexafluoride is readily adsorbed 

onto clinoptilolite, bedded tuff, and Topopah 

Spring Tuff, but that it does not appreciably adsorb 

onto gypsum cement, Tiva Canyon Tuff, or Pah 

Canyon Tuff. Retention equations, which were 

calculated as a function of the surface area of the 

tuffs, were similar for all but one (Tiva Canyon 

Tuff) of the tuffs. The similarity of the retention 

equations demonstrates that the surface area of a 

tuff is a good indicator of the sorptive capability of 

the tuff. The distribution coefficients and the surface areas of the tuffs show a correlation with the 

amount of zeolite in the tuff, providing evidence 

that zeolites are the principal mineral controlling 

the adsorption of sulfur hexafluoride.

INTRODUCTION

The U.S. Geological Survey (USGS) is conducting investigations to determine the geologic and hydrologic suitability of the Yucca Mountain area, Nye 

County, Nevada, as a potential site for a mined geologic repository for high-level nuclear wastes. These 

investigations are being conducted under the auspices 

of the U.S. Department of Energy, Nevada Operations Office, under Interagency Agreement

DE-AI08-92NV 10,874, and are part of the Yucca 

Mountain Site Characterization Project.

Effective utilization of gaseous tracers in geologic materials requires either that the tracer be truly 

conservative or that its transport properties be known. 

Although SF6 has previously been used as a gaseous 

tracer to study the tortuosity and sorption-affected 

porosity in the unsaturated zone at a low-level nuclear 

waste disposal site near Barnwell, South Carolina 

(Kreamer and others, 1988), and is currently being 

used in unsaturated zone studies at Yucca Mountain, 

Nevada, the transport properties of SF6 have not been 

rigorously evaluated.

The main objective of the work described here is 

to provide a model that accurately predicts the behavior 

of SF6 in the unsaturated zone of the Yucca Mountain 

area. Understanding how SF6 interacts with unsaturated zone materials will be helpful in evaluating the 

effectiveness of SF6 as a gas tracer for ongoing gascirculation studies, air-permeability testing, and drydrilling operations at Yucca Mountain.

Purpose and Scope

This report presents the results of batch experiments testing the adsorption of a gaseous tracer, sulfur 

hexafluoride (SF6), onto (1) crushed tuffs; (2) clinoptilolite, a principal zeolite mineral found in the tuffs; 

and (3) gypsum cement, a grout material used to stem 

instrumented boreholes at Yucca Mountain.

Location and Geologic Setting

Yucca Mountain is in Nye County, Nevada, 

approximately 145 km northwest of Las Vegas, on the 

Nevada Test Site (fig. 1). The Yucca Mountain area is 

underlain by a sequence of Miocene ash-flow tuffs that 

are separated by ash-fall bedded tuff. The eastern side 

of the mountain is dissected by five northwest-trending 

washes underlain by zones of right-lateral strike-slip 

faults (Scott and Bonk, 1984).

Abstract
116°45' 116°30'

Amargosa Valley (formerly 

Lathrop Wells)

36°45' -

Base from U.S. Geological Survey 

1:250,000, Death Valley, California

Nevada' 1970 0 5 KILOMETERS

EXPLANATION 

 DRILL HOLES AND IDENTIFICATION

Figure 1. Location of drill holes UE-25 UZ #4 and UE-25 UZ #5, and nearby geographic features in southern 

Nevada.

2 Adsorption of Sulfur Hexafluoride Onto Crushed Tuffs from the Yucca Mountain Area, Nye County, Nevada
Test holes UE-25 UZ #4 and UE-25 UZ #5 

(hereinafter called UZ4 and UZ5) are located in 

Pagany Wash on the eastern side of Yucca Mountain. 

These holes were drilled to a depth of about 110m into 

the unsaturated Paintbrush Group (Sawyer and others, 

1994). The members of the Paintbrush Group penetrated are, in descending order, the Tiva Canyon Tuff, 

Yucca Mountain Tuff, Pah Canyon Tuff, and Topopah 

Spring Tuff (Yang, 1992; Loskot and Hammermeister, 

1992). The stratigraphy and lithology of the geologic 

units penetrated at UZ4 and UZ5 are shown in fig. 2.

EXPERIMENT PROCEDURE

The adsorption experiment consisted of three 

general elements: selecting, preparing, and characterizing the samples; the adsorption test method; and the 

analytical instrument calibration and SF6 detection limits.

Samples

The tuffs used in the adsorption tests, except for 

the Tlva Canyon Tuff, were prepared from core samples obtained from UZ4 and UZ5. The sample from the 

Tiva Canyon Tuff was prepared from outcrop samples 

collected on top of Yucca Mountain. The core samples 

underwent triaxial-compression extraction of pore 

water before use for adsorption testing. Collection and 

preparation methods of cores for pore-water extraction 

are detailed in Yang and others (1988). The experiment 

samples included a representative from each tuff unit of 

the Paintbrush Group (table 1). With the exception of 

the Topopah Spring Tuff, the core intervals (or outcrop 

samples) from each tuff unit were combined to form 

one sample. In order to observe the change in sorptive 

capacity between a welded and nonwelded sample 

from the same tuff unit, the Topopah Spring Tuff was 

divided into two samples: one sample contained 

densely welded core from the UZ4 core intervals, and 

the second sample consisted of nonwelded core from 

the UZ5 core intervals. A sample of clinoptilolite, a 

major mineral phase of many of the tuffs found at 

Yucca Mountain, was provided by Lewis-Russ (1990). 

A sample of the grout material (gypsum cement) that 

will be used for stemming boreholes also was prepared 

for adsorption testing. The samples are abbreviated as 

follows: Tiva Canyon Tuff (TCT), Yucca Mountain 

Tuff (YMT), Pah Canyon Tuff (PCT), bedded tuff 

(BT), Topopah Spring Tuff (abbreviated as TST4 and 

TST5 to identify whether the sample is from borehole 

UZ4 or UZ5, respectively), clinoptilolite (CLINO), and 

gypsum cement (GC).

Sample Preparation

The sample cores were crushed, successively, 

with a sledgehammer, jawcrusher, and roller crusher. 

The sledgehammer was used to reduce the cores to 

about 5-mm-diameter fragments; the jawcrusher then 

reduced the fragments to particles of less than 1 mm; 

and the roller crusher reduced the particles to a size of 

less than 500 \im. The samples were then sieved to 

provide samples with particle sizes of 63-150, 

150-250, and 250-500 \im. An additional identifier 

was added to the sample abbreviations to reflect the 

sample particle size; for example, the Yucca Mountain 

Tuff samples are abbreviated as YMT 63-150, 

YMT 150-250, and YMT 250-500.

Sample Characterization

The adsorptive capacities of rock and grout 

material are controlled by their physical and chemical 

properties. Identification of those properties that 

directly influence the sorptive capability of rock and 

grout material will allow for development of a model 

predicting the sorptive behavior of SF6 at Yucca Mountain. Three physical and chemical properties of the 

samples were measured: cation-exchange capacity, 

surface area, and mineralogy. In addition, the bulk density and porosity of each sample was determined so that 

the solid volume of the sample used in the tests could 

be calculated.

Density and porosity

The bulk densities of the crushed samples 

(table 2) were determined by first measuring the volume of each sample in a volumetric beaker and then 

weighing the sample. The porosities of the crushed 

samples were calculated from:

pfc = p,(l-0) (1)

where,

pfc = bulk density of sample, g/cm3 ;

P£ = grain density of sample, g/cm3 ; and

0 = porosity of sample, dimensionless.

The grain densities used for calculating the sample 

porosities are from samples similar to those used in the 

adsorption experiment (Dayal and Klein, 1988; Ming 

and Mumpton, 1989; Flint and Flint, 1990; Flint and 

others, 1993).

The porosities of the crushed tuffs ranged from 

54 to 70 percent (includes both the inter- and intragrain porosities) with the nonwelded tuffs having 

higher porosities than the densely welded tuffs. The

EXPERIMENT PROCEDURE
UZ4 UZ5 LITHOLOGY

0

20

40

O)

o: LJJ h~ 

LJJ

Q_ LLJ 

Q

60

80

100

120

ALLUVIUM

-

CL 

D 

O

on

(D

X

~ c/)

D

on

DOhz

Q.

-

TIVA 

CANYON 

TUFF

BEDDED

TUFF

YUCCA 

MOUNTAIN

TUFF

BEDDED 

TUFF ,

PAH 

CANYON

TUFF

BEDDED 

TUFF

TOPOPAH 

SPRING 

TUFF

Q.

D 

O

on

(D

X 

0)

D

on

QQhz

a:

TIVA 

CANYON 

TUFF

BEDDED 

TUFF

If

YUCCA

MOUNTAIN 

TUFF

BEDDED 

TUFF

PAH

CANYON

TUFF

BEDDED

TUFF

-

TOPOPAH 

SPRING 

TUFF

TIVA CANYON TUFF

Ash-flow tuff with a partially to nonwelded 

vitric base and a moderately to densely 

welded devitrified top and interior.

YUCCA MOUNTAIN TUFF

Vitric ash-flow tuff with a nonwelded top

and base and a partially welded interior. 

PAH CANYON TUFF

Partially to nonwelded vitric ash-flow tuff.

BEDDED TUFF

Poorly sorted ash-fall tuff with a moderately 

to highly indurated top and base and a 

moderately consolidated interior.

TOPOPAH SPRING TUFF

Ash-flow tuff with a nonwelded top and 

base and a moderately to densely welded 

devitrified interior.

Figure 2. Diagram of the depth and extent of geologic units at boreholes UE-25 UZ #4 and UE-25 UZ #5. 

Lithologic descriptions are modified from Loskot and Hammermeister (1992).

4 Adsorption of Sulfur Hexafluoride Onto Crushed Tuffs from the Yucca Mountain Area, Nye County, Nevada
Table 1 . Sample collection depth and degree of welding of tuff samples

Sample UZ4 depth 

(meters)

Degree

of 

welding

UZ5 depth 

(meters)

Degree of 

welding

Tiva Canyon Tuff Collected on top of Yucca Mountain Moderate 

to dense

Yucca Mountain Tuff

Pah Canyon Tuff

Bedded Tuff

Topopah Spring Tuff

34.2-35.1

36.8-37.1

40.7^4.3

5.5-57.9

61.0-61.1

72.2-73.5

90.4-92.7

99.1-103.0

106.1-106.3

109.7-110.0

Non

Partly

Non

Non

Non

Non

Dense

Dense

37.9-38.0

66.1-68.0

75.3-82.0

86.4-88.0

96.0-96.7

98.3-101.4

101.5-101.7

102.4-102.6

104.7-104.8

Non

Non

Non

Non

Non

Non

Non

bulk densities of the crushed tuffs varied from 0.69 to 

1.05 g/cm3, with the densely welded Tiva Canyon Tuff 

and Topopah Spring Tuff, UZ4, samples having the 

highest densities (0.83 to 1.05 g/cm3) and the nonwelded Topopah Spring Tuff, UZ5; Pah Canyon Tuff; 

and bedded tuff samples having the lowest densities 

(0.69 to 0.88 g/cm3).

Cation-exchange capacity

The cation-exchange capacities were measured 

by Crystal Research Laboratories (Golden, Colo.) by 

saturating the samples with a solution of ammonium 

carbonate at a pH of 7.0. The samples were then heated 

to 100°C and analyzed for nitrogen gas (Lewis-Russ, 

1990; Maynard Slaughter, Crystal Research Laboratories, oral commun., 1994).

The range of cation-exchange capacities 

observed for the crushed samples (table 2) fall into 

three groups: (1) samples with cation exchangecapacities of less than 10 meq/100 grams (Tiva Canyon 

Tuff, Pah Canyon Tuff); (2) samples with cationexchange capacities between 10-50 meq/100 grams 

(Yucca Mountain Tuff; Topopah Spring Tuff, UZ4; 

gypsum cement); and (3) samples with cationexchange capacities of greater than 100 meq/100 grams

(Topopah Spring Tuff, UZ5; bedded tuff; clinoptilolite).

Surface area

Prior to (1.5 years) starting the SF6 adsorption 

tests, single-point surface-area determinations were 

made on all the crushed samples with a Quantasorb 

sorption analyzer (table 3). The single-point measurements were made at one partial pressure of adsorbate 

(nitrogen, N2) using a 30-70 percent N2-He (helium) 

mixture as the analysis gas. Samples were prepared for 

surface-area analyses by flowing 100 percent helium 

gas through the sample at outgassing temperatures of 

70° and 230°C for at least 3 hours. The reliability of the 

analytical instrument and procedure was tested by

comparing the measured surface-area (9.2 m2/g) of the 

KGa-1 kaolinite (KGa-1 kaolin, Washington County,

Georgia) with published values (10.1 m2/g, LewisRuss, 1990; 11.2 m2/g, Carroll-Webb and Walther, 

1988).

Immediately following the adsorption tests, 

multi-point surface-area determinations (measured at 

various partial pressures of adsorbate, N2) were made 

for samples used in the adsorption tests. The samples

EXPERIMENT PROCEDURE
Table 2. Densities, porosities, and cation-exchange capacities for crushed tuffs, gypsum cement, 

and clinoptilolite

[Bulk density measurements were repeatable within ±5 percent. TCT, Tiva Canyon Tuff; YMT, Yucca Mountain Tuff; 

PCT, Pah Canyon Tuff; BT, bedded tuff; TST4, Topopah Spring Tuff, UZ4; TST5, Topopah Spring Tuff, UZ5; GC,

gypsum cement; CLINO, clinoptilolite; 63-150, 150-250, and 250-500 are crushed rock sizes in micrometers; g/cm , 

gram per centimeter cubed; %, percent; meq/100 g, rnilliequivalents per 100 grams; ±, plus or minus; --, no data]

Sample

TCT 150-250

YMT 63-150

YMT 150-250

YMT 250-500

PCT 63-150

PCT 150-250

PCT 250-500

BT 63-150

BT 150-250

BT 250-500

TST4 63-150

TST4 150-250

TST4 250-500

TST5 63-150

TST5 150-250

TST5 250-500

GC 63-150

GC 150-250

GC 250-500

CLINO 150-250

Bulk density 

Pb(9/cm3)

1.00

0.90

0.93

0.85

0.76

0.77

0.73

0.72

0.69

0.81

0.89

0.83

1.05

0.77

0.83

0.88

0.82

0.89

-

0.87

Grain density1 

Pa (9/cm3)

2.4

2.3

2.3

2.3

2.4

2.4

2.4

2.3

2.3

2.3

2.3

2.3

2.3

2.3

2.3

2.3

2.2

2.2

2.2

2.2

Porosity2 

6(%)

58

61

60

63

68

68

70

69

70

65

61

64

54

67

64

62

63

60

-

62

Cation-exchange 

capacity3 

(meq/100 g)

2.1 ±2.1

17.84 ±6.9

17.13 ±6.7

9.28 ±4.3

9.28 ±4.3

5.71 ±2.8

7.14 ±3.4

103.5 ± 5.2

102.8 ±5.1

103.5 ±5.2

52.83 ±5.1

49.26 ±5.3

38.55 ±7.8

119.2 ±6.0

116.4 ±5.8

129.9 ±6.5

__

14.99 ±6.2

11.42 ±5.1

127.5 ± 10

^rain densities are from Hint and Hint, 1990; Hint and others, 1993; Ming and Mumpton, 1989; and 

Dayal and Klein, 1988.

Porosity values were calculated from bulk and grain densities. 

Analyzed by Crystal Research Laboratories, Golden, Colo.

6 Adsorption of Sulfur Hexaf luorlde Onto Crushed Tuffs from the Yucca Mountain Area, Nye County, Nevada
Table 3. Surface-area analyses1 (m2/g) of crushed tuffs, gypsum cement, clinoptilolite, 

and kaolinite

[ , no data]

Samples

Outgassing temperature, degrees Celsius (°C) 

Single-point Multi-point

70°C 230 C 150°C 230°C

TCT 150-2502'3 0.79 0.97 0.81 0.93

YMT 63-150

YMT 150-250

YMT 250-500

PCT 63-150

PCT 150-250

PCT 250-500

BT 63-1 50

BT 150-250

BT 250-500

TST4 63-150

TST4 150-250

TST4 250-500

TST5 63-150

TST5 150-250

TST5 250-500

GC 63-150

GC 150-250

GC 250-500

CLINO 150-2503

KGa-1 kaolin3

Kaolinite4

12.2

14.1

17.2

4.28

4.72

5.28

28.8 ± 0.30

32.6 ± 0.90

26.9 ±0.25

10.1

10.5

9.06

21.8

30.8

29.0

__

18.5

15.1

-

8.49

-

13.4 ±1.52

15.8 ±0.15

15.2 ±0.10

5.61 ±0.16

6. 10 ±0.13

6.90 ±0.01

32.9 ± 0.65

44.2 ±3.11

41.1 ±0.55

12.3 ±2.07

13.8 ±0.45

9.03 ±0.50

37.6 ±0.75

40.2 ±0.30

41.2 ±0.40

_

16.8 ± 2.67

11. 5 ±3.22

-

9.20

-

..

9.16 10.1

~

_

5.29 5.61

~

_

23.8 22.9

-

_

8.41 8.42

--

_

24.3 23.8

--

5.13 5.37

-

12.2 12.1

--

16.0

Single-point surface-area analyses were determined with a Quantasorb sorption analyzer manufactured by Quantachrome Corp., New York. Multi-point surface-area analyses were determined 

with a Micromeritics Gemini 2360 surface-area analyzer. Both instruments are maintained by the 

U.S. Geological Survey Water Quality Laboratory, Arvada, Colo.

Refer to table 2 for explanation of sample labels and symbols. 

3Single-point surface-area values (230°C) in Lewis-Russ (1990): TCT = 0.9 ± 0.17; 

Clinoptilolite = 17 ± 0.9; KGa-1 Kaolin = 10.1.

Tleference value of 16.10 ± 0.8 m /g from Micromeritics, Norcross, Georgia.

EXPERIMENT PROCEDURE
were outgassed in a helium environment at temperatures of 150° and 230°C and analyzed with a Gemini 

2360 surface-area analyzer using 100 percent N2 as the 

analysis gas. The accuracy of the instrument was 

determined by comparing the measured (16.0 m2/g) 

and reference (16.10 ±0.8 m2/g) value of a kaolinite 

reference material supplied with the instrument 

(Micromeritics, Norcross, Georgia).

The single-point surface-areas obtained when 

the outgassing temperature was 230°C were significantly higher than surface-areas obtained when the outgassing temperature was 70°C. This discrepancy is 

probably due to more efficient removal of contaminants from samples at higher temperatures (Ming and 

Mumpton, 1989). The lower single-point values do, 

however, compare favorably with the multi-point measurements of each sample. Because the multi-point 

surface-area measurements were determined immediately following the adsorption tests, these measurements are a better representation of the samples during 

the tests. Consequently, the multi-point measurements 

were used in our retention calculations (see below). On 

the basis of the multi-point measurements, the tuffs can 

be separated into three distinct groups: (1) samples

with surface areas less than 1 m2/g (Tiva Canyon Tuff);

(2) samples with surface areas between 5 and 10 m2/g 

(Pah Canyon Tuff; Yucca Mountain Tuff; Topopah 

Spring Tuff, UZ4); and (3) samples with surface areas

greater than 20 m2/g (Topopah Spring Tuff, UZ5; bedded tuff).

Mineralogy

The mineralogies of the crushed tuff, grout, and 

clinoptilolite samples (table 4) were determined by 

X-ray diffraction. The relative abundances of the minerals were estimated from the diffractometer patterns 

by comparing the patterns with peak intensities of 

internal standards (Klug and Alexander, 1974; 

Sheppard and Gude, 1982). This work was performed 

by Crystal Research Laboratories (Golden, Colo.) and 

the USGS Branch of Geochemistry (Lakewood, Colo.).

From the mineralogical analyses, most of the tuff 

samples can be categorized as containing either a primary or secondary assemblage of minerals. The unaltered tuffs contain a large percentage of unstable, 

primary phases such as glass, tridymite, and cristobalite (Vaniman and others, 1984). These constituents 

are major phases in the Tlva Canyon Tuff, Yucca 

Mountain Tuff, and Pah Canyon Tuff. Conversely, the 

bedded tuff and Topopah Spring Tuff, UZ5, samples 

are composed of approximately 50 percent secondary 

zeolites. The principal zeolite minerals in these samples are clinoptilolite and/or heulandite (Steve Sutley, 

USGS Branch of Geochemistry, written commun., 

1994; Maynard Slaughter, Crystal Research Laboratories, written commun., 1994). The densely welded 

Topopah Spring Tuff, UZ4, sample has an intermediate 

mineral assemblage as shown by its moderate amount 

of both cristobalite and zeolite. The high percentage of 

zeolites in the Topopah Spring Tuff samples (TST4, 

19 percent; TST5,50 percent) are not representative of 

the entire Topopah Spring Tuff, and are typically found 

only near the base of the unit (Vaniman and others, 

1984).

The percent of zeolite in a sample appears to 

have a large influence on the cation-exchange capacity 

and surface area of the sample. Samples with high percentages of zeolite have the highest cation-exchange 

capacities and surface areas (Topopah Spring Tuff, 

UZ4 and 5; bedded tuff), while samples with little or no 

zeolite have small cation-exchange capacities and surface areas (Tiva Canyon Tuff, Pah Canyon Tuff). This 

is not true of the Yucca Mountain Tuff, however, which 

does not contain any zeolite but does have a moderate 

cation-exchange capacity and surface area.

Adsorption Test Method

The adsorption test method includes the design . 

of the adsorption test apparatus, sample conditioning, 

and the experiment components.

Test Apparatus

The adsorption-test apparatus (fig. 3) consists of 

five integrated parts: (1) cylinders containing the test 

gas mixture; (2) the sample flasks; (3) a vacuum pump; 

(4) a pressure gage; and (5) a glass manifold. The 

apparatus parts were connected to the glass manifold, 

with the test-gas cylinder, sample flasks, and vacuum 

pump capable of being isolated using stopcocks.

The sample flasks (fig. 4) were connected to the 

glass manifold by a glass extension tube. Sample was 

added to the flask by removing the Teflon bushing and 

pouring the crushed material into the flask. The Teflon 

bushing was then reattached to the flask and formed a 

leakproof seal. Gas samples were collected with a 

syringe through a septum connected to the glass sleeve.

Sample Conditioning

Knowlton and others (1981) identified three 

types of water associated with the zeolite clinoptilolite: 

external water, loosely bound zeolitic water, and tightly 

bound zeolitic water, which are removed from clinop8 Adsorption of Sulfur Hexafluoride Onto Crushed Tuffs from the Yucca Mountain Area, Nye County, Nevada
0

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cs ! ro i rs o i o\ i

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a eft *rn C >s 1.3.1 ' & -^ M wa 

S3 e3 *O ta 2o 2>Q4 sl*dgSslsl

«j c

i

s-H

»i

8

s |

p fl> S3 "o

2 1

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1 a

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tti C

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a 2 >. 3 5 y <*- -s ? 

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EXPERIMENT PROCEDURE
I

o

f

<

I

TEST GAS 

CYLINDER PRESSURE 

GAGE

GLASS 

MANIFOLD STOPCOC

SAMPLE FLASKS

DRAWING NOT TO SCALE 

Figure 3. Schematic of adsorption test apparatus.
NYLON CAP

SEPTUM

TEFLON 

BUSHING

GLASS EXTENSION

TUBE TO GLASS

MANIFOLD

O-RING

DRAWING NOT TO SCALE 

Figure 4. Detailed diagram of sample flask.

EXPERIMENT PROCEDURE 11
tilolite at 75°, 171 °, and 271 °C, respectively. Although 

temperatures in excess of 300°C have been used by 

some researchers to dehydrate zeolitic tuffs (Sheppard 

and Gude, 1982; Ming and Mumpton, 1989), long-term 

heating at temperatures as low as 200°C will alter some 

clinoptilolites (Bish, 1990). To minimize alteration of 

the samples, they were dehydrated for 1 week at 150°C. 

The samples were then evacuated overnight at 0.13 Pa. 

Since the crushed samples were reused in successive adsorption tests, the SF6 adsorbed onto the samples from the previous test had to be removed. A series 

of desorption tests showed that baking the samples at a 

temperature of 150°C rapidly removed SF6 from the 

samples. To ensure that SF6 was completely removed, 

the samples were baked in an oven at 150°C for 

2 hours. The samples were then evacuated for 2 hours 

at 0.13 Pa.

Experiment Components

The adsorption experiment has several variable 

components, including: (1) the moisture content of the 

samples, (2) the equilibration time of the tests, (3) the 

particle size of the samples, (4) the amount of SF6 

injected into the flasks, and (5) the amount of sample 

tested. To determine adsorption isotherms, the amount 

of SF6 injected into the flasks must vary while the other 

components remain constant (Serne, 1992).

similar study by Houston and others (1989) determined 

equilibration times of less than 17 hours for several 

halogenated gaseous compounds with soils. Consequently, the adsorption tests were run for a minimum of 

41 hours and a maximum of 379 hours, with most test 

lengths ranging between 41 and 90 hours (table 5).

The adsorption of SF6 onto dry bedded tuff varied slightly (70 to 80 pmol/g) with particle size for particle sizes between 63 and 500 Jim (fig. 5). For samples 

of Pah Canyon Tuff, Yucca Mountain Tuff, and wet 

bedded tuff, particle size had no effect on the adsorption of SF6 within measurement error (fig. 6). Particle 

size also had no measurable effect on the cationexchange capacities or the surface-area measurements 

of the tuffs (tables 2 and 3). These results concur with 

studies undertaken at Los Alamos National Laboratory 

that investigated the sorption of radionuclides onto saturated Yucca Mountain tuffs (Thomas, 1987). Their 

results show that particle sizes of 75 to 500 Jim have 

similar sorptive capacities, but that particle sizes of less 

than 38 Jim could lead to larger sorption ratios. From 

the above information it was evident that the particle 

size of the crushed tuff samples would have no influence on the adsorption experiments; however, for consistency, only samples with a particle size of 150 to 

250 Jim were used in the adsorption tests (except for 

gypsum cement, which had a particle size of 

63-150 ^im).

Fixed components

The in-situ moisture content of the sample cores 

varied from 1 to 25 weight-percent. Because water 

competes with SF6 for available adsorption sites 

(Houston and others, 1989), all of the crushed samples 

were dried before being used in the tests. In tests with 

an initial flask atmosphere concentration of 1.94 parts 

per million by volume (ppmv) SF6, the amount of SF6 

adsorbed onto dry samples of bedded tuff (about 65 to 

80 picomoles per gram) decreased 15 to 35 percent (to 

50-55 pmol/g) when the tuff was wetted with 

10 weight-percent of deionized water (fig. 5).

The time required for SF6 to equilibrate between 

the flask atmosphere and test sample was determined 

by reaching a steady-state concentration of SF6 in the 

flask atmosphere. Initial tests indicated that the time 

required for the flask atmosphere, with a starting SF6 

concentration of 1.94 ppmv, to reach steady state was 

less than 25 hours when in contact with samples of dry 

bedded tuff and the Pah Canyon Tuff. With identical 

starting conditions, the flask atmosphere took less than 

50 hours to reach steady state with samples of wet bedded tuff and the Yucca Mountain Tuff (figs. 5 and 6). A

Variable components

The amount of SF6 injected into the flasks was 

calculated from the ideal gas law:

(2)

where,

nSF6 = number of moles of SF6, mol;

/>SF6= partial pressure of SF6, Pa;

V - volume of gas injected, m3 ;

R - gas constant, 8.314 Pa-m3-K" 1 -mol*1 ; and

T - temperature, constant at 293 degrees Kelvin. 

The partial pressure of SF6 is the product of the 

concentration of SF6 and gas pressure in the experiment flask. The concentrations of SF6 that were 

injected into the flasks were 0.103, 1.00, and 

1 .94 ppmv in a background of nitrogen. These concentrations span the concentration range of SF6 that may 

be present in the unsaturated zone from drilling operations at Yucca Mountain. The volume of gas injected 

into the flasks was calculated from the following equations:

12 Adsorption of Sulfur Hexafluoride Onto Crushed Tuffs from the Yucca Mountain Area, Nye County, Nevada
P 

CD

31dlAIVS dO 9dS dO 31OIAIOOId Nl 'NOIlddOSQV

i

T3

T3 

O

U_ 

CO

 8 E

s a

O 5= '"S. ® 

<o c

in

§>x

& (D

EXPERIMENT PROCEDURE 13
125

a

W

I

I

§ 

I

o

I

o

100

UJ

_j

Q_

<

CO

u_ 

O

LU

Q_ 

CO

u_

CO

u_

o

LU 

O

O 50 

O

0.

75

PCT 63-150 PCT 150-250 PCT 250-500 a PCT 250-500 b

 B ....... o~..... ----o-- ^-~-

YMT 63-150 YMT 150-250 YMT 250-500

40 60 80 100 120 140 160 

TEST DURATION, IN HOURS

180 2

Figure 6. Adsorption of sulfur hexafluoride (SF6) onto Pah Canyon Tuff (PCT) and Yucca Mountain Tuff (YMT), as a function of t

tration of sulfur hexafluoride in the flask of 1.94 ppmv.
Table 5. Adsorption test data

[fim, micrometers; Pa, pascal; pmol/g, picomoles per gram]

Test 

number

Mole fraction SF6

Initial Final

Test length 

(hours)

Total pressure 

(Pa)

Sorption 

(pmol/g)

Tiva Canyon Tuft

150-250 flm, 198 grams

TCT-1

TCT-2

TCT-3

TCT-4

TCT-5

TCT-6

TCT-7

TCT-8

TCT-9

TCT-10

TCT-1 1

1.03 x 10'7

1.03 x 10'7

l.OSxlO'7

1.03 x 10"7

1.00 xlO'6

l.OOxlO'6

1.00 xlO'6

1.00 xlO'6

1.94xlO'6

1.94X1Q-6

1.94xlO'6

1.004 xlO'7

1.005 x 10'7

1.026X10'7

1.027xlO-7

8.92 x 10'7

9.41 x 10'7

9.87 x 1Q-7

9.42 x 10-7

1.82X10"6

1.86X10"6

1.79X10'6

41

42

67

378

66

88

112

43

66

90

90

99,057

99,457

98,257

99,323

98,523

98,523

100,390

98,390

98,124

100,923

97,590

0.313

0.302

0.048

0.036

12.9

7.07

1.59

6.94

14.0

10.7

18.4

Yucca Mountain Tuft

150-250 fim, 119 grams

YMT-1

YMT-2

YMT-3

YMT-4

YMT-5

YMT-6

YMT-7

YMT-8

YMT-9

YMT-10

YMT-1 1

l.OSxlO'7

1.03 x 10'7

1.03 x 10'7

1.03xlO'7

l.OOxlO'6

l.OOxlO'6

l.OOxlO'6

l.OOxlO'6

1.94xlO'6

1.94xlO'6

1.94xlO'6

8.59 xlO'8

9.34 x 10'8

8.97 x 10'8

8.76 x 10'8

8.53 x 10'7

8.30 x 10'7

9.48 x 10'7

8.92 x 10'7

1.68X10"6

1.68xlO'6

1.63 x 10"6

41

42

67

378

66

88

112

43

66

90

90

99,590

99,190

99,323

99,590

99,057

98,523

99,990

98,523

97,587

100,790

96,790

3.69

2.06

2.86

3.33

31.6

36.3

11.3

23.1

54.3

56.6

65.9

Pah Canyon Tuft

150-250 fim, 200 grams

PCT-1

PCT-2

PCT-3

PCT-4

PCT-5

PCT-6

PCT-7

PCT-8

PCT-9

PCT-10

PCT-1 1

1.03 x 10'7

1.03X10'7

1.03X10'7

l.OSxlQ-7

l.OOxlQ-6

1.00 xlO"6

l.OOxlO'6

1.00 xlO'6

1.94xlO-6

1.94xlO'6

1.94xlO'6

9.24 x 10'8

9.68 x 10'8

9.71 x 10'8

9.48 x 10'8

9.64 x 10'7

8.36 xlO'7

8.69 xlQ-7

9.68 x 10-7

1.76X10"6

1.79X1Q-6

1.74X10"6

41

42

67

378

43

66

88

112

66

90

90

97,857

97,990

97,057

99,057

97,324

98,390

97,724

99,723

97,724

100,923

97,324

1.28

0.75

0.71

1.01

4.32

19.9

15.8

3.94

21.5

19.1

24.5

EXPERIMENT PROCEDURE 15
Table 5. Adsorption test data-Continued

Test 

number

BT-1

BT-2

BT-3

BT-4

BT-5

BT-6

BT-7

BT-8

BT-9

BT-10

BT-11

BT-1

BT-2

BT-3

BT-4

BT-5

BT-6

BT-7

BT-8

BT-9

BT-10

BT-11

TST4-1

TST4-2

TST4-3

TST4-4

TST4-5

TST4-6

TST4-7

TST4-8

TST4-9

TST4-10

TST4-11

TST4-1

TST4-2

TST4-3

TST4-4

Mole fraction SF6 Test length

Initial

l.OSxlO'7

l.OSxlO'7

1.03xl(r7

l.OSxlO-7

l.OOxlO-6

l.OOxlO-6

l.OOxlO-6

l.OOxlO-6

1.94x 10*

1.94 xlO"6

1.94X10"6

1.03X10'7

1.03xlO'7

1.03X10'7

1.03xlO'7

l.OOxlO-6

l.OOxlO-6

l.OOxlO-6

l.OOxlO-6

1.94 xlO"6

1.94 xlO"6

1.94X10"6

1.03X10'7

1.03X10'7

1.03xlO-7

1.03X10'7

l.OOxlO-6

LOOxlO"6

l.OOxlO-6

l.OOxlO-6

1.94X10"6

1.94X10"6

1.94X10"6

1.03xlO'7

1.03xlO-7

1.03X10'7

1.03X10'7

Final (houre>

Bedded Tuff

150-250 iim, 100 grams

7.09 x 10'8 41

7.37 x 10'8 42

6.87 x 10'8 67

6.36 x 10'8 379

6.81 x 10'7 42

6.84 xlO'7 66

7.13X10'7 88

7.09 xlO'7 112

1.38X10-6 65

1.32X10-6 90

1.41 x 10-6 91

Bedded Tuff

150-250 \im, 200 grams

5.72 x 10'8 41

4.79 x 10'8 42

5.07 x 10'8 67

4.54 x 10'8 379

5.27 x 10'7 42

5.26 x 10'7 66

5.51 x 10'7 88

5.44 xlO'7 112

1.13 xlO"6 65

1.07X1Q-6 90

l.OSxlO-6 91

Topopah Spring Tuft, UZ4

150-250 |im, 100 grams

8.66 xlO'8 41

8.63 x 10'8 42

8.27 x 10'8 67

8.05 x 10'8 378

8.17X10'7 43

7.58 x 10'7 66

7.96 x 10'7 88

8.07 xlO'7 112

1.59X10-6 66

1.64 xlO"6 90

1.64 xlO"6 90

Topopah Spring Tuff, UZ4

150-250 tun, 201 grams

7.69 x 10'8 41

7.35 x 10'8 42

6.76 x 10'8 67

6.39 x 10'8 379

Total pressure 

(Pa)

89,191

89,724

92,524

90,924

88,391

90,924

91,191

90,924

89,324

88,258

94,791

78,792

79,725

86,658

82,392

79,059

80,792

80,659

80,792

79,592

78,392

83,458

91,458

91,591

92,791

92,657

88,791

93,324

92,657

92,924

94,657

95,590

90,391

81,192

81,725

85,191

83,858

Sorptlon 

(pmol/g)

7.71

7.08

8.54

9.65

75.9

77.4

70.5

71.2

134

148

136

4.54

5.52

5.70

5.96

47.0

48.1

45.5

46.3

81.3

86.2

89.8

3.98

4.06

5.00

5.53

43.1

59.9

50.1

47.6

87.9

76.6

73.1

2.52

2.87

3.59

3.90

16 Adsorption of Sulfur Hexafluoride Onto Crushed Tuffs from the Yucca Mountain Area, Nye County, Nevada
Table 5. Adsorption test data-Continued

Test 

number

TST4-5

TST4-6

TST4-7

TST4-8

TST4-9

TST4-10

TST4-11

TST5-1

TST5-2

TST5-3

TST5-4

TST5-5

TST5-6

TST5-7

TST5-8

TST5-9

TST5-10

GC-1

GC-2

GC-3

GC-4

GC-5

GC-6

GC-7

GC-8

CLINO-1

CLINO-2

CLINO-3

CLINO-4

CLINO-5

CLINO-6

CLINO-7

CLINO-8

CLINO-9

CLINO-10

CLINO-1 1

Mole f faction SF6 Test length

Initial

l.OOxlO'6

1.00 xlO'6

1.00 xlO'6

1.00 xlO'6

1.94xlO'6

1.94X10"6

1.94X10-6

1.03 x 10'7

1.03 x 10'7

1.03 x 10'7

1.03X10'7

l.OOxlO-6

l.OOxlO'6

l.OOxlO'6

1.94 x ID"6

1.94 x lO"6

1.94 xlO"6

1.03X10'7

1.03X10'7

1.03xlO'7

1.00 xlO'6

1.00 xlO'6

1.00 xlO'6

1.94xlO'6

1.94X10'6

1.03X10'7

1.03 x 10'7

1.03X10'7

1.03X10'7

1.00 xlO'6

1.00 xlO'6

1.00 xlO'6

l.OOxlO'6

1.94 xlO'6

1.94 x 10'6

1.94 x ID"6

Final lhours)

Topopah Spring Tuff, UZ4

150-250 ^m, 201 grams-Continued

7.10X10'7 42

6.65 x 10'7 66

7.04 x 10'7 88

6.72 xlO'7 112

1.32X10"6 66

1.38X10-6 65

1.39 xlO"6 91

Topopah Spring "Riff, UZ5

150-250 iim, 199 grams

6.43 x 10'8 41

6.15xlO'8 42

6.50 x 10'8 67

5.64 x 10'8 379

6.43 x 10'7 42

6.45 x 10'7 88

6.30 xlO'7 112

1.31 xlO"6 65

1.24 xlO"6 90

1.34 xlO"6 91

Gypsum cement

63-150 lira, 188 grams

l.OOSxlO'7 41

l.OOSxlO'7 67

l.OOSxlQ-7 378

9.26 x 10'7 43

9.04 xlO'7 66

9.05 x 10'7 88

1.79 xlO"6 66

1.77 xlO"6 90

Clinoptilolite

150-250 ^m, 164 grams

6.40 x 10'8 41

5.95 x 10'8 42

6.33 x 10'8 67

5.04 xlO'8 379

6.14X10'7 42

6.21 x 10'7 66

6.77 x 10'7 88

6.05 xlO'7 112

1.28 xlO"6 65

1.45X10"6 90

LlSxlO"6 90

Total pressure 

(Pa)

81,459

83,058

83,192

83,592

81,459

81,725

87,191

76,926

77,192

82,392

78,925

77,326

79,592

79,325

77,726

75,459

83,592

97,724

100,123

99,457

98,523

90,391

88,791

98,257

97,724

77,326

77,726

90,390

90,258

76,392

82,525

81,992

83,592

80,659

84,658

77,992

Sorption 

(pmol/g)

28.1

33.1

29.3

32.6

59.9

54.0

57.5

3.52

3.79

3.70

4.35

32.6

33.4

34.7

58.3

62.5

59.0

0.307

0.314

0.312

9.15

10.9

10.6

18.2

20.5

4.74

5.31

5.64

7.46

46.3

49.1

41.6

51.9

83.8

64.9

92.6

EXPERIMENT PROCEDURE 17
V=Vf-V, (3)

(4)

where,

V = volume of gas injected, m3 ;

Vf = flask volume, m3 ;

Vs = sample volume, m3;

M = mass of sample, g;

pg = grain density of the sample, g/m3; and

0 = porosity of sample, dimensionless.

Due to the very small amount of SF6 that was adsorbed 

during a test, the volume of gas in the flask was 

assumed to be constant throughout the test.

In order to test whether the adsorption isotherms 

were independent of the amount of sorbent in the flask, 

experiments were conducted with 100-g and 200-g 

samples of bedded tuff and Topopah Spring Tuff, UZ4. 

The nearly identical adsorption isotherms for both the 

100-g and 200-g samples (fig. 7) demonstrated that the 

adsorption of SF6 is independent of the amount of sorbent in the flask.

The amount of SF6 adsorbed from the flask atmosphere per gram of sample was:

A =

(R) (T) (M) (5)

where,

A = amount of SF6 adsorbed, mol/g;

P = final gas pressure in flask, Pa;

V = volume of gas injected, m3 ;

R = gas constant, 8.314 Pa-m^K

T = temperature, constant at 293 K;

XSF6 initial mole fraction of SF6, dimensionless;

XSF6y= final mole fraction of SF6, dimensionless; 

and

M = grams of sample used in the experiment, g.

Of these values R and T are constants; XSF6| was a 

known quantity; Vwas calculated (equation 3); andP, 

and M were measured.

Analytical Instrument Calibration and Limits Adsorption Modeling

The gas samples collected from the flasks were 

analyzed using a Packard 439 gas chromatograph having a 6-foot by 1/8-inch column of 80/100 Porapak Q 

at 35°C, a nitrogen carrier gas flow of 30 cmVmin, and 

an electron capture detector at 100°C. The electron 

capture detector is capable of measuring SF6 at concentrations in the parts-per-trillion range (demons and 

Altschuller, 1966).

Calibration of the gas chromatograph was performed by analyzing National Institute of Standards 

and Technology certified standards before, during, and 

after a suite of samples was analyzed. The standards 

were chosen to span the range of concentration of the 

samples, and the analyses of each set of standards provided a calibration curve for the sample analyses. The

range of squared correlation coefficients (r2) of the calibration curves was 0.96 to 1.

RESULTS OF SULFUR HEXAFLUORIDE 

ADSORPTION EXPERIMENTS

The data collected during the adsorption experiments are shown in table 5, which lists the initial and 

final mole fraction of SF6 in the flask, the length of each 

test, the total gas pressure in the flask, and the amount 

of SF6 adsorbed onto the sample.

The data from the adsorption experiment were 

modeled using the Freundlich isotherm equation and an 

empirical retention equation. The Freundlich isotherm 

equation was used to calculate distribution coefficients 

for each sample, while the retention equation describes 

the moles of SF6 adsorbed per square meter of surface 

area as a function of the partial pressure of SF6. 

Because the tests used crushed and dried tuffs, they 

provide data for modeling the maximum amount of SF6 

that will be adsorbed onto the tuffs. In-situ adsorption 

will vary from the results of this experiment as a function of the core matrix and fracture permeability and 

moisture content of the rock or grout. In addition, these 

models are valid only for the range of SF6 concentrations actually tested.

Freundlich Isotherm Equation

The Freundlich isotherm equation (Freundlich, 

1926) can be used for modeling adsorption of trace 

concentrations of sorbate onto substrates (Freeze and 

Cherry, 1979; Serne, 1992). For a linear isotherm, the 

Freundlich isotherm equation has the form:

(6)

18 Adsorption of Sulfur Hexafluoride Onto Crushed Tuffs from the Yucca Mountain Area, Nye County, Nevada
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RESULTS OF SULFUR HEXAFLUORIDE ADSORPTION EXPERIMENTS 19
where,

X = amount of sorbate adsorbed per mass of 

solid (pg/g; note the conversion of units 

from table 5, where sorption is in units of 

(pmol/g);

C = sorbate concentration in solution (pg/cm3); 

Kd = distribution coefficient (cm3/g); and 

N =1 (dimensionless; N has a value other than 1

if the isotherm is not linear). 

By taking the logarithms of both sides of equation (6), the Freundlich isotherm equation can be transformed to a linear equation:

log X = log Kd + N log C, (7)

where N is the slope of the resulting line and log Kd is 

the y-intercept.

Values for N were determined from linear regressions that were fit through the log-transformed data 

points for each sample (figs. 7-9). The slope (N) varied 

between 0.87-1.04 (table 6) for all the samples except 

Tiva Canyon Tuff (1.69) and gypsum cement (1.50). 

Since N is close to 1.0 for all but two of the samples, 

the linear form of the Freundlich isotherm equation 

was assumed to accurately represent the adsorption 

data. The distribution coefficients (table 6) determined 

for the samples decrease in the order of clinoptilolite 

> BT > TST5 > TST4 > YMT > PCT > TCT and GC. 

These distribution coefficient values show that SF6 is 

abundantly adsorbed onto clinoptilolite, bedded tuff, 

and Topopah Spring Tuff (UZ5); moderately adsorbed 

onto Topopah Spring Tuff (UZ4) and Yucca Mountain 

Tuff; but that it does not appreciably adsorb onto Pah 

Canyon Tuff, Tiva Canyon Tuff, or gypsum cement.

The data from tests started with 0.103 ppmv SF6 

indicate that some of these tests may not have reached 

equilibrium. The data were corrected by using only the 

results that were collected from tests with a duration of 

379 hours. This correction increased the distribution 

coefficient values of bedded tuff (4.8 » 8.0), Topopah 

Spring Tuff, UZ4 (1.5 -» 3.4), Topopah Spring Tuff, 

UZ5 (3.3 -» 6.1), and clinoptilolite (6.2 -> 18).

Retardation factors (Freeze and Cherry, 1979; 

Fetter, 1994) were calculated for the dry, crushed tuffs 

from:

where,

Rf = retardation factor, dimensionless;

pb = dry bulk density of the crushed tuff, g/cm3 ;

0 = porosity of the crushed tuff, dimensionless; 

and

Kd = distribution coefficient, cm3/g.

The average linear fluid velocity relative to the average velocity of a chemical species can be determined 

from the retardation equation:

Rf=VJVc (9)

where,

Vx = the average linear velocity of the fluid, 

m/day; and

Vc = the average velocity of the chemical species, 

m/day.

The relative velocity of the chemical species is determined by taking the reciprocal of the retardation factor:

(10)

From the calculated relative velocities of SF6 for each 

tuff the percent retardation of SF6 could be determined. For instance, a tuff with a relative velocity of 

0.33 would indicate that SF6 is retarded by 67 percent 

in that tuff. The retardation factors and relative velocities (table 7) calculated from the adsorption tests indicate that: (1) the Tiva Canyon Tuff and gypsum 

cement do not retard the transport of SF6; (2) the transport of SF6 through the Pah Canyon Tuff is retarded by 

15-20 percent; and (3) the Yucca Mountain Tuff, bedded tuff, clinoptilolite, and Topopah Spring Tuff, UZ4 

and 5, retard the transport of SF6 by 60-90 percent.

In order to approximate in-situ conditions more 

closely, the retardation equations were recalculated 

using the distribution coefficients and bulk densities of 

the crushed tuffs, and the grain densities of the original 

core samples (Loskot and Hammermeister, 1992) for 

porosity calculations. The retardation factors and relative velocities (table 8) calculated with the core samples show that: (1) the Tiva Canyon Tuff still does not 

retard the transport of SF6; (2) the transport of SF6 

through the Pah Canyon Tuff is retarded by 33 percent; 

and (3) the Yucca Mountain Tuff, bedded tuff, clinoptilolite, and Topopah Spring Tuff, UZ4 and 5, retard the 

transport of SF6 by 75-96 percent. The increased calculated sorptivity results directly from the decreased 

porosity (and decreased flow path) in the cores. If fracture flow is the dominant flow path in welded tuffs, 

then the retardation of sorbates may be diminished by 

the larger flow path available in the fractures. For 

partly saturated conditions, with an insoluble species 

like SF6, retardation would decrease even more 

because water will compete with the sorbate for available adsorption sites.

20 Adsorption off Sulfur Hexaffluoride Onto Crushed Tuffs from the Yucca Mountain Area, Nye County, Nevada
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0 0.5 1 1.5 2 2.5 3 3.5 4 

CONCENTRATION, IN LOG (PICOGRAM OF SF6 PER MILLILITER OF AIR)

Figure 8. Adsorption isotherms for Yucca Mountain Tuff (YMT), Topopah Spring Tuff, UE-25 UZ #5 (TST5), and clinoptilolite.
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22 Adsorption of Sulfur Hexafluoride Onto Crushed Tuffs from the Yucca Mountain Area, Nye County, Nevada
Table 6. Values for constants (N, K& A), squared correlation coefficients (r2), and standard 

error of estimate (SEE) for the Freundlich isotherm equations and the retention equations

 5

[Kj is in units of cubic centimeters per gram, cm /g; AT is in units of picomoles per meter squared per millipascal,

pmol/m2 mPa; N is the slope of the line determined by the adsorption isotherm; the correlation coefficients and the 

standard error of estimate are the same for both equations; n is the number of experiments; --, no data]

Sample

TCT1 150-250

YMT 150-250

PCT 150-250

BT 150-250

200 grams

BT 150-250

100 grams

Combined BT

TST4 150-250

201 grams

TST4 150-250

100 grams

Combined TST4

TST5 150-250

GC 63-1 50

CLINO 150-250

Tuffs2

n

11

11

11

11

11

22

11

11

22

10

8

11

76

N

1.69

0.98

1.04

0.90

0.95

0.93

0.98

0.99

0.99

0.91

1.50

0.87

0.94

*d

0.0004

0.90

0.19

5.5

4.4

4.8

1.5

1.6

1.5

3.3

0.003

6.2

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K

0.003

0.037

0.017

0.064

0.061

0.062

0.068

0.075

0.070

0.038

0.002

0.120

0.050

r2

0.87

0.92

0.86

0.99

0.99

0.99

0.98

0.98

0.98

0.99

0.99

0.97

0.86

SEE

0.39

0.18

0.25

0.05

0.06

0.06

0.07

0.08

0.08

0.05

0.10

0.10

0.23

Refer to table 2 for explanation of sample labels. 

2Does not include TCT.

RESULTS OF SULFUR HEXAFLUORIDE ADSORPTION EXPERIMENTS 23
Table 7. Values for variables in the retardation equation Retention Equation

[p/,, bulk density in grams per cubic centimeter, g/cm ; 6, porosity, 

dimensionless; Kj, distribution coefficient in cubic centimeters per gram,

cm3/g; Rp retardation factor, dimension-less; Vc, average velocity of the 

chemical species in meters per day, m/day; V^ average linear velocity of 

the fluid in meters per day, m/day]

S.H.P*

TCT1 150-250

YMT 150-250

PCT 150-250

Combined BT

Combined TST4

TST5 150-250

A

1.72

1.55

1.13

0.99

1.30

1.30

** 

(cm'/g)

0.0004

0.90

0.19

4.8

1.5

3.3

*f

1.0007

2.4

1.2

5.8

3.0

5.3

 w

1.0

0.42

0.83

0.17

0.33

0.19

GC 63-150 1.30 0.003 1.004 1.0

CLINO150-250 1.40 6.2 9.7 0.10

Refer to table 2 for explanation of sample labels.

Table 8. Values of variables1 in the retardation equation 

using bulk and grain densities2 (for porosity calculations) of 

core samples

Core vcr^^ "g » 

sample (g/cm3) (cmj/g) f

TCT3

YMT

PCT

BT

TST4

TST5

18

3.3

2.4

2.3

18

2.8

0.0004

0.90

0.19

4.8

1.5

3.3

1.007

4.0

1.5

12

28

10

0.99

0.25

0.67

0.08

0.04

0.10

Refer to table 7 for an explanation of variables. 

2 Loskot and Hammermeister, 1992.

Refer to table 2 for explanation of sample labels.

The retention equation has the same form as the 

Freundlich isotherm equation (equation 6):

X = KC\ (11) 

but,

C 

K

N

= picomoles of sorbate adsorbed per meter

squared of sorbent (pmol/m2); 

= partial pressure of sorbate (mPa); 

= constant (pmol/m2 -mPa); and 

= constant (dimensionless).

Because the tuffs have similar values for the constants 

N and K, a retention equation was determined for the 

combined data from all of the tuffs (except the Tiva 

Canyon Tuff) (fig. 10, table 6). The combined equation is:

X=0.047C° 95 (12)

After correcting for the incomplete adsorption of some 

of the tests (see above), the equation changes to:

X=0.067C° 87 (13)

These two equations are plotted in figure 11 over the 

concentration range of SF6 used in the adsorption 

tests. At concentrations of 40 millipascals (mPa) of 

SF6 the values of X between the two equations vary by 

6 percent. The percentage difference between the two 

equations approaches zero at either end of the concentration range tested.

The retention equations can be used to predict 

the amount of SF6 that the tuffs will adsorb if the equilibrium concentration of SF6 is known or specified. 

The calculated value of X, determined from either the 

combined or individual retention equations, must be 

multiplied by the measured surface area of the appropriate tuff to give the predicted adsorption capacity of 

that tuff. The predicted adsorption capacities of the 

tuffs, calculated for both the combined and individual 

retention equations, and assuming that the tuffs were in 

equilibrium with air containing 100 mPa of SF6, are 

shown in table 9. The combined equation closely predicts the adsorption capacity for the Tiva Canyon Tuff, 

Yucca Mountain Tuff, and bedded tuff, but predicts too 

much adsorption for the Topopah Spring Tuff, UZ5, 

and Pah Canyon Tuff and too little adsorption for the 

Topopah Spring Tuff, UZ4 (fig. 10, table 9).

The adsorption of SF6 onto tuff is proportional to 

the surface area of the tuff (figure 10 and 11) and 

appears to be a function of the percent of zeolite in the 

tuff (fig. 12). Zeolites are secondary minerals in tuffs, 

so the degree of alteration that a tuff has undergone will 

affect the ability of that tuff to adsorb chemical species.

24 Adsorption of Sulfur Hexafluoride Onto Crushed Tuffs from the Yucca Mountain Area, Nye County, Nevada
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PRESSURE, IN LOG (MILLIPASCALS OF SF6)

2.4 

M Figure 10. Moles of sulfur hexafluoride adsorbed, per square meter of tuff, as a function of the partial pressure of sulfur hexaflu

01 sion through the data represents the combined retention equation for the tuffs (excluding the Tiva Canyon Tuff, TCT).
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26 Adsorption of Sulfur Hexafluorlde Onto Crushed Tuffs from the Yucca Mountain Area, Nye County, Nevada
SnflS3d dO UfldinS 3QldOmdVX3H NOIlddOSQV SlN3WIU3dX3 LZ 

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SAMPLE OF GRAM PER AIR OF MILLILITER IN (Kd), CONSTANT EQUILIBRIUM 

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TUFF VOLCANIC OF GRAM PER METER SQUARE IN AREA, SURFACE 
Table 9. Predicted adsorption of sulfur hexafluoride 

(pmolSF6/9tuff) onto tuffs usin9 the combined and individual 

retention equations

[The amount of adsorption was calculated for tuff in equilibrium with air 

containing 100 mpascals of SF$; pmol/g, picomoles per gram]

Adsorption Adsorption

Sample combined equation Individual equation

(pmol/g) (pmol/g)

TCT1 150-250

YMT 150-250

PCT 150-250

Combined BT

Combined TST4

3.1

35

20

90

32

5.8

31

11

110

56

TST5 150-250 92 61

Refer to table 2 for explanation of sample labels.

This is demonstrated by the Topopah Spring Tuff samples, where the welded sample from UZ4 has undergone less alteration than the nonwelded sample from 

UZ5, and consequently, contains less zeolite and has 

less sorptive capacity.

CONCLUSIONS

The adsorption experiment demonstrates that 

SF6 does not always behave conservatively. Although 

SF6 approximates conservative behavior (Kd » 0) when 

in contact with the Tiva Canyon Tuff and gypsum 

cement, it is slightly adsorbed by the Pah Canyon Tuff 

and strongly adsorbed (Kd > 0.9) by the Yucca Mountain Tuff, bedded tuff, and the Topopah Spring Tuff, 

UZ4 and UZ5. This adsorption will cause retardation 

of transport in these unsaturated zone materials, resulting in slower observed velocities than would be predicted from modeling of conservative tracer behavior.

The combined retention equation provides a 

direct means of predicting the amount of SF6 that may 

be adsorbed by tuffs with a known surface area, and a 

general application of the combined retention equation 

may be valid for estimating the sorptive capacity of any 

unsaturated zone silicates for SF6. The surface area of 

a material can be used to estimate the equilibrium 

adsorption constant (fig. 12) of the material, allowing a 

prediction of the relative transport velocity of SF6

through the material by using the retardation equation 

(equation 8).

REFERENCES

Bish, D.L., 1990, Long-term stability of clinoptilolite The 

development of a "B" phase: European Journal of 

Mineralogy, v. 2, no. 6, p. Ill-Ill.

Carroll-Webb, S.A., and Walther, J.V., 1988, A surface complex reaction model for the pH-dependence of corundum and kaolinite dissolution rates: Geochimica et 

Cosmochimica Acta, v. 52, p. 2609-2623.

demons, C.A., and Altschuller, A.P., 1966, Responses of 

electron-capture detector to halogenated substances: 

Analytical Chemistry, v. 38, no. 1, p. 133-136.

Dayal, Ramesh, and Klein, R., 1988, CO2/Grout interactions 

and their relevance to 14C attenuation in cementitious 

backfill materials: Radiochimica Acta, v. 44/45, pt. 2, 

p. 263-270.

Fetter, C.W., 1994, Applied hydrogeology (3d ed.): New 

York, Macmillan College Publishing Company, 691 p.

Flint, L.E., and Flint, A.L., 1990, Preliminary permeability 

and water-retention data for nonwelded and bedded tuff 

samples, Yucca Mountain area, Nye County, Nevada: 

U.S. Geological Survey Open-File Report 90-569,57 p.

Flint, A.L., Flint, L.E., and Hevesi, J.A., 1993, The influence 

of long-term climate change on net infiltration at Yucca 

Mountain, Nevada, in Proceedings of the Fourth International Conference on High Level Radioactive Waste 

Management, Las Vegas, Nevada, 1993: La Grange 

Park, Illinois, American Nuclear Society, Inc., and 

New York, American Society of Civil Engineers, 

p. 732-737.

Freeze, R.A., and Cherry, J.A., 1979, Groundwater: Englewood Cliffs, N.J., Prentice-Hall, 604 p.

Freundlich, Herbert, 1926, Colloid and capillary chemistry: 

New York, Dutton, 883 p.

Houston, S.L., and others, 1989, A batch-type testing 

method for determination of adsorption of gaseous 

compounds on partially saturated soils: Geotechnical 

Testing Journal, v. 12, no. 1, p. 3-10.

Klug, H.P., and Alexander, L.E., 1974, X-ray diffraction procedures for polycrystalline and amorphous materials 

(2d ed.): New York, John Wiley and Sons, 966 p.

Knowlton, G.D., White, T.R., and Mckague, H.L., 1981, 

Thermal study of types of water associated with clinoptilolite: Clays and Clay Minerals, v. 29, p. 403-411.

Kreamer, D.K., Weeks, E.P., and Thompson, G.M., 1988, A 

field technique to measure the tortuosity and sorptionaffected porosity for gaseous diffusion of materials in 

the unsaturated zone with experimental results from 

near Barnwell, South Carolina: Water Resources 

Research, v. 24, no. 3, p. 331-341.

28 Adsorption of Sulfur Hexafluoride Onto Crushed Tuffs from the Yucca Mountain Area, Nye County, Nevada
Lewis-Russ, Anne, 1990, Borate adsorption by some tuffs: 

Evaluation of borate as a potential tracer for unsaturated 

flow at Yucca Mountain: Golden, Colorado School of 

Mines, Ph.D. dissertation, 365 p.

Loskot, C.L., and Hammermeister, D.P., 1992, Geohydrologic data from test holes UE-25 UZ #4 and 

UE-25 UZ #5, Yucca Mountain Area, Nye County, 

Nevada: U.S. Geological Survey Open-File 

Report 90-369, 56 p.

Ming, W.M., and Mumpton, F.A., 1989, Zeolites in soils, in 

Dixon, J.B., and Weed, S.B., eds., Minerals in soil environments (2d ed.): Madison, Wisconsin, Soil Science 

Society of America, p. 873-911.

Sawyer, D.A., and others, 1994, Episodic caldera volcanism 

in the Miocene southwestern Nevada volcanic field: 

Revised stratigraphic framework, 40Ar/39Ar geochronology, and implications for magnetism and extension: 

Geological Society of America Bulletin, v. 106, 

p. 1304-1318.

Scott, R.B., and Bonk, J., 1984, Preliminary geologic map of 

Yucca Mountain with geologic sections, Nye County, 

Nevada: U.S. Geological Survey Open-File Report 

84-494,9 p., scale 1:12,000.

Seine, R.J., 1992, Current adsorption models and open

issues pertaining to performance assessment: Los Alamos National Laboratory Report LA-12325-C, 

p. 43-74.

Sheppard, R.A., and Gude, A.J., 1982, Mineralogy, chemistry, gas adsorption, and NH4+-exchange capacity for 

selected zeolitic tuffs from the western United States: 

U.S. Geological Survey Open-File Report 82-969,16 p.

Thomas, K.W., 1987, Summary of sorption measurements 

performed with Yucca Mountain, Nevada, tuff samples 

and water from well J-13: Los Alamos National Laboratory Report LA-10960-MS, 32 p.

Vaniman, D., and others, 1984, Variations in authigenic mineralogy and sorptive zeolite abundance at Yucca Mountain, Nevada, based on studies of drill cores 

USW GU-3 and G-3: Los Alamos National Laboratory 

Report LA-9707-MS, 71 p.

Yang, I.C., 1992, Flow and transport through unsaturated 

rock Data from two test holes, Yucca Mountain, 

Nevada, in Proceedings of the Third International 

Conference on High Level Radioactive Waste Management, Las Vegas, Nevada, 1992: La Grange Park, 

Illinois, American Nuclear Society, Inc., and 

New York, American Society of Civil Engineers, 

p. 732-737.

Yang, I.C., Turner, A.K., Sayre, T.M., and Montazer, Parviz, 

1988, Triaxial-compression of pore water from unsaturated tuff, Yucca Mountain, Nevada: U.S. Geological 

Survey Water-Resources Investigations 

Report 88-4189, 68 p.

REFERENCES 29

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