UCRL-AR-135949 Vol. 4 Ch. 6

Volume 4: Potential Ground and

Surface Water Impacts

Chapter 6: Environmental Transport

and Fate of Fuel Hydrocarbon Alkylates

Authors

Alfredo A. Marchetti*

Jeffrey I. Daniels*

David W. Layton*

Editor

G. Cannon*

December 1999

*Lawrence Livermore National Laboratory, Livermore, CA 94550

 University of California • Livermore, CA 94550

Lawrence Livermore National Laboratory

 Environmental Restoration Division
Work performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National

Laboratory under Contract W-7405-Eng-48.
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Table of Contents

6. Environmental Transport and Fate of Fuel Hydrocarbon Alkylates ..................................... 6-1

6.1. Introduction .................................................................................................................. 6-1

6.2. Background................................................................................................................... 6-1

6.3. Production of Alkylates ............................................................................................... 6-2

6.4. Chemical Composition and Properties of Alkylates.................................................. 6-3

6.5. Environmental Behavior and Fate of Alkylates ......................................................... 6-3

6.5.1. Air................................................................................................................... 6-4

6.5.2. Water .............................................................................................................. 6-6

6.5.3. Soil and Groundwater.................................................................................... 6-6

6.5.4. Biodegradation............................................................................................... 6-8

6.6. Summary ................................................................................................................................... 6-9

6.7. References............................................................................................................................... 6-10

List of Figures

Figure 6-1. Model of equilibrium distribution for isooctane between the phases of a

sandy soil (air, water, solid) ...................................................................................... 6-15

Figure 6-2. Model of equilibrium distribution for isooctane between the phases of a

clayey soil (air, water, solid) ..................................................................................... 6-16

List of Tables

Table 6-1. Representative components of a typical alkylate..................................................... 6-19

Table 6-2. Physical-chemical properties of representative alkylate compounds ..................... 6-20
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6. Environmental Transport and Fate

of Alkylates

6.1. Introduction

Alkylates—high-octane solutions of branched alkanes—are already components in gasoline,

but gasoline distributed in California after a phase-out of methyl tertiary butyl ether (MTBE) will

likely include an increased amount of alkylates. MTBE is a high-octane, anti-knock additive;

and to maintain octane levels in gasoline after MBTE is removed, additional amounts of

alkylates will be required. For example, although ethanol also has a high-octane level, its

oxygen content is about twice that of MTBE; consequently, less ethanol is required to meet a

specified oxygen content (e.g., 2.5 wt% oxygen). To compensate for the resulting octane deficit

that would be produced, petroleum companies could add more alkylates as the high-octane

blending components. If an oxygen requirement is eliminated altogether, even greater amounts

of high-octane components, such as alkylates, must be added to gasoline.

Assuming that California gasoline contains on average approximately 10 wt% MTBE and

approximately 14 wt% alkylate, and that the oxygen requirement is to remain fixed at about

2 wt%, ethanol would have to be added at the 5 wt% level. The octane rating of ethanol is close

to that of MTBE (approximately 110), and the octane rating of an alkylate solution is about 92.

To achieve an overall gasoline octane rating of 88 in the ethanol fuel, a linear calculation

indicates the need to increase the percentage of alkylate component from approximately 14 to

approximately 34 wt%. This is a rough estimation because there are other requirements that

gasoline formulations must meet besides octane rating, and many of the specification parameters

(including octane rating and vapor pressure) are not linear functions of the mass. However, it

illustrates that the percentage of alkylate in gasoline could increase significantly (approximately

20%) after a phaseout of MTBE.

The goal of this chapter of the report is to characterize the environmental transport and fate

of representative alkylate components, based on available information about their

physicochemical properties as well as on relevant studies conducted on their environmental

chemistry in soil, air, and water. Unlike ethanol or MTBE, the physicochemical properties of

alkylate components resemble those of other hydrocarbons present in gasoline, that is, very low

solubility in water, high octanol-water partition coefficient, high Henry’s law constant, etc.

Therefore, an increase in the alkylate percentage of gasoline may not effect significantly the way

gasoline behaves in environmental releases.

6.2. Background

Alkylation is an industrial synthetic process used to produce a high-octane solution of

branched alkanes (called isoalkanes). This solution is called alkylate and is used as a blending

component for gasoline. During World War II, alkylate production was increased to meet the

demand for high-octane gasoline used by fighter planes. After the war, the demand for highoctane components for aviation fuels diminished, and the use of alkylate shifted towards the
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automotive market (Owen, 1995; Kinnear, 1998). The production capacity remained more or less

constant during the 1950s and 1960s because of the competitive cost of other high-octane

blending components for automotive fuel, such as tetraethyl lead. In 1985, the United States

Environmental Protection Agency (US EPA) imposed a drastic reduction of lead in gasoline

formulations lowering it, from 1.1 to 0.5 g/gal, which increased the demand for alkylates.

Today, approximately 14% of the gasoline in the United States consists of alkylates (Miller,

1999). However, as noted above, this percentage may be increased significantly in the future to

compensate for the loss in octane number, resulting from either the removal of MTBE and use of

ethanol (EtOH) or no oxygenates in fuel.

Compared to products of other refining processes, alkylates have relatively low

Reid vapor pressure1

 as well as relatively low concentrations of aromatics, olefins, and sulfur.

From an environmental standpoint, these are some of the more desirable properties for a

blending component for gasoline. Therefore, as environmental regulations move toward cleaner

burning fuels, it is likely that the use of alkylates as blending stock for gasoline could rise.

However, alkylates have not been examined critically and systematically in terms of their

potential environmental, health, and safety impacts. This examination becomes particularly

important before they are used in greater quantities. Here we briefly examine some of the

relevant transport and fate issues related to alkylates.

6.3. Production of Alkylates

In the alkylation process, isobutane is combined with light olefins (double-bonded

compounds of 3 to 5 carbon atoms: C3 to C5) in the presence of a strong acid catalyst. The only

catalysts presently used for industrial production of alkylates are sulfuric and hydrofluoric acids

(H2SO4 and HF, respectively), with sulfuric acid accounting for about 46% of the world’s

production capacity (Peterson, 1999). Currently, research on alternative, solid catalysts is taking

place. In addition, there is some effort to convert alkylation units from hydrofluoric- to sulfuricacid catalysis because of the more serious health hazards associated with using hydrofluoric acid.

Studies on alkylation catalyzed by sulfuric acid have shown that there are at least four basic

reaction mechanisms. The first mechanism results in the direct alkylation of isobutane with C3

to C5 olefins to produce C7 to C9 isoalkanes, including isooctane (2, 2, 4-trimethylpentane). The

following equation shows a typical alkylation reaction by this process (that is, isobutene and

isobutane are combined in the presence of HF or H2SO4 at a temperature greater than 10°C):

 

1 Vapor pressure of a liquid measured by the American Society for Testing and Materials (ASTM) D323

procedure and generally applied to gasoline and gasoline components (Owen and Coley, 1995).
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 CH3 CH3 H2SO4 CH3 CH3

 | | (also applicable to HF) | |

CH3—C=CH2 + CH3—CH—CH3 CH3—C—CH2—CH—CH3(6-1)

 <10°C |

 CH3

Isobutene Isobutane Isooctane

(2-Methyl propene) (2-Methyl propane) (2,2,4-Trimethylpentane)

The second mechanism consists of polymerization-fragmentation reactions that produce less

branched paraffins, typically C5 to C10. The third mechanism is a polymerization reaction that

results in C10 and heavier isoalkanes. Finally, the fourth mechanism is a hydride transfer that

accounts for the production of isopentane and trimethylpentane (Kinnear, 1998; Peterson, 1999).

Kinnear (1998) indicates that a typical alkylate product can be composed of 100 to

200 paraffins ranging from C5 to C16. Calculations based on the analysis of an alkylate indicate

that more than 86 moles per 100 moles of alkylate correspond to the C8 family of isoalkanes (see

Table II in Albright et al., 1993). In addition, the average number of carbon atoms in an alkylate

molecule is shown to be between 7 and 8 under various alkylation conditions (Albright et al.,

1993). It is important to keep in mind that any current gasoline blend already does contain

alkylate hydrocarbons. Therefore, the concentration of any given hydrocarbon in gasoline will

be the result of its weighted contribution to the entire blended mixture. Only the alkylate

component is discussed in this chapter.

6.4. Chemical Composition and Properties of Alkylates

In 1963, Durrett et al. (1963) presented a detailed component analysis of alkylates, including

five commercial ones produced by either sulfuric- or hydrofluoric-acid catalysis. Based on the

yields (see Table V in Durrett et al., 1963), 13 compounds were chosen to represent, in principle,

more than 90% by mass of a typical alkylate; and these are listed in Table 6-1. (Note that

Table 6-1 is intended as a generic list; it is not a complete list and component yields of alkylates

vary depending on process parameters.) In the same reference, 2,2,4-trimethylpenatane

(isooctane) accounts for approximately 30–49% of the yield, and the isomers 2,3,4 and

2,3,3 account for approximately 18–32%. Miller (1999) indicated that a typical alkylate

produced in a refinery contains approximately 26% isooctane and approximately 25% of other

alkylated octanes, which agrees favorably with Durrett et al. (1963).

Table 6-1 summarizes some physical properties along with the Chemical Abstracts registry

numbers. Other physicochemical properties are presented in Table 6-2.

6.5. Environmental Behavior and Fate of Alkylates

Alkylates are produced in refineries and used as a high-octane blending stock for gasoline.

Therefore, typical scenarios for gasoline releases in the environment apply to alkylates as well.

Refueling, evaporative emissions, and incomplete combustion introduce alkylate compounds to

the atmosphere. In addition, accidental spills on land or water, or leaking underground fuel tanks
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(LUFTs) discharge alkylate compounds to the environment. Unlike MTBE or ethanol, however,

alkylate components behave very much like other gasoline hydrocarbons. The average molar

mass of an alkylate solution is close to that of gasoline (approximately 100 g/mol), and these

compounds have high Henry’s law constants, very low solubilities in water, and high octanolwater partition coefficients. If they are spilled into surface waters, they will float because of the

density contrast and form surface pools or films from which the hydrocarbons will evaporate into

the atmosphere. A small fraction will also dissolve in the water. These compounds will be

retained in the organic fraction of soils and sediments after release from LUFTs because of their

high octanol-water partition coefficients.

6.5.1. Air

The principal release mechanisms to the atmosphere include incomplete combustion, and

evaporative emissions from automotive-fuel systems and from direct spills on land and/or water.

For hydrocarbon solutions, such as light-crude oils and refined products, evaporation can account

for up to 75% of volume loss a few days after a spill occurs (Fingas, 1995). A similar behavior

should be expected for alkylates. The evaporation process of an alkylate (or any other

hydrocarbon solution) is difficult to describe because the vapor pressure is a function of the

changing composition of the solution. Qualitatively, as the solution evaporates, the vapor

composition becomes richer in the more volatile components, and the liquid composition

becomes richer in the less volatile components. As a result, the vapor pressure of the remaining

solution decreases as the evaporation progresses.

Fingas (1995) reviewed models used to predict evaporation from oil spills. One of the most

frequently used models is that of Stiver and MacKay (1984). In this model, it is assumed that a

liquid evaporates at a rate given by

N = k A P / (RT), (6-2)

where N (mol s–1) is the evaporative molar flux, A (m2) is the area, k (m s–1)is the mass-transfer

coefficient under the prevailing wind, P (Pa) is the vapor pressure of the bulk liquid,

R (8.314 Pa m3 mol–1 K–1) is the gas constant, and T(K) is the ambient temperature. Stiver and

MacKay (1984) rearranged this equation to give

dFv = H dθ (6-3)

where Fv is the volume fraction evaporated, H is a dimensionless Henry’s law constant, and θ is

defined as a dimensionless “evaporative exposure.” If the liquid is not pure, H is a function of

Fv, and Equation (6-3) can only be integrated if the vapor pressure is expressed as a function of

composition. In those cases, the evaporation rate slows, as expected, as the evaporation

proceeds.

In a recent study (Kirchstetter et al., 1999), vehicle emissions were measured in a heavily

used roadway tunnel (Caldecott tunnel) located in the San Francisco Bay Area. The researchers
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also measured the composition of liquid gasoline in samples from local service stations. In 1996,

the average local gasoline formulation contained, in weight percent, 3.64% isooctane

(2,2,4-trimethylpentane), 0.58% benzene, and 10.91% MTBE. (For purpose of comparison, only

the results for isooctane, benzene, and MTBE are discussed here.) From these values,

Kirchstetter et al. (1999) calculated a headspace-vapor composition of 1.21% isooctane,

0.36% benzene, and 16.83% MTBE. Their measurements of the non-methane, organic-carbon

emissions in the tunnel during the summer of 1996 were 2.67±0.20% isooctane,

3.34±0.29% benzene, and 5.47±0.74% MTBE. From this study, it is clear that isooctane, a

typical alkylate component, is already measurable in automotive emissions; and its concentration

should be expected to rise if the percentage of alkylate in gasoline increases. In addition, it is

important to note that the relative fractions measured in the tunnel show more isooctane and

benzene, and less MTBE than the values corresponding to the equilibrium vapor composition.

One possible explanation is that MTBE burns more efficiently than isooctane or benzene.

Once in the atmosphere, the isoalkanes representing the alkylate mixture can be destroyed by

chemical degradation and, to a lesser degree, removed by precipitation. The oxidation reactions

normally used to estimate the residence time of a chemical in the atmosphere are those involving

hydroxyl radicals (OH) and ozone. Alkanes react primarily with hydroxyl radicals to form alkyl

radicals:

RH + OH → R. + H2O. (6-4)

The alkyl radicals (R.) formed react subsequently with oxygen to form peroxyls. The

reaction of alkanes with ozone is relatively slow; thus, the reaction with OH is considered the

main initiation path for the oxidation of hydrocarbons in the atmosphere. The oxidized species

eventually undergo deposition and biodegradation. The rate of reaction (-d[RH]/dt) (see

Equation 6-4) is given by the product of a temperature-dependant rate constant (k(T)) and the

reactant concentrations:

 –[ ] ( ) d RH

dt = k T [OH] [RH]. (6-5)

For alkanes having between 2 to 8 carbons, the rate constant range is about

(0.3–9) × 10–12 cm3 molecule–1 s–1 at 298 K (Finlayson-Pitts and Pitts, 1986). Note that

methane has a very low hydroxyl-rate constant (0.0084 × 10–12 cm3 molecule–1 s–1) compared

to other alkanes and is, by far, the most abundant alkane in the atmosphere. For a typical akylate

component, such as isooctane, the rate constant is 3.66 × 10–12 cm3 molecule–1 s–1. For a mean

global concentration of OH of 1 × 106 molecules cm–3 (Prinn et al., 1995), the half-life

( 1/2

OH t = ln(2)/k(T)[OH]) for 2,2,4-trimethylpentane would be 2.2 days. Similar estimations of halflife for benzene, ethanol, and MTBE are 5.8, 2.7, and 2.7 days, respectively, based on

OH-reaction rates from Lyman (1990a) for benzene and ethanol, and from Jenkin and

Hayman (1999) for MTBE. It should be noted that the average concentration of hydroxyl radical
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may be 107 molecules cm–3 or higher in polluted atmospheres (Lyman, 1990a). Therefore,

under such conditions, the half lives may be reduced by an order of magnitude or more.

The formation of ground-level ozone is a serious pollution concern. Photochemical

interactions of volatile organic compounds in combination with oxides of nitrogen generate

ozone at ground level. The rate of reaction and mechanism will depend, among other

parameters, on the organic compound in question. Carter (1994) discusses the various ways of

quantifying the reactivity (the potential to form ozone) of a volatile organic compound. One of

them is the Maximum Incremental Reactivity or MIR scenario. Isooctane, benzene, and MTBE

have reactivities in the MIR scale of 0.93, 0.42, and 0.62 g of ozone per gram of organic

compound (gO3/goc), respectively (Carter, 1994). Other isoalkanes present in alkylates have

reactivities in the range 0.3–1.5 gO3/goc. Even though the ozone-forming capacity of some

alkylate components is higher than MTBE or benzene, it is much lower than those corresponding

to olefins that are already present in gasoline at a level between 2 and 3%, and they have

reactivities reaching as much as 10 gO3/goc.

6.5.2. Water

The solubility data in Table 6-2 consists of only three measured values, but it can be inferred

from the general solubility trends of hydrocarbons that these isoalkanes are sparingly soluble in

water, that is, in the range 10–4–10–5 M. The vapor pressures are in the range 103–104 Pa at

25°C (water-vapor pressure is 3.17 × 103 Pa at 25°C). In addition, the contrast in density will

cause the insoluble fraction of these compounds to float on water. Because of their low

solubility in water, they have relatively large Henry’s law constants (approximately

105 Pa m3 mol–1). This translates into a tendency for the compound to evaporate easily from

aqueous solutions. As a rule, compounds in aqueous solution with Henry’s law constants larger

than approximately 102 Pa m3 mol–1 are considered highly volatile (Thomas, 1990). In this case,

as long as the concentration in the gas phase remains negligible, the evaporation is controlled by

the liquid-phase transfer rate.

The concentration of alkylate in water from a precipitation event is expected to be negligible.

For example, a calculation based on an initial concentration in the atmosphere of 1 part per

billion (ppb[v])) predicted rainwater to contain 3.6 × 10–5 μg/L for isooctane, 0.17 μg/L for

MTBE, and 7.33 μg/L for ethanol (see Chapter 5, Equation [5-1] in this report). The other

isoalkanes will also have very low concentrations in rainwater.

6.5.3. Soil and Groundwater

As seen in Table 6-2, the components of an akylate have very high Kow; therefore, they will

tend to be retained by organic matter in the environment. Using a soil-partitioning model

(Jury et al., 1983), predictions can be made on the distribution of an organic compound in soil

phases. The model considers soil to have three phases (solid, water, and air), and the properties

of the soil are considered uniform. Some of these properties are water content θw, bulk density

ρb, porosity φ, air content θA, and organic carbon fraction foc. The total concentration of a given

compound in the soil is given by the sum of the contributions from each phase, as shown in the

following equation:
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CT = ρb CS + θw CL + θA CG, (6-6)

where CT, CS, CL, and CG, are the total, solid, water, and air concentrations, respectively. Using

linear-equilibrium approximations, Equation (6-6) may be written as

CT = RSCS = RLCL = RGCG (6-7)

where

RS = ρb + θw /KD + θA H/ KD (6-8)

RL = ρb KD + θw + θA H (6-9)

RG = ρb KD /H + θw /H + θA (6-10)

H is Henry’s law constant, and KD is the solid-liquid partition coefficient. The KD of isooctane

was calculated from

KD = foc Koc (6-11)

where Koc was calculated using the regression equation from Kenaga and Goring (1980), as cited

by Lyman (1990b).

log (Koc) = 0.544 log(Kow) + 1.377. (6-12)

Using this model and assuming an initial total concentration of 1 mg/L of the organic

compound in soil (isooctane, ethanol, MTBE, and benzene), we performed calculations for two

types of soil as defined by Jury et al. (1990):

a) Sandy soil (ρb = 1590 kg/m3, foc = 0.0075, φ = 0.4, and θw = 0.18) and

b) Clayey soil (ρb = 1320 kg/m3, foc = 0.025, φ = 0.5, and θw = 0.375).

The purpose was to see how isooctane, a typical alkylate component, compares to other gasoline

components. The results of the calculations are plotted in Figure 6-1 for the sandy soil and in

Figure 6-2 for the clayey soil. A distinctive feature of isooctane is its relatively high preference

for the air phase in both soils, even when compared to benzene. One-year evaporation losses

calculated for benzene in 1-m-deep soil are 34.3% for sandy soil and 0.01% for clayey soil
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(Jury et al., 1990). The rate of volatilization is somewhat proportional to the concentration in the

air phase; hence, isooctane would volatilize more rapidly than benzene. Therefore, volatilization

into the atmosphere could be the main mechanism determining the half-life of an alkylate

component in soil for surface contamination.

The limited distribution of the alkylate compounds in soil water and their high retention in

soils (see Figures 6-1 and 6-2) indicates that their mobility in infiltrating water will be quite

limited—particularly when compared to ethanol or MTBE. Alkylate compounds that are not

volatilized to air will probably undergo slow biodegradation based on the study by SolanoSerena et al. (1998), discussed in the next section.

6.5.4. Biodegradation

The biodegradability of a compound can be affected by many variables (pH, temperature,

substrate composition, oxygen availability, microorganisms present and their interaction, etc.).

Consequently, it is quite difficult to predict the actual biodegradation rates for individual

isoalkanes. A simple approach would be to consider the solution as a single compound; for

example, gasoline has been considered as a single compound that is moderately biodegradable

(Scow, 1990). However, an alkylate (and gasoline) is a complex solution, and the rate of

biodegradation of each component may be significantly different. Unfortunately, there have not

been many specific studies on the biodegradation of alkylate components. In general, n-alkanes

are degraded rapidly in the environment, but highly branched alkanes tend to be more resistant to

biodegradation because of their molecular structure. Under aerobic conditions, the oxidation of

alkanes normally proceeds by the oxidation of a terminal methyl group to form a carboxylic acid,

followed by β-oxidation of the acid. The presence of a quaternary carbon near the end of the

chain interferes with the β-oxidation step and the degradation stops. It is possible that later these

compounds are degraded by other, slower mechanisms (Alexander, 1994; Chapelle, 1993).

A recent laboratory study using unpolluted spruce-forest soil as substrate (Solano-Serena,

1998) has shown that all components of a model-gasoline solution were biodegraded below

detection limit after 28 days with the exception of 2,2,4-trimethylpentane (isooctane),

2,3,4-trimethylpentane, and cyclohexane. From Table 2 of Solano-Serena et al. (1998), the

percentages of degradation after 28 days of incubation at 30°C are 18% for

2,2,4-trimethylpentane, 9% for 2,3,4-trimethylpentane, and 100% for benzene. In addition,

mineralization yields (fraction of compound converted to CO2) for 23 individual components of

this gasoline solution were determined (see Table 1 in Solano-Serena, 1998) by measuring the

CO2 produced after 34 days of incubation at 30°C. The mineralization yield was 2% for

2,2,4-trimethylpentane and 0% for 2,3,4-trimethylpentane. On the other hand, the mineralization

yield for benzene was 56%. Note, it appears that the resistance to degradation is not due to the

inhibitory capacity of isoalkanes or cycloalkanes but rather to a population deficiency of the

naturally occurring microorganisms degrading these hydrocarbons.

Based on the previous values for 2,2,4-trimethylpentane, we can estimate a first-order

degradation constant of approximately 0.007 day–1 and a half-life of about 98 days. In the case

of 2,3,4-trimethylpentane, the first-order degradation constant is approximately 0.003 day–1 and

the half-life is approximately 206 days. Because benzene degraded completely, the same

constant cannot be determined for benzene from this data. In another experiment, the first-order

degradation constant of benzene in an aerobic aquifer was measured to be between 0.07 and
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0.5 day–1 (Nielsen et al., 1996). These values would predict over 85% degradation of benzene in

28 days, which generally agrees with the results of Solano-Serena et al., (1998) for this

compound. The half-life for mineralization of 2,2,4-trimethylpentane would be around three

years (compared to 25 days for the case of benzene).

A detailed laboratory study was conducted on gasoline biodegradation using microflora from

an urban-waste activated sludge (Solano-Serena, 1999). Based on the kinetics of oxygen

consumption, the authors concluded that there are two main degradation phases: a fast phase and

a slow phase. The rate of oxygen consumption is about 40 times lower during the slow phase.

Aromatic hydrocarbons were degraded mostly during the fast phase while linear, branched, and

cyclic alkanes were degraded mostly during the slow phase. After 25 days of incubation, the

undegraded compounds (6% of gasoline) were essentially branched alkanes with a quaternary

carbon and/or alkyl chains in consecutive carbons. For example, the undegraded fractions of

2,2,4-trimethylpentane, 2,3,3-trimethylpentane, and 2,3,4-trimethylpentane were 49.7%, 72.1%,

and 30.5%, respectively, after 25 days of incubation. One reason given for the relatively slow

biodegradation rate of some linear alkanes and isoalkanes is their low solubilities in water and

high Henry’s constants. The alkane in the gas phase of the culture flask is not readily available

to the microflora, and the rate-limiting step could be the transfer of the alkane from the gas to

aqueous phase. However, the incomplete biodegradation of some branched alkanes seems to

indicate that there is also a metabolic limit in this case.

6.6. Summary

Alkylates are solutions of highly branched alkanes produced in refineries by the reaction of

isobutane with light olefins, using either sulfuric or hydrofluoric acid as a catalyst. For many

years, alkylates have been used as gasoline-blend stock because of their high-octane number. In

the near future, the percentage of alkylate in gasoline may increase significantly to compensate

for the phase out of high-octane MTBE. The purpose of this chapter was to characterize the

transport and fate of alkylates in the environment. For simplicity, we concentrated on the

properties of isooctane as a typical alkylate component. Clearly, alkylates are complex solutions

of isoalkanes, and some properties like biodegradability may not be easily extrapolated to all

components. The low water solubility and high volatility of alkylate components indicate that

they would not exacerbate problems of ground or surface water contamination by gasoline in the

manner MTBE has done. Overall, it appears that alkylates would not effect dramatic changes in

the way gasoline behaves in the environment and, thus, in the treatment of accidental releases.

Unfortunately, there is no toxicity data on the chronic effects of isooctane in humans;

however, liver and kidney effects have been observed in rats that have been exposed chronically

(US EPA, 1991). Additionally, cancer risk and reproductive and developmental effects have not

been addressed. It should be noted that toxicity related to alkylate addition to gasoline is a

complex problem because any percentage increase of alkylate in gasoline composition must be

accompanied by a percentage decrease of other blending components (e.g., BTEX) in the final

formulation. Therefore, the toxicity of the final product should be evaluated in an integral

manner.

The physicochemical properties of alkylate components are typical of many hydrocarbons

and determine their behavior in the environment. They have very low solubility in water and
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very high octanol-water partition coefficient. They are less dense than water and have relatively

high Henry’s law constants. Surface releases of alkylate either on water or land will probably

result in the evaporation of most of the alkylate into the atmosphere. In the atmosphere, their

oxidation will be initiated mostly by the presence of atmospheric OH, with half-lives of about

2–3 days. Their potential to form ground-level ozone is about double that of MTBE in the MIR

scale. Alkylates will be strongly absorbed by the organic fraction of soils and sediments. Their

biodegradation rate is relatively slow compared to other organic compounds, and some

components may not biodegrade in reasonable time frames.

6.7. References

Albright, L.F., K.E. Kranz, and K.R. Masters (1993). Alkylation of isobutane with light olefins:

Yields of alkylates for different olefins. Ind. Eng. Chem. Res. 32: 2991–2996.

Alexander, M. (1994). Biodegradation and Bioremediation. Academic Press, San Diego, CA.

CambridgeSoft Corporation (1998a). ChemFinder Searching and Information Integration.

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Scientific Computing, Inc., Cambridge, MA. CS Chem3D 5.0 for Windows and Macintosh.

Carter, W.P.L. (1994). Development of ozone reactivity scales for volatile organic compounds.

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NY.

Durrett, L.R., L.M. Taylor, C.F. Wantland, and I. Dvoretzky (1963). Component analysis of

isoparaffin-olefin akylate by capillary gas chromatography. Anal. Chem. 35: 637–641.

Fingas, M.F. (1995). A literature review of the physics and predictive model of oil spill

evaporation. J. Hazard. Mat. 42: 157–175.

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Jury, W.A., D. Russo, G. Streile, and H. El Abd (1990). Evaluation of volatilization by organic

chemicals residing below the soil surface. Water Resour. Res. 26: 13–20.
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Kenaga, E.E., and C.A.I. Goring (1980). Relationship between water solubility, soil sorption,

octanol-water partitioning, and concentration of chemicals in biota. In Aquatic Toxicology,

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Phoenix, AZ (Accessed: December 6, 1999). Available: <http://www.stratco.

com/library/Seminar98/AlkyIntro/> (Updated: 1998).

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of soil microflorae for gasoline degradation. Biodegradation 9: 349–376.

Solano-Serena, F., R. Marchal, M. Ropars, J.M. Lebeault, and J.P. Vandecasteele (1999).

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mixtures. Environ. Sci. Technol. 18: 834–840.

Syracuse Research Corporation (SRC) Environmental Science Center (1999). Interactive

LogKow (KowWin) Demo, Syracuse Research Corporation (SRC) Environmental Science

Center, Syracuse, NY. Accessed: December 6,1999. <http://esc-plaza.syrres.com/

interkow/kowdemo.htm > (CAS=1634044, LogKOW's Log P Calculation for Methyl t-butyl

ether; and CAS=64175, LogKOW's Log P Calculation for Ethanol) , (Updated: 1999; see

also Meylan, W.M., and P.H. Howard (1995). Atom/fragment contribution method for

estimating octonal-water partition coefficients. J. Pharm. Sci. 84: 83–92).

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D.H. Rosenblatt, Eds. The American Chemical Society, Washington, DC.

Tucker, W.A., and L.H. Nelken (1990). Diffusion coefficients in air and water. In Handbook of

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Washington, DC.

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Information System Web Site for 2,2,4-Trimethylpentane (CASRN 540-84-1). United States

Environmental Protection Agency, National Center for Environmental Assessment,

Cincinnati, OH (E-mail RIH.IRIS@epamail.epa.gov), Accessed: December 21, 1999.

Available: < http://www.epa.gov/ngispgm3/iris/subst/0614.htm > (Updated: May 5, 1998).
11-99/Ethanol Ch. 6 6-13

UCRL-AR-135949 Vol. 4 Ch. 6

Figures
UCRL-AR-135949 Vol. 4 Ch. 6 Potential Ground and Surface Water Impacts December 1999

11-99/Ethanol Ch. 6 6-14
UCRL-AR-135949 Vol. 4 Ch. 6 Potential Ground and Surface Water Impacts December 1999

11-99/Ethanol Ch. 6 6-15

0

20

40

60

80

100

120

Isooctane Ethanol MTBE Benzene

Sandy Soil Air

Water

Solid

Figure 6-1. Model of equilibrium distribution for isooctane between the phases of a sandy soil

(air, water, solid). This partitioning model was developed by Jury et al. (1983); soil characteristics

were taken from Jury et al. (1990).
UCRL-AR-135949 Vol. 4 Ch. 6 Potential Ground and Surface Water Impacts December 1999

11-99/Ethanol Ch. 6 6-16

0

20

40

60

80

100

120

Isooctane Ethanol MTBE Benzene

Clayey Soil Air

Water

Solid

Figure 6-2. Model of equilibrium distribution for isooctane between the phases of a clayey soil

(air, water, solid). This partitioning model was developed by Jury et al. (1983); soil characteristics

were taken from Jury et al. (1990).
11-99/Ethanol Ch. 6 6-17

UCRL-AR-135949 Vol. 4 Ch. 6

Tables
UCRL-AR-135949 Vol. 4 Ch. 6 Potential Ground and Surface Water Impacts December 1999

11-99/Ethanol Ch. 6 6-18
UCRL-AR-135949 Vol. 4 Ch. 6 Potential Ground and Surface Water Impacts December 1999

11-99/Ethanol Ch. 6 6-19

Table 6-1. Representative components of a typical alkylatea.

Alkylate compound Formula

CAS

numberb

Molar mass

(g/mol)b

Melting

point (°C)b

Boiling

point

(°C)b

Density

(g/cm3)b

2,2,4-Trimethylpentane C8H18 540-84-1 114.23 -107 99.2 0.692

2,3,3-Trimethylpentane C8H18 560-21-4 114.23 -101 115 0.726

2,3,4-Trimethylpentane C8H18 565-75-3 114.23 -109 113 0.719

2,2,3-Trimethylpentane C8H18 564-02-3 114.23 -112 110 0.716

2,2,5-Trimethylhexane C9H20 3522-94-9 128.25 -106 124 0.707

2,3-Dimethylbutane C6H14 79-29-8 86.17 -128 58 0.661

2,3-Dimethylpentane C7H16 565-59-3 100.2 N/A 90 0.695

2,4-Dimethylpentane C7H16 108-08-7 100.2 -119 81 0.673

2,5-Dimethylhexane C8H18 592-13-2 114.23 -91 109 0.694

2,3-Dimethylhexane C8H18 584-94-1 114.23 N/A 116 0.712

2,4-Dimethylhexane C8H18 589-43-5 114.23 N/A 109 0.700

2,3,5-Trimethylhexane C9H20 1069-53-0 128.25 -128 131 0.722

2-Methylpentane C6H14 107-83-5 86.17 -154 62 0.653

a From Durett et al. (1963).

b Values estimated using ChemFinder Software (CambridgeSoft Corporation, 1998a).
UCRL-AR-135949 Vol. 4 Ch. 6 Potential Ground and Surface Water Impacts December 1999

11-99/Ethanol Ch. 6 6-20

Table 6-2. Physical-chemical properties of representative alkylate components.

Alkylate compounda

Vapor

pressureb

(Pa at

298K)

Solubilityc

(mg/L)

Henry’s

law

constantd

(Pa m3

mol–1) Kowe

LeBas

molar

volumef

(cm3 mol–1)

Dair

f

(m2 d–1;

at

298 K)

Dwater

f

(m2 d–1;

at

298 K)

2,2,4-Trimethylpentane 6.49×103 2.44 3.23×105 1.22×104 185 0.575 7.03×10-5

2,3,3-Trimethylpentane NA NA 4.17×105 1.22×104 185 0.575 7.03×10-5

2,3,4-Trimethylpentane 3.57×103 NA 1.83×105 1.12×104 185 0.575 7.03×10-5

2,2,3-Trimethylpentane NA NA 3.85×105 1.22×104 185 0.575 7.03×10-5

2,2,5-Trimethylhexane 2.22×103

NA

5.26×105

3.44×105 3.77×104 207.2 0.541 6.57×10-5

2,3-Dimethylbutane 3.13×104 NA 1.30×105 1.38×103 140.6 0.670 8.29×10-5

2,3-Dimethylpentane 9.18×103 NA 1.74×105 4.29×103 162.8 0.618 7.59×10-5

2,4-Dimethylpentane 1.31×104 4.06 3.00×105 4.29×103 162.8 0.618 7.59×10-5

2,5-Dimethylhexane 4.05×103 NA 3.33×105 1.33×104 185 0.575 7.03×10-5

2,3-Dimethylhexane 3.12×103 NA 3.85×105 1.33×104 185 0.575 7.03×10-5

2,4-Dimethylhexane 3.64×103 NA 3.57×105 1.33×104 185 0.575 7.03×10-5

2,3,5-Trimethylhexane NA NA 5.00×105 3.47×104 207.2 0.541 6.57×10-5

2-Methylpentane 2.82×104 13.8 1.71×105 1.64×103 140.6 0.670 8.29×10-5

a Representative components of a typical alkylate (Durett et al., 1963).

b Vapor pressure calculated using Antoine equation from National Institute of Standards and Technology (NIST)

(1999).

c Values were obtained using Chem3D Software (CambridgeSoft Corporation, 1998b).

d Henry’s law constant obtained from NIST (1999). Note: 2,2,5-trimethylhexane has two values.

e Octanol-water partition coefficients obtained using LogKow (KowWin) Demo software (SRC, 1999).

f Calculated according to methods described by Tucker and Nelken (1990).