Organic cosolvent effects on the sorption and transport of neutral organic chemicals

Cbmwsphere, Vol. 36, No. 8, pp. 1883-1892, 1998 Published by Elsevier Science Ltd All rights reserved. Printed in Great Britain 0045-6535/98 $19.C0+0.00

Pergamon

PII: s0045-6535(97)10065-0

ORGANIC

COSOLVENT

EFFECTS ON THE SORPTION AND TRANSPORT

OF NEUTRAL

ORGANIC CHEMICALS

Dermont C. Bouchard

U.S. Environmental

Protection Agency, Ecosystems

Research Division,

960 College Station Road, Athens, GA 30605, USA [email protected] (Received in USA 18 June 1997; accepted 6 August 1997)

ABSTRACT

Soil column cosolvent

miscible

(methanol)

phenantbrene,

displacement

techniques

on the sorption

were used to investigate

and transport

increased.

organic

chemicals;

and the herbicide diuron, through a sandy surface soil. A two-domain,

order mass transfer model described the experimental equilibrium

of three neutral

the effects

sorption

coeffkient

(K) decreased

of an organic naphthalene,

or bicontinuum,

first-

data well. For the three solutes used in this study, the

log-linearly

as the volume

fraction

of methanol

(f,)

The physical properties calculator of the SPARC computer model was used for generating solute

solubility profiles to estimate the slope of the Log K-f, relationship. Published by Elsevier Science Ltd

Key words:

soil sorption, desorption,

mixed solvents, contaminant transport, complex wastes

INTRODUCTION

Soil, sediment, and aquifer material pore waters near highly contaminated concentrations

of water-miscible

transport, of other, co-ocurring chemical aspects

solvents that can affect the aqueous solubility, contaminants.

sorption was initially described of the effects

established

sites may contain significant

of cosolvents

The influence of water-miscible

by Rao et al. [l] with subsequent

on contaminant

that the log-linear relationship

sorption

and transport

and thus the sorption and organic solvents on organic

investigations [2-41.

into differing

These studies have

between chemical aqueous solubility and the volumetric 1883

fraction

1884 of water-miscible

solvent can be extended

between water-cosolvent

to describe

the phase distribution

and the organic carbon phase of soils, sediments,

and aquifer materials.

has been the cosolvent of choice in most of these studies because it is completely a common constituent The objectives

utilize

SPARC

miscible with water and is

of the log-linear relationship

between the

(K) of three neutral organic solutes and the volume fraction of methanol (f,); and to

(SPARC

parameterizing

Methanol

of industrial waste streams.

of this study were to evaluate the applicability

sorption coefficients

of neutral organic solutes

Performs

a two-domain,

transport in mixed-solvent

Automatic

Reasoning

or bicontinuum,

in Chemistry)

first-order

IS] solubility

mass transfer

model

calculations

for simulating

for

solute

systems.

MATERIALS AND METHODS

Materials Naphthalene

and phenanthrene

(Aldrich Chemical Co.. Milwaukee,

(3,4-dichlorophenyl)-l,ldimethylurea] miscible

displacement

(Crescent

experiments

Chemical Co.. Hauppauge,

were reagent grade.

exhibit a wide range of hydrophobicity,

WI). and the herbicide

NY) used as solutes in the

These three solutes were selected

are of environmental

used in the soil columns and in HPLC analyses was HPLC grade

column system.

A glass preparatory-scale

i.d. with bed length (Psammentic

adjusted

Paleudults,

liquid chromatography

OC = 0.61%).

Pittsburgh,

PA)

Tritiated water, ‘H20 (Sigma Chemical

tracer to characterize

to 5.2 cm was dry-packed

because they

concern, and because they have been used

frequently as model solutes in solute transport studies 12-41. Methanol (Fisher Scientific,

Co., St. Louis, MO), was used as a nonsorbing

diuron [3-

hydrodynamic

dispersion

in the

column (Kontes, Vineland, NJ) 2.54 cm

with 1250~

fraction

of Eustis

fine sand

Bulk density and saturated water content of the packed column

were 1.654g cm-’ and 0.376 cm’ cm-‘; respectively.

Column Apparatus Experimental

and Methods

apparatus and methodology

were similar to those used previously oriented vertically

and connected

used in the soil column miscible

by the author 12.61. In these systems,

displacement

experiments

packed soil cohnnns

were

to two syringe pumps through an inert valve which allowed switching

between the mobile phases delivered by the pumps to the column.

One pump contained a mobile phase of

0.01 N CaCI,, or 0.01 N CaCl, and methanol, and the other contained mobile phase plus the naphthalene, phenanthrene,

diuron, or ‘H,O solutes.

The syringe pumps allowed for very accurate maintenance

of the

average pore water velocity at 37.8 cm h-l. Initially, the column was wetted slowly from the bottom with a degassed solution of 0.0 1 N CaCl, until the system appeared saturated by visual inspection.

Flow rate was then increased and a minimum

of 50

1885 pore volumes of degassed 0.01 N CaCl, were passed through the column to ensure saturation. The column was then prepared for the miscible displacement experiments by passing a minimum of 50 pore volumes of the experimental mobile phase through the column. Breakthrough curves (BTCs) were then conducted by introducing a pulse of the solute containing mobile phase into the column and monitoring column effluent until the solute effluent concentration, C (uMo1 L”), approached the intluent concentration, C, (nMo1 L-‘), i.e., C/C, = 1. This solution was then displaced from the column with the same mobile phase without solutes until C/C,, = 0 was approached.

Diuron, naphthalene, and phenanthrene concentrations in the

column effluents were determined by collecting and analyzing 4.8-mL fractions using HPLC. The HPLC system utilized a 4-u particle size C8 column with varying methanol-water mobile phases to achieve solute separation.

The UV analytical wavelengths were 251, 275, and 250 mn for diuron, naphthalene, and

phenanthrene, respectively.

Column effluent fractions (1.2 mL) collected from the ‘H,O BTCs used to

characterize column hydrodynamics were analyzed using liquid scintillation counting.

Data Analysis The Peclet value, P, which represents the dispersive-flux contribution to transport, was obtained from the 3H,0 BTC data by using a nonlinear, least-squares optimization program [7] to solve the advectivedispersive local equilibrium solute transport model.

PI

P=vL/D

where v is the average pore-water velocity (cm h-l); L is column length (cm), and D is the hydrodynamic dispersion coefficient (cm’ he’). A two-domain, or bicontinuum, first-order mass transfer model was used to analyze the data generated in the diuron, naphthalene,

and phenanthrene BTC experiments.

This model has been used with

considerable success to describe solute transport through soils, sediments, and aquifer materials [8-91. For the bicontinuum model sorption in domain 1 is assumed to be instantaneous, and sorption in domain 2 is rate limited and described by first-order reversible kinetics. Sorption in both regions is assumed to follow linear sorption isotherms. Five parameters (T,, P, R, 8, o) are required to run the bicontinuum model. The size of the input analyte pulse expressed in pore volumes (T,) is known from measurement, and the value for P is known from the 3H20 BTC data as described above.

Values for the retardation factor (R), a

measure of solute sorption during transport; 8, the fraction of total retardation attributed to sorption in the instantaneous domain (domain 1); and w, which represents the ratio between hydrodynamic residence time and the characteristic time of sorption, were estimated by using a nonlinear, least-squares optimization program [7] for the bicontinuum model under flux-type boundary conditions. sorption coefficient (K, cm3 g-‘), was then determined from:

The linear equilibrium

1886 R=l

where p is the bulk density

+(pi@K

PI

(g cm-“), and 8 is the volumetric

water content (cm’ cm-‘), of the porous

medium. The relationship

between the equilibrium

sorption coefficient

in aqueous systems (k,)

and in mixed

solvent systems (K,,,) may be expressed as [I]:

Log K,,, = Log K,, - cr.cr f,

where the parameter cr accounts for solute-cosolvent of the cosolvent,

and is an index of the solubilizing

power

f, is the volume fraction of cosolvent, and a is an empirical constant which embodies any

deviation of the sorption-f, interactions.

interactions

relationship

from the solubility-f,

relationship

arising from cosolvent-sorbent

The value of o has been shown to he correlated to solute properties,

such as molecular surface

area, and to solvent properties such as dielectric constant and bulk surface tension [IO]. For a given solute, o values may be approximated

from the slope of the solute solubility profile (Log solubility (S) vs f,). The

physical properties calculator of the SPARC computer model [S] was used to calculate solute solubilities aqueous

and in methanol-water

systems.

Values for cr were then determined

in

from the slope of the

regression of Log S on f,.

RESULTS AND DISCUSSION

Cosolvent Effects on Column Hydrodynamics

and Solute Retardation

Figure 1 contains the BTCs for ‘H?O at f, = 0.0, and for diuron at f, = 0.1. 0.2. and 0.3. The jH,O BTC in Figure 1. as well as jH,O BTCs run at varying f,, were all symmetric,

and the data were described well

by the advective-dispersive

Such symmetry

model fit is commonly equilibrium structure

local equilibrium

observed

solute transport

for sieved, unaggregated

model.

soil media and is indicative

and equilibrium of hydrodynamic

during transport; that is, that diffusionai mass transfer of solute into and out of the microporous

of the soil occurred

rapidly enough relative to solute resident

equilibrium

with bulk water transport.

tlsing the same size fractions

unsaturated

zone material, the author has observed such ideal conservative

times in the column to be at

of surface soil, and saturated

and

tracer behavior in prior studies

[2,6]. The constancy of the ‘H,O BTCs also indicated that the advective-dispersive the column equilibrium

remained

unchanged

over time, and as f, varied.

Fitting

transport characteristics

the advective-dispersive

of

local

solute transport model to the ‘H,O data yielded an average Peclet number of 73.5 with a range

1887

-Equilibrium

0.6

simulation

0

3

0.4

a

Diuron, fc = 0.1

. -.

Z-domain simulation

A

Diuron. fc = 0.2

-2-domain

simulation

Diuron, fc = 0.3

??

0.2

0 0

2

4

6

0

10

12

14

16

16

20

Pore Volumes

Figure 1. Breakthrough curves for ‘H,O at f, = 0.0 with equilibrium simulation, and diuron breakthrough curves at varying f, with 2-domain model simulations.

of 64.5-86.2. These Peclet numbers, which are a measure of hydrodynamic dispersion during solute transport, were then used in the two-domain model for simulating diuron, naphthalene, and phenanthrene transport through the column. The Peclet numbers measured in these experiments were consistent with the low dispersion expected for well-packed, tmaggregated soils, and are comparable to those measured by other researchers using similar column apparatus and sandy soil or aquifer materials [9,11]. The diuron BTCs in Figure 1 are representative phenanthrene.

of the BTCs observed for naphthalene

and

All BTCs exhibited decreasing retardation (i.e., earlier solute breakthrough) with increasing

methanol content of the eluting phase, and all solute BTC data were described well by the two-domain model. Retardation factors for diuron decreased from 5.15, 3.67, 2.69, to 2.08, as f, increased from 0.0 to 0.3.

BTCs (not shown) run prior to elution of the soil column with any organic solvent, and BTCs run at

the end of the 3 month experimental period, were virtually identical, thus indicating that the methanol-water solutions had not leached out any soil organic material that contributed significantly to soil sorptivity. Using soils with organic carbon contents as high as 3.9% and over an f, range from 0.0-l .O, Kizza et al. [12] did not observe any dissolved organic carbon effects on the K values of three neutral organic solutes. The effects

of methanol on solute transport in terms of the Log K-f, relationship are discussed further below.

1888 Solubility-f,

and Log K-f, Relationships

SPARC-calculated

solubility profiles for diuron, naphthalene,

2A and 2B. All of the plotted values are SPARC-calculated an experimental

calculated

except for the diuron value at f, = 0.0, which is

in binary solvent mixtures 1131. Regressing

(rL > 0.98) for naphthalene

and phenanthrene,

However.

Log S-f, relationship.

determination

are plotted in Figures

value. The log-linear nature of the plots is consistent with solution theory for hydrophobic

solute solubilization determination

and phenanthrene

more

Log S on f, yielded high coefftcients

indicating the high degree of linearity of the

data scatter,

as well as a lower

(2 = .92) for the log-linear model, was observed for diuron.

solute-solvent

interactions

for diuron

coefficient

of

This may be attributed to the

more complex chemical structure of the diuron molecule (Cl, N, and 0 heteroatoms), more complicated

of

and hence potentially

than for the hydrocarbons

naphthalene

and

phenanthrene. In Figures 3A and 3B Log K vs f, is plotted for diuron. naphthalene, equation 3, an inverse relationship > 0.99 for all solutes,

between Log K and f, was observed.

indicating

the strength

of the log-linear

and phenanthrene. Regressing

relationship.

As predicted by

Log K on f, yielded rr

This inverse

relationship

between Log K and f, is a result of the observed exponential

increase in solute solubility with f, (Figures

2A,B), and suggests

sorption from aqueous and methanol-water

that solvophobic

solutions for these three solutes.

interactions

dominate

In evaluating Log K-$ relationships

[12] observed that each soil-solute

combination

across a number of soils, Kizza et al.

yielded a unique slope, as observed

here, but that slopes

across soils did not vary for a given solute. The slopes (cr) of the SPARC-calculated o) of the Log K vs f, plots. phenanthrene experimental

respectively.

naphthalene

The values of w were 0.82, 1.13, and 1.06 for diuron, naphthalene, Using G values estimated

respectively.

methanol

from literature

The value of u c 1 observed

had a slightly

and phenanthrene

solute solubility. interactions.

with the slopes (a

solubility

and

data and using similar

apparatus, Wood et al. [2] reported a values of 0.88, 1.05, and 1.OO for diuron, naphthalene,

and phenanthrene cosolvent

solubility profiles were in close agreement

greater effect

on diuron

solubility

than on diuron

that the

sorption.

For

(a b 1). methanol had a slightly greater effect on solute sorption than on

In both cases, deviations of CI from unity may indicate the presence of cosolvent-sorbent

However,

given the small deviations

from unity observed for the data presented

the Wood at al. study [2], it is apparent that cosolvent-sorbent solute sorption

for diuron in this study indicated

in these mixed solvent systems.

Therefore.

interactions

here and in

did not have major effects on

the cr values estimated

from the solubility

profiles (Figure 2A,B) were used as slope estimates for Log K vs f, plots. The Log K vs f, plots using these estimated slopes are co-plotted with the experimental and 3B.

data and the associated regression

lines in Figures 3A

A

y = 2.22x + 2.40

3.2

?=0.98

,I’

eO’

, __+

data 2.4

,L_,

2.2

0.1

0

0.3

data

Phenanthrene

regr.

0.4

0.3

0.4

0.5

0.7

0.6

fc

fc

Figures 2A,B.

-I

t

Diuron regression 0.2

I. -

0.5

regr.

Phenanthrene

SPARC calculated solubility profiles for naphthalene,

diuron (A), and phenanthrene

(B).

0.6 .

0.4

A - - -

\L

Naphthalene data SPARC estimate

‘h.\ “.

0.2

\\

0.5

Y

0

Y

0

$

0”

-I

-I -0.2

-0.5

P = 0.99 -1

-0.6

-

“y

y=-4.69x+2.33

-0.4

\

B a.”

-0.8 0

0.1

0.3

0.2

0.3

0.4

0.5

0.7

0.6

0.4

%

Figure 3A,B.

Plots of the Log of the equilibrium

methanol (f,), with co-plots of SPARC estimations,

sorption coefficient for naphthalene,

(K) vs the volumetric

dim-on (A), and phenanthrene

fraction of (B).

1890 Using Estimated Kps for Transport Modeling Values of Log K from the estimated Log K-f, plots in Figures 3A,B may be used as input parameters to simulate solute transport at differing methanol fractions. naphthalene

Figure 4 contains the experimental

at f, = 0.3 and 0.4, as well as the BTCs simulated

simulations

using estimated

track the actual data quite well and provide a good approximation

mixed solvent systems.

As would be expected from the estimated naphthalene

3A, where the plot of the estimated

Log K values lies above the regression

BTC data for

Log K values.

The

for solute transport in these Log K values from Figure line for the actual data, the

simulated BTCs in Figure 4 are displaced to the right of the actual data, indicating that the simulated BTC over predicted naphthalene regression

Since the phenanthrene

estimated Log K values also lie above the

line for the actual data (Figure 3B), slight overestimation

simulated phenanthrene for the experimental simulated

retardation.

BTCs.

predict naphthalene

For diuron, however. estimated Log K values lie below the regression

data (Figure

diuron BTCs.

of retardation would also be evident for

3A), so slight underestimation

In terms of solute mobility

and phenanthrene

of retardation

in soil, this sorption

line

would be evident

estimation

technique

for

would

to be slightly less mobile, and diuron slightly more mobile, in soil

12

1

0.8

g

0.6

0

??

Data, fc = 0.3

??

Data, fc = 0.4

~-Simulation,

fc = 0.3

:,

0.4

0.2

0 0

10

5

15

20

25

Pore Volumes Figure 4. Naphthalene estimated equilibrium

breakthrough

curves at f, = 0.3 and 0.4 with 2-domain model simulations

sorption coefficients

(K).

using

1891 than is indicated by the experimental data. Since it is the nature (i.e., the slope, o ) of the relationship between Log K and f, that is being estimated, the BTC simulations will track the actual data more closely at lower f, values than at higher f, values. So, the simulations in Figure 4 would track the data even more closely at lower f, values.

SUMMARY AND CONCLUSIONS

The prolonged elution (over three months) of a soil column with varying methanol-water mixtures did not affect the hydrodynamic characteristics or sorptivity of the soil material.

The data presented above

demonstrate the applicability of the log-linear model for predicting solute sorption from methanol-water solutions.

For the diuron, naphthalene, and phenanthrene used in this study, the equilibrium sorption

coefficient (K) decreased log-linearly as the fraction of organic solvent (f,) increased. As observed here for diuron, and by Lee et al. [ 141 with neutral pentachlorophenol, this log-linear relationship is operative for both nonpolar and moderately polar solutes. The SPARC properties calculator was useful for generating solute solubility profiles for estimating the slope (a) of the Log K-f, relationship.

Using the estimated o

values with solute transport data collected at one cosolvent concentration should allow for a good first approximation of solute sorption and transport at other cosolvent concentrations.

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