Mechanisms of Slow Sorption of Organic Chemicals to Natural Particles docx

Critical Review
Mechanisms of Slow Sorption of Organic
Chemicals to Natural Particles
J O S E P H J . P I G N A T E L L O * A N D B A O S H A N X I N G
Department of Soil and Water, The Connecticut Agricultural Experiment Station,
P.O. Box 1106, New Haven, Connecticut 06504-1106
The use of equilibrium expressions for sorption to
natural particles in fate and transport models is often
invalid due to slow kinetics. This paper reviews
recent research into the causes of slow sorption and
desorption rates at the intraparticle level and how
this phenomenon relates to contaminant transport, bio-
availability, and remediation. Sorption kinetics are
complex and poorly predictable at present. Diffusion
limitations appear to play a major role. Contending
mechanisms include diffusion through natural organic
matter matrices and diffusion through intraparticle
nanopores. These mechanisms probably operate si-
multaneously, but the relative importance of each
in a given system is indeterminate. Sorption shows
anomalous behaviors that are presently not well
explained by the simple diffusion models, including
concentration dependence of the slow fraction,
distributed rate constants, and kinetic hysteresis.
Research is needed to determine whether adsorp-
tion/desorption bond energies may play a role along
with molecular diffusion in slow kinetics. The pos-
sible existence of high-energy adsorption sites both
within the internal matrix of organic matter and in
nanopores is discussed. Sorption can be rate-limiting
to biodegradation, bioavailablity, and subsurface

transport of contaminants. Characterization of mech-
anism is thus critical for fate and risk assessment.
Studiesareneededtomeasuredesorption kinetics under
digestive and respiratory conditions in receptor
organisms. Conditions under which the constraint of
slow desorption may be overcome are discussed,
including the addition of biological or chemical agents,
the application of heat, and the physical alteration
of the soil.
Introduction
Sorptionto naturalsolids is an underlying processaffecting
thetransport,degradation, andbiological activity of organic
compounds in the environment. Although often regarded
as instantaneous for modeling purposes, sorption may in
fact require weeks to many months to reach equilibrium.
It was not until the mid to late 1980s that serious study of
sorption kineticsinsoils andsedimentsbegan, despite early
circumstantial evidence going back to the 1960s that the
natural degradation of certain pesticides in the field slowed
or stopped after a while (1, 2). Sorption kinetics of
contaminants on airborne particles has just recently
received attention (3).
Fate, transport, and risk assessment models all contain
terms for sorption; therefore, an understanding of the
dynamics of sorption is crucial to their success. Ignoring
slow kinetics can lead to an underestimation of the true
extent of sorption, false predictions about the mobility and
bioavailability of contaminants, and perhaps the wrong
choice of cleanup technology. Kinetics can also be an
important mechanistic tool for understanding sorption

itself.
In this paper, we focus on updating our knowledge of
the causes of slow sorption and desorption. In addition,
we discuss its significance to bioavailability and the
remediation of organic pollutants. Much of the research
in this area has been carried out in batch systems where
particles are suspended in a well-mixed aqueous solvent.
Thus, we restrict discussion to phenomena occurring on
the intraparticle scale, that is, within individual soil grains
or within aggregates that are stable in water. We shall
exclude transport-related nonequilibrium behavior (“physi-
cal nonequilibrium”), which may also play an important
role in nonideal solute transport in the field and in some
experimental column systems. Physical nonequilibrium
is due to slow exchange of solute between mobile and less
mobile water, such as may exist between particles or
between zones of different hydraulic conductivities in the
soil column, and occurs for sorbing and nonsorbing
molecules alike. It can give rise to transport behavior
(plume spreading, “tailing” of the solute curve, etc.) that
looks much like sorption nonequilibrium. It is irrelevant
* Corresponding author telephone: (203) 789-7237;fax: (203)789-
7232; e-mail address:
0013-936X/96/0930-0001$12.00/0  1995 American Chemical Society VOL. 30, NO. 1, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1
to bioavailability per se, except that microbial populations
and/or activity may vary within the flow regime. Recent
papers discussing physical nonequilibrium are available
(4-9). We shall also exclude chemisorption involving

covalent bondsaswell as“bound residue” formation, which
is defined as any organic carbon remaining after exhaustive
extraction that results from degradation of the parent
molecule. It is safe to say that the mechanisms governing
sorption rates are not fully established. Thus, this paper
is partly speculative.
Slow Sorption and the Sorption Distribution
Coefficient
Research over the last decade or so has made it clear that
(1) the solid-phase to solution-phase distribution coef-
ficients(K
d
) routinelyarenotmeasuredattrueequilibrium;
(2) the use of equilibrium rather than kinetic expressions
forsorptionin manyfate andeffectsmodelsisquestionable;
and (3) the kinetics of sorption are complex and poorly
predictable.
Inmost cases, theuptakeor releaseoforganics bynatural
particles is bimodal in that it occurs in fast and slow stages.
The division between them is rather arbitrary, but in many
cases it occurs at a few hours to a few days. Hereafter, the
term slow will be used to describe the fraction sorbed or
desorbed in the slow stage. Adjectives such as resistant,
recalcitrant, rate-limiting, slowly reversible, and nonequi-
librium are also used in the literature.
The magnitude of the slowfraction is nottrivial, as many
long-term studies testify. Some recent examples appear in
Table 1. During uptake, the apparent sorption distribution
coefficient (K
d

app
) can increase by 30% to as much as 10-
fold between short contact (1-3 d) and long contact times.
The values listed in Table 1 should not be construed as
predictive nor necessarily representative. Data are sparse,
and our level of understanding is insufficient to make
predictions. During the slow uptake stage, experimentally
observed changes in solution-phase concentration can be
small over periods of many hours and are easily masked
by random analytical errors. Consequently, it has been
common in many routine sorption experiments to falsely
conclude that the system has come to equilibrium after 1
or 2 days.
Desorption likewise often reveals a major slow fraction
(10-96%) following a comparatively rapid release. Histori-
cally contaminated (aged) samples, where contact times
may have been months or years, can be enriched in the
slow fraction owing to partial dissipation or degradation of
more labile fractions before collection. The slow fraction
of some pesticides was found to increase with contact time
in the environment (10).
When thetotalcontaminantpresent must bedetermined
by extractionssuch as in field samples or in spiked samples
where uncertainlosses occurred during an experimentsthe
choice of extraction conditions is important to ensure
complete recovery of the analyte. Extraction methods are
TABLE 1
Recent Examples of Observed Slow Sorption or Desorption in Natural Sorbents
a
Uptake

contact period (d)
long short approx ratio
b
K
d
app
(long)/
K
d
app
(short) slow fraction
b,c
ref
PCE in aquifer sand material 10 1 3 0.67 33
TeCB in aquifer sand material 100 1 10 0.90 33
pyrene in lake sediments 180 3 2 0.50 107
phenanthrene in lake sediments 180 3 2 0.50 107
picloram in various soils 300 7 1.5-3.9 0.33-0.74 126
lindane in subsurface fine sand
(corrected for abiotic hydrolysis)
167 4.2 4 0.74 38
atrazine in soil 22 1 up to ∼0.3 127
metolachlor in peat 30 1 1.4 0.22-0.33
d
55
metolachlor in soil 30 1 1.6 0.31-0.37
d
55
1,3-dichlorobenzene in peat 30 1 1.3 0.14-0.39
d

55
1,3-dichlorobenzene in soil 30 1 1.4 0.19-0.48
d
55
Release
sparging or leaching time remaining slow fraction
b
ref
PCB-contaminated river sediments 7-d continuous removal 0.17-0.45 28
TCE-contaminated subsoil seven 1-d washings or 24 000 column PV
e
0.25-0.27 34
TCE-, PCE-, toluene-, xylene-contaminated soils 14 washings over 7 d 0.48-0.94 128
atrazine-contaminated soil 70-d leaching at 1 PV
e
/d 0.56 20
metolachlor-contaminated soil 70-d leaching at 1 PV
e
/d 0.59 20
naphthalene-contaminated soils 3-d gas purge 0.1-0.5 47
EDB-contaminated soil 10-d batch desorption 0.96 25
naphthalene-spiked soil (3-90 d contact) 3-d gas purge 0.1-0.2 47
simazine-spiked soil 35-d in the field 0.9 27
naphthalene-spiked soil (1-, 7-, 30-d contact) many 2-h to 7-d washings g0.6 15
phenanthrene-spiked soil (7-20-d contact) 10 washings over 178 d 0.62 15
TCE-spiked soil (2.5-, 5.5-, 15.5-mo contact) five 1-d washings 0.10, 0.25,0.45 34
PAHs on urban aerosols 28 d (130 m
3
of moist N
2

) 0.4-0.6 3
atrazine on soil (4-, 12-, or 24-d contact) six 6-d batch desorptions 0.35-0.55 127
a
PCE, tetrachloroethene; TeCB, 1,2,4,5-tetrachlorobenzene; picloram, 4-amino-3,5,6-trichloropicolinic acid; lindane, γ-1,2,3,4,5,6-hexachlorocy-
clohexane; PCB, polychlorobiphenyl congeners; EDB, 1,2-dibromoethane; TCE, trichloroethene; atrazine, 2-chloro-4-ethylamino-6-isopropylamino-
1,3,5-triazine;metolachlor,2-chloro-
N
-[2-ethyl-6-methylphenyl]-
N
-[2-methoxyethyl]acetamide;simazine,2-chloro-4,6-bis(ethylamino)-1,3,5-triazine].
b
Listed as estimates from graphs and tables in original work and may be rounded.
c
Slow fraction ) 1 -
K
d
app
(short)/
K
d
app
(long).
d
Concentration
dependent.
e
PV, column pore (void) volumes.
2
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 1, 1996

commonly validated with freshly spiked soil samples.
Unfortunately, validation is seldom performed on aged
samples that are enriched in resistant fractions. Hot
extraction with water-miscible solvents has been shown to
be superior for extracting resistant fractions compared to
traditionalmethods like solvent-shake atroomtemperature,
purge-and-trap, and Soxhlet techniques (11-15). It is well
known that recovery by nonmiscible solvents (e.g., hexane)
decreases with aging (16, 17). Supercritical CO
2
extraction
has not been fully investigated; but some reports indicate
that, even inthe presenceof organicsolvent modifiers which
increase its solvation power, it is inferior to hot solvent for
extracting resistant fractions (18, 19). In some studies, the
slow fraction is likely to have been underestimated due to
incomplete recovery. This can lead to erroneous conclu-
sions when some process of interest is being measured
against the mass of contaminant believed to be present.
For example, one may deem that biodegradation is suc-
cessful when actually loss of only the labile fraction has
been evaluated.
Since K
d
is time-dependent on a scale well beyond that
of most laboratory sorption experiments, the true extent of
sorption is known for just a few systems. Many reported
K
d
values represent principally the fast component rather

than overall sorption (20). Free energy correlations involv-
ing K
d
are thus brought into question. For example,
molecular structure-K
d
relationships rest on the assump-
tion of equilibrium or at least that all compounds have
attained the same fractional equilibrium. However, sorp-
tion rates can depend greatly on molecular geometry and
electronic properties. This is clearly evident in regard to
diffusionthrough a viscous mediumsuchasorganic matter
or a pore structure (see below). Moreover, Brusseau and
co-workers (21, 22) showed that a mass transfer coefficient
determined from soil column elution was inverse log-
linearly related to the octanol-water partition coefficient
for closely related compounds and that polarity in the
molecule caused an additional decline in the mass transfer
coefficient. Further research is needed to determine to
what degree nonequilibrium can influence free energy
relationships of sorption.
In general, the sorption equilibrium assumption in fate
andeffectsmodels isinvalid when the fate/transportprocess
of interest occurs over comparable or shorter time scales
thansorption. Given that, one canimaginemany processes
that might bemore sensitivetokinetic thanthermodynamic
sorption behavior; for example, uptake by an animal that
comes into brief or intermittant contact with the soil. The
equilibrium assumption has been found to fail in a growing
number of cases. There are numerous examples of long-

term persistence in soils of intrinsically biodegradable
compounds even when other environmental factors are
not limiting for microbial growth (2, 23-25). These are
backed by a laboratory study showing that aging of the
soil-contaminant mixture prior tothe additionof microbes
reduced bioavailability (26) and by a field study showing
that aging reduced herbicidal activity (27). Also, the fact
that bioremediation of soil often levels off after an initial
rapid decline [e.g., PCBs (28) and hydrocarbons (29)] is
believed to be due mostly, if not solely, to the unavailability
of a resistant fraction.
Finally, nonequilibrium sorption affects the hydrody-
namic transport of contaminants by causing asymmetrical
concentration vs time (elution) curves. In relatively
homogeneous soil columns, this asymmetry is exhibited
by early breakthrough, a decrease in peak breakthrough
concentration, breakthrough front tailing,and elution-front
tailing (5); whereas, nonsorbing solutes like
3
H
2
O or Cl
-
typically show little or no evidence of asymmetry. In more
heterogeneous media as exists in the field, the effect of
nonequilibrium sorption on transport is less distinct.
Vadose(30) and saturatedzone (4) studies reveal a decrease
in velocity and aqueous-phase mass of the contaminant
plume,relative to a nonsorbing tracer, withincreasing travel
time or distance. While this is consistent with a time-

dependent increase in K
d
app
due to rate-limiting sorption,
an interpretationis complicatedby permeability variations
in the flow field (physical nonequilibrium) as well as
variability in K
d
itself within the substrata (7, 8). Both of
these can lead to tailing via plume spreading. The relative
importance of sorption nonequilibrium and physical non-
equilibrium is likely to depend greatly on the heterogeneity
of the flow field and the type of particles that make it up.
Mechanisms
Possible Rate-Limiting Steps. The potential causesofslow
sorption are activation energy of sorptive bonds and mass-
transfer limitations (molecular diffusion). Sorption can
occurby physical adsorption on a surfaceor by partitioning
(dissolution) into a phase such as natural organic matter
(NOM). The intermolecular interactions potentially avail-
able to neutral organic compoundssvan der Waals (dis-
persion), dipole-dipole, dipole-induced dipole, and hy-
drogen bondingsare common to both adsorption and
partitioning. In solution these forces are fleeting. For
example, themeanlifetimeof theH
2
O‚‚‚NH
3
hydrogen bond
is 2 × 10

-12
s (31). Adsorption to a flat, unhindered, and
rigid surface is ordinarily unactivated or only slightly
activated and so should be practically instantaneous (32).
Desorption, however, is generally activated. The kinetic
energy of desorption (E
des
*) is the sum of the thermody-
namic energy of adsorption (Q)si.e., the depth of the
potential energy wellsand the activation energy of adsorp-
tion (E
ad
*) (32). A physisorbed molecule where E
ad
* ) 0
and Q e 40 kJ mol
-1
will have a lifetime on the surface of
e∼10
-6
s (32). For these reasons, most small compounds
might be expected to adsorb and desorb practically
instantaneously at the microscale. However, there may be
situations in which E
ad
* or E
des
* is much greater. Large or
longmoleculesthatcan interact simultaneously at multiple
points can be more difficult to desorb. There may be steric

hinderance to desorption or adsorptionsan ink bottle-
shaped pore is an example. Lastly, there may be a
cooperative change in the sorbent induced by the sorbate
that makes Q larger, as occurs in substrate binding to
enzymes. We must be open to these possibilities for
pollutant molecules in highly heterogeneous systems like
soil particles. It is noteworthy thatevensmall, weaklypolar
molecules like halogenatedmethanes,ethanes,and ethenes
exhibit slow sorption/desorption in soils (25, 33, 34). The
thermodynamic driving force for their sorption is hydro-
phobic expulsion from water, but their main interaction
with the surface is only by dispersion and weak dipolar
forces.
Most researchers, nevertheless, attribute slow kinetics
to some sortofdiffusion limitation. This is almost certainly
true because sorbing molecules are subject to diffusive
constraints throughout almost the entire sorption/desorp-
tion time course because of the porous nature of particles.
Diffusion is random movement under the influence of a
VOL. 30, NO. 1, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
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concentration gradient (35). Particles are porous by virtue
of their aggregated nature and because the lattice of
individual grains in the aggregate may be fractured.
Figure 1 is a conceptualization of a soil or sediment
particle aggregate showing possible diffusion processes.
Toreachall sorptionsites,diffusing moleculesmusttraverse
bulk liquid, the relatively stagnant liquid “film” extending
from the solid surface (film diffusion), pores within the

particle (pore diffusion),andpenetrablesolidphases (matrix
diffusion). Diffusion coefficients of organic molecules can
be expected to decrease along that same order, but except
forbulkaqueousdiffusion,fewdataare availableforrelevant
natural particle systems. The observed kinetics in any
region of the sorption vs time curve will reflect one or more
of these diffusive constraints, which may act in series or
parallel.
Themixing that takesplace in mostexperimentsensures
that bulk liquid or vapor diffusion is not rate-limiting.
Likewise, film diffusion is probably not rate-limiting. Film
diffusion of inorganic ions is reduced or eliminated with
vigorous mixing(36). Weberand Miller(37) and laterMiller
and Pedit (38) concluded that in well-mixed batch systems
film resistance of lindane and nitrobenzene on subsurface
materials was insignificant compared to intraparticle dif-
fusion, but may have been significant for nitrobenzene in
columns (39). Film diffusion is potentially rate-limiting
for the initial fast stage of sorption; but it is not likely to be
important in the long-term phenomena we have been
considering.
This leaves pore diffusion and matrix diffusion as likely
rate-limiting steps in slow processes. Diffusion in pores
canoccur inpore liquids oralong porewallsurfaces. Liquid
and surface diffusion may act concurrentlyand are difficult
to distinguish (40, 41). A model of hydrophobic sorption
to mineral surfaces (42) postulates that sorption occurs on
or in “vicinal” watersthe interfacial region consisting of
relatively ordered sorbed water moleculessrather than on
the bare surface itself. If this model is correct, liquid and

surface diffusion practically merge. Surface diffusion is
expected toincreaseinrelativeimportance: (i)invery small
pores wherefluids aremore ordered andviscous,andwhere
the sorbate spends a greater percentage of time on the
surface; (ii) at high surface concentrations. Surface dif-
fusion was invoked for porous resins (43) and activated
carbon (44, 45) because intraparticle transport appeared
to be faster than could be accounted for by liquid diffusion.
A surface diffusion model was used to simulate sorption-
desorption of lindane with some success (38). However,
it has been argued that surface diffusion is insignificant on
soil particles because of the discontinuity of the adsorbing
surface (33), if not the low mobility of the sorbate itself (46).
Kinetic Behavior. Proposed mathematical kinetic mod-
els include first-order, multiple first-order, Langmuir-type
second-order (i.e., first-order each in solute and “site”),
and various diffusion rate laws. The equations and their
incorporation into the advection-dispersion model for
solute transport are available in several good reviews (5, 6,
40). All except the diffusion models conceptualize specific
“sites” to/from which molecules may sorb in a first-order
fashion. Most sorption kinetic models fit the data better
by including an instantaneous, nonkinetic fraction de-
scribed by an equilibrium sorption constant . None of the
models are perfect, although diffusion models are more
successful than first-order models when they have been
compared (20, 41). First-order kinetics are easier to apply
to transport and degradation models because they do not
require knowledge about particle geometry. Fit to a
particular rate law does not by itself constitute proof of

mechanism. Nonmechanistic modelshavebeen employed
also. Pedit and Miller (41), on considering the inter- and
intraparticle heterogeneity of soil, modeled the months-
long uptake of diuron by a stochastic model, which treated
sorbate concentration (K
d
) and first-order rate constant as
continuously distributed random variables.
We call attention to three features of slow sorption
kinetics that, if fully explained, could lead to a deeper
understanding of the causesofslow sorption. First, a single
rate constant often does not apply over the entire kinetic
part of the curve (20, 46-48). In the elution of field-aged
residues of atrazine and metolachlor from a soil column,
a model with a single diffusion parameter underestimated
desorption at early times and overestimated desorption at
late times (20). Mass transfer coefficients obtained by
modeling elution curves depend on the contaminant
residence time in the columnsi.e., the flow rate (49). In
desorption studies, plots of the logarithm of fraction
remaining vs time tend to show a progressive decrease in
slope,indicating greaterandgreaterresistance todesorption
(47). Hence, desorption in natural particles seems to be,
kinetically speaking, a continuum. On considering that
soil may be a continuum of compartments ordered by their
desorption rateconstants,Connaughton et al. (47)modeled
the increasing desorption resistance of naphthalene by
assuming that the rate constant is distributed according to
a statistical Γ densityfunction,itself having two parameters.
The intrinsic heterogeneity of soils on many levelsse.g.,

polydisperse primaryand secondaryparticles,awiderange
FIGURE 1. Schematic of a soil particle aggregate showing the
different diffusion processes. Natural “particles” are usually
aggregates of smaller grains cemented together by organic or
inorganic materials. Porosity is due to spaces between grains and
fissures in individual grains.
4
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ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 1, 1996
of pore sizes, and spatial variations of mineral and organic
components on the micro-scale, etc.sis fully compatible
with continuous kinetics. An underlying problem in
studying slow sorption is that we are never dealing with a
homogeneous sorption/diffusion medium.
Second, the slow fraction (S
sl
) is inversely dependent,
often markedly, on the initial applied concentration, C
o
(14,46, 50-52), meaningthat itassumes greater importance
at lower concentration. Equilibrium considerations alone
may partly explain this: when the sorption isotherm is
nonlinearsthat is, when N in the Freundlich equation (C
s
) K
F
C
a
N
, where C

s
and C
a
are the sorbed and aqueous
concentrations),isless than unitysintraparticleretardation
will increaseastheconcentrationinsidetheparticledeclines
(38, 46, 48). However, in some studies it appears that the
concentration dependence is steeper than expected based
on equilibrium nonlinearity. In studies of TCE vapor
sorption to various porous particles at 100% relative
humidity, Farrell and Reinhard (46) showed that the slow
fraction remaining after N
2
gas desorption was highly
concentration-dependent and not well simulated by con-
sidering only equilibrium nonlinearity. In batch experi-
ments of a soil containing 1.26% OC (50), an empirical
nonlinear expression was used to relate “slow fraction”
(amount remaining after desorption to infinite dilution for
5 d) to initial concentration (S
sl
∝ C
o
n
). The exponent n
was found to be 0.90 for PCE, 0.73 for 1,2-dibromo-3-
chloropropane, and 0.49 for TCE. The isotherm of TCE in
the same soil was linear (N ) 1.01) (53). While the
Freundlich parameters were not measured for the other
two compounds, experience shows (25, 53, 54) that such

compounds give linear or slightly nonlinear isotherms (N
> ∼0.9) in soils that have a substantial amount of NOM.
Thus, for TCE at least, the concentration dependence of
the slow fraction is greater than the fast fraction. In a study
of metolachlor and 1,3-dichlorobenzene in two soils (55),
N was greater for sorption of a fast fraction (1 d contact
time) than a slower fraction (thedifference between 30 and
1 d contact times). This means that the slower fraction
becomes increasingly dominant as the total concentration
declines.
Third, sorption is often kinetically hysteretic, meaning
that the slow state appears to fill faster than it empties.
Further research must be done to validate this. Many
examples exist of apparent “irreversible” sorption of some
fractionsor at least exceedingly long times to achieve
desorptionsfollowing relatively short contact times (1, 5,
15, 56-59). Hysteresis may be caused by experimental
artifacts or degradation (1, 5, 56). Also, to fairly assess
hysteresis from the desorptive direction requires that
samples be at true equilibrium.
Kan and co-workers (15) sorbed naphthalene and
phenanthrene to a sediment (0.27% NOM). While uptake
appeared to reach equilibrium in a few days, successive
desorption stepssusually lasting 1-7 d and totalling as
long as 178 dsreleased less than 40% of chemical, even
from samples sorbed for only 1 d (Table 1). Good mass
balance was obtained upon soil extraction with CH
2
Cl
2

at
45 °C. Miller and Pedit (38) examined sorption of lindane
to a subsurface soil corrected for dehydrohalogenation
reactions. They found that an intraparticle diffusion model,
whose parameters were obtained from uptake, could
account for mostbutnot all of the hysteresis observed upon
sequential desorption. We note that the sorbed concen-
trations declined by only 2- or 3-fold after the three-step
desorption, and the model fit seems to worsen with step.
Hadfurther stepsbeen performedto uncover more resistant
fractions,itis possiblethatevenless of the hysteresis would
have been accounted for. Harmon and Roberts (48) found
theeffectivediffusioncoefficient of PCEin aquifersediment
to be 2-4 times smaller in the desorptive direction. They
cautioned that the sorptive diffusion coefficients were
obtained by others using a different technique. Inspection
oftheir datarevealsthat thetail endof thedesorption curves
tends to flatten out, indicating a substantial fraction of PCE
(∼20% of initital) that is overpredicted by the model, i.e.,
desorbs at a much slower rate.
The above three features of slow sorption suggest but
do not prove a departure from regular Fickian diffusion.
Fickian diffusion is symmetrical with respect to sorption
and desorption, and the diffusion coefficient is concentra-
tion-independent provided the sorbate does not alter the
sorbent properties (35). Further careful experiments are
needed to confirm whether sorption in soils truly deviates
from Fickian diffusion. If it does, one implication is that
the making/breaking of bonds may play a role along with
molecular diffusion insorption/desorption rate limitations,

even for classically “noninteracting” compounds like
aromatic hydrocarbons and chlorinated solvents. The
behaviorsabove are inlargemeasureasignature of sorption
to sites having a distribution of energies. If interaction
with an array of sites is responsible for sorption in the slow
state the following might be expected: (i) a distribution of
desorption rate constants corresponding to a distribution
of activation energies; (ii) inverse concentration depen-
dence of the slow fractionsat low applied concentration,
the higher energy sites (which are more important relative
tothe fast state) arepopulatedpreferentially; and(iii)kinetic
hysteresis since the activation energy of desorption is
normally greater than that of sorption from/to a specific
site.
We may better understand the meaning of these
observations in the context of the two models that have
been put forth as the most likely causes of slow sorption
in natural particles: the organic matter diffusion model
(OMD) and the sorption-retarded pore diffusion model
(SRPD). They are shown pictorially in Figure 2 and are
discussed below.
Organic Matter Diffusion. The OMD model postulates
diffusion through NOM solids as the rate-limiting step (5,
21). This is intuitively satisfying given the abundant
thermodynamic evidence that partitioning (dissolution) in
NOM is the primary mechanism of sorption when NOM
and water are sufficiently abundant (54). NOM can exist
as surface coatings or discreet particles. Supporting the
OMDmechanismare the following: (1) inverse correlations
between mass transfer parameters and NOM content (20,

28, 50, 60, 61); (2) organic cosolvents increase the rate in
accord with their ability to ‘swell’ NOM (62); (3) inverse
linear free energy correlations between rate constant and
K
d
or the octanol-water partition coefficient K
ow
(3, 21,
22); and (4) a decrease in rate for polar molecules capable
of hydrogen bonding to acceptor groups within NOM (22).
Yet these results are also consistent with SRPD if the active
sorbent material in pores is taken to be NOM coatings on
pore walls. Moreover, OMD is at odds with the observation
of slow sorption in zero or extremely low NOM materials
(33, 46, 63).
We may ask: Are diffusion length scales and diffusion
coefficients (D) in natural particles consistent with NOM
VOL. 30, NO. 1, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
5
as the diffusive medium? Desorption of resistant field-
aged pesticides (EDB, atrazine, metolachlor) in soil show
little particlesize dependencedowntotheclay-size fraction
(20, 25), suggesting that the upper limit diffusion length
scale is on the order of the clay particles (10
3
-10
2
nm). If
a single effective diffusivity (D

eff
) applies over this radial
length, D
eff
would equal ∼10
-17
cm
2
/s or less. Desorption
of PCBs from river sediments (28) also showed no particle
size effects and indicated diffusion length scales of ∼30
nm, corresponding to D
eff
of 10
-20
-10
-21
cm
2
/s. The true
dimensions of NOM are essentially unknown, but thick-
nesses of 30-1000 nm are not unreasonable for NOM
coatings or discreet NOM particles.
In regard to D
eff
values, the obvious analogy to NOM is
synthetic organic polymers. The polymer-phase concept
of humics is replete in the literature. Diffusion in polymers
occurs by either a place change mechanism, in which
movement is accomplished by cooperative interchange of

positionof polymersegmentsand thepenetrating molecule,
or by a defect mechanism where the penetrant may jump
between lattice defects, voids, pores, etc. (64). Polymers
are said to have glassy (condensed, rigid) or rubbery
(expanded, flexible) structures with respect to the order
andcohesiveforces of thepolymerchains. Likewise, humic
substances are described as having condensed and ex-
panded regions (65).
Choosing a polymer to model NOM is difficult because
NOM in situ is expected to be highly variable in its
properties, even within the same contiguous material.
Furthermore, structure of and sorption to NOM can be
strongly affected by soil minerals (66, 67). Attempts have
been made to estimate the cohesive forces holding the
humic polymer chains together in relation to their effects
on the diffusivity and solubility of sorbate molecules (28).
The true valuesswere it possible to determine themsare
likely to cover a wide range. ReportedD values in polymers
at 25-30 °C for a molecule like CCl
4
having a diameter of
0.55 nm range over many orders of magnitude, from 10
-7
cm
2
/s in rubbery polymers (polyethylene) to 10
-17
cm
2
/s

in glassy polymers (polyvinyl chloride) (64, 68). Diffusivity
is sensitive to the size and shape of the penetrant, much
more so for glassy than rubbery polymers. One might
expect a molecule to experience large changes in diffusivity
as it moves between expanded and condensed regions of
NOM. Accordingly, Carroll et al. (28) suggest that the
bimodal desorption vs time curves of PCBs from sediments
aredue todesorption fromthesetwo typesof phases. Future
work is needed on determining organic compound diffu-
sivities in NOM particles and on finding appropriate
polymer models.
Diffusion kinetics in polymers is widely variable de-
pending on polymer structure, particle size distribution,
diffusant structure, diffusant concentration, temperature,
andthe history ofexposure (64,69, 70). Mixturesof polymer
sphere sizes can lead to bimodal diffusion curves (71, 72).
Since the diffusion rate is inversely related to the square of
the radius, the proportion of fast and slow phases of the
uptake or release curve depends on the size distribution.
Obviously, the dimensions of NOM in a given soil will be
truly diverse.
Relatively high diffusant concentrations can cause
polymer swelling or crazing as the diffusant front advances
(69). These changes affect both the compound’s solubility
(partition coefficient) and diffusivity, which in turn dictate
the shape of the kinetic curve. Bimodal curves can result.
Pollutant concentrations in the environment may some-
times be high enough (e.g., a chemical spill) to swell or
soften NOM. Cosolvents can do the same thing. Methanol
cosolvent increased the desorption rate constant of diuron

and several PAHs (62).
Usually though, we are dealing with dilute contaminant
and no cosolvent. Sorption under dilute conditions in
rubbery polymers generally is linear and obeys Fick’s
lawsthat is, D is concentration-independent, and mass
transfer is symmetrical with respect to the forward and
reverse directions and proportional to the square root of
time (64, 69, 73). Sorption in glassy polymers on the other
hand is anomalous in that it is typified by nonlinear (N <
1) isotherms, concentration-dependent D, and a tendency
toward bimodal kinetics and sorption-desorption hyster-
esis (64,68,70, 72). This isreminiscentof sorption/diffusion
behavior of many compounds in soils.
Anomalous behavior in glassy polymers has been
attributed to dual-mode sorption. This was first proposed
FIGURE 2. Schematic of two models for slow sorption. (a) Organic
matter diffusion (OMD),illustrating diffusion through a rubberyphase
A,diffusionthrough a morecondensedglassyphaseB, and
ad
sorption
in a “Langmuir site” C (see text). (b) Sorption-retarded pore diffusion
(SRPD). Retardation by rapid-reversible sorption to pore walls, and
“enhanced adsorption” in pores of very small diameter due to
interaction with more than one surface.
6
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 1, 1996
for gaseous molecules like CO
2
and CH

4
(70) and later for
small organic molecules (68, 71, 72). Dual-mode sorption
is the sum of (a) normal linear partitioning taking place in
the bulk of the polymer and (b) a hole-filling mechanism
in which the incoming molecules undergo Langmuir-like
adsorption in voids internal to the polymer matrix. The
latter is the cause of isothermnonlinearity and non-Fickian
tendencies. Linear sorption can be restored by conversion
of the glassy state to the rubbery state by increasing the
temperature above the glass transition point (T
g
) or by
softening with organic solvents. The exact nature of the
voids is presently unknown. Solid-state
31
P-nuclear mag-
netic resonance spectroscopy showed that mobile and
immobile sorbed forms of tri-n-butyl phosphate exist in
glassy polystyrene (74). The relative mobility of the
immobile forms appeared to span a wide range.
Dual-mode sorption in NOM may rationalize qualita-
tively some behaviors of contaminants in natural parti-
clessnamely, nonlinear isotherms, competitive sorption,
and kinetichysteresis. Isotherms areoftennonlinear when
a sufficiently wide solute concentration range is used (37,
39,55, 75-77). Examplesinclude hydrophobiccompounds
in a peat soil composed almost entirely (93%) of NOM (55).
It might be expected that isotherms would linearize at high
concentrations as the adsorption sites became filled, but

this would depend on how the sites were distributed in
energy. Investigatorshave alsoshowncompetitivesorption
between nonpolar compounds in suspensions of soils (53,
76), the mentioned peat (53), and humic-coated clay (66).
Competitive sorption clearly indicates some measure of
site specificity (53, 76).
As shown in Figure 2, we may envision NOM as a bulk
partition medium consisting of rubbery (A) and glassy (B)
regions. Dispersed in the glassy regions are adsorption
sites(C)ofvarious energies, analogous to the voidsofglassy
polymers. In agreement with the dual-mode model,
phenanthrene isotherms became more linear with increas-
ing temperature in soil and shale samples where NOM was
believed to be the predominant sorbent (75). This is
consistent with a transition to a more rubbery state. The
nature of the adsorption sites is speculative. They could
be some type of inclusion complex between the guest
pollutant molecule and host subunit(s) on the NOM
macromolecule. Soil humic acid has condensed polyaro-
matic regions, even after extraction and reconstitution to
a particulate form (78, 79). It has been suggested that
polyaromatic structures provide adsorption sites (75).
Although it is far from certain at this time that the dual-
mode mechanism plays a role in slow kinetics, the
aforementioned results of ours (55), showing a decrease in
the Freundlich exponent N with time in NOM, are at least
consistent with it. Theexistence of high-energyadsorption
sites could account for kinetic hysteresis. It might be
expected that such sites would fill faster than they would
empty. Thus, it is plausible that desorption becomes at

some point rate limited by release from these sites, while
sorption is principally rate limited by diffusion through
bulk NOM. The nonlinear relationship between S
sl
and C
o
discussed above is also plausibly attributed to the presence
of sites.
Sorption-Retarded Pore Diffusion. The SRPD model
(Figure 2) postulates the rate-limiting process to be mo-
lecular diffusion in pore water that is retarded, chromato-
graphic-like, by local sorption on pore walls (80) (Figure 2).
Walls may or may not be composed of NOM. Assumptions
by most modelers are that local sorption is instantaneous,
particles are uniformly porous, and sorption parameters
K
d
and D
eff
in the pore are constant. According to the SRPD
model, rates are expected to be inversely dependent on the
square of the particle radius, on the tortuosity of pores
(bendingandtwisting,interconnectivity,presence of dead-
end pores), on the constrictivity (steric hindrance) in the
pores, and on the K
d
. The inverse dependence on K
d
does
not distinguish SRPD from OMD.

For natural particles, observations that point to SRPD
include faster rates after particle pulverization, which
reduces pore pathlength (25, 33,50), and afteracidification,
which was suggested to disagregate grains by dissolving
the inorganic oxide cements that hold the aggregates
together (50). Correlation of rate with particle size is only
qualitative at best. In one case where a rough correlation
was found (80), experiments took place over a few hours
at most. In another case (33) where coarse aquifer sand
particles equilibrated PCE and TeCB generally faster than
fine particles, the particles were calcite-cemented ag-
gregates thathad considerableinternalporosityand surface
area (81). In many systems, the particle size dependence
of desorption is altogether absent (20, 25, 28, 46, 82). For
example, desorption of field-aged pesticides in soil (20, 25)
and PCBs in river sediments (28) was not related to the
nominal particle radius down to the clay size fraction,
suggesting that the length scale of diffusion is proabably
less than 100 nm. The absence of size dependence might
be rationalized by assuming that most of the porosity exists
in an outer shell that is of similar thickness among size
fractions. Anotherpossiblereasonisthat sorption capacity
may not be uniformly distributed within the aggregate. Ball
and Roberts (33), for example, found that K
d
of PCE and
TeCB varied markedly among different size fractions and
a magnetically separated fraction of an aquifer material.
But in a study of desorption of TCE from silica with
monodisperse particle sizes and narrow pore size distribu-

tions, investigators found no particle size dependence (46).
Tortuosity and constrictivity are difficult to evaluate.
Both areexpectedto varyinverselywith pore size. However,
in a silica pore, diameters ranging from 6 to 30 nm had
little effect on TCE desorption (46). It is possible that this
effect shows up only in pores that are smaller than 6 nm.
Theanalyticaltoolsformeasuring nanoporecharacteristics
in natural materials are undeveloped, and the theoretical
foundations aretoo weak to incorporate them intothe SRPD
model (33, 46, 48). Thus, research is needed on character-
izing the geometry and spatial distribution of pores. The
kinetic continuum discussed earlier could be rationalized
by a heterogeneous pore structure in which there is a
distribution of diffusivities within the particle. Thus, we
may envision pores that fill and empty quickly, along with
those that do so slowly.
A potentially important influence on constrictivity in
the pore is the viscosity of water. Polar minerals have one
or more layers of water strongly sorbed on their surfaces
(83). Viscosity measurements of colloidal silica particles
in water indicate there is a monolayer more or less
immobilized on the surface (84). The water contained in
a pore of a few Angstroms in diameter may be ice-like and
therefore greatly restrict solute diffusion. Molecular mod-
eling can potentially give insight on the structure of water
in nanopores.
Thelong-term desorptionofEDBfromsoil towatercould
not be modeled by SRPD without invoking enormous
VOL. 30, NO. 1, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9

7
tortuosity or constriction in pores (25). Likewise, D
eff
’s for
desorption of alkanes and PAHs from urban atmospheric
particles were 10
6
times smaller than expected for sorption
retarded gaseous diffusion in pores (3). One rationale is
affordedby rejecting theSRPDassumptionof instantaneous
local equilibrium in the pore. For instance, a PCB required
hours to desorb from dissolved commercial humic acid
(85). From the standpoint of solutes, dissolved humic acid
macromolecules may well represent the smallest or most
penetrablehumic materialsexisting on wall surfaces. Also,
substituted benzenes required hours to desorb from
surfaces of alkyl-modified, nonporous silica gel particles
(86). Such slow stepwise desorption rates could strongly
retard transport through a pore compared to the instan-
taneous case. The mathematics of diffusion in systems
containing one diffusive medium in another have been
discussed (35).
An alternative explanation for the extraordinarily small
D
eff
has been offerred (46, 87). According to this enhanced
adsorption hypothesis, asthepore sizedecreases toroughly
theadsorbate diameter, the calculatedinteraction potential
increases up to 5-fold compared to the single surface case
owing to multipoint interaction of the adsorbate with pore

walls (Figure 2). Furthermore, since the pore volume is
small compared to the wall surface area, the adsorbate
spends less and less time in pore solution, ultimately being
restricted to diffusion along the surface, which may be
intrinsically slower. Farrell and Reinhard (46, 87) equili-
brated TCE vapors with a column of porous silica particles
at100% relativehumiditywhereall microporesare expected
to be filled with water. Desorption with a stream of
humidified N
2
proceeded in two distinct phases and was
incomplete after purging times lasting weeks. The silicas
behavedsimilarly tonatural sorbents inthat(1) thediffusion
length scale was not the nominal particle radius and (2) the
slow fraction was less than linearly related to initial TCE
concentration, which was attributed to a limited sorption
capacity in high-energy micropore sites. The authors
suggest that microporosity gives rise to both isotherm
nonlinearitysindicative of a distribution of site ener-
giessand slow desorption. Here again, we see the con-
sequences of energetic heterogeneity in the sorbent that
was referred to earlier in a general sense and in the context
of OMD. Cautioniscertainlycalledfor in interpreting these
experiments. Condensation of TCE in pores during the
equilibration period cannot be ruled out since vapor
concentrations were close to saturation. Removal of TCE
in a condensed phase from a pore may be slower. In
apparent contradiction of enhanced adsorption is the fact
that diffusion of small molecules through the micropores
(5-7 Å) of synthetic aluminosilicate zeolites is remarkably

faststhe time to reach equilibrium of small molecules like
hexane (88) and TCE (89) in micron-size particles being on
the order of 10
2
min (D ∼ 10
-12
cm
2
/s).
A form of pore diffusion that deserves more attention
is clay interlayer diffusion. Hydrated metal ion-exchanged
clays (e.g., with Ca
2+
) do not extensively sorb hydrophobic
molecules, but neither are such compounds excluded from
hydrated interlayer spaces. Na-montmorillonite in water
exhibited uptake of TCE lasting over 25 d (90). Desorption
of atrazine from some Ca
2+
-smectites revealed formation
of a tightly bound fraction (91). Smectites exchanged with
tetraorganoammonium cations haveamuch higher affinity
forhydrophobiccompounds (ref92and references therein),
but their kinetics have not been studied. Some evidence
suggests that clay interlayers are not important. Mont-
morillonite formed a much smaller fraction of slowly
released TCE than either silica or microporous glass beads
(46). Steinberg et al. (25) observed the lowest field residues
of EDB in the clay size fraction.
ConcludingRemarksRegarding Mechanism. Itisquite

likely that both OMD and SRPD mechanisms operate in
the environment, often probably together in the same
particle. OMD may predominate in soils that are high in
NOMand low inaggregation,while SRPDmay predominate
in soils where the opposite conditions exist. But this has
not been established. Slow desorption from an organic-
free silica, a substance so closely related to soil minerals,
is strong evidence that the mineral fraction is important.
Resolving the individual contributions of OMD and SRPD
in natural materials constitutes a challenge to future
investigators. We have seen that both mechanisms offer
the potential for high-energy adsorption sites to play a role.
These sites may be more rate-limiting in the desorptive
direction than the sorptive direction. Further research is
critical in this area. Evidence indicates that a decrease in
the rate constant occurswith increasing molecular size and
hydrophobicity. However, this is consistent with all of the
mechanisms discussed.
Significance of Slow Sorption Mechanism to
Bioavailability and Remediation
The bioavailability of chemicals in soil to microbes, plants,
and animals is important from the perspective of reme-
diation and risk assessment. Ex situ or in situ cleanup of
soil requires mass transport of contaminants through the
materials, which in turn depends on sorption kinetics.
Microbes take up substrates far more readily from the
fluid than the sorbed states (89, 93-96). Thus, it is no
surprise that aged chemicals are resistant to degradation
compared to freshly added chemicals (25, 27, 97, 98) and
that degradation of freshly added chemicals often tails off

to leave a resistant fraction (26, 98-100). Bioavailability
has been called a major limitation to complete bioreme-
diation of contaminated soils (29, 101). The soil-
contaminant-degrader system is dynamic and interde-
pendent. A mechanistic-basedbiodegradationmodelmust
be built on the mechanism(s) governing sorption/desorp-
tion, in addition to the biological mechanisms governing
cell growth and substrate utilization in the matrix. A
number of groups are now developing sorption-degrada-
tion kinetic models (26, 102-106). Both diffusion and two-
box (equilibrium and first-order kinetic compartments)
sorption concepts have been explored.
The bioavailability of pollutants to wildlife and humans
is also an area of critical importance. Pollutants can be
taken up in pore water, by dermal contact, by particle
ingestion, or by particle inhalation. The dynamics of
sorption are not currently incorporated into exposure and
risk models for organics. Availability in most cases is
assumed to be 100% (107). Recently, the following have
been demonstrated: (1) the time between spiking and
testing affects bioavailability (2, 108); (2) the kinetics of
desorption control bioaccumulation of historical contami-
nation (e.g., PAHs in benthic animals; 109); and (3)
historically contaminated soils are less toxic and/or lead to
lower body burdens than equivalent amounts of spiked
soils (110, 111).
In order to model bioavailability, it is crucial that we
understand sorption kinetics and the factors that influence
8
9

ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 30, NO. 1, 1996
rates under the conditions of exposure. Take particle
ingestion, for instance. The intestines of warm-blooded
animals are often at higher temperature than the soil being
ingested. Molecular diffusion through a viscous medium
like NOM and desorption from a surface are activated
processes and hence temperature sensitive. For example,
theapparentactivation enthalpy for desorption of historical
residues of EDB from soil into water was 66 kJ/mol,
corresponding to a 7-fold rate increase from 25 to 40 °C
(25). The application of heat increased the rate of desorp-
tion of PCBs from river sediment and reduced the resistant
fraction (28). Also, there is evidence that pH is important.
Acidification of a soil suspension to pH <∼2 accelerated
desorption of the slow-desorbing fraction of several ha-
logenated aliphatic hydrocarbons. The amounts desorbed
in 1 h ranged from 13 to 80% of the slow fraction (50), many
times more than the control at natural pH. The human
stomach can be highly acidic (pH 1.5-2) at times (112).
Soilingested by birdsissubjected to grindinginthegizzard,
which may release slow-desorbing contaminants if pore
diffusion isimportant. Furtherworkis needed to determine
what physiological conditions in both the digestive and
repiratory tracts may impact desorption, and further work
is needed on the design of experiments to accurately
simulate such conditions in the laboratory.
Vapor and water extraction methods (pump-and-treat),
which are widely used in remediation, are limited in part
by physical nonequilibrium and sorption nonequilibrium
(113-115). These processes both cause tailing of the

contaminant plume, whichincreases the time invested and
thevolumeofsparge air or waterneeded to achieve cleanup
(113, 115-117). Moreover, they act to resume contamina-
tion if pumping is ceased before all the contaminant is
removed (rebound) (118-120). Ways of experimentally
separating out the contributions of physical and sorption
nonequilibrium must be sought.
Experience is proving that the constraint of slow
desorption has to be overcome to achieve complete
remediation (29). We may consider the following conceiv-
able approaches to promoting desorption from the slow
state: (1) addition of biological agents capable of reaching
remote molecules; (2) application of heat; (3) addition of
chemical additives that displace the contaminant or alter
the soil structure; and (4) physical methods that alter the
soil structure.
Since cells, being g0.2 µm, are too large to fit in
nanopores orwithin theNOM matrix,it wouldseem unlikely
that strainsexist which candirectlyattackremote molecules.
Guerin and Boyd (106) isolated a Pseudomonad that,
compared to another degrader, appeared to enhance the
desorption of naphthalene by providing a steep concentra-
tion gradient at the particle surface. Some organisms,
especiallyfungi,metabolizecontaminantswith extracellular
enzymes. Enzymes may also be added to soil to remediate
it. However, enzymes are many times larger than con-
taminant molecules and probably diffuse far more slowly,
if at all, through micropores or NOM to reach resistant
molecules. Moreover, sorption of enzymes may reduce
their activity (121).

As mentioned, molecular diffusion through NOM and
desorption from high-energy sites are expected to be
strongly temperature dependent. Thermal desorption is
already in use in various remediation technologies for
contaminants of sufficient volatility. In batch application,
the soil is heated to temperatures ranging from 200 to 500
°C in a primary chamber, and the vapors are combusted
inasecondarychamber(122-124). Steam stripping (aform
of soil vapor extraction) can remove semivolatiles from the
vadose zone (125). Bioremediation in a compost mode
where temperatures reach 60 °C or more should prove
advantageous. The success of these methods requires a
fundamental understanding of kinetics. Research into
sorption kinetics in regard to steam stripping has been
initiated (125).
Alteration of the soil chemistry is another approach that
should be considered further based on preliminarystudies.
As mentioned, acidification promoted desorption (50), but
further work is needed to determine its scope and prac-
ticality. The use of surfactants targeted specifically to
removal of slow fractions has not yet been adequately
addressed in theliterature. Tobe effective, surfactants must
penetrate the intraparticle matrix (nanopores or NOM) to
either (i) solubilize the contaminant by micellization or (ii)
alter the intraparticle properties of the sorbent in such a
way as to promote desorption. The addition of surfactants
gave mixed results in stimulating biodegradation (95, 126).
The use of organic cosolvents is a promising approach
because cosolvents can increase desorption both thermo-
dynamically (by enhancing solubility) and kinetically (by

softening NOM) (62). Supercritical carbon dioxide extrac-
tion has been proposed for large-scale cleanup (19). It
probably would require up to 10% by weight of a polar
organic cosolvent to increase its solvation power. This is
an example where an understanding of sorption kinetics
would prove beneficial. Physical manipulation of the soil
such as grinding is known to be partially effective (25, 33,
50, 63) but would likely be impractical on a large scale.
Summary Remarks
Sorption and especially desorption in natural particles can
be exceedingly slow. The rate-limiting nature of sorption
has widespread implications but is poorly understood and
predicted. The importance of it is appreciated by con-
sidering that if sorption occurs on time scales of months
or longer, true equilibrium may exist in only limited
environments. It is hoped that researchers who deal in
fate and transport of contaminants are by now more aware
of the phenomenon itself as well as the potential for
misinterpretation that can result if kinetics are ignored.
Slow sorption has made complete remediation difficult.
However, there have been legitimate questions raised by
some (2, 29, 107) about whether we even need to be
concerned about residues that desorb so slowly and are
apparently largely bio-unavailable. At a minimum, it is
critical that we understand the factors that govern their
release. Sorption kinetics are extremely important in
modeling the transport of contaminants in the subsurface.
Understanding the causes of slow sorption/desorption
hasbeen hamperedbythe heterogeneity ofnaturalparticles
as a sorptive and diffusive medium. It is no wonder then

that rate parameters seem to depend in a complex way on
the soil, history of exposure, and even position along the
uptake vs time curve. But we can be confident that kinetics
studies will lead to a deeper understanding of sorption
mechanism itself. Future research should focus not only
on understanding andpredictingthe rates ofslow sorption/
desorption but also on overcoming the constraints of slow
desorption for remediation purposes.
VOL. 30, NO. 1, 1996 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
9
Acknowledgments
We thank the U.S. Department of Agriculture National
Research Initiative (Water Quality) for support and the
reviewers of the manuscript for their suggestions.
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Received for review November 2, 1994. Revised manuscript

received August 22, 1995. Accepted September 13, 1995.
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ES940683G
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Abstract published in Advance ACS Abstracts, November 1, 1995.
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