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rodu
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e Jo
urna
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ocie
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Colloid Movement in Unsaturated Porous Media:Recent Advances and Future Directions
Nicole M. DeNovio, James E. Saiers, and Joseph N. Ryan*
ABSTRACT (Buol and Hole, 1961; McKeague and St. Arnaud,1969; Matlack and Houseknecht, 1989).Investigations of colloid movement through geologic materials are
driven by a variety of issues, including contaminant transport, soil- How do these colloids become suspended in pore-profile development, and subsurface migration of pathogenic micro-water? Are they readily transported through the vadoseorganisms. In this review, we address recent advances in understandingzone? How rapidly are these colloids deposited backof colloid transport through partially saturated porous media. Specialonto soil surfaces? These questions can be addressed,emphasis is placed on features of the vadose zone (i.e., the presencein part, by examining processes of colloid depositionof air–water interfaces, rapid fluctuations in porewater flow rates and
chemistry) that distinguish colloid transport in unsaturated media and mobilization in saturated porous media (McDowell-from colloid transport in saturated media. We examine experimental Boyer et al., 1986; Ryan and Elimelech, 1996), but instudies on colloid deposition and mobilization and survey recent devel- this review, we focus on what is known about theseopments in modeling colloid transport and mass transfer. We conclude processes in unsaturated porous media.with an overview of directions for future research in this field. Three key features of the vadose zone play a critical
role in colloid movement: (i) the presence of air–waterinterfaces, (ii) transients in flow and chemistry, and (iii)
Mobile colloids are ubiquitous in the porewaters soil structure and heterogeneity (Fig. 1). First, the unsat-of vadose zone soils. Concentrations in excess of urated nature of the vadose zone introduces a third phase,
1 g L�1 have been reported during simulated and natural air, which affects colloid partitioning between water andrainfall events (Table 1). The colloids include mineral soil. Colloids of many types associate with the air–waterfragments, microbes, and plant decay debris, with min- interface (Wan and Wilson, 1994b; Sirivithayapakorneral fragments being the most plentiful in typical soils. and Keller, 2003), and the movement of these colloidsThe mineral fragments are derived mainly from the soil is affected by the movement of air bubbles (Gomez-itself, which contains a great abundance of particles in Suarez et al., 1999; Gomez-Suarez et al., 2001; Saiers etthe colloidal size range (Wu et al., 1993; Grout et al., al., 2003). Second, porewater flow and chemistry are1998; Posadas et al., 2001). The colloidal size range is highly transient in unsaturated porous media. Flow tran-about 10 nm to 10 �m, with the smallest colloids being sients, generated by rainfall and snowmelt events inter-those that are just larger than dissolved macromolecules, spersed by drying periods, can promote very rapid col-and the largest colloids being those that resist settling loid mobilization (El-Farhan et al., 2000). Chemicalonce suspended in soil porewaters. transients, often produced by the introduction of lowColloid movement in the vadose zone is of concern ionic-strength rainwater into the vadose zone, result infor four major reasons: destabilization of colloidal aggregates in soils and mobili-
1. The movement of mobile colloids may facilitate zation of colloids (e.g., Kaplan et al., 1993; Ryan et al.,the transport of some contaminants (Amrhein et 1998). Third, the soils of the vadose zone are usuallyal., 1993; de Jonge et al., 1998; Ryan et al., 1998; structured or physically heterogeneous to some extent.McGechan and Lewis, 2002). For example, macropores promote preferential flow
2. The movement of pathogenic microbes (“biocol- that has the potential to augment colloid mobilizationloids”) during wastewater reclamation and aquifer and reduce colloid deposition. Soil layering often inhib-recharge presents a public health risk (Hurst, 1980; its colloid movement by enhancing deposition of col-Powelson et al., 1993; Redman et al., 2001). loids mobilized in the upper soil horizons (Bond, 1986).
3. The deposition of mobile colloids may reduce soil In this review, we emphasize processes that controlpermeability (Quirk and Schofield, 1955; Frenkel the transfer of inorganic colloids between immobileet al., 1978; Baveye et al., 1998). phases of unsaturated porous media and moving pore-
4. The movement of colloids through the vadose zone water. Microbes and particulate organic matter are not(illuviation) is an important process in soil genesis considered in detail, nor are the effects of solution com-
position, soil composition, biota, and soil aggregatestructure on the dispersion and stability of soil colloids.N.M. DeNovio and J.N. Ryan, Department of Civil, Environmental,
and Architectural Engineering, University of Colorado at Boulder, These factors have been studied extensively (e.g., Ren-428 UCB, Boulder, CO 80309-0428; J.E. Saiers, School of Forestry gasamy et al., 1984; Pojasok and Kay, 1990; Brubakerand Environmental Studies, Yale Univ., Sage Hall, 205 Prospect
et al., 1992; Oades, 1993; Le Bissonnais, 1996), but usu-Street, New Haven, CT 06511. Received 22 Jan. 2004. Special Section:ally in batch systems that do not elucidate the mass-Colloids and Colloid-Facilitated Transport of Contaminants in Soils.
*Corresponding author (joseph.ryan@colorado.edu). transfer processes that occur during flow. We begin byexamining colloid deposition and mobilization in “ideal”Published in Vadose Zone Journal 3:338–351 (2004).soils, or unsaturated porous media composed of grains Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA of uniform size and shape (Table 2), and survey the
338
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www.vadosezonejournal.org 339
Tab
le1.
Exa
mpl
esof
stud
ies
onco
lloid
mob
iliza
tion
,dep
osit
ion,
and
tran
spor
tin
natu
rals
oils
duri
ngsi
mul
ated
rain
fall
inla
bora
tory
colu
mns
and
infi
eld
expe
rim
ents
.The
follo
win
gva
riab
les
are
used
inch
arac
teri
zati
onof
the
expe
rim
ents
:vol
umet
ric
disc
harg
e(Q
),sp
ecif
icdi
scha
rge
(q),
infl
uent
collo
idco
ncen
trat
ion
(C),
ioni
cst
reng
th(I
),m
oist
ure
cont
ent
(�),
spec
ific
cond
ucta
nce
(SC
),is
oele
ctri
cpo
int
(pH
iep)
,ele
ctro
phor
etic
mob
ility
(EM
),an
dze
tapo
tent
ial
(�).
Wat
erC
ollo
idC
ollo
idR
efer
ence
Por
ous
med
ium
chem
istr
yW
ater
flow
natu
reco
ncen
trat
ion
Maj
orre
sult
s
mg
L�
1
Pilg
rim
and
•in
situ
fiel
dex
peri
men
t•
natu
ral
rain
fall
•tw
ora
infa
llev
ents
•na
tura
lco
lloid
s10
0–53
00•
collo
idco
ncen
trat
ion
incr
ease
dw
ith
tim
edu
ring
rain
fall
Huf
f(1
983)
•G
avio
talo
am,
(9,2
5m
m)
•si
ze4–
8�
m•
mac
ropo
res
allo
wed
tran
spor
tof
larg
erco
lloid
sA
ltam
ont
clay
loam
•ir
riga
tion
spri
nkle
rs•
orga
nic
frac
tion
10%
•m
acro
poro
us(q
�10
.7m
mh�
1 )K
apla
net
al.
•3
�3
�1.
5m
tank
•ta
pw
ater
•ra
insi
mul
ator
•na
tura
lco
lloid
s:ka
olin
ite,
300–
1700
•co
lloid
conc
entr
atio
nan
dsi
zein
crea
sed
wit
hin
crea
sing
(199
3)pa
cked
wit
hso
il•
SC�
6�
Scm
�1
•q
�5.
1cm
h�1
quar
tz,v
erm
icul
ite,
gibb
site
,fl
owra
te•
Bla
nton
sand
•pH
6.1
•du
rati
on2
hfe
rric
oxid
es•
EM
decr
ease
dw
ith
incr
easi
ngfl
owra
te(A
pan
dE
hori
zons
)•
size
0.24
–0.4
2�
m•
clay
min
eral
cont
ent
incr
ease
dan
dm
etal
oxid
eco
nten
t•
EM
��
2.4
to�
3.4
�m
s�1
decr
ease
dw
ith
incr
ease
infl
owra
tecm
V�
1
Bid
dle
etal
.•
insi
tufi
eld
expe
rim
ent
•na
tura
lra
infa
ll•
two
natu
ral
rain
fall
•na
tura
lco
lloid
s:ill
ite,
13–2
90•
favo
rabl
eas
sess
men
tof
capi
llary
wic
kly
sim
eter
sfo
rso
il(1
995)
•tw
oso
ils:A
quic
even
tsof
sam
eka
olin
ite,
verm
icul
ite
colle
ctio
nH
aplo
xera
lf,M
ollic
dura
tion
and
End
oaqu
alf
inte
nsit
yJa
cobs
enet
al.
•in
tact
core
sin
•ta
pw
ater
•ra
insi
mul
ator
•ill
ite:
46%
�2
�m
,��
�13
mV
20–5
50•
part
icle
size
inef
flue
ntde
crea
sed
wit
hti
me
(199
7)la
bora
tory
colu
mns
•SC
�30
0�
S•
q�
11an
d30
mm
h�1
•ill
ite/
Ald
rich
hum
icac
id:
•gr
eate
rm
ass
reco
very
was
obse
rved
athi
gher
rain
fall
rate
•sa
ndy
loam
cm�
1•
nopo
ndin
g27
%�
2�
m,�
��
17m
V•
nosi
gnif
ican
tdi
ffer
ence
was
obse
rved
inth
etr
ansp
ort
of(m
orai
nede
posi
t);
•pH
7.0
hum
icac
id–c
oate
dill
ite
and
unco
ated
illit
eT
ypic
Hap
luda
lf•
collo
idsi
zede
crea
sed
wit
hti
me
•m
acro
poro
us•
collo
idm
ass
incr
ease
dw
ith
squa
rero
otof
tim
e(s
ugge
stin
gdi
ffus
ion-
cont
rolle
dki
neti
csof
mob
iliza
tion
)K
apla
net
al.
•3
�3
�1.
5m
tank
•na
tura
lra
infa
ll•
natu
ral
rain
fall
•na
tura
lco
lloid
s:ka
olin
ite,
•co
lloid
surf
ace
char
gew
ashi
ghly
nega
tive
(199
7)pa
cked
wit
hso
ilfe
rric
oxid
es,g
ibbs
ite,
•co
lloid
sor
igin
ated
from
surf
ace
hori
zons
•U
ltis
olqu
artz
,ver
mic
ulit
e•
smal
ler
collo
ids
mor
eab
unda
nt•
size
�1
�m
•ka
olin
ite,
ferr
icox
ide,
gibb
site
wer
een
rich
edin
smal
ler
•or
gani
cm
atte
r1%
collo
ids
•la
rger
part
icle
sw
ere
pref
eren
tial
lyre
mov
eddu
ring
tran
spor
tR
yan
etal
.•
loam
,Ari
dic
Arg
iust
oll
•ta
pw
ater
•ra
insi
mul
ator
•na
tura
lco
lloid
s:20
–980
0•
noco
rrel
atio
nw
asob
serv
edbe
twee
nco
lloid
conc
entr
atio
n(1
998)
•m
acro
poro
us•
SC�
30�
S•
q�
4.2–
16.7
cmh�
1m
ontm
orill
onit
e,qu
artz
,an
dfl
owve
loci
ty•
insi
tufi
eld
expe
rim
ent
cm�
1•
dura
tion
0.5–
2h
mic
rocl
ine,
ferr
ic•
collo
ids
mob
ilize
dde
crea
sed
wit
hsu
cces
sive
rain
falls
atox
yhyd
roxi
de5–
10d
inte
rval
s•
size
1–20
�m
•no
chan
gew
asob
serv
edin
part
icle
size
wit
hfl
owve
loci
ty•
pHie
p3–
4•
noco
rrel
atio
nw
asob
serv
edbe
twee
nco
lloid
sm
obili
zed
and
soil
grai
nsi
zeor
com
posi
tion
Læ
gdsm
and
et•
inta
ctco
res
in•
synt
heti
cra
in•
rain
sim
ulat
or•
natu
ral
collo
ids
3–45
•co
lloid
conc
entr
atio
nin
crea
sed
wit
hde
crea
sein
SCal
.(19
99)
labo
rato
ryco
lum
nsw
ater
•q
�1.
6–6.
5m
mh�
1•
orga
nic
Cfr
acti
on0.
3–12
%•
collo
idal
orga
nic
carb
onde
crea
sed
wit
hti
me
•sa
ndy
loam
,Alf
isol
,•
SC30
�S
cm�
1•
collo
idm
obili
zati
onra
tein
crea
sed
wit
hin
crea
sein
Typ
icH
aplu
dalf
•pH
4.6
outf
low
rate
•m
acro
poro
us•
mas
sof
mob
ilize
dco
lloid
sin
crea
sed
wit
hth
esq
uare
root
ofti
me
(sug
gest
ing
diff
usio
n-co
ntro
lled
mob
iliza
tion
kine
tics
)
Con
tinu
edne
xtpa
ge.
Rep
rodu
ced
from
Vad
ose
Zon
e Jo
urna
l. P
ublis
hed
by S
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cien
ce S
ocie
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eric
a. A
ll co
pyrig
hts
rese
rved
.
340 VADOSE ZONE J., VOL. 3, MAY 2004
Tab
le1.
Con
tinu
ed.
Wat
erC
ollo
idC
ollo
idR
efer
ence
Por
ous
med
ium
chem
istr
yW
ater
flow
natu
reco
ncen
trat
ion
Maj
orre
sult
s
mg
L�
1
El-
Far
han
etal
.•
insi
tufi
eld
expe
rim
ents
•�
0.1
mM
NaC
l•
pond
ing
•na
tura
lco
lloid
s7–
265
•m
ass
ofco
lloid
sm
obili
zed
decr
ease
dw
ith
(200
0)•
Fre
deri
ck,L
odi
solu
tion
•5–
20cm
init
ial
dept
h•
size
253–
270
nmsu
cces
sive
infi
ltra
tion
even
tssi
ltlo
am,T
ypic
•co
lloid
mas
sfl
uxw
asst
eady
amon
gin
filt
rati
onH
adlu
dult
even
ts•
high
lyst
ruct
ured
•pe
akco
lloid
conc
entr
atio
nsan
dfl
uxw
ere
obse
rved
duri
ngri
sing
and
falli
nglim
bsof
wat
erfl
ux(s
ugge
stin
gm
ovin
gai
r–w
ater
inte
rfac
esca
used
mob
iliza
tion
)N
oack
etal
.•
siev
edfr
acti
ons
pack
ed•
dist
illed
wat
er•
rain
sim
ulat
or•
Mul
oori
naill
ite,
size
0.07
�m
C�
100
•co
lloid
brea
kthr
ough
incr
ease
dw
ith
incr
ease
in(2
000)
inla
bora
tory
colu
mn
•SC
�10
�S
•q
�50
mm
h�1
•F
ithi
anill
ite,
two
size
frac
tion
s,m
oist
ure
cont
ent
•O
xiso
l,cl
ayte
xtur
ecm
�1
•�
�0.
28–0
.59
0.08
–0.2
,1–2
�m
•co
lloid
brea
kthr
ough
incr
ease
dw
ith
incr
ease
in•
Spod
osol
,san
dycl
ay•
pH5.
5•
soil
frac
tion
com
pose
dof
illit
e,ab
unda
nce
ofla
rger
pore
size
sin
poro
usm
ediu
mte
xtur
eka
olin
ite,
size
0.08
–0.2
�m
•co
lloid
brea
kthr
ough
incr
ease
dw
ith
decr
ease
inco
lloid
size
•co
lloid
surf
ace
char
gean
dca
tion
satu
rati
onha
dlit
tle
effe
cton
collo
idbr
eakt
hrou
ghG
amer
ding
er•
labo
rato
ryce
ntri
fuge
•de
ioni
zed
wat
er•
flow
driv
enby
•po
lyst
yren
ela
tex
C�
4.5
•in
crea
sed
collo
idbr
eakt
hrou
ghw
ith
decr
ease
dan
dK
apla
nco
lum
n•
carb
onat
e/ce
ntri
fuga
tion
mic
rosp
here
s,si
ze28
0nm
,io
nic
stre
ngth
(200
1)•
Yuc
caM
ount
ain
tuff
bica
rbon
ate
•�
�0.
06–0
.19
carb
oxyl
-mod
ifie
d•
incr
ease
dco
lloid
brea
kthr
ough
wit
hin
crea
sed
(0.2
5–0.
84m
m)
solu
tion
moi
stur
eco
nten
t•
I�
12m
M•
pH7.
8Sc
held
eet
al.
•in
tact
core
sin
•ta
pw
ater
•ra
insi
mul
ator
•na
tura
lco
lloid
s5–
4100
•co
lloid
mob
iliza
tion
dom
inat
edby
diff
usio
n,no
t(2
002)
labo
rato
ryco
lum
ns•
SC�
22�
S•
q�
11an
d30
mm
h�1
shea
r•
sand
ylo
am(m
orai
necm
�1
•in
terv
als
betw
een
•co
lloid
sour
cew
asm
odel
edas
unlim
ited
inth
ese
depo
sit)
,Typ
ic•
pH7.
8ra
infa
ll:0.
5h,
1d,
7d
expe
rim
ents
base
don
succ
essi
vera
infa
llH
aplu
dalf
sim
ulat
ions
•m
acro
poro
usC
herr
eyet
al.
•la
bora
tory
colu
mn
•si
mul
ated
•rai
nsi
mul
ator
•na
tive
collo
ids
from
Han
ford
C�
10•
mor
era
pid
collo
idbr
eakt
hrou
ghoc
curr
edat
low
er(2
003)
•si
eved
(�2
mm
)H
anfo
rdta
nk•
Q�
2–90
mL
min
�1
For
mat
ion
sedi
men
ts:
moi
stur
eco
nten
tfr
acti
onof
Han
ford
wat
er•
q�
0.00
5–4.
1cm
chlo
rite
,sm
ecti
te,o
ther
clay
•gr
eate
rdi
sper
sion
ofco
lloid
tran
spor
toc
curr
edat
For
mat
ion
•�
1.0
MN
aCl,
min
�1
min
eral
slo
wer
moi
stur
eco
nten
t•
coar
se,u
ncon
solid
ated
pH10
•�
�0.
16–1
.0•
mod
ifie
dco
lloid
sfr
om•
nove
loci
tyen
hanc
emen
t(w
hich
may
beat
trib
uted
•ai
r-dr
ied
(car
bona
te-
reac
ting
tank
wat
erw
ithto
pore
size
excl
usio
n)w
asob
serv
edbu
ffer
ed)
sedi
men
ts:c
ancr
init
e,so
dalit
e,•
pH10
othe
rcl
aym
iner
als
(car
bona
te-
•E
M�
�4.
0an
d�
4.5
�m
s�1 /
buff
ered
)V
cm�
1
•si
ze35
0–37
0nm
mea
n
Rep
rodu
ced
from
Vad
ose
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e Jo
urna
l. P
ublis
hed
by S
oil S
cien
ce S
ocie
ty o
f Am
eric
a. A
ll co
pyrig
hts
rese
rved
.
www.vadosezonejournal.org 341
Fig. 1. Processes affecting colloid movement in unsaturated porous media. Colloid deposition mechanisms include attachment to grains byphysicochemical filtration, attachment to immobile air–water interfaces (water flow is around bubble trapped in a pore), attachment bystraining in water-saturated pores, and entrapment in thinning water films during draining. Colloid mobilization mechanisms include colloiddispersion by chemical perturbation, expansion of water films during imbibition, air–water interface scouring during imbibition and drainage,and shear mobilization (soil profile from Tarbuck and Lutgens, 1997).
grain surface and on the probability that a colloid collisiondevelopment of mathematical models that describe col-with the mineral grain will succeed in attachment. Colloidsloid transport in these ideal soils. We then explore theare transported from the bulk fluid to the mineral grains byapplication of our understanding of colloid depositionBrownian diffusion, interception, and sedimentation (Yao etand mobilization in ideal soils to “nonideal” soils, oral., 1971). The transport rates due to these three mechanismsnatural and intact soils that are physically and geochemi-can be calculated for water-saturated media as functions ofcally heterogeneous (Table 1). We conclude with recom- the physical properties of the porous medium–water–colloid
mendations for future research. system, including colloid diameter and density, grain size, andflow velocity (Yao et al., 1971; Rajagopalan and Tien, 1976;Logan et al., 1995; Tufenkji and Elimelech, 2004). An analo-COLLOID MOVEMENT IN IDEALgous theory for water-unsaturated media is unavailable. ItsPOROUS MEDIAdevelopment relies on improvements in models for air–water
Colloid Transport and Deposition configuration in variably saturated porous media and, for natu-ral systems, on consideration of the effects of irregularities inMost experimental studies in ideal porous media have beenthe shapes of the mineral grains and colloids.conducted under conditions of uniform moisture content and
Attachment of colloids that strike the mineral grains issteady porewater velocity and have focused on elucidatingdetermined from the net-interaction potential, which can befactors that influence colloid deposition. The experimentalcalculated from DLVO theory as the sum of the electrostaticresults reveal that colloid deposition rates are sensitive todouble-layer force, the van der Waals force, and short-rangeseveral physical and chemical properties, including volumetricsolvation or steric forces (Derjaguin and Landau, 1941; Ver-moisture content, flow rate, porewater ionic strength, andwey and Overbeek, 1948; McDowell-Boyer et al., 1986; Ryancolloid size and composition (Wan and Wilson, 1994b; Wanand Elimelech, 1996). The magnitude and direction of theseand Tokunaga, 1997; Jewett et al., 1999; Gamerdinger andforces depend on the chemical and physical characteristicsKaplan, 2001; Saiers and Lenhart, 2003a). The variations inof the colloid and soil-grain surfaces and, for the electricalcolloid deposition rates with changes in these properties have
been attributed to interactions among three deposition mecha- double-layer force, the chemical composition of the porewater.nisms: mineral-grain attachment, air–water interface capture, At low ionic strength and for similarly charged colloids andand film straining (Fig. 1). soil grains, the net-interaction potential exhibits a repulsive
The kinetics of colloid deposition on mineral grains depends maximum that hinders the attachment of colloids that ap-on the rate of colloid transport from the bulk fluid to the proach the mineral-grain surface. With increasing ionic strength,
Rep
rodu
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from
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342 VADOSE ZONE J., VOL. 3, MAY 2004
Tab
le2.
Exa
mpl
esof
stud
ieso
nco
lloid
tran
spor
t,de
posi
tion
,and
mob
iliza
tion
inid
ealp
orou
smed
iaun
deru
nsat
urat
edco
ndit
ions
.The
follo
win
gva
riab
lesa
reus
edin
char
acte
riza
tion
ofth
eex
peri
men
tsan
dex
peri
men
talm
ater
ials
:flo
wve
loci
ty(v
),vo
lum
etri
cdi
scha
rge
(Q),
spec
ific
disc
harg
e(q
),in
flue
ntco
lloid
conc
entr
atio
n(C
),io
nic
stre
ngth
(I),
moi
stur
eco
nten
t(�
),an
del
ectr
opho
reti
cm
obili
ty(E
M).
Stud
ies
cond
ucte
dun
der
stea
dyfl
owfo
cus
onco
lloid
depo
siti
on,
whi
leth
ose
cond
ucte
dun
der
tran
sien
tfl
owfo
cus
onco
lloid
mob
iliza
tion
.
Ref
eren
ceP
orou
sm
ediu
mW
ater
chem
istr
yW
ater
flow
Col
loid
sM
ajor
resu
lts
Wan
and
Wils
on(1
994a
)•
poro
us-m
ediu
m•
NaN
O3
solu
tion
•Q
�1.
5–15
mL
h�1
•po
lyst
yren
ela
tex
mic
rosp
here
s:•
collo
ids
sorb
edto
air–
wat
erin
terf
ace
and
solid
–wat
erm
icro
mod
el,e
tche
dgl
ass
•I
�1
mM
•w
ater
-sat
urat
edca
rbox
ylat
e(9
50nm
),su
lfat
ein
terf
aces
•vi
sual
izat
ion
byop
tica
l•
pH6.
6•
unsa
tura
ted
(105
0nm
),am
idin
e(6
00nm
)•
hydr
opho
bic
collo
ids
(by
cont
act
angl
e)so
rbed
toa
mic
rosc
ope
•st
eady
flow
•m
ontm
orill
onit
e(≈
500
nm)
grea
ter
exte
ntth
anhy
drop
hilic
collo
ids
toai
r–w
ater
•po
rebo
dies
4–40
0�
m,
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cter
ia:h
ydro
phob
ican
din
terf
aces
pore
thro
ats
20–1
00�
mhy
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(abo
ut10
00nm
)•
sorp
tion
atai
r–w
ater
inte
rfac
ew
asir
reve
rsib
le•
C�
5�
107
collo
ids
L�
1
Wan
and
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on(1
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n•
NaN
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tion
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�10
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poly
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res:
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tent
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ofhy
drop
hilic
and
hydr
opho
bic
collo
ids
•qu
artz
sand
(0.2
1–0.
32m
m)
•I
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mM
•�
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23–0
.43
“hyd
roph
obic
”su
lfat
ein
crea
sed
wit
hpo
rous
-med
ium
air
cont
ent
•pH
6.6
•st
eady
flow
(220
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“hyd
roph
ilic”
•re
tent
ion
incr
ease
dw
ith
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eeof
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rfac
eca
rbox
ylat
e(1
90nm
)hy
drop
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city
•C
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lloid
sL
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•m
ovin
gai
r–w
ater
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rfac
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ayha
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fect
edco
lloid
tran
spor
tW
anan
dT
okun
aga
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bora
tory
colu
mn
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aCl
solu
tion
•q
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004
to•
poly
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ene
late
xm
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ghw
ith
incr
ease
d(1
997)
•qu
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:•
I�
1.0
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340
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114
,93,
280,
970
nmin
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eter
moi
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nten
t0.
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pH5.
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0.08
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5•
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10.1
�10
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lloid
sL
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lloid
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oval
dom
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low
0.21
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3m
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•st
eady
flow
moi
stur
eco
nten
t0.
43–0
.50
mm
•in
crea
sed
film
stra
inin
gef
fici
ency
wit
hde
crea
sed
grai
nsi
ze•
incr
ease
dfi
lmst
rain
ing
effi
cien
cyw
ith
incr
ease
dco
lloid
size
Scha
fer
etal
.(19
98)
•la
bora
tory
colu
mn
•ph
osph
ate-
buff
ered
•v
�1.
5–2.
cmm
in�
1•
two
bact
eria
lst
rain
s:•
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laf
fini
tyfo
rai
r–w
ater
inte
rfac
esw
asgr
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r•
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line
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tion
•�
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.41
Rho
doco
ccus
sp.C
125
and
P.
than
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eady
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dam
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tent
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hin
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mn
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ely
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h•
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Chu
etal
.(20
01)
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cked
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osph
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ered
•q
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ater
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tzsa
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line
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tion
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09–
0.33
bact
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inth
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ceof
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tive
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aces
•ir
onox
ide-
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nd•
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stea
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redu
ctio
nsin
moi
stur
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nten
tin
crea
sed
bact
erio
phag
ede
posi
tion
onto
solid
–wat
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terf
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Gam
erdi
nger
and
•la
bora
tory
cent
rifu
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deio
nize
dw
ater
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owdr
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by•
poly
styr
ene
late
xm
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crea
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collo
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ith
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nic
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lum
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lloid
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kthr
ough
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hin
crea
sed
•I
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mM
•st
eady
flow
moi
stur
eco
nten
t•
pH7.
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enha
rtan
dSa
iers
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bora
tory
colu
mn
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aHC
O3/N
aCl
wat
er•
v�
9–11
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h�1
•si
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(360
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lloid
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kthr
ough
incr
ease
dw
ith
incr
easi
ng(2
002)
•qu
artz
sand
(0.3
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tion
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ting
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ory
•pH
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min
ant
mec
hani
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idre
tent
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chan
ged
from
stra
inin
gto
air–
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capt
ure
asm
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and
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mn
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aHC
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•m
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ese
nsit
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ater
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reng
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Rep
rodu
ced
from
Vad
ose
Zon
e Jo
urna
l. P
ublis
hed
by S
oil S
cien
ce S
ocie
ty o
f Am
eric
a. A
ll co
pyrig
hts
rese
rved
.
www.vadosezonejournal.org 343
eter exceeded about one-twentieth to one-tenth the diameterof the porous media grains (Sakthivadivel, 1966; Herzig et al.,1970). More recent studies motivated by the need to betterunderstand the removal of protozoan cysts during riverbankfiltration have explored the pore straining of polystyrene mi-crospheres in uniform and poorly sorted porous media (Brad-ford et al., 2002, 2003). This recent work showed that porestraining can be modeled as first-order removal with a ratecoefficient that depends on the depth and mean grain diameterof the porous media. Pore straining may also contribute tocolloid immobilization within small, water-filled pore spacespresent within unsaturated porous media. In partially satu-rated pores with dimensions that exceed those of the colloids,film straining may remove colloids from the mobile phase.According to Wan and Tokunaga (1997), colloid immobiliza-tion by film straining depends on the probability of pendularFig. 2. Negatively charged latex colloids (0.95 �m) deposited prefer-
entially onto an air bubble trapped in a pore body of a porous- ring discontinuity and on the ratio of colloid size to film thick-medium micromodel (from Wan and Wilson, 1994a). ness (a pendular ring is water held by surface tension near
the contacts of adjacent mineral grains). The probability ofthe repulsive barrier decreases in magnitude, which increasespendular ring discontinuity increases from zero to unity asthe probability that a colloid-grain collision will succeed inthe capillary pressure decreases (i.e., as the porous mediumcolloid attachment. The repulsive barrier is absent for oppo-drains). As pendular rings disconnect, an increasing propor-sitely charge colloids and soil grains, in which case the deposi-tion of water flow and colloid transport is relegated to thetion rate is controlled by the rate at which colloids are trans-adsorbed films of water that envelop the mineral grains. Whenported from the pore fluid to the mineral-grain surface.film width is greater than colloid diameter, straining does notPredictions of colloid deposition that are based on DLVOoccur. When film width is similar to or less than the colloidtheory have not been published for water-unsaturated sys-
tems, but DLVO theory has been tested against measurements diameter, however, surface tension retains colloids against theof colloid deposition in water-saturated porous media. These mineral grain surfaces.evaluations show that theoretically determined deposition rates The relative importance of soil-grain attachment, air–watersubstantially underestimate corresponding measured values interface capture, and film straining to colloid deposition iswhen repulsive barriers exist between the colloids and mineral not constant, but varies as a function of porewater chemistry,grains (Elimelech et al., 1995). Agreement between DLVO- moisture content, and colloid characteristics. The work of Wanbased and laboratory-measured deposition rates has been im- and Tokunaga (1997) and Lenhart and Saiers (2002) suggestsproved through recent modifications to theory that account that film straining represents the most important depositionfor complexities associated with surface-charge heterogeneity, mechanism for hydrophilic colloids under conditions of lowgrain-scaled surface roughness, and deposition within the sec- ionic strength (�10�3 M) and low to intermediate moistureondary minimum of the net-interaction energy profile (Bhatta- content. As moisture content and ionic strength increase, thecharjee et al., 1998; Hahn and O’Melia, 2004). These modifi-
leading colloid deposition mechanism may transition from filmcations, although designed to improve descriptions of colloidstraining to air–water interface capture or soil grain attach-deposition in water-saturated media, should also be applicablement, depending on the surface characteristics of the colloidsfor quantifying colloid deposition reactions on mineral-grainand mineral grains (Saiers and Lenhart, 2003a).surfaces present within unsaturated porous media.
Like the soil surfaces, air–water interfaces present withinunsaturated porous media can serve as collectors of colloidal Modeling Colloid Transport and Depositionparticles (Fig. 1 and 2). Colloids that are transported to the
The observations reviewed above have been instrumentalair–water interface are retained by either capillary or electro-in guiding the development of mathematical models for colloidstatic forces; therefore, colloid capture at air–water interfaces
depends on pH, ionic strength, and colloid surface properties. transport and deposition within homogeneous granular mate-Increases in ionic strength reduce the magnitude of the repul- rials. Most of these transport and deposition models are basedsive energy barrier between the negatively charged air–water on the assumption of steady porewater flow and conceptualizeinterface and like-charged mineral colloids, leading to progres- the unsaturated porous medium as a three-component systemsively more favorable conditions for attachment and faster consisting of air, water, and mineral grains (e.g., Sim andrates of air–water interface capture (Wan and Wilson, 1994a; Chrysikopoulos, 2000). Colloids are transmitted through theSaiers and Lenhart, 2003a). Hydrophobic colloids, such as cer- water-filled sections of the porous medium by advection andtain bacteria, exhibit a greater affinity for air–water interfaces dispersion and are removed from the porewater by straining,than mineral colloids, which have comparatively hydrophilic air–water interface capture, and deposition onto soil–watersurfaces (Wan and Wilson, 1994b; Schafer et al., 1998; Lenhart interfaces. Film straining and air–water interface capture areand Saiers, 2002). Among clay-mineral colloids, the affinity treated as irreversible mass-transfer processes, a suitable ap-for the air–water interfaces depends on the colloid shape and
proximation provided that flow and porewater chemistry re-surface-charge distribution and varies inversely with colloidmain steady (Corapcioglu and Choi, 1996; Wan and Tokunaga,cation-exchange capacity. Kaolinite partitions more strongly1997). Colloid release from soil–water interfaces is often ac-to the air–water interface than illite, while bentonite and mont-commodated in unsaturated transport models, but is generallymorillonite exhibit negligible partitioning (Wan and Tokunaga,slow in the absence of hydrologic and chemical perturbations2002).(Schafer et al., 1998; Chu et al., 2001).Straining occurs within mobile-water conduits that are too
The advection–dispersion equation describes the move-narrow to permit colloids to pass (Fig. 1). Early studies onment of porewater colloids. The one-dimensional form of thisthe removal of colloids by pore straining in water-saturated
porous media showed that colloids were retained if their diam- equation is given by
Rep
rodu
ced
from
Vad
ose
Zon
e Jo
urna
l. P
ublis
hed
by S
oil S
cien
ce S
ocie
ty o
f Am
eric
a. A
ll co
pyrig
hts
rese
rved
.
344 VADOSE ZONE J., VOL. 3, MAY 2004
the attachment probability can be accurately determined on�C�t
��STR
�t�
c
Sw�fair
�AWI
�t� fsoil
�SWI
�t � � AL v�2C�z2
� v�C�z
[1]a theoretical basis, discernible trends between the magnitudeof kAWI and some system properties have been identified. In
where C is the porewater colloid concentration; STR, AWI, and particular, values of kAWI that quantify silica-colloid attach-SWI are immobile-phase colloid concentrations for removal by ment vary proportionately with the one-third power of thefilm straining (STR) air–water interface capture (AWI), and porewater velocity (kAWI � v1/3) (Lenhart and Saiers, 2002) andsoil–water interface deposition (SWI); t is time; c is the ratio increase linearly with porewater ionic strength (Saiers andof colloid mass to its effective cross-sectional area; Sw is water Lenhart, 2003a).saturation; fair is the air–water interfacial area per unit void The reciprocal of �AWI (�AWI
�1) defines the maximum attain-volume; fsoil is the soil–water interfacial area per unit void able surface coverage at the air–water interface. Estimates ofvolume; AL is the longitudinal dispersivity; v is the average
�AWI�1 increase with ionic strength because of a reduction in
porewater velocity; and z is the coordinate parallel to flow. repulsive electrical double layer forces between colloids. EvenThe concentration of strained colloids (STR) is expressed in at elevated ionic strengths, maximum surface coverages forterms of colloid mass per volume of porewater, while AWI both biocolloids and mineral colloids are low. For example,and SWI are expressed in terms of normalized surface cover- Abdel-Fattah and El-Genk (1998) reported �AWI
�1 values forages (i.e., area of attached colloids per area of interface). hydrophobic microsphere ranging from 0.012 to 0.08 for ionicSolution of Eq. [1] requires specification of the kinetics expres- strengths between 0.001 and 1 M, while Saiers and Lenhartsions for film straining, air–water interface capture, and depo- (2003a) reported �AWI
�1 values for silica colloids ranging fromsition onto soil–water interfaces. 0.001 to 0.03 for ionic strengths between 2 � 10�4 and 0.2 M.
Wan and Tokunaga (1997) quantified colloid straining in- The parameter �AWI�1 likely depends on hydrodynamic forces
side thin films with a first-order kinetics expression: in addition to forces between colloids (Ko and Elimelech,2000). Because hydrodynamic forces vary with position along�STR
�t� kSTR C [2] the air–water interface, colloid surface coverages are undoubt-
edly nonuniform, with some areas of the air–water interfacecompletely devoid of colloids (even at maximum surface cov-where kSTR, the rate coefficient for film straining, varies ac-erages), while other areas collect colloids in high concentra-cording totions. Estimates of �AWI
�1, then, should be regarded as a spatialaverage over the entire air–water interface.kSTR � P( )�d
w��
Nv(1�h) [3]Methods for quantifying soil–water interface reactions in
unsaturated media are largely based on approaches derivedIn Eq. [3], P( ) is the probability of pendular ring discontinu- from studies conducted in water-saturated systems. Severality (expressed as a function matric potential, ), d is the investigators have adopted a second-order reversible rate lawcolloid diameter, set w is the film thickness. h, N, and � are to describe colloid mass-transfer reactions with the solid phaseempirical parameters. Wan and Tokunaga (1997) employed (Corapcioglu and Choi, 1996; Schafer et al., 1998; Chu et al.,Eq. [2] and [3] to describe film straining rates in a suite of 2001):column experiments that were conducted at matric potentialsranging from �0.05 to �0.5 m and with microspheres ranging c
Sw
fsoil�SWI
�t� kSWI�SWI C �
c
Sw
fsoil kRSWI [5a]in diameter from 0.014 to 0.97 �m.Colloids traveling within relatively large water channels
(e.g., interconnected pendular rings) are not affected by film�SWI � 1 � �SWISWI [5b]straining, but they may diffuse to the air–water interface where
electrostatic or capillary forces retain them. A second-orderwhere kSWI is a rate coefficient for colloid deposition onto thekinetics expression has been invoked to describe the attach-mineral grains, kR is a rate coefficient for colloid release, andment of microspheres, bacteria, viruses, and mineral colloids�SWI is an excluded area parameter. Application of this kineticsat air–water interfaces present within porous media (Corapci-formulation to data on microsphere, virus, and bacteria trans-oglu and Choi, 1996; Schafer et al., 1998; Chu et al., 2001).port indicate that kR is small or zero, at least for conditionsThe formulation of this rate law varies slightly depending onof constant flow and porewater chemistry. Like their air–waterwhether the captured colloid mass is normalized by the volumeinterface counterparts, kSWI and �SWI are sensitive to porewaterof air or by air–water interfacial area. For the case of normal-chemistry, soil composition, and colloid type (Corapciogluization by interfacial area, the rate law is expressed byand Choi, 1996; Schafer et al., 1998; Chu et al., 2001). Thedeposition rate coefficient (kSWI) should exhibit an additionalc
Sw
fair�AWI
�t� kAWI�AWI C [4a] dependence on volumetric moisture content because changes
in air–water configuration that accompany variation in mois-where kAWI is a rate coefficient for air–water interface capture ture content will affect colloid trajectories around (and theand �AWI is a blocking function. The blocking function declines transport rate to) the mineral-grain surfaces.linearly as AWI increases: Equations [1], [2], and [4a] to [5b] with unknowns C, STR,
AWI, and SWI are suitable for simulating colloid transport,�AWI � 1 � �AWIAWI [4b] film straining, air–water interface capture, and mineral-grain
attachment in unsaturated, homogeneous porous media. Pub-where �AWI is an excluded area parameter equivalent to thelished models that incorporate one or more of these threeratio of blocked air–water interfacial area to the projectedmass-transfer mechanisms have successfully reproduced datacross-sectional area of the colloid. Inspection of Eq. [4a] andfrom laboratory experiments on the transport of both inor-[4b] shows that colloid capture rates vary linearly with C andganic and organic colloids in ideal porous media. Though verydecline as colloids accumulate on the air–water interface.encouraging, these results should not be taken as evidence thatThe magnitude of kAWI depends on the rate of colloid trans-the colloid-transport problem has been solved. The publishedport from the bulk fluid phase to the air–water interface andsimulations rely on adjustment of model parameters that can-on the probability that a colloid collision with the interface
will result in attachment. While neither the transport rate nor not be determined on a theoretical basis and hence the favor-
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able model-data agreement should not be considered defini- measuring the concentrations of colloids in water samplescollected in lysimeters installed within the soil profile (fortive proof of positive identification of the mechanisms that
govern colloid mass transfer. Alternative interpretations of field experiments) or at the base of the core (for laboratoryexperiments). Results of these studies have been instrumentalthe experimental observations are possible.in improving our understanding of factors that control themobilization of naturally occurring soil colloids.Colloid Mobilization
A salient characteristic of these field and intact soil labora-Few experimental or theoretical studies on colloid mobiliza- tory experiments is the consistent occurrence of a pulse of
tion within ideal unsaturated media are available. On the basis colloids at the beginning, and sometimes at the end, of aof studies with saturated porous media, we anticipate that rainfall event with an interlude of relatively steady colloidperturbations in porewater chemistry will promote colloid re- mobilization (e.g., Kaplan et al., 1993; Jacobsen et al., 1997;lease (Fig. 1). Ionic-strength reductions and pH increases are Ryan et al., 1998; El-Farhan et al., 2000). The colloid pulsesthe most common chemical perturbations that mobilize col- during imbibition and draining can be attributed to the effectloids in saturated systems (McDowell-Boyer, 1992; Ryan and of flow transients on colloid mobilization. The relatively steadyGschwend, 1994; Grolimund and Borkovec, 1999) and are colloid mobilization during the rainfall event can be attributedlikely to play an important role in colloid mobilization within to the gradual propagation of chemical (and perhaps someunsaturated systems. physical) perturbations through the soil column.
Physical perturbations in flow that characterize typical infil- The best example of colloid mobilization pulses coincidingtration events also drive colloid mobilization. Several mecha- with the beginning and end of a simulated rainfall event isnisms for this flow-induced mobilization have been proposed provided by the field experiments conducted by El-Farhan et(Fig. 1). Colloids trapped in narrow porewater conduits (by al. (2000). Infiltrating water was applied as water ponded onstraining) may be released into the pore fluid when these flow the soil surface. Peak colloid concentrations (up to 265 mgpaths expand during soil imbibition (Fig. 3; Saiers and Lenhart, L�1) were recorded in the first few and last few samples of2003b). Moving air–water interfaces associated with wetting water taken from zero-tension lysimeters at 25-cm depths (Fig.and drying fronts may scavenge colloids from mineral-grain 4). These peak concentrations were attributed to the passagesurfaces and facilitate their transport through the porous me- of colloid-scavenging air–water interfaces during imbibitiondium (Gomez-Suarez et al., 1999, 2001; Saiers et al., 2003). and draining. The experiments conducted by Saiers et al.Increases in shear stress that accompany porewater-velocity (2003) in ideal porous media reinforce this interpretation forincreases may cause colloids to roll along the surface to which the draining. In addition, some of the pulse of colloid mobiliza-they are attached, and these colloids may be released into the tion that occurs at the beginning of a rainfall event can beporewater upon encountering surface roughness that reduces attributed to the release of colloids into expanding of waterthe DLVO adhesion force (Hubbe, 1985). films (Saiers and Lenhart, 2003b).
Following the pulse of colloid mobilization typically ob-COLLOID MOVEMENT IN NONIDEAL served during imbibition, colloid concentrations are often rela-
tively steady (El-Farhan et al., 2000) or they gradually decreasePOROUS MEDIAwith time (Kaplan et al., 1993; Jacobsen et al., 1997; Ryan et al.,Findings from ideal systems have been used to identify 1998; Schelde et al., 2002). The colloid mobilization behaviorkey mechanisms that influence colloid-deposition kinetics in observed during steady rainfall infiltration has frequently beennatural vadose-zone environments and to define, at least quali- interpreted as control of colloid mobilization kinetics by col-tatively, how colloid mobility in soils and sediments responds loid diffusion. Colloid mobilization can be viewed as a two-to changes in measurable properties, such as moisture content, step process involving (i) detachment of colloids from soilporewater chemistry, and flow velocity. However, natural geo- grain and aggregate surfaces and (ii) diffusion of colloids fromlogic environments are more heterogeneous than ideal sys- the detachment site to the mobile porewater. The diffusiontems. Although the soils of some vadose-zone systems exhibit step may be envisioned as diffusive transport through a layera narrow distribution in pores sizes and are characterized by of immobile water in which diffusive transport of colloids isweak structure, abiotic and biotic processes lead to the cre- more important than advective transport (Ryan and Gschwend,ation of macropores (e.g., root channels, worm borrows, desic- 1994). The diffusion step can also be viewed as diffusioncation cracks) and aggregation of primary mineral particles through two regions, one being a soil “crust” representing soilin many near-surface soils. This soil structure complicates de- aggregates or soil matrix, and the other being the immobilescriptions of colloid transport because it produces nonunifor-water layer (Schelde et al., 2002).mity in the velocity of infiltrating water (Beven and Germann,
In nonideal porous media, there are indications that the1982; Selker et al., 1999). Therefore, mathematical modelsdetachment step is promoted by various chemical and physicaldeveloped for ideal porous media that are based on the as-perturbations (e.g., decreasing ionic strength, increasing pH,sumption of uniform flow cannot be used without modificationshear stress), with the addition of another chemical perturba-to quantify colloid movement through macroporous or aggre-tion, the detachment of colloids by dissolution of mineralgated soils. In addition to heterogeneity in porous-mediumcements that bind together various soil constituents (e.g., Har-physical properties, the geologic solids of real vadose-zoneris et al., 1987; Weisbrod et al., 2002). Despite these indica-environments exhibit substantial geochemical heterogeneity.tions, experiments in nonideal porous media have not yieldedConsequently, the distribution in the rates of colloid mass-much insight into detachment mechanisms because it is highlytransfer reactions may be broader than those measured inunlikely that the detachment kinetics would be the rate-lim-experiments with ideal porous media.iting step in an experiment in which a measurable amount ofcolloids were mobilized. Instead, most of these experimentsExperimental Findings show that kinetics of colloid mobilization during steady infil-tration appears to be limited by the diffusion step (JacobsenColloid movement through nonideal porous media has beenet al., 1997; Lægdsmand et al., 1999; Schelde et al., 2002).measured in small-scale field experiments and in laboratory
The key experimental result that supports an interpretationexperiments with intact soil cores. These experiments mostoften involve applying water to the surface of the soil and of diffusion-limited kinetics for colloid mobilization is a linear
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346 VADOSE ZONE J., VOL. 3, MAY 2004
Fig. 3. Model-computed results and those measured in duplicate experiments on silica-colloid mobilization from columns of quartz sand: (A,B)measured specific discharge at column boundaries, (C,D) measured moisture content (symbols) and modeled moisture content (lines) forthree positions along the 32-cm-long columns (z � 0 at column top), and (E,F) colloid breakthrough pulses generated by successive increasesin flow rate (from Saiers and Lenhart, 2003b).
relationship between the cumulative mass of mobilized col- may be dominating colloid mobilization. At high flow rates,loids and the square root of time (Fig. 5) following shear stress may affect colloid mobilization kinetics. In model
systems of spherical colloids attached to flat plates, the forceof hydrodynamic shear (FH) is proportional to the flow velocityMt � 4M∞l �Dct
�[6]
VR at the height of the colloid radius R and the radius of thewhere Mt is the cumulative mass of mobilized colloids as a colloids (O’Neill, 1968):function of time t, M∞ is the total mass of colloids that can be
FH � (1.7)6��RVR [7]mobilized in a sheet of thickness l, and Dc is the diffusioncoefficient of the colloid (Crank, 1975). Such linear relation-
where � is the dynamic viscosity of the fluid. The shear forceships were observed by Jacobsen et al. (1997), Lægdsmand etis opposed by an adhesive force, which is described by DLVOal. (1999), and Schelde et al. (2002) for intact soils in labora-interactions. Kaplan et al. (1993) and Lægdsmand et al. (1999)tory columns.found support for mobilization by shear in positive correla-Under some conditions, the linear relationship betweentions between mobilized colloid concentrations and flow ratecumulative mass and the square root of time has not been ob-by assuming that the velocity of infiltrating water is propor-served. For example, both Jacobsen et al. (1997) and Lægdsmandtional to flow rate and the concentration of colloids is propor-et al. (1999) noted deviations from the linear relationship fortional to the shear force. Similarly, Weisbrod et al. (2002)early time (during imbibition) and for high flow rates. Thesereported a power law relationship between the flow rate anddeviations indicate that processes other than diffusion maythe amount of colloids mobilized from a fractured chalk for-control colloid mobilization kinetics under these conditions.
During imbibition, colloid scavenging by air–water interfaces mation.
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www.vadosezonejournal.org 347
that approximates the average behavior of the actual macro-Modeling Colloid Mobilization in Nonideal Mediapore network. Water in the partially saturated macropore is
Efforts are just beginning to build a modeling framework assumed to occur as a thin film with mobile- and immobile-appropriate for describing the mobilization and transport of water portions. Colloids are generated from a “crust layer”colloids in nonideal, unsaturated porous media (Jarvis et al., near the macropore edge. These colloids presumably diffuse1999; Schelde et al., 2002). These colloid-transport models, across the stagnant portion of the water film and enter itslike those developed for ideal systems, ignore the effects of mobile-water portion, where flow is steady and the colloids arebiological processes (e.g., growth, decay, predation, and inacti- transported by advection and dispersion. Although Schelde etvation) and thus are most appropriately applied to the move- al. (2002) developed this model in the context of macroporousment of inorganic colloids. soils, it could be applied to describe colloid transport and mass
Schelde et al. (2002) developed a model capable of simulat- transfer in aggregated soils by conceptualizing the water ining the mobilization and transport of natural mineral colloids the aggregates as immobile water and the water in the interag-within macroporous soils cores (Fig. 6). This model is similar gregate pore spaces as the mobile water.in form to dual-porosity, mobile–immobile models for solute The model of Jarvis et al. (1999) shares the two-domaintransport in structured and aggregated porous media (Coats conceptualization embodied in the model of Schelde et al.and Smith, 1964; van Genucthen and Cleary, 1979; Nkedi- (2002), but accounts for transient porewater flow in both the
macroporous and microporous regions of the soil. This modelKizza et al., 1984). It accounts for an equivalent macropore
Fig. 4. Colloid mass flux (filled circles) and porewater flow rates (solid lines) measured during two ponded infiltration experiments. Colloidconcentrations peak during the passage of both wetting and drying fronts (from El-Farhan et al., 2000).
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348 VADOSE ZONE J., VOL. 3, MAY 2004
While progress has been made toward developing a capabil-ity to simulate the unsaturated transport of colloids in nonidealsystems characterized by porous-medium heterogeneity, thereis clearly a long way to go. Available models are very simpleand incorporate only a subset of the mass-transfer processesthat combine to influence colloid mobility in the vadose zone.Additional testing of models over a broader range of experi-mental conditions is needed. These model-data evaluationswill lead to model refinement by illuminating gaps in ourunderstanding of processes and will help to define quantitativerelationships between model parameters and measurable sys-tem properties.
FUTURE DIRECTIONS
To better understand colloid movement through the unsatu-rated zone, five major areas of research should be emphasized:(i) improved visualization of unsaturated flow and colloidtransport phenomena, (ii) continued investigation of transientflow (wetting and drying) conditions, (iii) further examina-tion of the effects of soil structure on colloid mobilizationand transport, (iv) better quantification of pore straining ofcolloids and its effect of soil clogging, and (v) assessmentof colloid mobilization under extreme conditions present at
Fig. 5. Cumulative colloid mobilization as a function of the square waste sites.root of time during leaching through intact macroporous soil cores.Using tools like light transmission through transparent mi-The linear relationship between these variables suggests that the
cro- and meso-models (Wan and Wilson, 1994a; Sirivithaya-kinetics of colloid mobilization were controlled by diffusion (frompakorn and Keller, 2003), magnetic resonance imaging, andJacobsen et al., 1997).X-ray computed tomography, efforts are underway to improveour understanding of flow and colloid transport in the un-is based on the assumptions that mineral colloids are only
mobilized at the soil surface, not within the soil profile, and saturated zone (Darnault et al., 2002; Nestle et al., 2002;Wildenschild et al., 2002; Weisbrod et al., 2003). As the resolu-that colloid deposition in both porewater domains can be
described by simple first-order kinetics expressions. Calcula- tion and capabilities of these visualization systems improve,it will be possible to test hypotheses regarding proposed mech-tions of the model of Jarvis et al. (1999) agree reasonably well
with colloid concentrations measured over an 80-d period in anisms of colloid mobilization and deposition, as well as toidentify new mechanisms that cannot readily be inferred fromsoil water samples collected from a tile-drained silty clay soil
in Sweden. analysis of column experiments. Visualization experiments
Fig. 6. Representation of a single equivalent macropore with colloid mass transfer between three phases: mobile water, immobile water, andcrust. The horizontal arrows indicate colloid diffusion between phases, and the vertical arrows indicate advective colloid transport (fromSchelde et al., 2002).
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www.vadosezonejournal.org 349
that permit air–water interface reactions to be unambiguously 1987; Liang et al., 1993; Schemel et al., 2000), must be assessedin vadose zones subject to these hazardous-waste environments.distinguished from solid–water interface reactions should be
particularly useful in guiding the development of mechanisticmodels for colloid deposition and mobilization. ACKNOWLEDGMENTS
Transients in flow conditions—the wetting and drying cyclesThis work was supported by National Science Foundationof soils—have recently been identified in field and laboratory
grants EAR-9909553 (JNR) and EAR-9909508 (JES) and De-experiments as key factors governing the mobilization of soilpartment of Energy grants DE-FG07-02ER63492 (JES) andcolloids (El-Farhan et al., 2000; Saiers and Lenhart, 2003b;DE-FG07-02ER63491 (JNR).Saiers et al., 2003). This transient flow–induced mobilization
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