DSpace at VNU: Clay dispersion and its relation to surface charge in a paddy soil of the Red River Delta, Vietnam

J. Plant Nutr. Soil Sci. 2009, 172, 477–486

DOI: 10.1002/jpln.200700217

477

Clay dispersion and its relation to surface charge in a paddy soil of the Red
River Delta, Vietnam
Minh N. Nguyen1*, Stefan Dultz1, Jörn Kasbohm2, and Duc Le3
1

Institute of Soil Science, Leibniz University of Hannover, Herrenhäuser Str. 2, 30419 Hannover, Germany
Institute of Geography and Geology, Greifswald University, Friedrich-Ludwig-Jahn Str. 17a, 17487 Greifswald, Germany
3 Department of Pedology and Soil Environment, Faculty of Environmental Sciences, University of Science, 334-Nguyen Trai, Hanoi, Vietnam
2

Abstract
Dispersion is an important issue for clay leaching in soils. In paddy soils of the Red River Delta
(RRD), flooding with fresh water and relatively high leaching rates can accelerate dispersion and
the translocation of clay. For the clay fraction of the puddled horizon of a typical paddy soil of the
RRD, the effect of various cations and anions as well as humic acid (HA) at different pH values
on the surface charge (SC) were quantified and the dispersion properties were determined in
test tubes and described by the C50 value.
In the <2 lm fraction, dominated by illite, the proportion of 2:1 vs. 1:1 clay minerals is 5:1. The
organic-C content of the clay fraction is 2.2%. Surface charge was found to be highly pH-dependent. At pH 8 values of –32 and at pH 1 of –8 mmolc kg–1 were obtained. Complete dispersion
was observed at pH > 4, where SC is > –18 mmolc kg–1. The flocculation efficiency of Ca strongly
depends on the pH. At pH 4, the C50 value is 0.33, 0.66 at pH 5, and 0.90 mmol L–1 at pH 6. At
pH 6, close to realistic conditions of paddy soils, the effect of divalent cations on the SC and flocculation decreases in the order: Pb > Cu > Cd > FeII > Zn > Ca > MnII > Mg; FeII was found to
have a slightly stronger effect on flocculation than Ca. An increase in concentrations of Ca, MnII,
and Mg from 0 to 1 mmol L–1 resulted in a change in SC from –25 to approx. –15 mmolc kg–1. In
comparison, the divalent heavy-metal cations Pb, Cu, Cd, and Zn were found to neutralize the

SC more effectively. At a Pb concentration of 1 mmol L–1, the SC is –2 mmolc kg–1. From pH 3 to
–
5, the dispersion of the clay fraction is facilitated rather by SO2À
4 than by Cl , which can be
explained by the higher affinity of SO2À
to
the
positively
charged
sites.
With
an
increase of the
4
amount of HA added, the SC continuously shifts to more negative values, and higher concentrations of cations are needed for flocculation. At pH 3, where flocculation is usually observed, the
presence of HA at a concentration of 40 mg L–1 resulted in a dispersion of the clay fraction.
While high anion concentrations and the presence of HA counteract flocculation by making the
SC more negative, FeII and Ca (C50 at pH 6 = 0.8 and 0.9 mmol L–1, respectively) are the main
factors for the flocculation of the clay fraction.
For FeII and Ca, the most common cations in soil solution, the C50 values were found to be relatively close together at pH 4, 5, and 6, respectively. Depending on the specific mineralogical
composition of the clay fraction, SC is a suitable measure for the determination of dispersion
properties and for the development of methods to keep clay particles in the soil in the flocculated
state.
Key words: paddy soil / surface charge / clay dispersion / heavy metals / anion effects / humic acid

Accepted February 18, 2008

1 Introduction
The Red River Delta (RRD) is typically used for the cultivation
of rice. In this area, high rainfall (1600–1900 mm y–1) and

human activities like tillage and irrigation can induce a dispersion of clay. Due to harvesting 2–3 times per year, there are
seasonal changes of the water regime causing a change of
oxidizing and reducing conditions. In the RRD, there are also
many so-called handicraft villages with a large number of
small metalworking factories, which have impacts on the
environment. The soil under investigation is in the vicinity of
such a handicraft village where, in addition, nonferrous–

heavy metal (HM) recycling has been carried out for many
decades. As a consequence, increased HM concentrations
are found in the soils and especially in the channels around
the village (Le and Nguyen, 2004). Increases of HM contents
below the puddled horizon were also reported by Nguyen et
al. (2006). Besides solute transport, leaching of HMs may
also be facilitated if they are adsorbed on clay minerals and
dissolved organic C (DOC) (Karathanasis, 2003; de Jonge et
al., 2004). The leaching of clay is a common observation in
paddy soils (Boivin et al., 2004; Nguyen et al., 2006).

* Correspondence: M. N. Nguyen; e-mail:

 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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478

Nguyen, Dultz, Kasbohm, Le

J. Plant Nutr. Soil Sci. 2009, 172, 477–486

For clay flocculation and the formation of stable aggregation,
type and concentration of cations in the soil solution are wellknown as decisive factors. The presence of di- and trivalent
cations in the electric double layer can decrease zeta potential by reducing net negative charge of the clay-minerals surfaces and thus accelerate flocculation more effectively than
monovalent cations. At a given pH, increased cation concentrations can make the surface charge (SC) of clay fraction
less negative and accelerate the flocculation. The cation concentration affects the thickness of the electric double layer,
which plays a decisive part, because the overlap of this layer
from two particles is a prerequisite for flocculation (Goldberg
and Forster, 1990; Sumner, 1993).
A decrease in SC to more negative values is known as the
primary factor for clay dispersion. The pH affects dispersion
by changing the SC of the clay particles through protonation
and deprotonation reactions of variable charged sites (Thellier et al., 1992; Kaya, 2006). Chorom and Rengasamy
(1995) reported a positive relationship between pH and the
percentage of dispersible clay. The increase in SC is also
positively correlated with pH.

persion of clay. Additions of lime and soluble salts from different sources such as fertilizers (Haynes and Naidu, 1998) and
the release of FeII and MnII under reducing conditions (Wada
et al., 1983; Saejiew et al., 2004) are expected to be factors
for clay flocculation. In this study, the SC and flocculation
properties of the clay fraction of the puddled horizon from a
typical paddy soil of the RRD were determined at different pH
and concentrations of cations and anions, which are found in
the soil solution. Soaking of a soil sample in water under
anaerobic conditions, representing reducing conditions in
paddy soils were conducted in order to determine decisive
factors for clay dispersion in the flooded period. As the
streaming potential, which is utilized to evaluate the zeta

potential, may show a weak reproducibility (Böckenhoff and
Fischer, 2001), the use of the SC, quantified by polyelectrolyte titration, as a parameter for clay dispersion was determined.

2 Materials and methods
2.1 Materials

In other studies on clay flocculation, it is tried to bracket the
equilibrium state between solution and the exchange complex either by presaturating the clay with the targeted SAR
(Goldberg and Forster, 1990) or by using both Na- and Casaturated clays (Saejiew et al., 2004) considering various
SAR and ionic strength. As the focus of this study was the
quantification of the effect of many different cations on SC
and flocculation, only the Na-saturated clay fraction was considered.

The soil used in this study was selected from a soil series
taken at the end of the dry season in March 2005 in flooded
rice fields, in direct vicinity of the handicraft village Tong Xa
with nonferrous-HM recycling activities, Red River Delta,
Vietnam (106°1′12″ E and 20°19′48″ N). The soil sample was
taken from the puddled horizon (0–25 cm depth). The sample
was air-dried and passed through a 2 mm sieve. The clayloam soil (sand: 22%, silt: 45%, clay: 33%) has a slight acidic
reaction (pH 5.6). The cation-exchange capacity (CEC) is
125 mmolc kg–1. Dithionite-soluble Fe is 1.7%. The organic-C
content is 2.2%, which is typically high for paddy soils. The
C : N ratio is 9.7. The DOC concentration, quantified with a
TOC-analyzer (elementar, liqui TOC trace) in a 1:10 aqueous
extract, is 186 mg kg–1. Soluble cations and anions (Tab. 1)
were extracted with de-ionized water (1:10) and detected by
inductively coupled plasma (ICP-OES) and anion chromatography (DIONEX ICS-90). In the solution, an abundance of
soluble Ca and Mg was observed while the concentration of
–

SO2À
4 was found to be much higher than that of Cl (Tab. 1).
In the charge balance, where the charge of dissolved organic
matter (DOM) is not considered, the sum of cations corresponds satisfactorily with the sum of anions (Tab. 1). Also at
the exchange sites (Ag-thiourea method performed at soil pH
according to Pleysier and Juo, 1980), Ca (62%) and Mg
(30%) are the most common cations.

In paddy soils, flooding and tilling for a new crop can facilitate
the dispersion of clay. In several periods of a year, a change
of pH due to the alternation of soil condition from dry to
flooded or in the reverse direction can also influence the dis-

For the preparation of soil solution, an experiment of soaking
the sample in water under anaerobic conditions was performed. After a period of 15 d, the FeII content released from
the sample to the solution is 3.6 mmol kg–1, which is approx.

The effect of inorganic and organic anions on clay dispersion
is also an important issue. Anions can be adsorbed on positively charged edge sites of clay minerals and counteract flocculation (Gu and Doner, 1993). Salts with multivalent inor3À
ganic anions such as SO2À
4 and PO4 were found to increase
the critical coagulation concentration of clay-mineral suspensions (Penner and Lagaly, 2001). Also the addition of small
amounts of humic acid (HA), citrate, formate, carbonate, and
silicate can increase the critical flocculation concentration of
kaolinitic soil clays (Frenkel et al., 1992). At pH 4, small additions of HA to kaolinite resulted in edge charge reversal from
positive to negative and substantially reduced coagulation
rates (Kretzschmar et al., 1998). As an effect of organic
anions, soil-structure stability was found to decrease (Goldberg et al., 1990; Tejada and Gonzalez, 2007).

Table 1: Soluble cations and anions in the 1:10 aqueous extract (pH 5.6) of the puddled horizon of a paddy soil from the Red River Delta,

Vietnam.
Na+

K+

NH4+

Ca2+

Mg2+

Mn2+

Fe2+

Cu2+

Zn2+

Cl–

SO42–

R cations

R anions

0.02

4.7

46.2

58.18

50.9

(mmolc kg–1)
8.0

0.66

2.9

31.4

15.0

0.14

0.05

 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

0.01

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J. Plant Nutr. Soil Sci. 2009, 172, 477–486

Clay dispersion and its relation to surface charge 479
tion and decantation. The suspension was flocculated with
NaCl, centrifuged, washed until salt-free, and freeze-dried. In
order to determine the effect of HA on dispersion, the clay
fraction was pretreated with H2O2 in order to remove organic
matter (OM).

0.334
|

Intensity (rel.)

d-values (nm)
0.717
|

1.00
|
1.43
|

0.358
|

0.499
| 0.473
| 0.426
|

2.2.2 Preparation of clay suspensions
a

0.485
|

b

c
|0.325
d

5

10

15
20
°2 Theta

25

a) Mg saturation

c) Mg saturation, ethylene glycol treatment

b) K saturation

d) K saturation, 550°C treatment

30

Figure 1: X-ray diffraction patterns of the clay fraction (<2 lm).
Saturation of the sample and pretreatment are given in the figure.

40 mol% from the sum of cations in the solution. This emphasizes the high importance of FeII for dispersion during the
flooded period.
The total K content (PHILIPS X-ray spectrometer PW2404)
of the clay fraction is 2.77% confirming the presence of relatively high amounts of illite in the sample, which is also observed in the XRD patterns (Fig. 1; PHILIPS X-ray diffractometer PW1390 with Cu Ka radiation, oriented samples on
glass slides). The presence of chlorite is indicated from the
1.43 nm spacing in the sample with 550°C treatment. As the
spacing at 1.0 nm appears higher than that at 0.72 nm at K
saturation in comparison with Mg saturation, there is indication for the contraction of 2:1 layer silicates on K treatment.
Ethylene glycol treatment causes some increase of the background at low 2° Theta, but no discrete interferences at
1.8 nm are observed. Also in the FTIR spectrum (Bruker,
Tensor 27), kaolinite and quartz are clearly detectable.
According to quantitative IR phase analysis, the content of
kaolinite and quartz is 15% and 8%, respectively. 2:1 layer
silicates, which have a total content of approx. 75% are much
more abundant than 1:1 layer silicates. The proportion of 2:1
vs. 1:1 clay minerals is 5:1. The presence of relatively high
amounts of permanent negatively charged clay minerals in
the sample, mainly illite, is also confirmed from the surfacecharge determinations (section 3.1, Fig. 2).

2.2 Methods
2.2.1 Separation of the clay fraction
Fine soil was dispersed by shaking overnight in de-ionized
water. The clay fraction (<2 lm was separated by sedimenta 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Solutions for the determination of the flocculation and SC

were prepared from pure analyzed salts from Merck KGaA
including CuCl2, Pb(NO3)2, ZnCl2, Cd(NO3)2, CaCl2, MgCl2,
MnCl2, AlCl3, FeSO4, KCl, NaCl. An amount of 20 mg of the
clay fraction was added to 10 mL of solutions where the concentrations were graded according to preliminary experiments: 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 mmol L–1
for Na and K; 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, and
1.0 mmol L–1 for FeII, Cu, Pb, Zn, and Cd; 0.2, 0.4, 0.6, 0.8,
1.0, 1.2, 1.4, 1.6, 1.8, and 2.0 mmol L–1 for Ca, MnII, and Mg;
0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, and
0.10 mmol L–1 for Al. The solutions were adjusted with 0.1 M
HCl and 0.1M NaOH to targeted pH values.
For the determination of anion effects, 20 mg of the clay fraction were added into 10 mL of solutions of NaCl and Na2SO4
(0–10 mmol L–1) respectively. At the same concentration of
Na, differences in flocculation can be attributed to the different anions Cl– and SO2À
4 . The dispersion properties were
determined at pH 3.0, 3.5, 4.0, 4.5, 5.0, 5.5, and 6.0, a pH
range which can be observed in paddy soils of the RRD. HCl
and H2SO4 were correspondingly used to adjust the pH.
For the determination of the effect of HA on SC and flocculation, 20 mg of the clay fraction were mixed with 10 mL of different concentrations of HA (0–100 mg L–1). The pH of the
suspensions was adjusted to 4, 5, and 6 by addition of 0.1 M
HCl. For the determination of the effect of FeII on clay flocculation in presence of HA, 20 mg of the clay fraction and 0.001
mg of HA were mixed with 10 mL of FeII solutions (0–1 mmol
L–1). The experiments were performed directly after mixing
and under N2 atmosphere in order to prevent oxidation of
FeII. 0.1 M NaOH and 0.1 M HCl were used to adjust the pH
of 4 and 5. The preparation was repeated with higher
amounts of HA (0.01 and 0.1 mg).
Humic acid was prepared according to the recommendations
of the International Humic Substances Society (Swift, 1996).
The soil sample used in this study was shaken overnight with
0.1 M NaOH (N2 atmosphere) with a soil-to-extractant ratio of

1:10. After separating the supernatant by centrifugation, the
pH was adjusted to 1 with 6 M HCl. The suspension was
allowed to stand overnight at room temperature. The fulvic
acid was separated from the coagulate by centrifugation.
Suspended HA was dissolved in a minimum volume of 0.1 M
KOH and 0.2 M KCl (total of 0.3 M K). The solution was again
purified from fulvic acid. For reducing the ash content <1%,
the HA precipitate was treated with 0.1 M HCl and 0.3 M HF
for 7 d. The purified HA was then washed several times with
de-ionized water and freeze-dried. Stock solutions of Na
humate were prepared by dissolving HA with 0.1 M NaOH
and subsequent dialyzing against de-ionized water.
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Nguyen, Dultz, Kasbohm, Le

2.2.3 Determination of flocculation properties
The flocculation properties were determined in test tubes
according to Lagaly et al. (1997). The suspensions of the clay
fraction prepared in section 2.2.2 were transferred to test
tubes and dispersed in an ultrasonic bath (Sonorex, RK 106)
for 15 s. After 2 h sedimentation at room temperature, 2 mL
of each suspension were sampled from the surface of the
suspension and the transmission was determined in a UVVIS photometer (Varian, Cary-50 Scan) at a wavelength of
600 nm. The ion concentration which results in a flocculation
corresponding to a transmission of 50% (C50 value) was used
for the comparison of the effectivity of different cations and
anions on the flocculation.
2.2.4 Quantification of surface charge

For SC determinations, a particle-charge detector (PCD;
Mütek PCD 03) combined with a titrator (Mettler DL25) was
used by employing a titration with the charge-compensating
cationic polyelectrolyte poly-DADMAC (poly-Dially-dimethylammonium chloride) for negative SC. In the cell of the PCD,
the streaming potential (Hunter, 1993; Müller, 1996), generated by a flow of liquid with an oscillating piston, is determined and utilized to evaluate the zeta potential. The streaming potential provides information about the weakly bound
fraction of counter-ions, which are from high importance for
the interactions between particles in suspensions. The titration with poly-DADMAC is performed across the point of zero
charge. This technique was reported to produce reasonable
results as long as the PCD signal is only used in combination
with polyelectrolyte titration to detect the sign of particle
charge and to indicate the point of zero charge (Böckenhoff
and Fischer, 2001). This method is already well-established
for the determination of SC properties of soil clay fractions
dependent on pH and Ca concentration (Böckenhoff et al.,
1997), metal complexation capacities of aquatic humic substances (Weiss et al., 1989), and adsorption properties of
tannic acid on modified montmorillonite (An and Dultz, 2007).
Clay suspensions, prepared in the same way as for the
experiments on the determination of flocculation properties
(10 mL suspension including 20 mg clay fraction; section
2.2.3), were dispersed by an ultrasonic treatment for 15 s and
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then transferred into the titration cell. The titration with the
polyelectrolyte, performed across the point of zero charge,
was terminated at the point of zero charge, where the electrokinetic potential is zero. The determined SC is a net value
from the sum of negative and positive charges.

3 Results and discussion
3.1 Effects of pH on flocculation
With a decrease of the pH of the clay suspension from 8 to 1,

the SC increases from –32 to –8 mmolc kg–1 (Fig. 2). The factor responsible for the less negative in SC at low pH is the
conversion of variable charged edge sites from negative to
positive (van Olphen, 1977). Even at pH 1, negative SC of
the clay fraction was detected, which confirms the presence
of relatively high amounts of permanent negatively charged
clay minerals in the sample. Between pH 4 and 3, the transmission of the suspension is increasing from 2% to 90%,
which indicates flocculation of the clay fraction. The C50 value
observed at pH 3.5, corresponding with a SC of
–16 mmolc kg–1, suggests that edge sites are at the point of
zero charge. At pH < 3.5, positively charged edge sites of the
clay minerals might favor the formation of edge-to-face structures, the so-called “card house” (van Olphen, 1977), which
produces voluminous aggregates of low density. At pH > 3.5,
due to the formation of negative edge charge, clay particles
tend to disperse. Flocculation might also be accelerated by
increasing cation concentrations, including Al3+ in the suspension, which are due to mineral-dissolution reactions at
low pH. For the pH range from 4.5 to 6, which are typical values found for paddy soils in the RRD, the SC is relatively low
(–24 to –27 mmolc kg–1). This might facilitate dispersion even
at high cation concentrations. At higher pH, OH– ions interact
with the edge sites of clay minerals and make them neutral or
negatively charged. As a consequence, SC of clay fraction
becomes more negative and the dispersion is facilitated
(Chorom and Rengasamy, 1995). Itami and Fujitani (2005)
recommended that keeping pH < 6–7 can prevent edge surfaces of kaolinite from dissociation, which is primarily important for the control of dispersion.
The flocculation efficiency of Ca strongly depends on the pH
(Fig. 3). At higher pH, the concentration of Ca needed for floc100
-30
r
Su

-20

e
arg
ch
e
fac

80
60
40

-10
20

Transmission (%)

Soil solutions were prepared in order to evaluate the flocculation properties of the clay fraction under flooding conditions of
the paddy soil. For this purpose, 500 g of the soil sample
were soaked in 1 L of de-ionized water in a glass bottle. Free
oxygen was removed by supplying N2 gas before the bottle
was tightly closed with a septum. The release of cations and
anions during the experiment was controlled by sampling the
supernatant (5 mL) every 24 h. The increases of the concentration due to losing of solution volume after each sampling
were also taken into account. The concentrations of Mg, Ca,
Fe, Mn, Cl–, and SO2À
4 were analyzed by AAS (Perkin Elmer,
Analyst 300) and anion chromatography (DIONEX ICS-90),
respectively. After 15 d, the supernatant was separated, analyzed, and diluted with de-ionized water in different ratios
from 1:10 to 1:1 and used for the determination of flocculation
properties.

J. Plant Nutr. Soil Sci. 2009, 172, 477–486

Surface charge (mmolc kg-1)

480

Transmission

0

0
1

2

3

4

5

6

7

8

pH
Figure 2: Surface charge and pH-dependent flocculation (measure:

transmission) of the suspension of the clay fraction.

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J. Plant Nutr. Soil Sci. 2009, 172, 477–486

Clay dispersion and its relation to surface charge 481

100

100

80

pH 3

60

Transmission (%)

Transmission (%)

a

pH 4

40

pH 5

20

pH 6

80
60
40
20
0

0

0.2

0.4

0.6

0.8

1.0

-1

Ca concentration (mmol L )
Figure 3: pH-dependent flocculation of the suspension of the clay
fraction at different Ca2+ concentrations.

culation is vigorously increased. At pH 4, the C50 value is

0.33, at pH 5 it is 0.66, and at pH 6 it is 0.90 mmol L–1. At
pH 4, only 30% of the Ca amount in solution is needed to
reach the C50 value in comparison with that at pH 6. At constant electrolyte levels, an increase in clay dispersion with
increasing pH was reported (Suarez et al., 1984). The fact
that 100% transmission is not measured in the experiments
after clay flocculation is probably due to the presence of
DOM in the supernatant. In soils, the competitive binding of
H+ and Ca2+ by soil functional groups affects SC and soilsolution chemistry. Calcium released or exchanged by H+ at
low pH can result in a change in SC to less negative values
and accelerate flocculation (Chorom and Rengasamy, 1995).
At pH 3, the clay fraction is flocculated in the whole concentration range from 0–1.0 mmolc kg–1. As mentioned above,
besides the formation of edge-to-face structures, this might
also be due to cation release by mineral dissolution. Even in
soils with high amounts of permanent negatively charged
clay minerals, the effect of pH on dispersion is a common
finding, but it is most pronounced in kaolinitic soils with relatively high amounts of variably charged sites (Kaya, 2006).

3.2 Cation effects
At pH 6, for the monovalent cations Na and K the flocculation
of the clay fraction started at a relatively high concentration
(C50 values at 40 and 27 mmol L–1, respectively). Also the SC
was found to be slightly affected. For all di- and trivalent
cations under investigation, an increase in transmission was
observed in the concentration range up to 1.0 mmol L–1
(Fig. 4a). The addition of AlCl3 solution was found to be most
effective on flocculation even at pH 6, where it can be
assumed that most of the Al is present as Al(OH)‡
2 species in
the solution. Flocculation was almost completed at the concentration of 0.2 mmol L–1 (C50 value at 0.11 mmol L–1). Concerning the divalent cations, Pb and Cu were most effective.
The strength of cations on flocculation at pH 6 based on the

C50 values is in the order (mmol L–1): Al (0.11) > Pb (0.3) >
Cu (0.45) > Cd (0.7) > Fe (0.8) > Zn (0.85) > Ca (0.9) > Mn
(1.1) > Mg (1.5) > K (27) > Na (40). This order is well-fitted to
the results of other studies (Arora and Coleman, 1978; Heil
and Sposito, 1993; Sumner, 1993) in which the strength of
cation was reported to be controlled by the valency, ionic
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Surface charge (mmolc kg-1)

0

-25

b

-20
-15
-10
-5
0
0

0.2

0.4

0.6

0.8

1.0

Cation concentration (mmol L-1)
Al
Pb
Cu

Cd
FeII
Zn

Ca
Mn
Mg

Figure 4: Flocculation (a) and corresponding surface charge (b) of
the clay fraction in dependence on the kind of cation at pH 6.

radius, and hydrated-ion size. In comparison with the findings
of Saejiew et al. (2004), slight differences in the effectiveness
of Ca and Fe were observed. However, there is a general difficulty when comparing the results, as Saejiew et al. (2004)
applied other methods (settling time of 24 h, different SAR
values, results are presented in grams of suspended clay).
An increase of the concentration of Ca, MnII, and Mg from 0 to
1 mmol L–1 results in a change in SC from –25 to approx.
–15 mmolc kg–1 (Fig. 4b). In comparison with these divalent
cations, the divalent heavy-metal cations Cu, Pb, Zn, and Cd
were found to neutralize the SC more effectively. Especially, the
presence of Pb can increase the SC, which is at a concentration of 1 mmol L–1 at –2 mmolc kg–1. The addition of Al can

completely neutralize the negative SC. The point of zero charge
is reached, when the Al concentration is 0.19 mmol L–1.
In comparison of the most common cations of the investigated soil, FeII has a slightly stronger effect on flocculation
than Ca. This is especially true at lower concentrations,
where the transmission of the suspension is increasing quite
earlier in presence of FeII than of Ca. From SC determination,
also the trend for a stronger increase of the value at the same
concentrations is observed. Despite the fact that Ca and FeII
(Tab. 1, section 3.5) are the most common cations in soil
solution, the pronounced effect of some heavy-metal cations
like Pb on flocculation and SC is an interesting issue. This
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Nguyen, Dultz, Kasbohm, Le

J. Plant Nutr. Soil Sci. 2009, 172, 477–486

might be of special importance for the channels around Tong
Xa village where in comparison with the adjacent soils highest concentrations of HMs are found (Le and Nguyen, 2004).

3.3 Anion effects
Whereas at pH 3 and 3.5, clay flocculation was observed in
the NaCl as well as in the Na2SO4 solution in the concentration range up to 10 mmol L–1, at pH 4, 4.5, and 5, flocculation
is observed only in case of addition of the NaCl solution
(Fig. 5a, b). From pH 3 to 5, the dispersion of the clay fraction
–
is facilitated by SO2À
4 rather than by Cl . Whereas at pH 3 the
change of transmission is the same for both anions, at higher

pH (3.5–5) the transmission of the suspension increases
quite earlier for Cl– than for SO2À
4 . This might be due to the
higher affinity of SO2À
to
the
positively charged sites.
4
Obviously, SO2À
4 can neutralize positive charges more effectively, which consequently counteracts flocculation. Finegrained permanent negatively charged muscovite that was
used to affirm this effect showed a similar trend. Furthermore,
–
in this experiment SO2À
4 much more than Cl facilitated the
dispersion at low pH. The stronger effect of multivalent
anions on dispersion was also confirmed by Penner and
Lagaly (2001), where the addition of SO2À
and PO3À
4
4
severely increased the critical flocculation concentration of
clay suspensions. Phosphate is known to form innersphere
complexes on surfaces, which decreases SC of the clay fraction and, as a consequence, facilitates dispersion (Jose et
al., 2000).
100

Transmission (%)

80

Cl
SO4

30

20

10

0
3.0

3.5

4.0

4.5

5.0

5.5

6.0

pH
Figure 6: pH-dependent C50 values of the suspensions of the clay
fraction for Cl– and SO2À
4 (Na salts added).

At pH 5, where the SC of the clay fraction decreased to

–17.5 mmolc kg–1 (Fig. 2), a change in the effect of Cl– and
SO2À
4 on dispersion is observed. From the C50 values at pH
5.5 and 6 it can be deduced that a higher amount of NaCl in
comparison with Na2SO4 is needed for flocculation (Fig. 6).
This means that the divalent SO2À
4 is more strongly attracted
at pH < 5, where increased protonation of functional groups
induces a higher density of positively charged sites. On the
other hand, the stronger attractive force for SO2À
4 can also
depend on the kind of functional groups protonated in this pH
region. At pH < 5, there is indication that the repulsion between the particles is accelerated if SO2À
4 is adsorbed on the
surfaces, especially at the edge sites. At pH 6, close to realistic conditions in paddy soils, SO2À
4 , the most common anion
in the soil solution (Tab. 1), accelerates flocculation more
strongly than Cl–.

60
a) Na2SO4

3.4 Effects of humic acid

b) NaCl

For both, purified HA as well as the clay fraction, where OM
was previously removed, an adjustment to different pH
resulted in a severe increase in SC at low pH (Fig. 7). In comparison with the SC of the original clay fraction (Fig. 2), at pH
< 4, a slightly higher SC was observed after the removal of

OM, and at pH > 5, the values were quite similar. For HA, the
point of zero charge was observed at pH 1.5. When the pH
was increased to 10, the SC of HA decreased to
–2720 mmolc kg–1. In comparison with the clay fraction, at pH
10 the SC of HA is two orders of magnitude higher.

40

20

0
100

80

Transmission (%)

40

C50 value (mmol L-1)

482

60

40

20

0

2

4

6

8

10

Na concentration (mmol L-1)
pH:

3

3.5

4

4.5

5

Figure 5: Effects of Na2SO4 and NaCl on the flocculation of the clay
fraction.

 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

In presence of HA, the SC of the suspension was observed
to be more negative. At pH 4, 5, and 6, an addition of HA in

amounts from 0 to 30 mg L–1 resulted in a decrease of the SC
of the clay suspensions from –17.6 to –29.8, from –19.2 to
–46.6, and from –31.3 to –52.1 mmolc kg–1, respectively. With
an increase of the amount of HA added, the SC continuously
shifted to more negative values. At pH 3, where flocculation
was usually observed (Fig. 3 and 5a, b), the presence of HA
at a concentration of 40 mg L–1 resulted in a dispersion of the
clay fraction (Fig. 8). Most probably, this is due to a reversal
of edge charge of the clay fraction from positive to negative in
the presence of HA (Kretzschmar et al., 1997), so that the forwww.plant-soil.com


Clay dispersion and its relation to surface charge 483

-40

100
Humic acid
Clay fraction,
organic matter removed

Transmission (%)

Surface charge: humic acid (dmolc kg-1)
-1
clay fraction (mmolc kg )

J. Plant Nutr. Soil Sci. 2009, 172, 477–486

-30

-20

-10

80
60
40
20
0

0
2

3

4

5

6

7

8

0

0.2

Figure 7: pH-dependent surface charge of humic acid and the clay
fraction, where organic matter was removed.

mation of edge-to-face structures (card houses) and the
resulting flocculation is prohibited.
By adding HA to the suspension, the amount of FeII needed
for flocculation is increased severely (Fig. 9). At a HA concentration of 20 mg L–1, the C50 values for FeII were found to
increase from 0.23 to 0.61 at pH 4 and from 0.49 to 0.82 at
pH 5. With increasing pH values, the SC of HA and clay
minerals becomes more negative. As a result, higher FeII
concentrations are required for the flocculation of the clay
fraction. Kretzschmar et al. (1998) reported that already small
additions of HA caused pronounced increases in colloidal stability and the coagulation rates became strongly dependent
on ionic strength. The increase in ionic strength generally
resulted in decreased colloidal stability indicating that the
suspensions were stabilized by electrostatic repulsive forces.
With an increase in pH from 4 to 6 and HA concentration from
0 to 6 mg L–1 (TOC), colloidal stability of clay suspensions
increased.

Transmission (%)

100
80
60
40
20
0
20

40

0.6

0.8

1.0

-1

Fe concentration (mmol L )

pH

0

0.4
II

60

80
-1

Humic acid concentration (mg L )
Figure 8: Effect of humic acid on the dispersion of the clay fraction at
pH 3.

 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Clay fraction:

pH:

Original
10 mg L-1 HA added
20 mg L-1 HA added

4
5

Figure 9: pH-dependent flocculation of the clay fraction in presence
of 10 and 20 mg L–1 HA and increasing FeII concentrations.

3.5 Flocculation by soil solutions
After 24 h soaking time of the soil in water under anaerobic
conditions (N2 atmosphere), a remarkable increase of the Ca
and Mg concentration was observed (1.7 and 0.3 mmol L–1),
which on the one hand might be due to soluble salts (Tab. 1)
and on the other hand to a desorption from cation-exchange
sites of clay minerals and OM. Up to day 10, increasing
amounts of Fe and Mn were determined in the supernatant.
Afterwards, only slight changes in the chemistry of the solution occurred. On day 15, the final concentration for Fe, Ca,
Mg, and Mn were 1.84, 2.01, 0.29, and 0.17 mmol L–1, respectively. During the experiment, an increase of the pH of
the solution from 5 to 7 was observed, which is most probably
due to the dissolution of Mn and Fe oxides under reducing
condition. The shift of pH can facilitate the dispersion of clay
and increase the critical flocculation concentration of cations.
From the transmission of the suspensions determined at different dilution ratios, it can be concluded that the amount of
cations corresponding to a dilution ratio 3:10 is sufficient to

induce flocculation (Fig. 10). The C50 value is reached at a
dilution ratio of 2:10 where the concentrations of FeII, Ca, Mg,
and MnII are 0.37, 0.40, 0.06, 0.04 mmol L–1, respectively. In
comparison with the C50 values of these cations at pH 6 (cf.,
section 3.2), it can be concluded that FeII (C50 value at
0.8 mmol L–1) and Ca (C50 value at 0.9 mmol L–1) are the
main factors for the flocculation of the clay fraction in the
selected paddy soil. The sum of the FeII and Ca concentration at a dilution ratio of 2:10 is 0.77 mmol L–1. This critical
concentration is smaller than that of Ca or FeII alone and is
most probably due to other cations like MnII and Mg in the soil
solution, even if they have low concentrations and relatively
high C50 values (1.1 and 1.5 mmol L–1, respectively; cf., section 3.2) and only slight effects on flocculation can be
assumed. Dissolved organic matter, which is 186 mg kg–1 in a
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484

Nguyen, Dultz, Kasbohm, Le

J. Plant Nutr. Soil Sci. 2009, 172, 477–486

Surface charge (mmolc kg-1)

Transmisson (%)

100
80
60
40

20
0

-15
-14
II

-13

s fo
alue
0v

r Fe

C5

-12
-11

C 50

-10

a
rC
s fo
e
u
val

pH 4
pH 5
pH 6

-9
-8

0

2/10

6/10

4/10

10/10

8/10

Dilution ratio (soil solution / deionized water)

0

0.8
Cation concentration (mmol L-1)
0.2

0.4

0.6

1.0

Figure 10: Flocculation of the clay fraction by a soil solution
representing reducing conditions.

Figure 12: pH-dependent C50 values and corresponding SC of the
clay fraction for FeII and Ca.

1:10 aqueous extract of the soil sample under investigation
(cf., section 2) and a common feature in paddy soils (Maie et
al., 2004; Chien et al., 2006), can facilitate clay dispersion
and increase the amount of cations needed for flocculation.

The saturation of the exchange sites by different cations
resulted in different SC at the C50 value. Therefore, the use of
SC to predict flocculation might be limited, if different cations
are present in the soil solution. For divalent cations, also the
formation of monovalent metal chloride ion pairs on the external surface of montmorillonite was reported (Sposito et al.,
1983). The possible sorption of FeIICl+ on clay-crystal edges
was confirmed with 57Fe Mößbauer experiments (Charlet and
Tournassat, 2005), which might affect SC and flocculation
properties. However, for FeII and Ca, the most common
cations in the soil solution of paddy soils, the SC at the C50 is
relatively close together (Fig. 11). The quantification of the
SC at the C50 value for FeII and Ca at pH 4, 5, and 6 shows
that at pH 4, the SC is highest (Fig. 12). At pH 4, SC becomes
less negative, which allows flocculation already at lower concentrations of FeII and Ca. An increase of the pH from 4 to 6
resulted in a change in SC from –10.3 to –13.1 mmolc kg–1

and from –10.4 to –14.6 mmolc kg–1 for FeII and Ca, respectively. At pH 4, 5, and 6, all the values for SC are relatively
close together in the FeII, Ca soil clay system and SC is—
under consideration of the pH—a valuable parameter for prediction of dispersion properties.

3.6 Relationship between surface charge and clay
dispersion

Surface charge (mmolc kg-1)

The effect of cations on SC and flocculation of the clay fraction is given by the relation between SC and C50 value
(Fig. 11). From the cations under investigation, highest SC of
the clay fraction at the C50 value is found for Al and Pb. The
SC has similar values (–5.0 and –5.9 mmolc kg–1, respectively). For these cations already at relatively low concentrations
the C50 value is reached (0.11 and 0.30 mmol L–1, respectively). The SC of Cu is –11 mmolc kg–1. From the other divalent
cations under investigation, SC at the C50 value is relatively
close together (–13.0 to –16.5 mmolc kg–1), but distinct differences in the cation concentration at C50 value are observed.
The highest value is obtained for Mg with 1.5 mmol L–1. The
strength of cations on flocculation (measure: C50 value) is
related with SC and has the order: Al > Pb > Cu > Cd > FeII >
Zn > Ca > MnII > Mg.

-16
C 50

s
lue
va

Al
Pb

Cu
Cd

-12

II

-8

Fe
Zn
Ca
Mn
Mg

-4

5 Conclusions

0
0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

Surface charge is highly dependent on the content of 2:1 and
1:1 clay minerals in the sample, which affects the proportion
of variable to permanent charge. Hence, results on SC determinations depend strongly on the kind of clay minerals present in a sample. The results reported in this study are valid
for the clay fraction under investigation, where the proportion
of 2:1 vs. 1:1 clay minerals is 5:1. With an increasing number
of variably charged sites due to higher amounts of 1:1 clay
minerals and/or secondary oxides, the effect of slight changes in pH and ion concentrations on SC is increasing.

1.6

Cation concentration (mmol L-1)
Figure 11: C50 values and corresponding SC for Al and different
divalent cations.

 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Leaching processes causing downward movement of clay
can be an important factor in rice fields with flooding irrigation
water and may result in environmental risks due to losses of
contaminants to the groundwater. It is well-known, that the
zeta potential is an important parameter for characterizing
clay dispersion. The physicochemical mechanisms of clay
www.plant-soil.com

J. Plant Nutr. Soil Sci. 2009, 172, 477–486
dispersion which is the major prerequisite for clay leaching
were reevaluated in this study for a paddy soil of the RRD.
The kind and concentration of cations, the pH, and, to a lower
extent also the presence of HA and certain anions, affect clay
dispersion in soils primarily by changing the negative SC of
the clay fraction. Inorganic ions and HA were found to have
strongly distinguishable effects on the SC of the clay fraction.
This demonstrates the high accuracy of the applied PCD
technique with polyelectrolyte titration for quantifying dissociated charge. For FeII and Ca, the most common cations in
paddy soils, the strength on flocculation and the related SC
were found for both cations from almost the same value at
certain pH. For such defined soil systems, SC quantified in
knowledge of the pH is another suitable measure for the
determination of dispersion properties and helpful for the
management of dispersive soils. A careful management such
as split dressing of chemical fertilizers can be a helpful tool in
addition to other agricultural practice like regular addition of
organic and Fe-supplying amendments, to decrease clay
leaching. Also predictions of particulate mediated transport of
contaminants can be ameliorated by considering decisive soil
properties for dispersion in modeling.

Acknowledgment
This work was granted by the Ministry of Education and
Training, Vietnam and the DAAD through their fellowship program.

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