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

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

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

Minh N Nguyen 1 *, Stefan Dultz 1 , Jörn Kasbohm 2 , and Duc Le 3

1 Institute of Soil Science, Leibniz University of Hannover, Herrenhäuser Str 2, 30419 Hannover, Germany

2 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

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 C50value

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-depen-dent At pH 8 values of –32 and at pH 1 of –8 mmolckg–1were obtained Complete dispersion

was observed at pH > 4, where SC is > –18 mmolckg–1 The flocculation efficiency of Ca strongly

depends on the pH At pH 4, the C50value is 0.33, 0.66 at pH 5, and 0.90 mmol L–1at pH 6 At

pH 6, close to realistic conditions of paddy soils, the effect of divalent cations on the SC and

floc-culation decreases in the order: Pb > Cu > Cd > FeII> Zn > Ca > MnII> Mg; FeIIwas 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–1resulted in a change in SC from –25 to approx –15 mmolckg–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 mmolckg–1 From pH 3 to

5, the dispersion of the clay fraction is facilitated rather by SO2 than by Cl–, which can be

explained by the higher affinity of SO2 to the positively charged sites With an increase of the

amount of HA added, the SC continuously shifts to more negative values, and higher

concentra-tions of caconcentra-tions are needed for flocculation At pH 3, where flocculation is usually observed, the

presence of HA at a concentration of 40 mg L–1resulted in a dispersion of the clay fraction

While high anion concentrations and the presence of HA counteract flocculation by making the

SC more negative, FeIIand Ca (C50at pH 6 = 0.8 and 0.9 mmol L–1, respectively) are the main

factors for the flocculation of the clay fraction

For FeIIand Ca, the most common cations in soil solution, the C50values were found to be

rela-tively close together at pH 4, 5, and 6, respecrela-tively 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

disper-sion 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:

minh@ifbk.uni-hanno-ver.de

For clay flocculation and the formation of stable aggregation,

type and concentration of cations in the soil solution are

well-known as decisive factors The presence of di- and trivalent

cations in the electric double layer can decrease zeta

poten-tial by reducing net negative charge of the clay-minerals

sur-faces and thus accelerate flocculation more effectively than

monovalent cations At a given pH, increased cation

concen-trations can make the surface charge (SC) of clay fraction

less negative and accelerate the flocculation The cation

con-centration 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

(Thel-lier 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

The effect of inorganic and organic anions on clay dispersion

is also an important issue Anions can be adsorbed on

posi-tively charged edge sites of clay minerals and counteract

floc-culation (Gu and Doner, 1993) Salts with multivalent

inor-ganic anions such as SO2 and PO3 were found to increase

the critical coagulation concentration of clay-mineral

suspen-sions (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

addi-tions 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

(Gold-berg et al., 1990; Tejada and Gonzalez, 2007).

In other studies on clay flocculation, it is tried to bracket the

equilibrium state between solution and the exchange

com-plex either by presaturating the clay with the targeted SAR

(Goldberg and Forster, 1990) or by using both Na- and

Ca-saturated 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

con-sidered

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-persion of clay Additions of lime and soluble salts from

differ-ent sources such as fertilizers (Haynes and Naidu, 1998) and

the release of FeIIand MnIIunder 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

polyelectro-lyte titration, as a parameter for clay dispersion was deter-mined

2 Materials and methods 2.1 Materials

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 clay-loam soil (sand: 22%, silt: 45%, clay: 33%) has a slight acidic reaction (pH 5.6) The cation-exchange capacity (CEC) is

125 mmolckg–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 chromato-graphy (DIONEX ICS-90) In the solution, an abundance of soluble Ca and Mg was observed while the concentration of

SO2 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 corre-sponds 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

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

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 + NH 4 Ca 2+ Mg 2+ Mn 2+ Fe 2+ Cu 2+ Zn 2+ Cl – SO 4 2– R cations R anions

(mmolckg –1 )

40 mol% from the sum of cations in the solution This

empha-sizes 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

rela-tively high amounts of illite in the sample, which is also

ob-served in the XRD patterns (Fig 1; PHILIPS X-ray

diffract-ometer PW1390 with Cu Karadiation, 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

indica-tion for the contracindica-tion of 2:1 layer silicates on K treatment

Ethylene glycol treatment causes some increase of the

back-ground 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

surface-charge 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-tion and decantasedimenta-tion 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 H2O2in order to remove organic matter (OM)

2.2.2 Preparation of clay suspensions

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 con-centrations were graded according to preliminary experi-ments: 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–1for 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–1for 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–1for 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 frac-tion were added into 10 mL of solufrac-tions of NaCl and Na2SO4 (0–10 mmol L–1) respectively At the same concentration of

Na, differences in flocculation can be attributed to the differ-ent anions Cl– and SO2 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 H2SO4were correspondingly used to adjust the pH

For the determination of the effect of HA on SC and floccula-tion, 20 mg of the clay fraction were mixed with 10 mL of dif-ferent 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 FeIIon clay floccu-lation in presence of HA, 20 mg of the clay fraction and 0.001

mg of HA were mixed with 10 mL of FeIIsolutions (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 (N2atmosphere) 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

°2 Theta

a

b

c

d

a) Mg saturation c) Mg saturation, ethylene glycol treatment

b) K saturation d) K saturation, 550°C treatment

|0.325

1.43

|

1.00

|

0.717

| 0.473

|

0.358

|

0.334

|

0.426

|

0.485

| d-values (nm)

Figure 1: X-ray diffraction patterns of the clay fraction (<2 lm).

Saturation of the sample and pretreatment are given in the figure.

Soil solutions were prepared in order to evaluate the

floccula-tion properties of the clay fracfloccula-tion under flooding condifloccula-tions 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 N2gas 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

concen-tration due to losing of solution volume after each sampling

were also taken into account The concentrations of Mg, Ca,

Fe, Mn, Cl–, and SO2 were analyzed by AAS (Perkin Elmer,

Analyst 300) and anion chromatography (DIONEX ICS-90),

respectively After 15 d, the supernatant was separated,

ana-lyzed, and diluted with de-ionized water in different ratios

from 1:10 to 1:1 and used for the determination of flocculation

properties

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

UV-VIS 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% (C50value) 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-dimethyl-ammonium chloride) for negative SC In the cell of the PCD,

the streaming potential (Hunter, 1993; Müller, 1996),

gener-ated by a flow of liquid with an oscillating piston, is

deter-mined and utilized to evaluate the zeta potential The

stream-ing potential provides information about the weakly bound

fraction of counter-ions, which are from high importance for

the interactions between particles in suspensions The

titra-tion 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

sub-stances (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

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 electro-kinetic 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 mmolckg–1(Fig 2) The fac-tor 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 trans-mission of the suspension is increasing from 2% to 90%, which indicates flocculation of the clay fraction The C50value observed at pH 3.5, corresponding with a SC of –16 mmolckg–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

struc-tures, 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 sus-pension, which are due to mineral-dissolution reactions at low pH For the pH range from 4.5 to 6, which are typical val-ues found for paddy soils in the RRD, the SC is relatively low (–24 to –27 mmolckg–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 sur-faces of kaolinite from dissociation, which is primarily impor-tant 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

floc-pH

l c kg

-1) -30

-20

-10

0

20 40 60 80 100

0

Surfa

ce ch arge

Transmission

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

transmission) of the suspension of the clay fraction.

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 C50value in comparison with that at pH 6 At

con-stant 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

soil-solution 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

concen-tration range from 0–1.0 mmolckg–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

rela-tively 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

(C50values 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 AlCl3solution 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

con-centration of 0.2 mmol L–1(C50value at 0.11 mmol L–1)

Con-cerning the divalent cations, Pb and Cu were most effective

The strength of cations on flocculation at pH 6 based on the

C50values 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

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

dif-ficulty 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 mmolckg–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 concentra-tion of 1 mmol L–1 at –2 mmolckg–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 investi-gated 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 FeIIthan 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

Ca concentration (mmol L-1)

0

20

40

60

80

100

0.2 0.4 0.6 0.8 1.0

0

pH 3

pH 4

pH 5

pH 6

Figure 3: pH-dependent flocculation of the suspension of the clay

fraction at different Ca 2+ concentrations.

0 20 40 60 80 100

Cation concentration (mmol L-1)

-1) -25 -20 -15 -10 -5 0

Al Pb Cu

Cd

FeII Zn

Ca Mn Mg

a

b

0

Figure 4: Flocculation (a) and corresponding surface charge (b) of

the clay fraction in dependence on the kind of cation at pH 6.

might be of special importance for the channels around Tong

Xa village where in comparison with the adjacent soils

high-est 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 Na2SO4solution in the

concentra-tion 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 SO2 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 SO2 This might be due to the

higher affinity of SO2 to the positively charged sites

Obviously, SO2 can neutralize positive charges more

effec-tively, which consequently counteracts flocculation

Fine-grained permanent negatively charged muscovite that was

used to affirm this effect showed a similar trend Furthermore,

in this experiment SO2 much more than Cl– facilitated the

dispersion at low pH The stronger effect of multivalent

anions on dispersion was also confirmed by Penner and

severely increased the critical flocculation concentration of

clay suspensions Phosphate is known to form innersphere

complexes on surfaces, which decreases SC of the clay

frac-tion and, as a consequence, facilitates dispersion (Jose et

al., 2000)

At pH 5, where the SC of the clay fraction decreased to –17.5 mmolckg–1 (Fig 2), a change in the effect of Cl– and

SO2 on dispersion is observed From the C50values 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 SO2 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 SO2 can also depend on the kind of functional groups protonated in this pH region At pH< 5, there is indication that the repulsion be-tween the particles is accelerated if SO2 is adsorbed on the surfaces, especially at the edge sites At pH 6, close to realis-tic conditions in paddy soils, SO2 , the most common anion

in the soil solution (Tab 1), accelerates flocculation more strongly than Cl–

3.4 Effects of humic acid

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 com-parison 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 mmolckg–1 In comparison with the clay fraction, at pH

10 the SC of HA is two orders of magnitude higher

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–1resulted 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 mmolckg–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–1resulted 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

for-Na concentration (mmol L-1)

0

20

40

60

80

100

0

20

40

60

80

100

a) Na2SO4

b) NaCl

pH:

Figure 5: Effects of Na2SO4and NaCl on the flocculation of the clay

fraction.

pH

C50

-1)

0 10 20 30 40

3.0 3.5 4.0 4.5 5.0 5.5 6.0

Cl SO4

Figure 6: pH-dependent C50values of the suspensions of the clay fraction for Cl – and SO 2 (Na salts added).

mation of edge-to-face structures (card houses) and the

resulting flocculation is prohibited

By adding HA to the suspension, the amount of FeIIneeded

for flocculation is increased severely (Fig 9) At a HA

concen-tration of 20 mg L–1, the C50values 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

sta-bility 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

3.5 Flocculation by soil solutions

After 24 h soaking time of the soil in water under anaerobic conditions (N2atmosphere), 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 solu-tion occurred On day 15, the final concentrasolu-tion for Fe, Ca,

Mg, and Mn were 1.84, 2.01, 0.29, and 0.17 mmol L–1, re-spectively 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 dif-ferent 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 C50value is reached at a dilution ratio of 2:10 where the concentrations of FeII, Ca, Mg, and MnIIare 0.37, 0.40, 0.06, 0.04 mmol L–1, respectively In comparison with the C50values 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 concentra-tion at a diluconcentra-tion 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 MnIIand Mg in the soil solution, even if they have low concentrations and relatively high C50values (1.1 and 1.5 mmol L–1, respectively; cf.,

sec-tion 3.2) and only slight effects on flocculasec-tion can be assumed Dissolved organic matter, which is 186 mg kg–1in a

pH

-1 )

-1 ) -40

-30

-20

-10

0

Humic acid Clay fraction, organic matter removed

Figure 7: pH-dependent surface charge of humic acid and the clay

fraction, where organic matter was removed.

Humic acid concentration (mg L-1)

0

20

40

60

80

100

Figure 8: Effect of humic acid on the dispersion of the clay fraction at

pH 3.

FeII concentration (mmol L-1)

0 20 40 60 80 100

Original

10 mg L-1 HA added

20 mg L-1 HA added

0.2 0.4 0.6 0.8 1.0 0

4 5 Clay fraction: pH:

Figure 9: pH-dependent flocculation of the clay fraction in presence

of 10 and 20 mg L –1 HA and increasing Fe II concentrations.

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

3.6 Relationship between surface charge and clay

dispersion

The effect of cations on SC and flocculation of the clay

frac-tion is given by the relafrac-tion between SC and C50 value

(Fig 11) From the cations under investigation, highest SC of

the clay fraction at the C50value is found for Al and Pb The

SC has similar values (–5.0 and –5.9 mmolckg–1,

respective-ly) For these cations already at relatively low concentrations

the C50value is reached (0.11 and 0.30 mmol L–1,

respective-ly) The SC of Cu is –11 mmolckg–1 From the other divalent

cations under investigation, SC at the C50value is relatively

close together (–13.0 to –16.5 mmolckg–1), but distinct

differ-ences in the cation concentration at C50value 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

The saturation of the exchange sites by different cations resulted in different SC at the C50value 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

exter-nal surface of montmorillonite was reported (Sposito et al.,

1983) The possible sorption of FeIICl+on clay-crystal edges was confirmed with57Fe 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 C50is relatively close together (Fig 11) The quantification of the

SC at the C50value for FeIIand 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 con-centrations of FeIIand Ca An increase of the pH from 4 to 6 resulted in a change in SC from –10.3 to –13.1 mmolckg–1 and from –10.4 to –14.6 mmolckg–1for FeII and Ca, respec-tively 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 pre-diction of dispersion properties

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 deter-minations depend strongly on the kind of clay minerals pre-sent 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

chang-es in pH and ion concentrations on SC is increasing

5 Conclusions

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

2/10 4/10 6/10 8/10 10/10

0

Dilution ratio (soil solution / deionized water)

0

20

40

60

80

100

Figure 10: Flocculation of the clay fraction by a soil solution

representing reducing conditions.

Cation concentration (mmol L-1)

l c

-1) -16

-12

-8

-4

0

Al Pb Cu Cd

FeII Zn Ca Mn Mg

0.2 0.4 0.6 0.8 1.0 1.2 1.4

C 50

value s

1.6 0

Figure 11: C50values and corresponding SC for Al and different

divalent cations.

Cation concentration (mmol L-1)

-14 -13 -12 -11 -10 -9 -8

0.2 0.4 0.6 0.8 1.0 0

pH 4

pH 5

pH 6

C50values f

or Fe II

C50values

forCa

Figure 12: pH-dependent C50values and corresponding SC of the clay fraction for Fe II and Ca.

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

disso-ciated charge For FeIIand 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

pro-gram

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