Surface charge and adsorption from water onto

quartz sand of humic acid

A. Jada

a,*

, R. Ait Akbour

a

, J. Douch

b

a

ICSI-CNRS-UHA, 15 rue Jean Starcky, B.P. 2488, 68057 Mulhouse, France

b

Faculte´ des Sciences d’Agadir, Universite´ Ibn Zohr, Agadir, Morocco

Received 17 June 2005; received in revised form 27 December 2005; accepted 28 December 2005

Available online 14 February 2006

Abstract

The surface charge of humic acid under different conditions of ionic strength, pH, and the presence of various cationic ions (Cu

2+

,

Zn

2+

, Ba

2+

, and Ca

2+

) was determined by a titration method using a cationic polyelectrolyte as titrant. Adsorption isotherms in batch

experiments of the polymer from water onto quartz sand were determined at 20

C, 40 C, and 60 C and under different conditions of

ionic strength, pH, and the presence of various cationic ions (Cu

2+

, Zn

2+

, Ba

2+

, and Ca

2+

). The data indicate significant decrease of

humic acid surface charge by decreasing the pH value from 10.0 to 4.1. Similar decrease of humic acid surface charge was observed
by increasing either the ionic strength or the affinity of the divalent cation toward the humic acid. At ambient temperature the adsorption
of humic acid on the quartz sand seems to be controlled mainly by electrical interaction between the organic particle and the solid sub-
strate. A correlation is found between the surface charge and the adsorbed amount of the polymer, the adsorbed amount increases when
the surface charge of humic acid decreases. The increase of the adsorbed amount with the temperature suggests that adsorption process is
endothermic.
2006 Elsevier Ltd. All rights reserved.

Keywords: Humic acid; Quartz sand; Environment; Adsorption; Streaming induced potential

1. Introduction

The adsorption of natural organic matter on inorganic

particles surfaces such as clay and quartz is an important
geochemical process that occurs in soil and aquatic media
(

Sposito, 1984; Stevenson, 1994; MacCarthy, 2001

). Such

adsorption is a function of the medium conditions and
the surface properties of the soil and organic matter. The
adsorption of the organic matter leads in turn to the
modification of the wettability, the surface charge, and
the size of the inorganic particles (

Tipping and Higgins,

1982; O’Melia, 1990; Kretzschmar and Sticher, 1997

).

The organic compounds naturally present in the soil in-
clude mainly humic and non-humic substances (

Thurman,

1985; Ochs et al., 1994

). Further, the humic and non-humic

substances can be classified on the basis of their solubility
in acid and base. Hence, fulvic acids are soluble in acid
and base, humic acids are soluble in base only and humin
is totally insoluble. The fulvic and humic acids are the
major natural organic matter (60%), which affect the soil
fertility and facilitate the transport of contaminants
through soils and aquatic media (

Ochs et al., 1994

). How-

ever, the physicochemical behaviour, the mobility, and the
accumulation in the soil of humic substances are function
of various parameters, such as the pH, the nature, and
the amount of metal ions present in the medium. Either
humic or fulvic acid contains mainly carboxylic and pheno-
lic acidic groups that are naturally oxidized, giving the
humic substance a negative surface charge in aqueous med-
ium. Further, if divalent cations such as Cu

2+

, Ba

2+

, Ca

2+

,

and Zn

2+

are present in the soil, formation of coordination

complexes between the humic substance acidic groups and

0045-6535/$ - see front matter

2006 Elsevier Ltd. All rights reserved.

doi:10.1016/j.chemosphere.2005.12.063

*

Corresponding author. Tel.: [phone]; fax: [phone].
E-mail address:

[e-mail]

(A. Jada).

www.elsevier.com/locate/chemosphere

Chemosphere 64 (2006) 1287–1295

the divalent cations may occur, affecting hence the mobility
through the porous medium of the organic colloid
(

Kretzschmar and Sticher, 1997; Ait Akbour et al., 2002

).

The complexing efficiency is a function of the nature of
divalent cation and humic substance content of oxygen-
containing functional groups, such as carboxylic, phenolic,
and carbonyl groups. In addition, others properties of
humic substances such as their conformation and aggrega-
tion states, are also function of their surface charge and the
composition of the surrounding media (

Senesi, 1999; Alva-

rez-Puebla et al., 2004

).

Since the adsorption of humic acid onto inorganic par-

ticles occurs in aqueous medium, the aim of the present
work is to investigate the effects of pH, the ionic strength,
the nature of the divalent cation present in the aqueous
phase, on the behaviour of humic acid (HA) in the absence
and the presence of quartz sand. Such knowledge will be
necessary to predict the geochemical processes that occur
in soil and aquatic media.

2. Experimental

2.1. Materials

The quartz sand used is Fontainebleau sand from

Alsace (France) and contains mainly quartz according to
the elementary analysis (Si = 45.03%; O = 52.18%; C <
0.3%; H < 0.3%; Ca = 100 mg l

1

; Al = 185 mg l

1

; Mg <

10 mg l

1

; Na < 50 mg l

1

and Fe = 150 mg l

1

). The mea-

sured BET surface area for this sample is 5.099 ±
0.09931 m

2

/g, while the pH

zpc

(zero point charge) was

below 3.78 as measured by potentiometric method.

The humic acid, sodium salt (HA) was purchased from

Aldrich and was used as received without further purifica-
tion, its molecular weight as determined by fluorescence
polarization study was around 2000 g/mol. The C, H,
N, O, and S contents of the sample were determined by
an elemental analysis and are C (38.02%), H (3.76%), N
(0.52%), O (57.4%), and S (<0.3%). In the elemental ana-
lysis, the procedure used for carbon and hydrogen con-
tents consist of heating the sample at 1050

C in oxygen

atmosphere and analysing the formed carbon dioxide
and water by electrochemical and infrared methods. Sim-
ilar procedure was used to measure the nitrogen content;
the sample was heated at 1050

C under helium and

oxygen atmosphere. After heating, the formed nitrogen
oxides were reduced to molecular nitrogen and then quan-
tified by thermal conductivity. To determine the sulphur
content, the sample was heated at 1350

C under oxygen

atmosphere and the sulphur-containing sample was then
transformed to sulphur dioxide, which was then analysed
by using infrared method. Finally, the oxygen content
was determined by heating the sample at 1080

C under

nitrogen atmosphere. The formed oxygen was trans-
formed to carbon monoxide by passing through active
carbon at 1120

C and then analysed by infrared detec-

tors.

2.2. Surface charge of quartz sand particles aqueous
dispersions

The zeta potential of the quartz sand particles aqueous

dispersions was measured at ambient temperature using
microelectrophoresis method as described elsewhere (

Jada

and Ait Chaou, 2003

). The apparatus used, converts the

electrophoretic mobility l into the zeta potential f accord-
ing to the Henry’s equation (

Shaw, 1980; Hiemenz, 1986

)

l

¼

2
3

e

g

ff

ðjaÞ;

ð1Þ

where g and e are, respectively, the viscosity and the
permittivity of the aqueous medium, j is the inverse of
the double layer thickness and f(ja) is a corrector factor
having values in the range 1–1.5. The pH values of the
aqueous dispersions were varied in the range 2.2–10.3 by
adding to the dispersions small amounts of sodium hy-
droxide (NaOH) or hydrochloric acid (HCl) aqueous
solutions.

2.3. Surface charge titration of HA in aqueous medium

A stock solution of the HA was prepared by solubiliz-

ing, at ambient temperature, the polymer in distilled water
in the absence or the presence of monovalent or divalent
chloride salts. The solution was then diluted with water
to give the desired final concentrations of HA and salt.

The HA surface charge under various experimental con-

ditions, was evaluated by titration of the negatively
charged humic aqueous suspensions with a cationic poly-
electrolyte

(Poly(diallyl-dimethyl-ammonium

chloride),

PDADMAC) and using a potential measuring device
(particle charge detection, PCD, Mu¨teck instrument). The
measuring cell is composed of a cylindrical poly (tetraflu-
oroethylene) (PTFE) container with a PTFE piston inside.
The titratable aqueous humic suspension (10 ml) was filled
into the gap (0.5 mm) between the container wall and the
piston, and various amounts of the aqueous PDADMAC
solution were then added. The resulting streaming induced
potential (SIP) was measured between two gold electrodes
located at the top and the bottom of the gap. During the
measurements, the piston moves sinusoidally up and down
at a frequency of 4 Hz and forces the aqueous suspension
to move and to stream through the gap along the container
wall. The SIP measured during the piston movement,
results from the separation of the counterions from the
humic particles adsorbed on the container wall. These
counterions are compensated by adding cationic polyelec-
trolyte PDADMAC that has opposite surface charge as
compared to humic particles. From the titration curve,
the point of neutral charge (PNC) or the endpoint of titra-
tion is determined, which allows the calculation of the
charge density of the humic particles. It should be men-
tioned that the PCD, Mu¨teck instrument allows the mea-
surement of only relative potential values and no
calibration with this apparatus is possible.

1288

A. Jada et al. / Chemosphere 64 (2006) 1287–1295

2.4. Adsorption of HA from water onto quartz sand particles

In order to study the effects of various parameters

(pH, salt, amount of the solid, and temperature) on the
adsorption isotherm of HA on quartz sand, various
series of quartz sand-HA aqueous dispersions were pre-
pared.

In the first series dealing with the effects of the ionic

strength, the nature of divalent cation and the pH, a weight
amount of grinded quartz sand (1.5 g) was placed in stop-
pered bottles and known volumes (50 ml) of salt (monova-
lent or divalent cation) aqueous solutions containing
various amounts of humic acid, (initial HA concentration,
C

initial

, ranging from 1 to 8 mg l

1

) were added. To study

the effect of ionic strength, samples containing various
NaCl concentrations (10

3

–10

1

M) at pH 6 were pre-

pared. The effect of divalent cation was studied by adding
various salts (CaCl

2

, CuCl

2

, BaCl

2

, ZnCl

2

) to the disper-

sions at fixed salt concentration = 10

3

M and at pH 6.

The effect of pH was investigated by adjusting the HA-
quartz aqueous dispersions to desired pH values (pH 3
and pH 6 ± 0.1) at NaCl = 10

3

M.

In the subsequent series dealing with the effect of the

solid content, various weight amounts (0.5 g and 2.5 g) of
grinded quartz sand were placed in stoppered bottles and
known volumes (50 ml) of monovalent (NaCl = 10

3

M)

aqueous solutions containing various amount of humic
acid, (initial HA concentration, C

i

, ranging from 1 to

8 mg l

1

) were added.

In all cases the resulted dispersions, having pH 6, were

agitated at 20

C for 24 h, then centrifuged at ambient tem-

perature at 15 000 rpm to settle HA-covered quartz sand
particles. A known volume of the supernatant was placed
in an optical cell and analysed at ambient temperature by
fluorescence spectroscopy to yield the residual HA concen-
tration, C

e

, expressed in mg l

1

. The fluorescence excitation

spectra were recorded on Shimadzu RF-5001 PC and
were measured at 20

C using the excitation wavelength

range 280–450 nm and the emission wavelength = 530 nm.
The adsorbed amount, C, as expressed in milligram of HA
per gram of the sand, was determined from the differ-
ence between the initial concentration (C

initial

) and the

residual concentration (C

equilibrium

), according to the

equation

C

¼

ð50 10

3

ÞðC

initial

C

equilibrium

Þ

m

;

ð2Þ

where m is the amount of the adsorbent, expressed in
grams, C

initial

and C

equilibrium

, are respectively, the HA

initial and the residual concentrations, expressed in
mg l

1

.

To study the effect of temperature on adsorption iso-

therm, the HA-quartz sand aqueous dispersions were agi-
tated at 20

C, 40 C, and 60 C and similar procedures

as described above were used for analysing the supernatant
and determining the HA adsorbed amount.

3. Results and discussion

3.1. Surface charge of quartz sand particles in water

Fig. 1

shows the variation of the zeta potential versus

the pH, for the quartz sand particles in NaCl = 10

3

M

aqueous dispersions. The main cause of quartz surface
charge generation at quartz–water interface is due to disso-
ciation of silanol groups at the interface. The H

+

and OH

ions are the potential determining ions, i.e. ionic species of
the aqueous medium that enter the inner part of the electri-
cal double layer and undergo specific interaction with the
surface. Hence, due to the amphoteric character of the sil-
anol groups the quartz surface may gain or lose a proton
depending on the pH value of the aqueous phase.

Fig. 1

indicates the isoelectric point (IEP) of the quartz sand par-
ticles, i.e. the pH at which the zeta potential, f = 0 occurs
at pH 2.44. Other authors observed similar IEP value of
quartz sands in aqueous media (

Kosmulski, 1997; Kosmul-

ski et al., 1999

). Below and above the IEP value, the quartz

particles are respectively, positively and negatively charged.

3.2. Factors affecting the HA surface charge

3.2.1. Effect of salt concentration

In the presence of CaCl

2

and before titrating the HA

with the PDADMAC, the counterions of the polyelectro-
lyte, i.e. the HA sodium ions, are exchanged by the calcium
ions. Such ion exchange is a function of the CaCl

2

concen-

tration and leads to the modification of HA polyelectrolyte
chain conformation and aggregation states. Thus, one can
expect that for a given HA concentration, a critical CaCl

2

1

2

3

4

5

6

7

8

9

10

11

-50

-40

-30

-20

-10

0

10

20

IEP=2.44

Zeta potential (mV)

pH

Fig. 1. Zeta potential of Fontainebleau quartz sand particles in water
(NaCl = 10

3

M, sample concentration = 0.2 wt%).

A. Jada et al. / Chemosphere 64 (2006) 1287–1295

1289

concentration exists, above which the HA polyelectrolyte
network collapses and aggregates. This critical salt concen-
tration is in turn a function of the viscosity and/or dimen-
sion of the HA polyelectrolyte free salt solution (

Fleer

et al., 1993; Dragan and Cristea, 2001

). In addition, it

can be assumed that either the rigid or the collapsed HA
polyelectrolyte behaves in water as a solid particle develop-
ing at its surface an electrical double layer (

Hunter, 1986;

Fleer et al., 1993

).

Fig. 2

shows the titration curves of HA by the PDAD-

MAC in the presence of various concentrations of CaCl

2

at pH 6. As can be seen in the figure, in all instances,
the amplitude of normalized streaming induced poten-
tial (SIP) decreases as the PDADMAC concentration,
C

PDADMAC

, increases up to the PNC, at which SIP =

0 mV. Beyond the PNC, the magnitude of the SIP increases
with the PDADMAC concentration and reaches a plateau
value. Furthermore,

Fig. 2

indicates that in the absence of

PDADMAC, the HA is negatively charged owing to the
ionization of the carboxylic surface groups.

All the titration curves shown in

Fig. 2

involve three sit-

uations of the net surface charge, Dr =

r

HA

+ r

PDADMAC

,

of HA polymer containing the adsorbed PDADMAC,
where r

HA

and r

PDADMAC

are, respectively, the surface

charge density of the HA polymer and the PDADMAC.
Thus, at low PDADMAC concentrations, i.e. C

PDADMAC

<

PNC, Dr < 0, the HA surface charge is under compensated,
while at C

PDADMAC

= PNC, Dr = 0, as resulting from

compensation of HA surface charge by PDADMAC macro-
molecules. Further, the increase of PDADMAC con-
centration beyond the PNC, leads to Dr > 0, i.e. an
overcompensation and charge reversal of HA surface charge
by the PDADMAC macromolecules. The mechanism by
which the PDADMAC modifies the surface charge density
of HA polyelectrolyte, results from ion exchange process.
In this process counterions of the HA polymer (i.e. the cal-
cium ions) move toward the solution and are replaced by
the PDADMAC polyion at the surface. The driving forces
are entropic; entropy loss due to polyelectrolyte and entropy
gain due to the liberation of counterions from the surface.
The amount of the PDADMAC exchanged and adsorbed
on the HA surface decreases when the CaCl

2

concentration

increases.

From the values of the PNC obtained at various values

of CaCl

2

concentrations and using the known equivalent

charge of PDADMAC, the HA equivalent charge was cal-
culated and are presented in

Table 1

. As can be seen in

Table 1

, the HA equivalent charge decreases from

2.41

· 10

4

to 1.35

· 10

4

eq g

1

by increasing the CaCl

2

salt concentration in the range 10

5

–5

· 10

4

M.

3.2.2. Effect of pH

Fig. 3

shows similar variation of the SIP versus

C

PDADMAC

and

Table 2

indicates the corresponding

decrease in the HA equivalent charge from 3.20

· 10

4

to

0.52

· 10

4

eq g

1

by decreasing the pH of the aqueous

phase in the range 10.0–4.1.

The decrease of the HA equivalent charge by increasing

the CaCl

2

concentration, at pH 6 is due to the increase of

complexes formed between the HA ionized carboxylic
groups and the Ca

2+

ions located on a Stern plane. The

increase of ionic state of the HA with the pH on the other
hand, results from the deprotonation first of the carboxylic
groups at low pH, followed by the phenolic groups at high
pH. From the data presented in

Tables 1 and 2

, it can be

seen that H

+

ions as compared to Ca

2+

, seem to be more

efficient in reducing the equivalent charge of HA polymer.
However, the HA is less charged (maximum surface charge
observed = 3.18

· 10

4

eq g

1

) as compared to other pH

dependent surface charge polyelectrolytes such as sodium
polyacrylate, PAA, and poly (ethylenimine), PEI. Reported
studies on charge/pH isotherms of the PEI (

Gill and Her-

rington, 1987

) have shown that the charge density of PEI

is highly dependent on the pH and falls from a value of
about 1.97 eq g

1

at pH 3 to zero at pH 11.1, in the pres-

ence of NaCl salt.

0.0

5.0x10

-5

1.0x10

-4

1.5x10

-4

2.0x10

-4

2.5x10

-4

3.0x10

-4

-1.2

-1.0

-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Normaliz

ed potential (a.u.)

[PDADMAC] (equivalent/liter)

CaCl

2

=5 10

-4

M

CaCl

2

=10

-4

M

CaCl

2

=10

-5

M

Fig. 2. Titration curves of HA by PDADMAC at various values of ionic
strength.

Table 1
Variation of HA equivalent charge with the CaCl

2

concentration

[Ca

2+

] (M)

10

5

5

· 10

5

10

4

3

· 10

4

5

· 10

4

[PDADMAC] (eq/l)

1.55

· 10

4

1.40

· 10

4

1.39

· 10

4

1.33

· 10

4

1.18

· 10

4

C

AH

(eq g

1

)

2.41

· 10

4

1.94

· 10

4

1.92

· 10

4

1.73

· 10

4

1.35

· 10

4

1290

A. Jada et al. / Chemosphere 64 (2006) 1287–1295

3.2.3. Effect of the nature of the divalent cation

Fig. 4

shows the SIP variation of HA versus the PDAD-

MAC concentration, in the presence of various divalent
cations (Ca

2+

, Ba

2+

, Zn

2+

, and Cu

2+

) at constant salt con-

centration = 3

· 10

4

M. The values of HA equivalent

charge, as calculated from the various PNC values are pre-
sented in

Table 3

. The curves of

Fig. 4

show marked differ-

ences in initial slopes and PNC values depending on the
nature of the HA adsorbed divalent cation. As can be
observed in the figure, divalent cations such as Cu

2+

and

Zn

2+

exhibit greater decrease of HA surface charge as com-

pared to Ca

2+

and Ba

2+

. This difference in behaviour of

HA polymer in the presence of various cations is related
to the electronic structure (

Kretzschmar and Sticher,

1997; Prado et al., 2005

), and the adsorptive affinity of

the cation (

Zhou et al., 2005

). Further, the mechanism by

which divalent cations affect the surface charge of HA
polymer involves contributing forces arising from electro-
static attraction, formation of coordination complexes
(

Baker and Khalili, 2005

) and solvent effect. Hence, in all

cases the adsorption of divalent cations such as Cu

2+

,

Zn

2+

, Ca

2+

, and Ba

2+

from bulk water onto the HA sur-

face reduces the thickness of the HA electrical double layer
and the zeta potential (i.e. the electrical potential at the

shear plane). In addition, in the presence of Cu

2+

and

Zn

2+

ions, the formation of coordination complexes

between these ions and the negatively charged HA surface
groups, decreases further the HA surface charge as shown
in

Table 3

.

3.3. Factors affecting HA adsorption onto quartz sand
particles

3.3.1. Effect of salt concentration

Fig. 5

represents the effect of NaCl concentration on the

adsorption isotherms of HA from water onto quartz sand
at ambient temperature and at pH 6. In all cases the uptake
of HA increases with increasing the HA initial concentra-
tion and reaches plateau value. Further, as can be seen in
the figure, the maximum adsorbed amount of HA increases
as the NaCl concentration increases in the range 10

3

–

10

1

M. It should emphasized that at pH 6 and in the pres-

ence of low salt concentration, both HA and the quartz
sand particles are negatively charged and are expected to
develop pronounced repulsive barriers (

Hunter, 1986;

Israelachvili, 1992; Elimelech et al., 1995

), which will lead

to reduction of HA adsorption. By increasing the salt con-
centration above a certain ionic strength, the repulsive
energy barriers of both the components are reduced due

0.0

1.0x10

-4

2.0x10

-4

3.0x10

-4

4.0x10

-4

-1.25

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

1.25

1.50

Normalized potential (a.u.)

[PDADMAC] (equivalent/liter)

pH=4.1

pH=5.2

pH=6.7

pH=10.0

Fig. 3. Titration curves of HA by PDADMAC at various pH of the
aqueous phase.

Table 2
Variation of HA equivalent charge with the pH of the aqueous phase

pH

10.0

6.7

5.2

4.1

[PDADMAC] (eq/l)

1.76

· 10

4

1.48

· 10

4

1.03

· 10

4

0.75

· 10

4

C

AH

(eq g

1

)

3.18

· 10

4

2.20

· 10

4

1.83

· 10

4

0.52

· 10

4

Normalized potential (a.u.)

[PDADMAC] (equivalent/liter)

0.0

5.0x10

-5

1.0x10

-4

1.5x10

-4

2.0x10

-4

2.5x10

-4

-1.25

-1.00

-0.75

-0.50

-0.25

0.00

0.25

0.50

0.75

1.00

1.25

1.50

CuCl

2

ZnCl

2

BaCl

2

CaCl

2

Fig. 4. Titration curves of HA by PDADMAC in the presence of various
divalent cations.

A. Jada et al. / Chemosphere 64 (2006) 1287–1295

1291

to decrease of the electrical double layer thickness, and this
will lead to an increase of HA adsorbed amount as shown
in

Fig. 5

.

To study colloidal adsorption or deposition in porous

medium, step-input or short-pulse columns are usually
used (

Kretzschmar and Sticher, 1997

). In these experiments

the deposition may be either reaction or transport limited,
at respectively, low or high ionic strength. Such colloidal
deposition processes occurring in step-input or short-pulse
column are likely involved in the batch experiments used in
the present work. However, in batch as well as in column
experiments, the deposition is a function of various para-
meters such as the solid particles size, the solid content
and its accessible surface area, the pH, the nature and the
concentration of the divalent cation present in the medium.
Similar parameters were also found to affect mercury
adsorption from water onto activated carbons surface
(

Zhang et al., 2005

).

3.3.2. Effect of the nature of the divalent cation

Fig. 6

shows the effect of divalent cation (Cu

2+

, Ba

2+

,

Zn

2+

, and Ca

2+

) on the adsorption isotherms of HA from

water onto quartz sand particles, at pH 6 and salt con-
centration = 3

· 10

4

M. As was mentioned above, the

divalent cations Cu

2+

and Zn

2+

form coordination com-

plexes with the negatively charged HA surface groups
and are more efficient than Ba

2+

and Ca

2+

in reducing

the HA surface charge. However, in investigating the HA
adsorption onto the quartz sand particles one should take
into account the effect of the divalent cation on the reduc-
tion of the surface charges and/or zeta potentials of both
quartz particles and HA polymer (

Elimelech and O’Melia,

1990a,b; Kihira et al., 1992; Litton and Olson, 1993

). Thus

the highest adsorption of HA on quartz particles in the
presence of Cu

2+

is likely due to strong reduction of the

HA surface charge and quartz particles zeta potential.
Adsorption to a lesser degree is expected to occur in the
presence of Zn

2+

as shown in

Fig. 6

. However, the higher

adsorbed amount observed in the presence of Ba

2+

as com-

pared to Zn

2+

or Ca

2+

is likely due to the preferential

adsorption of the Ba

2+

on the quartz surface as resulting

from it is high ionic radius.

3.3.3. Effect of pH

Fig. 7

shows the HA adsorption isotherms onto the

quartz sand particles at ambient temperature, and at pH
3 and pH 6. At pH 3, the adsorbed amount increases sev-
eral fold by increasing HA concentration and do not level

Table 3
Effect of the nature of cation Me

2+

on the HA equivalent charge

[Me

2+

] = 3.10

4

M

Ca

2+

Ba

2+

Zn

2+

Cu

2+

Ionic radius (nm)

0.099

0.135

0.074

0.069

[PDADMAC] (eq/l)

1.33

· 10

4

1.21

· 10

4

1.00

· 10

4

2.24

· 10

5

C

AH

(eq g

1

)

1.73

· 10

4

1.42

· 10

4

0.96

· 10

4

0.45

· 10

5

0

1

2

3

4

5

6

7

0.00

0.01

0.02

0.03

0.04

0.05

0.06

NaCl=10

-3

M

NaCl=5 10

-2

M

NaCl=10

-1

M

Γ

(mg/g)

C

equilibrium

(mg/l)

Fig. 5. Adsorption isotherms of HA on quartz sand particles at various
values of ionic strength.

C

equilibrium

(mg/l)

0

1

2

3

4

5

6

7

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

CaCl

2

ZnCl

2

BaCl

2

CuCl

2

Fig. 6. Adsorption isotherms of HA on quartz sand particles in the
presence of various divalent cations.

1292

A. Jada et al. / Chemosphere 64 (2006) 1287–1295

out, even at high polymer concentration. Such behaviour
indicates that the reduction of HA surface charge and
energy barrier of quartz particles leads to an exponential
increase of the deposition rate (

Ruckenstein and Prieve,

1976

). However, the HA adsorbed amount is at lesser

degree at pH 6, due to electrostatic repulsion between the
negatively charged HA polymer and quartz particles.

3.3.4. Effect of the solid content

Fig. 8

shows the effect of adsorbent mass on the HA

adsorption isotherm at pH 6, NaCl = 10

3

M and ambient

temperature. As was expected the adsorbed amount
decreases as the quartz sand content increases, due to the
increase of available surface area of the adsorbent. As
can be observed in

Fig. 8

, the maximum adsorbed amount

of HA polymer decreases more than 50% by increasing the
mass of the quartz sand by 5-fold. It should be emphasised
that the adsorption isotherms are obtained after 24 h of
contact between the solute and the adsorbent. As illus-
trated in

Fig. 8

, the adsorbed amount levels at a certain

HA concentration, due to saturation of available quartz
sand actives sites, its likely that the adsorption of HA on
quartz sand involves one or more contributing forces aris-
ing from electrostatic and hydrophobic interactions, hydro-
gen bonding, and solvent effects (

Jada and Ait Chaou,

2002

).

3.3.5. Effect of temperature

Fig. 9

shows the variation of the HA adsorbed amount

on quartz sand particles, versus the residual concentration,
at 20

C, 40 C, and 60 C and at fixed pH and ionic

strength values (pH 6, NaCl = 10

3

M). As can be

observed in the figure the HA uptake increases several folds

as the temperature increases, indicating an endothermic
process. Further, the adsorption curves at 40

C and

60

C as compared to the one obtained at 20 C do not

level out, even at high HA concentrations. Such adsorption
is similar to that obtained at pH 3 and T = 20

C (see

Fig 7

) and the exponential increase of the adsorption

shown in

Fig. 9

at 40

C and 60 C result likely from an

enhanced HA deposition rate due to an increased mobility

C

equilibrium

(mg/l)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.20

0.22

0.24

pH=3±0.1

pH=6±0.1

Fig. 7. Adsorption isotherms of HA on quartz sand particles at various
pH values of the aqueous phase.

C

equilibrium

(mg/l)

0

1

2

3

4

5

6

7

0.00

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

Amount of sand = 1 wt%
Amount of sand = 5 wt%

Fig. 8. Adsorption isotherms of HA on quartz sand particles at various
solid contents.

1

2

3

4

5

6

7

0.000

0.025

0.050

0.075

0.100

0.125

0.150

0.175

T=20

°C T=40 °C

T=60

°C

C

equilibrium

(mg/l)

Fig. 9. Adsorption isotherms of HA on quartz sand particles at various
temperatures.

A. Jada et al. / Chemosphere 64 (2006) 1287–1295

1293

or transport of HA polymer from bulk solution towards
the quartz surface. Similar increase with temperature of
the HA uptake on activated carbon was also observed by
other authors (

Khalil and Girgis, 1995; Daifullah et al.,

2004

). Accordingly, the increase of the adsorption with

the temperature may be attributed to the acceleration of
some originally slow adsorption steps or the creation of
some new active sites on the adsorbent surface. Finally,
the adsorption of HA from water onto quartz sand parti-
cles seems to involve mainly two steps: a transport step
during which HA polymer diffuses from bulk water onto
quartz surface and an attachment step during which it
adsorbs onto quartz surface. The kinetics of transport
and the attachment are under control of various factors
such as the ionic strength, the pH of the aqueous phase
and the temperature.

3.3.6. Mechanism of HA adsorption from water onto
quartz particles

Various mechanisms have been proposed to explain the

sorption of humic substances that occur in soils, river
sediments, and natural aquifer materials (

Juhna et al.,

2003

). Such mechanisms involve ligand exchange, electro-

static as well as hydrophobic attractions between humic
substances and solid surface. In the present study it is
shown that various factors such as pH, ionic strength, nat-
ure of divalent cations, and temperature, control the sur-
face charge and the adsorption mechanisms of HA on
quartz. The variation of these factors should affect the elec-
trical double layer structures of both HA and quartz
surfaces. Hence a decrease of HA surface charge by
increasing the ionic strength or decreasing the pH of the
aqueous phase is followed by an increase of HA adsorption
on quartz particles. Further, it should be emphasised that
HA contains both functional groups (carboxylic, phenolic,
and carbonyl groups) and hydrophobic moieties. In the
adsorption process the HA functional groups are mainly
involved in electrostatic interaction with the quartz hydro-
xyl surface groups, whereas the HA insoluble part (hydro-
phobic moiety) allows the polymer to accumulate on the
inorganic surface. Therefore, it is likely that electrostatic
as well as hydrophobic interactions are the most important
forces involved in the mechanism of HA adsorption on
quartz surface.

The adsorption isotherms of HA on quartz were found

to depend on agitation time (24 h or 48 h) and only the
data obtained at 24 h agitation time are presented in this
paper. Thus for a given agitation period various volumes
of HA aqueous solutions were equilibrated with a given
amount of quartz. In all instances the adsorption of HA
increased with an increase of HA initial concentration,
and depending on the experimental conditions the uptake
of HA may or not level out. The fact that HA adsorption
isotherms do not level out at low pH value (pH 3,

Fig. 7

) or

at high temperatures (T = 40

C and T = 60 C,

Fig. 9

)

may be explained by the polyelectrolyte nature of HA. In
these cases the adsorption may be determined not only

by the interaction between HA and quartz surface but also
by the lateral interaction that occur between the adsorbed
HA macromolecules themselves and the steric arrange-
ments of the macromolecules (i.e. association between
HA macromolecules at quartz surface, which is mainly
due to the decrease of their solubility in water) (

Kaiser

and Zech, 1997; Vermeer et al., 1998

).

4. Conclusion

In this work, the effects of various factors such as the

ionic strength, the nature of divalent cation, the pH, the
adsorbent content, and the temperature, on HA surface
charge and its adsorptive properties on quartz sand parti-
cles, were investigated. The HA surface charge reduction
results from surface complex formation and reduction of
the electrical double layer thickness. The adsorption of
HA on quartz sand involves two main steps, a first step,
which concerns the transport of HA polymer from bulk
water onto adsorbent surface followed by an adsorption
or retention step. The kinetics of these processes are under
control of the experimental conditions. Work is in progress
to determine the surface densities of the HA functional
groups and to investigate the effects of experimental condi-
tions on the adsorption kinetics of HA on quartz sand
particles.

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