Clays and Clay Minerals, Vol. 39, No. 1, 100-102, 1991.
FILTRATION-RATE TECHNIQUE FOR DETERMINING
ZERO POINT OF CHARGE OF IRON OXIDES
Key Words--Filtration rate, Goethite, Iron oxide, Potentiometric titration, Zero point of charge.
Interest in the preparation and use o f goethite and
other oxides as model sorbents in studies to explain
the reaction o f ions with soils, is increasing (Kuo, 1986;
Martin and Smart, 1987; Zasoski and Burau, 1988).
The zero point o f charge (ZPC), which is the p H value
o f a solution in equilibrium with a particle whose net
charge from all sources is zero (SSSA, 1987), is a characteristic frequently determined for such synthetic oxides. The ZPC o f oxides serves as a convenient reference for predicting how the surface charge and potential
on oxides depend on p H (Arnold, 1978).
The ZPC o f oxides can be determined by electrokinetic and equilibrium electrochemical methods.
Electrokinetic methods include microelectrophoretic
and streaming potential techniques (Johansen and Buchanan, 1957). Potentiometric titration is the equilibrium electrochemical technique most often used to determine the ZPC o f oxides and other materials (Bolt,
1957; Parks and de Bruyn, 1962). Potentiometric titration, however, requires special equipment. I f the
determination o f surface charge density is not required,
a simpler m e t h o d m a y be preferred to potentiometric
titration for determining the ZPC o f oxides.
The present paper describes a simple filtration setup and procedure for determining the ZPC o f synthetic
goethite and compares the results with those obtained
by the potentiometric titration technique.
MATERIALS AND METHODS
Oxide preparation
Goethite, was prepared in a polyethylene container
from 10% N a O H and 0.2 M Fe(NO3)3; all chemicals
were o f analytical reagent grade. The gel was aged with
continuous stirring for 10 days at r o o m temperature
at p H 11.7. The crystals were finally washed to p H 7
by repeated stirring in deionized distilled water, the
excess water at the end o f each washing cycle being
r e m o v e d with filter candles. The crystals had the following properties: surface area (ethylene glycol), 95.2
m2/g; principal X-ray powder diffraction peak, 4.17 ~ ;
ratio o f oxalate-soluble iron/total iron, 0.015; endothermic, differential thermal analysis peak, 285~ color, yellow (10 Y R 7/8); crystal shape, needlelike.
Filtration-rate technique
The set-up is presented in Figure 1. A suspension o f
the goethite was prepared with deionized distilled water
(1.0 g/liter) and passed through a 300-mesh sieve. The
Copyright 9 1991, The Clay Minerals Society
suspension was distributed in 100-ml portions into 125ml Erlenmeyer flasks. The p H was adjusted differently
in the flasks to cover the p H range 5 to 10, and the
suspensions were allowed to equilibrate for about 48
hr. Immediately before filtering the suspension in each
flask, the final p H was measured.
The suspension in the flask was mixed by a magnetic
stirrer (SU). The flask was tightly sealed with a rubber
stopper (RS 1) through which a narrow glass tube
reached to about 1 cm from the b o t t o m o f the flask.
The upper part of the glass tubing was connected by
plexiglas tubing (PT) to the upper part o f another glass
tube that passed through a second rubber stopper (RS
2), tightly sealing the mouth o f a Millipore tall-form
glass filter holder (FH). A Millipore micropore (0.22#m pore size) filter paper (FP) was placed at the lower
end o f the filter holder. The stem portion o f the filter
holder passed through a third rubber stopper (RS 3)
sealing the mouth o f a graduated receiving cylinder
(GC). Plexiglas tubing connected the upper part o f a
short glass tube passing through a second hole in the
stopper (RS 3) o f the receiving cylinder to another glass
tube passing through one o f two holes in a fourth rubber
stopper (RS 4) sealing the mouth o f a vacuum flask
(VF). The vacuum flask was connected through a manostat (Ms) to a vacuum line (Vc).
The system was evacuated, and the suspension was
drawn into the filter holder and filtered into the graduated cylinder. The time required to obtain a given
volume o f filtrate was recorded. The receiving cylinder
was emptied, the filter holder cleaned, and the micropore filter paper replaced between filtrations. The filtration time (minutes/volume) was plotted as a funct i o n o f s u s p e n s i o n pH. A t the Z P C , m a x i m u m
flocculation resulted in the fastest filtration.
Potentiometric titration
The ZPC o f the synthetic goethite was also determ i n e d by the potentiometric titration technique, as
described by Parks and de Bruyn (1962). In the present
study, the suspension concentration was 1 g o f oxide
in 300 ml o f NaNO3 solution having an ionic strength
o f 1.0 x 10 -3 M or 1.0 x 10 2 M. The p H range studied
was 5 to 11. Suspensions and blank solutions were
titrated with 0.1 M NaOH.
RESULTS A N D D I S C U S S I O N
A ZPC o f 7.9 was obtained for the synthetic goethite
(a-FeOOH) by both the filtration-rate and potentio100
Vol. 39, No. 1, 1991
Filtration-rate determination of zero point of charge
PT~ ~
30
RS 2
St
-g
20
m
15
o
101
/
,\
,T
AV" - ' ~ - > , ~ T ~
0
W
10
Spt---~
Ms~
Figure I. Experimental set-up for filtration-rate technique
for measuring zero point of charge. RS 1 = rubber stopper
n u m b e r l; RS 2 = rubber stopper number 2; RS 3 = rubber
stopper number 3; RS 4 = rubber stopper number 4; PT =
plexiglas tubing; Sp = oxide suspension; St = magnetic stirrer;
FH = filter holder; Sh = heat shield; SU = magnetic stirring
unit; FP = micropore (0.22/zm) filter paper; AV = air vent;
VF = vacuum flask (trap for filtrate overflow); Spt = support;
GC = graduated receiving cylinder; Vc = to vacuum; Ft =
filtrate; Ms = manostat.
I
l
I
I
6.0
7.0
80
90
SUSPENSION
9 f
10.0
pH
Figure 2. Determination of the zero point of charge of synthetic goethite by the filtration-rate technique. Filtration time
is plotted as a function of suspension pH. Suspension concentration = 0.10%.
w i t h electrolyte. T h e use o f a n a l y t i c a l r e a g e n t g r a d e
chemicals and repeated washing of the oxide in deionized w a t e r a t t h e p r e p a r a t i o n stage e n s u r e d its p u r i t y
f r o m N a § a n d o t h e r c o n t a m i n a n t ions. P r e l i m i n a r y
40
m e t r i c t i t r a t i o n t e c h n i q u e s (Figures 2 a n d 3, respectively).
E l e c t r o k i n e t i c m e t h o d s are b a s e d o n t h e d e t e r m i n a t i o n o f t h e zero p o i n t o f a p r o p e r t y t h a t d e p e n d s o n
t h e p r e s e n c e o f a n electric d o u b l e layer. T h e filtrationrate t e c h n i q u e is b a s e d o n t h e p r i n c i p l e t h a t at t h e Z P C ,
n o n e t c h a r g e exists o n t h e o x i d e particles to repel e a c h
o t h e r , r e s u l t i n g i n m a x i m u m f l o c c u l a t i o n o f particles;
t h u s , t h e fine p o r e s o f t h e filter p a p e r are n o t easily
b l o c k e d , giving t h e fastest filtration. L e w a n d o w s k i a n d
L i n f o r d (1972) u s e d a v a r i a t i o n o f t h e filtration s e t - u p
to e s t a b l i s h o p t i m a l c o n d i t i o n s for t h e b u l k filtration
o f ferric h y d r o x i d e s u s p e n s i o n s .
I n a d d i t i o n to p H , o t h e r factors, s u c h as t h e p r e s e n c e
b f electrolyte a n d t h e s u s p e n s i o n c o n c e n t r a t i o n (oxide
m a s s / v o l u m e ) , affect t h e rate o f filtration. T h e surface
c h a r g e d e n s i t y o n o x i d e s i n c r e a s e s ( b e c o m e s m o r e negative or positive) with an increase in concentration of
i n d i f f e r e n t electrolyte at a g i v e n p H (see F i g u r e 3),
t h e r e b y affecting t h e degree o f d i s p e r s i o n o r flocculat i o n and, t h u s , t h e rate o f filtration. A v e r y h i g h suspension concentration would quickly produce a thicker
o x i d e cake o n t h e filter p a p e r a n d r e t a r d o r e v e n stop
filtration. I n t h e f i l t r a t i o n - r a t e t e c h n i q u e , t h e s u s p e n s i o n was p r e p a r e d w i t h d e i o n i z e d distilled water, n o t
I
5-0
c
30
~
20
+,
10
E
0
A.
9
A
10-3 M
NaNO 3
|
10-2 M
NoNO 3
|
U.J
D
z
_.9.
-10
I-a.
~
-20
s:o
6:0
!
!
71o e.o
SUSPENSION
I
io'.o ;1.o
pH
Figure 3. Determination of the zero point o f charge of synthetic goethite by potentiometric titration. Adsorption density
of potential-determining ions on the gnethite is plotted as a
function o f p H and ionic strength, using NaNO3 as indifferent
electrolyte.
102
Ankomah
trials showed that the filtration rate was reasonable at
a suspension concentration of 1.0 g oxide/liter; the total
time for eight filtration runs was 2 hr (Figure 2).
Potentiometric titration is the technique most often
used to estimate the ZPC of oxides. This technique has
the advantage that the titration data can be used also
for quantitative estimates of the surface-charge density
of oxides at various pHs. A plot of charge density vs.
pH is not the same for different ionic strengths of the
same indifferent electrolyte, except at the ZPC. Surface
charge density may be expressed in such units as meq/
g, #eq/cm 2, and/zCoulombs/cm2. In this work, labelling
the Y-axis in Figure 3 in the usual charge density units
was not necessary; it was sufficient only to locate the
pH where the two curves intersect. The ordinate was
simply labelled as the difference between the volume
of base added to achieve any given pH increment in
the blank and that required to achieve the same pH
increment in the suspension for a given ionic strength.
The ZPC of oxides is influenced by the presence of
potential-determiningions (PDI) in addition to H § and
OH-. Parks (1967) distinguished between ZPC and the
isoelectric point (IEP) of oxides; the ZPC is the pH at
which the surface charge from all sources is zero,
whereas the IEP arises from the interaction of H +, OH-,
H20, and the solid alone. The values of ZPC and IEP
are equal only in the absence of adsorbed species different from the PDI: H § and OH-. For iron oxides,
Fe(III) hydroxo complex ions released from the solid
phase by dissolution are also commonly regarded as
PDI; however, Atkinson et al. (1967) pointed out that
the surface charge contribution offered by the transfer
of ferric hydroxo complex ions across the interface is
negligible compared with that from H § and OH- ion
transfer, because H § or O H - always greatly exceed the
complex ion concentration in solution in the pH range
3.5-11.0. The goethite was not prepared in a glass reaction vessel, because in a slurry of such a high pH
(11.7), silica could be solubilized out of the glass
(Schwertmann, 1988); the presence of silicon in goethite would lower the ZPC (Briimmer eta[., 1988).
The ZPC ofgoethite is also influenced by temperature. Parks (1965) cited literature that indicated that
the ZPC of natural goethite samples decreased from
6.7 at 25~ to 6.0 at 60~ and to 3.2 at 100~ The
present study was made at room temperature, which
did not vary over the 3-hr duration of the experiments.
Although goethite was used as the test material for
the filtration rate technique, the technique should be
suitable for the determination of ZPC of other oxides.
The filtration-rate technique is straightforward and utilizes simple and c o m m o n pieces of apparatus; it can
therefore be easily adopted for routine determinations
of ZPC of oxides, if a quantitative estimate of surface
charge densities is not required.
Clays and Clay Minerals
ACKNOWLEDGMENTS
I thank the University of Ghana, Legon, Ghana, for
the sponsorship of the study of which this work formed
a part.
Department o f Land, Air,
and Water Resources
University o f California
Davis, ~'a[ifornia 95616
AGYEMAN BOADI
ANKOMAn~
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(Received 28 September 1989; accepted 22 March 1990; Ms.
1952)
Present address: Soil Science Section, Department of Crop
Science, University of Science and Technology, Kumasi,
Ghana.