ARTICLE IN PRESS
WAT E R R E S E A R C H
40 (2006) 364– 372
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/watres
Arsenic removal from high-arsenic water by enhanced
coagulation with ferric ions and coarse calcite
S. Song, A. Lopez-Valdivieso, D.J. Hernandez-Campos, C. Peng, M.G. Monroy-Fernandez,
I. Razo-Soto
Instituto de Metalurgia, Universidad Autónoma de San Luis Potosı́, Avenida Sierra Leona 550, San Luis Potosı́, C. P. 78210, Mexico
ar t ic l e i n f o
A B S T R A C T
Article history:
Arsenic removal from high-arsenic water in a mine drainage system has been studied
Received 2 March 2004
through an enhanced coagulation process with ferric ions and coarse calcite (38–74 mm) in
Received in revised form
this work. The experimental results have shown that arsenic-borne coagulates produced by
3 April 2005
coagulation with ferric ions alone were very fine, so micro-filtration (membrane as filter
Accepted 26 September 2005
medium) was needed to remove the coagulates from water. In the presence of coarse
Available online 15 December 2005
calcite, small arsenic-borne coagulates coated on coarse calcite surfaces, leading the
Keywords:
settling rate of the coagulates to considerably increase. The enhanced coagulation followed
Coagulation
by conventional filtration (filter paper as filter medium) achieved a very high arsenic
Arsenic removal
removal (over 99%) from high-arsenic water (5 mg/l arsenic concentration), producing a
Coarse calcite
cleaned water with the residual arsenic concentration of 13 mg/l. It has been found that the
Ferric sulfate
mechanism by which coarse calcite enhanced the coagulation of high-arsenic water might
Colloidal stability
be due to attractive electrical double layer interaction between small arsenic-borne
coagulates and calcite particles, which leads to non-existence of a potential energy barrier
between the heterogeneous particles.
& 2005 Published by Elsevier Ltd.
1.
Introduction
In Mexico, hundreds of years of mining activities for gold,
silver and metallic sulfide minerals left a considerable
quantity of tailings, some of which have a high arsenic
content. Owing to rain, wind and other natural effects, the
arsenic dispersed into the nearest water courses or agricultural fields in the forms of arsenate ion (As (V)) and arsenite
ion (As (III)), leading to a high arsenic concentration in the
water courses around the mines. For example, in Matehuala
city, state of San Luis Potosı́, water from mine drainage
systems contained up to 7 mg/l As (Razo-Soto et al., 2005). As
it has been reported, serious health problems, such as
cancers, skin alteration, etc., have been linked to arsenic
ingestion even at a low concentration (20–50 mg/l) (O’Connor,
2002). The Secretary of Health (SAA) of Mexico recently has
Corresponding author. Tel./fax: +52 444 8254326.
E-mail address: [email protected] (S. Song).
0043-1354/$ - see front matter & 2005 Published by Elsevier Ltd.
doi:10.1016/j.watres.2005.09.046
implemented a new maximum contaminant level (MCL) for
arsenic in drinking water of 20 mg/l. Accordingly, the high
arsenic water from mine drainage systems would be a great
environmental concern through the contamination of shallow groundwater and surface water around the areas. The
water has to be treated to remove arsenic effectively, in order
to minimize the environmental effects.
Arsenic removal from water is an important subject
worldwide, which has recently attracted great attentions. A
variety of treatment processes has been developed for arsenic
elimination from water, including coagulation (precipitation)
(Wickramasinghe et al., 2004; Hering et al., 1997), adsorption
(Wang et al., 2002; Zhang et al., 2003; Katsoyiannis and
Zouboulis, 2002), ion exchange (Korngold et al., 2001),
membrane filtration (Sato et al., 2002), electrocoagulation
(Kumar et al., 2004; Arienzo et al., 2002), biological process
ARTICLE IN PRESS
WAT E R R E S E A R C H
(Katsoyiannis and Zouboulis, 2004), iron oxide-coated sand
(Thirunavukkarasu et al., 2003), high gradient magnetic
separation (Chiba et al., 2002) and natural iron ores (Zhang
et al., 2004), manganese greensand (Thirunavukkarasu et al.,
2005), etc. There are numerous review papers for the arsenic
removal technologies, some of which were recently made by
Jiang (2001), Bissen and Frimmel (2003), Dambies (2004) and
USEPA (2000). Coagulation and adsorption processes are most
promising for arsenic removal from high-arsenic water
because of the low cost and high efficiency, and are widely
used in the developing world. But, they have not been shown
to deeply eliminate arsenic from water and to produce
cleaned water with a very low arsenic concentration, say
10 mg/l. However, membrane filtration process could lower
arsenic concentration in water from 48 to 1–2 mg/l (Bissen and
Frimmel, 2003), and ion exchange process could remove
arsenic to levels lower than 5 mg/L for water with initial
arsenic concentration of 87 mg/L (Wang et al., 2002), which are
currently used in the developed world. The other processes
are still at a laboratory or pilot scale.
Coagulation process is traditionally realized by adding ferric
or aluminum ions (Hering et al., 1996). In this process, fine
particles in water first aggregate into coagulates because
added ferric or aluminum ions strongly reduce the absolute
values of zeta potentials of the particles. Then, arsenic ions
(arsenate or arsenite) precipitate with the ferric or aluminum
ions on the coagulates, and thus concentrate in the coagulates. After that, the coagulates are separated from water
through filtration, eliminating arsenic from the water. The
coagulates are termed arsenic-borne coagulates. Coagulation
with ferric ions for arsenic removal can be traced back to the
late 1960s in Taiwan to treat deep-well water with naturally
elevated arsenic concentrations (Shen, 1973). Gulledge and
O’Connor (1973) also reported that arsenic could be readily
removed from water to a high degree by conventional water
treatment using ferric or aluminum ions as coagulants. Since
then, there have been a lot of reports on coagulation process
for arsenic removal. It has been found that the coagulation is
much more effective for the removal of As (V) than As (III). In
the case when only As (III) is present, oxidation to convert As
(III) to As (V) is needed prior to coagulation. The effective pH
for arsenic removal was reported to be 5–7 with aluminum
ions, and 5–8 for ferric ions (Sorg and Logsdon, 1978). Besides
iron and aluminum compounds, manganese, calcium and
magnesium compounds are also of effective coagulants for
eliminating arsenic from water in neutral medium (Raje and
Swain, 2002; Jiang, 2001). Arsenic removal from water
achieved by coagulation process depends on initial arsenic
concentration in water (Thirunavukkarasu et al., 2005; Jiang,
2001). The arsenic removal could reach 99% (Jiang, 2001).
Recently, it was reported that modified coagulation/filtration
could give a residual arsenic concentration of 2 mg/l or less for
treated well water (Han et al., 2003).
The arsenic removal is also dependent on the pore size of
the membrane filter disks used for coagulation process (Han
et al., 2003), since coagulates smaller than the pore can pass
through the filter and remain in water. As it is known, in
filtration, larger the filter pores, lower the capital and
operation costs, and higher the separation efficiency. Therefore, there is a great significance in enlarging arsenic-borne
40 (20 06) 36 4 – 372
365
coagulates in order to improve coagulation/filtration process
for arsenic removal. However, there is little information in
this regard, although there are numerous reports on coagulation process for arsenic removal from water.
Usually, coagulations are enhanced by adjusting pH and
electrolyte concentration to reduce the absolute values of zeta
potentials of particles, and by optimizing coagulation kinetics. Besides, it can also be realized by adding appropriate
coarse particles, in which fine particles in suspensions coat
coarse particles in the form of multilayers. Such coagulates
consist of fine particles and coarse particles. This phenomenon was observed early in mineral flotation systems, and
was termed ‘‘slime coating’’ (Fuerstenau et al., 1979; Somasundaran, 1980). The coarse particle enhancement of coagulation of fine particles not only strongly reduces particle
numbers in suspensions, but also greatly increases coagulate
size. The mechanism of the coarse particle effect might be
attributed to the increase of aggregation rate, because
collision rate between coarse particle and fine particle is
much higher than that between fine particles themselves
(Song and Lopez-Valdivieso, 2002). The commercial uses of
this effect appeared in a flotation circuit for the purification of
kaolin early in 1960s, in which anatase fines aggregated with
coarse calcite, followed by the flotation of the coated calcite to
remove the anatase from kaolin (Green and Duke, 1962).
In the present study, we attempted to make use of the
coarse particle effect to improve the coagulation process for
arsenic removal from water. The objective is to develop a new
cost-effective technique for eliminating arsenic from higharsenic water in the mine drainage located in Matehuala city,
Mexico. The possibility and effectiveness of this new enhanced coagulation process were determined while ferric
sulfate was used as coagulant and calcite was used as the
coarse particle. In order to understand the coarse particle
effect in the enhanced coagulation, the zeta potentials of
arsenic-borne coagulates and calcite particles were determined, and the settling rates of arsenic-borne coagulates
before and after coating coarse calcite were measured. In
addition, arsenic-borne coagulates were observed by using a
scanning electronic microscope (SEM) and an atomic force
microscope (AFM).
2.
Experimental
2.1.
Materials
The high-arsenic water sample used in this work was
originally collected from the Cerrito Blanco channel, located
in Matehuala city, state of San Luis Potosi, Mexico. The water
was first sieved with a 400 mesh screen, and then was
filtrated with an Ahlstrom Grade 610 filter paper (2.5 mm
aperture) to remove solid contaminants. The water past the
filter paper was used for the tests, which gave the initial
arsenic concentration of 5071 mg/l. The chemical composition
and some properties of the water sample were listed in Tables
1 and 2, respectively.
Ferric sulfate (Fe2(SO4)3 5H2O) (J. T. Barker, analytic purity)
was used as coagulant; hydrochloric acid (HCl) (Fremont,
ARTICLE IN PRESS
366
WA T E R R E S E A R C H
40 (2006) 364– 372
Table 1 – Chemical composition of the high-arsenic water sample
Constituent
Ca
Mg
Na
K
Fe
Mn
As
Cl
NO
3
SO2
4
HCO
3
PO3
4
Concentration (mg/l)
566
77
37
8.9
0.11
0.01
5.07
142
0.25
1340
185
40.8
Table 2 – Some properties of the high-arsenic water sample
pH
Conductivity
(mS/cm)
Eh (mV)
Dissolved organic
carbon (mg/l)
Dissolved organic
materials (mg/l)
7.62
1960
414
60.5
121
analytic purity) and sodium hydroxide (NaOH) (J. T. Barker,
analytic purity) was used to adjust pH.
Calcite used in this work was obtained from the mine of
Minera de Las Curvas, Mexico. The chunk mineral was first
crushed with a hammer, and then purified by hand sorting.
After that, the mineral was ground with a porcelain mortar,
followed by a size classification with screens to obtain two
size fractions of 52–74 mm and 38–44 mm, which were determined to contain 99.2% and 99.0% CaCO3, respectively.
2.2.
Arsenic removal by coagulation with ferric ions
High-arsenic water (200 ml) was first mixed with a given
amount of Fe2(SO4)3 in a flask, and then was adjusted for pH
with HCl or NaOH by using a potential meter (Orion 720-A).
After that, it was stirred on a magnetic agitator (Digital hot
plate/stirrer 04644) at 240 rev/min for 30 min, while temperature was kept at 3070.2 1C. Arsenic-borne coagulates were
formed during the agitation. After that, the suspension was
filtrated through a Millipore white GSWP membrane (0.22 mm
aperture). The filtrate was sent for arsenic analysis. Each test
was duplicated. The arithmetic average result of the two tests
was reported in this paper.
2.3.
Arsenic removal by enhanced coagulation with ferric
ions and coarse calcite
High-arsenic water (200 ml) was first mixed with a given
amount of Fe2(SO4)3 and coarse calcite in a flask, and then pHadjusted and stirred in the same way as mentioned above.
After that, the suspension was filtrated through an Ahlstrom
Grade 610 filter paper (2.5 mm aperture). The filtrate was sent
for arsenic analysis. Each test was duplicated. The arithmetic
average result of the two tests was reported in this paper.
2.4.
Arsenic analysis
Arsenic concentration in water or filtrates was determined by
using a Perkin-Elmer Aanalyst 100 atomic absorption spectrometer coupled to a Perkin-Elmer FIAS 100 flow injection
system. Water or filtrate (20 ml) was first mixed with 2 ml
mixture of KI (10%) and ascorbic acid (5%) and 1 ml
concentrated hydrochloric acid at 75 1C for 5 min. This
treatment was to reduce As (V) to As (III) and to dissolve
solid particles in the water or filtrates. Then, the solution was
cooled to room temperature, from which a 10 ml solution was
taken for the analysis of As (III) concentration with the
instruments. Each analysis for arsenic concentration was
duplicated. The arithmetic average of the two analysis results
was reported in this paper. As soon as arsenic concentrations
were obtained, arsenic removal (E) was calculated by the
following expression:
E¼
C0 C
,
C0
(1)
where C0 and C are the arsenic concentration of the higharsenic water sample and a filtrate from arsenic removal
tests, respectively.
2.5.
Turbidity measurement
Turbidity of the high-arsenic water treated by coagulation
was determined by using a Bausch & Lomb Spectronic 20
spectrophotometer. First, coagulation was applied to water.
Then, 20 ml of the suspension was transferred to a tube for
turbidity measurement. Light transmissivity as a function of
settling time was obtained from this measurement, which
could provide information on settling rate of coagulates. At
the same settling time, a larger light transmissivity indicates
a higher settling rate, or stronger coagulation. Each measurement was tripled. The arithmetic average of the three
measurement results was reported in this paper.
2.6.
Microscope study
Coagulates from coagulations were observed by using a
Quesant Qscope 250 AFM and a Philips Microspec WDX SEM
in this work. AFM images and SEM micrographs of coagulates
were photographed. The samples for the observations were
prepared by putting a drop of coagulated suspension on a
metal dish for SEM study or on a mica for AFM study, and then
drying in room temperature. Before the SEM study, the
sample on the dish was coated by gold. In addition, an EDAX
energy dispersive spectrometer (EDS) attached to the SEM was
used to determine the existence of arsenic in coagulates.
2.7.
Zeta potential determination
A micro-electrophoresis meter (Riddick) was used to measure
the zeta potentials of arsenic-borne coagulates from the
ARTICLE IN PRESS
coagulation mentioned above and calcite fines in aqueous
solutions. The electrophoresis mobility of over 20 particles
was determined, and the average value was transformed to
zeta potential using the Smoluchowski equation.
2.8.
Particle size analysis
A Shimadzu SALD-1100 laser diffraction particle size analyzer
with semiconductor laser as the light source was used to
measure the size distributions of arsenic-borne coagulates
from the coagulation mentioned above in this study. This
instrument reports optical diameter instead of the equivalent
Stokes diameter. In order to protect coagulates from breaking,
ultrasonic was not applied to the suspensions during the
measurements. The measurement was tripled. The arithmetic average of the three measurement results was reported
in this paper.
3.
Results and discussions
3.1.
Arsenic removal by coagulation with ferric ions
367
40 (20 06) 36 4 – 372
100
CUMULATIVE UNDERSIZE, %
WAT E R R E S E A R C H
80
60
40
Arsenic-borne coagulates
20
100 mg/l Fe2(SO4)3 at pH 5
0
0
4
8
12
16
20
PARTICLE SIZE, µm
Fig. 2 – Size distribution of arsenic-borne coagulates from
the coagulation of high-arsenic water with 100 mg/
l ferric sulfate at pH 5.
500
400
300
Fig. 1 illustrates arsenic removal from the high-arsenic water
as a function of pH by coagulation with 100 mg/l ferric sulfate
and micro-filtration with the membrane. Error analysis for
the results of arsenic elimination presented in the graph was
made on the basis of the standard error of the coagulation/
filtration tests and arsenic analysis. The error was also
presented in the graph in the form of an error bar. As it can
be seen, in the acidic range, very high arsenic removal, about
99%, was achieved, while in the alkaline range, the arsenic
removal declined sharply with the increase of pH. This result
is in a good agreement with those obtained by coagulation
with ferric ions for low-arsenic concentration water
(20–100 mg/l), as reported elsewhere (Hering et al., 1996;
Gulledge and O’Connor, 1973). Obviously, coagulation/microfiltration process is very effective in eliminating arsenic from
high-arsenic water at acidic to neutral pH.
ARSENIC REMOVAL, %
100
95
90
85
100 mg/l Fe2(SO4)3
Stirring: 240 rev/min, 30 min
80
75
4
5
6
7
8
pH
9
10
11
12
Fig. 1 – Arsenic removal from the high-arsenic water as a
function of pH by coagulation with ferric ions and
filtration with the membrane.
200
515.9 nm
100
0
nm
0
5.0 µm
10.0 µm
15.0 µm
5.0 µm
10.0 µm
15.0 µm
0
Fig. 3 – AFM image of arsenic-borne coagulates from the
coagulation of high-arsenic water with 100 mg/l
ferric sulfate at pH 5.
Fig. 2 shows the size distribution of arsenic-borne coagulates from the coagulation of the high-arsenic water with
100 mg/l ferric sulfate at pH 5. Error analysis for the weight
percentage of each size fraction showed that the maximum
error was 70.6%. Since the error bars were too small to draw,
only the arithmetic average values of cumulative undersize
were presented in the graph. As it is noted, the coagulates
were very fine, ranging between 0.5 and 20 mm. There were
about 17% (weight percentage) coagulates smaller than
2.5 mm, and about 83% coagulates smaller than 10 mm.
Obviously, filtration with the membrane (0.22 mm aperture)
is qualified for a perfect separation of the coagulates from
water. However, filtration with the filter paper (2.5 mm
aperture) would lead to low arsenic removal, because 17%
coagulates would pass through the paper into filtrates. In
addition, this result suggests that the separation of the
coagulates from water cannot be realized by gravitational
sedimentation, as the coagulates are in colloidal size range.
The AFM image of coagulates from the coagulation of the
high-arsenic water with 100 mg/l ferric sulfate at pH 5 is
given in Fig. 3. It clearly shows the three dimensions of
ARTICLE IN PRESS
368
WA T E R R E S E A R C H
40 (2006) 364– 372
arsenic-borne coagulates. The coagulates have irregular
shape, and a size of 0.5–4 mm.
Enhanced coagulation with ferric ions and coarse calcite
followed by filtration with filter papers (2.5 mm aperture) was
tested to eliminate arsenic from the high-arsenic water. Fig. 4
illustrates arsenic removal from the water as a function of
calcite addition at pH 6 and 100 mg/l ferric sulfate addition.
The error bars obtained from the standard error analysis of
coagulation/filtration tests and arsenic analysis were also
given in the graph. Without coarse calcite, the arsenic
removal was only 85%, which was in correspondence with
the cumulative oversize of arsenic-borne coagulates as shown
in Fig. 2. Such a low arsenic removal should be attributed to
small arsenic-borne coagulates passing through the filter
papers. As the increase of coarse calcite addition, the arsenic
removal increased until it reached about 99%. This increase
was very sharp in the small calcite addition range, but was
mild after 2.5 g/l calcite addition. Indeed, the coagulation of
high-arsenic water could be greatly enhanced by adding
coarse calcite particles and, thus, arsenic can be effectively
removed from the water by filtration in a large pore filter
medium. Also, it can be observed from this graph that
3844 mm calcite enhanced arsenic removal more strongly
than 5274 mm calcite at the same amount of addition. The
mechanism might be due to the larger surface area on smaller
particles, leading to more coatings of arsenic-borne coagulates on calcite surfaces.
The effect of pH on arsenic removal from the high-arsenic
water by coagulation with 100 mg/l ferric sulfate and 5 g/l
coarse calcite and filtration with the filter papers is shown in
Fig. 5. The error bars of arsenic removal for the points
appeared in the graph were 70.2% to 70.7%. They were not
presented in the graph because most of the bars overlapped
with the corresponding points. As it is noted, the arsenic
removal vs. pH curve in the presence of coarse calcite is very
ARSENIC REMOVAL, %
100
95
100 mg/L Fe2(SO4)3
pH = 6.0
90
38 - 44 µm calcite
52 - 74 µm calcite
85
0
5
10
15
CALCITE ADDITION, g/L
Fig. 4 – Arsenic removal from high-arsenic water as a
function of coarse calcite addition by the enhanced
coagulation at pH 6 and filtration with filter papers.
ARSENIC REMOVAL, %
3.2.
Arsenic removal by enhanced coagulation with ferric
ions and coarse calcite
100
90
80
Coagulation: 100 mg/l Fe2(SO4)3
5 g/l calcite
70
38 - 44 µm calcite
52 - 74 µm calcite
60
5
6
7
8
9
10
11
12
pH
Fig. 5 – Arsenic removals from high-arsenic water as a
function of pH by the enhanced coagulation and
filtration with filter papers.
similar to that without calcite as shown in Fig. 1. High arsenic
removal was achieved in the range of pH 5–7. At the alkaline
pHs, the arsenic removal declined sharply with increasing pH.
Again, this graph also showed that 38–44 mm calcite improved
arsenic removal from the high-arsenic water by enhanced
coagulation more strongly than 5274 mm calcite.
At pH ¼ 5, the enhanced coagulation and filtration with
filter papers produced treated water with the residual arsenic
concentration of 13.2 mg/l, which is less than the Mexican MCL
for arsenic in drinking water. Compared with the results as
shown in Fig. 1, it is noted that the maximum arsenic removal
achieved by the enhanced coagulation was almost the same
as that produced by coagulation with the same amount of
ferric sulfate and filtration with membrane. It indicates that
the role played by coarse calcite in the enhanced coagulation
might be solely to increase the size of arsenic-borne
coagulates.
For high-arsenic water treatment, another effective process
is adsorption (Jiang, 2001; Bissen and Frimmel, 2003; Dambies,
2004; USEPA, 2000). Many kinds of materials, such as activated
alumina, activated carbon, granulated ferric hydroxide, zerovalent iron, natural iron minerals and metal-loaded polymers,
etc., are used as the adsorbent. Ahn et al. (2003) studied
arsenic removal from high-arsenic water in a mine tailing
containment system by using waste steels and zero-valent
iron, and produced treated waters with the residual arsenic
concentrations of 44 and 3 mg/l arsenic from waters initially
containing 4137 and 1400 mg/l arsenic, respectively. Jiang
(2001) reported that by activated carbon adsorption 96% of
arsenic can be removed from water with initial arsenic
concentration of 300 mg/l, and by coagulation with ferric ions
99% of arsenic can be removed from water with initial arsenic
concentration of 1100 mg/l. Clearly, the enhanced coagulation
process with ferric ions and coarse calcite showed a very high
efficiency for arsenic removal from high-arsenic water.
The results presented in this paper were obtained from
batch tests in laboratory. If the enhanced coagulation process
is applied to treat the high-arsenic water in pilot or plant
scale, the arsenic removal might be less than 99.5% that
was achieved in batch scale, and the residual arsenic
ARTICLE IN PRESS
concentration of cleaned water might be greater than the
Mexican MCL for arsenic in drinking water (20 mg/l As) and
much greater than the World Health Organization (WHO)
guideline value (10 mg/l As). Therefore, the enhanced coagulation process may be used as primary treatment for the higharsenic water from the mine drainage system. Another
process (which could be conventional coagulation, adsorption, ion exchange, etc.) has to be followed to deeply remove
arsenic from water in order to meet the Mexican MCL or WHO
guideline value.
3.3.
Mechanisms of the enhanced coagulation
100
Coagulation with 100 mg/l Fe2(SO4)3
pH 7
80
60
40
Without calcite
With 5 g/l calcite (38 - 44 µm)
20
0
0
Coagulates from the enhanced coagulation were photographed by using a SEM. Fig. 6 gives two SEM micrographs.
In the images, the large particles which were similar to cubes
were of calcite, and the small irregular particles were of
arsenic-borne coagulates. As it can be observed, the calcite
surfaces (right sides and lateral sides) were coated by a lot of
small coagulates, some of which were very fine (less than
1 mm). Because of the coating, the small arsenic-borne
coagulates behave similar to the coarse calcite in gravitational sedimentation and filtration, and thus are easily
removed from water. In addition, EDS attached to the SEM
was used to chemically analyze the small coagulates on the
calcite surfaces in the images. Numerous points on the
coagulates were chosen. Arsenic was found in the coagulates,
ranging from 0.8% to 5.3% As. Indeed, they were of arsenicborne coagulates.
Fig. 6 – SEM micrographs of arsenic-borne coagulates
coating coarse calcite particles.
369
40 (20 06) 36 4 – 372
LIGHT TRANSMISSIVITY, %
WAT E R R E S E A R C H
10
20
30
40
50
60
70
SETTLING TIME, min.
Fig. 7 – Sedimentation of arsenic-borne coagulates from
coagulation of high-arsenic water with ferric ions
at pH 7 in the absence or presence of coarse calcite
particles.
The enhanced coagulation was also studied through
sedimentation tests. Fig. 7 illustrates the light transmissivity
of the high-arsenic water treated by coagulation with 100 mg/l
ferric sulfate at pH 7 as a function of settling time, in the
absence and presence of coarse calcite (38–44 mm). Error
analysis for three tripled measurements showed that the
standard error for light transmissivity ranged from 0 to
70.8%. Such small error bars were difficult to be clearly
presented, so that only arithmetic average values of light
transmissivity appeared in the graph. As it is noted, without
calcite, the light transmissivity increased very slowly with
increasing settling time, indicating a slow coagulate sedimentation. Clean water in the sedimentation tube (100% light
transmissivity) was obtained after 22 h settling. However, 5 g/l
coarse calcite coagulates settled down much more rapidly,
leaving clean water in the tube after 14 min. These results
suggest that coarse calcite considerably enhanced the coagulation of the high-arsenic water, and greatly increased the
sedimentation of arsenic-borne coagulates.
According to Stokes equation of sedimentation, the terminal settling velocity of a particle in a liquid is proportional to
the square of the diameter of the particle. So, the coarse
calcite should have a much higher settling rate than small
arsenic-borne coagulates. If small coagulates coat on coarse
calcite particles, they would settle down in the same velocity
(or a little bit higher) as the calcite particles. This is the
mechanism by which the sedimentation of arsenic-borne
coagulates was considerably improved by adding coarse
calcite particles.
Fig. 8 shows light transmissivity of the high-arsenic water
treated by enhanced coagulation with 100 mg/l ferric sulfate
and 5 g/l coarse calcite as a function of settling time at various
pH. As it can be seen, the settling rate of arsenic-borne
coagulates decreased with the increase of pH. Light transmissivity of 100% appeared after 9 min at pH ¼ 6, compared
with 29 min at pH ¼ 9. These results might be attributed to
that coarse calcite had a stronger ability to carry small
arsenic-borne coagulates at pH ¼ 6 than at pH ¼ 9. In other
ARTICLE IN PRESS
370
WA T E R R E S E A R C H
geneous particles, the potential energy of electrical double
layer interaction between two heterogeneous spheres (VR) is
expressed by (Wang, 1991)
LIGHT TRANSMISSIVITY, %
100
80
VR ¼ 32 peo er
60
pH 6
pH 7
pH 8
pH 9
40
20
0
40 (2006) 364– 372
(2)
Coagulation: 100 mg/l Fe2(SO4)3
5 g/l calcite (38 - 44 µm)
0
5
10
15
20
SETTLING TIME, min
Fig. 8 – Effect of pH on the sedimentation of arsenic-borne
coagulates from the enhanced coagulation of higharsenic water.
where a1 and a2 are the radii of the particles 1 and 2,
respectively; eo is the vacuum dielectric permittivity; er is the
relative dielectric permittivity of the medium; k is the
Boltzmann constant; T is the absolute temperature; e is the
elementary charge, u is the ionic valence of electrolyte, c1 and
c2 are the outer Helmholtz plane (OHP) potentials or zeta
potentials of particles 1 and 2, respectively, k is the Debye
reciprocal length and h is the shortest separation between the
two particles. The potential energy of van der Waals interaction between two heterogeneous particles (VA) is expressed by
(Hiemenz and Rajagopalan, 1997)
VA ¼ 30
ZETA POTENTIAL, mV
10
0
-10
-20
Arsenic-borne coagulates
calcite
-30
2
4
6
8
pH
10
12
a1 a2 A132
,
a1 þ a2 6 h
(3)
where A123 is the Hamaker constant of particles 1 and 2 in
medium 3, which may be obtained by (Israelachvili, 1992)
pffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffi pffiffiffiffiffiffiffiffi
A11 A33 Þð A22 A33 ,
(4)
A132 ¼
0.01 mol/l KNO3
25°C
20
2a1 a2 kT 2
euc1
euc2
tanh
expðkhÞ,
tanh
a1 þ a2 eu
4kT
4kT
14
Fig. 9 – Zeta potentials of calcite particles and arsenic-borne
coagulates from the coagulation of high-arsenic
water with 100 mg/l ferric sulfate in 0.01 mol/l
KNO3 solution as a function of pH.
words, coarse calcite enhanced the coagulation more strongly
at acidic pH than at alkaline pH.
The zeta potentials of calcite particles and arsenic-borne
coagulates from the coagulation with 100 mg/l ferric sulfate in
0.01 mol/l KNO3 solution as a function of pH are illustrated in
Fig. 9. It shows that the isoelectric points (IEP) of the calcite
particle and the arsenic-borne coagulate were pH 9.3 and pH
4.9, respectively. In the range of pH 4.9–9.3, calcite particles
were positively charged, while arsenic-borne coagulates were
negatively charged.
The aggregative stability of colloidal suspensions is due to
the existence of a potential energy barrier between particles,
preventing the proximity of the particles. Such a study of
stability is known as the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory (Derjaguin and Landau, 1941; Verwey and
Overbeek, 1948). According to this theory, the potential energy
barrier arises as a result of interaction energies of electrical
double layer and van der Waals. In a suspension of hetero-
where A11, A22 and A33 are the Hamaker constants of particles
1, 2 and medium 3 in vacuum.
In the case of this work, the Hamaker constants of the
arsenic-borne coagulates and calcite are larger than water, i.e.
A114A33 and A224A33. So, A12340, and thus VAo0 according
to Eqs. (4) and (3), indicating an attractive interaction of van
der Waals between the two heterogeneous particles. In the
range of pH ¼ 4.9–9.3, c1o0 (arsenic-borne coagulates), and
euc2
1
c240 (calcite), leading to tanh euc
4 kT o0 and tanh 4 kT 40, and
thus VRo0 according to Eq. (1). It means that the electrical
double layer interaction was also attractive between the two
heterogeneous particles. Therefore, in the pH range, the total
potential energy of interaction between the arsenic-borne
coagulate and the calcite particle was attractive at every
distance. In other words, there was no potential energy
barrier between the two particles. A strong coagulation
should employ to the arsenic-borne coagulates and calcite
particles. This is the mechanism by which small arsenicborne coagulates coated on coarse calcite, as shown in Figs. 6
and 8.
In addition, there are interests to understand the impacts of
other factors on arsenic removal from the high-arsenic water
by the enhanced coagulation process, e.g., ions, organic
carbon and ionic strength in initial water, chemiosorption of
arsenate ions on calcite surfaces, etc., which are our current
and future research interests. Another further work in this
regard is to build up a pilot system for the enhanced
coagulation process to eliminate arsenic from the higharsenic water in the mine drainage system. The state
government of San Luis Potosı́ has approved financial support
for the pilot-scale tests.
The experimental results from this study have shown that
the enhanced coagulation process is a very cost-effective
technique for arsenic removal from high-arsenic water.
ARTICLE IN PRESS
WAT E R R E S E A R C H
Because of the high settling rate, the arsenic-borne coagulates
from the enhanced coagulation can be first concentrated
from water by gravitational sedimentation, and then separated from water by filtration with a large-pore filter medium,
producing a very low arsenic concentration of cleaned water.
Compared with conventional coagulation/filtration process
for arsenic removal, the enhanced coagulation process
features lower capital and operation costs, simpler in operation and lower task in maintenance. Since coagulation is a
commonly and widely used process for water treatment
worldwide, it will not be hard to realize the practical uses of
the enhanced coagulation process. Coarse calcite may be
added together with ferric sulfate and other chemical
reagents, which would not bring great difficulty. For the steps
of conditioning, sedimentation and filtration, the enhanced
coagulation could be the same as conventional coagulation/
filtration process. However, the disposal of the sludge from
the enhanced coagulation process has to be studied to meet
the Mexican regulation for toxic waste disposal, which will be
our further research subject together with the pilot-scale
studying.
4.
Conclusions
(1) Coagulation/filtration for arsenic removal from a higharsenic water with ferric ions as the coagulant could be
considerably enhanced by adding an appropriate amount
of coarse calcite (38–74 mm). This enhancement may be
attributed to the coating of small arsenic-borne coagulates
on calcite surfaces, improving the gravitational sedimentation and filtration of the coagulates greatly.
(2) The coating of small arsenic-borne coagulates on coarse
calcite may be due to the attractive electrical double layer
interaction between the coagulates and coarse calcite
because of the reverse zeta potential of the two particles,
which leads to non-existence of a potential energy barrier
between the two particles.
(3) The enhanced coagulation with ferric ions and coarse
calcite and conventional filtration (filter paper as filter
medium) achieved a very high arsenic removal (over 99%)
from a high-arsenic water in mine drainage system (5 mg/l
As), giving the residual arsenic concentration of 13 mg/l.
Acknowledgements
The financial support from the Consejo Nacional de Ciencia y
Tecnologı́a (CONACyT) of Mexico and the government of the
State of San Luis Potosı́ under the Grant FMSLP 2002-5537 is
gratefully acknowledged.
R E F E R E N C E S
Ahn, J.S., Chon, C.M., Moon, H.S., Kim, K.W., 2003. Arsenic
removal using steel manufacturing byproducts as permeable
reactive materials in mine tailing containment systems. Water
Res. 37, 2478–2488.
40 (20 06) 36 4 – 372
371
Arienzo, M., Adamo, P., Chiaenzelli, J., Bianco, M.R., de Martino, A.,
2002. Retention of arsenic on hydrous ferric oxides generated
by electrochemical peroxidation. Chemosphere 48, 1009–1018.
Bissen, M., Frimmel, F.H., 2003. Arsenic—a review. Part II:
oxidation of arsenic and its removal in water treatment. Acta
Hydrochim. Hydrobiol. 31 (2), 97–107.
Chiba, A., Okada, H., Tada, T., Kudo, H., Nakazawa, H., Mitsuhashi,
K., Ohara, T., Wada, H., 2002. Removal of arsenic from
geothermal water by high gradient magnetic separation. IEEE
Trans. Appl. Superconductivity 12 (1), 952–954.
Dambies, L., 2004. Existing and prospective sorption technologies
for the removal of arsenic in water. Separation Sci. Technol.
39, 599–623.
Derjaguin, B.V., Landau, L., 1941. Theory of molecular interaction.
Acta Physicochem. 14, 633–668.
Fuerstenau, D.W., Chander, S., Abouzeid, A.M., 1979. The recovery
of fine particles by physical separation methods. In: Somasundaran, P., Arbiter, N. (Eds.), Beneficiation of Mineral
Fines—Problems and Research Needs. AIME, New York,
pp. 3–59.
Green, E.W., Duke, J.B., 1962. Selective froth flotation of ultrafine
minerals or slimes. Trans. AIME 223, 389–395.
Gulledge, J.H., O’Connor, J.T., 1973. Removal of arsenic (V) from
water by adsorption on aluminum and ferric hydroxides.
J. AWWA 65 (8), 543.
Han, B., Zimbron, J., Runnells, T.R., Shen, Z., Wickramasinghe,
S.R., 2003. New arsenic standard spurs search for costeffective removal techniques. J. AWWA 95 (10), 109–118.
Hering, J.G., Chen, P.Y., Wilkie, J.A., Elimelech, M., Liang, S., 1996.
Arsenic removal by ferric chloride. J. AWWA 88 (4), 155–167.
Hering, J.G., Chen, P.J., Wilkie, J.A., Elimelech, M., 1997. Arsenic
removal from drinking water during coagulation. J. Environ.
Eng. 8, 800–807.
Hiemenz, P.C., Rajagopalan, R., 1997. Principles of Colloid and
Surface Chemistry, third ed. Marcel Dekker, New York.
Israelachvili, J.N., 1992. Intermolecular and Surface forces, second
ed. Academic Press, Inc., New York.
Jiang, J.Q., 2001. Removing arsenic from groundwater for the
developing world—a review. Water Sci. Technol. 44 (6), 89–98.
Katsoyiannis, I.A., Zouboulis, A.I., 2002. Removal of arsenic from
contaminated water sources by sorption onto iron-oxidecoated polymeric materials. Water Res. 36, 5145–5155.
Katsoyiannis, I.A., Zouboulis, A.I., 2004. Application of biological
processes for the removal of arsenic from groundwaters.
Water Res. 38, 17–26.
Korngold, E., Belayev, N., Aronov, L., 2001. Removal of arsenic
from drinking water by anion exchange. Desalination 141,
81–84.
Kumar, P.R., Chaudhari, S., Khilar, K.C., Mahajan, S.P., 2004.
Removal of arsenic from water by electrocoagulation. Chemosphere 55, 1245–1252.
O’Connor, J.T., 2002. Arsenic in drinking water. Part 2: human
exposure and health effects. Water Eng. Manage. 149 (3), 35–37.
Raje, N., Swain, K.K., 2002. Purification of arsenic contaminated
ground water using hydrated manganese dioxide. J. Radioanal.
Nucl. Chem. 253, 77–80.
Razo-Soto, I., Carrizales, L., Castro, J., Diaz-Barriga, F. and Monroy,
M., 2005. Arsenic and heavy metal pollution of soil, water and
sediments in a semi-arid climate mining area in Mexico.
Water, Air Soil Pollut., in press.
Sato, Y., Kang, M., Kamei, T., Magara, Y., 2002. Performance of
nanofiltration for arsenic removal. Water Res. 36, 3371–3377.
Shen, Y.S., 1973. Study of arsenic removal from drinking water.
J. AWWA 65 (8), 543.
Somasundaran, P., 1980. Role of surface chemistry of fine
sulphides in their flotation. In: Jones, M.J. (Ed.), Complex
Sulphide Ores. The Institute of Mining and Metallurgy,
London, p. 126.
ARTICLE IN PRESS
372
WA T E R R E S E A R C H
Song, S., Lopez-Valdivieso, A., 2002. Parametric aspect of hydrophobic flocculation technology. Miner. Process. Extr. Metall.
Rev. 23, 101–127.
Sorg, J.T., Logsdon, G.S., 1978. Treatment technology to meet the
interim primary drinking water regulations for inorganics.
Part 2. J. AWWA 70 (7), 379–392.
Thirunavukkarasu, O.S., Viraraghavan, T., Subramanian, K.S.,
2003. Arsenic removal from drinking water using iron oxidecoated sand. Water Air Soil Pollut. 142, 95–111.
Thirunavukkarasu, O.S., Viraraghavan, T., Subramanian, K.S.,
Chaalal, O., Islam, M.R., 2005. Arsenic removal in drinking
water—impacts and novel removal technologies. Energy
Source. 27, 209–219.
USEPA., 2000. Technologies y costs for removal of arsenic from
drinking water. EPA/815/R-00/028, Washington.
40 (2006) 364– 372
Verwey, E.J., Overbeek, J.G., 1948. Theory of the Stability of
Lyophobic Colloid. Elsevier, Amsterdam.
Wang, Q., 1991. Theoretical analysis of Brownian heterocoagulation of fine particles at secondary minimum. J. Colloid Interface Sci. 145, 305–313.
Wang, L., Chen, A.S.C., Sorg, T.J., Fields, K.A., 2002. Field
evaluation of as removal by IX and AA. J. AWWA 94, 161–173.
Wickramasinghe, S.R., Han, B., Zimbron, J., Shen, Z., Karim, M.N.,
2004. Arsenic removal by coagulation and filtration: comparison of groundwaters from the United States and Bangladesh.
Desalination 169, 224–231.
Zhang, Y., Yang, M., Huang, X., 2003. Arsenic(V) removal with a
Ce(IV)-doped iron oxide adsorbent. Chemosphere 51, 945–952.
Zhang, W., Singh, P., Paling, P., Delides, F., 2004. Arsenic removal from
contaminated water by natural iron ores. Miner. Eng. 17, 517–524.