Study on the hydrolysis/precipitation behavior of Keggin Al13 and Al30 polymers in polyaluminum solutions.

Abstract

The hydrolysis/precipitation behaviors of Al(3+), Al(13) and Al(30) under conditions typical for flocculation in water treatment were investigated by studying the particulates' size development, charge characteristics, chemical species and speciation transformation of coagulant hydrolysis precipitates. The optimal pH conditions for hydrolysis precipitates formation for AlCl(3), PAC(Al13) and PAC(Al30) were 6.5-7.5, 8.5-9.5, and 7.5-9.5, respectively. The precipitates' formation rate increased with the increase in dosage, and the relative rates were AlCl(3)>PAC(Al30)>PAC(Al13). The precipitates' size increased when the dosage increased from 50 microM to 200 microM, but it decreased when the dosage increased to 800 microM. The Zeta potential of coagulant hydrolysis precipitates decreased with the increase in pH for the three coagulants. The iso-electric points of the freshly formed precipitates for AlCl(3), PAC(Al13) and PAC(Al30) were 7.3, 9.6 and 9.2, respectively. The Zeta potentials of AlCl(3) hydrolysis precipitates were lower than those of PAC(Al13) and PAC(Al30) when pH>5.0. The Zeta potential of PAC(Al30) hydrolysis precipitates was higher than that of PAC(Al13) at the acidic side, but lower at the alkaline side. The dosage had no obvious effect on the Zeta potential of hydrolysis precipitates under fixed pH conditions. The increase in Zeta potential with the increase in dosage under uncontrolled pH conditions was due to the pH depression caused by coagulant addition. Al-Ferron research indicated that the hydrolysis precipitates of AlCl(3) were composed of amorphous Al(OH)(3) precipitates, but those of PAC(Al13) and PAC(Al30) were composed of aggregates of Al(13) and Al(30), respectively. Al(3+) was the most un-stable species in coagulants, and its hydrolysis was remarkably influenced by solution pH. Al(13) and Al(30) species were very stable, and solution pH and aging had little effect on the chemical species of their hydrolysis products. The research method involving coagulant hydrolysis precipitates based on Al-Ferron reaction kinetics was studied in detail. The Al species classification based on complex reaction kinetic of hydrolysis precipitates and Ferron reagent was different from that measured in a conventional coagulant assay using the Al-Ferron method. The chemical composition of Al(a), Al(b) and Al(c) depended on coagulant and solution pH. The Al(b) measured in the current case was different from Keggin Al(13), and the high Al(b) content in the AlCl(3) hydrolysis precipitates could not used as testimony that most of the Al(3+) was converted to highly charged Al(13) species during AlCl(3) coagulation.

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Study on the hydrolysis/precipitation behavior of Keggin Al13and Al30
polymers in polyaluminum solutions
Zhaoyang Chena, Zhaokun Luanb, Zhiping Jiab, Xiaosen Lia,*
aKey Laboratory of Renewable Energy and Gas Hydrate, CAS, Guangzhou Institute of Energy Conversion, Chinese Academy of Sciences, Guangzhou 510640, China
bState Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China
a r t i c l e i n f o
Article history:
Received 27 May 2008
Received in revised form
3 March 2009
Accepted 2 April 2009
Available online 5 May 2009
Keywords:
Al13and Al30species
Coagulant
Ferron method
Chemical speciation
Zeta potential
a b s t r a c t
The hydrolysis/precipitation behaviors of Al3þ, Al13and Al30under conditions typical for flocculation in
water treatment were investigated by studying the particulates’ size development, charge characteristics,
chemical species and speciation transformation of coagulant hydrolysis precipitates. The optimal pH
conditions for hydrolysis precipitates formation for AlCl3, PACAl13and PACAl30were 6.5–7.5, 8.5–9.5, and
7.5–9.5, respectively. The precipitates’ formation rate increased with the increase in dosage, and the
relative rates were AlCl3[PACAl30>PACAl13. The precipitates’ size increased when the dosage increased
from 50 mM to 200 mM, but it decreased when the dosage increased to 800 mM. The Zeta potential of
coagulant hydrolysis precipitates decreased with the increase in pH for the three coagulants. The iso-
electric points of the freshly formed precipitates for AlCl3, PACAl13and PACAl30were 7.3, 9.6 and 9.2,
respectively. The Zeta potentials of AlCl3hydrolysis precipitates were lower than those of PACAl13and
PACAl30when pH>5.0. The Zeta potential of PACAl30hydrolysis precipitates was higher than that of
PACAl13at the acidic side, but lower at the alkaline side. The dosage had no obvious effect on the Zeta
potential of hydrolysis precipitates under fixed pH conditions. The increase in Zeta potential with the
increase in dosage under uncontrolled pH conditions was due to the pH depression caused by coagulant
addition. Al–Ferron research indicated that the hydrolysis precipitates of AlCl3 were composed of
amorphous Al(OH)3precipitates, but those of PACAl13and PACAl30were composed of aggregates of Al13
and Al30, respectively. Al3þwas the most un-stable species in coagulants, and its hydrolysis was
remarkably influenced by solution pH. Al13and Al30species were very stable, and solution pH and aging
had little effect on the chemical species of their hydrolysis products. The research method involving
coagulant hydrolysis precipitates based on Al–Ferron reaction kinetics was studied in detail. The Al
species classification based on complex reaction kinetic of hydrolysis precipitates and Ferron reagent was
different from that measured in a conventional coagulant assay using the Al–Ferron method. The
chemical composition of Ala, Alband Alcdepended on coagulant and solution pH. The Albmeasured in
the current case was different from Keggin Al13, and the high Alb content in the AlCl3 hydrolysis
precipitates could not used as testimony that most of the Al3þwas converted to highly charged Al13
species during AlCl3coagulation.
? 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Coagulation/flocculation is commonly used as pretreatment
process in water and wastewater treatment for destabilizing dis-
solved and colloid impurities, and producing large floc aggregates
that can be removed from the water in subsequent sedimentation/
flotation and filtration processes. Coagulant is key to flocculation
efficiency. Inorganic polymer flocculants, especially partially pre-
hydrolyzed Al salts such as polyaluminum chloride, have been
developed and used worldwide since the 1980s (Jiang, 2001).
Aluminum hydrolysis reaction plays a very important role in
destablizing and aggregating the suspended particles and organic
matter in coagulation processes. Coagulant is usually added to the
water to be treated without dilution and pretreatment. After dosing
into water, it is diluted, and it undergoes a serial of hydrolysis,
polymerization, aggregation and precipitation processes, which
result in various metastable and transient species existing in bulk
solution.The hydrolysisof polymericAl coagulantsisquitedifferent
from that of monomeric Al coagulants. Generally, the conventional
Al(III) salt coagulants have lower coagulation efficiency than
* Corresponding author. Tel.: þ86 2087058468; fax: þ86 2087057037.
E-mail addresses: chenzy@ms.giec.ac.cn (Z. Chen), lixs@ms.giec.ac.cn (X. Li).
Contents lists available at ScienceDirect
Journal of Environmental Management
journal homepage: www.elsevier.com/locate/jenvman
0301-4797/$ – see front matter ? 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jenvman.2009.04.001
Journal of Environmental Management 90 (2009) 2831–2840

Page 2

polymeric Al coagulants because its hydrolysis is hard to be
controlled. However, the hydrolysis degree of polymeric Al coagu-
lants can be controlled during manufacture, and the formation rate
of hydroxide precipitates upon dilution is slowed, which conse-
quently allows the positively charged polymeric Al species to
remain for a greater duration, and thereby enhances the charge
neutralization capacity of polyaluminum coagulants (Tang and
Luan, 1996; Matsui et al., 1998; Duan and Gregory, 2003). Maxi-
mizing the active species content and optimizing the species
distribution were the guidelines for coagulant production.
In polyaluminum coagulants, the highly charged Al13species
([AlO4Al12(OH)24(H2O)12]7þ), due to its high charge neutralization
capacity and high species stability against hydrolysis, was consid-
ered to be the optimal active species for coagulation. So it has
received the most attention (Bi et al., 2004; Gao et al., 2005). Al30
species ([(AlO4)2Al28(OH)56(H2O)26]18þ) was first discovered in27Al
nuclear magnetic resonance (NMR) spectra about twenty years ago
(Akitt and Farthing,1981; Fu and Nazar,1991). But its structure and
stoichiometry were not determined, until 2000 it was character-
ized by Allouche et al. (2000) and Rowsell and Nazar (2000) using
X-ray diffraction and in situ27Al NMR. Al30, with 2 nm in length
(Phillips et al., 2003), was the largest Al polycation that was iden-
tified in polyaluminum solution. Al30formed by further polymer-
izing two Al13 and four Al monomers under hydrothermal
treatment (Allouche and Taulelle, 2003; Shafran and Perry, 2005a).
Al30species was more thermal resistant and more stable in low pH
solution than Al13species (Chen, 2006; Chen et al., 2007). It became
a dominant polymeric Al species in highly concentrated poly-
aluminum solutions (Shafran et al., 2005b; Chen et al., 2005). Our
recent research results indicated that Al30 species was more
effective for turbidity removal than Al13species. Compared with
Al13 species, Al30 achieved effective turbidity removal within
a broad dosage range and a wide pH range (Chen et al., 2006a).
However, the detailed coagulation mechanism of Al30species is
unclear.
Twomain flocculation mechanisms wereusually used to explain
the observed flocculation phenomena: (1) compression of electric
double layers of colloidal particles, adsorption–charge neutraliza-
tion and adsorptive-bridging by the positively charged Al species in
treated water; (2) precipitation–charge neutralization, floc enmesh
and sweep by coagulant hydrolysis precipitates (Duan and Gregory,
2003; Wang et al., 2002; Dentel, 1988). It was generally thought
that the conventional Al salts performed mainly through the
second flocculation mechanism due to their rapid hydrolysis and
precipitation, and the first flocculation mechanism played more
important role than the second one when polyaluminum coagu-
lants were used due to the high stability of polymeric Al species
(Luan, 1997). For a practical flocculation process, the flocculation
effects are the combination of the above various coagulation
mechanisms. The dominant flocculation mechanism depends on
the coagulant and dosage employed, the chemical compositions of
the treated water, etc. So the coagulation performance of poly-
aluminum coagulant is not only related to Al species distribution in
coagulants, but also closely related to the hydrolysis, polymeriza-
tion and precipitation behavior of coagulants after dosing into
water. The hydrolysis and polymerization rates, as well as the
structure, surface properties and charge characteristic of freshly
formed hydrolysis precipitates, are very significant to the coagu-
lation performance of polyaluminum coagulants.
Although there were some investigations on hydrolysis/
precipitation behavior and precipitates properties of Al salts and
polymeric Al species, little information was available regarding to
the hydrolysis/precipitation of Al13 and Al30 species in water
treatment flocculation process. Benschoten and Edzwald (1990)
studied the chemical aspects of alum and polyaluminum chloride
during coagulation. The results indicated that the precipitates
derived from alum and polyaluminum chloride possessed different
light scattering characteristics, electrophoretic mobility and solu-
bility. The solid phase of alum was composed of amorphous
Al(OH)3, and the precipitates of polyaluminum chloride retained
the polymeric structure. Solomentseva et al. (1999, 2004) studied in
detail the effect of working solution concentration, dosage, pH and
ionic strength on the surface properties and aggregation of basic
aluminum chloride/sulphate hydrolysis products. Wang et al.
(2004) studied the species transformation of polyaluminum coag-
ulants with different OH/Al ratios using Al–Ferron complex color-
imetry.Theresultsindicated
coagulants with high OH/Al ratio remained higher speciation
stability than coagulants with low OH/Al ratio under various
conditions.
In this work, the hydrolysis/precipitation behaviors of Al30and
Al13under conditions typical for flocculation in water treatment
were compared and studied. AlCl3was used as a reference. The
purposes of the research are: (1) to compare the hydrolysis rate and
species stability of Al30, Al13and AlCl3; (2) to study the differences
of morphology, structure and charge characteristics of hydrolysis
precipitates of Al13and Al30species; (3) to provide a theoretical
interpretationforthecoagulation
between Al13and Al30.
that polyaluminumchloride
performance differences
2. Materials and methods
2.1. Materials
The reagents used in this work were all of analytical grade
except those pointed out specifically. All solutions were prepared
using deionised water. The coagulant containing high content of
Al13(abr. PACAl13) was prepared by slowly neutralizing 1.0 M AlCl3
aqueous solutionwith 0.6 M NaOH solution at 80?C under vigorous
stirring until the Al hydrolysis ratio (B¼[OH]/[Al]) reached 2.4. The
coagulant containing high content of Al30 (abr. PACAl30) was
prepared by heating PACAl13at 95?C for 12 h under stirring and
refluxing. Both PACAl13and PACAl30had a total Al concentration
(AlT) of 0.2 M.
Two coagulants were stored at room temperature for 5 days
before analysis, characterization and experimentation. AlT was
measured by inductively coupled plasma-atomic emission spec-
troscopy (Vista-MPX ICP-AES, Varian). B value was measured by
chemical analysis according to the Chinese standard method (GB
15892-1995). The pH values were measured using a pH meter
(Orion 710A). The Al species distribution in these two coagulants
and 0.2 M AlCl3aqueous solution (abr. AlCl3) was determined by
the time-developed Al–Ferron complex colorimetry (abr. Al–Ferron
method) on UV–vis spectrophotometer (DR/4000U, Hach) and by
high-field27Al NMR method on Fast Fourier Transformation spec-
trometer (JNM-ECA600, JEOL). Based on the difference of the
dissociation and complex reaction kinetic rate between Ferron and
Al species, Al species in coagulants were divided into three types:
monomeric species (Ala) (reaction with Ferron within 1 min),
planar oligomeric and medium polymeric species (Alb) (reaction
with Ferron from 1 to 120 min), and three-dimensional species or
sol–gels (Alc) (reaction with Ferron after 120 min or non-reaction
with Ferron). Alcwas obtained by AlTminus Alaand Alb(Parker and
Bertsch, 1992). In27Al NMR analysis, the aluminum at 0 ppm was
assigned to Al monomer (Alm). The concentration for 62.5 ppm and
70 ppm signals was multiplied by 13 and 15, respectively. This is to
obtain the concentration of Al13 and Al30, respectively. The
concentration of Al species that cannot be clearlydetected (Alu) was
calculated by AlTminus Alm, Al13and Al30. The detailed coagulants’
preparation and characterization methods can be found in our
Z. Chen et al. / Journal of Environmental Management 90 (2009) 2831–28402832

Page 3

previous reports (Chen et al., 2006b, 2007). The detailed specifi-
cations of the three coagulants used in this work are listed in
Table 1. In order to avoid changing Al species arose from dilution, all
coagulants were used directly.
Synthetic water containing 5.0 mM NaHCO3and 5.0 mM NaNO3
was prepared by diluting a calculated amount of 0.5 M NaHCO3and
0.5 M NaNO3 aqueous solution with deionised water. Then the
turbidityof synthetic waterwasadjustedto about 2 NTUwith avery
smallamountofkaolinstocksuspensionpreparedaccordingtoChen
et al. (2006a). The kaolin particles in synthetic water were used as
nucleating agent, and were helpful to eliminate the uncertainty and
randomness during coagulants’ nucleation process, which thereby
enhanced the reproducibility of experimental results. The pH of the
synthetic water was adjusted to predetermined values with 0.1 M
HCl solution or 0.1 M NaOH solution before used.
2.2. Measurement of size development of coagulant hydrolysis
precipitates
The particle size development of hydrolysis precipitates after
coagulants were added into the synthetic water was measured on
a modified Laser Particle Size Analyzer (LPSA) (Mastersizer 2000,
Malvern). The inlet and outlet of the LPSA were connected to a jar
test flocculator with a single-paddle stirrer, and a peristaltic pump
was connected in the outputs tubes. The suspension in the beaker
of the flocculator was sucked into the LPSA by the peristaltic pump
continuously for online measuring particle size during coagulants’
hydrolysis process. The measurement procedures were as follows:
1000 mL synthetic water was added into the beaker. The stirrer was
adjusted to 200 rpm. The peristaltic pump was started up, and the
rotational speed and clamp of the peristaltic pump should be
adjusted carefully in case the hydrolysis precipitate was crushed by
peristaltic pump. After the instrument background data were
measured, a measured amount of coagulant was added into the
synthetic water to reach the target Al concentration. The solution
was stirred rapidly at 200 rpm for 2 min after coagulant dosed, and
followed by slow-stirring at 30 rpm for 15 min. The mean diameter
of the particles of hydrolysis precipitates was measured once every
20 s after coagulant addition. The pH value of the synthetic water
was depressed by the acidity and hydrolysis of coagulants. In order
to ensure the coagulant hydrolyzed under fixed pH condition, 0.1 M
NaOH solution was added to compensate the pH decrease using an
automatic titrator (716 DMS Titrino, Metrohm Co.). All experiments
were repeated at least three times in this work.
2.3. Measurement of Zeta potential of coagulant hydrolysis
precipitates
The experiments were carried out on a jar test flocculator with
a single-paddle stirrer. 1000 mL synthetic water sample was
added into the beaker, and a measured amount of coagulant was
pipetted into the synthetic water to give a certain Al concentration
under rapid stirring. The solution was stirred rapidly at 200 rpm
for 2 min after coagulant dosed, followed by slow-stirring at
30 rpmfor 1 min.Asamplewas taken usinga syringe
immediately after the 1 min slow-stirring for the measurement of
Zeta potential (Zetasizer 2000, Malvern). The pH decrease caused
by coagulants addition was compensated as the method described
in Section 2.2.
2.4. Measurement of Zeta potential of re-suspension of coagulant
hydrolysis precipitates
1000 mL synthetic water with pH¼7.5 was added into the
beaker of a jar test flocculator with a single-paddle stirrer. A
measured amount of coagulant was added into the water under
rapid stirring to give an Al concentration of 800 mM (calculated by
Al3þ). The solution was stirred rapidly at 200 rpm for 2 min after
coagulant dosed, followed by slow-stirring at 30 rpm for 15 min,
and then settled for 30 min. The fixed condition of pH¼7.5 was
controlled using the method described in Section 2.2. After 30 min
settlement, the supernatant liquid was decanted, and 200 mL
suspension of coagulant hydrolysis precipitates was collected and
divided into seven parts. Every part of the precipitates was diluted
with 200 mL deionised water. The pH of the dilution was adjusted
to the desired values with 0.1 M HCl solution or 0.1 M NaOH solu-
tion. Zeta potential of the solution was measured immediately on
Zetasizer 2000 after the pH adjustment.
2.5. Chemical speciation of coagulant hydrolysis precipitates
200 mL suspension of coagulant hydrolysis precipitates was
obtained according tothe jar testingprocedure described in Section
2.4 except the dosage of Al was 200 mM. Three methods were taken
to study the Al chemical speciation in the suspension using
Al–Ferron method: (1) The Al–Ferron analysis was carried out
without any pretreatment of the suspension; (2) The suspension
was adjusted to a pH of 4.0 with 0.1 M HCl solution, and then
immediately sampled for Al–Ferron analysis; (3) The suspension
was filtered with 0.45 mm microporous membrane, and the filtrate
was collected and sampled for Al–Ferron analysis.
The Ferron reagent was prepared referring to the methods
described by Parker and Bertsch (1992). The operation procedures
were as follows: 5 mL Al containing sample and 15 mL Ferron
reagent were added into a 50 mL colorimetric tube, and it was
diluted to the scale with deionised water. The solutions were mixed
vigorously and were pipetted into a 10 mm path-length quartz
cuvette. Then it was placed into a UV–vis spectrophotometer (DR/
4000U, HACH) to test its absorbance at a wavelength of 370 nm at
a frequencyof one reading per minute. All these operations finished
within 1 min. The pseudo first-order rate constant (k) of Al–Ferron
complex reaction within 1–30 min was calculated according to the
method described by Luan (1997).
2.6. Dilution experiment and measurement of chemical speciation
of hydrolysis precipitates
Generally, the dosage of polyaluminum coagulants is about
10?4mol/L orderof magnitude during flocculationprocess. In order
to investigate the Al species distribution after the coagulants were
dosed intowater, the coagulants werediluted to Al concentration of
2?10?4mol/L using model water under fixed pH condition, and
the Al species distribution in the diluent was measured by
Al–Ferron method at different dilution time. The pH decrease of the
model water caused by coagulants addition was compensated as
the method described in Section 2.2. The model water was
prepared by adjusting the alkalinity, electrolyte and pH of deion-
ised water to target values using 0.5 M NaHCO3, 0.5 M NaNO3, and
0.1 M HCl or 0.1 M NaOH solution, respectively.
Table 1
Al species distribution and pH values of coagulants used in this work.
SamplesAlT
(M)
BAl species distribution (%) pH
Coagulants Aging
time (d)
Ala
Alb
Alc
Alm
Al13
Al30
Alu
AlCl3
PACAl13
PACAl30
5
5
5
0.2
0.2
0.2
0
2.4
2.4
97.3
4.7
2.7
2.7
78.8
23.0
0.0
16.5
74.3
100.0
4.7
3.7
0.0
81.5
16.5
0.0
0.0
76.8
0.0
13.8
3.1
2.86
4.26
4.20
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3. Results and discussion
3.1. Effect of dosage on size development of coagulant hydrolysis
precipitates
Different dosages of AlCl3, PACAl13and PACAl30were added into
the synthetic water at fixed pH¼7.5. The particles’ size develop-
ments of hydrolysis precipitates measured by online LPSA are dis-
played in Fig. 1. As can be seen from Fig. 1, the particle diameter
(d50) increased rapidlyafter a certain inductionperiod (period from
the coagulant dosing to the beginning of rapid growth of precipi-
tates), and then leveled off or decreased slightly for all three
coagulants. The induction periods and the ultimate particle size
depended on the coagulant and dosage. The particles’ sizes were
very small during the induction period, and the tiny increase of
particle size was due to the coagulation of the small amount of
kaolin existed in synthetic water by charge neutralization.
For all three coagulants, the formation rate of hydrolysis
precipitates increased with the increase in dosage, and the induc-
tion periods reduced. When the dosage increased from 50 mM to
200 mM, the precipitates’ size increased, but when the dosage
increased to 800 mM, the precipitates’ size decreased instead for all
three coagulants, especially for AlCl3. The reason that the precipi-
tates’ size decreased at a dosage of 800 mM need further investi-
gated. This maybe resulted fromthe transitorylowpH shock caused
by high dosage of coagulants addition suddenly. The pH compen-
sation by automatic titration was hysteretic to the solution pH
decrease. The higher the coagulant added, the greater the hyster-
esis effect was. A large number of small precipitates formed initially
at low pH conditions possessed high charges and were difficult to
aggregate as dense particles when the solution pH restored 7.5.
Comparing the size development trajectory of hydrolysis
precipitates of the three coagulants, the precipitates’ size of AlCl3
was obviously larger than that of PACAl13and PACAl30. There was no
obvious difference on hydrolysis, aggregation and growth rate
between PACAl13and PACAl30under high dosage conditions (100–
800 mM). Al3þin AlCl3underwent rapid hydrolysis, aggregation and
precipitation, and ultimately converted to amorphous Al(OH)3
sediment, so the induction period was short, and the repulsion
among these precipitates was small due to its lowcharge. Although
the low charge and small repulsion favored the rapid aggregation
and growth of precipitates, the hydrolysis precipitates of AlCl3were
fragile because the combination between these particles was
physical aggregation. The broad and non-uniform particle size
distribution could be seen clearly during AlCl3 hydrolysis and
precipitation experiments. However, Al13 and Al30 species in
PACAl13 and PACAl30, respectively, were more stable and more
tolerant to alkaline hydrolysis and aggregation than Al3þafter
dosing into water. Al13and Al30species aggregated and grew up
slowly via by deprotonation of external water molecules of these
species. Because the hydrolysis precipitates derived from PACAl13
and PACAl30were mainly composed of repeated Keggin Al13and
Al30structural units (Bradley et al., 1993), their hydrolysis was not
complete. The hydrolysis products possessed high positive charge,
and the repulsive force among these precipitates was very large.
And correspondingly, the hydrolysis and aggregation of PACAl13and
PACAl30 became slow, so the size of hydrolysis precipitates of
PACAl13and PACAl30was relatively small as compared to that of
AlCl3.
3.2. Effect of pH on size development of hydrolysis precipitates
The pH of raw water has an important effect on the coagulation
performance of coagulants. Fig. 2 displays the particles’ size
development of hydrolysis precipitates when the three coagulants
were added into synthetic water with different pH values. As can be
seen from Fig. 2, the optimal pH value for AlCl3hydrolysis was
about 6.5–7.5, the particle size reached 350 mm at this pH range. But
the particles’ size decreased below 250 mm when the pH raised to
0
50 μM
100 μM
200 μM
800 μM
a
0200400 600800 1000
50
100
150
200
250
300
350
400
Particle size d50 (µm)
Time after coagulants dosing (s)
50 μM
100 μM
200 μM
800 μM
b
0
50
100
150
200
250
300
350
400
Particle size d50 (µm)
0200400600 8001000
Time after coagulants dosing (s)
c
50 μΜ
100 μM
200 μM
800 μM
0
50
100
150
200
250
300
350
400
Particle size d50 (µm)
02004006008001000
Time after coagulants dosing (s)
Fig. 1. Effect of dosage on precipitates’ size development when coagulants hydrolyzed
insyntheticwaterat fixedpH ¼7.5.
ðCNaHCO3¼ 5 mM; CNaNO3¼ 5 mMÞ.
(a)AlCl3;(b)PACAl13;(c)PACAl30.
Z. Chen et al. / Journal of Environmental Management 90 (2009) 2831–28402834

Page 5

8.5, and when the pH raised further and reached 9.5, the soluble
Al(OH)4
tion. The optimal pH for PACAl13hydrolysis and precipitation was
8.5–9.5. PACAl13did not produce precipitates in synthetic water
with pH¼6.5, which indicated that Al13 species in PACAl13
remained relative stable, and it hydrolyzed to form dissoluble Al13
?existed in solution, and there was no precipitates forma-
aggregates under this pH condition. PACAl30produced large size
particles when it was added into synthetic water with pH¼7.5–9.5.
It precipitated faster than PACAl13under pH¼7.5. After dosing into
synthetic water with pH¼6.5, PACAl30could build a certain size of
precipitates though the induction period was long. The above
results indicated that PACAl30is easier to hydrolyze and precipitate
than PACAl13in weak acidic and alkaline water.
3.3. Charge characteristics of coagulant hydrolysis precipitates
The charge characteristics of coagulant hydrolysis precipitates
are important to the coagulation performance of coagulants,
especially for the coagulation mechanisms of precipitation–charge
neutralization or absorption–charge neutralization. The effect of
the pH of synthetic water on the particles’ Zeta potential of coag-
ulant hydrolysis precipitates is shown in Fig. 3. Two experimental
methods had been adopted to investigate the variation of particles’
Zeta potential with the pH. In the first method, the hydrolysis
precipitates were prepared in synthetic water under fixed pH¼7.5,
then it was separated, diluted and the pH was adjusted to different
values for Zeta potential measurement (Fig. 3a). In the second
method, the Zeta potentials of hydrolysis precipitates were
Particle size d50 (µm)
0200400600800 1000
0
50
100
150
200
250
300
350
400
a
pH=6.5
pH=7.5
pH=8.5
pH=9.5
Time after coagulants dosing (s)
Particle size d50 (µm)
Time after coagulants dosing (s)
0200400600 800 1000
0
50
100
150
200
250
300
350
400
pH=6.5
pH=7.5
pH=8.5
pH=9.5
b
Particle size d50 (µm)
Time after coagulants dosing (s)
0
200
400600 8001000
0
50
100
150
200
250
300
350
400
c
pH=6.5
pH=7.5
pH=8.5
pH=9.5
Fig. 2. Effect of pH on precipitates’ size development when coagulants hydrolyzed in
synthetic water at fixed pH condition. (a) AlCl3; (b) PACAl13; (c) PACAl30. (Dosage
200 mM as Al, CNaHCO3¼ 5 mM; CNaNO3¼ 5 mM).
45678910 11
-30
-20
-10
0
10
20
30
40
50
Zeta potential (mV)
pH
AlCl3
PACAl13
PACAl30
a
Zeta potential (mV)
4567891011
-60
-40
-20
0
20
40
60
AlCl3
PACAl13
PACAl30
b
pH
Fig. 3. Effect of pH onparticulate Zeta potential of coagulants’ hydrolysis precipitates. (a)
Precipitates were prepared at fixed pH¼7.5, then the pH was adjusted to target values
using 0.1 mol/L HCl or NaOH aqueous solution before Zeta potential measurement; (b)
PrecipitateswereprepareddirectlyinsyntheticwaterwithdifferentpHvaluesatfixedpH
condition, and the coagulants’ dosage is 200 mM. ðCNaHCO3¼ 5 mM; CNaNO3¼ 5 mMÞ.
Z. Chen et al. / Journal of Environmental Management 90 (2009) 2831–28402835