Archives of

1. Introduction
Minor amounts of heavy metals such as nickel, manganese, lead, chromium, cadmium, zinc, copper, iron, and mercury are important components of most wastewater [1]. The presence of high amounts of each of these metals interferes with the beneficial uses of water in terms of toxicity [2]. Industrial effluents typically contain high concentrations of contaminants such as organics, heavy metals, and toxic compounds [2]. Unlike organic substances, heavy metals are non-biodegradable, and most of them are known to be toxic and carcinogenic metals, causing numerous environmental and health problems [3,4]. Therefore, wastewater contaminated with heavy metals must be treated prior to discharge [5].
Nickel is a hard silvery-white metallic element that is ductile. Its chemical symbol is Ni, its atomic weight is 58.96, its density is 8.9, and its valence is 0, 1, 2, and 3 [6]. Nickel is one of the toxic heavy metals that can cause headache, nausea, dry cough, chest pain, shortness of breath, and cyanosis in high concentrations [7]. It usually enters drinking water through pipes and fittings. In addition, nickel can penetrate into underground water through dissolution and rock erosion. It is used as an alloy in combination with other metals and non-metals. One of the properties of nickel alloys is strength and resistance to corrosion and heat [8]. Although inhalation of nickel metal compounds is carcinogenic, there is little evidence of carcinogenicity through ingestion. Perinatal mortality (senile) seems to be a major risk in drinking water containing 20 μg/L Ni as determined by the World Health Organization [9,10]. The adverse health effects of nickel are related to the heart and liver, and the European community has set a maximum value of 0.05 mg/L [6]. Although there is no information about the effects of nickel deficiency, nickel is essential for humans [8,11]. Divalent nickel compounds at the concentrations found around us are minimally toxic to humans. Adverse side effects caused by nickel exposure have been reported in cases of skin contact (causing contact dermatitis) and inhalation exposure (causing ductal irritation and asthma) in people exposed to this contaminant [12,13].
Various physicochemical processes such as chemical precipitation, flotation, membrane methods, ion exchange, electrochemical methods, and adsorption
The Efficiency of Polyaluminum Silicate Chloride Coagulants in Nickel Removal from Aqueous Solutions
Mohammad Javad Mohammadi1,2ID, Afshin Takdastan1,2*ID, Mehdi Zhoolanezhad3ID, Abdolkazem Neisi1,2ID
1Environmental Technologies Research Center, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran
2Department of Environmental Health Engineering, Faculty of Health, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran
3MSc Student in Environmental Health Engineering, Faculty of Health, Ahvaz Jundishapur University of Medical Sciences, Ahvaz, Iran
*Corresponding Author: Afshin Takdastan, Email: afshin_ir@yahoo.com
Received: January 21, 2022, Accepted: February 14, 2022, ePublished: December 29, 2022
https://jhygiene.muq.ac.ir/
10.34172/AHS.11.4.380.1
Vol. 11, No. 4, 2022, 264-271
Original Article
Arch Hyg Sci. Volume 11, Number 4, 2022 265
The Efficiency of Polyaluminum Chloride in Nickel Removal
have been used to remove heavy metals from wastewater
[8]. Heavy metal removal methods have limitations
such as high cost, incomplete removal of metals, the
need for reactive materials, large amounts of energy,
and generation of toxic substances or secondary sludge
that requires special treatment [9,10]. Coagulation and
flocculation are one of the main processes in water and
wastewater treatment. Coagulation works based on two
mechanisms: charge neutralization of colloidal particles
by hydrolyzed cationic coagulants and removal of
impurities by irregular hydroxide compounds [14,15].
Polyaluminum silicate chloride (PASiC) belongs to
a new generation of inorganic polymer coagulants.
Polyaluminum chloride and polysilicate (PSi) are
combined to form PASiC, which polymerizes trivalent
aluminum under special conditions to improve the
coagulation effect [16]. Considering the health and
environmental hazards of nickel and the strengths and
weaknesses of different heavy metal removal methods,
as well as reviewing study records, it seems necessary to
identify and use coagulant materials that can function
over a wider pH range without the need for the addition
of coagulant aid. Given the high efficiency of PASiC,
this study investigated the efficiency of this coagulant in
removing nickel from aqueous solutions. It also strived to
determine the effect of pH, amount of coagulant, initial
metal concentration, and settling time on the removal
efficiency by coagulation and flocculation methods
through PASiC coagulant and to determine the volume
of produced sludge.
2. Materials and Methods
This is an experimental study conducted on a laboratory
scale. PASiC coagulant concentration, pH, divalent nickel
metal concentration, and settling time were considered
in this study. Double distilled water was used for the
preparation of stock solutions and dilution. Chemicals
included nickel nitrate hexahydrate (Ni(NO3)2.6H2O),
water glass (SiO2), sodium carbonate (Na2CO3), and
hydrochloric acid 37%. Moreover, sodium hydroxide from
Merck and aluminum chloride (AlCl3) from Activechem
with laboratory grade were prepared. To make the nickel
stock solution, 4.95 grams of (Ni(NO3)2.6H2O) was reached
to 1 liter with twice-distilled water. This solution contained
1000 mg/L of divalent nickel ions. The current study used
pH meter devices (EUTECH - pH 1500), a digital scale with
an accuracy of 0.0001 (Sartorius TE214S), a 6-beaker test
jar (Phipps and Bird), a flame atomic absorption device
(AAS Vario 6), and a magnetic stirring device.
2.1. Synthesis of PASiC coagulant
PASiC was made according to the method proposed by
Gao et al using the co-polymerization technique at room
temperature with a molar ratio of Al/Si = 5 and OH/Al = 2
as follows:
First, 10.75 mL of water glass with a concentration of
3 M silicon dioxide was added slowly (1 mL/min) to 10
mL of 2 M hydrochloric acid under mixing conditions,
and the solution was called PSi. It has the properties of
silicon dioxide 1.5 M and pH = 2-2.5. In the second step,
2.5 M AlCl3 was mixed with 10 mL of fresh PSi solution
(about 2 hours old) to obtain a molar ratio of aluminum
to silicate equal to 5. Then, 1.5 M sodium carbonate
Na2CO3 solution was added slowly (0.1 mL/min) under
mixing conditions (700-800 rpm) to obtain a molar ratio
of hydroxide to aluminum equal to 2 and prepare PASiC
coagulant, which is a milky liquid [17]. Figure 1 shows
how to make and prepare PASiC.
2.2. Coagulation and flocculation tests
Experiments related to the process of coagulation and
flocculation were carried out by the Phipps and Bird
jar machine at a rapid mixing time of 1 minute with
200 revolutions per minute and a slow mixing period of
15 minutes with 45 revolutions per minute. Then, the
solution was allowed to settle for 30 minutes to remove
divalent nickel [18]. In the first phase of the first stage,
a solution containing nickel with a concentration of 10
mg/L was prepared from the stock solution to determine
the effect of pH on the efficiency of nickel removal, and
the pH of the solution was adjusted using hydrochloric
acid. Then, one-tenth normal sodium hydroxide [4-11]
was set at various values, and PASiC coagulant was
added to nickel-containing beakers at the rate of 75 mg/L
(in terms of aluminum). In the second phase, 15 mg/L
coagulant was added to the solution containing 100 mg/L
nickel, and the jar test was performed according to the
first phase. After the jar test, a sample was taken from a
height of 2 cm below the water surface of each beaker,
and the samples were filtered by a membrane. After
preparing the samples, the residual nickel was read using
a flame atomic absorption device. In the second stage, the
optimum amount of PASiC coagulant was determined
using the one-factor-at-a-time method. The solution
containing nickel with a concentration of 100 mg/L was
prepared from the stock solution, and the pH was adjusted
to the optimum values obtained from the previous stage
and neutral pH. Then, different concentrations of the
coagulant in the values of 0, 7.5, 15, 37.5, 56, and 75 mg/L
were added in terms of aluminum, respectively. After
performing the jar test according to the previous stage,
the remaining nickel was measured, and the best amount
of coagulant was determined.
In the third stage, the effect of the initial concentration
of nickel in 10, 25, 50, 100, 200, and 400 mg/L on the
removal efficiency was evaluated under optimal pH
and neutral pH conditions, and the optimal coagulant
concentration obtained from the previous steps was
tested and measured. In the fourth stage, to investigate
the effect of settling time on the removal efficiency,
Mohammadi et al
266 Arch Hyg Sci. Volume 11, Number 4, 2022
beakers containing the optimal concentration of PASiC
and nickel obtained from the previous stages were
tested at the pH rate of 7, and sampling was performed
at settling times 15, 30, 45, 60, and 90 minutes. Finally,
the amount of produced sludge with different amounts of
PASiC coagulant was investigated.
A flame atomic absorption device (AAS Vario
6) at a wavelength of 232 nm was used to measure
the concentration of divalent nickel ions. Nickel
concentration was measured after the coagulation and
flocculation process according to the acetylene-air direct
blowing method (B)-3111) of the standard method
of water and wastewater tests [19,20]. Further, 0.1 M
solution of hydrochloric acid and sodium hydroxide
was used to adjust the pH of the desired samples. The
amount of metal removal was calculated according to the
following formula:
(%) i f 100
i
R c
C
emoval eff c
C
C
i ien y
−
= ∗ (1)
where Ci and Cf are the initial and final concentration
of the metal, respectively. All tests were conducted at
room temperature, each stage of the test was measured
twice, and finally the average results were reported.
2.3. Statistical data analysis
The graphs were drawn using Microsoft Excel 2007. In
this study, SPSS 16 was used to perform statistical tests,
including between-groups ANOVA, regression, and t
test.
3. Results
3.1. Determining the effect of pH
The effect of pH on the removal efficiency of nickel
from synthetic samples is shown in Figures 2a and 2b. In
this stage, 75 mg/L of PASiC was added to the solution
containing 10 mg/L of nickel as a coagulant according to
Figures 2a, and the effect of pH in the range of 4-11 was
investigated based on the removal efficiency. The settling
time at this stage was 30 minutes. Results indicated that
the nickel removal efficiency increased from 4 to 7 pH
and increased from 5.06% to 58.63%. Moreover, in the
pH range between 8 and 11, the nickel removal efficiency
was always a constant value of 99%.
Then, according to Figure 2b, 15 mg/L of PASiC was
added to the solution containing 100 mg/L of nickel, and
the effect of pH in the range of 4-11 was investigated based
on the removal efficiency. As can be seen in Figure 2b,
the nickel removal efficiency increased with the change
in pH from 4 to 7 and increased from 20.13% to 52.54%,
then in the pH between 8 and 11, the nickel removal
efficiency was always a constant value of 99%. As a result,
pH equal to 8 with 99% removal efficiency was chosen as
the optimal pH for nickel removal.
3.2. Determining the effect of PASiC coagulant dose
Figures 3a and 3b present the effect of PASiC coagulant
dose on nickel removal efficiency from synthetic samples
at neutral pH and optimal pH. In this step, according to
Figure 3a, first, under neutral pH conditions, different
coagulant concentrations of 0, 7.5, 15, 37.5, 56, and 75
mg/L were added to the solution containing nickel with
a concentration of 100 mg/L in terms of aluminum.
The highest and lowest observed removal efficiencies
were 52.54% and 21.85% in doses of 15 and 75 mg/L,
respectively. Further, the results of the statistical test
comparing the mean of the single variable t test showed a
significant difference between the concentration of nickel
ions contacted and not contacted with the coagulant in
the process of coagulation and flocculation (P < 0.05).
Figure 1. Production of PASiC With a Molar Ratio of 5 = Al/Si, 2 = OH/Al. Note. PASiC: Polyaluminum silicate chloride; AL: Aluminum; Si: Silicate; OH: Hydroxide
Arch Hyg Sci. Volume 11, Number 4, 2022 267
The Efficiency of Polyaluminum Chloride in Nickel Removal
According to Figure 3b, in this stage, under the optimal
pH conditions obtained from the previous step (pH = 8),
different coagulant concentrations of 0, 7.5, 15, 37.5, 56,
and 75 mg/L were added to the solution containing nickel
with a concentration of 100 mg/L in terms of aluminum.
The results revealed that the highest and lowest observed
removal efficiencies of 99% and 66.12% were obtained
at doses of 15 and 7.5 mg/L, respectively. Moreover, the
results of the statistical test comparing the mean of the
single variable t test demonstrated a significant difference
between the concentration of nickel ions contacted
and not contacted with the coagulant in the process of
coagulation and flocculation (P < 0.05). Based on this,
the coagulant dose of 15 mg/L was chosen as the optimal
amount of nickel removal.
3.3. Determining the effect of initial metal concentration
At this stage, the effect of the initial concentration of
nickel was studied in values of 10, 25, 50, 100, 200, and
400 mg/L at neutral pH and the dose of PASiC equal to
15 mg/L. The highest and lowest observed nickel removal
efficiencies were obtained at 71.22% and 34.08% in
initial concentrations of 10 and 400 mg/L, respectively.
Figure 4a illustrates the trend of changes in nickel removal
efficiency parallel to the increase of initial concentration
at a pH rate of 7.
Then, the effect of the initial concentration of nickel
of 10, 25, 50, 100, 200, and 400 mg/L was studied at the
optimal pH of 8 and the dose of PASiC equal to 15 mg/L.
The results indicated that the highest removal efficiency
is obtained in the concentration range of 10-100 mg/L,
which is equal to 99%, and the removal efficiency was
53.48% and 30.84% in metal concentrations of 200 and
400 mg/L, respectively. Figure 4b depicts the trend of
nickel removal efficiency changes in parallel to increasing
initial concentration at an optimal pH of 8.
3.4. determining the effect of settling time
The effect of settling time on the amount of nickel
removal using PASiC is presented in Figure 5. At this
Figure 2. Effect of pH on nickel removal efficiency (a: 10 mg/L Ni = 75 mg/L PASiC, settling time 30 minutes and b: 100 mg/L Ni = 15 mg/L PASiC, settling time
30 minutes). Note. PASiC: Polyaluminum silicate chloride; Ni: Nickel
Figure 3. Effect of PASiC concentration on nickel removal efficiency (a: mg/L = 100 Ni, pH = 7, settling time 30 minutes and b: mg/L = 100 Ni, pH = 8, settling
time 30 minutes). Note. PASiC: Polyaluminum silicate chloride; Ni: Nickel
Mohammadi et al
268 Arch Hyg Sci. Volume 11, Number 4, 2022
stage, the coagulant concentration was 15 mg/L, the
nickel concentration was 100 mg/L, and the pH was 7.
The highest and lowest nickel removal efficiencies were
obtained at 54.98% and 43.26% in 15 and 90 minutes,
respectively. According to the obtained results, by
increasing the settling time up to 30 minutes, the removal
efficiency increased dramatically, the nickel removal
efficiency changed from 30 to 90 minutes, the slope of the
graph was almost constant, and no significant change was
observed in the removal rate. Accordingly, 30 minutes
was considered as the optimal settling time for nickel
removal.
3.5. Determining the volume of produced sludge
In this phase, the volume of sludge formed at the optimum
pH of 8 and 100 mg/L of nickel was studied with PASiC
doses of 15, 37.5, 56, and 75 mg/L, respectively. As can be
observed in Figure 6, the highest and lowest amounts of
produced sludge were 80 and 52 mL/L at coagulant doses
Figure 4. The effect of initial metal concentration on nickel removal efficiency (a: mg/L = 15PASiC, pH = 7, settling time 30 minutes and b: mg/L = 15PASiC,
pH = 8, settling time 30 minutes). Note. PASiC: Polyaluminum silicate chloride
Figure 5. The effect of the settling time on nickel removal efficiency
(100 Ni = mg/L, PASiC = 15 mg/L, pH = 7). Note. Ni: Nickel; PASiC:
Polyaluminum silicate chloride
of 75 and 15 mg/L, respectively.
4. Discussion
4.1. Determining the effect of pH
In order to determine the optimal pH of the coagulation
and flocculation processes in the removal of divalent
nickel from aqueous solutions by PASiC coagulant, this
stage was carried out in two phases. In the first phase, 10
mg/L nickel was added to the aqueous solutions, and after
adjusting the pH in the range of 4-11, 75 mg/L PASiC
was added to each beaker. Next, after conducting the jar
test, the residual nickel was measured. As can be seen
in Figure 2a, with increasing pH, the amount of nickel
removal increased, and at pH equal to and above 8, the
nickel removal efficiency was 99%. In the second phase
of determining the optimum pH, 100 mg/L of nickel and
15 mg/L of PASiC were added to beakers. According to
Figure 2b, the nickel removal efficiency increased with
increasing pH, and at pH equal to and above 8, the nickel
removal efficiency was 99%. Therefore, the optimal pH for
nickel removal by PASiC coagulant was obtained to be 8.
Regarding the relationship between pH and hydrolyzed
groups of aluminum, Yang et al reported that species
of aluminum hydrolysis at pH less than 5 include
monomeric, dimeric, and Al3 aluminum. At pH between
5 and 6, medium and large aluminum polymer species
with Al13 cores were dominant, and the flocs formed in this
case were relatively small. Moreover, the dominant form
of aluminum hydrolysis at pH between 6 and 7 consisted
of amorphous Al(OH)3 clots, and in this case, colloids
and natural organic substances were easily removed by
the mechanism of absorption and co-precipitation by
cationic species, aluminum hydroxide with low solubility,
and the top level. Further, at pHs higher than 7, Al(OH)4-
hydrolyzed species were dominant, and the stability of the
suspension increased [18]. Bakar and Halim found that
the removal rate of nickel from industrial wastewater by
polyaluminum chloride and coagulant aid at neutral pH
Arch Hyg Sci. Volume 11, Number 4, 2022 269
The Efficiency of Polyaluminum Chloride in Nickel Removal
was 63%. In the present study, the nickel removal rate at
neutral pH was less than 60%, indicating that high pH is
required to achieve high efficiency in nickel removal [21].
The results of Hu et al demonstrated that the removal
efficiency of cadmium increased with increasing pH, and
the removal rate at a pH of 8.5 was 93% [22]. Additionally,
in the study by Liu et al, the highest removal efficiency
of nickel from industrial wastewater was obtained by
polyaluminum chloride coagulant at a pH of 10 [23].
The results of Jaafarzadeh et al showed that the removal
efficiency increased with the increase of pH to 9 due to
the reduction of the positive charge of chitosan coagulant
and the cationic properties of nickel [24].
4.2. Determining the effect of PASiC coagulant dose
In the second stage of the study, the effect of different
dosages of coagulant on nickel removal efficiency was
investigated under neutral pH conditions and optimal
pH obtained from the first stage. As seen in Figures 3a
and 3b, the removal efficiency increased with the increase
of coagulant dose. According to Figure 3a, the highest
nickel removal efficiency at neutral pH was obtained
from PASiC at a concentration of 15 mg/L, which is
equivalent to 52.54%. According to Figure 3b, the highest
nickel removal efficiency was obtained at the optimal
pH of 8 for PASiC at a concentration of 15 mg/L, and
the removal efficiency was above 99%. The results of
Gyawali and Rajbhandari showed that increasing the
dosage of PASiC coagulant increased the amount of
turbidity removal. It was also observed that by decreasing
the ratio of aluminum to silicate, the molecular weight of
the coagulant increased, and by increasing the amount
of silicate in the structure of the coagulant, the growth
and development of flocs were faster due to the bridging
mechanism. Further, by increasing the ratio of hydroxide
to aluminum, the removal efficiency increased [25].
Therefore, given that in this study, a coagulant with a high
ratio of silicate to aluminum was used, nickel removal was
complete at alkaline pH. In a study, Jaafarzadeh et al found
that the removal efficiency of nickel increased from 40%
to 88% by changing the amount of chitosan from 10 to 100
mg/L [24]. In another study, Akbal and Camcı reported
that by changing the amount of alum and ferric chloride
coagulants from 100 to 2000 mg/L, the nickel removal
efficiency increased from 24.5% to 99% [26].
4.3. Determining the effect of initial metal concentration
A test was done to determine the effect of the initial
concentration on the removal efficiency at the optimal
dosage of coagulant of 15 mg/L, optimal pH of 8, and
neutral pH. As depicted in Figure 4a, under neutral
pH conditions, the removal efficiency decreased with
the increase in nickel concentration. The results of
Jaafarzadeh et al showed that the efficiency of nickel
removal by chitosan coagulant increased with the
increase of the initial concentration of nickel, which
was attributed to the insufficient amount of coagulant
to absorb the pollutant surface on the coagulant [24].
Further, the results of a study by Xu et al showed that
the removal efficiency decreased with the increase of the
initial concentration of cadmium due to the insufficient
absorption sites [27]. Moreover, according to Figure 4b
and at a pH of 8, it was observed that the amount of nickel
removal decreased to 200 mg/L with the increase in the
amount of nickel.
4.4. Determining the effect of settling time
This test was performed in the optimal concentration of
PASiC of 15 mg/L, nickel concentration of 100 mg/L, and
pH of 7. According to Figure 5, the removal efficiency at
settling times of 15, 30, 45, 60, and 90 minutes was 43.72%,
52.54%, 53.6%, 54.5%, and 54.98%, respectively. In Liu and
colleagues’ study entitled “Removal of phosphorus and
nickel from automobile industry wastewater by coagulation
and flocculation and combination with manganite, it was
found that the amount of removal increases by increasing
the settling time to 30 minutes [23].
5. Conclusion
In general, the results obtained from this study
indicated that the presence of active PSi compounds
in the coagulant polymer structure increased the
negative charge of PASiC in comparison with common
coagulants such as polyaluminum chloride, and this
negative charge facilitated the nickel removal, which had
cationic properties. The results also showed that in the
alkaline pH range, the removal efficiency increased due
to the production of Al(OH)4- hydrolytic species and the
decrease of nickel ion solubility. The results suggested
that PASiC at pH of 8, PASiC concentration of 15 mg/L
in terms of aluminum, settling time of 30 minutes, and
a concentration range between 10-100 mg/L related to
nickel had removal efficiency of above 99%. Therefore,
Figure 6. The Amount of Produced Sludge (mg/L: Ni = 100, pH = 8). Note.
Ni: Nickel
Mohammadi et al
270 Arch Hyg Sci. Volume 11, Number 4, 2022
the coagulation and flocculation processes using
cadmium-nickel were effective, and PASiC could be used
as an effective chemical method to remove nickel.
It is also recommended to study the use of PASiC
coagulant in industrial wastewater treatment and to
check its effectiveness in removing parameters such as
turbidity, suspended solids, biological oxygen demand,
chemical oxygen demand, oil, heavy metals, and the like.
Acknowledgments
We would like to express our gratitude to the Vice-chancellor of
Research and Technology Development and the Environmental
Technology Research Center of Ahvaz Jundishapur University
of Medical Sciences for their financial and spiritual support in
carrying out this research with the ethical code of ETRC-9408.
Author Contributions
M-JM, AT, MZ, and A-BN were principal investigators of the study
and drafted the manuscript. M-JM and AT were advisors of the
study. AT, MZ, and A-BN performed the statistical analysis. All
authors contributed to the design and data analysis and assisted in
the preparation of the final version of the manuscript. All authors
read and approved the final version of the manuscript.
Conflict of Interests
There is no conflict of interests according to the authors of this
study.
Funding
This research was funded by the Research and Technology
Development Vice-chancellor and Environmental Technology
Research Center of Jundishapur University of Medical Sciences,
Ahvaz.
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