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The role of sodium carbonate in PAM coagulation-
flocculation for oil acidized wastewater treatment
Jinyi Qin, Hainan Wang, Chuan Qin, Hailong Meng, Wengang Qu
and Hui Qian
ABSTRACT
The pH value of oil acidized wastewater is relatively low (pH ¼6.1), which seriously affects the
flocculation of polyacrylamide (PAM). NaOH was used to adjust the pH value but the maximum was
only 7.5. The regulation was limited due to the Ca
2þ
in aqueous phase up to 1,350 mgL
1
consumed
OH
. A novel formulation of Na
2
CO
3
þPAM was proposed to form CaCO
3
floc core to facilitate PAM
coagulation. When the concentration was above 400 mgL
1
, the PAM precipitation tended to be
maximum, followed by NaOH adjustment of pH to 8.0 that could enhance PAM flocculation
successively. The sewage sludge (SS) remained and residue oil reduced to 25 mgL
1
and 34 mgL
1
respectively. The analysis of the species and composition of fatty acids indicated that the
coagulation-flocculation selectively effected on the sedimentation of saturated fatty acids (SAT).
This provides a new idea for recovery of high value-added residual oil. The optimal additive of Na
2
CO
3
is expected as promising coagulant aid to improve the PAM coagulation-flocculation of oil acidized
wastewater.
Jinyi Qin (corresponding author)
Hainan Wang
School of Civil Engineering, Key Laboratory of
Water Supply & Sewage Engineering (Ministry of
Housing and Urban-Rural Development),
Chang’an University,
Xi’an 710054,
P.R. China
E-mail: jinyi.qin@chd.edu.cn
Chuan Qin
PetroChina Changqing Oilfield Company
Associated Gas Comprehensive Utilization
Project Department,
Xi’an 710018,
P.R. China
Hailong Meng
The Third Natural Gas Plant of PetroChina
Changqing Oilfield Constituent Company,
Erdos 017300,
P.R. China
Wengang Qu
Hui Qian
School of Environmental Science and Engineering,
Key Laboratory of Ministry of Education of the
Ecological Effect and Groundwater in Arid Areas,
Chang’an University,
Xi’an 710064,
P.R. China
Key words |Ca
2þ
,floc core, oil acidized wastewater, PAM
INTRODUCTION
The operation of acid fracturing that was used to boost yield
was common in oil and gas wells, so that oil extraction
wastewater possessed high acidity and strong corrosion. As
reported in literatures the wastewater contained large
amount of calcium chloride up to 3 ×10
3
∼2.8 ×10
4
mgL
1
(Wang et al. Q1 ). The Ca
2þ
in aqueous phase consuming
OH
results in the difficulty of pH adjustment and water
treatment. The conventional treatments so far are composed
of flocculation, neutralization, oxidation and AC sorption
(Wang et al. ). But the problem was still unsettled per-
fectly on account of wastewater characteristics of low pH.
In this study, a new approach to increase the PAM floccula-
tion during the Na
2
CO
3
added is investigated. The addition
of Na
2
CO
3
is not only as alkaline additive to improve the pH
of wastewater slightly, but also coagulates calcium ions to
form the insoluble calcium carbonate, the fine precipitates
absorb suspended solids effectively (Hassani et al. ).
A novel formulation of coagulation –flocculation pro-
cess was anticipated in wastewater treatment (Hassani
et al. ). Under the high pH conditions, the negative sur-
faces of calcium carbonate was modified by cationic PAM
significantly and increased the adsorption of humic acid
(Bob & Walker ). However, it has been long time to
argue with the role of calcium carbonate on coagulation –
flocculation. Apparently, the CaCO
3
can easily form a floc
core. The precipitates acted by the sweep coagulation mech-
anism affecting the sewage sludge (SS) removal, separating
from water to introduce precipitation of heavy metals, phe-
nolic compounds and long chain fatty acids efficiently
(Lee et al. ;Greenberg et al. ). The electrical
1© IWA Publishing 2018 Water Science & Technology |in press |2018
doi: 10.2166/wst.2018.224
Uncorrected Proof
double layer (EDL) between colloid particles was com-
pressed by dissolved calcium ion, which made colloid size
bigger and easier to form flocs (Sudoh et al. ). When
the low lime was used in drinking water treatment, the cal-
cium carbonate contributed to PAM coagulation and
settlement (Leentvaar & Rebhun ). Conversely, the
improper way of adding PAM will impede the precipitation
of calcium carbonate. The higher concentration induced the
more convex and concave presented on the surface of cal-
cium carbonate crystals (Peronno et al. ).
The lower pH value of oil acidized wastewater wea-
kened the effectiveness of PAM flocculation. To elucidate
whether adding Na
2
CO
3
assists the formation of CaCO
3
floc core and improves the PAM flocculation, the removal
of SS and residue oil in supernatant were examined. Com-
paring the effects of adding PAM and Na
2
CO
3
-PAM, it has
a new insight into the role of Na
2
CO
3
in coagulation-floccu-
lation. Moreover, the Na
2
CO
3
possibly selectively recycle
the high value-added residual oil from wastewater.
MATERIALS AND METHODS
Materials
In this investigation, wastewater was collected from
Changqing Oil Recovery Station located in the Inner
Mongolia, which was stored at 4 C prior to use. The
reagent of EDTA, Calcium red indicator, Methanol,
n-hexane, methyl tert-butyl ether and petroleum ether
were purchased from Sigma-Aldrich (MO, USA), respect-
ively. A novel formulation of Na
2
CO
3
coagulant (Merck,
Germany) PAM flocculants (Cationic C-100, SNF Co.,
China) was used for SS and oil removal. All the reagents
were of analytical reagent grade, and were dissolved in
water purified with both a deionizing-distilling apparatus
and a MilliQ apparatus (Millipore, USA). The solution
pH was measured with a pH meter (TOA DKK, Japan).
The concentration of calcium ion was determined by
EDTA titration.
Methods
Coagulation-flocculation experimental procedures
Experiments were carried out in a jar-test apparatus,
equipped with beakers of 500 mL volume. At the beginning
200 ml of oil acidized wastewater was taken, coupling with
pH adjustment, six strategies of NaOH-Na
2
CO
3
-PAM
added was adopted: In the absence of NaOH and PAM,
Na
2
CO
3
was added with the concentration of 0, 400, 800,
1,600 and 2,000 mgL
1
(i.e. 0, 4, 8, 15 and
19 mmolL
1
) respectively, for obtaining the CaCO
3
coagu-
lants; Since the oily wastewater with high polymer residue,
200 mgL
1
PAM was used as flocculants, the dosage selec-
tion was based on the practical application in oil field
(Zhao et al. ); Except for PAM, 0, 30, 60, 120 and
150 mgL
1
(i.e. 0, 0.8, 1.5, 3.0 and 3.8 mmolL
1
) of NaOH
blended with wastewater in the control; Instead of NaOH,
0, 400, 800, 1,600 and 2,000 mgL
1
of Na
2
CO
3
were
added respectively for each coagulation-flocculation exper-
iments; In the case of NaOH at 120 mgL
1
,the experiment
was carried out by addition of 0,400,800,1,600 and
2,000 mgL
1
Na
2
CO
3
separately; Last, when the Na
2
CO
3
was maintained at 800 mgL
1
,the experiment was con-
ducted by adding of NaOH at 0,30,60,120 and 150 mgL
1
respectively.
After rapidly mixing for 10 s at 150 rpm and slowly
mixing for 1 min at 30 rpm, the liquid was clarified for
10 mins (Amuda & Amoo ). 50 ml of supernatant was
taken for SS (in mgL
1
) gravimetrically determination,
40 ml of supernatant was taken for oil content UV analysis,
50 ml of supernatant was taken for fatty acids analysis,
100 μlofflocculate was picked out and diluted with
MilliQ water to observe the precipitate by 40x microscope
(BX61, Olympus, Japan). Unless otherwise stated, all exper-
iments were performed in triplicates and sampled after
10 mins.
The analysis of oil content
The 40 ml of supernatant from coagulation-flocculation
with 10 ml of petroleum ether were blended into 50 ml
of centrifuge tube. Sample extraction was conducted
with reciprocate shaker (incubator personal Lt, TAI
TEC, Japan) for 2 h, afterwards the mixture was centri-
fuged at 10,000 rpm for 10 mins to break emulsion. The
supernatant was withdrawn to measure the UV absor-
bance (UV 2450 PC, Shimadzu, Japan) at 235 nm (Mao
&Han).
The analysis of fatty acid
After saponification and methylation of 0.5 g precipitate,
1 ml of premixed solvent (n-hexane: methyl tert-butyl ether ¼
1: 1) was added for extraction, and the upper organic phase
was taken for the GC-MS analysis.
2J. Qin et al. |PAM coagulation-flocculation cooperated with sodium carbonate to clarify oil acidized wastewater Water Science & Technology |in press |2018
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RESULTS AND DISCUSSION
The formulations of NaOH and Na
2
CO
3
effects on the pH
value and Ca
2þ
removal
As shown in Figure 1(a), with the concentration of NaOH
increased, the pH value of wastewater rose to 7.9. A plateau
was observed at high equilibrium concentrations suggesting
the limited capacity of NaOH to adjust pH. Meanwhile, the
discrepancy of pH regulated by NaOH between 1 and
10 mins implied that a weak acid buffer system may con-
sume OH
i as time increased. The original calcium in
wastewater was up to 1,350 mgL
1
based on the titration
of EDTA in the solution. Addition of NaOH from
0.75 mmol to 3.75 mmol resulted in the consumption of
Ca
2þ
from 0.4 mmol to 2 mmol, which demonstrated the
consumption ratio of OH
:Ca
2þ
≈2:1. The Ca
2þ
in the oil
wastewater neutralized most of the OH
to form Ca(OH)
2. Although 30 mg L
1
NaOH theoretically provided
7.5 ×10
4
mol L
1
OH
, only 6 ×10
7
mol L
1
, 0.8 ‰
OH
was used to adjust the pH value within 1 min. Simi-
larly, there were 4 ×10
7
mol L 1 and 0.5 ‰OH
for
the regulation of pH within 10 mins. It is demonstrated
that Ca
2þ
and H
þ
compete to bind with OH
and the
greater reactivity of Ca
2þ
than the reaction of H
þ
.
Figure 1(b) presents the effect of sodium carbonate on
the pH value of oil acidized wastewater. The consumption
of Na
2
CO
3
and Ca
2þ
was 1:1. As the content of Na
2
CO
3
increased more than 15 molL
1
, the pH value of solution
reached to 7.4. Na
2
CO
3
was inherently a weak alkaline
reagent, the dissolved CO
3
2
in the wastewater was rapidly
captured by Ca
2þ
to form precipitated nuclei, and the rest
of Na
2
CO
3
reacted with H
þ
in liquid so as to present
partially capability of pH adjustment. As shown in Figure
1(a), 4 molL
1
Na
2
CO
3
cooperated with 0.75 mol L
1
NaOH significantly increased the aqueous pH to 8. Here
the ratio of CO
3
2
:Ca
2þ
¼1:1 and the CaCO
3
coagulation
was preferentially formed, the reaction of CO
3
2
to Ca
2þ
was stronger than that between OH
and Ca
2þ
. Conversely,
1‰OH
was used to regulate the pH value and the utiliz-
ation of NaOH was tripled.
The effect of precipitated CaCO
3
to the coagualtion and
flocculation
It can be seen from Figure 2(a), the PAM produced floc is a
loose group and light grey. The lower adsorption of PAM
attributed to the acidic pH of 6. Either the pollutant was
in a highly ionized state where the surface was close to
the point of zero charge or the expanded form of the PAM
Figure 1 |The effect of NaOH (a) & Na
2
CO
3
(b) on the pH value (solid line) and Ca
2þ
(dash line) depletion of oil acidized wastewater.
3J. Qin et al. |PAM coagulation-flocculation cooperated with sodium carbonate to clarify oil acidized wastewater Water Science & Technology |in press |2018
Uncorrected Proof
polymer coil which covered more surface area on adsorp-
tion (Besra et al. ). Additionally, in Figure 2(b) the pH
adjustment was done by NaOH, the number of clusters
increased, and the PAM floc became bigger and the settling
speed was accelerated. The colloidal particles from long dis-
tance were through the bridge to form loose floc with a large
internal water (Wu et al. ). But this looser structure was
susceptible to the exterior environment and easily broken up
(Jarvis et al. ).
Figure 2(c) shows the prominent coagulation of
Na
2
CO
3
in the wastewater. The CO
3
2
preferentially
bound with calcium ions and produces CaCO
3
floc core
(φ¼1–2μm). The coagulated nuclei adsorbed contami-
nants from the wastewater to form fine pellet
precipitates and shortened the settling time (Sudoh et al.
). At neutral pH of 7.4, CaCO
3
particles were neutral
or slightly positively charged. Thus, a high adsorption affi-
nity of the negatively charged pollutant was observed. In
Figure 2(d) PAM as bridge and CaCO
3
as coagulant aid
were applied to the wastewater. The flocs coiled around
the CaCO
3
were supposed to increase the volume and
weight of the settling sludge, showing a dark black core.
The CaCO
3
as the porous adsorbent (Sudoh et al. )
improved the cohesive force and made the faster liquid-
solid separation, consequently it was no longer vulnerable
under the action of hydrodynamic shear force (Gray &
Ritchie ). Comparing the results of Figure 2(d) to
Figure 2(a)–2(c),CaCO
3
as the coagulant aid and nuclei
was wrapped with coiled floc, which made the biggest
cluster, easier to be settled and hardly to breakup. As a
consequence, the supernatant was clearer and the hand-
ling result was more stable.
The effect of sole Na
2
CO
3
on coagulation-flocculation
As shown in Figure 3(a), with the increase of time, the
removal of SS and residue oil were improved. The maximal
clarifications were 37% and 84% respectively. Comparing
the processing effect between 40 and 10 mins, the removal
of SS was increased by 19% for 40 mins, while the differ-
ences of oil removal was negligible.
Figure 3(b) experiments showed that the high concen-
tration of Na
2
CO
3
increased the flocculation and raised
the corresponding pH value. As the concentration was
400 mgL
1
,Na
2
CO
3
adjusted the pH value to 6, and
the removal efficiency of SS was up to 22%. As the
Figure 2 |The coagulation-flocculation precipitates by the addition of 200 mgL
1
PAM (a), 120 mgL
1
NaOH þ200 mgL
1
PAM (b), 800 mgL
1
Na
2
CO
3
(c) and 800 mgL
1
Na
2
CO
3
þ
200 mgL
1
PAM (d) to the oil acidized wastewater.
4J. Qin et al. |PAM coagulation-flocculation cooperated with sodium carbonate to clarify oil acidized wastewater Water Science & Technology |in press |2018
Uncorrected Proof
concentration was 800 mgL
1
,theadjustmentofpH
value was up to 7, the removal of residue oil attained
the maximum of 68%. A plateau was observed at high
equilibrium concentrations, suggesting monolayer cover-
age on the calcium carbonate surface (Bob & Walker
). The isoelecteric point (IEP) for CaCO
3
particles
was around pH 8.1 (Thompson & Pownall ). When
the pH of solution equaled to 6 or 7, the calcium carbon-
ate particles were positively charged or uncharged, and
obtained highly adsorptive affinity to the negatively
charged pollutant. It can be seen, the increase of the pro-
cessing time and the amount of sodium carbonate can
promote the precipitation, so as to further purify the
water quality.
The effect of NaOH on the coagulation-flocculation of
Na
2
CO
3
-PAM
The result of Figure 4 demonstrates the influence of NaOH
on removal of SS and residue oil, by solely PAM and com-
bined Na
2
CO
3
-PAM respectively. In the absence of
NaOH, the SS residue after adsorption of PAM was
293 mg g
1
. In contrast, an equivalent of PAM and
800 mg L
1
of Na
2
CO
3
were used to remove SS, and SS resi-
due declined to 256 mg g
1
.
The adjustment of pH done by NaOH influenced on the
PAM flocculation and affected the PAM molecular chain
stretch (Besra et al. ). The higher pH gave rise to
higher efficiency flocculation. When the concentration of
Figure 3 |The effect of settling time (a) at 800 mgL
1
Na
2
CO
3
and varying Na
2
CO
3
concentration (b) on the removal of SS (square) and residue oil (circle).
5J. Qin et al. |PAM coagulation-flocculation cooperated with sodium carbonate to clarify oil acidized wastewater Water Science & Technology |in press |2018
Uncorrected Proof
NaOH approached to 60 mgL
1
, the PAM flocculation
attained the maximum. The SS residue was lowest at
217 mgL
1
and the removal ratio of oil was highest at
88%. However, a plateau was observed at high equilibrium
concentrations of NaOH, suggesting pH in a certain range
can promote PAM flocculation.
As can be seen, the removal of residue oil and SS by
PAM-CaCO
3
was better than the solely function of PAM.
Specifically, when the Na
2
CO
3
and NaOH were added at
400 mgL
1
and 120 mgL
1
respectively. The SS declined
to 10 mgL
1
and 97% of removal ratio, while the reduction
of residue oil was around 85%. This was consistent with the
values in the literature (Bob & Walker ), cationic poly-
acrylamide cooperated with calcium carbonate particles to
promote the adsorption, and so on to increase of the floccu-
lation effect. However, the removal was influenced both by
electrostatic interactions and chemical interactions between
contaminant particles and PAM-CaCO
3
. Petrovic et al.
pointed out that the ligand exchange played a role in adsorp-
tion of calcium (Petrovic
´et al. ). In this work, the
experimental environment was carried out at pH about
7.5, the net electrophoretic mobility of PAM-CaCO
3
par-
ticles was positive (Bob & Walker ), and facilitated the
attachment of SS and oil to PAM-CaCO
3
. However, it
cannot interpret an increasing removal of SS and oil corre-
sponding with the increase of NaOH. We assumed that
the higher pH had a more significant effect on the negative
charge of contaminant colloidal, which resulted in the
higher amount of adsorption by PAM-CaCO
3
.
The effect of Na
2
CO
3
on the PAM coagulation
As Na
2
CO
3
increased, the pH value rose to 7.4 and 7.7 in
the absence and presence of 120 mgL
1
NaOH respectively
(shown in Figure 1(a)). A steady stream of porous CaCO
3
was generated to attach more contaminants (Sudoh et al.
). Figure 5 demonstrated the performance of coagu-
lation-flocculation as a function of Na
2
CO
3
. Calcium ions
were homogeneously blended with SS and residue oil
beforehand in wastewater, it blocked the negatively col-
loidal particles and acted as bridges between functional
groups of the two adjacent molecules (Duan et al. ).
CaCO
3
acting as a coagulant aid by forming larger flocs
shortened the settling time for the removal of DOC
(Sudoh et al. ). The removal efficiency of SS and residue
oil by adding Na
2
CO
3
and PAM simultaneously was better
Figure 4 |Effect of PAM cooperated with 0 mgL
1
and 800 mgL
1
of Na
2
CO
3
on the removal of SS (histogram) and residue oil (linear) as a function of NaOH.
6J. Qin et al. |PAM coagulation-flocculation cooperated with sodium carbonate to clarify oil acidized wastewater Water Science & Technology |in press |2018
Uncorrected Proof
than successively (Figure S1), it once again proved that the
CaCO
3
as condensed nuclei coagulated with PAM flocs
facilitated the coagulation-flocculation.
In the presence of NaOH at 120 mgL
1
, the PAM floc-
culation enhanced as the increase of Na
2
CO
3
to
2,000 mgL
1
, the removal of residue oil approached to the
maximum of 90% and the residue SS declined to the mini-
mum of 25 mgL
1
. The dissolved Ca
2þ
compressing the
EDL between colloids at higher pH made SS and residue
oil cohesive and easily to sink (Iakovides et al. ). Conse-
quently, the effect of coagulation-flocculation was
cumulatively enhanced by adding NaOH to Na
2
CO
3
-PAM
system.
Effects of Na
2
CO
3
on selective deposition of fatty acids
for oil recovery
Only a few reports so far have focused on the species of fatty
acids in oil-containing wastewater. A complex mixture of
alkyl-substituted acyclic and cycloaliphatic carboxylic
acids in wastewater was named naphthenic acids (NAs).
NAs can be divided into Saturated Fatty Acid (SAT) and
Unsaturated Fatty Acid (UFA). Acyclic carboxylic acid as
the major part of naphthenic acid in Changqing oil
accounted for 48.08%,followed by cycloaliphatic carboxylic
acids of 34.11%, the low content of phenylalkanoic acid was
17.79% (Liu et al. ). SAT contained more energy than
UFA, and thus the ubiquitous NAs in oil wastewater
required to be removed and recovery efficiently. In raw oil
acidized wastewater (Figure 6) the fatty acid composition
were mainly composed of SAT and mono-UFA, accounting
for 95% and 5% respectively, but the content of polyunsatu-
rated fatty acids was small and negligible.
As shown in Figure 6, adding Na
2
CO
3
to the oil waste-
water, the UFA content decreased from 65% to 35% as the
increase of Na
2
CO
3
. Most of the UFA entered the super-
natant, indicating that CaCO
3
was selectively combined
with SAT in wastewater. The selective sedimentation of
SAT was conductive to the recovery of residue oil. Similarly,
a significant loss of C20:5 was detected in sludge when the
fatty acid was separated by flocculation. Borges et al. also
found that addition of PAM led to high levels of C14:0
and low content of C20:5, trapped by the flocculants
(Borges et al. ). The positive part of flocculent adhered
to the fatty acid and the negative formed bridges with
medium components, causing the UFA crawling to the
Figure 5 |Effect of PAM cooperated with 0 mg L
1
and 120 mgL
1
of NaOH on the removal of SS (histogram) and residue oil (linear) as a function of Na
2
CO
3
.
7J. Qin et al. |PAM coagulation-flocculation cooperated with sodium carbonate to clarify oil acidized wastewater Water Science & Technology |in press |2018
Uncorrected Proof
culture medium. Thus, the difference in the % of fatty acids
may be due to remaining of UFA in the supernatant, not the
entry into sedimentation during the flocculation (Martínez
et al. ).
Compared with the distribution of fatty acids in original
wastewater, the percentage of UFA in the supernatant gave
rise to 15% by solely PAM flocculation. It growing up to
54% in the presence of Na
2
CO
3
þPAM, indicated that
both of PAM and Na
2
CO
3
had a selectively sedimentation
on SAT. Na
2
CO
3
and PAM interacted with each other to
promote the coagulation-flocculation, both together had
stronger settling effect on the species of fatty acids. The
equivalent PAM along with different concentrations of
NaOH was used in Figure 6, the ratio of SAT/UFA basically
remained at 1.3. The ambiguous effect was possibly due to
the excessive calcium ion to disable the impact of NaOH
on pH value. However, at the addition of 800 mgL
1
Na
2
CO
3
, the ratio of SAT/UFA decreased gradually with
the increase of NaOH, the equilibrium state was at 0.5.
The residual concentration of SAT being 14 mgL
1
in
Figure 6 |Effect of PAM (grey histogram), Na
2
CO
3
þPAM (white histogram), NaOH þPAM (square) and 800 mgL
1
Na
2
CO
3
þNaOH þPAM (circle) on the removal of SAT ( grids) and UFA
(diagonal) in oil acidized wastewater (a), and the removal ratio related to the varies of SAT/UFA which derived from experimental data of Na
2
CO
3
þPAM, NaOH þPAM and
800 mgL
1
Na
2
CO
3
þNaOH þPAM (b).
8J. Qin et al. |PAM coagulation-flocculation cooperated with sodium carbonate to clarify oil acidized wastewater Water Science & Technology |in press |2018
Uncorrected Proof
supernatant confirmed that CaCO
3
precipitates promoted
the selective sedimentation of fatty acids species. In high
pH-induced flocculation–sedimentation researched on pro-
ductivity of bio-diseal, therein more UFA than SAT were
found, the increase of fatty acid unsaturation might be a
mechanism of adaption to environmental conditions
(Castrillo et al. ). Although an increase of UFA was
detected in supernatant at the pH value belowed 8, the
reason for it was unclear. Consequently, the association of
removal ratio to the SAT/UFA was investigated in Figure
6(b). As can be seen, the relationship was presented as
y¼86.4x 3.72 with correlation coefficient R
2
of 0.8
the selective deposition of FA was considered to follow a
uniform mechanism. Based on the tendency line, we sus-
pected that the distribution of fatty acids is highly related
to the degree of removal ratio in coagulation-flocculation.
CONCLUSIONS
The data presented in this research proposes one way to
improve the PAM flocculation of SS and residue oil by the
addition of Na
2
CO
3
. Oil acidized wastewater obtained pH
value at 6.1, herein the calcium ion was up to
1,350 mgL
1
and consumed NaOH at 99%. The released
CO
3
2
binding with Ca
2þ
faster than OH
suggested that
Na
2
CO
3
þNaOH enhanced the utilization of OH
to neu-
tralize. In the presence of pH 6.1–7.5, CaCO
3
particles
expressed positive charges under the IEP of 8.1, the electro-
static attraction was primary at the interface of particles and
contaminate, facilitating the SS and residue oil attached to
PAM-CaCO
3
. The increase of NaOH was beneficial to
PAM flocculation because of the stretch of PAM molecular
chain. Under the premise of Ca
2þ
homogeneously blending
with contaminants, the Na
2
CO
3
collected Ca
2þ
together to
generate CaCO
3
floc core, as coagulant aid to improve
coagulation-flocculation. Besides, the CaCO
3
and PAM
selectively combined with SAT to settle down in favor of
high-valued oil recovery. The optimal formulation was
suggested as 200 mgL
1
PAM, 120 mgL
1
NaOH and
400 mgL
1
Na
2
CO
3
. In this case, the content of SS and oil
was reduced to 25 mgL
1
and 34 mgL
1
respectively,
which almost met the requirements of MCLs and maxi-
mized the recycling of SAT. A novel hybrid technology
combing with Na
2
CO
3
coagulation and PAM flocculation
is highly recommended to effectively remove contaminants
in oil acidized wastewater. Future work is needed to explore
the size effect of precipitated CaCO
3
on coagulation-floccu-
lation. In addition, this study could expand to different other
coagulants as the floc core to decontaminate in acidized
environments.
ACKNOWLEDGEMENTS
This work was financially supported by the Fundamental
Research Funds for the Central Universities (Grant
No.310828171001) and Natural Science Foundation of
ShaanXi Province of China (Grant No.606211600026) and
Construction Technology Demonstration Project of Xi’an
(Grant No. SJW2017–15).
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First received 21 December 2017; accepted in revised form 8 May 2018. Available online 16 May 2018
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Author Queries
Journal: Water Science & Technology
Manuscript: WST-EM1824
Q1 As per style, if a references citation has more than two authors, names of first two authors must be listed followed by
et al. Hence we have changed accordingly. Please check.
Q2 As per the journal style, all author names should be included in the references list. So please list all author names for
references having et al.
Q3 Ref. Qin et al. (2015) is not cited in the main text. Please confirm where it should be cited, or delete the reference.
Q4 Please provide the volume number and page range for Qin et al. (2015).
Q5 Please provide the page range for Wang et al. (2016).
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