w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 1 7 7 e3 1 8 8
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Electrocoagulation versus chemical coagulation: Coagulation/
flocculation mechanisms and resulting floc characteristics
Tali Harif*, Moti Khai, Avner Adin
Soil & Water Sciences Department, Robert H. Smith Faculty of Agricultural, Food and Environmental Quality Sciences, The Hebrew University
of Jerusalem, POB 12, Rehovot 71600, Israel
article info
abstract
Article history:
Electrocoagulation (EC) and chemical coagulation (CC) are employed in water treatment for
Received 1 December 2011
particle removal. Although both are used for similar purposes, they differ in their dosing
Received in revised form
method e in EC the coagulant is added by electrolytic oxidation of an appropriate anode
29 February 2012
material, while in CC dissolution of a chemical coagulant is used. These different methods
Accepted 19 March 2012
in fact induce different chemical environments, which should impact coagulation/floccu-
Available online 29 March 2012
lation mechanisms and subsequent floc formation. Hence, the process implications when
choosing which to apply should be significant. This study elucidates differences in coag-
Keywords:
ulation/flocculation mechanisms in EC versus CC and their subsequent effect on floc
Electrocoagulation
growth kinetics and structural evolution. A buffered kaolin suspension served as a repre-
Aluminum
sentative solution that underwent EC and CC by applying aluminum via additive dosing
Floc
regime in batch mode. In EC an aluminum anode generated the active species while in CC,
Growth
commercial alum was used. Aluminum equivalent doses were applied, at initial pH values
Zeta potential
of 5, 6.5 and 8, while samples were taken over pre-determined time intervals, and analyzed
Scattering exponent
for pH, particle size distribution, z potential, and structural properties. EC generated fragile
flocs, compared to CC, over a wider pH range, at a substantially higher growth rate, that
were prone to restructuring and compaction. The results suggest that the flocculation
mechanism governing EC in sweep floc conditions is of Diffusion Limited Cluster Aggregation (DCLA) nature, versus a Reaction Limited Cluster Aggregation (RLCA) type in CC. The
implications of these differences are discussed.
ª 2012 Elsevier Ltd. All rights reserved.
1.
Introduction
1.1.
Coagulationeflocculation and factors determining
floc evolution
CoagulationeFlocculation in general is a two phase process
aimed at removing stable particles by forming larger aggregates that can be separated from the aqueous phase by
a subsequent separation step. The preliminary phase is the
coagulation phase in which destabilization is induced, either
by the reduction of repulsive forces between particles or by
the enmeshment in precipitates (Hogg, 2005). For insoluble
particles, such as many minerals (e.g. Kaolin), inter-particle
repulsion is usually due to electrical double layer interaction.
The addition of soluble ionic species will affect the surface
potential (electrical potential difference between the particle
surface and the bulk solution) of colloidal particles by either
adsorption to the particle surface or by double layer
compression. Ionic species that are specifically adsorbed at
the surface can include multivalent cations and anions, ionic
* Corresponding author.
E-mail addresses: [email protected] (T. Harif), [email protected] (M. Khai), [email protected] (A. Adin).
0043-1354/$ e see front matter ª 2012 Elsevier Ltd. All rights reserved.
doi:10.1016/j.watres.2012.03.034
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w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 1 7 7 e3 1 8 8
surfactants and, particularly in aqueous systems, hydrogen
and hydroxyl ions (Hogg, 2005). The type of coagulation
mechanism induced e be it surface adsorption or precipitate
formation (“sweep “floc”) e when using metal salts (such as
aluminum or iron) is governed by the solubility boundary of
the coagulant and therefore is highly dependent on pH, and
coagulant dose (Amirtharajah and O’Melia, 1990). Flocculation
is the second stage of the combined process and consists of
the aggregation of coagulated particles and/or precipitate
precursors in flocs. Successful inter-particle aggregation will
depend on the destabilization degree attained in the coagulation phase (the extent of a repulsive force barrier existing
between the particles), and on the collision rate between the
particles. The latter is a function of hydrodynamics dictated
by system geometry and therefore can greatly differ between
various systems (Bouyer et al., 2004). When referring to floc
growth, several phases can be identified (Tambo, 1991). After
a characteristic time, a steady state is reached between
aggregation and fragmentation, marked by a floc size distribution that is constant with time and is unique to the prevailing shear conditions of each system (Spicer and Pratsinis,
1996; Bouyer et al., 2004; Jarvis et al., 2005a). In general, the
overall floc growth rate (Rfloc) can be defined as (Jarvis et al.,
2005a):
Rfloc ¼ aRcol Rbr
(1)
Rcol ¼ rate of particle collision (frequency of collisions)
Rbr ¼ rate of aggregate breakage
a ¼ collision efficiency factor (fraction of collisions resulting in
attachment)
Floc growth will also be affected by the specific aggregate
structure because a more open structure is considered to have
a larger collision profile (Kusters et al., 1997). More porous flocs
are fragmented by fluid shear stresses more rapidly than
compact mass equivalent flocs (Potanin, 1991; Flesch et al.,
1999). Restructuring can also occur, by re-aggregation of
fragments or by shear interactions that rearrange the structure (Oles, 1992; Jarvis et al., 2005b). Two types of aggregation
have been identified: diffusion limited cluster aggregation
(DLCA) and reaction limited cluster aggregation (RLCA) (Tang
et al., 2000; Bushell et al., 2002). The former occurs when the
repulsive barrier between particles is low e forming looser,
tenuous structures; the latter occurs when the repulsive
barrier between particles is high e producing more compact
and stronger structures (Tang et al., 2000).
1.2.
Electrocoagulation in water treatment
Electrocoagulation (EC) over the past decade has gained
recognition as an effective process for various water treatment applications. Research into EC, although not extensive,
has examined colloidal and organic matter removal (Vik et al.,
1984; Matteson et al., 1995; Holt et al., 2002; Larue et al., 2003)
and explored a wide range of applications in urban and
industrial wastewater treatment (Ogutveren and Koparal,
1992; Alexandrova et al., 1994; Pouet and Grasmick, 1995;
Belongia et al., 1999; Mollah et al., 2001, 2004; Adin and Vescan,
2002). Additionally, EC has been found to be an effective pre-
treatment method prior to membrane filtration, both for
achieving flux enhancement (Harif et al., 2006; Ben Sasson and
Adin, 2010), and for optimizing virus removal (Zhu et al., 2005).
Despite the research interest in EC, the vast majority of
studies have been conducted at an applied level, rather than at
a mechanistic one. Sporadic studies have explored some
mechanistic aspects of the process, distinguishing between
various coagulant species (Ben Sasson et al., 2009;
Lakshmanan et al., 2009), and ascertaining how the unique
chemical environment impacts the structural properties of
flocs (Harif and Adin, 2007, 2011). Comparative studies with
chemical flocculation (CC) are scarce, and limited to removal
efficiency comparative assessments (Holt et al., 2002; Bagga
et al., 2008; Cañizares et al., 2009). Hence, a more fundamental approach may be required to be able to postulate the
relevance of the EC process in water treatment processes
versus the CC approach.
In EC the active coagulant species are generated in situ by
electrolytic oxidation of an appropriate anode material, thus
differing from CC in which chemical coagulants such as metal
salts or polymers and polyelectrolytes are used. The half-cell
electrochemical reactions occurring in the EC cell (using an
aluminum anode) and their corresponding standard electrode
potentials (written by convention as reductions) are:
Cathode:
2H2 O þ 2e / H2ðgÞ þ 2OH
ðaqÞ
E+ ¼ 0:83 V
(2)
Anode:
O2ðgÞ þ 4Hþ
ðaqÞ þ 4e /2H2 O
AlðaqÞ þ 3e /AlðsÞ
3þ
E+ ¼ þ1:229 V
E+ ¼ 1:662 V
(3)
(4)
When considering the half-reaction standard potentials,
anodic Al3þ dissolution yields a significantly lower Gibbs free
energy compared to anodic water reduction, therefore Al3þ
dissolution will ultimately dominate. Assuming that a minor
portion of the current will go toward anodic water reduction
the outcome is that hydroxyl ions (OH) will be formed at the
cathode in excess in relation to hydrogen ions (Hþ) at the
anode. Although Al(OH)3 precipitation can remove some OH
this is not suffice to counteract the overall OH production.
The rate of OH formation surpasses the rate of Al(OH)3
precipitation, even in optimal precipitation conditions,
particularly when considering that 1 mole of produced Al3þ
will divide into an array of hydrolyzed species, some of
which are soluble. Hence, the overall reactions result in
excess OH in solution, manifested in a transient pH rise
over time. This phenomenon has previously been reported
(Chen et al., 2000; Harif and Adin, 2007, 2011; Bagga et al.,
2008; Lakshmanan et al., 2009; Cañizares et al., 2009) and
differs greatly from CC, in which the pH decreases. Coagulation mechanisms depend primarily on pH and coagulant
dosage, which govern speciation of the active mononuclear
species (Amirtharajah and O’Melia, 1990; Sposito, 1996;
Letterman et al., 1999; Duan and Gregory, 2003), thus such
a difference is of pivotal importance when comparing both
processes. Disparities may also exist in the hydrolysis of the
w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 1 7 7 e3 1 8 8
aluminum species. In EC, as opposed to CC, negative
counter-ions are not added. Negative counter-ions can
influence the aluminum hydrolyzed species, because they
can replace the hydroxyl (Duan and Gregory, 2003) and their
absence in EC may affect the coagulation mechanisms. The
objective of this paper is to elucidate fundamental chemical
differences existing between EC and CC that would impact
coagulation mechanisms, flocculation behavior and subsequent floc characteristics.
1.2.1.
Electrocoagulation technology and coagulant types
The most basic EC reactor may be made up of an electrolytic
cell containing one anode and one cathode. The anode metals
most commonly used are aluminum or iron because when
electrochemically oxidized they produce the most commonly
used ionic coagulants, Al3þ and Fe3þ (or Fe2þ) respectively. The
dissolution of coagulant into solution is governed by Faraday’s
Law:
w¼
ItM
ZF
(5)
w ¼ metal dissolving (gr M/cm2)
I ¼ current intensity (A)
t ¼ time (s)
M ¼ molecular weight of metal (gr/mol)
Z ¼ number of electrons involved in the oxidation/reduction
reaction
F ¼ Faraday’s constant (96,485 C)
Aluminum hydrolysis and mononuclear species formation
following aluminum dosing has been discussed extensively in
Amirtharajah and O’Melia (1990) and Duan and Gregory (2003).
The solubility boundary of aluminum (0.03 mg/l Al3þ, at pH
6.3) denotes the thermodynamic equilibrium that exists
between the dominant aluminum species and amorphous
aluminum hydroxide, Al(OH)3, (the assumed solid form relevant in coagulation processes) at a given pH. Although
dimeric, trimeric and polynuclear hydrolysis products of Al3þ
can form, these can often be ignored, especially in dilute
solutions, and may not affect the overall speciation (Duan and
Gregory, 2003). As such, this paper will assume that mononuclear hydrolyzed species adequately predict Al(OH)3
precipitation.
1.3.
Floc structural interpretation
Characterization of structures can be done using various
techniques which are reviewed in Jarvis et al. (2005b). Static
light scattering will be used in this research, and is considered
advantageous due to the non-interfering nature of the technique (Thill et al., 2000).
1.3.1.
3179
aggregates. Calculating fractal structures derived from fractal
mathematics (Mandelbrot, 1982) is extensively discussed
elsewhere (Bushell et al., 2002; Jarvis et al., 2005b).
Using the fractal dimension as a measure of structural
measurements is applicable if the RayleigheGanseDebye
(RGD) approximation is met, for non-absorbing particles in the
limit that both:
jm 1j 1
(6a)
ð4pn=lÞL jm 1j 1
(6b)
where m is the relative refractive index of the scatterers
(primary particles) and L is the length of the scattering body
(the diameter of the primary particles).
For amorphous Al(OH)3 nucleation clusters (hereafter
termed “Al(OH)3 precursors”), a refractive index of 1.59 can be
used as an approximation, based on the refractive index range
of other Al(OH)3 solid forms (Li et al., 2005; Harif and Adin,
2011), thereby fulfilling the requirements in equations (6a)
and (6b).
The fractal dimension of an aggregate, assuming RGD
approximation, is acquired from the slope of the logarithmic
plot of the angular scattering intensity (I(q)) versus the
momentum transfer (q). However, due to variations in aggregate structures and sizes comprising the entire scattering
volume, the absolute slope of the logarithmic plot of I(q)
versus q will be referred to as the scattering exponent (SE), as it
may not necessarily represent the real mass fractal dimension
of all the aggregates. The SE, however, does still portray
structural properties, and higher SE values will indicate more
compact aggregates e and vice versa (Guan et al., 1998; Waite,
1999).
The system presented in this study represents a complex
water system, thus the light scattering results presented for
a mixture of Al(OH)3 precursors and kaolin should be interpreted with caution. However, for relatively narrow size
distributions, polydispersity effects are expected to be insignificant (Lawler, 1997; Bushell and Amal, 1998).
2.
Materials and methods
2.1.
Colloidal suspension
1.2 g of kaolin (AlSi2O5(OH)4, Aldrich Chemical Company Inc.
USA) was suspended in 20 L of distilled water (60 mg/l final
concentration), dispersed and homogenized using an Ultraturax 2000 (Ganke and Kunkel, GMBH). 1.66 g of NaHCO3 (BDH
Laboratory Suppliers, UK) was added (final concentration
83 mg/l), the pH was corrected to 5, 6.5 and 8 with NaOH or
H2SO4, and conductivity was increased to 1 mS/cm with
NaNO3 (Riedel-Dehaen, GMBH).
Light scattering techniques
Particle scattering patterns and intensities can be translated
into a measure of the particle size, according to available
approximations that were developed for particles of different
sizes and optical properties (Sorensen, 1997). The scattering
patterns are related to the scattering angle in a way which can
be used to obtain information also on the structure of the
2.2.
Coagulation cells
2.2.1.
Electrocoagulation cell
An EC batch cell was designed. It consisted of a plexyglass cap,
which could be fitted onto a 1 L chemical glass and to which
the electrodes were attached. The inner electrode, the
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cathode, was made from stainless steel and concentric in
form (H ¼ 10 cm, D ¼ 2.5 cm). It was fitted onto the arm of the
magnetic stirrer, and stationary throughout all the experiments. The outer electrode, the anode, was made from
aluminum, designed, also concentric in form with dimensions: H ¼ 10 cm, D ¼ 9.5 cm. The electrodes were connected to
a DC external power source.
2.2.2.
Chemical coagulation cell
To attain comparative chemical conditions between the
processes, for each time increment, alum was added to the
suspension for identical time intervals as current operation in
the EC cell. A 1 L chemical glass was used, fitted with the
identical plexyglass cap and magnetic stirrer used in the EC
cell, and a stock solution of commercial alum (Al2(SO4)318H2O,
SigmaeAldrich, Israel) was fed using a syringe pump (74900
series, ColeeParmer, USA).
2.3.
Floc size and structure analysis
Size distributions and structural information of kaolinAl(OH)3 flocs were determined as a function of time using
a Mastersizer Microplus (Malvern Instruments, UK), which
ascertains size by analysis of forward scattered light. The size
distribution data given by the instrument covers the size
range of 50 nme500 mm. Size distribution information was
obtained using supplied software which uses Mie theory to
develop a scattering pattern that matches the scattering
pattern of the sample being measured. Information on mean
distribution size is presented in this paper as the volume
mean diameter:
DðV; 0:5Þ ¼
X
X
ðVi di Þ=
Vi
i
(7)
i
Where Vi is the relative volume in size class i with mean class
diameter di.
Information on floc structure was obtained by measuring
the intensity of light (I ) at all detectors, for three consecutive
runs, and plotting log I versus log q. Information regarding the
angles of the detectors and intensity correction data, based on
the geometric configuration of the detectors, was supplied by
Malvern Instruments.
2.4.
the electrostatic forces of repulsion between charged particles
which changes with the addition of a coagulant.
2.6.
Experimental procedure
2.6.1.
Electrocoagulation
The EC apparatus was fixed onto a 1 L chemical glass containing 800 ml of kaolin suspension, the electrodes submerged
in the suspension, and magnetic stirrer set at a constant
gentle speed of 145 rpm. The stirring speed was selected to
induce floc growth over the defined experimental time range,
based on preliminary experiments. EC was conducted in galvanostatic mode: the current was set and the potential found
its own value dependent on the system’s overall resistance.
This ensured coagulant production at a pre-determined rate,
defined by Faraday’s law.
Preliminary experiments were conducted to ensure that
the aluminum dissolution could be calculated by Faraday’s
Law. Fig. 1 shows a comparison between the theoretical and
actual aluminum dissolution, as a function of time, in an EC
cell using an aluminum anode and stainless steel cathode.
Aluminum dissolution was measured by weighing the anode
at each time interval.
The graph shows a nearly ideal correlation between the
theoretical calculation, using Faraday’s Law, and the actual
values obtained. These values cover much higher aluminum
doses and longer time spans than applied in this research,
thus we can conclude that the set-up can generate a target aluminum dose. A stainless steel cathode was chosen to
minimize effects of cathodic dissolution that would lead to
a positive deviation from the theoretical Faradaic values. The
topic of cathodic dissolution in EC cells and ways to overcome
this phenomenon are discussed in Ivanshivili et al. (1987) and
Picard et al. (2000).
The mixing regime applied was chosen to enable discrete
monitoring of floc evolution during coagulation and initial
growth stages and provided the platform for analogous
fundamental evaluation. At various time intervals (0, 3, 6 and
10 min) samples were taken for analyses (pH, size distribution,
Image analysis
Image analysis was used in conjunction with scattering
measurements as a complementary analysis to ascertain
qualitatively floc properties over time. The flocculated
suspension was gently poured into a petri dish and the flocs
were photographed using a digital camera (model DP11,
Olympus, Japan) which was mounted on a Stereoscope (model
SZX12, Olympus, Japan). All photographs were taken at 3.0
mega pixel resolution, and magnification 50.
2.5.
z potential
The z potential of the suspension was measured using
a Zetamaster S (Malvern Instruments, UK). Each result was an
average of three readings. Its value determines the extent of
Fig. 1 e A comparison between theoretical and actual
aluminum anodic dissolution in the EC cell, using an
aluminum anode and stainless steel cathode.
w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 1 7 7 e3 1 8 8
z potential and image analysis). The complete dose of
aluminum was achieved at 10 min, for each applied current,
after which additional aluminum was not introduced into
solution. At optimal sweep floc conditions (initial pH 6.5)
samples underwent continuous mixing and measurements
were taken again at 15 min. For all size distribution measurements, the Mastersizer Microplus was operated at the lowest
possible pump speed of 400 rpm, so to minimize disruption of
floc structure. Qualitative analysis using image analysis
techniques corroborated relative floc sizes attained. The
currents used for the experiments were 0.042 A, 0.11 A and
0.22 A, yielding aluminum doses of 2.4 mg/l, 6.5 mg/l and
13 mg/l respectively. These doses are equivalent to aluminum
content in 30 mg/l, 80 mg/l and 160 mg/l commercial alum
(Al2(SO4)318H2O) used in the CC experiments (8.1% of the total
molecule).
2.6.2.
Chemical Coagulation
A 1 L chemical glass containing 800 ml of kaolin suspension
was fitted with the plexyglass cap and magnetic stirrer set at
145 rpm (as in all EC experiments). A stock solution of
10 103 mg/l commercial alum (Al2(SO4)318H2O) was prepared
and fed at a preset dosing rate of 0.24 ml/min, 0.64 ml/min and
1.28 ml/min, to achieve final doses of 30 mg/l, 80 mg/l, and
160 mg/l respectively. The alum was introduced into solution
over the identical time span as current operation in the EC cell
(10 min). In optimal sweep floc conditions (initial pH 6.5), the
suspension underwent continuous mixing for an additional
5 min. Samples were taken for analyses (pH, size distribution,
z potential and image analysis) at 0, 3, 6, 10 and in optimal
sweep floc conditions also at 15 min.
The mixing conditions applied both in the EC cell and CC
cell were aimed at inducing floc formation. A range of average
velocity gradients have been used in various studies examining the impact of shear stress on floc growth, and empirical
relationships developed (Jarvis et al., 2006). However, deviations between the average velocity gradient and resulting floc
sizes are apparent, while it is known that a specific floc growth
curve is unique to each system (Spicer et al., 1998; Bouyer
et al., 2004; Jarvis et al., 2005a). Furthermore, studies have
suggested that local velocity gradients have a more significant
impact then average velocity gradients leading investigators
to adapt the use of rpm instead of a calculated average shear
velocity (Jarvis et al., 2006). In light of this, the results obtained
from using a specific mixing regime, must be limited to the
set-up. In this study, the results are comparative and relevant
to a mixing regime that can be considered representative and
falls within the wide range of velocity gradients applied for
attaining floc growth.
3.
Results and discussion
3.1.
Size distributions and floc formation pH 5 and 8
Figs. S1 and S2 in supporting information show representative
growth histograms obtained with CC and EC respectively. In
CC, at pH 5, no substantial floc growth was observed. On the
other hand, in EC, at pH 5, all currents yielded flocs of
considerable size, with modal diameters reaching above
3181
300 mm. In CC, at pH 8, floc formation was observed only with
160 mg/l alum. The growth histogram indicates a very short
induction period (up to 3 min), after which slower growth is
observed, with final modal diameters reaching 250 mm. In EC,
at pH 8, floc formation did not occur, for all currents applied.
The histograms attained in EC at pH 5 show a long induction phase, and a paced type of growth. Interestingly, despite
this, the final modal diameters attained were larger than those
obtained in CC at pH 8 with 160 mg/l alum; 300 mm versus
250 mm, for EC and CC respectively.
The results suggest a low collision frequency and efficiency
for EC at pH 8 compared to CC, however at pH 5, this is
reversed. The collision frequency is a function of precursor
particle concentration (kaolin primary particle concentration,
and precipitation of Al(OH)3 precursors formed by addition of
aluminum coagulant into solution), aggregate geometry/
collision profile, and the latter a function of repulsive electrostatic barriers (Kusters et al., 1997; Chakraborti et al., 2003).
The pH and z potential measured over each time increment in
these experiments (Fig. 2) can shed light on the different
growth behavior observed between both types of coagulation.
The first most prominent difference observed between EC
and CC pertains to the pH change over time. In general, in EC
the pH rises, whereas in CC the pH decreases. This phenomenon and its significance have been discussed previously.
At pH 8, in EC, the pH did not change for the duration of the
experiments, for both currents applied. The z potential of the
solution also was stable, and maintained a value of approximately 40 mV. Both z potential and pH stabilization at pH 8
suggest the removal of excess OH from solution. Removal
could be attributed to the buffering capacity of the solution,
which was maintained due to bicarbonate predominating at
pH 8 under open atmosphere, and also to formation of
neutrally charged species, Al(OH)3. The formation of Al(OH)3 in
these conditions would be primarily dictated by Le Chatelier’s
principle, due to excess OH cathodic formation, which drives
the solubility equilibrium toward precipitation. The overall
removal of OH from solution, maintains the pH at a stable
value, inhibiting a pH rise and subsequent formation of
negatively charged species (Al(OH)4), which otherwise would
have lowered the z potential to a more negative value. The
coupled effect obstructs the growth of Al(OH)3 precursors into
larger flocs, due to the high repulsive electrostatic barrier
which exists in solution (reflected by the very negative z
potential). On the other hand, when applying CC at pH 8, the
pH is lowered, and reaches 5.8 for a dose of 160 mg/l alum. Floc
growth was not evident at pH 8 for lower doses of alum. The pH
value reached was not low enough to induce adequate positive
mononuclear species (AlOH2þ, Al(OH)þ
2 ) formation, which
would have lowered the electrostatic repulsive barrier, depicted by a shift in the z potential toward a more positive value.
Conditions in which both the electrostatic repulsive barrier is
adequately lowered (reflected by a more positive z potential
value of 10 mV), and Al(OH)3 precursor mass production is
suffice, occurs when applying a high alum dose. The induction
period is short, indicating high collision efficiencies at the early
stages, however the growth kinetics show that as the flocculation progresses, the rate of growth slows, suggesting that
a steady state between aggregation and fragmentation is being
reached (Oles, 1992; Spicer et al., 1998; Jarvis et al., 2005a).
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w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 1 7 7 e3 1 8 8
Fig. 2 e A. pH changes at initial pH 5, B. pH changes at initial pH 8, C. z potential changes at initial pH 5, D. z potential
changes at initial pH 8. Each z potential result is an average of three consecutive readings.
At pH 5 positive aluminum mononuclear species (AlOH2þ,
Al(OH)þ
2 ) govern the solution. In EC, the pH rises substantially
and conditions become favorable for Al(OH)3 precipitation
(transition into sweep-floc regime), subsequently causing an
acceleration in floc growth at later stages. The reason for pH
rise is excess cathodic production of OH, and in addition the
lowered buffering capacity of the solution at pH 5, due to
transition toward CO2 gas formation (originating from bicarbonate), which diffuses out of solution under atmospheric
pressure. The size distribution spreads indicate that smaller
particles are being formed simultaneously, and are able to
aggregate more efficiently into larger flocs at higher currents.
For both currents at initial pH 5, the z potential exhibits charge
reversal, with similar values reaching þ18 mV at the termination of the process (10 min). The rather large disparities in z
potential attained at initial pH 5 between EC and CC are most
likely due to the soluble negative ions introduced with the
chemical coagulant that mask the positive ion affect thus
limiting the z potential from reaching very positive values.
Despite the higher repulsive barrier, EC was able to generate
flocs, suggesting that floc growth at this pH is derived from
increased Al(OH)3 precipitate mass formation at higher
currents leading to higher collision frequencies. In CC, despite
attaining a lower repulsive barrier, reflected by a near isoelectric point z potential, precursor formation is limited, as
the pH values inhibit Al(OH)3 precipitation.
3.1.1.
pH 6.5
At initial pH 6.5, both CC and EC yielded sufficient floc growth
for all alum doses and all currents applied. At this pH, Al(OH)3
precursor formation is nearly optimal as the minimum solubility of Al(OH)3 occurs at pH 6.3 (Amirtharajah and O’Melia,
1990). As such, these conditions were chosen to serve as
a basis for structural comparisons between flocs obtained in
each process, and an additional 5 min mixing was used to
attain longer growth periods. Representative growth histograms are shown in Fig. 3.
Floc size evolution over time show for all applied currents
and lower alum doses growth up to modal diameters of
approximately 380 mm, with the size distribution spread
reaching the upper detection limit of the instrument. Previous
studies examining alum induced flocculation of colloidal
suspensions have documented modal diameters from 200 mm
(Bouyer et al., 2004) to 500 mm (Spicer et al., 1998), following
15 min of slow mixing. The floc sizes attained in this study are
well within that range, hence we can assume that the additive
dosing regime applied in CC was able to generate floc sizes
compatible to conventionally practiced methods. For a high
alum dose the modal diameter obtained at 15 min is lower,
and reaches 200 mm. For 0.042 A the induction phase is longer
than that observed for 0.22 A, but at 6 min accelerates, yielding
modal diameters similar to those attained with the high
current (above 300 mm). For 0.22 A, acceleration in growth at
6 min is also observed, resulting in near maximum modal
diameters by 6 min. The growth inhibition evident at 10 min
and above, is due to the enhanced breakage rate, dictated by
shear forces which restrict evolution into a larger floc.
EC produces narrower size distributions at 10 and 15 min
compared to CC, while both processes exhibit wide size
distributions up to 6 min. The wide size distributions indicate
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w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 1 7 7 e3 1 8 8
Fig. 3 e Growth histograms obtained with low and high alum doses and applied currents at initial pH 6.5. Coagulant
addition occurred over the first 10 min, following an additional 5 min mixing period, in aim of attaining longer growth
patterns. The size distribution at 0 min is that of the initial kaolin suspension.
pH
8
A
0.042A
7
0.11A
6
0.22A
30mg/l
5
80mg/l
4
0
5
0
30
20
ζ Potential (mV)
that while growth into larger floc is occurring, smaller particles are simultaneously being formed. For 0.22 A growth
acceleration occurs at 3 min, while for 0.042 A at 6 min, indicating a linear trend between current applied and floc
formation. CC, on the other hand, attains floc sizes similar to
those reached in EC at a slower rate (at 10 min and above as
opposed to 3 and 6 min in EC). These differences can be
explained by Al(OH)3 precursor formation, pH changes and z
potential changes over time. Fig. 4 shows the differences in z
potential and pH values for each time increment, between EC
and CC at initial pH 6.5.
The differences in pH and z potential values over time in
both processes underline the differences in the physicochemical environments both induce. For alum dosing of
80 mg/l and 160 mg/l the pH rapidly decreases at 6 min dosing,
reaching pH values below 5, by the end of the process. Dosing
of 30 mg/l alum, however, lowers the pH marginally, stabilizing at a value of 6 during the final stages of the process. In
these conditions a larger number of Al(OH)3 precursors can
form, as opposed to the higher alum doses, because all growth
stages occur within nearly optimal sweep-floc regime; hence
the increased growth and larger final sizes. Despite a stronger
repulsive barrier (reflected by a more positive z potential)
exhibited in CC versus EC, growth is appreciable, and for
30 mg/l reaches a maximum modal diameter of 380 mm. For
the higher alum doses transition out of sweep-floc regime
toward positive mononuclear speciation is pronounced,
limiting mass precursor formation in the overall process,
which in turn, limits floc growth.
10
0
B
5
10
Time (min)
Time (min)
10
15
160mg/l
15
0.042A
0.11A
0.22A
-10
30mg/l
-20
80mg/l
-30
160mg/l
-40
Fig. 4 e EC and CC at initial pH 6.5 A: Changes in pH values
over time for all currents and alum doses applied. B:
Changes in z potential values over time for all currents and
alum doses applied. Each z potential result is an average of
three consecutive readings.
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w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 1 7 7 e3 1 8 8
In EC, the z potential values following application of current
approach the iso-electric point, indicating a diminished electrostatic repulsive barrier. As such, the limiting factor for floc
growth in EC at initial pH 6.5 is apparently a decrease in collision frequencies, due to transition out of sweep-floc regime.
The pH rise, stemming from excess OH cathodic production
and lowered buffering capacity (due to CO2 gas diffusion out of
solution under atmospheric pressure following the initial pH
correction) indicates that the precipitation potential of Al(OH)3
decreases over time, resulting in less OH removal. The
enhanced production of Al(OH)3 precursors at the initial stages
of the process, most predominant at 0.22 A (exhibiting a very
short induction period), enables adequate precursor mass
formation that lay the foundation for appreciable floc evolution, despite the transition into unfavorable precipitation
conditions, as the process progresses.
3.2.
Floc growth profiles at pH 6.5
Gaining a deeper understanding of specific growth kinetic
patterns between both processes in sweep-floc range requires
plotting the volume mean diameter against time. Fig. 5 shows
the evolution of volume mean diameter for all currents and
alum doses applied at initial pH 6.5, and interestingly unveils
distinct differences in growth kinetics between EC and CC.
Significant variations between both processes are evident.
The particular growth patterns representative of each process
are a product of solution chemistry that gives rise to different
floc sizes, structures, strengths and subsequent collision
profiles, the importance of which has been presented elsewhere (Potanin, 1991; Oles, 1992; Spicer and Pratsinis, 1996;
Kusters et al., 1997; Flesch et al., 1999; Waite, 1999; Chakraborti
et al., 2003). EC shows distinctly faster growth rates, compared
to CC, for all applied currents and alum dosing concentrations.
In EC, a higher dose produces both a faster growth rate and
larger final floc sizes. In CC, on the other hand, a linear
correlation between growth and dose was not observed, and
for sweep floc conditions, the lowest dose yielded both faster
growth rates and larger final sizes. For 30 mg/l alum the final
volume mean diameters are similar to those attained for
0.22 A, but the path is somewhat different. For 0.22 A,
a distinct sigmoidal profile is observed, showing pronounced
fragmentation at 6 min, indicating transition into steady state
flocculation (Spicer et al., 1998; Bouyer et al., 2004; Jarvis et al.,
2005a). For 30 mg/l the growth profile is also sigmoidal, with
a mild deceleration observed around 12 min, indicating fragmentation only at later stages. The growth curves of the flocs
obtained from applying smaller currents show also sigmoidal
behavior e fragmentation occurring at 6 min and 10 min for
0.11 A and 0.042 A respectively. Alum dosing at higher doses
produced growth curves devoid of sigmoidal behavior, yet
suited to polynomial growth. In CC of a kaolin clay suspension
using alum, the rate determining step is dictated by interparticle collision (Matsui et al., 1998). It seems that in EC,
this step is shortened, due to higher Al(OH)3 precursor mass
production leading to a higher collision frequency. This,
coupled with a lower repulsive barrier, induces a rapid growth
rate. In CC, the floc growth is indeed slower, suggesting
smaller collision profiles. The flocs formed in EC apparently
possess a weaker strength and are more susceptible to fragmentation. For sigmoidal growth curves, transition into
steady state occurred in EC at approximately 220 mm, 180 mm
and 200 mm for 0.22 A, 0.11 A and 0.042 A respectively. This
occurred in CC (30 mg/l) at approximately 260 mm. Floc
strength is indicated by the size of a floc attained at a given
shear rate (Jarvis et al., 2005a). The general understanding is
that strength increases with increasing size, a relationship
found to be particularly appropriate for alumino-clay and
alumino-humic flocs (Bache and Gregory, 2010). The strength
referred to is for the given shear condition under which the
flocs were formed. In addition to floc size, floc structural
evolution can be indicative of floc strength, as the floc bonds
(numbers and strength) will dictate a floc’s propensity to
undergo restructuring. Hence, complementary structural
analysis was performed to attain a comprehensive picture of
specific flocculation mechanisms and floc structural behavior,
which are apparently individual to each process.
3.3.
Fig. 5 e Evolution of volume mean diameter for all currents
and alum doses at pH 6.5. The fitted curves show for all
currents and lowest alum dose a sigmoidal growth pattern,
indicating a fragmentation stage, and for higher alum
doses, a second order polynomial pattern, indicating no
fragmentation. All coefficients of determination (R2) are in
the range of 0.98e0.99.
Floc structural evolution at pH 6.5
Typical scattering plots (I(arb) versus q(nm1)) were used to
obtain the structural data. Interpretation of these plot types is
explained elsewhere (Thill et al., 2000; Bushell et al., 2002). For
all scattering graphs, one decade of linearity was observed for
1.4*104 < q < 1.6*103 (Figs. S3 and S4 in supporting information). This region was used to calculate the scattering
exponent (SE) from the graph slopes. Table 1 summarizes the
SE values obtained.
The flocs formed with all currents applied in EC, underwent compaction resulting in a final SE, at 15 min, which is
higher than that obtained during the initial 3 min. In some
cases, fluctuations in the structural evolution are apparent,
suggesting the flocs are of fragile nature, and are prone to
fragmentation and restructuring, and ultimately compaction.
In CC, on the other hand, for 30 mg/l alum, at which appreciable growth is observed, the structural evolution shows that
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w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 1 7 7 e3 1 8 8
Table 1 e Scattering exponent values for time increments over 3e15 min, obtained at pH 6.5 for all currents and alum doses
applied. The higher values indicate a more compact structure and the lower values a more porous structure.
Time (min)
Scattering exponent
30 mg/l Alum
3
6
10
15
1.99 0.03
1.91 0.02
1.83 0.01
1.78 0.02
0.042 A
1.83
1.93
1.89
1.93
0.04
0.01
0.02
0.01
80 mg/l Alum
1.87 0.02
1.89 0.01
1.82 0.01
1.84 0.01
at initial stages of flocculation, a more compact structure
exists that becomes more porous, as the flocculation progresses. Both favorable Al(OH)3 precipitation conditions and
surface forces enable growth into a larger floc, that exhibit
a stability not observed for equivalent sized flocs obtained in
EC. The SE values obtained for the higher alum doses change
insignificantly throughout the process, maintaining generally
a stable value, within a margin of error. These doses also
generated slow growth profiles, and smallest floc sizes, due to
smaller collision profiles, because of growth limitation
dictated by a high repulsive barrier and transition into inadequate Al(OH)3 precipitation conditions. Comparison of SE
values arising from conditions in which floc growth generated
comparable floc sizes (30 mg/l alum versus 0.22 A), indicates
different aggregation regimes, dictated by different collision
mechanisms. In EC, it seems that the flocs form by a DLCA
type of reaction, whereas in CC the aggregation is more of an
RLCA nature. DLCA occurs when there are negligible repulsive
forces between colloid particles, causing particles to “stick”
0.11 A
1.78
1.95
1.88
1.93
0.02
0.01
0.02
0.01
160 mg/l Alum
0.22 A
1.81 0.03
1.84 0.01
1.84 0.01
1.86 0.01
1.77 0.02
1.78 0.01
1.81 0.02
1.86 0.01
readily upon contact and form tenuous structures (Tang et al.,
2000). These structures have higher collision profiles, yet
are prone to fragmentation and restructuring. As such, in EC
the repulsive barrier is indeed negligible thus enabling accelerated growth. However, the large tenuous structure, and
subsequent large collision profile at the early stages, seems
fragile and more susceptible to the applied shear force, hence
floc restructuring occurs, leading to a more compact form by
the end of the process. The gaseous products (hydrogen and
oxygen) formed in EC process could also facilitate the adhesion of precursor particles together. Bubble nucleation rate
increases with increasing currents (Shahjahan Kaisar Alam
Sarkar et al., 2010), and indeed could impact the linear correlation existing between current applied and floc growth in EC.
RLCA occurs when a substantial repulsive force remains
between particles, so the “sticking” probability diminishes
and particles rely on many collision frequencies to form
a stable floc. In this case, the floc formed will exhibit a tighter
and more compact structure. In CC the repulsive barrier is
Fig. 6 e A visual comparison of floc images formed at 6 and minutes with 30 mg/l alum dosing and 0.22 A. Magnification
503, a comparative scale of 500 mm is shown in the lower right image.
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w a t e r r e s e a r c h 4 6 ( 2 0 1 2 ) 3 1 7 7 e3 1 8 8
indeed higher, leading to a slower growth curve, yet evolution
of a more strong and compact floc, able to withstand shear
forces. The result of this was structural evolution into a final
more porous structure than attained in EC.
Structural evolution, as floc size evolution, is dependent
not only on solution chemistry (dictated by the coagulant dose
and water characteristics) but also on specific flocculation
conditions (shear and time). Cycled shear flocculation and
tapered shear flocculation have shown to yield distinct
structural differences (Spicer et al., 1998). The mixing conditions in this study are assumed to be indicative of alum
induced flocculation at a constant applied shear. Therefore
the structural differences documented are relevant to the floc
growth phase and transition into steady state.
Qualitative image analysis of the flocs corroborated the
scattering structural analysis. The flocs formed by alum
dosing are indeed more compact, well defined and denser
compared to the flocs formed by applied current, which
appear “fluffy” and transparent. Fig. 6 shows visual differences in floc structure at 6 and 10 min, using 30 mg/l alum
versus 0.22 A.
4.
Concluding remarks
Although EC and CC are considered similar coagulation
processes, targeted toward particle removal, in fact substantial differences exist between them in terms of coagulationflocculation mechanisms and floc evolution patterns. The
conclusions presented are based on a comparative evaluation
of solution chemistry and kaolin-Al(OH)3 floc evolution
throughout growth stages. The authors acknowledge that the
choice of an additive dosing regime for CC is not common
practice, however, inducing time discrepancies between
dosing regimes would result in divergent coagulant concentrations, and subsequent coagulation-flocculation conditions
in which disparities would not be comparable at the mechanistic level. In effect, the applied CC dosing regime did
produce flocs comparable to other studies using the standard
method. In general, most studies exploring floc evolution
using chemical coagulation have limited the findings to the
unique experimental set-up because different systems have
shown to produce widely variable results with regard to floc
formation rate and structural characteristics (Spicer et al.,
1998; Bouyer et al., 2004; Jarvis et al., 2005a; Bache and
Gregory, 2010). Notwithstanding, we postulate that the
comparative supporting evidence provided can be projected to
larger systems. The overall distinct solution chemistry divergences observed e a function of the particular coagulant
addition method e are of a magnitude that heavily impact
coagulation mechanisms. As the coagulation mechanism
attained is the core driver of floc evolution in growth phase
conditions, the implications of using EC or CC should be
significant.
For equivalent coagulationeflocculation conditions, EC is
able to produce flocs over a wider range of pH values relevant
to water treatment, apparently at a more rapid rate. Floc
structural evolution in EC suggests the formation of structures
that are relatively porous and fragile, prone to restructuring
and compaction. Both the floc formation rate and structural
evolution patterns in EC, obtained in optimal sweep floc
conditions (initial pH 6.5) point to a DLCA mechanism versus
an RLCA type in CC. On a practical note, EC can diminish the
need for pH adjustment at low initial pH values, and could be
specifically suited to processes in which shorter flocculation
times are applied and initially a more porous (less dense)
aggregate is favored (i.e. dead-end/cake filtration, dewatering,
flotation). Smaller and more compact structures, less prone to
shear stress over longer flocculation times, like those generated in CC, would most likely be suited to processes incorporating higher shear environments. In line of this, particular
attention should be made when choosing to use either EC or
CC, depending on treatment process and overall objectives.
Acknowledgments
The work was partially supported by BMBF Germany and
Israeli Ministry of Science. We gratefully acknowledge Dr.
Rivka Amit and Mr. Yoav Nachmias from the Geological
Survey of Israel for assistance with the Malvern Mastersizer
Microplus, and additional instrumentation. Prof. Daniel
Mandler from the Hebrew University of Jerusalem is kindly
thanked for fruitful discussions.
Appendix A. Supporting information
Supplementary data related to this article can be found online
at doi:10.1016/j.watres.2012.03.034.
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