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8/14/2019 Magnetophoretic removal of microalgae from fishpond water Feasibility
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Accepted Manuscript
Magnetophoretic removal of microalgae from fishpond water: Feasibility of high gradient and low gradient magnetic separation
Pey Yi Toh, Swee Pin Yeap, Li Peng Kong, Bee Wah Ng, Chan Juinn ChiehDerek, Abdul Latif Ahmad, JitKang Lim
PII: S1385-8947(12)01240-5DOI: http://dx.doi.org/10.1016/j.cej.2012.09.051Reference: CEJ 9812
To appear in: Chemical Engineering Journal
Received Date: 15 August 2012Revised Date: 14 September 2012Accepted Date: 17 September 2012
Please cite this article as: P.Y. Toh, S.P. Yeap, L.P. Kong, B.W. Ng, C.J.C. Derek, A.L. Ahmad, J. Lim,Magnetophoretic removal of microalgae from fishpond water: Feasibility of high gradient and low gradient magneticseparation, Chemical Engineering Journal (2012), doi: http://dx.doi.org/10.1016/j.cej.2012.09.051
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customerswe are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, andreview of the resulting proof before it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
http://dx.doi.org/10.1016/j.cej.2012.09.051http://dx.doi.org/http://dx.doi.org/10.1016/j.cej.2012.09.051http://dx.doi.org/http://dx.doi.org/10.1016/j.cej.2012.09.051http://dx.doi.org/10.1016/j.cej.2012.09.0518/14/2019 Magnetophoretic removal of microalgae from fishpond water Feasibility
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ABSTRACT
Microalgae blooms in commercial fish production ponds resulting in a deficit in the
overall oxygen budget have posed serious challenges to aquaculture industry. In this
study, we demonstrate the feasibility of rapid microalgae separation in real-time from
fishpond water by magnetophoresis. By relying on the magneto-shape anisotropy of
rod-liked iron oxide magnetic nanoparticle (IONPs), overall separation efficiency of
microalgae cells up to 90% can be achieved in less than 3 minutes. The IONPs
employed, with a saturation magnetization at 113.8 emu/g, are surface functionalized
with cationic polyelectrolyte that promotes the attachment of these particles onto
microalgae cells via electrostatic interaction. Kinetic of magnetophoretic separation
process was monitored by suspension opacity measurements based upon a custom
built light dependent resistor (LDR setup) sensor. Whereas, the overall separation
efficiency of microalgae cells is determined spectrophotometrically at 685 nm
wavelength. Performance of both high gradient magnetic separation (HGMS) with
T/m and low gradient magnetic separation (LGMS) with T/m
were tested with varying particle concentration (50500 mg/l) and the results
obtained were interpreted in term of cooperative magnetophoresis theory. Cost
analysis was conducted to verify the feasibility for large scale implementation of
LGMS system with the cost involved at $0.13 for every one meter cube of treated
fishpond water.
Keywords: Magnetophoresis; Magnetic nanoparticle; Microalgae removal; Fishpond
water; Magnetic separation.
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1. Introduction
Aquaculture and fish farming is one of the solutions for the worldwide decline of
fisheries stocks of either marine or freshwater fish and also to fulfill the demand of
worlds growing population [1]. One major problem that plagues the freshwater fish
farm is microalgae blooms that occur in every fishpond during summer or tropical
country, such as Malaysia. Microalgae will naturally grow in fishpond water because
of the presence of nutrients, such as nitrogen, phosphorus, carbon source [2, 3], which
originated from the fish excretion, excess fish food and decaying organic matter.
Most of those nutrients promote the growth of microalgae are in organic matter form
and they can be quantified as Chemical Oxygen Demand (COD) with an ideal value
for fishpond at less than 50 mg/l [4]. Ironically, the growth of the microalgae
naturally is beneficial for the removal of the excess nutrient in the water to avoid
nutrient overloading as well as reducing the COD level. However, this benefit is
diminished once the microalgae start to grow excessively. For a typical freshwater
fishpond, the microalgae will keep growing as long as there are nutrient to
substantiate its growth. High microalgae concentration beneficial as oxygen source
through photosynthesis [5] and also provides shades for fishes from the sunlight.
However, the high concentration of microalgae will be disastrous, as their huge
amount will exhaust the oxygen supply through respiration and releasing carbon
dioxide during nighttime. Fish may be killed overnight through suffocation [6] when
dissolved oxygen (DO) is less than 2 mg/l [7]. In most cases, DO in fish pond should
be maintained at least 4 mg/l all the time [4]. At extremely high nutrient level
eutrophication will occur [7]. The nutrient will promote excessive growth of algae
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bloom in the presence of sunlight. The sunlight will be blocked totally by the dense
algae bloom to form a death zone beneath the bloom with drastic oxygen depletion
and hence deteriorate the fish production.
Conventionally, there are multiple well-practiced methods to maintain the
microalgae population within a small fishpond [8]. Effective management of
microalgae growth can be achieved naturally via few methods, such as, growing
aquatic plants around the fishpond to consume the nutrient and starving the
microalgae [9, 10], avoid over feeding and using high quality food to ensure complete
digestion of the food [11], and using barley straw to control algae growth in pond
[12]. Besides the natural treatment, by using the algaecides chemical, which contain
simazine, chelated copper and potassium permanganate, is also able to kill the algae
but it is harmful to living organisms and environment [13]. The quick death of all the
microalgae may increase ammonium concentration and decrease dissolve oxygen in
water and hence it is not favorable. Nevertheless, most of the standard practices
involved for microalgae removal were labor intensive and had limited efficiency [14].
Since microalgae biomass can be employed as third generation biofuel [15] and other
useful products, like nutrients in form of polyunsaturated fatty acid (PUFA) [16, 17,
18] or pigment [19, 20] with a robust removal technique without direct annihilation of
microalgae might be economically more attractive.
There are several microalgae separation methods which have been developed
to meet high microalgae separation efficiency. The most common microalgae
separation methods are filtration, centrifugation, flocculation and settling and ion
exchange [21, 22]. Flocculation and settling is a versatile method, which is suitable to
process large quantity of biomass, but it is time consuming [23]. Filtration method
has recorded high separation efficiency, however, this method is quite costly with the
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problem of blocking and fouling [21, 22]. Centrifugation is the most reliable method
but it is expensive and consumed large amount of energy during operation and is not
suitable to handle enormous quantity of effluents [24]. In addition to all the
separation techniques discussed so far, magnetic separation of microalgae from water
resources is a relatively new concept which was introduced in 1970s [25, 24]. This
method was recently revisited by us [26] and others [ 27, 28 ] due to its attractive
advantages such as high permeation fluxes, high removal efficiency, small land area
utilization and no clogging and fouling problems [ 29 ]. Moreover, magnetic
separation process can also be perform directly on raw samples that contain
suspended solid material due to its ease in capturing the targeted samples by using
surface functionalized magnetic particles [ 30 ].
In order to achieve magnetic separation of biological cell, tagging the cell
surface with a paramagnetic dipole moment is necessary since most cells are
irresponsive to applied magnetic force [ 31 ]. Microalgae cells membrane surface are
negatively charged because of the present of lipids, proteins and sugars, which have
functional groups like -SH, -OH and -COOH. Deprotonation of those ligands will
give a net negative charge on cell surface at natural pH of water [22, 32, 33, 34 ].
While for the magnetite, it is negative by charge when disperse in deionized water
[35 ], with isoelectric point between 6.30-6.85 [ 36, 37, 38 ]. Under this scenario, we
need a binder to immobilize the magnetic nanoparticles onto the microalgae cells.
The binder that is normally employed to serve this purpose is a positively charged
polyelectrolyte, where it can be adsorbed on the nanoparticle surface [26] through
direct method or link to the negative charged cell surface indirectly through [ 30 ]
electrostatic interaction [ 35 ]. After tagging the microalgae cells with magnetic
nanoparticles, cells can be separated magnetophoretically through either low gradient
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magnetic separation (LGMS), without magnetized matrix or high gradient magnetic
separation (HGMS) [ 30 ], which contains magnetized matrix [ 39 ].
In this work, we illustrated the engineering feasibility of LGMS and HGMS in
harvesting microalgae from fishpond water by direct method, which is tagging
surface functionalized iron oxide nanoparticle (IONPs) onto the microalgae cells
surface. Furthermore, we compared the separation efficiency between LGMS and
HGMS at various IONPs concentration corresponding to different kinetic behaviors.
Optical light intensity sensing system (LDR setup) is employed to measure the
overall separation efficiency and quantify its kinetic, while the UV-Vis spectrometer
is conducted to measure the specific cell separation efficiency. Cost analysis on
HGMS system for microalgae separation from fishpond water is conducted to provide
a guideline for different system setup and design preferences.
2. Experimental Methods
2.1. Materials
Rod-liked iron oxide magnetic nanoparticle (IONPs) were obtained from Toda
America, Inc. The 35 wt% very low molecular weight
poly(diallyldimethylammonium chloride) (PDDA) in water with molecular weight,
Mw < 100,000 g/mol was obtained from Sigma-Aldrich, Inc. Deionized water used
was obtained by reverse osmosis and further treated by the Milli-Q Plus system
(Millipore) to 18 M cm resistivity.
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2.2. Characteristic of microalgae in fishpond water
Fishpond water sample was collected from fish farm of Aik Lee Fishery, which is
located at Sungai Bakau, Parit Buntar, Perak, Malaysia. The samples were brought
back to the laboratory for analysis. Microalgae cells species were observed under
Olympus CX41RF microscope equipped with Image Pro Express 4.0.1 imaging
software. Chemical Oxygen demand (COD) of the samples was measured
spectrophotometrically by the DR 5000 TM UV-Vis Spectrophotometer with the use of
High Range Plus COD Reagent from HACH Company, USA. The pH of our
fishpond water was measured by using Eutech CyberScan pH 1500.
2.3. Nanoparticles attachment to microalgae
In this work, rod-like IONPs was used with physical dimension of ~ 20 nm in
diameter and 300 nm in length respectively [ 35 ]. The immobilized-on technique [26]
or direct method [ 30 ] was performed with the attachment of very low molecular
weight PDDA cationic polyelectrolyte onto the IONPs surface to form surface
functionalized IONPs. Firstly, 3408 l of PDDA was dispersed into 25 ml of
deionized water to obtain a concentration of 0.0458 g/ml, and sonicated for 1 hour.
This polyelectrolyte solution was left overnight to ensure complete dissolution of
PDDA. Next, 13 ml of IONPs at concentration of 0.01 g/ml was added into the
polyelectrolyte solution and sonicated for 1 hour. The final surface modified particle
suspension was left under mixing condition on an end-over-end rotating mixer at 37
rpm for 6 days. Electrophoretic mobility and spherical equivalent approximation of
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due to magnetophoretic collection, allowing more light to be detected by LDR. The
degree of separation achieved was quantified by monitoring the intensity of light
passing through the sample during cell separation in real time. Cell separation
efficiency based on the initial fishpond water opacity was calculated with following
equation:
Overall separation efficiency (%) = [V 0 V(t)] (V0 Vcentrifuged ) 100% Eq. (1)
where V0 represents initial voltage of fishpond water sample without adding surface
functionalized IONPs, V(t) represents voltage of sample at time t during
magnetophoretic separation, and the Vcentrifuged represented the voltage of the
centrifuged clear fishpond water sample (centrifuged for 20 minutes at ~ 4000g).
Since both the binding of surface functionalized IONPs to microalgae cells and the
LDR detection method is none specific, hence, this measurement will provide
information on the overall separation of all negatively charged objects out from the
fishpond water. To better quantify the cell separation efficiency, the absorbance of
our sample was measured spectrophotometrically by UVmini-1240 Shimadzu at
specific wavelength of 685 nm [ 40 ] (measured by Agilent Technologies Carry 60
UV-Vis). The cell separation efficiency was determined as
Cell separation efficiency (%) = [I 0 I(t)] (I0 Icentrifuged ) 100% Eq. (2)
where I0, I(t) and Icentrifuged are the absorbance intensity of microalgae suspension
initially, during magnetophoretic separation at time t and the clear centrifuged sample
respectively.
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2.5. High gradient magnetic separation (HGMS)
The HGMS system operated under continuous flow with magnetic field gradient >
1000 T/m were employed in this work. The HGMS column with internal diameter of
1.4 cm was packed with stainless steel wool with packing fraction of 14 vol% with
packed-bed height at around 3 cm. A pairs of N50-graded NdFeB permanent magnet
were employed to fully magnetize the packed-bed as shown in Fig. 2. The sample
solution was pumped into the HGMS column at a flow rate of 1.25 ml/min. Cell
separation efficiency was measured spectrophotometrically by using the same
procedure as discussed previously and the results were analyzed according to
equation (2).
2.6. Cost feasibility analysis
The costs of fishpond water treatment were estimated based on LGMS and HGMS as
shown in Table 1. The water treatment systems were made up of two unit operations
(Fig. 3) that were the mixer, for the mixing of fishpond water with surface
functionalized IONPs, and the LGMS/HGMS magnetic separator. All the
assumptions made were showed in Table 1 and further details regarding the system
employed are available from the supporting documents.
3. Results and Discussion
3.1. Characteristic of microalgae in fishpond water
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From the microscopic observation on the fishpond water sample, there are at least 6
species of microalgae identified in the sample. They are Scenedesmus sp., Spirulina
sp., Chlorella sp., Tetraedron sp., Haematococcus sp. and Dictyosphaerium sp. The
fishpond water sample contains 775 mg/l COD (Table 2) and is slightly alkaline with
pH ranged from 7.0 to 8.5.
3.2. Nanoparticles attachment to microalgae
The result (Table 3) showed the surface charge reversal of IONPs after addition of
PDDA which confirmed the attachment of cationic polyelectrolyte onto the initially
negatively charged IONPs. Furthermore, by making spherical equivalent
approximation [ 41 ], the colloidal stability of surface functionalized IONPs,
monitored by dynamic light scattering (DLS) (Malvern Instruments Nanosizer ZS)
showed an increment of IONPs hydrodynamic diameter from 374.0 139.3 nm to
474.4 35.1 nm after being coated with the PDDA with 1.76 0.6 nm in
hydrodynamic diameter.
3.3. Microalgae cell separation by using LGMS
Fig. 4 depicts the overall and cell separation efficiency of fishpond water after LGMS
collection induced by surface functionalized IONPs at various particle concentration.
In all cases, the overall removal efficiency measured by LDR setup is slightly lower
than the cell separation efficiency determined by UV-Vis absorbance measurement.
This observation is distinctively obvious when low surface functionalized IONPs
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concentration at 25 mg/l was employed. At this concentration, the differences
between both results is 36.88%, followed with 35.84%, 23.11%, 9.37% and 13.45%
for the subsequent surface functionalized IONPs concentration up to 500 mg/l. Since
the present of total solid suspension contributes directly to the opacity of fishpond
water, hence, the LDR measurement provides clear indication on the total removal
efficiency of all negatively charged objects within the fishpond water, including the
microalgae cells. The measurement obtained via the UV-Vis spectrophotometer
provides a more accurate reading of chlorophyll a (and b) of the green algae occurred
within 650 to 700 nm [ 42 ]. The large variation between the two measurements
especially at low concentration of surface functionalized IONPs, suggested that
electrostatic interaction induced particle attachment, even though is not target
specific, but is very effective to promote the magnetophoretic separation of the
microalgae cells from complex media.
At low concentration of surface functionalized IONPs, the recorded separation
efficiency of microalgae is low mainly due to the insufficient supply of surface
functionalized IONPs to impart magnetic properties to the microalgae cells. This
observation revealed the need to maintain high surface functionalized IONPs-to-cell
ratio in order to achieve better cell separation efficiency. At high surface
functionalized IONPs-to-cell ratio, there is higher tendency for more cells to be
decorated by the surface functionalized IONPs and thus favor the magnetophoretic
separation. This argument can be further generalized to justify the need of having
colloidally stable surface functionalized IONPs before its attachment on the
microalgae cells. Maintaining good dispersibility is vital to sustain high surface
functionalized IONPs-to-cell ratio without losing the freely suspended particles to
aggregation especially at high particles concentration [26]. Hence, by having an
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electrosteric hindrance layer around the nanoparticles, originated from PDDA
coating, led to the formation of a colloidally stable suspension which further aided the
magnetophoretic separation process [ 43, 44 ]. Furthermore, the short exposure time of
only 6 minutes is another factor that caused the higher cell separation efficiency
unfavorable at low concentration of surface functionalized IONPs. A much higher
separation efficiency of 79.12% for 25 mg/l surface functionalized IONPs has been
observed if the collection time was prolonged to 20 minutes and this is consistent
with our previous observation on single particle magnetophoresis [ 45 ].
The dependency of both overall and cells separation efficiency on surface
functionalized IONPs concentration as witnessed in Fig. 4 is the result of
magnetophoresis under low gradient magnetic field [ 46, 47 ]. The migration of
microalgae cells to the magnetic field source is mainly due to the cooperative
magnetophoresis of all particles attached to it. For such a rapid collection to happen
(Fig. 5), the formation of large aggregate under the influence of an external applied
magnetic field is necessary [ 48 ]. As the magnetically tagged microalgae cells
approaches the magnetic field source, the microalgae cells collide, leading to cells
chaining that further enhanced the magnetic removal rate [26]. Moreover, by
increasing the particle concentration, the chances for them to stay in close proximity
after attaching to microalgae cells surface will also increased. This in turn would
favor the formation of aggregates on the surface of microalgae cell that contribute to
rapid magnetophoresis.
3.4. Microalgae cell separation by using HGMS
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In conventional industry practice, the removal of magnetic responsive materials,
including the adsorbed biomaterials or pollutants, from its residing solution is
through high gradient magnetic separation (HGMS) process [ 49 ]. In a HGMS
operational unit, the magnetically responsive material is channeled through a pack-
bed column composed of stainless steel fibers with fine mesh size. These packed
fibers are responsible to generate inhomogeneous high magnetic field gradient within
the column after magnetized by an external source [ 50 ]. When the sample solution
flows through the magnetized stainless steel wool matrix, there are two dominating
forces imposed onto the magnetically seeded microalgae cells, namely,
magnetophoretic and viscous drag force [ 51, 52 ]. If the sample solution is well mixed
with low flow velocity, such as the one used in this work, diffusion force [ 51 ] and
hydrodynamic resistance [ 53 ] can be neglected. Balancing all these interactions is
non-trivial and becomes the main bottle neck to develop numerical and/or analytical
solutions to the magnetic separation problem [ 54 ].
From Fig. 6, it is obvious that the cell separation efficiency achieved by
HGMS shared a similar surface functionalized IONPs concentration dependency as
LGMS system. The cell separation efficiency of HGMS increased from 66% to 91%
by increasing the concentration of surface functionalized IONPs from 25 mg/l to 500
mg/l. For HGMS, the results obtained from cell counting through optical microscopy
observation has verified this spectrophotometry results indicating 90% separation of
microalgae cells has been achieved accompanying with 50% COD reduction (Table
2). This result is consistent with Cerff and coworkers observation in which the
cell separation efficiency up to 90% can be achieved by HGMS on harvesting
fresh water algae Chlamydomonas reinhardtii and Chlorella vulgaris [28].
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At low concentration of surface functionalized IONPs (< 150 mg/l), HGMS
out performed LGMS in term of cell separation efficiency mainly due to slow
magnetophoresis kinetic under low field gradient. We anticipated that for longer
collection time (>> 6 minutes) a much higher cell separation efficiency can be
achieved with the same surface functionalized IONPs concentration. Whereas for
high concentration of surface functionalized IONPs, both system performed relatively
well with negligible difference ( 150 mg/l). In addition to concentration of surface
functionalized IONPs, separation performance of HGMS depends on the particles
size and magnetic properties of the seeding materials as well [ 51, 53 ]. Since the
magnetophoretic force is directly proportional to the magnetic volume of the particle
[51, 55 ], hence, large particle will experience a much larger force and can be
collected in HGMS column much easily compared to smaller particle. However, the
utilization of larger particles beyond the superparamagnetic limit, for iron oxide
particles at around 50 nm [ 56 ], is not favorable as these particles has high tendency to
aggregate and settle out from the solution before their attachment to the microalgae
cells.
After magnetic treatment the previously greenish fishpond water turned to
crystal clear, especially when high particle concentration was used, indicating
effective removal of microalgae (Fig. 7). By eyes inspection, a hint of blackish
suspended solid can still be detected in the treated fishpond water after going through
LGMS process. This is very likely due to the present of trace amount of surface
functionalized IONPs in the treated fishpond water which is not fully recovered by
LGMS. Since there are ample experimental evidences suggesting the toxicity of
nanomaterials at cellular level [ 57, 58 ], thus, the implementation of LGMS unit for
fishpond water treatment needs to be conducted with a much sophisticated design
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which focuses on the full recovery of the particles used. Nevertheless, the surface
functionalized IONPs employed in this work are coated with cationic polyelectrolyte.
Thus, the positively charged particles should adsorb rapidly onto the surrounding soil
surface and losing their mobility [ 59 ].
3.5. Cost feasibility analysis
In order to demonstrate the economic feasibility of magnetic separation for small to
middle size fishpond water treatment, simple cost analysis was performed for LGMS
system with a process involves two unit operations as shown in Fig. 3 with all the
estimated expenses as shown in Table 1. This cost analysis was conducted based on
the water treatment requirement of Aik Lee Fishery where all our samples were
collected. The dimension of one fishpond there was around 22 m 25 m 1 m with a
total of 21 fishponds. In order to cope with the cleaning requirement and water
replacement rate of the fishpond during the microalgae blooming period, each
fishpond need to be treated once a week with 20% of water is being treated at a rate
of 48 m 3/h to ensure good water quality for fish to grow. Here wet drum-type low
gradient magnetic separator (Shanghai Lipu Heavy Industry Co., Ltd) is chosen due
to its satisfactory capacity to treat water up to 60 m 3 /h for an intermediate sized fish
farm. The fish farm of this scale is not uncommon in the northern part of Malaysia. A
two blades propeller agitator equipped mixer is needed here to avoid dead zones and
promote better mixing of surface functionalized IONPs suspension with the fishpond
water [ 60 ] before their introduction into LGMS.
From our analysis, the treatment cost for fishpond water by using LGMS at
the treatment rate of 48 m 3/h with 300 mg/l surface functionalized IONPs (0.519 g
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surface functionalized IONPs/g dry microalgae biomass) to achieve 90% cell
separation efficiency is estimated to be $6.03/h (= $0.13/m 3 pretreatment fishpond
water). Under this scenario, the magnetic materials and the polymer binder have
contributed almost 33% of the total cost involved. It is very unlikely that the current
market price of surface functionalized IONPs at $119/kg can be reduced significantly
in near future; hence, other replacements are needed. We envisage that in order for
magnetic separation to become more economically viable, magnetic materials from
scrap metal or mining residues might be a better candidate. However, the
environmental impact and engineering feasibility of these materials are still
unexplored and need further justifications. The cost of tap water for industrial usages
in Malaysia starts at $0.18/m 3 [61 ] and is slightly higher than the LGMS treatment
cost if the loss of surface functionalized IONPs is being tabulated at 0.07%. This is
the targeted value which we would like to achieve by further improvement of
separator efficiency involved in immediate future. Nevertheless, the charging of large
amount of fish farm effluents without treatment into the nearby river has caused
numerous problems to the local community and this makes magnetic separation
attractive.
The loss of surface functionalized IONPs for each cycle of LGMS treatment
was estimated at 0.07% with the use of 0.05 mg/l surface functionalized IONPs. This
value translated into ~ 0.15 mg/l of Fe (by assuming the surface functionalized
IONPs is 100% magnetite) is being introduced into the fishpond and is within the
ideal iron level at 0.01-0.30 mg/l to avoid bioaccumulation [4]. So, for the purposes
of cost reduction and safety issue, there is a pressing need to design a more effective
magnetic separation system to keep the surface functionalized IONPs leakage
minimum.
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We have also generalized our cost analysis to a HGMS system (See
supporting documents for the detail HGMS cost analysis in Table SII). The purchase
cost for high gradient magnetic filtration separator (HGMF) at $1,600,000 from the
literature [24] was used for cost estimation without cost index normalization. This is
an underestimated value and the cost involved will go up by a factor of four if the
cost index was taken into consideration. The treatment cost for 165.7 m 3/h of
fishpond water with 50 mg/l of surface functionalized IONPs (0.0866 g surface
functionalized IONPs/g dry microalgae biomass) to achieve about 90% cell
separation efficiency was approximated as $85.85/h (= $0.52/m 3 pretreatment
fishpond water) accordance to a process rate of 207 m 3/h (included feed of surface
functionalized IONPs) with the same working capacity of Yadida et al., 1977 [24].
This capacity is capable to treat every fishpond in Aik Lee Fishery for three times in
one week and it is comparable to the workload handled by 3 LGMS units. For HGMS
system, the huge expenses arise from the operation unit (contribute 52% of total cost)
with its high power consumption for magnetic power generation, pumping and
flushing power. This shortcoming, perhaps, can be counter-balanced by the use of
permanent magnet for the magnetization of inner matrix of HGMS unit as illustrated
by Hoffman and coworkers [64].
4. Conclusions
We have verified the feasibility of the microalgae separation from the fishpond water
through the application of the surface functionalized IONPs (very low molecular
weight PDDA coated TODA iron oxide) under low gradient magnetic separation
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technique (LGMS). The feasibility of LGMS was guaranteed as long as the amount of
surface functionalized IONPs introduced is sufficient to induce magnetophoresis for
tagged microalgae cells. In addition, the magnetophoretic separation of microalgae
from fishpond water was proven feasible by using high gradient magnetic separation
technique (HGMS). For both methods, LGMS and HGMS, microalgae removal
efficiency of more than 90% can be achieved depending on the amount of the surface
functionalized iron oxide nanoparticles (IONPs) used. Compared to HGMS, the
performance of LGMS is more sensitive toward the concentration of surface
functionalized IONPs, mainly due to its cooperative magnetophoresis nature. There is
a slight discrepancy in term of cell separation efficiency between HGMS and LGMS
systems at surface functionalized IONPs usages below 150 mg/l. At high particle
concentration, LGMS performed equally well as HGMS in microalgae separation. In
our study, the key advantages of LGMS system are its low energy consumption and
the ease of design by using permanent magnet arrays. These features are also
generally true for magnetic separator employed for industrial applications. By
monitoring the suspension opacity while undergoing magnetophoresis, we also
quantify the kinetic behavior microalgae removal by LGMS for 6 minutes. We
believe the removal time can be further reduced by increase the concentration of
surface functionalized IONPs. From our cost analysis, LGMS system is more cost
effective for microalgae separation, with an estimated cost of $0.13/m 3 water treated.
Here, the magnetic materials and the binding agents contribute the major expenses for
LGMS technique. For the implementation of LGMS for microalgae removal from
fish farm water, we envisage a simple process involved only two unit operations is
good enough for the uses of small to middle fisher industry.
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Acknowledgements
This material is based on work supported by Research University (RU) (Grant No.
1001/PJKIMIA/811165) from Universiti Sains Malaysia, Exploratory Research
Grants Scheme (ERGS) (Grant No. 203/PJKIMIA/6730011) from Ministry of Higher
Education of Malaysia, and International Foundation for Science (IFS) (Grant No.
304/PJKIMIA/6050232/I100). P. Y. Toh was supported by the My PhD scholarship
from Ministry of Higher Education of Malaysia. We thank Dr. B. S. Ooi from School
of Chemical Engineering, USM, Malaysia for providing invaluable help during the
experiment. All authors are affiliated to the Membrane Science and Technology
cluster USM.
Supporting Information Available
Tables showing the technical data and specification of each equipment for LGMS
unit, cost analysis of HGMS system based upon 90% of cell separation efficiency for
50 mg/l of surface functionalized IONPs.
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Highlights
Rapid magnetophoretic separation of mixed strains of microalgae from fishpond
water is feasible.
The concentration of surface functionalized IONPs affect the microalgae removal
efficiency.
LGMS and HGMS systems achieve high separation efficiency.
Kinetic behavior of both LGMS and HGMS systems are compared.
LGMS system more cost effective for small scale fishfarm water treatment.
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Table 1. Cost involved for microalgae removal by LGMS with 90% of cell separation efficiency
and 300 mg/l of surface functionalized IONPs used.
LGMS (Rate of f ishpond water being tr eated is 48 m 3 /h per un it L GM S) Cost ($)/h a
LGMS Separator (Wet Drum-type Magnetic Separator)
($ 3230) over 15 years 0.09
Mixer
($ 7177) over 15 years 0.20
Storage Tank
(Pretreatment surface functionalized IONPs& Sludge after treatment)
($ 30818) over 15 years 0.86
Feed Pump(Fishpond water & surface functionalized IONPs)
($ 2700) over 15 years 0.08
Power (LGMS separator, mixer and pumps) , c
(4.85 kW) 0.08
Installation(Instrallation cost, piping, instrumentation cost, electrical installation, yard improvement andmaintenance)
0.74
Labor e 2.00Raw Material 1.98
Total 6.03($ 0.13 /m pretreatment fishpond water)
aCost estimation based on the continuous treatment of fishpond water for 8 hours per day (6 working days/week; 50working weeks/year). There was 20% of water (110 m 3) of each pond (Based on a unit size of fishpond in fishingfarm of Aik Lee Fishery) will be treated. 21 fishponds will be treated in 6 working days.
bSee supporting documents for technical data and preference of each equipment showed in Table SI.
cCost estimation based on Malaysia electrical rate of Tenaga Nasional Berhad (2012) [62].
dReference: Book of Peter and Timmerhaus (1991) [63].
eCost estimation based on Malaysia labor rate.
f Cost estimation based on cost of Fe3O
4(Six C USA Co., Ltd, China) and cationic polyelectrolyte of chitosan
(Weifang Union Biochemistry Co. Ltd, Shandong, China) that are abundantly available in economic price andnormally use in industrial application. It was assumed that there was 0.07% lost of surface functionalized IONPs bywashout for each batch of water treatment and the surface functionalized IONPs was being recycled.
e(s)
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Table 2. Chemical Oxygen Demand (COD) and cell count of fishpond water before and after the
microalgae removal by HGMS.
Before After Decreasing (%)
COD (mg/l) a 775 364 c 53.03
Cell count (x10 cells/ml) 8267 825 90.02
a Measured by the DR 5000 UV-Vis Spectrophotometer.. b Counted on the Neubauer Improved Heamocytometer.c COD can further reduced by increase the fishpond water treatment rate up to appropriate capacity to meet thedesired value.
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Table 3. Electrophoretic mobility and spherical equivalent approximation of averaged
hydrodynamic diameter of IONPs, very low molecular weight PDDA and surface functionalized
IONPs.
Electrophoretic mobility
[mcm/ Vs]
Hydrodynamic diameter
[nm]
IONPs -1.757 0.001 374.2 139.3
PDDA 4.196 0.094 1.76 0.6
Surface functionalized IONPs 5.775 0.065 474.4 35.1
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Fig. 1. Schematic diagram of LGMS setup employed in this study. The LDR sensor was
employed to measure the light transmitted through the fishpond water sample. Magnified image
showed the magnetic field induction generated through the medium by a NdFeB magnet with
surface magnetization ~ 6000 Gauss measured by Alphalab, Inc. DC Magnetometer Model
GM2. This measurement verified the magnetic field working range of our system.
re(s)
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Fig. 2. Experimental HGMS setup with a cylinder column packed with stainless steel wool
matrix and expose to NdFeB permanent magnets. This arrangement is chosen to resemble our
LGMS setup for the ease of comparison.
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Fig. 3. Block flow diagram of the fishpond water microalgae cells separation for fish farm
application.
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Fig. 4. Separation efficiency achieved by LGMS as a function of surface functionalized IONPs
concentration measure by (i) LDR setup, and, (ii) UV-vis spectrometer respectively.
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Fig. 5. Real time overall separation efficiency of magnetically tagged microalgae cells from
fishpond water based on the initial fishpond sample after mixed with 150 mg/l surface
functionalized IONPs, presented in graph together with images, in the LGMS separation after 6
minutes exposure to NdFeB permanent magnet.
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Fig. 6. The separation efficiency of the microalgae cells from the fishpond water through (i)
LGMS, and, (ii) HGMS as a function of surface functionalized IONPs concentration, measure by
UV-Vis spectrometer.
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(i)
(ii)
Fig. 7. The images of the fishpond water sample, treated fishpond water by different surface
functionalized IONPs concentration, and centrifuged fishpond water for the (i) LGMS and (ii)
HGMS.
50 mg/l
IONPsFishpond
water25 mg/l
IONPs150 mg/l
IONPs300 mg/l
IONPs500 mg/l
IONPsCentrifuged
fishpond water
Fishpondwater
25 mg/lIONPs
50 mg/lIONPs
150 mg/lIONPs
300 mg/lIONPs
500 mg/lIONPs
Centrifugedfishpond
water