magnetophoretic removal of microalgae from fishpond water feasibility

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

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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.2012.09.051


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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

magnetophoretic removal of microalgae from fishpond water feasibility

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