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Synthesis of Maghemite Nano-particles using Micro-emulsion technique and its application in

removal of Heavy elements from Waste-Water Presenting R. Hitesh

1, M.Virendrasinh

2, B. Pushpajitsinh

3

1, 2, 3

Faculty of Engineering Technology & Research, Ta Bardoli Dist Surat

E-mail addresses: rhitesh98@yahoo.com,virendramahida23@gmail.com,baradpushpajit@ymail.com

Abstract: This paper investigates the synthesis and applicability of Maghemite( -Fe2O3) nanoparticles for the selective removal of toxic

heavy metals like Hexavalent chromium Cr(VI), Divalent Copper Cu(II) and Divalent Nickel Ni(II).The nanoparticles are formed by the

co-precipitation reaction of ferrous and ferric salts with organic base, cyclohexylamine and into a water-in-oil micro emulsion using

AOT(Aerosol-OT) as surfactant. Formation of Maghemite Nanoparticles is indicated by Red-Brown Color particles precipitated on

addition of cyclohexylamine. In contrast, cyclohexylamine does not avoid the aggregation of the particles during the synthesis. The

Maghemite nanoparticles of 5.615 nm (Hydrodynamic Diameter) were synthesized using Micro-Emulsion method. The Hydrodynamic

diameter of Maghemite Nanoparticles is characterized by DLS (Dynamic Light Scattering).Batch experiments were carried out to

determine the adsorption kinetics and mechanisms of Cr (VI), Ni (II) and Cu (II) by Maghemite nanoparticles. The adsorption process

was found to be highly pH dependent, which made the nanoparticles selectively adsorb these three metals from wastewater. The

adsorption of heavy metals reached equilibrium rapidly within 10 min. The formed Micro-emulsion is stable and characterized by

stability test using turbid scan for 12 Hrs.

Keywords: Maghemite Nano-particles, Micro-Emulsion, Characterization, Adsorption kinetics, Stability test

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1. Introduction:

The presence of heavy metals like Cr (VI) Ni (II) and Cu (II) in

wastewater and surface water is becoming a severe

environmental and public health problem [1] [2] Adsorption is

a conventional but efficient technique to remove heavy metals

or organics from aqueous solutions. The adsorbents used in

Adsorption are highly porous materials, providing adequate

surface area for adsorption. However, the existence of intra-

particle diffusion may lead to the decrease in the adsorption

rate and available capacity, especially for macro-molecules.

Thus, developing an adsorbent with large surface area and

small diffusion resistance is of great significance in practical

engineering applications.

With the latest development of Nanotechnology various types

of Nano-particles are synthesized using various methods like

Sol-gel, Co-precipitation, Thermal methods, Micro-emulsion

and other physical methods.

Micro-emulsions are a special class of ‘‘dispersions’’

(transparent or translucent) that actually have little in common

with emulsions. They are better described as ‘‘swollen

micelles’’. The term micro-emulsion was first introduced by

Hoar and Schulman [3] who discovered that by titration of a

milky emulsion (stabilized by soap such as potassium oleate)

with a medium-chain alcohol, such as pentanol or hexanol, a

transparent or translucent system was produced.

This technique can be applied in oil-recovery, as a fuel, in

agrochemicals, in synthesizing nanoparticles, in food industry

etc.

Maghemite has a cubic unit cell in which each cell contains 32

O ions, 21⅓ Fe3+

ions and 2⅔ vacancies. The cations are

distributed randomly over the 8 tetrahedral and 16 octahedral

sites [4].

In this study, monodisperse Maghemite Nanoparticles [5] are

synthesized using Micro-Emulsion Technique and is applied in

removal of selective Heavy metals like Cr (VI) [6], Ni (II), Cu

(II) from waste water [7].Maghemite nanoparticle as a novel

adsorbent is expected to offer an attractive and inexpensive

option for the removal of heavy metals by considering its

simple synthesizing method, high surface area, and magnetic

properties. Cr (VI), Cu (II), and Ni (II) were chosen as the

metal adsorbate because they commonly exist in the effluents

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of plating factories, petroleum, electrolytic refining plants, and

acid mining industries. Thus the objective of this study is:

Synthesis of nanoparticles using Micro-Emulsion technique

and using those nanoparticles as adsorbate for removal of

Heavy metal from waste water of Industrial Effluent.

2. Material and Methods:

The Maghemite Nano-particles are synthesized by Micro-

Emulsion technique. First metal precursor solution(water-

phase) is prepared using Ferric Chloride (0.5 M) , Ferrous

Sulphate (0.25 M) and Hydrochloric acid (0.1M). In a separate

beaker surfactant AOT (Aerosol-OT) is dissolved in

Cyclohexane (Oil-phase).The Metal precursor is added to

solution of Cyclohexane and AOT in proportion of 90/7/3 and

is constantly stirred. After that the mixture is purged with

Nitrogen Gas for 5-10 minutes and heated up to 50-55 oC with

constant stirring. The formation of Micro-Emulsion is observed

when the liquid becomes non-light scattering to light scattering

i.e., hazy to transparent. After formation of Micro-Emulsion the

precipitating agent like organic base Cyclohexylamine is added

to Micro-Emulsion Finally Maghemite Nanoparticles are

obtained as Red-Brown color particles and separated and

washed with acetone and then with ultra-pure water thrice and

dried. The Stability of Micro-Emulsion is tested using Turbid-

Scan for 12 Hours and the particles Hydro-dynamic Diameter

is characterized by DLS.

2.1 Adsorption Studies:

Synthetic solutions were prepared of K2Cr2O7, NiCl2 and

CuSO4 in ultrapure water. Adsorption studies were performed

by rotating 0.1 g maghemite nanoparticles with 20 mL of metal

solution in a glass vial at room temperature of 25°C To

investigate the effect of pH, 20 mL of 100 mg/L Cr (VI), Cu

(II), and Ni (II) ternary component systems with pH ranging

from 2.0–10.0 were prepared by dissolving desired metal salts

in ultrapure water. The above solutions at different pH were

mixed with 0.1 g Nano scale maghemite for 24 h to reach

equilibrium. For adsorption kinetic studies, 0.1 g Nano scale

maghemite was added into 20 mL of 100 mg/L Cr (VI), Cu (II),

and Ni (II) single-solute individually. The pH of the suspension

for Cr (VI), Cu (II), and Ni (II) systems was, respectively,

adjusted to 2.5, 6.5, and 8.5. Volumes of 3 mL of samples were

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taken for metal measurements at specific time intervals. The

measurement was carried on Atomic Spectroscopy for Cu (II)

and Ni (II) and Visible Spectroscopy for Cr (VI) using Quartz

Cuvette as a sample holder.

3. Results and Discussion

3.1 Characterization of Micro-Emulsion

The stability of Micro-Emulsion is tested using Turbid-Scan

(classic MA2000) for 12 Hours of time It was observed that BS

profiles for 12 hrs after interval of 15 min. were almost

superimposing. It indicated that the structure and average sizes

of the nanoparticles would slightly change with the progression

of time, but since the nanoparticles were stabilized by the

surfactant, no abrupt changes in BS data were observed.

Fig 3.1 BS Profile

3.2 Characterization of Maghemite nanoparticles

DLS (Malvern Zetasizer, Nano ZS 90, U.K.) measurements of

the cyclohexane/AOT/iron salt solutions were performed at

different temperatures to study the size of the microemulsion

nanodroplets. Fig. 2 shows the average size of nanodroplets as

a function of temperature. The average hydrodynamic diameter

of droplets is 30.48 nm at 45°C, 8.07 nm at 50° C, 8.23 nm at

55° C, 6.86 nm at 60° C and 5.65 nm at 63° C.

Fig 2: DLS graph at different tempratures

0

10

20

30

40

50

60

0 20 40 60 80 100

45° C

50° C

55° C

60° C

63° C

Num

ber

(%

)

Particle Size

Cyclohexylamine 12 hrs (23/11/13 17:43)

Back Scattering

0mm 20mm 40mm 60mm

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10%

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3.3 Effect of pH

The effect of solution pH on the removal of chromium, copper,

and nickel from ternary component systems during adsorption

process is shown in Figure below:

Fig 2: Effect of pH on % Removal Efficiency

As far as the metals are concerned, the removal efficiency was

highly pH dependent. The percentage of uptake of Cr (VI)

decreased gradually with an increase in pH, whereas the

percentage of removal of Cu (II) and Ni (II) increased with an

increase in pH. As observed, the maximum removal of Cr

occurred at about pH 2.5, while Cu and Ni were not at all

removed at this pH. With an increase in pH to 6.5, the removal

efficiency of Cu reached 92%; while only 16% of Ni was

removed. When the pH was further increased to 9.0, almost

92% of the Ni was removed. Thus, the selective removal of

these three metals can be verified by controlling the pH of the

solution. The dependence of metal removal on the pH can be

explained from the perspective of surface chemistry in an

aqueous phase; the surfaces of metal oxides are generally

covered with hydroxyl groups that vary in form at different pH

levels. The surface charge is neutral at the zero point of charge

pHpzc, which is 6.3 for maghemite. Below the pHpzc, the

adsorbent surface is positively charged, and anion adsorption

occurred by simple electrostatic attraction. Above the pHpzc,

the adsorbent surface is negatively charged, and cation

adsorption occurred. With an increase in pH, the uptake of Cr

(VI) ions decreased, which is apparently due to the higher

concentration of OH− ions present in the mixture that compete

with Cr (VI)species CrO42−

for adsorption sites. On the other

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10

% Removal

pH

Cr(VI)

Ni (II)

Cu(II)

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hand, as the adsorption surface is negatively charged pH

pHpzc, increasing electrostatic repulsion between negatively

charged Cr (VI) species and negatively charged nanoparticles

would also result in a release of the adsorbed HCrO4 −

and

CrO42−.

As far as Cu and Ni are concerned, the increase in

metal removal with pH is due to a decline in competition

between proton and metal species for surface sites; thereby

decreasing in positive surface charge and resulting in a lower

Columbic repulsion of the adsorbed metal. At a pH lower than

pHpzc, the percentage of adsorption of Cu and Ni should be

reduced to nearly zero; while it is not the case for Cu. It was

found that at a pH lower than pHpzc, the amount of Cu ions

was still adsorbed onto the maghemite, which suggested that

ion exchange between Cu2+

and H+ may play a role during this

pH range. This point will be further examined in the following

mechanism studies. Furthermore, at the same pH and initial

metal concentration, a higher percentage removal was recorded

for Cu (II) compared to Ni (II). In general, the preference of

common hydrous solids for metals has been related to the metal

electronegativity. Electronegativity values for Cu (II) and Ni

(II) are 2.00 and 1.91, respectively; hence Cu exhibited a

stronger attraction to maghemite than Ni [8]

3.4 Kinetics Studies

Since pH 2.5 and 6.5 are the optimal conditions for Cr (VI)

and Cu (II) adsorption, respectively, adsorption kinetics for Cr

and Cu were obtained by mixing 100 mg/L of single metal and

0.1 g maghemite nanoparticles at their optimal pH. As for Ni,

the maximum adsorption was found at pH 9.0. However,

heterogeneous precipitate of Ni is suspected to emerge on the

particle surface when the pH is higher than 9.0 .To ensure that

only an adsorption reaction occurs for Ni, the operating pH is

therefore defined to be 8.5. The effect of contact time on the

adsorption of Cr (VI), Cu (II) and Ni (II) at their optimal pH is

shown in figure. The metal removals seem to take place in two

phases. It is evident that, initially, that the rate of metal uptake

was significantly high, with much lower subsequent removal

rates that gradually approached an equilibrium condition. For

these three metals, the adsorption equilibrium was achieved

within 10 min. At equilibrium, the amount of Cr, Cu, and Ni

adsorbed was 27.4, 14.4 and 19 mg/g, respectively. The rapid

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adsorption of metals is perhaps due to external surface

adsorption. Since nearly all of the adsorption sites of

maghemite nanoparticles exist on the exterior of the adsorbent

compared to the porous adsorbent, it is easy for the adsorbate to

access the active sites; hence, a rapid approach to

equilibrium.[9],[10].

Fig 4 Adsorption capacity v/s Time

4. Conclusions:

The maghemite nanoparticles with a diameter of around 5.6 nm

were successfully synthesized using a Micro-Emulsion method

in a laboratory. The adsorption studies illustrated that the Nano

scale Maghemite was very effective for the removal of Cr (VI),

Cu (II) and Ni (II) from wastewater. Adsorption of metals by

Nano scale Maghemite reached equilibrium within 10 min and

the removal efficiency was highly pH dependent, which also

governs selective adsorption of metals from the solution. The

optimal pH for the selective removal of Cr, Cu, and Ni is 2.5,

6.5, and 8.5 respectively.

5. Reference:

[1] Salnikow, K.,and Zhitkovich, A.(2008). “Genetic and

epigenetic mechanisms in metal carcinogenesis and

cocarcinogenesis: nickel, arsenic, and chromium. “Chemical

Research in Toxicology 21(8), 28-44.

[2] Ochoa-Herrera, V., León G., Banihani, Q.;,Field,J.A., and

Sierra-Alvarez R.(2011). “Toxicity of copper(II) ions to

0

5

10

15

20

25

30

-30 20 70 120

Cu(II) pH=6.5

Ni(II) pH=8.5

Cr(VI) pH=2.5

Ad

sorp

tio

n C

apac

ity

(mg/

g)

Time(min)

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microorganisms in biological wastewater treatment systems.”

Science of Total environment. , 412, 380-385.

[3] T. P. Hoar, J. H. Schulman, Micro emulsion Theory and

Practice, Academic Press, New York, 1977, 102-152 .

[4] Gribanov, N.M., Bibik, E.E., Buzunov, O.V., Naumov,

V.N.(1990). “Physicochemical regularities of obtaining highly

dispersed magnetite by the method of chemical condensation.”

Journal of Magnetism and Magnetic material 85,7-10.

[5] Vidal-Vidal,J. Rivas, M.A. Lopez-Quintela, Synthesis of

monodisperse maghemite nanoparticles by the microemulsion

method Colloids and Surfaces A: Physicochemical Engineering

Aspects 288 (2006) 44–51 51.

[6] Hu,J. Chen,G. and Irene, Lo,M.C., and ASCE,M.(2006).

“Selective Removal of Heavy Metals from Industrial

Wastewater Using Maghemite Nanoparticle: Performance and

Mechanisms.” Journal of Environmental Engineering, 132(7),

709-715.

[7] Hu,J. Chen,G. and Irene, Lo,M.C.(2005) “Removal and

recovery of Cr(VI) from wastewater by maghemite

nanoparticles.” Water Research 39(18):4528-4536.

[8] Seco, A., Marzal, P., and Gabaldon, C. (1997).

“Adsorption of heavy metals from aqueous solutions onto

activated carbon in single Cu and Ni systems and in binary Cu–

Ni, Cu–Cd, and Cu–Zn systems.”Journal of Chemical

technology and Biotechnology , 68(1), 23-30.

[9] Brown, P. A., Gill, S. A., and Allen, S. J. (2000). “Metal

removal from wastewater using peat.” Water Research, 34

(16), 3907–3916.

[10] Lalvani, S. B., Hubener, A., and Wiltowski, T. S. (2000).

“Chromium adsorption by lignin.” Energy Sources, 22(1), 45–

56.

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