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Chapter 4
Synthesis, characterization and adsorption
behavior of MgO nanoparticles
Nanocrystalline materials exhibit a wide array of unusual properties and can be
considered as new materials that bridge molecular and condensed matter [1]. One of the
unusual features is enhanced surface chemical reactivity (due to increased surface area)
towards incoming adsorbates. Researchers have shown the use of nanocrystalline metal
oxides like MgO, CaO, TiO2, Al2O3, MnO2 and Zn–Al layered double hydroxides and
oxides as adsorbents for the removal of pollutants [2-5]. These nanomaterials were found
to adsorb polar organics in very high capacities and substantially outperform the activated
carbon samples that are normally utilized for such purposes.
Many years of research have clearly established the destructive adsorption
capability of nanoparticles towards many hazardous substances, including chlorocarbons,
acid gases, common air-pollutants, dimethyl methylphosphonate (DMMP), paraoxon, 2-
chloroethylethyl sulfide (2-CEES) and even some warfare agents. The enhanced chemical
reactivity suggests a two-step decomposition mechanism of the adsorbates on
nanoparticles (first step - adsorption of toxic agent on the surface by means of
physisorption, followed by the second step - chemical decomposition). This two-step
mechanism substantially enhances the detoxification abilities of nanoparticles because it
makes the decomposition less dependent on the rate of chemical reaction. The rate of
chemical reaction depends on the agent-nanoparticle combination; therefore, for some
agents the rate may be quite low. In addition, the reaction rate strongly decreases at lower
Chapter - 4
79
temperatures. For these reasons, any detoxification method that relies only on chemical
reactivity would not work for many toxic agents and would not be effective at low
temperatures. Reactive nanoparticles do not have this drawback because the surface
adsorption sites remain active even at very low temperatures. In fact, the physisorption of
the potential toxic agents is enhanced at low temperatures. In this way, the toxins are
trapped and eventually undergo “destructive adsorption”. It has been shown that
nanocrystalline metal oxides are particularly effective decontaminants for several classes
of environmentally problematic compounds at elevated temperatures; enabling complete
destruction of these compounds at considerably lower temperatures than that required for
incineration [6].
One of the most common metal oxides that have been synthesized in a range of
nanostructure morphologies is magnesium oxide (MgO) [7]. The MgO is considered as a
model system for solid state and surface studies because of its simple structure and ionic
bonding. Magnesium oxide is an important material for various applications including
catalysis, waste remediation, additives in refractory and paint products [8, 9]. It serve as
an effective chemisorbent for chlorocarbons, organophosphorus compounds and acidic
gases like SO2 and HCl [10-12]. MgO also acts as an anti-bacterial agent against
commonly found bacteria spores and viruses [13]. The other important environmental
remediation aspect of MgO includes its potential to scavenge fluoride from drinking
water. It has been established that the catalytic activity of MgO is due to a small number
of defect sites (steps, kinks, corners, etc.) with surface ions, particularly oxygen having
low coordination numbers. MgO is particularly interesting in nanoparticle form. This is
because higher surface area and increased adsorption capacities for different
Chapter - 4
80
contaminants are possible when the MgO crystallite size is in the nanometer scale.
Smaller the crystallite size, better will the adsorption efficiency. It has been possible to
prepare MgO with very high surface area and with very small crystallite sizes [14]. The
high surface areas and the intrinsically high surface reactivity allow these materials to be
especially effective as adsorbents [15].
Nano-sized alkaline earth metal oxides, in particular magnesium oxide (MgO), is
a very promising material for applications as adsorbent due to its destructive sorbent,
high surface reactivity and adsorption capacity compared to their commercial analogues
and the simplicity of its production from abundant natural minerals [16-18]. Destructive
adsorbents are molecules that adsorb another chemical onto its surface where a reaction
occurs that degrades the original chemical to compound(s) with lower toxicity. The MgO
nanoparticles have been shown to be capable of exhibiting such destructive adsorption
[19-21].
The present chapter on the synthesis, characterization and adsorption behavior of
MgO nanoparticles has been divided into three sub-chapters.
In chapter 4.1, the synthesis of MgO nanoparticles by precipitation method has
been discussed. It further deals with the characterization of synthesized product by
thermal analysis (TGA), X-ray diffraction measurements, infrared spectroscopy and
morphological studies by field emission scanning electron microscopy (FESEM).
In chapter 4.2, the adsorption of textile dyes like Levafix fast red CA (LFR) and
Indanthren blue BC (IB) on MgO nanoparticles has been discussed. The adsorption
studies were carried out by batch experiments. The parameters like effect of pH,
Chapter - 4
81
adsorption kinetics and adsorption thermodynamics have been discussed for both the
dyes.
In chapter 4.3, the adsorption of Acid Red 112 (AR 112) on MgO nanoparticles
has been discussed. The parameters like effect of pH, contact time and temperature have
been discussed. The kinetics and thermodynamics for the adsorption of AR 112 were
studied by using most popular models.
Chapter - 4
82
Chapter 4.1
Synthesis and characterization of magnesium oxide
nanoparticles
In the present work, magnesium oxide (MgO) nanoparticles were synthesized by
precipitation method by using polyvinyl pyrrolidone (PVP) as capping agent. The as
prepared product was characterized by thermal analysis. Later, the calcined product was
characterized by FTIR, N2 adsorption studies, X-ray diffraction and FESEM analyses.
4.1.1 Thermogravimetric analysis (TGA)
Figure-4.1: TGA curve of the as prepared Mg(OH)2.
In order to obtain MgO nanoparticles from its hydroxide precursor, it is essential
to know the temperature at which Mg(OH)2 tranforms into MgO. The thermal behavior of
the precursor Mg(OH)2 prepared by precipitation method was studied using TGA. The
Chapter - 4
83
precursor was subjected to heating in nitrogen atmosphere in alumina crucible at the rate
of 10° C/min. It can be seen from Figure 4.1 that, the precursor loses its weight in a
single step. The steep fall in the percentage weight loss is in the range of 300 - 350 °C.
This weight loss step may be attributed to the loss of water from Mg(OH)2 lattice
resulting in the formation of MgO. Thus the temperature above 350 ºC is considered as
an optimum calcination temperature for the formation of MgO nanoparticles from the
precursor.
4.1.2 X-ray diffraction studies
Figure-4.2: The XRD patterns of MgO prepared (a) with PVP and (b) without PVP.
The powder X-ray diffraction patterns of the products synthesized in the presence
and absence of polyvinyl pyrrolidone (PVP) are shown in Figure 4.2. All the diffraction
peaks matched well with the face centered cubic structure of periclase MgO (JCPDS No.
87-0653). The major peaks at 2θ values of 37.1º, 43.0º, 62.4º, 74.8º and 78.6º can be
Chapter - 4
84
indexed to the lattice planes of (111), (200), (220), (311) and (222) respectively.
Furthermore, no characteristic peaks from other crystalline impurities were detected by
XRD, suggesting that the product was of pure magnesium oxide.
The intense peaks in the XRD pattern of MgO prepared using PVP (Figure 4.2a)
revealed that the obtained product was more crystalline than that obtained in the absence
of it. Further, the XRD peaks obtained for the product prepared in the presence of PVP
were slightly broader than those of the product prepared in the absence of PVP. This is
clearly related to the crystallite size of MgO. The crystallite size as calculated from
Scherrer’s equation [22] for MgO obtained in the presence and absence of PVP were
found to be 27 nm and 53 nm respectively. The observed difference in the crystallite size
may be attributed to the capping behavior of PVP which prevents uncontrolled crystal
growth during MgO precursor formation. However, the components of PVP were
removed during calcination process leaving behind pure crystalline MgO product.
4.1.3 FESEM and Surface area analysis
Figure-4.3: The FESEM micrographs of MgO prepared (a) with PVP and (b) without
PVP.
(a) (b)
Chapter - 4
85
The surface and textural morphology of the product was studied by FESEM
analysis. As shown in Figure 4.3a, the average particle size of polyhedral MgO obtained
in presence of PVP is found to be 27 nm, while for the MgO obtained in absence of PVP
it is 100 nm (Figure 4.3b). Also the particles seem to be more agglomerated in the latter
case, which is a clear evidence for the capping and size controlling property of PVP.
Similar results were observed by Clifford Y. Tai et al. who synthesized MgO using same
precursors in a spinning disc reactor [23].
Figure-4.4: The N2 adsorption–desorption isotherm for nano magnesia prepared using
PVP.
The N2 adsorption–desorption isotherm for the prepared MgO sample is shown in
Figure 4.4. The nitrogen adsorption isotherm for the prepared sample could be classified
as type II with H3 hysteresis loop according to Brunauer–Deming–Deming–Teller
(BDDT) classification. The type H3 hysteresis loop, which does not clearly show any
adsorption plateau at relative pressures close to unity, is usually related to the existence
Chapter - 4
86
of slit-shaped pores in materials indicative of a pore size distribution extending to the
macropore range [24]. From BET analysis, the specific surface area of MgO sample was
found to be 22.1 m2
g-1
with corresponding total pore volume of 0.36 cm3
g-1
(p/p0=0.990).
4.1.4 Fourier Transform Infrared Spectroscopy (FTIR)
Figure-4.5: The FTIR spectrum of MgO prepared using 1 M MgCl2, cacined at 500 ºC for
2 hrs.
The FTIR spectrum of magnesium oxide nanoparticles was recorded by making
pellet with KBr. The spectrum obtained is as shown in Figure 4.5. The broad bands
centered in the range 410–503 cm-1
and 1461 cm-1
corresponding to characteristic Mg-O-
Mg deformation and stretching vibrations, respectively [25]. The broad absorption peak
at 3448 cm-1
may be attributed to the O-H stretching vibration of surface adsorbed water
molecules while, the peak at 1643 cm-1
corresponds to the bending vibration of these
water molecules [26, 27].
Chapter - 4
87
Chapter 4.2
Adsorption of Levafix fast red CA and Indanthren
blue BC from aqueous solution on MgO
nanoparticles
The dyes used in textile industries usually have complex structure and hence are resistant
to most of the degradative environmental conditions. Therefore the conventional
wastewater treatment methods remain ineffective [28]. At present, adsorption and
biological treatment are two major industrially viable techniques available for treating
dye wastewater. The biological process is difficult to start up and control [29] and the
intermediate products (aromatic amines) formed during anaerobic reduction of azo dyes
are known to be potential carcinogens [30]. Adsorption on the other hand, should be a
favorable procedure due to economic feasibility, simplicity of design, recycling of
adsorbent and nonexistence of harmful residues. Generally, the commercial activated
carbon is indeed effective for colour removal, but the high-cost of activated carbon has
restricted its widespread use. This compelled many researchers to search for other low-
cost and effective adsorbents for practical applications [31].
In the present work, magnesium oxide nanoparticles have been synthesized by
precipitation method using PVP as capping agent. Poly(N-vinyl-2-pyrrolidone) or PVP is
an organic polymer which is added during the wet-chemical synthesis for capping the
surface of the particles. The capping behavior of PVP may be clearly understood by
considering its structural details. PVP is structurally amphiphilic where the pyrrolidone
part (hydrophillic) acts as the head group, while the polyvinyl part (hydrophobic) acts as
Chapter - 4
88
the tail group. According to G. Ghosh et al., the role of the PVP is two folds: (a) either it
controls the growth of the particles by forming passivation layers around the particle core
via coordination bond formation between the nitrogen atom of the PVP and M2+
ion,
and/or (b) it prevents agglomeration by steric effect due to the repulsive force acting
among the polyvinyl groups (tail part) [32]. Therefore, the PVP encapsulation results in
small sized crystals with huge surface area. Hence the MgO prepared using PVP was
found to possess high adsorption capacity and was used for further studies.
The synthesized nano MgO was employed as adsorbent for the removal of two
commercial textile dyes procured from DyStar, India. Levafix fast red CA (LFR) and
Indanthren blue BC (IB) belonging to reactive and vat dye families respectively were
chosen as model organic water pollutant. The adsorption performance of the synthesized
MgO towards these dyes was tested. Batch adsorption experiments were carried out in an
incubator shaker at constant temperature. The effect of operating parameters such as
contact time, temperature and pH were studied and optimized. Also the kinetics and
thermodynamics of adsorption has been studied.
4.2.1 Effect of contact time
The effect of contact time was studied by taking 200 mL of 50 ppm dye solution at pH
7.0 and with 0.5 g L-1
of MgO. An aliquot (5 mL) of dye sample was withdrawn from the
flask at regular time intervals, centrifuged and the concentration of dye in the supernatant
solution was subjected for analysis. The variation of adsorption capacity of nano
magnesia (prepared with and without PVP) towards LFR and IB with time is depicted in
Figure 4.6. The results revealed that the adsorption capacity of MgO obtained without
PVP (towards both LFR and IB) is much lower compared to that of MgO prepared with
Chapter - 4
89
PVP. This may be attributed to the smaller particle size and larger surface area of MgO
particles obtained in the presence of PVP [33, 34]. The capping property exhibited by
PVP will result in small sized particles of MgO, as it evidenced by XRD and FESEM
results. The higher reactivity of smaller sized MgO particles is not only because of the
large specific surface area but also due to the high concentration of low-coordinated sites
and structural defects on their surface [35]. This clearly explains the high adsorption
capacity of MgO obtained in the presence of PVP. Several earlier workers have also
shown the use of PVP as capping agent for the synthesis of nanoparticles [36-39].
Figure-4.6: The Effect of contact time on the adsorption capacity of MgO prepared with
and without PVP towards (a) Levafix fast red CA (LFR) and (b) Indanthren blue BC (IB).
Figure 4.7 shows the variation of percentage dye removal by nano MgO with time
towards LFR and IB. The rapid initial adsorption is due to the high concentration gradient
of dye molecules between the surface active sites on the adsorbent and bulk solution. The
adsorption capacity reaches a limiting value after sometime where no more dye
adsorption is possible. This might be due to the saturation of surface active sites on the
adsorbent. It can be clearly seen from the figure that majority of adsorption from aqueous
solutions was completed within 45 min for LFR and 100 min for IB. The rapid and
(a) (b)
Chapter - 4
90
greater adsorption of LFR compared to IB may be attributed to the presence of more
number of negatively charged groups on LFR.
Figure-4.7: Effect of contact time on the percentage dye removal capacity of MgO
nanoparticles toward LFR and IB.
4.2.2 Effect of pH
Figure-4.8: Influence of pH on the adsorption capacity of MgO nanoparticles towards
LFR and IB dyes.
Chapter - 4
91
The solution pH is an important parameter that affects adsorption of dye
molecules. The effect of initial solution pH on the adsorption of LFR and IB onto nano
magnesia was assessed in the pH range of 3.0 - 12.0. The initial concentration of dye and
adsorbent dosage were set at 100 mg L-1
and 0.05 g respectively, for all the batch tests.
The variation of equilibrium adsorption capacity of nano MgO at different pH values for
LFR and IB is shown in Figure 4.8. The removal efficiency for LFR was found to be
increased with increase in solution pH upto 6.0, beyond which it decreased. However,
solution pH was found to have no influence on the adsorption of IB.
The effect of solution pH on the adsorption of LFR could be explained by
considering the surface charge of MgO and the dye molecule. At lower pH, the adsorbent
surface (MOH) will be completely covered by H+ ions (MOH2
+). At higher pH,
hydroxide ions react with the hydrous oxide to produce deprotonated oxide (MO-)
according to the following reactions [40].
2MOHHMOH (4.1)
OHMOOHMOH 2 (4.2)
OHMOOHMOH 22 22 (4.3)
Understanding the sorption of dye from aqueous solution on the oxides requires
knowledge of chemistry of the oxide/water interface. The increase in the removal
efficiency of LFR to its maximum value at pH 6.0 might be due to the electrostatic
attraction between the dye molecules (negatively charged) and MgO surface (positively
charged: pHzpc 12.4). However, an increase in the solution pH beyond 9.0 resulted in a
Chapter - 4
92
sharp reduction in the removal efficiency. This might be explained by the formation of
OH− and subsequent competition with the LFR molecules for adsorption sites.
Indanthren Blue BC (IB) is insoluble but highly dispersible in water. Thus it
remains non-ionic in aqueous solution. Although the solution pH influences the surface
charge of adsorbent, the surface properties of IB remain unchanged. So the change in
solution pH did not influence the adsorption of IB.
4.2.3 Adsorption isotherms
Adsorption isotherms are the equilibrium relations between the concentration of
adsorbate on the solid phase and its concentration in the liquid phase. The adsorption
process is normally described by Langmuir and Freundlich adsorption isotherms. The
experimental data fitted well with Langmuir isotherm for the adsorption of both LFR and
IB (Figure 4.9 and 4.10), suggesting the monolayer coverage of dyes on the adsorbent
surface. The Freundlich model showed very poor fit for the experimental data in case of
both the dyes and hence the corresponding isotherm parameters are not shown here. This
result indicated the homogeneous nature of sample surface, i.e., each dye molecule
adsorption has equal adsorption activation energy and it clearly demonstrated the
formation of monolayer coverage of dye molecules on the surface of adsorbent.
Langmuir adsorption isotherm parameters were calculated for the adsorption of
two dyes and are listed in Table 4.1 and 4.2. The correlation co-efficient (R2) values were
found very close to 0.99 for both the dyes, which clearly indicated the suitability of
Langmuir model to describe the adsorption process.
Chapter - 4
93
Figure-4.9: Langmuir adsorption isotherm plots at different temperatures for the
adsorption of LFR onto magnesium oxide nanoparticles.
Table-4.1: The parameters of Langmuir adsorption model for the adsorption of LFR onto
nano magnesia at different temperatures.
T (ºC) qmax (mg g-1
) KL (L mg-1
) R2 RL
25 92.16 0.220 0.994 0.025-0.154
35 55.83 0.230 0.996 0.024-0.148
45 39.34 0.142 0.996 0.038-0.219
55 28.88 0.095 0.995 0.057-0.174
Chapter - 4
94
Figure-4.10: Langmuir adsorption isotherm plots at different temperatures for the
adsorption of IB onto magnesium oxide nanoparticles.
Table-4.2: The parameters of Langmuir model for the adsorption of IB onto nano
magnesia at different temperatures.
T (ºC) qmax (mg g-1
) KL (L mg-1
) R2 RL
25 86.50 0.397 0.997 0.014-0.092
35 44.07 0.154 0.995 0.031-0.205
45 22.60 0.063 0.983 0.083-0.388
55 15.29 0.048 0.937 0.106-0.454
Chapter - 4
95
Table-4.3: Comparison of adsorption capacities of different adsorbents with MgO
nanoparticles toward reactive and vat dyes sorption.
Adsorbent
Dye qmax
(mg g-1
)
Reference
Amino functionalized
attapulgite
Reactive red 3BS 34.235
[44]
Dehydrated beet pulp Chemazol reactive red 195 58.0 [45]
Activated carbon Reactive red 24 64.94 [46]
Chitosan coated magnetic
nanoparticles
Reactive Yellow 145 47.62
[47]
Zn2SnO4 Reactive red 141 61.0 [48]
Polyurethane foam Reactive blue 21 8.31 [49]
MgO nanoparticles Lavfix Fast Rad CA 92.16 Present work
Smectite-rich clayey rock Vat blue 4 17.85 [50]
Cationic polymer/betonite
complex
Vat Scarlet R 16.43
[51]
Biosludge
Vat black 25
Vat yellow 1
40.0
49.3
[52]
MgO nanoparticles Vat blue 6 86.50 Present work
Adsorption capacities of different adsorbents towards various reactive and vat
dyes were compared with the MgO nanoparticles (Table 4.3). It is evident from the table
that the adsorption capacity of MgO nanoparticles is relatively higher than the most of the
other adsorbents reported earlier. The strong adsorption affinity of nano MgO towards the
dyes is due to its high specific surface area coupled with unusual surface morphologies.
Further it possesses more reactive surfaces due to the presence of high concentrations of
Chapter - 4
96
edge and corner sites, and other defects. This allows nanoscale MgO to exhibit unique
surface chemistry, as demonstrated by its high adsorption capacity [41].
For the Langmuir-type adsorption process, the influence of the isotherm shape on
whether adsorption is favorable or not can be classified by a dimensionless separation
factor RL, which is considered as a more reliable indicator of the adsorption capacity.
This constant is evaluated from the following equation:
oL
LCK
R
1
1 (4.4)
where, Co : The initial concentration of dye and
KL : The Langmuir adsorption constant (L/mg).
The values of RL indicate the shapes of isotherms to be either unfavorable (RL>1),
linear (RL=1), favorable (0<RL<1) or irreversible (RL=0) [42]. Favorable adsorption is
reported when the RL values are between 0 and 1 [43]. In the present work, the RL values
were in the range of 0.024 – 0.219 and 0.014 - 0.388 for LFR and IB respectively (Table
4.2.1 and 4.2.2), which shows that the adsorption of both the dyes is favorable.
4.2.4 Adsorption thermodynamics
Adsorption studies were carried out at four different temperatures (298, 308, 318 and 328
K) separately for each dye using 25 mg of nano MgO and dye concentration of 50 mg L-1
.
The experimental results showed that the adsorption capacity decreased with increase in
the solution temperature for both the dyes. This indicated that the adsorption of LFR and
IB on MgO is exothermic in nature. The decrease in the rate of adsorption with increase
Chapter - 4
97
in temperature is attributed to the tendency of dye molecules to escape from the solid
phase to bulk phase.
The values of thermodynamic parameters such as change in enthalpy (ΔH°),
change in entropy (ΔS°) and change in free energy (ΔG°) were determined by using the
van’t Hoff’s equation:
R
S
RTK
o
c
oH- ln (4.5)
e
Aec
C
CK (4.6)
where, Kc : The equilibrium constant,
CAe : The solid phase concentration at equilibrium (mg L-1
),
T : The temperature in Kelvin and
R : The gas constant.
The plot lnKc against 1/T gives a straight line with slope and intercept equal to -ΔH°/R
and ΔS°/R, respectively.
The values of ΔH° and ΔS° were calculated from Figure 4.11 and reported in
Table 4.4. The negative values of ΔH° indicated the exothermic nature of adsorption
process for both the dyes. The negative value of entropy change (ΔS°) corresponds to a
decrease in degrees of freedom of the adsorbed species. This clearly suggests the
decrease in concentration of adsorbate in solid–solution interface, thereby indicating an
increase in adsorbate concentration on the solid phase.
Chapter - 4
98
Figure-4.11: Van’t Hoff’s plot for the adsorption of LFR and IB onto MgO nanoparticles.
Energy of activation (Ea)
It is possible to calculate the activation energy for the adsorption of dyes carried out at
different temperatures. The values of activation energy (Ea) and sticking probability (S*)
were calculated using a modified Arrhenius equation related to surface coverage (θ) [53]:
RTEaeS/* )1(
(4.7)
The S* is a function of the adsorbate/adsorbent system under investigation, its value lies
in the range 0 < S* < 1 and is dependent on the temperature of the system. The value of θ
can be calculated from the following equation:
0
1C
Ce (4.8)
Chapter - 4
99
The value of Ea was calculated from the slope of the plot of ln(1−θ) versus 1/T [54]. The
values of Ea for the adsorption of LFR and IB onto MgO were found to be 77.72 kJ mol-1
and 57.37 kJ mol-1
respectively.
The Gibbs free energy of adsorption (ΔG°) was calculated from the following
relation and the values are given in Table 4.4.
ooo STHG (4.9)
The negative values of ΔG° indicated that the adsorption of both the dyes on nano
magnesia is a spontaneous process, whereby no energy input from outside of the system
is required. However, the values of ΔG° decreased with increasing temperature,
suggesting that the adsorption became less favorable at higher temperatures. As the
temperature increases, the mobility of dye molecules increases, causing the molecules to
escape from the solid phase to the liquid phase [55]. Therefore, the amount of dye that
can be adsorbed will decrease with increase in temperature. The increased mobility of
dye molecules at elevated temperature may also be reflected in the values of Kc (Table
4.2.4). As the temperature increased, the values of Kc decreased, indicating lower affinity
of the MgO nanoparticles towards the dye at higher temperatures.
Chapter - 4
100
Table-4.4: Thermodynamic parameters for the adsorption of LFR and IB (50 mg L-1
) on
MgO nanoparticles at pH 6.0.
Dye T (°C) ΔG°
(kJ mol−1
)
ΔS°
(J K−1
mol−1
)
ΔH°
(kJ mol−1
)
Kc Ea S*
LFR 25 -8.871 -0.276 -91.238 43.99 77.72 8.3 × 10-13
35 -6.107 7.512
45 -3.343 3.374
55 -0.579 1.427
IB 25 -5.820 -0.249 -80.171 13.74 57.37 9.6 × 10-10
35 -3.325 2.341
45 -0.830 1.228
55 1.665 0.665
4.2.5 Adsorption kinetics
Kinetics is an important tool for adsorption studies, because it can predict the rate at
which a pollutant is removed from aqueous solutions and it provides a valuable data for
understanding the mechanism of adsorption reactions. The adsorption kinetic experiments
were carried out in batch mode by taking 200 mL of 50 ppm dye solution and 0.5 g L-1
of
MgO at pH 7.0. An aliquot (5 mL) of dye sample was withdrawn from the flask at regular
time intervals, centrifuged and the concentration of dye in the supernatant solution was
analysed.
Chapter - 4
101
Figure-4.12: Pseudo-first-order plots for the adsorption of LFR and IB onto MgO
nanoparticles.
Figure-4.13: Pseudo-second-order plots for the adsorption of LFR and IB onto MgO
nanoparticles.
Chapter - 4
102
Table-4.5: The pseudo-first-order and pseudo-second-order kinetic parameters for the
adsorption of LFR and IB on MgO nanoparticles (Dye: 50 ppm; pH: 7.0; MgO: 0.5 gL-1
).
In order to investigate the mechanism of dye adsorption onto MgO nanoparticles,
pseudo-first-order and pseudo-second-order models were adopted. The pseudo-first-order
and pseudo-second-order plots for the adsorption of LFR and IB are shown in Figure 4.12
and 4.13. The rate constants, calculated equilibrium adsorption capacity qe(cal) and
experimental equilibrium adsorption capacity qe(exp) for the adsorption of LFR and IB
obtained using the pseudo-first-order and pseudo-second-order models are listed in Table
4.5. Although the correlation coefficients of both pseudo-first-order and pseudo-second-
order kinetic models were comparable, the qe values calculated from pseudo-first-order
kinetic model were too small compared to the experimental values for both the dyes.
However, the calculated qe from pseudo-second-order kinetic model were close to the
experimental values in both the cases. Therefore, it can be concluded that pseudo-second-
order equation is better in describing the adsorption kinetics of both the dyes on MgO
nanoparticles. Several earlier workers have also shown that pseudo-second-order model
fits well in describing the adsorption process [56-58]. The pseudo-second-order model
Dye
qe(exp)
(mg g-1
)
Pseudo-first-order model Pseudo-second-order model
qe(cal)
(mg g-1
)
k1
(min-1
)
R2 qe(cal)
(mg g-1
)
k2
(g mg-1
min-1
)
R2
LFR 96.84 63.79 56.5×10-3
0.993 103.84 1.45×10-3
0.997
IB 93.22 52.35 21.97×10-3
0.997 85.62 1.72×10-3
0.995
Chapter - 4
103
suggests that the adsorption depends on the adsorbate as well as the adsorbent and
involves chemisorption process in addition to physisorption [59].
The kinetic results were also analyzed by intra-particle diffusion model to gain
further insight into the adsorption behavior of dyes on MgO nanoparticles. The plots of qt
versus t0.5
for LFR and IB are shown in Figure 4.14. The values of kid and C were
calculated from the slope and intercept of plots of qt versus t0.5
and are summarized in
Table 4.6. Although, the plots for both the dyes were linear they do not pass through the
origin. Therefore it may be concluded that the boundary layer (film) diffusion is the rate
controlling step in the process of dye adsorption in the present case.
Figure-4.14: Intra-particle diffusion model for the adsorption of LFR and IB onto MgO
nanoparticles.
Chapter - 4
104
Table-4.6: The Intra-particle diffusion model parameters for the adsorption of LFR and
IB onto MgO nanoparticles (Dye: 50 ppm; pH: 7.0; MgO: 0.5 g L-1
).
Dye kid (mg g-1
min-½
) C (mg g-1
) R2
LFR 9.628 31.25 0.998
IB 6.071 32.52 0.999
4.2.6 Regeneration of adsorbent
The recyclability of adsorbent is one of the crucial factors for its field applications. The
nano MgO could be recovered by combustion at 500 °C and reused. Thus recovered MgO
could remove similar amounts of dyes even after the second and third regenerations.
Hongmin Chen and Junhui He have also employed combustion for the regeneration of
Manganese Oxide nanomaterial [60]. Thus in the present study, the dye removal ability
of MgO was found to be retained even after three adsorption-combustion cycles, which
will make it cost-effective.
Chapter - 4
105
Chapter 4.3
Adsorption of Acid Red 112 from aqueous solution
on MgO nanoparticles
Azo dyes constitute a largest group of compounds responsible for water pollution
worldwide. These dyes are invariably released as wastewater from textile, printing, paper,
cosmetic, food and pharmaceutical industries. The dyes used on commercial scale are
designed to resist environmental influences like heat, light, moisture, etc. Also the dye
containing wastewater has very low BOD/COD ratio since most of these dyes are toxic
and non-biodegradable. As a result they are hardly removed in the conventional methods
of treatment [61, 62]. If released without treatment, these dyes are not only harmful
themselves but can also yield dangerous byproducts through oxidation, reduction,
hydrolysis or other chemical reactions [63]. Dyes also affect the photosynthetic activity
of aquatic plants by reducing the penetration of sunlight and exchange of gases [64, 65].
Ponceau S (3-Hydroxy-4-(2-sulpho-4-[4-sulfophenylazo]phenylazo)-2,7-
naphthalene disulphonic acid sodium salt) also called Acid Red 112 (AR 112) is an acid
dye which is widely used in textile, leather and paper industries. This azo dye is anionic,
which is used in the textile industry for dyeing both natural and synthetic fibres. To a less
extent it is also used in a variety of applications such as in paints, inks and plastics. The
AR 112 is also used to prepare a stain for rapid reversible detection of protein bands
on nitrocellulose or PVDF membranes (Western blotting) [66]. However, its
biotransformation products have toxic effects against aquatic organisms and suspicious of
being carcinogenic for humans [67]. Very few researchers have worked on the removal of
Chapter - 4
106
AR 112. Meena et al. showed nearly complete decolorization of AR 112 on methylene
blue immobilized resin Dowex-11 photocatalyst [67]. Some researchers have reported the
degradation of AR 112 by electrochemical advanced Fenton oxidation [68], while the
photodegradation of this dye using nanosized niobium pentoxide has been reported
recently [69]. Besides, few biological methods have also been reported for the
decolorization of AR 112 azo dye in aqueous solutions [70, 71]. The microbial
degradation is inefficient due to low and incomplete decolorization/degradation [68].
During the last decade much efforts have been devoted towards the synthesis and
characterization of nanostructured materials because of their unique physical and
chemical properties. The development of nanotechnology in various fields has widened
the application in wastewater treatment. Compared to the micron-sized conventional
adsorbents, nano-sized carriers possess quite good performance due to the high specific
surface area with little internal diffusion resistance [72]. Materials like nanoscale calcium
amino diphosphonates [73], Starch/polyaniline nanocomposite [72], carbon nanotubes
[74], surface modified zinc oxide nanoparticle [75], cobalt oxide nanopowders [76],
mesoporous γ-alumina [77], calcium alginate/multi-walled carbon nanotube biocomposite
[78], barium phosphate nano-flake [79], graphene–Fe3O4 nanocomposite [80], cadmium
hydroxide nanowires [81] have been shown to posses high adsorption capacity for dyes.
Due to their high surface area, large numbers of highly reactive edges, corner
defect sites, unusual lattice planes and high surface to volume ratio, nanocrystalline metal
oxides have gained considerable interest among researchers as potential adsorbents for
decontamination of wastewater [82]. In the present chapter removal of AR 112 from
aqueous solution by magnesium oxide (MgO) nanoparticles has been discussed. The
Chapter - 4
107
influences of parameters like pH, contact time and temperature on the adsorption capacity
of MgO have been investigated. The adsorption isotherm studies have been carried out
using Langmuir and Freundlich models. The adsorption kinetics and thermodynamic
modeling has been also presented.
4.3.1 Effect of contact time
The variation of adsorption capacity of nano magnesia towards AR 112 with contact time
is depicted in Figure 4.15. It is evident from the figure that the adsorption was rapid
initially and later the dye removal percentage became almost constant. The initial rapid
adsorption is due to the presence of large number of active sites on the surface of MgO
nanoparticles and the high concentration gradient that exists between the MgO surface
and bulk solution. The adsorption equilibrium was attained within 120 min of contact
time with nearly 100% dye removal from the solution.
Figure-4.15: Effect of contact time on the adsorption capacity of MgO nanoparticles
towards AR 112.
Chapter - 4
108
4.3.2 Effect of pH
Figure-4.16: Influence of pH on the adsorption capacity of MgO nanoparticles towards
AR 112.
The effect of pH on the adsorption performance was studied by conducting the
batch experiments for 100 ppm dye solutions at different initial pH (3.0 – 11.0). The
experiment has been carried out with 1.0 g L-1
of nano MgO at 298 K. Figure 4.16 shows
the variation of adsorption capacity of nano magnesia at different pH values. It can be
seen from the graph that, the equilibrium adsorption capacity increased with increase in
solution pH, reached maximum at pH 7.0 and then decreased. The lower qe values in
acidic pH may be due to slight dissolution of MgO, while higher qe values observed at
neutral and slightly basic pH may be explained by considering the surface charge of MgO
and dye molecules. The charge of the dye molecules is highly negative due to the
presence of four SO3- groups on each molecule. Electrostatic forces of attraction between
the positively charged MgO (pHZPC = 12.4) and negatively charged AR 112 is
responsible for the high adsorption capacity observed below pH 9.0. However, a drastic
Chapter - 4
109
decrease in the dye removal efficiency is observed in highly basic conditions (above pH
9.0) which might be due to the formation of OH- and subsequent competition with the
dye molecules for adsorption sites [83].
4.3.3 Adsorption isotherms
Adsorption isotherms are important tools to describe the adsorption mechanism. The
equilibrium studies are useful to obtain the adsorption capacity of magnesium oxide
towards dye removal. An adsorption isotherm is characterized by certain constants that
express the surface properties and the affinity of the adsorbent towards dye molecules.
The equilibrium data for the adsorption of AR 112 on MgO nanoparticles were fitted into
two isotherm models at four different operating temperatures. In the present study,
Langmuir and Freundlich equilibrium models were used to investigate the mechanism of
AR 112 adsorption onto MgO nanoparticles.
Figure-4.17: Langmuir isotherm plots at different temperatures for the adsorption of AR
112 onto MgO nanoparticles.
Chapter - 4
110
Figure-4.18: Freundlich isotherm plots at different temperatures for the adsorption of AR
112 onto MgO nanoparticles.
The plots obtained for Langmuir and Freundlich isotherm models at different
temperatures are shown in Figure 4.17 and 4.18 respectively. The parameters calculated
for the two isotherm models are summarized in Table 4.7. The correlation coefficient
(R2) values obtained from Langmuir model were above 0.99 at all the temperatures
studied indicating a good agreement of the data. Therefore it may be concluded that
Langmuir model is suitable to describe the adsorption of AR 112 on nano magnesia,
which emphasizes the formation of monolayer coverage of dye molecules on the surface
of adsorbent. The calculated maximum adsorption capacities (qmax) of nano MgO were
found to be 93.02, 96.9, 102.9 and 125.3 mg g-1
at 298, 308, 318 and 328 K respectively.
Chapter - 4
111
Table-4.7: The parameters of Langmuir and Freundlich models for the adsorption of AR
112 onto MgO nanoparticles at different temperatures.
T (ºC) Langmuir Freundlich
qmax (mg g-1
) KL (L mg-1
) R2 RL Kf n R
2
25 93.02 0.416 0.993 0.029-0.074 53.70 7.479 0.969
35 96.90 0.438 0.995 0.022-0.071 58.99 8.733 0.981
45 102.9 0.896 0.998 0.012-0.027 76.82 13.82 0.971
55 125.3 0.504 0.996 0.021-0.038 77.13 7.99 0.976
Table-4.8: Comparison of adsorption capacities of different adsorbents with MgO
nanoparticles towards acid dye sorption.
Adsorbent
Dye qmax (mg g-1
)
Reference
Peat Acid Black 25 12.7 [84]
Modified bentonite
Acid Red 18
Acid Red23
69.8
75.4
[85]
Sawdust Acid Black 25 24.4 [86]
Activated bleaching earth Acid Orange 51 8.45 [87]
Sargassum glaucescens Acid Black 1 27.2 [88]
Modified feldspar
Acid Red 14
Acid Black 1
3.98
6.37
[89]
Chitosan-conjugated Fe3O4
nanoparticles
Acid Green 25 73.53
[90]
Activated carbon obtained
from pericarp of pecan
Acid Black 25 48.0
[91]
MgO nanoparticles Acid Red 112 93.02 Present work
Chapter - 4
112
Table 4.8 gives comparison of the adsorption capacities of some of the other
adsorbents with nano MgO towards acid dyes. The table reveals that the adsorption
capacity of MgO used in the present study is higher than most of the other adsorbents.
The strong adsorption affinity of nano MgO towards the dye is probably due to its high
specific surface area coupled with electrostatic forces of attraction involved.
The RL vaues within the range 0 < RL < 1 indicate a favorable adsorption. In the
present study, RL values obtained were in the range of 0.012 – 0.074, indicating favorable
adsorption of dye on nano magnesia.
4.3.4 Adsorption thermodynamics
Figure-4.19: (a) Effect of temperature on the adsorption capacity of MgO nanoparticles;
(b) van’t Hoff’s plot for the adsorption of AR 112 onto MgO nanoparticles.
The effect of temperature on the adsorption of AR 112 on MgO nanoparticles was
studied by performing the adsorption experiments at different temperatures (298, 308,
318 and 328 K). The results revealed that the adsorption capacity increased with increase
in temperature from 298 to 328 K (Figure 4.19a), indicating the endothermic nature of
dye adsorption. This supports chemisorption of AR 112 where there is an increase in the
(a) (b)
Chapter - 4
113
number of molecules acquiring sufficient energy to undergo chemical reaction with the
adsorbent at higher temperatures [97].
Table-4.9: Thermodynamic parameters for the adsorption of AR 112 (50 mg L-1
) on MgO
nanoparticles at pH 7.0.
T (°C) ΔG°
(kJ mol−1
)
ΔS°
(J K−1
mol−1
)
ΔH°
(kJ mol−1
)
Kc Ea
(kJ mol−1
)
S*
25 -2.053 0.147 41.78 2.597 34.89 2.4 × 10-7
35 -3.524 3.467
45 -4.995 6.373
55 -6.466 11.56
Figure 4.19b shows the van’t Hoff’s plot for the adsorption of AR 112 onto MgO
nanoparticles and the values of thermodynamic parameters obtained from the plot are
given in Table 4.9. The positive values of ΔH° and ΔS° showed that the adsorption
process is endothermic with the increase in randomness of the system [92]. The negative
values of free energy (ΔG°) indicated that the process of adsorption was spontaneous.
Further, the values of ΔG° became more negative at higher temperatures suggesting that
the adsorption became more favorable at higher temperatures.
Energy of activation
The values of activation energy (Ea) and sticking probability (S*) were calculated using
modified Arrhenius equation. The value of Ea was calculated from the slope of the plot of
ln(1−θ) versus 1/T. In the present case, the value of Ea was found to be 34.89 kJ mol-1
.
Chapter - 4
114
4.3.5 Adsorption kinetics
The pseudo-first-order and pseudo-second-order plots for the adsorption of AR 112 on
nano MgO are shown in Figure 4.20a and 4.20b respectively. The values of rate constants
and calculated equilibrium adsorption capacity qe(cal) obtained for pseudo-first-order and
pseudo-second-order models and experimental equilibrium adsorption capacity qe(exp)
values are listed in Table 4.10.
Figure-4.20: (a) Pseudo-first-order and (b) pseudo-second-order plots for adsorption of
AR 112 onto MgO nanoparticles.
Figure-4.21: Intra-particle diffusion model for the adsorption of AR 112 onto MgO
nanoparticles.
(a) (b)
Chapter - 4
115
Table-4.10: The pseudo-first-order and pseudo-second-order kinetic parameters for the
adsorption of AR 112 onto MgO nanoparticles (Dye: 50 ppm; pH: 7.0; MgO: 1.0 g L-1
).
Although high correlation coefficient values were obtained for both pseudo-first-
order and pseudo-second-order kinetic models, the qe value from pseudo-first-order is too
small compared to the experimental value. However, the qe value calculated from
pseudo-second-order kinetic model is close to the experimental value. Therefore, it can
be concluded that pseudo-second-order equation is better in describing the adsorption
kinetics of AR 112 on MgO nanoparticles. The pseudo-second-order model suggests that
the adsorption depends on the adsorbate as well as the adsorbent and involves
chemisorption.
The intra-particle diffusion model proposed by Weber and Morries was employed
to analyze the kinetic results. Figure 4.21 shows the plot of qt versus t0.5
for the
adsorption of AR 112. The plot is linear with high correlation coefficient (R2 = 0.999)
and it passes through the origin. This clearly indicated that during adsorption of AR 112,
intra-particle diffusion or pore diffusion is the rate limiting step. The value of kid was
found to be 5.54 mg g-1
min-0.5
.
qe(exp)
(mg g-1
)
Pseudo-first-order model Pseudo-second-order model
qe(cal)
(mg g-1
)
k1
(min-1
)
R2 qe(cal)
(mg g-1
)
k2
(g mg-1
min-1
)
R2
72.20 57.04 34.6×10-3
0.975 69.98 0.39×10-3
0.976
Chapter - 4
116
4.3.6 Recyclability of adsorbent
As discussed in chapter 4.2, nano MgO adsorbent could be recovered by combustion at
500 °C and reused. Thus recovered MgO could remove similar amounts of AR 112 even
after the second and third regenerations. Thus the dye removal ability of MgO was found
to be retained even after three adsorption-combustion cycles.
4.4 Conclusions
The magnesium oxide nanoparticles with an average crystallite size of 27 nm were
successfully synthesized by precipitation method. The suitability of MgO nanoparticles
as adsorbent was checked for the removal of Levafix Fast Red CA (LFR), Indanthren
Blue BC (IB) and Acid Red 112 (AR 112). The adsorption equilibrium was attained
within 45 min for LFR, 100 min for IB and 120 min for AR 112. The pseudo-second-
order kinetic model well fitted for all the dyes indicating the possible involvement of
chemisorption. The intra-particle model suggested that boundary layer (film) diffusion is
the rate controlling step for adsorption of LFR and IB while, pore diffusion was found to
be the rate controlling step for the adsorption of AR 112 on nano MgO. The experimental
data fitted with Langmuir adsorption isotherm suggesting the monolayer coverage of
dyes on the adsorbent surface. At 298 K, maximum adsorption of 92.16 mg g-1
, 86.50 mg
g-1
and 93.02 mg g-1
were achieved for LFR, IB and AR 112, respectively. The
adsorption of LFR and AR 112 were found to be maximum in the pH range 6.0-7.0 and
electrostatic forces of attraction were responsible for the high adsorption capacity of
MgO. However, the pH had little effect on the adsorption of IB. The adsorption was
found to be spontaneous and exothermic for LFR and IB while, the adsorption of AR 112
Chapter - 4
117
on nano MgO was found to be endothermic. The nano MgO is recyclable and hence may
serve as cost-effective adsorbent for dye removal from wastewater.
Chapter - 4
118
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