Adsorption Isotherm of Methylene Blue on KNbO3 Compound

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Analysis
Crystal structure and microstructure <strong>of</strong> the
samples were measured by using an X-ray
diffractometer (XRD, JDX-03530, JEOL, Japan),
and a scanning electron microscope (SEM, JEM-
2010, JEOL, Japan), respectively. Specific surface
area was measured by means <strong>of</strong> nitrogen
adsorption method (BET, Quantachrome Autosorb
Automated Gas Sorption).
Results and Discussion
Figure 1 shows XRD patterns <strong>of</strong> the starting
powder and the hydrothermal product. The starting
powder showed the XRD pattern corresponding
to the Nb2O5 phase (JCPDF # 37-1468). The
hydrothermal product showed the XRD pattern
corresponding to KNbO3 phase (JCPDF # 71-2171).
Intensity Intensity (a.u.) (a.u)
0 10 20 30 40 50 60 70
Figure 1. XRD patterns <strong>of</strong> the KNbO3 product and
starting Nb2O5 powder.
Figure 2 shows SEM images <strong>of</strong> the Nb2O5
powder and KNbO3 product. The Nb2O5 morphology
is a brick-shape particle (Figure 2 (a)). The KNbO3
product showed the brick-like morphology (Figure 2 (b)),
similar to that reported by Wang, et al. (3) The
specific surface area <strong>of</strong> the KNbO3 is 4.02 m 2 /g
which is ~ 3 times higher than that <strong>of</strong> the starting
Nb2O5 (Table 1).
Absorbent
2theta (degree)
KNbO3
Nb2O5
Table 1. <strong>Adsorption</strong> parameters and specific surface area.
SINTHAO, J. et al.
Figure 2. SEM images <strong>of</strong> the KNbO3 product and
starting Nb2O5 powder.
The equilibrium adsorption isotherms <strong>of</strong>
methylene blue onto the adsorbents were determined
by Langmuir and Freundlich isotherms. The Langmuir
equation assumes a finite number <strong>of</strong> coordination
sites on the surface <strong>of</strong> an adsorbent, whereas the
Freundlich equation is an empirical equation. (4) The
Langmuir isotherm is expressed as follow. (5)
q
e
qmK
LCe
=
1+
K C
L
e
(1)
where qe is the equilibrium adsorption capacity
(mg/g), Ce is the equilibrium concentration <strong>of</strong> the
methylene blue solution (mg/L), qm is the maximum
capacity <strong>of</strong> methylene blue (mg/g), and KL is the
Langmuir constant (L/mg).
The Freundlich isotherm is expressed as follow. (5)
q
e
= K C
(2)
F
1/
n
e
where KF is the Freundlich constant (mg/g (mg/L) n )
and 1/n is the heterogeneity factor.
Langmuir isotherm Freundlich isotherm
R 2 qm (mg/g) KL (L/mg) R 2 n KF (L/mg)
Nb2O5
KNbO3
BET (m 2 /g)
Nb2O5 0.993 0.4476 1.2384 0.935 5.6180 0.5798 1.33
KNbO3 0.914 1.4749 0.1799 0.912 1.6077 0.5428 4.02
3µm
3µm