Environmental Engineering Reference
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(B3),and2.88×10
−2
min
−1
(B4)forluorideadsorptionand3.99×10
−2
min
−1
(B1),
1.73 × 10
−2
min
−1
(B2), 1.50 × 10
−2
min
−1
(B3) and 2.16 × 10
−2
min
−1
(B4)forarsenite
adsorption. However, the validity of the first-order kinetics for arsenite is not good
because the equilibrium adsorption capacity,
q
e
, obtained from the plots deviated by
20.03-40.79%fromtheexperimentalvalue(Table
2
).
The second order kinetics (Ho and McKay
1999
) given by the equation
(
)
+
(
)
2
tq kq
/
=
1
/
1
/
qt
(4)
t
2
e
e
also yielded good linear plots of
t
/
q
t
vs
t
(
r
lies between + 0.92 and + 0.99 for fluoride
and is ~ +1.00 for arsenite). The second order rate coefficient, k
2
, has values of
6.73 × 10
−2
g mg
−1
min
−1
(B1), 1.75 × 10
−2
g mg
−1
min
-1
(B2), 2.06 × 10
−2
g mg
−1
min
−1
(B3) and 2.56 × 10
−2
g mg
−1
min
−1
(B4)forluorideand2.87×10
−3
g ʼg
−1
min
−1
(B1),
0.54×10
−3
g ʼg
−1
min
−1
(B2), 0.53 × 10
−3
g ʼg
−1
min
−1
(B3), 0.73 × 10
−3
g ʼg
−1
min
−1
(B4)forarsenite.Thedeviationsinthe
q
e
values (experimental and those obtained
from the slopes of the second-order plots) for arsenite adsorption are now quite
tolerable being in the range of −7.4 to -1.8 % (Table
2
). On the other hand the
deviation is more for fluoride adsorption.
These results indicate that fluoride adsorption follows Lagergren pseudo first
order kinetics, while arsenite adsorption follows second order kinetics.
Influence of Concentration
When the initial concentration of the anion adsorbate was increased at a constant
adsorbent amount, the amount adsorbed per unit mass of the adsorbent at equilib-
rium (
q
e
) also increased (Fig.
3
) as expected.
For example, by increasing fluoride concentration from 2.5 mg L
−1
to 20 mg L
−1
(adsorbent 1 g L
−1
), the amount of fluoride adsorbed per unit mass of adsorbent
increased from 0.56 mg g
−1
to 2.65 mg g
−1
for B1, 1.50 mg g
−1
to8.43mgg
−1
for B2,
1.67 mg g
−1
to 8.87 mg g
−1
for B3 and from 1.67 mg g
−1
to 9.03 mg g
−1
for B4.
Similarly by increasing arsenite concentration from 10 ʼg L
−1
to 100 ʼg L
−1
(adsor-
bent 1 g L
−1
), the amount adsorbed per unit mass increased from 3.31 ʼg g
−1
to
25.37 ʼg g
−1
for B1, from 7.16 ʼg g
−1
to 57.66 ʼg g
−1
for B2, from 7.79 ʼg g
−1
to
68.87 ʼg g
−1
forB3,andfrom8.93 ʼg g
−1
to74.17 ʼg g
−1
forB4.
Atlowinitialadsorbateconcentration,theratioofthenumberofanionstothenum-
ber of available adsorption sites on the adsorbent surface is small and consequently the
adsorption is independent of the initial concentration. With an increase in the number
of ions, the situation changes and the number of ions available per unit volume of the
solutionrises.Thisresultsinanincreasedcompetitionforthebindingsites(Ucunetal.
2003
), resulting in a higher uptake by unit mass and consequently an increase in
q
e
.A
good number of works related to fluoride and arsenite uptake have reported similar
results e.g. removal of fluoride from water by using granular red mud (Tor et al.
2009
),
adsorption of arsenite using natural laterite as adsorbent (Maiti et al.
2007
), etc.
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