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INTRODUCTION
Sorption isotherms obtained by using batch method have been traditionally
used for preliminary screening of the systems before running more costly tests. The
procedure are well known and give an indication, both of effectiveness of sorbents
and for removing specific impurities and maximum uptake that can be taken up by
a particular unit of sorbent material. The sorption studies by batch method have
been reported for many pollutants on many sorbents. However, the practical utility
of a sorbent in removing the pollutants from the wastewater is mainly judge by
column operations. Column operations have distinct advantage over batch method
as these operations allow more efficient utilization of sorbents for the sorption
capacity. In column operations, the sorbent at inlet end is contacted continuously by
the fresh solution of initial solute concentration. Consequently, the concentration of
the solution in contact with a given layer of sorbent in a column remains practically
constant. This procedure results in maximum loading of the sorbent at constant
solute concentration and is in contrast to continuously declining solute
concentration in batch method, thereby decreasing the effectiveness of the sorbent.
The efficiency of column can be explained by means of breakthrough
curves. A breakthrough curve is obtained by plotting column effluent concentration
versus volume treated or time of treatment. Breakthrough capacity, exhaustion
capacity and degree of column utilization are the important features of the
breakthrough curves. The breakthrough capacity is defined as the mass of sorbate
removed by the sorbent at break point concentration, which is also termed as
maximum acceptable concentration of the sorbates. The degree of column
utilization is defined as the mass sorbed at breakthrough point divided by the mass
sorbed at complete saturation. The exhaustion capacity is defined as the mass of the
sorbate removed by unit weight of the sorbent at saturation point.
The relation between the nature of breakthrough curves and fixed bed sorber
was nicely expressed by Weber et al. [221]. According to them, when feed water
(wastewater) is introduced through the inlet of the column, the solute is sorbed
most rapidly and effectively by the upper few layers of the fresh sorbent during the
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initial stage of the operations. These upper layers are in contact with the solution at
its highest concentration level (initial concentration, C0). The small amount of
solute which escape sorption in the first few layers of the sorbent are then removed
from the solution in the lower strata of the bed and essentially no solute escape
from the sorbent initially (effluent concentration, C=0). The primary sorption zone
(δ) is concentrated near the top or influent end of the column (Figure 6.1). As the
polluted feed water continues to flow into the column, the top layers of the sorbent
become practically saturated with the incoming solute and become less effective for
further sorption. Thus, the primary sorption zone now moves downward to regions
of fresher sorbent in the column. The wave like movement of this zone,
accompanied by a movement of initial concentration front, occurs at a rate which is
much slower than the linear velocity of the feed water. As the primary sorption
zone moves downwards, more and more solute trends to escape in the effluent, as
shown in Figure 6.1. The plots of C/C0 versus time or volume of effluent, for a
constant flow rate, depict the increase in the ratio of C/C0 as zone moves through
the column. In most of the case of the sorption by column method operation of
water and wastewater, breakthrough curves exhibit a characteristic ‘S’ shape but
with varying degree of steepness.
Figure 6.1 Representation of the movement of primary sorption zone and
formation of breakthrough curve
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DESIGNING OF FIXED BED SORBER
For the system involving feasible sorption process, a straight forward
approach may be adopted for the design of a fixed bed sorber [272]. The
breakthrough point is selected arbitrarily at some low value Cb (break point
concentration) for the effluent concentration and Cx (exhaustion point
concentration) closely approaching C0 (influent concentration of phenol), at which
the sorbent is considered to be essentially exhausted. Mass unit for C and Ve are
used to iilustrate the concept of mass balance in the sorption system.
For designing purpose two parameters are important to notice: (i) the total
mass i.e. quantity of effluent, Vb passed per unit cross section to the break point and
(ii) the nature of the curve between the value Vb and Vx. Here Vb is the volume of
effluent corresponding to Cb and Vx is the volume of effluent corresponding to Cx.
The parameters δ, tx, Fm, tδ, Ms, f, tf, and percent of saturation of column at break
point are evaluated using equations (6.1-6.10) [206, 272].
The primary sorption zone (PSZ) is the portion between exhaustion point
(Cx) and breakthrough point concentration of phenol (Cb) as shown in Figure 6.2.
PSZ is assumed to have a constant length or depth (δ). The total time, tx taken for
the primary sorption zone to establish itself move down the length of the column
and out of bed, can be obtained as follow,
m
xx
F
Vt (6.1)
where, Fm is mass flow rate. The time tδ, required for the movement of PSZ
downwards in the column is obtained by
m
bx
F
VVt
(6.2)
)()1( bxb
bx
xfx VVfV
VV
ftt
t
tt
t
D
(6.3)
where, D is the depth of the sorbent bed, tf is the time required for initial formation
of the PSZ.
The amount of phenol sorbed by the sorbent is presented by shaded portion
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(PSZ) in Figure 6.2. The quantity of, PSZ may be calculated by integrating the
quantity (C0 - C) over Ve within the limits of Vx and Vb. The fractional capacity (f)
of the sorbent in the sorption zone at breakpoint to continue to remove solute from
solution under the limiting conditions is given by Equation (6.4).
t
tf x1 (6.4)
The percent of saturation is obtained by Equation (6.5).
100)1(
%
D
fDsaturation
(6.5)
The breakthrough curves expressed in terms of C/C0 and total quantity of
phenol solution Ve, which passes through the column, is shown in Figure 6.3 and
Figure 6.4 for sorption of phenols on BFA and CZBFA respectively. The values of
Vb, Vx, Cx, and Cb obtained from this graph (Table 6.1) have been used to calculate
tx, tf, tδ, f, δ and percent saturation at breakpoint. The obtained results are presented
in Table 6.1 and Table 6.2. The results given in Table 6.2 reveal that the total time
(tx) required for the primary sorption zone to establish itself, move down the length
of the column and out of the bed is maximum for PNP and minimum for phenol
while for OCP it falls in between on BFA and CZBFA. The time taken for initial
formation of the primary sorption zone (tf) is between 1.5 to 2.5 h for the studied
phenols. The fractional capacity ‘f’ of the column in the sorption zone at breakpoint
to continue to remove solute from solution is 0.7, 0.6 and 0.5 for phenol, OCP and
PNP on BFA while it is 0.639, 0.571 and 0.500 for phenol, OCP and PNP on
CZBFA repectively. The length of the primary zone (δ) is maximum for phenol on
BFA and CZBFA and minimum for PNP on both the sorbents. The percent
saturation at breakpoint is 72.72, 75.0 and 79.41 for phenol, OCP and PNP
respectively on BFA while it is 69.77, 77.78 and 80.0 for phenol, OCP and PNP
respectively on CBZFA. The obtained values are comparable with the results
obtained by Gupta et al for phenolic wastewater [206]. From this observation a
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direct relationship between the length of the sorption zone (δ) and percent
saturation at break point can be developed. The smaller, the length of the sorption
zone, the higher is the percentage saturation which is in contrast with results
obtained by Gupta et al. [206]. The value of Vx - Vb is in the range 285-367mg/cm2
for studied sorbate - sorbents systems which indicates that the additional quantity of
sorbate of waste load per unit cross-sectional area that will result in complete
exhaustion of the capacity of sorbent.
The data of operational parameters obtained in these investigations give an
idea of the time required for breakthrough to occur and how much additional
solution loaded per unit cross sectional area of the sorber would result in complete
exhaustion of the capacity of the sorbent column. If applied on large scale, these
data can be useful for the design of fixed bed sorber for the treatment of known
phenol concentrations.
Figure 6.2 Ideal breakthrough curves
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Figure 6.3 Breakthrough curves of phenol, OCP and PNP sorption on BFA
Figure 6.4 Breakthrough curves of phenol, OCP and PNP sorption on CZBFA
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Table 6.1 Parameters of fixed bed sorber
Sorbent
Sorbate
C0
mg/cm3
Cx
mg/cm3
Cb
mg/cm3
Vx
mg/cm2
Vb
mg/cm2
Vx - Vb
mg/cm2
Fm (mg/
cm2min)
D
cm
BFA
Phenol 0.4 0.380 0.019 428.026 122.293 305.733 1.019 12
OCP 0.4 0.392 0.022 611.465 305.732 305.733 1.019 12
PNP 0.4 0.389 0.017 835.667 550.318 285.349 1.019 12
CZBFA
Phenol 0.4 0.395 0.022 570.701 203.822 366.879 1.019 12
OCP 0.4 0.391 0.019 672.612 387.261 285.351 1.019 12
PNP 0.4 0.391 0.019 917.198 611.465 305.733 1.019 12
Table 6.2 Parameters of fixed bed sorber
Sorbent
Sorbate
tx
min.
tδ
min.
tf
min. δ f % saturation
BFA
Phenol 420 300 90 10.91 0.700 72.72
OCP 600 300 120 7.500 0.600 75.00
PNP 820 280 140 4.941 0.500 79.41
CZBFA
Phenol 560 360 130 10.047 0.639 69.77
OCP 660 280 120 6.222 0.571 77.78
PNP 900 300 150 4.8 0.500 80.00
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BREAKTHROUGH CAPACITY
In the present study break point is considered at which C/C0 became
approximately equal to 0.02 i.e. effluent concentration is about 2 % of influent
concentration. The calculation of breakthrough capacity, exhaustion capacity and
degree of column utilization are done as follow in consideration of Figure 6.2
[240]:
01 CAreacapacitygh Breakthrou (6.6)
021 C)Area(Areacapacity Exhaussion (6.7)
100AreaArea
Area(%)n utilizatiocolumn of Degree
21
1
(6.8)
The column operations were carried out using solution of all three phenols
on column (cross sectional area: 0.19625 cm2, height: 12cm, mass of sorbent: 1g)
of BFA and CZBFA at a flow rate of 0.5mL/min. In case of FZBFA sorbent,
column was chocked after few hours due to presence of finest particle range as
shown in particle size analysis of FZBFA (chapter 5, Figure 5.10), so column study
was not achievable in the present operational conditions. The column operations for
BFA and CZBFA sorbents were continued till concentration of phenol in the
aliquot of effluent collected reached nearly 95% of the influent concentration, i.e.
C/C0 ~ 0.95. The breakthrough curves were obtained by plotting C/C0 against
volume of the effluent. The breakthrough curves of phenols (phenol, OCP and
PNP) on BFA and CZBFA are shown in Figure 6.3 and Figure 6.4 respectively.
The breakthrough capacity, exhaustion capacity and degree of column utilization
have been evaluated from these figures are given in Table 6.3.
It was observed that breakthrough capacity is less than the batch capacity for
BFA-Phenol and CZBFA-phenol system. This may be due to lesser contact
time/equilibration time of the phenol with sorbent which require longer time for
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equilibration and thus, inhibiting the utilization of column capacity. Similar results
were also reported by other workers [211] for the removal of chlorophenols by
bituminous shale. Further, it may be argued that phenol having high solubility
among three studied phenols, possess more tendencies to remain in the solution and
hence it requires longer time for equilibration. While, for BFA-OCP, BFA-PNP,
CZBFA-OCP and CZBFA-PNP sorbate – sorbent systems breakthrough capacities
are higher than that of batch capacities. A higher capacity of column operations was
established by a continuously large concentration gradient at the interface zone as it
passed through the column, while the concentration gradient decreased with time in
batch isotherm test. Similar results were reported by Gupta et al. [105] for
chlorphenols and by Amit Bhatnagar [123] for bromophenols.
It is further seen from Table 6.3, that the exhaustion capacity of column is
relatively higher than the batch capacity except BFA-phenol and CZBFA-phenol
systems, where exhaustion capacity is near about same. This appears due to
establishment of continuously larger concentration gradient at the interface zone as
the influent passes through the column. The concentration gradient generally
remains maintained because of fresh in flow of influent solution, whereas, in case
of batch experiments, the concentration gradient continuously decreases with time
resulting in smaller sorption capacity. Further, Table 6.3 shows that the degree of
column utilization lies in the range 55-86% for all the studied sorbate - sorbents
systems. The column capacities and degree of column utilization are higher in case
of PNP sorption by BFA and CZBFA. This may be due to its low solubility and
more affinity towards sorbents.
Thus, these results have shown that the columns of BFA and CZBFA
sorbent can be used to remove the phenols from wastewaters.
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Table 6.3 Column capacities and degree of column utilization data of phenols
sorption on BFA and CZBFA with batch sorption capacity
Sorbents Phenols
Batch
capacity
(mg/g)
Breakthrough
Capacity
(mg/g)
Exhaustion
Capacity
(mg/g)
Degree of
column
utilization
(%)
BFA
Phenol 47.19 29.64 47.46 62.45
OCP 52.52 61.02 81.57 74.80
PNP 55.04 105.63 123.84 85.29
CZBFA
Phenol 69.25 38.73 69.24 55.92
OCP 78.80 81.19 98.09 77.66
PNP 82.71 118.33 135.68 85.31
COLUMN DESORPTION
When the column bed is exhausted or the effluent coming out of the column
reaches the allowable maximum discharge level, the regeneration of the sorption
bed to recover the sorbed material and/ or to regenerate the sorbent becomes quit
essential. The regeneration could be accomplished by a variety of techniques such
as thermal desorption, steam washing, solvent extraction, etc. Each method has
inherent advantages and limitations. In this investigation several solvents were tried
to regenerate the sorption bed as shown in chapter-5 for desorption. According to
the batch desorption 0.5 M NaOH was found to be effective in desorbing and
recovering sorbates quantitatively from sorbent bed.
Regeneration of columns saturated with phenols was carried out by passing
0.5 M NaOH as an eluent at a fix flow rate of 1 ml/min at room temperature. To
evaluate the solvent recovery efficiency, the percent of phenols recovered is
calculated from the breakthrough and recovery curves. After exhaustion of the
column, it was washed with 25 mL double distilled water to remove unsorbed
phenols form the sorbent bed which was collected and analyzed for phenol amount.
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A Negligible amount was found in the water effluent. Then 0.5 M NaOH was
passed through column, it was collected in fraction of 10 mL and desorbed amount
of phenol was examined in the effluent. Figure 6.5 and Figure 6.6 shows desorption
curves of phenol. OCP and PNP from BFA and CZBFA respectively by 0.5 NaOH.
Desorption plots of phenols reveal that the first aliquot of 10 mL elutes more than
50% of phenol from the sorbents column and rest is desorbed in nine increments of
10 mL each. The percent recovery of all phenols is about 98% for BFA and
CZBFA column. From these desorption study it can be concluded that about 120
mL of 0.5 M NaOH is sufficient for almost complete desorption of phenols from
the sorbents.
Figure 6.5 Desorption of phenol, OCP and PNP from BFA with 0.5 M NaOH
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Figure 6.6 Desorption of phenol, OCP and PNP from CZBFA with 0.5 M NaOH
ECONOMIC EVALUATION
In India, the cheapest variety of commercially available carbon costs U.S.
$350 per tonnes. Commercial activated carbons of cheapest variety (generally used
for effluent treatment) cost ≈ US$ 2000 per tonnes in India [123]. The bagasse fly
ash is available for U.S. $25 per tonnes considering the cost of purchase, transport,
chemicals, electrical energy used in the sorbent development and labor required.
The cost of synthesized zeolite CZBFA and FZBA was estimated to be U.S. $150-
180 per tonnes which is lower compared to that of commercial activated carbon
available in the market. Since the cost of final sorbents prepared from bagasse fly
ash is less than the cost of activated carbons of cheapest variety, it is reasonable to
conclude that these materials can be fruitfully used as low-cost sorbents for the
treatment of phenolic wastewaters.
