Results change in pH values. The limited effect of

Results and Discussions:. Effect of pH:The
adsorption of PAHs on corncob activated carbon was studied at pH 1.5, 3, 4, 5
and 7 to determine the optimum pH for the adsorption of these PAHs. The amount
of PAHs removed was calculated by comparing the initial concentration of individual
PAHs to its residues in the solution after 1h of contact time. The analysis of
PAHs in a blank solution showed that there was no leaching of these compounds
from the adsorbent. Figure
(1) shows that, the variation in the value of pH has a different effect with
each individual of the PAHs. Benzo(a)pyrene, indo (1,2,3,c,d)pyrene and
dibenzo(a,h)anthracene were the least affected by this change. While the other
PAHs (Naphthalene, Flourene, Anthracene, Pyrene and Benzo (a)
anthracene) are moderately affected by change in pH values. The
limited effect of pH change is due to the properties of these chemical
compounds, PAHs are chemically inert and their bond linkages (C=C) give them chemical
stability. Furthermore, these compounds do not have ionizable groups that can
be influenced by the pH (Karimi-Jashni et al.1997; Kumar et al., 2007).  Figure (1) Effect of pH
on the removal of PAHs.
Effect of Contact Time

The
effect of contact time was studied using 500 ml PAHs solution at concentration
level of 100 µg/l with 0.5 g of activated carbon at pH?7 and applying different
retention times range from 10 to 180 minutes

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The
results presented in Figure (2) show that   an increase in the contact time can improve
the adsorption process of the studied 8 PAHs individuals till it reached equilibrium
after two hours. This could be due to increases in PAHs surface loading. The
driving force in mass transfer that occurred between solid- liquid phases can
led to increases in amount of PAHs concentration on the surface of
the adsorbent. These results seems to be consistent with naphthalene adsorption
on an activated carbons study (Cabal et al 2009) and the adsorption of PAHs
onto granular activated carbons and Macro net hyper- cross-linked polymers
(MN200) (Valderama et al., 2007). These also
agree with studies done by other researchers (Yang et al., 2008: Liu et al.,
2010).

Figure (2) Effect of contact
time on the removal of PAHs

. Effect of activated carbon dose:

An
improvement in amount of adsorbent should raise the certain number of active
sites and could lead to increase in the adsorption rate. Figure (3) shows that,
the percent removal of PAHs increases ascending with the increase of adsorbent
dosage till it reach 0.5 g of activated carbon. For doses higher than 0.5 g no
significant effect on PAHs adsorption rate. Therefore, the optimum dose of
adsorbent was around 0.5 g/500 ml this also consistent with
other studies (Valderrama et al., 2008; Valderrama et
al., 2009).

Figure
(3) Effect of activated Carbon Dose on the removal of PAHs

 

. Effect of Initial concentration

Different
initial concentrations of PAHs were tested to be adsorbed on the same amount of
activated carbon at a constant contact time and pH. It was observed that, an
increase in initial concentration of PAHs decreases the percentage binding as
shown in Figure (4). These observations can be explained by the fact that at
low concentrations of PAHs, the ratio of adsorptive surface area is high and
thus, there is a greater chance for PAHs removal. When PAHs   concentration      increased,   the adsorptive   sites become more quickly saturated since the
amount of adsorbent remains constant so at low initial PAHs concentrations, the
removal capacity is higher (Lamichane et al., 2016).

 

Figure (4) Effect of individual
PAHs Initial concentration on its removal at constant activated carbon dose,
time and pH   

Adsorption
isotherm models

The interactions between
the adsorbent and an organic adsorb ate can be assessed by Langmuir and
Freundlich isotherms (Lach et al., 2008; Yuan et al., 2010).  The 
Langmuir model   based   on  
monolayer  adsorption  on  
active  surface  sites, 
while  the Freundlich model  depends 
on heterogeneous  
adsorption.   However,   to evaluate the linearity of the
experimental data the two isotherm models were applied. The linear form of
Langmuir sorption isotherm equation. Equation (2);

                           (2)

Where Ce (mg/l) is the
equilibrium concentration of PAHs and qe (mg/g) is the amount of PAHs
adsorbed per unit weight of sorbent. Q and b are the Langmuir constants
indicating adsorption capacity and energy of adsorption respectively. The
values of various parameters obtained (Table 2).The favorability of adsorption
process is confirmed by the value of separation factor RL obtained
which is given by Equation (3); 

                                (3)

Where Co
is the optimum initial PAHs concentrations (mg/l) and b is the Langmuir
constant. The RL values were found to be between 0 and 1 and indicated
to the adsorption favorability. The linear form of the Freundlich sorption
isotherm equation. Equation (4);

                       (4)

Where   qe   is 
the  concentration  of  the  adsorbed 
solute (mg/g);  Ce   is 
the  concentration of  the 
solute  in  solution (mg/l);  K  is
related  to the  adsorption capacity  adsorbent and 1/n is related  to the surface heterogeneity. After fitting
the equilibrium adsorption data for multi-solute adsorption, Freundlich parameters
(1/n and K) were obtained from straight lines when log qe was
plotted against log Ce (Table 2). The adsorption process  of organic 
solutes  is determined by
various  parameters that govern  the relation between solute  and 
adsorbent, such  as Vander Waals,  electrostatic forces ,  dipole– dipole  interactions, ,  and 
weak  intermolecular  associations (Dabek et al 2009).  Since PAHs are nonpolar compounds, adsorption
must be governed mainly by hydrophobic interactions (Valderrama
et al., 2009).

Table (2): Values
of Langmuir and Freundlich parameters

PAHs

Freundlich Statistical Parameter (R2)

Langmuir Statistical Parameter (R2)

RL(Separation Factor)

Langmuir Constant

Freundlich Constants

Q

B

Log K

1/n

Naphthalene

0.985

0.972

0.004

27.4

5

0.7

0.7

Flourene

0.987

0.884

0.007

34.4

2.4

0.8

0.68

Anthracene

0.993

0.93

0.011

36.7

1.7

0.9

0.66

Pyrene

0.997

0.953

0.012

35

1.6

0.7

0.69

Benzoaanthracene

0.974

0.948

0.005

14.3

3.6

0.1

0.9

Benzoapyrene

0.977

0.935

0.012

7.9

1.5

1

1.3

Indo1,2,3,c,dpyrene

0.988

0.91

0.011

18.7

1.7

0

0.84

7,12
Dimethylbenzaanthracene

0.895

0.839

0.007

6.1

2.6

0.1

0.9

 

                                                                         

 

 

 

 

 

 

 

 

 

 

 

 

 

             

 

 

Figure (5) PAHs
adsorption models (Langmuir and Freundlich)

Experimental results of adsorption equilibrium
were obtained using
Langmuir and Freundlich isotherm
models. The linear plots of
isotherm models are shown in Figure (5). Linear regression analysis
shows that PAHs
adsorption followed by the Langmuir and then
Freundlich isotherm models. Freundlich isotherm models provided the best fit
with experimental data
(R2~ (0.90-0.99). Table 2 shows
isotherm constants for
PAHs adsorption onto activated carbon.

Several
models in the literature were described the adsorption isotherms (Hameed et al
2007). These models are preferred Langmuir and Freundlich models are most
commonly used models because of their simplicity and common uses (Srihari et al.,
2008; Xiao et. al., 2015). It is necessary to propose a suitable model that
describes the best isotherm models in the activated carbons adsorption (Rad et.
al., 2014).  Based on the results,
Freundlich isotherm models provided the best fit with experimental data (R2=
0.89 to 0.99). In Langmuir isotherm models, adsorption process and maximum
adsorption capacity have a constant slope (Long et al., 2008). These results
are in a good agreement with other studies carried out in PAHs
adsorption on activated carbons (Yakout
et. al., 2013; Xiao et. al., 2015).  This means that adsorption process
showed a strong correlation with Fraundlich isotherm models.