1. activated carbon. The organic phase containing the PAHs

1.      Materials and Methods

1.1  Chemicals:

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     All reagents used were
AR grade. PAHs stock solutions of 250 mg/1 containing 8 compounds (Naphthalene,
Flourene, Anthracene, Pyrene, indeo (1,2,3,c,d)pyrene, Benzo(a)pyrene, dibenzo(a,
h)anthracene, and Benzo (a)anthracene) were prepared
by dilution with n-Hexane (HPLC grade) of corresponding PAHs solution purchased
as certified standard from Accustandard. All spikes used in this paper were
prepared using Milli-Q water spiked with PAHs stock solutions. Other necessary
materials, anhydrous sodium sulfate extra pure and glass wool were purchased
from Fisher chemicals.

1.2  Adsorbent:

Agricultural waste (corncob) was collected from
Egyptian agriculture places.  These waste
material was washed  with deionized
water, and dried then crushed  and  grinded 
into small pieces then treated with phosphoric acid, ignited at 400°C
finally it was washed until reached pH 7 
and used as adsorbent. Physical properties of activated carbon were
measured Table 1. (Gamal et al., 2014).

1.3  Extraction method of PAHs:

The amount of PAHs in the solution before
and after the adsorption test was determined by liquid–liquid extractions
referred to the US-EPA 3510 C method. PAHs aqueous solutions were extracted
with n-Hexane three times in a separating funnel before and after the treatment
with activated carbon. The organic phase containing the PAHs was separated and
dried with Na2SO4 anhydrous. The final extract was prepared through
concentration using the rotary evaporator apparatus
to a volume of 5 ml and then to 1 ml under flow of clean nitrogen.

1.4    Sample Analysis using GC-FID

Analysis
PAHs was carried out using Agilent gas chromatography model 7980 equipped with
Flame Ionization Detector (FID).  The column used was a HP5 (30 m x 0.25 mm i.d x 0.25 µm film thickness); carrier
gas is Helium at a flow rate of 1.6 mL/min. Injections were made in the split
less mode at an injector temperature of 25 oC. The FID working
condition was: temperature 300 oC Hydrogen flaw (30 ml min-1),
Air flaw (400 ml min-1) and Make up N2 (25 ml min-1).
The temperature program employed was: initial oven temperature at 60 oC
(holding time 1 min) to 175 °Cat 6
°C min-1 (holding time 4 min) to 235 °C at 3 °C min-1 and finally to 300 °C at 8 °C min-1, keeping the final
temperature constant for 5 min. 1 µl injections of the
final extracts were auto-injected into the GC system.
(Jahin et al., 2015).

2.5 Adsorption studies

Batch
experiments were implemented by spiking known concentrations of standards
solutions of PAHs in deionized water., the adsorption studies were carried out to
illustrate the effect of pH, contact time, activated carbon dose and the
initial concentration of PAHs on the efficiency of adsorption processes .A
blank sample was prepared by adding 0.5 g of activated carbon to 500 ml of
organic free water and carried out through the same analytical procedure along
with each batch to identify any contamination from reagents, glassware,
equipment’s, etc.

2.5.1.
pH effect study

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. In a conical flask 0.5 g of activated carbon was added to 500 ml
organic free water spiked with PAHs standard solution at concentration of 100 µg/1.
The PAHs solutions were adjusted to the needed pH with concentrated HNO3
or NaOH. The solutions were shaken at 12 rpm for 6 h in a rotary mixer
(Heidolph Reax 20) at room temperature. The samples were then centrifuged (Medtronic
BL-S) and the supernatant were separated by decantation. Finally, the
supernatants were extracted and analyzed using the force mentioned method.

2.5.2.
Effect of contact time study

Batch experiment was conducted to estimate the time
needed to reach equilibrium. In a conical flask 0.5 g of activated carbon was
added to 500 ml organic free water spiked with PAHs standard solution at
concentration of 100 µg/1. The solutions were shaken at 12 rpm for various times:
10, 30, 60, 120, 180 min at room temperature, The PAHs residues in the solution
were extracted and analyzed using the previously mentioned procedure.

2.5.3.
Effect of activated carbon dose.

The
effect of adsorbent amount was studied on PAHs removal from aqueous solutions
by varying the amount of activated carbon from 0.1 to 1 g while keeping other conditions
constant pH?7, temperature ? 25 oC, initial concentrations of PAHs =100
ug/l for each PAHs individual and 1 h as contact time)

  The adsorption capacities
were calculated based on the differences between the concentrations of solutes
before and after the experiment according to the following Equation (1):

                                                   
                                       (1)                                                                         

Where   qe    is 
the  concentration  of 
the  adsorbed  solute (µg/g);  Co and Ce  are the initial and final concentrations of
the solute  in solution  (µg/l); 
V(ml)  is the volume  of the solution   and   
W(g)    is   the   
amount    of   the   
adsorbent. Adsorption  isotherms  (relationship 
between   the adsorption  capacities 
and  the  concentrations of  PAHs) were investigated   using 
the linear form of both Langmuir and 
Freundlich  sorption   isotherms 
equations.

2.5.4.
Effect of Initial concentration of PAHs

Different PAHs/activated carbon mass ratios were
prepared in glass flasks to determine the adsorption capacity of the adsorbent
material. These experiments were performed on solutions with PAHs individual concentrations
of 5, 25, 50, 100, 150 and 200 µg/ l and a constant dose of activated carbon (0.5
g) at 1 hour contact time. The PAHs residues in the solutions were then extracted
and analyzed using the same procedure as previously described.

2.      Results and Discussions: