In recent decades, population of humans has
grown significantly which increases the demand for safe water resources every
year. In addition climate change has changed
temperature and rainfall patterns worldwide. Today water resource security is a
very important issue for all of governments on the earth (Gao, 2014) (Evans A, 2009) (J.D.Miranda, 2011).
one of the widespread known contaminates of surface waters and can cause a
number of health problems for human being such as cyanosis or cancer if go into
the body by drinking water. Nowadays much attention has been paid to nitrate
elimination methods from water. (Yu Wang, 2007).
can be eliminated through several methods such as biological de-nitrification (Chunming Su,
reverse osmosis ( (Katri Häyrynen, 2009)) or physical
adsorption (Shikha Jain, 2015). Regarding to other
methods, sorption process seems to be simple, fast and cost effective. Generally
adsorption is the procedure of accumulating solvable components from a solution
on a suitable solid surface. The key practice in adsorption process is to find
a low-cost and effective sorbent. Various cheap materials have been tested for
removal of nitrate from aqueous solution such as modified clay minerals (Yunfei Xi, 2010) modified rice husk (Reza Katal a, 2012) natural zeolites (Y. Zhan,
widely grown in different areas of earth from China to United States and is a
very important product for both small and large profitmaking producers. Its 12th
most valuable cash crop grown in the United States with a farm value of over
one billion U.S. dollars. What remains after processing peanut is a hard shell
that doesn’t have a valuable price and usually will be used as a cheap
fertilizer for other farms and green houses (council, n.d.).In this paper after modifications, sorption abilities
of the peanut shell (PS) for adsorption
of dangerous nitrate ions were studied and sorption capacity, kinetics and
thermodynamic states were obtained. We found that MPS can be an effective
anionic sorbent and this offers a fast and economic friendly way for removing
of anionic pollutions.
Materials and methods
The peanut shell residue was obtained from regional farms and was modified
for elimination of nitrate ions from aqueous solution. It was dried at 90C for
60 min in an oven. Nitrate ions were formed by solving 1 gr sodium-nitrate salt
into 1 liter tri-distilled water as a stock solution and all of the required
concentrations in sorption studies were obtained by dilution.
The raw solid residue was washed two times with distilled water to remove dusts
and impurities. Next, the sample was
dried in an oven for 6h at 100C to remove any humidity and then milled into
powder meshing from 200 to 300 micrometers. Five grams of raw sample was added
to 60 ml N, N-dimethylformamide (DMF) in a 600 ml flask and was treated with
pyridine as a catalyst and finally 50 ml of dimethyl amine added and stirred
under 100C temperature for 5 hr. (Yu Wang, 2007)
The batch reactor was a flask inside an electric
heating jacket stirring by a magnetic stirrer at 400 rpm.to determine the
concentration of nitrate after finishing every batch experiment UV
spectrophotometry (camspec m501) was used at a wavelength of 200 nm. The
sorbent was characterized by using FTIR and Electron
Microscopy (SEM) method. FTIR spectra were
obtained to reveal the functional groups present on sorbent by a FTIR
spectrometer (Bruker Tensor 27) in
range of 400-4000 cm -1 with averaging 16 scans.
Surface morphologies of modified peanut shell were analyzed by SEM (LEO-1430VP)
To find the optimum
amount of sorbent various amounts of modified sorbent were added to 100 ml of
sodium-nitrate aqueous solution with initial concentration of 100 mg/lit at
constant temperatures. The optimum
amount of sorbent was 0.08. The sorption capacity at equilibrium state was
obtained as follows:
Where C0 is initial concentration (mg/L) , Ce is the equilibrium concentration of aqueous nitrate ion, V is the
liquid phase volume and m is the mass of unloaded modified sorbent.
It was found that the MPS
can be recycled after adsorption process by renewing active molecular sorption
sites under alkaline solutions. Desorption study was done by soaking loaded
sorbent into 100 ml of KOH solution in different concentrations and temperatures.
The KOH was selected in order to avoid common ion effect with NaNO3.
Results and discussion
The sorbent characteristics
3.1.1 Fourier transform
infrared (FTIR) spectroscopy
The surface morphology of MPS determined by SEM is shown in Fig. x. The MPS is made up of a porous uneven surface.
This surface can be seen to have high quantities of small pores representing
that this material grants good characteristics to be employed as a low cost
adsorbent for ionic uptake, as beforehand reported (Seyda Tasar *, 2014). It is apparent that
these pores provide easy contact and large surface area for the sorption of nitrate
on the sorption sites. Fig. x
Before the sorption
After the sorption
3.2.1 Effect of temperature
Fig.2 shows that the final adsorption
percentage of nitrate ion decreases with increasing temperature and in first 5
minutes the rate of sorption process is higher in warmer temperatures.
According to fig.2 removal percentage decreased from 95.91% to 71.78% by
varying temperature value from 10oC to 50oC.these data shows that nitrate
bio-sorption via MPS is exothermic and yields higher in lower temperatures.
Adsorption isotherms are used to explain absorbed material
and sorbent interaction regarding to sorption mechanism. The isotherm models are
widely used to calculate maximum adsorption capacity, which helps to understand
how effective is a specific sorbent (Seyda Tasar *, 2014) (X. Liu and L.
Several models are often applied to process equilibrium data while Langmuir and
Freundlich isotherms are the most commonly models among them.
The Langmuir model represents the monolayer
adsorption onto surface containing finite number of tantamount active sites
which is expressed as follows:
The linear form of Langmuir
equation can be rearranged as:
Where, qe is adsorption
capacity at equilibrium (mg/g), Ce is concentration of nitrate in the solution at equilibrium (mg/L), qm is the maximum adsorption
capacity (mg/g), and KL is the Langmuir
Constant related to the energy of
adsorption represents the degree of adsorption affinity the adsorbate
(L/mg). The parameters KL
qm were obtained
from the slope and intercept of the plot of Ce/qe against Ce . A plot of Ce
/ qe versus Ce should be a straight line with a slope of 1
While Langmuir isotherm speculates that enthalpy of
adsorption is independent of the loading of the sorbate, the practical
Freundlich equation, based on sorption on heterogeneous surface, can be derived
presuming a logarithmic decrease in the enthalpy of adsorption with the
increase in the fraction of occupied sites (al., 2012). The Freundlich
equation is entirely practical based on sorption on heterogeneous surface and
is given by:
qe = KFC1/n
where KF and 1/n are the Freundlich constants
demonstrating the biosorption capacity and biosorption intensity, respectively.
Eq. (x) can be linearized for the determination
Freundlich constants as follows:
The slope and the intercept correspond to (1/n) and KF,
respectively. It was revealed that the plot of log qe and log Ce yields a
straight line (Fig. 12). The results are indicated in Table 4. The parameter k related
to the adsorption density increased with a decrease of adsorbent amount.
According to following equation Gibbs
free energy changes can be calculated at various temperatures.
The equilibrium constant (Kc) is obtained from making
Langmuir Constant (Kl) dimensionless by multiplying it to 106.
the standard enthalpy and entropy variations were determined
by plotting ln Kc vs 1/T according to Van’t Hoff equation. The
calculated thermodynamic parameters are shown in Table
Kc x 106-
According to table x negative
values of ?G are obtained in all temperatures, revealing that the adsorption
nature of nitrate onto MPS is feasible and spontaneous. The negative value of ?H
changes indicate that the process is exothermic and the products are
energetically stable with a high binding of nitrate ions to the adsorbent sites.
The positive value of ?S suggests
increased degree of freedom while freedom of nitrate ions decreases . during
adsorption, water molecules that are formed hydration shell around nitrate ions
will be freed at the sorbate and solution interface with some structural
changes in the adsorbate and the adsorbent and an affinity of the adsorbents
toward nitrate ions during adsorption process.
3.3.1 Dosage of sorbent studies
The amount of sorbent required for the adsorption procedure
for each juncture is essential both in the design of the adsorption
and its usability on a
large scale. In order to obtain the favorable amount, different amounts of MPS
were tested in presence of nitrate soloution.in these series of experiments the
contact time was constant 10 minutes and the concentration had been fixed at 100
mg/L .according to fig.2 over 90% of sorbate was absorbed during the first 10
min and the effect of dose of sorbent decreased after that point in sorption process
thus the favorable amount was selected at 0.08 g.
3.3.2 Kinetic model
It is well-known
that the adsorption process is time dependent, thus it is essential to know the
rate of adsorption when designing a unit like biosorption reactor (Seyda Tasar
To this end, the adsorption data were analyzed with three kinetic models, i.e.,
pseudo first-order, pseudo second-order and Morris-weber kinetic models.
220.127.116.11 Pseudo first-order kinetic
The pseudo first-order
expression of Lagergren which is widely used for the sorption of a solute from
liquid solution is given as (W. Jianlong, 2001) (Lagergren, 1898):
This equation is linear
rate expression for pseudo-first order reaction, where qt (mg/g) Is the
capacity of sorbent at time t, K1 is rate constant of
pseudo-first-order adsorption (L min-1). k1 and qe as shown in Fig. 4
were calculated from the slope and intercept of the straight line plots of
log(qe?qt) against t..
18.104.22.168 Pseudo-second-order model
model is based on the assumption that the adsorption process has a chemical
base and it is generally given as follows (A.L. Ahmad, 2009):
Where qe is the amount of nitrate
adsorbed at equilibrium state; and k is the sorption rate constant of
pseudo-second order adsorption. The straight-line plots of t/qt
versus t at different temperatures (Fig. 4) indicate
the applicability of the above equation to nitrate adsorption on MPS. The values of kinetic parameters of
adsorption are summarized in Table 3
Pseudo first order
Pseudo second order
for indicating the reusability of the sorbent, the
adsorption–desorption progression was repeated six times for MPS. Adsorption
and desorption experiments were executed in the room temperature to simulate an
effective condition in industry. The adsorption and desorption capacities was nearly
the same even after six runs (Figs. X)