Rice straw (RS) has been activated using Na2CO3. Activated carbon samples (ARSC) were characterized using N2- adsorption, elemental analysis, surface fractional dimension and pore volume to support the adsorption of nitrate and nitrite ions. The effects of various parameters such as solution pH, adsorbent concentration, contact time, temperature and initial nitrate and nitrite concentrations were examined. Various kinetics models including the Pseudo-ﬁrst-order, Pseudo-second-order and intra particle diffusion models have been applied to the experimental data to predict the adsorption mechanism. The thermodynamics constants of the adsorption process, viz. ΔHo, ΔGo and ΔSo were evaluated. The results showed that the adsorption of nitrate and nitrite ions onto activated carbon was exothermic and non-spontaneous. The adsorption data followed second-order kinetics supporting that chemisorption process was involved. The obtained results show that ARSC can be used as an effective and natural low-cost adsorbent for the removal of nitrate and nitrite anions from wastewater.
|Activated carbon; Rice straw; Adsorption; Nitrate;
|Nitrogen (N) is an essential element for all living matter. Nitrogen
exists in different oxidation states such as NO3ˉ (+5), NO2ˉ (+3) and
ammonium NH4+ (-3). Among these, NO3ˉ, NO2ˉ and NH4
+ (-3) are
of more concern because they are soluble in water causing toxicity to
human health . Contamination of groundwater and surface water by
nitrate coming from non-point sources such as agricultural fertilization
has become a growing environmental problem so it becoming a
common concern of both industrial and developing countries .
Nitrate contaminations increasingly occurs due to the widespread
use of fertilizers containing nitrate and owing to poorly treated or
untreated human and animal wastes. Nitrate is a by-product of many
industrial processes, including paper and explosives manufacturing
and the production of nitro-organic and pharmaceutical compounds .
The increasing nitrate concentration in the groundwater causes a serious
health risk, responsible for the blue baby syndrome and a precursor
to carcinogenic nitrosamines . For these reasons, the European
Community limits nitrate and nitrite concentrations in drinking water .
|Nitrite and nitrate are also a form of non-radioactive waste present
in more radioactive waste; nitric acid is employed in processing
extremely toxic radioactive elements such as uranium, plutonium and
americium during the production of nuclear weapons and nuclear
fuel. However, if nitrate is also present in high concentrations, it may interfere with the formation of stable cement matrix, making it difficult
to use this process for the long-term disposal of various metal processing
wastes . Therefore, removal of nitrate and nitrite is of significant
importance from the health and environmental point of view.
|Conventional methods for removing nitrate and nitrite anions
from wastewater include reverse osmosis, ion exchange, combined
membrane bioreactor/powdered activated carbon adsorption, the
biofilm-electrode reactor (BER) and the BER/adsorption process . Most
of these methods suffer from some drawback, such as the high capital or
high operational cost or the disposal of the resulting sludge [7,8].
|Recently, numerous low-cost alternative adsorbents have been
examined for the removal of nitrate and nitrite anions from wastewater
|The present work was directed at improving the capability of ARSC
for removing nitrate and nitrite ions from wastewater. To achieve
this goal, the following studies were undertaken: (a) preparation and
characterization of ARSC, (b) establishment of the conditions under
which the maximum adsorption of nitrate and nitrite ions onto ARSC
occur using column technique, and (c) evaluation of the kinetic and
thermodynamic parameters for nitrate and nitrite ions adsorption onto
|Materials and methods
|Materials: Rice straw (RS) is a distinguished type of precursor
in comparison to other agricultural by-products. It obtained from
agricultural regions in Sharqia-Egypt
|Preparation of activated carbons: Half a kilogram of dried rice
straw is fed into the fluidity bed reactor, described elsewhere , at
a heating rate 50°C per 15 min in the presence of N2 flow (200 mL
min-1). Assess of the reactor was 5 mL min-1 when the furnace reached
350°C and the heating continued up to a final temperature of 650°C.
The hold time was 1 h and the furnace was stopped. Carbon given the
abbreviation. 1.0 g of carbon samples were mixed with 25 mL of 0.1
mol L-1 Na2CO3 solution for 72 h at 50°C, washed with water and stored
in stopper bottle.
|Samples characterization: Samples were characterized by using
nitrogen adsorption at [77 K] using (Quantachrome Instruments,
Model Nova1000e series, USA). The samples were outgased at 250°C
under N2 flow for 16 h. The pH of the samples was adjusted .
|Adsorption studies: Sample Selection: A fixed amount of both
non-oxidized and oxidized dry adsorbent (0.1 g) and 25 mL aliquots
of the stock solution of initial concentration Co=25 mgL-1, for each
solute were shaken for 24 h. Each mixture was filtered and the residual,
nitrate and nitrite content in the solution was determined for knowing
the effect of equilibrium time.
|The pH of the solutions was adapted from 2 to 11 by diluting NaOH
or HCl solutions. We used 20 mL of the pH adjusted solution and
30 mg adsorbent in batch experiments conducted at the determined
equilibrium time. The pH value provides the maximum anion removal
|Experiments were carried out by taking 20 ml of a nitrate-ionsspiked
aqueous solution of an initial concentration of 50 mg L-1 NO3ˉ
and carbon dose of 200 mg in the presence of different quantities of
Ca(HCO3)2 in the range of 0-400 mg L-1. Effect of shaking time was
done at 24 h at 25°C, 45, and 55°C.
|Metrohm 690 ion chromatography with column: 6.1006.000 anion
column super-sep, elluent: 2.5 mmolL-1 phathalic acid, 7% acetonitrile,
pH=4 with conductivity detector were used to determine anion
|Kinetic tests were conducted to study the effect of various parameters
on the adsorption efficiency of the anions (NO2ˉ and NO3ˉonto the
oxidized carbon sample, RS (ox.). In this respect, 50 mg of adsorbent
was shaken with 20 mL of the solution of an initial concentration
of nitrate 50 mg L-1 and 5 mg L-1 of nitrite for different intervals of
time. These are the concentration levels of nitrate and nitrite found in various aquaculture and industrial wastewaters . After the required
time intervals, the suspension was filtered through a Whatman No. 42
filter paper and analysed for residual anion concentration.
|Results and Discussion
|Activated carbon characterization
|The usual way of reporting oxygen content values from elemental
analysis is based on the difference between the percentage content of all
elements analysed with the residual ascribed to oxygen . RS (ox.)
sample, which show a considerable increase in oxygen content (from
0.92 to 5.7%) (Table 1).
|DFT pore size distributions of the adsorbents were studied in
Figure 1. The results of surface area and pore volume are given in Table
2. The microporous nature of carbons is demonstrated in Figure 1.
|Oxidation enhances pore volume and surface area of the adsorbents
without significant changes in the pore size distribution. The surface
area and pore volume of oxidized carbon RS(ox.) increased by about
43 and 35% respectively, compared to those of the unoxidized carbon
sample. Oxidized and unoxidized carbon samples possess a significant
amount of micropores with a maximum of 1 nm and mesopores in
the range 2-4 nm. The data also show that there was a widening of the
pores after the oxidation treatment. There are some enhancement in
the microregion and a slight reduction in the mesoporous range (2-4
nm). This may be due to a transition from pore width accommodating
one adsorbed layer to two, and two layers to three respectively .
|The fractinal dimensions (D) of two adsorbents are calculated from
Frenkel-Halsey-Hill (FHH) models. The unoxidized carbon sample
activated at 650°C has fraction dimension of D=2.1. This indicates that
the surface is very rough or irregular. Upon surface modification using
Na2CO3, the fractal dimension decreases (D=2.2).
|This suggests that the structure of modified carbon became more
ordered as the small crystallite and cross-linked structures were partially
decomposed. The reactivity of disorganized carbon is greater than that
of the crystallite carbon towards such type of reagent; therefore, the
carbon in cross-link was mainly consumed. The decomposition of the
cross-link leads to the release of plugged pores, which results in an
increase of surface area and pore volume, and a decrease of the fractal
dimension compared to parent carbon [15,16].
|Kinetic studies of anions adsorption
|Effect of agitation time: The time-profile of adsorption of NO3ˉ
and NO2ˉ, onto RS (ox.) carbon is presented in Figure 2. As agitation
time increases, anion removal also increases initially, but then gradually
approaches a more or less constant value, denoting attainment of
equilibrium. Obviously, equilibrium was attained after shaking for
about 10 h in both cases, beyond which there is no further increase in
|Nitrate adsorbed greater than nitrite. This is caused by nitrate
have high oxidizing strength. i.e., it is effectively able to extract enough
charge from RS (ox.) surface to form sorbed nitrate than in case of
|The kinetic curve of nitrate and nitrite adsorption in Figure 2
indicates that not only the surface of RS (ox.) can adsorb these ions,
but also, the inner surface is accessible for ions to diffuse. The former
rapid adsorption may be due to the ions adsorbed on the surface of
RS (ox.) directly and the latter shows adsorption mainly attributes to
long-range diffusion of anions in the inner surface of RS (ox.) where a
marginal increase in adsorption is observed up to time after which it is
|Kinetic rate parameters: The kinetic experimental data of anions
on RS (ox.) sorbent are simulated by the pseudo first-order and pseudo
second-order rate equation .
|Log(qe-q)=log qe-(K1/2.303)t 1st order (1)
|where qt and qe are the amount adsorbed (mgg-1) at time t and at
equilibrium time respectively and K1 and K2 are first and second-rate
constants of adsorption.
|The kinetic experimental data of nitrate and nitrite ions on RS (ox.)
are presented in Figure 3 and Table 3.
|The correlation coefficient R2 for the pseudo second-order
adsorption model has high value for the two anions and the calculated
equilibrium adsorption capacities qe is consistent with the experimental
|Remarkably, the kinetic data of the anions can be described well by
the pseudo-second-order rate equation (Table 3), the rate-limiting step
may be chemical sorption involving valency forces through sharing or
exchange of electrons between anions and adsorbent .
|Table 4 discuss the comparison between adsorption capacities of nitrate
and nitrite anions onto various adsorbents and showed that prepared has
higher monolayer adsorption capacity than the other ones.
|Intra-particle diffusion: The intra-particular diffusion rates (kp)
were determined from the plots of qe versus t0.5 as shown in Figure 4.
|It can be observed that the plots are not linear over the whole time
range and reflect a dual nature, with initial linear portion followed
by a plateau. This implies the anions are slowly transported via intraparticle
diffusion into the particles and is finally retained in the pores.
The rate constants of intraparticle diffusion were obtained from the
slopes of the straight lines and were found to be 0.62, and 0.3 mg g-1
h0.5 for nitrate, and nitrite respectively. However, the linear portion of
the curves not goes the origin (Figure 4) i.e., the pore diffusion is gets
another rate controlling step .
|Effect of pH
|Figure 5 shows that the effect of pH on the adsorption of nitrite
and nitrate is rather small in case of NO2 anions but amount adsorbed
increase gradually at pH 8 in case of nitrate anions. The broad pH range
(3-9) using RS (ox.) carbon makes it a promising adsorbent material
to remove nitrate and nitrite from water. Analogous results have been reported for the removal of nitrate on palladium-based catalysts
supported on activated carbons [21,22], nitrate by sepiolite , nitrate
by modified amine coconut coir , and nitrate and nitrite on ion
|Possible working mechanisms: The adsorption sites in active
carbons can be divided into two major types; these are
|(i) Hydrophobic surfaces comprising of the graphene layers; and
(ii) oxygen functional groups which are primarily hydrophilic. This
provides two main possibilities for nitrate and nitrite adsorption (a) adsorption by interaction between the p orbitals of the graphene layers
and anions; or (b) an ion exchange mechanism involving the functional
|The pH of the medium would definitely influence the course of the
2nd mechanism, but the 1st mechanism may operate over a large range
of pH without being affected much. The amount of nitrate and nitrite
adsorbed in the present work remained nearly constant in the pH range
of 2.0 -10.0 (Figure 6), and therefore, the adsorption of NO3ˉ and NO2ˉ
on RS (ox.) is expected to occur between the delocalised π-electrons of
the oxygen free Lewis basic sites and the free electrons of the anions
|Thermodynamic Studies for NO3 anions
|Temperature effect: The isotherm constants for the sorption of
NO3ˉ ions onto RS (ox.) carbon by the three models: Freundlich (F),
Langmuir (L), and Langmuir–Freundlich (LF) at different temperatures
are presented in Table 5.
|Based on the correlation coefficient values, R2, generally, LF
isotherm still found to be a good representative for the experimental
data over the whole concentration range with high correlation
|The qo values calculated from LF model were found to decrease
by increasing temperature. This is referred to the solubility of the
adsorbate. The solubility of KNO3 increases from 31.6 g per 100 g water
at 20oC to 110 g per 100 g water at 60oC.
|An alternative hypothesis would be due to an increase of
temperature causes a shift of the point of zero charge (pzc) of carbon
to low pH values and the surface charge density at a given pH becomes
more negative. This creates unfavorable conditions for adsorption of
|Thermodynamic parameters: The value of enthalpy change (ΔHo)
and entropy change (ΔSo) were obtained in Table 6.
|The thermodynamic equilibrium constant Ko equal to qo* b of
LF isotherm. The thermodynamic parameters determined for the
adsorption of NO3ˉ onto RS(ox.) carbon is given in Table 6 using the
following thermodynamic equations :
|where ΔGo is the standard free energy, R=Universal gas constant
(1.987 calmol-1 K-1 or 8.314 jmol-1 K-1) and T=Absolute temperature in
|The exothermic nature of adsorption is indicated by the negative
value of ΔHo. The entropy (ΔSo) value suggests no significant change
occurs in the internal structure of RS (ox.) carbon during the adsorption
. The sorption process takes place with an increase in the order
of the system. The positive value of ΔGo indicates the formation of
thermodynamically unstable adsorbed species. This positive value of
ΔGo indicates that better adsorption, is obtained at a lower temperature.
|Effect of hardness on nitrate removal
|The co-existing ions selected were calcium hydrogen carbonate that
is typically found in ground water and cause temporary hardness. It is
demonstrated in Figure 6. The nitrate removal is appreciably decreased
in the presence of Ca(HCO3)2. The inhibiting effect of these species can
be ascribed to the identical structures of NO3ˉ and HCO3ˉ ions. They
are both planar, and the angles between the N–O and C–O bonds are
identical and equal to 120°C. It can be thus reasonably believed that
HCO3ˉ anions competitively adsorb to the same active sites on the
surface of the RS (ox.) carbon [29,30].
|These results indicate that KMnO4 modified activated carbon
produced from rice straw is much better or even superior than any
of these sorbents. This is probably related to the sorption mechanism
which including both ion exchange and complexation.
|Indeed, one of the methods for decreasing the large volumes of waste
and toxic effluents produced by a variety of chemical processes is the
development of low-cost adsorbents, which is one of the major goals of
green chemistry. In this sense, the steam activated carbon derived from
rice straw and modified by potassium permanganate showed good
adsorption ability for nitrate and nitrite ins from aqueous solutions.
In this concern, Factors affecting the removal of nitrate and nitrite
ions in single-component systems were investigated: temperature, pH,
adsorbent concentration and carbon dosages. In the binary system
(NO3 and HCO3), the inhibitive effect of hardness on nitrate removal
is referred to the identical structure of NO3 and HCO3 ions and in the
system (NO3 and NO2), the nitrate sorption was suppressed due to the competitive effect whereas the nitrite sorption was promoted due to the
cooperative adsorption. In presence or absence of NOM, the adsorptive
capacity was similar, indicating that the affinity of nitrate for the carbon
surface is stronger than the attraction between nitrate and NOM.
|Many thanks to my friends in hot laboratories centre- Egyptian atomic energy
authority to help me in this research, also I want to thank all members in central
laboratory in college of science and humanities- Shaqra University for their great
efforts in this research.
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