CC BY
95
УДК 544.4
USE OF NATURAL ZEOLITE FOR AMMONIUM ION REMOVAL FROM AQUEOUS SOLUTION
© Munkhjargal Dashnyam, Doctor of Biological Sciences, Lecturer, Defency University of Mongolia, Department of Chemistry and Disaster Prevention
University Street 1, Ulaanbaatar, 14201, Mongolia, e-mail: [email protected] © Oyuntsetseg Janchiv, Doctor of Chemical Sciences, senior researcher, Mongolia, Institute of Chemistry and Chemical Technology MAS
51, Peace Ave., Ulaanbaatar, Mongolia, e-mail: [email protected] © Ulambayar Rinchinnorov, researcher, Institute of Chemistry and Chemical Technology MAS 51, Peace Ave., Ulaanbaatar, Mongolia, e-mail: [email protected]
© Ganbaatar Jamsranjav, Doctor of Biological Sciences, senior researcher, Institute of Chemistry and Chemical Technology MAS, Mongolia, Ulaanbaatar Peace Ave., 51, e-mail: [email protected]
This study assesses the potential of natural Mongolian clinoptilolite for ammonium ion removal from aqueous solution. The effect of relevant parameters, such as pH, adsorbent dosage, contact time and initial ammonium concentration was examined, respectively. The results show that pH contributes to ammonium removal efficiency as it can influence both the character of the exchanging ions and clinoptilolite itself, ammonium removal by clinoptilolite occurs rapidly within the first 15 minutes of contact time, the ammonium removal capacity of clinoptilolite increases with the increase of the initial ammonium concentration. Langmuir and Freundlich models were applied to describe the equilibrium isotherms of ammonium uptake. Freundlich model correlates very well with the experimental data. Based on the results, it can be concluded that natural zeolite is suitable for the removal of NH4+ ions in sewage. Keywords: clinoptilolite, natural zeolite, ammonium ion, aqueous solution, Langmuir and Freundlich models, adsorbent.
ИСПОЛЬЗОВАНИЕ ПРИРОДНОГО ЦЕОЛИТА ДЛЯ УДАЛЕНИЯ ИОНОВ АММОНИЯ ИЗ ВОДНОГО РАСТВОРА
Мунхжаргал Дашниям, доктор технических наук, преподаватель Университета Монголии, факультет химии и предотвращение стихийных бедствий
Монголия, 1420, Улан-Батор, ул. Университетская, 1, e-mail: [email protected] Оюунцэцэг Жанчив, доктор химических наук, Институт химии и химической технологии Монгольской академии наук
Монголия, Улан-Батор, просп. Мира, 51, e-mail: [email protected]
Уламбаяр Ринчинноров, научный сотрудник, Институт химии и химической технологии Монгольской академии наук
Монголия, Улан-Батор, просп. Мира, 51, e-mail: [email protected]
Ганбатор Жамсанжив, доктор биологических наук, Институт химии и химической технологии Монгольской академии наук
Монголия, Улан-Батор, просп. Мира, 51, e-mail: [email protected]
Проведено исследование, позволяющее оценить потенциал природных монгольских клиноптилолитов для удаления иона аммония из водного раствора. Изучены влияние рН, дозировка адсорбента, время контакта и начальная концентрация аммония. Из результатов видно, что рН оказывает влияние на эффективность удаления аммония и на характер обмена ионов в клиноптилолите. Удаление аммония из клиноптилолита происходит быстро, в течение первых 15 мин. Время контакта для удаления аммония из емкости клиноптилолита увеличивается с ростом начальной концентрации аммония. Для описания равновесной изотермы использованы модели Ленгмюра и Фрейндлиха. Модель Фрейндлиха очень хорошо согласуется с экспериментальными данными. Результы показывают, что природный цеолит можно использовать для удаления NH4+-hohob в сточных водах.
Ключевые слова: клиноптилолиты, природный цеолит, аммоний, модели Ленгмюра и Фрейндлиха, адсорбента.
Ammonia nitrogen contributes to accelerated eutrophication of lakes and rivers, dissolved oxygen depletion and fish toxicity in receiving water. The most widely used traditional processes of ammonia removal are air stripping, ion exchange and biological nitrification-denitrification.
The efficiency of the process of air stripping, and biological nitrification-denitrification is significantly impaired by the low temperature in winter. Ion exchange, therefore, is more competitive because of little influence of the low winter temperature in Mongolia and particularly its relative simplicity of application and operation.
Clinoptilolite, one of natural zeolites, has been found very effective in removing ammonia from water by means of its excellent ion exchange capacity since the seventies of the last century. Natural zeolite is porous material with high cation exchange capacity (CEC), cation selectivity, higher void volume and great affinity for NH44 [1].
General formula of zeolite is: (Mx+, My2+)(Al(x+2y)Sin_(x+2y)O2n)-mH2O, where M+ and M2+ are monovalent
and divalent cations such as Na , K and Ca , Mg , Ba respectively. They are called the exchangeable cations. Al3+ and Si4+ are known as the structural cations, and they make up the framework of the structure with O [1]. In several studies, the authors have also reported the use of natural zeolite as a sorbent for trace metals, N compounds and cations [2-6].
The main objectives of this study were to investigate the effect of pH, dosage of adsorbent and shaking time on ion exchange of NH4+ by the natural Mongolian (Urgun) zeolite from aqueous solution and to determine the equilibrium isotherms.
In our country, an excess of the permissible level of wastewater treatment plant effluent of ammonium is controlled during most of the year by the Environment and Metrology laboratory which data are presented in the article [10]. Therefore, it is necessary to reduce ammonium contents.
Materials and methods
Clinoptilolite used as ion exchanger in the experiments was obtained in the province of Dornogovi, Mongolia. The chemical composition of clinoptilolite used in the study is shown in Table 1.
Table 1
Chemical composition of clinoptilolite
Component S1O2 T1O2 AI2O3 Fe2O3 CaO MgO Na2O K2O P2O5 LOI
( %) 65.78 0.36 13.7 2.25 1.44 0.96 2.99 2.68 0.09 9.64
Natural zeolite samples were crushed in a mortar and sieved using 100 ^m sieves. The crushed samples were dried in an oven at 105°C for 6 hours before being used in the experiments. Preliminary experiments were conducted to optimize general pattern of NH4+ ion removal from aqueous solution; pH of solution, dosage of adsorbent and shaking time. The ion exchange of NH4+ ion for zeolite was carried out by the batch method. The batch experiments were conducted with 0.5 g of adsorbent in 50 ml of solution in the range of
1-30 mg/l of initial NH+ concentrations. Analytical grades of ammonium chloride salt (NH4Q) and deion-ised water were used to prepare stock NH4+ solutions. The stock solutions were diluted to prepare working solutions. The conical flasks containing sorbate and sorbent were placed in a shaker and shaken at room temperature. After equilibrium time, samples were filtered through whatman 42 filter paper. The equilibrium concentrations of ammonium were determined analyzing samples after filtration in the laboratory by the col-orimetric method using Nessler solution. The removal efficiency (%) and the amounts of exchanged NH4+
ion (Qe) by zeolite were computed using eqs. (1) and (2), respectively:
---- (1)
m
where Qe is the amount of exchanged ammonium ions (mg/g), C0 and Ce are the initial and equilibrium concentrations of ammonium in solution (mg/L), respectively. V is the solution volume (L) and m is the adsorbent weight (g).
Results and discussion
Effect of exchangeable cations
For the purpose of increase in the cation - exchange capacity of natural zeolite was activated by solutions 0.1N HCl and 0.1N NaCl. Two replicates of 10 g of natural zeolites were shaken with 200 ml of 0.1N HCl and NaCl salt for 7 h. The exchanged forms were washed with distilled water and dried in an electric oven at 105oC for 2-3 h before being used for adsorption purpose.
The adsorbed amounts were determined using eqs. 1: 0.25 mg/g, 0.48 mg/g and 0.63 mg/g - natural zeolite, zeolite that formed its exchangeable cations to Na, and zeolite that formed its exchangeable cations to H, respectively (fig. 1).
Fig. 1. The effect of exchangeable cation (N-Z - natural zeolite, Na-Z - Na form zeolite, H-Z - H form zeolite, NH4+ ion concentration - 10 mg/L, adsorbent dosage - 0,5 g; shaking time - 180 min, temperature - 20 oC)
It can be seen that the adsorption capacity of zeolite increased when the exchangeable ions of zeolite were replaced with the same type of ions.
Effect of pH
The removal of NH4+ ion from aqueous solution using natural zeolite was studied at pH values 2-10 and the data obtained were given in fig. 2.
The removal efficiency of NH+ ions by zeolite increased with the increase of zeolite amount, and plateau occurred at 1.0 g of adsorbent. Fig. 3 also indicated that NH+ ion removal was negligible at higher than 1.0 g amounts of adsorbent. This may be attributed to the formation of aggregates at higher solid/liquid ratio or precipitation of particles.
Equilibrium studies
Effect of initial ammonium
Ammonium exchange by natural clinoptilolite was studied at different initial NH4+ concentrations in the range of 5-30 mg/L. As shown in fig. 3, ammonium exchange capacity increased with the increase of initial
NH+ concentration that is the result of driving force increase. The rate of sorption to the surface should be proportional to the driving force, time and area. The driving force is the solution concentration and the area is the amount of bare surface [8]. For lower initial concentration of NH4+, equilibrium time was lower than higher concentration because of the increased competition for the active sites with increasing in NH4+ concentration. This is consistent with the ion exchange surface becoming increasingly saturated with ammonium ion.
Fig. 2. The effect of pH on NH4+ ion removal using natural zeolite (NH4+ ion concentration - 10 mg/L, adsorbent dosage - 0.5 g, shaking time - 180 min, temperature - 200C)
Fig. 3. The effect of adsorbent dosage on NH4+ ion removal using natural zeolite (NH4+ ion concentration - 10 mg/L, shaking time - 180 min, temperature - 20oC)
Effect of contact time
Fig. 4 shows that ammonium ion removal by clinoptilolite is high in the initial 15 min, but thereafter the rate significantly levels off and eventually approaches zero when equilibrium is attained. These changes in the rate of ammonium removal might occur due to the fact that initially all adsorbent sites were vacant and the solute concentration gradient was high. Afterwards ammonium uptake rate by clinoptilolite decreased significantly, due to the decrease in adsorption sites. A decreasing removal rate, particularly at the end of the experiment, indicates a possible monolayer of ammonium ions on the outer surface and pores of the clinoptilolite and pore diffusion onto the inner surface of clinoptilolite particles through the film due to continuous shaking maintained during the experiment.
1.6 1.4 1.2 1
0.8 0.6 0.4 0.2 О
■ 5mg/L
■ 10mg/L - 30mg/L
100 150 200
Time, min.
Fig. 4. The effect of contact time on NH4+ ion removal using natural zeolite (NH4+ ion concentrations - 5 mg/L, 10 mg/L and 30 mg/L, adsorbent dosage - 0.5 g, pH - 6; temperature - 20oC)
Adsorption isotherm
Two important physico-chemical aspects to evaluate adsorption process as a unit operation are the equilibria of the adsorption and the kinetics. Equilibrium studies give the capacity of the adsorbent [9]. The equilibrium relationships between adsorbent and adsorbate are described by adsorption isotherms, usually the ratio between the quantities of the adsorbed and the remained ones in the solution at a fixed temperature at equilibrium. There are two types of adsorption isotherms: Langmuir adsorption isotherms and Freundlich ones.
Langmuir isotherm
Irving Langmuir, an American chemist who was awarded the Nobel Prize for Chemistry in 1932 for "his discoveries and researches in the realm of surface chemistry", developed a relationship between the amount of gas adsorbed on the surface and its pressure. Such equations are now referred to as Langmuir adsorption isotherms, a theoretical adsorption isotherm in the ideal case. The Langmuir adsorption isotherm is often used for the solute adsorption from liquid solution. The Langmuir adsorption isotherm is perhaps the best known one to describe adsorption and is often expressed as: [10]
K ■ C
C
1/
■ = Q max •
C
a + C
(3)
0=0 1 + kc = o / k + c
NC Ncmax Ncmax
Qmax - maximum adsorption capacity (mg/g); K - an adsorption constant (L/g) and a=1/K; C - metal initial concentration
Then, by raising both members of the previous equation at -1 it is possible to obtain the following expression:
1 _ a + C _( a \ 1 1
Q Q max • C ^ Q max j C Q max (4)
Considering the axes y=1/Q and x=1/C, it is possible to have a linear function of the type y=dx+b, where d=a/0max and b=1/0max. Then 1/0 max is equal to the intercept of this straight line on the vertical axis, while a/Q max is the slope of the straight-line equation. The correlation coefficient (R2) and b and d values have been obtained from the linear equation. By this procedure Q max and a have been derived for each tracer in each soil.
Fig. 5. Langmuir adsorption isotherms of ammonium ion on zeolite
0
50
250
300
Freundlich isotherm
Herbert Max Finley Freundlich, a German physical chemist, presented an empirical adsorption isotherm for non-ideal systems in 1906. The Freundlich isotherm is the earliest known relationship describing the adsorption equation and is often expressed as: [10]
A plot of lnCe against lnQe yielding a straight line indicates the confirmation of the Freundlich isotherm for adsorption. The constants can be determined from the slope and the intercept.
Qe = Kf • Ce1n (5),
where: Qe - is the adsorption density (mg of adsorbate per g of adsorbent), Ce - is the concentration of adsorbate in solution (mg/l), Kf - and n are the empirical constants dependent on several environmental factors and n is greater than one. This equation is conveniently used in the linear form by taking the logarithmic of both sides as:
log(Qe) = log(Kf) + 1/nlog(Ce) (6) The isotherms are compared based on the parameter values with experimental data 303 K as shown in fig. 7. Freundlich isotherm shows better fit than Langmuir isotherm.
0.3 0.2 0.1
-0-0.1 -0.2 -0.3
-0.4
05 -0.6 -0.7
y = 0.5741x - 0.3811 R2 = 0.9907
LogCe
Fig. 6. Linear Freundlich isotherm of ammonium ion sorption on zeolite
Adsorption isotherm of zeolite
Table
T (K) Langmuir constant Freundlich constant
Qmax (mg/g) K (l/mg) R2 Kf (mg/g) n R2
303 1.64 0.345 0.99 0.4158 1.74 0.9907
Fig. 7. Adsorption isotherms at pH 6.0 and T=303 K
0.5
0
1.5
Conclusion
The experimental parameters such as solution pH, contact time, and adsorbent dosage influence NH4+ ion removal from aqueous system by zeolite. Freundlich model yields a much better (R2=0.9907) fit than that of the Langmuir model (R2=0.99).
Based on the results, it can be concluded that the natural Mongolian (Urgun) zeolite is suitable for NH4+ ion removal from aqueous solution. Moreover, zeolite can be recommended for wastewater treatments and agricultural purposes in terms of sustainability of environmental quality.
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