CC BY
92
KMYA PROBLEML9RÎ № 1 2018
ISSN 2221-8688
73
UDC 544.723.22
ADSORPTION OF CADMIUM IONS ISOTHERM FROM WATER INTO POLYMER
NANOCOMPOSITE
J.R. Imanova, A.A. Azizov, A.M. Nabiev, F.G. Khalilova, R.M. Alosmanov
Baku State University
Z. Khalilov str., 23, Baku AZ1148, Azerbaijan Republic : e-mail: r_alosmanov@rambler.ru
Polymer nanocomposite with zerovalent iron particles was used to remove Cd2+ from water. The adsorption of Cd2+ was examined as a function of initial metal ion concentration. Also, the equilibrium data were analyzed on the basis of various adsorption isotherm models, specifically Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich. The results showed that the adsorption process was in agreement with the Langmuir isotherm model. Keywords: polymer nanocomposite, adsorption, Langmuir isotherm, cadmium ions
INTRODUCTION
Cadmium is a toxic heavy metal of particular environmental and occupational importance [1]. It is discharged into the environment through the combustion of fossil fuels, metal production, application of phosphate fertilizers, electroplating, and the manufacturing of batteries, pigments, and screens [2-4]. This heavy metal leads to serious contamination of both soil and water. Cadmium has been classified as a human carcinogen and teratogen affecting lungs, kidneys, liver and reproductive organs [1,5]. The World Health Organization (WHO) has set a criterion of maximum concentration of 0.003 mg/L for Cd in drinking water [6]. Given the pervasive cadmium contamination and the low drinking water basis, there is an upsurge of interest in the development of
techniques to remove cadmium from contaminated water. Thus, scholars are focusing on the removal and recovery of cadmium, including various methods such as chemical precipitation [7], reduction-oxidation [8], ion-exchange [9], forward osmosis [10], biological process [11] which have been introduced for removal of cadmium from the aqueous solution. Among the different methods described above, adsorption process is attractive in consideration of its efficiency, economy and ease of operation, especially as lots of studies on the theme have been carried out [12-14].
The objective of the research is to inquire into the adsorption isotherm models and the cadmium removal by polymer nanocomposite (PNC).
MATERIALS .
Preparation of PNC
For preparation of PNC, commercial, sulfonated styrene-divinylbenzene and cross-linked copolymer has been used as polymer.
Note that the synthesis of PNC is based on
2+
borohydride reduction of Fe . Polymer (1.0 g) has been added to the stirred ethanol solution of FeCl2-4H2O (0.4 M, 300 ml). Also, 1M NaBH4 solution has then been added drop-wise to the Fe-polymer mixture with continuous stirring of the resulting solution. After the addition of NaBH4 solution, the mixture has been stirred for additional 20 min,
*iD METHODS
PNC collected and washed thrice with iso-propanol to prevent oxidation. Deionized deoxygenated water (sparged with nitrogen) used to prepare aqueous solutions. Adsorption studies
Batch adsorption experiments have been
carried out to enable an accurately weighted
2+
amount of PNC reach equilibrium with Cd aqueous solutions of various initial concentrations between 20 and 400 mg/L and temperatures ranging around 25 °C. Besides, the weighed samples of sorbent (0.1 g) have been placed into bottles and filled up with
solutions (0.3 L). The bottles underwent filtrate determined. The adsorption capacity (q, stirring by a temperature-controlled shaker mg/g) and adsorption degree (R, %) has been (IKA, Germany). In 24 hours the solution has calculated by the following equations:
2+
been filtrated and the concentration of Cd in
q = (Co - Ceq)• - (1)
m
(C0 - Ceq )
R = —-^ • 100% (2)
C
^ n
"0
2+
where C0, Ceq - initial and equilibrium concentration of Cd in solution respectively, mg/L, V- solution volume and m - sorbent dose (g).
All experiments have been performed in duplicate.
RESULTS AND DISCUSSION
2+
Effect of initial Cd concentration on adsorption process.
Table 1 shows that the equilibrium adsorption capacity of sorbent rises together
with the increase of initial concentration of
2+
Cd up to 420 mg/L. This is explained as being due to the fact that the initial
concentration of solute provides the major driving force to overcome the mass transfer
resistance. Rise in loading capacity of PNC at
2+
high initial concentrations of Cd may also be due to the appreciable interaction between metal ions and sorbent surface.
Table 1. Equilibrium uptake capacities and adsorption degree
2+
at different initial concentrations for Cd
C0, mg/L qeq, mg/g R, %
20 5.9 89.00
40 11.5 86.00
60 16.4 82.17
80 21.5 80.75
100 26.2 78.60
125 31.4 75.36
150 37.1 74.20
175 43.0 73.66
200 47.8 71.70
225 52.8 70.36
250 56.5 67.76
275 60.3 65.75
300 62.4 62.43
325 64.2 59.26
350 67.0 57.40
380 67.7 53.47
400 69.2 51.90
420 69.8 49.88
The maximum value of adsorption degree was identified as being due to 89.00 % at 20.0 mg/L. The results demonstrate a
tendency toward decrease in adsorption degree
2+
as the initial Cd concentration rose. At low initial concentrations all sorbate from the
adsorption medium tend to interact with binding sites of the sorbent leading to higher degrees of adsorption. In contrast, at high initial concentrations of Cd2+, low adsorption degrees became apparent due to the saturation of adsorption sites.
Adsorption isotherms
Equilibrium data may be analyzed using well-known adsorption isotherms which provide for the basis of adsorption systems. The most widely used isotherm equation for modeling the adsorption data is the Langmuir equation which is valid for monolayer sorption on the surface with a finite number of identical sites defined by Equation (3).
q max K L Ceq
q-=t+kc;- (3)
where KL is the adsorption equilibrium constant representing the affinity of binding sites (L/g) and qmax is the maximum amount of AR per PNC unit weight to form a complete monolayer on the surface (mg/g). It presents a practical limiting adsorption capacity in case where the surface is fully covered with metal ions. qmax and K can be determined from the linear plot of Ceq/qeq versus Ceq [15].
The Freundlich model is an empirical equation based on sorption on the heterogeneous surface. It is given as:
qeq = Kf (4)
where Kf and n are the Freundlich constants that indicate relative capacity and adsorption intensity, respectively. The Freundlich equation can be linearized by taking logarithms and constants [16].
Temkin isotherm is another adsorption model considering adsorbent-adsorbate interactions. The model assumes that these interactions cause a decrease in the heat of adsorption of molecules in the layer and the binding energies show a uniform distribution
in the adsorption process [17]. This isotherm can be expressed by the following formula: qe = B ln A + B ln Ceq (5)
In Equation (5) constant A denotes Temkin constant used to examine adsorbate-adsorbate interactions and B is the constant related to adsorption heat. A and B can be determined from plot of qe vs. lnCe.
The Dubinin-Radushkevich isotherm model is a semi-empirical equation where adsorption follows a pore filling mechanism. It assumes that the adsorption has a multilayer character, involves van der Waals forces and is applicable for physical adsorption processes. The Dubinin-Radushkevich [18] equation has the following form
qe = qm e
-ße2
(6)
where qm is the theoretical isotherm saturation capacity (mg/g), / is a constant related to the sorption energy, and e is the Polanyi potential which is related to the equilibrium concentration as follows e = RT ln(1 + c-) (7)
where R is the gas constant (8.314 J mol-1 K-1) and T is the absolute temperature.
The constant / gives the mean free energy, E, of sorption per molecule of the sorbate when it is transferred to the surface of the solid from infinity in the solution and can be computed using the relationship
E = (8)
The /, and qm parameters can be determined from plot of lnCeq versus e .
All obtained isotherm constants and correlation coefficients are listed in Table 2.
Table 2. Langmuir, Freundlich, Temkin, and Dubinin-Radushkevich isotherm model parameters _and correlation coefficients for adsorption of cadmium on PNC_
Langmuir Parameters
Kl, L/g qmax, mg/g r2
0.022 90.91 0.9940
Freundlich Parameters
Kf n r2
4.677 1.842 0.9740
Temkin Parameters
A B r2
0.3878 16.47 0.9570
Dubinin-Radushkevich Parameters
qm, ß106, E, kC/mol r2
44.36 2.0 0.5 0.5570
As the four systems reviewed, Langmuir isotherm correlates (r2 > 0.9900) rather with the experimental data of adsorption equilibrium of Cd2+ on the PNC than Freundlich, Temkin and Dubinin-Radushkevich isotherms. The adsorption data of metal ions in accordance with the Langmuir isotherm shows that the binding energy of the entire surface of PNC is uniform. By the way, the whole surface has an identical adsorption activity. In line with Langmuir isotherm, the adsorption data of Cd2+ also show that the adsorbed ions do not interact or compete with each other to form a monolayer. This phenomenon also indicates that chemisorption
has the principal removal mechanism in the
adsorption process. The maximum adsorption
2+
(#max) values for Cd are in good accordance with the experimentally obtained values.
As seen from Table 2, according to Freindlich model the n value was found to make up above >1.0. The value of E as set forth in Dubinin-Radushkevich model is less than 1 kC/mol. From this it follows that the physical adsorption process prevails. However, in line with this model the value of correlation coefficient the regression parameter r (0.5570) showed that this isotherm model did not provided a very good fit to the experimental data.
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ИЗОТЕРМА СОРБЦИИ ИОНОВ КАДМИЯ ИЗ ВОДЫ НА ПОЛИМЕРНОМ
НАНОКОМПОЗИТЕ
Дж.Р. Иманова, A.A. A3U3oe, A.M. Набиев, Ф.Г. Халилова, P.M. Алосманов
Бакинский государственный университет AZ1148 Баку, ул. З.Халилова, 23; e-mail: r_alosmanov@rambler. ru
Для удаления ионов Cd2+ из воды использовали полимерный нанокомпозит с нульвалентными частицами железа. Адсорбцию Cd2+ исследовали в зависимости от начальной концентрации иона металла. Данные о равновесии анализировали на основе различных моделей изотерм адсорбции, а именно, Ленгмюра, Фрейндлиха, Темкина и Дубинина-Радушкевича. Результаты показали, что процесс адсорбции согласуется с моделью Ленгмюра.
Ключевые слова: полимерный нанокомпозит, адсорбция, изотерма Ленгмюра, ионы Cd2+
kadmíum íonlarinin sulu mohl ullardan polímer nanokompozítlo
sorbsíya ízotermí
C.R. Imanova, A.3. 3zizov, A.M. Nabiyev, F.Q. Xalilova, R.M. Alosmanov
Baki Dövlat Universiteti AZ 1148 Baki, Z.Xalilov küg., 23; e-mail: r_alosmanov@rambler. ru
Kadmium ionlarinin sudan kanarla^dirilmasi ügün tarkibinda sifir valentli damir hissaciklari olan polimer nanokompozit istifada olunmu^dur. Kadmium ionlarinin sorbsiyasi metal ionlarinin ilkin qatiligindan asili olaraq tadqiq edilmi^dir. Sorbsiya izotermi Lenqmür, Freyndlix, Temkin va Dubinin-Radu^kevig modelhrih i^hnmi^dir. Müayyan olunmu^dur ki, sorbsiya prosesi Lenqmür izoterm modeli ila yax^i tasvir olunur.
Agar sözfor: polimer nanokompozit, adsorbsiya, Lenqmür izotermi, kadmium ionlari
Received 27.01.2018.
