ISSN 1067-8212, Russian Journal of Non-Ferrous Metals, 2009, Vol. 50, No. 1, pp. 9–12. © Allerton Press, Inc., 2009. Original Russian Text © Yu.S. Syrykh, V.I. Dudarev, T.Yu. Afonina, Yu.N. Moskaeva, 2009, published in Izvestiya VUZ. Tsvetnaya Metallurgiya, 2009, No. 1, pp. 14–17.

METALLURGY OF NON-FERROUS METALS

The Adsorption Extraction of Nickel(II) Ions from Aqueous Solutions Yu. S. Syrykh*, V. I. Dudarev**, T. Yu. Afonina***, and Yu. N. Moskaeva Irkutsk State Technical University, ul. Lermontova 83, Irkutsk, 664074 Russia *e-mail: [email protected] **e-mail: [email protected] ***e-mail: [email protected] Abstract—Processes of the sorption of nickel(II) ions from aqueous solutions on carbon adsorbents synthesized based on fossil coals are investigated. The maximal adsorption is observed in a neutral medium. Adsorption isotherms are obtained under static conditions in various temperature modes. It is shown that they are described satisfactorily by the Langmuir isotherm. The decrease in the value of adsorption as the temperature increases indicates that the process is exothermic. The kinetics of the process is investigated and thermodynamic parameters of adsorption are determined. DOI: 10.3103/S1067821209010039

Granulometric composition, % of the particles to the sizes

The production and waste waters of basic nickelproducing metallurgical enterprises contain high concentrations of Ni(II) ions. Processing such solutions by sorption methods can be economically sound. It is possible to use coal adsorbents with high selectivities and sorption capacities for this purpose. The sorption process is well-controlled and automated, which is doubtless an advantage.

Granulometric composition, % of the particles to the sizes: <0.5 mm 0.5–2.0 mm >2.0 mm Specific surface, m2/g Mechanical strength (by GOST (State Standard) 16188-70), % Summary volume of pores by water, cm3/g Sorption activity by iodine, % Apparent density, g/cm3

The purpose of this work is to investigate the adsorption capacity of new carbon sorbents of the AD-05-02 grade (obtained from cannel coals) with respect to the nickel(II) ions in complex aqueous solutions [1–2] with the help of isotherms and kinetic curves of adsorption. In this work we used methods of variable charges and concentrations, volumes, and their combinations. Adsorption from solutions was performed under static conditions. In preliminary kinetic experiments, we determined the time for that the equilibrium was established in the system “coal adsorbent– solution of metal salt.” Adsorption isotherms of metals at temperatures differing from room temperature were recorded in the temperature-stabilized installations, and the experiments at room temperature were performed using a mechanical shaker. We used model aqueous solutions of the chemically pure-grade (kh. ch.) nickel sulfate as the adsorbates in the experiments. The ion concentrations in the solutions were monitored using the standard methods of the quantitative analysis [3].

≤5 ≥90 5 ≥550 ≥68 ≥ 0.6 ≥50 0.55

It was established that the sorption is noticeably affected by the acidity of the medium. The experiments to determine the optimum pH value were performed under static conditions. The adsorbent charge of 0.5 g was placed into a solution of the salt of the corresponding metal with a volume of 100 ml. The value of pH varied from 1 to 11, and the metal concentration in the solution was 30 mg/l. The solution was stirred until the adsorption equilibrium was established; the time that this took varied from 30 to 60 min. It was revealed that the maximum adsorption of nickel(II) is observed in a neutral medium at pH = 7. One important factor determining the adsorption equilibrium is temperature. We obtained the sorption isotherms (Fig. 1) at T = 293, 313, and 333 K and various charges of the adsorbate. The adsorption isotherms of nickel(II) ions are described by the mechanism of monomolecular adsorption. A high sorption capacity of the carbon adsorbent is explained by the fact that active

Carbon adsorbents of the AD-05-2 grade are irregularly shaped black granules with an average particle size of 0.5–2.0 cm and have the following technical characteristics: 9

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Concentration on the sorbent, mg/g 90 80 70 60 50 40 30 20 10 0

293 K 313 K 333 K

accepted to be constant and equal to 0.1–0.5. Therefore, Eq. (1) is valid only for the range of initial and medium concentrations. The Freundlich constants were found graphically (Fig. 2), substituting Eq. (1) in logarithmic coordinates in the form of linear dependence log A = log k + ( 1/n ) log C .

The segment cut by the straight line on the ordinate equals log k, and the slope tangent of the plot to the abscissa equals 1/n. We show the result below. It indicates that as the temperature increases, the values of k decrease and the values of n increase.

5 10 15 20 25 Equilibrium concentration in the solution, mg/l

T, K k n

Fig. 1. Adsorption isotherms of nickel(II) ions at various temperatures and adsorbent charges.

centers can exist on the surface of pores which enhance its cation-exchange properties. Currently, there is no unique equation for describing adsorption from solutions. To process the experimental data that correspond to the medium part of the adsorption isotherm, the Freundlich equation is widely used: A = kC

1/n

,

(1)

where A is the value of adsorption, mol/g; C is the equilibrium concentration of the solution, mol/l; and k and 1/n are the constants. The constant k is the value of A at the adsorbate concentration C = 1 mol/l. The index 1/n is a proper fraction and characterizes the degree of approaching the adsorption isotherm to the straight line. In investigating the adsorption from solutions, the value of 1/n is log A, [mol/g] 2.5 2.0 1.5 1.0 0.5

(2)

(a)

293 15.8 1.52

313 13.5 1.56

333 13.2 1.67

The experimentally obtained adsorption isotherms were further described by the Langmuir equation of the adsorption isotherm, which is adequate for the process of attaining the limiting value of adsorption (A∞): KeC A = A ∞ -------------------, 1 + KeC

(3)

where K is the constant of adsorption equilibrium. Dividing the unity per the first and second members of expression (3), we acquire the equation of the straight line in the coordinates 1/A = f (1/C) 1 1 1 1 --- = ------ + ------------- ---- , A∞ A∞ K e C A

(4)

which allows us to graphically determine the constants A∞ and Ke in the Langmuir equation (Fig. 3).

log A, [mol/g] 3 (b) 2 1

0 0.5 1.0 1.5 0 log C , [mol/l] log A, [mol/g] 2.5 (c) 2.0 1.5 1.0 0.5 0 –0.5 0 0.5 1.0 1.5 log C, [mol/l]

0.5

1.0 1.5 log C, [mol/l]

Fig. 2. Isotherms of the sorption of nickel(II) ions by the AD-05-2 sorbent in the Freundlich coordinates at (a) T = 333, (b) 313, and (c) 293 K. RUSSIAN JOURNAL OF NON-FERROUS METALS

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THE ADSORPTION EXTRACTION OF NICKEL(II) IONS 1/A, [mol/g]–1 0.08 0.06 0.04 0.02 0

1/A, [mol/g]–1 0.08 (b) 0.06 0.04 0.02

(a)

0.5 1.0 1/C, [mol/l]–1

0

0.5 1.0 1/C, [mol/l]–1

1/A, [mol/g]–1 0.08 (c) 0.06 0.04 0.02 0 0

11

0.5 1.0 1/C, [mol/l]–1

Fig. 3. Linear dependence of adsorption on the concentration in the Langmuir coordinates at (a) T = 333, (b) 313, and (c) 293 K.

The segment cut on the ordinate equals 1/A∞, and by the slope tangent of the straight line we find the constant of the adsorption equilibrium Ke: tan α = 1/ ( A ∞ K e ).

(5)

The obtained results indicate that the limiting value of adsorption decreases as the temperature increases: T, K A∞ , mg/g Ke

293 133 0.128

313 125 0.120

333 111 0.120

– ∆G = RT ln K e ,

(6)

where CMe is the equilibrium concentration of the metal in the solution, mg/l; Q is the isosteric differential heat of adsorption, kJ/mol; and R is the gas constant, kJ/(mol K). From here, it follows that ∂ ln C Me⎞ . Q = – R ⎛ ----------------⎝ ∂ ( 1/T ) ⎠ A = const

T, K –∆G, J/mol

293 5000

313 4900

333 4600

CONCLUSIONS Processes of sorption of the nickel(II) ions from aqueous solutions on coal adsorbents synthesized on the basis of fossil coals were investigated. The kinetics

(7)

For most adsorbents, the sorption heat over the whole surface varies: its largest values are observed for the most active sites of the surface, where the molecules are adsorbed in the first turn because the exchanged functionally active groups of hydroxyl, carbonyl, and carboxyl are arranged there. As they are filled, the less active zones start to operate, and heat gradually decreases. By the slope angle of the isostere (Fig. 4), based on three experimentally obtained isotherms for RUSSIAN JOURNAL OF NON-FERROUS METALS

(8)

where Ke is the equilibrium constant. The results are presented below:

In the evaluation of the nature of adsorption, the values of thermal phenomena can serve as an important criterion. The integrated heat (called the sorption heat) is of practical interest. According to the Clapeyron– Clausius equation, ln C Me⎞ Q ⎛ ∂----------------= – ---- , ⎝ ∂ ( 1/T ) ⎠ A = const R

temperatures 293, 313, and 333 K (see Fig. 3), we calculated the value of Q for nickel(II), which was 2.76 kJ/mol, according to Eq. (7). Using the Arrhenius equation, we determined the activation energy of sorption Ea = 1.32 kJ/mol from the graphic dependence log K = f (1/T). The variation in the Gibbs energy was calculated by the formula

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logC, [mol/l] 1.352 1.350 1.348 1.346 1.344 1.342 1.340 1.338 0.0029 0.0030 0.0031 0.0032 0.0033 0.0034 0.0035 1/T, K–1 Fig. 4. Isostere that was constructed based on three experimental isotherms for T = 293, 313, and 333 K (see Fig. 3). No. 1

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of the process was investigated and thermodynamic parameters of sorption were determined. The adsorption isotherms were obtained under static conditions in different temperature modes. The results of the investigations showed that the carbon sorbents of the AD-05-2 grade are able to sorb the nickel(II) ions. Applying them in the sorption method of purification can be economically sound, and the obtained materials are promising for the selective extraction of the nickel(II) ions from industrial waste waters.

REFERENCES 1. Dudarev, V.I., Randin, O.I., Oznobikhin, L.M., and Leonov, S.B., Carbon Sorbents, Materialy Mezhdunarodnogo seminara (Proc. Int. Seminar), Kemerovo: KIUiU, 1997, p. 24. 2. Leonov, S.B., Elshin, V.V., Dudarev, V.I., et al., Uglerodnye sorbenty na osnove iskopaemykh uglei (Carbon Sorbents Based on Fossil Coals), Irkutsk: IrGTU, 2000. 3. Bulatov, M.I. and Kalinkin, I.P., Prakticheskoe rukovodstvo po fotokolorimetricheskim i spektrofotometricheskim metodam analiza (Practical Manual on Photocolorimetric and Spectrophotometric Methods of Analysis), Leningrad: Khimiya, 1985.

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