Carbon, Vol. 33, No. 2, pp. 215-220, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved ooO8-6223/95 $9.50 + .OO
ADSORPTION OF DYSPROSIUM IONS ON ACTIVATED CHARCOAL FROM AQUEOUS SOLUTIONS and JAVED HANIF Science and Technology, P.O. Box 13.56, Islamabad
RIAZ QADEER Pakistan
(Received 13 July 1994; accepted in revised form 4 October 1994) Abstract-The adsorption of dysprosium ions onto activated charcoal from aqueous solution has been investigated in relation to pertinent variables, such as shaking time, pH, concentration of dysprosium ions, and temperature. The conditions leading to maximum adsorption have been established. The adsorption of dysprosium ions obeys the Langmuir and the Dubinin-Radushkevich isotherm equations. Thermodynamic quantities, namely AH and AS, have been calculated from the slopes and intercepts of plots of In (Ko) versus l/T. The results indicate that the adsorption of dysprosium ions on activated charcoal is an endothermic process. The influence of different cations and anions on the adsorption of dysprosium ions has been examined. The adsorption of other metal ions on activated charcoal has been measured un-
der specified conditions to evaluate their selectivity. Approximately 98% of the dysprosium adsorbed on the activated charcoal was recovered using 40 ml of 3M HNOs solution. Key Words-Activated
charcoal, dysprosium ions, adsorption,
Activated charcoal is a widely used material that is well known for its adsorption properties. Activated charcoals are non-hazardous, processed carbonaceous products having an intricate porous structure and a large internal surface area that is contained predominantly within micropores. The adsorbent properties of activated charcoals are essentially attributed to their large surface area, high degree of surface reactivity, universal adsorption effect, and favorable pore size distribution. The latter makes their internal surface accessible and enhances their adsorption rate without sacrificing their mechanical strength[ I]. Due to these properties, activated charcoals have found many applications in industry for separation and purification purposes, such as the removal of toxic and healthhazardous particles and ions from solutions. We have previously used activated charcoal for the adsorption of strontium, samarium, gadolinium, europium, thorium, uranium, and cerium ions from aqueous solutions. The present work describes our investigations of the adsorption of dysprosium ions on activated charcoal. The adsorption of dysprosium ion on solids is important for its preconcentration, because it has numerous applications in nuclear and metallurgical industries. Due to its thermal neutron adsorption cross-section (930b) and high melting point (1412”C), dysprosium along with special steel is used to make nuclear reactor control rods. Moreover, in combination with vanadium and other rare earth, dysprosium has been used in making laser materials. Previously, a few reports have been published on the adsorption of dysprosium ions on tungsten[9-lo], mo-
aqueous solution, selectivity, elution.
lybdenum[l l] and copper[l2] surfaces. No data are available on its adsorption on activated charcoal.
The chemicals used in this study were dysprosium nitrate (Rare Earth Product, 99.999%) and a commercial activated charcoal (M/S British Drug House, BDH; item No. 33032). The B.E.T. surface area, determined by nitrogen adsorption, was found to be 980 m2/g. The porosity and pore volume were found to be 75.74% and 1.43 cm3/g, respectively, and average particle size was 3.7 + 0.2 pm.
2.2 Instruments An energy dispersive X-ray fluorescence (EDXRF) spectrometer, XR-500 from Links System, U.K., equipped with an 860 analyzer, a Si(Li) detector and an X-ray tube K5012 SV with tungsten anode, was used for measuring the dysprosium ion concentration in the solutions, with an error within 2.0%. A Hetofrig shaker from Heto Birkerod, Denmark, was used for the adsorption studies. The surface area of the activated charcoal was determined using Quantasorb Sorption System (M/S Quanta Chrome Corporation, N.Y.). An Autoscanmercury porosimeter (M/S Quantachrome Corporation, N.Y.) was used to measure the pore volume and porosity of the activated charcoal. Sub Sieve Sizer-95 supplied by Fischer Scientific Company was used to carry out the particle size analysis of the activated charcoal.
2.3 Procedure Correspondence should be addressed Senior Scientific Officer, PINSTECH, Islamabad Pakistan.
Adsorption measurements were carried out via a batch technique at room temperature (22 + O.SOC), except where otherwise specified. Accordingly, 10 ml of
to: Dr. Riaz Qadeer, P.O. Box No. 1356,
R. QADEER and J. HANIF
dysprosium-containing solutions of known concentration were shaken with about 0.1 g of dry activated charcoal in 250-ml reagent glass bottles for a given time period. The solutions were then filtered through Whatman filter paper No. 40 (circular, 14.0 cm). The first 2-3 ml portion of the filtrate was rejected because of the adsorption of dysprosium ions on the filter paper. The concentration of dysprosium ions with mearing filtrate was determined by means of the EDXRF spectrometer and was corrected for losses due to adsorption on the walls of the glass bottles by running blank experiments (i.e., without activated charcoal added). The percentage adsorption and distribution coefficient (K,,) were computed using the following equations:
(C; - (7)
x loo (1)
c, - C, v (Ko) = ___ x -M C,
where C, and C’ denote the initial and final concentration of dysprosium ions in the solution, respectively. In eqn (2), V is the volume of the solution and M is the amount of the activated charcoal used.
Preliminary investigations were conducted to ascertain the time required to reach an equilibrium between dysprosium ions and the activated charcoal. This exploratory work involved shaking 10 ml of a solution containing 2.0 g/l of dysprosium (pH = 3.66) with 0.1 g dry activated charcoal for different intervals of time from 2 to 120 minutes. Figure 1 shows the result-
ing variation in percentage adsorption and in the distribution coefficient (KD) with shaking time. It is clear that, under the conditions chosen, the adsorption of dysprosium ions is rapid in its initial stages, and that gradually the process slows down and subsequently attains a constant value after about 60 minutes (i.e., when adsorption equilibrium is established). For this reason, a shaking time of 60 minutes was selected for all subsequent experiments. The adsorption rate constants for the two stages of dysprosium ion uptake on activated charcoal are determined using the eqn[l3]: [l - F(t)]
where F( t ) is the fraction of dysprosium ions adsorbed at time t on activated charcoal, k, and k2 are the rate constants for the first (I) and second (II) stages of adsorption, respectively, and Ai and AZ are constants. Plots of In [ 1 - F( t )] versus t for dysprosium ion adsorption on activated charcoal are given in Fig. 2. The values of k, , kZ, A, and A2 obtained from these curves are 9.22 x 10p2/min, 6.24 x 10-4/min, 0.15, and 0.36, respectively. The influence of pH on the adsorption of dysprosium ions onto activated charcoal was studied with the dysprosium ion concentration, shaking time, and amount of activated charcoal fixed at 2.0 g/l, 60 minutes, and 0.1 g, respectively. Figure 3 shows the influence of pH on the adsorption of dysprosium ions on activated charcoal. The percentage adsorption and the distribution coefficient (KD) increase with increasing pH up to a value of 4 and then start to decrease. Maximum adsorption occurs at a pH of 4 and, hence, a buffer with a pH of 4 (Fluka, item No. 82560) was used in all subsequent experiments. The behaviour of dysprosium ions in aqueous solution is a complex phenomenon, in the sense that dysprosium ions may be present as ions exhibiting
t (mini pS
Fig. 1. Adsorption of dysprosium on activated a function of shaking time.
Fig. 2. Plot of In [ 1 - F(t)] versus t for dysprosium sorption on activated charcoal.
Adsorption of dysprosium ions
.E ._ z
.S -400 20
Fig. 3. Influence of pH on the adsorption of dysprosium ions on activated charcoal.
Fig. 4. Distribution of various hydrolyzed species of dysprodifferent compositions and different degrees of activity. Therefore, it is necessary to ascertain the nature of dysprosium ions in solution in order to understand their adsorption behaviour towards activated charcoal. The percentages of hydrolyzed species of dysprosium ions at a total dysprosium concentration of 2.0 g/l were calculated from the following equilibria and their respective hydrolysis constants[ 121: _ Dy3+ + H,O = Dy(OH)*+
K I = [email protected]
+ Hz0 + Dy(OH);
K2 = 1O-8.2 Dy(OH):
+ Hz0 = Dy(OH);
+ H,O = Dy(OH), K
K3 = 1O-8.5 Dy(OH),
+ H+, 4
The percentages of all ionic species, namely Dy3+, *+, Dy(OH):, Dy(OH)$ and Dy(OH), at difDY (OH) ferent pH values, have been estimated and are plotted versus pH in Fig. 4. It is evident from this figure that at a pH between 1 and 4 the predominant species is Dy3+. At a pH of 5, the species Dy(OH)*+ has a value of 0.037% but, at a pH of 8, it has a maximum value of 11.3%. However, above a pH of 8, its value starts decreasing. Dy(OH): is calculated at a pH of 5 to be < 1 .O%, having a maximum value at a pH of 8 (14.65%) and then decreasing to 1.49% at a pH of 10. Whereas the species Dy(OH)i is insignificant at a pH of 6, its extent is 71.28% at a pH of 9 and 63.60% at a pH of 10. At pH values below 8, Dy(OH), is negligibly small; however, its percentage at a pH of 10 is 33.40%. Based on the above calculations, it is concluded that from a pH of 1 to 4, the Dy3+ ions exist as such in aqueous solutions. In this pH range, the
sium ions in solution
competitive adsorption of H30+ and Dy3+ ions varies in accordance with the acidity of the solutions. As the pH of solution increases from 1 to 4, the adsorption of H30+ ions decreases, and that of Dy3+ ions goes up. Above a pH of 4, the Dy3+ ions start to get hydrolyzed, resulting in the formation of hydroxide ions such as Dy(OH)‘+, DY (OH):, DY (OH)!, and Dy(OH),. These hydroxide ions are weakly adsorbed as compared to Dy 3f ions; therefore, the adsorption of dysprosium starts decreasing above pH 5 and up to 8. Above a pH of 8, the adsorption process could not be followed because of the formation of insoluble dysprosium complexes in aqueous solution. The adsorption of dysprosium ions on the activated charcoal was studied as a function of total dysprosium ion concentration in the range of 1 .O to 7.0 g/l on a buffer solution at pH = 4. The results, depicted in Fig. 5, show that the percentage adsorption and the distribution coefficient (KD) decreased as the dysprosium ion concentration increased, thereby indicating that energetically less favourable sites become involved in the adsorption process with increasing dysprosium concentration. The data concerning the dependence of the extent of adsorption on the dysprosium concentration were subjected to examination by means of the Freundlich, Langmuir, and Dubinin-Radushkevich (D-R) isotherm equations. The Freundlich equation was used in the form:
q = AC””
where q is the amount of dysprosium ions adsorbed per gram of the activated charcoal, C is the equilibrium concentration of dysprosium ions in solution, and A and n are constants that can be related to the strength of the adsorptive bond and the adsorption bond distribution, respectively. The Freundlich plot
and J. HANIF
In q = In qm - K’t2
Fig. 5. Effect of dysprosium ion concentration on its adsorption onto activated charcoal.
of log q versus log C shown in Fig. 6(a) demonstrates the non-validity of the equation over the whole range of dysprosium ion concentration studied here. The Langmuir equation was applied in the form:
where E = R T ln( 1 + l/C), K’ is the constant related to the adsorption energy (mo12/kJ2), R is the gas constant, and T is the absolute temperature. The quantities q, qm, and C have their previously defined meanings. A straight line is obtained upon plotting In q versus e2 as shown in Fig. 6(c), indicating that dysprosium ion adsorption onto activated charcoal also obeys the D-R isotherm equation. Values of qm and K’ calculated from the intercept and the slope of the plot were 0.29 (g/g) and 0.0247 (mo12/kJ2), respectively. The influence of temperature on the adsorption of dysprosium ions was also investigated. For such studies, the dysprosium concentration in the solutions used were 2.0, 3.0, 4.0, 5.0, and 6.0 g/l and the temperature was varied from 10°C to 60°C in 10°C steps while other parameters were kept constant. Figure 7 shows that KD values increase with increasing temperature. The thermodynamic quantities AH, AS and AC of dysprosium ion adsorption were calculated from the KD values using the following relations:
where C and q have been already defined, qm is a measure of the monolayer capacity, and K is a constant related to the heat of adsorption. A straight line was obtained by plotting C/q against C, Fig. 6(b), indicating the conformity of the data to the Langmuir equation in the whole concentration range. Values of constants qm and K from the slope and intercept of the plot in Fig. 6(b) were 0.294 (g/g) and 8.94 (l/g), respectively.
The values of AH and AS were computed from the slopes and intercepts of the linear variation of In KD with the reciprocal of temperature, Fig. 8, and are given in Table 1 along with the values of AC determined from eqn (12). The positive values of AH show that the adsorption of dysprosium ions on activated
=: ," " 1 s\ D s
Dy soln. cont. (g/l
03.0 n 6.0
02.0 05.0 0
c (g /II
Fig. 6. (a) Freundlich, (b) Langmuir, and (c) D-R isotherm plots for the adsorption of dysprosium ions onto activated charcoal.
Fig. 7. Effect of temperature on the adsorption sium ions on activated charcoal.
for the adsorption
ions on activated charcoal
AG (kJ/mol) (kJ%ol) 58.11 28.71 24.23 19.98 13.68
2.0 3.0 4.0 5.0 6.0
-15.47 -13.74 -12.56 -11.15 -8.96
-18.07 -15.24 -13.86 -12.25 -9.76
(kJ/dtzmol) 0.26 0.15 0.13 0.11 0.08
charcoal is an endothermic process, a fact that is quite contrary to the usual observations of adsorption exothermicity. A possible explanation for the endothermic heat of adsorption was given in our earlier communications[ 16,171. The values of AC are negative, as expected for a spontaneous process. The decrease in AC values with increasing temperature shows that the adsorption of dysprosium ions on activated charcoal is more favourable at higher temperatures. Since the adsorption process is endothermic, it follows that, under these conditions, the process becomes spontaneous because of a positive entropy change. The effect of various cations such as Cs+, Na+, Li+, Sr’+, Ca2+, Zn2+, Co2+, Ce3+, Sc3+, and Cr3+ on the adsorption of dysprosium ions on activated charcoal has also been examined. The concentration of added cations and dysprosium ions were fixed at 1.O g/l and 2.0 g/l, respectively. The results of these investigations are given in Table 2. It is obvious from this Table that the presence of additional cations in solution reduces the adsorption of dysprosium ions on activated charcoal. The adsorption of dysprosium is lowered because these cations are coadsorbed along with dysprosium ions. Table 2 also demonstrates that the cations with larger charge density (Z/r) values reduced the adsorption of dysprosium ions on activated
9 D Y + 6
-20.67 -16.74 -15.16 -13.35 -10.56
-23.27 -18.24 -16.46 -14.45 -11.36
-25.87 -19.74 -17.76 -15.55 -12.16
charcoal more effectively than the cations with smaller Z/r values. We have also examined the adsorption behaviour of dysprosium ions on activated charcoal in the presence of CH,COO-, S,O:-, I-, Br; Cl-, NO;, and EDTA. The concentration of added anions and dysprosium ions were fixed at 1.Og/l and 2.0 g/l, respectively. The results are shown in Table 3. The presence of additional anions also induced a negative effect on dysprosium adsorption. The degree of reduction in the adsorption of dysprosium ions in the presence of anions was in the order EDTA > NO; > Cl- > Br- > I- > S20:- > CH,COO-. It may be inferred that the affinity of the complexes of anions is low towards the activated charcoal surface as compared to that of free Dy3+ ions. The adsorption of other metal ions was measured under the optimum adsorption conditions employed for dysprosium ions to check the selectivity of the activated charcoal for dysprosium ions. The results, presented in Table 4, indicate that Eu, Sm, Er, U, La, Y, Ce, and Ba exhibit considerably higher values for the percentage adsorption and K,; hence, would be coadsorbed beside dysprosium onto activated charcoal. In contrast, Cr, Cd, Cs, Co, Ni, Zn, Mn, V, Sr, Cu, and Rb are poorly adsorbed; hence, the separation of dysprosium from these metals may be achieved. The separation factor for dysprosium is large in the presence of Zn, Mn, V, Sr, Cu, and Rb because these metals have much lower KD values. The feasibility of using activated charcoal for the preconcentration/separation of dysprosium was as-
Cations Nil cs+ Na+ Li+ Sr’+
lo'/ T IK)
$: Fig. 8. Plots of lnKD versus l/T for the adsorption prosium ions on activated charcoal.
2. Effect of different cations on the adsorption dysprosium ions on activated charcoal Z/r
0.5988 1.0309 1.4706 1.7857 2.0202 2.7027 2.7778 2.9013 4.0984 4.7619
% Adsorption 91.90 91.00 90.00 88.50 86.30 85.45 84.25 83.85 80.50 77.65 74.95
KD (ml/g) 1134.57 1011.11 900.00 769.57 629.93 587.29 534.92 519.20 412.82 347.43 299.20
R. QADEER and J. HANIF 3. Effect of different anions on the adsorption dysprosium ions on activated charcoal
Nil CH,COOs,o:IBrclNO; EDTA
91.90 91.85 88.45 80.20 16.10 71.29 60.65 49.28
1134.51 1127.00 765.80 405.05 329.20 250.26 154.13 97.04
a glass col-
(30 cm x 0.7 cm i.d.) containing a known amount of dry activated charcoal on a glass wool support was used. The column was loaded with a solution containing 2.0 g/l of dysprosium at a pH of 4 and kept for 60 minutes. About 90% dysprosium was adsorbed on the activated charcoal. The elution of adsorbed dysprosium was carried out with 3M HN03 solution at a flow rate of 1 .O ml/min. Figure 9 shows the elution profile of dysprosium. About 98% of the adsorbed dysprosium on activated charcoal was recovered by elution with 40 ml of 3M HN03 solution. umn
Based on the above data, it is concluded that activated charcoal can be used for the preconcentration/ separation of dysprosium ions from aqueous solutions.
Volume of eluont
Fig. 9. Elution of dysprosium ions adsorbed onto activated charcoal by means of 3M HNO,.
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DY Eu Sm Er U La Y Ce Ba Cr Cd cs co Ni Zn Mn V Sr cu Rb *Concentration
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