Investigations of the adsorption of palladium on carbonaceous adsorbents modified with dimethylglyoxime—I. The adsorption of dimethylglyoxime on selected carbonaceous adsorbents

Carbon Vol. 2X. No I. pp. 27-34. Pnnted m Great Britain

1990

0008-62231YO $3.00 + .(I0 CopyrIght 0 1990 Pergamon Press plc

INVESTIGATIONS OF THE ADSORPTION OF PALLADIUM ON CARBONACEOUS ADSORBENTS MODIFIED WITH DIMETHYLGLYOXIME-I. THE ADSORPTION OF DIMETHYLGLYOXIME ON SELECTED CARBONACEOUS ADSORBENTS H.-U. FOERSTERLING Central Institute of Isotope and Radiation Research, Academy of Sciences of the GDR, Leipzig, GDR (Revised 8 July 1988; accepted in revised form 20 February 1989) Abstract-The adsorption of dimethylglyoxime (DMG) on six commercial activated carbons and on lignite was investigated with the aim to manufacture a modified adsorbent which is capable of separating palladium selectively from metal ion mixtures in nitric acid solutions. The DMG adsorption isotherms and the palladium adsorption behaviour of the modified and unmodified carbonaceous adsorbents indicate strong interaction forces which bring about the adsorption of the reagent. These interactions are probably based on hydrogen bonds. Key Words-Carbonaceous

adsorbents,

adsorption, dimethylglyoxime,

1. INTRODUCTION for our investigation was the search for various possibilities for the separation of palladium from nitric acid solutions of nuclear fuel wastes. For this, among other things, the use of activated carbons modified with the complexing agent dimethylglyoxime (2,3-di(hydroxyimino)butane; abbr.: DMG) has been suggested by Moore[l]. However, there is still no adequate knowledge to allow estimation of the prospects for a practical use of such modified adsorbents. It is just as difficult to draw a conclusion from the patent data regarding the effects of the application of different carbonaceous adsorbents on the adsorption of DMG and palladium and on the selectivity of the palladium separation. DMG was selected as a modifying reagent because of its well known selectivity for palladium in acid solutions, and activated carbons were investigated as substrate because of their large internal surface area resulting in a high DMG sorption capacity. In addition to six commercial activated carbons, lignite was included in our investigations. Lignites are fossil woods which are found in great quantities in brown coal layers[2]. Little is known about the mechanism of the DMG adsorption on carbonaceous adsorbents. There is also considerable confusion in the literature concerning the sorption of other polar organic substances. Hydrogen bonds between acid surface oxides and molecules of the adsorbate, but also between individual molecules in the adsorbate, as well as the sole or additional action of electrostatic and dispersion forces are under discussion[3-91. Indications of the characteristics and mechanism of adsorption can be derived, among other things,

The motivation

palladium.

from adsorption isotherms. The variety of such curves, described in the literature, is attrributable to a few main classes. The system of classification of Giles et a/.[101 appeared suitable for our investigations. It is based on four main classes (S, L, H and C) with up to five subgroups, which allows us to discuss the different interactions occurring in the course of adsorption.

2. EXPERIMENTAL

The carbonaceous adsorbents (Table 1) were ground in a beater mill, sized, washed with distilled water until they were free of dust and dried to constant weight over CaCl,. The grain size fraction of 200-400 km was selected for the adsorption experiments. To characterize the texture of the carbonaceous adsorbents their BET surface areas, SBET,were calculated via nitrogen adsorption at 77 K CD/do< 0.35) using a conventional volumetric apparatus. The total pore volumes, VP, of these adsorbents were predominantly determined pyknometrically by means of the mercury and cyclohexane densities. Also samples of the modified activated carbon mA2 with different DMG contents were analyzed in this way. Additionally, the pore distribution of these samples were determined in the pore radius range from 7.5 to 7500 nm by mercury porosimetry using a Carlo Erba AG 65 porosimeter. For the texture investigations of the different modified mA2 samples lower temperatures were chosen for the pretreatment of the adsorbents than in the analyses used to characterize the different carbonaceous adsorbents (Table 2). This was necessary to 27

28

H.-U. FOERSTERLING Table 1. Characterization

Symbol Al

A2 A3 A4 A5 A6 L

BETSurface Area (m’ig)

Total Pore Volume (cm’jg)

N.V. Norit-Vereeniging, NL

1756

Farbenfabrik Bayer AG, FRG CFW Premnitz, GDR CFW Premnitz, GDR CFW Premnitz, GDR CFW Premnitz, GDR -

1398

Carbon NoritRkd3special SSE B4 :23 R4 Lignite’)

of the carbonaceous adsorbents used

Manufacturer

1210 1185 1035 993 unmeasurably small

Ash Content”) (%)

Oxygen Contentb) (%)

1SOd’

6.1 f 0.33

7.5 t 0.71

1.02d)

2.4 2 0.54

5.8 + 0.69

0.87d’ 0.59”’ 0.49” 0.59d’ -

3.4 9.8 4.9 7.9 1.2

f r + f f

0.70 1.15 0.66 0.77 0.19

8.1 9.3 9.5 10.1 31.7

* r ? r ?

0.48 0.85 0.62 1.09 1.64

a) Incineration in an air stream at 5oo”C, b) 14 MeV neutron activation analysis: 160(n,p) 16N,c) Variety X3 according to classification of Suess[ll] and Roselt[l2], d) Determined from cyclohexane and mercury densities, e) Determined by nitrogen adsorption using the BET equation.

avoid significant losses of DMG. Preliminary tests showed for instance that three hours of heating of mA2-2937 at 130°C causes a loss of about 3 wt.% in the form of desorbed complexing agent. After heating to 200°C this loss already amounts to about 9 wt.%. The DMG adsorption took place in aqueous solution using cylindrical stoppered tubes, which were rotated end-over-end at ca. 5 r.p.m. At 70°C the adsorption was certainly completed within a contact time of 90 hours. A sediment, existing in the beginning, was completely dissolved in the course of the adsorption experiment. At the end of the adsorption reaction the modified carbons were filtered off with a fritted disk funnel and washed with distilled water. Carbon samples to be used for further analyses or for palladium adsorption investigations were first dried over night at 50-60°C and subsequently over CaCl, until constant weight was reached; they were stored over this desiccant till use. To estimate the

Table 2. Pretreatment Sample Different unmodified carbonaceous adsorbents (cf. Table 1)

Unmodified and modified activated carbons A2 with different DMG contents (cf. Table 5)

DMG uptake the DMG concentration in the adsorptive solution was determined at the end of each experiment either gravimetrically or by means of extraction photometry with nickel ammonium sulfate as reagent. For the investigation of the palladium adsorption, 3 mol/l HNO, solutions of palladium nitrate were used, prepared as described by Gorski et a1.[13]. The palladium uptake of the sorbents was determined by means of radiometrical (labeling with la3Pd; tin = 17 d) or photometrical[l4] analysis of the aqueous phase.

3. RESULTS AND DISCUSSION

3.1 Methods of modification The preparation of the modified carbonaceous adsorbents is, among other approaches, possible by batch adsorption of the dissolved reagent[lS18]$.

of the carbonaceous adsorbents for the texture analyses Analysis Method

Pretreatment

NZ adsorption (BET)

Heating from room temperature to 300 “C at about 7 Pa in the apparatus

Density measurement

Heating for 3 hours at 200°C; cooling and storage over P,O,

N2 adsorption (BET)

Heating for 6 hours at 120°C; cooling and storage over P205; heating for 1 hour at 100°C and about 7 Pa in the apparatus Heating for 6 hours at 120°C; cooling and storage over P,O,

Density measurement and mercury oorosimetrv

tUsed abbreviations: mA2-293--modified activated carbon A2 with a DMG content of 293 mg DMGig activated carbon.

$Suggestions for other methods of modification been described in ref. [I] and [15].

have

Adsorption of palladium

The alternatives

in the selection of the solvent are:

l Use of a good solvent for the complexing agent to offer the carbon a constant high DMG concentration, l Use of a relatively poor solvent to minimize its competing action during the adsorption.

The two methods were compared under defined conditions (Table 3). Because of the clearly higher DMG uptake during adsorption from water the further investigations were carried out with this solvent[17,18]. 3.2 DMG adsorption isotherms DMG adsorption isotherms of different shapes were observed with the selected carbonaceous adsorbents (Fig. 1). The adsorbents A2, AS, A6, and L can be classified without reservation as one of the types described by Giles et al. [lo]. For the carbons Al and A4 H isotherms were obtained which show a strictly linear dependence of the uptake, x, on the concentration of the adsorptive solution, c, behind the vertical branch of the isotherm. Such behavior is characteristic of C isotherms[21]. The evaluation of the isotherm of carbon A3 is not possible because of the strong scatter of the experimental values. The capacity of the DMG monolayer follows from the position of the first plateau or of the first inflection (“knee”) in the isotherm (Table 4). 3.3 Texture analyses of modified activated carbons mA2 with different DMG contents

Changes in the texture following from the modification of the activated carbon were studied with mA2 samples with different DMG contents. A linear dependence of the BET surface area, SBET,and of the total pore volume, V,, on the DMG uptake, xDMG,was found (Fig. 2). For the transition from the unmodified activated carbon to the modified carbon of low DMG content mA2-155 the decrease of the pore volume with increasing uptake is almost exclusively effected by DMG sorption in micro- and mesopores with radii smaller than 10 nm. For modified carbons mA2 with a DMG uptake high enough to reach or exceed the monolayer capacity the decrease

Table 3. DMG adsorption from ethanol and water. Experimental conditions: 2 g adsorbent; 250 mg DMG; 30 h; 50°C Uptake (mg DMGig adsorbent) Absorbent

Solvent: 25 ml Ethanol

Solvent: 70 ml Water

A4 A5 L

67 49 21

125 105 49

Solubilities, c,, of DMG in water: c, (20°C) = 0.62 mg/ml; c, (50°C) = 1.60 mgiml [19], ethanol: c, (20°C) = 13.92 mg/ml[20].

29

of the total pore volume with further increase of the DMG content is largely due to sorption of the complexing agent in meso- and macropores with radii larger than 50 nm. It cannot be excluded that DMG is not only adsorbed on the pore walls but also crystallized from such parts of the adsorptive solution that were not completely washed out of these pores. The differences between the texture data for the unmodified A2 which were obtained, on the one hand, in the investigations of all carbonaceous adsorbents (Table 1) and, on the other hand, with the analyses discussed just now (Fig. 2) can be explained by the different heating temperatures prior to the texture investigations (Table 2). The A2 carbon samples analyzed in this series very likely contained incompletely desorbed atmospheric moisture or chemisorbed oxygen[22] resulting in a decrease of the BET surface area and the total pore volume as compared to a thoroughly heated sample. Another cause of these differences could be variations between the texture variations of the A2 charges used in the two series. 3.4 Palladium adsorption on unmodified and DMG modified carbonaceous adsorbents

The unmodified and DMG ceous adsorbents show distinct palladium adsorption relating to tion as well as palladium uptake

modified carbonadifferences for the the rate of adsorp(Figs. 3 and 4):

0 With the unmodified activated carbons complete palladium adsorption is attained after a contact time, t, of maximally 7 hours, whereas with lignite over 100 hours is needed. With the modified adsorbents, complete palladium adsorption takes place within a few hours. l In the case of the unmodified activated carbons a decrease of the nitric acid concentration of the feed solution results in an increase of the palladium uptake. On the other hand, lignite adsorbs more palladium from solutions containing 3 mol/l nitric acid than from 1 mol/l nitric acid so1utions.t l As compared with the unmodified activated carbons the modified carbons mA1, mA2 and mA3 adsorb more palladium. In contrast, the palladium capacity of the other carbons is only little affected by DMG adsorption. l In the investigated intervals of DMG uptake, XnMG,no significant dependence of the palladium uptake, xpd, on the DMG content of the modified carbonaceous adsorbents was observed throughout a contact time of one hour. The molar DMG:Pd ratios differ clearly. At higher DMG contents of the modified carbons the stoichiometric DMG capacity of all the adsorbents is not fully exhausted (Table 5).

tMore detailed information will be given in part III of this paper.

30

H.-U.

Cmg/gl

FOERSTERLING

Al

/

I

I

I

I

I

I

0.5 1.0 15 2.0 clmg/ml I

I

I

I

I

0.5 10 15 20 2.5 c

A3

t I

I

I

I

I

0.5 I#0 a5 20 2,s c X

A5

I

I

I

I

I

0.5 1.0 15 20 2.5 c Fig. 1. DMG adsorption isotherms for selected carbonaceous adsorbents. Experimental conditions: 500 mg adsorbent; 30-275 mg DMG; 25 ml water, 290 h; 70°C. Solubility, c,, of DMG in water: c, (70°C) = 2,75 mg/ml[l9].

4.

THE MECHANISMOF

DMG ADSORPTION

No strict correlations exist between the BET surface areas and the total pore volumes of the different activated carbons investigated and their ability to adsorb DMG. But for the modified activated carbon mA2 a linear dependence of these texture data upon the DMG uptake was found.

The BET surface areas of all carbons are only partly covered with the complexing agent. If we assume a specific molecular area of 0.25 nm*t for DMG it is possible to estimate that in the plateau region of the DMG adsorption isotherm the complexing

tCalculated by means of neutron diffraction data[23].

31

Adsorption of palladium

1000

-

900

-

800

1

1 100

1

1 200

I

I I I 300 xoMJmg/gl

50

100

I50

200

250

300 XDMG[mg/gl

0.60.5-

0.3 0.2 0

Fig. 2. Dependence of the BET surface area and the pore volumes on the DMG uptake for the modified activated carbon A2. Symbols: 0 total pore volume, l pore volume for pore radii Cl0 nm, 0 pore volume for pore radii from 10 to 50 nm, W pore volume for pore radii HO nm.

32

H.-U.FOERSTERLING c/&_$[md~l

CL [m9/4 3.15

4

-

3.0 2.4 1.0

3.15 3.15

X

60

40

20 40 I

I

,

2 X

QO

80 I

:

I

4

mgl91

6

t[h]

0 :

I

I

X

A3

60

I

I

I 4

i

oo thl

WI

I

I

:

6

tbl

A4

60

I

I

2

x [Pa91

2

I

I ,

4

I 1

I ,

6

:

t[hl

x pw91

A5

4

2

6

thl

2

4

6

tU-4

A6

4

Fig. 3. Kinetics of palladium adsorption on unmodified carbonaceous adsorbents. ditions: 50 mg adsorbent; 3 ml Pd solution; room temperature.

6

t[hl

Experimental

con-

Adsorption of palladium

xPd

33

!mg/sl

120 mA1 +

++

+

100 A

A

80

A

m

AA.

A-m A2f A

A

A

60 40 20 I

I

I

I

I

120

40

I

I

200

I

280

I

360

I

I

I

I

*

x DMG Zmgigl

Fig. 4. Palladium adsorption on modified carbonaceous adsorbents with different MG uptakes, Experimental conditions: 150 mg adsorbent; 8 ml Pd solution; (3,2 mg Pdiml 3 molil HNO,); 1 h; room temperature. Symbols: + mA1, n mA2, A mA3, n mA4, 0 mA5, q mA6, 0 mL. agent utilizes only betwen about II4 and 112 of the BET surface area of the activated carbons A2, A5 and A6. At the end of the vertical branch of the DMG isotherms of the activated carbons Al and A4 even not more than l/6 of the BET surface area is covered with DMG. It is not possible to call such a DMG uptake “monolayer coverage” in the strict sense of the Langmuir concept. The use of the term “monolayer,” however, is possible if, following Giles et al., we define by this term the adsorbate quantity adsorbed first on the most active surface sites in the course of the adsorption reaction. The formation of

Table

4. Characterization of the isotherms

DMG

isolated clusters of adsorbate molecules in the “monolayer” cannot be ruled out. The term “monolayer” is used in our paper in this sense. The DMG quantity which is adsorbed on lignite in the plateau region of the isotherm would as a monomolecular layer require a specific surface area of 130 m*/g lignite. Because of the non-developed internal surface of the adsorbent (Table 1) this value is to be taken as too high. This discrepancy is ex-

Table 5. Molar DMG:Pd ratio for the adsorption of palladium on modified carbonaceous adsorbents with different DMG contents. The experimental conditions are the same as for Fig. 4

adsorption

Adsorbent

Al

A2

A4

A5

A6

L

Isotherm class according to Giles et a[. [lo] Monolayer coverage (mg DMGig adsorbent)

H/C

S3

H/C

L2

L2

S2

240

260

150

360

355

100

Adsorbent

Molar DMG:Pd Ratio (mol DMG/mol Pd)

mA1 mA2 mA3 mA4 mA5 mA6 mL

2.4-3.7 2.7-3.4 2.1-3.8 5.3-8.8 5.8-7.1 5.3-8.1 3.7-4.2

34

H.-U. FOERSTERLING

plicable only if we assume that strong forces at the grain surface bring about the formation of DMG multilayers or of DMG clusters. The assumption of strong interaction forces between the adsorbate molecules is confirmed by the S-shape of the DMG isotherm. Relatively strong adsorbate-adsorbate (A2) and adsorbent-adsorbate (Al and A4) interactions are also probable for the adsorption of DMG on the activated carbons Al, A2 and A4 by reason of the shape of the isotherm. In the case of the carbons Al and A4 the strictly linear rise of the isotherm behind its vertical branch can be explained with the formation of several DMG layers. Hence, the surface area suitable for further DMG adsorption would be unchanged over a larger range of DMG uptake[21]. Hitherto it has not been possible to precisely characterize these relatively strong adsorbate-adsorbate and adsorbent-adsorbate interactions. In view of the bonding conditions in solid DMG-the DMG molecules are connected by hydrogen bonds in trans configuration[23-24]-it is likely that similar to the adsorption of water on activated carbons[25,26] DMG clusters exist in the adsorbed phase, which are stablized by hydrogen bonds. Also the fixation of these clusters on the adsorbent is supposed to take place, at least in part, due to hydrogen bonds. After all, interactions of the solvent, water, with the adsorptive, DMG, and with the adsorbents are imaginable. A volume filling of micropores during the DMG adsorption cannot be excluded for all activated carbons. This mechanism should be extensive in the case of the activated carbons A5 and A6, whose adsorption isotherms obey the concepts of Langmuir and Dubinin-Radushkevich and whose palladium capacities are only little affected by the modification with DMG. But in the case of the activated carbons Al, A2 and A3 this adsorption mechanism cannot be dominant. The texture analyses of mA2 with varying DMG content showed that with increasing DMG uptake an increasing part of the complexing agent is sorbed in meso- and macropores. After modification with DMG all three activated carbons are able to adsorb distinctly more palladium than in the unmodified state. This can only be explained with the action of the complexing agent in incompletely filled pores, which are still accessible to the metal. Therefore we may suppose that during the modification an essential part of DMG is adsorbed in such a manner that the complexing oxime groups remain free for the reaction with palladium.

Acknowledgement-I thank Mr. M. Kraft for texture analyses and Dr. E. Maul for the oxygen analyses of the carbonaceous adsorbents, Prof. Dr. R. Vulpius for the determination of the variety of the examined lignite, Dr. B. Gorski, Dr. K.-H. Radeke and Dr. H. Wolf for valuable discussions, and Miss Hoehndorf for her experimental assistance. REFERENCES 1. United States Atomic Energy Commission, R. H. Moore, US-Pat., 3848048, Nov. 12, 1974. 2. H. Lehmann, Freiberger Forsch. Heffe A148,9 (1959). 3. J. T. Cookson, In Carbon Adsorption Handbook (Edited by P. N. Cheremisinoff and F. Ellerbusch), p. 241, Ann Arbor Sci. Publ., Ann Arbor (1978). 4. A. V. Kiselev, Quart. Rev. 15, 99 (1961): 5. M. Rozwadowski, J. Siedlewski, and R. Wojsz, Carbon 17, 411 (1979); 19, 383 (1981). 6. S. J. Mattson and H. B. Mark, Activated Carbon, p. 159. Marcel Dekker. New York (1971). 7. I. Abe, K. Hayashi; M. Kitagawa, and T. Urahata, Bull. Chem. Sot. Jpn. 52, 1899 (1979). 8. M. Manes, In Activated Carbon Adsorption of Organits from Aqueous Phase (Edited by J. J. Suffet and M. J. McGuire), Vol. 1, p. 43, Ann Arbor Sci. Publ., Ann Arbor (1980). 9. M. J. Kamlet, R. M. Doherty, M. H. Abraham, and R. W. Taft, Carbon 23, 549 (1985). 10. C. H. Giles, T. H. MacEwan, S. N. Nakhwa, and D. Smith. J. Chem. Sot. 1960. 3973. 11. M. St&, Freiberger Forsch. Hefte A148, 14 (1959). 12. G. Roselt, Freiberger Forsch. Hefte C335, 114 (1978). 13. B. Gorski, L. Ku&, and H. Petrzilova, J. Radioanal. Nucl. Chem. Artic. 91. 305 (1985).

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19. Be&reins Handbuch der Organischen Chemie, Suppl. Vol. 2, Vol. 1, p. 826. Springer Verlag, Berlin (1941). 20. A. K. Babko and P. B. Mikhel’son. Ukr. Khim. Zh. 21, 388 (1955). 21. G. D. Parfitt and C. H. Rochester, In Adsorptionfrom Solution at the SolidlLiquid Interface (Edited by G. D. Parfitt and C. H. Rochester). L-I.12. Academic Press, London, New York, Paris (1983). 22. M. Kamishita, 0. P. Mahajan, and P. L. Walker, Fuel 56, 444 (1977).

23. W. C. Hamilton, Acta Cryst. 14, 95 (1961). 24. V. M. Peshkova, V. M. Savostina, and E. K. Ivanova, Oksimv. D. 30. Nauka. Moscow, (1977). 25. M. M.‘Dubinin, In Chemistry and Physics of Carbon (Edited by P. L. Walker, Jr.), Vol. 2, p. 51. Marcel Dekker, New York (1960). 26. R. C. Bansal, T. L. Dhami, and S. Parkash, Carbon 16, 389 (1978).