COLLOIDS AND A

Colloids and Surfaces SURFACES

ELSEVIER A: Physicochemical and Engineering Aspects 137 (1998) 231-242

Ion-exchange processes of lead and cobalt ions on the surface of calcium-montmorillonite in the

presence of complex-forming agents 1 I. The effect of EDTA on the sorption of lead and

cobalt ions on calcium-montmorillonite

No6mi M. Nagy *, J6zsef K6nya Isotope Laboratory, Kossuth University, Debreeen, H-4010, Hungary

Received 30 June 1997; accepted 24 November 1997

Abstract

The sorption of lead and cobalt ions on the surface of calcium-montmorillonite is studied in the presence and absence of a complex-forming agent. The ion exchange processes are greatly influenced by the composition of the solution (pH, complex-forming agent). However, when the sorption can take place in different ways these also affect the results. © 1998 Elsevier Science B.V.

Keywords: Calcium-montmorillonite; Cobalt ions; Complex formation; EDTA; Ion exchange; Lead ions

1. Introduction microelements. We have intended to separate the different interfacial processes and to examine the

Nowadays the heavy metal concentration of effect of the composition of the solution (especially soils can increase as a result of anthropogenic complex-forming agent) on the sorbed quantity of activity. Heavy metal ions exchange the essential microelements. macro- and micro-elements on the surface of soils In this work the sorption of lead and cobalt and clay minerals so they can be toxic. Radioactive ions is studied, especially in the presence of com- fallouts contaminate the soils, too. Consequently, plex-forming agents. the thermodynamics and mechanism of sorption Lead is usually toxic for plants because it forms and desorption of these contaminations must be many toxic lead organic compounds. In addition, studied, the amount of lead in the environment is steadily

In the last few years we have systematically increasing by anthropogenic activities. It associates studied the interfacial processes between calcium- mainly with clay minerals, different oxides and montmorillonite and different ions [1-8], mainly hydroxides of soil and organic matter [9].

The interactions of lead with clay minerals have * Corresponding author. Tel: 3652 310122; fax: 3652 210122.

been studied by several authors, e.g. [10-12] (and 1 This work was presented at the 7th Conference on Colloid Chemistry in memoriam Alad~ir Buz~tgh, Eger (Hungary), Ref. [13] cited in Ref. [9]). In some papers, how- September 23-26, 1996. ever, the experimental conditions are confused (pH

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232 N.M. Nagy, J. Krnya / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 231-242

values above lead hydroxide precipitation, chloride Part I the interfacial reaction of lead perchlorate content [10]), the competition of other cations are and calcium-montmorillonite in aqueous solutions neglected [ 11] or the results are in contradiction at different pH values is studied in the presence of [ 12] (and Ref. [ 13 ] cited in Ref. [9]). ethylene-diamine tetra-acetic acid (EDTA). In Part

Cobalt is an essential microelement (e.g. vitamin II the effect of diethyl-triamine penta-acetic acid B12) [14]. However, an excess of cobalt is toxic as (DTPA), tartaric acid (Tart) and citric acid (Cit) a heavy metal and its radioactive isotope (6°Co), on the sorption of lead ions on calcium-montmoril- formed from the structural materials of nuclear lonite is investigated. power plants, is very dangerous because of its hard The concentration of different metal ions (lead, 7-radiation and long half-life, cobalt, calcium) in montmorillonite and in solution

The interaction of cobalt ions and soils/clay was determined by radioisotopic labelling. The minerals has been studied by several authors [15- isotopes applied have different radiation types and 24]. The change of entropy, free energy, enthalpy, energies, which makes it possible for the simulta- equilibrium constant and selectivity orders related neous measurement of several components. to other metal ions as well as the effect on clay coagulation have been determined. In a previous paper we have studied the interaction of cobalt 2. Reactions possible in metal ion-complex-forming ions with calcium-montmorillonite at different pH agent-caicium-montmorilioalte systems values [6].

The role of complex-forming agents in the inter- The reactions in the Me(CIO4)2, HaO + solu- action between soil and metal ions is very impor- tion-complex-forming agent-calcium-montmoril- tant because these agents can, to a high degree, lonite system are illustrated in Fig. 1. The solid influence the amount and ratio of different species phase in Fig. 1 (boxes 1 and 3) can be relatively of trace elements in soil solution. Similarly, the simple because only hydrated metal ions and cat-

ionic complexes are supposed to be sorbed on the study of the effect of complex-forming agents on

surface of montmorillonite. Among the complex- the processes in soils can answer the question: how forming agents applied, only Cit and Tart can can the polluting metal ions or radioactive fallouts form positive complexes, but their concentrations, be washed out of soil? As a first step, we have as will be discussed later, are very low. In addition, developed a principal model, which is shown else- they have lower charges and greater size than where [7]. hydrated cations; this means that they sorb to a

The basic principles [25,26] for the interactions lesser degree by the Schulze-Hardy rule. So the of cation exchange and complex formation suggest different complexes have to be taken into account that the anionic and neutral complexes formed do only in the solution (boxes 3 and 4). not sorb on cation exchangers. The cationic com- In Fig. 1 iron(III) ions are also written. These plexes are suggested not to sorb by Schubert's get into solution (box 4) by the destruction of the method [25] because they sorb to a lesser degree montmorillonite crystal lattice under the influence than typical hydrated cations owing to their of acidic media and complex-forming agents. This smaller charge and greater size. The sorption of will be discussed in detail later. cationic complexes is taken into consideration by The solution is more complicated: all the equilib- Fronaeus's model [25]. The models are valid only ria possible in the solution can influence the resul- at constant pH values. Since the hydrated cation tant equilibrium between the solid and solution. mainly takes part in the cation exchange reaction and its concentration is inversely proportional to complex stability, the sorbed amount decreases as 3. Reactions possible in metal ion-EDTA-calcinm- the complex stability increases [26]. montmoriUonite systems

In this paper the interaction of lead, as well as cobalt, ions with calcium-montmorillonite in the The reactions in this system can be classified as presence of a complex-forming agent is studied. In follows. Eqs. (1), (2a), (2b), (3), (4a), (4b), (5),

N.M. Nagy, J. K6nya / Colloids Surfaces A." Physicochem. Eng. Aspects 137 (1998) 231-242 233

Box 1 Box 2

Solid initial: Solution initial: 2+

Ca-montmorilionite (Fe) Me , C104, H +, HzO, complex-forming agent

Box 3 Box 4

Solid equilibrium: Solution equilibrium:

Ca 2+, Me 2+, H +, Fe 3÷

Ca-complexes ¢:>

Me-complexes

Me, CaM, cationic complexes- Fe-complexes

montmorillonite (Fe) protonated ligands

Fig. 1. The scheme of the equilibria in the calcium-montmorillonite-Me(C104)2-complex-forming agent systems.

(6a), (6b)-(9) , (10a), (10b) and (11) mean the C o - m o n t + 2 H + ~ 2 H - m o n t + C o 2+ (6b) reactions between solid and solution (boxes 3 and 4). Among them, Eqs. (1) , (2a), (2b) and (3) 3Ca-m°n t+2Fe3+~2Fe-m°n t+3Ca2+ (7)

represent the dynamic heterogeneous exchange F e _ m o n t + 3 H + ~ 3 H _ m o n t + F e 3+ (8) with regard to each ion between montmorillonite and solution. If the ions are labelled by radioactive In addition the different complex formation and isotopes, as in our case, they mean heterogeneous protonation equilibria with the complex-forming isotope exchange as well: agent have to be taken into account. They are

present only in the solution (box 4) [27]. For Ca-mont +45Ca2+~45Ca-mont+Ca 1+ (1) example, in case of EDTA the complex formation

P b - m o n t + 2 1 2 p b 2 + < : > 2 1 2 p b - m o n t + P b 2+ (2a) reactions can be characterized by Eqs. (9), (10a), (10b)-(12), (13a) and (13b)-(15):

Co-mont + 6°Co2 + ¢~6°Co-mont + Co 2 + (2b) Ca 2 + + EDTA 4 - <:>CaEDTA / - (9)

The exchange of hydrogen ions between the solu- tion and montmorillonite is also possible: P b 2 + + EDTA 4-<::-PbEDTA 2- (10a)

H-mont + H+c>H-mont + H + (3) C o 2+ + E D T A 4 - ~ :>CoEDTA 2- (10b)

The equilibria of Eqs. (4a), (4b), (5), (6a) and Fe 3+ + E D T A 4 - ~ F e E D T A - (11) (6b)- (8) are ion-exchange reactions between the surface of montmorillonite and the solution: Ca2++ H ++ E D T A 4 - ~ C a H E D T A - (12)

Ca-mont + pb2+c>Pb-mont + Ca z+ (4a) Pb 2+ + H + + EDTA4-<:>PbHEDTA - (13a)

Ca-mont + Co/+¢>Co-mont + C a 2+ (4b) Co 2+ + H + + EDTA4-¢>CoHEDTA - (13b)

Ca-mont +2H+¢>2H-mont + Ca 2+ (5) Fe 3+ + H + + EDTA4-<:>FeHEDTA (14)

Pb-mont + 2 H + ~ 2 H - m o n t + P b z+ (6a) Fe 3+ + E D T A 4- + O H - ~ F e E D T A O H 2- (15)

234 N.M. Nagy, J. Krnya / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 231-242

Table 1 Company, Hungary. According to the manufactur- Logarithm of stability constants of different complexes and pro- er's certification the thermoanalytical, X-ray tonated ligands in the presence of EDTA [27] diffraction and ethylene glycol adsorption studies

Species log K show 96% montmorillonite content. The cation- exchange capacity was determined by heteroge-

PbEDTA 2 - 18 nous isotope exchange and by the ammonium HPbEDTA - 20.8

acetate method [28] and it was found to be CaEDTA z- 10.69 52 mmol/100 g for calcium ions (that is CaHEDTA 13.18 104 mmol/100 g for hydrogen ions).

A dual radioisotopic tracer method was used FeEDTA- 25.1 for the study of the interfacial process. Lead was FeHEDTA 26.4 labelled with the 212pb isotope, cobalt was FeEDTAOH z - 17.61

labelled using the 6°Co isotope and calcium was HEDTA 3 10.24 labelled using the 45Ca isotope. The radiochemical H2EDTA 2- 16 .40 properties of the isotopes are as follows. H3EDTA 19.06 2a2pb: half life 10.64h; decay, 13 (0.17MeV, H4EDTA 21.06 0.35 MeV, 0.889 MeV), 7 (0.115 MeV, 0.176 MeV,

CoEDTA 2- 16.49 0.239 MeV, 0.300 MeV, 0.415 MeV). 6°Co: HCoEDTA- 19.49 half-life, 5.2years; decay, 13 (0.312MeV and

1.48 MeV), 7 (1.132 MeV and 1.172 MeV). 45Ca: half-life, 164days; decay, 13 (0.254MeV). The

Finally, we can described the protonation equi- y-activity of 212pb and 6°Co was measured by an NaI(T1) scintillation crystal. The quantity of

libria of EDTA (Eqs. (16)-( 19): 4SCa was measured on the basis of 13-activity using

EDTA 4- + H ÷ ~ H E D T A 3- (16) a Wallac 1409 liquid scintillation counter. The

EDTA 4 + 2H ÷ ¢~H2EDTA 2- (17) composition of the scintillator was as follows: 4.0 g PPO, 257cm 3 Triton-X 100, 37cm 3 ethylene

EDTA 4- +3H÷c~H3EDTA - (18) glycol, 106 cm 3 ethanol, diluted to 1000 cm 3 with

EDTA 4- + 4H + ¢,H4EDTA (19) xylene. 6°Co interferes with the measurement of 45Ca

The stability constants of the complexes and because of its own 13-radiation. The degree of protonated ligands are summarized in Table 1 [27]. interference was determined as a function of

The resultant equilibrium state of the system is 7-activity and it was taken into account as back- affected by the equilibria shown by Eqs. (1), (2a), ground radiation in the measurement of the (2b), (3), (4a), (4b), (5), (6a), (6b)-(9) , (10a), fl-radiation. At the same time the 13-spectra were (10b)-(12), (13a) and (13b)-(19). In our work cut into two portions: one belongs to only the we determined the total amount of different ions cobalt isotope, the other one contains refers to the (calcium, lead or cobalt) in solid and in solution cobalt and calcium isotopes together. The ratio of by radioisotopic labelling, measured the pH, then the two portions was determined for the pure we computed the amount of different species in cobalt isotope and the cobalt+calcium spectrum solution on the basis of total amounts and stability portion was divided by the mean of this ratio. The constants (Table 1 ). results obtained by these two methods were the

same within the experimental error. Such interference was not observed in the case

4. Experimental of 212pb because of its small radioactivity. 212pb

isotope was obtained by a Hahn's emanation Calcium-montmorillonite is a product of the source [29] made by us. Since the daughter element

Central Laboratory of the National Mining of 212pb (212Bi) is also radioactive, the activity

N.M. Nagy, J. Krnya / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 231-242 235

measurement of lead was made after reaching the of acidic media and a complex-forming agent was transient equilibrium between 2X2pb and 212Bi, that measured by a Pye Unicam SP 1900 atomic absorp- is a day after the experiments (the half-life of tion spectrometer. The amount of the dissolved 212Bi is 60.6 min). aluminium could not be detected. The amount of

Calcium-montmorillonite was labelled by the dissolved iron was determined as a function of heterogeneous isotope exchange with 45Ca isotope, pH and the measured concentrations were used as In this way we can simultaneously study the two total iron concentration of the solution [7]. directions of the ion-exchange reaction, i.e. we can The concentrations of different species in the determine the quantity of metal ions entering and equilibrium solution were calculated from the total leaving the solid, concentration of the ions, complex-forming agent,

In the experiments with lead ions, 20.0 mg of pH and the stability constants of the complexes air-dried calcium-montmorillonite labelled with with the aid of the program PSEQUAD [30]. 45Ca was measured into a beaker, and 20.0 cm 3 of The structural modification of calcium-montmo- bidistilled water or HCIO4 solution of different rillonite during the cation-exchanges with lead was concentration was added and the mixture stirred studied by X-ray diffraction. For this, cobalt or at constant speed for 30 min. Experience has lead ion-exchange montmorillonites were produced shown that this time is sufficient to reach solubility from calcium-montmorillonite by several ion equilibrium between the phases. The solution con- exchanges. The conditions are summarized in taining lead perchlorate or lead perchlorate and Table 2. EDTA in molar ratios 1:1 and 1:4 respectively was Lead and cobalt contents of the cation- then added to the system. In equilibrium (after exchanged montmorillonite were determined by 60 min) the phases were separated using a mem- X-ray fluorescence analysis. It was shown that the brane filter (0.45 rtm pore size) and the T-radiation ion exchange was successful. of the solid and liquid, the [3-radiation of the liquid X-ray diffractograms of lead- and cobalt-mont- and the pH were measured, morillonites were made using a computer-con-

In the experiments with cobalt ions, 10.0 mg of trolled Philips PW1710 diffractometer with a Cu air-dried calcium-montmoriUonite labelled with anticathode, operating at 30 mA and 40 kV and 45Ca isotope was applied and cobalt perchlorate with graphite monochromator. The scanning rate solution labelled with 6°Co and EDTA in 1:1 molar was 2 ° 20 min- 1. ratio was added to the system. The other experi- mental conditions were the same as in the lead ions-calcium-montmorillonite system.

The experiments were carried out at 24°C. The 5. Results and discussions constant temperature was maintained by an ultrathermostat. 5.1. Lead ions--wi thout complex-forming agent

The average standard deviation of the measure- ment for Co ions was _+ 3%; for Pb and Ca ions For the ion exchange reaction in Eq. (4a) the it was + 5%. equilibrium distribution of lead ions between the

The amount of aluminium and iron dissolved phases at constant temperature can be evaluated from the lattice ofmontmorillonite under the effect by Langmuir's representation applied to ion

Table 2 Experimental conditions in the production of lead and cobalt ion-exchanged montmorillonites

Mass Ca-mont (g) Solution pH Number of treatments Drying temp. (°C)

Co-mont 5 60 cm 3 0.01 mol dm -3 Co(C104)z 7 7 105 Pb-mont 5 10 cm 3 0.1 tool dm -3 Pb(CIO4)2 2.9 3 105

236 N.M. Nagy, J. K6nya / Colloids Surfaces A: Physieochem. Eng. Aspects 137 (1998) 231-242

exchange reactions [3,31-35]: The application of dual labelling makes it pos- Cpb 1 ( Kpb c ~ sible to compare the amount of lead introduced to

= - Cpb + - - ca (20) the surface of montmorillonite and the amount of apb ¢ \ /(ca f calcium leaving the surface. The differences are

where Cpb and apb mean the concentration of lead about 10-20% of the cation-exchange capacity of montmorillonite. It is in the range of edge charges ions in solution or solid in equilibrium respec- of montmorillonite [37], which depends on pH. tively, ¢/mol g-1 is the number of active sites, Cca

is the concentration of calcium ions in solution, Similarly, the lead adsorption-desorption mecha- and Kpb and/(ca are the isotherm parameters for nism involving ion exchange was interpreted on the individual ions and can be determined by the surface of 7-A1203 (see Ref. [38] in Ref. [39]). heterogeneous isotope exchange. Experience shows The octahedral layers of montmorillonite have

similar surface characteristics. that in certain ion-exchange systems there are concentration ranges where (Kpb/Kc~)Cca is found The X-ray diffractogram of lead-montmorillon- to be nearly constant, ite is shown in Fig. 3. The base reflection (001) of

From the experimental results obtained at pH-- lead-montmorillonite is 1.254 nm, which is similar 3.8 (where lead is thermodynamically stable as to the value characteristic ofmonovalent montmo- Pb z+ ions [36]) isotherms are constructed in the rillonite (1.241 nm). However, this does not mean usual way, i.e. apb is plotted as a function of the that lead is sorbed on the surface ofmontmorillon- equilibrium concentration of lead. The isotherm ite as monovalent cations, since the parameters of obtained is shown in Fig. 2. lead determining the distance between the layers

The parameters ~ =4.6 x 10 -4 mol g-1 and (hydration entropy, charge/ion radius value, water K=3.5 x 10 -4 tool dm -3. It can be seen that at content in the interlayer space) are between the 24°C the number of active sites is close to the ion- values for bivalent and monovalent cations. The exchange capacity of montmorillonite. (The iso- temperature of drying (105°C) can cause the therms obtained at lower and higher temperatures, decrease of the distance of the layers. (Another however, are different to that obtained at 24°C; paper [40] has been submitted on the X-ray and this will be presented in another paper--the experi- DTA studies of different cation-exchange mont- ments are being made.) morillonites.)

0,0006

0,0004 •

0,0002 '[ O

i[' 0 ~ - T ~ - ---~ .... ~ . . . . . . . . . ~ . . . . . . . . ~ . . . . . . . . ~ . . . . . . ~ . . . . . . . ~ - - ' - - t

O, OOE+O0 5,00E'04 1,00E"03 1,50E"03 2,00E'03 2,50E'03 3,00E'03 3,SOE'03 tl,OOE*03 4,50E'03 5,00E-03

Ct~ (moLdm4!

Fig. 2. Langmuir isotherm applied to ion exchange in lead ion-calcium-montmorillonite system: pH = 3.8, T=24°C.

N.M. Nagy, Z K6nya / Col~ids Surfaces A." Physicochem. Eng. Aspects 137 (1998) 231-242 237

F~leN~me Sampl,e Zaent. : . . . . . Pete: W~velen Tube kV mA Step Range Hax . I

1700ER P b - m o n t n o r t ] l o n t t 0 4 - 0 i - 9 6 1,54184 Cu 40 30 0,02 2 ,0 -62 ,0 642

]00 . . . . . . . . . .

gO-

80 -

70-

60-

0,I

~ ~o- ~,~ 40-

20 .

10

t) ~ "- --"-T -~ "'"" '~ "''" " I ' ~ ~ ' I r -'-'T"

~0 20 30 40 50 60

T~O Theta [deg] / d - sp:.~cing

Fig. 3. X-ray diffractogram of Pb-exchanged montmorillonite.

5.2. Lead ion--with complex-forming agents orders of magnitude than the other species and so they were neglected. The highest concentrations

5.2.1. The effect of EDTA of FeEDTA- and FeEDTAOH 2- are about The equilibrium fractions of lead, calcium and 10-6 mol dm-3, which is less than 1% of the small-

hydrogen ions on the surface of montmorillonite est EDTA concentration, so the different iron(Ill) versus pH in the presence of EDTA at 1:1 and 1:4 complexes slightly influence the concentrations of Pb:EDTA ratios are shown in Fig. 4Fig. 5. For the different lead and calcium complexes. comparison, the results obtained without complex- In case of Pb:EDTA = h l the concentration of forming agent are also plotted (Fig. 6). the different EDTA species is less than 1% of the

At the same time the quantities of the different total amount of EDTA; their quantity, of course, species in the solution were computed on the basis increases at Pb:EDTA= 1:4. of the total concentrations of lead, calcium, The equivalent fractions of lead ions on the iron(III) ions, EDTA and pH. The concentrations surface of montmorillonite and the ratios of of the following species were computed: Pb 2÷, different Pb-EDTA complexes in the solutions PbEDTA 2-, HPbEDTA-, Ca 2+, CaEDTA 2-, show similar tendencies, so the results obtained at CaHEDTA- Fe 3÷, FeEDTA-, FeHEDTA, Pb:EDTA= 1:4 are shown in Fig. 7. Similarly, the FeEDTAOH 2-, EDTA 4-, HEDTA 3-, equivalent fractions of calcium ions on the surface H2EDTA 2-, H3EDTA-, H4EDTA. The concen- of montmoriUonite and the ratios of different trations of EDTA 4-, CaHEDTA-, Fe 3÷ and Ca-EDTA complexes in the solutions are shown FeHEDTA were, in all cases smaller, by several in Fig. 8 at Pb:EDTA= 1:4.

238 N.M. Nagy, J. K6nya / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 231-242

1

0,9

0,8.

0,7-

0,6 •

0,4

0,3

0,2

0,1 ~

1 2 3 4 5 6 7

pH

Fig. 4. The equivalent fraction of lead, calcium and hydrogen ions on the surface of montmorillonite as a function ofpH; P b : E D T A = 1:1, the total initial concentrations of lead and EDTA are 5 × 10 -4 tool d m -3.

0,8 [ A

0,7

0,6

0,5

x 0,4

0,3

0,2

o,1

d i I i I I I 1 2 3 4 $ 6 7

pfl

Fig. 5. The equivalent fraction of lead, calcium and hydrogen ions on the surface of montmorillonite as a function of pH; Pb:EDTA = 1:4, the total initial concentration of lead is 2.5 x 10 -4 mol d m -3 and of E D T A is 10 -3 mol d m -3.

It can be seen from the comparison of Figs. 4- caused by the decrease of the concentration of 6 that the equivalent fraction of lead increases hydrated, positive lead(II) ions, as with manga- with the increase ofpH in the absence and presence nese ions [8]. In the case of lead, however, there of EDTA. EDTA, however, decreases the equiva- is not a such strict relation between the sorbed lent fraction of lead on the surface of montmoril- quantity and the hydrated ion concentration; the lonite, the decrease is greater when the ratio of quantity of the sorbed lead never decreases to Iead:EDTA is 1:4, that is when EDTA is in excess, zero. This is probably caused by the formation of The slope of the Xpb-pH curve is less than without Pb-O on the edges of layers, that is on the EDTA. Fig. 7 shows that lead is present as negative pH-dependent charges (see Ref. [38] in Ref. [39]). complexes. The decrease of the sorbed lead is The sorption is less influenced by complex-forma-

242 N.M. Nagy, J. K6nya / Colloids Surfaces A: Physicochem. Eng. Aspects 137 (1998) 231-242

[ 14] A. Kabata-Pendias, H. Pendias, Trace Elements in Soils [28] L.A. Richards, Diagnosis and Improvement of Saline and and Plants, CRC Press, 1985, pp. 238-246. Alkaline Soils, US Dept. Agr. Handbook, vol. 60, 1957.

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