Arab J. Nucl. Sc. & Appl.; 39 (1) 2006

Sorption / Desorption Studies of Some Heavy Elementson Soil by Different Techniques.

E. Metwally, M. K. K. Shehata and H. S. OrabiNuclear Chemistry Department, Hot Labs. Center, AEA, Cairo, Egypt.

ABSTRACT

Different soil samples were collected from industrial area (Mostorod) near Cairo, Egypt. Soil samples were sieved and characterized to investigate the adsorption / desorptionbehavior of the selected elements onto contaminated and uncontaminated soil samples. The factors affecting the sorption behavior (e.g. pH variation, contact time, competing ion, etc.) were studied. It is generally agreed that the uptake of ions by soil is an ion exchange process involving one or more mechanisms. Spectrophotometric technique was used for determination of the studied heavy elements: Cr(VI), Pb(II) and Ni(II) ions while radiometric technique was used for tracing 65Zn, 152+154Eu, 134Cs and 60Co isotopes in aqueous solutions. Results are discussed to through more light on the absorbability of heavy metals and their retention on soils.

Key Words : Sorption / Desorption / Heavy elements / Soil.

INTRODUCTION

Sorption studies of radionuclides on various materials are of great importance and significance in evaluating the feasibility of aparticular natural material for its using for the decontamination purposes and volume reduction. Many natural occurring materials exhibit one or more of the useful sorption or chemical reactions with radioactive or stable trace elements. Sorption of various radionuclides on soils and clay minerals has been studied(1). Zinc is an essential trace element, but a large concentration of zinc in biological system was proved to be toxic(2). Adsorption of Zn ions on different absorbent surface has been studied earlier(3). Distribution coefficient of 134Cs, 60Co, 65Zn and 51Cr in different

contaminated soils affected by organic waste disposal have been studied(4). Also, diffusion kinetics of some radioactive ions adsorbed onto local clay soils has been determined(5). Adsorption–desorption of copper Cu(II) at contaminated levels in two red soils from China was investigated(6). Selectivity sequence and competitive adsorption of heavy metals by soils from Brazil have been investigated(7). Sixty-four surface soil samples taken in the vicinity of Al-Ain landfill at Arab Emirates were analyzed for cadmium, chromium, copper, nickel, lead and zinc by inductively coupled plasma spectroscopy(8). In which, extraction techniques were used to establish the association of the total concentrations of thesix metals in the soil samples with their contents in the exchangeable, carbonate, iron/manganese oxides, and residual fractions. The behavior of trace metals during selective extractionswas examined(9). Where, selective extraction protocols were developedfor the attempt to optimize both the bulk phase dissolution and the selectivity of some trace metals hosted by particular phases(9). A four-step novel sequential extraction procedure was developed to assess Hg fractionation and mobility in three highly contaminated soils from color-alkali plants at Montreal in Canada(10). The mechanisms of thorium migration in some semiarid soils at USA were investigated(11). The influence of silicic acid on the adsorption kinetics of chromate ions on goethite at different soil pH values hasbeen studied1(2). Adsorption of some heavy elements as Pb(II) and Ni(II) has been investigated on some glass fiber filters(13). Effect of cobalt sorption on metal fractionation in anaerobic granular sludge has been determined(14). Speciation studies of copper in contaminated soil(15) and the effect of pH on dissolution of trace elements contaminant from soil samples at USA have been studied(16). Leaching characteristics of heavy metals from sewage sludge were studied(17). Sorption-desorption of Pb(II), Hg(II) and Cd(II) by modelassociations of soil colloids and different fractions of soils wereestimated(18,19). Also, other adsorption treatments on different soil samples have been determined(20-24). In the present study sorption desorption of Pb(II), Ni(II), Cr(VI) ions and 65Zn, 152+154Eu, 134Cs and 60Co isotopes were conducted on different soil samples to through light on the absorbability and the retention of these elements on soils and their probable pollution to the environment. The importance of these ions is focused as Zn(II) and Cr(VI) ions could be produced as contaminant in the fuel cycle. Also, Pb(II), Ni(II), Zn(II) and Cr(VI) ions could be produced from industrial activities in the environment. While 152+154Eu, and 134Cs are fission product isotopes which could be produced from nuclear activities.

EXPERIMENTAL

Chemicals of analytical grade and bidestilled water were used in preparing the solutions in the present study. Soil samples used were collected from different locations of an industrial area north of greater Cairo (Mostorod area). Industrial waste water and sewage effluent were the main sources of environmental pollutant problem ofthe studied zone. Soil samples S1, S2 and S3 were collected from thefollowing regions: Petroleum and Ceramic industries; Plating and Electronic Cable industries; and Detergents and Glasses industries, respectively. While, soil sample S4 was collected from area far awayfrom other industrial activities (El-Fayom area). All soil samples were collected from the surface layer (0-10 Cm deep), air dried, grounded and sieved with a siever of diameter = 0.335 mm. Spectrophotometric technique was used in the present study for determination of Pb(II), Ni(II) and Cr(VI); while radiometric technique was used for determination of 65Zn, 152+154Eu, 134Cs and 60Co ions. Batch adsorption technique was applied at ambient room temperature for all the experiments. Also, V/m ratio of a value 100 ml/g was maintained constant. The uptake percent of an element on soil sample and the distribution coefficient Kd in addition to desorption percent were expressed as follows: Ao – (At + Aw) Uptake % = ------------------------- x 100 (1) Ao – Aw

Ao - At

Kd = -----------------------x V/m (2) At

Ad - xAt

D % = ---------------------x 100 (3) Ao - At

where,Ao , is the initial activity in the aqueous solution.At , is the final activity in the aqueous solution at time t.Aw , is the activity adsorbed on the walls of the tube.Ad , is the total activity of desorbed element.X , is the volume fraction of solution left initially with soil.

RESULTS AND DISCUSSION Physical Properties of Soil Samples.

Soil samples collected for the present study were characterizedby the standard soil testing laboratories(25) according to Table (1). The data obtained from the physical analysis imply that soil sample S4 (control sample) has the highest clay percent that corresponding to the highest cation exchange capacity (CEC).Table(1): Physicochemical properties of soil samples used in the present study.

SampleGrains size fraction %

Soil texture

CECm.eq/100g.

Moisture content

pH

Sand Silt Clay

S1 23.0 30.0 47 clayly 29.35 7.2

7.2

S2 25.85

25.15

49 clayly 31.19 9.1

7.6

S3 26.87

29.13

44 clayly 27.71 6.4

6.9

S4 22.0 24.0 54 clayly 39.46 8.1

7.3

Adsorption of Pb(II) Ions.

Absorbability of Pb(II) onto different soil samples has been studied. Maximum adsorption of Pb(II) on soil samples occurs at pH =5 with uptake percent of 90, 95, 88 and 100 for S1, S2, S3 and S4 respectively using initial lead concentration of 2.4 x 10-4 M as shown in Fig. (1). These results indicate that adsorption of Pb(II) is poor in highly acidic solutions and with increasing pH values more than 5 it causes a gradually decrease Pb(II) adsorption. Adsorption of Pb(II) increases with increasing shaking time, Fig. (2).

0 1 2 3 4 5 6 7 8 90

20

40

60

80

100

soil(1) soil(2) soil(3) soil(4)

upta

ke p

erce

nt

pH

Fig.(1): Effect of pH variation on adsorption of Pb (II) using different soil samples. Uptake % of Pb(II) reaches 94 % for S4 sample after 3 hours shaking time. So, this time is sufficient to the equilibration time for Pb(II) adsorption experiments, Fig.(3). In adsorption isotherm of Pb(II) in nitrate solution straight lines were obtained for all soil samples with slope equals to unity. This result, on lead ion concentration range 10-3 – 10-4M, agrees with Freundlish type isothermindicating that monolayer adsorption was occurred, Fig.(4).

From the above data, it is clear that Pb(II) ions is highly retained on soil at pH =5, and it may cause a lead contamination tothe environment at this condition. Also, its migration into the environment could be occurred at highly acidic or higher pH values.

0 5 10 15 20 25

40

45

50

55

60

65

70

75

80

85

90

95

soil(1)soil(2)soil(3)soil(4)

upta

ke p

erce

nt

Tim e (h)

Fig.(2): Effect of contact time on adsorption of Pb (II) from 1.44x10-3 M of lead nitrate using different soil samples

0 5 10 15 20 2530

40

50

60

70

80

90

9.6x 10-5M9.6x 10-4M14.4x 10-4M19.2x 10-4M

upta

ke p

erca

nt

Tim e (h)

Fig.(3): Effect of contact time on adsorption of Pb(II) from lead nitrate solutions using soil sample (S1) at different initial Pb concentration at pH=5.

1E -4 1E-30.0 1

0 .1

soil(1)soil(2)soil(3)soil(4)

log

(X/m

)

log (C )

Fig.(4): Effect of ion concentration on adsorptionof Pb(II) from lead nitrate solution using different soil samples Studying the effect of Co(II) ion concentration as competing ion on the extractability of Pb(II) ions onto different soil sampleswas achieved using initial Pb(II) ion concentration of 2.4 x 10-4 M at pH =5, as shown in Fig.(5).

0.0000 0.0002 0.0004 0.0006 0.0008 0.0010

0

20

40

60

80

100

soil 1 soil 2 soil 3 soil 4

Upt

ake

perc

ent

C o(II) concenteration (M )

Fig.(5): Effect of Co(II) concentration on adsorptionof lead (II) from 0.85x 10-3M lead solution using different soil samples at pH=5.

Desorption of Pb(II) Ions.

Different concentrations of EDTA solutions were used to elute Pb(II) ions adsorbed on soil sample (S1). More than 80 % of Pb(II) ions could be removed from the loaded soil sample using 10-3M EDTA solution after 3 hours of shaking time, as shown in Fig.(6). Also, more than 65% and 55% of Pb(II) ions could also be removed using 10-2

and 10-1 M EDTA solutions respectively.

0 5 10 15 20 25

35

40

45

50

55

60

65

70

75

80

85

0.1M E D T A 0.01 E D T A 0.0001M E D T A

deso

rptio

n pe

rcen

t

Tim e (h)

Fig.(6): Effect of contact time on desorption of pb(II) from soil sample (S1) using different concentration of EDTA at pH=10.

Adsorption of Ni(II) Ions.

Effect of pH variation on the uptake % of Ni(II) ions from chloride solution onto different soil samples is shown in Fig.(7). Maximum adsorption of Ni(II) ions occurs at pH =6. Where, the uptake% reaches 72, 78, 65 and 84% for S1, S2, S3 and S4 soil samples respectively using initial concentration 0.85 x 10-3M Ni(II) ions. Nickel ions form different chloride species with increasing pH value. At highly acidic solutions, the formed species is very difficult to be adsorbed(26), where the uptake % of Ni(II) is very low. Increasing pH value above 6 leads to the formation of soluble Nickel species, which is difficult to be adsorbed. Maximum adsorption % of Ni(II) is attained after 3h of shaking time using initial Ni (II) ions concentration of 1.7 x 10-3M at pH = 6, as shownin Fig.(8). From the above results, the control soil sample S4 has the highest uptake percent.

Effect of contact time on the uptake percent of Ni(II) onto soil sample S1 using different initial ion concentration is shown inFig.(9). Results indicate that uptake percent of Ni(II) ions

decreases with increasing its initial concentration on the range 0.42 x 10-3 – 2.65 x 10-3 M. Adsorption isotherm of Ni(II) is presented in Fig.(10). The plots in this Figure give straight lines of slope close to unity. So, the adsorption process agree with Freundlish type isotherm, which means that adsorption of Ni(II) takes place through the formation of a monolayer. The linearity of the isotherm disappear at higher concentration that may refer to theformation of more than one layer of Ni(II) ions adsorped on soil.

0 1 2 3 4 5 6 7 8 9

0

10

20

30

40

50

60

70

80

90

soil(1)soil(2)soil(3)soil(4)

upta

ke p

erce

nt

pH

Fig.(7): Effect of pH variation on the adsorption of Ni(II) using initial Ni(II) concentration of 0.85x 10-3M on different soil samples.

0 5 10 15 20 25

30

40

50

60

70

80

90

soil(1)soil(2)soil(3)soil(4)

upta

ke pe

rcen

t

Tim e (h)

Fig.(8): Effect of contact time on adsorptionof Ni(II) from 1.7x10-3 M of Ni(II) solution using different soil samples at pH=6.

0 5 10 15 20 25

20

30

40

50

60

70

0.85x 10-3M 1.7x 10-3M 2.65x 10-3M

Upt

ake

perc

ent

Tim e (h)

Fig.(9): Effect of contact time on adsorption of Ni(II) from solutions of different initial Ni (II) concentrations using soil sample (S1) .

1E-30.01

0.1

soil(1) soil(2)soil(3)soil(4)

log (X

/m)

log (C )

Fig.(10): Effect of ion concentration on adsorption of Ni(II) using different soil samples Effect of competing ion on the uptake percent of Ni(II) has been studied using Fe(III) ions and the results are shown in Fig. (11). With increasing Fe(III) ions concentration a continual decrease of the uptake % of Ni(II) ions on different soil samples occurred. The competing between both ions to be adsorbed on soil samples may be attributed to the close values of their ionic radius (Fe3+ = 0.64 Ao , Ni2+ = 0.78 Ao). Also, the adsorption tendency of the trivalent ions on soil is very likely higher than divalent ions(27). From the above data, it is clear that Ni(II) ions is highly retained on soil at pH =6, and it may cause a nickel contamination to the environment at this condition. Also, its migration into the environment could be occurred at highly acidic condition.

Desorption of Ni(II) Ions.

Desorption of loaded Ni(II) ions from soil was tested using 0.1M NaOH, 10-3 – 10-1M EDTA and 0.1M HCl at various contact time, theresults are shown in Fig.(12). The used NaOH and EDTA solutions do not elute Ni(II) from soil samples. While 0.1M of HCl solution give desorption value reaching 75 – 80 % after approximately 3 - 4 hoursof contact time.

0.000 0.002 0.004 0.006 0.008 0.010

10

20

30

40

50

60

70 soil(1)soil(2)soil(3)soil(4)

upta

ke p

erce

nt

Iron concentration [M ]

Fig.(11): Effect of Fe(III) ion concentration on the adsorption of Ni(II) by different soil samples at pH=6.

0 5 10 15 20 250

10

20

30

40

50

60

70

80

Am m onium sol. pH =10 A cidified w ater pH =1 0.001M E D TA 0.1M E D TA

Des

orpt

ion

perc

ent

Tim e (h)

Fig.(12): Effect of contact time on desorption of Ni(II) using different aqueous solutions.

Adsorption of Cr(VI) Ions.

Effect of pH variation on adsorption of Cr(VI) ions from initial K2Cr2O7 concentration of 9.6 x 10-5 M on soil samples have been experimented. As shown in Fig.(13) there is a sharp decrease ofCr(VI) adsorption during pH range of 1–3. Chromium ions are present

as Cr2O72- at acidic condition, which is adsorbed due to the exchange

process taking place in the soil matrix. As the pH value increases the net positive charge decreases and consequently the adsorption ofthe anion species decreases. So, the uptake percent of Cr(VI) ions is very low over the pH range of 3-8. Effect of contact time on adsorption of Cr(VI) from aqueous K2Cr2O7 solution of pH =1onto different soil samples are presented in Fig.(14). There is a sharp increase of adsorption by increasing shaking time. The highest uptake % of Cr(VI) reaches 75, 57, 62 and 55 % for S1, S2, S3 and S4soil samples respectively after 4 hours of shaking time.

0 1 2 3 4 5 6 7 8 90

20

40

60

80

100soil(1)soil(2)soil(3)soil(4)

upta

ke pe

rcen

t

pH

Fig.(13): Effect of pH variation on adsorption of Cr(VI) from initial concentration 9.6 x10-5 M of K2Cr2O7 solution using different soil samples.

0 5 10 15 20 2520

30

40

50

60

70

80

soil(1)soil(2)soil(3)soil(4)

upta

ke p

erce

nt

Tim e (h)

Fig.(14): Effect of contact time on adsorption of Cr(VI) from 1.92 x10-4 M K2Cr2O7 solution using different soil samples Effect of contact tie on adsorption o Cr(VI) from different concentration o K2Cr2O7 on soil sample S1 is presented in Fig.(15). As the initial chromium ion concentration decreases its uptake percent increases. Adsorption isotherm of Cr(VI) is presented in Fig.(16). The plots in this Figure give actually two straight lines. The first line of slope equal nearly unity up toion concentration of 2.9 x10-4 M. In which it confirms Freundlish isotherm type. While the second line has higher slope than one, it means that the adsorption fits Langmuir model. So, the adsorption o Cr(VI) by the soil is morecomplex than the adsorption o either Pb(II) or NI(II) and could be analyzed by applying more than one isotherm model(28-30). From the above data, it is clear that Cr(VI) ions is highly retained on soil at pH =1, and it may cause a chromium contamination to the environment at this condition. Also, its migration into the environment could be occurred at highly pH values.

0 5 10 15 20 25

40

50

60

70

80

90

100

9.6x 10-5M 19.2x 10-5M 24x 10-5M 28.8x 10-5M

upta

ke p

erce

nt

Tim e (h)

Fig.(15): Effect of contact time on adsorption of Cr(VI) from different concentration of K2Cr2O7 using soil sample (S1).

1E-4

0.01 soil(1) soil(2) soil(3) soil(4)

log(X

/m)

log(c)

Fig.(16): Effect of ion concentration on adsorption of Cr(VI) using different soil samples.

Dsorption of Cr(VI) Ions.

Desorption of loaded Cr(VI) from soil samples has been tested using different aqueous solutions which are presented in Figures (17& 18). The used eluting solutions include (NH4)3PO4, NH4Cl, NaOH, H3PO4, Na2PO4 and ethyl acetate. All these solutions give passive

results except Na2PO4 and ethyl acetate solutions which give positiveresults, where the amount of desorbed Cr(VI) ions does not exceed than 18%.

0 5 10 15 20 250

5

10

15

20

(N H 4)2PO 4 NH 4C l H 3PO 4 NaO H

deso

rptio

n pe

rcen

t

Tim e (h)

Fig.(17): Effect o contact time on desorption of Cr(VI) using different aqueous solutions.

0 5 10 15 20 250

10

20

30

40

50

N a2H P O 4 E thyl actate

Des

orpt

ion

perc

ent

Tim e (h)

Fig(18): Effect of contact time on desorption of Cr(VI) using sodium hydrogen phosphate and ethyl acetate aqueoussolutions.

Adsorption of 65Zn Ions.

Effect of pH variation on the adsorption of 65Zn ions on different soil samples was experimented from chloride solution and was shown in Fig.(19). Adsorption of 65Zn ions increases with increasing pH values through the studied pH range of 1–5. Effect of contact time on the adsorption of 65Zn ions was studied at pH =4. Maximum uptake % of 65Zn was in the range of 35-50% after shaking time of 3 hours, as shown in Fig.(20).

1 2 3 4 5

0

10

20

30

40

50

60

soil1 soil2 soil3 soil4

upta

ke p

erce

nt

pH

Fig.(19): Effect of pH varition on adsorption zinc(II) from chloride solution using different soil samples.

0 5 10 15 20 25

10

15

20

25

30

35

40

45

50

55

soil(1) soil(2) soil(3) soil(4)

Upta

ke p

erce

nt

Tim e (h)

Fig.(20): Effect of contact time on adsorption of Zn(II) from chloride solution of pH =4 using different soil samples. Adsorption isotherm of Zn(II) ions was given in Fig,(21). Freundlish type isotherm was applied on the first straight part of the plot of the graph. Which indicate to the formation of monolayer adsorption in this part of the plot where its slope is near unity. With increasing ion concentration the slope changes of the plot as an indication of another type of the isotherm could be occurred. From the above data, it is clear that Zn(II) ions is highly retainedon soil at pH =4, and it may cause a zinc contamination to the environment at this condition. Also, its migration into the environment could be occurred at highly acidic condition. Dsorption of 65Zn Ions.

Elution studies were done using either pure distilled H2O or 0.1M HCl solution for various contact time. From the results given in Fig.(22), it is obvious that 20 minutes were enough to achieve complete elution of 65Zn ions using 0.1M HCl solution.

10 100

1000

soil(2)soil(4)

log(X

/m)

log(C )

Fig.(21): Effect of zinc ion concentration on dsorption ofZn(II) using different soil samples

0 20 40 60 80 100 1200

10

20

30

40

50

60

70

80

90

100

0.1M H C L H 2O

Des

orpti

on %

T im e (h)

Fig.(22 ): Effect of contact time on desorptionof Zn(II) from soil sample (S2).Adsorption study on a mixture of Cs(I), Co(II), Zn(II) Eu(III) and Cr(VI) ions.

Effect of pH variation on adsorption of different cation species from actual radiowaste solutions on soil samples S1, S2 and from simulated radiowaste solution on soil sample S4 were presented in Figures 23, 24 and 25 respectively.

Fig. (23) Effect of pH variation on adsorption of different cation species from their actual radioactive waste mixture using soil sample (S1)

Fig. (24) Effect of pH variation on adsorption of different cation species from their actual radioactive waste mixture using soil sample (S2)

Fig. (25) Effect of pH variation on adsorption of different cation species from their simulated mixture using soil sample (S4) Separation Factors. The separation factor for each couple of elements is obtained as the ratio between the adsorption coefficient (Kd) for the two elements under the same conditions. All separation factors are listed in Table (2). Maximum value of the calculated separation factors at pH =7 was DCs/Co =5.33 for soil samples S4. The values of separation factors were enough to attain a fessible separation for the studied ions. Where, it gives a very acceptable idea to the direction on possibility of separation of the studied elements from environmental waste solution containing the mixed ions(31).

Table (2): Separation factors for [Eu, Co, Cr, Cs and Zn] in their mixture in sulfate solution.

Separation factor

pH S1 S2 S4

DEu(III)/Cs(I) 3 26.25 29.5 8DEu(III)/ Zn(II) 3 1.25 1.33 1.9DEu(III)/ Co(II) 3 21 23.6 8.57DEu(III)/Cr(VI) 3 - - 7.5

DZn(II)/Co(II) 3 16.8 17.8 1.5DCr(VI)/Cs(I) 7 - - 20DCs(I)/Co(II) 7 1.14 5.33 -DCo(II)/Cs(I) 6 0.42 0.5 21.8

CONCLUSION

It is generally agreed that the uptake of the ions by soil is an ion exchange processes. The capacity of soil samples for the uptake of the studied ions increases with increasing initial pH value until reaching its maximum percent. Freundlich isotherm was found to fit the data of uptake of Pb(II), Ni(II) and Zn(II) by soilsamples, while the corresponding data for Cr(VI) was described with langmuir model. Soil can be used as adsorbent for some heavy elements and some other ions. pH value is a controlling factor for retention of the studied elements on soil.

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، ي دروج� د ال�هن م ...(. وق�� دام ت�� خ� ة� اس�ت� �ئ ن� ق5 ل ب�� خلن ي ال�ب5 ف� ي¥نt ال�طث ع�ن ث§ ر� ل�ث� د ��ق ز� وب�� ت ��ات� ث��رك ���وب ت� cاصز ا ال�دراس�ة� م�خل ال�عن��روم��اص ، ) ال�ك ���كل ، ال�زص ي ي ال�ي� ل ( ف� ة� ال�مخال�ن ����لف ت� م . ك�ما ال�مخ� دام ت�� خ� ت� ���س cة� ا ��ئ ن� ق5 ل ب�� خلن �ت�زي ال�ب5�وم ت§ ي¥نt ال�زاد �ن �ع ل�ث�

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