Journal of Scientific & Industrial Research Vo1.58, November 1 999, pp 883-892

Adsorption Equilibria of Hg(II) on Clays in Presence of Organic Materials

Beena T Abraham & T S Anirudhan •

Department of Chemistry, University of Kerala, Trivandrum 695 58 1 , India-

Revised received: 1 8 February 1 999; accepted: 23 April 1 999

Adsorption of Hg(II), organic l igands (HA, NTA, EDTA and CDTA) and Hg(II) in the presence of organic ligands on montmorillo­nite, illite and kaolin from water environment is reported. The maximum adsorption of Hg(lI) onto montmorillonite and kaolin lakes place in the pH range 5- 10. The maximum removal of Hg(Il) by illite is observed at pH 8. The efficiency for the removal of Hg(/I) by adsorption is in the order: montmoril lonite > illite > kaolin. The adsorption isotherm data follows both the Langmuir and Freundlich isotherm equations. The maximum removal of organic ligands by adsorption is observed at low pH and is in the order: HA >NTA > EDTA > CDTA. The monovalent species of EDT A and CDTA have lower adsorption capacity than divalent species of NTA on the clay surface at pH 3.0. In ligand titration experiments, if the Hg and clay concentrations are kept constant and the ligand concentration is allowed to vary, conditions under which ligand enhances and inhibits Hg adsorption with the solid phase are noted. The experimental equilibrium data for the adsorption of Hg(II) in the presence of ligand at constant UM ratio are analysed using Langmuir isotherm model. The enhanced uptake of metal in the presence of organic ligands, at least at certain circumstances, may be due to the formation cf an adsorbed organic layer on the clay providing new adsorption sites for metal at the surface.

Introduction Water is a major carrier of metal pollutants which can

be transported from a disposal site in dissolved and par­ticulate forms. The concentration and transport of many metals in natural waters are, to a great extent, controlled by sorption process at sediment surfaces 1 . Clay miner­als and hydrous metal oxides are the adsorbent species that are mostly present in sediments and suspended ma­terials . The adsorption of metal cation onto these solid surfaces has been the subject of many studies during re­cent years2•3. Various adsorbent materials reported for the removal of mercury from aqueous solutions are on­ion skin, waste tyre rubber, peanut skin, humic acid, tree bark, ferric oxide, a-quartz, peanut husk carbon and mixed oxide gels4• 1 I • Recently, the removal of mercury from water by surface modified cellulose, sawdust and hydrous titanium(IV) oxide gel has been reported 1 2 . 1 4.

Most experimental studies of the adsorption behaviour of trace metals, however, have been conducted in simple electrolyte systems in the absence of organic ligands.

S ignificant concentration of natural and synthetic or­ganic l igands are present in many aquatic systems. The adsorbed organic ligands on solid surface may interact with metal ions, leading to their accumulation on par­ticle surface 1 5. 1 6 . The extent to which organic ligand

• Author for correspondence

covers the surface of solid surface is unknown. The met­als in natural bodies are in part present as dissolved metal­ligand complexes. Therefore, the examination of metal adsorption characteristics in the presence of organic ligands is necessary to understand the fate of metals in soil and sediment components. Singh et al. 17 have stud­ied the influence of citric acid and glycine, which are common in fresh waters, on the kinetics of the adsorp­tion of Hg(II) by Kaolin under various pH conditions. In the present investigation, adsorption characteristics for the removal of Hg(II) in the presence of organic ligands by clay minerals such as montmorillonite, ill ite and kaolin have been studied. The organic l igands used in t h i s study are : n i tr i lo tr iacet ic acid (NTA) ethylenediaminetetraacet i c ac id (EDTA) , cyclohexylenedinitrilotetraacetic acid (CDTA) and hu­mic acid (HA). We chose these adsorption systems be­cause (i) the actual behaviour of Hg(II) in natural sys­tem is not well understood and Hg is an environmen­tally important toxic metal, (ii) a better understanding of the interactions in the ternary system of heavy met­als, inorganic solid surface (clays) and organic l igands in aquatic media is of great importance for refinement of the transport models of metals in waterbodies and for predicting the environmental effects of increased pol­lutant loads in aquatic environment, and (iii) NTA is used as a substitute for sodium tripolyphosphate in household

884 J SCI IND RES VOL.58 NOVEMBER 1 999

Table I - Surface and physical properties of the investigated clay samples

Parameter Clay Montmorillonite Illite Kaolin

Particle size, mesh size -80+230 Surface area, m2/g 291 .5 Porosity, mUg 0.56 Apparent density, glmL 2. 1 Moisture content, % 2.7 Cation exchange capacity, 0.80 meqlg pHzpc 4.6

detergents and EDTA and CDTA as industrial cleaning agents. Further areas of applications are: conditioning of cooling water, scale inhibition, corrosion and oxida­tion prevention and masking of heavy metals.

Materials and Method All the chemicals used were of analytical reagent grade

and were obtained from BDH, Merck and Fluka. Or­ganic l igands NTA, EDTA and HA were of Fluka Chemi­cals and CDTA was of Aldrich Chemical, which were used without further purification. The clay minerals such as kaolin from Loba Chemie, Bombay, montmoril lonite and ilFte from BDH (UK) were used as received. Prior to use, clays were treated with conc. HCI and washed with distilled water until free of chloride. The washed product was dried and sieved to . �80 + 230 mesh size. The suface area of clay particles was obtained from BET method with a Gemini 2360 analyser, using physical adsorption technique with nitrogen at different pressures at 77 K18 . The cross-sectional area of the nitogen mol­ecule was taken to be 1 6.2x W-2° m2. The surface area, S (m2/g) was calculated from the BET isotherm slope and intercept using Eq.( l )

S = Vm a N x 1 0-20 224 1 4 m A . . . . ( 1 )

where V is the volume of the gas required fo form a m monolayer on the solid, NA is the Avogadro's number and a is the average area occupied by a molecule of

m

adsobate. V is calulated from the slope and intercept of m the BET isotherm plot.

The zero point of charge (pH2P) is defined as the pH of the suspension at which surface charge density (ao) is zero. A potentiometric titration methodl9 was used to

-80+230 -80+230 106.5 1 6.8 0.41 0.36 1 .9 2.4 1 .8 2.6

0.23 0.09

4.9 3.4

determine the surface charge as a function of pH and ionic strength. About 0.5g of the c lay was suspended in 1 00 mL of electrolyte solution (O.OOI M, O.OIM, 0. 1 M) and equilibrated for 4 h.The titration was performed at 30"C by successive 20 to 50 ilL increment of O. I M HN03 or O. I M NaOH. Following each addition of titrant, the pH of the suspension was measured after 5 min when emf drift was usually less than 0.3 mY/min using a Il­processor systronic pH meter. The ao as a function of pH was calculated by taking the difference in the amount of base or acid taken up in the titration of suspension and the corresponding blank electrolyte solutions . The ao (C/cm2) was calculated by Eq.(2).

. . . (2)

where F is the Faraday constant; A, the area of the suspension ( cm21L) ; CA and CB are the concentra­tions of acid and base after each addition during the

titration and [H+] and [OR] are equivalents of H+ and OR- bound to the suspension surface (eq/cm2). The physical and surface properties of the clays were determined by standard methods and are given in Table ! .

Batch adsorption studies were performed by shak­ing O. l g of clay with 50 mL of Hg(II) solution of varying concentration in 1 00 mL stoppered conical flasks. The pH of the solution was adjusted using O. l M Hel or O. IM NaOH. Isotherm studies were conducted at pH 5.0 with varying concentration of Hg(II) at different temperatures. The contents of the flask were shaken in a temperature-controlled wa­ter bath shaker. At the end of predetermined time

. .

ABRAHAM & ANIRUDHAN : ADSORPTION EQUILIBRIA OF Hg(II) 885

intervals, the contents were centrifuged and super­natant analysed using Mercury Analyser (Perkin­Elmer, model MAS -50A).

To examine the effect of pH on organic ligand adsorption by clays, batch adsorption tests were per­formed at 30°C. About O. l g of clay was shaken with 50 mL solution of organic ligands of required con­centration in different stoppered bottles. The ionic strength of the medium was kept constant through­out.by using O.O IM NaCl and pH using O. IM HCI or O . lMNaOH. After equilibration, the sorbate was centrifuged and the final concentration of organic ligands in supernatant was analysed using uv-vis­ible spectrophotometry2o.

To study the effect of organic ligands on Hg(II) adsorption, the following ligand titration experiment was conducted. In a titration vessel known quantity of clay mineral (2 gIL) was mixed with 0.0 1M NaCI solution at constant pH of 3 .0. The l igands tested were HA, NTA, EDTA and CDTA. Sufficient mer­cury was added to produce a tota] concentration of 1 5 mg/L and RN03 and N aOH were added to main­tain the pH at 3 .0 . The slurry was then mixed in a water bath shaker for 6 h . Aliquots of the solutions were centrifuged and the total concentration of dis­solved Hg(II) was measured.

Results and Discussion

The effect of pH on Hg(II) adsorption on clays was studied by changing the pH from 2.0 to 10.0, using the initial concentration of 1 0 mg/L (Figure 1 ) . The adsorption of Hg(II) onto mont­morillonite and kaolin was negligible below pH 4.0 but abruptly increased at pH 5.0 to 6.0. The increase of pH above 6.0 showed a marginal in­crease in the uptake of Hg(II). Three distinct re­gions were observed in the case of i ll ite: (i) below pH 4.0, where little or no adsorption occurred, (ii) pH 3.0 to 8 .0, where an increase in metal ad­sorption was observed, and ( i i i ) above pH 8 .0, where adsorption decreased. Significant adsorption did not occur at low pH region because the net clay surface charge is positive in this pH region and adsorption of cat ionic Hg(II ) species i s unfavourable or because of hydrolysis, Hg(II) is aq

� 0

';j � e Go> =:

100 Initial Hg(Il) concn : 1 0 mgIL

90 Temperature : 30"C Clay mineral : + Monunorillonite

80 • Illite

70 A Kaolin

60 SO 40 30 20 10 O +---�---+--�----�--�--�

o 2 4 6 8 10 12 pH

Figure I-Effect of pH on the removal of metal ions by clay minerals

not significant. Metal adsorption is manifested par­ticularly when the metal ion undergo hydroly­sis2 1 .22 . The slight increase in adsorption above pH 7 .0 in the case of montmoril lonite and kaolin im­plies that besides adsorption, another process may be involved, namely retention of Hg(OH)2 species in the inner pores of the clay surfaces . It is evident that kaolin and montmorillonite are effective for the quantitative removal of Hg(II) over the pH range 5.0 - 1 0.0. However, illite is effective for the quan­titative removal of Hg(IT) within the narrow range of pH 4.0 to 8 .0. The order of efficiency for the removal of metal ion was: montmorillonite > il­lite > kaolin.

The properties of montmorillonite differ signifi­cantly from those of illite and kaolin. The high sur­face area of 29 1 .5 m2 /g and high cation-exchange capacity of 0.80 meq/g are the characteristics of montmorillonite. The montmorillonite used is three layer aluminosilicates with a 2: I type lattice, i .e. it contains-two sheets of silicon - oxygen tetrahedra and one octahedra sheet is located between them. Although a certain amount of anion exchange may occur on the edges of montmorillonite, most ex­change adsorption is cationic in nature, with posi­tively charged cations from solution displacing inorganic cations23. Egozy24 assumed that the adsorp­tion of cation by montmorillonite was due to two classes of sites, one by constant charges sites, and

886 J SCI IND RES VOL.58 NOVEMBER 1 999

25

20

15

10

5

0 25

20 ... Ol. E 15 r:i

10

5

0 25

20

15

10

5

0 0

Kaolin

mIte

Temperature : +4O"C • 3S'C A 3O"C x 2S'C

Temperature : +40'C . . 35'C A 3O"C x 25'C

Montmorillonite

Temperature : +4O"C . 3S'C A 3O"C x 25'C

25 50 75 100 125 . 150

C" mg/L

Figure 2---PloIS of qe vs Co for the adsorption of Hg(I1) by clay minerals

the other by constant potential (oxide l ike edge ) sites. The montmorillonite becomes highly expan­sive in the presence of water as a result of hydra­tion of water molecules by the exchangeable cat­ions25. The cation exchange capacity of illite and kaolin is 0.26 and 0. 1 2 meq/g, respectively. The negative charge is associated with OH-group on the surface of the kaolin and i l lite. The magnitude of charge depends on the pH of the medium. The pH dependent charge of the clay is assumed to be lo­cated in the hydroxyl groups formed in the broken crystal edges. With a decrease in pH, dissociation of surface OH-groups decreases and negative charge decrease. The pHzpc of montmori llonite, i l lite and kaolin was found to be 4.6, 4.9 and 3.4, respectively, and above this pH, the surface charge of the adsor-

bent is negative. The perusal of Hg(IJ) specIatIOn diagramS clearly indicates that in the range of high­est sorption efficiency (pH 5 .0 - 8 .0) , the dominant species were M2+ and M(OHY . As the adsorbent su�ace is �egatively cha�ged (ab�ve pHzpc) as wel l , the Increasmg electrostatIc attractIOn between posi­tive sorbate species and sorbent particles, would lead to increased adsorption of metal ions.

The study of adsorption isotherm is helpful in de­termining the adsorption capacities of various clay minerals for the removal of metal ions at certain tem­peratures. Figure 2 shows the variation of C the e' amount of metal ions in solution after attainment of equilibrium and qe' the amount adsorbed at equilib­rium at different temperatures for the concentration ranges between 5 and 1 50 mg/L. According to a category of the l iquid adsorption from aqueous so­lution by Giles et aI. 26, the curve of the adsorption isotherm belongs to C-type, indicates that the num­ber of adsorption sites remains constant irrespec­tive of the quantity of the sorbed substance. When the initial concentration of Hg(IJ) increased from 5 to 1 50 mg/L, the amount of uptake increased from 2.38 to 1 7 .7 1 mg/g by montmorillonite, from 2 . 1 8 to 1 5 .83 mg/g by ill ite, and from 1 .95 to 1 4.73 mg/ g by kaol in at 25°C and the corresponding values at 40°C increased from 2. 1 6 to 1 4.04 mg/g by mont­morillonite, from 1 .84 to 1 2 .6 1 mg/g by ill ite and from 1 .69 to 1 1 .03 mg/g by kaolin. An increase in temperature results in decreased adsorption, the mechanism of adsorption in the systems under study may be purely physical in nature.

The FTIR spectra (50-600 cm-' and 500-4000 cm- ') of montmoril lonite and Hg(IJ) adsorbed mont­moril lonite were plotted in Figure 3. The broad asymmetric absorption bands at 3448 cm-' for mont­morillonite and 3463 cm- ' for Hg(ll) adsorbed mont­morillonite are attributable to the sum of the contri­bution from adsorbed or hydrated water and hy­droxyl group of octahedra27. Absorption b ands formed from adsorbed water is also observed at 1 622 cm- ' for montmoril lonite and J 608 cm- ' for Hg(II) adsorbed montmori llonite. The peak at 1 025 cm- I for mon tmori l l onite and 1 003 c m - ' for Hg(II) adsorbed montmoril lonite is caused by the vibra-

,.

yII>-

ABRAHAM & ANIRUDHAN : ADSORPTION EQUILIBRIA OF Hg(II) 887

0 � 0

, co

0 o.D

0 B 0 .:# � .:1' .

.. 0 0 «II N . N ' g ,, 0 - -.'� S � '':;1

18 _ .< 2 , ' ' 4000, :'3500 � ,3000 2500 2000 1500 1000 500 600 550 500 450 400 33) 300 250 200 '50 100 50 ' . ' , ' ' ' -1

Wawnu", btr) 'C III

Figure 3-FTIR spectra of (A) montmorillQnite and (B) Hg(lI) adsorbed montmorillonite

tions of the O-Si-O valence bond. The absorption band of H-O-AI for montmorillonite appears at 773 cm- I . The bands at 538 and 457 cm-i are due to the deformation oscillations of Si-O and Si-O-AF7.28 . The absorption band observed at 3448 cm-i on montmorillonite and its shift to 3463 cm- I on Hg(IJ) adsorbed montmorillonite indicates the presence of hydroxyl group on montmorillonite for Hg(IJ) ad­sorption. The additional peak at 266 cm-! on Hg(II) adsorbed montmorillonite indicates the presence of Hg-O bond. It is therefore clear that Hg(II) is adsorbed on montmorillonite surface.

Experimental data were fitted to the linearised Langmuir equation [Eq.(3)] :

C. 1 C. - = -- + -qe Q"b Q"

. . . (3)

where C is the equilibrium concentration; q , the e e amount adsorbed per unit weight of adsorbent; QO, the maximum adsorption capacity, and b is the bind­ing energy or stability constant. The Langmuir equa­tion was tested by plotting C/qe versus Ce for dif­ferent temperatures (Figure 4) . The straight line plots indicate the applicability of the above model for the present system. The Langmuir model agrees well

with these data, assuming all the adsorption sites have equal energy, while the percentage removal versus pH curve shown earlier clearly suggested different site energies. The Langmuir equation is also valid in many cases where different site energies are known to possess on the surface29. The langmuir equation has been used successfully to describe sorp­tion of Pb by pure minerals and soils30. The values of QO and b were calculated from the plots through regression analysis and are given in Table 2. The values of oa and b decreased with increase of tem­perature indicating the exothermic nature of adsorp­tion. The Langmuir constant, b related to the equi­l ibrium constant or energy of adsorption of metal ions to the clay surface was smaller for kaolin than illite and montmorillonite. This strongly suggests that in kaolin metal ions were more looseiy bound to the surface.

The C-type isotherm observed in the present study is described well by the Freundlich equation27 [Eq.(4)] :

1 log qe = log K F + - log C.

n . . . (4)

where KF and l in are Freundlich constants rei<lted

888 J SCI IND RES VOL.58 NOVEMBER 1 999

to adsorption capacity and intensity of adsorption respectively. Figure 5 represents the l iner plots of logqe versus 10gCe at different tempratures, which indicate the applicability of the Freundlich equation. The Freundl ich constants along with correlation coefficents are presented in Table 3. The decrease in the values of Kr at higher tempratures shows that the adsorption rate decreases with rise in tempera­ture. When the sorption data fol low both Langmuir and Freundlich isotherms, i t is preferable to use the Langmuir isotherm because the sorption maximum can be calculated.

The removal of organic l igands from aqueous so­lution by sorption is highly dependent on pH of the solution which affects the surface charge of the clay and degree of ionization and the solubility of or­ganic l igands. Figure 6 shows the effect of pH on the removal of organic ligands from aqueous solu­tion by clays. It was observed that the decrease in pH leads to increased uptake of organic l i gands by clays, the lower the pH the higher the adsorption. The maximum amount decreases in the order: HA > NTA > EDTA > CDTA.Increasing pH increase

� ". �

U

12 .

10 Kaolin

8

6

4

Temperature : + 40'C 2

0

. 35'C ... 30'C t-_+---_.---+--_.-< __ x--j25'C

10

9 ; 8

7

6

5 4

2

. Illite

Temperature : + 4O'C . 35'C ... 30'C )( 25'C o +-_�_ .. ___ .. . _�_� 10 1

9 8

7

6 .

5

4 .

3

2

Montmorillonite •

Temperature : + 40'C . 35'C ... 30'C x 25'C

o �. __ � __ � __ � __ � __ � __ � o 25 50 75 100 125 150

C.- mglL

the ionization of HA(HA + H P � H /0 + A- ) and hence the concentration of negatively charged an­ion A

-. The increasing elec trostatic repulsion be­

tween A-

and negatively charged clay particles would lead to reduced adsorption of HA at high pH3 l . The undissociated H A molecu les dominating at low pH are more hydrophobic and more adsorbable than the ionized form32. The adsorption of NTA at pH

Figure 4-Langmuir plots for the adsorption of Hg(lI) on clay minerals

Table 2-Langmuir constanls for Ihe adsorplion of metal ions on clay minerals

2S"C 30"C 35"C 40"C

Clay QO b r QO b r QO b r Q" b r mg/g Llmg mg/g/g Llmg mg/g Llmg mg/g Llmg

Montmorillonite 1 8 .87 0.096 0.993 1 6.04 0.057 0.948 1 5 .6 3 0.046 0.95 1 1 4 .44 0.039 0.998 I l lite 1 7 .24 0.070 0.9 8 1 1 5 .07 0.045 0.959 1 4 .85 0.03 8 0.938 1 3 .96 0.03 1 0 .983 Kaolin 1 5 .04 0.04 1 0.952 1 4 .40 0.03 1 0 .943 1 3 .64 0.028 0.949 1 2 .83 0.02R 0.953

T a b le 3 - Fre u n d l i c h c o n s t an t s for the a d s o rp t i o n o f H g ( l I ) o n c l a y m i n e ra l s

2 5 "C J O"C 3 5 "C 4 0 " C

C la y K r I /n K " 1 /1/ K ,. I /n K " I /n

M o n lm o ri ilo n ite 3 . 1 7 0 . 3 4 8 0 . 9 4 1 2 . 5 5 0 . 3 S Q 0 . 9 5 2 2 . 3 8 0 . 3 8 3 0 . 9 8 3 2 .04 0 . 3 8 � 0 . 943

I l l i te 2 . 3 5 0 . 4 0 K 0 . 9 1 9 1 . 9 7 O A 2 5 0 . 9 8 2 1 . 5 8 0 . 4 5 5 0. 9 1 1 1 . 3 6 O A 7 2 0 . 940 K ao l i n 1 . 76 0 .446 0 . 9 2 5 1 . 5 2 0 . 4 6 0 0 . 9 3 1 1 . 2 7 0 . 4 8 9 0 . 9 4 2 Ll 3 0 . 4 9 9 0 . 9 5 8

./ ,j

ABRAHAM & ANIRUDHAN : ADSORPTION EQUILIBRIA OF Hg(II) 889

of tlI)

,.g

-1

2 .,-1 .8 1 .6 1 .4 1 :2

1 0.8 .

0.6

�

2 T 1 .8 t 1 .6 � 1 .4 1 .2

0.8

2 1 .8 t 1 .6 1 .4 1 .2

1 0.8

a

Temperature : + 40"C . WC .& 30"C x 25°C

Kaolin Temperature ",: + 40°C

• 35°C .& 30"C x 25°C

I l lite Temperature : + 40°C

. 35OC .& 30"C x 25°C

Montmori llonite 2

log e.

3

Fi';Ure 5-Freundlich plots for the adsorption of Hg(H) on clay minerals

3 .0, which is essentially present as divalent anion, U- (NTA: pKI = 1 .97, pK2=2.57 \nd pK3 =9.8 1 ) un­der these conditions on positively charged clay par­ticles (less than pH of clay particle) reaches maxi-. zpc mum. The increasing electrostatic attraction between ionized acid species and positively charged clay par­ticles would lead to increased adsorption of NTA. EDTA and CDTA have four -COOH functional groups with pK I of 2 . 1 4 and 2.30, pK 2 of 3.2 and 3 .6, pK 3 of 6.3 and 6.2 and pK 4 of 1 0.3 and 1 1 .2, respectively. Hence, at pH 3 .0, any one -COOH group from foW -COOH groups present in EDTA and CDTA ionizes and these monovalent species

10

9 8 .

7 6

5 4 3 2

0

10

9 8 -

tlI) e 7

� e 6 -; 5 1; e 4 .

.� 3 2 .

a 10

9 8

7 . 6

5 4 3 2

Concn. of organic ligands : 25 mgIL Temperature : 30"C Organic ligands : + HA

. NTA .& EDTA x CDTA

- Concn: of organic ligands : 2S-mgIL .

I l l ite

Temperature : 3O"C Organic ligands : + HA

. NTA .& EDTA x CDTA

Concn. of organic ligands : 25 mgIL Temperature : 30°C

rganic ligands : + HA . NTA .& EDTA x CDTA

Montmorillonite O +-__ � __ � ____ � __ � __ � __ �

a 2 4 6 8 pH

1 0 12

Figure 6-Effect of pH on the removal of organic l igands by clay minerals

have lower adsorption capacity than divalent spe­cies of NTA on positively charged surface of clays. The clay surface carries a net negative charge at higher pH and thus negatively charged acid species cannot be adsorbed onto the clay surface.

Figure 7 shows the results of the l igand titration conducted at the constant total Hg concentration of 1 0 mglL. The results have been expressed as the

890 J SCI IND RES VOL.58 NOVEMBER 1 999

�, u·

10 ;-----------------------� Initial conen : 1 0 mgIL 9 pH : 3.0

Kaolin 8 . Organic ligands : + HA • NT A

7 6

5

4

3

2

0

.A EDTA x CD

Hg-HA Hg-NTA Hg-EDTA Hg-CDTA

UM : 2.5 : 2.0 : 1 .5 : 0.5

10 Initial conen : 10 mgIL Illite 9 pH : 3.0

8 Organic ligands : + HA • NT A .A EDTA x CDT

7 6 .

5

4

3 Hg-HA 2 Hg-NTA

Hg-EDTA Hg-CDTA

0 10 Initial conen : 10 mgIL

LIM : 1 .5 : \ .5 : 1 .0 : 0.5

9 pH : 3.0 Montmorillonite 8 Organic ligands : • HA . • NT A

.A EDTA x CDT 1 6

5

4

3 UM

2 Hg-HA : 2.0 Hg-NTA : 1 .5

1 . Hg-EDTA : 1 .5

0 Hg-CDTA : 0.5

0 50 100 150 200 250 Ligand concentration, mgIL

Figure 7-Equilibrium dissolved Hg(JI) at varying initial organic ligand concentrations

remaining concentration of metal ions in solution versus initial concentration of l igands. Decreased values of Hg(II) concentration in solution represent increasing adsorption of Hg(II) on clay mineral s . Adsorption of Hg(II) on al l clays drastical ly in­creased on the addition of small amount of HA and NTA. At very low concentrations of HA and NTA, the adsorption of Hg(II) on clay surface enhanced. At higher concentration of these l igands, adsorption of Hg(U) was found to be decreasing. The concen-

tration of Hg(II) ions in overlying solution decreased from 2.40 to 1 .03 mgIL for montmori l lon i te, from 3 . 1 0 to 1 .30 mg/L for i l l i te, and from 4.2 1 to 1 .80 mg/L for kaolin with the i ncrease of HA concentra­tion from 0 to 20.0 mg/L, 0 to 1 5 .0 mg/L and 0 to 25.0 mg/L, respectively and thereafter, the concen­tration of Hg(II) increased with the increase of HA concentration. According to the surface chemistry theory, when two phases such as mineral particles and sorbate molecules are i n contact, they are bound to be surrounded by an electric double l ayer due to e lectrostatic interactions33• The dcrease in Hg(II) adsorption at higher concentration of l igands may be due to extended e lectric double layers surround­ing both clay and Hg - l igand complex34• The LIM ratio for maximum uptake of Hg(II) i n the presence of HA onto montmori l lonite, i ll ite and kaolin was found to be 2.0, 1 .5 and 2.5, respecti vely. On the other hand, the remaining concentration of Hg(II) in so­lution decreased from 2.40 to 1 .42 mg/L for mont­morillonite, from 3 . 1 1 to 1 .8 1 mg/L for i l l i te, and from 4. 1 6 to 2.33 mg/L for kaolin , by increasing the concentration of NTA from 0 to 1 5.0 mg/L, 0 to 1 5 .0 mg/L and 0 to 20.0 mg/L, respectively, and the cor­responding LIM ratio for the maximum uptake of Hg was found to be 1 .5 for montmori l lon ite and i l­l i te and 2.0 for kaol in . The max imum uptake of Hg(II) in the presence of EDTA was observed at LIM ratio of 1 .5 for montmoril lonite and kaolin and 1 .0 for i l lite. The behaviour of CDTA was different in the l igand titration studies , where there was a mild increase in surface concentration of Hg(II) onto clays (LIM ratio = 0.5) fol lowed by a complete re­versal of sorption at h igher concentrations of CDTA. In the case of HA and NT A, there appeared to be a more definite pattern of enhancement of Hg bind­ing by clay at low l igand concentration, fol iowed by complete reversal .

The adsorption isotherm experiments were con­ducted at constant l igand: metal dose ratio. The ex­perimental procedure was similar to that adcpted in the ·preceeding experiments, except that the init ia l concentrations of metal and l igand were simulta­neously varied. The isotherm data has been pro­cessed in accordance with Langmuir isotherm , It

ABRAHAM & ANIRUDHAN : ADSORPTION EQUILIBRIA OF Hg(II)

has already been demonstrated that the Langmuir equation gives adequate results in many cases where surface heterogeneity is known to be present35 • Its validity in the system is in order as adsorbent for Hg(II), consists of clay and preferentially adsorbed l igands possessing different surface energies. Fig­ure 8 presents the results of these experiments in comparison to the isotherm recorded for the system in the absence of ligand. The values of Langmuir constants (! and b were calculated from the slopes and intercepts of the plots and presented in Table 4. The values of (! obtained from the slope of the plots indicated that CDTA and blank isotherms were iden­tical while EDTA, NTA and HA had a statistically significant higher isotherm. In all the cases, isotherm in the presence of HA shows lower slopes. The re­sults of these studies lead to a working hypothesis for metal binding to clay surface as influenced by soluble organic matter. .

The surface of the clay minerals carries a net posi­tive charge at pH 3 .0, thus mercury - ligand com­plexes can be readily adsorbed by clays. Also, HA, NTA and EDTA are adsorbed onto clay surface and the residual -COOH group (PK 1 > 3 .0) of the or­ganic ligand may serve as new adsorption sites for mercury. Banjamin and Leckie36 suggested that the functional groups of an adsorbed ligand may serve as new adsorption sites for trace metals at the solid surface. The enhancement of metal binding in the presence of HA is possible at l igand : metal ratios of 2.0 for montmorillonite, 1 .5 for ill ite, and 2.5 for kaolin. The adsorption of Hg(ll) on clays in the pres­ence of CDTA is minimum. The amount of CDTA adsorbed by clay is also minimum. The environ-

Organic ligands : . No ligand Kaolin 10 • CDTA • EDTA x NTA • HA 9 8

7

6

5 4 3 2

Hg-HA Hg-NrA Hg-EDTA

Hg-CDTA

UM : 2.S : 2.0 : I .S : 0.5

0 r-�----�------�--____ � 10 Organic ligands : . No ligand Illite 9 • CDTA • EDTA x NTA • HA

8

7 Hg-HA Hg-NrA

6 . Hg-EDTA

5 Hg-CDTA

4 3 2

UM

l� r=M-o-n-bn--O-r�i'I-lo-n-i-te----r-------�

9 Hg-HA 8 Hg-NrA 7 Hg-EDTA

.6

5 4 3 2

Hg-CDTA

40

UM : 2.0 : i .S

C" mgIL

Figure 8-Langmuir plots for the adsorption of Hg(II) on clay minerals at a fixed UM dose ratio

Table 4-Langmuir constants for the adsorption of Hg(II) on clay minerals at constant l igand: metal dose ratio

Organic Montmoril lonite Illite Kaolin l igands

QO b r QO b r QO b r

mg/g Umg mg/g Umg mglg Umg

No ligand 1 5.73 0.059 0.965 14.92 0.05 1 0.98 1 1 3 .58 0.032 0.982 CDTA 1 6.69 0.067 0.975 1 6.44 0.056 0.983 1 5.60 0.032 0.972 EDTA 1 7.64 0.074 0.998 1 6.83 0.059 0.99 1 1 5 .77 0.03 1 0.968 NTA 1 8.32 0.079 0.995 1 6.92 0.065 0.99 1 1 5.85 0.034 0.974 HA 1 8.98 0.082 0.996 1 7.70 0.069 0.989 1 6.80 0.030 0.966

89 1

892 fSCI IND RES VOL.58 NOVEMBER 1 999

mental implication of these results is that enhance­ment of metal uptake on clay mineral by organic l igands is possible, at least in certain circumstances.

Conclusions The mercury removal from water environment us­

ing clay minerals such as montmoril lonite, i l l ite and kaolin, has been found to be pH dependent. The adsorption isotherm is also affected by temperature as the adsorption capacity decreases by raising the temperature from 25 to 40°C. The experimental ad­sorption data fits both Langmuir and Freundlich iso­therm equations. The adsorption of organic l igands such as HA, NTA, EDTA and CDTA is pH depen­dent and decreases with increase in pH up to 1 1 .0. The conditions under which l igands enhance and inhibit Hg association at clay surfaces have been investigated. Studies at constant Hg: l igand ratio have confi rmed that, at least in certain c ircum­stances, enhancement of uptake on clays occurred. Specifically HA, NTA and EDTA, all of which are adsorbable, have been found to enhance Hg uptake on clays. The results of these studies lead to a work­ing hypothesis for metal binding to clay surfaces as influenced by soluble organic matter. The enhance­ment of uptake in the presence of organic l igands may be due to the formation of an adsorbed organic layer on the clays serving as a sol id phase l igand.

Acknowledgement The authors are thankfu l to the Head, Department

of Chemistry, University of Kerala, for providing the laboratory faci lities.

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