The adsorption of Cu, Pb, Zn, and Cd on goethite from major ion seawater

Geochimica ei Cosmochimico Actn Vol. 46, pp. I253 0 Pergamon Press Ltd. 19RZ. Printed in U.S.A.

to 1265

The adsorption of Cu, Pb, Zn, and Cd on goethite from major ion seawater L. S. BALISTRIERI and J. W. MURRAY Department of Oceanography, (Received

September

University of Washington, Seattle, WA 9819.5

4, 198 1; accepted in revised form March 2, 1982)

Abstract---The adsorption of Cu, Pb, Zn, and Cd on goethite (nFeOOH) from NaNO, solutions and from major ion seawater was compared to assess the effect of the major ions of seawater (Na, Mg, Ca, K, C!, and SO,) on the adsorption behavior of the metals. Magnesium and sulphate are the principal seawater ions which enhance or inhibit adsorption relative to the inert system. Their effect, as determined from the site-binding model of Davis et al. (1978), was a combination of changing the electrostatic conditions at the interface and decreasing the available binding sites. The basic differences between the experimental system of major ion seawater and natural seawater were examined. It was concluded that: I) although the experimental metal concentrations in major ion seawater were higher than those found in natural seawater, estimates of the binding energy of Cu, Zn, and Cd with aFeOOH for natural seawater concentrations could be made from the data, 2) Cu. Pb, Zn, and Cd showed little or no competition for surface sites on goethite, and 3) the presence of carbonate, phosphate, and silicate had little or no effect on the adsorption of Zn and Cd on goethite. INTRODUCTION

of as ligands. From this perspective ion adsorption can be considered analogous to the formation of soADSORPTION by solid surfaces has long been considlution complexes (Stumm et al., 1976). The imporered an important mechanism for controlling certain tant difference, however, is the development of an trace metal concentrations in natural waters (see reelectrical double layer at the oxide-solution interface views by Turekian, 1977; Murray and Brewer, 1977). which depends on pH and ionic strength and which This has led to intense studies defining and quantigreatly affects ion adsorption. fying the interactions between solids, in particular 5) Oxide surfaces may be composed of different metal oxides, and solution species. At this time the types of sites with each site having a characteristic following generalizations can be made concerning the binding energy. This is analogous to a multi-ligand surface chemistry behavior of oxides with respect to solution system. At present much work remains to ion adsorption. be done on determining the quantity, distributions, 1) There is a narrow pH range where the adsorpand effect of different types of sites on the adsorption tion of a cation goes from near zero adsorption to behavior of metals (Benjamin and Leckie, 1980, 100% adsorption. This is commonly called the ad1981). sorption edge. Cation adsorption is anaiogous to In addition to these qualitative observations, varmetal hydrolysis in solution in that the adsorption ious mathematical models have been developed for or hydrolysis of a cation increases as the pH is indescribing ion adsorption on oxide surfaces. The creased and is accompanied by the release of protons models differ in their representation of ion speciation (James and Healy, 1972). and location as well as the formulation of the elec2) Anion adsorption is a mirror image of cation trical double layer terms (for reviews, see Parks, adsorption, i.e., adsorption is strongest at low pH 1975; James et ai., 1975; Westall and Hohl, 1980; with a decrease in adsorption as the pH is increased. James and Parks, in press). The models also require The adsorption of an anion is accompanied by the knowledge about some of the basic properties of the uptake of protons or alternately by the release of oxide such as the surface area, the total number of hydroxide ions (Davis and Leckie, 1979, 1980; exchangeable sites, and the acid/base characteristics Stumm et al., 1980; Benjamin and Bloom, 198 1; of the surface. Benjamin and Leckie, 198 I ). The above generalizations and models concerning 3) Ternary complexes (organic or inorganic) may ion adsorption by oxides have been primarily deduced adsorb which are either metal-like or ligand-like in from work done in simple systems. We are interested their behavior. Their adsorption characteristics can in applying these generalizations to quantify the adbe stronger than, equal to, or weaker than the adsorption of Cu, Pb, Cd, and Zn on goethite in a sorption behavior of the individual metal ion or iicomplex solution resembling seawater. in this paper gand (Davis and Leckie, 1978; Bourg and Schindler, we shall focus on what effect the major seawater ions 1978, 1979; Bourg et al., 1979; Benjamin and Leckie, have on the adsorption of trace metals on goethite. 1981). We need to determine if the effects are due to the 4) The surface sites of an oxide can be thought influence of the major ions on the electrostatic conditions at the interface, a competition between the University of Washington Contribution Number 1255. major and minor ions for sites on the surface or the 1253

L. S. BALISTRIERI

1254 Cu

on Goethite X 3.2x

10“M

k~t.6X

lO+M

.

in

AND J. W. MURRAY

NaN03

Cu

on Goethite

in

Seawater

X 3.1 1 lo-7Y

A

3.1 x 10-5M

2.5 x IO-~

M

n 3.1 x 1o-5 M

AA

IOO

xAx.

A Xm

xA

Ai

A

80

.

l

.

A

xx

d

n

A

: 60 P G

X

Xrn

: t ; 40 t

bA

&’

X

m

. XA

. 0

0 3

4

5

6

3

4

5

PH

6

7

PH

FIG. 1. The adsorption of copper on goethite as a function of pH and total copper concentration from 0.1 M NaN03 and major ion seawater. The goethite concentration is 28.5 m’/l. 1981). Briefly, the oxide was synthetically prepared ac. cording to the procedure of Atkinson el al. (1967) and identified by X-ray diffraction using Cu radiation (Burns and Burns, 1977). A surface area of 51.8 m’/g was determined by N2 adsorption (Brunauer et al., 1938). The total number of exchangeable sites (N,) is 0.22 moles/kg of oxide as determined by titrations in 1 M NaCl to pH 1 I. Potentiometric titrations and the solid addition method determined the pH at the point of zero charge [pH(PZC)] to be 7.5 in NaCl solutions and 7.1 in major ion seawater. The adsorption of Cu, Pb, Zn, and Cd on goethite as a function of pH from 0.1 M NaNO, solutions and from syn-

formation of solution complexes each with their own adsorption behavior. Finally, we shall evaluate how our synthetic experimental system compares with real seawater solutions. EXPERIMENTAL

METHODS

A thorough discussion of the preparation and identification of goethite (c~Fe00H) and its basic surface characteristics in simple salt solutions and in major ion seawater was presented in a previous paper (Balistrieri and Murray, Pb

on

Goethite

0 6.5 x 10:; X 7.6x10 A 3.4 x lo+ .2.7x10-SM

in NaN03

Pb

on Goethite -7

M M M

01.4x10 x 6.6X 10-7 A 3.2 x ld6 8 2.9 x IO-’

60

A

X X 80

0

: P ; ::

A

8

Pm 0 ae

n

0

60

0

0

l-

3

l

A XA

40

. .

X .

.

. s:a

. 5

PH

.

20

.

4

A

A

A

. I--

:

XX

OA 20

A

X

X

.

x0

aA

XA

.

I-.

1..

00

a,

.

X

Seawater

M M M M

0

044 AX x0 co

in

6

0 -I3

t4

5

6

7

P”

FIG. 2. The adsorption of lead on goethite as a function of pH and total lead concentration from 0. I M NaNO, and major ion seawater. The goethite concentration is 28.5 m2/l.

ADSORPTION

OF Cu, Pb, Zn AND

thetic solutions containing the major seawater ions (Na, Mg, Ca, K, Cl, and SO,) at their natural concentrations (Wilson, 1975) was measured as the decrease in the ion concentration in solution after the addition of the oxide. Each point on the adsorption curves represents an individual experiment. The samples were allowed to equilibrate overnight at 25°C after pH adjustment with 0.1 M HNO, or NaOH. Preliminary kinetic experiments indicate that equilibrium was reached after 2.5 hours. After equilibration the pH of each slurry was measured with an Orion-model 801 A pH meter. The samples were centrifuged and then filtered through 0.45 pm Millipore filters and analyzed by flame or flameless atomic absorption (Perkin-Elmer model 306 Atomic Absorption Spectrophotometer with HGA carbon furnace). Standards for the analyses were prepared in the same matrix as the samples. RESULTS

of Cu, Pb. Zn, and Cd on aFeOOH. of the major ions of seawater

A) Results The

effect

of the

metal removal

major

ions on trace

seawater

by cuFeOOH

was determined

by com-

in an inert electrolyte and in major ion seawater. We chose NaN03 as our inert electrolyte because nitrate does not significantly complex cations and because sodium and nitrate do not strongly interact with the goethite surface. A comparison of the adsorption data in the two systems will determine the importance of strongly adsorbing major electrolyte ions, such as Mg and SO,, and solution complexes such, as PbCl+ or Cd@, on the adsorption of the trace ions. The adsorption data for Cu, Pb, Zn, and Cd on LvFeOOH as a function of pH and total metal concentration in 0.1 M NaN03 and major ion seawater paring

the trace

are illustrated in Figs. 1-4. The adsorption data for 3 X 10e5 M Cu, Pb, Zn, and Cd in 0.1 M NaNOX compares very well with the data of Forbes et al. ( 1976) whose experimental conditions were similar to ours. Qualitatively, as the system changes from an inert electrolyte to major ion seawater, for a given pH and total metal concentration: 1) there is an overall increase in Cu adsorption; 2) there is an increase in Pb adsorption at low pH and a slight decrease in Pb adsorption for the higher concentrations at higher pH; 3) there is an overall decrease in Zn adsorption; and 4) there is a significant decrease in Cd adsorption.

AND DISCUSSION

I) Adsorption

Effect

1255

Cd

metal

adsorption

Zn

behavior

on Goethite

X 5.6X10 A 2.9x1O-6 , 2.9x10-5

-7

in

The above observations indicate that if the adsorption edge of a trace metal ion in NaNOj occurs in the pH range of -3-6, then the adsorption of the trace ion from seawater is enhanced relative to that in the inert system whereas if the adsorption edge of the trace metal in NaNO1 occurs in the pH range of -6-9, then the adsorption of the trace ion from seawater is suppressed relative to that in NaN03. Therefore, the adsorption behavior of the trace metal ion appears to partially depend on the pH range of its adsorption edge. The location of the adsorption edge for a given total trace metal concentration depends on the solid to solution ratio in the system. The adsorption edges illustrated in Figs. 1-4 can be shifted to lower pH values by increasing the solid to solution ratio and to higher pH values by decreasing the solid to solution ratio. By shifting the adsorption edges for Zn to lower pH values we observe, in contrast to the previous results in a higher pH range, an enhanceZn on

NaN03

Goethite -7 x 1o_6

X 5.4 A2.9xlO . 2.9x10-5

M M M

lOO--

M M M

loo-.

Xd

xx

in Seawater

A8

A

A

60 ..

.

60 -.

A

X

.

A

n

xA

.

X . A

A

X

A X

.

X

’

A

. A

oL 5

’ .

H 20--

.

A..

20--

x c:

.

4.

4 6

7 P”

6

o’:x:

I

5

6

7

6

P”

FIG. 3. The adsorption of zinc on goethite as a function of pH and total zinc concentration M NaNOS and major ion seawater. The goethite concentration is 28.5 m2/1.

from 0.1

1256

L. S. BALISTRIERI Cd

in

on Goethite

AND

J. W. MURRAY Cd

NaN03

on Goethils

I”

Ssawate,

-8

IOO-

00 .

.

100..

l

0

0

SO--

XA .

SO--

X

A

0

z 80-E ;:

X

0”

*I

:: ac

X

a

8” X

20--

BO--

xA

2

..

.

8 dp

40.-

X

n

P mm

20--

AA AA

a’

“-

A

2% D

A A

40”.

1

X

X*

“?

’ l=

!?6’

PH

. .

a .,B

mm n

n

i

d

t

PM

FIG. 4. The adsorption of cadmium on goethite as a function of pH and total cadmium from 0. I M NaNO, and major ion seawater. The goethite concentration is 28.5 m*/l.

ment in adsorption in seawater relative to that in NaNO, (Fig. 5). This emphasizes first that the pH range of the adsorption edge depends on the concentration of the solid present and second that the particular adsorption behavior of a trace metal in seawater is influenced by the pH range of its adsorption edge, which is determined by the solid concentration. The particular seawater ions which cause the changes in the adsorption curves were determined by successively adding the major ions to our system, starting with chloride, until some combination reproduced the adsorption results obtained in seawater. The data for 3 X 10e6 M total metal concentrations are illustrated in Figs. 6-9 where once again the adsorption data from NaN03 are used as a reference point.

.

A

concentration

CM The presence of chloride does not alter the adsorption behavior of Cu on aFeOOH. The adsorption data in seawater are reproduced when SO4 is added at natural seawater concentrations (Fig. 6). Pb Chloride does not influence the adsorption behavior of Pb. The addition of SO4 at natural seawater concentrations accounts for the enhancement in Pb adsorption at low pH values while the further addition of Mg accounts for the decrease in adsorption at higher pH values (Fig. 7). These results contrast with those obtained by Swallow et al. (1980) for am. Fe(OH)3. These workers found that Cu adsorption on am - Fe(OH), was not affected by changing the background electrolyte from NaC104 to synthetic seawater. Pb adsorption on am - Fe(OH)X was decreased only in the presence of chloride; presumably by competing solution complexation. It is interesting that the adsorption behavior of Cu and Pb from synthetic seawater is markedly different for the two forms of iron oxide, although this may, in part, be due to the concentration of oxide used and the pH range of the metal’s adsorption edge. Zn and Cd

FIG. 5. The adsorption behavior of 3 X 10e6 M Zn on goethite in 0.1 M NaNOl and major ion seawater as a function of pH and goethite concentration.

Chloride does not influence the adsorption behavior of either of these ions. The small decrease in Zn adsorption (Fig. 8) and the dramatic decrease in Cd adsorption (Fig. 9) from seawater relative to that in NaNOS are reproduced when Mg is added to the system.

ADSORPTION

OF Cu. Pb, Zn AND

-6 2.5x10

M

A

0.53

M

0

0.53

M

I 100

Cu

on Goethite

/f \ d NaCl NaCl

+

/

t

0.028

M

1251

Cd -6

M Pb

on

Goethite

3.3

x 10

A

0 53 M NaCl

0

0.53

M NaCl

+ 0.028M

Na2S04

0.53

M NaCl

+ 0.026M

Na2S04

Na2S04

-K/

q

’

Seawater

0.1 M

NaN03

80

I \I

I I

d

I

q’

Seawater

3’

20

LJ

0 3

4

5

6

0-L

7

3

5

4

PH

FIG. 6. The adsorption of 2.5 X IOU6 M copper on goethite as a function of pH in natural seawater concentrations of sodium, chloride, and sulphate. The dashed and solid lines represent the adsorption data illustrated in Fig. I for 2 X 10e6 M Cu in 0.1 M NaNO, and major ion seawater. The goethite concentration is 28.5 m2/l.

B) interpretation

have established

7

FIG. 7. The adsorption of 3.3 X lO-6 M lead on goethite as a function of pH in natural seawater concentrations of sodium, chloride, sulphate, and magnesium. The dashed and solid lines represent the adsorption data illustrated in Fig. 2 for 3 X lOL6 M Pb in 0.1 M NaNO, and major ion seawater. The goethite concentration is 28.5 m’/l.

of Results

The above observations ative to the inert system:

6

PH

2.7 x10

that rel-

1) chloride

has no effect on the adsorption behavior of the trace ions; 2) the enhancement in metal adsorption is due to SO,; and 3) the suppression in adsorption is due to Mg. The adsorption of the trace ions from 0.53 M NaCl and 0.1 M NaNOj is experimentally the same even though the ionic strength is different in the two systems. The results of Swallow et al. (1980) for Cu and Pb adsorption on am * Fe(OH)X in 0.005 M, 0.05 M, and 0.5 M NaClO, also indicate that ionic strength does not significantly influence the adsorption of these ions. This may be due to the fact that the adsorption process results in no net change in charge and, therefore, is not susceptible to changes in surface charge caused by ionic strength variations. The effect of Mg and SO4 on the adsorption behavior of the trace ions can be attributed to one or a combination of three factors. First, the electrostatic conditions at the goethite-seawater interface may be different from those at the goethite-NaNO, interface due to the specific adsorption of certain major seawater ions. The major ions in an oxide-solution system are thought to control the electrical aspects of

-6

M

A

0.53M

NaCl

0

0.53M

NaCl

Zn

+

on Goethite

0.054M

MgCl2

100

80

0.1 M

5

NaN03

6

d O,’

7

8

PH

FIG. 8. The adsorption of 2.7 X lo-” M.zinc on goethite as a function of pH in natural seawater concentrations of sodium, chloride, and magnesium. The dashed and solid lines represent the adsorption data illustrated in Fig. 3 for 3 X 10e6 M Zn in 0.1 M NaNO, and major ion seawater. The goethite concentration is 28.5 m*/l.

L. S. BALISTRIERI

1258 2.8 x10S6M

Cd

A

0.53

M NaCl

0

0.53

M NaCt

O.lM NaNe3

on Goethite

+ 0.054

M MgC12

/

O/

O/

9’

/

\Seawater

0 / /

0-d ! 5

A i 0

6 6

7

qualitatively assessed by examining the speciation of the goethite surface and the speciation of the trace metals in our systems. Our previous work (Balistrieri and Murray, 198 1) determined the quantity of goethite surface sites bound by particular ions in simple systems and in major ion seawater using the triple layer model of Davis et al. (1978). A comparison of the surface speciation of crFeOOH in NaNOj and in major ion seawater indicates that Mg and SO, play a significant role in determining the electrical characteristics of goethite in seawater and that the available adsorption sites (SOH) are greatly diminished in seawater relative to those in NaN03 due to the strong adsorption of S04, Mg, and Ca (Fig. IO).’ In NaN03 solutions the trace metal ions are not significantly complexed by nitrate and, therefore, for the pH range of their adsorption edges they exist primarily as the free metal ions. As the system changes to major ion seawater, the solution speciation of Pb and Cd is dominated by chloro complexes (~94%) for the pH range of their adsorption edges while copper and zinc exist primarily as free metal ions (60-70%) and chloro complexes (18.-27%) (Table 1). It is important to note that the solution speciation does not necessarily control the charge of the adsorbed species or the type of surface complex that is formed because the resulting surface speciation depends on pH, on the concentration of surface ligands, solution ligands, and trace metal, and on the binding energies between the various ligands and metal (see Fig. 7 in Davis and Leckie, 1978). We can quantitatively evaluate the change in electrostatic conditions and site availability by using the triple layer model (Davis et al., 1978) to fit the data for one total metal concentration (3 X 10m6M) from the simple system and then predict the adsorption behavior from major ion seawater and major ion mixtures. The adsorption data were modeled using the following adsorption reactions:

_i”A 0

0

AND J. W. MURRAY

8

9

PH

FIG.9. The adsorption of 2.8 X IOe6 M cadmium on goethite as a function of pH in natural seawater concentrations of sodium, chloride, and magnesium. The dashed and solid lines represent the adsorption data illustrated in Fig. 4 for 3 X low6 M Cd in 0.1 M NaNO, and major ion seawater. The goethite concentration is 28.5 m*/l.

the interface. The trace ions are, therefore, susceptible to the electrostatic conditions imposed by the major ions. However, not only are the quantity and types of ions that are adsorbed important, but the speciation of the adsorbed ions is also important. The binding of one ion can affect the double layer potential so as to increase, decrease, or have no effect on the adsorption of another ion. The direction and extent of the effect, in part, depends on the charges of the adsorbed ions. Second, the competition between major and minor ions for particular sites on the oxide surface may influence the adsorption behavior of trace metals from seawater. The resulting surface speciation depends on the relative energies of interaction between the ions and the surface, the relative concentrations of the ions available for adsorption and the pH. The third factor may be the formation of solution complexes. It has been shown by a number of workers that metal-ligand complexes have their own adsorption behavior; that is, they can adsorb in a metal-like or a ligand-like fashion as well as stronger, weaker, or the same as the free metal ion or ligand (Davis and Leckie, 1978, 1979; Bourg and Schindler, 1978, 1979; Bourg et al., 1979; Benjamin and Leckie, 1981). The above three factors-electrostatic effects, site availability, and solution complex formation-can be

SOH + Me’+ Kkle --. SOMe++H’ SOH + Me2+ + Hz0 Ky” SOMeOH

(1) + 2H ’

(2)

where SOH is an oxide surface group and Me2+ is the divalent metal cation. The equilibrium constants describing the adsorption of the major ions of seawater with goethite were previously determined using the triple layer model (Balistrieri and Murray, 198 I ). Since sodium and chloride account for -85% of the

I The speciation depicted in Fig. 10 assumes that the total exchangeable sites (N,) are 0.22 moles/kg and that one site is covered by each adsorbed ion. The percent of sites occupied by particular ions would change if we had used the value for the total number of surface sites as determined by the tritium exchange technique of 1.35 moles/kg (Yates, 1975) and if we had assumed that each adsorbed SO, occupies three surface sites (Davis and Leckie, 1980). The smaller titration value was used to simplify our modeling by eliminating estimates on the number of sites covered by each adsorbing ion.

ADSORPTION

OF Cu, Pb, Zn AND

1259

Cd

PH

PH

FIG. 10. A comparison of the fraction of goethite surface sites occupied by particular ions in 0.1 M NaNO, and major ion seawater. The results are based on previous work (Balistrieri and Murray, 1981) using the triple-iayer model (Davis et al., 1978).

charge in the bulk solution of major ion seawater (Garrels and Thompson, 1962), we assumed they were the ions responsible for the charge in the diffuse layer in our model calculations. Using the aquo ion stoichiometry depicted in reactions 1 and 2, the triple layer model predicts no difference in the adsorption behavior of Cu and Zn in NaCl and NaN03. On the other hand, Pb and Cd adsorption is predicted to be diminished in NaCl relative to NaN03. This model result occurs because Pb and Cd are strongly complexed in solution by chloride, that is, chloride ligands successfully compete with surface ligands for metal ions. Experimentally the data for Pb and Cd, as well as Cu and Zn, adsorption in NaCl and NaN03 are the same. The experimental results, in conjunction with the model results, suggest that Pb and Cd chlorocomplexes and Pb and Cd aquo ions adsorb with equal intensity. However, in order to simplify our modeling efforts, the adsorption data for Cu, Pb, Cd, and Zn in NaCl were fit simply using the aquo ion stoichiometry (reactions 1 and 2). This approach requires larger binding constants for the Pb and Cd aquo ion stoichiometricies in NaCl relative to NaN03 indicating that chlorocomplexes adsorb with binding energies similar to those for the aquo metal ion on goethite. The model fit for the metal adsorption data in NaCl and the model predictions for the seawater-type solutions are illustrated in Figs. 11-14. Cu and Pb The model correctly predicts Cu adsorption from NaCl/Na$SO., for pH > 5 (Fig. 11). Below pH 5 in this system the model predicts enhanced Cu adsorption relative to the NaCl system; but much greater than what is observed. On the other hand, the model underestimates Cu adsorption in major ion seawater. The results for Cu contrast with those for Pb as the enhanced adsorption of Pb in NaC1/Na2S04 relative to that in NaCl is correctly predicted by the model

(Fig. 12). The enhanced adsorption can be attributed to the influence of adsorbed SO4 on the electrostatic conditions at the interface. The model also successfully predicts Pb adsorption from major ion seawater and from the major ion mixture of NaC1/Na2S04/ MgClz for pH > 5. The decrease in adsorption for these conditions can be attributed to a decrease in available binding sites and by opposing electrostatic forces caused by the adsorption of Mg. For the same conditions, but pH < 5, the influence of SOa on the electrostatic conditions and on the adsorption behavior of Pb is not as inhibited by Mg as the model indicates. It also should be noted that the combined effects of electrostatic interactions and competition Solution speciation of Cu, Pb, Cd, and Zn in major ion seawater.

Table 1

pH3-6L

pH

Cd2+

2.1

2.7

CdOH+

0

0

Cd(OH$

0

0

CdCl+

33.4

33.4

CdCl;

41.2

41.2

;;;:i-

';I;

CdSO;

0.4

14.4 7.9 0.4 -

Pb2+

6.1

5.6

zn2+

65.0

PbOH+

0

8.6

ZnOH+

0

Pb(OH);

0

0.3

Zn(OH);

0

PbCl+

26.9

24.5

znc1+

18.4

17.0

PbCl;

37.3

33.9

znc1;

4.8

4.4

PbCl;

25.9

23.6

znc1;

2.6

2.4

PbClt-

3.2

2.9

znc1;-

2.1

1.9

F'bSOg

0.4

0.4

znso;

6.4

5.9

*

-

-

60.1 3.7 3.8

The % of total metal as a particular species as determined from the equilibrium constants compiled by Ruppert (1980) for I = 0.67, T = 25'C, and P = 1 atm.

1260

L. S. BALISTRIERI

2.5 x 10

q

NaCt

As

loo--

-6

M

Cu

on

AND J. W. MURRAY

correctly may be due to a problem with the adsorbed speciation of Cu. The model is very sensitive to the charges of the adsorbed ions as well as the quantity of strongly adsorbed ions. The speciation of adsorbed ions determined from mathematical models depends on the type of reactions that are chosen to fit the data, although the choice of reactions has been somewhat constrained by the results of proton release experiments (Loganathan and Burau, 1373; Forbes er nl., 1976; Benjamin and Leckie, 1981), thermodynamic arguments (Davis and Leckie, 1979), and spectroscopic methods (Parfitt and Russell, 1977: Murray and Dillard, 1979).

Goethite

/ Na2SOq

eawater

Zn and Cd

_Ic _____

0,

5

4

3

mode\

.

I

6

7

PH

FIG. Il. The triple layer model fit for the adsorption of 2.5 X 10e6 M copper on goethite in 0.53 M NaCl and the model predictions for the adsorption data in 0.53 M NaCI/ 0.028 M Na,SO, and major ion seawater using p*K~~-=‘c = 3.0 and p*l(;;&$~Nsrc = 7.0,

for surface sites in major ion seawater are reproduced in a system containing only NaClfNa2S0,/MgC12. The inability of the model to predict Cu adsorption 3.3 x IO+

q ,

M

The triple layer model correctly predicts the adsorption of Zn (Fig. 13) and Cd (Fig. 14) on goethite from major ion seawater and from the major ion mixture of NaCI/MgCIZ. The decrease in adsorption for both ions relative to that in NaN03 can again be attributed to a decrease in available surface sites and by opposing electrostatic forces due to the strong adsorption of Mg in this pH range. Our model results suggest that the dramatic decrease in Cd adsorption in seawater compared to that of Zn or Pb probably occurs because adsorbed Cd behaves as a divalent (i.e., Cd+‘), rather than a monovalent ion. Specific adsorption of Mg creates a positive surface and thus the adsorption of a doubly charged cation is more difficult than the adsorption of a singly charged cation. The triple-layer model cannot predict the shift in

Pb on Go%thite

ASeawater

or

Zn

2.7x10-6M

NaCl / Na2S04 NaCl

/N&$0,,

1

M&l9

0 NaCl

100 I

on

Goethita

/ M&I2

ASeawater

t 80 NaCl/ D

2

60

z f

j

0 3

4

5 PH

6

7

The triple layer model fit for the adsorption of 3.3 X 10e6M lead on goethite in 0.53 M NaCl and the model predictions for the adsorption data in 0.53 M NaCl/ 0.028 M Na2S04, 0.53 M NaC1/0.028 M Na~O$O.054 M MgCl, and major ion seawater usingp*Kb~TR’NS’ = 1.8 ‘YTR’NS’C = 5.0. The model predictions for major and p*Kpbo~ ion seawater and NaCI/Na2S04/MgC12 are the same. FIG.

12.

‘:i$A+@ ;; i

5

6

7

8

P+t

FIG. 13. The triple layer model tit for 2.7 X IO + M zinc on goethite in 0.53 M NaCl and the model predictions for the adsorption data in 0.53 M NaCl 0.054 M MgCIZ and d" = 9.15. major ion seawater using p*Kit&tiN

ADSORPTION

OF Cu. Pb, Zn AND

1261

Cd -6

the adsorption edge for different total metal concentrations using one set of adsorption equilibrium constants. Benjamin and Leckie (1981) have suggested this occurs because surface sites are heterogeneous rather than homogeneous as assumed in the triplelayer model. Therefore, equilibrium constants determined from the triple-layer model at one total metal concentration are not necessarily valid at other total metal concentrations. This will be briefly discussed in the next section and more fully addressed in a separate paper.

2.8~10

M

0

NaCl

/

A

Seawater

Cd

on

Goethite

MgCt2

II) Adsorption of Cu. Pb, 231, and Cd on CuFeOOH: Major ion seawater versus natural seawater Our experimental major ion seawater system and natural seawater differ in, at least, the following respects: 1) The actual concentrations of Cu, Pb, Zn, and Cd are much lower in seawater than in our major ion seawater experiments (Bewers et al., 1976; Boyle et al., 1976, 1977; Bruland et al., 1978a, 1978b), 2) the trace metal ions are present simultaneously in seawater rather than individually as in our experiments, 3) natural seawater contains other “minor” major ions-such as bicarbonate and carbonate-which may strongly interact with goethite, and 4) organic matter is present in natural seawater (- 1 mg org C/l) and not in our experimental system. Below we will evaluate how these differences affect the adsorption of Cu, Pb, Zn, and Cd on aFeOOH. The recent work of Benjamin and Leckie (1980, 1981) proposes that oxide surface sites have a spectrum of binding energies. When the surface sites of all energy types are in excess, then the fractional adsorption of an ion at a given pH is independent of the total metal concentration and the surface appears to behave as if all sites were equal. However, as the surface binds more ions, the highest energy sites may become limiting and then the overall binding energy of the surface decreases. This results in a shift of the adsorption edge of the metal to a more alkaline pH region as the total metal concentration is increased. Benjamin and Leckie (1981) also propose to describe the adsorption of a divalent metal ion on a heterogeneous surface by an average binding constant represented by the following reaction: SOH,

+ Me2+ 2 SOMe’*-“”

+ xH+

(3)

where K, is the average equilibrium constant, x is the average number of protons released per metal ion adsorbed, and SOH, is the average concentration of surface sites bound by protons. K, is a composite equilibrium constant; that is, it includes contributions from various chemical and electrical interactions and, therefore, it is not a thermodynamic equilibrium constant. The value of K, will be a constant only if all sites are available in excess, i.e., when fractional

5

6

FIG. 14. The triple layer mium on goethite in 0.53 M for the adsorption data in and major ion seawater p*KfCNd;;Ns’C = 9.35.

7 PH

8

g

model fit for 2.8 X 10e6 M cadNaCl and the model predictions 0.53 M NaCl/0.054 M MgC12 using ,D*K&~‘~‘~~‘~ = 1.3 and

adsorption is independent of total metal concentration. When the highest energy sites become limited (reflected by the shift in the adsorption edge), then the value of K, will decrease. For our experimental conditions, we assume that the adsorption edges for Cu, Zn, and Cd at their natural seawater concentrations would fall on those edges which are independent of the total metal concentration as illustrated in Figs. 1,3, and 4 and, therefore, the average binding energy for those metals at their natural seawater concentrations can be estimated from our experimental data at higher metal concentrations. The second difference is that our experimental system contains only one trace metal rather than an array of trace metals as in natural seawater. Benjamin and Leckie (1980) have also shown that there is competition between metals for sites on some oxide surfaces (yFeOOH and rA120,) even though the available surface sites are in great excess. In other words, the stronger adsorption of one trace metal ion over another ion can reduce the quantity of high energy sites specifically available to the two trace ions. In contrast there appears to be little or no competition between metals for the surface sites of am Fe(OH),. An experiment involving the simultaneous adsorption of Cu, Pb, Zn, and Cd on aFeOOH in major ion seawater revealed that there is little or no competition for surface ligands for Cu, Pb, or Zn while Cd tends to show a slight suppression in adsorption when the other metals are present (Fig. 15). Therefore, the effect of trace metal competition for

L. S. BALISTRIERI

1262

AND J. W. MURRAY

cu

0

ACu

0 Pb in presence

A

of Pb, zn,Cd

A@ A

100

Pb

in prermee

of Cu. Zn,Cd

100..

L1 .

60

60--

0

A

0

ii

.

0”

40 -.

be

.

4

I

20

O-

A

l

20

A

00 t 0%

3

4

5

6

3

4

5 PM

PH 0 Zn

in prassncs

Azn

of Cu,Pb,Cd

A

100

.

Cd

A

Cd

6

in presence

ol

7

Cu,Pb,Zn

A, loo .

A .

80

. 60 I

1 D 2

. 60--

G l

I N’ s?

A

40--

A

;: a4

. 20

4

:*

5

bA

.

. I

A

’

20-w

0

A

40

I

0

.‘r,

0, 6

7

6

PH

5

A

6

7

8

9

PH

FIG. 15. A comparison of the adsorption of 2.5 X IO-6 M Cu. 3.0 X 10e6 M Pb, 3.0 X 1O--bM Zn, and 3.0 X 10e6 M Cd individually and simultaneously on goethite in major ion seawater. The concentration of goethite is 28.5 m*/l.

particular sites on goethite should be minimal in natural seawater. The third difference is the presence of additional anions, such as carbonate, phosphate, and silicate, in natural seawater. These ions may influence the adsorption of the trace ions by strongly interacting with goethite and, thereby, altering the electrostatic conditions at the interface or by forming solution complexes with the metals. We can use the data of Stumm et al. (1980) to compare qualitatively the binding energies of phosphate and silicate with sulfate as well as estimate the binding constants for carbonate and bicarbonate with goethite. The energy of exchange of the anion for the hydroxyl group of water (H-OH) and of a goethite surface site (Fe-

OH) is compared in Fig. 16. The relationship between the equilibrium constants for sulphate, fluoride, phosphate, and silicate yields estimates for the equilibrium constants describing bicarbonate and carbonate interactions with goethite. Our estimate for log&z: agrees with that obtained by Stumm et al. ( 1980) from a comparison of the tendency to form goethite surface complexes with the tendency to form solution complexes with Fe3+. The data of Fig. 16 indicate that the binding energy of phosphate, silicate, bicarbonate, and carbonate are large compared to sulphate but the relative concentrations of those ions as well as the pH are also important in determining the goethite surface speciation in natural seawater. Therefore, it appears that other anions besides

ADSORPTION

OF Cu, Pb, Zn AND

2-m g + E ._ r

0..

H2S ‘04

-2. CO3

PO4

H3SI04 n

i

r:

-10 ; ;:

-12

log KIN’ Fe-OH

+ anion-

+

Fe-anion

+ OH-

FIG. 16. A comparison of the equilibrium constants which define the exchange of sulphate, fluoride, phosphate, and silicate for the hydroxyl group of a goethite surface site (Fe-OH) (data of Stumm et al., 1980) and the hydroxyl group of water (H-OH).

in particular bicarbonate and carbonate-may be important in determining the surface properties of goethite in natural seawater. However, the adsorption of Zn and Cd from Puget Sound seawater collected from a depth of 40 m shows little or no difference from their adsorption in major ion seawater (Fig. 17) suggesting that 1) Mg and SO., are the dominant ions which control the conditions at the sulphate-and

2.5 x10-6M 0

Puget

0 major

ion

goethite-seawater interface or 2) the conditions at the interface are similar in synthetic seawater and real seawater but may be caused by Mg, S04, and HC03/C03 rather than just Mg and SO+ Finally, the fourth and probably most important difference is the presence of organic matter in natural seawater. Organic matter has been shown to complex certain trace metals strongly, in particular Cu (Mantoura et al., 1978) and to adsorb on surfaces suspended in seawater (Neihof and Loeb, 1972, 1974; Hunter and Liss, 1979; Hunter, 1980; Tipping, 1981). The latter work strongly suggests that solids suspended in seawater lose their individual surface characteristics and acquire the chemical properties of organic matter whose major functional groups are carboxylic acids (-COOH) and phenols (-OH). Defining the effect of organic matter on trace metal adsorption by goethite in seawater will be the goal of our future work. CONCLUSIONS

1) Magnesium and sulphate are the principle ions of seawater which influence the adsorption of Cu, Pb, Zn, and Cd on oFeOOH in seawater. The adsorption of sulphate on goethite changes the electrostatic conditions at the interface so as to enhance the adsorption of trace metal ions. Magnesium adsorption tends to suppress the adsorption of trace metals by decreasing the quantity of available binding sites and by making the electrostatic conditions at the interface unfavorable for further cation adsorption. At 3.0x10%.4

Zn Sound

1263

Cd

Seawater

Seawater

Cd

0

puget

Sound

l

major

ion Seawater

Seawater

100..

100 --

cl

00-m

20..

6

6

7 PH

9

FIG. 17. A comparison of the adsorption of 2.5 X 10m6 M zinc and 3.0 X 10e6 M cadmium on goethite as a function of pH in synthetic (major ion) seawater and natural seawater from Puget Sound. The concentration of goethite for the zinc experiments is 7.5 m2/1 and for the cadmium experiments is 28.5 m2/1.

1264

L. S. BALISTRIERI

the pH of natural seawater, only the effects caused by Mg would be significant in influencing the adsorption of trace ions. 2) The pH range of the adsorption edge of a trace ion is a function of the concentration of the solid. Since the electrostatic conditions imposed by the major ions at the oxide-solution interface are not a function of the solid’s concentration (moles of major ions % moles of surface sites), then the particular adsorption behavior of trace ions in seawater as determined by the pH range of their adsorption edges will depend on the concentration of the solid in the system. 3) The triple layer model results for Pb and Cd adsorption on goethite in NaCl and NaN03 using aquo ion stoichiometry indicate that adsorption of these ions in NaCl should be suppressed relative to that in NaN03 due to competing complexation by chloride. The experimental evidence indicates that adsorption of Pb and Cd in the NaCl and NaNO, systems is the same. This suggests that Pb and Cd chlorocomplexes adsorb in addition to Pb and Cd aquo ions. Further experiments need to confirm this hypothesis. 4) The apparent average equilibrium constant (K,) describing the interaction of Cu, Zn, and Cd with goethite in seawater can be estimated for natural seawater concentrations from our data because the fractional adsorption of these ions is independent of total metal concentrations which are less than 3 X low6 M. K, is a constant because all types of sites are available in excess for our experimental conditions. However, whether it is possible to apply the constant K. values determined for Cu, Cd, and Zn from our experiments to natural seawater conditions can not be assessed from this data alone because it is not clear that at natural particulate matter concentrations there are excess sites available. 5) Cu, Pb, Zn, and Cd show little or no competition with each other for particular sites on goethite in seawater. 6) Carbonate, phosphate, and silicate appear to have little or no effect on the adsorption of Zn and Cd on goethite in seawater. manuscript has benefitted greatly from discussions with Dr. M. Benjamin and from the critical reviews given by Dr. J. 0. Leckie and an anonymous reviewer. B. Fulton patiently typed the many revisions of this manuscript. The research was supported by NSF Grant OCE 80-18335.

Acknowledgments-This

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ADSORPTION

OF Cu, Pb, Zn AND

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