The equilibration of clay minerals with sea water: exchange reactions

Geochimica et Cosmochimm

Acta,1917.

Vol. 41, pp. 951 to 960.

Pergamon Press. Printed inGreatBritain

The equilibration of clay minerals with seawater: exchange reactions* F. L. SAYLESand P. C. MANGELSDCIRF, JR. Department of Chemistry, Woods Hole Oceanographic

Institution, Woods Hole, MA

02543, U.S.A. (Received 7 October 1976; accepted in revised form 4 Feb~~~~ 1977)

Abstract-Studies of seawater-sediment and seawater4ay mineral exchange equilibria demonstrate that rinsing procedures employed in many previous studies have grossly shifted the exchange equilibria away from the true seawater conditions. Exchange complements have been determined here by measurement of compositional changes in seawater that result from reaction with clays, thereby avoiding rinsing. These data show that exchangeable Nar is normally greater than or equal to exchangeable Mg2+ on clays and sediments in exchange equilibrium with seawater. On introduction to seawater, fluvial clays are shown to give up their exchangeable Cazt for Na+, a process of importance in the geochemical budget of Na+.

INTRODUCTION

and in K as the composition of the bound complement changes, will complicate the picture. In general, ION EXCHANGEreactions between clay minerals and however, dilution leads to the selective uptake of seawater have been of interest to a broad spectrum cations of higher valence relative to those of lower of scientists, dating back at least to Aristotle in about valence. 330 B.C. This interest has led to a number of investigaThe literature of soil chemistry abounds in tions of the equilibrium exchange composition of examples of the successful use of distilled water to clays, soils and sediments in seawater. However, the flush out interstitial electrolyte. However, such rinsing results of many of these studies are suspect or demonis truly safe only if a single species of metal cation strably incorrect, chiefly as a result of a common prois present in solution and bound on the clay, and cedural step in the analyses-the rinsing of samples if there is no anion exchange capacity. Rinsing may with distilled water prior to the determination of be safe if, as is generally the case in soils, the initial exchange complements (MANGELSDORFand SAYLES, content of interstitial electrolyte is much less than the 1975). exchange capacity of the sediment. Rinsing seems to Equilib~a between ideal ion exchangers and simple be relatively safe also in instances where the cation aqueous salt solutions have been found to be demixture includes only ions of the same valence. None scribed quite well by Donnan equilibrium concepts of these criteria are met when the interstitial solution (HELFFEIUCH,1962; WIKLANDER, 1964). Donnan conis seawater. cepts have further been shown to apply qualitatively The effects of rinsing dilution were first encounto ion exchange between sediments and seawater by tered in our laboratory in 1969 by Michael Fuiler, MURTHY and FERRELL(1972). Donnan equilibrium who was studying exchange reactions between fluvial predicts that in a heterovalent mixed salt solution sediments and seawater directly by measuring such as one containing Na+ and Ca2+ the following changes in seawater composition and, as a check, inwill hold directly by conventional NH&l d~spla~ment of exchangeable ions. The two kinds of measurements disagreed drastically and systematically. We eventually realized that the discrepancies could be explained if where XT* and Xcl; denote the equivalent fractions bound Na+ and K’ were displaced by interstitial of the bound species, the gammas are activity coeffiCa’+ and Mg2+ during the distilled water rinse. cients, brackets denote solution concentrations, and These are precisely the qualitative effects predicted K is an equilibrium constant having the units of inby Donnan equilibrium and observed by MURTHY verse concentration. and FERRELL(1972). Fuller’s results, thus interpreted, On dilution of the interstitial solution both are summarized in Table 1. X&[Na+] and X,J[Ca’+] will increase, but the A series of experiments was carried out to confirm second tends to increase as the square of the first. the effects of rinsing. Clay and sediment samples were Changes in the activity coefficients of the bound ions subjected to a prolonged seawater wash followed by a thorough distilled water rinse and were then re*Woods Hole Oceanographic Institution Contribution No. 3853. introduced into seawater, the entire cycle being 951

F. L. SAYL~Sand P. C. MANGI:LSDOW.JK.

951

I. Exchangeable

Table

ions on fuvlal ca++

sediments

Mg++

Na+

K+

Before sea water exposure

2.75

.99

.54

.38

During sea water exposure

1.18

2.22

2.14

1.06

After sea water removal by rinse with distilled water

1.62

3.50

.50

.52

Median Hathaway

values

for 32 fluvial

sediment

samples. collected

by Dr. John

of the U.S. Geological Survey

from river banks and flood plains along the Atlantic seaboard from Maine to Georgia (HAWAWAY. 1967). analyzed by Michael J. Fuller. All values in meq/lOOg of dry sediment. Table

2. Change

in ion complement of sediment on re-entering sea water after interim distilled water (all values in meq,/lOO g of sediment)

Sample

stage

Mg++

Cat+

Nat

washing

with

K+

USGS H-15 Graham Lake, Me. CEC = 6.8

1st w-entry 2nd w-entry

-l.O,-1.2 -.9, -.9

-3.5,-3.4 -6.3,-6.9

+3.5,+3.5 +f5.7,+7.4

+1.1,+1.1 +1.0,+1.0

USGS H-29 Delaware R., Pa. CEC = 5.0

1st w-entry 2nd w-entry

+.a +.0

-. 2

-.2 -.2

t.5 +.5

USGS H-41 Roanoke R., N.C. CEC = 7.4

1st 2nd

USGS H-52 santee R., s.c CEC = 10.3

1st w-entry 2nd w-entry

Flood sediment Susquehanna R., Pa Sample A ,I 8,

1st w-entry 2nd w-entry

1st w-entry 2nd re-entry

Sample B CEC = 7.9

re-entry re-entry

-.2

-.6 -_5

-3.7 -4.4

+3.5 t4.2

+1.0 +.a

-.5, -.5 -1.0

-5.5,-5.5 -7.2

+4.9,+4.9 +7.6

+1.1,+1.1 t.9

-1.7,-1.9 -1.0,.l.l,-1.5

+.8,+.9 +.4,-.3,+.a

-.l,-.l -.2,+.6,-.l

+1.0,+1.2 +.9,+.7,+.a

-1.5 -1.2

t.7 t.6

-.l -.0

t.9 +.8

+25.7,+26.5 +24.2,+24.3

+1.4,+1.6 +1.7,+1.7

Montmorillonite Polkville, Miss CEC = 62.3

1st w-entry 2nd i-e-entry

-8.2,-8.9 -4.3,-4.6

-18.7,-19.0 -22.2,-21.5

Kaolinite Bath, S.C. CEC = 4.3

1st w-entry 2nd re-entry

-l.l,-1.1 -. 9

-1.9,-1.8 -2.0

For these experiments each treated for 6 hr with 100 ml of water (lOOmI for 1 hr, 50ml of step 2); (5) washed (repeat

+z.fi,+z.fj +3.2

+.4,+.4 +.4

2g sample was (1) washed with distilled water, ground and dried; (2) sea water, centrifuged and decanted; (3) washed three times with distilled for I hr, 50ml for another hour); (4) treated with sea water (repeat of step 3); (6) treated with sea water (repeat of step 2).

repeated several times. The results of these dilution experiments are summarized in Table 2. When these samples re-entered seawater after the prior seawater equilibration had been disrupted by a distilled water rinse, substantial exchange reaction occurred. These re-entry reactions, although different in character from those occurring on initial exposure, often involved more than SOo/, of the total bound equivalents (CEC). After distilled water rinsing the clays were often very nearly as far from seawater equilibrium as they had been to begin with. The effects were quite reproducible bhen the cycle was repeated. In all cases where the sediment exchange capacity was large enough that the Na/‘Mg results were unambiguous. the effect of distilled water rinsing on the seawater equilibrated sediment was to remove Nap’ and K ’ and to add Mg*+ and Ca” as the Donnan model predicts. Working from the Fuller data we developed a ruleof-thumb for determining whether data reported in

the literature represented true equilibrium or had been affected by rinsing. An equilibrated sediment should have (in equivalents) exchangeable Na+ comparable to or greater than Mg2’ and should also have K+ comparable to Cd’+. For the Fuller data the median values of the ratios were Na/Mg = I.5 and K/Ca = 0.85. The use of organic solvents or mixtures for removing seawater is equally subject to rinsing errors. In view of the embarrassing early history of artifacts produced by alcohol treatment of marine sediments. specifically the “Bathybios” blunder committed by no less a scientist than T. H. Huxley (BUCHANAN. 1876; DEACON. 1971). treatment of sediment-seawater mixtures with organic solve+ seems especially illadvised. As noted earlier. many important experimental determinations of clay mineral and sediment exchange equilibria in seawater carried out in recent decades have specifically included a rinsing step.

Exchange reactions either with distilled water or with a distiiled waterorganic solvent mixture (POTTS, 1959; CARROLLand STARKEY, 1960; MCCRONE, 1967; ~&CRONE and KOCH, 1968; DOBBINS,RAGLANDand JOHNSON,1970; RUSSELL,1970; DREVER,1971; ROBERX)N,1974). Ion

~omplem~ts determined in such studies are in error although the total exchange capacities need not be. Perhaps the most striking example of the misleading consequences of the rinsing step is the peculiar Na+ behavior dwelt upon by MCCRONEand KOCH (1968). BI~CHOFFet al. (1975) have attempted to minimize rinsing effects through reducing pore water volume by squeezing. The generally low Na/Mg ratios they report (average = 0.41) suggest that this approach was not successful for most samples. On the other hand, some of the Russian workers, notably Zaytseva, have anticipated the rinsing problem and assiduously avoided it (ZAYTSEVA, 19.58, 1962, 1966). The purpose of the present paper is to report experimental data for the equilibrium cation exchange ~om~sition of a number of reference clays in seawater and in river water. The seawater data have been obtained without use of a rinsing step. The study has been limited to the four major seawater cations, plus hydrogen ion. METHODS Analysis mixtures

of exchangeable

ions in seawater-clay

mineral

Our determina~ons of exchangeable ions in clay mineral-seawater systems are based on measurements of the compositional changes produced in seawater by the clay minerals. We use for this purpose the method of difference chromatography, which was especially developed for determining small changes in the major-ion composition of seawater (MANG~S~RF and WILSON, 1971). This is a type of ion exchange chromatography which senses compositional differences between a small sample (1 or 2 ml) and the stream of reference solution into which the sample is injected. We use Sargasso Sea surface water both as the reference and as the reagent seawater in our experiments. The method, as used in the clay mineral experiments reported here, can resolve ~omposjtional changes of less than 0.1% for Naf, Mg’+ and K+ and 0.5% for Ca2+. Calculation of the equilibrium exchange composition is based upon a mass balance. It is assumed that the quantity

953

of each eminent available for exchange is constant throughout the experiment. Thus -0 meqj + meqy = &$ + meqy where the superscripts 0 and F refer to the conditions prior to (original) and after (final) reaction, respectively. meqj and meqj are. respectively, the exchangeable and solution milliequivalents ofj. We determine i%$, the concentration per liter of bound ions on the seawater-equilibrated clay, from the expression &$

= %$’

where Ameqj is meq: - meqy, the change in ion composition of the seawater, measured directly by Difference Chromatography. Ameqj itself, is also a quantity of fundamental geochemical interest if the clay was originally equilibrated with river water. ZAYTSEVA (I 958,1962) has described a different displacement technique which studiously avoids dilution. Sediment is squeezed to extract the pore waters and reduce interstitial volume, a measurement is made of the water content of the pressed cake, and NH: displacement of interstitial and exchangeable ions is then carried out. Correction is made for interstitial salt on the basis of composition and water content with allowance for the amount of non-interstitial bound water. This procedure calls for unusual precision since a large fraction of the total salt is present in the pore solution, necessitating a large correction for Na+ and Mgzc, particularly in low CEC sediments. The consistency of her data and the agreement between cation sums and independently determined total capacity reported by ZAYTSEVA (1966) attest both to Zatseva’s abilities and to the feasibility of the method. Zatseva’s method is a proven one and a more generally available alternative to the approach we have taken. Both methods appear accurate for relatively high CEC exchangers. For low CEC exchangers our seawater measurement avoids the serious obstacle posed by the practically irreducible content of residual seawater remaining in a squeezed sediment. Direct seawater measurement also avoids any possible effects of compaction on exchange equilibria such as those suggested by HANSHAW(1964) in his experimental studies of clay membranes. Experiments,

materials

and methods

Two sets of experiments were carried out in this study: (a) the equilibration of dry clay mineral standards with seawater, referred to as the dry-clay series, and (b) the equilibration of clays previously conditioned in artificial river water, referred to as the river-clay series. Both experiments yield determinations of equilibrium exchange compositions of the clay minerals in seawater.

Table 3. Clay minerals used in seawater and river water equilibration S0WTe

Kaolinite - Bath, S.C.

Ward's'

7.0

Illite - Fithian, Ill.

H. Starkey

__-

Mixed Layer - High Bridge, Ky.

H. Starkey

__-

Montmorillonite - Cameron, Ariz.

Ward's'

8.3

Montmorillonite - Belle Fourche, S.D. Ward's'

8.8

Montmorillonite- Osage, Wyo.

studies

pH in River Water

Sample

Ca-Mon~orillonite - Texas

- Ameqj

Source Clay Mineral Repository2 Ii.Starkey

7.4 -__

’ Ward’s Natural Science Establishment, Rochester, N.Y. ‘Source Clay Mineral Repository, Dept. of Geology, Univ. of Missouri, Columbia, MO.

F. L. SAYLESand

954

P. C. MANG~LSDORF.JR

Table 4. Composition of “Mean World River” water after LIVINGS.~OF;T(1963a) used in the river-clay series M/1.x

Mq/l,

Ion Nat K+

Mg2+ cl-

lo3

6.29

0.27

2.35

0.06 0.37

15.0

ca*+

clays (JACKSON, 1968). Soluble salts were determined separately by rinsing with 8Op6 methanol (JACKSON, 1968) and were taken into account in the calculations of both -0 meq, and Ameqi. Aqueous NH,Cl (1N) was used for the clays pretreated with river water (river-clay series). In all cases three successive washings over a 96-hr period were used to displace the cations. The three washings were analyzed separately by A.A. and the exchangeable ions calculated from the sum in the three aliquots. The quantity meqy was then calculated in proportion to the weight of clay used in the seawater experiment. The quantity Ameqj was determined directly by difference chromatography. with allowance made for any change in total normality. Total exchange capacity was separately determined by displacement of exchangeable NH: from the NH; loaded clays. In most cases displacement was by 2M NaCl (0.005 N HCl). but displacement with seawater was used in a few cases and is considered more consistent with the rest of our experiment. By using changes in seawater composition as the basis of our analysis we introduce the possibility that our results include the effects. not only of ion exchange, but also of any other reactions occurrmg during seawater exposure. The same is true of a displacement by NH:, but the presence of a variety of cations in seawater enhances the possibility of irreversible reactions. Also, the greater duration of our exposure times than is customary in NH: displacement could permit greater effect of either dissolution or non-exchange reactions. Consequently, the two methods (NH, vs seawater) might yield somewhat different results even without rinsing artifacts. To ascertain whether dissolution significantly aifccts OLIIresults, the fraction of reaction occurring in the time interval 24-72 hr and 48-96 hr was determined for the seawater and NH,Cl experiments. The results are presented in Table 5. The percent reaction occurring after 24 hr (seawater) and 48 hr (NH&I) is generally small demonstrating that dissolution of the minerals is not a problem. The only real exception to this is the kaolinite which was found to release Na’ at a fairly constant rate. Irreversible fixation of cations on exchange sites results in a decrease in NH: determined CEC and can be identilied on this basis. CARROLL and STARKEY (1960) measured NH4 CEC on untreated samples as well as the same minerals after IO days exposure to seawater. The determined changes in CEC values were within normal error limits except for a kaolinite which showed a definite increase. We have made similar comparisons of NH4 exchange capacity on clay minerals exposed to seawater

4.10

0.17

7.78

0.22

s042-

11.2

0.12

HC03-

58.9

0.97

The clay minerals used were standard reference materials obtained either from Ward’s, from the University of Missouri. or were kindly supplied by Harry Starkey of the U.S. Geological Survey, Denver. Colorado (see Table 3). The clays were ground in an agate mortar and passed through a 200 mesh sieve. The equilibrations utilized 0.5-2 g of each mineral. Each clay sample was added to 50ml of Sargasso Sea surface water having a salinity of 36.0”‘:,. In the dry-clay series the clays were added directly to the seawater. In the river-clay series the clays were first equilibrated with a “world mean river water” (Table 4). Equilibration was accomplished by repeated washing (six times) with the river water over a period of a week. Through oversight the pH of the solutions in contact with the clays was not adjusted to a common value. Consequently, the pH of the equilibrating river water was partially determined by the clays and is different for each mineral. The pH data are included in Table 3. The progress of the reaction with seawater was monitored at time intervals of 1. 3 and 12 days for some of the experiments. All of the river clay series were sampled at 7 days and all of the dry-clay series at 12 days. Sampling involved centrifugation, withdrawal of 5 ml of solution. filtration (0.45 11) of the sample and immediate measurement of pH. Alkalinity was measured within a day; cation analysis of all samples was carried out at the conclusion of the cxpcriment. The exchangeable cations bound on the clay prior to reaction (@p) were measured by 1N NH,Cl displaccment. For the dry-clay series the displacement was carried out in NH,CL---RO”,; MeOH to minimize dissolution of the Table

5. Extent

Montmorillonite ,I

Dakota

4

II

Texas

II

Wyoming Layer Kaolinite

Illite

ions in hours as total

!!!I

ca

cl

1


el

1


bearina

shale

change in bound ions in the period 24-72 hours as a percent of the total change in 72 hours (negative values indicate overshoot at 24 hours)

5

8

1



23

-4

10

5

1

cl

3

-4


Sl

5

2



9

0

0

6

44 8

3



3

3


5

10

-
4

II

Wyoming

Bath

with time Seawater

release of bound the period 48-96 a percent of the released

Na

Mixed

reaction

1M NH,Cl

Clay Mineral

Arizona

of exchange

-9 II

Exchange

for 48 hr NH&EC.

and, The

ring on the same time scale as the more likewise, find no significant change in observations of CARROLLand STARKEY defined exchange reactions involving Ca2+,

(1960) and of RUSELL (1970) that the sum of exchangeable Ca + Mg + K + Na decreases on exposure to seawater is puzzling. Since NH: is taken up reversibly to the same extent before and after exposure to seawater, it would appear that something was missing from the measured sums. If dissolution and fixation are ruled out in our experiments, any other non-exchange reactions which could affect our results would have to occur rapidly, independent of NH., exchange sites, when our clays are first put in seawater. Such reactions seem unlikely to us, but we intend to check on this possibility by following the total normality of the seawater. RESULTS Clay mineral

955

reactions

exchange

AND

DISCUSSION

complements

The original complements of the clay minerals prior to introduction into seawater are summarized in Table 6. We have included H+ in the cation sum (C) presented here although such has not been customary in the soils literature. This inclusion of HC presents some conceptual difficulties because the buffering capacities of clays extend across a broad range of pH (CARROLLand STARKEY,1960). We are arbitrarily defining the clay to have zero bound protons when in seawater equilibrium. This convention is supported by observations that seawater is even more effective in displacing H+ than is 1N NH,Cl at a pH of 7. In experiments with clays loaded with NHf at pH 7 we have found that, in addition to NH:, a small amount of H+ is released upon introduction of these clays into seawater. We are not concerned with whether or not the proton exchange takes place on permanent exchange sites produced by charge deficiencies resulting from lattice substitutions. The proton reactions are rapid, occur-

Table

6. Original

Na+ and K+ and cannot be distinguished experiments. In the river-clay experiments the inclusion of H+ in the sum is geochemically significant because the release of bound H+ from clays must affect the net alkalinity input to the ocean by rivers. More practically, the exchange of a seawater cation for a bound H+ also reduces the total normality of seawater through titration of alkalinity. This decrease must be allowed for in all our calculations of Ameqj since we have not directly determined the normality change by difference chromatography. Our estimates of Ameq,, and Ameq,, are particularly affected by this normality change simply because their equivalent fractions in sea salt are so large (0.7741 and 0.1747 respectively). The inclusion of H+ in our sum also improves the match between Z and the independently determined NH, CEC in all but two cases. In only one instance is the resulting mismatch greater than our expected error. The Bath kaolinite equilibration with river water was duplicated and one of these yields a sum that is significantly larger than the NH4 exchange capacity. The source of this discrepancy is not known. The agreement between C and NH4 CEC is generally good, differences usually being less than 5%. The discrepancy in the case of the Dakota montmorillonite is too large to be anything but an analytical error. Different experiments with this clay over a range of pH values have yielded equivalent fractions that do not differ significantly from those of Table 4. Consequently, we feel that the Xi values given are correct and that the error in this case lies in the initial amount of clay used for the river water equilibration.

exchange

Exchangeable Ion (meq/lOO q) N&

Mg

Ca

K

H'

compositions NH4C13

H Cations*

E.C.

Equivalent Fractions4 %a

'Mg

'Ca

'K

'H

At-iZOll.3 Montmorillonite5

1.6

26.8

48.8

1.0

1.3

79.5

78.1

.02

.34

.61

.Ol

.02

Dakota Montmorillonite'

1.9

15.2

35.8

0.10

1.3

54.3

85.0

.03

.2B

.66

.002

.02

Texas Montmorillonite'

2.3

18.4

57.6

0.6

2.0

80.9

83.6

.03

.23

.71

,007

.OZ

Wycming Montmorillonite5

2.6

19.4

43.0

0.3

3.4

68.7

69.3

.04

.28

.63

,004

.05

Bath Kaolinite'

0.4

1.1

3.2

0.05

1.5

6.2

4.5

.06

.18

.51

,008

.24

Bath Kaolinite5

0.3

0.9

2.2

0.05

1.2

4.6

4.5

.06

.20

.49

.Ol

.25

33.2

.05.

Wyoming Montmorillonite6

strictly Mg2+, in these

13.2

19.6

1.2

.18

.27

.02

9.0

15.1

2.9

71.1, 70.2 38.5

.46

0.22

3.9, 3.0 11.3

69.3

Mixed Layer6

35.0

.Ol

.23

.39

.08

Illite6

0.10

0.22

12.5

2.5

1.5

16.8

16.1

.Ol

.Ol

.74

.15

’ HC = hydrogen ion displaced on addition to seawater. Additional H+ could be present. 2 Sum includes H+. 3 NH&l E.C. is exchange capacity as determined in 1M NH,CI at pH = 8.0. 4 Equivalent fractions based on ;T. cations. ’ Clays equilibrated with river water (“river-clay series”). 6 Dry clay, composition prior to addition to seawater (“Dry-clay series”).

.og

Table 7. Equilibrium compositions -....

_Eguivalent Fractionsi ----

Exchangeable Components (meq/lOOQ)

K

!.

111seawater -_____..

NH&

Exper1 ment

Duration (Days)

%a

%

.47

.?4

.04

.50

-22

.?h

.flz

”

7

83.6

.55

.?2

!Y

.u3

u

7

71.1

69.3

.55

.40

111

2.5

?0.3

69.3

.53

-39

.I5

2.7

30.5

35.0

.56

-32

1.8

2.8

16.8

16.1

.47

.2J

.I1

1.5

0.4

6.3

.38

.3T

:J

Sanple

Na

e3

Ca

Arizona Montmorillonite

19.4

37.7

19.1

3.2

19.4

78.1

.24

Dakota Montmoriilonite

27.1

12.2

14.1

1.0

54.4

as.0

Texas Montmorillonite

44.3

18.1

15.7

2.7

80.8

wyomiog Montmarillonite

39.2

28.2

1.0

2.7

Wyoming Montmorillonite

37.4

27.5

2.9

Mixed Layer

21.6

14.2

Illite

7.9

4.1

Bath &aoiinite

2.4

2.0

- 2

cation: -_--_-

E.C

4.5

‘Na ‘Mg - -. _--_

I)4

River-clay Srriei

Dry-clay series

7

12

.04

u

3

.lj7

"

12

.17

"

12

.I!6 uiuer-clay series

7

’ X values based on Z of cations. ZValue calculated as negative, assumed 0 Ibr X;, The data indicate little variation in the relative abundances of ions on the clays equilibrated with river water. As has been demonstrated previously (MCCRONE, 1967; MCCRONE and KOCH, 1968; Rcsand is followed by SELL, 1970), Ca2 + dominates Mg”. Na+ and K’ are minor components. The H+ data are interesting in that the quantity bound is not correlated with total exchange capacity. Both the montmorillonites and the kaolinites exhibit low vatues of I %I.3 rn~~l~ g. There seems little question that. within the pH range of our experiments. H’ is a relatively more abundant ion on the clays with lower exchange capacity, the equivalent fraction being 0.02 to 0.05 on the montmorillonites and 0.25 on the kaolinite. This presumably reflects the association of H’ largely with lattice edge sites found on both types 01‘ clay as opposed to i&et-layer sites of the montmorillonite. The mixed-layer mineral was equilibrated with river water in a later experiment and the alkalinity loss on reaction with seawater measured. The river equilibrated sample yielded an H+ value of only 3.0 meqjlO0 g rather than the inconsistent I I .3 of the dry-clay. The exchange compositions of the clays equilibrated in seawater are summarized in Table 7. The most important feature of these data is the dominance of Na- on the clays in all but one case. For most of the clays Na + comprises very close to 5OY;,of the exchangeable cations. The kaolinite is somewhat lower but Na )- is still the most abundant ion. Mg” is the next most abundant ion with XugL+ usually falling in the range 0.20 to 0.40. Ca” ranges from 0.01 to 0.26 and K+ from X, f = 0.02 to 0.17. The clay minerals differ somewhat from the sediments studied by FL:LLI'R in that for three montmorillonites and the kaolinite G > Ti: whereas K was comparable to ?% on the sediment samples. Because of the con-

stancy in .I\, and the small amounts of exchangeable K* generally observed. the sum of the divalents is quite constant. Consequently, there is a rough negative correlation between XHB1. and Xc,.: I One inconsistency is readily apparent in Table 7: the departure of the Arizona montmoriIlonite from the rather consistent general trend of all the other clays. The equivalent fraction of Mg2” is about twice that of Na+ whereas for all the rest the ratio of Mg’+ to Nat ranges from about 0.7 to 0.4. This deviation has all the characteristics of a rinsed clay. but this can only be a coincidence. We have no reasonable explanation for this. and have included the data because we have been unable to discover any reason for discounting them. We believe the result is due to an error but cannot find the source of it.

Some of the experiments of this study were designed to provide ;I basis of comparison between results sub,ject to rinsing error and our results. Our “dry clays” are those used by CARROLL and STAKKIII (1960) in their investigation of clay-seawater exchange equilibria. The results ohtained by Carol1 and Starkey and those we have presented are compared graphically in Fig. I. The samples used include the Wyoming montmorillonite. the mixed layer mineral, the illite and the Bath kaolinite. The data are presented as equivalent fractions based upon the sum of the cations cxclusivc of I-l I. The results from Carol1 and Starkey arc for the natural claqs put into seawater rather than the H-form clays and thus are derived from an experimental design very similar to our “dryclay” scrics. It is rcadilq ~~pI~~ir~lltthat there are gross differenccs in the determined equivalent fractions. These differences are qualitatively those predicted on the basis of Donnan theory. The tnajor difference lies in

Exchange

951

reactions

I

THISSTUDY

CARROLL a STARKEY(i960)

MONTMORILLONITE

di co

p

6

Ml:XEC )-LAYER

ILLITE

KAOLINITE

composition of four clays as determined by CARROLLand STARKEY (1960) and by seawater measurement. ND is not detected or not determined as the component was believed to be below detection. The equivalent fractions are based upon the sum of exchangeable Na+ + K+ + Ca2+ + Mg*+. Fig. 1. Comparison

of exchange

the Na+ and divalent cations. Carroll and Starkey report that either Ca’+ or Mg2+ dominates whereas we find that in each case Naf is dominant. Carroll and Starkey did not find detectable K’ where analysis was done; we do find it to be present although still in minor proportions, usually at levels similar to Ca2+ on these clays. Differences similar in character are found when comparison is made with other studies that employ rinsing. Handling (grinding, sieve size etc.) and slight differences in the clays could introduce small discrepancies, but cannot explain those exhibited by Fig. 1. The differences are real and originate as an artifact of the rinsing procedure used by Carroll and Starkey. Our results may also be compared with the work of Zaytseva, reported in a series of articles (ZAYTSEVA, 1958, 1962, 1966). The data presented are for sediment samples from the Pacific Ocean and Black Sea. The methods used have been described above and are en-

Table

8. Comparison

Sample

tirely different from ours. The results obtained, however, are very similar (Table 8). Since Zaytseva studied sediments rather than minerals, the comparison with any specific mineral is not strictly valid. The clays of the region from which Zaytseva’s samples were obtained are predominately illite (40-80%), chlorite (lO-30%) and montmorillonite (t&20%) according to the regional mineralogical studies reported by LISITZIN (1972). Lacking data for chlorite we have calculated from our results the equivalent fractions corresponding to a mixture of 80% illite and 20% montmorillonite. The montmorillonite is itself an average value of the Wyoming and Texas montmorillonites. The results are not particularly sensitive to the proportions used. It is important that illite is the dominant mineral primarily in that exchangeable K+ is more abundant on illite than for any of the other minerals we have studied. Considering the differences in the samples, the agreement is

of NH,Cl determined exchange complements 1966) with seawater determinations

'Na

xM9

'Ca

'K

Pacific

sediment'

.39

.29

.18

.14

Pacific

sediment'

.40

.27

.17

.16

Black Mineral

Sea

sediment3

mixture4

.37

.28

.22

.13

.52

.28

.ll

.lO

(ZAYTSEVA,

Source

Zaytseva

(1966)

II

,I

I,

II

This

study,

’ Average value for seven sediment surface samples. ’ Average value for five samples from a 7.5 m core. 3 Average value for five samples from the Black Sea. 4 Value calculated from a mix of 80% illite and 20% montmorillonite has average composition of the Texas and Wyoming montmorillonite.

which

F.

95x

L. SA~.LH

and P. C‘. MANGELSDOKI..

Table 9. Net reactlolls 01‘Huv~al clay minerals Mineral

JK.

with seawater

AXNat

Ax@

AXCaB

bXK+

aXH+

Experiment

Arizona Montmorillonite

+.22

t.13

-.37

+.03

-.02

River-clay series

Dakota Montmorillonite

+.47

-.03

-.44

+.02

-.D2

'1

'I

Texas Montmorillonite

+.52

-.Ol

-.52

t.02

_,02

”

(’

Wyoming Montmorillonite'

+.50

c.12

-.61

+.04

-.05

Bath Kaolinite

t.32

+.14

-.27

+.05

_.25

Natural-clay series 18

11

Data presented clay mineral, - =

’

Average

as change in equivalent fraction. Sign convention is + = gain by loss by clay mineral. of two measurements.

quite good. The general features are the same. The results differ in detail in that our Cal+ values are higher. lower and our Na’ values correspondingly The values for Mg2+ are identical and those for K + only slightly different.

Net reaction

hrtwtvn

jluoiul

c/uys urld seuwatcr

The data presented can be used to evaluate the net changes in composition that occur when river supplied clays are introduced into seawater. The net changes in exchangeable ions are summarized in Table 9. In the sign convention used here a positive change denotes uptake by the clay. Thus the values correspond to - Ameqj of the mass balance equation. It is apparent that the primary change occurring is a substitution of NaC for exchangeable Ca’+. K’. Mg2+ and HC undergo smaller changes. On average, half of the exchange sites of clays in seawater are occupied by Na+, little of it being inherited from the river clay. HOLEMAN (1968) estimates that 183 x 1014g of suspended sediment are delivered annually to the oceans by rivers. KENNEDY (1965) reports values for the exchange capacity of suspended material in rivers of the continental U.S. that range from 5.6 to 47.4 meq,/lOO g. For the sake of estimation we have used a median value of 25 meq/100 g, yielding a total Na + uptake of 2.3 x 10’ ’ meq/yr. This represents about 25’;/, of the total Na+ supplied annually by rivers and nearly 4076 of the Na+ from continental weathering (i.e. corrected for atmospheric recycling) according to the data of GAKKELS and MACKENZIE (p. 108, 1971). LIVINGSTON~ (1963b) and MACKDJZIE and GARRELS (1966) have estimated that 65-80>; of the Na+ supplied to the oceans each year may be accounted for by burial in pore waters, halite deposition and atmospheric recycling. The data we have presented indicate that much of the remainder may be removed as exchangeable Na+. This suggestion was made by LIVINGSTONE(1963b) but was at apparent odds with much of the exchange data then available. The Ca” displaced by Na’ is probably of more

significance in the proton balance of the oceans than in the Ca2+ budget. Using the above estimates for total sediment and average exchange capacity, Ca2+ released through exchange increases the flux of riverborne Ca’.’ by slightly less than 10%. If we follow the scheme of MACKENZIE and GAKRELS (1966) and eventually remove this additional Ca2+ from the ocean as biogenic CaCO,. we are, in effect, removing both alkalinity and Na’ without recourse to “reverse weathering” as postulated by Mackenzie and Garrels for Na+ removal. The readily accountable losses of alkalinity through exchange are, primarily, the Ca2+ displacement by Na+ plus a small amount of H+ also released. The sum amounts to about 2.5 x IO’ ’ meqjyr or only 8’;: of the HCO; brought to the oceans by rivers. While this is only a small part of the total HCO; flux, it is a significant fraction (2&25’?J of the “excess” HCO; that led MACKENZIE and GAKRELS (1966) to postulate the concept of reverse weathering. The preceding estimates of the influence of exchange reactions on mass balances strictly apply only to the clay minerals studied. Natural variations in pH. clay mineralogy, organic matter and temperature may complicate the picture, but almost certainly will not alter the magnitude of our estimates. We found only minor dependence of selectivity on mineralogy for the most common clays of marine sediments. Changes in pH will modify exchange complements only slightly and. as regards the calculations presented, will result only in Na + exchange for H’ as well as Ca”. The role of organic matter is, at this point. an open question. We know little about the composition of exchangeable ions in sedimentary organic matter and the contribution organic matter makes to total exchange capacity. The Fuller data, which are for sediments containing organic matter. seem to show a somewhat greater proportion of bound Mg’+ when exposed to seawater than do the clays studied. On the other hand, the data of Zaytseva for the Black Sea sediments, also organic-rich, are closely comparable to the data for our clays and for

Exchange reactions

959

her organic-poor clay sediments of the Pacific (Table NOTE ADDED IN PROOF 8). Finally, the equilibrium composition will be F. T. Manheim recently called our attention to the docaltered on those clays ultimately reaching the deeptoral thesis of VANDERMOLEN(1958) showing that Dutch soil scientists were aware of the dilution problem and were sea floor where temperatures on the’order of 2°C prevail. Temperature of squeezing studies (MANGELSDORF handling it correctly as long ago as 1938. (ZUUR, 1938). We have not seen a copy of Zuur’s paper. et al., 1969; BI~CHOFFet al., 1970; SAYLES et al., 1973) show that this will lead to increased uptake of Na+ and K+ and loss of exchangeable Mg’+ and Ca’+. REFERENCES For the montmorillonite studied by MANGEZSD~RFet al. (1969) we calculate the shift caused by a drop in ARISTOTLE(330 B.C.). Works 7, 933b, Clarendon Press temperature from 21 to 2°C to be +15x for Na+, (1927). BISCHOFFJ. L., GREER R. E. and LUISTROA. 0. (1970) +lO% for K+, -3% for Ca*+ and, by difference, Composition of interstitial waters of marine sediments: -25% for Mg2+ (+ = uptake by the clay). The temperature of squeezing effect. Science 167, 1245changes might be significant for the Naf budget, but 1246. without far more adequate data further evaluation BISCHOFFJ. L., CLANCYJ. J., and BERTHJ. S. (1975) Magnesium removal in reducing marine sediments by cation does not seem justified. SUMMARY Modification of equilibrium cation exchange complements of clay minerals in seawater as a result of dilution render much of the available data grossly incorrect. The influence of rinsing may often be detected as exchangeable Mg*+/Na+ ratios that are significantly greater than one and by divalent/monovalent ratios greater than one. In contrast to many previous reports, in seawater Na+, rather than Mg*+, is the major exchangeable cation of the clays studied, and, probably, of most natural exchangers. Mg*+ is also an important component. Exchangeable H+, contributed to seawater by the clays, is independent of total exchange capacity, being relatively constant at l-3 meq/lOO g. The net reaction between fluvial clays and seawater is primarily an exchange of seawater Na+ for bound Ca*+. Calculations suggest that this uptake of Na’ may be responsible for the removal of much of the annual supply of Na+ not accounted for by pore water burial, halite deposition and atmospheric recycling.

exchange. Geochim. Cosmochim. Acta 39. 559-568. BUCHANANJ. Y. (1876) Preliminary report to Professor Wyville Thomson, F.R.S., Director of the Civilian Scientific Staff on Work (Chemical and Geological) done on Board H.M.S. Challenger. Proc. Roy. Sot. London 24. 593423. CARROLLD. and STARKEYH. (1960) Effect of sea water on clay minerals. Clays Clay Minerals, Proc. 7th Natnl. Conf 80-101. DEACONM. (1971) Scientists and the Sea, 1650-1900; A Study of Marine Science, pp. 352-353. Academic Press. DOBBINS D. A., RAGLANDP. C. and JOHNSONJ. D. (1970) Water-clay interactions in N. Carolina’s Pamlico Estuary. Environ. Sci. Technol. 4, 743-748. DREVERJ. I. (1971) Early diagenesis of clay minerals, Rio Ameca Basin, Mexico. J. Sediment. Petrol. 41. 982-994. GARRELSR. M. and MACKENZIEF. T. (1971) Euolution of sedimentary rocks. W. W. Norton. HANSHAWB. B. (1964) Cation exchange constants for clays from electrochemical measurements. Clays C2a)j Minerals, Proc. 12th Conf: 12. 397421.

HATHAWAY J. C. (1967) Data file, Continental Margin Program, Atlantic Coast of the United States. Vol. 1. Sample Collection Data, Supplement 1; Woods Hole Oceanographic Institution, Tech. Report 67-21, in cooperation with the U.S. Geological Survey, Unpublished manuscript. HELFFERICH F. (1962) Ion Exchange. McGraw-Hill. HOLEMANJ. N. (1968) The sediment yield of maior rivers of the world. Water Resources Res.-4. 732-747: JACKSONM. L. (1968) Soil Chemical Analvsis-Advanced Acknowledgements-Our study of sediment-seawater interCourse, 2nd edition ipublished by the au&or) Dent. Soil. action has evolved over a number of years and conseSci., Univ. Wisconsin, Madison,. Wis. quently has progressed through the efforts of a number KENNEDYV. C. (1965) Mineralogv and cation exchange v of people. We wish to acknowledge the contributions of capacity of sediments from selectkd streams. U.S. GeoloM. J. FULLERand T. R. S. WILWN in our earliest studies gical Surv. Prof. Paper 433-D. of river sediment-seawater reaction and JOHN HATHAWAY LISITZINA. P. (1972) Sedimentation in the world ocean. who supplied the sediment samples as well as valuable Sot. Econ. Paleontologists Mineralogists, Spec. Puhl. 17. advice. The studies of sediment-seawater reaction after Chap. 6. prior seawater equilibration and rinsing were carried out LIVINGSTONE D. A. (1963a) Chemical composition of rivers by WEN CHANGof our laboratory. These experiments were and lakes. In Data of Geochemistry (edited by Fleischer, conducted in laboratory facilities generously made availM.), U.S. Geol. Survey Prof. Paper No. 440, Chap. G, able by FRANKMILLEROof the University of Miami. PainsGl-G64. taking analyses at various stages of our studies were carLIVINGSTONE D. A. (1963b) The sodium cycle and the age ried out by R. W. ZUEHLKE,CHARLESOLSONand BRUCE of the ocean. Geochim. Cosmochim. Acta 27. 1055-1069. (TURKEY)JENKINS;their efforts were essential to the proMACKENZIEF. T. and GARRELSR. M. (1966) Chemical gress of the investigation. F. T. MANHEIMcalled our attenmass balance between rivers and oceans. Am. J. Sci. 264. tion to the work of ZAYTSEVA. The manuscript has bene507-525. fited from review by R. M. GARRELS,J. I. DREVER,J. L. MANGELSDORF P. C. JR., WILSONT. R. S. and DANIELL BISCHOFF and H. STARKEY. Financial support has been proE. (1969) Potassium enrichments in interstitial waters of vided through the Atomic Energy Commission, iater recent marine sediments. Science 165, 171-174. ERDA. (Contract ATll-3119) and the National Science MANGELSD~RFP. C. JR. and SAYLESF. L. (1975) Ion Foundation (Grants DES 03do2 and DES 10277) and is exchange between sediments and seawater. Trans. Amer. gratefully acknowledged. Geophys. Uniojl 56. 1004 (Abs.).

960

F. L. S~u~.ts and P. C. MANG~LSIXIKI,. .II<.

MANGLLSUOKFP. C. .IK and WILSON T. R. S. (197 I) Difference chromatography of seawater. 1. Phys. Chm. 75.

1418-1425. McCRO~*~~A. W. (1967) The Hudson River Estuary: sedimentary and geochemicdl properties between Kingston and Havershaw. New York. J. Srdime~t. P&w/. 37. 475-486. MCCRON~ A. W. and Ko(‘H R. T. (1968) Natural and experimental sodium uptake in Hudson River sediments. Kingston to Manhattan. New York. .I. S&r)~~rt. Petrol. 38. 654-660. MURTHY A. S. P. and FI;RRI-I.L R. E. JK. (1972) C’omparative chemical composition of sediment interstitial waters. Clay C/u!. Minrruls 20. 3 I7 -32 I. POTTS R. H. (1959) Cationic and structural changes in Missouri River clavs when treated with sea water. Unpublished Masters’.Thesis, University of Missouri. RO~JERWN H. E. (1974) Early diagenesis: expansible soil clay-sea water reactions. J. Scdirtlctrlt. Petrol. 44. 341-449. R~JSSELLK. L. (1970) Geochemistry and halmyrolysls 01 clay minerals. Rio Ameca. Mexico. C;c,ochin~. Cosfnochi~~~.

SAYLES F. L., MANHUM F. T. and

Interstitial Init.

water studies

Reps Drrp

WATERMAN L. S. (1973) on small core samples, Leg 15.

Seu DriUinq

Prqjrct

20. 33-54.

VAN DER MOLE~VW. H. (1958). The exchangeable

cations in soils flooded with sea water. Doctoral thesis, Landbouwhogeschool, Wageningen. Published by Staatsdrukkerij. ‘s-Gravenhage; 167 pages. WIKLANLXR L. (1964) Cation and anion exchange phenomena. In Chernistr~~ of t/w Soil (edited by F. E. Bear) 2nd edition. Reinhold. ZAYTSEVAE. D. (1958) Cation exchange capacity of marine sediments and methods of determination. Tr. Inst. Okrauol. 46. I 8I ~204. ZAYTSEVA E. D. (1962) Exchangeable cations in sediments of the Black Sea. Tr. Inst. Okemol. 54. 48-82. ZAYIXVA E. D. (1966) Capacity of exchange and exchange cations of sediments of the Pacific Ocean. In Khimiy Tikhqo Okctm (Chemistry of the Pacific Ocean) (edited by S. V. Brujewic7). I/d. “Nauka”. ZI’IIH A. .I. (I 93X).In : Trams. Secmd Comm. arId Alkali-Suhcomm. I~zr. Conorrss of Soil Sci. Helsinki B, 66--67.