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Temperature and pH controls over isotopic fractionation during adsorption of boron on marine clay
I, pp.
0016.7037/87/53.00
Gwchrrmca n Cosmorhrmrca Acta Vol. 5 23 19-2323 0 Pergamon Journals Ltd. 1987. Ptinted in U.S.A.
+ .oO
Temperature and pH controls over isotopic fractionation during adsorption of boron on marine clay M. R. PALMER, A. J. SPIVACK* and J. M. EDMOND Department of Earth, Atmospheric and Planetary Sciences, E34-201, MIT.,
Cambridge, MA 02139, U.S.A.
(Received July 25, 1986; accepted in revisedform June 8, 1987) Abstract-The variation of adsorption constants and isotope fractionation with pH and temperature during the adsorption of B from seawater onto marine clay have been examined. The controls over adsorption are similar to those exhibited by pure clay minerals (BASSETT,1976; KEREN and MEZUMAN,198 I). The isotope fractionations are the result of equilibrium processes, not kinetic effects. Variations in the measured frac-
tionation factor with pH arise from the differencesbetween the isotope fractionation associatedwith adsorption of B(OH)s and B(OH); and the pH dependence of B speciation. The implications of these results for the distribution of B isotopes in seawater and sediment porewaters are briefly discussed. INTRODUCI’ION IT Is WELL KNOWN that boron is adsorbed by clay minerals when they are introduced into seawater by rivers. Initial interest in this process centred around the use of boron concentrations of clay minerals (particularly illite) as palaeo-salinity indices (HARDER, 1970). However, subsequent work demonstrated that the boron contents of marine and freshwater estuarine sediments are indistinguishable when normal&d to the percentage of clay present (DEWiS et al., 1972). Adsorption of boron by clay has also been postulated as a mechanism for regulating the boron content of the oceans (HARRISS, 1969). Boron undergoes isotopic fractionation during adsorption, with the light isotope “B being preferentially incorporated into the adsorbed phase. AGYEI ( 1968) and SCHWARCZ et al. ( 1969) suggested that this process may act to buffer the boron isotopic composition of seawater. Recent developments in analytical techniques have resulted in important advances in our understanding of boron isotope geochemistry (SPIVACK and EDMOND, 1986, 1987; SPIVACK, 1986; SPIVACK et al., 1987) and as part of our investigations of the boron geochemical cycle we have taken a new look at adsorption and isotopic fractionation of boron by marine clay.
sorbed boron had been removed. The sediment consists predominantly of mixed layer illite-smectite, discrete illite and kaolinite, together with lesser amounts of quartz, feldspar, mica and organic matter. Constant size fractions of the sediment were pipetted as a slurry into 20 ml centrifuge tubes. A known weight of open ocean water was added, together with HCl or NaOH to adjust the pH to the final measured value. The sample was placed in a constant temperature water bath (precise to + 1“C over the course of the experiment) and agitated regularly. KERENand MEZUMAN(198 1) suggest the adsorption reaction reaches equilibrium within 2 hours. BASSEIT (1976) allowed 48 hours for the pH to reach equilibrium before adding boric acid to sediment slurries, and then waited a further 24 houn for the adsorption reaction to equilibrate. A 72 hour reaction time was employed here. The samples were then centrifuged and replaced in the water bath for 30 minutes. An aliquot was passed through a 0.45 pm Millipore filter for use in the concentration and isotopic analyses. The final pH was determined on the remaining solution at the in situ temperature. During all experiments parallel controls were set up without sediment to determine whether boron was lost to, or gained from, the container walls. In every case the final concentrations were identical to those at the start of the experiment (within analytical error). Boron concentrations were determined by a modified version of the curcumin technique (UPPSTROEM,1968). Precisions were better than + 1I. The boron isotope ratios were determined by thermal ionisation mass spectrometry of caesium metaborate (SPIVACK and EDMOND,1986; RAMAKUMAR et al., 1985). The results are expressed as:
METHODS
6”B=([+=]-
The adsorption properties of standard clay minerals have teen examined under a variety of pH, temperature and concentration conditions (BASSET, 1976;KERENand MEZUWN, 198 1). These data are of some value in predicting the adsorption behaviour of boron in Seawater, but it is necessary to confirm that natural sediments, which contain a variety of inorganic and organic phases, behave similarly to standard clay minerals. Hence, our approach has been to use marine sediment and seawater so as to mimic more closely the natural environment. Mississippi delta sediment was rinsed repeatedly with distilled and deionised water at room temperature until all ad-
1)X 103-0.19
where, -0.19 is the oxygen isotope correction. The NBS SRM is 4.04558. Internal precisions on 95 1 “B/‘% ratio (R--) individual runs were as low as +O. 15%, but the reproducibility (the true measure of analytical precision) is +0.35%. The accuracy and precision of the pH measurements is approximately +0.5 pH units. The precision of the adsorption constants and the isotopic fractionation factors is largely determined by the precision of the concentration measurement. RESULTS
The results are listed in Table 1 and illustrated in Fig. 1. The distribution coefficient is expressed as:
* Present address: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, U.S.A. 2319
Kd= K;;;Iy).
M. R. Palmer, A. J. Spivack and J. M. Edmond
2320
Table 1: Experimental Results
Temperature (oC)
pH
Kd
"
7.15 7.40 7.45 7.70 7.90 8.05 8.10 8.15 8.45
2.15±0.05 2.00±0.05 l. 95±0 .10 2.25±0.05 2.55±0.10 3.05±0.10 2.75±0.15 2.85±0.05 3.60±0.15
0.970±0.003 O. 972±0. 003
15
6.65 7.50 7.65 7.80 8.05
1.60±0.10 2.05±0.10 2.30±0.10 2.70±0.10 3.30±0.15
0.968±0.002 0.973±0.001 O. 972±0. 002 0.974±0.003 0.9775±0.0015
25
6.75 7.25 7.40 7.60 7.75 7.80
l. 40±0 .10
0.968±0.003 0.971±0.003
6.70 7.25 7.40 7.50
0.75±0.05 l. 10±0. 05 l. 25±0 .10 1.20±0.20
40
2.05±0.05 2.05±0.05 2.20±0.10 2.45±0.20 2.60±0.05
0.973±0.002
0.975±0.002 0.977±0.0015
0.975±0.002 0.967±0.004 0.970±0.003 O. 9715±0. 002
KEREN and MEZUMAN (1981) demonstrated that adsorption is reversible under the experimental conditions employed here. Hence, the fractionation factor between the adsorbed and dissolved species can be expressed as:
where (lIIBi is the initial (lIIB of the solution (+39.5%0 in every case), (lIIBds the final blIB ofthe solution and F is the ratio of the final to initial boron concentrations in solution. DISCUSSION
The increase in Kd with pH over the range considered here has been observed in previous studies (BASSETT, 1976; KEREN and MEZUMAN, 1981). Both sets of authors used standard clay minerals, but the experimental conditions employed (including temperature, clay mineral surface area and boron concentrations of the solutions) and the manner in which the data are reported do not allow quantitative comparison of the absolute Kd values measured in the two studies. Nevertheless, the nature of the relationship between the pH of the solution and the Kd is identical (over the pH ranges coincident in the studies) in both the experiments using standard clay minerals and in this study using natural sediments, although on this study we have not determined the pH at which Kd reaches a maximum. The fall in Kd values at temperatures above 25°C shown here has been observed by BASSETT (1976). These similarities strongly suggest that the processes that control adsorption of boron on pure clay minerals from artificial solutions are also responsible for determining the distribution of boron between sea-
water and that adsorbed on natural sediments. KEREN and MEZUMAN (1981) related the causes of the pH dependence of Kd in terms of the speciation of dissolved boron (see Fig. 2). At pH values below 7 Kd is low because the affinity of clay for B(OHh is relatively weak. Kd rises with pH because clays have a stronger adsorption affinity for B(OH)4' . At pH values beyond the upper range of those considered in this study Kd falls due to competition for sites with OH-. A plot of Kd versus the fraction of boron present as B(OH)4' (see Fig. 3a) yields a zero intercept of 1.68 ± 0.01 (excluding the data obtained at 40°C), giving the Kd for the adsorption of B(OHh. It is important to note that the pKa of boric acid has been adjusted for the different temperatures according to GIESKES (1974), but using the pKa determined by HERSHEY et al. (1986). From the change of Kd and boron speciation with pH the Kd for the adsorption of B(OH)4 is calculated to be 7.82 ± 0.03. The ratios of the two Kd values (4.67 ± 0.05) is considerably lower than those measured by KEREN and MEZUMAN (1981) for pure separates of montmorillonite (10) and illite (16). Boron is adsorbed on clay minerals by displacement of H 20 and OHfrom surface sites on a-Ah03 (HINGSTON et at., 1972; BASSETT, 1976). This process quantitatively explains the difference in Kd values between clay minerals in terms of the relative concentration of a-Ah03 surface sites produced by the different lattice structures of the clays (BASSETT, 1976). The heterogeneous nature of natural sediments will produce a variety of a-Ah03 adsorption sites, including coatings of non-clay minerals. It is possible that boron is also adsorbed by other
a
+
t
~
-J+
Kd
-¢-_ . .
'*'+
-0
-o--?-
-0
t.
7.0
...
.. ,
7.'
pH
b
.98',..--------------,
+ ,975
a
1+
tff
910
.'01..
1
70
+
It
7.'
-?
0.'
.,
pH FIG. 1. a. Boron adsorption distribution coefficient as a function of pH. (0) 5°C, (6,) 15°C, (+) 25°C, (0) 40°C. b. Boron adsorption isotopic fractionation factor as a function of pH. Symbols as above.
2321
Boron adsorption on clay
.2 0
1
_-----. 7
9
8
601
/
50
/
B(OH), 40
i
/ / 7
8
9
PH FIG.2. a. Boron co-ordination at 25°C and latm. as a function of pH (plotted using data from HERSHEYet al., 1986). b. Boron isotopic composition of B(OHh and B(OH)i at 25°C and latm. as a function of pH (using the theoretical fractionation factor calculated by KAK~HANA et al., 1977).
phases, e.g. Fe-oxyhydroxides. Therefore, some small differences between natural sediments and pure clay minerals are to be expected (e.g. SPIVACKet al., 1987, measured Kd values of 4.2- 1.2 in deep sea sediments), although the basic controls over the adsorption process are the same in both instances. Compared with the number of studies of the adsorption behaviour of boron comparatively little is known about the extent of boron isotope fractionation during this process. SHERGINA and KAMINSKAYA ( 1967) and SCHWARCZet al. ( 1969) made preliminary studies of this problem and showed that “B is preferentially adsorbed from solution relative to “B. The application of their results to the natural environment is limited to a qualitative interpretation because of the experimental procedures used. Both sets of workers employed solutions containing more than 20 times the boron concentration of seawater. At these levels boron forms polynuclear species (INGN et al., 1957) that alter the adsorption behaviour of boron (BASSETT, 1976; KEREN and MEZUMAN, 198 1). Polynuclear boron species are not present in significant concentrations in seawater (HERSHEY et al., 1986). Neither group reported the pH of the systems they studied, which (as shown above) is of critical importance in the adsorption process and has an important effect on the isotopic fractionation factor (see Fig. 1b). Finally, the analytical precision of the techniques employed in these early
studies has been improved by over an order of magnitude in this present study. Studies of boron adsorption on ion exchange resins from water have shown that the isotope fractionation factor between the dissolved and adsorbed species is dependent on temperature, with the greater degrees of fractionation associated with lower temperatures (KAKIHANAet al., 1977). The results presented here appear to contradict those findings as the fractionation factors obtained at 40°C are analytically indistinguishable from those at 5°C. Although the adsorption characteristics of natural sediments are likely to be different from those of ion exchange resins, we also note that the difference between the fractionation factors between dissolved and adsorbed boron at 40°C and 15°C calculated by KAIUHANAet al. (1977) is only 0.00 16 which is within the analytical precision of our technique. The isotope fractionation factor between the adsorbed and dissolved boron also varies with pH, showing less extreme fractionation at higher pH. The individual fractionation factors for the two boron species can be obtained from a plot of (Yagainst the fraction of boron present as the anion (see Fig. 3b) in the same manner that the individual Kd values were obtained (for consistency the data obtained at 40°C are again omitted). The data define a curve for which the best fit is obtained using fractionation factors (a = Ra&&RJotution) of 0.969 +- ,002 and 0.992 + .002 for B(OHh and B(OH);, respectively. These LYvalues describe the fmctionations between each individual species in so-
a
“1
/4
Kd
1
t
I 0
4
.l
.2
.3
.4
.l
.2
.3
.4
b .0607
.966” 0
f B(OH);
FIG. 3. a. Adsorption distribution coefficient as a function of ^ fraction of_dissolved boron present as B(OH)i . b. Isotope fracuonation factor as a function of fraction of dissolvedboron present as B(OH)i. Symbols are the same as those in Fig. 1.
2322
M. R. Palmer, A. J. Spivack and J. M. Edmond
lution with the respective adsorbed species. The boron isotopic compositions of the two species in solution vary with pH; e.g. see Fig. 2b. It is thought that the aAlzO,-boron complex is tetrahedrally coordinated for both species adsorbed from solution (BASSETT, 1976). This hypothesis is consistent with the two fractionation factors calculated here as it is to be expected that the trigonally co-ordinated B(OHh will undergo a greater degree of fractionation during formation of the tetrahedrally coordinated boron adsorption complex than will the already tetrahedrally co-ordinated B(OH);. The results of this study are in good agreement with measurements of the isotopic composition of boron adsorbed on deep sea sediments. SPIVACKet al. ( 1987 ) obtained an average 6”B of +14.2L for this phase. Seawater has a constant 6”B of +39.5%0 (SPIVACK, 1986). Thistmnslates to fractionation factor ((Y= w of 0.976 + 0.001 which is in good agreement R scpwater) with the calculated value of 0.975 using the isotopic fractionation factors and distribution coefficients calculated here and a pH of 8.2 for seawater. All the evidence presented here, together with the studies of BASSETT(1976) and KEREN and MEZU~WAN (198 1) indicate that boron adsorption is an equilibrium process and the isotope fractionations are not the result of kinetic effects. There are a number of implications of this data which we wilI briefly examine here. Fresh mid-ocean ridge basalts (MORB) contain approximately 0.3 ppm of boron (SPIVACK,1986). During low temperature weathering boron is taken up from seawater leading to concentrations of up to I50 ppm in the weathering products (THOMPSONand MELSON, 1970). The b”B of the basalts increases from -3.2k in fresh MORB to as high as +9.2% in altered basalts dredged from the sea floor (SPIVACK, 1986). The average fractionation factor (cu= RbustJR,,,,) between boron in sea floor weathered basalts and seawater is 0.968 -t 0.002 (SPIVACK, 1986). Unpublished results from “B NMR studies indicate that in clay minerals boron is principally tetrahedrahy coordinated, probably substituting for silicon (S. SOMMER, pers. commun., 1986). These observations are compatible with the hypothesis that the first step in the process of boron uptake during weathering of basalts is adsorption onto a surface with the same resultant isotopic fractionation as is exhibited during adsorption of boron by marine sediments. A relatively smaller degree of isotopic fractionation would then occur during incorporation of the boron into the clay mineral lattice. The slightly higher degree of isotopic fractionation observed during weathering of basahs (0.968) compared to that measumd during adsorption of boron on deep sea sediments (0.976) could be interpreted to imply a lower pH than open ocean water for the fluids in the cracks and grain boundaries of the weathering basalt. This is not surprising as both proton producing and consuming reactions occur during weathering which will change the
pH of the solution (HONNOREZ, 1981). It should be noted that potentiai differences in the reaction mechanisms may complicate
this first order model.
Adsorption of boron on clay minerals IS. together with low temperature weathering of the oceanic crust. one of the major mechanisms removing boron from the oceans (SPIVACK, 1986). The dependence of the adsorption constant, Kd, and the isotope fractionation factor, cy,on pH suggests that the boron concentration and isotope composition of the oceans (possibly as recorded in evaporite deposits) may be a sensitive recorder of the pH of seawater (HARRIS& 1969; SPIVAC~. 1986). During low temperature weathering of oceanic basalts boron is taken up from seawater and overlying sediment interstitial waters resulting in boron concentrations of up to 150 ppm in the weathering products. compared to only 0.3 ppm in fresh basalts (SPIVAC’K and EDMOND,1987; SPIVACKet al.. 1987). This uptake of boron continues after the basalt is covered with sediments and will lead to concentration gradients in deep sea sediment porewaters. Boron adsorption on clal minerals will lower the diffusion rate. Because isotopic fractionation occurs during adsorption the two boron isotopes will have different diffusion rates. This will result in an isotopic gradient in sediment porewaters in addition to the concentration gradient. Models based on non-steady state diffusion theory (MCDUPP and GIESKES, 1976) suggest that measurements of boron concentrations and isotope ratios in porewaters from mid-ocean ridge crest and Ilank sediments and accretionary prisms will provide information concerning; a) the nature of hydrologic regimes in sediment ponds (essential for accurate interpretation of heat llow data). b) the time of cessation of hydrothermally driven convection through layer I sediments and c) the thermal dewatering of descending slabs during subduction (PALMERet al., 1985; SPIVACKet ul.. 1987). CONCLUSIONS We have measured the variation of adsorption constants (Kd) and isotope fractionation factors (a) with pH (6.65-8.45) and temperature (54O’C) during adsorption of boron from seawater onto marine clay. The variations of Kd are consistent with previous studies of boron adsorption on pure clay minerals which demonstrated that B(OH); is adsorbed more strongly than B(OH)s leading to increasing Kd at higher pH values over the range considered in this study. These results demonstrate that adsorption of boron on marine sediments is also controlled by surface (Y-AI~OJsites in a similar manner to the behaviour of pure clay minerals. The isotope fractionation factors are the result of equilibrium processes and are not due to a kinetic isotope effect. Adsorption of B(OHh is accompanied by a greater degree of fractionation than B(OH); which is consistent with the formation of a tetrahedrally coordinated adsorbed species. The difference in isotope fractionation factors between the two boron species leads to a pH dependence of the measured isotope fractionation with less fractionation occurring at higher pH values. The results of this study have implications for controls over seawater boron concentrations and
Boron adsorption on clay
2323
(1997) Equilibrium studies of polyanions. Polyborates in NaClO* medium. Acta Chem. &aid. 11, 1034-1058. KAKIHANAH., KOTAKA M., SATOH S., NOMURA M. and OKAMOTOM. (1977) Fundamental studies on the ion-exAcknowledgements-We thank S. R. Hart for his generous change separation of boron isotopes. Bull. Chem. Sot. Japan provision of mass spectrometer facilities (supported by NSF 50, 158-163. EAR 83-08809). M.R.P. acknowledges the support of a KERENR. and MEZUMANV. (I 98 I) Boron adsorption by N.A.T.O. post doctoral fellowship. A.J.S. thanks the John and clay minerals using a phenomenological equation. Clays Fannie Hertz Foundation for provision of a graduate student Clay Minerals 29, 198-204. fellowship. This work was partially supported by NSF OCE McDuff R. E. and GIESKESJ. M. (1976) Calcium and mag83-08876. We are grateful to J. Gieskes, H. Schwartz, J. Lawnesium profiles in DSDP interstitial waters: diffusion or rence and G. Swihart for their thoughtful reviews. reaction? Earth Planet. Sri. Lett. 33, I- 10. Editorial handling: H. P. Schwartz PALMERM. R., SPIVACKA. J. and EDMONDJ. M. (1985) Adsorption of boron on marine sediments. Trans. .4mer. Geophys. Union 66, 9 16. REFERENCES RAMAKUMAR K. L., PARABA. R., KHODADEP. S.. ALMAULA AGYEI E. K. (1968) Isotopic and elemental composition of A. I., CHITAMBARS. A. and JAINH. C. (1985) Determiboron in meteorites, tektites and terrestrial material. Ph.D. nation of isotopic composition of boron. J. Radioanal. Nucl. thesis, McMaster Univ., 161~. Chem. Lett. 94, 53-62. BASSETT R. L. (1976) The geochemistry of boron in geotherSCHWARCZH. P., AGYEIE. K. and MCMULLENC. C. (1969) mal waters. Ph.D. thesis, Stanford Univ., 220~. Boron isotopic fractionation during adsorption from seaDEWISF. J., LEVINSONA. A. and BAYLISSP. (1972) Hydrowater. Earth Planet. Sci. Lett. 6, 1-5. geochemistry of the surface waters of the MacKenzie River SHERGINAY. P. and KAMINSKAYA A. B. (1967) Experimental drainage relationships in modem deltas. Geochim. Cossimulation of the natural separation of boron isotopes. mochim. Acta 36, 1359-1375. Geochimiya 10, 111 l-l 115. GIESKESJ. M. (1974) The alkalinity-total carbon dioxide SPIVACKA. J. (1986) Boron isotope geochemistry. Ph.D thesis, system in seawater. In The Sea, vol. 5 (ed. E. D. GOLDBERG), M.I.T./W.H.O.I., 184~. pp. 123-151. SPIVACKA. J. and EDMONDJ. M. (1986) Determination of HARDERH. (1970) Boron content of sediments as a tool in boron isotope ratios by thermal ionization mass spectromfacies analysis. Sediment. Geol. 4, 153- 175. etry of the dicesium metaborate cation. Anal. Chem. 58, HARRISSR. C. (1969) Boron regulation in the oceans. Nature 31-35. 223,290-29 1. SPIVACKA. J. and EDMONDJ. M. (1987) Boron isotope exHERSHEYJ. P., FERNANDEZM., MILNE P. J. and MILLERO change between seawater and oceanic crust. Geochim. CosF. J. (I 986) The ionization of boric acid in Na-Cl, Na-Camochim. Acta 51, 1033-1043. Cl and Na-Mg-Cl solutions at 25°C. Geochim. Cosmochim. SPIVACKA. J., PALMER M. R. and EDMONDJ. M. (1987) Acta 50, 143-148. The sedimentary cycle of the boron isotopes. Geochim. HONNOREZJ. (1981) The ageing of the oceanic crust at low Cosmochim. Acta 51, 1939-1950. temperature. In The Sea, vol. 7 (ed. E. D. GOLDBERG),pp. UPP~TROEML. R. (1968) A modified method for the deter525-588. mination of boron with curcumin and a simplified water INGRI N., LAGERSTROM G., FRYDMANM. and SILLENL. G. elimination procedure. Anal. Chim. Acta 43. 475-486. isotom comDositions and for the distribution isotopes in deep sea sediment porewaters
of boron
0016.7037/87/53.00
Gwchrrmca n Cosmorhrmrca Acta Vol. 5 23 19-2323 0 Pergamon Journals Ltd. 1987. Ptinted in U.S.A.
+ .oO
Temperature and pH controls over isotopic fractionation during adsorption of boron on marine clay M. R. PALMER, A. J. SPIVACK* and J. M. EDMOND Department of Earth, Atmospheric and Planetary Sciences, E34-201, MIT.,
Cambridge, MA 02139, U.S.A.
(Received July 25, 1986; accepted in revisedform June 8, 1987) Abstract-The variation of adsorption constants and isotope fractionation with pH and temperature during the adsorption of B from seawater onto marine clay have been examined. The controls over adsorption are similar to those exhibited by pure clay minerals (BASSETT,1976; KEREN and MEZUMAN,198 I). The isotope fractionations are the result of equilibrium processes, not kinetic effects. Variations in the measured frac-
tionation factor with pH arise from the differencesbetween the isotope fractionation associatedwith adsorption of B(OH)s and B(OH); and the pH dependence of B speciation. The implications of these results for the distribution of B isotopes in seawater and sediment porewaters are briefly discussed. INTRODUCI’ION IT Is WELL KNOWN that boron is adsorbed by clay minerals when they are introduced into seawater by rivers. Initial interest in this process centred around the use of boron concentrations of clay minerals (particularly illite) as palaeo-salinity indices (HARDER, 1970). However, subsequent work demonstrated that the boron contents of marine and freshwater estuarine sediments are indistinguishable when normal&d to the percentage of clay present (DEWiS et al., 1972). Adsorption of boron by clay has also been postulated as a mechanism for regulating the boron content of the oceans (HARRISS, 1969). Boron undergoes isotopic fractionation during adsorption, with the light isotope “B being preferentially incorporated into the adsorbed phase. AGYEI ( 1968) and SCHWARCZ et al. ( 1969) suggested that this process may act to buffer the boron isotopic composition of seawater. Recent developments in analytical techniques have resulted in important advances in our understanding of boron isotope geochemistry (SPIVACK and EDMOND, 1986, 1987; SPIVACK, 1986; SPIVACK et al., 1987) and as part of our investigations of the boron geochemical cycle we have taken a new look at adsorption and isotopic fractionation of boron by marine clay.
sorbed boron had been removed. The sediment consists predominantly of mixed layer illite-smectite, discrete illite and kaolinite, together with lesser amounts of quartz, feldspar, mica and organic matter. Constant size fractions of the sediment were pipetted as a slurry into 20 ml centrifuge tubes. A known weight of open ocean water was added, together with HCl or NaOH to adjust the pH to the final measured value. The sample was placed in a constant temperature water bath (precise to + 1“C over the course of the experiment) and agitated regularly. KERENand MEZUMAN(198 1) suggest the adsorption reaction reaches equilibrium within 2 hours. BASSEIT (1976) allowed 48 hours for the pH to reach equilibrium before adding boric acid to sediment slurries, and then waited a further 24 houn for the adsorption reaction to equilibrate. A 72 hour reaction time was employed here. The samples were then centrifuged and replaced in the water bath for 30 minutes. An aliquot was passed through a 0.45 pm Millipore filter for use in the concentration and isotopic analyses. The final pH was determined on the remaining solution at the in situ temperature. During all experiments parallel controls were set up without sediment to determine whether boron was lost to, or gained from, the container walls. In every case the final concentrations were identical to those at the start of the experiment (within analytical error). Boron concentrations were determined by a modified version of the curcumin technique (UPPSTROEM,1968). Precisions were better than + 1I. The boron isotope ratios were determined by thermal ionisation mass spectrometry of caesium metaborate (SPIVACK and EDMOND,1986; RAMAKUMAR et al., 1985). The results are expressed as:
METHODS
6”B=([+=]-
The adsorption properties of standard clay minerals have teen examined under a variety of pH, temperature and concentration conditions (BASSET, 1976;KERENand MEZUWN, 198 1). These data are of some value in predicting the adsorption behaviour of boron in Seawater, but it is necessary to confirm that natural sediments, which contain a variety of inorganic and organic phases, behave similarly to standard clay minerals. Hence, our approach has been to use marine sediment and seawater so as to mimic more closely the natural environment. Mississippi delta sediment was rinsed repeatedly with distilled and deionised water at room temperature until all ad-
1)X 103-0.19
where, -0.19 is the oxygen isotope correction. The NBS SRM is 4.04558. Internal precisions on 95 1 “B/‘% ratio (R--) individual runs were as low as +O. 15%, but the reproducibility (the true measure of analytical precision) is +0.35%. The accuracy and precision of the pH measurements is approximately +0.5 pH units. The precision of the adsorption constants and the isotopic fractionation factors is largely determined by the precision of the concentration measurement. RESULTS
The results are listed in Table 1 and illustrated in Fig. 1. The distribution coefficient is expressed as:
* Present address: Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, U.S.A. 2319
Kd= K;;;Iy).
M. R. Palmer, A. J. Spivack and J. M. Edmond
2320
Table 1: Experimental Results
Temperature (oC)
pH
Kd
"
7.15 7.40 7.45 7.70 7.90 8.05 8.10 8.15 8.45
2.15±0.05 2.00±0.05 l. 95±0 .10 2.25±0.05 2.55±0.10 3.05±0.10 2.75±0.15 2.85±0.05 3.60±0.15
0.970±0.003 O. 972±0. 003
15
6.65 7.50 7.65 7.80 8.05
1.60±0.10 2.05±0.10 2.30±0.10 2.70±0.10 3.30±0.15
0.968±0.002 0.973±0.001 O. 972±0. 002 0.974±0.003 0.9775±0.0015
25
6.75 7.25 7.40 7.60 7.75 7.80
l. 40±0 .10
0.968±0.003 0.971±0.003
6.70 7.25 7.40 7.50
0.75±0.05 l. 10±0. 05 l. 25±0 .10 1.20±0.20
40
2.05±0.05 2.05±0.05 2.20±0.10 2.45±0.20 2.60±0.05
0.973±0.002
0.975±0.002 0.977±0.0015
0.975±0.002 0.967±0.004 0.970±0.003 O. 9715±0. 002
KEREN and MEZUMAN (1981) demonstrated that adsorption is reversible under the experimental conditions employed here. Hence, the fractionation factor between the adsorbed and dissolved species can be expressed as:
where (lIIBi is the initial (lIIB of the solution (+39.5%0 in every case), (lIIBds the final blIB ofthe solution and F is the ratio of the final to initial boron concentrations in solution. DISCUSSION
The increase in Kd with pH over the range considered here has been observed in previous studies (BASSETT, 1976; KEREN and MEZUMAN, 1981). Both sets of authors used standard clay minerals, but the experimental conditions employed (including temperature, clay mineral surface area and boron concentrations of the solutions) and the manner in which the data are reported do not allow quantitative comparison of the absolute Kd values measured in the two studies. Nevertheless, the nature of the relationship between the pH of the solution and the Kd is identical (over the pH ranges coincident in the studies) in both the experiments using standard clay minerals and in this study using natural sediments, although on this study we have not determined the pH at which Kd reaches a maximum. The fall in Kd values at temperatures above 25°C shown here has been observed by BASSETT (1976). These similarities strongly suggest that the processes that control adsorption of boron on pure clay minerals from artificial solutions are also responsible for determining the distribution of boron between sea-
water and that adsorbed on natural sediments. KEREN and MEZUMAN (1981) related the causes of the pH dependence of Kd in terms of the speciation of dissolved boron (see Fig. 2). At pH values below 7 Kd is low because the affinity of clay for B(OHh is relatively weak. Kd rises with pH because clays have a stronger adsorption affinity for B(OH)4' . At pH values beyond the upper range of those considered in this study Kd falls due to competition for sites with OH-. A plot of Kd versus the fraction of boron present as B(OH)4' (see Fig. 3a) yields a zero intercept of 1.68 ± 0.01 (excluding the data obtained at 40°C), giving the Kd for the adsorption of B(OHh. It is important to note that the pKa of boric acid has been adjusted for the different temperatures according to GIESKES (1974), but using the pKa determined by HERSHEY et al. (1986). From the change of Kd and boron speciation with pH the Kd for the adsorption of B(OH)4 is calculated to be 7.82 ± 0.03. The ratios of the two Kd values (4.67 ± 0.05) is considerably lower than those measured by KEREN and MEZUMAN (1981) for pure separates of montmorillonite (10) and illite (16). Boron is adsorbed on clay minerals by displacement of H 20 and OHfrom surface sites on a-Ah03 (HINGSTON et at., 1972; BASSETT, 1976). This process quantitatively explains the difference in Kd values between clay minerals in terms of the relative concentration of a-Ah03 surface sites produced by the different lattice structures of the clays (BASSETT, 1976). The heterogeneous nature of natural sediments will produce a variety of a-Ah03 adsorption sites, including coatings of non-clay minerals. It is possible that boron is also adsorbed by other
a
+
t
~
-J+
Kd
-¢-_ . .
'*'+
-0
-o--?-
-0
t.
7.0
...
.. ,
7.'
pH
b
.98',..--------------,
+ ,975
a
1+
tff
910
.'01..
1
70
+
It
7.'
-?
0.'
.,
pH FIG. 1. a. Boron adsorption distribution coefficient as a function of pH. (0) 5°C, (6,) 15°C, (+) 25°C, (0) 40°C. b. Boron adsorption isotopic fractionation factor as a function of pH. Symbols as above.
2321
Boron adsorption on clay
.2 0
1
_-----. 7
9
8
601
/
50
/
B(OH), 40
i
/ / 7
8
9
PH FIG.2. a. Boron co-ordination at 25°C and latm. as a function of pH (plotted using data from HERSHEYet al., 1986). b. Boron isotopic composition of B(OHh and B(OH)i at 25°C and latm. as a function of pH (using the theoretical fractionation factor calculated by KAK~HANA et al., 1977).
phases, e.g. Fe-oxyhydroxides. Therefore, some small differences between natural sediments and pure clay minerals are to be expected (e.g. SPIVACKet al., 1987, measured Kd values of 4.2- 1.2 in deep sea sediments), although the basic controls over the adsorption process are the same in both instances. Compared with the number of studies of the adsorption behaviour of boron comparatively little is known about the extent of boron isotope fractionation during this process. SHERGINA and KAMINSKAYA ( 1967) and SCHWARCZet al. ( 1969) made preliminary studies of this problem and showed that “B is preferentially adsorbed from solution relative to “B. The application of their results to the natural environment is limited to a qualitative interpretation because of the experimental procedures used. Both sets of workers employed solutions containing more than 20 times the boron concentration of seawater. At these levels boron forms polynuclear species (INGN et al., 1957) that alter the adsorption behaviour of boron (BASSETT, 1976; KEREN and MEZUMAN, 198 1). Polynuclear boron species are not present in significant concentrations in seawater (HERSHEY et al., 1986). Neither group reported the pH of the systems they studied, which (as shown above) is of critical importance in the adsorption process and has an important effect on the isotopic fractionation factor (see Fig. 1b). Finally, the analytical precision of the techniques employed in these early
studies has been improved by over an order of magnitude in this present study. Studies of boron adsorption on ion exchange resins from water have shown that the isotope fractionation factor between the dissolved and adsorbed species is dependent on temperature, with the greater degrees of fractionation associated with lower temperatures (KAKIHANAet al., 1977). The results presented here appear to contradict those findings as the fractionation factors obtained at 40°C are analytically indistinguishable from those at 5°C. Although the adsorption characteristics of natural sediments are likely to be different from those of ion exchange resins, we also note that the difference between the fractionation factors between dissolved and adsorbed boron at 40°C and 15°C calculated by KAIUHANAet al. (1977) is only 0.00 16 which is within the analytical precision of our technique. The isotope fractionation factor between the adsorbed and dissolved boron also varies with pH, showing less extreme fractionation at higher pH. The individual fractionation factors for the two boron species can be obtained from a plot of (Yagainst the fraction of boron present as the anion (see Fig. 3b) in the same manner that the individual Kd values were obtained (for consistency the data obtained at 40°C are again omitted). The data define a curve for which the best fit is obtained using fractionation factors (a = Ra&&RJotution) of 0.969 +- ,002 and 0.992 + .002 for B(OHh and B(OH);, respectively. These LYvalues describe the fmctionations between each individual species in so-
a
“1
/4
Kd
1
t
I 0
4
.l
.2
.3
.4
.l
.2
.3
.4
b .0607
.966” 0
f B(OH);
FIG. 3. a. Adsorption distribution coefficient as a function of ^ fraction of_dissolved boron present as B(OH)i . b. Isotope fracuonation factor as a function of fraction of dissolvedboron present as B(OH)i. Symbols are the same as those in Fig. 1.
2322
M. R. Palmer, A. J. Spivack and J. M. Edmond
lution with the respective adsorbed species. The boron isotopic compositions of the two species in solution vary with pH; e.g. see Fig. 2b. It is thought that the aAlzO,-boron complex is tetrahedrally coordinated for both species adsorbed from solution (BASSETT, 1976). This hypothesis is consistent with the two fractionation factors calculated here as it is to be expected that the trigonally co-ordinated B(OHh will undergo a greater degree of fractionation during formation of the tetrahedrally coordinated boron adsorption complex than will the already tetrahedrally co-ordinated B(OH);. The results of this study are in good agreement with measurements of the isotopic composition of boron adsorbed on deep sea sediments. SPIVACKet al. ( 1987 ) obtained an average 6”B of +14.2L for this phase. Seawater has a constant 6”B of +39.5%0 (SPIVACK, 1986). Thistmnslates to fractionation factor ((Y= w of 0.976 + 0.001 which is in good agreement R scpwater) with the calculated value of 0.975 using the isotopic fractionation factors and distribution coefficients calculated here and a pH of 8.2 for seawater. All the evidence presented here, together with the studies of BASSETT(1976) and KEREN and MEZU~WAN (198 1) indicate that boron adsorption is an equilibrium process and the isotope fractionations are not the result of kinetic effects. There are a number of implications of this data which we wilI briefly examine here. Fresh mid-ocean ridge basalts (MORB) contain approximately 0.3 ppm of boron (SPIVACK,1986). During low temperature weathering boron is taken up from seawater leading to concentrations of up to I50 ppm in the weathering products (THOMPSONand MELSON, 1970). The b”B of the basalts increases from -3.2k in fresh MORB to as high as +9.2% in altered basalts dredged from the sea floor (SPIVACK, 1986). The average fractionation factor (cu= RbustJR,,,,) between boron in sea floor weathered basalts and seawater is 0.968 -t 0.002 (SPIVACK, 1986). Unpublished results from “B NMR studies indicate that in clay minerals boron is principally tetrahedrahy coordinated, probably substituting for silicon (S. SOMMER, pers. commun., 1986). These observations are compatible with the hypothesis that the first step in the process of boron uptake during weathering of basalts is adsorption onto a surface with the same resultant isotopic fractionation as is exhibited during adsorption of boron by marine sediments. A relatively smaller degree of isotopic fractionation would then occur during incorporation of the boron into the clay mineral lattice. The slightly higher degree of isotopic fractionation observed during weathering of basahs (0.968) compared to that measumd during adsorption of boron on deep sea sediments (0.976) could be interpreted to imply a lower pH than open ocean water for the fluids in the cracks and grain boundaries of the weathering basalt. This is not surprising as both proton producing and consuming reactions occur during weathering which will change the
pH of the solution (HONNOREZ, 1981). It should be noted that potentiai differences in the reaction mechanisms may complicate
this first order model.
Adsorption of boron on clay minerals IS. together with low temperature weathering of the oceanic crust. one of the major mechanisms removing boron from the oceans (SPIVACK, 1986). The dependence of the adsorption constant, Kd, and the isotope fractionation factor, cy,on pH suggests that the boron concentration and isotope composition of the oceans (possibly as recorded in evaporite deposits) may be a sensitive recorder of the pH of seawater (HARRIS& 1969; SPIVAC~. 1986). During low temperature weathering of oceanic basalts boron is taken up from seawater and overlying sediment interstitial waters resulting in boron concentrations of up to 150 ppm in the weathering products. compared to only 0.3 ppm in fresh basalts (SPIVAC’K and EDMOND,1987; SPIVACKet al.. 1987). This uptake of boron continues after the basalt is covered with sediments and will lead to concentration gradients in deep sea sediment porewaters. Boron adsorption on clal minerals will lower the diffusion rate. Because isotopic fractionation occurs during adsorption the two boron isotopes will have different diffusion rates. This will result in an isotopic gradient in sediment porewaters in addition to the concentration gradient. Models based on non-steady state diffusion theory (MCDUPP and GIESKES, 1976) suggest that measurements of boron concentrations and isotope ratios in porewaters from mid-ocean ridge crest and Ilank sediments and accretionary prisms will provide information concerning; a) the nature of hydrologic regimes in sediment ponds (essential for accurate interpretation of heat llow data). b) the time of cessation of hydrothermally driven convection through layer I sediments and c) the thermal dewatering of descending slabs during subduction (PALMERet al., 1985; SPIVACKet ul.. 1987). CONCLUSIONS We have measured the variation of adsorption constants (Kd) and isotope fractionation factors (a) with pH (6.65-8.45) and temperature (54O’C) during adsorption of boron from seawater onto marine clay. The variations of Kd are consistent with previous studies of boron adsorption on pure clay minerals which demonstrated that B(OH); is adsorbed more strongly than B(OH)s leading to increasing Kd at higher pH values over the range considered in this study. These results demonstrate that adsorption of boron on marine sediments is also controlled by surface (Y-AI~OJsites in a similar manner to the behaviour of pure clay minerals. The isotope fractionation factors are the result of equilibrium processes and are not due to a kinetic isotope effect. Adsorption of B(OHh is accompanied by a greater degree of fractionation than B(OH); which is consistent with the formation of a tetrahedrally coordinated adsorbed species. The difference in isotope fractionation factors between the two boron species leads to a pH dependence of the measured isotope fractionation with less fractionation occurring at higher pH values. The results of this study have implications for controls over seawater boron concentrations and
Boron adsorption on clay
2323
(1997) Equilibrium studies of polyanions. Polyborates in NaClO* medium. Acta Chem. &aid. 11, 1034-1058. KAKIHANAH., KOTAKA M., SATOH S., NOMURA M. and OKAMOTOM. (1977) Fundamental studies on the ion-exAcknowledgements-We thank S. R. Hart for his generous change separation of boron isotopes. Bull. Chem. Sot. Japan provision of mass spectrometer facilities (supported by NSF 50, 158-163. EAR 83-08809). M.R.P. acknowledges the support of a KERENR. and MEZUMANV. (I 98 I) Boron adsorption by N.A.T.O. post doctoral fellowship. A.J.S. thanks the John and clay minerals using a phenomenological equation. Clays Fannie Hertz Foundation for provision of a graduate student Clay Minerals 29, 198-204. fellowship. This work was partially supported by NSF OCE McDuff R. E. and GIESKESJ. M. (1976) Calcium and mag83-08876. We are grateful to J. Gieskes, H. Schwartz, J. Lawnesium profiles in DSDP interstitial waters: diffusion or rence and G. Swihart for their thoughtful reviews. reaction? Earth Planet. Sri. Lett. 33, I- 10. Editorial handling: H. P. Schwartz PALMERM. R., SPIVACKA. J. and EDMONDJ. M. (1985) Adsorption of boron on marine sediments. Trans. .4mer. Geophys. Union 66, 9 16. REFERENCES RAMAKUMAR K. L., PARABA. R., KHODADEP. S.. ALMAULA AGYEI E. K. (1968) Isotopic and elemental composition of A. I., CHITAMBARS. A. and JAINH. C. (1985) Determiboron in meteorites, tektites and terrestrial material. Ph.D. nation of isotopic composition of boron. J. Radioanal. Nucl. thesis, McMaster Univ., 161~. Chem. Lett. 94, 53-62. BASSETT R. L. (1976) The geochemistry of boron in geotherSCHWARCZH. P., AGYEIE. K. and MCMULLENC. C. (1969) mal waters. Ph.D. thesis, Stanford Univ., 220~. Boron isotopic fractionation during adsorption from seaDEWISF. J., LEVINSONA. A. and BAYLISSP. (1972) Hydrowater. Earth Planet. Sci. Lett. 6, 1-5. geochemistry of the surface waters of the MacKenzie River SHERGINAY. P. and KAMINSKAYA A. B. (1967) Experimental drainage relationships in modem deltas. Geochim. Cossimulation of the natural separation of boron isotopes. mochim. Acta 36, 1359-1375. Geochimiya 10, 111 l-l 115. GIESKESJ. M. (1974) The alkalinity-total carbon dioxide SPIVACKA. J. (1986) Boron isotope geochemistry. Ph.D thesis, system in seawater. In The Sea, vol. 5 (ed. E. D. GOLDBERG), M.I.T./W.H.O.I., 184~. pp. 123-151. SPIVACKA. J. and EDMONDJ. M. (1986) Determination of HARDERH. (1970) Boron content of sediments as a tool in boron isotope ratios by thermal ionization mass spectromfacies analysis. Sediment. Geol. 4, 153- 175. etry of the dicesium metaborate cation. Anal. Chem. 58, HARRISSR. C. (1969) Boron regulation in the oceans. Nature 31-35. 223,290-29 1. SPIVACKA. J. and EDMONDJ. M. (1987) Boron isotope exHERSHEYJ. P., FERNANDEZM., MILNE P. J. and MILLERO change between seawater and oceanic crust. Geochim. CosF. J. (I 986) The ionization of boric acid in Na-Cl, Na-Camochim. Acta 51, 1033-1043. Cl and Na-Mg-Cl solutions at 25°C. Geochim. Cosmochim. SPIVACKA. J., PALMER M. R. and EDMONDJ. M. (1987) Acta 50, 143-148. The sedimentary cycle of the boron isotopes. Geochim. HONNOREZJ. (1981) The ageing of the oceanic crust at low Cosmochim. Acta 51, 1939-1950. temperature. In The Sea, vol. 7 (ed. E. D. GOLDBERG),pp. UPP~TROEML. R. (1968) A modified method for the deter525-588. mination of boron with curcumin and a simplified water INGRI N., LAGERSTROM G., FRYDMANM. and SILLENL. G. elimination procedure. Anal. Chim. Acta 43. 475-486. isotom comDositions and for the distribution isotopes in deep sea sediment porewaters
of boron