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Micro Volume Changes Due to Pb(II) and Cu(II) Sorption on Amorphous Fe(III) Hydroxide
Journal of Colloid and Interface Science 249, 489–491 (2002) doi:10.1006/jcis.2002.8276, available online at http://www.idealibrary.com on
NOTE Micro Volume Changes Due to Pb(II) and Cu(II) Sorption on Amorphous Fe(III) Hydroxide MATERIALS AND METHODS Micro volume changes due to Pb(II) and Cu(II) sorption on amorphous Fe(III) hydroxide (AFH) were determined by a dilatometer at pH 4.50. Volume change is attributed to change in hydration status of dissolved and/or suspended substances. The volume of the system increased due to Pb(II) and Cu(II) sorption, suggesting that water molecules hydrated around Pb(II) or Cu(II) ions and AFH were released during sorption. Volume increases due to Pb(II) and Cu(II) sorption were smaller than those due to bulk precipitation of Pb and Cu hydroxides. Precipitation of Pb(II) and Cu(II) was not likely to occur at pH 4.50 in the presence of AFH. In conclusion, Pb(II) and Cu(II) formed an inner-sphere complex on AFH at pH 4.50, keeping hydrated water on the adsorbed species. Adsorbed Cu(II) kept more hydrated water than adsorbed Pb(II) on AFH. C 2002 Elsevier Science (USA) Key Words: dilatometry; hydration; copper(II); lead(II); sorption; precipitation.
INTRODUCTION Sorption is one of the most important processes for heavy metal removal from an aquatic environment in contact with soil or suspended solids. Sorption processes include some solute accumulation processes onto a solid surface such as adsorption, precipitation, and polymerization (1). The sorption mechanism strongly depends on the type of mineral phase present, sorbate/sorbent ratio, and contact period of solute with sorbent. To understand the fate and mobility of heavy metals in soil and aquatic environments, molecular-scale information of sorbed heavy metal species is necessary. When ions are sorbed to a solid surface, the structure of hydrated water around the ions and sorbents changes. If the sorbed ions and/or the sorbent release their hydrated water to the bulk, the water molecules become sparse, resulting in an increase of system volume. Yamaguchi et al. (2, 3) determined volume changes due to oxoanion adsorption on amorphous Fe(III) hydroxide (AFH) using a dilatometer designed by Hashitani (4) and experimentally proved the release of 2− 2− hydrated water due to inner-sphere complexation of H2 PO− 4 , SO4 , and SeO4 . These oxoanions lost most of their hydrated water to be adsorbed on the AFH surface (3). In this study, we determined volume changes due to Pb(II) and Cu(II) sorption on AFH. Recent studies by EXAFS (extended X-ray absorption fine structure) revealed that Pb(II) and Cu(II) adsorbed as inner-sphere complexes on Fe(III) oxide surfaces (5, 6). Nevertheless, the hydration status of sorbed Pb(II) and Cu(II) has not been investigated thus far. The purposes of this study are (1) to determine volume changes caused by Pb(II) and Cu(II) sorption and (2) to estimate the degree of hydration for Pb(II) and Cu(II) sorbed on AFH.
AFH was synthesized by the method of Okazaki et al. (7). No peak was observed for the synthesized Fe(III) oxide by XRD; therefore, the Fe(III) hydroxide was designated as amorphous. AFH was weighed (2.5 g for Pb(II) and 3 g for Cu(II)) and suspended in 25 ml of 0.1 mol L−1 NaNO3 solution. The suspension pH was adjusted to 4.50 by dilute HNO3 . Sorbate solutions were prepared by dissolving Pb(NO3 )2 or Cu(NO3 )2 in 0.1 mol L−1 NaNO3 solution. Their pHs were adjusted to pH 4.50 using dilute HNO3 solution in a N2 atmosphere. Initial concentrations of Pb(NO3 )2 and Cu(NO3 )2 are 4.9 and 7.8 mmol L−1 , respectively. Volume changes due to Pb(II) and Cu(II) sorption were determined using a dilatometer designed by Hashitani (4). The dilatometer, made from Pyrex glass, is divided into two compartments (ca. 60 cm3 and 50 cm3 for lower and upper compartments) by two optically polished glass discs. A capillary tube with an inside diameter of 0.5 mm was fused to the upper compartment. Volume changes were determined as differences in solution heights in the capillary tube before and after mixing the solutions (or suspension) in a thermostat system at 298.0 ± 0.0001–0.005 K. An AFH–0.1 mol L−1 NaNO3 suspension and sorbate solution were poured into the lower and upper compartments, respectively. The detailed procedure for determining volume change is described elsewhere (2). After a 5-h reaction, the suspensions were filtered through 0.22-µm Millipore filters, and then pH and total ion concentrations of the filtrate were determined. Concentrations of Pb(II) and Cu(II) were determined by EDTA titration methods with XO (xylenol orange, 0.1% in water) and TAR (thiazolylazo resorcinol, 0.1% in methanol) indicators (8). The moles of ions sorbed on AFH were calculated from the differences in ion concentrations in the solutions before and after sorption. Volume changes due to Pb(II) and Cu(II) hydroxide precipitation were determined by mixing 0.01 mol L−1 of Pb(NO3 )2 or Cu(NO3 )2 (upper compartment) and 0.02 mol L−1 NaOH solutions (lower compartment) using a dilatometer with a capillary tube of 1-mm inside diameter. Initial Pb(NO3 )2 and Cu(NO3 )2 solution pHs were 4.69 and 4.66, respectively. Amounts of Pb and Cu precipitated were calculated from the differences in dissolved Pb(II) and Cu(II) concentrations before and after reaction. Each experiment was repeated at least five times.
RESULTS AND DISCUSSION
Volume Changes Due to Pb(II) and Cu(II) Sorption on AFH The volume of the system was increased due to Pb(II) and Cu(II) sorption on AFH (Table 1), suggesting that water molecules hydrated around the sorbate ion and/or sorbent were released to bulk as free water. Volume changes per mole of sorbed ion (Vsorb ) were +32 cm3 mol−1 for Pb(II) > +27 cm3 mol−1 for Cu(II), indicating that more hydrated water was apparently released to the bulk during Pb(II) sorption than during Cu(II) sorption.
Volume Changes Due to Pb(II) and Cu(II) Bulk Precipitation When 0.01 mol L−1 Pb(NO3 )2 or Cu(NO3 )2 solution was mixed with 0.02 mol L−1 NaOH, white or blue precipitate was observed and the system 489
0021-9797/02 $35.00
C 2002 Elsevier Science (USA)
All rights reserved.
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NOTE
TABLE 1 Volume Changes Due to Pb(II) and Cu(II) Sorption (∆Vsorb ) and Hydroxide Precipitation (∆Vprep )
Pb(II) Cu(II)
pHfinal
Vobs (10−3 cm3 )
Sorption amounts (mmol)
Vsorb (cm3 mol−1 )
Vpre (cm3 mol−1 )
3.83 ± 0.03 4.08 ± 0.02
+7.16 ± 0.14 +4.11 ± 0.46
0.227 ± 0.007 0.155 ± 0.013
+32 ± 1.1 +27 ± 2.0
+52 ± 1.0 +47 ± 1.0
volumes were increased. The pHs of the suspension were 6.00 for Pb(II) and 5.83 for Cu(II), respectively. Neutralization of H+ simultaneously occurred with Pb(II) and Cu(II) precipitation. The contribution of the volume change due to − 3 −1 H+ neutralization (H+ aq + OHaq → f.w. + 21 cm mol , where f.w. represents free water (2)) was subtracted from the total volume change. Volume changes due to Pb(II) and Cu(II) hydroxide precipitation were +52 and +47 cm3 /mol of precipitate formed, respectively (Table 1).
Hydration Status of Pb(II) and Cu(II) Sorbed on AFH The decrease in Pb(II) and Cu(II) concentration after mixing Pb(II) or Cu(II) solution with AFH can result from either Pb(II) and Cu(II) adsorption or precipitation. Volume changes due to Pb(II) and Cu(II) sorption on AFH were smaller than those due to Pb(II) and Cu(II) hydroxide precipitation (Table 1). This fact indicated that Pb(II) and Cu(II) did not form a precipitate phase on AFH at pH 4.5. Hence, Pb(II) and Cu(II) were likely adsorbed on AFH at pH 4.5. For the precipitation of Pb(II) or Cu(II) hydroxide (Me2+ + 2OH− → Me(OH)2 ), hydrated water is released from both Pb2+ or Cu2+ and OH− . Volume change due to OH− dehydration is +10 cm3 mol−1 (2). Assuming OH− releases all hydrated water to form precipitate, volume changes due to Pb2+ and Cu2+ dehydration for precipitation were estimated by subtracting the contribution of OH− dehydration from Vprep . The volume changes due to Pb(II) and Cu(II) dehydration to form precipitation were estimated as +32 and +27 cm3 mol−1 , respectively. Note that OH− , Pb2+ , and Cu2+ would not lose all of their hydrated water for precipitation since fresh metal hydroxide usually keeps hydrated water. For the formation of an inner-sphere complex of Pb(II) and Cu(II) on AFH, hydrated water should be released from both the Pb(II) or Cu(II) and AFH surface. The volume changes caused by the dehydration of both adsorbate (Pb(II) or Cu(II)) and adsorbent (AFH) corresponded to those caused by dehydration of Pb(II) or Cu(II) to form hydroxide precipitation. If the ions were adsorbed as an outer-sphere surface complex, dehydration from ions and sorbents was not significant. Volume changes due to Ca and Mg adsorption on AFH at pH 7 were smaller than the detection limit of the dilatometer (2 × 10−5 cm3 ). It was not likely that adsorbed Pb(II) and Cu(II) formed an outer-sphere surface complex on AFH since substantial amounts of hydrated water were released during adsorption. Therefore, it was concluded that Pb(II) and Cu(II) formed an inner-
sphere surface complex on AFH at pH 4.5. Spectroscopic approaches to Pb(II) and Cu(II) sorption mechanisms on Fe(III) hydroxide concluded the formation of an inner-sphere complex (5, 6), in agreement with our result. Several adsorption sites of different geometries have different hydration status. It should be noted that the volume change determined in this study was an average value for the formation of Pb(II) and Cu(II) inner-sphere complexes on several different adsorption sites. Table 2 lists the hydration entropies and hydration numbers for Pb2+ and Cu2+ (9, 10). Greater −Shø indicates stronger ion hydration ability. Copper ion has a stronger ability of hydration than lead ion. Despite the fact that aqueous Cu2+ keeps more hydrated water than aqueous Pb2+ , the release of hydrated water for Cu(II) adsorption was smaller than that due to Pb(II) adsorption. Copper(II) ion, which has stronger ability of hydration than Pb(II), can retain more hydrated water in the form of sorption complexes or hydroxide precipitates. Therefore, Cu(II) released less hydrated water than Pb(II) during sorption or precipitation processes. Aqueous Pb(II), which keeps less hydrated water than aqueous Cu(II), approaches the adsorbent surface easier than aqueous Cu(II). Adsorbed Pb(II) and Cu(II) on AFH still retained their ability of hydration. In contrast, Yamaguchi et al. (3) proved that phosphate, which adsorbed as an inner-sphere complex, released most of its hydrated water to the bulk. Hydration ability of oxoanion, which is larger in size, is not so strong as Pb(II) and Cu(II). Therefore, an oxoanion loses its ability of hydration by adsorbing on an AFH surface. Ion hydration ability in an aqueous solution can be a measure of hydration ability of adsorbed ion.
SUMMARY Volume changes due to Pb(II) and Cu(II) adsorption were determined by a dilatometer. Neither Pb(II) nor Cu(II) precipitation was likely to have occurred at low reaction pH (pH <4.5) and short reaction period (5 h) in this study. Pb(II) and Cu(II) formed an inner-sphere surface complex on AFH while keeping hydrated water. Adsorbed Cu(II) kept more hydrated water than adsorbed Pb(II) on amorphous Fe(III) hydroxide.
ACKNOWLEDGMENTS TABLE 2 Hydration Properties for Pb2+ and Cu2+
Pb2+ Cu2+ a
−Shø a (J mol−1 K−1 )
Hydration numberb
198 309
8 12
Friedman and Krishman (9). b Estimated from entropies of hydration: −S ø /(−25 J mol−1 K−1 ) Hashitani h and Tamamushi (10).
This work was partly supported by JSPS Research Fellowships for Young Scientists. We express our gratitude to Prof. T. Hashitani for his generous assistance for the use of the dilatometer and discussion and to Dr. M. Mizoguchi of the University of Tokyo for his critical reading of our manuscript.
REFERENCES 1. Sparks, D. L., “Environmental Soil Chemistry.” Academic Press, San Diego, 1995. 2. Yamaguchi, N., Hashitani, T., Okazaki, M., and Takeda, T., J. Colloid Interface Sci. 183, 280 (1996).
491
NOTE 3. Yamaguchi, N. U., Okazaki, M., and Hashitani, T., J. Colloid Interface Sci. 209, 386 (1999); doi: jcis.1998.5900. 4. Hashitani, T., “Annual Report of the Faculty of General Education,” Vol. 14, p. 84. Tokyo Univ. of Agriculture & Technology, Tokyo, 1978 [in Japanese]. 5. Parkman, R. H., Charnock, J. M., Bryan, N. D., Livens, F. R., and Vaughan, D. J., Am. Mineral. 84, 407 (1999). 6. Roe, A. L., Hayes, K. F., Chisholmbrause, C., Brown, G. E., Parks G. A., Hodgson K. O., and Leckie J. O., Langmuir 7, 367 (1991). 7. Okazaki, M., Takamido, K., and Yamane, I., Soil Sci. Plant Nutr. 32, 523 (1986). 8. Ueno, K., “Methods of Chelatometric Titration.” Nankodo, Tokyo, 1972 [in Japanese]. 9. Friedman, H. L., and Krishman, C. V., in “Water” (F. Franks, Ed.), p. 55. Plenum, New York, 1973. 10. Hashitani, T., and Tamamushi, R., in “Ions and Solvents” (The Chemical Society of Japan Eds.), Chap. 5, p. 83. Academic Press Center, Tokyo, 1976 [in Japanese].
Noriko U. Yamaguchi∗,1 Masanori Okazaki† ∗ Graduate School of Agriculture and Life Science The University of Tokyo 1-1-1, Yayoi, Bunkyo Tokyo, 113-8657 Japan †Graduate School of Bio-Applications and Systems Engineering Tokyo University of Agriculture and Technology Koganei, Tokyo, 184-8657 Japan
Received November 2, 2001; accepted January 31, 2002; published online April 10, 2002
1
To whom correspondence should be addressed. E-mail: [email protected].
NOTE Micro Volume Changes Due to Pb(II) and Cu(II) Sorption on Amorphous Fe(III) Hydroxide MATERIALS AND METHODS Micro volume changes due to Pb(II) and Cu(II) sorption on amorphous Fe(III) hydroxide (AFH) were determined by a dilatometer at pH 4.50. Volume change is attributed to change in hydration status of dissolved and/or suspended substances. The volume of the system increased due to Pb(II) and Cu(II) sorption, suggesting that water molecules hydrated around Pb(II) or Cu(II) ions and AFH were released during sorption. Volume increases due to Pb(II) and Cu(II) sorption were smaller than those due to bulk precipitation of Pb and Cu hydroxides. Precipitation of Pb(II) and Cu(II) was not likely to occur at pH 4.50 in the presence of AFH. In conclusion, Pb(II) and Cu(II) formed an inner-sphere complex on AFH at pH 4.50, keeping hydrated water on the adsorbed species. Adsorbed Cu(II) kept more hydrated water than adsorbed Pb(II) on AFH. C 2002 Elsevier Science (USA) Key Words: dilatometry; hydration; copper(II); lead(II); sorption; precipitation.
INTRODUCTION Sorption is one of the most important processes for heavy metal removal from an aquatic environment in contact with soil or suspended solids. Sorption processes include some solute accumulation processes onto a solid surface such as adsorption, precipitation, and polymerization (1). The sorption mechanism strongly depends on the type of mineral phase present, sorbate/sorbent ratio, and contact period of solute with sorbent. To understand the fate and mobility of heavy metals in soil and aquatic environments, molecular-scale information of sorbed heavy metal species is necessary. When ions are sorbed to a solid surface, the structure of hydrated water around the ions and sorbents changes. If the sorbed ions and/or the sorbent release their hydrated water to the bulk, the water molecules become sparse, resulting in an increase of system volume. Yamaguchi et al. (2, 3) determined volume changes due to oxoanion adsorption on amorphous Fe(III) hydroxide (AFH) using a dilatometer designed by Hashitani (4) and experimentally proved the release of 2− 2− hydrated water due to inner-sphere complexation of H2 PO− 4 , SO4 , and SeO4 . These oxoanions lost most of their hydrated water to be adsorbed on the AFH surface (3). In this study, we determined volume changes due to Pb(II) and Cu(II) sorption on AFH. Recent studies by EXAFS (extended X-ray absorption fine structure) revealed that Pb(II) and Cu(II) adsorbed as inner-sphere complexes on Fe(III) oxide surfaces (5, 6). Nevertheless, the hydration status of sorbed Pb(II) and Cu(II) has not been investigated thus far. The purposes of this study are (1) to determine volume changes caused by Pb(II) and Cu(II) sorption and (2) to estimate the degree of hydration for Pb(II) and Cu(II) sorbed on AFH.
AFH was synthesized by the method of Okazaki et al. (7). No peak was observed for the synthesized Fe(III) oxide by XRD; therefore, the Fe(III) hydroxide was designated as amorphous. AFH was weighed (2.5 g for Pb(II) and 3 g for Cu(II)) and suspended in 25 ml of 0.1 mol L−1 NaNO3 solution. The suspension pH was adjusted to 4.50 by dilute HNO3 . Sorbate solutions were prepared by dissolving Pb(NO3 )2 or Cu(NO3 )2 in 0.1 mol L−1 NaNO3 solution. Their pHs were adjusted to pH 4.50 using dilute HNO3 solution in a N2 atmosphere. Initial concentrations of Pb(NO3 )2 and Cu(NO3 )2 are 4.9 and 7.8 mmol L−1 , respectively. Volume changes due to Pb(II) and Cu(II) sorption were determined using a dilatometer designed by Hashitani (4). The dilatometer, made from Pyrex glass, is divided into two compartments (ca. 60 cm3 and 50 cm3 for lower and upper compartments) by two optically polished glass discs. A capillary tube with an inside diameter of 0.5 mm was fused to the upper compartment. Volume changes were determined as differences in solution heights in the capillary tube before and after mixing the solutions (or suspension) in a thermostat system at 298.0 ± 0.0001–0.005 K. An AFH–0.1 mol L−1 NaNO3 suspension and sorbate solution were poured into the lower and upper compartments, respectively. The detailed procedure for determining volume change is described elsewhere (2). After a 5-h reaction, the suspensions were filtered through 0.22-µm Millipore filters, and then pH and total ion concentrations of the filtrate were determined. Concentrations of Pb(II) and Cu(II) were determined by EDTA titration methods with XO (xylenol orange, 0.1% in water) and TAR (thiazolylazo resorcinol, 0.1% in methanol) indicators (8). The moles of ions sorbed on AFH were calculated from the differences in ion concentrations in the solutions before and after sorption. Volume changes due to Pb(II) and Cu(II) hydroxide precipitation were determined by mixing 0.01 mol L−1 of Pb(NO3 )2 or Cu(NO3 )2 (upper compartment) and 0.02 mol L−1 NaOH solutions (lower compartment) using a dilatometer with a capillary tube of 1-mm inside diameter. Initial Pb(NO3 )2 and Cu(NO3 )2 solution pHs were 4.69 and 4.66, respectively. Amounts of Pb and Cu precipitated were calculated from the differences in dissolved Pb(II) and Cu(II) concentrations before and after reaction. Each experiment was repeated at least five times.
RESULTS AND DISCUSSION
Volume Changes Due to Pb(II) and Cu(II) Sorption on AFH The volume of the system was increased due to Pb(II) and Cu(II) sorption on AFH (Table 1), suggesting that water molecules hydrated around the sorbate ion and/or sorbent were released to bulk as free water. Volume changes per mole of sorbed ion (Vsorb ) were +32 cm3 mol−1 for Pb(II) > +27 cm3 mol−1 for Cu(II), indicating that more hydrated water was apparently released to the bulk during Pb(II) sorption than during Cu(II) sorption.
Volume Changes Due to Pb(II) and Cu(II) Bulk Precipitation When 0.01 mol L−1 Pb(NO3 )2 or Cu(NO3 )2 solution was mixed with 0.02 mol L−1 NaOH, white or blue precipitate was observed and the system 489
0021-9797/02 $35.00
C 2002 Elsevier Science (USA)
All rights reserved.
490
NOTE
TABLE 1 Volume Changes Due to Pb(II) and Cu(II) Sorption (∆Vsorb ) and Hydroxide Precipitation (∆Vprep )
Pb(II) Cu(II)
pHfinal
Vobs (10−3 cm3 )
Sorption amounts (mmol)
Vsorb (cm3 mol−1 )
Vpre (cm3 mol−1 )
3.83 ± 0.03 4.08 ± 0.02
+7.16 ± 0.14 +4.11 ± 0.46
0.227 ± 0.007 0.155 ± 0.013
+32 ± 1.1 +27 ± 2.0
+52 ± 1.0 +47 ± 1.0
volumes were increased. The pHs of the suspension were 6.00 for Pb(II) and 5.83 for Cu(II), respectively. Neutralization of H+ simultaneously occurred with Pb(II) and Cu(II) precipitation. The contribution of the volume change due to − 3 −1 H+ neutralization (H+ aq + OHaq → f.w. + 21 cm mol , where f.w. represents free water (2)) was subtracted from the total volume change. Volume changes due to Pb(II) and Cu(II) hydroxide precipitation were +52 and +47 cm3 /mol of precipitate formed, respectively (Table 1).
Hydration Status of Pb(II) and Cu(II) Sorbed on AFH The decrease in Pb(II) and Cu(II) concentration after mixing Pb(II) or Cu(II) solution with AFH can result from either Pb(II) and Cu(II) adsorption or precipitation. Volume changes due to Pb(II) and Cu(II) sorption on AFH were smaller than those due to Pb(II) and Cu(II) hydroxide precipitation (Table 1). This fact indicated that Pb(II) and Cu(II) did not form a precipitate phase on AFH at pH 4.5. Hence, Pb(II) and Cu(II) were likely adsorbed on AFH at pH 4.5. For the precipitation of Pb(II) or Cu(II) hydroxide (Me2+ + 2OH− → Me(OH)2 ), hydrated water is released from both Pb2+ or Cu2+ and OH− . Volume change due to OH− dehydration is +10 cm3 mol−1 (2). Assuming OH− releases all hydrated water to form precipitate, volume changes due to Pb2+ and Cu2+ dehydration for precipitation were estimated by subtracting the contribution of OH− dehydration from Vprep . The volume changes due to Pb(II) and Cu(II) dehydration to form precipitation were estimated as +32 and +27 cm3 mol−1 , respectively. Note that OH− , Pb2+ , and Cu2+ would not lose all of their hydrated water for precipitation since fresh metal hydroxide usually keeps hydrated water. For the formation of an inner-sphere complex of Pb(II) and Cu(II) on AFH, hydrated water should be released from both the Pb(II) or Cu(II) and AFH surface. The volume changes caused by the dehydration of both adsorbate (Pb(II) or Cu(II)) and adsorbent (AFH) corresponded to those caused by dehydration of Pb(II) or Cu(II) to form hydroxide precipitation. If the ions were adsorbed as an outer-sphere surface complex, dehydration from ions and sorbents was not significant. Volume changes due to Ca and Mg adsorption on AFH at pH 7 were smaller than the detection limit of the dilatometer (2 × 10−5 cm3 ). It was not likely that adsorbed Pb(II) and Cu(II) formed an outer-sphere surface complex on AFH since substantial amounts of hydrated water were released during adsorption. Therefore, it was concluded that Pb(II) and Cu(II) formed an inner-
sphere surface complex on AFH at pH 4.5. Spectroscopic approaches to Pb(II) and Cu(II) sorption mechanisms on Fe(III) hydroxide concluded the formation of an inner-sphere complex (5, 6), in agreement with our result. Several adsorption sites of different geometries have different hydration status. It should be noted that the volume change determined in this study was an average value for the formation of Pb(II) and Cu(II) inner-sphere complexes on several different adsorption sites. Table 2 lists the hydration entropies and hydration numbers for Pb2+ and Cu2+ (9, 10). Greater −Shø indicates stronger ion hydration ability. Copper ion has a stronger ability of hydration than lead ion. Despite the fact that aqueous Cu2+ keeps more hydrated water than aqueous Pb2+ , the release of hydrated water for Cu(II) adsorption was smaller than that due to Pb(II) adsorption. Copper(II) ion, which has stronger ability of hydration than Pb(II), can retain more hydrated water in the form of sorption complexes or hydroxide precipitates. Therefore, Cu(II) released less hydrated water than Pb(II) during sorption or precipitation processes. Aqueous Pb(II), which keeps less hydrated water than aqueous Cu(II), approaches the adsorbent surface easier than aqueous Cu(II). Adsorbed Pb(II) and Cu(II) on AFH still retained their ability of hydration. In contrast, Yamaguchi et al. (3) proved that phosphate, which adsorbed as an inner-sphere complex, released most of its hydrated water to the bulk. Hydration ability of oxoanion, which is larger in size, is not so strong as Pb(II) and Cu(II). Therefore, an oxoanion loses its ability of hydration by adsorbing on an AFH surface. Ion hydration ability in an aqueous solution can be a measure of hydration ability of adsorbed ion.
SUMMARY Volume changes due to Pb(II) and Cu(II) adsorption were determined by a dilatometer. Neither Pb(II) nor Cu(II) precipitation was likely to have occurred at low reaction pH (pH <4.5) and short reaction period (5 h) in this study. Pb(II) and Cu(II) formed an inner-sphere surface complex on AFH while keeping hydrated water. Adsorbed Cu(II) kept more hydrated water than adsorbed Pb(II) on amorphous Fe(III) hydroxide.
ACKNOWLEDGMENTS TABLE 2 Hydration Properties for Pb2+ and Cu2+
Pb2+ Cu2+ a
−Shø a (J mol−1 K−1 )
Hydration numberb
198 309
8 12
Friedman and Krishman (9). b Estimated from entropies of hydration: −S ø /(−25 J mol−1 K−1 ) Hashitani h and Tamamushi (10).
This work was partly supported by JSPS Research Fellowships for Young Scientists. We express our gratitude to Prof. T. Hashitani for his generous assistance for the use of the dilatometer and discussion and to Dr. M. Mizoguchi of the University of Tokyo for his critical reading of our manuscript.
REFERENCES 1. Sparks, D. L., “Environmental Soil Chemistry.” Academic Press, San Diego, 1995. 2. Yamaguchi, N., Hashitani, T., Okazaki, M., and Takeda, T., J. Colloid Interface Sci. 183, 280 (1996).
491
NOTE 3. Yamaguchi, N. U., Okazaki, M., and Hashitani, T., J. Colloid Interface Sci. 209, 386 (1999); doi: jcis.1998.5900. 4. Hashitani, T., “Annual Report of the Faculty of General Education,” Vol. 14, p. 84. Tokyo Univ. of Agriculture & Technology, Tokyo, 1978 [in Japanese]. 5. Parkman, R. H., Charnock, J. M., Bryan, N. D., Livens, F. R., and Vaughan, D. J., Am. Mineral. 84, 407 (1999). 6. Roe, A. L., Hayes, K. F., Chisholmbrause, C., Brown, G. E., Parks G. A., Hodgson K. O., and Leckie J. O., Langmuir 7, 367 (1991). 7. Okazaki, M., Takamido, K., and Yamane, I., Soil Sci. Plant Nutr. 32, 523 (1986). 8. Ueno, K., “Methods of Chelatometric Titration.” Nankodo, Tokyo, 1972 [in Japanese]. 9. Friedman, H. L., and Krishman, C. V., in “Water” (F. Franks, Ed.), p. 55. Plenum, New York, 1973. 10. Hashitani, T., and Tamamushi, R., in “Ions and Solvents” (The Chemical Society of Japan Eds.), Chap. 5, p. 83. Academic Press Center, Tokyo, 1976 [in Japanese].
Noriko U. Yamaguchi∗,1 Masanori Okazaki† ∗ Graduate School of Agriculture and Life Science The University of Tokyo 1-1-1, Yayoi, Bunkyo Tokyo, 113-8657 Japan †Graduate School of Bio-Applications and Systems Engineering Tokyo University of Agriculture and Technology Koganei, Tokyo, 184-8657 Japan
Received November 2, 2001; accepted January 31, 2002; published online April 10, 2002
1
To whom correspondence should be addressed. E-mail: [email protected].