Environ. Sci. Technol. 1982, 16, 617-624
Divalent and Trivalent Ion Exchange with Zeolite A Brandon H. Wlers, Robert J. Grosse, and William A. Cllley*t The Procter & Gamble Company, Miami Valley Laboratorles, Cincinnati, Ohio ~~
The ion-exchange isotherm for Ca2+ exchange with sodium zeolite A was determined at 25 OC by using a novel solid uptake analytical scheme and a Langmuir adsorption model, resulting in an improved value for the thermodynamic equilibrium constant. Ion-exchange isotherms and approximate thermodynamic equilibrium constants were also determined for calcium zeolite A exchange with Pb2+, Cd2+,and Cu2+. Pb2+and Cd2+exchanges were found to be reversible, whereas Cu2+exchange was not, probably due to Cu(OH),(s) precipitation upon or within the zeolite. Ion exchange was attempted but found to be irreversible with C$+, Hg2+,Fe3+,and A13+. Except for Cr3+,these ions caused zeolite A structure degradation, probably through extensive proton exchange. Cr3+ exchanged partially and then formed a precipitate. Data obtained in this work provide an improved understanding of the possible environmental water effects of zeolite A. Introduction Isotherms for divalent ion exchange with sodium zeolite A (NaA) were first reported by Breck et al. (I). Isotherms plus thermodynamic equilibrium constants have subsequently been reported by Barrer et al. (2, 3), Ames ( 4 ) , Sherry et al. (5, 6 ) , Gal et al. (7-9), and Rees ( I O ) . Concurrently, there have been numerous less quantitative studies reported by Barrer and Meier (II), Wolf et al. (12-14), Zhdanov et al. (15),Dubinin et al. (16),and Seff et al. (17-24). The divalent ions that have been observed to displace Nat in zeolite A to date are Mg2+,Ca2+,Sr2+,Ba2+,Mn2+, Fez+,Co2+,N P , Cu2+,Zn2+,Cd2+,Hg2+,Pb2+,and Eu2+. Of this group, Ni2+,Fez+,Hg2+,and Cu2+have been observed to cause partial or total loss of zeolite A crystal structure, either during the course of the exchange reaction or after. This is in contrast to the behavior of natural aluminosilicates such as clays and natural zeolites, which retain structural integrity in the presence of such cations. Divalent-divalent ion exchanges have been studied much less thoroughly. A study by Ames (4) of the Sr2+ exchange with CaA and a study by Lomic and Gal (9) of the Cd2+and Zn2+exchanges with ZnA and CdA, respectively, are the only ones in which both isotherms and equilibrium constants were determined. Schwuger et al. (25) studied exchanges of Cu2+,Zn2+,Co2+,Ni2+, Cd2+, Pb2+,and Mn2+with a mixed Ca2+/Na+zeolite A but did no thermodynamic analysis. Trivalent ion exchange with sodium zeolite A is reported to have been unsuccessful with Ce3+(I), destructive with Fe3+ ( I ) , and partially successful with Cr3+(26)and Ti3+ (27). There have been no reported isotherms or equilibrium constant determinations for these exchanges or any reports of trivalent-divalent exchanges. The present work was undertaken in order to resolve discrepancies found in calcium ion exchange thermodynamic data, to obtain new data for di- and trivalent ionexchange reactions, and to provide a firmer basis for future studies of the possible effects of zeolite A in environmental water systems. This is of interest due to the introduction Author to whom inquiries should be directed at Ivorydale Technical Center, The Procter & Gamble Co., Cincinnati, OH 45217. f
0013-936X/82/0916-0617$01.25/0
of sodium zeolite A into detergent products for purposes of water hardness control. Ions selected for investigation in this work were Pb2+, Cd2+,Hg2+,Cu2+,A13+, Fe3+,and Cr3+. The selection of these ions was based on their occurrence in natural surface waters and regulatory concerns therewith. Little or no data existed in the ion-exchange literature for them when this study was initiated. Experimental Section The zeolite raw material used in this work was a commercial sample of sodium zeolite A, unit cell formula Na12(A102)12(Si02)12~27Hz0, obtained from J. M. Huber Corp.: analytical formula in hydrated state, Nall.9(A102)12.8(Si02)11.,-27H20; mass-median equivalent spherical diameter, 2.85 pm; external specific surface of the cubic crystals, 1.9 m2/g; ion-exchangecapacity (anhydrous basis), 6.90 mequiv/g (7.04 mequiv/g theoretical); electrokinetic ({) potential in 0.001 M NaCl at 25 OC, -62 mV; electrokinetic (4.5. Therefore, a 0.1 M ferric nitrate solution in methanol was used to make Fe3+ ions available for exchange. Sodium zeolite A was stirred in this solution for 1day and washed repeatedly with methanol and finally with water. Analysis for iron showed 15% exchange had taken place. However, X-ray diffraction analysis showed no type A crystallinity, in agreement with previous aqueous solution results ( I ) , and analysis for sodium showed only 10% of the original amount remaining. Exchange with protons released from waters of hydration or from MeOH2+ (29) probably accounts for the sodium loss. The crystal degradation results from -Si-0-AI- bond breaking, which occurs for proton exchange greater than 33% (28). Similar hydrolysis-induced crystal degradation is postulated for Fe2+( 2 0 ~and ) Ti3+ (27) interactions with zeolite A. Hydrous oxide precipitation problems also led to using a 0.1 M aluminum nitrate solution in methanol to effect AP+ exchange with sodium zeolite A. After an exposure of 1 day to this solution, the zeolite was washed with methanol and then water and analyzed for sodium. Results showed only 12% of the sodium remaining in the sample. X-ray diffraction showed loss of all crystallinity. Again, this is probably due to proton exchange from MeOH2+and hydrolyzed hydrate water, followed by -Si-0-A1- bondbreaking. NaA was stirred for 10 min in a 0.01 M aqueous solution of CrC1,.6H20. A short exposure time was used because the pH of this solution was 4.50. The resulting zeolite showed about 30% exchange of Cr3+ for Na+ and also exhibited both type A crystallinity by X-ray diffraction 618
Envlron. Sci. Technol., Vol. 16, No. 9, 1982
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Cas Flgure 1. Ca2+-NaA exchange isotherm (filled circles) for 25 OC, N , = 0.1. The solM curve (SW) is from Sherry and Walon (5)the dashed curve (BRW) from Barrer, Rees, and Ward (3) and the dotted curve (DW) from Danes and Wolf (13). (Ca,) and (Ca,) denote equivalent fractions of Ca2+ in the zeolite and solution phase, respectively.
and a type A infrared spectrum. The 30% exchange surpassed the previously reported (26) 12% and approached the theoretical limit (33%) €or complete site I1 (octagonal pore) occupancy. (c) Procedures for Isotherms. Isotherm data were obtained by acid (1.0 N HC1) dissolution of solids followed by atomic absorption analysis for both ions involved in the exchange, plus aluminum. All three cations were determined for the dissolved solids, rather than merely the final solution concentration of the exchanging cation. The latter method, which may be referred to as a “solution depletion” method, is the method used in the majority of previous studies. The present method, referred to as “solid uptake”, provides improved accuracy as well as otherwise unavailable information about effects such as precipitation within the zeolite and cation hydrolysis induced zeolite dissolution. The isotherms for Pb2+,Cd2+,and Cu2+exchange with CaA employed 0.01 g of CaA and 0.10 isonormal solutions of M(N03), in which the equivalent fraction of each cation varied from zero to unity. The metal-exchanged zeolites prepared separately were used to determine reversibility of the exchange reactions. No attempt was made to obtain isotherms for trivalent ion exchanges because of the crystal degradation or incomplete exchange phenomena described previously.
Results Calcium-sodium exchange results are plotted in Figure 1 and summarized in Table I. Figure 1 shows that the present data, represented by filled circles, agree very well with the solution depletion results of Sherry and Walton ( 5 ) , represented by the solid curve. The good agreement probably arises from the fact that the Sherry and Walton material was a Linde 4-A lot that had never been calcined and was known to have a lower than customary level of excess aluminum. The zeolite A material used in this work had these features in common with the Sherry and Walton material. Though not shown in Figure 1, recent results by Rees (IO)also agree with present data and are based on a similarly prepared zeolite A material. The dashed and dotted curves in Figure 1 are reproductions of isotherms previously published by Barrer, Rees, and Ward (3) and by Danes and Wolf (13),respectively. Both of these were determined by solution depletion procedures, and both differ significantly from our data and the Sherry and Walton isotherm. An earlier isotherm reported by Wolf and Furtig (12) is even lower than that
Table I. Experimental Results for CaZ+-NaExchange initl soln concn, mN sample
NaCl
CaCl,
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
100 99 98 96 94 92 90 85 80 75 70 60 50 40 30 20
. o 1 2 4 6 8 10 15 20 25 30 40 50 60 70 80
equilibrium composition of solids, mN totala Caz+ Na+
2.12 1.392 1.164 1.196 0.821 0.674 0.647 0.587 0.522 0.506 0.462 0.3 53 0.272 0.299 0.250 0.223
