Patent Number: 045377103
Section: description

EXAMPLES 1-8 A number of tobermorites were synthesized from different starting materials at 80.degree. C. to 200.degree. C. Zeolites, such as clinoptilolite, phillipsite and Linde 3A were used as some of the silica-alumina sources. Other materials such as amorphous SiO.sub.2 (-400 mesh); Na.sub.2 SiO.sub.3.9H.sub.2 O; AlCl.sub.3.6H.sub.2 O; and NaOH were also used. Teflon capsules were used for synthesis at 85.degree. C. and 180.degree. C. at saturated steam pressure in a pot furnace; polyethylene bottles were used for synthesis at 80.degree. C. in an oven and gold capsules were used at 180.degree. C. and 200.degree. C. in cold-seal vessels under a confining pressure of 30 MPa. The solids were removed from the capsules or bottles and washed with deionized water prior to cation exchange and cesium sorption measurements. The cation exchange capacity (CEC) of each sample was measured by washing a known weight (10 to 50 mg) twice with 3N KCl, then washing three times with 0.01N KCl to remove excess KCl and to prevent hydrolysis of K.sup.+ from the exchange sites (a correction was made for excess 0.01N KCl, which was determined by weighing); the K.sup.+ was displaced from the exchange sites with four washings of 1N CsCl (1/2 hour equilibration for each). K.sup.+ was determined by atomic emision spectroscopy. When NH.sub.4 Cl was used to displace K.sup.+ from the exchange sites, lower CEC's were obtained probably because of H.sup.+ bonding and poor diffusion of NH.sub.4.sup.+ into the structure. Selective cesium sorption by the various tobermorites was determined by adding 10 ml of 0.02N CaCl.sub.2 or 0.02N NaCl containing 0.0002N CsCl (Ca.sup.2+ /Cs.sup.+ or Na.sup.+ /Cs.sup.+ equivalent ratio is 100 to 1) to 20 mg of sample, equilibrating for 1 day in glass vials, centrifuging to separate solid and solution phases, and analyzing Cs.sup.+ in solution by atomic absorption spectrophotometry. Cesium sorption as expressed K.sub.d (in milliliters per gram) values (K.sub.d is a distribution coefficient defined as the ratio of the amount of cesium sorbed per gram of solid to the amount of cesium remaining per milliliter of solution). The results are given in Table 1 below: TABLE I __________________________________________________________________________ Temperature Cation Cesium sorption K.sub.d (ml/g) (.degree.C.) pressure: exchange from solution Sample duration (days) capacity 0.02N 0.02N No. Initial sample mixture of treatment (meq/100 g) CaCl.sub.2 NaCl __________________________________________________________________________ 1 420 mg Na.sub.2 SiO.sub.3.9H.sub.2 O + 39.6 mg 80.degree. C.; SS; (approx. 182 4,144 .+-. 626 2,683 .+-. 1 AlCl.sub.3.6H.sub.2 O + 76.8 mg CaO 1 MPa); 1 2 420 mg Na.sub.2 SiO.sub.3.9H.sub.2 O + 39.6 mg 85.degree. C.; SS; 4 164 756 .+-. 7 654 .+-. 9 AlCl.sub.3.6H.sub.2 O + 76.8 mg CaO 3 210 mg Na.sub.2 SiO.sub.3.9H.sub.2 O + 5.3 mg 180.degree. C.; 30 MPa; 14 146 14,240 kaolinite + 38.4 mg CaO 4 45.9 mg phillipsite + 16 mg amor. 200.degree. C.; 30 MPa; 28 148 20,370 .+-. 1,985 8,930 SiO.sub.2 + 38.4 mg CaO 5 22.2 mg Linde 3A + 34.8 mg amor. 200.degree. C.; 30 MPa; 28 158 6,328 .+-. 258 3,670 SiO.sub.2 + 38.4 mg CaO 6 50.7 mg clinoptilolite, Idaho, + 200.degree. C.; 30 MPa; 28 80 9,150 4,412 .+-. 157 10.4 mg amor. SiO.sub.2 + 38.4 mg CaO 7 2.22 g Linde 3A + 3.48 g amor. 80.degree. C.; SS; 22 144 461 .+-. 15 484 .+-. 18 SiO.sub.2 + 3.84 g CaO + 24 g NaOH 8 5.04 g CaO + 5.4 g amor. 80.degree. C.; SS; 12 130 42 51 .+-. 2 SiO.sub.2 + 32 g NaOH __________________________________________________________________________ *Plus or minus denotes standard deviation (N weighting) based on three or four replicates. Average of duplicate determinations. Abbreviations: SS, saturated steam; amor., amorphous. As can be seen from the results in Table 1, all of the tobermorite samples exhibit ion-exchange properties. However, the tobermorite not substituted by aluminum of sample 8 has little or no selectivity for the large and less hydrated Cs.sup.+ from 0.02N CaCl.sub.2 or 0.02N NaCl. The aluminum-substituted tobermorites exhibit selectivity for Cs.sup.+, i.e. high cesium sorption K.sub.d from 0.02N CaCl.sub.2 or 0.02N NaCl. Sample 1 has a CEC of 182/meq 100 g and sorbed 9 meq of cesium per 100 grams from 0.02N CaCl.sub.2 (the equivalent ratio of CEC to cesium sorbed is 20 to 1) even though the Ca.sup.2+ /Cs.sup.+ ratio in solution is 100 to 1. Thus the cesium sorption is in excess of what can be expected from the total exchange capacity of the Al substituted tobermorite and the Ca.sup.2+ /Cs.sup.+ equivalent ratio in the equilibrating solution. The high selectivity for Cs.sup.+ arises from the amount of aluminum substitution for silicon and possibly from resulting changes in the structure. Aluminum-substituted tobermorites synthesized at higher temperatures and pressure have greater selectivity for cesium than those synthesized at lower temperatures and saturated steam pressure. This difference is attributed to the greater substitution of aluminum for silicon along with sodium or potassium in the structure synthesized at higher temperatures and pressures. As can be seen, the aluminum-substituted tobermorites exhibit a high selectivity for cesium and other ions. The similarity of the tobermorite to the cementitious matrix in concrete will provide a new material for the long-term storage of cesium and other radioactive ions.