2012 Volume 50 Issue 3 Pages 154-161
Zeolite A with Mg-Al layered double hydroxide (LDH) on its surface is prepared by a precipitation of LDH on zeolite A to be used for simultaneous removal of nutrients (ammonium ion and phosphate ion) from rivers and lakes. The zeolite A/LDH composite containing 35 wt% LDH showed ammonium adsorption up to 51 % in 5 mM ammonium chloride solution at pH 5.6 and phosphate adsorption up to 52 % in 0.5 mM sodium phosphate solutions at pH 7.0. Zeolite A covered with a hydroxyapatite on its surface is prepared simply by cation exchange of calcium loaded zeolite A with ammonium phosphate solution. This reaction forms a homogeneous layer on the zeolite A surface, and the resultant hydroxyapatite is a tightly bound to the zeolite A surface. This can be used for immobilization of radioactive strontium ion. The zeolite/hydroxyapatite composite showed high strontium adsorption ability. The maximum capacity was determined to be 0.40 mmol・g^<-1>, which was comparable to the value for zeolite A alone; this was because of the loose arrangement of hydroxyapatite nanocrystals on the surfaces. The composites obtained were then heated at 800℃ to 1,200℃ to improve the solution stability of adsorbed strontium ions. The elution of strontium ions in sintered composites was compared with those of sintered zeolite A in an alkaline solution (pH 10.2). Stability of strontium ions in sintered composites for solutions was improved with sintering temperatures over 950℃ as compared with those of sintered zeolite A because zeolite A, which forms feldspar phase after the sintering, is completely covered with hydroxyapatite crystals, which forms tricalcium phosphate after the sintering. Thus, we obtained a material which forms a barrier for elution and can be transformed into a stable material for storage of strontium. Zeolite composites with LDH and hydroxyapatite showed superior performance than zeolite alone in environmental purification applications.
