Bulletin of the Society of Sea Water Science, Japan

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In order to build up a utilization system of seawater resources based on the desalination and salt production process and to prevent scaling in reverse osmosis（RO）and electrodialysis（ED）units, a recovery and upgrading method for calcium（Ca）and magnesium（Mg）from brine solution was studied. From the viewpoint of solubility of salts, the synthesis of calcium carbonate（CaCO₃）by reactive crystallization between the dissolved Ca^2＋ in brine solution and carbon dioxide（CO₂）can be considered as an effective separation/recovery method. The obtained CaCO₃ is able to upgrade to hydroxyl apatite（HAP）with treatment of phosphoric acid and hydroxide. Moreover, the remaining Mg^2＋ can be recovered from the Ca^2＋ removed brine by reactive crystallization of magnesium carbonate（MgCO₃）. From the results obtained in this study, at a solution pH of 8.0 and a solution temperature of 313 K, the crystallization phenomena of MgCO₃ caused by CO₂ bubble injection and minimizing bubble size can be summarized as follows: 1）when Na₂CO₃ solution was continuously supplied to the Ca^2＋ removed brine, only basic magnesium carbonate（4MgCO₃・Mg（OH）₂・4H₂O）with an average size of about 25 µm was produced; 2）in the case where CO₂ bubbles with an average diameter of 2000 µm were injected, magnesium carbonate tri-hydrate（MgCO₃・3H₂O）was selectively crystallized within 30 min, and 4MgCO₃・Mg（OH）₂・4H₂O was obtained at 60 min; 3）by minimizing bubble diameter to 20 µm, the produced moles of MgCO₃・3H₂O and 4MgCO₃・Mg（OH）₂・4H₂O increased and the obtained particles were downsized to around 4.5 µm. Furthermore, the effects of solution pH and solution temperature on the reactive crystallization of MgCO₃ were investigated under CO₂ minute-bubble injection. Consequently, 5）at a solution pH of 7.0, the induction period for nucleation increased remarkably and only MgCO₃・3H₂O was produced, 6）when the solution temperature was increased to 333 K, 4MgCO₃・Mg（OH）₂・4H₂O with high-selectivity was crystallized irrespective of crystallization time.

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Sodium titanate was impregnated onto a commercially available 6-nylon fiber by means of radiation-induced graft polymerization of dimethyaminopropyl acrylamide（DMAPAA）and subsequent chemical modifications. A peroxo complex of titanium anions was bound onto the DMAPAA-grafted fiber before the bound titanium species was converted to sodium titanate through precipitation with sodium hydroxide. Impregnation percentage of sodium titanate of the fiber was constant at 20 ％ in the range of sodium hydroxide concentration in a mixture of methanol and water at a volume fraction of methanol of 80 ％ of 0.001 to 1 M, whereas the removal percentage of strontium from seawater leveled off at 80 ％ above a sodium hydroxide concentration in water of 0.1 M. Determination of adsorption isotherms in seawater demonstrates that the sodium-titanate-impregnated fiber with an impregnation percentage of 10 ％ exhibited 2.6-fold higher amount of strontium adsorbed in seawater per g of sodium titanate（8.8 mg-Sr/g）than a commercially available granular adsorbent for strontium, SrTreat^®（3.4 mg-Sr/g）.

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