No abstract

Rapid collection and removal of gram-negative heterotrophic bacterial strain, Escherichia coli, was investigated by utilizing the interfacial interaction between cell and solid substratum. Both E.coli collection from stable suspension and its successive removal were performed by a column bed packed with fibrous ferro-nickel slag, which was employed as the collector media of bacterial cells. In the cell collection tests, little cell recovery was obtained at neutral pH condition where both E.coli and FS had negative surface potential. On the other hand, E.coli collection gradually increased as the pH decreased. These findings were explained by the suppression of electrostatic repulsive interaction between E.coli and FS led to an improvement in cell attachment to FS surface. Cell collection capacity Γ max exponentially increased as the interfacial interaction energy minimum V min became lower, indicating that surface characteristics played crucial roles in cell attachment mechanism. Moreover, part of the E.coli cells that had adhered to FS surface were effectively removed when the eluting solutions were alkaline conditions; hence reversible cell detachment from FS was possible by controlling the electro-repulsive force. Cyclic E.coli collection/removal tests demonstrated that cell collection and successive removal were repeatedly carried out al least six times, although about 40 mg of E.coli cell made a firm and irreversible attachment on FS. E.coli collection/removal behaviors in the present experiments were generally in good agreement with electrokinetic properties of cell and FS, suggesting that cell recovery based on surface characteristics is a promising method, especially for stable bacterial suspension.

We have developed a non-water gel comprised of polyethylene glycol (PEG) and hydroxypropylcellulose (HPC) as a gelling agent. We investigated the gelation mechanism of the HPC/PEG system on the basis of a stress-controlled rheometer and an X-ray diffractometer. From these results, we found that two gelation mechanisms coexisted in the HPC/PEG system; the entanglement of interacting HPC polymer chains which behave like a flexible polymer and the cross-linking of the microcrystalline domains of the HPC main chains. In the lower HPC concentration range, the gelation is dominated by the cross-linking of the microcrystalline domains of the HPC main chains. In the higher HPC concentration range, on the other hand, the gelation is dominated by the entanglement of interacting HPC polymer chains.

1．緒 言 フッ化水素 (HF) はフルオロカーボン製造，アルミ工業，ガラ ス工業および無機化製品などに使用されており，我が国の需要は 年間約 15 万トンにのぼる 1) 。フッ素の過剰摂取は骨硬化症，脂 質代謝障害，糖質代謝障害と関連があるとされており 2) ，環境 への排出が厳しく規制されている。産業排水としてのフッ素含有 水は，半導体や液晶のエッチング液およびその洗浄水，ステンレ スや特殊鋼の酸洗水，フロン破壊工程からのガス吸収水などがあ る。また，温泉や坑廃水など地質に由来するフッ素汚染水も存在 する。水質汚濁に関するフッ素の環境規制としては，環境基準と して 0.8 mg/L 以下 3) ，水質汚濁防止法の一律排水基準値として 8 mg/L ( 海域 15 mg/L) が設定されており，一部の業種については 暫定基準が適用されている 4) 。自治体によっては上乗せ基準値 を設けているところがあり，対象となる事業所ではより厳しい濃 度規制を遵守する必要がある。 最も一般的なフッ素排水の処理法は， 水中のフッ化物イオン (F -) を難溶性のフッ化カルシウム (CaF 2 ) として不溶化させて分離する CaF 2 生成法である 5) 。CaF 2 生成法は，炭酸カルシウム (CaCO 3 ) や消石灰 (Ca(OH) 2 ) 等を添加して CaF 2 を生成する凝集沈殿法を 基本とし，粒状炭酸カルシウムによる回収 6) ，生成スラリーを反 応槽に循環する応用型がある 7-9) 。 反応晶析による CaF 2 生成では， フッ酸原料として活用できる高純度な CaF 2 汚泥が回収される 10) 。 CaF 2 生成法では，その溶解度の関係から，処理水に 10 ～ 20 mg/ L 程度のフッ素が残留することが多く，排水基準 (8 mg/L) を満た すためには高度処理が必要となる。共沈法は，水酸化アルミニウ ム等の金属水酸化物の表面にフッ素を吸着して除去する方法であ り，安価にフッ素濃度を低減することができる 11, 12) 。しかし， かさ高く脱水性に乏しい汚泥が大量に生成するため，汚泥処理に 負担がかかる。選択的かつ低濃度までフッ素を除去することができ る吸着剤が市販されており，ヒロドキシアパタイト系 13) ，酸化セ リウム系 14) ，ジルコニウムフェライト系 15) ，キレート樹脂 16, 17) などがあるが，ランニングコストが高いため導入できる例は限ら れる。排水基準，さらに環境基準といった低濃度までフッ素を低 減するためには，より簡便で安価な処理技術が求められる。 フッ素等の陰イオン成分を吸着する粘土鉱物として層状複水Removal of fluorine in effluent with low concentration is difficult technique and an effective method has not been established. This paper describes a novel fluorine-removal process that composed the following processes, addition of Al(III) source to feed water, conditioning of the slurry with magnesium oxide (MgO), neutralizing reaction, returning the settled slurry, and gravitational settling of the solids. This process allows the formation of a mixed Mg(II)-Al(III) hydroxide precipitate, which gradually develops into Layered Double Hydroxides (LDH) compound. MgO, which serves as both the neutralizer and the Mg(II) source, contributes to both the excellent settling and dewatering characteristics of the resulting solids. The sludge returning technique, well-known as High Density Sludge (HDS) method, is employed to prolong the sludge retention time to dissolve as much MgO as possible. Repeated batch-wise fluorine-removal tests showed that the residual fluorine concentration gradually decreased according as the number of batch test increased. Optimum fluorine removal ratio reached 96 %, where initial fluorine concentration 20 mg/L decreased below the national environmental standard, 0.8 mg/L. Powder X-ray diffraction analysis indicated the presence of both LDH phase and Periclase phase in the formed solids. Considering the good settling and dewatering characteristics of solids, the proposed process served as alternative treatment technique for low concentrated fluorine-containing effluents.