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（２）水銀の安定同位体分析による媒体間動態の検討
国立研究開発法人国立環境研究所
環境計測研究センター 柴田 康行
環境計測研究センター 基盤計測化学研究室 武内 章記
平成26～28年度累計予算額：30,745千円（うち平成28年度：10,055千円）
予算額は、間接経費を含む。
［要旨］
水銀の多媒体モデルの構築と、海洋生物に蓄積していることが知られているメチル水銀の発生 源を推定するためには、詳細な水銀の環境動態を把握する必要がある。特に、近年の分析技術の 向上により、水銀の安定同位体比を高精度に計測することが可能となり、天然の追跡指標と反応 機構解析指標として有用されている。そこで本研究では水銀の安定同位体分析技術の高度化から 開始して、遠洋と沿岸魚類に蓄積している水銀、特にメチル水銀の発生源推定および反応機構の 解析を行った。外洋の表層回遊魚の筋肉中の202Hgは約0.2から1.3‰で、199Hgは約2.0から3.0‰で あった。また外洋の中深層海水回遊魚の筋肉中の202Hgは約-0.1から1.3‰で、199Hgは約1.7から
2.1‰の範囲であった。202Hg変動は微生物による脱メチル化反応と、紫外線や波長の短い可視光
による脱メチル化反応の痕跡を示し、さらに無機水銀の起源の差を示した。得られた199Hgの変動 幅は光による脱メチル化反応の影響を受けた水銀しか示さないために、光脱メチル化反応の痕跡 を示した。遠洋域では、得られた水銀同位体比の鉛直構造から、中深海水層でのメチル水銀の生 成を示す結果となった。その一方、沿岸域に生息する魚類の水銀同位体比は、水深30メートル以 下の水俣湾で、202Hgが約-0.75から0.4‰、199Hgが約0.0から0.6‰、そして水深200メートル以下の 玄界灘で、202Hgが約0.25から1.3‰、199Hgが約0.8から1.6‰であった。これらの地域では同魚種 でも水銀同位体比が異なることから、それぞれの環境で蓄積している水銀の起源、または生物移 行における反応機構が異なることを示唆している。最後に遠洋および沿岸域の底質中の水銀同位 体比は、202Hgが約-0.4から-1.3‰、199Hgが約0から0.1‰であった。上記のことから、生物に蓄積 している水銀は、生息環境毎に同位体比が異なり、沿岸と遠洋では水銀の起源が異なることに加 えて、生物移行プロセスにおける反応機構が異なる事を示す。
［キーワード］
水銀同位体、分析技術、多重検出器型誘導結合プラズマ質量分析装置、起源、反応機構解析
１．はじめに
近年、水銀は地球環境汚染物質として対策および研究が進められている1)。水銀は環境中で複数 の物理的な状態と化学形態で存在しており、自然界でおこる酸化還元反応によって循環している2)。 水銀は一般的に有害金属として知られているが、特に有害なメチル水銀はその循環の過程で、微 生物による生物地球化学的なプロセスの影響を受けて生成される2)。そのために水銀の全球多媒体 モデルを構築するためには、自然界でおこる様々なプロセスを把握する必要がある。