海水に対する酸性赤色土の化学的インパクト Chemical impact of the acidic red soil on seawater

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Abstract

海域に流入する酸性赤色土が、海水に対して化学的にどんなインパクトを与えるかを研究した。特にpH, Na<sup>+</sup>, K<sup>+</sup>, Ca<sup>2+</sup>, Mg<sup>2+</sup>, Al<sup>3+</sup>, 溶存ケイ酸に注目した。海水のpHは赤土量と共に減少し、4.06まで下がった。Al濃度はpHの低下に伴って、通常海水の0.0256meqL<sup>-1</sup>から5.95meqL<sup>-1</sup>まで増加した。H<sup>+</sup>とAl濃度が増加するにつれて、K<sup>+</sup>, Ca<sup>2+</sup>, Mg<sup>2+</sup>は減少した。ただ、Na<sup>+</sup>はそれほど変化しなかった。増加したH<sup>+</sup>とAlの合計濃度と減少したNa<sup>+</sup>, K<sup>+</sup>, Ca<sup>2+</sup>, Mg<sup>2+</sup>の合計濃度の間には、強い相関があった。このことは、赤土と海水間でイオン交換反応が起っていることを示している。それからpH低下につれて、溶存ケイ酸濃度も増加した。このことは、赤土自身が自分から出ていくH<sup>+</sup>によって風化を進めていることを意味している。赤土の海域への流入は、物理的悪影響だけでなく、化学的にも海域生態系に悪い影響を与えているといえる。

Red soil contamination into marine ecosystems such as coral reefs and estuaries is a prevalent environmental problem in the Okinawa Islands. This study was conducted to examine the chemical impact of Okinawan red soil on seawater. The different soil weights were mixed and shaken with 100mL of seawater for four hours to obtain the extracts. The pH, concentrations of Na<sup>+</sup>, K<sup>+</sup>, Ca<sup>2+</sup>, Mg<sup>2+</sup>, Al<sup>3+</sup> and dissolved silica were then determined. The soil is acidic with a pH value of 5.03, dominated by SiO<sub>2</sub> and Al<sub>2</sub>O<sub>3</sub>. The pH of the Zampa cape seawater was 8.23 and decreased with soil weight to the lowest value of 4.06. The Al concentration in the seawater was 0.0256meqL<sup>-1</sup> and increased with soil to solution ratio to the maximum value of 5.95meqL<sup>-1</sup>. The base metal concentrations decreased from the seawater by 23.5%, 8.04%, and 3.59% in extracts for K<sup>+</sup>, Ca<sup>2+</sup> and Mg<sup>2+</sup>, respectively, while Na<sup>+</sup> was relatively stable to change. The dissolved SiO<sub>2</sub> increased from 0.430mgL<sup>-1</sup> in the seawater to the maximum value of 20.0mgL<sup>-1</sup> in ground soil. The Summation of decreasing K<sup>+</sup>, Ca<sup>2+</sup> and Mg<sup>2+</sup> concentrations and increasing Al<sup>3+</sup> and H<sup>+</sup> in the extracts showed a high correlation coefficient (P<0.001), which largely suggests an exchange process. The significant correlations between logarithmic concentrations of Al<sup>3+</sup> and silica vs. pH (P<0.001) also suggest the importance of H<sup>+</sup> to release soluble Al as well as hydrogen-promoted weathering of silicates. Physical features such as sedimentation and turbidity do affect coral reefs, but possibly chemical effects such as the elevated Al concentration observed in coastal seawaters may also interfere with the coral's well being at significant red soil-affected areas in the Okinawa Islands.

Journal

  • Journal of the Japanese Coral Reef Society

    Journal of the Japanese Coral Reef Society 8(1), 13-23, 2006

    The Japanese Coral Reef Society

Codes

  • NII Article ID (NAID)
    130003670762
  • Text Lang
    ENG
  • ISSN
    1345-1421
  • Data Source
    J-STAGE 
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