Japan Geoscience Union Meeting 2016

Presentation information


Symbol B (Biogeosciences) » B-CG Complex & General

[B-CG09] Interrelation between Life, Water, Mineral, and Atmosphere

Mon. May 23, 2016 5:15 PM - 6:30 PM Poster Hall (International Exhibition Hall HALL6)

Convener:*Kentaro Nakamura(Department of Systems Innovation, School of Engineering, University of Tokyo), Tsubasa Otake(Division of Sustainable Resources Engineering, Faculty of Engineering, Hokkaido University), Yohey Suzuki(Graduate School of Science, The University of Tokyo), Ken Takai(Extremobiosphere Research Center, Japan Agency for Marine-Earth Science & Technology), Yuichiro Ueno(Department of Earth and Planetary Sciences, Tokyo Institute of Technology), Takeshi Naganuma(Graduate School of Biosphere Science), Takeshi Kakegawa(Graduate School of Science, Tohoku University), Tadashi Yokoyama(Department of Earth and Space Science, Graduate School of Science, Osaka University), Fumito Shiraishi(Department of Earth and Planetary Systems Science, Graduate School of Science, Hiroshima University)

5:15 PM - 6:30 PM

[BCG09-P05] Reaction rate of ferric hydroxide formation at pH 2–4

Sagakuni Nagasaki1, *Tadashi Yokoyama1 (1.Department of Earth and Space Science, Graduate School of Science, Osaka University)

Keywords:Ferric hydroxide, Reaction rate

Iron hydroxides widely exist in the Earth’s environment, and their formation processes, adsorption properties, and transport behavior have been attracting great attention. Dissolved Fe3+ reacts with water and transforms to ferric hydroxide (Fe(OH)3) as time passes. We focused on the early process of ferric hydroxides formation and evaluated the reaction rate constants and activation energies under a wide range of solution conditions (pH 2–4, initial Fe concentration 5–300 ppm, temperature 5–55 °C, and dissolved anion species Cl, NO3, SO42−). Aqueous solutions containing ferric ions were prepared by dissolving one of the following FeCl3, Fe(NO3)3·9H2O, and Fe2(SO4)3·nH2O in pure water. As dissolved ferric ions change to ferric hydroxide, the pH of the solution gradually decreases. The time variation of pH was monitored under the constant temperature, and the rate constant k was determined by converting the pH change to the change in the concentration of dissolved ferric species (assumed to be a 1st order reaction as with Grundl and Delwiche, 1993) using a geochemical code PHREEQC (Parkhurst and Appelo, 1999). Comparison of the k values of ferric hydroxide formation at pH 2–3 under the presence of the different anion species revealed that the k values for Cl and NO3 were almost the same and that for SO42− was approximately one half to one fourth of the values for Cl and NO3. Despite the dependence of the rate constants on the anion species, activation energies were almost the same between Cl, NO3, and SO42− (~120 kJ/mol). In the experiment using FeCl3 solution, the k value increased as the initial pH increased from 2 to 4. On the basis of the pH dependence of k at 25 °C, an equation to predict k for various pH was obtained.