Malmstrom(oの頭に¨) & Banwart(1997)による〔『Biotite dissolution at 25゜C: The pH dependence of dissolution rate and stoichiometry』(2779p)から〕


【フロー型反応器(thin-film flow-through reactor)による黒雲母の溶解実験】
【溶解速度式:R = kH [H+]m + ko + kOH[H+]n (moles m-2 h-1)】

 The rate and stoichiometry of biotite dissolution were studied in the pH range 2-10 using thin-film continuous flow reactors. The release of interlayer K is relatively fast and becomes diffusion-controlled within a few days. The release rates of framework ions (Mg, Al, Fe, Si) are much slower and reach an apparent steady-state within ten days. The stoichiometry and rate of dissolution vary greatly with pH. Consistent with surface reaction control of release rates, an empirical rate law, R = kH [H+]m + ko + kOH[H+]n (moles m-2 h-1) describes proton- and hydroxyl-catalysed dissolution for each ion.
  Si Fe Mg Al
log kH -4.45 -5.10 -4.93 -5.31
m 0.91 0.51 0.57 0.40
log ko -7.31 - - -
log kOH -11.29 -15.36 -10.57 -12.58
n -0.48 -0.81 -0.29 -0.54

 Rapid K+ release provides a tracer for the extent of the hydrated reacting layer on the biotite surface and within interlayers. An altered reaction layer composition, calculated from mass balances for released ions, results from preferential leaching of some ions and is consistent with that of vermiculite. X-Ray powder diffractometry confirmed the formation of both vermiculite and kaolinite during the weathering reaction. The pH dependence of release rates, normalised to the corresponding ion concentrations in the reacting layer, correlate with those for the respective binary oxides (SiO2, Al2O3, Fe2O3, MgO).
 Release rates for Al, Mg, and Fe at neutral pH are much slower when the mineral has been previously reacted at low pH where these ions are released rapidly. Model simulations suggest that, for ions that initially dissolved rapidly, release rates will decrease as the ion is depleted in the reacting layer. Rates will eventually approach those of the most slowly dissolving ion. At 25゜C and pH 7, this process would lead to stoichiometric dissolution within 50 y.』

 黒雲母の溶解の速度と化学量論性についての研究を、薄膜連続式フロー型反応器を使ってpH2〜pH10の範囲で行った。層間のKの放出は比較的速く、数日で拡散律速になる。フレームワーク・イオン(Mg,Al,Fe,Si)の放出速度はかなり遅く、10日で見かけの定常状態に達する。溶解の化学量論性と速度は、pHによって大きく変動する。放出速度の表面反応律速と矛盾なく、経験的な速度則、R = kH [H+]m + ko + kOH[H+]n (moles/m2/時間) は各イオンに対するプロトンおよび水酸基により触媒された溶解を記述できる。
  Si Fe Mg Al
log kH -4.45 -5.10 -4.93 -5.31
m 0.91 0.51 0.57 0.40
log ko -7.31 - - -
log kOH -11.29 -15.36 -10.57 -12.58
n -0.48 -0.81 -0.29 -0.54
 K+ 放出が急速であることから、黒雲母表面および層間の水和した反応層の広がりを追える。放出されたイオンについての質量平衡から計算された、変質した反応層の組成は、いくつかのイオンの優先的な溶脱で形成され、バーミキュライトの組成に一致する。X線粉末回折により、風化反応の間にバーミキュライトとカオリナイトが形成されたことを確認した。放出速度のpH依存性は、反応層中の相当するイオン濃度にノーマライズすると、それぞれの2成分酸化物(SiO2, Al2O3, Fe2O3, MgO)に対するものと相関する。

1. Introduction
 1.1. Background
 1.2. Biotite structure and composition
 1.3. The weathering of biotite: mechanism and kinetic studies
 1.4. Objectives
2. Experimental methods and specimen
 2.1. Mineral sample
 2.2. Experiments
  2.2.1. Chemicals
  2.2.2. Experimental conditions
  2.2.3. Experimental procedure - the thin-film flow-through reactor
 2.3. Calculation of dissolution rates and estimates of experimental uncertainties and reproducibility
 2.4. Solid phase analysis
3. Results and discussion
 3.1. Dissolution behaviour in the thin-film flow-through reactor
  3.1.1. Framework ions
  3.1.2. Potassium
 3.2. Alteration of the biotite - solid phase analyses
  3.2.1. Observations using XPD
  3.2.2. Observations using SEM and FTIR
  3.2.3. Observations using Mossbauer(oの頭に¨) analysis
  3.2.4. Observations on the specific surface area
4. Modelling
 4.1. A kinetic model for the dissolution of biotite
  4.1.1. Modelling the pH dependent rate of multisite dissolution
  4.1.2. Ion-exchange of interlayer potassium
  4.1.3. Progression to congruent dissolution - an extended conceptual model
 4.2. Vermiculite stoichiometry
5. Conclusions and implications for groundwaters