Drever,J.I. and Stillings,L.L.(1997): The role of organic acids in mineral weathering. Colloids and Surfaces A: Physicochemical and Engineering Aspects, 120, 167-181.


 Organic acids and their anions (for brevity we shall use the term “acids” to include both) may affect mineral weathering rates by at least three mechanisms: by changing the dissolution rate far from equilibrium through decreasing solution pH or forming complexes with cations at the mineral surface; by affecting the saturation state of the solution with respect to the mineral; and by affecting the speciation in solution of ions such as Al3+ that themselves affect mineral dissolution rate. In this paper we review the effects of organic acids on the dissolution rates of silicate minerals, particularly feldspars, under conditions approximating the natural weathering environment - 25℃, pH 4-7 - and with concentrations of organic acids comparable to those measured in soil solutions.
 Feldspar dissolution rates far from equilibrium increase with decreasing pH below pH 4-5. They appear to be independent of pH between pH 4-5 and about 8, and above pH 8 feldspar dissolution rates increase with increasing pH.
 Small chelating ligands such as oxalate appear to accelerate feldspar dissolution through complexation of Al at the surface of the mineral. Feldspar dissolution rates in the presence of 1 mM oxalic acid show effects ranging from no enhancement to enhancements of a factor of 15, depending upon the data set, pH, and aluminum content of the mineral: there is a great deal of scatter in the available data. In general, concentrations of oxalate of the order of 1 mM are necessary to cause a significant effect. Humic acids do not appear to increase feldspar dissolution rates significantly.
 Dissolution rates must decrease as the solution approaches saturation with respect to the primary phase (the chemical affinity effect). Organic acids will influence chemical affinity by complexing Al (and possibly other elements) in solution and hence decreasing the chemical activity of Al3+. There are essentially no data on the effect of chemical affinity on feldspar dissolution rate at 25℃ and mildly acid pH, so it is hard to evaluate the importance of organic acids in accelerating silicate dissolution through the chemical affinity effect. The effect of complexation of dissolved Al does not appear to be an important determinant of silicate dissolution rates in nature.
 Observed rates of silicate weathering in the field are typically much slower than predicted from laboratory experiments far from equilibrium, suggesting control by transport of solutes between “micropores” and “macropores” (“micropores” include fractures and crystal defects within grains). If such transport is rate-controlling, analysis of the effect of organic acids on weathering rates in nature in terms of dissolution rates far from equilibrium may be misleading.

Keywords: Feldspar; Kinetics; Organic acids; Silicates; Weathering』

 有機酸とそれらの陰イオン(簡単のために、我々は両者を含めて「酸」という用語を用いることにする)は、少なくとも3つのメカニズムにより鉱物の風化速度に影響を与えるであろう:溶液pHを減少させて平衡から離れた溶解速度を変化させること、あるいは鉱物表面で陽イオンと錯体を形成することによる;鉱物に関して、溶液の飽和状態に影響を与えることによる;そして、それ自身が鉱物溶解速度に影響する Al3+ のようなイオンの溶液での種形成に影響を与えることによる。本論文で我々は、天然の風化環境に類似し(−25℃、pH 4〜7)、土壌溶液で測定される値に匹敵する有機酸濃度である条件下で、有機酸が珪酸塩鉱物とくに長石の溶解速度に与える影響をレビューしている。
 平衡から離れた長石溶解速度は、pH 4〜5 以下でpHの減少とともに増加する。それらはpH 4〜5と約8の間ではpHに関係しないように見え、pH 8以上では長石溶解速度はpHの増加とともに増加する。
 シュウ酸塩のような弱いキレート化配位子は、鉱物表面でのAlの錯体化により長石溶解を促進させると思われる。1 mM のシュウ酸が存在する場合の長石溶解速度は、一連のデータ・pH・鉱物のアルミニウム成分に応じて、変化のないものから15倍促進されるものまで異なる影響を示す:利用できるデータに大きなばらつきがある。一般に、1 mMの桁のシュウ酸濃度は、重要な影響を与えるために必要である。フミン酸は長石溶解速度を著しく増加させるとは思われない。

1. Introduction
2. Surface complexation far from equilibrium
 2.1. Review of the model
 2.2. Effect of pH
 2.3. Simple organic acids

Table 2 Effect of oxalate on silicate dissolution rates
表2 珪酸塩溶解速度に対するシュウ酸塩の影響
pH Release ratea
Albite 10-3 3.3 4.8 [21]
Oligoclase 10-3 4-9 1 [54]
10-3 2.8 2.5 [21]
10-3 5.6 9.6 [21]
Andesine 10-3 4 and 5 2 [87]
10-3 3.0 2.3 [21]
10-3 5.6 16 [21]
Labradorite 10-3 3.1 2-4 [53]
Bytownite 10-3 3.0 1.3-3.6 [53]
10-3 4 1 [87]
10-3 4.5 1.7 [87]
10-3 5 3 [87]
10-3 4.6 12 [21]
Anorthite 10-4 4 2.2 [15]
K-feldsparb 2×10-2 3.6 1.6 [88]
Microcline 10-3 3.3 2.6 [21]
Olivine 10-3 4.5 23 [89]
Amphibole 10-3 3-9 1 [54]
Kaolinite 10-3 4 1.4 [18]
10-2 4 2.3 [18]
2×10-4 3.7-4.9 1 [50]
10-3 3.7-4.9 1.7-2.3 [50]
5×10-3 3.7-4.9 2.7-6.3 [50]
Quartz 10-3 7 1 [17]
2×10-3 7 1.4 [17]
2×10-2 7 3.5 [17]
a Rate with ligand divided by rate without ligand.
b At 70℃.