Here, we demonstrate the potential advantages of using isotope tracers to test hypotheses of reaction mechanisms near-equilibrium. Using non-traditional stable Si isotopes as tracers, we measured albite unidirectional dissolution rates (r₊) across a range of Gibbs free energy of reaction (Δ_rG) close to equilibrium (−26 to −2 kJ/mol). Thirteen batch experiment series were conducted at 50 °C and pH ∼ 8 ± 0.25. Different distances from equilibrium were achieved by a stepwise increase of concentrations of Si (0–600 μM), Al (0–10 μM), and Na (0–1000 μM). The temperature, pH, sample preparation, and reaction duration were kept identical to isolate the Δ_rG effect. Secondary phase precipitation, which is difficult to avoid in near-equilibrium, near-neutral pH experiments renders the rate measurement method based on changes in Si and Al concentration unworkable, but it should not impact the Si isotope ratios-based rates.

The resulting r₊ values were nearly constant in the experimental Δ_rG range, signaling no major Δ_rG-related switch of reaction mechanisms. Our results suggest that the switch from etch pit opening at far-from-equilibrium to step retreat at near-equilibrium does not operate under circum-neutral pH in low-temperature systems; this mechanism switch was proposed based on experimental data in alkaline solutions at hydrothermal temperatures. The nearly constant r₊ values at pH 5–8 also suggest that an H₂O-catalyzed reaction mechanism dominant at circumneutral pH, in addition to the H⁺- and OH^–-catalyzed reaction mechanisms dominant at acidic and alkaline pH, respectively.

The experimental results have implications for geochemical modeling of low-temperature geological and environmental processes. The results suggest that a term of H₂O-catalyzed reaction mechanism should be included in rate laws and that the parallel rate law with a mechanism-switch is not applicable in the pH range of 5–8.

在这里，我们证明了使用同位素示踪剂来测试接近平衡的反应机理假说的潜在优势。使用非传统的稳定Si同位素作为示踪剂，我们在接近平衡（-26至-2 kJ / mol）的一系列吉布斯反应自由能（_Δr G）范围内测量钠长石的单向溶出度（r ₊）。在50°C和pH约为8±0.25的条件下进行了13批实验。通过逐步增加Si（0–600μM），Al（0–10μM）和Na（0–1000μM）的浓度，可以达到距平衡的不同距离。保持温度，pH，样品制备和反应持续时间相同以分离_Δr G影响。在接近平衡，接近中性的pH实验中难以避免的次生相沉淀使得基于Si和Al浓度变化的速率测量方法不可行，但不会影响基于Si同位素比率的速率。

所得的r ₊值在实验_Δr G范围内几乎恒定，表明反应机理没有与_Δr G相关的主要变化。我们的结果表明，在低温系统中，在环境中性pH值下，从远离平衡点的蚀刻坑开口切换到接近平衡点的台阶撤退操作不起作用。根据在水热温度下碱性溶液中的实验数据提出了这种机理转换。的几乎恒定的- [R ₊在pH值5-8还表明，一个H ₂ O形催化的反应机理在circumneutral主导pH值，除了向H ⁺ -和OH ^--催化的反应机理分别在酸性和碱性pH下占优势。

实验结果对低温地质和环境过程的地球化学建模具有重要意义。结果表明在速率定律中应包含H ₂ O催化反应机理的术语，并且具有机理转换的平行速率定律不适用于5-8的pH范围。