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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 r G) 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 r G 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 r G range, signaling no major Δ r G -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 H 2 O-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 H 2 O-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. [ABSTRACT FROM AUTHOR]

• Novel Si isotope tracer method deconvoluted feldspar dissolution and precipitation. • Feldspar backward reaction is insignificant at low temperatures. • Recommend forward dissolution rates only in low-temperature kinetics models. • This approach for weathering, geological carbon sequestration, and diagenesis. Here we show that the isotope tracer experimental method for kinetic studies, aided by the recent advance and accessibility of multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS) analysis for non-traditional stable isotopes, can provide unidirectional dissolution rates at near-equilibrium conditions. For a long time, the only rates available at near-equilibrium were net reaction rates—dissolution rates minus precipitation rates. This is because the conventional experimental method of kinetic studies is based on element concentrations and can only provide net rates. The availability of unidirectional rates allows us to re-examine some fundamental concepts and practices of modeling weathering in geochemistry. In this study, we used the 29Si isotope tracer to conduct albite and K-feldspar dissolution experiments at near-equilibrium conditions in near-neutral pH solutions at 50 °C. Results show that the saturation indices (SI) of solutions approached zero with respect to albite and K-feldspar after ∼240–360 h (h), but 29Si/28Si ratios of the experimental solutions indicated continual dissolution for another 720–1440 h. The rates of total Si precipitation were much smaller than the rates of Si dissolution. The experimental solutions were supersaturated with respect to amorphous Al(OH) 3 , gibbsite, quartz, allophane, imogolite, and kaolinite. The SI of the solutions remained constant with respect to these phases while Al concentrations slightly decreased and Si concentrations slightly increased, indicating the coupled feldspar dissolution and precipitation of secondary phases, such as albite → amorphous Al(OH) 3 + quartz or albite → solution + Al-Si phase(s), instead of significant albite and K-feldspar precipitation (the reverse reaction) at 50 °C. Reaction path modeling of the temporal evolution of Si, Al, Na, and pH revealed that albite dissolution (without significant backward reaction) coupled with the precipitation of a secondary phase with a Si:Al ratio of ∼2:1 can successfully match the experimental data. Given the negligible feldspar precipitation reactions in low-temperature systems (e.g., T < 100 °C), we recommend modeling feldspar weathering using unidirectional forward rates together with secondary phase precipitation rates in near-equilibrium, feldspar-undersaturated systems. This can be accomplished with minor modifications in geochemical modeling software or input files. The coupled feldspar dissolution with secondary phase precipitation arrests the system in a near-equilibrium steady state. Using affinity-based rate equations such the classical linear Transition State Theory rate law or the Burch empirical relation together with far-from-equilibrium rate data will predict significant feldspar precipitation in solutions undersaturated but close to equilibrium with respect to feldspars, which is unlikely at near ambient temperatures. [ABSTRACT FROM AUTHOR]

We measured barium attachment and detachment fluxes from and to barite crystal surfaces in solutions with [Ba]/[SO 4 ] ratios from 0.06 to 52. The doping of 137Ba in the initial experimental solutions enabled us to measure unidirectional fluxes near solubility equilibrium (Q / K ≈ 1.0) in batch reactors. Both attachment and detachment fluxes increase with [Ba]/[SO 4 ] ratios. As expected, since the solutions were near solubility equilibrium ( S I ≈ 0.0 n), the attachment and detachment fluxes were nearly equal. The experimental data can be fit accurately using the Zhang and Nancollas (1998) (ZN98) process-based AB crystal growth model, which describes crystal growth and dissolution through nucleation and propagation of kink sites. Simultaneous fitting step velocity data of Kowacz et al. (2007) significantly reduced the number of non-uniqueness solutions. The remarkable fit implies that barite recrystallization at room temperature near equilibrium and moderate supersaturation occurs via ion-by-ion addition and removal, rather than the formation of amorphous precursors or aggregation of particles. The model is best fit using different detachment frequencies for Ba and SO 4 , suggesting the ZN98 assumption that detachment frequencies should be the same for the constituent ions does not apply to barite. Based on experimental data and model fitting, Ba attachment and detachment fluxes at a fixed saturation state increase with [Ba]/[SO 4 ] ratios monotonically. This has potentially significant implications for interpreting isotope and trace element cycling in natural environments. [ABSTRACT FROM AUTHOR]