非饱和带溶质运移与化学风化作用的耦合机理研究

Chemical weathering has significant impacts on solute concentration of surface and ground water, soil properties and global climate change. Due to the favorable reactant mass transfer condition, chemical weathering rate in the unsaturated zone has potentially significant impact on the overall chemical weathering rate of the entire critical zone. Chemical weathering rate under unsaturated condition can be dominated by the coupling effect of several pore-scale processes including two-phase flow, interfacial mass transfer, convective and diffusive mass transfer of solutes, chemical reaction on the mineral surface and the reaction induced solid particle property change. However, the detailed coupling mechanism these complicated pore-scale processes has not been fully understood. In this project, to comprehensively investigate the pore-scale mechanism of chemical weathering under unsaturated condition, the unsaturated flow path, air-water interface, solute concentration, mineral particle morphology and surface chemical composition will be directly visualized via micromodel experiments and numerically simulated using Lattice Boltzmann method. Based on the quantitative pore-scale simulation results, an up-scaled effective reaction rate model applicable to the unsaturated condition can be derived. The up-scaled reaction rate model will be included in a continuum-scale reactive transport model and verified by the comparison with the column-scale experimental results. The quantitative prediction on field-scale chemical weathering rate can be improved by the pore-scale mechanism and reaction rate model obtained in this project.

非饱和带溶质运移与化学风化作用的耦合效应对整个地球关键带的化学风化速率有显著影响，因而间接影响矿物质元素迁移转化、土壤性质演化及气候变化等重要过程，其孔隙尺度耦合机理是化学风化作用研究的关键科学问题。本项目拟围绕非饱和水流路径、气-水界面及溶质浓度的孔隙尺度分布对化学风化反应速率的影响机理，及长期风化过程中固相形貌与表面化学性质演化引起的动态反馈机制等两方面具体问题，结合微观模型可视化实验与基于格子Boltzmann方法的孔隙尺度两相流动-传质-反应耦合模拟，精细刻画流体界面、水流路径、溶质浓度、颗粒形貌及表面化学成分的微观分布及其动态变化，揭示非饱和溶质运移与化学风化反应的孔隙尺度耦合机理，建立适用于非饱和带的连续介质尺度反应动力学模型，并根据连续介质尺度模拟与柱实验的对比，验证微观耦合机理及反应动力学模型。研究成果将为非饱和带乃至整个地球关键带宏观化学风化速率的定量模拟提供理论基础。

化学风化作用对土壤性质演化、碳循环与气候变化等重要过程均有着深刻的影响，是环境地球化学领域研究的热点问题之一。而化学风化过程常发生在非饱和带的非均质介质中，风化介质中的多相流动、溶质运移与水岩反应过程的耦合反馈机制复杂，制约了野外化学风化速率的定量估算。本项目结合微流控芯片原位可视化技术、孔隙/裂隙网络模型、连续介质尺度反应性溶质运移模拟及理论分析等多种方法，系统研究了非饱和流动、水相溶质运移和矿物风化反应在单孔隙尺度、孔隙网络尺度和剖面尺度的耦合机理与动态反馈规律。孔隙尺度的原位可视化结果表明，连续入渗条件下残余气不连通，气相扩散传质贡献可以忽略，非饱和流动对化学风化反应的主要影响机制为残余气泡水水相流动传质路径的改变；在水相饱和区域，但孔隙/裂隙内部的浓度梯度与传质速率，对于局部的矿物溶解速率影响显著。基于孔隙尺度的反应-传质耦合机理认识，进一步构建了孔隙/裂隙网络模型，全面刻画了水相饱和度、孔隙/裂隙网络连通性、裂隙开度、宏观水动力条件及风化反应本身的微观反应动力学特征等多种因素对化学风化速率及其动态反馈行为的定量影响规律，基于平均无量纲水力停留时间Da数实现了宏观反应速率的粗略估算。进一步开展剖面尺度风化过程的模拟，探究了非饱和带的水动力边界条件差异引起的来流-入渗掺混比对流场分布及风化速率的影响，同时定量刻画了宏观剖面中的风化介质非均质性对反应速率的影响。最终，基于水力停留时间分布函数的理论分析，提出了适用于非均质与瞬态变化等复杂流动条件的风化反应速率估算方法，极大地简化了野外介质的非均质性表征需求。研究成果加深了多相/瞬态/非均质复杂流动-溶质运移-化学风化反应的多尺度耦合机制与规律认识，同时丰富了地球化学过程多尺度耦合过程的研究方法。相关成果以第一/通讯作者在环境、地学领域中科院1区权威期刊发表学术论文3篇。