| The study of sorption-desorption and dissolution-precipitation in the natural subsurface is of fundamental interest in many areas of scientific, industrial and engineering processes, including environmental contaminant transport, leaching of agrochemicals from soil surface to groundwater, chemical weathering, enhanced oil or gas recovery and CO2 sequestration. The natural subsurface is highly heterogeneous with minerals distributed in different spatial patterns. Knowledge of how mineral spatial distributions regulate sorption and dissolution processes is important for understanding and modelling the transport and fate of chemicals. However, most published studies about the sorption and dissolution reactions were carried out in well-mixed batch reactors or uniformly packed columns, few data are available on the effects of spatial heterogeneities on the overall reaction rates. The objective of this work is 1) to examine the largely unexplored role of illite spatial distribution patterns in dictating sorption of Cr(VI), a ubiquitously occurring contaminant in Hanford, Oak Ridge, Los Alamos and other sites, 2) to systematically understand and quantify the effects of calcite spatial patterns on its dissolution rates under various reactivity conditions.;Flow-through experiments were carried out at 0.1-18.5 m/day using columns packed with the same illite or calcite and quartz mass however with different patterns and permeability contrasts. Two-dimensional reactive transport modeling was used to reproduce the experimental data and to extrapolate the model under a wide range of conditions. For Cr(VI) sorption, at 0.6 and 3.0 m/day, well-connected low permeability illite zone oriented in the flow-parallel direction leads to diffusion-controlled mass transport limitation for accessing sorption sites. This results in up to 1.4 order of magnitude lower macrocapacity and macrorates compared to those in minimally-connected columns with well-mixed illite and quartz. At 15.0 m/day, the effects of spatial heterogeneities are less significant (up to a factor of 2.8) owing to the close to chemical kinetics-controlled condition. Additional patterns with the same permeability mean but different sigma 2lnK (variance of lnK) of 4.5 and 0.2 were generated by Sequential Gaussian Simulation (SGS) at different correlation lengths and column lengths. Sorption capacity and rates decrease with correlation length and transport connectivity, quantitative measures of heterogeneity characteristics. For calcite dissolution, calcite dissolution rates in the 1-zone columns are lower than those in the Mixed columns for all conditions due to the mass transport limitation. The spatial patterns make negligible effects under too low or too high flow velocities due to the equilibrium or kinetic-controlled regimes. At high local dissolution rate conditions (pH <4.0, large surface area or fast dissolving mineral), the "critical" flow region where the effects of spatial heterogeneities are significant is broad and locates at high flow conditions (>10.0 m/d). In contrast, the "critical" region is narrow and locates at low flow conditions (< 0.1 m/d).;Mineral spatial distributions have profound impacts on sorption and dissolution rates. Understanding these processes are of great importance for many applications in environmental and geological systems. Insights gained here bridge gaps between laboratory and field application in hydrogeology and geochemistry fields, and advance predictive understanding of reactive transport processes in the natural heterogeneous subsurface. Furthermore, identifying the key geochemical conditions where spatial heterogeneities influence the sorption and dissolution rates most provides useful information for adsorbent industrial application, landfill design, chemical weathering and acidization design during energy extraction. |
