| 04 Aug 2026
Transient electroosmotic transport under evolving pH and contrasting background electrolyte concentrations: coupled controls of interfacial charge and pore structure
Abstract. Electroosmotic flow (EOF) can transport porewater and dissolved neutral solutes through fine-grained porous media, but its magnitude may evolve as electrode reactions alter pH and electrolyte conditions at mineral–water interfaces. Here, we use 24 two-dimensional pore-scale simulations to quantify how interfacial-charge parameterization, background electrolyte concentration, and pore architecture regulate transient electroosmotic displacement. The model couples incompressible flow, multicomponent Nernst–Planck–Poisson transport, time-dependent electrolysis boundary forcing, and chemistry-dependent electroosmotic slip. Zeta potential is calculated from the local aqueous chemistry using either a one-pK electric-double-layer (EDL) parameterization or a two-pK extended triple-layer (ETL) parameterization. The simulations were conducted in four uniformly oriented packings and two composite pore geometries. Under a matched background electrolyte concentration of 10 mM, the geometry-averaged fraction of tracer depleted from the contaminated medium zone after 30 min was 0.998 for the EDL cases and 0.918 for the ETL cases, while the corresponding late-time tailing indices were 0.002 and 0.094. Within the ETL cases, increasing the background electrolyte concentration from 1 to 100 mM reduced the mean 30 min depletion fraction from 0.963 to 0.613 and increased the tailing index from 0.037 to 0.465. None of the 100 mM cases reached 90 % depletion within 30 min. Pore architecture did not alter this group-level ordering but systematically reorganized directional transport and plume geometry within each electrochemical regime. In the uniformly oriented packings, rotation redistributed flow between the gradient-aligned and transverse directions and produced systematic plume reorientation, although fixed-time depletion varied non-monotonically with angle. In the composite geometries, the heterogeneous directional geometry maintained stronger domain-scale directional organization and reached its case-specific early and intermediate progress levels earlier than the non-directional geometry, whereas their fixed-window depletion ranking depended on the electrochemical setting. Comparisons at case-specific relative progress levels separated plume reorganization from differences in transport timescale. These results reveal a two-level control on transient electroosmotic transport: interfacial-charge representation and background electrolyte concentration determine transport magnitude and timescale, whereas pore structure redistributes flow pathways and plume geometry within the resulting regime.
Competing interests: At least one of the (co-)authors is a member of the editorial board of Hydrology and Earth System Sciences.
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General Assessment
This manuscript presents a coherent pore-scale numerical investigation of transient electroosmotic transport in low-permeability porous media. The coupled treatment of aqueous chemistry, interfacial charge, electroosmotic slip, and pore structure is scientifically meaningful, and the comparison between the one-pK EDL and two-pK ETL formulations is particularly useful. The 24-case simulation matrix is well designed, and the mesh, time-step, and solver-sensitivity tests support the numerical robustness of the results.
The main conceptual contribution is clear: the interfacial electrochemical setting primarily controls transport magnitude and timescale, whereas pore architecture mainly redistributes flow pathways and plume geometry. The manuscript is publishable in principle. I recommend minor revision, mainly to sharpen the interpretation of the metrics, clarify model limitations, and avoid over-generalization.
Main Comments
1.Clarify the meaning of CMZ depletion.
The main bulk metrics are defined with respect to the contaminated medium zone (CMZ). Depletion from the CMZ does not necessarily mean removal from the whole computational domain, because tracer transferred into the buffer zones is already counted as depleted from the CMZ. Please state this distinction more explicitly in the Abstract, Results, and Conclusions. If convenient, a whole-domain mass or cumulative outflux metric could be added as a complementary measure.
2.Qualify the EDL–ETL comparison more clearly.
The large difference between EDL and ETL is an important result, but it reflects both model-form and parameter-set differences. The comparison therefore demonstrates sensitivity to the selected interfacial representation rather than proving that one model is universally superior. This point should be emphasized earlier. A limited parameter sensitivity test would strengthen the conclusion, although a clearer discussion may be sufficient.
3.Strengthen the discussion of chemical simplifications.
The model updates zeta potential from local surface speciation but does not include finite surface-site inventories or surface-reaction source/sink terms in the aqueous mass balance. In addition, activity coefficients are set to unity, which is increasingly restrictive at 100 mM. Please explain more explicitly how these assumptions may affect pH propagation, interfacial charge, and the quantitative interpretation of the high-electrolyte cases.
4.Keep the pore-structure conclusions appropriately limited.
The SROE rotation series also introduces modest changes in porosity and local pore geometry, so the results should continue to be described as orientation-associated structural effects rather than pure orientation effects. Likewise, the HD–ND comparison is based on one deterministic realization of each geometry. The authors should avoid implying a universal ranking of directional and non-directional structures.
5.Emphasize the scope of the neutral conservative tracer.
Org is used as a neutral, conservative tracer without electromigration, sorption, degradation, or reaction. This is appropriate for isolating electroosmotic advection–diffusion, but the resulting depletion should not be interpreted directly as remediation efficiency for ionic, sorbing, or reactive contaminants. A concise statement in the Discussion and Conclusions would improve the environmental interpretation.
6.Briefly justify the relative-progress analysis.
The stage-aligned plume comparison is a useful feature because it separates transport-rate differences from plume reorganization. Please add a short rationale for the selected progress levels (0.25, 0.60, and 1.00) and state whether the main structural conclusions are expected to be robust to moderate changes in these thresholds.
Minor Comments
1.“Contaminated medium zone” first appears in the Abstract without introducing the abbreviation CMZ, while “CMZ depletion” is subsequently used in the main text before CMZ is formally defined in Sect. 2.3.2. Please define “contaminated medium zone (CMZ)” at its first occurrence and use the terminology consistently throughout.
2.For the 100 mM cases, emphasize that the results are most reliable for qualitative concentration-dependent trends rather than absolute values.
3.If possible, include the post-processing scripts for CMZ metrics and plume descriptors in the final public repository to improve reproducibility.
Recommendation
The manuscript is scientifically sound and the major conclusions are supported by the simulations. The remaining issues are primarily matters of clarification, scope, and interpretation rather than fundamental methodological flaws. I therefore recommend Minor Revision / Accept after Revision.