REFINING DIFFUSION COEFFICIENT MEASUREMENTS IN POROUS MEDIA: A NOVEL APPROACH TO MINIMIZING ERROR IN QUASI-STEADY STATE TESTS

Accurate quantification of diffusion coefficients in porous media is critical for environmental engineering, geotechnical engineering, and petrochemical industries, particularly in applications involving contaminant transport, groundwater remediation, and energy storage systems. Traditional quasi-steady state (QSS) diffusion setups, while widely utilized, often suffer from significant errors due to premature assessments and suboptimal configurations. These errors, previously unquantified, can substantially impact the reliability of transport predictions in porous media applications. This study optimizes the QSS setup through a systematic investigation of measurement errors, particularly during the critical early phases of diffusion tests. A finite element model, specifically designed for QSS diffusion tests, systematically investigates errors associated with nonlinear concentration gradients, enabling precise determination of steady-state onset. Our findings reveal significant deviations in diffusion coefficients during initial stages (<i>t</i> < 255 hr, error > 33%), attributed to inadequate diffusion flux relative to the imposed concentration gradient. These errors systematically decrease to below 5% at 605 hr, coinciding with the establishment of linear concentration profiles. Statistical validation demonstrates exceptional consistency across multiple timescales (mean absolute percentage error 3.8-4.1%, <i>n</i> = 4, σ = 0.4-0.5%), with mass flux consistency validated to within 3.2 × 10<sup>-6</sup> mmol · m<sup>-2</sup> · s<sup>-1</sup> . Establishing a linear concentration gradient stabilizes diffusion coefficients (CoV < 2.1%), highlighting the necessity for methodological precision. The model accurately predicts the timeframe for achieving reliable steady-state conditions (R<sup>2</sup> = 0.992), reducing unnecessary extended test durations while maintaining measurement accuracy. This research provides a comprehensive framework for optimizing diffusion test setups, enhancing accuracy and efficiency in measuring diffusion phenomena in porous media. The insights are crucial for improving diffusion metrics reliability in environmental engineering and energy sectors, with direct applications in contaminant transport prediction, subsurface characterization, and energy storage system design. By identifying and quantifying errors in premature QSS assessments, this study advances precise diffusion measurement, significantly contributing to both scientific understanding and practical applications in porous media transport phenomena.