Estimating specific surface area: Incorporating the effect of surface roughness and probing molecule size

The pore–solid interface and its characteristics play a key role in chemical interactions between minerals in the solid soil matrix and the liquid in pore space and, consequently, solute transport in soils. Specific surface area (SSA), typically measured to characterize the pore–solid interface, depends not only on the particle size distribution (PSD) but also on particle shapes and surface roughness. In this note, we investigate the effects of surface roughness and probing molecule size on SSA estimation, use concepts from fractals, and theoretically estimate SSA from PSD and the water retention curve (WRC). The former is used to characterize the particle sizes and the latter to approximately quantify the pore–solid interface roughness by determining the surface fractal dimension. To evaluate our approach, we use five Washington and 21 Arizona soils for which both PSDs and WRCs were accurately measured over a wide range of particle sizes and matric potentials. Comparison with the experiments show that the proposed method estimates the SSA reasonably well, with RMSE = 16.8 and 30.1 m² g^–1 and average relative error = –56 and –35% for the Washington and Arizona datasets, respectively.