Surface Area of Soil Organic Matter Reexamined
1995
Pennell, K. D. | Abriola, L. M. | Boyd, S. A.
The specific surface area of soil organic matter has traditionally been considered to range from 550 × 10³ to 800 × 10³ m² kg⁻¹, based on the change in ethylene glycol (EG) retention before and after treatment of soils with H₂O₂. Recent studies, however, indicate that when N₂ is used as the adsorbate, the surface area of soil organic matter is approximately 1 × 10³ m² kg⁻¹. To investigate this large discrepancy, the surface areas of four H₂O₂-treated soils were determined by EG retention and the N₂-Brunauer, Emmett, and Teller (BET) method. After exposure of the soils to H₂O₂, retention of EG decreased, while N₂ adsorption increased. The surface area of soil organic matter calculated from the reduction in EG retention ranged from 65 × 10³ to 481 × 10³ m² kg⁻¹ and was dependent on the initial organic C content of the soil. In contrast, the N₂-BET data yielded negative estimates of soil organic matter surface area that varied from −13 × 10³ to −410 × 10³ m² kg⁻¹. These results demonstrate that techniques based on the removal of organic C fail to provide consistent and reliable values for the surface area of soil organic matter. Due to the complex nature of interactions between soil organic matter and EG, including cation solvation, partitioning, and surface adsorption, it is recommended that EG retention be regarded as an estimate of the total uptake capacity of soils for polar liquids, rather than a measure of surface area.
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