An improved method for determining the specific surface areas of topsoils with varied organic matter content, texture and clay mineral composition

  • Theng B
  • Ristori G
  • Santi C
 et al. 
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Measuring the specific surface area (SSA) of soils that contain much
organic matter (OM) is problematic. The adsorption of p-nitrophenol
(pNP) from xylene at room temperature yielded realistic values for
the SSA of a wide range of clays, oxides and subsoils. Here we have
extended the same measurement to some topsoils with varied OM content,
texture and clay mineral composition. Specifically, we have compared
the surface areas measured by adsorption of N2, and, applying the
BET equation, with the values obtained by adsorption of pNP, before
and after treatment of the samples with hydrogen peroxide. In all
instances, the removal by H2O2 of organic matter � albeit in part
only � led to a marked increase in the SSAs measured by nitrogen
because of the exposure of micropores previously blocked or covered
by OM. The surface areas measured by pNP were appreciably larger
than those obtained by the standard BET equation, and showed little
change after removal of organic matter. However, the surface area
of two smectite-rich samples measured by pNP increased substantially
after peroxidation, presumably because smectite crystals decomposed
during treatment with H2O2. The results suggest that, under the experimental
conditions used, pNP could diffuse without hindrance into and through
organic matter, enabling it to adsorb on to micropore surfaces within
clay aggregates (domains). In keeping with this suggestion, the relation
between the surface areas measured by pNP and the corresponding values
calculated from the clay and OM contents, and clay mineral composition,
of the soils was close to 1:1. An even stronger relation was observed
between the measured and calculated values for cation exchange capacity.

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  • B. K. G. Theng

  • G. G. Ristori

  • C. A. Santi

  • H. J. Percival

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