Measurement of specific surface areas of soils by p-Nitrophenol adsorption

Abstract

The adsorption of p-Nitrophenol (pNP) was used for measuring the specific external and total surface area of Na + - and Ca 2+ -saturated soil samples. Adsorption isotherms at 20°C from a pNP solution in xylene were obtained after heating the samples at (1) 160°C or (2) 90°C. With soils not containing swelling clay minerals L-type isotherms were obtained independently of the pretreatments. The final plateau was assumed to indicate the completion of pNP monolayer coverage of the surface of the samples. The derived specific surface area (SSA) was in good agreement with the specific area obtained by N 2 sorption. In the presence of swelling phyllosilicates, only the Na + -saturated samples preheated at 160°C showed L-type isotherms quite similar to those typical of the first group of soils. Under the other adsorption conditions, isotherms having a first branch of L-type and a second branch of C (constant partition)-type were obtained. The C-branch was found to reveal the penetration of pNP in the interlayer spaces of swelling clay minerals. Only the curves relative to Ca 2+ -saturated samples heated at 90°C terminated in a plateau, assumed to be the result of the complete monolayer coverage of both external and internal surfaces. The onset of a linear segment (point X) was considered to indicate the pNP complete coverage of the external surface. The SSA calculated at this point agrees well with values obtained by N 2 sorption. The total (internal + external) area measured at the plateau is lower than the EGME (ethylene glycol monoethyl ether) area, but it was considered more realistic. The influence of the pretreatments on the behaviour of swelling soils was ascribed mainly to the different polarizing power of the monovalent and divalent exchange cations.