The retention of hydrophobic organic compounds by humic acid

Abstract

A humic acid, extracted from the Ah horizon of a Black Chernozem soil was found to fix or strongly retain significant amounts of hydrophobic organic compounds. Only approximately 8 per cent of the total alkanes and fatty acids occurring in the humic acid could be removed by exhaustive extraction with organic solvents. Another 19 and 11 per cent of the two classes of compounds could be removed by organic solvents after methylation of the humic acid, but extraction of the bulk of these compounds required extensive chromatographic separations of the methylated humic acid. Alkanes and fatty acids were estimated to account for up to 0.34 and 0.47 per cent of the weight of the original humic acid, respectively. Other hydrophobic organic compounds extracted were dialkyl phthalates, some of which are toxic, and di-sec-butyl adipate, which were estimated to account for about 1 per cent of the initial weight of the humic acid. The organic esters were shown not to be artifacts but are considered to be of biosynthetic origin. The data reported herein show that 100 g of humic acid can firmly retain up to 2 g, and possibly more, of hydrophobic organic compounds by a mechanism that most likely involves adsorption on external surfaces and in internal voids of a molecular sieve-type structural arrangement. The adsorption behaviour of humic substances for organic compounds, some of which may be toxic pollutants, should be of interest to those concerned with environmental and geochemical problems.