Measurement of molecular weight distribution of humic substances with untreated Sephadex G-15 and ammonia-treated Sephadex G-15

Abstract

Gel chromatography of the humic substances in coexisting inorganic salts using untreated and ammonia-treated Sephadex G-15 was investigated. The fulvic acid found in treated sewage and the fulvic and humic acids found in leaf mold were used as samples. When untreated Sephadex G-15 was used, the elution of the fulvic acid in treated sewage could be elucidated by Posner's theory which concerns the depression of electric repulsion between the humic substances and gel by inorganic salts. The fulvic acid in leaf mold had a higher molecular weight than the fulvic acid in treated sewage. Most of the former was eluted around V 0, but some of it was adsorbed and eluted later. The humic acid in leaf mold had a much larger molecular weight and was expected to be eluted near V 0. However, it was adsorbed and eluted later according to the pH decrease caused by the ion-exchange between the sodium ion of the solution and the hydrogen of the carboxyl groups in the gel. On the other hand, when ammonia-treated Sephadex G-15 was used, the fulvic acid in treated sewage was eluted as sharp peaks in the order of the molecular weights being exchanged by the inorganic anions. Most of the fulvic acid in leaf mold was eluted around V 0. Most of the humic acid in leaf mold had a much higher molecular weight and was eluted at V 0. Consequently, it is considered that ammonia-treated Sephadex G-15 is more useful than untreated Sephadex G-15 for the measurement of the molecular weight distribution of aquatic humic substances.