Analytical Sciences

Separation of Fulvic Acid from Soil Extracts Based on Ion-Pair Formation with a Cationic Surfactant

Masami FUKUSHIMA,* Atsunori KIKUCHI,* Kenji TATSUMI,* and Fumiko TANAKA**

*National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba-West, Tsukuba 305-8569, Japan
**National Food Research Institute (NFRI), 2-1-12 Kannondai, Tsukuba 305-8642, Japan

A novel method for separating fulvic acid (FA) from soil extracts is proposed. The FA, defined as the acid-soluble fraction of an alkaline extract of soil, was separated based on the precipitation of an ion-pair with a cationic surfactant, such as domiphen bromide. The precipitate was dissolved in aqueous HCl to produce H⁺-type FA and a surfactant chloride (SUR-Cl). SUR-Cl, in the aqueous solution, was removed by extraction with CHCl₃. After the aqueous phase was passed through a cation-exchanger (H⁺-type), H⁺-type FA (FA-SUR) was obtained as a powder by lyophylization. The chemical characteristics of FA-SUR were compared with an FA sample separated according to the method of the International Humic Substances Society using a DAX-8 resin (FA-DAX). The oxygen content, O/C atomic ratio and total acidity of the FA-SUR were significantly larger than the corresponding values for FA-DAX. The solid-state CPMS ¹³C NMR spectra indicated that the higher oxygen content of the FA-SUR could be due to alcoholic hydroxyl groups and polysaccharides as well as carboxylic groups. These results show that FA-SUR is more polar and hydrophilic than FA-DAX.

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