Fine fractionation and purification of the fulvic acid fraction using adsorption and precipitation procedures

A new scheme for fine fractionating and purifying a crude fulvic acid fraction was proposed. The fractionation procedure includes an adsorption process of the fulvic acid fraction on a hydrophobic resin (e.g. XAD-8 and DAX-8) and a sequence of successive elution processes. This procedure yields a non-adsorbed fraction (FA-1), a 0.1 mol L^-1 HCl eluted fraction (FA-2), a distilled water eluted fraction (FA-3) and a 0.1 mol L^-1 NaOH eluted fraction (FA-4). To remove salts and water, the fractionated organic molecules were co-precipitated with Al hydroxides at pH 5 and collected by centrifugation. Recoveries of carbon from the crude fulvic acid fraction of an Andosol were 42, 20, 19 and 14% for FA-1, FA-2, FA-3 and FA-4, respectively, and 96% of carbon was recovered in total. Optical properties and ¹³C nuclear magnetic resonance spectra indicated that the chemical properties of FA-1 were similar to those of FA-2 and were hydrophilic, while the chemical properties of FA-3 were very close to those of FA-4 and were hydrophobic (estimated hydrophobicity: humic acid >> FA-4 ≥ FA-3 >> FA-2 ≥ FA-1). A fulvic acid that can be isolated using the International Humic Substances Society method would be, in practice, a mixture of hydrophilic fulvic acid (FA-2) and hydrophobic fulvic acid (FA-3 + FA-4). The method developed in the present study will provide more detailed, quantitative and complete information of the fulvic acid fraction of soils.