Isolation and purification of hydrophilic fulvic acids by precipitation

Abstract

Fulvic acids play an important role in the behavior of metals and hydrophobic organic chemicals in soil and water environments. The isolation and purification of the fulvic acids have been difficult to achieve, however, because these compounds are soluble in both alkaline and acidic solution, resulting in difficulty of dehydration and demineralization. We propose here a new procedure for isolating fulvic acids as precipitates. Our procedure includes pH-adjustment of the fulvic acid solution to weakly acidic to neutral pH range (4 to 7). In an Andisol, recoveries of the fulvic acids we prepared by precipitation at equilibrium pH of 5.0 were 86% (dissolved total organic carbon basis) and 97% (absorbance basis, 400 nm), whereas recoveries of fulvic acids adsorbed on XAD-8 resin (hydrophobic fulvic acids) were 14% and 28%, respectively. Recoveries of the fulvic acids were further increased in our procedure by adding Al. The mechanism forming the precipitates includes a complexation reaction of carboxylic groups of the fulvic acids with Al (ligand exchange reaction); this mechanism is identical to that for the retention of fulvic acids in many soils. Therefore, it is likely that our preparation procedure is appropriate for separating the fulvic acids stabilized in soils. Solid-state cross polarization and magic angle spinning 13C nuclear magnetic resonance spectra showed that the fulvic acids prepared by our precipitation procedure were relatively rich in O-alkyl carbons and poor in aromatic and alkyl carbons compared with those forms from the hydrophobic fulvic acids adsorbed on XAD-8 resin. A new scheme for preparing the hydrophobic and hydrophilic fulvic acids is also proposed here. The hydrophilic fulvic acids are likely to maintain high solubilities in water even after the formation of complexes with metals and may influence on their behavior.