Sulfur in biosolids-derived fulvic acid: Characterization by XANES spectroscopy and selective dissolution approaches

To better understand the chemistry of S-containing ligands in biosolids-der ived fulvic acid (BFA), we used selective dissolution analysis and X-ray ab sorption near-edge structure (XANES) spectroscopy to estimate major classes of S (oxidized and reduced S) and oxidation states of S in BFA, respective ly. The BFA was fractionated into two classes-total hydrophilic and hydroph obic acid fractions-using XAD-8 resin and into molecular-weight classes by dialysis. The selective dissolution analyses show that oxidized S was the d ominant class of S in the total hydrophilic fraction, whereas reduced S was dominant in the hydrophobic acid fraction. The XANES analyses also indicat ed that oxidized forms of S were dominant in the total hydrophilic fraction , whereas reduced forms of S were dominant in the hydrophobic acid fraction . BFA fractions separated by XAD-8 resin and dialysis contained distinct cl asses and oxidation states of S. The XANES analyses not only corroborated t he traditional selective dissolution approaches to characterizing S in comp lex organic materials but also provided insight concerning the most likely types of S groups within the broad categories of oxidized and reduced S.