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-derived fulvic acid (BFA), we used selective dissolution analysis and X-ray absorption near-edge structure (XANES) spectroscopy to estimate major classes of S (oxidized and reduced S) and oxidation states of S in BFA, respectively. The BFA was fractionated into two classes-total hydrophilic and hydrophobic acid fractions-using XAD-8 resin and into molecular-weight classes by dialysis. The selective dissolution analyses show that oxidized S was the dominant class of S in the total hydrophilic fraction, whereas reduced S was dominant in the hydrophobic acid fraction. The XANES analyses also indicated 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 classes and oxidation states of S. The XANES analyses not only corroborated the traditional selective dissolution approaches to characterizing S in complex organic materials but also provided insight concerning the most likely types of S groups within the broad categories of oxidized and reduced S.