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 (XAN ES) 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 Sin complex organic materials but also provided insight concerning the most likely types of S groups within the broad categories of oxidized and reduced S.
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