Adsorption of fulvic acid on clay subfractions isolated from mineral horizons of peat-podzol-gley soil

We studied the adsorption of fulvic acid (FA) obtained from the H horizon of peaty-podzolic-gleyic soil on sludge subfractions isolated from the ELG and Ecng horizons of the same soil: 0–0.2 µm (I), 0.2–0.06 µm (II) , 0.06–0.02 µm (III) and <0.02 µm (IV). It has been established that, in terms of unit mass, more FAs are sorbed by subfractions III and IV, which have a larger surface area. In terms of per unit surface area, an inverse relationship is observed: the larger the fraction, the more FA is sorbed on it. All subfractions of sludge isolated from both horizons sorb predominantly hydrophobic components of FA, but in the finer subfractions, which practically do not contain kaolinite, the contribution of hydrophilic components in total sorption increases. Under the experimental conditions, FA molecules with a molecular weight of 20 kDa were not adsorbed in micropores with an average size of ≈ 3.7 nm. The main mechanism of FA sorption on sludge subfractions is hydrophobic interactions. The hydrophilic components of FA are sorbed through electrostatic interactions, through ligand exchange on lateral cleaved clay minerals and with the formation of bridging bonds with the Ca 2+ ion occupying exchange positions in clays.