Adsorption properties of soil humic and fulvic acids by hematite

The adsorption isotherms and their size fractionations of soil humic and fulvic acids (HA and FA, respectively) were determined at pH 7, 9.2 and 11, where the surfaces of the hematite are positively, neutrally and negatively charged, respectively. The M-w, M-n and polydispersity in the solution after adsorption were measured in order to investigate the preferential adsorption trend of the HA and FA. The measured adsorption was consistent with the adsorption from Langmuir isotherm fitting. The adsorption of HA onto the hematite was more favorable than that of FA due to the preferential adsorption of high molecular weight fractions and the greater hydrophobicity. As the solution pH increased, the preferential adsorption for the high molecular weight fractions partially deviated with higher polydispersity. This indicates that the adsorption of the HA and FA onto hematite was affected by the molecular conformation, and the electrostatic repulsion between the HA and FA and the hematite lead to the kinetically sluggish adsorption. Further, it implies that the adsorption properties of soil organic matter may affect the adsorption of metal and metalloid onto the adsorbent like iron-oxide in the geochemical environments.