Sorption and molecular fractionation of biochar-derived dissolved organic matter on ferrihydrite

Molecular fractionation of dissolved organic matter (DOM) induced by the sorption on soil minerals is a common geochemical process in soil, which has been well documented on natural DOM. Biochar is an emerging soil amendment and can continuously release DOM into the soil. However, reports regarding the interactions of soil minerals and biochar-derived DOM are limited. Herein, the molecular fractionation of DOMs derived from biochar samples obtained by pyrolyzing maize straw (MS) at 300 and 500 °C (M3 and M5) and MS (selected as the control) after sorption on ferrihydrite, a common soil mineral, was investigated for the first time. Results showed that biochar-derived DOMs contained greater aromatic and less aliphatic and O-containing structure compared to MSDOM. The sorption affinities of ferrihydrite for the three DOMs were in the order of M5DOM > M3DOM > MSDOM. A greater decrease in zeta potential and more external CO species were observed in ferrihydrite after sorption of M5DOM than after sorption of MSDOM or M3DOM. DOM components with high molecular weights and high contents of unsaturated oxidized or aromatic structures were preferentially sorbed by ferrihydrite via hydrophobic partition, H-bonding and electrostatic interactions. Polycyclic aromatics and polyphenols in MSDOM and M3DOM were the primary components sorbed by ferrihydrite, while the highly unsaturated and phenolic compounds in M5DOM showed stronger affinity to ferrihydrite. The oxygen-containing functional groups and unsaturation level are the primary parameters controlling the molecular fractionation of biochar-derived DOM on ferrihydrite. The results of this study help to get deep insight on the environmental behavior of straw- and biochar-derived DOMs in soil.