Combined effects of ferrihydrite coating and ionic type on the transport of compost-derived dissolved organic matter in saturated porous media

Field application of manure compost introduces a large quantity of dissolved organic matter (DOM), which can affect the migration of DOM-associated contaminants. In this study, the transport of humic acid (HA) and compost-derived dissolved organic matter (CDOM) in two porous media under various conditions, including ionic types, ionic strength, and influent concentrations, were investigated by column experiments and modeling analysis. Increasing Na+ concentration did not affect the transport of CDOM and HA in quartz sands, but inhibited CDOM transport in ferrihydrite (Fh)-coated sands. The retention recoveries of CDOM in Fh-coated sands were not changed with increasing NaCl concentration, suggesting that the adsorption of CDOM on Fhcoated sands caused by increasing NaCl concentration was a reversible process. Ca2+ could reduce the mobility of CDOM and HA through bridge bonding and electrostatic interaction. CDOM had a higher mobility than HA in quartz sands under CaCl2 conditions because the aggregation stability of CDOM was stronger than that of HA. The ferrihydrite coating increased the roughness of sand surface, resulting in lower mobility of CDOM in the Fh-coated sands than in quartz sands. A part of CDOM adsorbed onto Fh-coated sand was strongly bound through ligand exchange-surface complexation. The pore volume of CDOM required to saturate adsorption sites onto the Fh-coated sand depends on the influent CDOM concentration. The influent CDOM with higher concentration required less pore volume to achieve adsorption equilibrium. Modeling analysis suggested that the types of deposition sites provided by Fh-coated sand are mainly irreversible sites. Our findings demonstrated that CDOM transport in the sand columns may change the porous medium's physicochemical properties and alter the hydrochemistry conditions. Therefore, these factors mentioned above should not be ignored when evaluating the environmental risks of CDOM.