Regulatory role of DOM-coupled ferrihydrite nanoparticles on migration of Cu(II) across variable-charged porous matrices.

The transport of colloids and their associated heavy metals in the environment is strongly influenced by dissolved organic matter (DOM). The present study explores how ferrihydrite nanoparticles (FNPs) affect Cu(II) mobility in columns packed with kaolinite- or layered double hydroxides (LDH)-coated quartz sand in the presence of humic/fulvic acids. High concentrations DOM reversed the surface charge of FNPs, generating negatively charged DOM-FNPs composites. Consequently, Cu(II) breakthrough accelerated in kaolinite columns because of electrostatic repulsion, whereas it was retarded in LDH columns because of enhanced electrostatic attraction. Due to the strong adsorption of DOM-FNPs onto LDH, DOM-FNPs acting as carriers to adsorb Cu(II) onto LDH, thereby enhancing the adsorption of Cu(II) by LDH (increased by 4.03/2.69 times). However, due to the weak adsorption of kaolinite toward DOM-FNPs, a competitive adsorption relationship for Cu(II) was formed between DOM-FNPs and kaolinite, resulting in a reduction in Cu(II) adsorption by kaolinite (decreased by 84.4%/63.1%). And DOM enhanced the adsorption stability of Cu in the solid medium. The strength of these adsorption interactions directly governs the migration rate. The excessive DOM induces preferential complexation of Cu-free DOM by solid media, while competitive adsorption reduces overall Cu(II) retention, elucidating the molecular mechanism behind DOM-regulated Cu(II) migration. The findings provide critical insights for assessing the ecological risks of composite pollutants and directly inform remediation strategies for contaminated soils and the design of engineered barriers.