The effects of formation modes of ferrihydrite-low molecular weight organic matter composites on the adsorption of Cd(ii)†

The interactions between iron oxides and organic matter (OM) play vital roles in the geochemical cycle of cadmium (Cd). However, the effects of the formation modes of ferrihydrite (Fh)-low molecular weight OM (e.g., fulvic acid (FA)) composites on Cd(II) adsorption remain poorly understood. The immobilization mechanisms of Cd(II) on Fe-OM composites formed by adsorption and coprecipitation at varying C/Fe molar ratios were investigated by means of adsorption batch experiments, two-dimensional correlation spectroscopy, and surface complexation models (SCMs). The composites formed by adsorption or coprecipitation exhibited a crystal structure similar to that of Fh. Ligand exchange and hydrogen bonding were identified as the primary mechanisms between components in adsorption composites and coprecipitates, respectively. Compared to coprecipitates, the adsorption composites showed a higher adsorption capacity and formed ternary complexes (Fh-FA-Cd). In coprecipitates, Cd(II) primarily interacted with the carboxyl and hydroxyl groups of FA and the hydroxyl groups of Fh. With increasing C/Fe molar ratios, the FA functional group (R-COOH) in adsorption composites responded more quickly to Cd(II). However, the order of functional group reactions in coprecipitates was unaffected by C/Fe molar ratios, which is due to the irregular distribution of C and Fe elements. SCM calculation results indicated that Cd(II) distribution on Fh in adsorption composites was higher than that in coprecipitates. The molar ratios of C/Fe and Cd(II) concentrations influenced the distribution of Cd(II) on the composites, with the highest proportion of Cd(II) on Fh reaching about 70%. These findings contribute to understanding Cd behavior in environments with periodically fluctuating redox conditions.