Ferrihydrite–organo composites are a suitable analog for predicting Cd(II)–As(V) coexistence behaviors at the soil solid-liquid interfaces

Organomineral assemblages are building units of soil micro-aggregates and exert their essential roles in immobilizing toxic elements. Currently, our knowledge of the adsorption and partitioning behaviors of coexisting Cd–As onto organomineral composites is limited. Herein, we carefully studied Cd–As cosorption onto ferrihydrite organomineral composites made with either living or non-living organics, i.e., bacteria (Delftia sp.) or humic acid (HA), using batch adsorption and various spectroscopies. Batch results show that As(V) only enhances Cd(II) sorption on pure Fh at pH < 6 but cannot promote Cd(II) sorption to Fh–organo composites. However, Cd(II) noticeably promotes As(V) sorption at pH>~5–6. Synchrotron micro X-ray fluorescence indicates that Cd(II) adsorbs predominately to the bacterial fraction (Cd versus P, r = 0.924), whereas As(V) binds mainly to the Fh fraction (As versus Fe, r = 0.844) of the Fh–bacteria composite. On Fh–HA composite, however, Cd(II) and As(V) are both primarily sorbed by the Fh fraction (Cd/As versus P, r > 0.8), based on the scanning transmission electron microscopy-energy disperse spectroscopy analyses. Elemental distribution characterization also manifests the co-localization of Cd(II) and As(V) within the organomineral composite, particular in Fh–HA composite (Cd versus As, r = 0.8), which is further identified as the Fh–As–Cd ternary complex based on the observations (higher frequencies at ~753–761 cm⁻¹) of attenuated total reflection Fourier-transform infrared spectroscopy. Moreover, this ternary interaction is more pronounced in Fh–HA than in Fh–bacteria. In summary, our results suggest that Cd–As coadsorption behaviors on Fh–organo composites are different from those on pure minerals, and the presence of bacteria/HA can significantly affect metal (loid)s speciation, distribution, and ternary interaction. Therefore organomineral composites are a more suitable analog than pure mineral phases to predict the mobility and fate of Cd–As in natural environments.