In the soil environment, iron oxides co‐occur commonly with different types of organic constituents to produce iron oxide–organic composites that play an important role in the biogeochemical cycling of trace metals. We investigated copper (Cu) adsorption on synthetic goethite, Pseudomonas putida (CCTCC M209319) bacterial cells, humic acid (HA) and their binary and ternary composites with batch adsorption experiments coupled with isothermal titration calorimetry (ITC). Morphological characterizations show that the three components can form closely combined and heterogeneous aggregates with one another. The kinetics of sorption of Cu to these composite materials conforms to the pseudo‐second‐order model, whereas the Cu sorptivities deviate from linear additivity. Specifically, Cu sorptivities on the binary goethite–P. putida and P. putida–HA and ternary goethite–P. putida–HA composites are less than expected assuming additivity, whereas the opposite is seen for the binary goethite–HA composite. There is considerable masking of adsorption sites in the ternary goethite–P. putida–HA system, but the binding sites on bacteria are not completely covered, as shown by the adsorption enthalpy and entropy (19.60 kJ mol−1 and 120.9 J mol−1 K−1, respectively) for the ternary composite. We conclude that the binding affinity of Cu for the binary goethite–humic acid composite is larger than that for the bacterial composites with goethite or humic acid, or both. This research indicates that bacteria, iron oxide and humic substances exhibit different behaviour in the sequestration of heavy metals when they form various binary and ternary complexes in natural environments. Highlights How does mineral–organic interaction affect the binding behaviour of trace elements? First investigation of Cu sorption to goethite–bacteria–humic acid composites. Goethite–HA shows enhanced Cu adsorption, whereas the opposite is true for bacterial complexes. Various bacteria–iron oxide–humic substance composites behave differently in metal sequestration.

The mobility and fate of heavy metals in soil depend strongly on their interactions with naturally occurring ligands and sorbents such as bacteria–mineral complexes, which are among the most important bioactive components in soil. However, the mechanism through which the various ligands affect heavy metal adsorption on bacteria–mineral complexes remains unclear. In this study, the effects of several inorganic and organic ligands on Cd adsorption on Pseudomonas putida–montmorillonite (Mont) and Pseudomonas putida–goethite mixtures were examined with measurements of macroscopic sorption. The results showed that citrate and humic acid enhanced Cd adsorption on P. putida–Mont and P. putida–goethite. Conversely, oxalate suppressed Cd adsorption. Phosphate showed both limited and increased Cd adsorption, depending on the types of mixture. Cadmium adsorption was increased on P. putida–goethite, but was reduced on P. putida–Mont. This study provided the first compelling evidence that the effects of ligands on bacteria–mineral mixtures cannot simply be inferred from the effects on the individual components of such mixtures. The positive, neutral or negative effects also depend on the types and concentrations of ligands as well as the mineral species that form the bacteria–mineral mixtures. Highlights Little information is available on metal adsorption at organo–mineral interfaces affected by ligands. Effect of ligands on metal adsorption to bacteria–mineral complexes is elucidated. Ligands can form surface Cd complexes on the mineral component of the bacteria–mineral mixture. The effects of ligands on the mixture depend on the interactions between individual components.