Process-dependent inhibitory effects of zinc-based metal-organic frameworks on nitrifying microbial consortia

Zinc-based metal-organic frameworks (Zn-MOFs) are attractive for environmental applications; however, their impact on key microbial processes, such as nitrification, remains poorly understood. In this study, we evaluated the effects of MOF-5, Zn(II)-MOF, dissolved Zn(II), and their corresponding organic ligands on ammonium oxidation (AO) and nitrite oxidation (NO) using microrespirometric assays. Structural and physicochemical characterization confirmed the successful synthesis of both MOFs and revealed distinct morphologies, crystal structures, and colloidal behavior in aqueous media. For the AO process, all Zn-based compounds induced concentration-dependent inhibition, with maximum inhibition values of 30, 57, and 62% for MOF-5, Zn(II)-MOF, and Zn(II), respectively. Kinetic analysis showed a significant decrease in maximum oxygen uptake rate (OUR_max) without changes in substrate affinity constant (K_S), indicating a non-competitive inhibition pattern mainly associated with the bioavailability of free Zn²⁺, consistent with Zn(II) solubility and speciation results. In contrast, the NO process was strongly inhibited by MOF-5, whereas Zn(II)-MOF and Zn(II) showed limited effects. For MOF-5. The simultaneous reduction in OUR_max and K_S for MOF-5, suggests an apparent uncompetitive-type response, likely associated with the material as a whole rather than with Zn²⁺ release or ligand effects individually. Overall, these findings demonstrate that the inhibitory effects of Zn-MOFs on nitrification are process-dependent and controlled by the combined contributions of Zn²⁺ bioavailability, ligand properties, and intrinsic material characteristics.