Unveiling the synergistic mechanisms of multi-ionic removal in microbial desalination cells: Ion transfer, electrochemical behavior and microbial response

Microbial desalination cells (MDCs) have traditionally employed simplified NaCl solutions as feedwater for synchronous desalination and bioenergy recovery. Nevertheless, the specific mechanisms by which MDCs remove complex multi-ions from saline wastewater remain obscure. This study thoroughly investigated ion migration, bioelectrochemical dynamics, and microbial ecological responses across three distinct configurations: monovalent ions − PMDC, divalent cations − CMDC and anions − AMDC. Results showed desalination efficiencies exceeding 97 % among all bioreactors, with CMDC outperforming AMDC yet lower than PMDC. The transmembrane behavior of ions with varying charges was regulated by electric field gradients, with cation migration hierarchies following Na⁺ > Ca²⁺ > Mg²⁺ and anion transfer prioritizing CO₃^2– > SO₄^2- > Cl^-. A mathematical model further elucidated variations in ion diffusion and salt transfer. The CMDC exhibited peak power density (927 mW/m²), achieving 1.57 and 1.99 times that of PMDC and AMDC, due to reduced ohmic resistance and improved cathodic kinetics. Prolonged operation exposed critical limitations: multivalent cations induced 21.4 % desalination efficiency loss and 38.3 % power decline via membrane scaling and biofouling accumulation. Microbial community analysis highlighted Lentimicrobium (26.7 %), Desulfomicrobium (12.3 %), and Oscillochloris (9.2 %) as dominant electroactive genera, with their synergistic interactions and functional metabolic pathways (carbon fixation, nitrogen metabolism, and sulfur reduction) critically enhancing organic degradation and bioelectricity generation in multi-ionic MDCs. This comprehensive study bridges critical gaps in understanding electrochemical and microbial responses in MDCs treating hypersaline wastewater, advancing their practical implementation.