Periplasmic transport channels to accelerate the proton motive force for efficient groundwater bioelectrocatalytic Cr(VI) reduction

Groundwater remediation presents significant challenges, while microbial electrocatalytic remediation is promising but suffers from low efficiency due to low periplasmic electron transfer. Unlike traditional methods, which involve directly improving the conditions for electron transfer, herein, we first developed an electron-enhanced proton motive force (PMF) strategy for effective Cr(VI) removal. The proton is used as a new mutagenic source to mutate microorganisms by coupling with a micro-electric field. Interestingly, the dominant bacterial species of Rhizobium can directly capture protons (H + ) and electrons from the cathode as energy sources, resulting in greater than 97% reduction and immobilization of Cr(VI) in groundwater. Since the increase in the proton concentration gradient across the cytoplasmic membrane is due to the free diffusion of protons through new proton transfer channels, the extracellular electron transfer (EET) process is accelerated significantly, and Cr(VI) can also shuttle inside the cell to be reduced. 13 C isotope tracer technology and genetic volcano maps were used to further monitor the active transmembrane ETC transport pathway. The accumulated H + transfers electrons through reduced coenzymes, such as glycoside hydrolases (GH3, GH13-40, and GH5-15) and beta-galactosidase (CBM43), establishing a new transmembrane transfer pathway in the periplasmic space. In addition, unlike conventional bioremediation techniques, the proton gradient intensifies the motive force (PMF-H + ) to accelerate the synthesis of adenosine triphosphate, which provides energy for the bioremediation of Cr(VI), leading to a high bioelectrocatalytic rate. The new PMF-H + strategy successfully restored Cr(VI)-contaminated groundwater in situ to meet the water quality standard, indicating great potential for groundwater remediation.