Heavy metal pollution is an environmental and social problem that needs to be solved urgently. Heavy metal elements in nature will form a variety of minerals with different states of valence and show different physical and chemical properties, which has important implications for the treatment of heavy metal pollution. The valence state of heavy metal ions can be regulated through electron transfer and electron absorption from microbe, electrode, and semiconductor mineral photocatalysis. This review analyzes the reduction effect of heavy metal ions by microbes, microbial electrolysis system (MES), photoelectron and photoelectron-microorganism synergistic effect. The redox reaction and the energy utilization mechanism during the heavy metal ions reduction process are illustrated based on oxidation reduction potential principle. Detailed transmembrane electron transfer process and molecular networks are elaborated for direct/indirect electron transfer pathways during reduction of heavy metal ions by microorganism. The electron transfer from microorganism cells to electrode and reverse electron absorption-utilization process are also expounded for heavy metal ion valence state regulation by microbial electrolysis system and photoelectron-microorganism synergistic effect. Finally, the interaction network and energy utilization mechanism are proposed to elucidate the regulation of heavy metal ion valence state by microorganisms, electrodes, semiconductor minerals and light. This review will provide a reference for the further study regarding the microbial extracellular electron transfer, the photoelectron utilization by microorganism and the valence state regulation of heavy metal ions by microbe-photoelectron coordination as well as its environmental significance.
Contents
1 Introduction
2 Reduction and immobilization of heavy metal ions by microorganisms and electrochemical systems
2.1 Reduction of heavy metal ions by microorganisms
2.2 Reduction of heavy metal ions by microbial electrolysis system
3 Reduction of heavy metal ions by microorganism and photoelectron coordination
3.1 Reduction of heavy metal ions by photoelectron
3.2 Reduction of heavy metal ions by microorganism and photoelectron coordination
3.3 Mineralization, transformation and interface interaction during reduction of heavy metals induced by microorganisms and photoelectrons
4 Electrochemical mechanism of reduction of heavy metal ions by microorganisms, electrochemical systems and photoelectrons
5 The role of microbial electrode as well as electron transport and transfer during reduction of heavy metal ions
5.1 Electron transfer from microorganism to heavy metal ions
5.2 Electron transfer from microorganism to electrode
5.3 The reverse transfer of electrons:from electrode to microorganism
6 Conclusion and outlook