Bio-electrochemical systems for the remediation of metal-ion effluents

Abstract

The roots of this study stem from the applied sciences of microbiology and electrochemistry to form the exciting new field of bio-electrochemistry. Our aim here being the application of bioelectrochemical processes for social and environmental value in toxic metal ion remediation and recovery from the discharge of aqueous mine and industrial effluents. This within a broader vision of reducing the present burden caused by industrial and mining anthropogenic activity on the planet we inhabit. These processes we have explored within a green chemistry philosophy with the application of chemical engineering principles. Our aims being (i) to further the scientific state of art and (ii) conceptualize the feasible engineering of novel metal remediation strategies, with the lucrative application of bacterial cells as green “nanofactories” and recovery of metallic biogenic nanoparticles with application in the ever growing field of nanotechnology. The proof of principle has been evaluated with a systematic study of Au3+, Co2+ and Fe3+ metallic cationic species (Co < 500 ppm) dissolved in acidic (pH < 3) aqueous electrolytes and their removal by microbiological (chapter 3) and bioelectrochemical (chapter 4) processes. Electrochemical remediation as described by electronation charge transfer at an electrified interface for various potentials causes the electrodeposition of metal ions upon electrode surfaces and hence separation by phase transformation. Of note, base cations such Co2+ and Fe3+ co-deposited with the evolution of hydrogen gas could be applied as electron donors for chemolithotrophic bacteria as part of dissimilatory respiration. Microbial biosorption of metal ions by means of ionized groups located on the outer membrane of the outer lipopolysaccharide leaf of gram negative bacteria, with some evidence of bio-reduction via dissimilatory and redox resistance mechanisms, with biogenic nanoparticles produced as a consequence. Bio-electrochemistry formed by the collaboration of these two processes where electroactive bacteria such as that of the Shewanella genus are known to respire by the application of cathodic currents directly via bio-nanowires or indirectly using in-situ electron mediators or in-situ hydrogen production. The effects of bacteria on electronation thermodynamics were investigated in chapter 4 with observed positive shifts in reversible potentials (Er) for AuCl3- electrodeposition.