Bio-electrochemical system-a novel technology for metal recovery

Metals like platinum, silver, nickel, cadmium, chromium, copper, zinc, gold etc. are released in the environment through the discharge of industrial wastewaters. Wastewater containing these metals poses severe health hazards to the human being and ecosystem, if discharged without proper treatment. Recovery of metals from the wastewater is one of the most sustainable approach to reduce their discharge in the environment. Conventional technologies for metal recovery include coagulation-flocculation, chemical precipitation and membrane processes, but they are generally energy and chemical intensive. Disposal of hazardous sludge further aggravate the problem due to land requirement. To overcome the constraints of existing technology, bio-electrochemical technology provides a novel and alternative approach for metal recovery. Bio-electrochemical reactor converts chemical energy of organic compounds to electrical energy through catalytic reactions of microorganisms. Bio-electrochemical technology reduces metal ions into metal by accepting the electron in the cathode chamber generated through the oxidation of organic substrate in the anode chamber. This paper describes various facets of Bio-electrochemical system with special emphasis on its efficiency, optimization of process parameters, cost and the efficacy of various electrodes. The metal recovery efficiency of the bio-electrochemical technology varies depending upon the initial concentration of metal ions, substrate utilised, reaction time and reactor configuration and found to vary from 70% to 99% depending upon the metal concentration, substrate utilised, reactor configuration and duration of the reactor. The distance between the electrodes also plays a major role. The studies revealed that minimizing the distance between electrodes enhances the performances of the system. Among different type of electrodes, graphite electrodes were the best in enhancing the performance of the system than aluminium, stainless steel and iron electrodes. The current density generation depends upon the microbial community in the anode chamber, type and concentration of substrate utilised. When acetate was utilized as substrate, current density generated was 0.8 mA/cm² using pre-acclimated microbial culture as inoculum whereas current density generated was 2.05 mA/cm² when Sodium fumarate was used as substrate with microbial inoculum of Geobacter. Sulfurreducens. The paper also highlights various drawbacks and constraints with focus on the application of this technology at field scale.