Sulphur from a contaminated pond has been successfully recovered and used in a high-performance battery. This waste-to-wealth feat was achieved by a group of researchers from CSIR-Central Electrochemical Research Institute (CECRI), Karaikudi, in Tamil Nadu.
Published recently in the journal Separation and Purification Technology, this is the first time that the sulphur recovery process was done by an integrated approach of biological and electrochemical oxidation process.
Water from a pond contaminated by sodium dithionate-processing industry was collected and studied. Sodium dithionate salt is used in many textile industries to remove the excess dye and unintended colours, thereby improving overall colour quality. It is also used in processes in leather, certain food and plastic industries. The effluents from these industries can cause a range of health and environmental hazards. Removal or reduction of the sulphur in the waste water has always been a challenge.
Bio-electrochemical process
Sulphate-reducing bacteria (SRB), which have a natural ability to convert sulphate to sulphide, were used in the biological treatment process. The bacteria are capable of using sulphate instead of oxygen for their energy source. Due to reduced nutrients, the conversion rate to sulphide was very low in the pond.
After 72 hours of incubation in lab conditions with additional supply of nutrients, three dominant strains— Stenotrophomonas maltophilia, Bacillus cereus, and Bacillus licheniformis —in the pond were identified. These bacteria are already used in many industries for treatment of their effluents before discharge.
When the researchers simulated the micro-enviroment where oxygen supply is less by keeping the bacteria without oxygen for 20 days and added iron powder, the bacteria liberated hydrogen sulphide gas. The gas was collected and dissolved in sodium hydroxide to form sodium sulphide. The sulphide was further oxidised to elemental sulphur using an electrochemical process.
A double-compartment cell was constructed, and on passing current, the elemental sulphur precipitated at the electrodes. Though the bacteria are used to treat industrial wastes, this is the first time an electrochemical approach is applied to further convert sulphide to elemental sulphur. This sulphur can be used in various applications such as production of sulphuric acid and liquid sulphur dioxide. Since the cost of pure sulphur is high, the new approach can help recover sulphur from waste and turn it into a resource.
When the recovered sulphur was used as cathode in lithium sulphur (Li-S) battery, a current of 1050 mAh/g was produced. After 10 cycles the current produced reduced to 840 mAh/g. The researchers are planning to conduct more studies to improve the conductivity of the sulphur in order to get higher discharge capacity.
