Biotechnological recycling of hazardous waste PCBs using Sulfobacillus thermosulfidooxidans through pretreatment of toxicant metals: Process optimization and kinetic studies

The present study dealt with the restricted microbial tolerance for lead and tin during bioleaching of waste printed circuit boards (WPCBs) and lower extraction yields of valuable metals. Pretreatment of WPCBs in 4.0 mol/L HNO3 at 90 degrees C for 180 min duration prominently dissolved the toxicant metals before the mi-crobial mobilization of valuable metals. Acid pretreatment followed the first-order kinetics that exhibiting an intermediate-controlled mechanism with the apparent activation energy determined to be E-a(Pb), 25.1 kJ/mol and Ea(Sn), 21.9 kJ/mol. Thereafter, the parametric optimization of aeration rate, O-2-enrichment, external CO2 supply, temperature, and time for bioleaching of ground WPCBs was examined using Sulfo-bacillus thermosulfidooxidans (strain RDB). A favourable condition for Cu-bioleaching under higher oxida-tive environment in comparison to Ni and Zn exhibited the auto-catalytic behaviour of Cu2+ in the biological system. More than 92% of valuable metals were extracted under the optimal condition of aeration rate, 0.5 L/min; O-2-enrichement dosage, 30%; external CO2 supply, 0.1%; temperature, 55 degrees C; and time, 18 days. The bioleaching kinetics followed shrinking core model that exhibiting the shifting of mass transfer from chemically-controlled to the diffusion-controlled mechanism. This process offers two-fold advantages that restoring the valuable metals with low-emission biotechnological route for waste valorization.

Automated summary

This study focused on improving the extraction yield of valuable metals during bioleaching of waste printed circuit boards (WPCBs) by acid pretreatment and parametric optimization. The research demonstrated the auto-catalytic behavior of Cu and the transition from chemically-controlled to diffusion-controlled mechanism in bioleaching kinetics.