Ultrasound-assisted nitric acid pretreatment for enhanced biorecovery of platinum group metals from spent automotive catalyst

Abstract

Recovery of platinum group metals (PGMs; Platinum, Palladium, and Rhodium) from spent automotive catalysts (SAC) is important due to the need for resource conservation. SAC contains precious metals, along with bulk concentration of base metals in their native or alloy form. The presence of these non-target metals in the waste reduces the recovery of PGMs. In this study, the effect of ultrasound-assisted nitric acid pretreatment on enhancing the leaching of PGMs by cyanogenic bacteria was investigated. SAC samples were first pretreated using ultrasonication and acid to remove competing metals that otherwise interfere in the subsequent PGM-cyanidation during bioleaching. The optimal parameters in the pretreatment were found to be ultrasound power 80%, nitric acid concentration 6M, ultrasound duration 50 min, and ultrasound frequency 37 kHz. Mesophilic bacteria Pseudomonas fluorescens and Bacillus megaterium were used to bioleach PGMs from untreated and pretreated SAC. Pretreated SAC samples showed a higher recovery of PGMs. Using two-step bioleaching at a pulp density of 1% (w/v) and at pH 9, P. fluorescens yielded a maximum recovery of Pt (38%), Pd (44%), and Rh (91%) for pretreated samples. The corresponding values for B. megaterium were Pt (35%), Pd (41%), and Rh (82%). Ultrasound-assisted nitric acid pretreatment preceding bioleaching by cyanogenic bacteria is a promising biorecovery technique for the extraction of PGMs from spent automotive catalysts.