Bioleaching of E-Waste: Influence of Printed Circuit Boards on the Activity of Acidophilic Iron-Oxidizing Bacteria

Abstract

Bioleaching is a promising strategy to recover valuable metals from spent printed circuit boards (PCBs). The performance of the process is catalyzed by microorganisms, which the toxic effect of PCBs can inhibit. This study aimed to investigate the capacity of an acidophilic iron-oxidizing culture, mainly composed of Leptospirillum ferriphilum, to oxidize iron in PCB-enriched environments. The culture pre-adapted to 1% (w/v) PCB content successfully thrived in leachates with the equivalent of 6% of PCBs, containing 8.5 g L^-1 Cu, 8 g L^-1 Fe, 1 g L^-1 Zn, 92 mg L^-1 Ni, 12.6 mg L^-1 Pb, and 4.4 mg L^-1 Co, among other metals. However, the inhibiting effect of PCBs limited the microbial activity by delaying the onset of the exponential iron oxidation. Successive subcultures boosted the activity of the culture by reducing this delay by up to 2.6 times under batch conditions. Subcultures also favored the rapid establishment of high microbial activity in continuous mode.

Keywords: Leptospirillum ferriphilum; bioleaching; iron-oxidizing bacteria; pre-oxidation phase; printed circuit boards; subculturing.

FIGURE 1

Scheme of the successive subculturing procedure applied on 2.2 L-STR in batch conditions. Each culture contained 3Cm medium and 2% (w/v) PCBs.

FIGURE 2

Microbial activity at various concentrations of the chemical leachate produced with 10% (w/v) PCBs: (A) Bacterial concentration and (B) redox potential. The experiments were performed in shake flasks and started with 8 g L^{– 1} Fe(II) and 10⁷ cells mL^{− 1}. The error bars represent the range of all values obtained under fixed operating conditions. Additional information on experiments with 80% (v/v) leachate the abiotic control is available up to day 25, although no significant changes in either redox behavior or bacterial concentration were observed.

FIGURE 3

Follow up of redox potential in bioleaching batch subcultures in 2.2 L-STRs: (A) subcultures SO, SI, and S2, (B) subcultures S3, S4, and S5. The first stage involves the microbial oxidation of the 3 Cm medium while the second stage starts with the addition of 2% (w/v) PCBs.

FIGURE 4

Follow-up of (A) redox potential and (B) bacterial concentration overtime during the startup of continuous flow PCB bioleaching experiments in 2.2 L-STRs. Three scenarios of the startup were tested: I was only inoculated with the non-adapted culture; II performed a batch test on the adapted culture before starting the continuous flow process; III performed two successive batch subcultures on the adapted culture prior to the continuous flow process. Conditions of continuous flow bioleaching were set at 48 h of HRT and 1% (w/v) PCBs.