Base metals recovery from waste printed circuit board leachate using biogenic hydrogen sulfide gas

Abstract

Electronic waste, or e-waste, represents one of the rapidly expanding categories of waste worldwide. By 2019, the global production of e-waste had surged to 53.6 million tons. Due to its substantial metal content, e-waste holds significant financial value, estimated at US$57 billion globally in 2019, predominantly concentrated in printed circuit boards (PCBs). Previous studies have explored hydrometallurgy techniques to extract base metals from PCBs, but effectively recovering these solubilised metals remained a challenge. This research sought to assess metal recovery from PCB waste leachate by utilising hydrogen sulfide generated through a consortium of sulfate-reducing bacteria (SRB) in a fluidised bed reactor (FBR). Both lactate and glycerol were examined as potential organic electron donors for the sulfate reduction. With lactate (1 g L⁻¹) as the electron donor, the FBR achieved an average sulfate reduction efficiency of 62%, with a hydrogen sulfide (H₂S) production rate of 250 mg H₂S-S L⁻¹ d⁻¹ and H₂S-S concentration of 300 mg L⁻¹ in the effluent. When glycerol was the organic electron donor, the average sulfate reduction efficiency was 49%, H₂S production rate was 210 mg H₂S-S L⁻¹ d⁻¹ and H₂S-S concentration was 260 mg L⁻¹. Desulfovibrio, Desulfococcus and Desulfomicrobium were the dominant sulfate reducers in the FBR. The resulting dissolved hydrogen sulfide was employed to recover metals from e-waste leach liquor. Utilising biogenic sulfide and NaOH, a notably high precipitation efficiency (>99%) was attained for aluminum, nickel, copper, and zinc. Additionally, iron, utilised in the e-waste leaching process, was also recovered with an efficiency exceeding 99%. The precipitation of metals occurred within a pH range from 1.5 to 8.5. Overall, this process facilitated the formation of valuable mixed-metal precipitates from waste PCB-derived leachate. These precipitates could undergo further purification or serve as raw material for subsequent processes.