*[1]Soliu, A. A., 1Bajepade, T. T., and [2]Alade, B. M.
1Department of Microbiology, Faculty of Pure and Applied Science, Kwara State University, Malete, Kwara State, Nigeria
2Department of Science Laboratory Technology, Institute of Applied Sciences, Kwara State Polytechnic, Ilorin, Nigeria
*Correspondence to: Soliuabdulgafar018@gmail.com; ORCID: 0009–0009–3417–3325
Abstract:
Background: Informal handling and disposal of electronic waste can release heavy metals into soil and select for metal-tolerant microorganisms. This study aimed to isolate and identify heavy metal-tolerant bacteria from soils around two informal e-waste handling sites in Ilorin, Nigeria, assess changes in dissolved metal concentrations during 21 days of sequential enrichment with waste printed circuit boards (WPCBs), and evaluate the isolates as potential candidates for future bioleaching studies.
Methodology: In this study, samples of soils in two metal-polluted markets (Sango Kulende and Ipata Oloje Spare Parts) and a non-market control zone at Ilorin, Nigeria, were collected at 5 sub-locations in each site. Powdered waste printed circuit boards (WPCBs) were added to the soil samples and cultured in Luria Bertani (LB) broth at successively growing pulp densities (1-5 per cent) for 21 days. Aliquots of the cultures were diluted in 0.85% NaCl solution and plated on nutrient agar plates, which were then incubated for 48-96 hours at 30oC. Pure isolates were identified by morphological and biochemical characteristics. Atomic absorption spectrophotometry (AAS) was used to confirm the presence of iron (Fe), copper (Cu), nickel (Ni), and zinc (Zn) in the WPCBs before and after soil enrichment.
Results: Four morphologically different isolates were obtained on nutrient agar following serial dilution of enriched cultures and morphological and biochemical characterisation identified them as Lactobacillus plantarum, Bacillus subtilis, Proteus vulgaris and Staphylococcus aureus. Monitoring AAS of culture supernatants in three replication weeks demonstrated the steady rise of dissolved metal concentration mainly during the first and third weeks with a significant rise of Cu and Ni concentrations, suggesting a continuous mobilization of metals by the bacterial isolates. After week three, the concentration of Cu in Ipata soil + powdered e-waste (IS +PEW) cultures was 14.429ppm, and the concentration of Ni in Sango soil + general e-waste (SS +GEW) cultures was 6.699ppm. The isolates, especially those of B. subtilis and P. vulgaris, exhibited strong tolerance to copper and nickel, which increased with increased exposure.
Conclusion: These results suggest that soil bacteria used in informal e-waste recycling locations can be utilized as a bioleaching source and should be more explored regarding their resistance and scalability.
Keywords: Electronic waste, bioleaching, heavy metal tolerance, printed circuit boards, bioremediation
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