Bioleaching of Metals Cu Au and Ag from Waste Computer Printed Circuit Boards using Cyanogenic Microorganisms

Abstract

The present work has addressed an important global issue the electronic waste (e-waste). The bioleaching process was applied to treat waste computer printed circuited boards (CPCBs) for recovery of Cu, Au, and Ag using indigenous bacterial strains isolated from an abandoned gold mine and e-waste recycling facility. The tolerance levels of bacterial strains towards e-waste toxicity was examined using pulverized (particle size and#8804;150 and#956;m) waste CPCBs per liter (L) of the culture medium. The toxicity assessment and dose-response analysis of waste CPCBs showed EC50 values of 325.7, 128.9, 98.7, and 90.8 g/L for Pseudomonas balearica, Bacillus sp. SAG3, Bacillus megaterium SAG1 and Lysinibacillus sphaericus SAG2, respectively, whereas,for Chromobacterium violaceum EC50 was 83.70 g/L. To maximize precious metals dissolution; optimization was conducted using both one-factor-at-a-time (OFAT) and central composite design of response surface methodology (CCD-RSM). The process parameters such as initial pH, e-waste pulp density, temperature, and precursor molecule (glycine) were optimized to enhance metal mobilization. The maximum metals recovery under OFAT optimized conditions occurred at 10 g/L pulp density, 9.0 pH, 5 g/L glycine concentration, and 30°C temperature for 7 days by C. violaceum; was 87.5% and 73.6% of Cu and Au, respectively. Whereas, Ag (33.8%) mobilization was maximum by P. balearica. The kinetic modeling results showed that bioleaching using cyanogenic microorganisms followed the first-order reaction kinetics, where the rate of metal solubilization from CPCBs depends upon microbial lixiviant production. The CCD-RSM optimization extracted 81.7, 73.9 and 41.6% of Cu, Au, and Ag by P. balearica at pulp density 5 g/L, glycine concentration 6.8 g/L, initial pH 8.6, and temperature 31.2°C, respectively. The CCD-RSM proposed three polynomial quadratic models which can be used as an effective tool to predict bioleaching of Cu, Au, and Ag from e-waste using cyanogenic microorganisms.