Bioremoval of heavy metals from recycling industry electronic waste by a consortium of moderate thermophiles: process development and optimization

The aim of the current work is to develop an environmentally friendly process for the removal of heavy metals (Cu, Zn, Ni, Cd, Al, Cr, Pb) from recycling industry electronic waste with a consortium of Sulfobacillus thermosulfidooxidans and Thermoplasma acidophilum. The performances of commercial S0 powder and biogenic S0 sludge as substrates for the bio-removal of heavy metals were compared. Empirical models for the bioleaching process based on a statistical analysis were developed to evaluate the individual and combined effects of critical variables including S0 dosage, particle size, pulp density and bacterial feed formulation (inoculum size and inoculation style) in shaken flasks while specifying the effective variable ranges. Upscale feasibility experiments in a stirred tank reactor demonstrated a maximum metal bio-removal efficiency (92%) at a 1% dosage of biogenic S0 sludge and 2% dosage of commercial S0 powder (82%), given a 15% pulp density and 150 μm particle size with an intermittent low-concentration addition of inoculum (1ｘ106 cells/mL). Biogenic S0 sludge showed a higher degree of S0 oxidation (95%) in a shorter time period (12 days) compared to commercial S0 powder (82% in 24 days), thereby reducing the process cost. Risk assessments of discarded electronic wastes before and after bioremediation by the toxicity characteristic leaching procedure (TCLP), waste extraction test (WET), synthetic precipitation test (SPLP) and total threshold limit concentration (TTLC) indicated that the leaching/toxicity of bio-remediated residue was well within the regulatory limits.