Effective methods for extracting extracellular polymeric substances from Shewanella oneidensis MR-1.

Extracellular polymeric substances (EPS) play crucial roles in bio-aggregate formation and survival of bacterial cells. To develop an effective but harmless method for EPS extraction from Shewanella oneidensis MR-1, five extraction methods, i.e. centrifugation (control), heating (40, 45, 50, and 60 °C), and treatments with H2SO4, ethylenediaminetetraacetic acid (EDTA) and NaOH, were examined, respectively. Results from scanning electron microscope and flow cytometric analyses indicate that MR-1 cells were severely broken by H2SO4, NaOH and heating temperature ≥45 °C. Proteins and polysaccharides in EPS extracted by heating at 40 °C were 7.12 and 1.60 mg g-1 dry cell, respectively. Although EDTA treatment had a relatively lower yield of EPS (proteins and polysaccharides yields of 5.15 and 1.30 mg g-1 dry cell, respectively), cell lysis was barely found after EPS extraction. Three peaks were identified from the three-dimensional excitation-emission matrix spectrum of each EPS sample, suggesting the presence of protein-like substances. Furthermore, the peak intensity was in good accordance with protein concentration measured by the chemical analysis. In short, heating (40 °C) and EDTA treatments were found the most suitable methods for EPS extraction considering the cell lysis and EPS content, composition and functional groups together.

Enhanced bioleaching efficiency of metals from E-wastes driven by biochar.

Electronic wastes (E-wastes) contain a huge amount of valuable metals that are worth recovering. Bioleaching has attracted widespread attention as an environment-friendly and low-cost technology for the recycling of E-wastes. To avoid the disadvantages of being time-consuming or having a relatively low efficiency, biochar with redox activity was used to enhance bioleaching efficiency of metals from a basic E-waste (i.e., printed circuit boards in this study). The role of biochar was examined through three basic processes: Carbon-mediated, Sulfur-mediated and Iron-mediated bioleaching pathways. Although no obvious enhancement of bioleaching performance was observed in the C-mediated and S-mediated systems, Fe-mediated bioleaching was significantly promoted by the participation of biochar, and its leaching time was decreased by one-third compared with that of a biochar-free system. By mapping the dynamic concentration of Fe(II) and Cu(II), biochar was proved to facilitate the redox action between Fe(II) to Fe(III), which resulted in effective leaching of Cu. Two dominant functional species consisting of Alicyclobacillus spp. and Sulfobacillus spp. may cooperate in the Fe-mediated bioleaching system, and the ratio of these two species was regulated by biochar for enhancing the efficiency of bioleaching. Hence, this work provides a method to improve bioleaching efficiency with low-cost solid redox media.