| E-wastes, especiallywaste printed circuit boards, are becoming a source of valuable materials due to the quantity of metals and they also havegreat recycling value.Recently, the bioleaching technology attract wide attention. Nowadays, researches focused on the factors influencing bioleacmhing for more soluble elements from waste PCBs, such as initial concentrates of Fe2+,initial pH, the dosage of metal powder, inoculation quantityand so on. However, the bioleaching mechanism pertaining to metal extraction from waste PCBs via acidophilic bacteria could refer to the achievements in metal sulfides area. But metals in printed circuited boards are mainly zero valence and the bioleaching mechanism has been little investigated, especially the adsorption behavior, the adsorption kinetic and the adsorption meachanisms need further studies. Based on the contact bioleaching mechanism, the adsorption behavior between bacteria and printed circuit boards powder and the adsorption kineticwere estimated, the effect of LPS was identified in the process of adsorption and the possible adsorption mechanism was explored. The result was shown as the following:(1) The mechanism of bioleiaching from printed circuit boards was identified and the effect of adsorption in the contact bioleaching mechanism was explored via experiments of chemical leaching, different initial concentrations of Fe2+ anddialysis bags. The results indicated that the chemical leaching efficiency was just 2.3%, compared with 83.8% of bioleaching. Due to dialysis bags which can prevent bacteria from contacting with metal powder, the extraction efficiency of copper was only 47.9% in the encapsulated system while was 81.43% in the bioleaching system. It shown that the bioleaching system was consist of contactand non-contact mechanisms and the contact mechanism played a major role in the leaching process. It was considered that adsorption was the first step of contact bioleaching and there was adsorption between A. ferrooxidans and circuit boards.(2) Under different initial concentrations of Acidithiobacillus ferrooxidans(A. ferrooxidans), the adsorption behavior of copper leaching from circuit boards by A. ferrooxidans was investigated, the adsorption of A. ferrooxidans onto the surface of circuit boards was estimated and the kinetic characteristics of adsorption behavior were analyzed.The results shown that there was the adsorption behavior between A. ferrooxidansand the surface of circuit boards and theadsorption equilibrium of A. ferrooxidans onto the surface of circuit boards was reached in 5h. The adsorption rates and attached biomass increased with the increase of the initial concentration of bacteria. On the condition that the initial concentrations were 2.4×105, 2.4×107 and 2.4×109 cells/mL,the individual maximum sorption capacity were 1.8701, 2.3552 and 2.9833 mg/g, the percentage of adsorbed biomass of the total reached 43.75%, 53.97% and 55.94%. The adsorption behavior between A. ferrooxidans and circuit boards confirmed to the pseudo-first order kinetic model.(3) By the treatment of EDTA, the removal effect of lipopylysaccharide(LPS) was discussed and the change of bacteria activity was also explored. The influence of LPS during adsorption between A. ferrooxidans and metal powder was verified.The content of lipopylysaccharide on the surface of A. ferrooxidanswas determined by Phenol-sulfuric acid method. The results implied that the content of the untreated and treated with EDTA were 50.14μ g/1010 cells and 563.14ug/1010 cells, respectively. At the same time, the removal effects of lipopylysaccharide were identified further via Ruthenium red adsorption method and the adsorbance of the untreated and treated with EDTA achieved 27.9mg/1012 cells and 7.46mg/1012 cells, respectively. The results of T-test of statisticalsoftware indicated that the lipopylysaccharides of A. ferrooxidans could be effectively stripped off via the treatment of EDTA(P<0.5). Though bacteria activity could be weaken through the treatment of EDTA, it recovered after the development of a period of time.(4) The influence of LPS on the adsorption between A. ferrooxidans and printed circuit boards powder were studied by Zeta and FTIRvia experiments of the untreated, treated with EDTA and treated with EDTA+LPS. The Zeta-potentialsexhibited changes from 2.5 to 6.3, from 3.2 to 4.2 and from 2.8 to 5.2 in the three systems of the untreated, treated with EDTA and treated with EDTA+LPS, respectively. Till 24 h, the i.e.p.s of all the three systems were about 6.0.It illustated thatelectrostatic effect was greater than hydrophobic effect in the adsorption process between A. ferrooxidansand printed circuit boards powder and the process was mainly driven by electrostatic force.The FTIR spectra obtained shown the presence of-CH3,-CH2,-CH,-CONH, COO-,-NH3,-NH2and-NH on the surface of all the cells in the three systems and. It implied there was a amount of proteinon the surface in all the systems. At 0h, the CH3CO- group was only found on the surface of the untreated. But at 5h, there was no found of theCH3CO- group on the surface of all the cells. At 24 h, there was the presence of the CH3CO- group on the surface of all the cells. It indicated that LPS played a role in A. ferrooxidansadhension to powder surfaces in the adsorption process and the CH3CO- group was described and may took an important part. Besides, proteins may also took effect in the process of A. ferrooxidansadhension to powder surfaces. |
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