Magnetic Bioeffect And Its Mechanism Of Cr(VI) Removal For Wastewater Treatment

The treatment of heavy metal pollution by biological process is one of the most active research fields in recent years. Cr(â…¥)-removal by the reduction and absorption of the predominated strains-containing bio-system is one of the most prospective methods, and to apply the beneficial magnetic bio-effect in improving the stability and anti-impaction of the bio-system is one of the most effective methods of pushing and widening the application of biological process in heavy metal wastewater treatmentFirstly, based on the screening and optimizing of the predominated strains, the Cr(â…¥)-containing wastewater was treated in batch by a bio-filter, in which the predominated strains inoculated and cultured, then several main running conditions and their effect on Cr(â…¥) / total Cr removal were studied. Secondly, based on the selection of optimum magnetic density, the action modes of magnetic field were also compared by analyzing the variety of the anaerobic sludge activity. And then the Cr(â…¥) / total Cr removal efficiencies of the magnetic field-bio-system containing the predominated strains were tested by inflowing the factory wastewater. Finally, by instrument analysis (such as XPS, TEM-EDS, SEM-EDS) and experimental design, the absorption-reduction efficiency of different cellular structures, Cr settlement site, the influence of magnetic field on the composition of anaerobic biogas etc were researched. Thereout the mechanism of the predominated strains and the magnetic field on the Cr(â…¥) bio-removal were concluded and summarized.Two predominated strains numbered 2-1^# and 5^# were obtained, which could stand the Cr(â…¥) concentration of 200mg/L and were identified with Bacillus cereus and Brevibacillus parabrevis, respectively. For the bio-filter containing the predominated strains, the COD_Cr value of about 140mg/L, the pump flow rate of 47mL/min and the addition of trace metal elements were the optimum running conditions, and the system showed good stability. The continuous Cr(â…¥) absorption capability of the sludge containing predominated strains was 27-28mg/g dry sludge, and the absorption capability could increase by 29.68% after regeneration. By the SEM, the Bacillus were observed as the dominant of the bio-film and the cocci such as Methanosaricina sp. and Methanithrix sp. also presented themselves in the sludge of the bottom bio-filter.By determining the predominated strains' growth and propagation rate under different magnetic densities, 6.0 mT was selected as the optimum magnetic density ultimately and applied in the study henceforth. The batch treatment was identified as the best ideal process for Cr(â…¥) bio-removal, but high sludge concentration was one of the necessary conditions. The magnetic field could increase the sludge continuous absorption-regeneration capability of Cr(â…¥) by 123.17%. And it took less 1-3 hours for the magnetic-bio-system in treating the factory wastewater containing Cr(â…¥) to meet the state one class standard of effluent discharge than the control. It was found that both direct and indirect magnetic fields could increase the U_max-CH4 and decreasethe U_maxCODCr of the sludge.XPS spectras for the superstratum and underlayer biofilm show that the ratio between Cr(â…¥) and Cr(â…¥) is about 1:4,and the state of Cr(â…¢) includes Cr₂O₃ and Cr(OH)₃, while K₂Cr₂O₇ is showed as the state of Cr(â…¥). The tests on the Cr(â…¥) reduction-absorption contribution of strains' EPS,cell wall and sphaeroplast illuminate that even the sphaeroplast can reduce and absorb more Cr(â…¥) than the intact strain, the EPS still is the most contributive construction. The introduction of magnetic field decrease the output of NH3,H₂S and VOCs, while increase the output of CO₂ and CH₄. By TEM-EDS, Cr could detect only in extracellular granules. That is to say, the influence of the magnetic field on Cr(â…¥)-biosorption site was not obvious.Based on the experimental researches and literature analysis, the Cr(â…¥) reduction and removal mechanism of the sludge system containing predominated strains and the mechanism of the magnetic bioeffect on Cr(â…¥) reduction and removal were concluded. The Cr(â…¥) reduction and removal mechanism of the sludge system containing predominated strains included the extracellular reduction-settlement process (mechanismâ… ) , extracellular direct bond-settlement process (mechanismâ…¡) and intercellular reduction-metabolism process (mechanismâ…¢) . The contribution of each mechanism could vary with the environmental and operational conditions. And the magnetic field can improve the Cr(â…¥) removal efficiency of the predominated strains contained bio-system by increasing the activity of non-methane-production bacteria, the methane output and system stability.The research harvests may provide some useful experiment basis and academic foundation for the popularization and application of the magnetic field-predominated strains combined technique.