| Blast furnace dust (BFD) is a solid waste produced by the iron and steel industries with a general composition of iron oxides (approximately 50%) and small amounts of refractory oxides, e.g., aluminum, silicon, calcium and magnesium. In recent years, China’s steel output ranks first in the world, the crude steel production has exceeded 700 million tons by 2012. And the annual yield of BFD is more than 10 million tons.Monosodium glutamate (MSG), a sodium salt of L-glutamic acid, is used as flavor enhancer for foods. After the extraction of MSG, Monosodium glutamate wastewater (MSGW) has high levels of COD, BOD, sulfate, ammonium-N, and a low pH of 2-3. If discharged without treatment, it can cause severe environmental pollution and damage the ecological balance, which is a major obstacle for the development of MSG production.In this work, polyaluminum ferric chloride (PAFC) coagulant was produced using the BFD waste as the main raw material. We examined the impact of the molar ratio of aluminum to iron (Al/Fe) and basicity (B) of PAFC, which is [OH-]/[Al+Fe], on the content of (Al+Fe)b. Suitable conditions for separation and purification of (Al+Fe)b from PAFC by organic solvents precipitation method were explored, the structure characteristics of PAFC and (Al+Fe)b were investigated. The results of this study will promote the research progress of (Al+Fe) b species in PAFC, and then provide reference information for studies of PAFC coagulation effect on the removal of contaminants and its coagulating mechanism.The culture of Bacillus subtilis to produce y-PGA, mixed culture of Bacillus subtilis and Lipomyces starkeyi to produce microbial flocculant (MBF) and microbial lipid in MSGW were also studied in this paper. The purpose of this research was to determine the optimal cultivation conditions for Bacillus subtilis and Lipomyces starkeyi to produce MBF and microbial lipid using MSGW as a culture medium, which was to provide a method for economical production of y-PGA as well as a comprehensive utilization of MSGW. Moreover, the flocculation performances of culture supernatant and MBF were also investigated. The main conclusions were summarized as follows:(1) Fifty grams of blast furnace dust reacted with 150mL hydrochloric acid diluted solution for 3h at 90 ℃ and 10 grams of aluminum dross reacted with 150mL hydrochloric acid diluted solution for 2.5h under room temperature; In co-polymerization stage, the Al/Fe molar ratio=7:3, basicity B=1.8, polymerization temperature 60-70℃, and reaction time (pre-polymerization time of aluminum+ copolymerization time of aluminum and iron) 1.0h+3.0h.The coagulation tests indicated that self-made PAFC outperformed commercial PAC in turbidity and color removal. PAFC (Al/Fe=7:3, B=1.8) achieved above 99% for turbidity removal at dosage of 5mg/L (Al+Fe), and 77% for color removal at dosage of 60mg/L (Al+Fe). Self-made PAFC and flocs generated in water treating process were measured by Scanning Electron Microscopy. Surface charge characteristics of PAFC were measured by Zetasizer 3000HS. Charge neutralization was a very important coagulation mechanism of PAFC in turbidity removal process. And there were other coagulation mechanisms besides charge neutralization such as bridge connection and sweep flocculation, playing significant roles in decolorization process.(2) Both the Al/Fe molar ratio and basicity have significant influence on (Al+Fe)b content. The (Al+Fe)b content in PAFC peaked at 36.67% with Al/Fe=7:3, and basicity(B)=1.8. The optimal conditions for purifying (Al+Fe)b by organic solvents precipitation method were determined as follows:organic solvents (ethanol, propylene glycol and acetone mixed solution) volume ratio=2:1:7.50mL, 100mL and 50mL of organic mixed solution was added into lOmL of 0.3M PAFC solution to extract (Al+Fe)c, (Al+Fe)b and (Al+Fe)a, respectively.The (AI+Fe)b content in the separated samples reached over 75% with the yield higher than 50% of the total PAFC. Infrared spectrum of the extracted (Al+Fe)b, PAFC and commercial PAC showed that the content of hydroxyl bridge linked aluminum/iron in the extracted (Al+Fe)b sample was higher than that in PAC and PAFC. The transmission electron microscope images of the extracted (Al+Fe)b suggested 3D network structure of the (Al+Fe)b with rough surfaces, which benefits adsorption and sweep of contaminants.(3) The findings of the experiments suggested that the MSGW was an ideal medium for B. subtilis cultivation to produce γ-PGA. Single-factor experiments showed that B. subtilis growth and γ-PGA production were affected by cultivation conditions such as initial MSGW concentration, initial culture pH, inoculation concentration, and cultivation time. The maximum γ-PGA productivity (53.51 ±0.92 g L-1) was obtained by using Box-Behnken design (BBD) experiments to optimize the cultivation conditions. Simultaneously, results of coagulation tests showed that the turbidity and active red removal efficiency of crude y-PGA was about 80% and 55%, respectively.(4) Microbial flocculant (MBF) and microbial lipid were obtained through mixed cultivation of Bacillus subtilis and Lipomyces starkeyi using MSGW as culture medium. The biomass of mixed cultivation was higher than that of pure culture of single species, which indicated that the growth of Bacillus subtilis and Lipomyces starkeyi had mutual promoted effects. The initial MSGW concentration, initial culture pH, cultivation temperature, inoculation concentration, and cultivation time had significant influences to the MBF and microbial lipid production, as well as the COD, NH3-N removal efficiency of MSGW. The optimal conditions of mixed cultivation were as follows:4-fold diluted MSGW with pH 5.5, culture temperature 30℃, the inoculation volume of Bacillus subtilis and Lipomyces starkeyi was 12mL. The production of MBF and microbial lipid were 78.25g/L and 2.91 g/L, respectively. And the COD and NH3-N removal efficiencies of MSGW were 83.67% and 55.38%, respectively.The MBF produced by Bacillus sublilis and Lipomyces starkeyi had high removal efficiencies for turbidity and active red. The removal efficiencies for turbidity and active red were 87% and 58% approximately. MBF produced by mixed cultivation outperformed than that of pure culitivation in coagulation tests.(5) The γ-PGA content had significance influence to the species distribution in PAFC when compositing PAFC and γ-PGA. There was higher (Al+Fe)b content in PAFC and the coagulation performance was satisfied when y-PGA/(Al+Fe)=10%. The removal efficiencies of turbidity and active red reached peaks when y-PGA/(Al+Fe)=5-10%. The removal of active red was 80.15% treated by the compound coagulant PAFC-(y-PGA) with the coagulant doage of 50mg/L. PAFC (Al/Fe=7:3, B=1.8) produced using BFD was composited with the supernatant of Bacillus subtilis cultivation (concentration of γ-PGA was 41.86 g/L) under 20min of ultrasonic stirring. The turbidity removal efficiency improved 15.59% than that of the supernatant when (Al+Fe)/(γ-PGA)=20%, and active red removal efficiency improved 9.18% than that of the supernatant when (Al+Fe)/(y-PGA)= 15%.PAFC (Al/Fe=7:3, B=1.8) produced using BFD was composited with MBF produced in the mixed cultivation process of Bacillus subtilis and Lipomyces starkeyi under 20min of ultrasonic stirring. The turbidity and active red removal efficiencies of PAFC-MBF were 10.21% and 10.29% higer than that of PAFC, respectively. PAFC (Al/Fe=7:3, B=1.8) produced using BFD was composited with the mixed cultivation supernatant of Bacillus subtilis and Lipomyces starkeyi under 20min of ultrasonic stirring. The turbidity and active red removal efficiencies of PAFC-MBF were 17.55% and 9.81% higer than that of mixed cultivation supernatant.A small quantity addition of MBF or γ-PGA to PAFC could promote the molecular weight of the inorganic coagulant which enhanced the coagulation performance of PAFC. Meanwhile, less quantity of PAFC composited with the supernatant of cultivation impoved the surface charge of MBF, thus, the coagulation performance of the cultivation supernatant was promoted. |
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