|其他摘要||In Xinjiang arid and semi-arid regions, water environments are facing combined pollutions including heavy metals and persistent organic matters. It is also one of hot environmental issues which should be faced and resolved in the development of Xinjiang Uygur Autonomous Region. Biogenic Mn oxides produced by Mn(II) oxidation bacteria, namely, Mn(II) oxidation bacteria can oxidize dissolved Mn(II) into insoluble Mn(IV) oxides under aerobic conditions. Due to the special metabolic pathways and metabolic products, this process provides a new approach for the remediation of heavy metals and organic combined pollutants in water environments. The aims of this research were to investigate the formation mechanisms of extracellular biogenic Mn oxides generated by Pseudomonas putida strain MnB1 and the removal efficiency of the targeted pollutants (e.g., arsenic, antimony, tetracyclines and chlorophenols) through batch experiments, electrochemical experiments and bioreactor experiments. The main conclusions were as follows:
(1) The structural feature of biogenic Mn oxides was determined by SEM-EDS, XPS and Zeta potential. Superoxide radical produced by P. putida strain MnB1 played an important role in oxidation of Mn(II). Meanwhile, we also clarified the production characteristics of H2O2 during bacterial growth and oxidation of Mn(II). Moreover, the addition of H2O2 (≤100 μM) showed no obvious effect on bacterial Mn(II) oxidation process. EPS from P. putida strain MnB1 accelerated the bacterial Mn(II) oxidation process, and this indicated that EPS played a key role in bacterial Mn(II) oxidation process. Also, Mn(II) oxidation process was not significantly influenced by addition of humic substances and treatment of light.
(2) Through batch experiments, electrochemical experiments, SEM-EDS and FTIR analysis, it was confirmed that bacteria EPS played a vital role in increasing the dissolution of natural rhodochrosite, and the specific mechanism were as follows. P. putida strain MnB1 cells could dissolve natural rhodochrosite effectively and subsequently oxidize liberated Mn(II) ions to form Mn oxides. Bacterial EPS increased the dissolution rate of natural rhodochrosite, and the dissolution rate was obviously influenced by water chemistry factors, such as pH, ionic strengths, EPS concentrations and rhodochrosite dosages. Functional groups like N-H, C=O and C-H in the polysaccharide or proteins of EPS were involved in the dissolution of natural rhodochrosite. In addition, EPS was confirmed to play a key role in increasing dissolution of natural rhodochrosite mineral based on electrochemical methods such as Tafel and EIS.
(3) Biogenic Mn oxides and δ-MnO2 could remove tetracycline hydrochloride (TC) and As(III) simultaneously, and both materials had a potential application for wastewater treatment. The removal efficiency of TC using biogenic Mn oxides was 3-6 times higher than δ-MnO2 and commercial MnO2. The removal efficiencies of TC and As(III) were positively increased with the initial Mn oxides dosage, however, high concentrations of TC and As(III) competed with each other for oxidation or adsorption sites and thus affected the removal efficiencies of both contaminants. The intermediates and products of TC were identified by high performance liquid chromatography-mass spectrometry (HPLC-MS), and the degradation pathway of TC by δ-MnO2 was proposed.
(4) Compared with process of bacterial growth and bacterial Mn(II) oxidizing process, it was that the latter one played a key role in 2, 4-dichlorophenol (DCP) removal. The removal efficiency of DCP was increased with biogenic Mn oxides concentrations. Biogenic Mn(II) oxidizing process could significantly accelerate the removal of DCP in sediments from the bioreactor experiments. Meanwhile, this process reduced the exchangeable fraction of arsenic and increased the fraction of Fe-Mn oxides bound. This mean that bacterial Mn(II) oxidizing process could increase the stability of arsenic. According to the HPLC-MS analysis, it was found that DCP could be directly dechlorinated and hydroxylated to hydroxyhydroquinone (HHQ), and then HHQ were further decomposed to small molecule acids.
(5) By comparison of Sb(III) and Sb(V) sequestration on biogenic manganese oxides, it could be conclude as follows. Varied concentrations of Sb(III) was quickly oxidized within 10 min in the presence of biogenic Mn oxides, while Sb(V) sequestration on biogenic Mn oxides was completely inhibited. X-ray photoelectron spectrometer analysis revealed that Sb(V) was the dominant species in the solid phase after the reaction of Sb(III) with biogenic Mn oxides. Chemical extraction experiments showed that Sb(V) was mainly as Mn oxides bound and more stable fraction. X-ray diffraction results suggested that the oxidation of Sb(III) by biogenic Mn oxides led to the formation of manganese antimonate mineral (Mn2Sb2O7). Morevoer, bacterial activity in biogenic Mn oxides showed an apparent sequestration effect on Sb(III), but not on Sb(V).
(6) By comparison of Sb and As removal efficiencies on quartz sand load biogenic manganese oxides and biochar, it could be conclude as follows. Quartz sand load biogenic manganese oxides column can transform Sb(III) and As(III) to Sb(V) and As(V), and reduce the toxicity of antimony and arsenic. Meanwhile, biogenic Mn oxides exhited a higher sequestration rate to antimony and arsenic at the early stage. The removal efficiency of As by biochar column increased with reaction time, but not effective for Sb removal. Overall, the removal efficiency of As using quartz sand load biogenic manganese oxides and biochar column was higher than Sb, and the two phases reactors have better removal effect to As over the period of the experiments.|