| In recent years,the Chinese livestock and poultry breeding industry has developed rapidly.The large amount of pollutants discharged from livestock and poultry farming and the high share of pollution have led to pollution of livestock and poultry farming as an important source of environmental pollution.The main method for treating livestock waste is anaerobic fermentation.The residual biogas slurry after treatment is rich in organic matter,N,P,K and other nutrients,making it a good organic fertilizer,directly applying biogas slurry.The entry of heavy metals into farms poses ecological and environmental risks.Biological manganese oxide(BioMnO_x)synthesized by microorganisms has high adsorption and oxidation ability for many metal ions due to its special structure and properties.In this paper,the effects of Mn(II)on the growth of the strain,the concentration of Mn(II)and the incubation time on the oxidation rate of manganese oxide bacteria were studied by screening the strains with high manganese oxidation activity.On the basis of this,the manganese oxide bacteria were cultured in our laboratory to prepare BioMnO_x.At the same time,two other manganese oxidizing bacteria provided by the other laboratory were cultured to prepare BioMnO_x and an artificial manganese oxide(Chem-MnO_x)was synthetized in our laboratory,too.The adsorption kinetics and isothermal adsorption experiments of heavy metals were carried out by using four manganese oxides,respectively.The effects of pH,ionic strength and temperature on adsorption were studied.The structural properties of manganese oxides after heavy metal adsorption and the contribution of oxygen-containing functional groups to adsorption capacity were analyzed.The ability of bio-manganese oxide to remove heavy metals was evaluated by comparing the adsorption differences of heavy metals by Chem-MnO_x and BioMnO_x.Finally,the manganese oxide with better comprehensive performance is selected for the removal of heavy metals in the actual biogas slurry.The main results were as follows:(1)A strain of manganese oxidizing bacteria Bacillus sp.ZM-1 has high manganese oxidation activity,and its optimum growth pH is 7 and its resistance to manganese is 12mmol/L.The growth of ZM-1 in the medium containing 6 mmol/L manganese was inhibited in a small quantity compared with the growth in the manganese-free medium,but the overall effect was not significant.The optimum initial manganese concentration in the culture solution was 6 mmol/L when the manganese oxide was formed by help of ZM-1.The ammount of manganese oxide and the oxidation rate of manganese at this time were both maximum,i.e.,1.92 mmol/L and 32%,respectively.(2)XRD analysis showed that the three kinds of bio-manganese oxides mediated by manganese oxidation strains Bacillus sp.ZM-1,Sphingopyxis sp.4-15 and Lysinibacillus sp.M14 contained different types of crystalline manganese oxides,but some diffraction peaks are not obvious and widened,indicating that the biological manganese oxide is a weak crystalline structure,and the artificial manganese oxide diffraction peak corresponds to the characteristic peak of the sodium bicarbonate.FITR analysis showed typical Mn-O bonds in all four manganese oxide structures.SEM-EDX analysis showed that the surface of several manganese oxides is porous.The bio-manganese oxide mainly contains C,O,N,and Mn elements,while the artificial manganese oxide mainly contains C,O,and Mn elements.The specific surface areas of ZM-1,4-15,M14 bio-manganese oxide and artificial manganese oxide are 43.27,41.49,15.34 and 233.55 m~2/g,respectively.XPS analysis showed that most of the manganese elements in the four manganese oxides were tetravalent manganese.(3)In the range of pH 2-6,the adsorption capacity of manganese oxide for Cu and Zn increases with the increase of pH.When the pH is 5,the adsorption capacity on Cr by bio-manganese oxide is the strongest.The adsorption of As by bio-manganese oxide is not obviously affected by pH.The adsorption capacity of artificial manganese oxide to As is weakened with the increase of pH.With the increase of ionic strength,the adsorption capacity of Cu,Zn and Cr by four kinds of manganese oxides is slightly weakened,and the adsorption of As is not interfered by ionic strength.With the increase of temperature,the adsorption capacity of Cu and Zn is enhanced by four kinds of manganese oxides,indicating that the adsorption reaction of Cu and Zn is an endothermic reaction.The adsorption capacity of Cr showed a fluctuating change with the increase of temperature,and the adsorption capacity of As decreased slightly with the increase of temperature,indicating that the adsorption of As on manganese oxide is an exothermic reaction.The isothermal adsorption results of ZM-1 bio-manganese oxide and artificial manganese oxide on four heavy metals are consistent with the Langmuir model,and the adsorption kinetics process is basically in accordance with the quasi-second order kinetic model,indicating that the adsorption is mainly controlled by chemisorption.(4)After adsorption of heavy metals,there are corresponding heavy metal elements on the surface of manganese oxide,and XPS analysis shows that the relative content of low-valence manganese(Mn(II)and Mn(III))in manganese oxides is reduced,and high-valence manganese(Mn(IV))relative content and average manganese oxidation degree increase.The results of FTIR analysis show that there is no significant changes in the peak position of the main functional group in the manganese oxide after heavy metal adsorption,but the peak of the Mn-O vibrational band broadened,indicating that the Mn-O bond in the structure participated in the adsorption reaction.After masking the hydroxyl functional groups in ZM-1 bio-manganese oxide,the adsorption amounts of four heavy metals Cu,Zn,Cr and As were reduced by 5.07%,31.76%,4.79%and 50.89%,respectively,indicating that the hydroxyl group has a certain contribution to the adsorption.(5)The removal effect of heavy metals by adding 1:1 mixture of the bio-manganese oxide and the artificial manganese oxide to the biogas slurry is better than that of the two manganese oxides alone.Considering the removal rate of different heavy metals,the optimal dose of manganese oxides was selected as 2 mg/L.The total concentrations of Cu,Zn,Cr,and As in the biogas slurry after treatment with manganese oxides under the dose of 2 mg/L were 2.05,1.38,0.45,and 0.37 mg/L,and the corresponding removal rates were 80.79%,92.27%,69.44%and 83.25%,respectively.The optimal reaction time was 7h and the optimum reaction temperature is 25-35~°C.Under the optimal reaction conditions,manganese oxides can effectively remove the heavy metals in the biogas slurry while retaining the nutrients in the biogas slurry. |
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