| The rapid development of industrialization process has promoted the advancement of human society.However,with the improvement of living standards,the amount of sewage has increased rapidly,and the components of sewage have become more complicated.Ammonium is being a hot attention in water treatment,and the coexistence of ammonium and hexavalent chromium also presents new challenges for water treatment.The traditional sewage treatment process includes aerobic nitrification part and anaerobic denitrification part.The nitrification and denitrification tasks are respectively completed by two different types of microbial community.The infrastructure constructions need a large site area,the operation cost is high,and the microorganisms are sensitive to the impact of heavy metals.Now,it has been found that heterotrophic nitrification-aerobic denitrification(HNAD)bacteria can independently complete the nitrification and denitrification processes under aerobic conditions.Besides,some HNAD bacteria can remove hexavalent chromium while degrading ammonia nitrogen in water.However,the strains usually face two problems including the adverse environmental conditions and the loss of functional bacteria.This is also the reason why most of the current researches have found strains with high efficiency of nitrogen and chromium removal,but the strains are difficult to realize the application.Therefore,it is paramount to develop the special materials and methods to enhance the removal and application performance of efficient strains for the high-efficiency and low-cost removal of target pollutants in wastewater.Based on the above problems and considerations,in this work,two kinds of HNAD bacteria Pseudomonas stutzeri XL-2 and Acinetobacter baumannii AL-6(isolated by the research team at previous working)were domesticated using a special method.They possessed the abilities of ammonium removal or combined pollutants(ammonium and hexavalent chromium)removal,efficiently.Walnut shell biochar(BC)and modified walnut shell biochar(MBC)were prepared using walnut shells as raw materials.And the coupling-system of bacteria and biochar constructed using a simple"symbiotic adsorption"method.The removal performance of single pollutants(ammonium)and combined pollutants(ammonium and hexavalent chromium)by the coupling-system were discussed,and the preliminary application of coupling-system for pollutants removal was tried in the sequencing batch reactor(SBR).Further,the synergistic effects between bacteria and biochar in coupling-system were analyzed.Meanwhile,the synergetic mechanism of enhancing removal of target pollutants by coupling-system was revealed.The main research contents and conclusions are as follows:(1)After domestication,the characteristics of strain XL-2 and AL-6 on degradation ammonium showed that the maximum removal efficiency of ammonium by bacteria XL-2 and AL-6 was 71.10%and 88.45%,respectively.The COD removal efficiency was 75.53%and 77.89%.In the whole process,the accumulation amount of nitrate-nitrogen was very small,while the nitrite-nitrogen was almost not accumulated.The Monod equation was used to analyze the process of degradation,and the ratio of the maximum specific growth rate(?max)and half-saturation constant(Ks)were 0.00326mg·L-1·h-1 and 0.00993 mg·L-1·h-1,indicating that both bacteria possessed a good ammonium degradation performance,and the efficiency of strain AL-6 is higher than that of strain XL-2.When the initial ammonia-nitrogen concentration was 100 mg·L-1,the maximum adsorption capacities of BC and MBC for ammonia-nitrogen were 0.99mg·g-1 and 3.29 mg·g-1,respectively.The adsorption process of ammonium by BC and MBC was mainly chemical adsorption,which was proved by the characterization of functional group using FT-IR.The dominant strain AL-6 coupled with BC and MBC,and the removal performance of AL-6&MBC was better than that of AL-6&BC.While the dominant MBC coupled with strain AL-6 and strain XL-2,the better coupling-system was XL-2&MBC.After comparison,it was determined that XL-2&MBC was the optimal coupling-system.Substantially reducing the amount of MBC seriously affected the ammonium removal performance of XL-2&MBC.In different initial ammonia-nitrogen concentrations and p H conditions,the coupling-system XL-2&MBC exhibited a stronger removal performance than bacteria and biochar.In the SBR,ammonium-rich wastewater was treated with XL-2&MBC and the results showed that the efficiency and stability of the coupling-system were much higher than that of the individual bacteria in the reactor.(2)Compared with single ammonia-nitrogen pollutants,the combined pollutants of ammonia-nitrogen and hexavalent chromium is a new challenge,and the removal process is also more complicated.In the Cr BM medium,the removal characteristics of ammonia-nitrogen and hexavalent chromium by strain XL-2 and strain AL-6 showed that the simultaneous removal efficiency of ammonia-nitrogen and hexavalent chromium were 64.85%and 70.80%(strain XL-2),82.01%and 56.49%(strain AL-6),respectively.Compared with the medium without hexavalent chromium,the maximum removal efficiency of ammonia-nitrogen by strain XL-2 and strain AL-6 decreased from71.10%to 64.85%and from 88.45%to 82.01%,respectively.The analysis result of Monod equation showed that the ratio of?max and Ks were 0.00183 mg·L-1·h-1 for strain XL-2 and 0.00829 mg·L-1·h-1 for strain AL-6.Model analysis showed that the relationship between the cell growth and the hexavalent chromium removal of strain XL-2 and strain AL-6 was more in line with the polynomial model,and fitting parameters reach 0.9107 and 0.9831,respectively.According to the analysis of chromium distribution,the hexavalent chromium in the solution was mainly removed in the form of reduction in the supernatant,and a spot of chromium was distributed on the surface or inside of cell.And the XPS scan results further confirmed that the hexavalent chromium was reduced to trivalent chromium.At initial ammonia-nitrogen concentration of 100 mg·L-1 and hexavalent chromium concentration of 3 mg·L-1,the maximum removal efficiency of ammonia-nitrogen by BC and MBC were 8.58%and36.71%,the removal efficiency of hexavalent chromium were 58.97%and 17.32%,respectively.Strain XL-2 coupled with BC and MBC to removal ammonia-nitrogen and hexavalent chromium,respectively.Results displayed that coupling-system XL-2&BC and XL-2&MBC had a similar ammonia-nitrogen removal performance,however,hexavalent chromium removal performance by XL-2&BC was significantly higher than XL-2&MBC.The dominant BC coupled with strain XL-2 and strain AL-6 for ammonia-nitrogen and hexavalent chromium removal.The results showed that AL-6&BC had a higher performance on combined pollutants removal and a lower accumulation of toxic nitrogen-containing metabolic products.Therefore,AL-6&BC was determined as the preferred coupling system.Increasing the amount of BC almost did not affect the removal effect of AL-6&BC for ammonia-nitrogen and hexavalent chromium removal.Under the different initial ammonia-nitrogen concentration and hexavalent chromium concentration,AL-6&BC showed a significant improvement on ammonia-nitrogen and hexavalent chromium removal than individual bacteria and biochar.The application performance of AL-6&BC for treatment of ammonia-nitrogen and hexavalent chromium in SBR was significantly better than that of strain AL-6.(3)For the ammonium-rich wastewater,the coupling-system constructed by strain XL-2 and MBC showed an excellent synergistic effect on ammonia-nitrogen removal.Especially,the synergistic effect was stronger under the adverse conditions containing high ammonia-nitrogen concentration and alkalinity.The synergy mechanism was as follows:1)biochar could absorb the ammonium and free ammonia from the solution,reducing the toxic effect of free ammonia for bacteria.2)The oxygen uptake rate(OUR)showed that biochar could improve and keep the bacteria in a high metabolic activity throughout the growth period.3)Further research found that the release of Mg2+from MBC could significantly improve the AMO enzyme(key enzyme of ammonia-nitrogen degradation)activity of strain XL-2.4)The large specific surface area of MBC enhanced the connection between bacteria and the target pollutants,which improved the efficiency of taking up the target pollutants by hydrophobic bacteria.5)In addition,the large specific surface area,rich porosity and a large number of hydrophilic functional groups distributed of biochar could protect and enhance the colonization of bacteria on biochar.Therefore,the coupling-system could reduce the large loss of strain XL-2,ensuring an efficient and stable operation in SBR.(4)Compared to the ammonia-nitrogen pollutant,the nature and removal mechanism of water body containing combined pollutants of ammonia-nitrogen and hexavalent chromium was more complication.At different initial ammonia-nitrogen concentration and hexavalent chromium concentration,the coupling-system of strain AL-6 and BC exhibited a synergistic effect on ammonia-nitrogen and hexavalent chromium removal.And the synergistic effects involved multiple mechanisms.Firstly,similar with ammonia-nitrogen removal study,the adsorption characteristics of BC to ammonia-nitrogen and hexavalent chromium in water,decreasing the concentration of free ammonia and hexavalent chromium ions,reducing the toxic effect to strain AL-6,improving the removal efficiency of combined pollutants.Secondly,the energy spectrum analysis showed that there were rich Mg,K and Ca elements on BC,which could significantly improve the AMO enzyme activity of the strain AL-6,and improve the degradation efficiency of ammonia-nitrogen.Thirdly,the oxygen-containing functional groups distributed on biochar could promote the extracellular electron transfer,and the Si-O functional groups could further enhance the electron transfer efficiency.In addition,according to the electrochemical analysis of the reaction system displayed that there was an oxidation-reduction reaction between biochar and hexavalent chromium,and the biochar could improve the ability of electron transfer in the reaction system.It provided a convenient channel for the electron transfer among hexavalent chromium,bacteria and biochar in the system,and enhanced the reduction and adsorption of hexavalent chromium.Finally,the large specific surface area?rich mesoporous pore distribution and a large number of hydrophilic functional groups enhanced the connection between bacteria AL-6 and target pollutants,improving the ingesting efficiency of target pollutants,meanwhile,protected the colonization of strain AL-6 on biochar,reducing bacteria biomass loss in suspension solution.Overall,the coupling-system has a great application prospect in ammonium-rich wastewater,ammonium and hexavalent-chromium combined wastewater treatment,providing a reference approach for the upgrading of traditional activated sludge wastewater treatment technology. |
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