Isolation Of Alkali-tolerant Microorganisms And Their Improvement Of Bauxite Residue

Aluminum is a strategic metal for national economic development and national defense,which is widely used in various fields.Alumina is the main raw material in producing aluminum products,and the production process of alumina includes bayer process,sintering process and joint process.Among which,bayer process is the widely used production process now.One ton aluminum production can consumes about two tons of alumina,and one ton alumina production can produce one to two tons of bauxite residues.The impurity mineral,such as SiO₂,Fe₂O₃,TiO₂ and the additive（lime）aregoing into the bauxite residues in the alumina production process..Due to the strong alkaline and high salt content,it is difficult to realize the comprehensive development of bauxite residues.The main disposal method of bauxite residuespiled up.The accumulated bauxite residues over the world were over 4 billion tons in 2015 and it still increased with120 million tons per year.China is the largest producer and consumer of alumina all over the world,and the largest alumina industrial system also built in China.More than 400 million tons of bauxite residues were piled up in China and the production volume still exceed 60 million tons per year.The environmental safety of bauxite residues disposal siteis a serious threat to the sustainable development of China’s alumina industry.Great deals of researches about the disposal of bauxite residue have been conducted,the main methods includes addition of remediation agents,resource utilization and natural recovery,which are expensive,have unpredictable potential risks or time-consuming.Besides,just few reportsabout improvement of bauxite residue disposal site have been reported and their mechanism is still unclear.Improvement by microbial can reduce the investment and enhance the improve efficiency.Besides that,the environmental risk brought by microbiology improvement is smaller than other methods.Therefore,it is of great practical significance and ecological benefit to carry out the isolation of salt tolerant microoranisms and their application in improvement of bauxite reside disposal site.In this study,alkali-tolerant bacterial strains were isolated from the bauxite residue disposal site.The organic acid species and production amounts by these bacterial strains were determineed.The potential of alkali-tolerant bacterial strains in improving bauxite residue disposal site were also assessed.The main conclusions are as follows:（1）22 alkali-tolerant bacteria strains were successfully isolated from the bauxite residue disposal site,amonge which two alkali-tolerant bacterial strains（named as W1 and W22）with high yield of organic acids were selected for further research.Identify by 16S rRNA gene showed that they both belonged to the genus Bacillus sp.（2）Optimal medium for the production of organic acid of W1 and W22 were identified by orthogonal test.The results showed that the optimal medium for the production of organic acid of W1 was glucose8g·L^-1,yeast extract 3g·L^-1,potassium dihydrogen phosphate 0.3g·L^-1,magnesium chloride 0.3g·L^-1.The optimal medium for the production of acid of W22 was glucose 5g·L^-1,yeast extract 4g·L^-1,potassium dihydrogen phosphate 0.3g·L^-1,magnesium chloride 0.3g·L^-1.Under the optimal condition,a large amount of tartaric acid was produced during the growth of W1,which was also the main reason for the decrease of pH the medium.W22 produced citric acid,succinic acid and tartaric acid in the culture process,which result in the decreaseof the medium pH value.W1 decreased the pH value of culture medium from 12.0 to about 6.0 through producing organic acids,the time needed for shaking culture is shorter than that of static culture.W22 decreased the pH value of culture medium from 12.0 to about 6.5 by organic acidproducing.Under shaking culture and static culture conditions,there was a great difference between thegrowth status and acid production of W1 and W22,indicating that oxygen supply had a significant impact on their growth and W22 had a higher requirement for oxygen supply.（3）W1 and W22 were used to leaching bauxite residue,the leaching efficiency of sodium ion in each step increased silightly compared with leaching by water,and the total leaching efficiency of sodium ion increased significantly.At the same time,the total soluble salts increased significantly after the treatment of W1 and W22.After the treatment of W1 and W22,the pH of bauxite residue decreased from 11.57 to about9 within 45 days and remained stable.The organic matter content,catalase activity,microbial biomass carbon and dehydrogenase activity all increased significant,indicating the addition of W1 and W22improved the physicochemical properties of bauxidte residue.（4）After 30 days treatment by W1 and W22,the stability of aggregates in bauxite residue was significantly improved,and the improvement after the treatment of W22 was more obvious.W1 and W22were can improve the physical structure of bauxite residue and increase the stability aggregates,and the influence of W22 was more obvious.The change of the main minerals composition of the bauxite residue after the treatment of W1 and W22 is not obvious.The main minerals were trachynite,hydrating garnet,illite,chlorite,calcite,hematite,perovskite,and gypsum.（5）Bacterial community analyzeshowed that the relative abundance of W1 and W22 in bauxite residue increased from 5.06%and 2.71%to 23.52%and 12.03%,respectively,in the improvement process.Exogenous carbon source was used torecovery of W1 and W22 on the 30th day,the amount and relative abundance of W1 and W22 further increased after the addition of exogenous carbon source,indicating the alkali-tolerant microorganism can be successfully recoveried after a period of time.