Studies Of The Changes And Effects Of Microorganisms In The Soil Formation Of Bauxite Residues

Bauxite residue is a solid waste generated in the process of alumina industrial production.Although many research has been conducted on the utilization of bauxite residue,the utilization rate of which was low due to its characteristics.Soil formation of bauxite residue and in situ bioremediation of bauxite residue disposal areas are considered as one of the most promising ways to solve the environmental problems of bauxite residue.In the present study,soil as the microbial inoculant and amendment,and phosphogypsum and leaf litter as the amendments,were used in a 2-year laboratory study for restoring the microbial community and accelerating the soil formation of bauxite residue.The characteristics of microbial community structure and species of bauxite residue treated with different amendments in different incubation time were identified,and the changes of the main physical and chemical properties of bauxite residue were examined.Furthermore,the interrelation between microbial community and physical and chemical properties of bauxite residue was explored,and the effects of microbial community on the soil formation of bauxite residue was revealed.Meanwhile,a more reasonable and effective measure for soil formation of bauxite residue was proposed.The main research results are as follows:（1）The addition of amendments could significantly affect the richness,diversity,and structure of microbial community and enzyme activities in bauxite residue.When amendment was added alone to bauxite residue,leaf litter showed the most significant effect,followed by soil and phosphogypsum.The combined effects of different amendments,especially the combined effects of leaf litter and other amendments,were significantly higher than those of the amendments alone,and were significantly affected by incubation time.Under the combined addition of different amendments,the OTU number and diversity index of the bacterial and fungal communities always remained at a high level,the microbial community structure was more stable,and the enzyme activities in bauxite residue were also maintained at a high level.After 24 months of incubation,the number of OTU,Chao1 index,Shannon index and Simpson index of bacterial community in treatments GLS₁ and GLS₂increased by 1609.70%and 1830.30%,1607.27%and 1824.79%,91.25%and 105.26%and12.36%and 14.11%,respectively,compared with the control,and the number of OTU,Chao1 index,Shannon index and Simpson index of fungal community increased by 339.54%and 535.73%,239.43%and 481.97%,1913.85%and 1943.65%,2200.61%and 2159.60%,respectively.The activities ofβ-glucosidase,urease and alkaline phosphatase increased by27.06 units and 37.96 units,555.18 units and 981.23 units,23522.19 units and 16533.63units,respectively,compared with the control.And more notably,the relative abundance of dominant bacteria.（2）The physicochemical properties of bauxite residue were significantly affected by amendments and incubation time.When amendment was added alone,phosphogypsum could effectively reduce the alkalinity of bauxite residue and increase the content of available phosphorus,but significantly increase the conductivity of bauxite residue.Leaf litter could improve the bulk density of bauxite residue and increase the content of organic carbon,available nitrogen and other nutrients and macroaggregates.The effect of soil on physical and chemical properties of bauxite residue was slightly smaller than phosphogypsum and leaf litter.The combined addition of different amendments exhibited more significant effects on physicochemical properties of bauxite residue,which could effectively reduce the p H and bulk density of bauxite residue（by 15.38%-18.40%and 8.77%-12.78%,respectively,compared with the control）,increase the contents of organic carbon,available nitrogen and available phosphorus in bauxite residue（by 765.10%-1124.01%,1333.33%-2333.33%and481.94%-834.94%compared with the control respectively）,but conductivity of bauxite residuewas increased by phosphogypsum compared with the control（conductivity:3.31m S/cm³～3.75 m S/cm³）.With the increase of incubation time,p H showed a trend of first decreased and then slightly increased in all the treatments.In the treatment containing phosphogypsum,the conductivity increased first and then decreased.In other treatments,the conductivity showed no significantly changed with incubation time.The bulk density decreased with time at the beginning of incubation stage and stabilized after 9 months of incubation.In the treatments with leaf litter alone or combined with other amendments,the organic carbon and available nitrogen content increased firstly and then decreased.In other treatments,the changes of organic carbon and available nitrogen contents were relatively small with incubation time.In the treatments with phosphogypsum,the contents of available phosphorus showed a decreased trend,while in other treatments,the content of available phosphorus exhibited no significant change.（3）Although soil and phosphogypsum could increase the functional genes richness of bacterial community in bauxite residue,showed no significantly effect on the KEGG enzyme and Meta Cyc pathways.The effect of leaf litter for the main function,KEGG enzyme and Meta Cyc pathways genes richness of bacterial community in bauxite residue showed significantly change over time.After the addition of amendments,the changes of the microbial community were significantly correlated with the physicochemical properties of bauxite residue.The richness,functional groups and diversity of microbial community and enzyme activities were significantly correlated with p H,organic carbon,available nitrogen and available phosphorus contents in bauxite residue.The results showed that the microbial community could play an important role in reducing the alkalinity of bauxite residue,decomposing organic matter and increasing nutrient content,which could regulate the alkalinity of bauxite residue and improve the physical and chemical properties and microenvironment of bauxite residue.At the same time,the growth and development of microorganisms were also affected by environmental factors.The main physical and chemical factors driving the changes of microbial community were p H,organic carbon and available nitrogen contents.（4）Based on the above research results,in consideration of the economy,availability and feasibility of materials and other factors,the strategy with phosphogypsum,leaf litter and soil is relatively low-cost and efficient and could be used in soil formation process of bauxite residue.The results obtained in the present study would provide important theoretical basis and support for the bioremediation of bauxite residue disposal areas.