| Microorganisms are the most early life forms on the earth and an important part of the geological ecosystem.Microorganisms play an important role in the weathering of minerals,the formation of soils and the degradation of harmful substances.Soil fluoride is the main source of groundwater fluoride,among which,soil water-soluble fluoride is the key factor that directly affects groundwater fluoride concentration.The purpose of this paper is to study the effect of microorganisms on fluoride release in the soil matrix to evaluate the role of bacteria in the process of soil fluoride dissolution in high fluoride areas,This study improve the formation theory of fluoride pollution in soil and shallow groundwater,the main finding are as following:Investigation of soil physicochemical properties in Xianghuangqi.The survey results showed that:in the soil samples collected from Xianghuangqi,the contents of water-soluble potassium,calcium,magnesium,silicon and fluoride in the soil were 0.1-2.5mg/kg,0.42-23.6mg/kg,0.575-3.875mg/kg,4.31-47.45mg/kg,1.0-3.9mg/kg respectively.The content of water-soluble fluorine increased with the increase of soil depth,p H and conductivity,and had a very significant correlation with p H(P<0.01),and a significant correlation with conductivity(P<0.05).Microbial community structure analysis.The bacteria in the soil at different heights and depths of the sampling site were isolated and purified,and 225 colonies were randomly selected as the representative of local culturable bacteria.The 16S r RNA genes of the isolates was sequenced,and they were divided into 10 Operational Taxonomic Units(OTUs)according to the sequence identity threshold of 97%.The results showed that Bacillusaccounted for 95.06%of the total cultured colonies.In addition,Arthrobacter accounted for 0.45%,Paenarthrobacter accounted for 0.9%,Acinetobacte accounted for 0.9%,and Lactococcus accounted for 2.69%.In order to deeply investigate the distribution of local soil microorganisms,high-throughput sequencing of 16S r RNA gene was performed.The results showed that Bacillus was still the dominant species in the local microbial community,accounting for31.47‰,Arthrobacter accounted for 53.209‰,Acinetobacter accounted for 2.049‰,Lactococcus accounted for 2.079‰.One strain was randomly selected from the above 10 OTUs as the representative strain of this OTU,including strains 7C4,7C5,7C9,1B16,1B18,1C12,1C24,4A24,4B21,and 1A25 for subsequent experiments.The influence of soil representative bacteria on the dissolution and precipitation of common minerals.The dissolution and precipitation effect of 10 representative bacteria on common minerals such as biotite,orthoclase,calcite and fluorite were studied in soil solution.The results show that compared with the bacteria-free control,the representative bacteria can accelerate the dissolution of potassium ions from biotite and orthoclase contain solutions,and also promote the dissolution from fluoride of fluorite.Although the representative bacteria can increase the dissolution of potassium ions in calcite,the concentration of calcium in the bacteria-free control system containing calcite is higher than that of the bacteria-containing system,which may be related to the secondary precipitation of calcium ions caused by the microorganisms.Ten representative bacteria were cultured in the soil solution without minerals amendment to assess the bacterial effect on the p H of soil microenvironment.It was found that the p H of the soil solution decreased first and then increased during the cultivation process,indicating that the bacteria may release acidic substances during early growth phases,and cause alkalization during late growth phase.Calcium precipitation potentials of the representative bacteria were assessed.The 10 representative soil bacteria significantly decrease the concentration of calcium in B4 medium.Meanwhile,formation of white precipitates were observed in the culture medium.The precipitate was separated and subjected to XRD analysis.The result showed that its main component was calcite.Scanning electron microscope analysis showed that the calcite precipitate was closely attached to the surface of the bacteria.Study on the mechanism of calcium precipitation showed that representative bacteria may induce calcium carbonate precipitation by alkalizing the microenvironment,providing coagulation nucleation and carbonic anhydrase activity.The 10 representative bacteria were cultured in sterile soil,and their effects on water-soluble fluoride in soil were discussed.The results showed that 8 of the 10 representative bacteria significantly increased the content of water-soluble fluoride in soil,and 4 of them significantly increased p H of soil.All the 10 strains reduced the content of water-soluble calcium ions in soil.The content of water-soluble fluoride in soil was positively correlated with p H(P<0.01,R~2=0.3086)and negatively correlated with calcium ion content(P<0.05,R~2=0.1431).It is suggested that the increase of soil water-soluble fluoride content might be related to the increase of p H as well as the calcium precipitation by these bacteria. |
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