|其他摘要||In recent years, uranium pollution has gained much attention. According to reports, high concentration of uranium was released into the soil resulted from the coal mining in Yili region, Xinjiang thus increasing the radioactivity level of the surrounding area (An Huimin et al, 1982). The aim of the present work is to investigate the on-site level of uranium in this coal mining area and U(VI) remediation by microbially induced Fe(III) oxide precipitation and microbially induced calcium carbonate precipitation (MICP).
The comparative studies of U(VI) adsorption by synthetic Fe(III) oxide and biogenic Fe(III) oxide produced by Clostridium sp. PXL2 was investigated. SEM-EDX analysis showed that synthesis and biogenic Fe(III) oxides both were amorphous. SRD analysis confirmed that the main components of both Fe (III) oxides are ferrihydrite. The acid-base titration data have shown that there were more biding sites and functional groups such as carboxyl, sulfydryl, phosphoryl groups on the surface of biogenic Fe(III) oxide which might be involved in U(VI) adsorption resulting in the occupation of these binding sites by U(VI) ions.ITC data showed that U(VI) adsorption by both Fe(III) oxides is a spontaneous and exothermic process. Isotherm data of U(VI) adsorption for both synthetic and biogenic Fe(III) oxides was well fitted with both Langmuir and Freundlich equation. By calculation, the maximum U(VI) adsorption capacity was 8.788 and 7.898 mg/g, and U(VI) adsorption difficulty constant (1/n) was 0.658 and 0.380 for synthetic and biogenic Fe(III) oxides, respectively, implying that biogenic Fe(III) oxide has slightly lower U(VI) capacity than synthetic Fe(III) oxide, but more easily adsorbs U(VI) than synthetic Fe(III) oxide. With the increase in the salinity and pH (4-7), rate of U(VI) removal by both types of Fe(III) oxides increased due to the formation of (UO2)3(OH)5+ and UO2Cl42-, which can be more effectively adsorbed by both Fe(III) oxides. The point of zero charge of these two types of Fe(III) oxides suggested that the U(VI) adsorption process by these Fe(III) oxides belongs to electrostatic attraction.
Further, Clostridium sp. PLX2 was applied to remediate both U(VI) and NO3- in groundwater simultaneously. U(VI) immobilization and nitrate reduction rates in groundwater samples inoculated with this bacterium reached up to 75.1% and 55.7% respectively, under neutral condition. Increasing the initial Fe(II) concentration resulted in the further increase of U(VI) immobilization (84.5%), however reduced nitrate reduction. Addition of sodium humate and sodium carbonate suppressed U(VI) immobilization and nitrate reduction to varying degrees. Scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) showed that U(VI) immobilization was mainly due to sorption to amorphous ferric oxides.
In addition, urease-producing bacteria Halomonas sp. SBC20 was chosen to study its effect on CaCO3 formation and U(VI) removal. QCM results showed that CaCO3 precipitation and U(VI) removal decreased as the concentration of EPS increased explaining that the organic substances in EPS formed strong complexes with Ca2+ ions inhibiting CaCO3 precipitation and U(VI) removal. SEM-EDX analysis showed that CaCO3 sediments formed with EPS participation had crystal structure defects compared to the control CaCO3 sediments without EPS further explaining that the involvement of EPS Inhibited the CaCO3 precipitation process. EEM test results showed that complexation constants for peak A and peak C (protein substances in EPS) with Ca2+ ions were 2.37 and 2.13 respectively, meaning that bacterial EPS has strong complexation ability with Ca2+. ITC results indicated that Ca2+ ions mainly interact with carboxyl and phosphoryl groups as inner-sphere complexes on EPS molecules. The complexation is a spontaneous and exothermic process.
Last, Halomonas sp. SBC20 bacterium was applied to immobilize U(VI) in coal ash. After 3 weeks of incubation, 95.8 % of exchangeable U(VI) in bacterially treated coal ash bricks was immobilized while only 6.67 % was immobilized in the control bricks. With the increase in the initial concentration of CaCl2, the compressive strength intensified as a whole. However, higher concentrations up to 50 and 80 mM CaCl2 led to the increase in the leaching of U(VI) concentration. With the increase of cell concentration (OD600), the compressive strength generally intensified which was in consistent with the decrease in U(VI) concentrations released from the bricks. Addition of high percentage stimulating factor A (>2%) was not good for the U(VI) immobilization in coal ash bricks. Emulation test of extreme weather conditions showed that the bioimmobilized coal ash bricks were able to withstand extreme weather conditions during high and low temperatures. However, other factors such as snowmelt caused by frequent changes in high and low temperatures led to the decrease of compressive strength and increase of U(VI) mobility in coal ash bricks. FTIR, XRD and SEM-EDX analysis confirmed the role of MICP on CaCO3 precipitation and U(VI) immobilization.
Key Words: Clostridium sp. PXL2, biogenic Fe(III) oxide, microbially-induced Fe(III) oxide precipitation, Halomonas sp. SBC20, microbially-induced calcium carbonate precipitation (MICP)|