The Mechanism Of Simultaneous Removal Of Fluoride By Microbial Induced Calcium Precipitation

A calcium precipitating and denitrifying bacterium H12 was used to investigate the F^-removal performance and mechanism.The results showed that the strain H12 reduced85.24%(0.036 mg·L^-1·h^-1)of F^-,62.43%(0.94 mg·L^-1·h^-1)of Ca²⁺,and approximately100%of NO₃^-over 120 h in continuous determination experiments.The response surface methodology analysis demonstrated that the maximum removal efficiency of F^-was 88.98%(0.062 mg·L^-1·h^-1)within 72 h under the following conditions:the initial Ca²⁺concentration of 250.00 mg·L^-1,p H of 7.50,and the initial C₄H₄Na₂O₄·6H₂O concentration of 800.00 mg·L^-1.The scanning electron microscopy images,the X-ray photoelectron spectroscopy,and X-ray diffraction results suggested the following removal mechanism of F^-:（1）the bacteria,as the nucleation site,were encapsulated by bioprecipitation to form biological crystal seeds;（2）Biological crystal seeds adsorbed F^-to form Ca₅（PO₄）₃F and CaF₂;（3）Under the induction of bacteria,calcium,fluoride and phosphate coprecipitated to form Ca₅（PO₄）₃F and CaF₂.In addition,the gas chromatography data indicated that F^-had little or no effect on the gas composition during denitrification,and the fluorescence spectroscopy analysis also proved that the extracellular polymeric substance（protein）is the site of bioprecipitation nucleation.Using Acinetobacter H12 as the nucleation template,the biological crystal species with bacteria as the core were established.The factors affecting the removal of fluoride（F^-）and Ca²⁺by the seed with Acinetobacter H12 as the growth core were studied.The study reflected that the fluoride removal increased significantly with the increasing temperature and time.On the other hand the fluoride adsorption capacities（Q_e）were found to decrease with the further increasing p H while the Ca²⁺removal ratio gradually increased.The adsorption kinetics more fit with pseudo-second-order（R²>0.999）in compare to pseudo-first-order（R²≈0.99）.Based on kinetic equations,the maximum rate of F^-reduction was 0.762 mg/g/h at the initial fluorine concentration is 3mg/L.The adsorption process is closer to Freundlich isotherm（R²>0.99）.Adsorption thermodynamic parameters(gibbs free energy（ΔG<0）,enthalpy（ΔH>0）and entropy（ΔS>0）indicates the spontaneous and endothermic reactions of the adsorption process.The slope and intercept of Arrhenius equation indicated the activation/adsorption energy（Ea）of 2.81 k J/mol and frequency factor（A）of 10.88 1/h.Three-dimensional fluorescence,SEM,XRD,XPS and other methods used to explore the mechanism of removing fluorine and calcium from calcium precipitation:fluorine adsorbed and calcium induced to form precipitation.The packed biofilm reactor and quartz sand biofilm reactor were established by Acinetobacter.sp H12.In the Yulong packing biofilm reactor,the effects of different initial fluoride（F^-）concentration（3mg/L,5mg/L,10mg/L）,different initial p H（6.0,7.0,8.0）and different HRT（6h,9h,12h）on the biological removal of fluoride（F^-）,Ca²⁺and nitrate were studied.At the same time,Yulong packing group without membrane was used as the control group.The results show that when the initial concentration of F^-is 3mg/L,the initial p H is 8.00,and HRT is 12 h,the removal efficiency of F^-,Ca²⁺and nitrate is the best(the removal efficiency of nitrate is about 99.99%,Ca²⁺is 67.66%,and the removal efficiency of F^-is 81.91%).At the same time,SEM images showed that Acinetobacter H12 was successfully attached to the Yulong packing,and most of the biological precipitation was observed on it.XRD and XPS analysis showed that fluoride（F^-）was removed by biological precipitation.The results showed that 99.61%of the gas was nitrogen,and no N₂O was detected in the stable period of the reactor.The effects of different C/N ratio（5:1,4:1,3:1）,initial Ca²⁺concentration（180mg/L,144mg/L,108mg/L）and HRT（4h,6h,8h）on the removal of F^-,Ca²⁺and nitrate in sand filter membrane reactor were studied.Meanwhile,quartz sand without membrane was used as the control group.The results showed that when the ratio of carbon to nitrogen was 5:1,the initial concentration of F^-was 5mg/L,and HRT was 8h,the removal efficiency of F^-,Ca²⁺and nitrate was the best.However,with the continuous operation of the reactor,when the ratio of carbon to nitrogen is 3:1,it also has a good removal effect and can meet the effluent standard.At the same time,SEM,XPS,XRD data show that the removal mechanism is the same as before,and the nucleation efficiency of biological seeds is improved with quartz sand as carrier.