Study On The Removal Of Phosphorus From The Water Body Of Dianchi Lake By Iron Salt And New AI-MOFs And Its Mechanism

In recent years,eutrophication has become one of the water pollution problems attracting much attention in the environmental field.Eutrophication limits the development of fisheries,industry and the supply of drinking water,and may cause harm to animal and human health.Most studies have shown that phosphorus in water is the main factor causing eutrophication.At the same time,the position of phosphorus in agricultural development cannot be replaced,and it is also a kind of non-renewable resource.Therefore,the recovery and reuse of phosphorus resources is becoming more and more important.Adsorption is considered as a promising technology for water treatment due to its ecological safety,low cost and simple operation.Fe and Al have been widely used in water dephosphorization due to their abundant reserves in nature,strong affinity for phosphorus,and ecological safety.The research content of this paper is mainly divided into two parts.Part I:The efficiency of removing phosphorus in Dianchi Lake water by FeCl₃ and the stability of the Fe-P co-precipitate formed by FeCl₃ in the process of phosphorus removal were explored,and the mechanism of phosphorus removal by FeCl₃ was explored through modern analytical means;The optimal dosage of phosphorus in Dianchi Lake sediments passivated by FeCl₃ and the influence of FeCl₃ on the content of various forms of phosphorus in Dianchi Lake sediments were investigated,and the feasibility of passivating phosphorus in Dianchi Lake sediments by FeCl₃ was evaluated.PartⅡ:By adjusting the species of organic ligands,the dosage of FeCl₃ and the species of metal salts in the precursor solution,a series of new metal-organic frameworks（MOFs）were synthesized which were used to remove and recover phosphate in water.In addition,the influences of initial p H,temperature,initial concentration and other factors on the phosphorus removal efficiency of MOFs were examined,and the phosphorus removal mechanism of MOFs was studied.The main research contents are summarized as follows:1.The efficiency of phosphorus removal from Dianchi Lake water by FeCl₃,the stability of Fe-P co-precipitates formed by FeCl₃ in the process of phosphorus removal and the mechanism of phosphorus removal by FeCl₃ were studied.The results show that when the dosage of FeCl₃ is only 20 mg/L,the removal efficiency of phosphorus in Dianchi Lake water is more than 90%,which can reduce the concentration of phosphorus in Dianchi Lake from 0.193 mg P/L to 0.016 mg P/L.Under the temperature,p H and the disturbance of the Dianchi lake,the Fe-P co-precipitates can exist stably;the increase of the concentration of CO₃^2-in water body can partially desorb the phosphorus in the Fe-P co-precipitate,but the amount of desorption is very small.Even when the concentration of CO₃^2-in water body was as high as 200 mg/L,only 5.31%phosphorus in Fe-P co-precipitate was desorbed.The feasibility analysis of removing phosphorus from Dianchi Lake water by FeCl₃ showes that even if Fe-P co-precipitates were stored in Dianchi Lake water as long as 280 d,the phosphorus in the sediments was hardly released.In addition,the research results of modern analytical methods such as SEM,TEM,FTIR and XPS show that dephosphorization mechanism of FeCl₃ is mainly through co-precipitation,chemical adsorption and ligand exchange.2.The efficiency of passivating phosphorus in Dianchi Lake sediments by FeCl₃and the effect of FeCl₃ on phosphorus species in Dianchi Lake sediments were studied,and the feasibility of passivating phosphorus in Dianchi Lake sediments by FeCl₃ was evaluated.When the mud-water ratio is 1:100（g/m L）,the concentration of phosphorus in the overlying water should not exceed 0.020 mg P/L（National Water Quality Standard of GradeⅠ）,and the dosage of FeCl₃ in the covered state and mixed state is3.2 mg/g and 8.8 mg/g（calculated in mud）;in addition,the immobilization efficiency of phosphorus by flocs formed by FeCl₃ hydrolysis increases with the increase of contact time.The feasibility analysis result of phosphorus immobilization in Dianchi Lake sediments by FeCl₃ shows that FeCl₃ has the dual effects of simultaneously removing phosphorus in Dianchi Lake water and immobilizing phosphorus in the sediments of Dianchi Lake.In the practice of lake treatment,FeCl₃ should be added in small amount and multiple times.FeCl₃ can convert the unstable Ex-P,Fe-P and Or-P in the sediments of Dianchi Lake into more stable O-P,especially from Fe-P to O-P,which can reduce the release of endogenous phosphorus.These above results indicate that FeCl₃ has the potential to be applied in the Dianchi Lake eutrophication treatment project.3.Adjust the microstructure of Al-MOFs by changing the species of organic ligands to achieve efficient phosphorus removal.The results show that by changing the species of organic ligands in the precursor solution,Al-MOFs with different microstructures can be synthesized.The results of the adsorption isotherm showed that Al-BDC had the largest adsorption capacity for phosphorus（97.5 mg P/g）.In addition,when the concentration of phosphorus in the water was less than 1.0 mg P/L,the removal efficiency of phosphorus was more than 90%by adding Al-MOF to the water at the dose of 0.1 g/L.After the phosphorus removal of the synthesized Al-MOFs in water with p H ranging from 5.0-9.0,the residual Al concentration was only0.001-3.036 mg/L.Considering the phosphorus removal efficiency of Al-MOFs and the possible influence of residual Al on aquatic ecosystem,the optimal p H of the synthesized Al-MOFs for phosphorus removal is 5.0-9.0.Studies on the phosphorus removal stability of Al-MOFs showed that Al-MOFs loaded with phosphorus can stably exist in 0.01 mol/L Na Cl background electrolyte solution.When the dosage of Al-MOFs is only 0.04 g/L,the concentration of phosphorus in Dianchi Lake water can be reduced from 0.080 mg P/L to 0.020 mg P/L.After 5 adsorption-desorption cycles,the phosphorus removal efficiency of Al-BDC was still higher than 80%.Using 1.0mol/L Na HCO₃ can recover more than 70%of the phosphorus in Al-BDC.Electrostatic adsorption and coordination reaction are the main phosphorus removal mechanisms for the synthesized Al-MOFs.4.By changing the dosage of FeCl₃ in the precursor solution,it is possible to control the synthesis of Al-MIL-101 analogs with different microstructures and apply them to remove phosphorus in water.The results show that Al-MOFs with different microstructures can be synthesized by adjusting the dosage of FeCl₃ in the precursor solution.The results of the adsorption isotherm showed that when the molar ratio of Fe to Al in the precursor solution was 0.5,the synthesized Al-MOFs（Fe/Al（0.5））had the highest phosphorus adsorption capacity（90.0 mg P/g）.The Langmuir model can better describe the adsorption process of phosphorus on Al-MOFs.In addition,the dephosphorization data of Al-MOFs can be well fitted by the quasi-second-order kinetic model.The optimum p H of the synthesized Al-MOFs for phosphorus removal is 3.0-9.0,and it has the potential to remove phosphorus from natural water.The synthesized Al-MOFs can be regenerated with 1%Na Cl solution（w/v）,so as to recover the phosphorus adsorbed by Al-MOFs;even after five cycles of adsorption-desorption,the phosphorus removal efficiency of Fe/Al（0.5）is still higher than 76%.The results of FT-IR and XPS showed that Fe/Al（0.5）mainly removes phosphorus from water through chemical adsorption and coordination reaction.5.MOFs with different microstructure can be synthesized to selectively remove phosphate in water by adjusting the species of ferric and aluminum salts in the precursor solution（FeCl₃ and Al Cl₃,Fe（NO₃）₃ and Al（NO₃）₃ or Fe₂（SO₄）₃ and Al₂（SO₄）₃）.The results show that MOFs with different microstructures can be synthesized by adjusting the species of ferric and aluminum salts in the precursor solution.When the ferric and aluminum salts in the precursor solution are nitrate,the synthesized MOFs（Fe/Al（NO₃^-））have the largest adsorption capacity for phosphorus（130.0 mg P/g）.In addition,the synthesized MOFs are suitable for the phosphorus removal in water with wide p H（3.0-11.0）and have the potential to remove phosphorus in real waters.The results of studies on the influence of ionic strength show that the synthesized MOFs are suitable for phosphorus removal in water with high salinity.After repeated applications for 5 times,the adsorption efficiency of Fe/Al（NO₃^-）to phosphorus is still higher than 70%.According to the analysis results of FTIR and XPS,the possible mechanism of removing phosphate from water by Fe/Al（NO₃^-）is chemical adsorption and coordination reaction.