Basic Study On Application Of Inorganic Ferric Salt And Fe-base MOFs To Remove Phosphate From Eutrophic Water Body

Eutrophication of lakes and reservoirs has seriously threatened the limited freshwater resources on the earth.It is well known that phosphorus is the main controling factor of eutrophication,and reducing the concentration of phosphorus in the water is an important measure to control eutrophication.Adsorption technology is recognized as an economic,efficient and flexible method of water treatment.Based on the affinity ability of Fe to P,iron adsorbent is gradually emerging in the application research of removing phosphorus pollution from eutrophic water because of its low cost,abundant raw material sources,friendly environment and harmless.Based on the existing research conditions and experimental exploration of our research group,this paper selected two types of ferric adsorption materials—traditional inorganic ferric salt and Fe-based Metal-organic frameworks materials?Fe-based MOFs?,to study the effect of phosphate removal,influencing factors and adsorption mechanism in simulated phosphate water sample,moderate eutrophication landscape lake?Cuihui Lake?and severe eutrophication natural lakes?Xingyun Lake?.The morphology,chemical composition and phase transformation of two kinds of adsorbent P loaded sediment were studied by modern instrumental analysis methods.Stability of P loaded precipitation was studied by single extraction method.Meanwhile,the effect of functional modification Fe-based MOFs on phosphate removal in eutrophic water bodies was preliminarily explored.The main research contents are listed as follows:1.The effects of two kinds of adsorbents on phosphate removal were studied in simulated water samples of eutrophic water bodies such as lakes and reservoirs with low total phosphorus concentration(TP<1.0mg·L^-1).The results show that:Three inorganic ferric salt FeCl₃,FeSO₄ and PFS can adsorb and remove phosphate efficiently in simulated phosphate water sample(NaH₂PO₄,c₀=0.6mg?L^-1;NaCl=0.005mol?L^-1;pH=7±0.1;V=0.2L).The optimum dosage of ferric salt was 60mg?L^-1,and the corresponding phosphate removal rate was 98.33%,98.17%and 98.67%respectively;Three Fe-based MOFs adsorbent mil-101?Fe?,mil-100?Fe?,mil-53?Fe?can also selectively remove phosphate from simulated phosphate water sample.The optimum dosage was also 60mg?L^-1,and the corresponding phosphate removal rate was 92.50%?91.33%?86.17%.After phosphate removal by two types of adsorbents,the supernatant meets the requirement of water quality standard of surface water I^II Category in China.Both types of adsorbents have a wide pH range of phosphate removal,and no preregulation of pH is required.The adsorption behavior of two adsorbents was rapid reaction,and the adsorption behavior was consistent with the Langmuir and Freundlich adsorption model,and their adsorption process was easy to be carried out.The maximum phosphate adsorption capacity was 166.39mg?g^-1?147.71mg?g^-1?153.37mg?g^-11 and 107.07mg?g^-1?102.88mg?g^-1?97.28mg?g^-1,respectively.The presence of NO₃^-?SO₄^2-?Ca²⁺and Mg²⁺had little effect on the phosphate removal of these two kinds of adsorbents,while the presence of HCO₃^-and CO₃^2-had a certain inhibitory effect on their phosphate adsorption.2.Two kinds of weak alkaline eutrophic water bodies,cuihu lake(c₀=0.283mg?L^-1,pH=8.36)and Xingyun lake(c₀=1.352mg?L^-1,pH=8.87),were studied for the effect of phosphate removal by two kinds adsorbents.The results show that:Dosage of adsorbents are only 20 mg?L^-1,two types of adsorbents can achieve the selective efficient phosphate removal in weak alkaline water.In the water samples of Cuihu Lake,the phosphate removal rate was 96.82%,95.05%,96.11%,91.87%,86.22%and 83.39%respectively,which could reach the requirements of China's surface water II water quality standards.The phosphate removal rate in Xingyun Lake was 94.38%,94.82%,88.39%,91.49%,85.58%and 82.54%,respectively,which can meet the requirements of the surface water III quality standard in China.In addition,the phosphate removal rate increased with the increase of adsorbent dosages,and the residual iron concentration was not detected in the supernatant after phosphate adsorption by two adsorbents.Since large amounts of HCO₃^-in these two lakes,the equilibrium pH value can be maintained in the neutral range.The phosphate adsorption process in the actual lake all belongs to the fast reaction,and adsorption equilibrium of ferric salt was achieved within 0.5min,and those of Fe-based MOFs achieved within 30min.Besides,stirring rate had little effect on the adsorption performance,and the best stirring speed was 200r?min^-1.P loaded precipitation and sediment after phosphate adsorption by inorganic ferric salts have a high specific surface area and total pore volume,and still have a high adsorption capacity for phosphate.Fe-baseds MOFs adsorbents can be regenerated and reused.After four repeated use,the phosphate removal rate remained above 80%.In summary,two kinds of adsorbents have the advantages of high phosphate removal rate,simple operation and ecological safety.3.The composition,morphology and the chemical existence form of phosphorus in P loaded precipitation?sediment?of two types of adsorbents were analyzed by analysis methods such as SEM?BET and XPS etc.,and the stability of phosphorus in sediment was also studied by single extraction method.The results show that:The phosphate anions were firmly adsorbed on the surface of two types of adsorbents by chemical adsorption,and were encased by a large number of iron?hydrogen?oxides.XPS analysis showed that in two kinds of P loaded precipitation?or sediment?,Fe,O and P are all involved in the coordination.It further revealed that the main mechanism of phosphate removal is the combination of electrostatic interaction and coordination.The adsorption product was a single base and nuclear or double base and nuclear complex containing Fe-O-P bonds on the surface of iron?hydrogen?oxygen compound.The P adsorbed on the surface may be two chemical forms of HPO₄^2-or H₂PO₄^-.Due to the difference of phosphate adsorbed on the sediment surface and the initial phosphate concentration,the chemical form of phosphorus is different.Single extraction experiments showed that no matter in neutral CaCl₂ system,weak alkaline HCO₃^-system and strong reducing HONH₃Cl system,the desorption rate of phosphorus was very low.In the system of strong acidity?pH 2 or so?,the phosphorus desorption rate of three kinds of P loaded ferric salts were 0.50%,9.74%and 6.20%respectively,and the phosphorus desorption rate of three P loaded Fe-based MOFs were 13.83%,2.72%and13.60%respectively.In the strong alkaline system?pH>11?,the phosphorus desorption rates of P loaded ferric salts were 18.35%,26.96%and 33.23%,respectively.The phosphorus desorption rates of three kinds of P loaded Fe-based MOFs were 45.44%,42.48%and 48.57%,respectively.It can be proved that phosphorus in the P loaded precipitate is stable and will not be released and cause"secondary pollution".4.The amino-functionalized material NH₂-MIL-101?Fe?was applied to the study of phosphate removal from simulated water samples and actual eutrophic water samples.The regeneration and reuse of amino-functionalized material and the stability of P loaded adsorbent were also studied.The results indicate that:NH₂-MIL-101?Fe?can efficiently remove phosphate in simulated water samples,and the optimal dosage was60mg?L^-1,corresponding removal rate reached 94.67%.NH₂-MIL-101?Fe?has also made a very well phosphate removal effect in CuihuLake and Xingyun Lake water samples.The removal rate reached 95.47%and 90.97%respectively.The adsorption mechanism and adsorption selectivity of NH₂-MIL-101?Fe?are basically consistent with that of unfunctionalized MIL-101?Fe?,and their adsorption mechanism is mainly composed of electrostatic interaction and coordination.Amino-functionalized can provide new adsorption sites,which is helpful to improve the NH₂-MIL-101?Fe?of phosphate adsorption,and the maximum phosphate adsorption capacity of Langmuir fitted was 124.38mg?g^-1.Moreover,the phosphate adsorption by NH₂-MIL-101?Fe?can also be reused for reutilization.Compared with MIL-101?Fe?,due to existence of amino functional groups,the phosphate adsorption of NH₂-MIL-101?Fe?was more stable and difficult to be desorbed.5.By means of magnetic functional design for NH₂-MIL-101?Fe?,surface modification of nano hollow magnetic composite Fe₃O₄@NH₂-MIL-101?Fe?derived from metal-organic frameworks was prepared,and its physical and chemical properties were characterized.Its effect of adsorption and removal of phosphate in simulated water was preliminarily studied.The results demonstrate that:Fe₃O₄@NH₂-MIL-101?Fe?exhibited a remarkable selective removal of phosphates from aqueous solution.In simulated water samples,dosage of 60mg?L^-1,the phosphate removal rate can reach91.33%.The selective and efficient phosphate removal performance of the composites mainly originates from the surface layer of NH₂-MIL-101?Fe?.Kinetic studies revealed that the adsorption process of Fe₃O₄@NH₂-MIL-101?Fe?was fast reaction.The adsorption process followed the rule described by pseudo-second-order reaction kinetic model,which belonged to chemisorption.The adsorption process conforms to the Langmuir adsorption model and belongs to monolayer chemisorption.The presence of Cl^-?Br^-?NO₃^-?SO₄^2-?Ca²⁺and Mg²⁺did not hinder the adsorption of phosphate to Fe₃O₄@NH₂-MIL-101?Fe?,while the presence of HCO₃^-and CO₃^2-had a certain inhibitory effect on its phosphate adsorption.The adsorption of phosphate by Fe₃O₄@NH₂-MIL-101?Fe?can be regenerated and reused,and the solid-liquid separation and recovery can be realized simply and quickly.