| A large number of industrial and agricultural waste containing phosphorus is produced,resulting in high phosphorus content in water,eutrophication in water becomes more serious,threatening ecological balance.Therefore,it is particularly necessary to remove excess phosphorus in water efficiently.The existing phosphorus removal methods include chemical precipitation,biological treatment and adsorption,among which the adsorption treatment is simple and easy to operate,and the variety of adsorbents has attracted extensive attention.Fly ash(FA),a waste material from a wide range of sources,has complex elemental composition and potential application in water treatment,but poor adsorption effect.Layered double hydroxides(LDHs)composed of divalent and trivalent metals have a higher phosphorus adsorption capacity,but the phosphorus adsorption rate is slow and the adsorption selectivity is not ideal.Therefore,based on the characteristics of fly ash and layered double hydroxides,different systems were designed to increase phosphorus adsorption capacity,improve phosphorus adsorption rate and adsorption selectivity.Firstly,calcium/aluminum(magnesium/iron)metal salts etc.,were co-precipitated to prepare LDHs with different metal compositions.The differences in structure and phosphorus adsorption performance were compared to optimize LDHs used for phosphorus adsorption.The results showed that under the same preparation conditions and the same adsorption environment,calcium-based LDHs had better phosphorus adsorption performance than magnesium-based LDHs.Among them,the thicker Ca/Al-LDH lamellar crystals have obvious morphology.Between p H 3 and p H 10,0.4 g/L Ca/Al-LDH can remove more than 90% of phosphorus.The adsorption interference of carbonate to phosphorus is great in different anion environment.The maximum adsorption capacity of Ca/Al-LDH for phosphorus is 155 mg/g,which accords with Freundlich model as well as Elovich kinetics model.In order to improve the effect of LDHs lamellar stacking on phosphorus adsorption,Ca/AlLDH was loaded on FA surface(Ca/Al-LDH@FA)to improve phosphorus adsorption capacity.Ca/Al-LDH was intercalated with pyromellitic dianhydride(PMDA)(Ca/Al-LDH-PMDA)to improve the selective adsorption performance of phosphorus.Synchronous loading and intercalation can obtain Ca/Al-LDH-PMDA@FA,which has the above advantages and comprehensively improves the phosphorus adsorption effect.The results showed that Ca/AlLDH-PMDA@FA could adsorb 96% of phosphorus within 10 min,realizing rapid phosphorus removal,and the maximum phosphorus adsorption capacity of 0.4 g/L Ca/Al-LDHPMDA@FA could reach 203 mg/g.Under the interference of 80 mg/L carbonate,90%(90mg/g)phosphorus can still be removed,and the phosphorus adsorption capacity is nearly 30%higher than Ca/Al-LDH.Ca/Al-LDH-PMDA@FA has the characteristics of Pseudo secondorder and Temkin adsorption models.Through alkali melt treatment and co-precipitation process,inert silicon aluminate contained in FA was activated,and the calcium-aluminum layered double hydroxides composite system(Ca/(Al-FA)-LDH)with FA as aluminum source was prepared.The results show that Ca/(Al-FA)-LDH has both lamellar morphology and a variety of Si-Al crystals,among which Ca/(Al-FA)-LDH2 has a better lamellar morphology.Ca/(Al-FA)-LDH2 can remove 97% phosphorus at the dosage of 0.8 g/L between p H 2 and p H 12,further broadening the application range of p H.And the phosphorus removal efficiency of Ca/(Al-FA)-LDH2 is no less than 80% under the interference of 80 mg/L carbonate.The adsorption characteristics of Ca/(Al-FA)-LDH2 are in accordance with Pseudo first-order kinetics model and Temkin thermodynamic model,thus,the adsorption process is heterogeneous adsorption.The mechanism of phosphorus adsorption by Ca/(Al-FA)-LDH2 is more complex,including ion exchange,metal complexation and ligand exchange.By combining the characteristics of fly ash and layered dihydroxides,phosphorus composite adsorbent was constructed to achieve high selectivity and high adsorption rate for phosphate... |
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