Combined Removal Of Nitrogen And Phosphorus In Eutrophic Water By Modified Clay Minerals And Their Reuse In Soils

Nitrogen and phosphorus are the main limiting factors of water eutrophication,with the continuous collection of nitrogen and phosphorus nutrient elements in the water body,the problem of water eutrophication is becoming increasingly serious,and the exploration of cost-effective eutrophication water treatment technology has become a research hotspot in recent years.In this study,three kinds of clay minerals,montmorillonite,illite and kaolin,were selected,and sodium-based and calcium-based modification treatments were used as controls.Hydroxyl iron and hydroxyl aluminum modified clay minerals were prepared and loaded with potassium humate to obtain organic-inorganic composite modified clay minerals.Through the analysis of the morphological structure characteristics of modified clay minerals,combined with batch adsorption experiments and simulated eutrophication wastewater adsorption experiments,the combined removal effect and mechanism of nitrogen and phosphorus by composite modified clay minerals were studied.The modified clay minerals with the best adsorption capacity for nitrogen and phosphorus were selected,and the pot experiment of ryegrass was used to verify the reuse potential after adsorbing nitrogen and phosphorus.The main results are as follows:（1）Compared with modified illite and modified kaolin,modified montmorillonite has larger specific surface area,interlayer spacing and cation exchange capacity.The specific surface area of clay minerals modified by hydroxyl iron and hydroxyl aluminum and clay minerals modified by hydroxyl iron humic acid and hydroxyl aluminum humic acid increased to varying degrees.Among them,Na M-Fe-HA had the largest specific surface area of298.91 m²·g^-1.The XRD results showed that the d₀₀₁ value of sodium montmorillonite modified by hydroxyl iron and hydroxyl aluminum increased significantly,indicating that hydroxyl aluminum and hydroxyl iron successfully formed pillars between montmorillonite layers.The interlayer spacing of Na M-Al-HA increased by 1.78 nm,which was 0.54 nm higher than that of Na M.Fourier transform infrared spectroscopy results showed that the enhancement of-OH absorption peak and the emergence of COO-symmetric stretching vibration peak of Na M-Fe-HA and Na M-Al-HA indicated the success of pillared modification and the successful loading of potassium humate.（2）The adsorption kinetics model showed that the adsorption of nitrogen and phosphorus by 30 modified clay minerals reached adsorption equilibrium within 360 min,and most of the adsorption process was completed within the first 30 min.The pseudo-second-order adsorption kinetics can better describe the adsorption process of nitrogen and phosphorus by30 clay minerals.The intra-particle diffusion model shows a triple linear relationship.The adsorption process of phosphorus and ammonia nitrogen by modified clay minerals is mainly chemical adsorption.It can be divided into three stages:rapid adsorption stage,slow adsorption stage and equilibrium stage.The adsorption rate is mainly controlled by external model diffusion and intra-particle diffusion.The isothermal adsorption model showed that the adsorption capacity of hydroxyl aluminum pillared modified sodium montmorillonite Na M-Al to phosphorus was the largest,reaching 25.39 mg·g^-1,followed by hydroxyl iron humic acid modified sodium montmorillonite Na M-Fe-HA,and the adsorption capacity of phosphorus was 24.04 mg·g^-1.The adsorption capacity of hydroxyl iron humic acid modified sodium montmorillonite Na M-Fe-HA to ammonia nitrogen is the largest,reaching21.64 mg·g^-1.Freundlich model can better describe the adsorption process of hydroxyl iron humic acid modified clay minerals and hydroxyl aluminum humic acid modified clay minerals to nitrogen and phosphorus.The other modified clay minerals are more in line with the Langmuir model.Hydroxyl iron humic acid modified sodium montmorillonite Na M-Fe-HA had the best combined removal ability of nitrogen and phosphorus.The removal rates of ammonia nitrogen and phosphorus in simulated eutrophic wastewater(P=1 mg·L^-1,N=5 mg·L^-1)with low concentration of nitrogen and phosphorus were 90.03%and62.57%,respectively.The removal rates of phosphorus and ammonia nitrogen in simulated eutrophic wastewater(P=3 mg·L^-1,N=25 mg·L^-1)could reach 91.55%and 51.10%.In the simulated eutrophic wastewater,the removal rate of phosphorus by Na M-Fe-HA decreased first,then increased,and then decreased with the increase of p H in the range of5.5-9.5,and reached the highest at p H 8.5.The removal rate of ammonia nitrogen by Na M-Fe-HA increased with the increase of p H,and reached the highest at p H 9.5.The inhibitory effect of different cations on the adsorption of ammonia nitrogen by Na M-Fe-HA was Al³⁺>Ca²⁺>Na⁺,and with the increase of the concentration of coexisting cations,the inhibitory effect of coexisting anions on the adsorption of phosphorus by Na M-Fe-HA was SO₄^2->Cl^-.The removal rate of phosphorus and ammonia nitrogen in the three treatments with different initial concentrations of nitrogen and phosphorus was between 95.11%and97.30%,and the removal rate of ammonia nitrogen was between 64.31%and 78.51%,which had the application potential of joint removal of nitrogen and phosphorus from eutrophicized water.（3）Adding 1%modified clay mineral Na M-Fe-HAL1 after adsorbing nitrogen and phosphorus can make ryegrass obtain the maximum biomass dry weight,which is148.83g·m^-2.The nitrate nitrogen content and available phosphorus content in the soil treated with Na M-Fe-HAL1 were the highest,which were 106.01%and 54.42%higher than CK treatment.The application of hydroxyl iron humic acid potassium modified sodium montmorillonite Na M-Fe-HA after adsorption of nitrogen and phosphorus can effectively increase the biomass of landscape crops,improve soil fertility,and reduce the loss of nitrogen and phosphorus.In summary,this study prepared organic-inorganic composite modified sodium-based and calcium-based clay minerals by loading potassium humate after hydroxyl iron and hydroxyl aluminum modified clay minerals,which significantly improved the adsorption performance of clay minerals for nitrogen and phosphorus.Hydroxyl iron humic acid potassium modified sodium montmorillonite Na M-Fe-HA can simultaneously achieve a high removal rate of nitrogen and phosphorus in simulated eutrophic water,and the appropriate amount of Na M-Fe-HA after adsorption of nitrogen and phosphorus can promote the growth and development of ryegrass and improve soil fertility.It can not only solve the problem of combined removal of nitrogen and phosphorus in eutrophic water to a certain extent,but also has certain reuse potential as a soil regulator for landscape plants after adsorption of nitrogen and phosphorus.