Study On The Control Of Nitrogen And Phosphorus Release From Sediment Based On Calcium Nitrate And Covering Improved Material Combination Technology

Under the effective control of external pollutants,the release of internal pollutants of sediment will further aggravate the deterioration of water quality.Therefore,controlling the release of endogenous nitrogen and phosphorus in sediment has become the key to controlling eutrophication of water bodies.Both calcium nitrate?Ca?NO₃?₂?injection technology and in-situ mulching technology are important methods to control the endogenous release of sediment.However,there are still some disputes over the use of the two.The effects of this need to be further studied.This topic mainly focuses on two techniques of calcium nitrate treatment and in-situ sediment capping.In view of the shortcomings of using calcium nitrate injection technology alone,having a systematic investigated of the combined technology of calcium nitrate injection and anion exchange resin?AER?capping,calcium nitrate/zeolite/anion exchange resin combined technology,natural zeolite and zirconium modified zeolite?Zr MZ?combined covering technology,calcium nitrate/denitrifying bacteria?DNB?/natural zeolite/Zr MZ/AER combined technology for simultaneously controlling phosphorus and nitrogen release from sediment result and the improvement effect of zirconium-modified zeolite on the sediment,as well as evaluated the risk of NO₃^--N release after nitrate injection.Four combinations of technologies that have not been reported in the literature were developed,which were useful for control the release of nitrogen and phosphorus in the sediment.And through the comparison of the influence of each single treatment technology on each other,combining the theory of adsorption and ion exchange,the mechanism and feasibility of each treatment technology are discussed.The main conclusions are as follows:?1?Adopting the combination technology of calcium nitrate injection and anion exchange resin capping for controlling phosphorus release from sediment and reduce the risk of nitrate release.The results showed that The combined application of Ca?NO₃?₂addition and AER capping could tremendously reduce the amount of soluble reactive phosphorus?SRP?in the overlying water,with SRP reduction rates of 75.9%–98.7%.Furthermore,it could cut down the contents of high-resolution diffusive gradients in thin films?DGT?-liable phosphorus in the sediments,resulting in the formation of static layer in the upper sediments.The combined treatment using Ca?NO₃?₂ and AER had a slight effect on the contents of mobile phosphorus,Al-bound phosphorus and Ca-bound phosphorus in the sediments,but it could increase the amount of residual phosphorus in the top 30 mm sediments?increased by 27.7%–42.9%?.The amount of NO₃^--N?1.7-6.5mg/L?in the overlying water under the action of the combined treatment method using Ca?NO₃?₂ and AER was much lower than that under the action of the single Ca?NO₃?₂treatment?5.2-8.3 mg/L?.?2?Based on the combined use of calcium nitrate,natural zeolite and anion exchange resin to control the release of nitrogen and phosphorus in the sediment and reduce the risk of nitrate release.The results showed that the joint use of Ca?NO₃?₂ injection,zeolite capping and AER-contained floating system suspending not only could effectively suppress the release of SRP and NH₄⁺-N from sediments into OL-waters simultaneously,but also had much less risk of NO₃^--N releasing into OL-waters?removal rate is 29.3%-71.1%?as compared to the single Ca?NO₃?₂ injection method and the combined Ca?NO₃?₂/zeolite method.The inhibition of the reductive dissolution of the P-bound Fe?III?oxides/hydroxides by the presence of nitrate and the adsorption of ammonium on the zeolite played very important roles in the interception of SRP and NH₄⁺-N releasing into OL-waters by the Ca?NO₃?₂/zeolite/AER method.After the sediment remediation using the Ca?NO₃?₂/zeolite/AER approach,the increase in the content of residual P in the sediment layer of 0-50 mm,the decrease of mobile P in the sediment layer of 0-10 mm and the increased NH₄⁺-N adsorption capacity for the sediment layer of 0-10 mm would be conductive to the interception of SRP and NH₄⁺-N liberation in the long run.?3?Effect of zirconium-modified zeolite on sediment improvement.Results showed that although the Zr MZ amendment resulted in the increase in the phosphate sorption capacity for the sediment,the presence of physical disturbance reduced the phosphate sorption capacity and rate for the Zr MZ-amended sediment.Based on the Langmuir isotherm model,the maximum phosphate monolayer sorption capacity for the Zr MZ-amended sediment after incubation under physical disturbance condition was 743 mg·kg^-1,which was 18%lower than that under static condition(902 mg·kg^-1).Furthermore,the presence of physical disturbance changed the P speciation and bioavailablity of the Zr MZ-amended sediment.After physical disturbance,the contents of Na₂S₂O₄/Na HCO₃extractable P?P-BD?and Na OH extractable P?P-Na OH?in the Zr MZ-amended sediment decreased,but that of residual P?P-Res?increased.Moreover,the contents of bioavailable P?BAP?including Na HCO₃ extractable phosphorus?Olsen-P?and algal available phosphorus?AAP?in the Zr MZ-amended sediment also decreased after physical disturbance.Results of this work indicate that the physical disturbance causes the decrease of the phosphate adsorption ability for the Zr MZ-amended sediment but causes the increase in the stability of P in the Zr MZ-amended sediment.?4?A combination of natural zeolite and zirconium-modified zeolite was used to control the release of internal nitrogen and phosphorus.Results showed that in an anaerobic environment,nitrogen and phosphorus in the sediment are easily released into pore water and overlying water.The removal rate of SRP in the overlying water body can reach 80.4%-98.2%and 88.3%-98.8%by the combined coverage of granular natural zeolite and zirconium modified zeolite as well as the combined coverage of powder natural zeolite and zirconium modified zeolite,respectively.And the reduction effects of the two on NH₄⁺-N in overlying water are 30.4%-86.1%and 48.9%-84.7%,respectively.In addition,the control effect was significantly improved compared with the natural zeolite capping alone.The addition of natural particulate zeolite or natural powder zeolite alone can promote the conversion of potentially mobile phosphorus,P-Na OH,and P-HCl to P-Res in the surface sediment.When the modified zeolite is added to the surface sediment,it can promote the conversion of more potentially mobile phosphorus and P-HCl in the surface sediment to P-Na OH and P-Res.In addition,the addition of each group of materials could lead to an increase in the P-Res content of the sediments 10-20 and 20-30 mm below the surface sediment.?5?Based on the combined use of calcium nitrate,anion exchange resin,zeolite,zirconium modified zeolite and denitrifying bacteria for simultaneously controlling phosphorus and nitrogen release from sediment and reduce the risk of nitrate release.It shows that in the Ca?NO₃?₂/DNB group and the Ca?NO₃?₂/DNB/zeolite/Zr MZ/AER group with denitrifying bacteria,the DO content in the overlying water was quickly consumed and reduced to below 0.35 mg/L during the initial culture period?4-13 d?,the release rate and concentration of SRP in the two groups were significantly higher than those in the other groups.After two weeks,the SRP concentration in the Ca?NO₃?₂/DNB group and the Ca?NO₃?₂/DNB/zeolite/Zr MZ/AER group began to decline and eventually remained stable at low concentration levels.During the culture period,the average concentration of NH₄⁺-N in the overlying water on each reaction column in descending order was Ca?NO₃?₂/DNB group>Ca?NO₃?₂group>Control group>Ca?NO₃?₂/DNB/zeolite/Zr MZ/AER group>zeolite/Zr MZ group.The average concentration of NO₃^--N in the overlying water was 47.54 mg/L of the Ca?NO₃?₂/DNB group,which is close to half that of the Ca?NO₃?₂ group with 80.97 mg/L.while the average concentration of NO₃^--N in the Ca?NO₃?₂/DNB/zeolite/Zr MZ/AER group was only 14.72 mg/L,which is less than half of the Ca?NO₃?₂ group.that is,the combined use of calcium nitrate and denitrifying bacteria can better reduce the release of NO₃^--N.