Study On Phosphorus Release Potential Of Sediments In Urban River Network Region And In-situ Phosphorus Immobilization Materials

The circulation of river phosphorus in the water environment system of the river basin had attracted wide attention,especially the phosphorus existence in sediments developed an important impact in the regional environmental changes.With the advancement of urbanization and industrialization,the external input of the urban river network in the Taihu Lake Basin,as well as the small hydraulic slope,dense and slow flow rate of its own river courses,had resulted in serious sediment deposition in the river network and increased endogenous pollution.In order to study the phosphorus release potential of the sediments in the urban river network area and the discussion relevant technical countermeasures,this study selected the urban river network area of??Jiaxing as the study object.On the basis of clarifying the phosphorus release potential of the study region,two types of phosphorus immobilization materials were prepared to reduce the load of water and sediments.On the basis of considering the in-situ coverage mode and coverage strength on the effect of phosphorus immobilization,calcium nitrate was combined with the material to immobilise sediments phosphorus and provide data support for the management of endogenous phosphorus in rivers and lakes.The main conclusions are as follows:（1）The highest concentrations of TP and TN in the water body in the research area were1.145 mg·L^-1and 8.312 mg·L^-1,respectively.The average TP content in the sediments was 993.13mg·kg^-1,showing a spatial pattern of the north>west>southern river network area.Based on the the single-factor index ecological risk assessment standard,the proportions of samples with grades II,III,and IV samples were 10.91%,25.45%,and 63.64%,respectively.The concentrations of phosphorus species in sediments generally showed TP>inorganic phosphorus（IP）>calcium bound state phosphorus（HCl-P）>iron-aluminum bound state phosphorus（NaOH-P）>organic phosphorus（OP）,and the sediment bioavailable phosphorus（BAP）content was in the order of NaHCO₃extracted phosphorus（Olsen-P）>algae available phosphorus（AAP）>water-soluble phosphorus（WSP）>easily desorbable phosphorus（RDP）.In addition,the phosphorus species content of the vertical section of the sediments decreased with the depth as a whole.The phosphorus in the study area was mainly contributed by the incoming water quality,with a contribution rate of 40.8%.The contribution of potential active phosphorus release from sediments in different regions on long and short time scales was still high.（2）Two kinds of phosphorus immobilization materials were studied in this paper:Fe₃O₄/La（OH）₃modified attapulgite（MT-LHMT）and Fe₃O₄/La₂（CO₃）₃modified attapulgite（MT-LCMT）.The results showed that the adsorption kinetics of phosphorus on the materials were better fitted with the pseudo-second-order model and the Elovich model,respectively,and the adsorption isotherm behaviors were better fitted with the Sips model.The two materials showed good adsorption efficiency at p H 3～8 or in the presence of electrolytes(Cl^-,NO₃^-,SO₄^2-,K⁺,Ca²⁺,Mg²⁺).The phosphorous species bound by the two materials was mainly HCl-P.Mainly,the proportion of mobile phosphorus（Mobile-P）was very low.Coordination exchange,inner-sphere complexation,electrostatic interaction and surface precipitation were P adsorption mechanisms of two materials.（3）The results of in-situ covering experiments showed that MT-LHMT under anaerobic conditions can significantly reduce the content of mobile state phosphorus（Mobile-P）in sediment,soluble orthophosphate（SRP）concentration in overlying water and DGT available phosphorus concentration(Labile-P_DGT)in sediment-water profile in different covering modes and covering intensities,thereby reducing the risk of phosphorus release from the sediment.The MT-LHMT capping with high cover strength（2X）had a more positive effect on inhibiting the release of sediment phosphorus into the overlying water.The fabric-wrapped capping columns had a very low Labile-P_DGTconcentration(0.008 mg·L^-1)and also significantly reduced the risk of nitrogen release from the sediment.（4）The experimental results of the combination of materials and calcium nitrate showed that the reduction rate of SRP concentration in the overlying water of each treatment group showed that the combined use group（70.18%～97.95%）>MT-LHMT single use group（71.21～98.23%）>CN single use group（58.32～92.48%）.The combined use group significantly reduced different reactive phosphorus(SRP,Mobile-P,Labile-P_DGT)levels of in the sediment-water system,and its extremely low Labile-P_DGTconcentration(0.009 mg·L^-1)of 0～-40 mm static layer was the key to high phosphorus control efficiency.Both calcium nitrate addition and combination group promoted nitrogen migration at the sediment-water interface（SWI）,and the concentration of NO₃^--N in the overlying water of calcium nitrate addition group increased with time and then gradually decreased to a lower level(6.13 mg·L^-1and 28.58 mg·L^-1,respectively),and the NH₄⁺-N concentration in the overlying water first increased and then decreased to a lower level in each treatment group.The concentration of SO₄^2-in the overlying water of calcium nitrate addition group and combination group increased,and the reduction rates of acid-volatile sulfide（AVS）in the sediment surface were 61.17%and 58.8%,respectively.