Influence Of Submergence/drainage On The Release Performance Of Sediment Phosphorus In The Fluctuating Zone Of Nansi Lake And Its Control Measures

Phosphorus is one of the main elements affecting the primary productivity of lake water,and the excessive content of phosphorus is one of the main causes of eutrophication.Lake Nansi is an important water storage and regulation lake in the eastern route of South-to-North Water Diversion Project.It belongs to the type of moderately eutrophic lake.During the water transfer period,the fluctuation of water level in the lake area causes the formation of a large area of fluctuation zone along the lake.Understanding the mechanism and key influencing factors of phosphorus release in sediment in the water-level-fluctuation zone is of great scientific significance for controlling the endogenous pollution of Lake Nansi and ensuring the water quality safety of the eastern route.In this thesis,the sediment in the water-level fluctuation zone of Lake Nansi was taken as the research subject,and a laboratory simulated release test was designed to study the phosphorus release characteristics of sediment under different periods of flooding/drying.The control mechanism of iron and manganese on the phosphorus release performance of sediment in the water-level fluctuation zone was investigated,and the release mechanism of phosphorus from the sediment in the water-level fluctuation zone was revealed.The results showed that:The release rate of phosphorus from sediment in the water-leveling zone increased significantly at the initial stage of inundation,and then decreased after reaching the peak value,and then tended to be stable.The duration of submergence/drying has an effect on the release of phosphorus from sediment in the water-fluctuation zone,and frequent alternation of submergence/drying can promote the release of phosphorus from sediment.The ratio of dissolved total phosphorus(DTP)to TP in the overlying water increased with the increase of the frequency of inundation/drying cycles(69%in the first inundation period,65%in the second inundation period,and 73%in the third inundation period).In DTP,the contribution of dissolved inorganic phosphorus(DIP)to TP was higher than that of dissolved organic phosphorus(DOP),and the proportion of DIP/TP in overlying water was 45%,51%and 48%in three inundation stages,respectively.In the process of submergence/desiccation,the main physical and chemical properties of sediments are different,which affects the release of phosphorus from sediments.In the process of inundation,the p H of overlying water fluctuated at the initial stage,and stabilized at neutral at the later stage.It was positively correlated with TP,DTP,PP and DOP(P<0.01),and with DIP(P<0.05).ORP showed a downward trend,and was negatively correlated with TP,DTP,PP,DIP and DOP(P<0.01).DO was significantly positively correlated with TP(P<0.05),and IP,DIP and DOP were significantly positively correlated(P<0.01).TOC generally showed an upward trend in different inungnation periods,and it was positively correlated with TP,DTP,PP and DOP(P<0.01).Sediment?_LOIshowed a downward trend in the flooded stage and an upward trend in the drying stage,and it had a significant positive correlation with TP(P<0.05)and OP(P<0.01).Iron and manganese can effectively inhibit the release of phosphorus from sediment in the water-fluctuation zone.The inhibition effect of Fe and Mn on P release increased with the increase of Fe and Mn content,and the inhibition effect was strongest when Fe and Mn were present simultaneously.When the dosage of Fe and Mn was 0.1 mol,the mean TP values of the overlying water in three successive submersion/drying cycles were 1.66 mg/L,1.97 mg/L and 1.71 mg/L,respectively.All of them were significantly lower than those without Fe/Mn salt(the mean TP values of overlaying water were 2.34 mg/L,2.83 mg/L and 2.04 mg/L,respectively).After adding iron and manganese,the percentage of DTP in TP decreased,and the percentage of PP in TP increased.During the process of submergence/desiccation,the sediment particle size,surface functional groups,crystal structure and organic composition changed.Specifically,the sediment particle size gradually changed from 10 nm to 100 nm during the submergence process.According to the results of Fourier transform infrared spectroscopy,after the addition of Fe Cl₃and Mn Cl₂to the bottom mud,the peak intensity of hydroxyl vibration at 3432 and 1630 cm^-1and Ca CO₃vibration at1432 cm^-1changed.It is speculated that the addition of Fe and Mn makes the content of functional groups adsorbable phosphorus in sediment remain stable or increase,thus inhibiting the release of phosphorus.The XRD patterns of sediments in different inundation/drying periods are similar,with regular peaks,but the intensity of the peaks is slightly different,and the crystal structures are also different.The parallel factor analysis was used to analyze the results of three-dimensional fluorescence spectra,and it was found that the main categories of dissolved organic compounds in sediments/overlying water/intercalation water were basically the same in different periods of flooding/drying,but the contents of different kinds changed.