Sediment Characteristics,effects Of Aquatic Plant On Sediment And Phosphorus-locking Research In Xi’an Moat

The severe eutrophication of Xi’an Moat during summer causes water to become black and smelly,which results in a negative impact on the city’s landscape water environment.Hence,in order to control eutrophication and recover the water body,the sediments and Potamogeton crispus(P.crispus)growth were tracked and investigated in different areas of Xi’an Moat.Furthermore,the high internal phosphorus load in moat water was controlled systematically.In this paper,the sediments were tracked and sampled in different areas of Xi’an Moat firstly,which were from the Northwest corner,Shangwu gate,Shangde gate,Northeast corner and Chaoyang gate.Then the seasonal changes of carbon,nitrogen,phosphorus,and heavy metals in different sediments of Xi’an Moat with and without P.crispus growth were monitored.Finally,to lower phosphorus concentration in Xi’an moat,four different phosphorus-locking materials,namely,calcium nitrate(CN),sponge-iron(SI),fly ash,and silica alumina clay were selected in this experiment to study their effects on water quality and sediment.Based on that,the combination effect of SI and CN on phosphorus(P)control was explored in laboratory and field experiments.The main results were as follows:(1)The average content of total nitrogen(TN)in sediment of the Northwest corner,Shangwu gate,Shangde gate,Northeast corner and Chaoyang gate were 10.99mg/g,8.20 mg/g,5.73 mg/g,8.99 mg/g and 6.55 mg/g,respectively.All the TN content in the above sites got to the largest value in spring and the smallest value in autumn.The average content of total phosphorus(TP)in sediment of these five areas were 3.31mg/g,2.09mg/g,2.34mg/g,3.35mg/g and 2.69mg/g,respectively.All the TP content in the above sites had no obvious seasonal variations.The average content of total organic carbon(TOC)in sediment of these five areas were 118.79mg/g,101.83mg/g,73.22mg/g,108.98mg/g and 88.40mg/g,respectively.All the TOC content were higher in summer and winter and lower in spring and autumn in addition to the Northwest corner,Northeast corner and Shangwu gate,as the seasonal variations of TOC content in sediment of these areas were not obvious.(2)The seasonal variations of inorganic phosphorus(IP)content in sediment of the Northwest corner,Shangwu gate,Shangde gate,Northeast corner and Chaoyang gate were similar to those of TP,while organic phosphorus(OP)content in sediment of these five areas all got to the largest value in summer and the smallest value in autumn in addition to the Northwest corner,as the seasonal variation of TOC content in sediment of Northwest corner got to the largest value in spring and the smallest value in winter.The seasonal variations of Fe/Al-bound phosphorus(Fe/Al-P)content in sediment of these five areas were obvious,while the seasonal variations of Ca-bound phosphorus(Ca-P)were not significant.From the spatial distribution point of view,the average content of IP and Fe/Al-P in sediment got to the largest value in Northwest corner and the smallest value in Shangwu gate,while the average content of OP and Ca-P in sediment got to the largest value in Northwest corner and the smallest value in Shangde gate.(3)The average content of TN and TOC in sediment of P.crispus growth area was 10.70mg/g and 104.41mg/g,which were 1.66 and 1.23 times of that in sediment without P.crispus growth,respectively.Moreover both TN and TOC content in sediment got to the largest value in spring and the smallest value in autumn.However,the content of TP in sediment of P.crispus growth,2.01mg/g averagely,was 85.9% of that in sediment without P.crispus growth and was affected little by seasonal variation.On the other hand,Fe/Al-P content in sediment of P.crispus area was only 65.7% of that in sediment without P.crispus.Being part of the OP,the mean content of stable organic phosphorus attained 49.4% of OP in sediment of P.crispus growth area.In addition,the mean value of main heavy metals were 68.03 ug Cr/g,100.63 ug Mn/g,66.12 ug Cu/g and 295.19 ug Zn/g in sediment of P.crispus growth area,which were 88.8%、65.7%、56.2% and 110.9% of that in sediment without P.crispus growth,respectively.(4)The influence of CN was the most obvious with respect to the lower phosphorus concentration in overlying water and interstitial water.At the end of the experiment,the phosphorus-locking efficiency for SRP in overlying water and interstitial water were 99.1 and 95.5 %,respectively.The phosphorus-locking efficiency for SRP in overlying water and interstitial water were 78.5 and 77.9 % with the addition of SI,respectively.The fly ash had a limited phosphorus-locking efficiency relatively and the silica alumina clay almost had no adsorption capacity for phosphorus.(5)The addition of SI and CN at different mass ratio of SI and CN(MSI: MCN)significantly reduced SRP concentration in the overlying water,which the efficiency of SRP locking reached a maximum value of 98.7% with MSI: MCN of 4.Moreover,the optimum environmental conditions were p H of 8.9 and temperature of 15℃ for SRP locking with MSI: MCN of 4.(6)Considering the environmental effect and economic investment required,treatment with the mass ratio of 4 and 1.4g/L combined SI and CN is a better choice for the in situ restoration of a water body with high internal P load.At 1.4g/L combined SI and CN,the efficiency of SRP locking was 62.2% in overlying water and 62.3% in interstitial water after 56 days.Under this treatment,TP,OP,and Ca-P contents in sediment increased by 7.7%,15.2%,and 2.4%,respectively.