River Sediment Pretreatment And Its Application In Sand Greening

River sediment is the sediment of rivers,which is rich in nutrients such as nitrogen,phosphorus and organic matter,as well as harmful substances such as heavy metals and pathogenic microorganisms.With the increasing of sediment production,how to control river sediment has become an urgent environmental problem.The research shows that the utilization of river sediment is a promising treatment method,but the sediment must be treated harmlessly before entering the soil to remove heavy metals and other pollutants.Therefore,this paper takes the river sediment of Xinkai River and Nujiang Bridge section in Shenyang City as the research object.Firstly,harmless pretreatment is carried out on the river sediment to study the influence of different factors（initial mass of sediment,freezing time,centrifugal time and centrifugal speed）on the effect of freeze-thawing centrifugal treatment of river sediment and determine the optimal treatment conditions.The differences between the effects of freeze-thaw,centrifugation and drying alone and the effects of different combinations were discussed.According to different ratios（0%,10%,20%,30%,40%,50%,60%,80%,100%,according to the dry mass ratio of sediment and sand）,the river sediment under the best treatment conditions was mixed with sand soil,and the pot experiment was carried out to cultivate ryegrass,and the influence of the river sediment application on the properties of sand soil and the growth of ryegrass was analyzed.To clarify the distribution characteristics of heavy metals between soil and plants under the condition of application of river sediment.The main research results are as follows:（1）The influences of different factors（initial mass of sediment,freezing time,centrifugal time and centrifugal speed）on the dewatering rate of river sediment and turbidity of supernatant were analyzed.The results show that with the increase of freezing time,centrifugation time and centrifugation speed,the dewatering of sediment first increases and then stays flat,and the turbidity of supernatant decreases first and then tends to balance.The initial mass was inversely correlated with the change of sediment dewatering rate and positively correlated with the change of turbidity of supernatant.The optimum conditions were freezing time 5 h,initial mass60 g,centrifugation time 20 min and centrifugation speed 6000 r/min.Under these conditions,the dewatering rate of sediment increased from 34.58%to 61.09%.The turbidity of supernatant decreases from 181.2 NTU to 14.43 NTU by 92.04%.（2）According to the above optimal treatment conditions,the effects of different treatment methods（air drying,oven drying,freeze-thaw-air drying,freeze-thaw-oven drying,centrifugal-air drying,centrifugal-oven drying,freeze-thaw-centrifugal-air drying and freeze-thaw-centrifugal-oven drying）on the nutrient and heavy metal contents of river sediment were studied.At the same time,the index content of the supernatant was determined,and the ultraviolet-visible spectrum（UV-vis）and three-dimensional fluorescence spectrum（3D-EEM）of organic matter in the supernatant were analyzed.The results showed that freeze-thaw-centrifugal-oven drying had the best effect on reducing organic matter,total nitrogen,electrical conductivity,total phosphorus and bacteria in the sediment,which decreased by6.63%,4.60%,45.72%,32.91%and 97.33%,respectively,compared with direct drying.freeze-thaw-centrifugal has the best removal effect on heavy metals（copper,cadmium,lead,zinc,nickel and chromium）,and the removal rate is 11.62%～15.10%compared with that of air drying.The sludge and water separation effect of freeze-thaw-centrifugal is the best.Compared with before treatment,the liquid volume of supernatant increases by 97.06%,turbidity of supernatant decreases by85.12%,conductivity of supernatant increases by 40.30%,and COD_Crcontent of supernatant increases by 149.04%.UV-vis showed that the supernatant of sediment was mainly fulvic acid before treatment,after freeze-thaw and after freeze-thaw-centrifugal,but with the combination of freeze-thaw and centrifugation,the proportion of fulvic acid in the supernatant increased.The 3D-EEM showed that the fluorescence peak of tyrosine-like substance and tryptophan-like substance appeared in the supernatant after natural standing of the bottom mud,the fluorescence peak of tyrosine-like substance disappeared and the fluorescence peak of fuligenous acid substance appeared in the supernatant after standing of the bottom mud after freeze-thaw treatment,and the fluorescence peak of fuligenous acid substance disappeared and the fluorescence peak of tryptophan-like substance existed in the supernatant after freeze-thaw-centrifugation treatment.（3）Pot experiment showed that:1.The mixed application of river bottom mud-sand soil（10%～100%）increased soil fertility,organic matter increased by1.2-13.6 times,total nitrogen increased by 4.2～59.8 times and total phosphorus increased by 0.7～14.8 times.2.The contents of copper（Cu）,cadmium（Cd）,lead（Pb）,zinc（Zn）,nickel（Ni）and chromium（Cr）in different mixed application（10%～100%）soil ranged from 5.78～63.08 mg/kg,0.04～5.32 mg/kg,7.83～49.18 mg/kg,13.51～47.17 mg/kg,1.03～72.71 mg/kg and 3.86～55.04 mg/kg.When the proportion of ryegrass applied in river sediment was 100%,the growth of ryegrass was the best,but combined with the technical requirements of heavy metal content in green planting soil（heavy metal content should not exceed the range ofⅣ）in《Green Planting Soil》（CJ/T 340-2016）,it was determined that the proportion of river sediment applied should be lower than 40%.3.The contents of Cu,Cd,Pb,Zn,Ni and Cr in ryegrass cultured in different treatments ranged from 2.53 to 41.39 mg/kg,0.02to 3.85 mg/kg,2.40 to 23.29 mg/kg,6.24 to 22.58 mg/kg and 0.09 to 6.43,respectively mg/kg and 1.38 to 15.38 mg/kg.The transport coefficients of ryegrass for Cu ranged from 0.15 to 0.24,for Cd,Pb,Zn and Cr ranged from 0.06 to 0.14,and for Ni was less than 0.04.Under different treatment conditions,the enrichment coefficients of Cu,Cd and Pb in ryegrass increased with the increase of heavy metal contents in each treatment,and the enrichment coefficients ranged from 0.437 to0.565,from 0.355 to 0.725 and from 0.284 to 0.474,respectively.The enrichment coefficients of Zn and Ni changed little with the heavy metal content of each treatment at about 0.465 and about 0.087,respectively.The enrichment coefficient of Cr decreased with the increase of heavy metal content in each treatment,ranging from0.280 to 0.357.Ryegrass showed good transport and enrichment effects on Cu,Cd,Pb,Zn and Cr,but poor transport and enrichment effects on Ni.Compared with the control,the transport capacity of 10%to 100%ryegrass for Cu,Cd,Pb,Zn and Ni increased,while the transport capacity for Cr decreased.The enrichment capacity of Cu,Cd,Pb and Ni in ryegrass cultured under 10%to 100%treatment is enhanced,while the enrichment capacity of Cr is weakened.The enrichment capacity of Zn in ryegrass cultured under 10%to 60%treatment is weakened,and the enrichment capacity of Zn in ryegrass cultured under 80%to 100%treatment is enhanced.