In-situ Release And Ex-situ Removal Of Nitrogen And Phosphorus In River Sediment By The Addition Of Activated Persulfate

In recent years,remarkable progress has been made in river pollution control,butthere are still potential problems such as secondary pollution caused by re-release of nitrogen and phosphorus from river bottom mud.The restoration methods of excessive disturbance of river bottom sediment,such as dredgings,often lead to the rapid and high intensity release of nitrogen and phosphorus pollutants to the overburden water,resulting in the rapid deterioration of water quality and even irreversible damage to the river ecological environment.To explore a reasonable way to safely treat and dispose of pollutants deposited in river sediment,research is conducted with the aim of low disturbance,high efficiency,green environment and cost saving,this paper takes the sediment of"Zao he River"as the research object to explore the feasibility of using activated persulfate in situ to promote the release of nitrogen and phosphorus in the sediment and artificial wetland ectopic enhancement removal as the removal of river sediment pollutants.To provide a new way to gently and quickly restore sediment habitat and eliminate potential environmental risks.The main research content is to activate sodium persulfate with Fe Cl₂and biochar at normal temperature to release nitrogen and phosphorus from the river bottom mud in situ into the overlying water,and to enhance the removal of nitrogen and phosphorus from the overlying water by ectopic enhanced ryegrass constructed wetland induced by root redundancy.The main results and conclusions are as follows:（1）The effect of sodium persulfate,ferrous iron and biochar on the release of nitrogen and phosphorus from the substrate:the addition of sodium persulfate promoted the release of nitrogen and phosphorus from the substrate;the addition of excess ferrous iron would inhibit the release of nitrogen and phosphorus;the addition of biochar promoted the release of nitrogen and inhibited the release of phosphorus.The best amount of persulfate is 5%-10%,and a small amount of persulfate will lead to insufficient oxidation.The release of total nitrogen and total phosphorus from the substrate is 15.89 mg/L and 1.35 mg/L when 5%sodium persulfate is added,which is57.3%and 101.5%higher than the release of total nitrogen and total phosphorus from the original mud（no agent is added to the substrate）of 10.1 mg/L and 0.67 mg/L;Fe²⁺The amount of Fe²⁺added exceeded 6%,and ferrous iron would make the water less acidic,resulting in lower nitrogen release and adsorption and chemical reaction on total phosphorus in the system,which led to a continuous decrease of total phosphorus content;different doses of biochar were added to promote nitrogen release,while excessive biochar had an inhibitory effect on phosphorus release.（2）Characteristics of nitrogen and phosphorus release from ferrous iron and biochar-activated sodium persulfate substrate:ferrous iron and biochar-activated sodium persulfate could promote the release of nitrogen and phosphorus from the substrate.When 4%ferrous iron+7%sodium persulfate was used,the total nitrogen release from the substrate was 35.45 mg/L,which was 2.5 times,2.1 times and 1.5times higher than that of the original mud group,ferrous iron group and sodium persulfate group,respectively;the total phosphorus release was 4.83 mg/L,which was6.2 times,4 times and 2.6 times higher than that of the original mud group,ferrous iron group and sodium persulfate group,respectively.2%biochar+7%sodium persulfate The release of total nitrogen was 31.1 mg/L,which was 2.1 times,1.5 times and 1.2 times higher than that of the original mud group,biochar group and sodium persulfate group,respectively;the release of total phosphorus was 4.22 mg/L,which was 5.2 times,7.5 times and 3.0%higher than that of the original mud group,biochar group and sodium persulfate group.In contrast,the release of total nitrogen and phosphorus was 19.23 and 5.05 mg/L,respectively,after 8%biochar+7%sodium persulfate was added to the substrate,which was 45.8%and 16%lower than that of2%biochar+7%sodium persulfate,indicating that excessive biochar activation of sodium persulfate was not conducive to the release of nitrogen and phosphorus from the substrate.（3）The effect of water flow disturbance on the nitrogen and phosphorus release characteristics of the substrate:the release of nitrogen and phosphorus from the raw mud under high water flow disturbance,low water flow disturbance and stationary conditions were comparable.When activated persulfate was added to the substrate,the release of nitrogen and phosphorus from the substrate under the current disturbance condition was significantly increased,but there was a large variability of nitrogen and phosphorus release between different water flow disturbances,and the release of total nitrogen was 56%higher for high water flow disturbance than low water flow disturbance,but the release of total phosphorus was 80%lower.（4）Effect of pH on the release of nitrogen and phosphorus from substrate:acidic（pH<5）and alkaline（pH>9）conditions contribute to the release of nitrogen and phosphorus from the original mud,and the optimal pH conditions for the release of nitrogen and phosphorus from ferrous iron and biochar-activated sodium persulfate substrates are 7 and 11,respectively.In the test of nitrogen and phosphorus release from biochar-activated sodium persulfate substrate,the total nitrogen and total phosphorus release from pH=11 substrate were 23.24 and 3.22 mg/L,respectively,which increased by 31%,19%,12%,and 56.5%compared with pH=3,5,and 7,respectively.19%and 12%and 56.3%,33.1%and 10.6%,respectively,compared with pH=3,5 and 7.（5）Nitrogen and phosphorus in water bodies were treated in allotropic enhancement:ryegrass surface flow artificial wetland and ryegrass surface flow artificial wetland with limited root cultivation both had good removal effect on nitrogen and phosphorus,and both had comparable treatment efficiency in phosphate,but the latter had more advantages than the former in ammonia and nitrate nitrogen removal.The effluent concentrations of ammonia nitrogen,nitrate nitrogen and phosphate from the ryegrass surface flow artificial wetland were 10.58 mg/L,2.76mg/L and 3.1 mg/L,and the removal rates reached 47%,45%and 38%,which were8.1%,11%and 3%higher than those of the conventional surface flow artificial wetland.