Study On Phosphorus Release From Surface Sediments Of The Yellow River Riparian Zone By Passivation With Concrete Slag

The Yellow River riparian zone is a critical ecological barrier for water resources protection in the Yellow River basin.In the Yellow River riparian zone,surface sediments are widely distributed by natural and human activities,which are affected by both aquatic and terrestrial environments,and play an important role in the material-energy cycle of the environment and the deposition of pollutants.For the overall Yellow River water environment,how to effectively improve the retention of phosphorus in the Yellow River riparian zone and the surface sediments of the Yellow River riparian zone is essential to reduce phosphorus pollution and protect the water ecological environment.Therefore,this study takes concrete slag as the research object to study the mechanism and process of phosphorus adsorption and fixation,and the effect and law of phosphorus adsorption and fixation under different influencing factors of particle size,hydraulic disturbance,pH and temperature,and also analyze the characteristics of phosphorus desorption and release of concrete slag and the effect of phosphorus desorption and release under different pH,hydraulic disturbance and temperature.The effect of concrete slag on phosphorus desorption and release under different pH,hydraulic disturbance and temperature conditions was also analyzed to verify its phosphorus retention capacity.Finally,the overall phosphorus fixation and passivation effect was investigated by covering the surface sediment of the Yellow River riparian zone with concrete slag as a phosphorus passivation material in situ.It was found that concrete slag as a passivation material to inhibit the release of phosphorus from surface sediments can treat"waste"with"waste",with good phosphorus adsorption and fixation effect and good retention capacity of phosphorus,and reduce the secondary pollution caused by the release of phosphorus from surface sediments.The risk of secondary pollution caused by the release of phosphorus from surface sediments is reduced.The results of the study are as follows:（1）It was found that concrete slag has a complex surface structure and high Ca content.By experimental analysis,the equilibrium state of phosphorus adsorption by concrete slag was reached at 12 h.The equilibrium adsorption amounts were 1137.184 mg·Kg^-1,2497.184 mg·Kg^-1 and 4716.361 mg·Kg^-1 at different phosphorus concentrations,respectively.15 min rate of phosphorus adsorption by concrete slag reached 60.365 mg·（kg·min）^-1,which is a fast rate.The results of its adsorption process better fit the quasi-secondary kinetic equation,the degree of fit R² were above 0.95,indicating that the process process of phosphorus adsorption by concrete slag contains a variety of adsorption processes such as surface adsorption and ion exchange adsorption.By kinetic analysis,the adsorption equilibrium of concrete slag reached after 24 h in the static state,and the adsorption amount was 2948.851 mg·Kg^-1.（2）The results of phosphorus adsorption by concrete slag with different particle sizes at different temperatures fitted the Langmuir isotherm equation well,and the R² of the fit was greater than 0.98.The theoretical maximum saturation adsorption capacity of concrete slag with less than 100 mesh at 150 r·min^-125°C was 21,200 mg·Kg^-1.It was found that the phosphorus adsorption process of concrete slagΔG^?is less than 0,which is a spontaneous process.The enthalpy change of phosphorus adsorptionΔH^?for concrete slag with different particle sizes is less than zero,which is an exothermic process.The process of phosphorus adsorption by concrete slag materials withΔS^?>0 is an entropy increasing process.Experiments show that at a certain concentration,the phosphorus pollutant concentration increases,the phosphorus adsorption increases,and the concrete slag with small particle size,its adsorption increases fast and the adsorption is large.Under the conditions of lower concentration of phosphorus,the particle size has less effect on the adsorption of concrete slag,and under the conditions of high concentration of phosphorus,the smaller the particle size,the larger the adsorption of concrete slag and the good adsorption and fixation effect.（3）Through the experiments on the effect of concrete slag on phosphorus adsorption and fixation under different temperatures,hydraulic perturbation and pH,it was found that the effect of concrete slag on phosphorus adsorption and fixation gradually increased with the increase of temperature,and the effect on its adsorption and fixation amount was more obvious.When the perturbation frequency was<150 r·min^-1,the adsorption amount of concrete slag increased with the increase of hydraulic perturbation,and when the perturbation frequency was>150 r·min^-1,its effect on phosphorus adsorption and fixation was not significant.The effect of pH on the concrete slag material was also less as the particle size decreased.When pH<9.0,pH increases,the amount of phosphorus adsorption by concrete slag decreases,when pH=9.0 the adsorption amount is the smallest,and when pH>9.0,the amount of phosphorus adsorption changes less.pH decreases are beneficial to phosphorus adsorption and fixation by concrete slag.（4）By analyzing the characteristics of phosphorus desorption and release characteristics of concrete slag and the effects on desorption and release under different temperature,hydraulic disturbance and pH conditions,it was found that it reached desorption equilibrium within 120min and the desorption kinetic process was more in line with the quasi first-order kinetic equation.The desorption rate was fast and the desorption rate was low,and the desorption rate was only about 4.03%.The desorption volume increases with the increase of temperature.When the hydrodynamic perturbation frequency was between 50 r·min^-1 and 150 r·min^-1,the increase of perturbation increased the desorption of concrete slag,and when the perturbation frequency was below 50 r·min^-1 and above 150 r·min^-1,the effect of the change of hydrodynamic perturbation on the desorption volume was not significant.pH was smaller the greater the desorption volume,and pH was larger the smaller the desorption volume.The desorption release rate ranged from 1.14%to 9.19%.Overall,the concrete slag desorption release rate was small,the adsorption fixation rate was high,and the retention capacity of phosphorus was good.（5）During the experimental period,the phosphorus concentrations in the overlying water of the non-addition group were all greater than those of the addition group,and the TP and PO₄^3--P concentrations in the overlying water were significantly higher than those of the non-addition group.In the added group,the PO₄^3--P and TP concentrations in the overlying water were maintained below 0.06 mg·L^-1 and 0.07 mg·L^-1,respectively,and the passivation efficiency W of concrete sludge on phosphorus in the surface sediment was 82%in 60 days.The pH and DO concentrations of the overlying water in the experimental setup added and unadded groups showed a decreasing trend,with DO concentrations decreasing to 0.33 mg·L^-1and 0.47 mg·L^-1 on day 60,and pH decreasing by 0.2 and 0.5,respectively.Concrete slag overlying maintained the pH of the water column during a certain period and slowed down the pH decrease.Overall,the phosphorus concentration of the non-added control group on day 60was five times higher than that of the in situ overlay added group,and the inhibition rateηof concrete slag on phosphorus in the surface sediment of the Yellow River was 80.1%.The measured heavy metal contents in the overlying water of the added group were lower than the limits of surface water environmental quality standard III and the blank group.It was found that the surface sediment of Yellow River was dominated by inorganic phosphorus,accounting for89.6%of the total phosphorus content.By phosphorus graded extraction method,hydrochloric acid extracted state phosphorus（HCl-P）accounted for the largest proportion of 69.4%.The residual state phosphorus（Res-P）accounted for the smallest percentage of 2.6%.By experimental comparison,the total phosphorus TP of the sediment in the non-addition group was reduced by 50.6%,while the total phosphorus TP of the sediment in the addition group was reduced by only 40.2%,and the top-covered concrete had a good effect of inhibiting the release of the surface sediment.The content of different forms of phosphorus in the concrete top-covered added group was higher than that in the unadded group,and the concrete slag top-covered in situ could reduce the release of different forms of phosphorus from the surface sediment,and the study proved that concrete slag could be used as a phosphorus passivation material.