Preparation Of Modified Fly Ash Composite Material And Its Control Of Nutrients In Water Sediment

Sediment is an important element in the water environment,and the release of nitrogen and phosphorus nutrients in the sediment is an important cause of water eutrophication.The research on controlling the release of nutrient salts from sediments is mainly to achieve effective removal and fixation of nutrients at the mud-water interface by adding a material covering layer with adsorption activity and using the dual effects of physics and chemistry.Fly ash is one of the most accumulated solid wastes in our country,which has potential adsorption activity and utilization value.The resource utilization of fly ash in water treatment is an important content in the field of environmental protection and solid waste utilization.Based on the fly ash of a coal-fired power plant in Jinan,this study analyzes its internal components and removes excess impurities and pollution risks through experimental methods.A variety of modification methods are proposed for fly ash,and the optimal modification process and parameters are studied through theoretical analysis,beaker experiments,data statistics and microscopic characterization.Further composite with auxiliary materials,study the application effect of modified fly ash composite material to control nutrients in sediment,and obtain the following results:(1)The preparation of modified fly ash was pretreated,soaked with NaOH and calcined with CaO.The pores on the surface of the modified fly ash became larger,the proportion of C,Fe,Si,and Al elements decreased,and the proportion of Na,Ca,and O elements increased,and the characteristic peaks of sodium-type zeolite appeared.After 22 days of adsorption under the conditions of dosage of 25 g/L,p H=6 and temperature of 20°C.The NH₄⁺-N concentration was reduced from 50 mg/L to 24.09 mg/L,and the TP concentration was reduced from 5 mg/L to 1.23 mg/L.The removal rates were 44.62%and 75.4%,which were25.26%and 39.3%higher than the original fly ash,respectively.(2)The cement was used as the cementitious material,and the mixing amount was 10%.After mixing with the modified fly ash,it was made into 3mm particles through the mold,and the modified fly ash composite material was obtained after curing.Its porosity,hydrochloric acid solubility,and 1 h water absorption can all meet the standard limit requirements,the strength is 1.63 MPa,and the wear rate is 5.3%?6%,so the material integrity is strong.And the cement hydration reaction will not only produce gel products such as C-S-H,C-A-S-H and N-A-S-H,but also stimulate the active reaction of silica-alumina oxide again to enhance the adsorption performance of the material.The adsorption of NH₄⁺-N by the composites fits the Freundich adsorption isotherm.In the early stage,the adsorption capacity was high up to 1.129mg/g,while in the later stage,the adsorbate with weak adsorption force from the outer layer increased,and the adsorption capacity decreased to 0.728 mg/g.The amount of adsorption will increase again over time,showing a periodic change.The adsorption of TP was consistent with the Langmuir adsorption isotherm equation,and the adsorption amount quickly reached the equilibrium state of 0.191 mg/g,which was a fast and stable process.From the Weber-Morris intraparticle diffusion model,it was concluded that the adsorption of NH₄⁺-N by the composite material was the result of the simultaneous action of surface adsorption and pore diffusion,and the surface heterophasic and heterogeneous adsorption were the key factors.There was a plateau in the adsorption of TP,which was mainly controlled by intraparticle diffusion.(3)When the volume ratio of water and sediment was 5:2,the inhibition rates of NH₄⁺-N,TN,and TP in the sediment by the composite material directly covering the thickness of 2 cm were 90.91%,75.06%,and 90.63%,respectively.The control mechanism of composite materials on sediment nutrients was compared and analyzed by three methods:direct cover,covered with gauze and hang with gauze.It was found that the inhibition rates of NH₄⁺-N,TN,and TP in the gauze group were 89.85%,69.71%,and 87.79%,respectively;the inhibition rates of NH₄⁺-N,TN,and TP in the hanging group were 78.35%,41.17%,and 77.09%,respectively.The control effects of direct cover and covered with gauze were better.In the hanging group,the interface between the sediment and the overlying water could exchange materials freely,and the treatment effect was greatly affected by the amount of nutrients released from the sediment.Therefore,the physical barrier effect is crucial to the effect of composite materials on inhibiting nitrogen and phosphorus nutrients in sediments.Relying on adsorption alone is difficult to achieve water restoration of polluted sediments.The dynamic balance of nutrient concentration during the covering process is the result of the dual effects of physical barrier and chemical adsorption.