Modification Of Gasification Coarse Slag And Its Mechanism Of Purifying Phosphate In Water

With the growth of the national economy,the use of phosphorus-containing materials has increased.Owing to this,excessive phosphate content in found in water bodies and eutrophication occurs,posing a threat to the ecological environment and human health.In this regard,the adsorption method has been favored because of its simple operation,high efficiency,and wide range of applications.However,traditional adsorbents have drawbacks,such as limited availability of raw materials,complex pretreatment processes,and high costs.Therefore,the development of new adsorbents with diverse raw-material sources,large stocks,low costs,and high efficiency is the key to promoting the further development of phosphate removal technology via adsorption.Although the rapid development of the coal chemical industry in China brings economic benefits,a large amount of gasification slag is also produced,posing potential threats to the ecological environment.The resource utilization of gasification slag has become a focus and hot issue for the government and scholars.Based on this,this study proposed four gasification slag-based adsorption materials based on the structure and material characteristics of CGCS while fully utilizing the phosphate removal beneficial components（metal oxides）in CGCS.Different characterization methods were used to analyze the microstructure,specific surface area,phase composition,and other internal structures and component characteristics of gasification slag-based materials.The mechanism of phosphate removal using the gasification slag-based adsorption material was revealed by analyzing the change in the composition structure before and after phosphate adsorption.Static and dynamic experiments were used to investigate the adsorption properties of the obtained adsorption materials on simulated phosphate wastewater and real phosphate wastewater.The main conclusions of this thesis are summarized as follows:（1）A hematite–silicon mesoporous composite（HSMC）was prepared in situ via acid leaching and roasting to fully utilize the beneficial phosphorus-removal components（metal oxide）in the gasification slag.The preparation mechanism of the HSMC was revealed for the first time.HCl is believed to primarily react with the calcium,aluminum,and iron silicates in CGCS to produce metal chlorides of calcium,aluminum,and iron or hydrate metal chloride.These substances undergo oxidation and decomposition reactions under the action of roasting,yielding crystalline hematite,amorphous alumina,and calcium hydroxide.The HSMC has a specific surface area that is nearly 19 times that of CGCS,and its adsorption capacity is more than 28 times that of CGCS.The adsorption isotherm model and kinetic model analysis show that the phosphate adsorption using the HSMC was a single-layer adsorption and chemisorption process.The adsorption rate was influenced by boundary layer diffusion（membrane diffusion）and intraparticle diffusion.The phosphate removal mechanism of the HSMC mainly involves electrostatic adsorption,ligand exchange,inner-layer complexation,and surface chemical precipitation.（2）Zirconium-modified biochar,activated carbon,synthetic zeolite,natural minerals,and other composite materials used in phosphate removal have been studied in greater detail.However,the limited sources of raw materials,high costs,and relatively complex preparation processes of these carrier materials limit their large-scale application.Therefore,to improve the phosphate removal performance of CGCS and develop zirconium-modified adsorption materials,zirconium was introduced into a cheap and easily available carrier of CGCS,and a new zirconium-modified CGCS adsorbent was prepared using a more efficient one-step precipitation method.CGCS-Zr4 was determined to be the best adsorption material in a series of zirconium-modified CGCS adsorption materials.The mesoporous material CGCS-Zr4 has a large specific surface area（100 m2/g）.The results of competitive ion experiments showed that CCGCS-Zr4had good phosphate adsorption selectivity.Langmuir’s isothermal model can well describe the phosphate adsorption behavior of CCGCS-Zr4.Phosphate adsorption by CCGCS-Zr4 was a spontaneous endothermic reaction.The kinetic adsorption process was more consistent with the Elovich model,and the adsorption rate was mainly controlled by membrane diffusion and intraparticle diffusion.The adsorption mechanism of CCGCS-Zr4 on phosphate can be summarized as the electrostatic adsorption between positively charged CCGCS-Zr4 and negatively charged phosphate ions.The inner-sphere complex was formed through ligand exchange between the hydroxyl groups on the surface of CCGCS-Zr4 and phosphate ions.（3）Lanthanum and magnesium,which have a high phosphate adsorption capacity,were added to further improve the adsorption performance of CGCS,and the Mg–La-modified CGCS adsorption material was prepared using a coprecipitation method.The molar ratio of Mg/La was 1:2,and the mass ratio of CGCS to Mg Cl2·6H2O and La Cl3·7H2O was 5:3,indicating that Mg–La-modified CGCS had the best material ratio.At 25°C and p H 6,the maximum adsorption capacity of ML2-CGCS was 39.38 mg/g;the phosphate with an initial concentration of 35 mg/L can reach the adsorption equilibrium within 180 min,indicating that Mg–La-modified CGCS（ML2-CGCS）has a high efficiency for phosphate adsorption.The phosphate adsorption capacity of CGCS modified using Mg–La was significantly improved,which is of practical significance for developing La-modified materials and the reduction of related costs.Phosphate adsorption by ML2-CGCS mainly involved surface precipitation,ligand exchange,and inner-sphere complexation.（4）Magnesium slag（MS）was used as a calcium source to modify coal gasification coarse slag（CGCS）to prepare a novel phosphate adsorbent（MS-CGCS）.Its formation mechanism is mainly as follows:Ca₂Si O₄ in MS reacts with Na OH during the high-temperature synthesis process,with sodium displacing a part of the calcium content in Ca₂Si O₄ and entering the mineral lattice to form Na₂Ca Si O₄.Hydroxide ions reacted with calcium in Ca₂Si O₄ to generate Ca（OH）₂ and decomposed into Ca O at high temperature.The adsorbent MS-CGCS had a high p H_PZC（8.9）and a phosphate adsorption capacity of50.14 mg/g,indicating a strong adsorption capacity.The results of the metal leaching test show that the treatment of phosphate wastewater in water by MS-CGCS does not cause secondary water pollution.The main phosphate removal mechanisms are electrostatic interaction,the inner-sphere complexation,and the precipitation,precipitation contributes to～32%of the phosphate removal ability.（5）This study used dynamic adsorption experiments to investigate the actual phosphate wastewater treatment capacity of the four different CGCS-based adsorption materials.Under the same dynamic adsorption conditions,the penetration times of CGCS-Zr4,HSMC,ML2-CGCS,and MS-CGCS to treat the real phosphate wastewater were approximately 600,650,800,and 950 min,respectively,and the dynamic adsorption capacities were 1.83,1.99,2.41,and 2.90 mg/g,respectively.The MS-CGCS absorbent had the best treatment capacity for the real phosphate wastewater,so it was selected to investigate the impact of dynamic experimental parameters on the treatment of actual phosphate wastewater.Results show that increasing the concentration and flow rate could reduce the capacity of MS-CGCS to treat phosphate wastewater,and increasing the bed height could improve the phosphate wastewater treatment capacity of MS-CGCS.In this study,the commonly used Adams–Bohart,Thomas,and Yoon–Nelson models were selected for the fitting analysis of dynamic experimental data.Results showed that the Adams–Bohart model can describe the dynamic adsorption processes of the four CGCS-based adsorbents under specified conditions and the dynamic adsorption processes of MS-CGCS under different parameters（initial concentration,bed height,and liquid flow rate）.The adsorption capacity q_m of the four CGCS-based adsorbents obtained using the Thomas model was close to the actual adsorption capacity q_e.Theτcalculated using the Yoon-Nelson model was consistent with the 50%of the actual penetration times of the four CGCS-based sorbents and MS-CGCS with different parameters.