Study On Modified Magnesium Desulfurization Residue For Phosphorus Removal: Performance And Mechanism

With focus on resource utilization of magnesium desulphurization residues（MDR）in thermal power plant and the research of phosphorus removal from wastewater,and combined with the idea of waste disposal,a tentative idea of modifying MDR into a new adsorbent for phosphorus removal was put forward.Then the activation and modification methods for preparing phosphorus removing adsorbent（A-MDR）by MDR were studied,the phosphorus removal performance by adsorbent of A-MDR and the influencing factors of adsorption process were investigated and the mechanism of phosphorus removal was studied.The results show that:（1）The main component of MDR is diatomite and magnesium oxide,there are 50%⁶0% of silicon oxide,about 40% magnesium containing compounds and about 5% metal oxides（Such as Al,Ca and Fe）.Arrangement of modification methods based on the adsorption capacity of phosphorus is that: H₂SO₄ modification > Supported iron oxide at high temperature > NaOH solution activation > High temperature activation.Finally,the modification process for the preparation of A-MDR is high temperature activation（600 ℃）and H₂SO₄ modification.The Modification condition is 2.0mol/L of acid concentration,1:3.5 of solid to liquid ratio,180 min of modification time and 65℃ of bath temperature.A large number of magnesium salt crystals attached to the surface of MDR after modification.Specific surface area and pore volume of A-MDR increased by 46.6% and 28%,respectively.And there are functional groups such as Mg-OH on the surface,which play a key role in the adsorption process of phosphorus.The experimen of XRD shows that most of the magnesium oxide in MDR was activated as magnesium salts such as Magnesium sulphate and the surface activity was enhanced.（2）The phosphorus removal rate of 50mg/L phosphorus sastewater by A-MDR can reach 98.4% under the condition that the initial solution pH was 9,the dosage was 12g/L and the reaction time is 120 min.At the same time,A-MDR has the potential to treat high concentration phosphorus wastewater,the ability to resist the influence of coexisting anions(such as Cl-、SO₄^2-and NO₃^2-)in solution and the ability of reuse（regeneration）.The adsorption process of phosphorus by A-MDR is more in accordance with the pseudo two order kinetics equations;it shows that the process is mainly chemical adsorption.The fitting results of the internal diffusion equation show that the adsorption rate of A-MDR is controlled by the diffusion process.The adsorption isotherms of A-MDR adsorption were more in line with the Langmuir model,which indicated that the adsorption type of A-MDR was a single molecular layer adsorption.（3）The maximum adsorption capacity of A-MDR is 27.548mgP/g,which is higher than that of most modified waste residue.It showed that it is a new type of phosphorus removal adsorbent which has the advantages of low cost,simple preparation process,large specific surface area,strong surface activity,good adsorption performance,good desorption performance,easy regeneration,high adsorption efficiency,large adsorption capacity,etc.It’s mechanism of phosphorus removal is: At first,A-MDR enters the waste water containing phosphorus.Then,a concentration gradient of phosphorus occurs between the adsorption sites on the surface and the liquid film at the solid-liquid interface,the phosphate in water rapidly migrates to the adsorption site of A-MDR surface by the gradient force.The Highly active Mg²⁺ and Mg-OH contained in these adsorption sites have strong ability to adsorb phosphorus.Therefore,phosphate can combine with Mg²⁺ to form chemical bonds and is firmly fixed on the surface of A-MDR.On the other hand,phosphate is adsorbed by ligand exchange with the hydroxyl of Mg-OH functional group.Final,the phosphate in water is transferred from the liquid phase to the solid phase and is removed by the solid-liquid separation process.