Study On Optimization Of Modification Method Of Hydrated Cement Particles And Behavior Of Phosphate Adsorption

As one of the main factors causing water eutrophication,the control of phosphorus content is an important measure to prevent water eutrophication.Adsorption removal of phosphorus has been widely concerned due to safe and reliable operation,simple procedure and cost effectiveness.However,low adsorption capacity,slow initial adsorption rate and high production cost limit the widely application of the adsorption method.In this study,hydrated cement was used as the raw material to prepare phosphorus adsorbent.Different modification methods were analyzed and compared according to the intensity and phosphorus removal effect.The effects of initial concentration,contact time,initial p H,coexisting ions and ionic strength on the adsorption performance of the best modification method HPC(namely HPC_0.5ASand HPC_T700)were studied.Through XRD,FTIR,BET,SEM and other characterization techniques,the surface morphology and internal structure characteristics and adsorption mechanism were analyzed,and the parameters of HPC_0.5ASindustrial preparation were optimized.The purpose is to develop a new type of phosphorus removal adsorbent that is efficient,inexpensive and suitable for large-scale production and application.The results of the study are as follows:（1）The physical and chemical properties of hydrated portland cement had changed significantly with 0.5 mol/L Al₂（SO₄）₃modification and 700℃high temperature modification.X-ray diffraction（XRD）analyzes indicated that after activation at 700℃,the characteristic peak of C₃AH disappeared.And there were peak of Ca SO₄and increasement of C₃AH with modification with 0.5 mol/L Al₂（SO₄）₃.The surfaces of HPC_0.5ASand HPC_T700became rough,with porosity increased.The specific surface area became larger,reaching 22.05 m²/g and 19.81 m²/g,respectively.High temperature modification destroyed the structure of HPC to a certain extent.Fourier transform infrared spectroscopy（FTIR）analysis indicated that the symmetrical shrinkage peak of Al-O-Si was enhanced and gypsum was generated after Al₂（SO₄）₃modification.（2）The phosphate adsorption capacity of HPC_0.5ASand HPC_T700in 4 h were 5.86mg/g and 6.26 mg/g,respectively.The adsorption capacities of HPC_0.5ASand HPC_T700in72 h is 25.31 mg/g and 30.52 mg/g（C₀=100 mg/L,adsorbent dose 2 g/L,T=25°C,p H=4.8）,and the quality loss rates was 4.25%and 5.84%,respectively.The adsorption performance showed that the most suitable modification method of hydrated Portland cement particles is 0.5 mol/L aluminum sulfate and 700℃thermal modification.（3）The adsorption of phosphate by HPC_0.5ASand HPC_T700reached an equilibrium in 72 h.The adsorption capacity of HPC_0.5ASdecreased first and then increased with the increase of p H,lowest adsorption was obtained under neutral conditions.On the other hand,the adsorption effect of HPC_T700decreased with the increase of p H.The order of the influence of anions on the adsorption of phosphate by HPC_0.5ASin water was:HCO₃^->CO₃^2->SO₄^2->NO₃^-,which is similar to the order by HPC_T700:CO₃^2->HCO₃^->SO₄^2->NO₃^-.The adsorption of phosphate by HPC_0.5ASwas not affected by the ionic strength.By contrast,with the increase of ionic strength,the adsorption by HPC_T700gradually increased and tended to saturation finally.（4）The adsorption kinetic of HPC_0.5ASand HPC_T700was fitted better with the pseudo-second-order model.The isothermal adsorption process was described better by the Freundlich equation.Thermodynamic results indicated that the adsorption is a spontaneous process with endothermic and entropy increasing.Combined with the changes of the crystal structure,chemical bonds,and surface morphology of HPC_0.5ASand HPC_T700before and after the adsorption of phosphate,it is determined that chemical adsorption is main the adsorption mechanism.the removal of phosphate could be attributed to both inner-sphere complexation and surface precipitation process.（5）The optimal conditions for the industrial production of HPC_0.5ASwere determined.The best modification method was selected with the time of 6 h,and the solid-liquid ratio of 1:10.The modified liquid can be used repeatedly.The adsorption capacity of the modified liquid at the sixth use is 17.66 mg/g in 72 h.After 3 times regeneration with 0.5 mol/L Al₂（SO₄）_3,the adsorption capacity of HPC_0.5ASwas still greater than HPC.It indicated that HPC_0.5ASis an effective phosphorus removal adsorbent for practical application.Therefore,HPC_0.5ASis an excellent phosphorus removal adsorbent with cheap raw materials,easy preparation,large adsorption capacity,stable performance,and regeneration.It has good application prospects.