Study On The Fluorine Removal Performance Of Activated Magnesium Oxide Modified By Iron And Calcium Salts

In China,the fluorine content in groundwater exceeds the standard in many places,which brings inconvenience to the production and life of local residents.In addition,industrial development will also produce a large number of fluorine-containing wastewater,if not properly treated will not only lead to serious environmental pollution,but also to human health threats.Therefore,it is of great significance to explore the effective removal of F^-in water.Active magnesium oxide is a kind of high performance adsorbent,which has the advantages of high specific surface area,large adsorption capacity,non-toxic,harmless,green and safe.It has a good development prospect in the field of fluoride removal.But at the same time,the adsorption efficiency,anti-interference ability and regeneration adsorption performance of active magnesium oxide need to be further strengthened.Therefore,FeCl₃,Fe₂(SO₄)₃ and CaCl₂ were selected as the modified reagents of active magnesium oxide.FeCl₃-MgO,Fe₂(SO₄)₃-MgO,CaCl₂-MgO,FeCl₃-CaCl₂-MgO and Fe₂(SO₄)₃-CaCl₂-MgO were prepared by impregnation-roasting method,respectively.The optimal preparation conditions of these five adsorbents were explored,and the adsorption performance of the modified adsorbents was explored from four aspects:adsorption conditions(dosage,temperature,p H),adsorption kinetics,adsorption isotherms,and the influence of coexisting ions.The adsorption mechanism was analyzed by combining the characterization results(XRD,SEM,FTIR).Finally,the regeneration adsorption performance of adsorbent was investigated.The main results are as follows:(1)Under the optimum preparation conditions,FeCl₃-MgO(concentration of FeCl₃ was0.05 mol/L,calcination temperature was 400?)and Fe₂(SO₄)₃-MgO(concentration of Fe₂(SO₄)₃ was 0.01 mol/L,calcination temperature was 500?)were prepared.The optimum p H range,adsorption efficiency and overall anti-interference ability of the two adsorbents were better than those of active magnesium oxide.Among them,FeCl₃-MgO had a wider suitable p H range(p H 3?11),and its fluorine removal rate remained above 95%at p H 11.Fe₂(SO₄)₃-MgO had a higher fluorine removal efficiency,and the F^-concentration of 100 m L simulated fluorine-containing wastewater could be reduced from 20 mg/L to less than 1 mg/L after 0.2 g of the adsorbent was treated for 0.5 h.The selective adsorption capacity of Fe₂(SO₄)₃-MgO was stronger,and the adverse degree of fluoride removal by co-existing ions(fluoride removal rate was less than 80%)could be ranked as Si O₃^2->HPO₄^2->H₂PO₄^-.The adsorption process of FeCl₃-MgO and Fe₂(SO₄)₃-MgO was more consistent with Langmuir adsorption isotherm model and quasi-second-order kinetic model.Compared with active magnesium oxide,the saturated adsorption capacity of FeCl₃-MgO had little difference,while that of Fe₂(SO₄)₃-MgO decreased.According to the mechanism analysis,FeCl₃-MgO and Fe₂(SO₄)₃-MgO mainly realized the removal of F^-through ion exchange and complexation.(2)CaCl₂-MgO was prepared under the optimum preparation conditions(concentration of CaCl₂ was 0.01 mol/L,calcination temperature was 400?).The optimum p H range,adsorption efficiency,overall anti-interference ability and regeneration adsorption capacity of CaCl₂-MgO were better than those of active magnesium oxide.The suitable p H range of CaCl₂-MgO(p H 3?10)and adsorption efficiency(0.2 g of the adsorbent could reduce the F^-concentration of 100 m L simulated fluorine-containing wastewater from 20 mg/L to less than1 mg/L after 2 h treatment)were similar to Fe₂(SO₄)₃-MgO and FeCl₃-MgO respectively.But the overall anti-interference ability was weaker than that of active magnesium oxide modified by ferric salt.The desorption and regeneration capacity of CaCl₂-MgO were improved,and the regeneration rate of CaCl₂-MgO could reach more than 90%after desorption by water,acid and alkali and roasting at high temperature.The fluoride removal process of CaCl₂-MgO was more consistent with Langmuir adsorption isotherm model and quasi-second-order kinetic model,and the saturated adsorption capacity of CaCl₂-MgO was slightly higher than that of active magnesium oxide,which could reach 109.54 mg/g.According to the mechanism analysis,CaCl₂-MgO mainly achieved the removal of F^-through ion exchange.(3)Under the optimum preparation conditions,FeCl₃-CaCl₂-MgO(molar ratio of FeCl₃to CaCl₂ was 1:3,calcination temperature was 400?)and Fe₂(SO₄)₃-CaCl₂-MgO(molar ratio of Fe₂(SO₄)₃ to CaCl₂ was 1:1,calcination temperature was 500?)were prepared.The suitable p H range,adsorption efficiency and overall anti-interference ability of the two adsorbents were better than those of active magnesium oxide,but they were not further improved compared with single solution modification.Among them,FeCl₃-CaCl₂-MgO could combine the advantages of selective adsorption of FeCl₃ and CaCl₂ alone,and the regeneration rate was more than 90%,and the adsorption capacity recovery was good.Fe₂(SO₄)₃-CaCl₂-MgO could solve the problem of the decrease of adsorption capacity caused by Fe₂(SO₄)₃ modification and realized the synergistic effect of iron and calcium on the basis of maintaining high adsorption efficiency.In conclusion,the iron and calcium salt modified active magnesium oxide prepared in this thesis has obvious advantages in fluoride removal,which can effectively solve the adsorption defects of active magnesium oxide itself and achieve different functional improvements.The cost of activated magnesium oxide,iron and calcium salt is low,and the preparation is convenient,which improves the practical value of this kind of adsorbent.