Two-Stage Fluoride Removal By Flocculation Precipitation Method And Modified Magnesium Oxide Adsorption Method

In the past few decades,due to human’s overuse of earth’s resources,fluoride ion infiltration into groundwater caused by mineral exploitation,or fluoride wastewater produced by industrial production,has posed a great threat to nature and human beings themselves.Therefore,fluorine pollution is widely regarded as a serious environmental health hazard worldwide.According to the Comprehensive Sewage Discharge Standard（GB8978-1996）,the fluorine ion primary discharge standard should be less than 10 mg·L^-1,and according to the Drinking water hygiene Standard（GB5749-2006）,the concentration of fluoride ion in drinking water should be less than 1 mg·L^-1.Therefore,the treatment of fluorinated water has become a hot topic for scientists at home and abroad.Now the common industrial fluorine-containing water treatment method is lime precipitation method,which can often reduce the concentration of fluoride ions in water to 30-40 mg·L^-1.In order to further treat fluorine-containing water,this paper adopts two-stage method to treat fluorine-containing water(fluoride ion concentration is about 40 mg·L^-1).In the process,the fluorine removal capacity of different flocculants is tested.Then,the fluorine-containing water(10 mg·L^-1)is deeply treated by modified magnesium oxide,so as to meet the fluoride ion concentration standard in drinking water.At the same time,the changes of surface morphology and adsorption mechanism of magnesium oxide before and after modification were studied by a variety of characterization methods.The main results are as follows:（1）It was found in the study of fluoride removal by flocculation and precipitation:It is found that the fluorine removal ability of aluminum flocculant is far superior to that of iron flocculant.The addition of silicate in PASC destroys the composition of Al₁₃ and affects the fluorine removal efficiency.After purification of PAFC,it is found that the interaction between aluminum and iron is physical.The decrease of fluoride removal ability caused by excessive dosage of aluminum flocculant was due to the increase of aluminum monomer.The optimal dosage of purified PAC and purified industrial PAC is 5 mmol·L^-1,and the minimum concentration of fluoride ion in solution can be reduced to below 10 mg·L^-1.Because the complex complex produced by iron flocculant fluoride removal is more compact for separation,the volume ratio of purified industrial PAC and PFC is 9:1 under the total dosage of 1 m L·L^-1,and the fluorine removal effect meets the requirements.Among the influencing factors,the suitable p H range of fluorine-containing water was between 4 and 10,and the effect of fluoride removal had no significant change.However,the significance of influencing factors in interfering ions was as follows:CO₃^2->HCO₃^->PO₄^3->SO₄^2-≈NO₃^-≈Cl^-(interfering ion concentration=100mg·L^-1).（2）In the fluoride removal experiment of lanthanum-modified spheroidal magnesium oxide,SEM images show that the surface of spheroidal magnesium oxide is rough and there are many pore structures.After BET determination,the specific surface area of spheroid magnesium oxide（SMO）is 216.76 m²·g^-1,and the surface area of spheroid magnesium oxide modified by 15%mass lanthanum nitrate（15LSMO）is increased to 232.41 m²·g^-1.The suitable p H range of fluorine-containing water is between 2-11,and the fluorine removal effect has no obvious change,while the interfering ions have little effect on the fluorine removal of the material(interfering ion concentration=100 mg·L^-1).For 1 g·L^-1 adsorbent dosage,its fluoride removal effect increased from 82.7%to 93.1%,so as to meet the drinking water standard.The adsorption kinetics model of the two sorbents is closer to the pseudo-second-order kinetics model,indicating that the adsorption mechanism of the adsorbent is mainly chemical adsorption.The adsorption thermodynamic model is closer to the Langmuir model,indicating that the adsorption on the surface of the adsorbent is closer to the homogeneous adsorption.According to FT-IR,XPS and other characterization analysis of the main adsorption mechanism of fluorine:hydroxyl substitution,carbonate substitution and metal bond adsorption.（3）In the fluoride removal experiment using CTAB modified spheroidal magnesium oxide（CSMO）,SEM images show that the Mg O precursor powder impregnated with CTAB solution forms CSMO after specific calcination conditions,thus forming a tighter spheroidal magnesium oxide and smaller pore structure.BET measurement showed that the specific surface area of CSMO was 163.54 m²·g^-1compared with SMO,and the pore size and volume were also reduced,but the Zeta potential of CSMO surface was significantly higher than that of SMO.The optimum p H range of fluorine-containing water is between 2 and 10,and the fluoride removal effect does not change significantly,while the interference ions have little influence on the fluoride removal of materials(interference ion concentration=100 mg·L^-1).Compared with SMO,the fluoride removal efficiency of CSMO is obviously more affected by alkaline environment.It is inferred that the increase of positive charge on the material surface leads to the enrichment of OH^-on the material surface,so alkaline environment is more likely to affect the fluoride removal efficiency.For 1g·L^-1 adsorbent dosage,CSMO fluoride removal rate is 93.3%,so as to meet the drinking water standard.The adsorption kinetics model of CSMO is closer to the pseudo-second-order kinetics model,indicating that the adsorption mechanism of adsorbent is mainly chemical adsorption,and the adsorption thermodynamic model is closer to Langmuir model,indicating that the adsorption on adsorbent surface is closer to homogeneous adsorption.According to FT-IR and XPS characterization,the main fluorine removal mechanisms of CSMO are hydroxyl substitution,carbonate substitution and metal bond adsorption.