| With the rapid development of industry and the increase of human demand worldwide,the number of enterprises,as well as the total production,have increased.However,the effluent discharge of all kinds of industrial wastewater containing toxic and harmful pollutants,such as heavy metals,organics and fluoride,comes to increase year by year.The pollutants has resulted in the deterioration of water quality,which has aroused high concern of domestic and foreign researchers engaged in the field of environmental protection.A lot of studies attempted to remove heavy metal ions and fluoride from wastewater,which are difficult to degrade and durable,by using removal technologies,methods and materials.Compared with other treatment technologies,adsorption is generally considered to be the most effective in removing heavy metal ions and fluoride ions from wastewater.The key problem to limit this practical application of adsorption method is the selection of adsorption materials.Therefore,it is of great practical significance to develop cost-effective,environmentally friendly adsorption materials with rapid and high efficient removal of pollutants.The natural xonotlite(Ca6(Si6O17)(OH)2,CSH)is hydrated calcium silicate compounds mainly composed of calcium,silicon and oxygen.It is widely applied in construction industry,insulation materials,as well as in adsorption material.Due to its limited reserves in nature,the practical application of CSH mainly depends on artificial synthesis.Although the CSH had the characteristics of adsorption material and certain adsorption property to the pollutants from aqueous solution,it has a low adsorption capacity and slow adsorption rate,which limit its promotion and application in the field of adsorption material.Therefore,to solve the lack of raw materials and low adsorption capacity of synthetic CSH as adsorption material,the new CSH was synthesized by using discarded egg shell as the raw material with hydrothermal method through adjusting the calcium-silicon ratio of the raw material and reaction temperature and combining with doping and grafting modification technology.The synthesized products have a larger specific surface area and rougher surface and more pores and more adsorption sites.The composition and morphology of the new CSH samples were analyzed by BET surface area,fourier transform infrared spectroscopy(FTIR),X-ray diffraction spectroscopy(XRD),scanning electron microscopy(SEM),energy disperse spectroscopy(EDS)and thermogravimetric analysis(TGA)etc.Then,they were applied to remove wastewater containing heavy metal ions(Pb2+,Cd2+,Cu2+)and F-ions.The effect factors including pH,initial concentration of the Pb2+,Cd2+,Cu2+and F-ions,adsorbent dosage,adsorption time and adsorption temperature were investigated.The adsorption mechanism for Pb2+,Cd2+,Cu2+and F-ions adsorbed on new CSH were investigated according to the macroscopic experimental results and the microscopic theoretical fitting models(isothermal adsorption models,adsorption dynamics models,thermodynamics).This study provided theoretical basis for the practical promotion and application of the new CSH.The main contents and results of this research are listed as follows:(1)Preparation of sodium-doped xonotlite and its adsorption properties for heavy metal ions(Pb2+,Cd2+,Cu2+)and fluoride ions.According to the preparation procedure of CSH,sodium-doped xonotlite(NaCSH:NaCSH1,NaCSH2and NaCSH3)were synthesized from a mixture of raw materials(n(Ca)/n(Si)=1:1)and sodium chloride solutions of three different concentrations(n(Na)/n(Ca+Na)=1:6,2:6,3:6),and were used to removal wastewater containing heavy metal ions(Pb2+,Cd2+,Cu2+)and F-ions.According to the adsorption experiment,NaCSH1had the best adsorption effect on heavy metal ions(Pb2+,Cd2+,Cu2+)under the same adsorption conditions,indicating that the n(Na)/n(Ca+Na)ratio of raw material of synthetic NaCSH for 1:6 was optimum.Under the same adsorption conditions,NaCSH2had the best adsorption effect on F-ions,indicating that the n(Na)/n(Ca+Na)ratio of raw material of synthetic NaCSH for 2:6 was optimum.NaCSH(NaCSH1and NaCSH2)and CSH were used to adsorb heavy metal ions(Pb2+,Cd2+,Cu2+)and F-ions from aqueous solution in static adsorption experiments,respectively.The results showed that the adsorption effect of NaCSH on the above ions was significantly better than that of CSH synthesis.Under the optimized experimental conditions,the maximum adsorption capacity of NaCSH with the optimal sodium-doped content for Pb2+,Cd2+,Cu2+and F-ions could come to 425.22,158.41,189.27and 24.32 mg/g,respectively,which were superior to the adsorption effect of similar adsorption materials reported.The correlation coefficient(R2)of the quasi-second-order kinetic equation all were basically above 0.99,which could well describe the adsorption behavior of NaCSH for Pb2+,Cd2+,Cu2+and F-ions from aqueous solution.The maximum adsorption capacity of theoretical calculation and equilibrium adsorption capacity were very close.Thermodynamic parameters showed that the adsorption reactions of NaCSH for the four kinds of ions were spontaneous and endothermic,and the adsorption mechanisms mainly included ion exchange,solution-precipitation and electrostatic attraction.(2)The adsorption mechanisms of NaCSH for heavy metal ions(such as Pb2+,Cu2+)and F-ions were studied based on the first principles of DFT.The CASTEP was used to set up the calculation model of NaCSH for heavy metal ions and fluoride ions,and the adsorbed existence forms of heavy metal ions(Pb2+,Cu2+)and F-ions on NaCSH were analyzed by the first principles.The results showed that the configuration model of NaCSH obtained by structural optimization was compared with the Ca-O,Ca-Si and Ca-Ca bond lengths of Nd(III)or Eu(III)doped on tobermullite and CSH by EXAFS(extended X-ray absorption fine structure spectroscopy),and the calculation method was considered reasonable,indicating that sodium ions could replace some calcium ions in CSH to form NaCSH.To deeply analyze the adsorption mechanism of heavy metal ions(Pb2+,Cu2+)and fluoride ions on NaCSH,the geometric structures of Pb2+,Cu2+and F-replacing NaCSH and of NaCSH adsorbing Pb2+,Cu2+and F-were designed,respectively.The calculated substitution energy indicated that NaCSH had ion exchange reactions to Pb2+,Cu2+and F-ions.The adsorption energy showed that it was feasible for NaCSH to adsorb Pb2+,Cu2+or F-ions in solution.The result of state density orbital contribution analysis identified the more active adsorption sites of NaCSH,and the result of Bader charge analysis found that there was charge transfer between some atoms in NaCSH and Pb2+,Cu2+and F-,further confirming the adsorption mechanism of Pb2+,Cu2+and F-ions on NaCSH.(3)Preparation of aluminum-doped xonotlite and its adsorption properties for heavy metals(Pb2+,Cd2+,Cu2+)and fluoride ions from aqueous solution.According to the preparation procedure of CSH,aluminum-doped xonotlite[hereinafter referred to as Al-CSH]was synthesized from a mixture of raw materials(n(Ca)/n(Si)=1:1)and partial sodium aluminate of four different concentrations(n(Al)/n(Si+Al)=2%?5%?10%?15%),and was used to removal wastewater containing heavy metal ions(Pb2+,Cd2+,Cu2+)and F-ions from aqueous solution under same conditions.After comparing the adsorption effect of four kinds of Al-CSH prepared with different aluminum contents on the above ions,it was found that the aluminum content(n(Al)/n(Si+Al))in the raw material was 5%,and its adsorption effect was the best.Al-CSH prepared with this amount(n(Al)/n(Si+Al)=5%)was applied to remove wastewater containing Pb2+,Cu2+,Cd2+and F-ions from aqueous solution.The results showed that the sodium and aluminum ions as foreign metal ions replaced part of the calcium and silicon of CSH and entered its crystal structure and changed its composition and morphology by SEM and EDS and XPS characterizations.Compared with the adsorption effects of Al-CSH and NaCSH on Pb2+,Cd2+,Cu2+and F-ions,the former was slightly better than the latter,indicating that the adsorption capacity of Al-CSH modified by Na Al O2was higher than that of NaCSH.Under optimized adsorption conditions,the maximum adsorption capacities of Al-CSH for Pb2+,Cd2+,Cu2+and F-ions got up to 449.74,190.21,224.28 and 30.90 mg/g,respectively,which were superior to those of similar adsorption materials reported previously.The adsorption mechanisms of Pb2+,Cd2+,Cu2+and F-ions on Al-CSH were basically the same as that of NaCSH with ion exchange,solution-precipitation and electrostatic attraction.(4)Study on the adsorption of Hg2+from aqueous solution on CSH modified by coumarin derivatives of fluorescent small molecules(COUSH).The interaction of COUSH with Hg2+could form fluorescent-enhanced substances and the interference ions such as Na+,Ca2+,Pb2+,Cu2+and Cd2+had little effect on the detection.COUSH had certain selectivity for mercury ion detection.It indicated that COUSH had certain selectivity for detection of mercury ion from aqueous solution.CSH-COUSH was synthesized by COUSH modification of CSH and used to remove Hg2+from aqueous solution as new adsorbent.The results showed that the removal of Hg2+by CSH-COUSH was less affected by pH,and the removal rates were about 98%in a wide range of pH from 3.0 to 8.0.The fitted correlation coefficients of Langmuir model were all higher than 0.99,which could better describe the adsorption.The maximum adsorption capacity(Qm)at 293,298 and 303K were 112.37,109.66 and 110.56mg/g,respectively.The correlation coefficients(R2)of the quasi-second-order kinetic equations were basically above 0.99,which could well describe the adsorption behavior of CSH-COUSH to Hg2+from aqueous solution,and the maximum adsorption capacity of theoretical calculation(Qe,cal)and equilibrium adsorption capacity(Qe,exp)were very close.Thermodynamic parameters showed that the adsorption reactions of CSH-COUSH for Hg2+ions were spontaneous and endothermic,and the adsorption mechanisms mainly included ion exchange,solution-precipitation and electrostatic attraction. |
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