Preparation Of Amended Sugarcane Bagasse Biochar Composites And Its Adsorption Of Arsenate And Lead From Aqueous Solution

In this study,the preparations of the nanometer iron oxide/bagasse biomass carbon composite adsorbent(N-Fe2O3-BC),hydroxyl apatite/bagasse biomass carbon composite adsorbent(HAP-BC)and magnetic iron oxide/bagasse biomass carbon composite adsorbent(M-Fe3O4-BC)were achieved by the precipitation one-step carbonization heating chemical modification method with bagasse as the main raw material.The preparation conditions and technological process of the three composite adsorbents were optimized.The charactiristics of the three adsorbents had been characterized by scanning electron microscope(SEM),X-ray diffraction(XRD),X-ray photoelectron spectrometer(XPS)and Fourier transform infrared spectrometer(FT-IR).The adsorption performance and mechanism of the three adsorbents to the As(V)or Pb(?)in aqueous solution were also investigated.The main results of the study were presented:1.Optimization of preparation conditions and structure characterizations of N-Fe2O3-BC,HAP-BC and M-Fe3O4-BCThe preparation conditions of N-Fe2O3-BC is as follows:the concentration of FeCl3 is 0.15 mol/L,iron-to-bagasse mass ratio is 1:6,the soaking time is 36 h,and the pH of the precipitation reaction ending is 7.4,the carbonization temperature is 450? and the carbonization time is 4 hours.For HAP-BC:the concentration of Ca(CH3COO)2 is 0.2 mol/L,the concentration of NH4H2PO4 is 0.06 moI/L,calcium-to-phosphorus-to-bagasse ratio is 1:0.29:6.25,the soaking time is 24 hours,and the pH of the precipitation reaction ending is 7.5,the carbonization temperature is 500? and the carbonization time is 4 hours.For M-Fe3O4-BC:the concentration of FeSO4 is 0.15 mol/L,iron-to-bagasse mass ratio is 1:6,the soaking time is 36 hours,and the pH of the precipitation reaction ending is 8.5,the carbonization temperature is 500? and the carbonization time is 5 hours.SEM,XRD and FT-IR analysis indicated that the N-Fe2O3-BC was mainly composed of?-Fe2O3 and C and the grain diameters reached nanometers.The HAP-BC was mainly composed of Ca5(PO4)3(OH)and C.The M-Fe3O4-BC was mainly composed of ?-Fe2O3,Fe3O4 and C with magnetic.The specific surface areas of N-Fe2O3-BC,HAP-BC and M-Fe3O4-BC were 48.94 m2/g,89.52 m2/g and 81.94 m2/g,respectively,and the point of zero charge values(pHzpc)of the three adsorbents were 4.9,7.1 and 6.2,respectively.2.Adsorption performance of As(?)onto N-Fe2O3-BC,HAP-BC and M-Fe3O4-BCThe adsorption equilibrium times of N-Fe2O3-BC,HAP-BC and M-Fe3O4-BC to remove As(?)from aqueous solution were increased with the increase of initial concentration of As(?).The pH values of the solutions had great influence on the adsorption of As(?)onto the three adsorbents,and the pH range for the optimal As(?)adsorption was between 4 and 9(N-Fe2O3-BC),3 and 9(HAP-BC),3 and 7(M-Fe3O4-BC),respectively.The adsorptions of As(?)onto N-Fe2O3-BC,HAP-BC and M-Fe3O4-BC were affected by PO43-,NO3-,SO42-and Cl-coexisted in the solution,and PO43-affected it the most.Besides,temperature had different influence on the adsorption of As(?)on N-Fe2O3-BC,HAP-BC and M-Fe3O4-BC.With the increase of temperature,there is almost no change of the adsorption of As(?)on N-Fe2O3-BC,but the adsorption of As(?)on HAP-BC increased with increasing temperature,and the adsorption of As(?)on M-Fe3O4-BC increased at the beginning and then decreased with the increase of temperature.Langmuir isotherms were more suitable for fitting the isothermal adsorption processes of As(?)onto N-Fe2O3-BC,HAP-BC and M-Fe3O4-BC.The maximum As(?)adsorption capacity for N-Fe2O3-BC,HAP-BC and M-Fe3O4-BC were 9.33,6.76,and 3.61 mg/g at 25?,respectively,and the maximum As(?)adsorption capacity for three adsorbents was much higher than that of bagasse biochar.There was no correlation between specific surface area of three adsorbents and the maximum As(?)adsorption capacity for three adsorbents.The adsorptions of As(?)onto N-Fe2O3-BC,HAP-BC and M-Fe3O4-BC were a spontaneous adsorption process of entropy,and could well be described by the Pseudo-second-order kinetic equation.The dynamic adsorption process of three adsorbents would be promoted with low velocity,high influent As(?)concentration and high packing layer.In addition,the regenerated adsorbents still maintained a high adsorption capacity for As(?).3.Adsorption mechanisms of As(?)onto N-Fe2O3-BC,HAP-BC and M-Fe3O4-BCSEM-EDS,XRD,FT-IR and XPS analysis of the N-Fe2O3-BC,HAP-BC and M-Fe3O4-BC before and after As(?)adsorption showed that the adsorption mechanism of As(V)onto N-Fe2O3-BC,HAP-BC and M-Fe3O4-BC included surface diffusion,electrostatic attraction,complexation on solid phase surface and dissolving-coprecipitation.When pH<pHzpc various forms of As(?)ions were absorbed by N-Fe2O3-BC,HAP-BC and M-Fe3O4-BC through surface diffusion and electrostatic attraction,while iron oxide[Fe(OH)2+ and Fe(OH)2+]on the solid phase surface of N-Fe2O3-BC or M-Fe3O4-BC created a complexation reaction on surface with H2AsO4-and HAsO42-,generating Fe(H2AsO4)3 and Fe2(HAsO4)3 and other components.And a small amount of arsenic acid radical reacted with dissolved Fe(?),generating the micro crystal structure of arsenic acid iron.When pH>pHzpc,various forms of As(?)ions were adsorbed through the process of surface diffusion and surface complexation,and oxygen-containing functional groups on the surface of N-Fe2O3-BC or M-Fe3O4-BC were involved in the N-Fe2O3-BC or M-Fe3O4-BC and As(?)complexation reaction.The adsorption mechanism of As(?)onto HAP-BC included surface diffusion,electrostatic attraction and solid surface complexation.When pH<pHzpc,HAP-BC removed As(?)from the solution through surface diffusion,electrostatic attraction and solid surface complexation.When pH>pHzvc,HAsO42-and AsO43-in solution were adsorbed through surface diffusion.The total number of surface sites available for adsorption of N-Fe2O3-BC,HAP-BC and M-Fe3O4-BC were 10.56,4.56 and 4.24 ?C/cm2(I=10-2 M NaCl),respectively.Surface complex formation models of N-Fe2O3-BC,HAP-BC and M-Fe3O4-BC with As(?)were=S-HAsO4-and = S-H2AsO4,=S-HAsO4-and = S-AsO42-,=S-HAsO4-,respectively.The deducing results of surface complex formation models were matched with XPS analysis results,and Stumm surface complex formation model can well describe the surface complexation reaction of As(V)in aqueous solution with the three adsorbents.4.Adsorption performance of Pb(?)onto N-Fe2O3-BC and HAP-BCThe initial concentration of Pb(?)had different effects on the adsorption equilibrium time of adsorption of Pb(?)onto N-Fe2O3-BC and HAP-BC.With the increase of the initial Pb(?)concentration,the adsorption equilibrium time of N-Fe2O3-BC for Pb(?)was shortened,while the adsorption equilibrium time of HAP-BC for Pb(?)was extended.The pH value had a great influence on the adsorption of Pb(?)onto N-Fe2O3-BC and HAP-BC,and the optimal initial pH value ranged from 7 to 10(N-Fe2O3-BC)and 4 to 8(HAP-BC),respectively.Temperature had less effect on the adsorption of Pb(?)onto N-Fe2O3-BC,but had great influence on adsorption of Pb(?)onto HAP-BC,as the temperature increased,the adsorption of Pb(?)gradually increased.Langmuir isotherms was more suitable for fitting the isothermal adsorption process of Pb(?).Langmuir and Freundlich isotherms were more suitable for fitting the isothermal adsorption process model of Pb(?)onto HAP-BC.The maximum adsorption capacity of Pb(?)on N-Fe2O3-BC and HAP-BC were 17.57 m2/g and 209.63 mg/g at 25?,respectively.The adsorption of Pb(?)was a spontaneous adsorption process of entropy.The Pseudo-second-order kinetic model can well describe the dynamics of adsorption of Pb(?)onto N-Fe2O3-BC.The dynamics of adsorption of Pb(?)onto HAP-BC can be better described by the Intra-particle diffusion model.5.Adsorption mechanism of Pb(?)onto N-Fe2O3-BC and HAP-BCSEM-EDS,XRD,FT-IR and XPS were used to monitor structural changes of the two adsorbents before and after removing Pb(?)from the solution.The experimental results showed that adsorption mechanism of Pb(?)onto N-Fe2O3-BC included surface diffusion,electrostatic attraction and solid phase surface complexation.The oxygen-containing functional groups on the surface of N-Fe2O3-BC were involved in the adsorption process of Pb(?).The mechanism of HAP-BC removing Pb(?)in the solution mainly were surface diffusion,electrostatic attraction,surface complexation,ion exchange and dissolving-coprecipitation reaction.When pH<pHzpc,Pb(?)ions were mainly absorbed through surface diffusion and surface complexation,simultaneously ion exchange and dissolving-coprecipitation reaction.When pH>pHzpc,Pb(?)ions were mainly absorbed through surface diffusion,electrostatic attraction and surface complexation.The oxygen-containing functional groups of HAP-BC was involved in adsorbing Pb(?).The surface complex formation models of N-Fe2O3-BC and HAP-BC with Pb(?)were=SO-Pb+,=SO-Pb+ and =SO-Pb(OH),respectively.The deducing results of surface complexation models consisted with the XPS analysis results and the Stumm surface complexation model,which can also well describe the surface complexation reaction of N-Fe2O3-BC and HAP-BC with Pb(?)in aqueous solution.