Preparation Of Lignin-Based Carbon Foams Based On Transplantation Foaming And Adsorption Properties For Heavy Metals

As a type of new carbon materials with three-dimensional network structure,carbon foams exhibit a wide application prospects in the water pollutant treatment,catalyst carrier,thermal management materials and electrode materials.However,poor controllability in the microstructure and great performance fluctuation have greatly restricted the industrial production and application of carbon foams.In view of this,in this paper,we put forward the ideas of transplantation foaming with Walnut shell liquefaction product resin(WLP-PF resin)with low viscosity as precursor,and porous solid particles loaded with blowing agent as foaming core.Further,starting form the selection of carbonaceous precursor and the dispersion of foaming agent,the preparation of carbon foams with controllable microstructure and stable macro-performace is achieved.On this basis,the studies on the removal ability of heavy metal ions from water by carbon foams were performed,so as to provide basic data and scientific basis for the carbon foams in water pollution control.The main results are as follows:The process conditions of preparing carbon foams by transplantation foaming were investigated,from preparation of transplantation cores,synthesis of WLP-PF resin,the optimization of foaming process and carbonization temperature.It is found that the preparation of carbon foams can be achieved with low viscosity WLP-PF resin as precursor and using activated carbons with n-hexane loading of about 36%as foaming cores.The better particle size of transplantation core is 100?200 mesh(H/AC2),the better carbon precursor is WLP-PF resin with a material liquid ration of 1:4?1:5 and viscosity of 285.1?497.2m Pa·s.The better foaming process is foaming pressure of0.05?0.1MPa(gauge pressure),H/AC2 amount of 3?7%,tween-80 dosage of 8%and HCl amount of 5%.In addition,carbonization temperature has no effect on the foaming properties and uniformity of internal structure of carbon foams.However,an increase in temperature will reduce the pore size and the thickness of the ligament and nod of carbon foams,resulting in the volume shrinkage of carbon foams.The microstructure of carbon foams obtained tends to be stable at carbonization temperature of 800?.Using the better process conditions,the carbon foam exhibits bulk density of 0.27g/cm³,compressive strength of 5.15MPa,well-foaming performance,uniformity in internal structure,the smooth cell wall surface and basically spherical pore in shape,the average pore size of157?m,the cell density of 0.28×10⁴/cm²,the thickness of ligament and nod of 29.5?m and the opening rate of 87.50%.Afterwards,the foaming mechanism of carbon foams prepared by transplantation foaming was analyzed by SEM.The results show that the nucleation mechanism in the transplantation foaming is the gasification of n-hexane loaded on the surface and inside of the activated carbon,followed by the bubbling nucleation of n-hexane and heterogeneous surface nucleation at solid-liquid interface between the activated carbon and resin.Moreover,the mechanism of bubble growth and setting is the process of adjacent bubbles merging and continuous growth,and the matching of bubbles growth rate with curing rate of resin matrix is the key to determinate the microstructure of carbon foams.The characterization and analysis of carbon foams obtained at different carbonization temperature showed that the surfaces of all carbon foams obtained in the range of 400?900?were negatively charged.The water contact angle were more than90o,which was a hydrophobic material.The main phase was amorphous carbon,and graphitization degree increased slightly with the increase of carbonization temperature.The high temperature carbonization was favorable for improving the specific surface area and micropore volume,and reducing pore size of the micropores.The specific surface area and micropore volume of the carbon foam obtained at carbonization temperature of700?was the largest,and its average pore size was the smallest,which were 383.8 m²/g?0.1593 cm³/g?2.50nmThe better process conditions of removing Pb²⁺and Cd²⁺from water by carbon foams were obtained by single factor experiments,which included the solid-liquid ration of 1:100,using carbon foams obtained at the carbonization temperature of 700??800?with particle size of 0.425?0.125mm as removal agent,the p H_Initialof more than or equal to 4,the adsorption temperature of 25?.And then,the removal rate of 263ppm Pb²for4h and 190ppm Cd²for 16h by carbon foams were 67.12%and 64.10%,respectively.The characterization and analysis of carbon foams obtained at different carbonization temperature showed that carbon foams obtained were in the range of400?900?,and their Zeta potential were negative.The water contact angle were more than 90o,so the carbon foams were hydrophobic material.Moreover,with the,volume shrinkage rate,the bulk density and compressive strength of carbon foams increased,while average pore size decreaseandd from 240?m to 157?m and pore density increased from 1.5×10³/cm²to 2.8×10³/cm².The specific surface area and micropore volume of the carbon foam obtained at carbonization temperature of 700?was the largest,and its average pore size was the smallest,which were 383.8 m²/g,0.1593 cm³/g and2.50nm,respectively.Self activation should be the main reason for the increase of specific surface area and micropore volume of carbon foams at high temperature.The research on kinetics,thermodanymic and adsorption isotherm of the removal of heavy metal ions from water by carbon foams was performed.The results showed that the adsorption process satisfied pseudo second-order kinetic model,Sips isotherm model,and was endothermic and entropy increment.Moreover,saturated adsorption capacity obtained by isotherm model were 47.52mg·g^-1and 43.53mg·g^-1,respectively.When Pb²⁺and Cd²⁺ions coexited,Pb²⁺ions could antagonize the removal of Cd²⁺ions.When the filter residue obtained from removal of Pb²⁺and Cd²⁺ions was regenerated with deionized water with p H value of 1,the release rate of lead and cadmium reached 92.5%and 70.9%,respectively,indicating that carbon foams had good renewability.By means of SEM,SEM-EDS,FT-IR,XRD and XPS,it is proposed that the removal of Pb²⁺and Cd²⁺ions from water by carbon foams is mainly composed of precipitation,accompanied by compound action process of surface physical adsorption,electrostatic attraction and complexation.