Study On Adsorption Of The First Category Metal Pollutants By Magnetic Graphene Composite Material

The treatment of the first heavy category metal pollutants in water is of great concern now. Adsorption has been widely used in the removal of pollutants, but the traditional adsorbent is difficult to recover. In this study, efficient adsorption of the first category metal pollutants were performed based on magnetic nanomaterials. Magnetic composite material(Fe3O4-GS) was prepared by compositing graphene sheet with ferroferric oxide, and it was used to study the adsorption of the first category metal pollutants. The main research contents and results are as follows:(1) Fe3O4-GS was prepared by compositing graphene sheet with ferroferric oxide by hydrothermal method and characterized by SEM, XRD, BET, FTIR, Zeta potential and magnetic separation effect characterization. The results indicate that the ferroferric oxide was attached to the surface of the graphene sheet, and the Fe3O4-GS was successfully prepared.(2) The Fe3O4-GS was used for the study of metal oxyanion, Cr(Ⅵ) was selected as a representative to consider the optimum adsorption conditions(such as pH value, adsorption time, adsorbent dose). Adsorption mechanism was investigated by adsorption kinetic, adsorption isotherm and adsorption thermodynamics. The results showed:the best amount of sorbent was8mg, the best adsorption time was240min, and the best pH value was under acidic condition (pH=1～3.5). Removal efficiency could reach above95%. The adsorption kinetics data well fit the pseudo-second-order equation, which indicated the adsorption process was mainly affected by the rate control. The theoretical saturation adsorption capacity calculated by the pseudo-second-order equation was17.29mg/g. At different temperatures, adsorption of Cr(Ⅵ) onto Fe3O4-GS all agreed well to the Freundlich adsorption model. Thermodynamic studies illustrated the adsorption process was endothermic and spontaneous. The temperature increasing was favorable to the adsorption reaction.(3) The Fe3O4-GS was used for the study of metal cation, such as Pb2+、Ni2+、Cd2+and Hg2+. The optimum adsorption conditions was considered, and the adsorption mechanism was investigated by adsorption kinetic, adsorption isotherm and adsorption thermodynamics. The results showed:the best amounts of sorbent were8mg (Pb2+、Hg2+) and10mg (Cd2+、Ni2+) respectively, the best adsorption time was120min, and the best pH value was under slightly acidic condition (pH=5.5～7.0).The adsorption kinetics data of the four heavy metal cations fit the pseudo-second-order equation well, which indicated the adsorption process was mainly affected by the rate control. The theoretical saturation adsorption capacities of Pb2+、Hg2+、 Cd2+、Ni2+adsorption onto Fe2O4-GS calculated by the pseudo-second-order equation were27.95mg/g,27.83mg/g,23.03mg/g and22.07mg/g respectively. At different temperatures, adsorption of these cations onto Fe3O4-GS all agreed well to the Freundlich adsorption model, which illustrated the heterogeneous adsorption process on the suface. The calculated Kf values showed the order of adsorption capacities was in the following:Pb2+> Hg2+> Cd2+> Ni2+, which was consistent with the data obtained in the dynamics calculations. Thermodynamic studies of the four metal cation all showed:ΔG<0,ΔH>0. Which illustrated the adsorption process was endothermic and spontaneous, and higher temperature was beneficial to the phosphate adsorption. The positive values of ΔS suggested that the entropy turned larger and the disturbance degree was higher during the adsorption process.(4) By comparison, the adsorption conditions of the heavy metal cations are milder. They can achieve better results under slightly acidic conditions, and the saturation adsorption capacities are all far larger than that of metal oxyanion. In contrast, under strong acidic conditions(pH<3.5), the adsorption efficiency of the heavy metal oxyanion onto Fe3O4-GS is better.(5) The effect of different types of desorption agent with the desorption of metal cations and metal oxyanion were studied. Cr(Ⅵ) and Pb(Ⅱ) were selected as representative, NaOH and HCl were chosen to be desorption agent respectively. The desorption efficiencies went higher with the increase of their concentration. Considering the practical application,0.2mol/L NaOH and0.01mol/L HCl solution were selected. In the recycling experiment of Fe3O4-GS. With the increase of the cycle, the adsorption efficiencies of the two heavy metals onto Fe3O4-GS decreased, but still considerable. Therefore, it is feasible to select the two desorption agents in practical applications.