Preparation And Application Of Graphene Oxide Modifed Materials For Adsorption Of Heavy Metal Ions

Graphene oxide(GO) was obtained by the chemical oxidation of graphite. GO could be used as an excellent adsorbent in the removal of heavy metal ions from effulents because its surface had abundant oxygen-containing functional groups. However, GO as an adsorbent had some shortages, such as high dispersibility in aqueous solution, easy aggregation after absorption of heavy metal ions and hard separation by traditional method. Further, the adsorption capacity of graphene oxide for heavy metal ions was not very high. To solve these problems, some modification of GO for adsorption of heavy metal ions should be studied.Three new types of materials were prepared by functional modification of GO with different small organic molecules. The morphology and structure of these sorbents were characterized by FTIR, SEM, XRD, and TGA. These materials were applied to the adsorption of heavy metal ions. Various variables affecting the adsorption of heavy metal ions, including pH, time, initial concentration and temperature, were systematically explored. Different kinetic, isotherm and thermodynamic models of the adsorption were introduced to fit and analyze the experiment datas. The main results were given as follows:(1) L-glutamic acid functionalized GO(L-Glu/GO) was prepared by the nucleophilic substitution reaction between L-glutamic acid(L-Glu) and GO. The SEM result indicated that L-Glu/GO was a nano material with the size of about 50-400 nm. The adsorption experiments for various heavy ions by L-Glu/GO demonstrated that adsorption performance for Pb(II) was better than other ions. The experimental results indicated that the maximum adsorption capacity and equilibrium time of Pb(II) on L-Glu/GO were 513.4 mg·g-1 at pH 5.0 and 40 min, respectively. The sorption kinetics and isotherm fitted well with the pseudo-second-order model and Langmuir model, respectively. The sorption mainly was a chemical process. Thermodynamic studies revealed that the adsorption was a spontaneous and exothermic process. The adsorbent could be regenerated with HCl solution. Hence, it was suggested that the L-Glu/GO could be applied in the removal of Pb(II) from wastewaters.(2) Using graphene oxide as raw material, functional GO adsorbent(DTC-ED-GO) was prepared by the reaction of GO with ethylene diamine and carbon disulfide and was applied to the removal of Hg(II). The FTIR characterization confirmed that the dithiocarbamate groups have been successfully grafted to the surface of GO. The adsorption results indicated that the maximum sorption capacity was found to be 941.6 mg·g-1 at pH 6.0 and the adsorption could reach equilibrium in 120 min. The adsorption process comformed to the pseudo-second-order kinetic model and Langmuir isotherm model, respectively. The adsorption of Hg(II) by DTC-ED-GO was a chemical process. Thermodynamic studies showed that the sorption capacity of Hg(II) decreased with temperature rise, meaning that the sorption processes were exothermic and spontaneous. The Hg(II)-loaded sorbent could be recycled with EDTA, indicating that DTC-ED-GO had a good reused performance.(3) Thiosemicarbazide-modified graphene oxide(TSC-GO) was prepared by the nucleophilic substitution reaction of epoxide groups of GO with amine groups of thiosemicarbazide. The adsorption study of TSC-GO for Cr(VI) revealed that the largest capacity of 218.2 mg·g-1 was achieved at pH 2.5. Various adsorption kinetic models were used to fit the adsorption process of Cr(VI) ion on TSC-GO. The results showed that the adsorption process was well-described by pseudo-second-order kinetic model. Furthermore, the equilibrium capacity increased as the rise of Cr(VI) concentration. The Langmuir isothermic model was better fitting with the adsorption experiment datas than other isotherm model, indicating that the adsorption of Cr(VI) was a chemical process. The adsorption thermodynamic studies showed that the equibrium adsorption capacity increased with the increase of temperature, indicating that the adsorption of Cr(VI) was an endothermic process. The adsorbent could be recycled with NaOH soltion. After five adsorption/desorption cycle experiments, the uptake of Cr(VI) was 82.0% of initial adsorption capacity, showing that TSC-GO had a good regenerated performance.