Study On The Preparation Of Amino Acid Functionalized Graphene Oxide And Their Adsorption Performances Towards Heavy Metal Ions In Water

Heavy metal pollution caused by the indiscriminate disposal of wastewater from industries such as metallurgical, mining and chemical manufacturing has posed a risk to both ecological environment and human beings. Adsorption has been widely used in heavy metal removal for its simplicity and economical efficiency. The uptake of heavy metal ions by adsorption is largely ascribed to the interaction of intermolecular, chemical bond and electrostation and it is strongly dependent on the surface area and binding sites of adsorbents. Graphene, an advanced carbon material, has a two-dimensional honeycomb carbon lattice structure and large surface area. Graphene oxide(GO), the exfoliation of graphite after oxidation, has a wide range of binding and reaction sites besides its large surface area. It is often taken as an adsorbent for heavy metal removal and can be modified to increase binding sites. Amino acid, a kind of environmentally friendly material, can be used to modify the surface of GO to enhance adsorption capacity under weakly acidic condition and it has a great significance in application.Since lead and copper are the most common metal contaminants, in this work, graphene oxide and amino acid functionalized graphene oxide were prepared from the raw material of graphite to remove Pb2+ and Cu2+ and their adsorption performances were also investigated.(1) Graphene oxide was prepared by modified Hummers and electrochemical oxidation methods(marked as GO-1 and GO-2 respectively) and exfoliated by ultrasound. Their structures and contents of oxygen-containing groups were characterized and analyzed by attenuated total reflection fourier transform infrared spectroscopy(ATR-FTIR), ultraviolet and visible spectroscopy(UV-Vis), raman spectroscopy(Raman), X-ray diffraction(XRD), and X-ray photoelectron spectroscopy(XPS) and their oxidation degree were also compared. Field emission scanning electron microscope(SEM) was also used to analyse the micromorphology of GO-2. Moreover, the dispersibility and stability of GO-1 and GO-2 were compared by observing their ultrasonic disperisions in water, methanol, ethanol, ethylene glycol, dimethylcarbinol, glycerol and butanol. The results showed that graphene oxide were obtained by modified Hummers and electrochemical oxidation methods. GO-2 had more oxygen-containing groups than GO-1 and its oxidization degree was higher. The micromorphology of GO-2 was multilayered sheets. The dispersibility and stability of GO-2 was better than that of GO-1.(2) Amino acids were used to modify the surface of GO and amino acid functionalized graphene oxide(marked as GO/L-Trp, GO/L-Lys, GO/L-Arg, GO/L-Cys and GO/L-Ala respectively and GO/L-AA collectively) were prepared by reactions between L-tryptophan(L-Trp), L-cysteine(L-Cys), L-lysine(L-Lys), L-arginine(L-Arg), L-alanine(L-Ala) and GO-2. Their structures were characterized by ATR-FTIR, XRD, Raman and XPS and their performances for Pb2+ and Cu2+ adsorption were also compared by a set of experiments. The results showed that all the GO/L-AA were obtained by the nucleophilic substitution reaction between amine groups of amino acid and carboxyl groups of GO-2 and their adsorption abilities towards Pb2+ and Cu2+ were GO/L-Trp>GO/L-Cys>GO/L-Ala>GO/L-Lys>GO/L-Arg.(3) The performances of GO/L-Trp for Pb2+ and Cu2+ adsorption, including the effects of mass of adsorbent, initial p H of solution, ionic strength, contact time, initial concentrations of Pb2+ and Cu2+ and temperature, were investigated. Moreover, the adsorption kenetics, isotherms, thermodynamics and reuse of GO/L-Trp were also inverstigated. The adsorption capacities for Pb2+ and Cu2+ were compared between GO/L-Trp, activated carbon and carbon nanotube. The results showed that GO/L-Trp exhibited enhanced adsorption performances towards Pb2+ and Cu2+, with 95% and 98% being removed in the condition that optimum mass of GO/L-Trp was 10 mg and p H were 4 and 5 for Pb2+ and Cu2+ respectively. The influence of ionic strength was negligible and it could be ignored in the adsorption processes. The adsorption reactions were almost completed within 40 min and followed Lagergren pseudo-second-order model which was based on the assumption that the rate determining step was chemical interaction. The adsorption processes were monolayer coverage and the adsorption capacities were found to be 222mg/g for Pb2+ and 588mg/g for Cu2+, which were higher than other caobon materials. Thermodynamic studies revealed that adsorption were exothermic and spontaneous. Moreover, it also showed that the adsorption capacity variations of GO/L-Trp were not more than 5% after reused for three times, indicating that GO/L-Trp could be employed as a recyclable adsorbent for the removal of heavy metal ions.