The Preparation And Characterization Of Amino-functionalized Graphene Oxide As Heavy Metal Adsorbents

The increase in industrial activities has not only brought about advancedeconomy, but also intensified environmental pollution and the deterioration of someecosystems, with the accumulation of pollutants especially heavy metals. For thethreaten to human’s health, growing attention has been given to the control of heavymetal pollution. Conventional methods for removing heavy metal ions fromwastewater include chemical precipitation, chemical oxidation or reduction, filtration,ion exchange, electrochemical treatment, application of membrane technologyevaporation recovery and adsorption. The adsorption method has been widely studiedfor the advantages of economic, convenient and effectivity.In this study, an economic, convenient and high performance sorbent, wasprepared through interface functionalize of GO sheets. The morphology, structure andproperty of the sorbent were determined by several characterizations. Moreover, theadsorption study of heavy metal on the sorbent was presented in this paper. Theadsorption mechanism was also elucidated. Details are given as follows:(1) One milliliter of3-aminopropyltriethoxysilane (AS) was dropwise added into250mL deionized water with constantly stirring at316K to getpoly3-aminopropyltriethoxysilane (PAS) oligomer. The sorbent PAS-GO get throughthe mixture of PAS oligomer and graphene (GO). The morphology, structure andproperty of the PAS-GO composite were determined by scanning electron microscope(SEM), transmission electron microscope (TEM), Fourier transform infrared (FTIR),X-ray diffractometer (XRD), Brunauer-Emmett-Teller (BET), thermogravimetricanalysis (TGA) and X-ray photoelectron spectroscopy (XPS). It was found that PASchains provided more functional groups available for binding metal ions andmulti-arm PAS oligomer chains prevented GO sheets from aggregation, giving metalions easier access to binding sites and endowing PAS-GO with the larger BET surfacearea. Compared PAS-GO to3-aminopropyltriethoxysilane functioned graphene oxide(AS-GO) and GO, found that the maximum Pb(II) adsorption capacities of PAS-GO,AS-GO and GO at303K were312.5mg/g,119.05mg/g and204.08mg/g,respectively, calculated by the slopes of Langumir adsorption isotherm. The highestadsorption capacity of PAS-GO should be ascribed to the special physical andchemical properties of the sorbent. (2) When increasing the pH to4.0, the effect of protonation became weak andthe adsorption efficiency reached nearly100%much higher than those at low pHvalues of2.0and3.0(11%at pH2.0and27%at pH3.0). When further increasing pHto6.0, the adsorption efficiency still kept nearly100%. In fact, the pH of practicalPb-containing wastewater is below7.0because Pb ions in basic solution willprecipitate. So PAS-GO adsorbent can be used at a wide pH range of about4.0-7.0fortreating wastewater containing Pb ions.Various adsorption isotherms and kinetics were used to fit the adsorption processof Pb ions on PAS-GO. It reveals that the process was well-described by pseudosecond-order kinetic model. And lower initial concentration brought about shorteradsorption equilibrium time. Furthermore, higher concentration helped to increase theequilibrium capacity. The better fitting of Langmuir isotherm model indicated thehomogeneous adsorption surface with all the adsorption sites having equal adsorptionaffinity. The maximum adsorption capacity qmaxcan be calculated by the slopes ofLangmuir adsorption isotherm. By increasing the temperature from293K to313K,qmaxincreased from200.0mg/g to344.8mg/g. This phenomenon indicates theadsorption process in the study would be an endothermic process. Increasing thetemperature is helpful for adsorption of Pb(II). And the adsorption thermodynamicstudy found that the negative ΔG0values, the positive value of ΔH0and ΔS0alsoindicates the spontaneous and endothermic process of Pb(II) sorption under theconditions applied. The decrease of ΔG0with increase temperature indicates moreefficient sorption at high temperature.The main sorption mechanism of PAS-GO is chemical adsorption. The zetapotential of GO was negative while the zeta potential of PAS-GO was positive in thepH range from2.0to6.0, which might attribute to the introduction of amine groups onthe surface of PAS-GO where the amine groups were protonated. Obviously, thepositive zeta potential of PAS-GO sorbent repulsed the positively charged metal ionsthrough electrostatic force. So the electrostatic interactions were adverse rather thanstimulative to the sorption. FTIR spectra and XPS analysis also applied tocharacterize the changes of functional groups before and after adsorption, and provedthat metal ions strongly bound to electron-rich hydroxyl or amino groups by formingcoordination bonds with oxygen or nitrogen atoms.(3) Because of the difference of chelation ability of different metal species,preferential adsorption might happen in a competition environment. In the selectiveadsorption test of heavy metal on PAS-GO, the removal percentages of Pb(Π) and Cu(Π) reached to over97%, much higher than the other species. When dealing withpractical industrial effluent, PAS-GO exhibited effective adsorption ability of Pb, Cuand Fe ions. After the desorption process, PAS-GO showed a good re-utilizing. Themaximum adsorption capacities of PAS-GO in the second and third using time are232.6mg/g and208.3mg/g, respectively.