Fabrication of terminal amino hyperbranched polymer modified graphene oxide and its prominent adsorption performance towards Cr(VI)

In order to increase the density and quantity of functional groups on adsorbent, three terminal amino hyperbranched polymer modified graphene oxide adsorbents including GO-HBP-NH₂-DETA, GO-HBP-NH₂-TETA and GO-HBP-NH₂-TEPA with N-containing functional group density of 7.21 wt%, 10.20 wt% and 12.43 wt%, respectively, were prepared and used for the adsorption and reduction of toxic hexavalent chromium Cr(VI) to less toxic Cr(III). The morphology and structure of obtained adsorbents were characterized by FT-IR, SEM, XRD, Raman, BET, XPS and zeta potential. The density of receptor sites (N_m) of the three adsorbents calculated from statistical physics model with one energy site were found to be 456.62, 604.54 and 636.03, respectively. Adsorption experiments demonstrated that the high adsorption capacities of the three adsorbents obtained from Langmuir isotherm model towards Cr(VI) were 245.01, 257.26 and 300.88 mg/g, respectively, suggesting that GO-HBP-NH₂-TEPA had better adsorption ability. The density functional theory calculation (DFT) indicated that GO-HBP-NH₂-TEPA was more likely to adsorb HCrO₄^- rather than Cr₂O₇^2- at lower pH conditions. A possible adsorption mechanism was also proposed where electrostatic interaction between Cr(VI) (HCrO₄^- or Cr₂O₇^2-) and the N functional group (+) on GO-HBP-NH₂-TEPA dominated the adsorption of Cr(VI) and reduction mechanism dominated the reduction of Cr(VI)-Cr(III).