Polyethyleneimine cross-linked graphene oxide for removing hazardous hexavalent chromium: Adsorption performance and mechanism

It is essential and important to understand the adsorption mechanism and identify the removal efficiency when applying an adsorption technique to remove toxic heavy metals from wastewater. A facile and cost-effective route to preparing polyethyleneimine cross-linked graphene oxide (GO-PEI) is developed and reported herein. With significantly enhanced adsorption capacity, the obtained GO-PEI materials found promising prospects as an effective adsorbent toward Cr(VI) in comparison with untreated GO and rGO. Their structure and morphology were characterized with XRD, FT-IR, XPS, SEM and TEM techniques, and detailed mechanism for the adsorption behavior was systematically investigated in terms of the optimal molar ratio, pH effect, kinetics, isotherm models, and interference. Experiments suggested that PEI molecular weight at 70 k combined with GO and the ratio of 1: 2 were conducive to the optimal adsorption efficiency. The prepared GO-2PEI exhibited a high capacity of 436.20 mg g(-1) for adsorption toward Cr(VI) anions, which guaranteed eligible discharge at the final Cr(VI) concentration. According to XPS, Cr(VI) were reduced partially to Cr(III), and these trivalent ions would be chelated on GO-2PEI surface for simultaneous immobilization during Cr(VI) adsorption. The kinetic process of Cr (VI) adsorption fitted better with the pseudo-second-order model and was subject to multi-step influence. Adsorption mechanisms consisted of electrostatic interaction, reduction, and chelation. This work not only provides a simple and environmental-friendly synthetic route to GO-PEI, but also offers valuable clues for wastewater treatment.