Selective adsorption and determination of hexavalent chromium ions using graphene oxide modified with amino silanes

Novel adsorbents are described for the preconcentration of chromium(VI). Graphene oxide (GO) was modified with various amino silanes containing one, two, or three nitrogen atoms in the molecule. These include 3-aminopropyltriethoxysilane (APTES), N-(3-trimethoxysilylpropyl)ethylenediamine (TMSPEDA), and N¹-(3-trimethoxysilylpropyl)diethylenetriamine (TMSPDETA). The resulting GO derivatives were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, and energy-dispersive X-ray fluorescence spectrometry (EDXRF). Adsorption studies show that these GO based sorbents are highly selective for Cr(VI) in the presence of Cr(III) at pH 3.5. Although the amino silanes applied in modification of GO contain different numbers of nitrogen atoms, the maximum adsorption capacities of GO derivatives are very similar (13.3-15.1 mg·g^-1). Such results are in accordance with spectroscopy studies which show that the amount of amino silanes attached to GO decreases in the order of APTES > TMSPEDA > TMSPDETA. The APTES-modified GO was applied to selective and sensitive extraction of Cr(VI) ions prior to quantitation by low-power EDXRF using the Cr K_α line. The Cr(VI) ions need not be eluted from the solid adsorbent. The method has a 0.17 ng·mL^-1 detection limit, and the recovery is 99.7 ± 2.2% at a spiking level of 10 ng·mL^-1. The method was successfully applied to the determination of Cr(VI) in water samples. Graphical abstractGraphene oxide adsorbents modified with various amino silanes are described for the preconcentration and speciation of trace and ultratrace levels of chromium ions.