Determination of heavy metal ions by energy dispersive X-ray fluorescence spectrometry using reduced graphene oxide decorated with molybdenum disulfide as solid adsorbent

Reduced graphene oxide (rGO) decorated with molybdenum disulfide (MoS2) was synthesized by hydrothermal method. The physicochemical properties of the nanoadsorbent were characterized by different spectroscopic techniques, namely Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). MoS2-rGO was used as a solid nanoadsorbent in a dispersive micro-solid phase extraction (DMSPE) for the preconcentration of heavy metal ions from different types of waters prior to energy dispersive X-ray fluorescence spectrometry (EDXRF) determination. To optimize extraction process parameters such as sample pH, contact time and sample volume, were examined. The determined adsorption capacities for Cr(III), Co(II), Ni(II), Cu(II), Zn(II) and Pb(II) ions were 242, 112, 145, 417, 550 and 498 mg g(-1), respectively. Under optimal conditions (pH = 6.5, sample volume 50 mL, and sonication time of 10 min) the calibration curves showed linear response between analytes concentration and intensity of fluorescence radiation in the range 1-200 ng mL(-1) for Cr(III), Cu(II), Zn(II) and Pb(II) ions, and 1-100 ng mL-1 for Co(II), Ni(II) ions. The limits of detection were 0.11, 0.12, 0.10, 0.07, 0.08, and 0.09 ng mL(-1) for Cr(III), Co(II), Ni(II), Cu(II), Zn(II) and Pb(II), respectively. The precision of the method determined at two concentration levels, e.g. 2 and 20 ng mL(-1) varied from 3% for Cu(II) to 5.1% for Co(II) and 1.4 for Cu(II) to 3.6% for Ni(II), respectively. The DMSPE-EDXRF procedure was suitable for determination of target analytes at ultra-trace level even in high salinity samples. The accuracy of the method was proved using certified material NIST 1640a (Trace elements in natural water). The 'greenness' of the procedure was calculated with the Analytical Eco-Scale tool.