Energy dispersive X-ray spectrometric determination of Cr(III), Cu(II), Zn(II) and Pb(II) and chromium speciation after preconcentration on graphene oxide chemically modified with mercapto-groups

A novel derivative of graphene oxide, referred as a GO-S, was used as an effective adsorbent for preconcentration of heavy metal ions from water samples and chromium speciation prior to energy dispersive X-ray spectrometric (EDXRF) determination. At optimal conditions GO-S quantitatively adsorbs Cr(III), Cu(II) Zn(II) and Pb(II) ions from aqueous samples. The determined adsorption capacity of GO-S for Cr(III), Cu(II), Zn(II) and Pb(II) ions are 191.5, 410.7, 265.6, and 487.3 mg g-1, respectively. The experimental data are fitted better to the Langmuir isotherm model which indicates that the adsorption has a chemical character. The utility of GO-S as a solid adsorbent was confirmed by developing procedure based on combination of dispersive micro-solid phase extraction (DMSPE) with EDXRF. The linear response for the method was obtained in 1200 ng mL-1 range for each of the examined metal ion. The uncertainty of the proposed procedure did not exceed 3.8%. The detection limits are found to be in 0.060.10 ng mL-1 range and they are much lower than the upper concentration limits for Cr(III), Cu(II), Zn(II) and Pb(II) in drinking water established by the World Health Organization. The method offers impressive enrichment factors ranging between 520 and 3120 for the 50 mL-in-volume samples and it is resistant to high concentration of a large number of anions, alkali and alkaline earth cations. The application of the described procedure to the determination of Cr(III), Cu(II), Zn(II) and Pb(II) ions in lake, river, spring, mineral and sea waters confirms the usefulness of the GO-S as a solid adsorbent. Moreover, chromium speciation using GO-S is also possible due to the fact that the carrier selectively retains Cr(III) on its surface while Cr(VI) ions remain in the aqueous sample. The Cr(VI) concentration is calculated as a difference between total chromium content (after reduction Cr(VI) to Cr(III)) and Cr(III) content.