Study on multi-walled carbon nanotubes on-line separation/preconcentration of chromium(III) and chromium speciation

Using multi-walled carbon nanotubes (MWCNTs) as an adsorbent has been established for the on-line separation and preconcentration Cr(III) and chromium speciation. The surface functional groups and negative charges of MWCNTs are beneficial to the adsorption of Cr(III). At pH 3.0 - 6.0, a discrimination of Cr(III) and Cr(VI) is achieved on the MWCNTs surface. Cr(III) ions are adsorbed onto the oxidized MWCNTs surface, while Cr(VI) has no affinity for the MWCNTs. The adsorbed Cr(III) is quantitatively eluted by 10% (v/v) nitric acid with detection by flame atomic absorption spectrometry. By loading a 6.0-ml sample solution, an enrichment factor of 22, a detection limit (3σ) of 1.15 μg l(-1) and a precision of 1.7% RSD at the 30 μg l(-1) level (n = 7) are achieved for Cr(III) within a linear range of 5 - 200 μg l(-1) (r = 0.9994). After Cr(VI) has been reduced to Cr(III) with hydroxylamine hydrochloride, the total amount of chromium is obtained, and the content of Cr(VI) is given by subtraction. The procedure is validated by analyzing chromium in a certified reference material, and is further applied for the speciation of chromium in electroplating wastewater samples with satisfactory results.