Polyelectrolyte-modified multi-walled carbon nanotubes for the adsorption of chromium(vi)

Abstract

Multi-walled carbon nanotubes (MWNTs) are noncovalently wrapped by a cationic polyelectrolyte of poly(diallyldimethylammonium chloride) (PDDA) via π–π interactions between the unsaturated impurity in PDDA and MWNTs. The modified PDDA-MWNTs are characterized by FT-IR and surface charge analysis. The PDDA-MWNTs are packed into a mini-column and used for the adsorption of Cr(VI) in a sequential injection system with detection by electrothermal atomic absorption spectrometry (ETAAS). A 32% improvement on the adsorption efficiency of Cr(VI) by the PDDA-MWNTs at pH 6 is achieved with respect to that obtained by the bare MWNTs. The retained Cr(VI) is afterwards recovered by using 80 μL of ammonium nitrate solution (0.1 mol L⁻¹) as stripping reagent producing an elution efficiency of 81%. The presence of a certain amount of Cr(III) within a Cr(III)/Cr(VI) concentration ratio of 6 : 1 causes no interfering effect for the adsorption of Cr(VI) by the PDDA-MWNTs mini-column. A sample loading volume of 1000 μL creates an enrichment factor of 8.6, along with a detection limit of 0.016 μg L⁻¹ (3σ, n = 7) and a RSD of 3.9% (0.5 μg L⁻¹) within a linear calibration range of 0.05–1.5 μg L⁻¹. The procedure has been validated by analyzing a certified reference material of GBW08608 and a series of water samples.