Selective preconcentration and determination of chromium(VI) using a flat sheet polymer inclusion sorbent: potential application for Cr(VI) determination in real samples

A method for sorption preconcentration of Cr(VI) from aqueous samples was developed using a polymer inclusion sorbent (PIS). The PIS used in this method was prepared by physical inclusion of Aliquat-336 in the matrix formed by cellulose triacetate and 2-nitrophenyl octyl ether. This sorbent was found to be stable, cost-effective, efficient for preconcentration of Cr(VI) present in the aqueous samples, and amenable to direct quantitative analysis of Cr(VI) held in it by neutron activation analysis and spectrophotometry. The quantifying of Cr(VI) in PIS by spectrophotometry was carried out by developing color directly on the PIS after reacting it with 1,5-diphenylcarbazide. The distinct color developed on the PIS even at very low concentrations of Cr(VI) suggests its possible use for field determination of Cr(VI). The composition of PIS was optimized to obtain maximum uptake of Cr(VI) without sacrificing uniformity in terms of thickness and distribution of ion-exchange sites, stability, and time required for quantitative sorption of Cr(VI) from aqueous samples. The Cr(VI) species held in the PIS, mainly HCrO4- and CrO4(2-), were found to vary as a function of pH of the aqueous samples from which Cr(VI) was preconcentrated. A close agreement was found in the abundances of Cr(VI) species held in the PIS with those reported in the literature for aqueous solutions at different pH. The variation of Cr(VI) species as a function of pH was found to have a significant impact on the tolerance to anions on the uptake of Cr(VI) in the PIS. The high selectivity of PIS toward Cr(VI) from aqueous solution at pH = 2 was explained on the basis of hydration of anions. The uptake of Cr(VI) was found to be fairly constant (88 +/- 3%) up to nearly complete exchange of counterions present in the PIS. The method developed in the present work was successfully used for the preconcentration of Cr(VI) from tap water and seawater samples containing low levels of Cr(VI).