Egg-shell membrane (ESM) is a unique cell surface with various functional groups, providing the potential for bio-sorption of metal species. We have demonstrated the selective retention of chromium(VI) via a reductive sorption process, while Cr(III) is virtually not retained. This includes adsorption of Cr(VI) onto the ESM surface while charge transfer on the ESM results in the reduction of Cr(VI) to Cr(III) through transient, unstable species of Cr(V) and Cr(IV). A novel procedure for chromium speciation was thus developed in a sequential injection system with detection by electrothermal atomic absorption spectrometry. The procedure includes the separation and preconcentration of Cr(VI) on ESM at pH 2 and its subsequent detection after elution, followed by conversion of Cr(III) to Cr(VI) and total chromium analysis, Cr(III) finally being obtained by subtraction. With a sampling volume of 1000 ml, an enrichment factor of 13.3 was achieved. A linear range of 0.05-1.25 mg l À1 for Cr(VI), along with a detection limit of 0.01 mg l À1 and a precision of 3.2% at the level of 0.5 mg l À1 , were obtained. Chromium speciation was performed by using a certified reference material of riverine water (GBW08608) and cave water.

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The potential of biological cell-sorption for separation/pre-concentration of ultra-trace heavy metals was exploited by immobilizing Chlorella vulgaris and Saccharomyces cerevisiae cells onto silica beads and using them for cadmium sorption. FT-IR investigations showed that when C. vulgaris was used, the functional groups on the cell wall, i.e., hydroxyl and ether, were both involved in the sorption, while for S. cerevisiae, hydroxyl, amide and acetyl served as binding sites, but ether was not active. Because both cells contribute to the sorption, a significant improvement in retention efficiency was observed by immobilizing a mixture of C. vulgaris and S. cerevisiae cells onto silica used for sorption, with respect to those obtained by a single type of cell. A novel procedure for cadmium pre-concentration was developed based on this observation with detection by electrothermal atomic absorption spectrometry (ETAAS), employing cell immobilized silica beads for packing a micro-column in a sequential injection system. The cadmium retained on the column was eluted with a small amount of nitric acid and quantified with ETAAS. Within a range of 0.005-0.2 mg l À1 and a sample volume of 1000 ml, the retention efficiencies achieved by using mixed cells, individual cells of C. vulgaris and individual cells of S. cerevisiae were 97%, 74% and 65%, respectively, with respect to 32% with pure silica. When a cell mixture was employed, an enrichment factor of 38.6, a limit of detection of 1.0 ng l À1 , along with a sampling frequency of 20 h À1 , was attained, leading to a precision of 2.3% RSD (0.05 mg l À1 ). The procedure was validated by analyzing cadmium in a certified reference material of riverine water and spike recovery in a lake water sample.

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