Iron phosphate as a novel sorbent for selective adsorption of chromium(iii) and chromium speciation with detection by ETAAS

Iron phosphate (FePO4) is for the first time used as a novel sorbent for chromium(III) adsorption. The prepared FePO4 was characterized by means of FT-IR, SEM and surface charge analysis, and used for selective adsorption of chromium(III). At pH 5.9, ca.100% of Cr(III) (<5 μg L−1) was retained on the surface of the adsorbent, while at the same conditions the adsorption of Cr(VI) was negligible (<2%). Cr(III) was adsorbed with an ca. 62/1 selectivity over Cr(VI). The adsorption of Cr(III) fits Langmuir model, corresponding to a maximum adsorption capacity of 8.12 mg g−1. The retained Cr(III) could be readily recovered by 200 μL of aqueous mixture of 0.1% H2O2 + 0.05 mol L−1 NH3 as stripping reagent, giving rise to a recovery of 96.5%. The chromium in the eluate is quantified with detection by electrothermal atomic absorption spectrometry. A sample volume of 2000 μL creates an enrichment factor of 8.7, along with a detection limit of 0.02 μg L−1 (3σ, n = 9) and a RSD of 2.5% (0.5 μg L−1) within a linear calibration range of 0.05–2.5 μg L−1. Total chromium was determined after reduction of Cr(VI) to Cr(III) by hydroxylamine hydrochloride as a reducing reagent. The content of Cr(VI) was achieved by difference. The method was validated by analyzing chromium content in a certified reference material (GBW08608, Trace Elements in Water) and speciation of Cr(III) and Cr(VI) in sea water, tap water, spring water and ground water samples was conducted.