Mobility of chromium in sediments dominated by macrophytes and cyanobacteria in different zones of Lake Taihu

To study the mechanisms of chromium (Cr) mobilization in sediments of lakes with different ecotypes, seasonal sampling was performed in the macrophyte-clominatal East Taihu (MDET) and cyanobacteria-clominatal Meiliang Bay (CDMB) in Lake Taihu. Concentrations of labile Cr(VI) and dissolved Cr were assessed using cliffusivc gradients in thin films (DGT) and high-resolution dialysis passive sampling devices, respectively. Results indicated that in pore water the dissolved Cr concentrations and in sediments total Cr and Cr fractions concentrations (dissolved, exchangeable and carbonate fraction (F1), Fe-Mn oxide fraction (F2), organic sulfide fraction (F3)) were lower in MDET than in CDMB. However, the highly toxic DGT-labile Cr(VI) concentrations were higher in MDET sediments than in CDMB sediments. In the two lake zones, the seasonal variations in concentrations of dissolved Cr and DGT-labile Cr(VI) were significant, while total Cr and Cr fractions showed negligible changes (except Fl fraction). In summer, the high mobility of dissolved Cr (MDET: 103.42 +/- 10.82 mu g,L; CDMB: 108.99 +/- 424 mu g/t) were mainly caused by dissolved organic matter complexing with Cr(III). In winter the high mobility of dissolved Cr (MDET: 10027 +/- 22.04 mu g/L; CDMB: 102.01 +/- 8.81 mu g/L) and DGT-labile Cr (VI) (MDET: 2826 +/- 3.73 mu g. L; CDLZ: 25.82 +/- 2.26 mu g/L) were primarily caused by the oxidization of Cr (III) by Mn(III/IV) oxides. This study establishes the mechanisms for seasonal variation of Cr mobilization in different lake ecological zones, highlighting the urgent need for remediation of Cr pollution, especially in macrophyte-dominated lake zones. (C) 2019 Published by Elsevier B.V.