Changes in mobility and solubility of the redox sensitive metals Fe, Mn and Co at the seawater-sediment interface following CO ₂ seepage

The impact of CO ₂ seepage on the solubility and distribution of the redox-sensitive metals iron, manganese, and cobalt in seawater and sediment pore water has been studied in experiments in laboratory-scale 0.6 m ³ chambers. The mobility and solubility of Fe, Mn and Co were investigated in seawater, membrane filtered seawater, and DGT samplers deployed in water and sediment during a 26 day CO ₂ seepage study.

During the first phase of the experiment of CO ₂ seepage (0–16 days), total acid-leachable (pH 1) and "dissolved" (<0.2 μm) concentrations of Fe, Mn and Co (DFe, DMn and DCo) in the seawater increased significantly; the ratios of concentrations of DFe, DMn and DCo in the CO ₂ chamber to the corresponding values in the control chamber ( R_DFe, R_DMn and R_DCo) were as high as 6, 65 and 58, respectively. The second phase of experiment consisted of an additional 10 days of incubation, where the concentrations of all the metals studied still increased but at reduced rates for DMn and DCo. The highest values of R_DFe, R_DMn and R_DCo were about 3 for all metals during this part of the experiment.

DGT (diffusive gradients in thin film) labile fractions denoted Fe _DGT, Mn _DGT and Co _DGT were, respectively 50, 25 and 22 times higher in the CO ₂ seepage chamber than in the control chamber in the first phase of the experiment. During the second phase, all DGT labile metal concentrations still increased considerably, most notably for Fe. The ratio of Fe _DGT in the CO ₂ chamber to that in the control ( R_DGT-Fe) was still high, about 5, in the second phase of the experiment, whereas the increase in Mn _DGT and Co _DGT slowed down. Our results indicate that acidification following CO ₂ seepage enhances the mobility and solubility of Fe Mn and Co in sediment and overlying water with contribution of changing in redox conditions and seepage related re-suspension.