Arsenic, iron and sulfur co-diagenesis in lake sediments

摘要

Profiles of porewater pH and dissolved As, Fe, Mn, sulfate, total sulfide (RSII), total zero-valent sulfur (RS0), organic carbon and major ion concentrations, as well as those of solid As, acid-volatile sulfide (AVS), total S, Fe, Mn, Al, organic C, 210Pb and 137Cs were determined in the sediment of four lakes spanning a range of redox and geochemical conditions. An inverse modeling approach, based on a one-dimensional transport-reaction equation assuming steady-state, was applied to the porewater As profiles and used to constrain the net rates of reactions involving As (RAs net). The model defines depth intervals where As is either released to (positive RAs net) or removed from (negative RAs net) the porewaters. At two of the sites, whose bottom water were oxygenated at sampling time, a production zone (RAs net = 12 1018 mol cm3 s1–71 1018 mol cm3 s1) is inferred a few cm below the sediment–water interface, coincident with sharp porewater As and Fe peaks that indicate an intense coupled recycling of As and Fe. This process is confirmed by solid As and Fe maxima just below the sediment surface. In these two lakes a zone of As consumption (RAs net = 5 1018 mol cm3 s1 to 53 1018 mol cm3 s1), attributed to the slow adsorption of As to authigenic Fe oxyhydroxides, occurs just above the production zone. A second-order rate constant kAs ads of 0.12 ± 0.03 cm3 mol1 s1 is estimated for this adsorption reaction. Such features in the porewater and solid profiles were absent from the two other lakes that develop a seasonally anoxic hypolimnion. Thermodynamic calculations indicate that the porewaters of the four lakes, when sulfidic (i.e., RSII P 0.1 lM), were undersaturated with respect to all known solid As sulfides; the calculation also predicts the presence of AsV oxythioanions in the sulfidic waters, as suggested by a recent study. In the sulfidic waters, the removal of As (RAs net = 1 1018 mol cm3 s1 to 23 1018 mol cm3 s1) consistently occurred when saturation, with respect to FeS(s), was reached and when AsV oxythioanions were predicted to be significant components of total dissolved As. This finding has potential implications for As transport in other anoxic waters and should be tested in a wider variety of natural environments