Many natural aquatic systems fluctuate between oxic and anoxic conditions due to changes in water saturation levels, microbial activity, and supply and dynamics of nutrients and organic carbon. With oscillating redox conditions, trace metals undergo speciation and mobility changes in response to transformations of iron and sulfur-bearing minerals, and organic matter. We investigated the effect of multiple cycles of oxic and anoxic conditions on trace metal (Cu, Ni, Co, and Zn) mobility in soils and sediments collected from two different natural aquatic systems: a riparian wetland and a stream. The total solid-phase trace metal concentrations in both the systems were similar to or below geological background levels. The water-saturated soils and sediments were incubated under three cycles of anoxic-oxic conditions (τ<sub>anoxic</sub>:τ<sub>oxic</sub> = 3) spanning 24 days. The change in pH and dissolved oxygen was monitored atleast twice a day, and the dissolved trace metal concentrations were determined at the end of each fluctuation. Additionally, after each anoxic and oxic fluctuation, bioavailable concentrations of trace metals were estimated using the diffusive gradients in thin films (DGT) technique. For both riparian wetland and stream sites, less than 1% of total solid-phase associated trace metals were released during redox fluctuations. In both systems, Co and Zn were mobilized under anoxic conditions, which corresponded with the reductive dissolution of iron oxides. In contrast, Cu was mobilized under oxic conditions and correlated positively with the formation of sulfate, suggesting that oxidation of sulfide moieties increased Cu mobility in the selected sites. In riparian wetland soils, Ni solubility increased under anoxic conditions, however, in stream sediments, the Ni solubility was greater under oxic conditions. The contrasting Ni behavior at two sites was due to different solid-phase speciation; the amount of Ni bound to sulfides is substantially higher in stream site. Redox oscillations increased the bioavailability of all four trace metals in stream samples, however, for wetland soils, after three cycles of redox fluctuations, the bioavailability of Zn and Cu increased, whereas bioavailable Ni and Co decreased. This study indicated that the trace metal mobilization/sequestration upon oscillating redox conditions in natural environments is strongly coupled to the solid-phase speciation of trace metals and the redox status of the recent past.

Impacts of oscillating redox conditions on trace metal mobilization in natural aquatic systems

ACS Spring 2022

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