Release of toxic elements in fishpond sediments under dynamic redox conditions: Assessing the potential environmental risk for a safe management of fisheries systems and degraded waterlogged sediments

Waterlogged soils and sediments contaminated with potentially toxic elements (PTEs) constitute a complicated case of degraded areas; their management requires understanding of the dynamic redox-driven PTE mobilization. Such studies about PTE redox-induced dynamics in fishpond sediments are still scarce, but of great importance concerning environmental and human health risk. We studied the redox potential (E_H)-induced impacts on the solubility of As, Co, Cu, Mo, Ni, Se, V, and Zn in the sediments of a fish farm in the Nile Delta, Egypt, using an automated apparatus of biogeochemical microcosm. We assessed the fate of elements as affected by the E_H-induced changes in pH, Fe, Mn, SO₄ ²⁻, Cl⁻, and the dissolved aliphatic (DOC) and aromatic (DAC) organic carbon. Sediment redox ranged from −480 mV to +264 mV. Flooding the sediments caused a significant decrease in pH from 8.2 to 5.7. Dissolved concentrations of As, Co, Ni, Se, and Zn, as well as DOC, Fe, and Mn increased under the reducing acidic conditions. The release of As, Co, Ni, Se, and Zn could be attributed to the decrease of E_H and the subsequent decrease of pH, as well as to the increase of DOC, and/or the dissolution of Fe–Mn oxides caused by redox reactions. Dissolved concentrations of Cu, Mo, and V increased under oxic conditions and were significantly positive correlated with E_H, pH, DAC, and SO₄ ²⁻. This enhancement might be caused by the E_H-dependent increase of pH under oxic conditions (particularly for Mo and V), which also led to DAC increase. Sulfide oxidation and the release of the associated elements may have also had a contribution, particularly in the release of Cu. Therefore, the release dynamics of dissolved Cu, Mo, and V in the sediments were controlled, to a certain extent, by the changes of E_H/pH, DAC, and sulfur chemistry. We conclude that the biogeochemical differences in the behaviour of the studied elements under variable redox regimes substantially affected the fishponds via possible enhancement of PTE mobilization. Our work shows that the potential environmental risks related to PTE mobilization and fish food security should be taken into consideration for the management of degraded aquaculture systems and waterlogged soils and sediments.