Biochar as a sustainable amendment to mitigate mercury mobility at the sediment-water interface: evidences from incubation experiments using benthic chambers

Mercury (Hg) contaminated sediments may act as a secondary source of contamination, especially during anoxic events that occur naturally in shallow, semi-confined aquatic systems such as fish farms. Indeed, anoxia and high organic matter (OM) production may promote the release of Hg species into the water column, thus posing a risk to marine life and human health. In this context, sorbent amendments may represent an economical and ecologically sustainable approach to mitigate Hg mobility and its availability to be methylated [Gilmour et al., 2013, 2018]. This study aims at evaluating the ability of biochar from vineyard pruning residues in reducing Hg mobility at the sediment-water interface (SWI) in one of the most productive fish farms of the Marano and Grado Lagoon, affected by Hg contamination [Covelli et al., 2012]. Laboratory incubation experiments were conducted using two benthic chambers to compare biochar-amended and untreated sediments. Temporal changes in dissolved Hg, dissolved oxygen (DO), redox potential, Fe, Mn, H2S N-NO3, N-NO2, N-NH4, and P-PO4 were monitored for 35 days. At early stage, a faster DO consumption and more reductive conditions were observed, especially in the untreated system, as well as an increase in dissolved N-NH4, P-PO4, Fe and Mn, as a result of intense OM mineralisation processes and reductive dissolution of oxy-hydroxides, respectively. Once anoxia was reached, the control system showed the highest Hg concentrations (46.7 ± 11.1 ng/L) whereas notably lower concentrations were observed in the biochar-amended chamber (12.7 ± 3.8 ng/L). After re-oxygenation, oxic conditions were rapidly restored in the amended system with respect to the control chamber where anoxia and elevated Hg concentrations (37.2 ± 11.5 ng/L) lasted for a further 6 days. The results suggest that biochar is effective in reducing Hg mobility at the SWI, limiting the development of anoxic conditions and promoting re-oxygenation.