Biochar as a sustainable amendment for mitigating mercury and methylmercury mobility in contaminated lagoon sediments: insights from incubation experiments

Mercury (Hg) contaminated sediments may represent a secondary source of contamination, especially under anoxia which promotes the release of Hg, including its most toxic form methylmercury (MeHg), into the water column, posing a risk to marine life and human health. As such, sorbent amendments added to the sediment could be a sustainable approach to mitigate Hg methylation and mobility. This study aimed to evaluate the effectiveness of biochar in reducing Hg and MeHg effluxes at the sediment-water interface (SWI) in a Hg-contaminated fish farm. Incubation experiments compared biochar-amended and untreated sediment using two benthic chambers, monitoring temporal changes in physico-chemical parameters (dissolved oxygen - DO, ORP), DHg, DMeHg, Fe, Mn, H₂S and nutrients (NO₃^-, NO₂^-, NH₄⁺, and PO₄^3-) during oxic-anoxic transition and re‑oxygenation. In the early stage, a faster DO consumption was observed, especially in the untreated system, alongside increases of NH₄⁺, PO₄^3-, Fe and Mn, due to the intense remineralisation of organic matter and reductive dissolution of oxy-hydroxides. In the untreated mesocosm, the highest DHg (64.3 ng L^-1) and DMeHg (53.1 ng L^-1) levels were observed under anoxia. Conversely, the biochar-amended chamber maintained lower DHg (12.7 ± 3.8 ng L^-1) and DMeHg (4.26 ± 1.26 ng L^-1) concentrations, with an average reduction of 75 and 90 %, respectively. Following re‑oxygenation, oxic conditions were rapidly restored in the biochar-amended system whereas anoxia lasted for 6 days in the untreated chamber. Results suggest that biochar effectively mitigated Hg and MeHg fluxes at the SWI, reducing methylation potential, limiting the development of anoxia and promoting re-oxygenation.