Mobilization of mercury species under dynamic laboratory redox conditions in a contaminated floodplain soil as affected by biochar and sugar beet factory lime

Mercury and its species are toxic and therefore strategies to immobilize them or to impede the formation of bioaccumulative MeHg are a hot topic of ongoing research. Biochar (BC) and sugar beet factory lime (SBFL) are suggested to have the potential to meet these goals. However, their ability to restrain the mobilization of total Hg (Hg_t), methylmercury (MeHg), and ethylmercury (EtHg) or the formation of MeHg and EtHg has not been examined to date. Moreover, the effect of systematically altered redox conditions on the release dynamics of Hg_t, MeHg, and EtHg in a contaminated floodplain soil as affected by these soil amendments has not been studied. Therefore, we investigated the impact of pre-defined redox conditions on the release dynamics of Hg_t, MeHg, and EtHg in a contaminated floodplain soil (CS) and the soil amended with either BC (CS+BC) or SBFL (CS+SBFL). The mobilization of Hg_t, MeHg, and EtHg was generally higher at low redox potential (E_H) and decreased with increasing E_H, irrespective of soil treatment. Both BC and SBFL diminished the release of Hg_t from soil but not the methylation and ethylation of Hg. In CS+SBFL approximately half of Hg_t was found in solution compared to CS. However, higher methylation efficiency (MeHg/Hg_t ratio) was found in CS+SBFL counterbalancing this benefit. Abundances of specific phospholipid fatty acids suggest the presence of sulfate-reducing bacteria, which are considered as primary Hg methylators. The results indicate that both BC and SBFL have the potential to curtail the release of Hg_t from inundated soils, while SBFL was more efficient. However, these amendments had no marked effect on the MeHg and EtHg concentrations. Therefore, further research should be conducted to identify soil additives that are capable to reduce the release and formation of these Hg species.