Biogeochemical factors affecting mercury methylation rate in two contaminated floodplain soils

Abstract

<jats:p>Abstract. An automated biogeochemical microcosm system allowing controlled variation of redox potential (EH) in soil suspensions was used to assess the effect of various factors on the mobility of mercury (Hg) as well as on the methylation of Hg in two contaminated floodplain soils with different Hg concentrations (approximately 5 mg Hg kg−1 and &gt;30 mg Hg kg–1). The experiment was conducted under stepwise variation from reducing (approximately −350 mV at pH 5) to oxidizing conditions (approximately 600 mV at pH 5). Results of phospholipid fatty acids (PLFA) analysis indicate the occurrence of sulfate reducing bacteria (SRB) such as Desulfobacter species (10Me16:0, cy17:0, 10Me18:0, cy19:0) or Desulfovibrio species (18:2ω6,9), which are considered to promote Hg methylation. The products of the methylation process are lipophilic, highly toxic methyl mercury species such as the monomethyl mercury ion [MeHg+], which is named as MeHg here. The ln(MeHg/Hgt) ratio is assumed to reflect the net production of monomethyl mercury normalized to total dissolved Hg (Hgt) concentration. This ratio increases with rising dissolved organic carbon (DOC) to Hgt ratio (ln(DOC/Hgt) ratio) (R2 = 0.39, p&lt;0.0001, n= 63) whereas the relation between ln(MeHg/Hgt ratio and lnDOC is weaker (R2 = 0.09; p&lt;0.05; n = 63). In conclusion, the DOC/Hgt ratio might be a more important factor for the Hg net methylation than DOC alone in the current study. Redox variations seem to affect the biogeochemical behavior of dissolved inorganic Hg species and MeHg indirectly through related changes in DOC, sulfur cycle, and microbial community structure whereas EH and pH values, as well as concentration of dissolved Fe3+/Fe2+ and Cl– seem to play subordinate roles in Hg mobilization and methylation under our experimental conditions.</jats:p>

Comment on bg-2020-466

“…Our experiment shows that redox oscillations (flooding-draining-flooding cycles) of a polluted floodplain soil are likely to induce pulses of both Hg and MeHg to the downstream ecosystems. This is supported with earlier studies (Poulin et al, 2016;Frohne et al, 2012;Hofacker et al, 2013). In NOM poor agricultural soils, the application of additional NOM in form of manure may reduce the mobilization and contribute to the transformation of Hg towards less mobile species, especially during low flow conditions.…”

“…The fate of Hg in soils is still not well characterized, and its mobilization and sequestration in soil depends on a variety of factors and mechanisms. The release of Hg to the soil solution and its further transport has been associated with the mobilization of NOM (Kronberg et al, 2016;Eklöf et al, 2018;Åkerblom et al, 2008), copper (Cu) nanoparticles (Hofacker et al, 2013) or the reductive dissolution of Fe/Mn-oxyhydroxides (Frohne et al, 2012;Gygax et al, 2019;Poulin et al, 2016). Earlier studies reported an immediate decrease of dissolved Hg after its release upon flooding in various riparian settings (Hofacker et al, 2013;Poulin et al, 2016;Gygax et al, 2019).…”

“…However, only few studies focused on Hg methylation and mobility in temperate floodplain soils (Frohne et al, 2012;Hofacker et al, 2013;Weber et al, 2009;Gilli et al, 2018;Poulin et al, 2016). As well, few studies have examined the effect of flooding and/or land use (NOM addition in the form of animal manure) in polluted soils with respect to Hg release and methylation potential (Tang et al, 2018;Gygax et al, 2019;Zhang et al, 2018;Hofacker et al, 2013;Frohne et al, 2012). Furthermore, most of these studies were focusing on soils with rather high OC https://doi.org/10.5194/bg-2020-466 Preprint.…”

Mercurio en Un Arroyo Altoandino Con Alto Impacto Por Minería Aurífera Artesanal (La Rinconada, Puno, Perú)

“…Las concentraciones de mercurio sí se correlacionaron significativamente con otros parámetros físicos y químicos (Cuadro III), resaltando la correlación inversa significativa entre el Hg del agua y el pH (r = -0.752; p = 0.005), por lo que se infiere que bajos valores de pH propiciaron mayores niveles de mercurio. La mayor acidez del agua induce mayor movilidad y solubilidad del mercurio sobre todo por debajo de 3 (Spence y Barton 2003, Wu et al 2011; además, la materia orgánica a bajo pH incrementa su disponibilidad para la metilación bacteriana, transformándolo en metilmercurio (Frohne et al 2012). Es decir, aguas con bajo pH presentan mayor tasa de metilación y por tanto mayor bioacumulación (Jensen 1988, Shastri y Diwekar 2008; por ejemplo, lagos relativamente ácidos propician altas concentraciones de mercurio en peces (Jensen 1988), situación que eventualmente debe ocurrir en los ecosistemas implicados en esta evaluación.…”

Assessment of the relationships between elements and organic matter in water after a large landslide in the Bureyskoye reservoir (Russia)

“…The risk of mercury contamination depends on many factors, including how this metal is found in the environment. The most toxic form of mercury, methylmercury (CH 3 Hg + ), is formed in the presence of methyl radicals in the environment [21]. In our case, the presence of methyl-containing VOCs (methanol, toluene and xylenes) could affect the mobility of mercury.…”