Abstract

Mercury is a toxin that causes neurological impairments in adults, is particularly harmful for fetuses and children, and is deadly in severe cases, making it a worldwide health concern. Methylmercury (MeHg) is the environmentally relevant form of mercury (Hg) because it biomagnifies along the food chain. Methylmercury is mainly produced in aquatic sediments via methylation of inorganic Hg (Hg(II)) and transformed back via demethylation. Because transformation rates determine MeHg concentrations, quantification of methylation and demethylation rates is needed to inform management of MeHg. Published rate constants for Hg(II) methylation ($k_m$) and MeHg demethylation ($k_d$) vary greatly, stemming partly from differences in experimental methods. We conducted a comprehensive review of rate laws, evaluated published rate constants, and performed biogeochemical simulations to assess variability in reported $k_m$ and $k_d.$ Based on selected studies employing the same pseudo-first-order rate law and similar experimental methods, we found that $k_m=0.04\pm 0.03\mathrm{ }{\mathrm{d}}^{-1}$ is a reasonable range for wetland sediments. Over a number of environments, maximum $k_d$ was smaller at sites without Hg source ($k_d=0.5\mathrm{ }{\mathrm{d}}^{-1}$) than at sites with identified Hg source ($k_d=1.8\mathrm{ }{\mathrm{d}}^{-1}$). Larger variability and higher uncertainty in $k_d$ compared to $k_m$ highlight the need for more research on MeHg demethylation rates. This critical review: (a) aids the design of future experimental studies of $k_m$ and $k_d;$ (b) provides guidance for comparing rate constants from different studies; (c) presents a biogeochemical reaction model to assess rate constants; and (d) informs selection of $k_m$ and $k_d$ values from the literature for use in model simulations.

摘要

汞是一种毒素，会导致成人神经功能障碍，对胎儿和儿童尤其有害，严重时会致命，使其成为全球健康问题。甲基汞 (MeHg) 是与环境相关的汞 (Hg) 形式，因为它会沿着食物链进行生物放大。甲基汞主要通过无机汞 (Hg(II)) 的甲基化在水生沉积物中产生，并通过去甲基化转化回来。因为转化率决定了甲基汞的浓度，甲基化和去甲基化率的量化需要为甲基汞的管理提供信息。已发表的 Hg(II) 甲基化速率常数 (${ķ}_{米}$) 和甲基汞去甲基化 (${ķ}_d$) 差异很大，部分源于实验方法的差异。我们对速率定律进行了全面审查，评估了已发表的速率常数，并进行了生物地球化学模拟以评估报告的可变性${ķ}_{米}$和${ķ}_d.$基于采用相同的伪一阶速率定律和类似实验方法的选定研究，我们发现${ķ}_{米}=0.04\pm 0.03\mathrm{ }{\mathrm{d}}^{-1}$是湿地沉积物的合理范围。在许多环境中，最大${ķ}_d$在没有汞源的地方较小（${ķ}_d=0.5\mathrm{ }{\mathrm{d}}^{-1}$) 比在已确定 Hg 源的地点 (${ķ}_d=1.8\mathrm{ }{\mathrm{d}}^{-1}$）。更大的可变性和更高的不确定性${ķ}_d$相比${ķ}_{米}$强调需要对甲基汞去甲基化率进行更多研究。这篇批判性评论：（a）有助于设计未来的实验研究${ķ}_{米}$和${ķ}_d;$(b) 为比较不同研究的速率常数提供指导；(c) 提出一个生物地球化学反应模型来评估速率常数；(d) 通知选择${ķ}_{米}$和${ķ}_d$用于模型模拟的文献中的值。