Abstract Sorption processes control the solubility of toxic thallium (Tl) in soils and thereby its potential leaching into groundwater or uptake by plants. Micaceous clay minerals and Mn oxides are considered to be key sorbents for Tl in soils. We studied the sorption and speciation of Tl in 36 geogenically Tl-rich topsoil materials from the Swiss Jura Mountains by combining chemical extractions, isotope exchange experiments, adsorption experiments, X-ray absorption spectroscopy (XAS), and sorption modelling. We demonstrate that the relation between exchangeable and soluble Tl determined in batch extractions of soils with only geogenic and with freshly spiked Tl matches adsorption isotherms of freshly spiked Tl, and that this relation can be described with a published 3-site cation exchange model for Tl adsorption onto illite. Complemented with XAS data, the results show that micaceous clay minerals control the short-term solubility of Tl via cation exchange, but also the long-term sequestration of most geogenic soil Tl (>90%) via structural fixation. Adsorption competition with K+ and NH4+ at the frayed edges of micaceous clay minerals greatly affects Tl solubility. Increases in the dissolved concentrations of K and NH4 in soil pore water may therefore lead to the release of Tl into solution. The fractions of geogenic Tl associated with Mn oxides were about half as high as the fractions of exchangeable Tl. This Mn-associated Tl is not readily mobilized by cation exchange, but could be released during periodic water logging and soil reduction. In (periodically) reducing environments, the potential of Mn oxides for long-term Tl sequestration is therefore limited. In conclusion, the results from this study highlight the importance of micaceous clay minerals for Tl cycling in soils and sediments, and suggest that concepts developed to assess the sorption of (radio)caesium onto micaceous clay minerals in soils and sediments are transferable to Tl.

中文翻译：

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土壤中铊的吸附和物种形成：云母粘土矿物和锰氧化物的作用

摘要 吸附过程控制有毒铊 (Tl) 在土壤中的溶解度，从而控制其渗入地下水或被植物吸收的可能性。云母粘土矿物和锰氧化物被认为是土壤中 Tl 的关键吸附剂。我们通过结合化学提取、同位素交换实验、吸附实验、X 射线吸收光谱 (XAS) 和吸附建模，研究了来自瑞士侏罗山脉的 36 种富含 Tl 的地质成因表土材料中 Tl 的吸附和形态形成。我们证明了在仅具有地质成因和新添加的 Tl 的土壤的批量提取中确定的可交换和可溶 Tl 之间的关系与新添加的 Tl 的吸附等温线相匹配，并且这种关系可以用已发布的 3 位 Tl 阳离子交换模型来描述吸附到伊利石上。与 XAS 数据相辅相成，结果表明云母粘土矿物通过阳离子交换控制 Tl 的短期溶解度，但也通过结构固定控制大多数地质土壤 Tl (>90%) 的长期固存。在云母粘土矿物的磨损边缘与 K+ 和 NH4+ 的吸附竞争极大地影响了 Tl 溶解度。因此，土壤孔隙水中溶解的 K 和 NH4 浓度的增加可能导致 Tl 释放到溶液中。与 Mn 氧化物相关的地质成因 Tl 的分数约为可交换 Tl 分数的一半。这种与 Mn 相关的 Tl 不容易通过阳离子交换移动，但可以在周期性水涝和土壤减少期间释放。在（定期）还原环境中，Mn 氧化物长期封存 Tl 的潜力因此受到限制。