Chappell Mark A, Laird David A, Thompson Michael L, Li Hui, Teppen Brian J, Aggarwal Vaneet, Johnston Cliff T, Boyd Stephen A
Department of Agronomy, Agronomy Hall, Iowa State University, Ames, Iowa 50011, USA.
Environ Sci Technol. 2005 May 1;39(9):3150-6. doi: 10.1021/es048942h.
Smectites, clay minerals commonly found in soils and sediments, vary widely in their ability to adsorb organic chemicals. Recent research has demonstrated the importance of surface charge density and properties of exchangeable cations in controlling the affinity of smectites for organic molecules. In this study, we induced hysteresis in the crystalline swelling of smectites to test the hypothesis that the extent of crystalline swelling (or interlayer hydration status) has a large influence on the ability of smectites to adsorb atrazine from aqueous systems. Air-dried K-saturated Panther Creek (PC) smectite swelled less (d(001) = 1.38 nm) than never-dried K-PC (d(001) = 1.7 nm) when rehydrated in 20 mM KCl. Correspondingly, the air-dried-rehydrated K-PC had an order of magnitude greater affinity for atrazine relative to the never-dried K-PC. Both air-dried-rehydrated and never-dried Ca-PC expanded to approximately 2.0 nm in 10 mM CaCl2 and both samples had similar affinities for atrazine that were slightly lower than that of never-dried K-PC. The importance of interlayer hydration status in controlling sorption affinity was confirmed by molecular modeling, which revealed much greater interaction between interlayer water molecules and atrazine in a three-layer hydrate relative to a one-layer hydrate. The entropy change on moving atrazine from a fully hydrated state in the bulk solution to a partially hydrated state in the smectite interlayers is believed to be a major factor influencing sorption affinity. In an application test, choice of background solution (20 mM KCl versus 10 mM CaCl2) and air-drying treatments significantly affected atrazine sorption affinities for three-smectitic soils; however, the trends were not consistent with those observed for the reference smectite. Further, extending the initial rehydration time from 24 to 240 h (prior to adding atrazine) significantly decreased the soil's sorption affinity for atrazine. We conclude that interlayer hydration status has a large influence on the affinity of smectites for atrazine and that air-drying treatments have the potential to modify the sorption affinity of smectitic soils for organic molecules such as atrazine.
蒙脱石是一种常见于土壤和沉积物中的黏土矿物，其吸附有机化学物质的能力差异很大。最近的研究表明，表面电荷密度和可交换阳离子的性质在控制蒙脱石对有机分子的亲和力方面具有重要作用。在本研究中，我们诱导蒙脱石晶体膨胀出现滞后现象，以检验晶体膨胀程度（或层间水化状态）对蒙脱石从水体系中吸附阿特拉津能力有很大影响这一假设。在20 mM KCl中再水化时，空气干燥的K饱和黑豹溪（PC）蒙脱石膨胀程度较小（d(001)=1.38 nm），而从未干燥的K-PC蒙脱石膨胀程度较大（d(001)=1.7 nm）。相应地，空气干燥后再水化的K-PC对阿特拉津的亲和力比从未干燥的K-PC高一个数量级。在10 mM CaCl2中，空气干燥后再水化的Ca-PC和从未干燥的Ca-PC都膨胀到约2.0 nm，且两个样品对阿特拉津的亲和力相似，略低于从未干燥的K-PC。分子模拟证实了层间水化状态在控制吸附亲和力方面的重要性，该模拟表明相对于单层水合物，三层水合物中层间水分子与阿特拉津之间的相互作用要大得多。阿特拉津从本体溶液中的完全水合状态转移到蒙脱石层间的部分水合状态时的熵变被认为是影响吸附亲和力的一个主要因素。在应用测试中，背景溶液（20 mM KCl与10 mM CaCl2）的选择和空气干燥处理显著影响了三种蒙脱石土壤对阿特拉津的吸附亲和力；然而，这些趋势与参考蒙脱石观察到的趋势不一致。此外，将初始再水化时间从24小时延长至240小时（在添加阿特拉津之前）显著降低了土壤对阿特拉津的吸附亲和力。我们得出结论，层间水化状态对蒙脱石对阿特拉津的亲和力有很大影响，并且空气干燥处理有可能改变蒙脱石土壤对阿特拉津等有机分子的吸附亲和力。
