Li Hui, Teppen Brian J, Laird David A, Johnston Cliff T, Boyd Stephen A
Environmental Science and Policy Program, and Department of Crop and Soil Sciences, Michigan State University, East Lansing, Michigan 48824, USA.
Environ Sci Technol. 2004 Oct 15;38(20):5393-9. doi: 10.1021/es0494555.
Pesticide adsorption by soil clays can be dramatically influenced by the exchangeable cations present. Among the common exchangeable base cations in soils (Ca2+, Mg2+, K+, and Na+), K+-saturated clays frequently demonstrate the strongest affinity for pesticides. In the presence of multiple exchangeable cations in the system, we hypothesize that the magnitude of pesticide sorption to soil minerals is proportional to the fraction of clay interlayers saturated with K+ ions. To test this hypothesis, we measured sorption of three pesticides with different polarities (dichlobenil, monuron, and biphenyl) by homoionic K- and Ca-smectite (SWy-2) in KCl/CaCl2 aqueous solutions. The presence of different amounts of KCl and CaCl2 resulted in varying populations of K+ and Ca2+ on the clay exchange sites. The sorption of dichlobenil and, to a lesser extent monuron, increased with the fraction of K+ on clay mineral exchange sites. Ca- and K-SWy-2 displayed the same sorption capacities for nonpolar biphenyl. X-ray diffraction patterns indicated that at lower fractions of K+-saturation, exchangeable K+ ions were randomly distributed in clay interlayers and did not enhance pesticide sorption. At higher populations of K+ (vs Ca2+), demixing occurred causing some clay interlayers, regions, or tactoids to become fully saturated by K+, manifesting greatly enhanced pesticide sorption. The forward and reverse cation exchange reactions influenced not only K+ and Ca2+ populations on clays but also the nanostructures of clay quasicrystals in aqueous solution which plays an important, if not dominant, role in controlling the extent of pesticide sorption. Modulating the cation type and composition on clay mineral surfaces through cation exchange processes provides an environmental-safe protocol to manipulate the mobility and availability of polar pesticides, which could have applications for pesticide formulation and in environmental remediation.
土壤黏土对农药的吸附会受到存在的可交换阳离子的显著影响。在土壤常见的可交换碱金属阳离子（Ca2+、Mg2+、K+和Na+）中，K+饱和的黏土通常对农药表现出最强的亲和力。在系统中存在多种可交换阳离子的情况下，我们假设农药对土壤矿物质的吸附量与被K+离子饱和的黏土层间比例成正比。为了验证这一假设，我们在KCl/CaCl2水溶液中，测量了三种不同极性的农药（敌草腈、灭草隆和联苯）在同离子K - 和Ca - 蒙脱石（SWy - 2）上的吸附情况。不同量的KCl和CaCl2的存在导致黏土交换位点上K+和Ca2+的数量不同。敌草腈的吸附，以及程度稍轻的灭草隆的吸附，随着黏土矿物交换位点上K+的比例增加而增加。Ca - 和K - SWy - 2对非极性联苯表现出相同的吸附能力。X射线衍射图谱表明，在较低的K+饱和度分数下，可交换的K+离子随机分布在黏土层间，并未增强农药吸附。在较高的K+（相对于Ca2+）含量下，发生了分层现象，导致一些黏土层间、区域或准晶体完全被K+饱和，农药吸附显著增强。正向和反向阳离子交换反应不仅影响黏土上K+和Ca2+的数量，还影响水溶液中黏土准晶体的纳米结构，这在控制农药吸附程度方面发挥着重要（如果不是主导）作用。通过阳离子交换过程调节黏土矿物表面的阳离子类型和组成，为控制极性农药的迁移性和有效性提供了一种环境安全的方法，这可能在农药配方和环境修复中具有应用价值。
