Elucidating mechanisms of pesticide sorption and degradation by compound specific isotope analysis in conjunction with advanced mathematical transport modelling

生物地球化学界面在延缓和消除有机农药方面发挥着关键作用：(1)作为保留的吸附媒介；(2)滋生促进降解的微生物。在没有质量平衡的情况下，在自然条件下追踪这些过程是困难的。特别令人担忧的是极性和阴离子杀虫剂，因为它们具有很强的流动性。这项提议旨在为更好地理解过程提供两个重要的新贡献，每个贡献都集中在一个博士项目上：(1)测量化合物特有的13C和15N同位素分馏，以调查农药的吸附和降解。我们将首次(A)测量苯达松和MCPA的降解相关同位素分馏，作为检测它们在自然系统中转化的新方法；(B)利用中性和阴离子形式的苯达松之间的同位素效应，直接表征哪种形式(中性和离子)优先保留在地球化学表面(纯矿物、人造和天然土壤)。(2)结合尖端数学模型进行柱研究，以评估固定水在吸附和降解方面的作用。我们的目标是测试以下假设：(A)在静止(=停滞，近地表)的水中，当更多的物质存在时，杀虫剂的表面相互作用更明显，以及(B)在不饱和的条件下，这一比例更大。为此，(I)在不同水分含量下的示踪剂试验(~3H，溴)将量化不同饱和程度下流动水和静止水的比例；(Ii)将建立一个高级数学模型，包括在静止水带中的缓慢、不可逆吸附和快速吸附平衡，以首次描述静止水对农药吸附和降解的影响。

Biogeochemical interfaces play a key role for retardation and elimination of organic pesticides: (i) as sorption medium for retention; (ii) by harbouring microorganisms that facilitate degradation. Tracing these processes under natural conditions in the absence of mass balances is difficult. Of particular concern are polar and anionic pesticides because they are highly mobile. This proposal aims to provide two important, new contributions towards a better process understanding, each focussed in one Ph.D. project:(1) Measurements of compound-specific 13C and 15N isotope fractionation to investigate pesticide sorption and degradation. We will for the first time (a) measure degradation-associated isotope fractionation for bentazone and MCPA, as a new approach to detecting their transformation in natural systems; (b) use the isotope effect between the neutral and anionic form of bentazone to characterize directly which form (neutral versus ionic) is preferentially retained at geochemical surfaces (pure minerals, artificial and natural soils).(2) Column studies in conjunction with cutting-edge mathematical modelling to assess the role of immobile water on sorption and degradation. We aim to test the hypotheses that (a) surface interactions of pesticides are more pronounced when more of the substances are present in immobile (= stagnant, near-surface) water, and (b) that this proportion is greater under unsaturated conditions. To this end, (i) tracer tests (3H, bromide) at different degrees of water content will quantify the proportions of mobile versus immobile water at different degrees of saturation; (ii) an advanced mathematical model including slow, irreversible sorption, and rapid sorption equilibrium in the immobile water zone will be developed to describe for the first time the influence of immobile water on pesticide sorption and degradation.