匹配阴离子影响下咪唑类离子液体的土壤吸附行为

Imidazolium-based ionic liquids (IBILs) are room temperature molten salts composed of alkyl-substituted imidazolium cations and inorganic/organic anions. According to the previous studies, the adsorption of IBILs in soils mainly depends on the electrostatic interaction between IBIL cations and soil clays as well as the hydrophobic interaction between cationic alkyls and soil organic matters (SOMs), and rarely involves any contribution from IBIL anions. However, evidences showed that some anions can form ion pairs with imidazolium cations through ion association, thereby effectively changing the water solubility and the hydrophobicity of IBILs. To clarify the impact of different types of IBIL anions on the adsorption of IBILs in soils, lots of IBILs matched with inorganic/organic, fluoric/fluoride-free, aromatic-ring/non-aromatic ring, and long-chain/short-chain anions were selected in the present study. The project will likewise investigate the IBILs' cation/anion associations and disclose how the octanol-water partition coefficient (Kow) of IBILs in response to changes of IBIL anions. Moreover, the study will compare the adsorption/desorption of series of IBILs compounds which characterized with the same imidazolium cation but the different ions in varied soils (with different CEC and different content of SOMs) and varied soil components (clay minerals, metal oxide minerals, humic acids, etc.), discuss the influence degree and influence mechanism of IBIL anions in different adsorption/desorption systems combined with various instrumental analyses, and comprehensively disclose the role of IBIL anions in the adsorption of IBILs in soils.

咪唑类离子液体（ILs）是由烷基咪唑阳离子和无机/有机阴离子组成的室温熔盐。以往研究认为，咪唑类ILs的土壤吸附行为主要取决于阳离子的静电吸附作用和烷基的疏水分配作用，很少涉及ILs阴离子的贡献。然而有证据表明，某些阴离子能通过离子缔合作用与咪唑阳离子形成离子对，并有效改变ILs的溶解度和疏水性。为澄清不同类型阴离子对土壤吸附咪唑类ILs的影响，本项目将以多种（无机/有机、含氟/非氟、芳环/非芳环、长链/短链）阴离子匹配的咪唑类ILs为研究对象，研究不同阴离子和咪唑阳离子间的缔合能力，揭示ILs的Kow值随阴离子变化的规律，比较阴离子不同的咪唑类ILs在多种土壤（阳离子交换容量不同、有机质含量不同）和多种土壤介质（粘土矿物、金属氧化矿物、腐殖酸）上的吸附/脱附行为，结合仪器表征手段讨论不同吸附/脱附体系中ILs阴离子影响的程度与机理，综合阐明土壤吸附咪唑类ILs过程中阴离子的角色与作用。

本项目研究了匹配阴离子影响下，咪唑类离子液体在土壤和土壤介质（黑碳和蒙脱石）上的吸附行为和规律。发现离子液体在多种土壤上的吸附系数Kd值均很低（<几十L/kg），土壤阳离子交换容量（CEC）是影响吸附的主要因素。土壤矿物（如蒙脱石）能为离子液体提供较强吸附，层间阳离子交换是吸附的主要作用力；蒙脱石的吸附能力随碳链增长而增加。但总体而言，阴离子对土壤和蒙脱石吸附离子液体的影响均不显著。然而，环境黑碳对离子液体的吸附却严重受制于阴离子的影响。不同阴离子影响下，咪唑离子液体的Kd范围为59.8-344.3 L/kg之间；二苯并咪唑类离子液体的Kd范围为253.4-253.4 L/kg之间；[Tf2N]-和[PF6]-阴离子能显著提升离子液体的吸附能力。吸附影响总体排序为：[Tf2N]- > [PF6]- > [CF3SO3]- > [BF4]- ≈ [CF3COO]- > [CH3SO3]- ≈ [CH3COO]-，这主要得益于不同阴离子与咪唑或苯并咪唑阳离子间缔合能力不同。我们通过离子对缔合常数（KIP）大小的测定，定量评价了上述阴/阳离子缔合能力的强弱；并且通过辛醇/水分配系数（Kow）大小的测定，定量评估了离子对的疏水性。发现Kd与KIP，以及Kd与Kow之间，存在很好的线性正相关。这说明：阴离子与阳离子的缔合强弱，能影响离子对的疏水性，进而影响离子液体在黑碳上的吸附。