水溶性离子液体在生物炭上的吸附增效机制与调控原理

The widely used short-chain imidazolium-type ionic liquids (ITILs) are water-soluble and have aquatic toxicity, therefore potentially imposing a risk to water bodies. The hydrophilic biochar is an effective adsorbent for removal of ITILs due to its high affinity to water-soluble ITILs. However, the unclear sorption mechanism prevent the further improvement of adsorption capacity of ITILs on biochar. In this study, biochars will be prepared from various biomasses. By controlling the pyrolysis temperature and pyrolysis time, the project will obtain the optimized biochar with abundant surface groups, excellent pore size distribution and perfect desorption characteristics for efficiently removal of water-soluble ITILs, and will discuss the enhanced sorption mechanism combined with various instrumental analyses. Moreover, to improve the sorption of ITILs on biochar, the project will further investigate the opportunity of adjusting the hydrophilicity of water-soluble ITILs based on "ITILs’ ion-pair effect". To achieve this goal, specific measures will be taken, which include the change of "ion pair formation constant" of ITILs, discussing the inherent relationship between the ITILs association and ITILs hydrophobicity, screening the strongly associated ITILs anion types. After adding such anionic salt to water, which can form strongly associated ion-pair with ITILs cation, the project will achieve the higher sorption of water-soluble ITILs on the optimized biochar.

短链的咪唑类离子液体（ITILs）易溶于水，具备水生毒性，在工业应用过程中存在着污染水体的风险。生物炭结构亲水，对水溶性ITILs具有较强的亲和力，是一种潜在的优质吸附剂。然而其吸附机理还有待揭示，吸附能力也有待提升。为此，本研究将通过生物质筛选、热解温度和时间的综合调控，来优化生物炭的结构性质，制备表面基团丰富、孔径分布合理、脱附性能优异的生物炭，用于水溶性ITILs的吸附去除。进而结合仪器表征手段，讨论其吸附增效机制。在此基础上，利用ITILs的“离子对效应”对水溶性ITILs的亲水性进行调控，以实现进一步的吸附增效。具体措施包括：探索ITILs“离子对形成常数”的变化规律；讨论ITILs离子缔合行为和ITILs疏水性之间的内在联系；筛选出与ITILs阳离子强缔合的阴离子类型。通过外加此类阴离子盐，促进水溶液中ITILs“离子对”的形成，进而实现水溶性ITILs在生物炭的整体吸附增效。

本项目研究发现，水溶性离子液体在生物炭上的吸附增效机制主要取决生物炭的结构性质和离子液体阴/阳离子的缔合能力。提升生物炭的比表面积、微孔体积和表面亲水性，能有效提高其对水溶性离子液体的吸附能力。离子液体阴/阳离子形成的“离子对”越稳定，则越容易被生物炭吸附。因此，调控吸附体系中的阴离子类型，选用能与咪唑阳离子（或吡啶阳离子）缔合更强（离子对缔合常数KIP更大）的[BF4]、[PF6]和[NTf2]等阴离子，可帮助形成更稳定的“离子对”，从而促进水溶性离子液体逐渐向疏水性离子液体转化，进而提升水溶性离子液体的吸附能力。上述阴离子促进吸附的效果呈现[BF4]<[PF6]<[NTf2]的规律。此外，在咪唑环上引入羧基、1-甲氧乙基、1-(乙氧羰基)甲基等含氧官能团，可增强了[Xmim]阳离子与[NTf2]阴离子之间的氢键作用，从而减弱两者之间的静电缔合。密度泛函理论（DFT）计算证实，上述极性取代基上的负电性氧有助于与水分子形成氢键。强氢键可诱导离子液体离解并导致吸附被削弱。而非极性基团(丁基、烯丙基和苄基)修饰的咪唑离子液体则有着较高的辛醇-水分配系数和KIP值，因而具有较大的吸附系数。因此，在咪唑阳离子上增加非极性基团也是提升水溶性离子液体吸附的有效途径。