A series of zirconium-based metal–organic frameworks (Zr-based MOFs) (UiO-66, UiO-66-NH₂, UiO-66-OH, UiO-66-(OH)₂ and UiO-67) with different surface charge properties and geometric dimensions were tested to decrease the concentration of graphene oxide (GO) nanosheets in aqueous solutions. Based on the experimental results and density functional theory (DFT) calculations, UiO-67 showed the highest adsorption capacity of all Zr-based MOFs studied herein. The π–π interaction/stacking, hydrogen bonding and Lewis acid–base interactions were the main cause for the removal of GO by negatively charged UiO-66-OH and UiO-66-(OH)₂. Electrostatic attractions governed the association between GO and positively charged Zr-based MOFs (UiO-66, UiO-66-NH₂ and UiO-67) via heteroaggregation. All adsorption and desorption isotherms of GO on UiO-66, UiO-66-NH₂ or UiO-67 followed the linear model, and the obvious intercept (27–51 mg g⁻¹) between adsorption–desorption isotherms disclosed that the GO adsorption over these MOFs was irreversible. This irreversible phenomenon was associated with a type of specific sheet–particle configuration, in which the particles of Zr-based MOFs were wrapped by GO nanosheets to form multilayered GO–MOF heteroaggregates with high geometric stability. The DFT calculations showed that the most stable adsorption structures were the geometries with the para-site of the linked ligand. Given the low-cost and simple preparation of Zr-based MOFs, it is clear that Zr-based MOFs could potentially act as coagulants for the efficient elimination of GO from aqueous solutions. This experimental evidence provides valuable information for the understanding of the interaction between GO and coagulants, and the potential fate, toxicity and migration of GO under natural conditions in aquatic environments, as well as in soils and sediments.

中文翻译：

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氧化石墨烯在Zr基金属-有机骨架上的异质聚集行为：组合的实验和理论研究

一系列具有不同表面的锆基金属有机骨架（Zr基MOF）（UiO-66，UiO-66-NH ₂，UiO-66-OH，UiO-66-（OH）₂和UiO-67）测试了电荷性质和几何尺寸以降低水溶液中氧化石墨烯（GO）纳米片的浓度。根据实验结果和密度泛函理论（DFT）计算，UiO-67在本文研究的所有Zr基MOF中显示出最高的吸附容量。π-π相互作用/堆积，氢键和路易斯酸碱相互作用是带负电荷的UiO-66-OH和UiO-66-（OH）₂去除GO的主要原因。静电引力控制着GO和带正电的Zr基MOF（UiO-66，UiO-66-NH ₂和UIO-67）经由杂凝聚。U在UiO-66，UiO-66-NH ₂或UiO-67上的所有吸附和解吸等温线均遵循线性模型，吸附-解吸等温线之间明显的截距（27-51 mg g ^-1）表明GO吸附这些MOF是不可逆的。这种不可逆的现象与一种特定的片-颗粒结构有关，其中基于Zr的MOF颗粒被GO纳米片包裹，形成了具有高几何稳定性的多层GO-MOF杂聚集体。该理论计算表明，最稳定的吸附结构均与几何形状对的配体的位点。鉴于基于Zr的MOF的低成本和简单制备，很明显，基于Zr的MOF可以潜在地充当凝结剂，以从水溶液中有效消除GO。该实验证据为了解GO和混凝剂之间的相互作用以及在自然条件下在水生环境以及土壤和沉积物中GO的潜在命运，毒性和迁移提供了有价值的信息。