Abstract In this study, a combined process of bio-inspired modification and magnetic treatment is presented for the preparation of a polydopamine (Pdop)-modified graphene (Pdop-G)-based adsorbent which incorporates ultra-small, active Fe3O4 nanoparticles (with an average size of 6.5 nm). Not only can the nanoparticles impart superparamagnetism to the modified graphene adsorbent but also enhance the adsorption performance. The ultra-small size of Fe3O4 nanoparticles allows the exposure of a high proportion of low-coordinated sites such as corners and edges. Additional active sites can thus be provided to bind methylene blue molecules, in addition to the active Pdop-G surface with catechol and amine groups which induce hydrogen bonding, electrostatic attraction, and π-π stacking interactions. The Pdop interface wraps graphene and immobilizes Fe3O4, endowing the magnetic Pdop-G (MPG) with high adsorption capacity, easy recyclability, and excellent reusability for the organic pollutant removal. In stark contrast, the counterpart without the interfacial Pdop layer suffers from severe Fe3O4 aggregation, causing its adsorption performance inferior to that of MPG. The MPG-based adsorption obeys the pseudo-second-order kinetics, and the intraparticle diffusion model also indicates the complex adsorption pathway, including the external and intraparticle mass transfer. The Langmuir isotherm can better fit the experimental data than the Freundlich isotherm, with the theoretical maximum adsorption capacities estimated to be 131.6, 140.3, and 152.0 mg/g at 30, 40, and 50 °C, respectively. The adsorption is endothermic and spontaneous, along with an increase in the randomness at the solid-solution interface. The separation factor (RL) reveals the favorable adsorption process with MPG. The superparamagnetism imparted via the Fe3O4 composition makes MPG easily recyclable. Furthermore, the removal rate can be maintained at about 90% after 5 runs of repeated usage of MPG. This study opens up a new avenue to the magnetization of adsorbents for enhancing adsorption performance in addition to imparting magnetism.

中文翻译：

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聚多巴胺界面封装石墨烯并固定超小活性 Fe3O4 纳米颗粒用于有机染料吸附

摘要 在本研究中，提出了一种仿生改性和磁处理的组合工艺，用于制备聚多巴胺 (Pdop) 改性石墨烯 (Pdop-G) 基吸附剂，该吸附剂包含超小活性 Fe3O4 纳米颗粒（具有6.5 nm 的平均尺寸）。纳米粒子不仅可以赋予改性石墨烯吸附剂超顺磁性，还可以提高吸附性能。Fe3O4 纳米颗粒的超小尺寸允许暴露高比例的低配位点，例如角和边缘。因此，除了具有邻苯二酚和胺基团的活性 Pdop-G 表面之外，还可以提供额外的活性位点来结合亚甲蓝分子，这些基团会诱导氢键、静电引力和 π-π 堆积相互作用。Pdop 界面包裹石墨烯并固定 Fe3O4，赋予磁性 Pdop-G (MPG) 高吸附能力、易回收性和良好的有机污染物去除重复使用性。与之形成鲜明对比的是，没有界面 Pdop 层的对应物遭受严重的 Fe3O4 聚集，导致其吸附性能不如 MPG。基于 MPG 的吸附服从准二级动力学，颗粒内扩散模型也表明了复杂的吸附途径，包括外部和颗粒内的传质。Langmuir 等温线比 Freundlich 等温线更符合实验数据，在 30、40 和 50 °C 下，理论最大吸附容量估计分别为 131.6、140.3 和 152.0 mg/g。吸附是吸热和自发的，随着固溶界面随机性的增加。分离因子（RL）揭示了 MPG 的有利吸附过程。通过 Fe3O4 成分赋予的超顺磁性使 MPG 易于回收。此外，MPG重复使用5次后，去除率可保持在90%左右。这项研究为吸附剂的磁化开辟了一条新途径，除了赋予磁性外，还可以提高吸附性能。