Synthesis and utilization of zinc-based metal-organic framework from spent batteries and polyethylene terephthalate plastic waste

Recently, the synthesis of metal-organic frameworks (MOFs) using recycled materials has become increasingly attractive to many researchers because it can be a strategy to support a circular economic system. In this work, Zn-based MOFs were synthesized utilizing zinc extracted from the electrode of spent alkaline batteries, along with the terephthalic acid (TPA) organic ligand derived from the acid hydrolysis of polyethylene terephthalate (PET) plastic waste. The investigation involved two different synthesis approaches, namely the one-pot synthesis and the two-step synthesis. The one-pot synthesis was performed by using the zinc acid-leached solution for depolymerizing PET and also for synthesizing MOF in one pot. The depolymerization reaction condition was first conducted at 130 °C for 6 h at the metal: ligand (M:L) mole ratio of 3:1. After 6h, the pH was adjusted at three different pH levels (not adjusted, 5, and 6) and continued heating for 12 h to synthesize the MOF. In the two-step synthesis, it involved adding the zinc leached solution after PET has been depolymerized at 130 °C for 12 hours. The solution was then adjusted to five different pH levels (not adjusted, 4, 6, 9, and 12) and continued heating for 12h to produce the MOF. All synthesized MOF Zn-TPA products were characterized using a Fourier-transform infrared spectrometer, powder X-ray diffraction, scanning electron microscopy, and a zeta potential analyzer, and were subsequently evaluated for their antibiotic adsorption applications. The synthetic antibiotic of the quinolone class, enrofloxacin (ENR), was employed to represent an accumulated antibiotic in the environment that can contribute to bacterial antibiotic resistance. In the adsorption study, various parameters such as adsorption kinetics, adsorption isotherms, the influence of pH, and interference of salts were investigated. The results revealed that the two-step synthesis products exhibited a higher adsorption capacity compared to the one-pot products. The MOF Zn-TPA exhibited efficient ENR absorption at pH 8 and 10 but decreased at pH 6. At pH 4 and 12, explicit adsorption was not observed. Among all MOF Zn-TPA products, the MOF Zn-TPA synthesized using the two-step method at pH 9 displayed characteristic MOF 69-c, demonstrating a high adsorption capacity across a wide pH range (6-10) and stability in moisture, making it an optimized Zn-TPA MOF. When tested for ENR adsorption in canal water, the two-step synthesized products maintained a high adsorption capacity, in contrast to the low capacity observed in simulated seawater due to the presence of high ion concentration. While our synthesized MOF was not able to regenerate, it can be reused for many cycles until its active sites are completely utilized.

近年来，利用再生材料合成金属有机框架（metal-organic frameworks, MOFs）的研究日益受到众多研究者的关注，因其可为循环经济体系的构建提供可行策略。本研究以废弃碱性电池电极中提取的锌为锌源，辅以由聚对苯二甲酸乙二醇酯（polyethylene terephthalate, PET）塑料废弃物酸解得到的对苯二甲酸（terephthalic acid, TPA）有机配体，合成了锌基MOFs。本研究采用两种不同的合成路径，即一锅法与两步法。一锅法合成中，以锌酸浸出液同时实现PET解聚与MOF合成：先在130 ℃、金属与配体（M:L）摩尔比3:1的条件下进行解聚反应6 h；6 h后将体系pH调节至三个梯度（未调节、5、6），继续加热12 h以完成MOF合成。两步法合成则为先在130 ℃下将PET解聚12 h，随后加入锌浸出液；之后将体系pH调节至五个梯度（未调节、4、6、9、12），继续加热12 h得到MOF产物。所有合成得到的Zn-TPA型MOF产物均通过傅里叶变换红外光谱仪（Fourier-transform infrared spectrometer）、粉末X射线衍射（powder X-ray diffraction）、扫描电子显微镜（scanning electron microscopy）及Zeta电位分析仪进行了表征，并进一步评估了其在抗生素吸附领域的应用潜力。本研究选用喹诺酮类合成抗生素恩诺沙星（enrofloxacin, ENR）作为模型污染物，其作为环境中累积的抗生素可诱导细菌产生抗生素耐药性。吸附实验中，系统考察了吸附动力学、吸附等温线、pH影响及盐离子干扰等多项参数。研究结果表明，两步法合成的产物相较于一锅法产物具备更高的吸附容量。Zn-TPA型MOF在pH 8与10条件下展现出高效的ENR吸附性能，但在pH 6时吸附能力下降；而在pH 4与12条件下未观察到明显吸附。在所有Zn-TPA型MOF产物中，采用两步法在pH 9条件下合成的Zn-TPA MOF具有MOF 69-c的特征晶型，其在宽pH范围（6~10）内均具备较高吸附容量，且在潮湿环境中稳定性良好，为最优的Zn-TPA MOF产物。在运河水中进行ENR吸附测试时，两步法合成的产物仍保持较高的吸附容量；而由于模拟海水中离子浓度较高，该类产物的吸附容量显著降低。尽管本研究合成的MOF无法实现再生，但其活性位点被完全消耗前可多次循环使用。