The development of antibiotics is of significant benefit to humanity. Among the existing antibiotics, tetracycline (TC) is one of the most prescribed antibacterial agents for various applications, including agriculture and human treatment. This can be partly attributed to its low production cost and broad spectrum. Unfortunately, a significant amount of TC is often directly discharged into water environment, with some being discharged into wastewater as metabolites due to insufficient bioavailability. TC accumulation in natural water not only causes serious water pollution but is also a big threat to health and environmental security. Thus, developing effective methods for treating TC-polluted wastewater is urgently required.
Although numerous methods for removing organic pollutants in wastewater have been developed, each of them has their own limitations. For example, advanced oxidation method is relatively expensive, difficult to operate and unsuitable for complete purification of wastewater despite being environment friendly. The main drawback of membrane limitation is its low separation efficiency. Unlike these methods, adsorption is commonly used to remove heavy metal ions and organic substances in sewage due to its high removal efficiency and low operation cost. It is also widely used to remove pollutants from water. Through the interaction between metal ions and organic compounds in an appropriate system, synergistic removal of metal ions and organic pollutants can be achieved.
Traditional materials like graphene oxide have been used for collaborative treatment of organic and metal ions pollutants. Compared to these materials, MXenes are two-dimensional layered nanomaterials with excellent inherent characteristics like hydrophilicity, biological compatibility and high chemical stability. Therefore, MXene is a promising adsorbent candidate for removing pollutants from wastewater. Surface functionalization or modification is an effective method for improving the performance of MXene-based adsorbents. It is important to understand whether the complex effects between MXene and heavy metal ions is synergistic or antagonistic, which is lacking. Additionally, the complex composition of industrial wastewater could affect TC removal efficiency.
On this account, Tianjin University researchers: Mr. Xuan Dao, Professor Hongxun Hao, Dr. Jingtao Bi, Ms. Shiyu Sun and Professor Xin Huang studied the adsorption behaviors of alkalized MXene (ALK-MXene) on TC in a coexistence system consisting of TC and various metal ions. The hydroxyl-rich surface ALK-MXene was synthesized using alkaline intercalation technique to facilitate the removal of TC from wastewater. The adsorption thermodynamics and correlation coefficients of the model were determined. Finally, the effects of various metal ions, like Cu(II), Cd(III) and Ni(II), on the adsorption of TC by ALK-MXene adsorbent and possible adsorption mechanisms were discussed. Their work is currently published in the journal, Industrial & Engineering Chemistry Research.
From the batch adsorption experiments, the research team showed that introducing metal ions significantly improved the TC adsorption capacity by adjusting the ALK-MXene surface with metal ions in the water environment. The alkalization process was responsible for increasing the MXene interlayer spacing while maintaining the layered structure. Consequently, the transformation of surface -F to -OH was observed. As a result, a 750% increase in the removal capacity when coexisting with Ni(II) was obtained. The significant increase in TC adsorption was attributed to three main reasons: adsorption at the hydroxyl site on the ALK-MXene surface, surface modification by metal ions and further improvement by complexation.
In summary, the new study demonstrated that surface complexation enhanced TC adsorption via ALKMXene. The adsorption mechanism was investigated using three different adsorption isotherm models: Freundlich model, Langmuir model and monolayer model. The fitting calculation results implied that the adsorption followed the Freundlich model. To realize synergistic removal of various contaminants in wastewater, it was advisable to utilize special components and highly active surfaces. In a statement to Advances in Engineering, Professor Xin Huang opinioned that their findings would provide the much-needed theoretical guidance for synergistic removal of inorganic ions and organic pollutants from wastewater.
Dao, X., Hao, H., Bi, J., Sun, S., & Huang, X. (2022). Surface Complexation Enhanced Adsorption of Tetracycline by ALK-MXene. Industrial & Engineering Chemistry Research, 61(17), 6028-6036.