Synthesis and application of bimetallic ZIF-11 as an adsorbent for tetracycline: understanding the performance-enhancing role of cobalt in the framework

In this study, bimetallic zeolitic imidazolate frameworks with a ZIF-11 structure with different molar ratios (Zn/Co: 3, 2, 1, 0.5, 0.33) were synthesized. Then, tetracycline (TC) adsorption was examined and compared between bimetallic and monometallic samples. Techniques such as XRD, FTIR, FESEM, EDS, and BET were used to determine their characteristics. A slight shift in peak position on XRD patterns for samples with Zn/Co ratios of 1, 2, and 3 was observed, resulting from the introduction of cobalt instead of zinc. FESEM images show that the bimetallic samples, like the monometallic ZIF-11, have a characteristic dodecahedral crystal structure. According to the obtained results and especially in the comparison between single and bimetallic samples, ZIF-11 (Zn/Co:3) appears to have the greatest surface area of 580.4 m2.g− 1. Based on the findings, ZIF-11 (Zn/Co:3) is identified as the best synthetic sample, bearing a 95.1% adsorption efficiency for TC (5 ppm) in 60 min at pH 7, along with a maximum adsorption capacity of 294.1 mg/g. The adsorption efficiency with the bimetallic sample (ZIF-11 (Zn/Co:3)) was about 2.2 times that of the monometallic sample (ZIF-11), which indicated that the addition of cobalt greatly improved the adsorption capacity, probably due to the large supply of metal sites provided by cobalt. According to the pHPZC of ZIF-11 (Zn/Co:3) equal to 7.3, the adsorption process was investigated at normal pH. The best kinetic and isotherm models for TC adsorption on ZIF-11 (Zn/Co:3) are Elovich (R2 = 0.97) and Langmuir (R2 = 0.99), respectively. Adsorption mechanisms of ZIF-11(Zn/Co:3) include π-π interactions and hydrogen bonding. The structural properties of the ZIF-11 bimetallic mixture have been improved compared to its monometallic type, which has subsequently been effective in its performance in the adsorption process.