MXene/polyaniline/sodium alginate composite gel: Adsorption and regeneration studies and application in Cu(II) and Hg(II) removal

The imperative to efficiently remove heavy metals from water has heightened the demand for adsorbents capable of co-adsorbing heavy metals and offering robust reusability. A novel composite adsorbent (PANI@SA-SNM) was synthesized with amine/sulfur-modified MXene, in-situ polymerized polyaniline (PANI) and sodium alginate (SA). Cu(II) and Hg(II) were used as model pollutants to test the adsorption capacity and stability of the adsorbent. Results showed that this adsorbent illustrated remarkable stability and adaptability across a pH range of 3 to 7 due to the proton transfer effect in PANI. The pseudo-second-order kinetic and Langmuir model were suitable for describing the physical and chemical adsorption process of Cu(II) and Hg(II). Thermodynamic analysis confirmed the adsorption process to be spontaneous and endothermic. The maximum adsorption capacity calculated by the Langmuir model was 255.81 mg g−1 for Cu(II) and 352.76 mg g−1 for Hg(II) (C0 = 50 ∼ 1000 mg L−1, pH = 4, Dos = 0.02 g/50 mL and T = 303.15 K), outperforming most of the reported adsorbents. In the study of binary systems, we found that the adsorbent exhibited effective co-adsorption capacity, and the differential competitive relationship between two ions was regulated by complex factors. Detailed characterizations revealed the adsorption mechanism, which involved physical adsorption, electrostatic attraction, ion exchange, complexation, and redox reaction. Moreover, the adsorbent maintained high desorption and adsorption efficiency for both ions over eight cycles thanks to strong protonation, Ca2+'s chelation and PANI's reversibility. This research not only provides a new method for improving gel reusability, but also offers insights for ions co-adsorption study.