Adsorption behaviors and mechanisms of Cu²⁺, Zn²⁺ and Pb²⁺ by magnetically modified lignite

The study aims to solve the problems of limited capacity and difficult recovery of lignite to adsort Cu²⁺, Zn²⁺ and Pb²⁺ in acid mine wastewater (AMD). Magnetically modified lignite (MML) was prepared by the chemical co-precipitation method. Static beaker experiments and dynamic continuous column experiments were set up to explore the adsorption properties of Cu²⁺, Zn²⁺ and Pb²⁺ by lignite and MML. Lignite and MML before and after the adsorption of heavy metal ions were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Fourier transform infrared spectrometer (FTIR). Meanwhile, the adsorption mechanisms of Cu²⁺, Zn²⁺ and Pb²⁺ by lignite and MML were revealed by combining the adsorption isotherm model and the adsorption kinetics model. The results showed that the pH, adsorbent dosage, temperature, initial concentration of heavy metal ions, and contact time had an influence on the adsorption of Cu²⁺, Zn²⁺ and Pb²⁺ by lignite and MML, and the adsorption processes were more in line with the Langmuir model. The adsorption kinetics experiments showed that the adsorption processes were jointly controlled by multiple adsorption stages. The adsorption of heavy metal ions by lignite obeyed the Quasi first-order kinetic model, while the adsorption of MML was chemisorption that obeyed the Quasi second-order kinetic model. The negative ΔG and positive ΔH of Cu²⁺ and Zn²⁺ indicated the spontaneous and endothermic nature reaction, while the negative ΔH of Pb²⁺ indicated the exothermic nature reaction. The dynamic continuous column experiments showed that the average removal rates of Cu²⁺, Zn²⁺ and Pb²⁺ by lignite were 78.00, 76.97 and 78.65%, respectively, and those of heavy metal ions by MML were 82.83, 81.57 and 83.50%, respectively. Compared with lignite, the adsorption effect of MML was better. As shown by SEM, XRD and FTIR tests, Fe₃O₄ was successfully loaded on the surface of lignite during the magnetic modification, which made the surface morphology of lignite coarser. Lignite and MML removed Cu²⁺, Zn²⁺ and Pb²⁺ from AMD in different forms. In addition, the adsorption process of MML is related to the O-H stretching vibration of carboxylic acid ions and the Fe-O stretching vibration of Fe₃O₄ particles.