Efficient recycle of palladium from wastewater by modified fumed nano-silica composites: Synthesis, adsorption, kinetics and mechanisms

In this paper, two adsorbents based on hydrophilic and hydrophobic fumed silica carriers for efficient recovery of palladium were prepared using 1,4,7,10-tetraazacyclododecane as a functional group. These functional adsorbents were characterized by FT-IR, SEM and BET. The effects of pH (2-6), contact time (1-480   min), and temperature (25-55℃) on the adsorption performance were investigated. The concentration of palladium(II) after adsorption was determined by flame atomic absorption spectrometry and the data were analyzed by kinetic, thermodynamic and isotherm fitting. At pH = 4, the adsorption conformed to Pseudo-second-order kinetic model, with rapid adsorption within 2   h and able to reach equilibrium within 5   h. Meanwhile, the Langmuir model was able to better describe the isotherm data, with the maximum adsorption exceeding 140   mg·g -1 . Moreover, the thermodynamic analysis indicated that the adsorption was a heat-absorbing spontaneous process. The combination of XPS and DFT calculations elucidated the adsorption and desorption mechanisms, emphasized the potential of these adsorbents for the recovery of Pd(II) from aqueous solutions, and contributed to the development of sustainable and effective adsorption technologies.