Adsorption of silver from aqueous solution onto pre-treated bentonite clay: complete batch system evaluation

Abstract

In this work, thermally modified bentonite clay (Verde-lodo) was used for batch adsorption of silver from aqueous solution. The adsorption rate was evaluated by a kinetics study based on the following models: pseudo-first order, pseudo-second order, intra-particle diffusion, Mass Transfer in External Film and Boyd model. Among these models, the Mass Transfer in External Film model presented better agreements between experimental and calculated data, revealing that the process is mainly controlled by external transport. The equilibrium isotherms were obtained at four different temperatures (283, 293, 313 and 333 K) and were adjusted by three equilibrium models: Langmuir, Freundlich and Dubinin–Radushkevich. According to the correlation coefficient and Relative Standard Deviation, Langmuir model has presented the best adjustment results. The maximum adsorption capacities verified for Verde-lodo clay at 283 K and 293 K were 61.48 mg g−1 and 55.55 mg g−1, respectively. Furthermore, the increasing trend of adsorption capacity for higher equilibrium temperatures indicates an exothermic behaviour for this process. This fact was also verified by the evaluation of thermodynamic parameters, which has concluded that the process is spontaneous and governed by physical adsorption. In order to bring scientific advances related to silver recovery from wastewater, batch desorption essays were performed using different eluents. Among the components, nitric acid presented a high desorption capacity and was selected for future experiments. The characterization techniques of regenerated clay revealed that the desorption process do not significantly affect the thermal stability and chemical and crystalline structure of Verde-lodo clay. However, some deformities were verified on the adsorbent's surface by the Scanning Electron Microscopy.