Methyl orange adsorption from aqueous solutions on 3D hierarchical PbS/ZnO microspheres

In this study, three-dimensional (3D) hierarchical PbS/ZnO heterojunction microspheres were successfully fabricated by combining a facile hydrothermal method and an improved successive ionic-layer adsorption and reaction (SILAR). As an adsorbent for the first time, the adsorption activities for the methyl orange (MO) of the prepared 3D hierarchical PbS/ZnO heterojunction microspheres were investigated. By optimizing the loading of PbS nanoparticles (NPs), the 3D hierarchical PbS/ZnO heterojunction microspheres show enhanced adsorption ability for MO with a high removal rate of more than 91% and a maximum adsorption capacity of 159 mg g^-1 at equilibrium time within less than 40 min. The kinetics, thermodynamics, and isotherm investigations for the adsorption process of MO over the 3D hierarchical PbS/ZnO heterojunction microspheres experimentally demonstrate that the adsorption process follows a pseudo-second-order model with the nature of spontaneous and endothermic and is well fitted with the normal Langmuir isotherm model. Based on the comparative investigations and the experimental results of kinetics, thermodynamics, and isotherm studies, a reasonable adsorption mechanism of MO over the 3D hierarchical PbS/ZnO heterojunction microspheres was proposed. This work not only describes a new semiconductor heterojunction adsorbent but also offers a new idea for the fabrication of other semiconductor heterojunction adsorbents.