Photoinduced adsorption of Cr(VI) ions in nano‐zinc oxide and nano‐zinc oxide/polypyrrole composite

Abstract Zinc oxide‐polypyrrole composites (ZnO‐PPY) are novel materials that have attracted significant attention in recent years. These materials have the potential to remove hazardous pollutants from water effectively. Here, we explore facile synthesis, characterization, and adsorption capacity of hexavalent chromium anions, by nano‐ZnO and nano‐ZnO‐PPY composite. Cold precipitation of ZnO using sodium dodecyl sulfate as an additive prevents nanoparticle growth, which permits the composite preparation with PPY. The results demonstrated that the ZnO‐PPY structure was different than ZnO, more amorphous, less porous, and with a different morphology. New Fourier transform infrared spectroscopy vibrations observed in ZnO‐PPY hinted an interaction with ZnO, leading to a more acidic surface. This information could point to the composite formation driven by interactions between oxygen groups and pyrrole nitrogen. Both materials performed better under light, indicating a photoinduced adsorption phenomenon favored by oxygen vacancies. The new composite demonstrated a reduced adsorption capacity compared to ZnO at neutral and slightly acidic pHs due to electrostatic repulsion of the negatively charged surface and Cr(VI) anions. At alkaline pHs, the pyrrolic nitrogen's is able to interact with partially charged Cr‐complex and to carry out a ligand interchange even in a not favorable charge environment. The ZnO‐PPY composite, outperformed the zinc oxide capacity.