Cobalt–Aluminum Layered Double Hydroxides And Their Hybrid Composites With Olive-Stone-Derived Biochar And Zif-8 For Dye Removal From Wastewater Via Integrated Adsorption And Photocatalysis

Abstract

For advanced wastewater treatment using integrated adsorption–photocatalysis, this study describes the preparation, structural analysis, and functional evaluation of cobalt–aluminum layered double hydroxides (Co/Al-LDHs) and their composite materials integrated with olive-stone-derived biochar (OSBC600) and ZIF-8 metal–organic frameworks. SEM-EDX, FT-IR, XRD, and TGA were used to characterize the materials, which were synthesized using hydrothermal, co-precipitation, and mechanochemical methods. These analyses confirmed a heterogeneous morphology with well-dispersed LDH nanosheets anchored on the porous biochar matrix that improved interfacial contact and stability. Initial experiments on dye removal using carmine, methylene blue, methyl orange, safranin O, and titan yellow showed that the Co/Al-LDHs@OSBC600 composite exhibited superior performance compared to Co/Al-LDHs@ZIF-8. Of the dyes examined, safranin O and titan yellow experienced the greatest removal efficiency and were therefore selected for subsequent investigations. The selected dye–adsorbent system was further optimized using a Box–Behnken experimental design, considering pH, starting dye concentration, adsorbent dosage, and contact time as the key variables. Statistical evaluation and regression analysis confirmed the robustness of the model, with the most important parameters being dye concentration and adsorbent dosage. At low pH and high adsorbent dosages, the Co/Al-LDHs@OSBC600 achieved maximal removal efficiency, repeatedly outperforming OSBC600 alone. The Freundlich and Redlich-Peterson models provided the best fit to the adsorption equilibrium data, demonstrating heterogeneous and multilayer adsorption. Electrostatic interaction with the positively charged LDH layers allowed TY to exhibit a greater adsorption capacity and a stronger affinity than SFO. According to kinetic modeling, SFO followed the pseudo-second-order model, which suggests surface-controlled uptake, whereas TY followed the Elovich model, which suggests chemisorption on heterogeneous surfaces. Lastly, Co/Al-LDH@OSBC600 showed impressive photocatalytic activity under UV light, decolorizing TY by about 97% and SFO by approximately 75% in just 10 minutes. The LDH framework and the conductivity of the biochar were shown to assist the production of reactive oxygen species and effective electron–hole separation, which were responsible for the increased activity. In summary, the hybrid Co/Al-LDH@OSBC600 composite showed itself to be a highly effective and thermally stable adsorbent–photocatalyst, providing a sustainable method for the combined removal of organic contaminants from wastewater through adsorption and photocatalysis.