Thermal synthesis of rGO and rGO-Co3O4 and their application as adsorbents for anionic dye removal

A composite material composed of thermally reduced graphene oxide (rGO) and nanostructured cobalt oxide (Co3O4) was prepared and used as an adsorbent for the anionic dyes Indosol Blue and Indigo Carmine. A thermally reduced rGO was also prepared for comparison. Surface areas for rGO and for rGO-Co3O4 were approximately 246 and 92 m(2)/g, respectively. Despite the decrease in surface area due to the incorporation of the metal oxide, the adsorbent performance was significantly improved. The point of zero charge indicated that the best pH for anionic dye adsorption was acidic, and the adsorption experiments were carried out at pH 3.5. Due to better initial results, Indosol Blue was used for the subsequent, more detailed adsorption studies. Elovich, pseudo second-order, pseudo-first order, and intraparticle diffusion models were used to fit the adsorption kinetic data. The adsorption isotherms at 300, 308, and 318 K were then investigated. A liquid phase BET model was used to fit the equilibrium data, suggesting that adsorption occurs in multiple layers - although, as expected, adsorption on the first layer is favored. The results demonstrate that anionic dye adsorption by the composite is favored in the relatively high concentrations and low pH values studied here.