Contents lists available at ScienceDirect Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa Thermal synthesis of rGO and rGO-Co 3 O 4 and their application as adsorbents for anionic dye removal Aluiz M.A.D. Assis a , Karolyne S. da Silva a , Maysa K.S. Araújo a , Deivson C.S. Sales b , Maryanne C. Ferreira c , Ana Cláudia V. de Araújo c , Walter M. de Azevedo d , Eduardo H.L. Falcão d, * a Universidade Federal de Pernambuco, Materials Science Post-Graduate Program, Recife, 50740-560, Brazil b Universidade de Pernambuco, Polytechnic School of Pernambuco, Recife, 50720-001, Brazil c Universidade Federal Rural de Pernambuco, Unidade Acadêmica do Cabo de Santo Agostinho, Cabo de Santo Agostinho, 54518-430, Brazil d Universidade Federal de Pernambuco, Departamento de Química Fundamental, Recife, 50740-560, Brazil GRAPHICAL ABSTRACT ARTICLE INFO Keywords: Graphene Metal oxide nanoparticles Adsorption Dye removal Cobalt oxide ABSTRACT A composite material composed of thermally reduced graphene oxide (rGO) and nanostructured cobalt oxide (Co 3 O 4 ) 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-Co 3 O 4 were ap- proximately 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. 1. Introduction Graphene-based materials have been of interest to the scientific community because of their various specific functionalities and applications, such as capacitors, electrodes, batteries, sensors and so on [1,2]. Structurally, graphene is a two-dimensional material composed of honeycomb-arranged sp 2 carbons [3–7]. In the top-down method of graphene production, mechanical and/or chemical exfoliation of https://doi.org/10.1016/j.colsurfa.2020.124837 Received 16 January 2020; Received in revised form 7 April 2020; Accepted 7 April 2020 ⁎ Corresponding author. E-mail addresses: eduardo.lfalcao@ufpe.br, efalcao.ufpe@gmail.com (E.H.L. Falcão). Colloids and Surfaces A 599 (2020) 124837 Available online 30 April 2020 0927-7757/ © 2020 Elsevier B.V. All rights reserved. T