journal of environmental sciences 98 (2020) 151–160 Available online at www.sciencedirect.com w w w . e l s e v i e r . c o m / l o c a t e / j e s Simultaneous removal of Congo red and cadmium(II) from aqueous solutions using graphene oxide–silica composite as a multifunctional adsorbent Ebrahim Mahmoudi 1,∗ , Sepehr Azizkhani 2 , Abdul Wahab Mohammad 1,4 , Law Yong Ng 3 , Abdelbaki Benamor 5 , Wei Lun Ang 1,4 , Muneer Ba-Abbad 5 1 Department of Chemical and Process Engineering, Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor Darul Ehsan, Malaysia 2 Department of Chemical and Environmental Engineering, Faculty of Engineering, Universiti Putra Malaysia, 43400 Serdang, Selangor, Malaysia 3 Department of Chemical Engineering, Lee Kong Chian Faculty of Engineering and Science, Universiti Tunku Abdul Rahman, Jalan Sungai Long, Bandar Sungai Long, Cheras, 43000 Kajang, Selangor, Malaysia 4 Centre for Sustainable Process Technology (CESPRO), Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor Darul Ehsan, Malaysia 5 Gas Processing Centre, Qatar University, P.O. Box 2713, Doha, Qatar a r t i c l e i n f o Article history: Received 8 December 2019 Revised 2 May 2020 Accepted 7 May 2020 Available online 18 June 2020 Keywords: Graphene-oxide Multifunctional adsorbent Competitive removal Heavy metal -dye removal a b s t r a c t Graphene oxide is a very high capacity adsorbent due to its functional groups and π −π interactions with other compounds. Adsorption capacity of graphene oxide, however, can be further enhanced by having synergistic effects through the use of mixed-matrix com- posite. In this study, silica-decorated graphene oxide (SGO) was used as a high-efficiency adsorbent to remove Congo red (CR) and Cadmium (II) from aqueous solutions. The effects of solution initial concentration (20 to 120 mg/l), solution pH (pH 2 to 7), adsorption du- ration (0 to 140 min) and temperature (298 to 323 K) were measured in order to optimize the adsorption conditions using the SGO adsorbent. Morphological analysis indicated that the silica nanoparticles could be dispersed uniformly on the graphene oxide surfaces. The maximum capacities of adsorbent for effective removal of Cd (II) and CR were 43.45 and 333.33 mg/g based on Freundlich and Langmuir isotherms, respectively. Langmuir and Fre- undlich isotherms displayed the highest values of Q max for CR and Cd (II) adsorption in this study, which indicated monolayer adsorption of CR and multilayer adsorption of Cd (II) onto the SGO, respectively. Thermodynamic study showed that the enthalpy (H) and Gibbs free energy(G) values of the adsorption process for both pollutants were negative, suggesting that the process was spontaneous and exothermic in nature. This study showed active sites of SGO (π -π , hydroxyl, carboxyl, ketone, silane-based functional groups) contributed to an enormous enhancement in simultaneous removal of CR and Cd (II) from an aqueous solu- ∗ Corresponding author. E-mail: ebi.mahmoudi@siswa.ukm.edu.my (E. Mahmoudi). https://doi.org/10.1016/j.jes.2020.05.013 1001-0742/© 2020 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V.