Removal of mercury(ii) and methylene blue from a wastewater environment with magnetic graphene oxide: adsorption kinetics, isotherms and mechanism

Abstract

To enhance the adsorption capacity and affinity of graphene oxide (GO) for heavy metals and dyes, a magnetic graphene oxide composite (MGO) was synthesized with magnetic Fe₃O₄ and graphene oxide and used to study the adsorption performance for the heavy metal Hg(II) and the dye methylene blue (MB). The adsorbents were characterized by XRD, FTIR, zeta potential, BET, SEM-EDS, magnetic properties, Raman and XPS analyses. Batch adsorption experiments were performed to evaluate the adsorption conditions and reusability. The results show that the as-prepared MGO had a much higher Langmuir surface area of 1259.9 m² g⁻¹. The kinetic data well fitted the pseudo-second order and intra-particle diffusion equations. The adsorption isotherm of MGO for Hg(II) and MB were best described by the Langmuir model with a maximum adsorption capacity of 71.3 and 306.5 mg g⁻¹, respectively, which surpassed the adsorption capacities of many other materials. And this result was also much higher than the adsorption capacity of GO for Hg(II) and MB of only 32.7 and 216.7 mg g⁻¹. Overall the adsorption processes of Hg(II) and MB onto MGO were controlled by intra-particle diffusion and involved some chemisorption. The thermodynamics indicated that the adsorption process of MGO for Hg(II) and MB was endothermic and spontaneous in nature. Moreover, the adsorption capacity of MGO was still over 80% of the initial saturation adsorption capacity after being used five times. Lastly, we found out that the as-synthesized MGO was efficient for Hg(II) removal from real chloralkali wastewater. Results of this work suggest that the magnetic GO nanoparticles may be a promising adsorbent for the adsorption of heavy metals and dyes from wastewater.