An anionic metal-organic framework and its r-graphene oxide composite for selective adsorption and separation of methyl blue from aqueous solution

Abstract

AA mono-metallic MOF (Mn-FM) and r-GO composite of the MOF (rGO@Mn-FM) were synthesized with simple and cost-effective method. The MOF contains Mn2+ metal centre with in situ generated ligand from the decomposition of DMF used as a reaction medium under solvothermal condition. The unit cell also contains in situ generated dimethyl ammonium (DMA) cation entrapped inside the pores of the framework. Due to the presence of DMA cation, MOF exhibits selective adsorption of anionic dye (namely methyl blue, methyl orange) through ionic interaction. Again, upon removal of DMA cation, adsorption was significantly enhanced owing to the formation of higher charge density species in the framework. Water stability of the MOF was significantly improved through composite formation with r-graphene oxide (namely rGO@Mn-FM) which in turn improved the adsorption of methyl blue as well. The adsorption efficiency could be achieved up to 84% within 40 minutes. Moreover, the composite was found to be highly sensitive and capable of adsorbing trace amount of the MeB dye (upto 1 ppm) from aqueous solution. The composite follows pseudo-second order kinetics and Langmuir isotherm model signifying monolayer chemisorption with homogenous sites. The releasing/desorption of adsorbed methyl blue from rGO@Mn-FM surface establishes the reusability of dye and subsequently the stability and potential of the adsorbent to reuse for multiple cycles.