Mechanistic aspects for the removal of Congo red dye from aqueous media through adsorption over N-doped graphene oxide nanoadsorbents prepared from graphite flakes and powders

Abstract

This work reports the mechanistic models responsible for the adsorptive removal of Congo red (CR) dye molecules by nitrogen-doped graphene oxide nanosheets (NGO) prepared via a one-pot modified Hummer's method using either graphite powder (to form NGOp) or graphite flakes (to form NGOf) as starting materials. The microstructural properties, surface chemistry and thermal stability of the produced NGO adsorbents were ascertained using a range of characterization techniques. The effects of adsorbent dosage, CR dye initial concentration, contact time and solution pH on the removal of CR dye were studied. The NGO adsorption process was highly dependent on initial dye concentration, contact time, adsorbent dosage, pH, NGO surface area and nanosheet size. The NGOp nanoadsorbents exhibited excellent selectivity towards the removal of CR dye molecules than NGOf. NGOp adsorbents achieved maximum removal efficiency of 98–99% at a pH of 2 compared to NGOf that achieved 96–98%. The adsorption kinetics and isotherms for CR dye on the NGO adsorbents were well fitted to the pseudo second-order kinetic model. The Langmuir isotherm model indicated that the adsorption behavior occurred via chemisorption and in a homogeneous monolayer on the surface of NGO. Our findings suggest that the adsorption mechanism involved was due to electrostatic interactions between the negatively charged (oxygen-containing) and positively charged (nitrogen-containing) groups on the NGO adsorbents and the positively charged (amino and cationic azo-linkages) and negatively charged (sulfonic (SO3−)) groups on the anionic CR dye molecules. The electrostatic interactions between oxygen and nitrogen containing functional groups present on the NGO surface and functional moieties of CR molecules are necessary for the enhancement of CR adsorption over NGO surface (based on the existing pH dependence). NGOp nanosheets have shown to have higher removal potential and adsorption capacity toward CR dye compared to undoped GO and NGOf adsorbents. In particular, this work indicates that NGOp can serve as good adsorbents in the removal of anionic dyes in wastewater treatment processes.