Influence of flake size and electrolyte conditions on graphene oxide adsorption of ionic dyes

Two graphene oxide (GO) materials were characterised and their performance as adsorbents for common ionic dyes was investigated. The GOs were demonstrated to possess significantly different flake sizes; here termed Smaller Flake GO (SFGO) and Larger Flake GO (LFGO). Kinetics and isotherm studies using both cationic Methylene Blue (MB) and anionic Methyl Orange (MO) showed adsorption behaviour followed second order kinetics and Langmuir monolayer isotherms in all cases. For MB, adsorption capacities were 526.3 mg/g (SFGO) and 384.6 mg/g (LFGO) with interaction attributed primarily to electrostatic attraction, where the smaller, higher surface area flake led to significantly greater adsorption. In comparison for MO, adsorption was lower at 113.6 mg/g (SFGO) and 149.3 mg/g (LFGO). Here, while electrostatically repulsive, adsorption occurred via π-π interactions and hydrogen bonding, with the LFGO having more favourable surface chemistry. Both GOs were observed to aggregate significantly when dispersed in the dyes, while the introduction of sodium (Na+) or magnesium (Mg2+) sulphates adversely affected adsorption. For SFGO, MB adsorption reduced by 5.1% (Na+) and 32.4% (Mg2+). Similarly, for LFGO, MB adsorption capacity reduced by 7% (Na+) and 17.3% (Mg2+). Adsorption of Mg2+ onto vacant binding sites and charge neutralisation were the principal antagonistic mechanisms. These findings demonstrate the potential usage of commercial GO for dye removal and separation and will be of significance to industrial effluent applications.