Adsorption of phenol and chromium (VI) pollutants in wastewater using exfoliated graphite

Abstract

Phenol and Cr(VI) are two of the most common organic and heavy metal-based pollutants found in industrial effluents. Both pollutants pose considerable health risks if left untreated. Activated carbon adsorption is generally used for the physical removal of these types of pollutants during wastewater treatment. This work investigates the use of thermally exfoliated graphite as an alternative adsorbent material for the removal of phenol and Cr(VI) in wastewater. The well-developed surface pore structure and high adsorption capacities reported in literature make this material an ideal candidate for investigation. The effectiveness of the exfoliation process was characterised using x-ray diffraction whilst the particle morphology, surface structure and adsorption surface area were determined using scanning electron microscopy and Brunauer-Emmett-Teller (BET) specific surface area measurements. Upon expansion, the particle morphology of expandable graphite changed from flakes to worm-like, accordion structures. This change was accompanied by an increase in BET surface area from 2.4 to 22.4 g/m2. Batch experiments using simulated wastewater revealed that expandable graphite had negligible adsorption affinity towards both pollutants. However, exfoliated graphite had adsorption capacities of 0.73 mg/g and 0.55 mg/g for Cr(VI) and phenol. The equilibrium adsorption isotherms for both pollutants were best described by the Langmuir adsorption model and had adsorption constants of 0.84 and 0.32 L/mg for the two pollutants. The adsorption capacities obtained were much lower than those reported for the same pollutants when using activated carbon as an adsorbent. This is most likely due to the high BET surface areas recorded for activated carbon. Additional modification of the exfoliated graphite is required before satisfactory adsorption capacities can be achieved for large-scale wastewater treatment applications.