Effects of expanded graphite’s structural and elemental characteristics on its oil and heavy metal sorption properties

Abstract

Expanded graphite has promising potential environmental applications due to its porous structure and oleophilic nature, which allow it to absorb large quantities of oil. The material is produced by intercalating graphite and applying heat to convert the intercalant into gas to cause expansion between the layers in the graphite. Using different intercalants and temperature conditions results in varying properties of expanded graphite. This work has proven that the sorption properties of commercial expanded graphite differ significantly due to the material’s structural and elemental characteristics, which can be attributed to the intercalation method. This resulted in various degrees of exfoliation of the graphite and possible functionalisation of the graphene sheets within the structure. This affected the material's sorption capacity and its affinity for heavy metal sorption by incorporating selectivity towards the sorption of certain metals. It was found that sample EG3, which underwent a less harsh expansion, exhibited lower porosity than EG1, and thus, the sample absorbed less oil at 37.29 g/g compared to the more expanded samples EG1 and EG2 with 55.16 g/g and 48.82 g/g, respectively. However, it was able to entrap a wider variety of metal particles compared to EG1 and EG2, possibly due to its smaller cavities allowing for a capillary effect between the graphene sheets and greater Van der Waals forces. A second possibility is that ionic or coordination complexes could form with certain metals due to the possible functionalisation of the expanded graphite during the intercalation process. This would be in addition to coordination between the metals and expanded graphite carbon atoms. The findings suggest that there is evidence of functionalisation as determined by XRD and elemental analyses. However, further investigation is necessary to confirm this hypothesis. The findings in this work suggest that the first mechanism of sorption was more likely to be related to the degree of expansion of the expanded graphite. Various metals are present in used oil, and their removal can be challenging. Some metals in oil are not considered heavy since they have a relatively low density but can be associated with heavy metals in terms of toxicity.