Hexadecylamine functionalised graphene quantum dots as suitable nano-adsorbents for phenanthrene removal from aqueous solution

Abstract

In this study, three novel hexadecylamine graphene quantum dots (hexadecyl-GQDs) with varying moieties on the surface were synthesised and characterised to examine the effect of surface functionalisation on their adsorption efficiency. Introducing hydrophobic moieties via in situ generation of an amide bond by coupling hydrophilic carboxylic acid-functionalised GQDs with hexadecylamine improved the adsorption capacity of GQDs. The AFM and Raman spectroscopy results revealed the formation of multilayered GQDs with an average diameter of 10.0, 9.8, and 7.0 nm for C₁₆-GQDs, C₁₆-NGQDs and C₁₆-SNGQDs, respectively. According to the sum of squared error (SSE) values obtained from fitting phenanthrene adsorption experimental data to two- and three-parameter models, the Sips hybrid isotherm model best described the adsorption of sorbate to the doped hexadecyl-GQDs (0.55 ≥ SSE ≤ 0.72), with R² values >0.9. Furthermore, doping of the GQDs favoured a multilayer adsorption mechanism over monolayer adsorption, enhancing adsorption capacities (K_F & K_d). Hexadecylamine sulphur and nitrogen co-doped GQDs (C₁₆-SNGQDs) gave the highest maximum adsorption capacity (q_m) of ∼1377 mg g⁻¹. Pristine GQDs interact with phenanthrene endothermically, while the adsorption of phenanthrene onto modified GQDs is spontaneously exothermic. The adsorption of phenanthrene is most affected by the presence of sulfate and phosphate anions and is best at ambient temperature and acidic pH. Co-doping with nitrogen and sulphur groups and modification of hydrophilic to hydrophobic form enhanced the physicochemical and adsorption performance of C₁₆-SNGQDs, making them potentially suitable as materials for the extraction of hydrophobic organic pollutants from water.