Adsorption properties of graphene materials for pesticides: Structure effect

In recent years, the application of graphene materials to the adsorption of pesticides has attracted great research interests. However, to the best of our knowledge, there has been no report about the specific adsorption mechanism of graphene materials for pesticides with different structures. This study investigated the potential adsorption mechanism of graphene oxide (GO) and reduced graphene oxide (rGO) for three pesticides with different structures [Methomyl (Met; without aromatic rings), Acetamiprid (Ace; one aromatic ring) and Azoxystrobin (Azo, three aromatic rings)]. The operating variables including time, temperature and initial pesticide concentration were estimated and optimized with face-centered composite design (FCCD) through response surface methodology (RSM). Under optimal conditions, the calculated adsorption capacity of GO for Met, Ace and Azo was 106.22, 285.96 and 2896.84 mg/g; while that of rGO was 96.86, 357.65 and 2818.04 mg/g, respectively. The adsorption kinetics follows the pseudo second-order and Elovich models, while the adsorption isotherm can be well described by the Sips model. Thermodynamic study revealed that the adsorption process of the three pesticides onto GO or rGO was an exothermic and physical process. The adsorption was deduced to be synergistically driven by (1) the π-π conjugation between the sp2 region of adsorbent and the aromatic ring of pesticide molecules, (2) H-bonding and electrostatic interaction between heteroatoms in pesticide molecules and oxygen-containing functional groups on adsorption materials, and (3) H-bonding and π-π conjugation between pesticides. In general, this study provides important practical and theoretical implications for understanding the molecular mechanism underlying the adsorption of pollutants by GO or rGO and its practical application.