Mechanism and thermodynamics of adsorption of diclofenac on graphene-based nanomaterials

The widely used anti-inflammatory diclofenac (DCF) is one of the pharmaceutical drugs most frequently detected in the environment as it is not fully degraded by conventional water treatment plants. Carbon-based adsorbent nanomaterials are attractive for the treatment of wastewater contaminated by pharmaceuticals, due to their high affinity for organic pollutants, high loading capacity, and the efficient regeneration. To obtain insight at molecular level into factors affecting the adsorption of DCF by carbon nanomaterials, molecular dynamics simulations are carried out considering the pure (pG), hydroxyl- (G-OH) and epoxy- (G-COC) functionalized graphene. Also, a material presenting both functional groups is considered (GOX). DCF is adsorbed spontaneously by all materials, but with different types of interactions: pi-pi and CH-pi interactions are present for pG and G-COC, while also hydrogen bonds are formed in the case of G-OH and GOX. While DCF is adsorbed on pG it can translate, this is not the case for the other materials. Spontaneous aggregation leading to the formation to small clusters is observed in all cases when several DCF molecules are present in the system. Free energy calculations give Delta G(ads) which are close to experimental data and follow the order pG < G-COC similar to G-OH. While for pG adsorption is favored by a negative enthalpy, for the other materials is entropy-driven. We show that surface desolvation plays a major role in determining the thermodynamic affinity order for DCF adsorption. This study provides a theoretical basis for the design and optimization of adsorbent materials with high affinity for DCF.