Role of surface functional groups in the adsorption behavior of microcystin-LR on graphene surfaces

Biochars are good adsorbents for removing microcystin from water but the molecular interactions responsible for microcystin adsorption are not understood. In this work, adsorption behavior of microcystin-LR (MC-LR) on three model surfaces that mimic biochar (bare graphene, graphene with ammonium, and with phosphate functional groups) is studied using well-tempered metadynamics in atomistic simulations. MC-LR is found to strongly adsorb on all the three surfaces. The adsorption free energy is most favorable for the bare graphene surface. On both bare graphene and the surface with phosphate groups, MC-LR adsorbs with its ring parallel to the surface. On the surface with ammonium groups, MC-LR adsorbs with its ring tilted with respect to the surface because of favorable Coulombic interactions between the ammonium groups and the glutamic acid in the MC-LR ring. On the bare graphene surface, the phenyl ring of the pendant Adda group shows a bimodal distribution with peaks at 0° and 40° with the surface normal, indicating that the phenyl ring forms π-π interactions with graphene in some adsorbed configurations. Such π-π interactions are not observed on the surfaces with ammonium and phosphate groups. Favorable adsorption free energy of MC-LR on the charge-neutral (bare graphene), positively charged (ammonium) and negatively charged (phosphate) surfaces suggest that the adsorption is dominated by van der Waals and hydrophobic interactions. Coulombic and π-π interactions affect the orientation of MC-LR in the adsorbed state.