What governs the nature of fouling in forward osmosis (FO) and reverse osmosis (RO)? A molecular dynamics study

Membrane fouling is a performance hampering phenomenon, which impacts forward osmosis (FO) and reverse osmosis (RO) differently. Experiments have found that the fouling layer structure for FO and RO is very different, but the reasons are not yet very clear, and hence a mechanistic understanding of the fouling in FO and RO is indispensable. Here we used molecular dynamics (MD) simulations to characterize the nature of fouling in FO and RO. Lysozyme and layered graphene oxide (GO) were used as typical representatives of the model foulant and desalination membrane, respectively. It was found that protein-solvent and protein-ion interactions are at the core of the structural differences between RO and FO. In particular, we suggest hydration repulsion and charge screening as probable mechanisms, which lead to different fouling layer structures in FO and RO. Also, a probable mechanism of lysozyme adsorption on the GO surface is proposed, which is based on the transport of protein towards the surface due to hydraulic pressure and Coulombic interactions induced between basic residues of lysozyme (arginine, lysine) facing the surface and oxygen-rich functional groups present on the GO surface. Both hydraulic pressure and Coulombic interactions acted synergistically, which led to lysozyme adsorption on the GO surface. Furthermore, the effect of initial protein orientation on protein-membrane interaction was also explored and was found to be an important factor in determining the nature of interaction and the time scale within which an adsorption event could be observed. This study facilitates the current understanding of the fouling in FO and RO and provides a probable molecular mechanism of how fundamental forces such as hydration repulsion and electrostatic interaction make fouling structurally different in FO and RO.