(393j) Polyoxometalate-Based Membranes for Water Purification: A Molecular Dynamics Study

Rising population and pollution levels have spurred an increased need for access to clean water across the globe. The inadequate access to clean water has led to a high risk of tumors, congenital disorders, and annual economic losses. Polyoxometalate-based porous material shows great promise as filtration membranes for removing heavy metal ions and other toxic and hazardous chemicals from the contaminated water Polyoxometalates are anionic nanoclusters composed of oxo ligands and early transition metals (V, Mo, W or Ta) in their highest oxidation state. Besides, they have shown a wide range of applications in the field of medicine, catalytic converters, electromagnetism, and energy storage materials. Motivated by the excellent efficacy demonstrated by polyoxometalate membranes in size-based separation of cationic, anionic and neutral dyes by our experimental collaborators, in this study, we use non-equilibrium classical molecular dynamics to study the permeation of water and industrial dyes through the system composed of polyoxometalate anion [P8W48O184]^40− and tetraheptyl ammonium counterions (C7H15). The polyoxometalate membrane is held fixed to allow greater degree of freedom for contaminants and water molecules. The study involves the use of ESP(CHELPG) and AM1-BCC partial charges generated separately using DFT calculations for polyoxometalate and the organic entities in the system respectively. The non-bonded interactions between the membrane, counterions, water molecules and model contaminants are modeled using the Lennard-Jones potential and Coulombic interactions. Two different configurations of the polyoxometalate clusters are considered, namely, ordered (periodic arrangement of pores with pore features modeled to reproduce experimental observations) and disordered (where pores are placed randomly). The study elucidates mechanisms of water permeation through the polyoxometalate membrane at different pressure ranges and calculates the variation in water flux as a function of pressure. Furthermore, rejection of toxic contaminants such as common organic dyes are studied, and the removal efficiency are correlated with the pore and contaminant size and the nature of interactions with the polyoxometalate membrane. Detailed structural (via radial distribution functions) and transport (via diffusivities) properties of the contaminant and water are analyzed and the efficacy of polyoxometalate membranes in water treatment are established.