I want to put forward my research statement in advancing the fundamental knowledge and application-orientated research applied to Energy, Health-care, and Environmental. In the next five years, I would like to focus on experimental and numerical work on separation processes. My proposed research involves Separation Processes and Molecular Simulations. Molecular dynamics can be used as model candidates for understanding the behavior of chemical and biological systems. By modeling the molecular structure by the computational techniques in this kind of specific problems, the number of efforts of the synthesis would be significantly reduced. This kind of computational studies will contribute to establishing guiding principles for designing the molecular structure of the adsorbent at the micro/nanometer scale.
The simplicity, flexibility, and high adsorption capacity of nanostructured materials make it more attractive for the selective removal of toxic pollutants from industrial wastewaters. Therefore, these materials have the potential to be one of the most versatile and reliable materials for future wastewater treatment applications. The effective use and experimental demonstration of these materials for the removal of organic and inorganic pollutants from wastewater still remain a challenge. The nanostructured materials used as adsorbent materials for the removal of various aquatic pollutants, such as metal ions and other organic pollutants from aqueous solutions using molecular dynamics simulations. Nanostructured materials are a promising alternative to activated carbon and other adsorbent materials that are currently being considered for wastewater treatment. I have outlined below a few of the problems I would like to take up in the next few years.
1. Adsorption of metal ions on the nanomaterials
Past research conducted the adsorption behavior of divalent metal ions viz., Cd2+, Cu2+, Pb2+, and Hg2+ ions onto bare and functionalized CNT surfaces from aqueous solution. The armchair (6, 6) CNT is functionalized with carboxyl (-COO-), carbonyl (-CO) and hydroxyl (-OH) groups to study the metal ion adsorption. The density profiles of metal ions, which explain the ion distribution around CNTs. The number of metal ions adsorbed on the CNT surface is enhanced in the presence of a carboxylic (-COO-) functional group compared to the hydroxyl and amide functional groups (-OH and -CONH2) on the CNT surface for five different concentrations. Moreover, the functionalized CNTs shows higher residence time and lower diffusion coefficients with increasing ion concentration compared to the bare CNT. The adsorption amount of heavy metals ions on the CNT surface, with and without functionalization, is found to increase with increasing the concentration of the metal ion in an aqueous solution, which is well supported by the potential of mean force (PMF) calculations. In general, CNT with functionalization of the carboxylic group (-COO-) is a better adsorbent than other functional groups (-OH and –CONH2).
Also, I investigated the adsorption behavior of metal ions using graphene, graphene oxide, and polyamidoamine (PAMAM) dendrimer grafted graphene and graphene oxide surfaces. Explained the fundamental mechanisms governing the interactions between metal ions and different surfaces, as listed above, using molecular dynamics. The system is analyzed through the structural and dynamical properties of metal ions in the presence of different surfaces. The results show that the adsorption capacity of the metal ion is improved significantly using a dendrimer grafted graphene surface. The adsorption mechanism of metal ions on nanostructured materials is discussed with the help of microscopic interactions between metal ion and surface.
2. Study of fluid flow through nanochannels
I examined the desalination performance of nanoporous graphene (NPG) membranes for different pore sizes, chemical functionalization (hydroxyl (-OH), nitrogen (N), and fluorine (F)) of the pores and applied hydrostatic pressure. I quantify the effect of functionalization, with nitrogen, fluorine, and hydroxyl (-OH) groups, on the ability to reject Pb(NO3)2, Cu(NO3)2, Cd(NO3)2, Co(NO3)2 and Zn(NO3)2 using free-energy profiles, water permeability, and salt rejection calculations. The mechanism of water transport and ion rejection is explored using PMF calculations. Moreover, DFT calculations to corroborate the PMF results obtained using classical MD simulations. PMF results for NPG functionalized with different functional groups show significantly higher energy barriers for ions than that for a water molecule. This indicates that the water molecules pass through the pores more easily than the ions. The water flux and PMF results obtained for water using MD simulations are also in agreement with the DFT calculations. Even at high pressures like 500 MPa, the ion rejection is not less than ~90%, and the minimum permeability is ~270 L/cm2-hr-bar. These values of permeability are 4–5 orders more than the values obtained using the existing technologies with very high salt rejection. This study showed that the NPG membrane is a promising material for water purification from industrial waste.
Also, I explored the selectivity of ethanol and water mixtures inside the slit-shaped pores of hexagonal boron nitride (hBN) and graphene sheets with variable width 7 to 13 Å. Confinement of fluids in nanoscale systems gives rise to changes in the fluid-structure and properties. This has attracted considerable research interest in recent times due to its important role in the design and development of nanofluidic devices. The removal of organic pollutants is studied using the structural and dynamical behavior of ethanol-water mixtures in slit pores. However, the selectivity of ethanol is relatively higher for hBN pores compared to the graphene pores, for all the considered pore widths. The diffusion coefficients of water and ethanol molecules substantially decrease with a decrease in pore width for both graphene and hBN surfaces. The residence time of water and ethanol molecules decreases with an increase in the slit-width. The behavior of water and ethanol molecules inside the slit pores are also analyzed using PMF for water and ethanol molecules on the graphene and hBN surfaces, which are determined by the umbrella sampling technique. MD computations are employed to understand the atomic-level mechanisms of fluid flow through nanotubes and nanochannels. This provides an insight into the removal of inorganic and organic pollutants using nanostructured materials that can play a valuable role in water purification.
3. Adsorption of metal ions using the ligand and resin.
Separation of heavy metal ions from contaminated wastewater is a significant and challenging problem in the world. The high-level radioactive waste contains the Lanthanide ions such as Gadolinium (Gd3+) and Uranyl (UO2), which are toxic in nature, and thus extraction of these ions from nuclear waste is warranted. One commonly used separation method is liquid-liquid extraction, where a suitable extractant is used to bind the metal ion in the form of complexation. How the thermodynamics of metal ion binding on to the resin-grafted on the surface of soft materials in organic solvents affects the separation efficiency of metal ions using a liquid-liquid extraction process. These questions remain unaddressed, which may play a vital role in the selectivity of suitable resins and other molecules present in the solution. The selection of suitable ligand and resin for the extraction of metal present in the solution is a challenging task, which usually very costly and risky to perform the experiments at the lab scale. The recent advancement of computational power would enhance us to perform the molecular simulations for these material screening and further leading to designing.
4. Extraction of rare-earth elements using ionic liquid
The demand for green energy alternatives is booming globally, which leads to an increase in demand for rare earth elements (REEs) because they provide critical functionalities in various applications and used in key technologies such as photovoltaics, fuel cells, and wind turbines. Rare earth elements are also important in high tech applications like luminescence, electronics, magnetism, catalysis, therapeutic application, and so on. Solvent extraction is the most commonly used industrial separation technology for REEs. The solvent exaction process involves a large amount of hazardous and volatile organic solvents, which results in serious environmental concerns. Ionic liquids (ILs) can be used as an alternative for volatile organic compounds in the extraction process because of the low volatility and combustibility, having excellent thermal and radiation stability.
Despite numerous experimental studies on the exaction of REEs, the molecular level understanding is still missing. The advancement of computational technology and molecular dynamic simulation tools can provide a better understanding of the separation process at the molecular scale. This project will provide insight exaction process of REEs to develop novel extractant using ionic liquids. To do so, it is desired to understand the structural, equilibrium, and thermodynamical properties of REEs and ILs systems at the molecular scale. In this work, we would like to study of the REEs and ILs using theoretical approach starting from classical molecular dynamics methods would reveal the affinity of the ILs extractants for the separation of REEs from aqueous solution thereby providing necessary details for the appropriate selection for ionic liquids for selective extraction of rare-earth elements.