Molecular dynamics study of water transport and reverse solute diffusion mechanisms across a polyamide membrane during forward-osmosis-driven dewatering of microalgae

Date of Publication

2017

Document Type

Master's Thesis

Degree Name

Master of Science in Physics

College

College of Science

Department/Unit

Physics

Thesis Adviser

Nelson B. Arboleda, Jr.

Defense Panel Chair

Michelle T. Natividad

Defense Panel Member

Maria Carla F. Manzano
Ramon Christina P. Eusebio
Glenn V. Alea

Abstract/Summary

Microalgae are believed to be the futures potential novel sources of biological nutrients due to their interesting physical and chemical compositions. However, the substantial existence of water in the culture of microalgae poses a major challenge for their production, specifically the dewatering procedure, which is highly energy demanding. Forward-osmosis-driven (FO) process is a promising alternative to conventional methods of dewatering microalgae and understanding the system mechanisms at the molecular scale could provide substantial insights on system optimization.

Molecular dynamics (MD) simulations were performed in this study to investigate the mechanisms of water transport and reverse solute diffusion across the polyamide membrane during forward-osmosis-driven dewatering of microalgae. A series of nanoscale MD simulations mimicking the FO-driven dewatering process were performed. Dipalmitoylphosphatidylcholine (DPPC) lipid bilayer with water molecules on both sides was modeled to represent microalgae suspension in the feed solution of the FO system. The draw solution was modeled as solutions of water molecules and ions. Varying concentrations of NaCl and MgCl2 ions were used to observe the relationship of the rate of water transport and the occurrence of reverse solute diffusion to draw solution types and concentrations and to observe other differences in the behavior of the system. Polyamide (PA) thin film composite was used and modeled as the membrane material separating the feed and draw solutions.

Results of the calculations show that no water molecules have permeated the DPPC lipid bilayer and thus, extraction of water from inside of the cell membrane has not occurred in the simulations. On the other hand, the rate of extraction of water molecules (water flux) outside of the cell membrane showed strong dependence on the type and concentration of draw solutions used in the simulations. Higher draw solution concentration led to higher water fluxes for different FO systems with NaCl and MgCl2 in the draw solution region. The observation of reverse solute diffusion also showed strong dependence on the type and concentration of draw solutions used. Higher accumulation of ions on the surface of and inside the membrane was evident at higher draw solution concentrations. It was found then that NaCl ions from the NaCl draw solutions exhibited higher ionic permeability than MgCl2 ions from the MgCl2 draw solutions. Distinct transport behavior of water and ions across the polyamide membrane was also discovered and compared with other studies which allowed the understanding of the local structure of the membrane during the simulations of FO systems. The calculated diffusion coefficients and hydration numbers of water and ions in the system were found to be helpful in providing in-depth insights on the mechanisms of water transport and reverse solute diffusion across the polyamide membrane and on the behavior of water and ions in bulk solutions.

Abstract Format

html

Language

English

Format

Electronic

Accession Number

CDTG007828

Shelf Location

Archives, The Learning Commons, 12F Henry Sy Sr. Hall

Physical Description

1 computer disc ; 4 3/4 in.

Keywords

Microalgae; Osmosis; Polyamide membranes

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