A method for the simultaneous determination of transport and structural parameters of forward osmosis membranes

We present a simple and rapid methodology to characterize the water and solute permeability coefficients (A and B, respectively) and structural parameter (S) of forward osmosis (FO) membranes. The methodology comprises a single FO experiment divided into four stages, each using a different concentration of draw solution. The experimental water and reverse salt fluxes measured in each stage are fitted to the corresponding FO transport equations by performing a least-squares non-linear regression, using A, B, and S as regression parameters. Hand-cast thin-film composite (TFC) FO membranes and commercial TFC FO, TFC reverse osmosis (RO), and cellulose acetate-based asymmetric FO membranes are evaluated following this protocol. We compare the membrane properties obtained with our FO-based methodology with those derived from existing protocols based on an RO experiment followed by an FO experiment. For all membranes, the FO-based protocol gives more accurate predictions of the water and salt fluxes than the existing method. The numerical robustness of the method and the sensitivity of the regression parameters to random errors in the measured quantities are thoroughly analyzed. The assessment shows that confidence in the accuracy of the determined membrane parameters can be enhanced by simultaneously achieving close fitting of the predicted fluxes to experimental measurements (i.e., high R2 values) and constant water to salt flux ratios in each stage. Additionally, the existing and proposed approaches yield consistently dissimilar results for some of the analyzed membranes, indicating a discrepancy that might be attributed to the different driving forces utilized in RO and in FO that should be further investigated

Desalination and Reuse of High-Salinity Shale Gas Produced Water: Drivers, Technologies, and Future Directions

In the rapidly developing shale gas industry, managing produced water is a major challenge for maintaining the profitability of shale gas extraction while protecting public health and the environment. We review the current state of practice for produced water management across the United States and discuss the interrelated regulatory, infrastructure, and economic drivers for produced water reuse. Within this framework, we examine the Marcellus shale play, a region in the eastern United States where produced water is currently reused without desalination. In the Marcellus region, and in other shale plays worldwide with similar constraints, contraction of current reuse opportunities within the shale gas industry and growing restrictions on produced water disposal will provide strong incentives for produced water desalination for reuse outside the industry. The most challenging scenarios for the selection of desalination for reuse over other management strategies will be those involving high-salinity produced water, which must be desalinated with thermal separation processes. We explore desalination technologies for treatment of high-salinity shale gas produced water, and we critically review mechanical vapor compression (MVC), membrane distillation (MD), and forward osmosis (FO) as the technologies best suited for desalination of high-salinity produced water for reuse outside the shale gas industry. The advantages and challenges of applying MVC, MD, and FO technologies to produced water desalination are discussed, and directions for future research and development are identified. We find that desalination for reuse of produced water is technically feasible and can be economically relevant. However, because produced water management is primarily an economic decision, expanding desalination for reuse is dependent on process and material improvements to reduce capital and operating costs

COMPUTATIONAL STUDIES OF CONFINED, INTERFACIAL AND HYDRATION WATER

In this dissertation, we use molecular simulation to investigate systems in which symmetry breaking, due to an interface or confining geometry, modifies the structural, dynamic and thermodynamic properties of water. We focus on two types of aqueous systems: confined and interfacial water films; and hydrophobic solvation. Throughout, the common denominator is the existence of a hydration layer, a three-dimensional (3D) region within which water's properties depart from those of the bulk liquid. The objective of this study is twofold: (i) to characterize water's structural and dynamic properties within this layer; (ii) to investigate the effect of water's hydration layer thermodynamics on conformational transitions of a hydrophobic, protein-like oligomer. In the first part of the dissertation, we use molecular dynamics (MD) simulations to investigate confined and interfacial water. The former category includes systems in which two solid surfaces confine a water film to a nanoscopic geometry; in the latter, an adsorbed film of water exists between solid-liquid and liquid-vapor interfaces. We first study the effect of surface polarity on confined water's translational and rotational dynamics. Our results show that water dynamic properties exhibit a non-monotonic dependence on surface polarity, a phenomenon explained by the different water structures observed on polar and apolar interfaces. Next, considering hydrophilic surfaces, we investigate the effect of confinement length scale (i.e., inter-surface separation, d) on confined water's molecular dynamics. We show that translational dynamics are surface-dominated (hence, slower than in the bulk) within ~1 nm from the nearest interface, while rotational dynamics exhibit slowing down within ~0.5 nm from the interface. We also find that, for d >= 1.0 nm, both the local in-plane diffusion coefficient and translational relaxation time collapse onto d-independent curves. Next, we investigate the properties of interfacial water on hydrophilic substrates. We evaluate the effect of water-surface (W-S) and water-water (W-W) interactions on film molecular structure, finding that W-S interactions determine film structure in 1-monolayer (ML) films. W-W interactions become equally important in thicker films, but without disrupting W-S interactions. Interface-induced modifications to the water structure propagate throughout films <= 4 ML in thickness. In the second part of the dissertation, we study oligomer conformational stability using a 3D lattice model in explicit water-like solvent, numerically solved with flat-histogram Monte Carlo simulation. The model incorporates the entropic penalty and enthalpic bonus that characterize water-water interactions in the hydrophobic hydration layer. We first focus on the effect of density and temperature on the stability of a flexible hydrophobic oligomer. We show that the model qualitatively reproduces features of protein systems, including cold, thermal and high-density unfolding (a phenomenon akin to pressure unfolding). Next, we exploit the 3D nature of the model to incorporate elements of secondary structure into the oligomer Hamiltonian. We show that a minimalist model incorporating meaningful inter-monomer energetics and the thermodynamics of hydrophobic solvation suffices to describe the thermal stability of helical oligomers