Santiago Romero-Vargas Castrillón

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.

Effect of Surface Polarity on the Structure and Dynamics of Water in Nanoscale Confinement

We present a molecular dynamics simulation study of the structure and dynamics of water confined between silica surfaces using beta-cristobalite as a model template. We scale the surface Coulombic charges by means of a dimensionless number, k, ranging from 0 to 1, and thereby we can model systems ranging from hydrophobic apolar to hydrophilic, respectively. Both rotational and translational dynamics exhibit a nonmonotonic dependence on k characterized by a maximum in the in-plane diffusion coefficient, D||, at values between 0.6 and 0.8, and a minimum in the rotational relaxation time, tauR, at k=0.6. The slow dynamics observed in the proximity of the hydrophobic apolar surface are a consequence of beta-cristobalite templating an ice-like water layer. The fully hydrophilic surfaces (k=1.0), on the other hand, result in slow interfacial dynamics due to the presence of dense but disordered water that forms strong hydrogen bonds with surface silanol groups. Confinement also induces decoupling between translational and rotational dynamics, as evidenced by the fact that tauR attains values similar to that of the bulk, while D|| is always lower than in the bulk. The decoupling is characterized by a more drastic reduction in the translational dynamics of water compared to rotational relaxation.

In situ surface chemical modification of thin-film composite forward osmosis membranes for enhanced organic fouling resistance

Forward osmosis (FO) is an emerging membrane-based water separation process with potential applications in a host of environmental and industrial processes. Nevertheless, membrane fouling remains a technical obstacle affecting this technology, increasing operating costs and decreasing membrane life. This work presents the first fabrication of an antifouling thin-film composite (TFC) FO membrane by an in situ technique without postfabrication treatment. The membrane was fabricated and modified in situ, grafting Jeffamine, an amine-terminated poly(ethylene glycol) derivative, to dangling acyl chloride surface groups on the nascent polyamide active layer. Surface characterization by contact angle, Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS), zeta potential, atomic force microscopy (AFM), and fluorescence microscopy, confirms the presence of Jeffamine on the membrane surface. We demonstrate the improved fouling resistance of the in situ modified membranes through accelerated dynamic fouling FO experiments using a synthetic wastewater feed solution at high concentration (250 mg/L) of alginate, a model macromolecule for the hydrophilic fraction of wastewater effluent organic matter. Our results show a significantly lower flux decline for the in situ modified membranes compared to pristine polyamide (14.3 ± 2.7% vs 2.8 ± 1.4%, respectively). AFM adhesion force measurements between the membrane and a carboxylate-modified latex particle, a surrogate for the organic (alginate) foulant, show weaker foulant-membrane interactions, further confirming the enhanced fouling resistance of the in situ modified membranes.

Influence of active layer and support layer surface structures on organic fouling propensity of thin-film composite forward osmosis membranes.

In this study, we investigate the influence of surface structure on the fouling propensity of thin-film composite (TFC) forward osmosis (FO) membranes. Specifically, we compare membranes fabricated through identical procedures except for the use of different solvents (dimethylformamide, DMF and N-methyl-2-pyrrolidinone, NMP) during phase separation. FO fouling experiments were carried out with a feed solution containing a model organic foulant. The TFC membranes fabricated using NMP (NMP-TFC) had significantly less flux decline (7.47 ± 0.15%) when compared to the membranes fabricated using DMF (DMF-TFC, 12.70 ± 2.62% flux decline). Water flux was also more easily recovered through physical cleaning for the NMP-TFC membrane. To determine the fundamental cause of these differences in fouling propensity, the active and support layers of the membranes were extensively characterized for physical and chemical characteristics relevant to fouling behavior. Polyamide surface roughness was found to dominate all other investigated factors in determining the fouling propensities of our membranes relative to each other. The high roughness polyamide surface of the DMF-TFC membrane was also rich in larger leaf-like structures, whereas the lower roughness NMP-TFC membrane polyamide layer contained more nodular and smaller features. The support layers of the two membrane types were also characterized for their morphological properties, and the relation between support layer surface structure and polyamide active layer formation was discussed. Taken together, our findings indicate that support layer structure has a significant impact on the fouling propensity of the active layer, and this impact should be considered in the design of support layer structures for TFC membranes.

Non-monotonic dependence of water reorientation dynamics on surface hydrophilicity: competing effects of the hydration structure and hydrogen-bond strength.

The reorientation dynamics of interfacial water molecules was recently shown to change non-monotonically next to surfaces of increasing hydrophilicity, with slower dynamics next to strongly hydrophobic (apolar) and very hydrophilic surfaces, and faster dynamics next to surfaces of intermediate hydrophilicities. Through a combination of molecular dynamics simulations and analytic modeling, we provide a molecular interpretation of this behavior. We show that this non-monotonic dependence arises from two competing effects induced by the increasing surface hydrophilicity: first a change in the hydration structure with an enhanced population of water OH bonds pointing toward the surface and second a strengthening of the water-surface interaction energy. The extended jump model, including the effects due to transition-state excluded volume and transition-state hydrogen-bond strength, provides a quasi-quantitative description of the non-monotonic changes in the water reorientation dynamics with surface hydrophilicity.

Bacterial Adhesion to Ultrafiltration Membranes: Role of Hydrophilicity, Natural Organic Matter, and Cell-Surface Macromolecules

Insight into the mechanisms underlying bacterial adhesion is critical to the formulation of membrane biofouling control strategies. Using AFM-based single-cell force spectroscopy, we investigated the interaction between Pseudomonas fluorescens, a biofilm-forming bacterium, and polysulfone (PSF) ultrafiltration (UF) membranes to unravel the mechanisms underlying early stage membrane biofouling. We show that hydrophilic polydopamine (PDA) coatings decrease bacterial adhesion forces at short bacterium-membrane contact times. Further, we find that adhesion forces are weakened by the presence of natural organic matter (NOM) conditioning films, owing to the hydrophilicity of NOM. Investigation of the effect of adhesion contact time revealed that PDA coatings are less effective at preventing bioadhesion when the contact time is prolonged to 2-5 s, or when the membranes are exposed to bacterial suspensions under stirring. These results therefore challenge the notion that simple hydrophilic surface coatings are effective as a biofouling control strategy. Finally, we present evidence that adhesion to the UF membrane surface is mediated by cell-surface macromolecules (likely to be outer membrane proteins and pili) which, upon contacting the membrane, undergo surface-induced unfolding.