Salinity of brackish groundwater in the San Joaquin Valley (SJV) is typically in the range of about 3,000 - 30,000 mg/L total dissolved solids (TDS) In recent years, there has been a growing interest in the potential use of membrane desalination technology for reducing the salinity SJV brackish water. Membrane desalination of SJV brackish water would have to be carried out at relatively high water recovery in order to reduce the volume of generated RO concentrate. However, at high water recoveries the concentration of mineral salt ions on the feed-side of the membrane may increase to levels that exceed the solubility limits of various sparingly water-soluble mineral salts (e.g., calcium sulfate, calcium carbonate, and barium sulfate). The ensuing crystallization of these minerals results in scale build-up that leads to permeate flux decline, shortening of membrane life, and thus a reduction in process efficiency and increased operational cost. Therefore, process strategies must be designed to enhance product water recovery, while reducing the potential for mineral salt scaling. Accordingly, the principal objective of the present study was to evaluate the feasibility of RO desalting of SJV brackish water.

The present project focused on a systematic evaluation of: (a) the recovery limits for RO desalination of SJV AD water that are imposed by mineral salt scaling, and (b) the integration of accelerated precipitation (AP) of mineral salts with RO desalting to mitigate scaling and enable high RO recovery. AP treatment would serve to demineralize and desupersaturate the RO primary or secondary feed with respect to mineral salt scalants. In the first phase of the project, a systematic analysis based on multi-electrolyte thermodynamic solubility calculations was carried to determine the recovery limits due to mineral scaling. Subsequently, a detailed theoretical analysis and laboratory bench-scale studies were carried out with field water samples to assess mineral scaling propensity for a number of specific SJV water sources. In the second phase of the study, the integration of accelerated precipitation with RO desalting was investigated as a potential approach to lowering source water scaling propensity to enable enhanced water product water recovery.

Analysis of historical water quality data and of recently obtained water field samples, from various locations in the San Joaquin Valley, demonstrated a significant variability of water salinity and scaling propensity with respect to calcite, gypsum, barite, and silica. The above analysis and experimental RO scaling tests suggested that the expected range of product water recovery by RO desalting across the SJV can be in the range of 50%-70% for most of the sites, with the exception of the ERR site for which a much higher recovery was estimated (in excess of 90%). The integration of accelerated precipitation with RO desalting was shown to be technically feasible for the range of brackish water quality in the San Joaquin Valley. In this process, the concentrate from primary RO (PRO) desalting would be treated by accelerated precipitation softening (i.e., chemical demineralization) or desupersaturated to lower the scaling propensity of this stream, followed by secondary RO (SRO) desalting. Overall recovery of up to ~90%-95% could be achieved at an estimated cost of $0.56 -$0.98 per m3 product water, with the ACP process accounting for about 15%-25% of the overall water production cost.

The present study demonstrated provided a framework for assessing RO recovery limits and thus identifying potential hurdles that should be addressed in pilot studies and ultimately in the design and implementation of large-scale RO desalting processes. It is expected that, the methods developed in the present study for scale characterization, evaluation of accelerated precipitation effectiveness and RO process performance analysis will significantly advance the knowledge base needed to arrive at optimal design and deployment of future RO brackish water desalination strategies for the San Joaquin Valley.

Reverse osmosis (RO) membrane desalination is increasingly used for production of potable water from seawater and brackish water and in municipal and industrial water reuse applications. However, RO processes at high recovery are impacted by membrane mineral scaling and the upper pressure barrier of RO elements, pumps and the associated energy expenditure. Membrane surface scaling decreases membrane water permeability and may reduce membrane lifetime. Membrane mineral scaling can be partially mitigated via antiscalant dosing of the RO feed, but this is at an added cost. An alternate approach to scale mitigation can be achieved via RO Feed flow reversal (FFR) which is a chemical-free method. By periodically reversing the direction of the raw RO feed, mineral scale that develops at the membrane stage exit can be removed via dissolution, thereby restoring the membrane permeability. Accordingly, an evaluation of the FFR process in a spiral-wound RO pilot system was carried out, without antiscalant dosing, with gypsum as a model scalant whereby the efficacy of the process was monitored via real time membrane surface monitoring.

The recovery of RO process depends on both the ability to mitigate scaling and overcoming the pressure barrier, particularly for highly saline source water. However, the upper pressure limit constraint on membrane elements and the added energy cost often place a practical limit on the achievable recovery. In order to overcome the pressure limitation, the present work explored the deployment of a hybrid RO-NF configuration. In this arrangement the RO membrane enable balancing the rejection of the target mineral salts while the NF membranes allow for further concentration of the RO concentrate, which is particularly beneficial for source water high in concentration of divalent ions. The NF membrane permeate is then recycled and directed to the RO membranes. The attained recovery range and energy utilization for the above process were explored demonstrating that a higher recovery can be attained at a lower applied pressure relative to conventional RO system configuration.

Surface tethering of hydrophilic polymer brush layers is a popular approach for membrane surface modification with the target of overcoming and mitigating various challenges such as perm-selectivity tradeoff, removal of specific contaminants, and membrane fouling and mineral scaling. In the present study, a systematic investigation of membrane surface structuring with tethered polyacrylic acid (PAA) layers was conducted to tune both reverse osmosis (RO) and ultrafiltration (UF) membranes performance in terms of water permeability, solute rejection, molecular weight cutoff (MWCO), fouling resistance, scaling propensity and cleaning efficacy. Surface nano-structured (SNS) PAA brush layers were synthesized onto the base polysulfone (PSf) UF and polyamide (PA) thin-film composite (TFC) RO membranes via membrane surface activation with different atmospheric pressure plasma (APP) types (i.e., Air, He/O2, and He), followed by graft polymerization (GP) of acrylic acid (AA). Effective tuning of SNS-PAA-PSf UF membrane performance in terms of hydraulic permeability and molecular weight cutoff (MWCO) was feasible by adjustments of the APP and graft polymerization conditions. It was shown, for the first time, that SNS-PAA-PSf membranes can be synthesized with a range of hydraulic permeability (spanning a factor of 1.1-2.6 in magnitude) for a given MWCO, or a range of MWCO (spanning a factor of 1.5-2.3 in magnitude) for a given hydraulic permeability, thereby overcoming the hydraulic permeability-MWCO tradeoff. The SNS-PAA-PSf membrane characteristics (surface hydrophilicity, intrinsic membrane resistance, and PEG MWCO) were responsive to pH and ionic strength due to the conformational change (i.e., swelling/collapse) of the surface tethered PAA chains. Within the tested range of pH (3-11) and ionic strength (0-547 mM), the SNS-PAA-PSf membrane demonstrated self-regulated membrane performance (i.e., Rm 0.74 - 2.29�1013 m-1, and MWCO 1.8 - 15.0 kDa) and surface hydrophilicity (i.e., surface energy -114.5 to -139.2 mJ/m2). UF fouling stress tests with bovine serum album (BSA) and alginic acid in high salinity water and post-cleaning with D.I. water demonstrated reduced flux decline (by ~11.3%) and improved permeability recovery (by ~34%) for the SNS-PAA-PSf membrane relative to the native PSf membrane. The surface tethered PAA chains also improved polyamide (PA) RO membrane removal of nitrate, boron, As (III), and As (V), with rejection of 98.0%, 90.7%, 96%, and 99.6%, respectively, relative to 76.8-84.9%, 87.3-92.1%, and 94.5-97.2% for the tested commercial RO membranes. The increased membrane removal of the specific contaminants is attributed to the surface tethered PAA layer sealing of microscopic defects in the polyamide membrane active layer. The SNS-PAA-PA membrane also exhibited lower flux decline for both gypsum and calcium carbonate scaling tests compared to the tested commercial RO membranes and 100% and 94% permeability recovery post D.I. water flushing, respectively. Scale up of the membrane surface nano-structuring approach, atmospheric pressure plasma-induced graft polymerization (APPIGP), was developed for SNS-PAA-PA membrane sheets of size sufficient for fabrication of 2.5 inch � 21 inch spiral-wound RO elements. Laboratory testing of 18 membrane coupons (~2” x 4”) extracted from different locations of the SNS-PAA-PA membrane sheet, in terms of water and salt permeability coefficients and intrinsic membrane rejection, demonstrated the similar or higher performance uniformity level compared to Base-PA. SNS-PAA-PA spiral wound elements, fabricated with the above SNS-PAA-PA membrane sheets, outperformed the commercial Dow SW30 element exhibiting lower flux decline and 100% permeability recovery in fouling tests of both BSA and sodium alginate model foulant solutions. Results of the present study suggest that the APPIGP approach can be scaled up to fabricate commercial scale spiral-wound RO elements of superior antifouling properties.

Reverse osmosis (RO) desalination of inland brackish water can replenish dwindling water supplies in various regions around the world. However, successful implementation of RO technology requires high product water recovery (>85%) in order to minimize the volume of generated concentrate brine. Therefore, brine management is a critical aspect of inland water desalination. At high water recovery, dissolved mineral salts (e.g. CaSO4, CaCO3) may concentrate above their solubility limits and may crystallize, potentially blocking or damaging RO membrane surfaces, reduce water permeate flux, and shorten membrane life. Therefore, it is essential to reduce the propensity for mineral scaling in order to increase the potential for high product water recovery. Attaining high recovery for inland water desalination, while avoiding membrane mineral scaling, can be achieved via an intermediate concentrate demineralization (ICD) method that utilizes two-step chemically-enhanced seeded precipitation (CESP) process. In the CESP approach, primary RO concentrate is first treated via partial lime softening in which residual antiscalant in the PRO concentrate is scavenged by precipitating calcium carbonate (CaCO3). The filtered lime treated PRO concentrate is then treated in a seeded gypsum (CaSO4•2H2O) precipitation step whereby, gypsum crystal seeds promote rapid crystal growth. As a consequence, the treated PRO stream is desupersaturated with respect to gypsum and upon filtration step; a secondary RO desalting step is carried out to increase the overall product water recovery.

Development of the ICD approach as a continuous process suitable for deployment in RO desalting is the focus of the present study. Accordingly, a novel system for continuous chemically enhanced seeded precipitation (CCESP) pilot was developed and constructed consisting of an alkaline chemical softening flocculation tank followed by a vertical static mixing bed reactor for seeded precipitation. The overall feasible feed slow rate for the pilot CCESP system was 0.026 – 0.25 gpm. Evaluation of the continuous ICD process performance was undertaken with a range of solutions that mimic PRO concentrate produced from desalination of San Joaquin Valley brackish water at a recovery of 63%. The major salts in the PRO concentrate feed to the CCESP included CaCl2 (30.7 mM), Na2SO4 (145.4 mM), MgSO4 (31.2 mM), NaHCO3 (11.4 mM), and NaCl (20.3 mM). Antiscalant (Flocon 260, 5 mg/L) was introduced to the PRO concentrate in order to assess the feasibility for residual antiscalant (typically present in PRO concentrate) removal so as to avoid retardation of the subsequent gypsum desupersaturation step. The CCESP system enabled continuous gypsum desupersaturation by purging spent gypsum seeds and recycling a portion of the seeds or introducing fresh seeds to the fluidized bed. Various gypsum seeds were tested, with a focus on industrial sources for gypsum (e.g. mining, drywall, food, agriculture) due to their availability and low cost. The purity of the gypsum seeds was found to be a key factor, where gypsum seeds with >98% purity were found to be most effective. Using the synthetic PRO concentrate, each of the two steps of the process were first evaluated individually to determine the optimal operating conditions and subsequently combined to evaluate the complete continuous operation. In the CCESP, lime softening occurs in a flocculation tank with recirculation, solids removal from the lime treated stream is via an inline centrifugal separator, and the gypsum seeded precipitation takes place in a fluidized bed. It was found that CCESP treatment of the PRO concentrate with 5.75 mM lime enabled up to 68% removal of the residual antiscalant. Subsequent gypsum seeded precipitation (initial seed loading of 240 g/L gypsum) reduced the PRO concentrate gypsum supersaturation index (SIg) level from 2.36 to nearly unity. The above level of gypsum desupersaturation was assessed to be sufficient for carrying out a secondary RO desalting that would enable increased recovery from 63% at the PRO step to an overall recovery of about 85% and possibly higher.

The present study successfully developed a continuous ICD process and demonstrated its technical feasibility. The present results are encouraging and support the merit of evaluating the process under field conditions. Overall, it is expected that deployment of the CCESP process will enable high recovery desalting of challenging inland water of high mineral scaling propensity.

A systematic investigation of membrane surface nano-structuring (SNS) with surface tethered hydrophilic polymer layers was conducted in order to mitigate fouling of reverse osmosis (RO) and ultrafiltration (UF) membranes and tune membrane performance in seawater desalination. Surface tethered hydrophilic polymer (i.e., polyacrylic acid (PAA)) layers were synthesized onto base polysulfone (PSf) UF and polyamide (PA) thin film composite (TFC) RO membranes via membrane surface activation with an atmospheric pressure plasma (APP), followed by graft polymerization (GP) of acrylic acid. Both APP surface activation and GP conditions impacted the structure of the synthesized tethered PAA layers and RO membrane desalination performance. Detailed characterization of tethered PAA chain extension length via atomic force microscopy (AFM) based force spectroscopy (FS) revealed that the number average molecular weight of the synthesized tethered PAA chains was estimated to be in the range of ∼13,000–35,000 with chain-chain separation of 1.5–2.4 nm. Extension of the tethered PAA chains in DI water was significantly greater in DI water than in high salinity aqueous environment. UF fouling stress tests with alginic acid in high salinity water and post-cleaning with non-saline water demonstrated permeability restoration of up to 90–100% for a surface nano-structured (SNS)-PAA-PSf membrane relative to 50–70% for the native PSf membrane. Synthesized SNS-PAA-PA BWRO and SWRO membranes having water and salt permeability coefficient in the range of 2.3 – 3.4 L�m-2�h-1�bar-1 and 0.15 – 0.54 L�m-2�h-1 revealed that membrane surface structuring with a tethered PAA layer enabled tuning membrane performance (in terms of Lp and B) to achieve water/salt selectivity (evaluated as Lp:B ratio) that was significantly higher (by up to 56%) relative to the base PA membranes. It was demonstrated that, depending on APP surface activation and GP conditions, PA TFC membranes could be tuned to have essentially the same salt rejection over a wide permeability range or a given permeability over a range of salt rejection. It was also shown that membrane performance could be achieved that overcomes the permeability-selectivity trade-off. Lastly, the present study developed an approach to scale up the APP surface activation and GP process by performing membrane surface nano-structuring for large PA TFC flat sheet membranes that are suitable for fabricating 2.5” x 21” spiral wound RO elements.

Reverse osmosis (RO) membrane desalination is the primary technology for seawater and brackish water desalination, agricultural drainage desalting, as well as municipal wastewater recycling for potable water reuse applications. RO membranes achieve high salt rejection (>95%) and in principle should provide a complete physical barrier to nanosize pathogens (e.g., waterborne enteric viruses). However, in the presence of imperfections and/or membrane damage, membrane breaches as small as 20-30 nm in diameter can allow nanosize pathogens to pass through the membrane and contaminate the product water stream. In order to demonstrate that the level of removal of these contaminants by RO processes will assure rigorous public health protection, there is a need for real-time RO membrane integrity monitoring (MIM). However, at present, reliable real-time RO membrane MIM methods are unavailable for the detection of membrane or module breaches.

Given the above needs, a Pulsed Marker Membrane Integrity Monitoring (PM-MIM) approach was developed for real-time assessments of RO membrane integrity, based on characterization of fluorescent molecular marker passage across an RO membrane. This approach involves monitoring the dynamic change in marker concentration in the RO permeate (product) stream in response to a pulsed marker injection into the RO feed stream. The presence and characteristics of membrane integrity loss is identified by decoupling marker diffusive and convective transport through the RO membrane. The approach was first evaluated the passage of a molecular marker through intact and compromised RO membranes (with micron-size membrane breaches) in a bench-scale, plate-and-frame (PFRO) system. It was demonstrated that while marker passage through intact RO membranes was governed by diffusive transport, enhanced marker passage for compromised membranes was associated with convective transport through the breached areas. Therefore, detection of enhanced molecular marker passage was indicative of membrane integrity loss.

One of the potential causes of RO membrane integrity loss in RO plants is oxidation of the polyamide RO membrane surface by chlorinated disinfectants, which are typically introduced into the RO feed stream to prevent biofouling. The suitability of the PM-MIM approach for quantification of the extent of membrane integrity loss due to oxidation was evaluated for flat-sheet RO membranes that were degraded via exposure to sodium hypochlorite (NaOCl) solution at various exposure conditions (i.e., NaOCl concentration and exposure time). Upon membrane exposure to 50-200 mg/L NaOCl for a period of 2.5 to 10 hours, there was a significant increase in marker solution-diffusion and convection across the RO membrane as a result of changes in membrane surface physicochemical properties (surface roughness, chemical composition, and wettability) and structural damage (as evident from the presence of micron-size breaches). The severity of membrane integrity loss, represented by an equivalent breach size, increased with chlorine feed concentration and exposure time, with the latter having a more pronounced impact on membrane integrity.

In order to implement the PM-MIM approach in full-scale RO plants, which typically consist of multiple spiral-wound RO (SPRO) elements, an analytical framework was developed for characterization of marker passage through RO membranes in each individual SPRO element in multi-element SPRO systems. The approach is based on monitoring and analyzing the dynamic change in the marker concentration in the combined permeate stream (from multiple SPRO elements), in response to a pulsed marker injection into the RO feed. Subsequently, marker convective transport, across the RO membrane in each SPRO element, was be resolved via data fusion of dynamic marker response with online water flow, feed salinity, and pressure data, combined with the estimated marker residence time for each SPRO element. Experimental evaluation, which was carried out with intact and compromised SPRO elements (with mechanically induced membrane breaches), demonstrated enhanced marker passage for compromised SPRO elements, with increased marker passage that correlated with increased breach size. The PM-MIM approach was shown to be both technically and economically suitable for multi-element Ro system and through the proposed protocol for its implementation can serve to quantify the severity and identify the approximate location of membrane integrity loss in a multi-element SPRO plant.

Improvements in system and process design has enabled reverse osmosis (RO) membrane technology to gain foothold in various water treatment and desalination applications. However, energy consumption and mineral scaling on membrane surfaces remain impediments to high recovery operation of RO desalination. Accordingly, research efforts have been devoted to process optimization to improve permeate water productivity, reduce energy cost, and mitigate membrane mineral scaling. In this regard, RO concentrate recycling is an effective approach for enhancing product water recovery, reducing system footprint, and lowering installation as well as operating costs of RO desalination systems and plants. Energy consumption and membrane mineral scaling propensity in RO processes are impacted primarily by the osmotic pressure magnitude and the level of supersaturation of mineral scalants at the membrane surface, respectively, which are, in turn, governed by the operational strategy. Optimization of RO processes with concentrate recycle require fundamental models of RO processes with concentrate recycle under both steady and unsteady-state operation,

To date, only simple models have been proposed which are of limited applicability to practical systems given the use of various simplifying assumptions of complete energy recovery, neglect of the efficacy of concentrate flushing in unsteady-state semi-batch RO (SBRO), and omission of concentration polarization. Previous studies have not provided experimental data to corroborate conclusions regarding system performance based on oversimplifications. Moreover, SBRO operation was not assessed relative to steady state RO (SSRO) with partial recycle (SSRO-PR) which is also suitable for high recovery operation with a small footprint. Accordingly, in the present work a fundamental quantitative modeling framework was developed and implemented for the design and operation of SSRO-PR and SBRO. Modeling RO desalting, including at the limit of the thermodynamic restriction, was undertaken to evaluate the minimum energy consumption as a function of product water recovery. SBRO process analysis revealed a significant increase in the salinity range during the RO filtration period and a progressive rise in the initial filtration period salinity until the stable cycle-to-cycle operation is reached. SBRO performance is highly dependent on the efficacy of concentrate discharge and flushing during the SBRO flushing period. For the condition of concentrate flushing under ideal plug flow, energy consumption in SBRO was assessed to be lower than for single-pass RO (SPRO) operation at the same level of overall product water recovery. However, for the practical range of expected concentrate flushing efficacy energy consumption in SBRO could be significantly above that which would be attained in SPRO.

Using the direct real-time membrane surface optical imaging mineral scaling was experimentally evaluated in both SBRO and SSRO-PR pilot systems, with respect to the efficacy of concentrate flushing with the undersaturated raw feed water, at given levels of supersaturation or product water recovery. The predicted RO element feed stream osmotic pressure and solution supersaturation levels (for the target mineral scalant) at the membrane surface, for a given product water recovery, were higher on average in SBRO relative to SSRO-PR as corroborated by experimental data. The experimental data revealed mineral scaling propensity, which was significantly higher in multi-cycle SBRO operation, relative to SSRO-PR at the same water recovery. However, the rates of crystal nucleation and growth were similar at when both systems are compared at the same level of average supersaturation, although the product water recovery was lower in SBRO compared to SSRO-PR.

The present theoretical modeling framework, validated by experimental data, provide a fundamental approach to assessing the performance of SBRO and SSRO-PR desalting systems with respect to energy consumption and mineral scaling propensity. The presented approach provides the means necessary for optimizing these low footprint technologies for high recovery RO desalination.

Engineered nanomaterials (ENMs), which may be released to the environment due to human-related activities, can move across environmental phase boundaries and be found in most media. Given the rapid development and growing applications of nanotechnology, there is concern and thus the need to assess the potential environmental impact associated with ENMs. Accordingly, a modeling platform was developed to enable evaluation of the dynamic multimedia environmental distribution of ENMs (MendNano) and the range of potential exposure concentrations of ENMs.

The MendNano was based on a dynamic multimedia compartmental modeling approach that was guided by detailed analysis of the agglomeration of ENMs, life-cycle analysis based estimates of their potential release to the environment, and incorporation of mechanistic sub-models of various intermedia transport processes. Model simulations for various environmental scenarios indicated that ENM accumulation in the sediment increased significantly with increased ENMs attachment to suspended solids in water. Atmospheric dry and wet depositions can be important pathways for ENMs input to the terrestrial environment in the absence of direct and distributed ENM release to soil. Increased ENM concentration in water due to atmospheric deposition (wet and dry) is expected as direct ENM release to water diminishes. However, for soluble ENMs dissolution can be the dominant pathway for suspended ENM removal from water even compared to advective transport. For example, simulations for Los Angeles showed that dry deposition, rain scavenging, and wind dilution can remove 90% of ENMs from the atmospheric airshed in ~100-230 days, ~2-6 hrs, and ~0.5-2 days, respectively. For the evaluated ENMs (metal, metal oxides, carbon nanotubes (CNT), nanoclays), mass accumulation in the multimedia environment was mostly in the soil and sediment. Additionally, simulation results for TiO₂ in Los Angeles demonstrates that the ENM concentrations in air and water increases rapidly to reach steady state, in 72 hrs and 8 days after the start of source release, respectively. After termination of source release, ENM concentrations would decrease by 90% in ~1 and ~4 days. In contrast, steady state for ENM concentrations in soil would not be expected to occur until after about 10 years. MendNano was further integrated with a sub-model of lifecycle environmental assessment for the release of ENMs (LearNano). Estimation of the releases of various ENMs and their environmental distributions in various regions in the U.S. and countries throughout the world revealed that the exposure concentrations for most ENMs (e.g., metal, metal oxides and carbon nanotubes) are expected to be in the range of 0.0003 - 30 ng m^-3 (air), 0.006 - 150 ng L^-1 (water), 0.01 - 40 μg kg^-1 (soil), and 0.005 - 100 mg kg^-1 (sediment).

It is important to note that the environmental transport of ENMs is governed by particulate transport processes; and thus the transport rates of ENMs are dependent on their particle size distribution. Accordingly, a computational constant-number Direct Simulation Monte Carlo (DSMC) model was also developed to assess the ENM agglomeration in aqueous systems, by solving the Smoluchowski coagulation equation coupled with particle-particle interaction energies provided by the classical Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and non-DLVO hydration repulsion interaction energy. Prediction of ENM agglomerate PSDs demonstrated excellent agreement with experimental measurements for TiO₂, CeO₂, α-Fe₂O₃, SiO₂, and C₆₀ ENMs over a wide range suspension conditions. Simulations also demonstrated, in quantitative agreement with DLS measurements, that nanoparticle agglomerate size increased both with ionic strength (IS) and as the solution pH approached the isoelectric point (IEP). Additionally, evaluation of experimental DLS measurements for TiO₂, CeO₂, SiO₂, and α-Fe₂O₃ (hematite) at high IS (up to 900 mM) or low |ζ-potential| (>=1.35 mV) revealed that non-DLVO hydration repulsion energy can be above electrostatic repulsion energy such that the increased overall repulsion energy (contributed by hydration repulsion energy) can significantly lower the agglomerate diameter relative to the classical DLVO prediction. The classical DLVO theory, which is reasonably applicable for agglomeration of NPs of high |ζ-potential| (~>35 mV) in suspensions of low IS (~<1 mM), can overpredict agglomerate sizes by up to a factor of 5 at high IS (~>1 mM) or low |ζ-potential| (~< 40 mV) conditions.

In summary, the MendNano-LearNano integrated modeling platform was implemented as a web-based software application that enables rapid "what-if?" scenario analysis, in order to assess the response of environmental system to various scenarios of ENM releases, investigate the impact of geographical and meteorological parameters on ENM distribution in the environment, compare the impact of ENM production and potential releases on different regions, as well as estimate source release rates based on monitored ENM concentrations. It is envisioned that the present integrated modeling platform can serve as a decision support tool to rapidly and critically assess the potential environmental implications of ENMs and thus ensure that nanotechnology is developed in a productive and environmentally responsible manner.

Surface nano-structuring of polysulfone (PSf) membrane surrogate surfaces was accomplished by grafting hydrophilic polymers, poly(methacrylic acid) (PMAA) and poly(acrylic acid) (PAA), using the atmospheric pressure plasma-induced graft polymerization (APPIGP) approach. Atmospheric pressure (AP) plasma was used to activate the polysulfone substrate surface, and subsequent graft polymerization of hydrophilic monomers was carried out with an aqueous monomer solution. Optimization of generating the surface initiation sites was guided by water contact angle (WCA) measurements of the PSf surface after plasma treatment at different gas compositions, radio frequency (RF) power, and the treatment time. Optimal surface activation of the PSf substrate surface was achieved for AP helium plasma of RF power of 50 W and treatment time of 15 s. Graft polymerization was carried out at initial monomer concentration in the range of 5 - 20 vol% and reaction time of 0.5 - 2 h at 60°C and 70°C for grafting PMAA and PAA, respectively. Compared to the PSf surrogate surface (WCA: approx. 97 - 100º), the surface hydrophilicity of the grafted surfaces was improved for all grafting conditions, with the water contact angles of the PMAA- and PAA-grafted polysulfone surfaces decreasing by 28.8% and 34.0%, respectively, both at the lowest initial monomer concentration of 5 vol% and the longest graft polymerization time of 2 hours. Surface topography of the grafted surfaces was evaluated by atomic force microscopy (AFM). Both the surface feature number density and root-mean-square surface roughness (Rrms) increased with initial monomer concentration, from 5 - 20 vol%, suggesting that a higher density of surface grafted chains with a wider chain size distribution was achieved with increased initial monomer concentration. All polymer grafted polysulfone surfaces were significantly more hydrophilic than the original polysulfone surface, and high grafting densities were observed. However, relative to lower initial monomer concentration (5 vol%), increased roughness of the surface at high initial monomer concentration (20 vol%) resulted in somewhat reduced surface hydrophilicities quantified by 15.9% and 18.2% contact angle decrease for PMAA- and PAA-grafted PSf, respectively, after 2 h of graft polymerization. Results of the present study are encouraging in indicating that the APPIGP methodology could be beneficial for surface modification of commercial polysulfone membranes for improving the surface hydrophilicity and hence fouling mitigation.

Surface wettability (or surface hydrophilicity) is of considerable importance in a variety of applications, including membrane separations, lubrication, fibers (e.g., textiles), and biomedical applications. Alteration of surface wettability to the desired level can be of significant benefit in the above applications. Accordingly, the present study focused on a systematic investigation of the modification of surface hydrophilicity via the synthesis of hydrophilic surface tethered polymers. This approach to surface nanostructuring (SNS) was achieved by a two-step process, whereby surface activation is achieved using atmospheric pressure plasma (APP) followed by graft polymerization with a suitable vinyl monomer. The resulting polymer layer consists of chains that are terminally and covalently attached to the underlying surface, being polyamide in the present study. Polyamide (PA) was the selected substrate given the importance of this polymer in various applications (e.g., reverse osmosis (RO) membranes, clothing, body armor, etc.). The degree of surface hydrophilicity imparted to the PA surface was evaluated with respect to the conditions of APP surface activation (i.e., hydrogen, oxygen, and helium as plasma source gases, and exposure time) and graft polymerization (i.e., reaction time, 2-hydroxyethyl methacrylate, acrylamide, acrylic acid, n-vinylpyrrolidone, methacrylic acid, and vinylsulfonic acid monomers, and initial monomer concentration). Helium APP was found to be most effective for the synthesis of tethered polymers on the PA surface leading to an increase in hydrophilicity as quantified by a 15 – 51% reduction in the free energy of hydration (ΔGiw) of the underlying PA substrate. In particular, polymer-water affinity of the nanostructured PA surfaces, as quantified by the polar component of the surface energy, was a factor of 2.2 – 6.5 higher than for the native PA surface. Overall, surface hydrophilicity increased with increasing tethered polymer layer surface roughness, volume, and thickness; the above trend is consistent with the expected corresponding increased water sorption capacity by the grafted water soluble polymers.

The hydrophilic polymer brush layer effectiveness in reducing biofouling propensity and improving surface cleaning post-biofouling (i.e., decreasing surface-solute affinity) was demonstrated for SNS-PA thin-film composite (TFC) RO membranes. SNS-PA RO membranes with polyacrylamide (PAAm), poly(acrylic acid) (PAA), poly(methacrylic acid) (PMAA), and poly(vinylsulfonic acid) (PVSA) brush layers were synthesized, yielding permeability and salt (NaCl) rejection ranges of 4.8 – 6.7 L/m2∙h∙bar and 95.2 – 96.6%, respectively. Performance testing of the SNS-PA-TFC membranes was carried out using secondary wastewater from a municipal wastewater treatment (MWT) plant. Performance tests with the PMAA-SNS-PA and PAAm-SNS-PA membranes (highest and lowest ranking with regard to hydrophilicity, respectively) demonstrated measurable resistance to biofouling. Biofilm layer thickness was up to 4.7 times lower for the above SNS-PA membranes relative to a commercial TFC membrane of similar salt rejection. Moreover, up to 89% of the SNS-PA membrane permeability was recovered after water cleaning, and complete restoration of membrane permeability was attained after chemical (Na2∙EDTA) cleaning. In summary, the present approach for tailor-designing surfaces is effective for wettability control through adjustment of the brush layer topography and chemistry which affect the polymer-water affinity. Such hydrophilic tethered polymer surface layers can reduce the biofouling propensity of surfaces, and in particular, increase the biofouling resistance and improve cleaning efficiency of RO membranes.