Interfacial Interactions of Uranium and Arsenic with Microplastics: From Field Detection to Controlled Laboratory Tests.

We studied the co-occurrence of microplastics (MPs) and metals in field sites and further investigated their interfacial interaction in controlled laboratory conditions. First, we detected MPs in freshwater co-occurring with metals in rural and urban areas in New Mexico. Automated particle counting and fluorescence microscopy indicated that particles in field samples ranged from 7 to 149 particles/L. The urban location contained the highest count of confirmed MPs, including polyester, cellophane, and rayon, as indicated by Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) spectroscopy analyses. Metal analyses using inductively coupled plasma (ICP) revealed that bodies of water in a rural site affected by mining legacy contained up to 332.8 µg/L of U, while all bodies of water contained As concentrations below 11.4 µg/L. These field findings motivated experiments in laboratory conditions, reacting MPs with 0.02-0.2 mM of As or U solutions at acidic and neutral pH with poly(methyl-methacrylate), polyethylene, and polystyrene MPs. In these experiments, As did not interact with any of the MPs tested at pH 3 and pH 7, nor U with any MPs at pH 3. Experiments supplied with U and MPs at pH 7 indicated that MPs served as substrate surface for the adsorption and nucleation of U precipitates. Chemical speciation modeling and microscopy analyses (i.e., Transmission Electron Microscopy [TEM]) suggest that U precipitates resemble sodium-compreignacite and schoepite. These findings have relevant implications to further understanding the occurrence and interfacial interaction of MPs and metals in freshwater.

Group Contribution Method to Predict the Mass Transfer Coefficients of Organics through Various RO Membranes.

Reverse osmosis (RO) is a membrane technology that separates dissolved species from water. RO has been applied for the removal of chemical contaminants from water for potable reuse applications. The presence of a wide variety of influent chemical contaminants and the insufficient rejection of low-molecular-weight neutral organics by RO calls for the need to develop a model that predicts the rejection of various organics. In this study, we develop a group contribution method (GCM) to predict the mass transfer coefficients by fragmenting the structure of low-molecular-weight neutral organics into small parts that interact with the RO membrane. Overall, 54 organics including 26 halogenated and oxygenated alkanes, 8 alkenes, and 20 alkyl and halobenzenes were used to determine 39 parameters to calibrate for 6 different RO membranes, including 4 brackish water and 2 seawater membranes. Through six membranes, approximately 80% of calculated rejection was within an error goal (i.e., ±5%) from the experimental observation. To extend the GCM for a reference RO membrane, ESPA2-LD, 14 additional organics were included from the literature to calibrate nitrogen-containing functional groups of nitrosamine, nitriles, and amide compounds. Overall, 49 organics (72% of 68 compounds) from calibration and 7 compounds (87.5% of 8 compounds) from prediction were within the error goal.

The role of interaction between low molecular weight neutral organic compounds and a polyamide RO membrane in the rejection mechanism.

Reverse osmosis (RO) is a membrane technology that separates dissolved species from water. RO has been applied for the removal of chemical contaminants from water and is employed in wastewater reclamation to provide an additional barrier to improve the removal of trace organic contaminants. The presence of a wide variety of influent chemical contaminants and the insufficient rejection of low molecular weight neutral chemicals by RO calls for the need to develop a comprehensive model that predicts the rejection of various chemicals in RO. Yet the role of the interaction between neutral organic compounds and a RO membrane, and how the functional groups of organic compounds affect the interaction have not been fully elucidated. In this study, we first constructed a molecular model for a reference polyamide (PA) membrane. We then investigated the impact of explicit water molecules and PA membrane functionality on the membrane structure using quantum mechanical calculations. We examined solvent-membrane interactions and then solvent-membrane-solute interactions using two neutral test solutes, arsenic and boron, by comparing the theoretically calculated aqueous-phase free energies of interaction with their experimental values. Finally, the validated PA membrane model was used to calculate the free energies of interaction for a wide variety of organic compounds such as haloalkanes, haloalkenes, alkylbenzenes and halobenzenes, which correlated with the experimentally obtained mass transfer coefficients. The correlation indicates that the interaction between organic compounds and PA membranes plays a critical role in the rejection mechanism.

Effect of Functional Chemistry on the Rejection of Low-Molecular Weight Neutral Organics through Reverse Osmosis Membranes for Potable Reuse.

Potable reuse facilities must be designed and operated to minimize the presence of contaminants of emerging concern (CECs) and other trace organics in the product water. Reverse osmosis (RO) is incorporated into the process train of many potable reuse facilities and has been demonstrated to achieve excellent removal of many, but not all, organic compounds. Organics that may be poorly removed by RO include low-molecular weight (MW) neutral compounds. This laboratory study examined the rejection of 73 low-MW neutral organics through a commercial RO membrane that is commonly used in potable reuse applications. The organics were selected using a reductionist approach so that the effect of individual functional groups on rejection could be ascertained. The research demonstrated that halogens, carbonyl functional groups, C═C double bonds, and aromaticity decrease rejection, that methyl and hydroxyl functional groups increase rejection, and that the position of functional groups in structural isomers has a significant effect on rejection. The results help explain the discrepancies and inconsistencies in RO rejection of neutral organics that are observed when considered from the conventional perspective of molecular size and hydrophobicity.

Boron Can Be Used to Predict Trace Organic Rejection through Reverse Osmosis Membranes for Potable Reuse.

Potable water reuse is a viable option for communities with extreme water scarcity. Improvements in measurement capabilities and greater occurrence of contaminants of emerging concern (CECs) have made the investigation of the removal of CECs through advanced treatment facilities essential for further reuse considerations. Reverse osmosis (RO) has been demonstrated to remove many CECs, but poor removal has been observed for many low molecular weight (MW), neutral organic compounds. With the availability of many RO membrane products on the market, it is increasingly important to be able to predict organics rejection through different products without detailed information about the RO membrane's properties or structure. This laboratory-scale study investigated the rejection of low-MW, neutral organics, boron, and sodium chloride by six RO membrane products. The experimental results were used to develop a correlation between the removal of organics and boron. If the rejection of boron and a neutral organic through one reference membrane is available, then the rejection of that organic through any other membrane product can be estimated using the rejection of boron through that membrane.

Supercapacitive microbial desalination cells: New class of power generating devices for reduction of salinity content.

In this work, the electrodes of a microbial desalination cell (MDC) are investigated as the positive and negative electrodes of an internal supercapacitor. The resulting system has been named a supercapacitive microbial desalination cell (SC-MDC). The electrodes are self-polarized by the red-ox reactions and therefore the anode acts as a negative electrode and the cathode as a positive electrode of the internal supercapacitor. In order to overcome cathodic losses, an additional capacitive electrode (AdE) was added and short-circuited with the SC-MDC cathode (SC-MDC-AdE). A total of 7600 discharge/self-recharge cycles (equivalent to 44 h of operation) of SC-MDC-AdE with a desalination chamber filled with an aqueous solution of 30 g L-1 NaCl are reported. The same reactor system was operated with real seawater collected from Pacific Ocean for 88 h (15,100 cycles). Maximum power generated was 1.63 ±â€¯0.04 W m-2 for SC-MDC and 3.01 ±â€¯0.01 W m-2 for SC-MDC-AdE. Solution conductivity in the desalination reactor decreased by ∼50% after 23 h and by more than 60% after 44 h. There was no observable change in the pH during cell operation. Power/current pulses were generated without an external power supply.

Spatial and temporal evolution of organic foulant layers on reverse osmosis membranes in wastewater reuse applications.

Advanced treatment to remove trace constituents and emerging contaminants is an important consideration for wastewater treatment for potable reuse, and reverse osmosis (RO) can be a suitable technology to provide the necessary level of treatment. However, membrane fouling by biological and organic matter is a concern. This research examined the development of the RO membrane fouling layer using a bench-scale membrane bioreactor operating at different solids retention times (SRTs), followed by a custom-designed RO test cell. The RO test cell contained stacked plates that sandwich five sheets of RO membrane material, which can be extracted for autopsy at separate times over the course of an experiment without disturbing the remaining membranes. The MBR-RO system was run continuously for 2 weeks at each SRT. The RO membranes were stained for live and dead cells, protein, and carbohydrate-like materials, and visualized using confocal laser scanning microscopy. Images of the stained foulant layers were obtained at different depths within the foulant layer at each time point for all SRT conditions. As the RO foulant layer developed, changes occurred in the distribution and morphology of the live cells and carbohydrates, but not the proteins. These trends were similar for all three SRT conditions tested. RO membrane fouling increased with increased MBR SRT, and the highest SRT had the highest ratios of live to dead cells and carbohydrate-like material to dead cells. The autopsied membranes were also analyzed for protein and carbohydrate content, and it was found that the carbohydrate concentration on the membranes after 14 days increased as the SRT increased.

Effect of membrane bioreactor solids retention time on reverse osmosis membrane fouling for wastewater reuse.

The effect of the solids retention time (SRT) in a membrane bioreactor (MBR) on the fouling of the membranes in a subsequent reverse osmosis (RO) process used for wastewater reuse was studied experimentally using a pilot-scale treatment system. The MBR-RO pilot system was fed effluent from the primary clarifiers at a large municipal wastewater treatment plant. The SRT in the MBRs was adjusted to approximately 2, 10, and 20 days in three experiments. The normalized specific flux through the MBR and RO membranes was evaluated along with inorganic and organic constituents in the influent and effluent of each process. Increasing the SRT in the MBR led to an increase in the removal of bulk DOC, protein, and carbohydrates, as has been observed in previous studies. Increasing the SRT led to a decrease in the fouling of the MBR membranes, which is consistent with previous studies. However, the opposite trend was observed for fouling of the RO membranes; increasing the SRT of the MBR resulted in increased fouling of the RO membranes. These results indicate that the constituents that foul MBR membranes are not the same as those that foul RO membranes; to be an RO membrane foulant in a MBR-RO system, the constituents must first pass through the MBR membranes without being retained. Thus, an intermediate value of SRT may be best choice of operating conditions in an MBR when the MBR is followed by RO for wastewater reuse.

Ozone and biofiltration as an alternative to reverse osmosis for removing PPCPs and micropollutants from treated wastewater.

This pilot-scale research project investigated and compared the removal of pharmaceuticals and personal care products (PPCPs) and other micropollutants from treated wastewater by ozone/biofiltration and reverse osmosis (RO). The reduction in UV254 absorbance as a function of ozone dose correlated well with the reduction in nonbiodegradable dissolved organic carbon and simultaneous production of biodegradable dissolved organic carbon (BDOC). BDOC analyses demonstrated that ozone does not mineralize organics in treated wastewater and that biofiltration can remove the organic oxidation products of ozonation. Biofiltration is recommended for treatment of ozone contactor effluent to minimize the presence of unknown micropollutant oxidation products in the treated water. Ozone/biofiltration and RO were compared on the basis of micropollutant removal efficiency, energy consumption, and waste production. Ozone doses of 4-8 mg/L were nearly as effective as RO for removing micropollutants. When wider environmental impacts such as energy consumption, water recovery, and waste production are considered, ozone/biofiltration may be a more desirable process than RO for removing PPCPs and other trace organics from treated wastewater.

Fouling indices for low pressure hollow fiber membrane performance assessment.

This study evaluated the use of fouling indices to describe low pressure membrane fouling. One critical aspect of this study was the use of a bench-scale hollow fiber membrane system that imitated full-scale operation (constant flux with automatic hydraulic backwash and chemical cleaning). Fouling indices were based on a resistance-in-series model. Two different hollow fiber membrane types (membrane A and B) were tested with water from two water utilities (A and B) and three other natural sources (oligotrophic, algal bloom impacted, and wastewater impaired). The bench-scale testing included use of the same membrane as utilized at Utility B. Most fouling was reversible by hydraulic backwash and chemical cleaning. Specific flux and fouling indices for the bench-scale system were higher than those determined from full-scale data but fouling index ratios were comparable, suggesting a similar fouling nature. At similar organic loading, fouling was specific to water source and membrane type, i.e., no generalization on the impact of water source was possible. Full-scale data were compared with bench-scale data to validate the use of fouling indices. Fouling indices based on a resistance-in-series are useful tools to describe membrane performance data for both raw and pretreated water, for different water sources, and different membrane types.

Effect of membrane configuration on bench-scale MF and UF fouling experiments.

Hollow fiber and flat sheet membranes were compared in side-by-side bench-scale experiments to evaluate whether the configuration has an impact on the rate of membrane fouling. Both microfiltration (MF) and ultrafiltration (UF) membranes were evaluated. In general, flat sheet membranes fouled more rapidly than hollow fiber membranes. Pretreatment such as coagulation generally affected both configurations similarly, but in some cases coagulation reduced fouling on hollow fiber membranes but increased fouling on flat sheet membranes. Prefiltration to remove foulants above 1microm in size had a consistent effect on both configurations. A bench-scale apparatus employing a single-fiber module that allows testing over multiple filter runs with integral backwashing capabilities was demonstrated to provide more detailed information about fouling, which can be applied to full-scale applications. When bench-scale tests are to be used to screen treatment options for full-scale applications, the use of a backwashable hollow fiber system is recommended.

Effect of coagulation on the size of MF and UF membrane foulants.

Microfiltration (MF) and ultrafiltration (UF) have become common water treatment technologies for the removal of particles from natural waters. Many water utilities are now integrating MF/UF with other treatment processes to provide treatment for nonparticulate contaminants. Research is needed to understand the impact that other processes have on MF/UF performance. This study was conducted to investigate the interactions between water quality, coagulation, and membrane fouling. The study examined the fouling of MF/UF membranes by natural waters with and without coagulation by specific fractions of constituents in natural water, separated by size. This research found thatthe component of natural organic matter (NOM) smaller than 100 kDa contributes relatively little to fouling during filtration of either raw or coagulated water. The fraction between 1 microm and 100 kDa contributes a significant portion of the fouling. After coagulation pretreatment, fouling due to various size fractions in the feedwater can change.

Fouling of microfiltration and ultrafiltration membranes by natural waters.

Membrane filtration (microfiltration and ultrafiltration) has become an accepted process for drinking water treatment, but membrane fouling remains a significant problem. The objective of this study was to systematically investigate the mechanisms and components in natural waters that contribute to fouling. Natural waters from five sources were filtered in a benchtop filtration system. A sequential filtration process was used in most experiments. The first filtration steps removed specific components from the water, and the latter filtration steps investigated membrane fouling by the remaining components. Particulate matter (larger than 0.45 microm) was relatively unimportant in fouling as compared to dissolved matter. Very small colloids, ranging from about 3-20 nm in diameter, appeared to be important membrane foulants based on this experimental protocol. The colloidal foulants included both inorganic and organic matter, but the greatest fraction of material was organic. When the colloidal fraction of material was removed, the remaining dissolved organic matter (DOM), which was smaller than about 3 nm and included about 85-90% of the total DOM, caused very little fouling. Thus, although other studies have identified DOM as a major foulant during filtration of natural waters, this work shows that a small fraction of DOM may be responsible for fouling. Adsorption was demonstrated to be an important mechanism for fouling by colloids.