Tutorial review of error evaluation in experimental water research at the example of membrane filtration.

Experimental water research lacks clear methodology to estimate experimental error. Especially when natural waters are involved, the characterization tools bear method-specific artifacts while the varying environmental conditions prevent regular repeats. This tutorial review identifies common mistakes, and proposes a practical procedure to determine experimental errors at the example of membrane filtration. Statistical analysis is often applied to an insufficient number of repeated measurements, while not all error sources and contributions are considered. This results in an underestimation of the experimental error. Variations in relevant experimental parameters need to be investigated systematically, and the related errors are quantified as a half of the variation between the max and min values when standard deviation is not applicable. Error of calculated parameters (e.g. flux, pollutant removal and mass loss) is estimated by applying error propagation, where weighing contributions of the experimental parameters are considered. Appropriate judgment and five-time repetition of a selected experiment under identical conditions are proposed to validate the propagated experimental error. For validation, the five repeated data points should lie within the estimated error range of the error bar. The proposed error evaluation procedure is adaptable in experimental water research and intended for researchers to identify the contributing factors of an experimental error and carry out appropriate error quantification and validation. The most important aim is to raise awareness of the necessity to question error methodology and reproducibility of experimental data, to produce and publish high quality research.

Removal of strontium by nanofiltration: Role of complexation and speciation of strontium with organic matter.

Strontium (Sr) removal from water is required because excessive naturally occurring Sr exposure is hazardous to human health. Climate and seasonal changes cause water quality variations, in particular quality and quantity of organic matter (OM) and pH, and such variations affect Sr removal by nanofiltration (NF). The mechanisms for such variations are not clear and thus OM complexation and speciation require attention. Sr removal by NF was investigated with emphasis on the role of OM (type and concentration) and pH (2-12) on possible removal mechanisms, specifically size and/or charge exclusion as well as solute-solute interactions. The filtration results show that the addition of various OM (10 types) and an increase of OM concentration (2-100 mgC.L-1) increased Sr removal by 10-15%. The Sr-OM interaction was enhanced with increasing OM concentration, implying enhanced size exclusion via Sr-OM interaction as the main mechanism. Such interactions were quantified by asymmetric flow field-flow fractionation (FFFF) coupled with an inductively coupled plasma mass spectrometer (ICP-MS). Both extremely low and high pH increased Sr removal due to the enhanced charge exclusion and Sr-OM interactions. This work elucidated and verified the mechanism of OM and pH on Sr removal by NF membranes.

Interplay of the forces governing steroid hormone micropollutant adsorption in vertically-aligned carbon nanotube membrane nanopores.

Vertically-aligned carbon nanotube (VaCNT) membranes allow water to conduct rapidly at low pressures and open up the possibility for water purification and desalination, although the ultralow viscous stress in hydrophobic and low-tortuosity nanopores prevents surface interactions with contaminants. In this experimental investigation, steroid hormone micropollutant adsorption by VaCNT membranes is quantified and explained via the interplay of the hydrodynamic drag and friction forces acting on the hormone, and the adhesive and repulsive forces between the hormone and the inner carbon nanotube wall. It is concluded that a drag force above 2.2 × 10-3 pN overcomes the friction force resulting in insignificant adsorption, whereas lowering the drag force from 2.2 × 10-3 to 4.3 × 10-4 pN increases the adsorbed mass of hormones from zero to 0.4 ng cm-2. At a low drag force of 1.6 × 10-3 pN, the adsorbed mass of four hormones is correlated with the hormone-wall adhesive (van der Waals) force. These findings explain micropollutant adsorption in nanopores via the forces acting on the micropollutant along and perpendicular to the flow, which can be exploited for selectivity.

Removal of glyphosate (GLY) and aminomethylphosphonic acid (AMPA) by ultrafiltration with permeate-side polymer-based spherical activated carbon (UF-PBSAC).

Glyphosate (GLY) is the most commonly used herbicide worldwide, and aminomethylphosphonic acid (AMPA) is one of its main metabolites. GLY and AMPA are toxic to humans, and their complex physicochemical properties present challenges in their removal from water. Several technologies have been applied to remove GLY and AMPA such as adsorption, filtration, and degradation with varied efficiencies. In previous works, an ultrafiltration membrane with permeate-side polymer-based spherical activated carbon (UF-PBSAC) showed the feasibility of removing uncharged micropollutants via adsorption in a flow-through configuration. The same UF-PBSAC was investigated for GLY and AMPA adsorption to assess the removal of charged and lower molecular weight micropollutants. The results indicated that both surface area and hydraulic residence time were limiting factors in GLY/AMPA adsorption by UF-PBSAC. The higher external surface of PBSAC with strong affinity for GLY and AMPA showed higher removal in a dynamic process where the hydraulic residence time was short (tens of seconds). Extending hydraulic residence times (hundreds of seconds) resulted in higher GLY/AMPA removal by allowing GLY/AMPA to diffuse into the PBSAC pores and reach more surfaces. Enhancement was achieved by minimising both limiting factors (external surface and hydraulic residence time) with a low flux of 25 L/m2.h, increased PBSAC layer of 6 mm, and small PBSAC particle size of 78 µm. With this configuration, UF-PBSAC could remove 98 % of GLY and 95 % of AMPA from an initial concentration of 1000 ng/L at pH 8.2 ± 0.2 and meet European Union (EU) regulation for herbicides (100 ng/L for individuals and 500 ng/L for total herbicides). The results implied that UF-PBSAC was able to remove charged micropollutants to the required levels and had potential for application in wastewater treatment and water reuse.

Nitrogen compounds removal from brackish water by electrodialysis at fixed electric potential and dynamic current density operations.

Nitrogen (N) compounds can occur in water resources from natural and anthropogenic activities. It is ideal that these contaminants be removed before water consumption. As water quality has been affected by increased salinity and pH variation, more advanced and robust technologies such as electrodialysis (ED) can be considered for simultaneous desalination and pollutant removal. In this context, the removal of N-species (NO3-, NO2-, NH4+, and CH4N2O) from brackish water by ED was investigated for different feed water quality, considering increased salinity (0 - 10g/L NaCl) and pH variation (3 - 11), under limit current density (LCD) at fixed electric potential condition. The applied electric potential (5 - 25V) under, at, and over the LCD at fixed electric potential and dynamic current density (DCD), as a percentage of LCD (0.4 - 1.2), were analyzed to improve the process. In addition, energy efficiency in the form of specific energy consumption (SEC) and current efficiency (CE) were assessed for ED at fixed electric potential and DCD. The results showed that, at extreme pH of the feed water, the removal of NO2- and NH4+ can be affected, while NO3-was the most stable compound with pH variation. An increase in feed water salinity just slightly impacted the removal of N-compounds, due to the similar characteristics of the ions in the water. The increase in electric potential at fixed electric potential or DCD increased the removal and molar flux of N-compounds. However, operating over the LCD increased the SEC of the ED process while changes in removal were not significant. DCD procedures resulted in higher CE and shorter run time of the experiments. Therefore, ED proved to be a suitable treatment technique to produce fresh water due to the selective removal of the studied ions, especially at 15V (fixed electrical potential) and 0.8 LCD (DCD) related to removal, molar flux, and run time to achieve guidelines.

Adsorption of uranium (VI) complexes with polymer-based spherical activated carbon.

Adsorption processes with carbon-based adsorbents have received substantial attention as a solution to remove uranium from drinking water. This study investigated uranium adsorption by a polymer-based spherical activated carbon (PBSAC) characterised by a uniformly smooth exterior and an extended surface of internal cavities accessible via mesopores. The static adsorption of uranium was investigated applying varying PBSAC properties and relevant solution chemistry. Spatial time-of-flight secondary ion mass spectrometry (ToF-SIMS) was employed to visualise the distribution of the different uranium species in the PBSAC. The isotherms and thermodynamics calculations revealed monolayer adsorption capacities of 28-667 mg/g and physical adsorption energies of 13-21 kJ/mol. Increasing the surface oxygen content of the PBSAC to 10 % enhanced the adsorption and reduced the equilibrium time to 2 h, while the WHO drinking water guideline of 30 µgU/L could be achieved for an initial concentration of 250 µgU/L. Uranium adsorption with PBSAC was favourable at the pH 6-8. At this pH range, uranyl carbonate complexes (UO2CO3(aq), UO2(CO3)22-, (UO2)2CO3(OH)3-) predominated in the solution, and the ToF-SIMS analysis revealed that the adsorption of these complexes occurred on the surface and inside the PBSAC due to intra-particle diffusion. For the uranyl cations (UO22+, UO2OH+) at pH 2-4, only shallow adsorption in the outermost PBSAC layers was observed. The work demonstrated the effective removal of uranium from contaminated natural water (67 µgU/L) and meeting both German (10 µgU/L) and WHO guideline concentrations. These findings also open opportunities to consider PBSAC in hybrid treatment technologies for uranium removal, for instance, from high-level radioactive waste.

Influence of organic matter on the photocatalytic degradation of steroid hormones by TiO2-coated polyethersulfone microfiltration membrane.

Water treatment in photocatalytic membrane reactors (PMR) holds great promise for removing micropollutants from aquatic environments. Organic matter (OM) that is present in any water matrix may significantly interfere with the degradation of steroid hormone (SH) micropollutants in PMRs. In this study, the interference of various OM types, humic acid (HA), Australian natural organic matter (AUS), worm farm extract (WF), tannic acid (TA), and gallic acid (GA) with the SH degradation at its environmentally relevant concentration (100 ng/L) in a flow-through PMR equipped with a polyethersulphone-titanium dioxide (PES-TiO2) membrane operated under UV light (365 nm) was investigated. Results of this study showed that OM effects are complex and depend on OM type and concentration. The removal of ß-estradiol (E2) was enhanced by HA at its levels below 5 mgC/L while the enhancement was abated at higher HA concentrations. The E2 removal was inhibited by TA, and GA, while no significant interference observed for AUS, and WF. The data demonstrated diverse roles of OM that acts in PMRs as a light screening agent, a photoreactive species scavenger, an adsorption alteration trigger, and a photosensitizer. The time-resolved fluorescence measurement showed that HA, acting as a photosensitizer, promoted the sensitization of TiO2 by absorbing light energy and transferring energy/electron to the TiO2 substrate. This pathway dominated the mechanism of the enhanced E2 degradation by HA. The favorable effect of HA was augmented as increasing the light intensity from 0.5 to 10 mW/cm2 and was weakened at higher light intensities due to the increased scavenging reactions and the limited amount of HA. This work clarifies the underlying mechanism of the OM interference on photocatalytic degradation of E2 by the PES-TiO2 PMR.

Chromium (III) and chromium (VI) removal and organic matter interaction with nanofiltration.

Chromium (Cr) is a toxic inorganic contaminant for drinking water, in which the concentration has to be controlled for human health and safety. Cr retention was investigated with stirred cell experiments using sulphonated polyethersulfone nanofiltration (NF) membranes of different molecular weight cut-off (MWCO). Cr(III) and Cr(VI) retention follow the order of the MWCO of the studied NF membranes; HY70-720 Da > HY50-1000 Da > HY10-3000 Da with a pH dependency, especially for Cr(III). The importance of the charge exclusion was highlighted when Cr(OH)4- (for Cr(III)) and CrO42- (for Cr(VI)) was the predominant species in the feed solution. In presence of organic matter, namely humic acid (HA), Cr(III) retention increased by 60 %, while no influence of HA was observed for Cr(VI). HA did not induce major modifications on the membrane surface charge for these membranes. Solute-solute interaction, in particular Cr(III)-HA complexation, was the responsible mechanism for the increase in Cr(III) retention. This was confirmed by asymmetric flow field-flow fractionation, coupled with inductively coupled plasma mass spectrometry (FFFF-ICP-MS) analysis. Cr(III)-HA complexation was significant at HA concentrations as low as 1 mgC/L. The chosen NF membranes were able to achieve the EU guideline (25 µg/L) for Cr in drinking water for a feed concentration of 250 µg/L.

Adsorption of glyphosate and metabolite aminomethylphosphonic acid (AMPA) from water by polymer-based spherical activated carbon (PBSAC).

Glyphosate (GLY) is the most commonly used herbicide worldwide, and aminomethylphosphonic acid (AMPA) is its main metabolite. Their occurrence in ground and surface waters causes diseases in humans, while complex physico-chemical properties hinder detection and effective removal. Polymer-based spherical activated carbon (PBSAC) can adsorb many micropollutants efficiently and, hence, overcome the shortfalls of conventional treatment methods. The static adsorption of a mixture of GLY and AMPA by PBSAC was investigated with varying PBSAC properties and relevant solution chemistry. The results show that PBSAC can remove 95% GLY and 57% AMPA from an initial concentration of 1 µg/L at pH 8.2. PBSAC properties (size, activation level, and surface charge) have a strong influence on herbicide removal, where surface area plays a key role. Low to neutral pH favors non-charge interactions and results in good adsorption, while higher temperatures equally enhance GLY/AMPA adsorption by PBSAC. The work demonstrated the effective removal of GLY to meet the European guideline concentration (0.1 µg/L), while AMPA could not be removed to the required level.

Water quality of The Gambia River: A prospective drinking water supply.

Drinking water in The Gambia is mostly derived from boreholes that could potentially be contaminated. The Gambia River, a major river in West Africa that covers 12 % of the country's area, could be more exploited for drinking water supply. During the dry season, the total dissolved solids (TDS), ranging from 0.02 to 33 g/L in The Gambia River, decreases with the distance to the river mouth with no major inorganic contamination. The freshwater (<0.8 g/L TDS) starts from Jasobo at approximately 120 km from the river mouth and extends by about 350 km to the eastern border of The Gambia. With a dissolved organic carbon (DOC) ranging from 2 to 15 mgC/L, the natural organic matter (NOM) of The Gambia River was characterised by 40-60 % humic substances of paedogenic origin. With such characteristics, unknown disinfection by-products could be formed if chemical disinfection, such as chlorination, was implemented during treatment. Out of 103 types of micropollutants, 21 were detected (4 pesticides, 10 pharmaceuticals, 7 per- and polyfluoroalkyl substances (PFAS)) with concentrations ranging from 0.1 to 1500 ng/L. Pesticides, bisphenol A and PFAS concentrations were below the stricter EU guidelines set for drinking water. These were mainly confined to the urban area of high population density near the river mouth, while the quality of the freshwater region of low population density was surprisingly pristine. These results indicate that The Gambia River, especially in its upper regions, would be well suited as a drinking water supply when using decentralised ultrafiltration treatment for the removal of turbidity, as well as, depending on pore size, to a certain extent microorganisms and DOC.

Poly(vinylidene fluoride) membrane with immobilized TiO2 for degradation of steroid hormone micropollutants in a photocatalytic membrane reactor.

The lack of effective technologies to remove steroid hormones (SHs) from aquatic systems is a critical issue for both environment and public health. The performance of a flow-through photocatalytic membrane reactor (PMR) with TiO2 immobilized on a photostable poly(vinylidene fluoride) membrane (PVDF-TiO2) was evaluated in the context of SHs degradation at concentrations from 0.05 to 1000 µg/L under UV exposure (365 nm). A comprehensive investigation into the membrane preparation approach, including varying the surface Ti content and distribution, and membrane pore size, was conducted to gain insights on the rate-limiting steps for the SHs degradation. Increasing surface Ti content from 4 % to 6.5 % enhanced the 17ß-estradiol (E2) degradation from 46 ± 12-81 ± 6 %. Apparent degradation kinetics were independent of both TiO2 homogeneity and membrane pore size (0.1-0.45 µm). With optimized conditions, E2 removal was higher than 96 % at environmentally relevant feed concentration (100 ng/L), a flux of 60 L/m2h, 25 mW/cm2, and 6.5 % Ti. These results indicated that the E2 degradation on the PVDF-TiO2 membrane was limited by the catalyst content and light penetration depth. Further exploration of novel TiO2 immobilization approach that can offer a larger catalyst content and light penetration is required to improve the micropollutant removal efficiency in PMR.

Membrane-organic solute interactions in asymmetric flow field flow fractionation: Interplay of hydrodynamic and electrostatic forces.

The structure and size characterization of organic matter (OM) using flow field-flow fractionation (FFFF) is interesting due to the numerous interactions of OM in aquatic systems and water treatment processes. The estimation of hydrodynamic and electrostatic forces involved in the fractionation of OM over different molecular weight cut-off (MWCO) membranes is vital for a better understanding of the FFFF process. This work aims to understand the membrane-OM interactive forces with respect to membrane MWCO, solute molecular weight, flow rates, solution pH and ionic strength. Polystyrene sulfonate sodium salt (PSS) of molecular weights 10, 30 and 65 kDa were used as model organic solutes for fractionation over ultrafiltration (UF) membranes of MWCO 1-30 kDa. Maximum fractionation of PSS was achieved by using a tight membrane of 1 kDa MWCO at the conditions of high permeate flow rate (1.5-2.0 mL·min-1), low concentrate flow rate (0.2-0.3 mL·min-1) and low ionic strength (10 mM). The better fractionation corresponds to high permeate drag force and low concentrate drag force. A low membrane-solute DLVO interaction is favourable for the retention of a small solute. This study illustrated that FFFF characteristics can be analyzed based on membrane-solute interactive forces controlled by selected flow, size and charge parameters.

Energy Barriers for Steroid Hormone Transport in Nanofiltration.

Nanofiltration (NF) membranes can retain micropollutants (MPs) to a large extent, even though adsorption into the membrane and gradual permeation result in breakthrough and incomplete removal. The permeation of MPs is investigated by examining the energy barriers (determined using the Arrhenius concept) for adsorption, intrapore diffusion, and permeation encountered by four different steroid hormones in tight and loose NF membranes. Results show that the energy barriers for steroid hormone transport in tight membrane are entropically dominated and underestimated because of the high steric exclusion at the pore entrance. In contrast, the loose NF membrane enables steroid hormones partitioning at the pore entrance, with a permeation energy barrier (from feed toward the permeate side) ranging between 96 and 116 kJ/mol. The contribution of adsorption and intrapore diffusion to the energy barrier for steroid hormone permeation reveals a significant role of intrapore diffusive transport on the obtained permeation energy barrier. Overall, the breakthrough phenomenon observed during the NF of MPs is facilitated by the low energy barrier for adsorption. Experimental evidence of such principles is relevant for understanding mechanisms and ultimately improving the selectivity of NF.

Renewable energy powered membrane technology: Implications of adhesive interaction between membrane and organic matter on spontaneous osmotic backwash cleaning.

Organic matter (OM) in surface and ground waters may cause membrane fouling that is laborious to clean once established. Spontaneous osmotic backwash (OB) induced by solar irradiance fluctuation has been demonstrated for early mineral scaling/organic fouling control in decentralised small-scale photovoltaic powered-nanofiltration/reverse osmosis (PV-NF/RO) membrane systems. However, various OM types will interact differently with membranes which in turn affects the effectiveness of OB. This work evaluates the suitability of spontaneous OB cleaning for eleven OM types (covering low-molecular-weight organics (LMWO), humic substances, polyphenolic compounds and biopolymers) regarding adhesive interactions with NF/RO membranes. The adhesive interactions were quantified by an asymmetric flow field-flow fractionation coupled with an organic carbon detector (FFFF-OCD). The underlying mechanism of OM-membrane adhesive interactions affecting OB cleaning was elucidated. The results indicate that humic acid (a typical humic substance) and tannic acid (a typical polyphenolic compound) induced stronger adhesive interaction with NF/RO membranes than biopolymers and LMWO. When the mass loss of an OM due to adhesion was below a critical range, the spontaneous OB is most effective (>85% flux recovery); and above this range, the OB becomes ineffective (<50% flux recovery). Polyphenolic compounds and humic substances resulted in lower OB cleaning efficiency, due to their higher aromatic content, enhancing hydrophobic interactions and hydrogen bonding. Calcium-facilitated adhesion of some OM types (such as humic substances, polyphenolics and biopolymers) increased irreversible organic fouling potential and weakened OB cleaning, which was verified by both FFFF-OCD and membrane filtration results. This work provides a guidance to formulate strategies to enhance spontaneous OB cleaning, such as first identifying the adhesion of OM in feedwater (surface and ground waters) using FFFF-OCD, and then removing "sticky" OM using suitable pre-treatment processes.

Photocatalytic degradation of steroid hormone micropollutants by TiO2-coated polyethersulfone membranes in a continuous flow-through process.

Micropollutants in the aquatic environment pose a high risk to both environmental and human health. The photocatalytic degradation of steroid hormones in a flow-through photocatalytic membrane reactor under UV light (365 nm) at environmentally relevant concentrations (50 ng l-1 to 1 mg l-1) was examined using a polyethersulfone-titanium dioxide (PES-TiO2) membrane. The TiO2 nanoparticles (10-30 nm) were immobilized both on the surface and in the nanopores (220 nm) of the membrane. Water quality and operational parameters were evaluated to elucidate the limiting factors in the degradation of steroid hormones. Flow through the photocatalytic membrane increased contact between the micropollutants and ·OH in the pores. Notably, 80% of both oestradiol and oestrone was removed from a 200 ng l-1 feed (at 25 mW cm-2 and 300 l m-2 h-1). Progesterone and testosterone removal was lower at 44% and 33%, respectively. Increasing the oestradiol concentration to 1 mg l-1 resulted in 20% removal, whereas with a 100 ng l-1 solution, a maximum removal of 94% was achieved at 44 mW cm-2 and 60 l m-2 h-1. The effectiveness of the relatively well-known PES-TiO2 membrane for micropollutant removal has been demonstrated; this effectiveness is due to the nanoscale size of the membrane, which provides a high surface area and facilitates close contact of the radicals with the very small (0.8 nm) micropollutant at an extremely low, environmentally relevant concentration (100 ng l-1).

Selenium species removal by nanofiltration: Determination of retention mechanisms.

Selenium (Se) is a dissolved oxyanion drinking water contaminant requiring appropriate removal technologies. The removal of selenite (SeIV) and selenite (SeVI) with nanofiltration (NF) was investigated with an emphasis on the role of Se speciation and membrane charge screening on the retention mechanisms. The pH (2 to 12) showed strong pH dependence of Se retention, which was due to the speciation. No significant impact of salinity was observed by increasing NaCl concentration from 0.58 to 20 g/L. Application of the Donnan steric pore partitioning model with dielectric exclusion (DSPM-DE) showed that Donnan exclusion was the dominant retention mechanism for the oxyanions Se species. Nine different organic matter (OM) types were investigated at 10 mgC/L to determine if OM affects Se retention. Only OM characterised by negatively charged fractions, such as humic acid (HA), enhanced Se retention with NF270 of up to 20% for SeIV and 10% for SeVI. This was explained by enhanced Donnan exclusion. NF270 was effective in removing Se from real water (Gahard groundwater, Ille et Vilaine, France). The EU guideline (20 µg/L) of Se in drinking water was achieved with comparable performance to OM-free experiments using synthetic waters.

Removal of arsenic(III) via nanofiltration: contribution of organic matter interactions.

The removal of arsenic(III) (As(III)) with nanofiltration (NF) was investigated with emphasis on the role of salinity, pH and organic matter on retention mechanisms. While no measurable impact of salinity on As(III) retention with NF membranes (NF270 and NF90) was observed, a significant increase in As(III) retention occurred from pH 9 to pH 12. This was explained by As(III) deprotonation at pH > 9 that enhanced Donnan (charge) exclusion. Of the five different organic matter types investigated at 10 mgC/L, only humic acid (HA) increased As(III) retention by up to 10%. Increasing HA concentration to 100 mgC/L enhanced As(III) retention by 40%, which was attributed to As(III)-HA complexation. Complexation was confirmed by field-flow fractionation inductively coupled plasma mass spectrometry (FFF-ICP-MS) measurements, which showed that the bound As(III) increased with HA concentration. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) showed that NF90, which exhibited lower permeability reduction than NF270, has accumulated a lower amount of As(III) in the presence of HA, where As(III)-HA complex was formed in the feed solution. This finding implies that As(III) retention with NF technology can be enhanced by complexation, instead of using other methods such as oxidation or pH adjustement.

Organic matter interference with steroid hormone removal by single-walled carbon nanotubes - ultrafiltration composite membrane.

Composite adsorbent-ultrafiltration membranes for micropollutant removal are an interesting concept, although water matrix compounds, such as organic matter (OM), may interfere with micropollutant adsorption. To prevent this interference, the adsorbents such as single-walled carbon nanotubes (SWCNTs) can be deposited on the permeate side of an ultrafiltration membrane (UF) that can retain OM. In this study, the 'shielding' effect by different UF MWCOs of such an SWCNT-UF composite was evaluated for nine different types of OM. All nine OM types competed with 17ß-estradiol (E2) for adsorption to varying extents, and tannic acid (TA, 1.7 kDa) reduced E2 adsorption by SWCNT-UF most drastically. TA interference mechanisms included direct competition with hormones for adsorption sites, and indirect competition via TA-hormone interactions. TA was not fully retained by the composite membrane with an MWCO range of 3-100 kDa, resulting in ineffective shielding. In contrast, humic acid was mostly retained by the composite with low MWCO of 3-10 kDa and allowed high E2 adsorption. This study demonstrates that tailoring composite membranes by carefully choosing UF MWCO can prevent interference of OM types in micropollutant adsorption by permeate-side adsorbents.

Interactions between carbon-based nanoparticles and steroid hormone micropollutants in water.

The occurrence of micropollutants (MPs) including steroid hormones is a global environmental and health challenge. Carbon-based nanoparticles can be incorporated with water treatment processes to allow MP removal by adsorption. The aim was to compare the suitability of such nanoparticles (graphene, graphene oxide, carbon nanotubes and C60) to adsorb steroid hormones for later incorporation in membrane composites. All nanoparticles displayed fast kinetics; carbon nanotubes and graphene showed high adsorption capacities for hormones undeterminable in isotherm studies (over 10 mg/g). External surface adsorption appears to be the most prominent factor impacting adsorption performance. Structure, conformation, geometry and surface charge of nanoparticles can influence the accessibility of surface area through colloidal instability in aqueous solution. Mechanism inspection shows that adsorption initiates at long ranges (up to 10 nm) through hydrophobic and electrostatic interactions. At relatively short ranges (0.2-0.5 nm), adsorption is enhanced by π/π stacking, XH / π (X = C, O) interactions, van der Waals forces and hydrogen bonding. Both long- and short-range forces transporting hormones from the liquid bulk into the adsorbed phase could control the rate. With relatively short residence time required and high adsorption capacity, carbon nanotubes and graphene are promising for incorporation in a membrane composite.