Hyperspectral Imaging Microscopy of Acetaminophen Adsorbed on Multiwalled Carbon Nanotubes.

In this study, enhanced dark-field hyperspectral imaging (ED-HSI) was employed to directly observe acetaminophen (AAP), a model pharmaceutical and personal care product (PPCP), adsorbed on multiwalled carbon nanotubes with large diameters (L-MWCNT) and small diameters (S-MWCNT) under equilibrium conditions. The ED-HSI results revealed that (1) AAP molecules primarily adsorbed onto the external surfaces, rather than the internal surfaces of L- and S-MWCNT aggregates, (2) or on sidewall of the dispersed tubes, but not at their end caps. Besides, ED-HSI images showed that the surface coverage ratio of AAP/S-MWCNT is smaller than that of AAP/L-MWCNT (1.1 vs 3.4), indicating that there are more available adsorption sites on S-MWCNT than L-MWCNT when the adsorption reached equilibrium. This finding was consistent with the adsorption capacities of S-MWCNT and L-MWCNT (252.7 vs 54.6 mg g-1). Direct visualization of sorption sites for PPCP molecules provides new insights into the heterogeneous structures and surface properties of MWCNT and helps elucidate the adsorption mechanisms that are fundamental to the design of functional adsorbents for PPCP contaminants.

Highly efficient and mild electrochemical mineralization of long-chain perfluorocarboxylic acids (C9-C10) by Ti/SnO2-Sb-Ce, Ti/SnO2-Sb/Ce-PbO2, and Ti/BDD electrodes.

The electrochemical mineralization of environmentally persistent long-chain perfluorinated carboxylic acids (PFCAs), i.e., perfluorononanoic acid (C8F17COOH, PFNA) and perfluorodecanoic acid (C9F19COOH, PFDA) was investigated in aqueous solutions (0.25 mmol L(-1)) over Ti/SnO2-Sb-Ce (SnO2), Ti/SnO2-Sb/Ce-PbO2 (PbO2), and Ti/BDD (BDD) anodes under galvanostatic control at room temperature. Based on PFCA decay rate, total organic carbon (TOC) reduction, defluorination ratio, safety, and energy consumption, the performance of PbO2 electrode was comparable with that of BDD electrode. After 180 min electrolysis, the PFNA removals on BDD and PbO2 electrodes were 98.7 ± 0.4% and 97.1 ± 1.0%, respectively, while the corresponding PFDA removals were 96.0 ± 1.4% and 92.2 ± 1.9%. SnO2 electrode yielded lower PFCA removals and led to notable secondary pollution by Sb ions. The primary mineralization product, F(-), as well as trace amounts of intermediate PFCAs with shortened chain lengths, were detected in aqueous solution after electrolysis. On the basis of these results, a degradation mechanism including three potential routes is proposed: via formation of short-chain PFCAs by stepwise removal of CF2; direct mineralization to CO2 and HF; conversion to volatile fluorinated organic compounds. The results presented here demonstrate that electrochemical technique exhibits high efficiency in mineralizing PFNA and PFDA under mild conditions, and is promising for the treatment of long-chain PFCAs in wastewater.

In-situ cathodic electrolysis coupled with hydraulic backwash inhibited biofilm formation on a backwashable carbon nanotube membrane.

Electro-coupled membrane filtration (ECMF) is an innovative and green technology for water and wastewater treatment. However, the dynamics of biofouling development in the ECMF system has yet been determined. This fundamental question was systematically investigated in this study through laboratory dead-end ECMF experiments. It was found that the ECMF process with an applied voltage of 3 V and a backwash interval of 60 min was capable of completely eradicating membrane biofouling in an extended filtration time of 1450 min. In contrast, membrane biofouling was much severer with a longer backwash interval of 720 min or without backwash. The complemental permeate analysis and membrane characterization results revealed that biofouling during ECMF involved two sequential stages. During the first stage, dead bacteria and their degradation debris formed a loose deposit layer on the membrane surface. The continuous accumulation of this layer decreased the electrochemical performance of the membrane cathode. As such, bacteria in the top deposit layer proliferated and secreted extracellular polymeric substances, which led to irreversible fouling in the second stage. Therefore, timely removal of the initial deposit layer by hydraulic backwash was crucial in preventing irreversible membrane biofouling. These findings provided novel insights into the synergistic effects of cathodic electrolysis and hydraulic backwash for biofouling mitigation.

Silica mitigated calcium mineral scaling in brackish water reverse osmosis.

Although the autopsies of reverse osmosis (RO) membranes from full-scale, brackish water desalination plants identify the co-presence of silica and Ca-based minerals in scaling layers, minimal research exists on their formation process and mechanisms. Therefore, combined scaling by silica and either gypsum (non-alkaline) or amorphous calcium phosphate (ACP, alkaline) was investigated in this study for their distinctive impacts on membrane performance. The obtained results demonstrate that the coexistence of silica and Ca-based mineral salts in feedwaters significantly reduced water flux decline as compared to single type of Ca-based mineral salts. This antagonistic effect was primarily attributed to the silica-mediated alleviation of Ca-based mineral scaling. In the presence of silica, silica skins were immediately established around Ca-based mineral precipitates once they emerged. Sheathing by the siliceous skins hindered the aggregation and thus the morphological evolution of Ca-based mineral species. Unlike sulfate precipitates, ACP precipitates can induce the formation of dense and thick silica skins via an additional condensation reaction. Such a phenomenon rationalized the notion concerning a stronger mitigating effect of silica on ACP scaling than gypsum scaling. Meanwhile, coating by silica skins altered the surface chemistries of Ca-based mineral precipitates, which should be fully considered in regulating membrane surface properties for combined scaling control. Our findings advance the mechanistic understanding on combined mineral scaling of RO membranes, and may guide the appropriate design of membrane surface properties for scaling-resistant membrane tailored to brackish water desalination.

Mechanisms of membrane fouling control by integrated magnetic ion exchange and coagulation.

Colloidal natural organic matter (NOM) is an important foulant to low-pressure membranes (LPMs) employed in drinking water treatment. Removal of colloidal NOM by magnetic ion exchange (MIEX), coagulation, and integrated MIEX and coagulation was investigated in this study to determine the relationship between colloidal NOM removal and membrane fouling reduction. The results showed that coagulation did not selectively remove colloidal NOM and the optimal coagulant dose was primarily determined by the concentration of humic substances. Comparatively, MIEX pretreatment preferentially removed humic substances and reduced the coagulant dose needed for colloidal NOM removal as a result of coagulation stoichiometry. A matched-pair analysis showed that integrated MIEX and coagulation pretreatment at much lower coagulant doses was as effective as coagulation in reducing membrane fouling. It is concluded that integrated MIEX and coagulation is potentially a viable pretreatment approach to reduce membrane fouling and in general removal of colloidal NOM in feedwater is an effective approach for membrane fouling control and should be considered in the research, development, and application of novel LPM-based treatment processes.

Refractory fluorescent dissolved organic matter in conventional and membrane-based drinking water treatment processes.

Fluorescent dissolved organic matter (fDOM) has been generally considered a refractory DOM component for drinking water treatment. However, this judgement is made without clear understandings on the removal behaviors of individual fDOM fractions. Therefore, the removals of high, medium and low molecular weight (MW), as well as hydrophobic fDOM fractions in a natural surface water were determined in this study for selected bench- and full-scale water treatment processes. The results showed that low MW (<1000 Da) and hydrophobic fractions of protein-like fDOM were more refractory than other fractions and even released during coagulation and ozonation processes. The corresponding removal efficiencies ranged -25.7%-68.6%. Besides, similar-sized, tyrosine- and tryptophan-like fDOM (F-Tyr and F-Trp) fractions exhibited distinct removal behaviors. Coagulation and powdered activated carbon (PAC) adsorption were ineffective in removing both types of fractions. Ozonation and ion exchange (IX) more effectively removed F-Trp, while F-Tyr fractions were more prone to nanofiltration (NF). Moreover, the integration of coagulation and IX pretreatment moderately enhanced F-Trp removal, but not F-Tyr removal by NF. However, the release of protein-like substances during ozonation, coagulation, and activated carbon-sand filtration adversely affected fDOM removal in a full-scale treatment plant. These findings highlighted the persistency of protein-like fDOM fractions in drinking water treatment processes.

Electrocoagulation pretreatment of pulp and paper wastewater for low pressure reverse osmosis membrane fouling control.

Low pressure reverse osmosis (LPRO) has been increasingly used in advanced treatment of pulp and paper wastewater (PPWW) for the purpose of water reuse. However, membrane fouling is a major problem encountered by full-scale RO systems due to the organic and inorganic contents of the feedwater. Electrocoagulation (EC) as an effective treatment for foulants removal can be applied in pre-filtration. Therefore, the LPRO membrane fouling mechanism and the membrane fouling control performance by EC treatment were investigated in this study. EC pretreatment could reduce the membrane fouling and improve the membrane flux by 31%, by effectively removing and/or decomposing the organic pollutants in PPWW. Fluorescent spectrometry analyses of the feedwater and the permeate revealed that humic acid-like and fulvic acid-like organics in PPWW were the major foulants for the LPRO membranes. Fourier transformation infrared spectrometry results confirmed that the organic foulants contained benzoic rings and carboxylic groups, which were typical for organic substances. EC effectively removed organic pollutants containing functional groups such as carboxylic acid COH out-of-plane bending, olefin (trans), and NH3+ symmetrical angle-changing. Moreover, the extended Derjaguin-Landau-Verwey-Overbeek model suggested that the membrane filtered 30-min EC-treated PPWW had the strong repulsion force to foulants due to the higher cohesion energy (12.1 mJ/m2) and the lower critical load, which theoretically explained the reason of EC pretreatment on membrane fouling control.

Effects of water matrix on the rejection of neutral pharmaceutically active compound by thin-film composite nanofiltration and reverse osmosis membranes.

Thin-film composite (TFC) nanofiltration (NF) and reverse osmosis (RO) membranes have been widely used to remove pharmaceutically active compounds (PhACs) from water and wastewater. However, limited information is available to present the rejection of neutral PhACs under complex water matrices. In this study, we used acetaminophen (AAP) as a representative neutral pollutant to study the effects of feedwater matrices on the rejection of neutral PhACs by NF and RO membranes. The results showed that the permeation of solutes and water through NF and RO membranes followed the classical solution-diffusion model. The corresponding permeability coefficients of AAP for the RO membrane showed good consistency, with average values ranging between (6.19-7.56) × 10-6 µm s-1 in fresh and brackish feedwater. Meanwhile, the NF membrane exhibited stable AAP and NaCl fluxes as the applied pressure increased from 4.8 to 7.6 bar, suggesting an insignificant influence of convection on solute transport. In addition, a 10-fold increase in NaCl concentration reduced the average AAP permeability coefficient of the NF membrane by 57% (i.e. from 2.8 × 10-5 m s-1 to 1.2 × 10-5 m s-1), highlighting the relevance of co-existing ions to AAP transport. Furthermore, organic fouling resulted in enhanced AAP rejection by both NF and RO membranes at neutral pH level and medium applied pressure (i.e. 5.8 bar). Overall, this study provided important insights into the separation mechanism of TFC membranes for neutral PhACs, as well as the complex effects of the water matrix on the solute permeation processes.

Use, exposure, and environmental impacts of pesticides in Pakistan: a critical review.

The excessive use of pesticides is posing major threats to humans and the environment. However, the environmental exposure and impact of pesticides in Pakistan have yet been systematically reviewed, despite the country's leading role in pesticide use. Therefore, this study identified and then reviewed 85 peer-reviewed scientific publications on the topic. It was found that, compared to the worldwide average, Pakistan had high consumptions of pesticides, with an alarming increase of 1169% in the last two decades. The quantities of pesticides used followed an order of pyrethroids > organophosphates > organochlorines > carbamates, but organochlorines were the most problematic due to their environmental occurrence, the ability to transport across the media, and identified human and ecological toxicities. Additionally, the misuse or overuse of pesticides by farmers is prevailing due to insufficient knowledge about the risks, which leads to high risks in occupational exposure. These issues are further aggravated by the illegal use or continuous impacts of banned organochlorine pesticides. For the future, we suggested the establishment of organized monitoring, assessment, and reporting program based on environmental laws to minimize contamination and exposure to pesticides in Pakistan. Remediation of the contaminated areas to mitigate the adverse environmental-cum-health impacts are recommended in the most affected regions.

Effects of solution chemistry on the adsorption of ibuprofen and triclosan onto carbon nanotubes.

Single-walled carbon nanotubes (SWCNTs), multiwalled carbon nanotubes (MWCNTs), and oxidized MWCNTs (O-MWCNTs) were studied for the adsorption of ibuprofen (IBU) and triclosan (TCS) as representative types of pharmaceutical and personal care products (PPCPs) under different chemical solution conditions. A good fitting of sorption isotherms was obtained using a Polanyi-Manes model (PMM). IBU and TCS sorption was stronger for SWCNTs than for MWCNTs due to higher specific surface area. The high oxygen content of O-MWCNT further depressed PPCP sorption. The sorption capacity of PPCPs was found to be pH-dependent, and more adsorption was observed at pHs below their pK(a) values. Ionic strength was also found to substantially affect TCS adsorption, with higher adsorption capacity observed for TCS at lower ionic strength. In the presence of a reference aquatic fulvic acid (FA), sorption of IBU and TCS was reduced due to the competitive sorption of FA on carbon nanotubes (CNTs). Sorption isotherm results with SWCNTs, MWCNTs and O-MWCNTs confirmed that the surface chemistry of CNTs, the chemical properties of PPCPs, and aqueous solution chemistry (pH, ionic strength, fulvic acid) all play an important role in PPCP adsorption onto CNTs.

Efficient removal of bisphenol-A from water and wastewater by Fe2O3-modified graphene oxide.

Bisphenol-A (BPA) has been widely used as a plasticizer in modern society and persistently released into aquatic environments. Herein, a novel Fe2O3-graphene oxide (GO) hybrid containing 22.8% of GO was prepared to enhance BPA removal from contaminated water and wastewater. This hybrid material afforded outstanding BPA adsorption capacities of 3293.9 mg g-1 under optimized conditions, which led to 1.9 times and 1.2 times of BPA removal as compared to GO and reduced GO (rGO), respectively. In addition, Fe2O3-GO showed higher thermal stability, greater solid/liquid separation performance, and better anti-fouling performance. Moreover, the coexistence of natural or effluent organic matter caused 6.7-16.8% decline in BPA adsorption capacity of Fe2O3-GO, which was lower than those of GO and rGO (11.8-39.4%). Further characterization experiments revealed that BPA removal by Fe2O3-GO was enhanced because of the formation of Lewis acid-base (AB) interactions between the active sites on Fe2O3 (Lewis base) and BPA anions (Lewis acid). The existence of the AB interaction is beneficial for practical application considering the low environmental concentrations of BPA in water and wastewater. Besides, the distinctly lowered GO content of the hybrid saved 77.2% of the adsorbent cost. In conclusion, this study demonstrated the potential of Fe2O3-GO as a novel material for the treatment of BPA-contaminated water and wastewater.

[Removal Behavior of Protein-like Dissolved Organic Matter During Different Water Treatment Processes in Full-Scale Drinking Water Treatment Plants].

Protein-like dissolved organic matter (pDOM), which is ubiquitous in natural waters, is a critical precursor of nitrogenous disinfection byproducts. Recently, the control and elimination of pDOM have been a growing concern during drinking water treatment processes. In this study, a high-performance size exclusion chromatography system coupled with photo-diode array, fluorescence detector, and online organic carbon detector (HPSEC-PDA/FLD/OCD) was used to determine the removal behaviors of different-sized pDOM from two full-scale drinking water treatment plants (DWTPs). Coagulation and activated carbon adsorption were selected for bench-scale experiments to further assess the removal behavior of pDOM during conventional water treatment processes. The results showed that different-sized pDOM fractions exhibited different removal characteristics. Pre-oxidation can effectively remove some tyrosine-like and tryptophan-like components with high MW, and as the oxidization effect was enhanced, more high MW fractions decomposed into low MW ones. Conversely, some aliphatic pDOM fractions in high MW (e.g., aliphatic proteins) were not subject to pre-oxidation removal. The coagulation-sedimentation unit was efficient in removing high MW fractions, specifically tryptophan-like fractions. Additionally, some pDOM components may be released during coagulation. pDOM with low MW and high hydrophobicity were easily removed during activated carbon filtration. However, long-term operation of the activated carbon filter may breed microorganisms, resulting in the partial release of pDOM fractions. Moreover, UV disinfection processes promoted the degradation of low MW pDOM components. Due to the complex water quality and uncontrollable microbial activities, the aforementioned water treatment units did not exhibit a synergistic effect on pDOM removal. In comparison with humic-like substances, pDOM was susceptible to water quality changes, and its removal was limited in the surveyed DWTPs. Therefore, DWTPs must strengthen pDOM monitoring in influent and effluent and adjust the operating parameters of different treatment units in a timely manner. Moreover, the combination of advanced water treatment processes, such as ozone-biological activated carbon process and nanofiltration, should also be considered to strictly control pDOM component removal.

Silica scaling of reverse osmosis membranes preconditioned by natural organic matter.

Reverse osmosis (RO) membranes were preconditioned in this study with humic acid, sodium alginate, or bovine serum albumin, and subsequently examined for silica scaling using the water matrix representative of concentrated brackish groundwater. The results suggested that water matrix combined with organic foulants affected silica scaling. High ambient pH favored the moderate silica ionization and thus the silica homogeneous polymerization to potentially form low molecular weight silica oligomers. The resulting scaling layer was dense and highly impermeable. Under the high Ca proportion at a given hardness, membrane scaling was enhanced through the Ca-induced silica scaling and the formation of intermolecular bridges between adjacent silica species. In contrast, high Mg hardness may facilitate the sustainable growth of silica oligomers to form the ringed high molecular weight oligomers by reducing the required energy for chain deformation. The deposition of these oligomers caused a loose scaling layer with reduced hydraulic resistance to water permeation. During the scaling tests under similar water matrix, the membranes slightly fouled by organics suffered severe flux decline due to an available space provided by the pre-existing organic fouling layer for subsequent silica scaling.

Comparison of emerging contaminant abatement by conventional ozonation, catalytic ozonation, O3/H2O2 and electro-peroxone processes.

The abatement of several emerging contaminants (ECs) in groundwater by conventional ozonation and three ozone-based advanced oxidation processes (AOPs) - catalytic ozonation with manganese dioxide (MnO2), conventional peroxone (O3/H2O2), and electro-peroxone (EP) - was compared in this study. The addition of MnO2, H2O2, or electro-generation of H2O2 during ozonation enhanced ozone transformation to hydroxyl radicals to different extent. These changes did not considerably influence the abatement of ECs with moderate to high ozone reactivities ( [Formula: see text] ), whose abatements were similar with >90 % during all four processes. In comparison, the abatements of ozone-refractory ECs (kO3< 15 M-1s-1) were lower during conventional ozonation (∼40-85 % abatement), but could be enhanced by ∼10-40 % during the three ozone-based AOPs. Besides enhancing ozone-refractory EC abatement, the three AOPs, especially the O3/H2O2 and EP processes, reduced considerably bromate formation compared to conventional ozonation. These results demonstrate that the EP process performs similarly as catalytic ozonation and O3/H2O2 processes in terms of EC abatement and bromate control. Considering its more convenient, flexible, and safer way of operation, the EP process may provide an attractive alternative to the two more traditional AOPs for water treatment.

Data gaps in evidence-based research on small water enterprises in developing countries.

Small water enterprises (SWEs) are water delivery operations that predominantly provide water at the community level. SWEs operate beyond the reach of piped water systems, selling water to households throughout the world. Their ubiquity in the developing world and access to vulnerable populations suggests that these small-scale water vendors may prove valuable in improving potable water availability. This paper assesses the current literature on SWEs to evaluate previous studies and determine gaps in the evidence base. Piped systems and point-of-use products were not included in this assessment. Results indicate that SWES are active in urban, peri-urban and rural areas of Africa, Asia and Latin America. Benefits of SWEs include: no upfront connection fees; demand-driven and flexible to local conditions; and service to large populations without high costs of utility infrastructure. Disadvantages of SWEs include: higher charges for water per unit of volume compared with infrastructure-based utilities; lack of regulation; operation often outside legal structures; no water quality monitoring; increased potential for conflict with local utilities; and potential for extortion by local officials. No rigorous, evidence-based, peer-reviewed scientific studies that control for confounders examining the effectiveness of SWEs in providing potable water were identified.

Dependence of initial silica scaling on the surface physicochemical properties of reverse osmosis membranes during bench-scale brackish water desalination.

Silica scaling of reverse osmosis membranes in brackish water desalination is less understood than hardness scaling due to the complex silica behaviors at the membrane/water interface. In this study, -COOH, -SO3H, -NH2 and -OH functional groups were introduced onto polyamide membranes to create distinct surface physicochemical properties. The resulting membranes were further studied under similar scaling conditions to yield temporal flux loss data that were empirically interpreted by a logistic growth model. The scaled membranes were also characterized by complementary analytical techniques. It was found that permeate flux loss was strongly correlated to the initial silica layer formed by direct interaction between reactive silanol (Si-OH) and reciprocal groups on the membrane surface, rather than the entire scaling layer. Importantly, membrane surface properties dictated the initial silica layer formation through three possible mechanisms, i.e., electrostatic repulsion, competitive adsorption, and interfacial energy change. Of these, electrostatic repulsion was identified as the primary one. Therefore, by modifying the membrane surface properties, the three aforementioned mechanisms may be enhanced to favor the formation of a loose, disordered initial silica scaling layer. Accordingly, membrane flux loss may be mitigated. This finding provided important insights into the design heuristics of scaling-resistant reverse osmosis membrane for brackish water desalination.

Effective adsorption of trace phosphate and aluminum in realistic water by carbon nanotubes and reduced graphene oxides.

In this study, carbon nanotube (CNT) and reduced graphene oxide (rGO) were studied for their potentials as novel adsorbents for trace concentrations of phosphorus and aluminum in water and wastewater. Static adsorption results demonstrated that CNT and rGO employed in this study removed up to 65.6% of total dissolved Al and 98.9% of P from a natural surface water and a secondary wastewater effluent. Hydrogen-bonding interactions between CNT/rGO and oxyanions were hypothesized to contribute to the adsorption process. Accordingly, acetaminophen (AAP), a pharmaceutical compound known to form hydrogen bonding with CNT, was spiked into the real water as a competitor for P and Al adsorption. Subsequent sorption results showed that the presence of AAP reduced Al and P adsorption by CNT and rGO by 9.3%-18.4% and 11.2%-18.2%, respectively. These results suggest that hydrogen bonding interactions with CNT/rGO influenced the adsorption of P and Al species. In addition, pH effect investigation on Al/P removal further verified the above opinion. Overall, this study provided important evidence and insights into CNT/rGO adsorption of P and Al species from water and wastewater, which expanded our understanding on the ability of carbonaceous nanomaterials for advanced water and wastewater treatment.

Factors affecting microbiological quality of household drinking water supplied by small-scale ultrafiltration systems: A field study.

Small scale ultrafiltration (UF) systems have been increasingly used in rural areas for drinking water supply, but their effectiveness in guarantying microbiological water safety at household level has rarely been assessed. Therefore, this study surveyed six representative villages where UF was utilized for full-scale drinking water supply for at least four years. At each village, the influent and the effluent from every stages of the treatment, as well as household tap water, were sampled and analyzed for microbiological indicating parameters, including total coliform count, Escherichia coli count, and heterotrophic plate count. The results were further assessed against current drinking water quality guidelines and standards. It was found that: (1) the qualification rate of household tap water samples varied substantially in the studied villages (0-75%), mainly due to the lack of post-disinfection and the occurrence of fecal contamination during water distribution; (2) UF appeared to be effective in controlling microbial contamination for small-scale systems with high-quality source water, while for systems using inferior source water, fecal contamination during water distribution necessitated continuous post-disinfection; and, (3) existing monitoring of membrane operational parameters cannot ensure microbial quality of treated water, and therefore, routine monitoring of microbial indicators in household water is recommended.

Ar/O2 plasma treatment of carbon nanotube membranes for enhanced removal of zinc from water and wastewater: A dynamic sorption-filtration process.

In this study, pristine multi-walled carbon nanotubes (MWCNTs) were functionalized by using Ar/O2 plasma treatment technique, which enhanced adsorptive membrane filtration of zinc ions from water and wastewater. The XPS analysis showed that plasma treatment largely increased the surface oxygen groups content of MWCNTs from 2.78% to 6.79%. This change increased the surface negative charged, dispersion and adsorption properties of MWCNTs without causing any damages to the integrity of the nanotube pattern. Pressure-driven filtration of plasma-treated MWCNT (P-CNT) dispersion formed a stable layer inside the lumen of a hollow fiber membrane. The contact angle analysis demonstrated that after incorporation of P-CNT into the HF membrane increased membrane hydrophilicity. The P-CNT membrane effectively removed almost 100% of zinc from synthetic waters and approximately 80% of zinc from a wastewater effluent by surface complexation reaction. A follow-up regeneration study demonstrated that the adsorptive removal of zinc by the CNT membrane was reversible under selected conditions, thus making it possible to repeatedly use the membrane for long-term zinc filtration. This study suggests that HF membranes modified with P-CNTs possess superb adsorption properties for metal ions, allowing the operation of CNT membranes for water treatment.

Challenges and opportunities in functional carbon nanotubes for membrane-based water treatment and desalination.

Environmental applications of carbon nanotubes (CNTs) have grabbed worldwide attentions due to their excellent adsorption capacities and promising physical, chemical and mechanical properties. The functionalization of CNTs, which involves chemical/physical modification of pristine CNTs with different types of functional groups, improves the capabilities of CNT for desalination and/or removals of waterborne contaminants. This paper intends to provide a comprehensive review of functional CNT materials (f-CNT) and their existing and potential applications in membrane-based water treatment and desalination processes, with focuses on critical evaluation of advances, knowledge gaps and future research directions. CNT nanocomposite membranes have been studied at bench scale to efficiently remove a variety of waterborne contaminants and salts, while future improvement is under way with development in CNT functionalization techniques. The CNT-based membrane applications are found to possess a variety of advantages, including improve water permeability, high selectivity and antifouling capability. However, their applications at full scale are still limited by their high cost. Finally, we highlight that f-CNT membranes with promising removal efficiencies for respective contaminants be considered for commercialization and to achieve holistic performance for the purpose of water treatment and desalination.