Electrostatic-induced ion-confined partitioning in graphene nanolaminate membrane for breaking anion-cation co-transport to enhance desalination.

Constructing nanolaminate membranes made of two-dimensional graphene oxide nanosheets has gained enormous interest in recent decades. However, a key challenge facing current graphene-based membranes is their poor rejection for monovalent salts due to the swelling-induced weak nanoconfinement and the transmembrane co-transport of anions and cations. Herein, we propose a strategy of electrostatic-induced ion-confined partitioning in a reduced graphene oxide membrane for breaking the correlation of anions and cations to suppress anion-cation co-transport, substantially improving the desalination performance. The membrane demonstrates a rejection of 95.5% for NaCl with a water permeance of 48.6 L m-2 h-1 bar-1 in pressure-driven process, and it also exhibits a salt rejection of 99.7% and a water flux of 47.0 L m-2 h-1 under osmosis-driven condition, outperforming the performance of reported graphene-based membranes. The simulation and calculation results unveil that the strong electrostatic attraction of membrane forces the hydrated Na+ to undergo dehydration and be exclusively confined in the nanochannels, strengthening the intra-nanochannel anion/cation partitioning, which refrains from the dynamical anion-cation correlations and thereby prevents anions and cations from co-transporting through the membrane. This study provides guidance for designing advanced desalination membranes and inspires the future development of membrane-based separation technologies.

Electrochemical Opening of Impermeable Nanochannels in Laminar Graphene Membranes for Ultrafast Nanofiltration.

Reduced graphene oxide (rGO) could be theoretically used to construct highly permeable laminar membranes with nearly frictionless nanochannels for water treatment. However, their pristine (sp2 C-C) regions usually restack into impermeable channels as a result of van der Waals interactions, resulting in a much low permeance. In this study, we demonstrate that the restacked regions could be electrochemically expanded to form ultrafast water transport nanochannels by providing a low positive potential (e.g., +1.00 V vs SCE) to the rGO membrane. Experimental investigations indicate that the structural expansion is attributed to the intercalation of water molecules into the restacked regions, driven by hydrogen bond interactions between water molecules and hydroxyl groups that are electrochemically produced on edges of rGO nanosheets. The structural expansion could be promoted by weakening the graphene-OH- interactions through intermittent application of the potential. As a result of more ultrafast water transport nanochannels available, the electrochemically treated rGO membranes could have a permeance 2 orders of magnitude higher than that of the pristine one and ∼3 times higher than that of graphene oxide membranes. Because of their smaller average pore size, the rGO membranes also have a higher ionic/molecular rejection performance than graphene oxide membranes.

Improving the Performance of the Lamellar Reduced Graphene Oxide/Molybdenum Sulfide Nanofiltration Membrane through Accelerated Water-Transport Channels and Capacitively Enhanced Charge Density.

Graphene is promising in the construction of next-generation nanofiltration membranes for wastewater treatment and water purification. However, the application of graphene-based membranes has still been prohibited by their deficiencies in permeability and ion rejection. Herein, regulating the 2D channel and enhancing the charge density are co-adopted for simultaneous enhancement of the water flux and salt rejection of reduced graphene oxide (rGO) membranes through the intercalation of molybdenum sulfide (MoS2) nanosheets and external electrical assistance. The fabricated rGO/MoS2 membranes possess expanded nanochannels with less friction and a higher water molecule transport velocity gradient (from 8.57 to 14.07 s-1) than those of rGO membranes. Consequently, their water permeance increases from 0.92 to 34.9 L m-2 h-1 bar-1. Meanwhile, benefiting from the high capacitance and negative potential of -1.1 V versus the saturated calomel electrode given to the membranes, their rejection rates toward NaCl reach 87.2% and those toward Na2SO4 reach 93.7%. The Donnan steric pore model analysis indicates that the capacitively and electrically increased surface charge density make great contributions to the higher ion rejection rate. This work gives new insights into membrane design for high water flux and salt rejection efficiency.

Scaling behavior of iron in capacitive deionization (CDI) system.

This study investigated the fouling and scaling behaviors in a capacitive deionization (CDI) system in the presence of iron and natural organic matter (NOM). It was found that the salt adsorption capacity (SAC) significantly decreased when treating Fe-containing brackish water, with higher Fe concentrations leading to severer SAC reduction. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis demonstrated that Fe2O3 appeared to be the predominant foulant attached on the electrode surface, which was difficult to be removed via backwashing, indicating the irreversible property of the foulant. Further characterizations (e.g., N2 sorption-desorption isotherms, electrochemical impedance spectroscopy and cyclic voltammetry) revealed that the CDI electrodes suffered from obvious deterioration such as specific surface area loss, resistance increase and capacitance decline with the occurrence of Fe scaling. While the presence of NOM alleviated the Fe scaling through NOM-Fe complexing effects, NOM itself was found to have negative impacts on CDI desalination performance due to their strong interactions with the carbon electrodes.

Insight into Fe(II)/UV/chlorine pretreatment for reducing ultrafiltration (UF) membrane fouling: Effects of different natural organic fractions and comparison with coagulation.

Fe(II)/UV/chlorine was promoted as a pretreatment strategy for UF membrane to mitigate membrane fouling induced from different organic fractions. This treatment could be an emerging alternative prior to UF process attributed to the coupled role of oxidation and coagulation. To obtain a comprehensive understanding of fouling reduction, the influence of Fe(II)/UV/chlorine process on the characteristics of various feed solutions was inspected, including humic acid (HA), bovine serum albumin (BSA), sodium alginate (SA) and their mixture (HSB). The results suggested that Fe(II)/UV/chlorine process exhibited notable performance on membrane fouling control compared to Fe(II) coagulation alone. With the UV exposure of 720 mJ/cm2, the certain dose of Fe(II) and chlorine (15 µM and 2 mg/L) effectively prevented the rapid development of fouling caused by the single organic fractions and their mixture. And the increased dosage promoted the performance of membrane fouling mitigation. The reduction of organic loadings and characteristics change of feed water took the main responsibility for the fouling alleviation. The properties of membrane fouling and their correlation with feed water qualities were analyzed. The results and insight analysis were supposed to evaluate and predict the effectiveness of fouling control when the feed solutions were pretreated by Fe(II)/UV/chlorine process according to various compositions and characteristics of the organic fractions.

Ordered Mesoporous Cobalt Containing Perovskite as a High-Performance Heterogeneous Catalyst in Activation of Peroxymonosulfate.

An ordered mesoporous perovskite, La2CoMnO6-δ (MLCMO), was synthesized for the first time using a facile method of evaporation-induced self-assembly. The N2-sorption, scanning electron microscopy, and transmission electron microscopy measurements indicated that the optimized MLCMO possessed a high specific surface area (58.7 m2/g) and was uniformly mesoporous (11.6 nm). The MLCMO exhibited superior catalytic performance in peroxymonosulfate (PMS) activation for atrazine (ATZ) degradation. From a comparison view, the catalytic activity of the mesoporous MLCMO outperformed that of the bulk La2CoMnO6-δ (LCMO) and other common PMS activators, including α-MnO2, Co3O4, and CoFe2O4. The mechanisms of PMS activation by the MLCMO were investigated by X-ray photoelectron spectroscopy, electron spin resonance, and quenching tests. SO4•-, •OH, 1O2, and O2•- were identified as main reactive oxygen species generated from PMS activation. The Co and Mn in MLCMO were the active sites responsible for active radical generation. The lattice oxygen reversible redox sites (OL-/OL2-), which were involved in the electron transfer of the MnIII/MnIV cycle, were demonstrated as redox partners to the cation active sites. In addition, the SO4•-/•OH radical conversion was promoted at pH 11, which accelerated the consumption of PMS and seriously inhibited the degradation of ATZ.

Organic matter removal and membrane fouling mitigation during algae-rich surface water treatment by powdered activated carbon adsorption pretreatment: Enhanced by UV and UV/chlorine oxidation.

In this work, UV and UV/chlorine (UV/Cl) were employed to enhance powdered activated carbon (PAC) adsorption pretreatment prior to ultrafiltration process for algae-contaminated surface water treatment. Their performance on membrane fouling mitigation and organic pollutant rejection was systematically evaluated. A comparative experiment was conducted under varying pollution degrees of algal extracellular organic matter (EOM) contamination in surface river water. The results indicated that UV/PAC and UV/Cl/PAC pretreatment effectively enhanced the removal of dissolved organic carbon (DOC) and UV-absorbing at 254 nm (UV254). The characteristics of feed water after pretreatments were investigated through apparent molecular-weight (MW) distribution and fluorescence parallel factor analysis (PARAFAC). In regard to membrane fouling mitigation, UV/Cl/PAC noticeably decreased reversible and irreversible fouling resistance simultaneously and UV/PAC preferred reducing reversible membrane fouling. Combined fouling modeling was operated to scrutinize the fouling mitigation mechanisms and standard pore blocking was proved to be dominant during the filtration process. Moreover, the UV/Cl and UV/Cl/PAC pretreatments were proved positive for emerging micropollutants degradation and disinfection by-products formation potential reduction. The results suggested that UV and UV/Cl are likely strategies to enhance the efficiency of PAC adsorption pretreatments prior to ultrafiltration during algae-contaminated water treatment.

Combined effects of coagulation and adsorption on ultrafiltration membrane fouling control and subsequent disinfection in drinking water treatment.

This study investigated the combined effects of coagulation and powdered activated carbon (PAC) adsorption on ultrafiltration (UF) membrane fouling control and subsequent disinfection efficiency through filtration performance, dissolved organic carbon (DOC) removal, fluorescence excitation-emission matrix (EEM) spectroscopy, and disinfectant curve. The fouling behavior of UF membrane was comprehensively analyzed especially in terms of pollutant removal and fouling reversibility to understand the mechanism of fouling accumulation and disinfectant dose reduction. Pre-coagulation with or without adsorption both achieved remarkable effect of fouling mitigation and disinfection dose reduction. The two pretreatments were effective in total fouling control and pre-coagulation combined with PAC adsorption even decreased hydraulically irreversible fouling notably. Besides, pre-coagulation decreased residual disinfectant decline due to the removal of hydrophobic components of natural organic matters (NOM). Pre-coagulation combined with adsorption had a synergistic effect on further disinfectant decline rate reduction and decreased total disinfectant consumption due to additional removal of hydrophilic NOM by PAC adsorption. The disinfectant demand was further reduced after membrane. These results show that membrane fouling and disinfectant dose can be reduced in UF coupled with pretreatment, which could lead to the avoidance of excessive operation cost disinfectant dose for drinking water supply.

Effect of sulfate radical-based oxidation pretreatments for mitigating ceramic UF membrane fouling caused by algal extracellular organic matter.

Algal extracellular organic matter (EOM) released from Microcystis aeruginosa can cause severe membrane fouling during algae-laden water treatment. To solve this problem, three typical sulfate radical-based advanced oxidation processes (SR-AOPs), i.e., ferrous iron/peroxymonosulfate (Fe(II)/PMS), UV/PMS and UV/Fe(II)/PMS, were employed as membrane pretreatment strategies. Their performance on mitigating EOM fouling of a ceramic UF membrane was systematically investigated and compared in the present study. The results indicated that SR-AOPs pretreatments could promote the reduction of DOC and UV254, and the removal performance showed an apparent regularity of UV/Fe(II)/PMS > Fe(II)/PMS > UV/PMS. The pretreatments were very effective for decomposing high-MW biopolymers (>20,000 Da) into low-MW humic substances (1000-20,000 Da), thus reducing the accumulation of high-MW biopolymers on membrane surface. With respect to membrane fouling control, Fe(II)/PMS significantly mitigated both reversible and irreversible membrane fouling, whereas UV/PMS only reduced reversible fouling, and exhibited little effect on irreversible fouling. By contrast, UV/Fe(II)/PMS showed the best performance for fouling reduction due to the synergistic effect of UV and Fe(II) for PMS activation. The dominating fouling mechanism was governed by both pore blockage and cake filtration, likely due to the bimodal MW distribution of EOM, and SR-AOPs pretreatments delayed the transition from pore blockage to cake filtration. In addition, SR-AOPs prior to UF membrane were also very effective to improve the removal of micropollutants (i.e., ATZ, SMT and p-CNB). These results demonstrate the potential application of SR-AOPs as pretreatment for membrane fouling control during algae-laden water treatment.

Application of Fe(II)/peroxymonosulfate for improving ultrafiltration membrane performance in surface water treatment: Comparison with coagulation and ozonation.

Coagulation and ozonation have been widely used as pretreatments for ultrafiltration (UF) membrane in drinking water treatment. While beneficial, coagulation or ozonation alone is unable to both efficiently control membrane fouling and product water quality in many cases. Thus, in this study an emerging alternative of ferrous iron/peroxymonosulfate (Fe(II)/PMS), which can act as both an oxidant and a coagulant was employed prior to UF for treatment of natural surface water, and compared with conventional coagulation and ozonation. The results showed that the Fe(II)/PMS-UF system exhibited the best performance for dissolved organic carbon removal, likely due to the dual functions of coagulation and oxidation in the single process. The fluorescent and UV-absorbing organic components were more susceptible to ozonation than Fe(II)/PMS treatment. Fe(II)/PMS and ozonation pretreatments significantly increased the removal efficiency of atrazine, p-chloronitrobenzene and sulfamethazine by 12-76% and 50-94%, respectively, whereas coagulation exerted a minor influence. The Fe(II)/PMS pretreatment also showed the best performance for the reduction of both reversible and irreversible membrane fouling, and the performance was hardly affected by membrane pore size and surface hydrophobicity. In addition, the characterization of hydraulic irreversible organic foulants confirmed its effectiveness. These results demonstrate the potential advantages of applying Fe(II)/PMS as a pretreatment for UF to simultaneously control membrane fouling and improve the permeate quality.

[Effects of Pretreatment on Hydraulic Irreversible Membrane Fouling During Ultrafiltration Short Process: A Pilot Study].

Pilot-scale performances for ultrafiltration of Dongjiang river water without and with micro-coagulation and coagulation-adsorption-sedimentation pretreatments were conducted to investigate the effects of pretreatments on hydraulic irreversible fouling of ultrafiltration short processes. Scanning electron microscope (SEM) imaging, high performance size exclusion chromatography (HPSEC) and fluorescence excitation-emission matrix (EEM) were employed to identify foulants responsible for irreversible fouling during ultrafiltration with and without pretreatments. The results showed that the hydraulic irreversible fouling rate was 0.16 kPa·d-1 with micro-coagulation pretreatment at 10 L·(m2·h)-1, while the rate without pretreatment was 0.32 kPa·d-1 at the same flux. Meanwhile, the fouling rate with coagulation-adsorption-sedimentation pretreatment was almost zero at the same flux. Furthermore, when the flux increased to 17 L·(m2·h)-1, the irreversible fouling was not observable with coagulation-adsorption-sedimentation pretreatment (backwashing every 24 hours). Properties of foulants from external membrane and internal membrane after backwashing showed that the protein-like and fulvic-like substances were responsible for irreversible fouling in Dongjiang river water. Furthermore, structural alteration of cake layer resulted in less cake resistance and higher backwash efficiency after micro-coagulation pretreatment, leading to less irreversible fouling. Coagulation-adsorption-sedimentation pretreatment decreased irreversible fouling with higher removal efficiency of protein-like and fulvic substances, which were the foulants responsible for irreversible fouling during ultrafiltration. Both micro-coagulation and coagulation-adsorption-sedimentation pretreatments decreased irreversible fouling during ultrafiltration short process.