Modification of the distribution of humic acid complexations by introducing microbubbles to membrane distillation process for effective membrane fouling alleviation.

Membrane fouling caused by inorganic ions and natural organic matters (NOMs) has been a severe issue in membrane distillation. Microbubble aeration (MB) is a promising technology to control membrane fouling. In this study, MB aeration was introduced to alleviate humic acid (HA) composited fouling during the treatment of simulative reverse osmosis concentrate (ROC) by vacuum membrane distillation (VMD). The objective of this work was to explore the HA fouling inhibiting effect by MB aeration and discuss its mechanism from the interfacial point of view. The results showed that VMD was effective for treating ROC, followed by a severe membrane fouling aggravated with the addition of 100 mg/L HA in feed solution, resulting in 45.7% decline of membrane flux. Analysis using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and zeta potential distribution of charged particles proved the coexistence of HA and inorganic cations (especially Ca2+), resulting in more serious membrane fouling. The introduction of MB aeration exhibited excellent alleviating effect on HA-inorganic salt fouling, with the normalized flux increased from 19.7% to 37.0%. The interfacial properties of MBs played an important role, which altered the zeta potential distributions of charged particles in HA solution, indicating that MBs adhere the HA complexations. Furthermore, this mitigating effect was limited at high inorganic cations concentration. Overall, MBs could change the potential characteristics of HA complexes, which also be used for other similar membrane fouling alleviation.

Preparation of SGO-modified nanofiltration membrane and its application in SO42- and Cl- separation in salt treatment.

The lack of fresh water in the world makes the search for an effective method to decontaminate water an urgent priority. An important step is to remove different multivalent ions in salt treatment. Nanofiltration (NF) has been used for treating water containing different kinds of salts. In this work, sulfonate group-modified graphene oxide (SGO) was prepared, and added during the interfacial polymerization (IP) reaction to prepare SGO-modifiedNF membranes (PA-SGO). The chemical composition, structure and surface properties of PA and PA-SGO membranes were characterized by FT-IR, XPS, SEM, AFM, contact angle and zeta potential measurements. Their water flux, salt rejection and anti-fouling abilities were investigated systematically. The testing results showed that the water flux of PA-SGO (0.03% SGO) was 45.85 LMH under a pressure of 0.2 MPa, and the salt rejection varied in the order of Na2SO4 (98.99%) > MgSO4 (91.25%) > MgCl2 (42.27%) > NaCl (21.96%). An anti-fouling experiment indicated that the PA-SGO membrane had good anti-fouling properties because of its decreased roughness and increased hydrophilicity and electronegativity. The PA-SGO membrane has good potential for use in removing salt ions from water.

Zwitterion-like, Charge-Balanced Ultrathin Layers on Polymeric Membranes for Antifouling Property.

Zwitterions of charge-balanced units have super-low fouling properties induced by ionic solvation, but their extensive applications in polymeric substrates are strictly constrained by current constructing strategies. A zwitterion-like, charge-balanced ultrathin layer with high antifouling capacity was covalently constructed on delicate aromatic polyamide (PA) reverse osmosis (RO) membranes via a mild and solvent-free grafting-to strategy. Two oppositely charged commercial short-chain carbonyl alkenes, 2-acrylamido-2-methylpropanesulfonic acid (AMPS) and methacryloxyethyltrimethylammonium chloride (DMC), were directly mixed-grafted with amino groups on PA RO membrane surface via Michael addition. Under ambient temperature and pressure, these oppositely charged compounds were assembled into a zwitterion-like, charge-balanced ultrathin layer. The dynamic fouling experiments indicated that the modified membrane exhibited strong antifouling properties and excellent permeation recovery abilities. Surface characterization revealed that the selective layer thickness and surface roughness were not measurably changed. More meaningful is that the typical ridge-and-valley surface structure and the excellent separation performance were both well preserved after modification. This opens a universal avenue to construct a zwitterion-like, ultrathin antifouling layer on the delicate polymer substrate without compromising its original matrix structure and performance, which has promising application in areas of biosensors, tissue engineering, and biomaterials.

Functionalization of seawater reverse osmosis membrane with quorum sensing inhibitor to regulate microbial community and mitigate membrane biofouling.

Membrane biofouling is a challenge to be solved for the stable operation of the seawater reverse osmosis (SWRO) membrane. This study explored the regulation mechanism of quorum sensing (QS) inhibition on microbial community composition and population-level behaviors in seawater desalination membrane biofouling. A novel antibiofouling SWRO membrane (MA_m) by incorporating one of quorum sensing inhibitors (QSIs), methyl anthranilate (MA) was prepared. It exhibited enhanced anti-biofouling performance than the exogenous addition of QSIs, showing long-term stability and alleviating 22 % decrease in membrane flux compared with the virgin membrane. The results observed that dominant bacteria Epsilon- and Gamma-proteobacteria (Shewanella, Olleya, Colwellia, and Arcobacter), which are significantly related to (P ≤ 0.01) the metabolic products (i.e., polysaccharides, proteins and eDNA), are reduced by over 80 % on the MA_m membrane. Additionally, the introduction of MA has a more significant impact on the QS signal-sensing pathway through binding to the active site of the transmembrane sensor receptor. It effectively reduces the abundance of genes encoding QS and extracellular polymeric substance (EPS) (exopolysaccharides (i.e., galE and nagB) and amino acids (i.e., ilvE, metH, phhA, and serB)) by up to 50 % and 30 %, respectively, resulting in a reduction of EPS by more than 50 %, thereby limiting the biofilm formation on the QSI-modified membrane. This study provides novel insights into the potential of QSIs to control consortial biofilm formation in practical SWRO applications.

A comprehensive evaluation of the temporal and spatial fouling characteristics of RO membranes in a full-scale seawater desalination plant.

The fouling of seawater reverse osmosis (SWRO) membranes remains a persistent challenge in desalination. Previous research has focused mainly on fouling separately; however, organic, inorganic, and biofouling can coexist and influence each other. Hence, in-depth study of the spatiotemporal changes in actual combined fouling in full-scale seawater desalination will provide more effective information for fouling investigation and control. In this study, we monitored (i) the operational performance of a full-scale desalination plant for 7 years and (ii) the development and characterization of membrane and spacer fouling at different locations of spiral-wound membrane modules sampled after 2.5-, 3.5-, and 7-year operation. The findings showed that (i) operational performance indicators declined with time (normalized flux 40 % reduction, salt rejection 2 % in 7 years), with a limited effect of the 20-day cleaning frequency, (ii) fouling accumulation in the membrane module mainly occurred at the feed side of the lead module and the microbial community in these area exhibited the highest diversity, (iii) the dominant microbial OTUs belonged mainly to Proteobacteria (43-70 %), followed by Bacteroidetes (10-11 %), (iv) Phylogenetic molecular ecological networks and Spearman correlation analysis revealed that Chloroflexi (Anaerolineae) and Planctomycetes were keystone species in maintaining the community structure and biofilm maturation and significantly impacted the foulant content on the SWRO membrane, even with low abundance, and that (v) fouling accumulation was composed of polysaccharides, soluble microbial products, marine humic acid-like substances, and inorganic Ca/Fe/Mg/Si dominate the fouling layer of both the membrane and spacer. Overall, variation partitioning analysis quantitatively describes the increasing contribution of biofouling over time. Ultimately, the organic‒inorganic-biofouling interaction (70 %) significantly contributed to the overall fouling of the membrane after 7 years of operation. These results can be used to develop more targeted fouling control strategies to optimize SWRO desalination plant design and operation.

Insights into graphene oxide/ferrihydrite adsorption as pretreatment during ultrafiltration: Membrane fouling mitigation and disinfection by-product control.

In this study, a novel adsorbent of graphene oxide (GO) incorporated ferrihydrite (FH) was fabricated and integrated with ultrafiltration (UF) to remove natural organic matter (NOM), the crucial cause of membrane fouling and major precursor of disinfection by-products (DBPs). Compared with FH and powdered activated carbon (PAC), GO/FH exhibited superior removal for high molecular weight (HMW) humic- and fulvic-like substances and low molecular weight (LMW) protein. The cake layer formed by GO/FH alleviated the deposition of NOM on membrane surface or inside membrane pores. Therefore, GO/FH reduced 89% and 95% total fouling resistance and irreversible membrane resistance, respectively, together with the lowest increment of transmembrane pressure. Pearson correlation analysis indicated that DOC, rather than specific ultraviolet absorbance (SUVA) and UV254, was significantly correlated to the formation of trihalomethanes (THMs) and haloacetic acids (HAAs) when SUVA was below 4 L/mg-C.m. Whilst the HMW NOM (1-20 kDa) was highly related to dibromochloromethane (DBCM) (r = 0.98-1), the LMW fraction (< 1 kDa) was correlated with dibromochloromethane (TCAA) and dichloroacetic acid (DCAA) (r = 0.88-0.98). Inspiringly, GO/FH-UF reduced 90% of carbonaceous DBPs, the concentrations of which well met the WHO Guidelines. In summary, GO/FH-UF substantially alleviated membrane fouling and dramatically reduced DBP formation potential.

Efficient removal of Cs(I) from water using a novel Prussian blue and graphene oxide modified PVDF membrane: Preparation, characterization, and mechanism.

The Prussian blue (PB) blending membranes are promising candidates for the removal of trace radionuclide Cs+. Constructing a membrane with high flux and selectivity are challenging in its practical application. Here, a novel polyvinylidene fluoride (PVDF)-PB-graphene oxide (GO) modified membrane was fabricated via phase inversion for trace radionuclide cesium (137Cs) removal from water. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM) were used to analyze chemical composition and morphology of the membrane, and the properties in terms of water flux and Cs+ removal were studied under different PB dosage, pH and co-existing ions conditions. It was observed that the addition of GO improved the dispersion of PB, and the PVDF-PB-GO membrane presented the highest Cs+ removal efficiency (99.6 %) and water flux (1638.2 LMH/bar) at pH = 7 with 0.1 wt% GO and 5 wt% PB. In addition, Langmuir and pseudo-second-order kinetics models fitted well for Cs+ adsorption by the PVDF-PB-GO membrane, illustrating that the Cs+ was removed via chemical adsorption dominated by Fe(CN)64- defect sites of PB and the oxygen groups of GO. Furthermore, the membrane showed a significant selectivity and reusability towards trace radioactive cesium, even in the presence of excess co-existing ions and in real water, which strongly verified that the modified membrane had application potential.

Combined effects of Pseudomonas quinolone signal-based quorum quenching and graphene oxide on the mitigation of biofouling and improvement of the application potential for the thin-film composite membrane.

Biofouling caused by the growth of the biofilm is the main bottleneck that limits the effective operation of thin-film composite (TFC) membrane in the forward osmosis (FO) process. This study investigated the combined effects of graphene oxide (GO) immobilized thin-film nanocomposite (TFN-S) membrane and Pseudomonas quinolone signal (PQS)-based quorum quenching on biofouling mitigation, especially under the operation of pressure-retarded osmosis (PRO) mode, and the influence of methyl anthranilate (MA) inhibitor on the composition and structure of biofilm was also evaluated. Synthetic wastewater was used as the feed solution, in which the model strain Pseudomonas aeruginosa was added to simulate biofouling. The results showed that GO modification and MA addition both efficiently mitigated flux decline and EPS secretion, but the interference of PQS pathway on biofouling control was better than GO embedding. TFN-S membrane with MA addition exhibited superior anti-biofouling performance based on the combined effects of GO and MA. The alleviated concentration polarization and enhanced hydrophilicity of the TFN-S membrane reduced the flux decline in the early stage. Additionally, the antibacterial property of GO inhibited the viability of the attached bacteria (under PRO mode) and MA further mitigated the EPS secretion and biofilm development in the later stage. In the presence of PQS inhibitor MA, live/total cells ratio was 15% and 13% higher than that of TFC membrane in FO and PRO modes, respectively. Furthermore, exogenous addition of MA led to a relatively loose biofilm structure, resulting in high membrane permeability in the biofouling formation process.

Removal of levofloxacin through adsorption and peroxymonosulfate activation using carbothermal reduction synthesized nZVI/carbon fiber.

Nano zero-valent iron (nZVI) is widely used for decontamination. The main issues associated with nZVI are agglomeration and oxidation in the long term. In this study, the carbothermal reduction of cotton fiber was conducted for the synthesis of nZVI supported on cotton carbon fiber (nZVI/CF) to address the agglomeration and oxidation of nZVI. Synergistic adsorption and peroxymonosulfate (PMS) activation using nZVI/CF for removing levofloxacin (LEV) are reported herein. The nZVI concentration and morphology were conveniently adjusted by soaking cotton fiber in ferric nitrate solutions of various Fe3+ concentrations. The carbothermal reduction of the cotton fiber at 900 °C contributed to the reduction of Fe3+ into nZVI. A nZVI/CF-900-0.3 system was obtained through the carbothermal reduction of cotton fiber soaked in 0.3 M ferric nitrate. Favorable adsorption of nZVI/CF-900-0.3 to LEV facilitated LEV degradation under PMS activation. Approximately 93.83% of LEV (C0 = 20 ppm) was removed within 60 min with 0.2 g/L of the catalyst and 1 mM PMS. It was preferable to use nZVI + CF-900 to activate PMS for degrading LEV, thus confirming the favorable effect of LEV adsorption on further degradation. The nZVI/CF-900-0.3 exhibited excellent long-term stability given that it was able to activate PMS after it was stored for 6 months. ·SO4- played an important role in LEV degradation in the presence of PMS.

Removal of pharmaceuticals by fouled forward osmosis membranes: Impact of DOM fractions, Ca2+ and real water.

This study systematically investigated the impact of dissolved organic matters (DOM) fractions, Ca2+, membrane orientation and real water matrix on the membrane fouling and the subsequent pharmaceutical retention in forward osmosis (FO). Ca2+ increased the removal of carbamazepine (CBZ) through steric effect, while it reduced sulfamethoxazole (SMZ) removal due to reduced electrostatic repulsion and enhanced external concentration polarization for three organic foulants. The study of operating mode showed that the pharmaceutical removal in pressure retarded osmosis (PRO) mode were lower than those in FO mode for both the baseline and HA fouling, which was attributed to the concentrative internal concentration polarization caused by long-term accumulation of pharmaceuticals or HA in support layer. In terms of the real water tests, the secondary effluent used as feed solution caused higher hydrophilicity and negative charge of fouled FO membrane, leading to increased removal of pharmaceuticals. Seawater used as draw solution also caused severe fouling in the support layer of FO with humic acid-like material as major foulants, increasing the removal of SMZ because of enhanced steric hindrance and electrostatic repulsion. However, the combined effects of increased adsorption and steric effect resulted in little change for the CBZ removal. This study gave implications on the practical application of FO process for pharmaceutical removal.

Assessment of biomass demineralization on gasification: From experimental investigation, mechanism to potential application.

Considering the advantages (e.g. agglomeration mitigation) and disadvantages (e.g. inorganic species catalysis removal) of biomass demineralization, it is valuable to investigate its effects on gasification performance, thus assessing its necessity prior to performing gasification. To accomplish this, corn straw (CS) was demineralized to different degrees with H2O and HCl, respectively. H2O and HCl demineralization behaved different abilities to inorganic species removal. Cellulose and hemicelluloses content decreased, while lignin content increased, especially with HCl demineralization. The experiments were investigated by using a bench-scale downdraft fixed-bed gasifier at 600-800 °C and were further analyzed via thermogravimetric coupled with Fourier transform infrared spectrometry. Demineralization demonstrated a positive effect on gasification at lower temperatures (600-700 °C) for a dominant effect of lignin content and an insignificant effect of inorganic species removal. However, the catalysis of inorganic species increased as the temperature increased, resulting in the highest H2 (11.30 vol%) and CO (16.02 vol%) production of raw CS compared to demineralized CS at 800 °C. Inorganic species had a dual positive effect on CO generation, promoting both CO2 and char generation leading to a higher CO yield following Boundouard reaction, and increasing the formation of active intermediates thus producing more CO. These effects enhanced when the gasification temperature increased. Additionally, inorganic species catalyzed the aromatic rings rearrangement to generate more H2O, thus driving the endothermic Primary water-gas to produce H2. This was also positively correlated with gasification temperature. Therefore, raw CS demonstrated higher H2 and CO production than demineralized CS at a higher gasification temperature. Moreover, the promotion effect of inorganic species on thermal devolatilization of methoxyl groups and Methanation reaction led to the higher CH4 production of raw CS. This research clarifies the effects of biomass demineralization on its gasification and suggests the potential application.

Removal of Co, Sr and Cs ions from simulated radioactive wastewater by forward osmosis.

The removal of Co, Sr and Cs ions form simulated radioactive wastewater using forward osmosis (FO) process was investigated. The effect of various factors on nuclide transport was examined, including membrane orientation, NaCl concentration, flow velocity, and the main factors were identified by correlation analysis. The mechanisms of nuclides transfer through membrane were explored. The results indicated that the active layer facing draw solution (AL-DS) had higher nuclide flux than AL-FS. At AL-FS mode, the highest flux of Co, Sr and Cs were only 1.54, 10.22 and 15.63 mg m-2 h-1 respectively by cellulose triacetate with embedded polyester screen support (CTA-ES) membrane. At AL-DS mode, the flux of Co and Cs increased when NaCl concentration and flow velocity increased. Convection, diffusion and electrostatic interactions were found to influence the nuclide transport all together. The Pearson correlation and partial correlation analysis identified that the diffusion coefficient of nuclides and reverse NaCl flux were the most important factors affecting nuclide flux through cellulose triacetate membrane. The water flux, NaCl concentration, flow velocity and partition coefficient were not the main affecting factors for nuclide flux.

Impacts of mono/divalent cations on the lamellar structure of cross-linked GO layers and membrane filtration performance for different DOM fractions.

This study evaluated the effects of co-existing cations (Na+ or Ca2+) on the lamellar structure of cross-linked graphene oxide (GO) layers and GO modified membrane performance in terms of their fouling behaviours and retention for single-model organic matter, namely, bull serum albumin (BSA), sodium alginate (SA), humic acid (HA) and tannic acid (TA). In the absence of co-existing cations, the GO layers mitigated membrane fouling for large molecules (SA, BSA, and HA) but led to severer pore blocking for small molecules (TA) compared with pristine membrane. Na+ and Ca2+ altered the performance impacts of the GO modified membrane due to different interactions with the cross-linked GO layers. Low concentrations of Na+ (<0.4 mM) enlarged interlayer spacing of the GO layers and caused a decrease in flux after physical cleaning, but the GO layers maintained the uniform lamellar structure. High concentrations of Na+ (>0.4 mM) promoted the aggregation of cross-linked GO layers through charge shielding and reduced the uniformity of lamellar structure, which weakened the antifouling performance for large molecules and promoted the passage of small molecules through the membrane. However, Ca2+ complexed with GO sheets and reinforced the uniform lamellar structure of the GO layers, leading to a better antifouling performance for the filtration of large molecules than the pristine membrane but aggravated TA fouling.

Microbubble aeration enhances performance of vacuum membrane distillation desalination by alleviating membrane scaling.

Membrane fouling, especially inorganic fouling due to salt crystal formation and deposition on the membrane surface, is still a major technical issue in membrane distillation (MD) applications. In this study, microbubble aeration (MBA) was included in a laboratory-scale vacuum membrane distillation (VMD) rig and its effect on a desalination process was examined. Without MBA, serious membrane scaling occurred during desalination of simulated high-salinity sea water (100 g.L-1 salt concentration), which resulted in a dramatic reduction of permeate flux to essentially zero after 120 min. Scanning electron microscopy showed that a layer of large cuboid salt crystals uniformly covered the membrane surface. However, membrane scaling was mitigated with the introduction of MBA, resulting in the improved VMD desalination performance, which was positively correlated with pump pressure in the microbubble (MB) generator. Results showed that the effective processing time of the VMD desalination processing cycle was respectively prolonged to 150, 180, and more than 300 and 360 min (cf. 120 min without MBA) when the pump pressure was respectively at 0.1, 0.2, 0.3 and 0.4 MPa, leading to the increase of cumulative water production. Further studies found that larger numbers of MBs of smaller size were produced at higher pump pressure, which are more beneficial for increasing water vapor production and alleviating salt precipitation. The difference in zeta potential between the MBs in distilled water (about -30 mV) and that in SW100 solution (about -2 mV) demonstrated that MBA not only effectively mitigated the negative effect of concentration polarization by enhancing the surface shear rate at the membrane surface, but also reduced salt precipitation probably due to the MBs attracting counterions to the gas-water interface. Finally, energy consumption analysis of the modified VMD desalination process revealed that MBA, while itself only adding about 3% to the total energy consumption at varied pump pressures, was able to improve the specific energy consumption, especially at higher pump pressures. Together, these results demonstrate that MBA is an effective way of improving the performance of VMD desalination of water.

The effects of organic fouling on the removal of radionuclides by reverse osmosis membranes.

The removals of cesium (Cs) and strontium (Sr), two hazardous and abundant radionuclides in aquatic environment, were assessed with their isotopes in a synthetic water containing Suwannee River natural organic matter (SRNOM), a natural surface water (SW) and a wastewater effluent (WW) by two different types of ultra-low pressure RO membranes (M1 and M2). The rejections of Sr by the membranes M1 and M2 were higher than 97.5% and 96.0%, respectively, and the rejections of Cs exceeded 90.0% and 85.0%, respectively, in the filtration of real water. The membrane M1 exhibited a more significant flux decline in the filtration of the SRNOM solution, while more severe flux declines were observed with the membrane M2 in the filtration of SW and WW. Protein-like materials with relatively high molecular weight were the main contributors to the flux decline, and humic-acid-like compounds had little effect on the flux decline. Donnan exclusion and size exclusion by humic-acid-like compounds improved the rejections by the membrane M2 with weaker hydrophilicity, while the cake-enhanced concentration polarization reduced the rejections of Cs and Sr by the membrane M1 with stronger hydrophilicity. The ionic strength in the real water resulted in the mitigation of membrane fouling. This study provided important insights into foulant characterization and the mechanisms of organic-fouling-enhanced rejections of Cr and Sr by ultra-low pressure RO membranes.

Effects of feed solution chemistry on low pressure reverse osmosis filtration of cesium and strontium.

The objective of this study was to identify the removal mechanisms of radionuclides by reverse osmosis (RO) membranes under conditions relevant to full-scale water treatment. For this purpose, the effects of feed solution chemistry on the removal of Cs and Sr by a low pressure RO system was investigated by systematically varying membrane surface charge, ionic composition, and organic matter concentrations. The results showed that the effects of solution chemistry on the filtration of Cs and Sr were related to their hydrated ionic radius, resulting in the predominance of the Donnan's effect and electrostatic interactions, respectively. Consequently, the rejection of Cs increased more pronouncedly than Sr with the increases of feed concentration. Due to the Donnan's effect, different anions decreased the rejection of Cs to different extents in accordance to the order of anions' radii as SO4(2-)>Cl(-)>NO3(-)>F(-). The variations in Sr rejection were influenced by the electrostatic interactions between Sr(2+) and the membrane. In addition, humic acid (HA) lowered the rejection of Cs and caused significant membrane flux decline, but did not change the rejection of Sr. Sr also aggravated HA fouling of the membrane.