Which Surface Is More Scaling Resistant? A Closer Look at Nucleation Theories for Heterogeneous Gypsum Nucleation in Aqueous Solutions.

Developing engineered surfaces with scaling resistance is an effective means to inhibit surface-mediated mineral scaling in various industries including desalination. However, contrasting results have been reported on the relationship between scaling potential and surface hydrophilicity. In this study, we combine a theoretical analysis with experimental investigation to clarify the effect of surface wetting property on heterogeneous gypsum (CaSO4·2H2O) formation on surfaces immersed in aqueous solutions. Theoretical prediction derived from classical nucleation theory (CNT) indicates that an increase of surface hydrophobicity reduces scaling potential, which contrasts our experimental results that more hydrophilic surfaces are less prone to gypsum scaling. We further consider the possibility of nonclassical pathway of gypsum nucleation, which proceeds by the aggregation of precursor clusters of CaSO4. Accordingly, we investigate the affinity of CaSO4 to substrate surfaces of varied wetting properties via calculating the total free energy of interaction, with the results perfectly predicting experimental observations of surface scaling propensity. This indicates that the interactions between precursor clusters of CaSO4 and substrate surfaces might play an important role in regulating heterogeneous gypsum formation. Our findings provide evidence that CNT might not be applicable to describing gypsum scaling in aqueous solutions. The fundamental insights we reveal on gypsum scaling mechanisms have the potential to guide rational design of scaling-resistant engineered surfaces.

Ultrahigh resistance of hexagonal boron nitride to mineral scale formation.

Formation of mineral scale on a material surface has profound impact on a wide range of natural processes as well as industrial applications. However, how specific material surface characteristics affect the mineral-surface interactions and subsequent mineral scale formation is not well understood. Here we report the superior resistance of hexagonal boron nitride (hBN) to mineral scale formation compared to not only common metal and polymer surfaces but also the highly scaling-resistant graphene, making hBN possibly the most scaling resistant material reported to date. Experimental and simulation results reveal that this ultrahigh scaling-resistance is attributed to the combination of hBN's atomically-smooth surface, in-plane atomic energy corrugation due to the polar boron-nitrogen bond, and the close match between its interatomic spacing and the size of water molecules. The latter two properties lead to strong polar interactions with water and hence the formation of a dense hydration layer, which strongly hinders the approach of mineral ions and crystals, decreasing both surface heterogeneous nucleation and crystal attachment.

Eggshell membrane derived nitrogen rich porous carbon for selective electrosorption of nitrate from water.

Nitrate (NO3-) is a ubiquitous contaminant in water and wastewater. Conventional treatment processes such as adsorption and membrane separation suffer from low selectivity for NO3- removal, causing high energy consumption and adsorbents usage. In this study, we demonstrate selective removal of NO3- in an electrosorption process by a thin, porous carbonized eggshell membrane (CESM) derived from eggshell bio-waste. The CESM possesses an interconnected hierarchical pore structure with pore size ranging from a few nanometers to tens of micrometers. When utilized as the anode in an electrosorption process, the CESM exhibited strong selectivity for NO3- over Cl-, SO42-, and H2PO4-. Adsorption of NO3- by the CESM reached 2.4 × 10-3 mmol/m2, almost two orders of magnitude higher than that by activated carbon (AC). More importantly, the CESM achieved NO3-/Cl- selectivity of 7.79 at an applied voltage of 1.2 V, the highest NO3-/Cl- selectivity reported to date. The high selectivity led to a five-fold reduction in energy consumption for NO3- removal compared to electrosorption using conventional AC electrodes. Density function theory calculation suggests that the high NO3- selectivity of CESM is attributed to its rich nitrogen-containing functional groups, which possess higher binding energy with NO3- compared to Cl-, SO42-, and H2PO4-. These results suggest that nitrogen-rich biomaterials are good precursors for NO3- selective electrodes; similar chemistry can also be used in other materials to achieve NO3- selectivity.

High efficiency electrochemical disinfection of Pseudomons putida using electrode of orange peel biochar with endogenous metals.

The electrochemical disinfection efficiency of Pseudomons putida was studied using ruthenium iridium coated titanium (RICT) electrode as anode and carbonized orange peel biochar (OPB) or graphite as the cathode. The results indicated that RICT/OPB system induced 6.5 and 7.0 log of P. putia inactivation after 60 s at 2 V and 45 s at 10 V, respectively. RICT/OPB system showed better efficiency than RICT/graphite system. The energy consumption of OPB cathode (17.5 Wh m-3 per log) was significantly lower than that of graphite cathode (23.09 Wh m-3 per log). Both anode and cathode played great roles on the disinfection. The anode absorbed electric energy to generate electrical hole, which can oxidize chloride ions to chlorine free radicals. The continuous porous structure of OPB can provide more adsorption sites and reduce electrolyte transport resistance, resulting in more Cl· production. Moreover, P. putia was much easier adsorbed to the anode surface in the RICT/OPB system because of the stronger electrostatic repulsion between cells and OPB cathode. As a result, P. putia was more easily inactivated by the Cl· produced on the anode. Besides chlorine active species, superoxide radical (O2·ï¹£) produced on surface of cathode may also result in P. putia inactivation. The endogenous CuO in OPB can induce persistent free radicals (PFRs) production during pyrosis process. O2·ï¹£ can be produced by O2 activation through the function of Cu2O/CuO and PFRs existed in OPB cathode. The more superoxide radical production led to the better disinfection effect than the graphite cathode. As a consequence, OPB electrode showed high efficiency electrochemical disinfection of P. putida.

Electrochemically-active carbon nanotube coatings for biofouling mitigation: Cleaning kinetics and energy consumption for cathodic and anodic regimes.

Biofouling is a major obstacle in engineered systems exposed to aqueous conditions. Many attempts have been made to engineer the surface properties of materials to render them resistant to biofouling. These modifications typically rely on passive antimicrobial or anti-adhesive surface coatings that prevent the deposition of bacteria or inactivate them once they reach the surface. However, no surface modification strategy completely prevents biofilm formation, and, over time, surfaces will be fouled and require cleaning. In this work, we demonstrate the capacity of electrochemical carbon nanotube coatings in dispersing biofilms formed on the surface. A systematic analysis of the biofilm removal kinetics in function of applied current density is made to identify the optimal current conditions needed for efficient surface cleaning. Operating the electrochemically active surface as a cathode produces superior results compared to when it is operated as an anode. Specifically, the 5.00 A m-2 and 2.50 A m-2 cathodic conditions produced rapid cleaning, with complete biofilm dispersal after 2 min of operation. Surface cleaning is attributed to the generation of microbubbles on the surface that scours the surface to remove the adhered biofilm. Energy consumption analyses indicate that the 2.50 A m-2 cathodic condition offers the best combination of cleaning kinetics and energy consumption achieving 99% biofilm removal at an energy cost of ~$ 0.0318 m-2. This approach can be competitive compared to the current chemical cleaning strategies, while offering an opportunity for a more sustainable and integrated approach for biofouling management in engineered systems.

A novel MnOOH coated nylon membrane for efficient removal of 2,4-dichlorophenol through peroxymonosulfate activation.

2,4-Dichlorophenol (2,4-DCP) is a highly toxic water contaminant. In this study, we demonstrate a novel catalytic filtration membrane by coating MnOOH nanoparticles on nylon membrane (MnOOH@nylon) for improved removal of 2,4-DCP through a synergetic "trap-and-zap" process. In this hybrid membrane, the underlying nylon membrane provides high adsorption affinity for 2,4-DCP. While the immobilized MnOOH nanoparticles on the membrane surface provide catalytic property for peroxymonosulfate activation to produce reactive oxygen species (ROS), which migrate with the fluid to the underlying nylon membrane pore channels and react with the adsorbed 2,4-DCP with a much higher rate (0.9575 mg L-1 min-1) than that in the suspended MnOOH particle system (0.1493 mg L-1 min-1). The forced flow in the small voids of the MnOOH nanoparticle coating layer (< 200 nm) and channels of nylon membrane (~220 nm) is critical to improve the 2,4-DCP adsorption, ROS production, and 2,4-DCP degradation. The hybrid MnOOH@nylon membrane also improves the stability of the MnOOH nanoparticles and the resistibility to competitive anions, due to much higher concentration ratio of the adsorbed 2,4-DCP and produced ROS versus background competitive ions in the membrane phase. This study provides a generally applicable approach to achieve high removal of target contaminants in catalytic membrane processes.

Publisher Correction: Multifunctional nanocoated membranes for high-rate electrothermal desalination of hypersaline waters.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Predominant Effect of Material Surface Hydrophobicity on Gypsum Scale Formation.

Scale formation is an important challenge in water and wastewater treatment systems. However, due to the complex nature of membrane surfaces, the effects of specific membrane surface characteristics on scale formation are poorly understood. In this study, the independent effect of surface hydrophobicity on gypsum (CaSO4·2H2O) scale formation via surface-induced nucleation and bulk homogeneous nucleation was investigated using quartz crystal microbalance with dissipation (QCM-D) on self-assembled monolayers (SAMs) terminated with -OH, -CH3, and -CF3 functional groups. Results show that higher surface hydrophobicity enhances both surface-induced nucleation of gypsum and attachment of gypsum crystals formed from homogeneous nucleation in the bulk solution. The enhanced surface-induced nucleation is attributed to the lower nucleation energy barrier on a hydrophobic surface, while the increased gypsum crystal attachment results from the favorable hydrophobic interactions between gypsum and more hydrophobic surfaces. Contrary to previous findings, the role of Ca2+ adsorption in surface-induced nucleation was found to be relatively small and similar on the different SAMs. Therefore, increasing material hydrophilicity is a potential approach to reduce gypsum scaling.

Multifunctional nanocoated membranes for high-rate electrothermal desalination of hypersaline waters.

Surface heating membrane distillation overcomes several limitations inherent in conventional membrane distillation technology. Here we report a successful effort to grow in situ a hexagonal boron nitride (hBN) nanocoating on a stainless-steel wire cloth (hBN-SSWC), and its application as a scalable electrothermal heating material in surface heating membrane distillation. The novel hBN-SSWC provides superior vapour permeability, thermal conductivity, electrical insulation and anticorrosion properties, all of which are critical for the long-term surface heating membrane distillation performance, particularly with hypersaline solutions. By simply attaching hBN-SSWC to a commercial membrane and providing power with an a.c. supply at household frequency, we demonstrate that hBN-SSWC is able to support an ultrahigh power intensity (50 kW m-2) to desalinate hypersaline solutions with exceptionally high water flux (and throughput), single-pass water recovery and heat utilization efficiency while maintaining excellent material stability. We also demonstrate the exceptional performance of hBN-SSWC in a scalable and compact spiral-wound electrothermal membrane distillation module.

A Hybrid Metal-Organic Framework-Reduced Graphene Oxide Nanomaterial for Selective Removal of Chromate from Water in an Electrochemical Process.

Hexavalent chromium Cr(VI) is a highly toxic groundwater contaminant. In this study, we demonstrate a selective electrochemical process tailored for removal of Cr(VI) using a hybrid MOF@rGO nanomaterial synthesized by in situ growth of a nanocrystalline, mixed ligand octahedral metal-organic framework with cobalt metal centers, [Co2(btec)(bipy)(DMF)2]n (Co-MOF), on the surface of reduced graphene oxide (rGO). The rGO provides the electric conductivity necessary for an electrode, while the Co-MOF endows highly selective adsorption sites for CrO42-. When used as an anode in the treatment cycles, the MOF@rGO electrode exhibits strong selectivity for adsorption of CrO42- over competing anions including Cl-, SO42-, and As(III) and achieves charge efficiency (CE) >100% due to the strong physisorption of CrO42- by Co-MOF; both electro- and physisorption capacities are regenerated with the reversal of the applied voltage, when highly toxic Cr(VI) is reduced to less toxic reduced Cr species and subsequently released into brine. This approach allows easy regeneration of the nonconducting Co-MOF without any chemical addition while simultaneously transforming Cr(VI), inspiring a novel electrochemical method for highly selective degradation of toxic contaminants using tailor-designed electrodes with high affinity adsorbents.

Electrically Tuning Ultrafiltration Behavior for Efficient Water Purification.

Conventional ultrafiltration (UF) technology suffers from membrane fouling and limited separation performance. This work demonstrates a novel electrical tuning strategy to improve the separation efficiency of the UF process. An electrically enhanced UF (EUF) system with two sets of oppositely placed membrane-electrode modules was set up. A series of multicycle treatment experiments were conducted to reveal the performance and tuning mechanism of the EUF system. The applied electrical tuning operation brought about an up to 68% reduction of average transmembrane pressure increasing rate (Rp), indicating a strong capability in inhibiting membrane fouling. This fouling reduction can be mainly ascribed to the applied electrophoretic force, changes in solution chemistry, and generation of peroxide, which repulses foulants away from the membrane, hampers foulant adsorption owing to enhanced electrostatic repulsion, and degrades foulants, respectively. The 1.2 V voltage was identified as an effective voltage for stably inhibiting membrane fouling. Besides, the electrical tuning operation led to an up to ∼32% increase in foulant retention rate (φ) owing to both non-Faradaic effects (including electrosorption and electrophoretic repulsion) and Faradaic oxidative degradation. Moreover, the electrical tuning operation allowed a remarkable desalination capability with a significantly higher desalination rate and an up to ∼43% greater salt adsorption capacity as compared with a conventional capacitive deionization process. Additionally, the EUF system achieved a good performance in removing heavy metals (Ag, Cu, Pb, Se, and Sb). The overall enhanced EUF performance suggests promising prospects for practical applications.

Microbial vanadate and nitrate reductions coupled with anaerobic methane oxidation in groundwater.

Vanadate contaminant in groundwater receives increasing attentions, but little is known on its biogeochemical transformation with gaseous electron donors. This study investigated bio-reduction of vanadate coupled with anaerobic methane oxidation and its relationship with nitrate reduction. Results showed 95.8 ±â€¯3.1% of 1 mM vanadate was removed within 7 days using methane as the sole electron donor. Tetravalent vanadium compounds were the main reduction products, which precipitated naturally in groundwater environment. The introduction of nitrate inhibited vanadate reduction, though both were reduced in parallel. Accumulations of volatile fatty acids (VFAs) were observed from methane oxidation. Preliminary microbial community structure and metabolite analyses indicated that vanadate was likely reduced via Methylomonas coupled with methane oxidation or through synergistic relationships between methane oxidizing bacteria and heterotrophic vanadate reducers with VFAs served as the intermediates.

Removal of calcium ions from water by selective electrosorption using target-ion specific nanocomposite electrode.

Technologies capable of selective removal of target contaminants from water are highly desirable to achieve "fit-for-purpose" treatment. In this study, we developed a simple yet highly effective method to achieve calcium-selective removal in an electrosorption process by coating the cathode with a calcium-selective nanocomposite (CSN) layer using an aqueous phase process. The CSN coating consisted of nano-sized calcium chelating resins with aminophosphonic groups in a sulfonated polyvinyl alcohol hydrogel matrix, which accomplished a Ca2+-over-Na+ selectivity of 3.5-5.4 at Na+:Ca2+ equivalent concentration ratio from 10:1 to 1:1, 94 - 184% greater than the uncoated electrode. The CSN coated electrode exhibited complete reversibility in repeated operation. Mechanistic studies suggested that the CSN coating did not contribute to the adsorption capacity, but rather allowed preferential permeation of Ca2+ and hence increased Ca2+ adsorption on the carbon cathode. The CSN-coated electrode was very stable, showing reproducible performance in 60 repeated cycles.

A novel operational strategy to enhance wastewater treatment with dual-anode assembled microbial desalination cell.

This study introduced a novel dual-anode assembled microbial desalination cell to enhance the performance of domestic wastewater treatment. Two parallel units were fabricated with two anodes and one cathode, which is separated by two ion exchange membrane stacks. A hollow fiber membrane module was inserted in the cathode to intercept suspended solids and microbes. Based on preliminary experiments where synthetic wastewater was utilized, anode hydraulic retention time of 10 h and cathode aeration rate of 0.16 m3/h were chosen as the operating conditions. By innovatively connecting four membrane stacks in cascades, which multiplied flow rate without adding extra circulation pumps, the desalination rate of the system was improved 214.8% compared with single membrane stack mode. When modified domestic wastewater was applied, the average removal efficiencies of chemical oxygen demand, ammonia nitrogen, total nitrogen and total phosphorous reached 96.9%, 99.0%, 98.0% and 98.3%, respectively.

Novel Composite Electrodes for Selective Removal of Sulfate by the Capacitive Deionization Process.

Capacitive deionization (CDI) can remove ionic contaminants from water. However, concentrations of background ions in water are usually much higher than target contaminants, and existing CDI electrodes have no designed selectivity toward specific contaminants. In this study, we demonstrate a selective CDI process tailored for removal of SO42- using activated carbon electrodes modified with a thin, quaternary amine functionalized poly(vinyl alcohol) (QPVA) coating containing submicron sized sulfate selective ion exchange resin particles. The resin/QPVA coating exhibited strong selectivity for SO42- at Cl-: SO42- concentration ratios up to 20:1 by enabling preferential transport of SO42- through the coating, but had no negative impact on the electrosorption kinetics when the coating thickness was small. The cationic nature of the coating also significantly improved the charge efficiency and consequently the total salt adsorption capacity of the electrode by 42%. The resin/QPVA coated CDI system was stable, showing highly reproducible performance in more than 50 adsorption and desorption cycles. This work suggests that addition of selective ion exchange resins on the surface of a carbon electrode could be a generally applicable approach to achieve selective removal of target ions in a CDI process.

Aqueous-Processed, High-Capacity Electrodes for Membrane Capacitive Deionization.

Membrane capacitive deionization (MCDI) is a low-cost technology for desalination. Typically, MCDI electrodes are fabricated using a slurry of nanoparticles in an organic solvent along with polyvinylidene fluoride (PVDF) polymeric binder. Recent studies of the environmental impact of CDI have pointed to the organic solvents used in the fabrication of CDI electrodes as key contributors to the overall environmental impact of the technology. Here, we report a scalable, aqueous processing approach to prepare MCDI electrodes using water-soluble polymer poly(vinyl alcohol) (PVA) as a binder and ion-exchange polymer. Electrodes are prepared by depositing aqueous slurry of activated carbon and PVA binder followed by coating with a thin layer of PVA-based cation- or anion-exchange polymer. When coated with ion-exchange layers, the PVA-bound electrodes exhibit salt adsorption capacities up to 14.4 mg/g and charge efficiencies up to 86.3%, higher than typically achieved for activated carbon electrodes with a hydrophobic polymer binder and ion-exchange membranes (5-13 mg/g). Furthermore, when paired with low-resistance commercial ion-exchange membranes, salt adsorption capacities exceed 18 mg/g. Our overall approach demonstrates a simple, environmentally friendly, cost-effective, and scalable method for the fabrication of high-capacity MCDI electrodes.

Self-Driven Desalination and Advanced Treatment of Wastewater in a Modularized Filtration Air Cathode Microbial Desalination Cell.

Microbial desalination cells (MDCs) extract organic energy from wastewater for in situ desalination of saline water. However, to desalinate salt water, traditional MDCs often require an anolyte (wastewater) and a catholyte (other synthetic water) to produce electricity. Correspondingly, the traditional MDCs also produced anode effluent and cathode effluent, and may produce a concentrate solution, resulting in a low production of diluate. In this study, nitrogen-doped carbon nanotube membranes and Pt carbon cloths were utilized as filtration material and cathode to fabricate a modularized filtration air cathode MDC (F-MDC). With real wastewater flowing from anode to cathode, and finally to the middle membrane stack, the diluate volume production reached 82.4%, with the removal efficiency of salinity and chemical oxygen demand (COD) reached 93.6% and 97.3% respectively. The final diluate conductivity was 68 ± 12 µS/cm, and the turbidity was 0.41 NTU, which were sufficient for boiler supplementary or industrial cooling. The concentrate production was only 17.6%, and almost all the phosphorus and salt, and most of the nitrogen were recovered, potentially allowing the recovery of nutrients and other chemicals. These results show the potential utility of the modularized F-MDC in the application of municipal wastewater advanced treatment and self-driven desalination.

Reducing aeration energy consumption in a large-scale membrane bioreactor: Process simulation and engineering application.

Reducing the energy consumption of membrane bioreactors (MBRs) is highly important for their wider application in wastewater treatment engineering. Of particular significance is reducing aeration in aerobic tanks to reduce the overall energy consumption. This study proposed an in situ ammonia-N-based feedback control strategy for aeration in aerobic tanks; this was tested via model simulation and through a large-scale (50,000 m(3)/d) engineering application. A full-scale MBR model was developed based on the activated sludge model (ASM) and was calibrated to the actual MBR. The aeration control strategy took the form of a two-step cascaded proportion-integration (PI) feedback algorithm. Algorithmic parameters were optimized via model simulation. The strategy achieved real-time adjustment of aeration amounts based on feedback from effluent quality (i.e., ammonia-N). The effectiveness of the strategy was evaluated through both the model platform and the full-scale engineering application. In the former, the aeration flow rate was reduced by 15-20%. In the engineering application, the aeration flow rate was reduced by 20%, and overall specific energy consumption correspondingly reduced by 4% to 0.45 kWh/m(3)-effluent, using the present practice of regulating the angle of guide vanes of fixed-frequency blowers. Potential energy savings are expected to be higher for MBRs with variable-frequency blowers. This study indicated that the ammonia-N-based aeration control strategy holds promise for application in full-scale MBRs.

A Single-Use Paper-Shaped Microbial Fuel Cell for Rapid Aqueous Biosensing.

The traditional chamber-based microbial fuel cell (MFC) often has the disadvantages of high ohmic resistance, large volume requirements, and delayed start-up. In this study, paper-shaped MFCs utilizing a porous carbon anode, a solid Ag2 O-coated carbon cathode, and a micrometer-thin porous polyvinylidene fluoride (PVDF) separator are investigated to address the classical MFC issues. The Ag2 O-coated cathode has a low overpotential of 0.06 V at a reducing current of 1 mA compared to a Pt-air cathode. Rapid inoculation by filtration results in an instantaneous power density of 92 mW m(-2) with an internal resistance of 162 Ω. Integrated current over the first 30 min of operation has a linear relation with microbial concentration.

Carbon filtration cathode in microbial fuel cell to enhance wastewater treatment.

A homogeneous carbon membrane with multi-functions of microfiltration, electron conduction, and oxygen reduction catalysis was fabricated without using noble metals. The produced carbon membrane has a pore size of 553nm, a resistance of 6.0±0.4Ωcm(2)/cm, and a specific surface area of 32.2m(2)/g. After it was assembled in microbial fuel cell (MFC) as filtration air cathode, a power density of 581.5mW/m(2) and a current density of 1671.4mA/m(2) were achieved, comparable with previous Pt air cathode MFCs. The filtration MFC was continuously operated for 20days and excellent wastewater treatment performance was also achieved with removal efficiencies of TOC (93.6%), NH4(+)-N (97.2%), and total nitrogen (91.6%). In addition, the carbon membrane was much cheaper than traditional microfiltration membrane, suggesting a promising multi-functional material in wastewater treatment field.