Photothermal Membrane Water Treatment for Two Worlds - Accounts

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Cite This: Acc. Chem. Res. XXXX, XXX, XXX−XXX

Photothermal Membrane Water Treatment for Two Worlds Published as part of the Accounts of Chemical Research special issue “Water for Two Worlds: Urban and Rural Communities”. Young-Shin Jun,*,† Xuanhao Wu,† Deoukchen Ghim,† Qisheng Jiang,‡ Sisi Cao,‡ and Srikanth Singamaneni*,‡ †

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Department of Energy, Environmental & Chemical Engineering, Washington University in St. Louis, St. Louis, Missouri 63130, United States ‡ Department of Mechanical Engineering and Materials Science, Washington University in St. Louis, St. Louis, Missouri 63130, United States CONSPECTUS: In meeting the increasing need for clean water in both developing and developed countries and in rural and urban communities, photothermal membrane water treatment technologies provide outstanding advantages: For developing countries and rural communities, by utilizing sunlight, photothermal membrane water treatment provides inexpensive, convenient, modular, decentralized, and accessible ways to clean water, which can reduce the consumption of conventional energy (e.g., electricity, natural gas) and the cost of clean water production. In developed countries and urban communities, photothermal membrane water treatment can improve the energy efficiency during water purification. In these water purification processes, the light absorption and light-to-heat conversion of photothermal materials are important factors in determining the membrane efficacy. Nanomaterials with well-controlled structure and optical properties can increase the light absorption and photothermal conversion of newly developed membranes. This Account introduces our recent work on developing scalable, cost-effective, and highly efficient photothermal membranes for four water purification applications: reverse osmosis (RO), ultrafiltration (UF), solar steam generation (SSG), and photothermal membrane distillation (PMD). By utilizing photothermal materials, first, we have demonstrated how sunlight can be used to improve the membrane’s resistance to biofouling in RO and UF processes by photothermally induced inactivation of microorganisms. Second, we have developed novel SSG membranes (i.e., interfacial evaporators) that can harvest solar energy, convert it to localized heat, and generate clean water by evaporation. This desalination approach is particularly useful and promising for treatment of highly saline water. These new interfacial evaporators utilized graphene oxide (GO), reduced graphene oxide (RGO), molybdenum disulfide (MoS2), and polydopamine (PDA). The solar conversion efficiency and environmental sustainability of the interfacial evaporators were optimized via (i) novel and versatile bottom-up biofabrication (e.g., incorporation of photothermal materials during bacterial nanocellulose (BNC) growth) and (ii) easy and cost-effective top-down preparation (e.g., modification of natural wood with photothermal materials). Third, we have developed membranes for PMD that incorporate photothermal materials to generate heat under solar irradiation, thus providing a higher transmembrane temperature difference and higher driving force for effective vapor transport, making the membrane distillation process more energy-efficient. Lastly, this Account compares the photothermal membrane applications, summarizes current challenges for photothermal membrane applications, and offers future directions to facilitate the translation of photothermal membranes from the laboratory to large engineered systems by improving their scalability, stability, and sustainability.

I. IMPORTANCE OF PHOTOTHERMAL MEMBRANE WATER TREATMENTS Water scarcity has been a top challenge for human society in both developing and developed countries and rural and urban communities.1 Currently, one-third of the world’s population lives in areas with limited access to adequate fresh water.2 Water treatment techniques such as filtration, thermal distillation, and electrodialysis have been implemented to generate clean water and alleviate global water stress. In recent years, significant technological developments in water treat© XXXX American Chemical Society

ment have enabled better desalination of seawater that comprises ∼98% of the water on the Earth’s surface.3 At present, purification of abundant saline water by thermal distillation or membrane processes is one of the most common approaches. To date, reverse osmosis (RO) is considered the most efficient desalination process, accounting for 60% of global Received: January 7, 2019

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DOI: 10.1021/acs.accounts.9b00012 Acc. Chem. Res. XXXX, XXX, XXX−XXX

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Accounts of Chemical Research desalination capacity.4 Even so, membrane fouling in the RO process reduces the membrane flux, lowers the energy efficiency, and leads to frequent membrane replacements. Furthermore, current desalination techniques, such as RO, multistage flash, and multieffect distillation, strongly rely on large-scale systems, indicating that higher capital costs are required for energy-efficient desalination. In developing countries and rural areas, the absence of large power plants to provide electricity hinders the implementation of conventional desalination systems. Another emerging challenge is the highly saline water generated by conventional and unconventional oil and gas recovery practices, which lead to saline water disposal and subsequent environmental concerns. For example, the saline water generation rate at oil and gas well sites can reach 1000 m3/day, with as high as 300 000 mg/L total dissolved solids (TDS),5 which will be difficult to be treated by current water treatment processes. In view of these concerns about both seawater and highly saline wastewater treatments, there is an urgent need for a fast, economical, and sustainable process to treat water, one that conserves both energy and other resources. The ideal process would have a modular configuration, be performed off-grid, have zero liquid discharge (ZLD), and yield fresh water sufficiently. To address this need, highly abundant, accessible, and sustainable solar energy is an outstanding energy source for next-generation water purification techniques, especially in remote areas and developing countries. The solar energy flux on the earth’s surface per hour is estimated to be greater than annual global energy consumption.6 After attenuation by the atmosphere, the normal earth surface irradiance is around 1 kW/m2 at sea level on a cloudless day (thus, we call it one sun in this Account).7 Efficient harvesting of solar energy is a key to using it as a sustainable energy source for water treatment. Compared with photovoltaic (PV) power, which uses solar panels to convert sunlight into electricity, the use of direct solar energy is more cost-effective (for materials, fabrication, and maintenance), requires a smaller footprint, and is easier to operate for water treatment in both developing and developed countries. Hence, photothermal-membrane-based desalination using direct solar conversion can be less energy-intensive, lower-cost, and more decentralized than conventional desalination processes. For this reason, photothermal membrane processes have been developed significantly in recent years (Figure 1).8−10 Mostly, as Figure 1A shows, two photothermal water treatment techniques, photothermal solar steam generation (SSG) and photothermal membrane distillation (PMD), have been investigated. Among many materials, carbon-based materials have been mainly employed (Figure 1B). This Account introduces our recent work on developing photothermal membranes for four water purification applications: RO and UF (Figure 2A), SSG (Figure 2B), and PMD (Figure 2C).

Figure 1. (A) Publication trends in research on photothermal membranes related to SSG and other water treatments. Other photothermal membrane water treatments include PMD, oil−water separation, and water filtration. The annual numbers of publications were taken from the Web of Science, spanning from 1999 to 2018. (B) Photothermal materials utilized for SSG and other membranebased water treatments in 2018.

inorganic materials (noble metals14 and semiconductors21) and organic materials (carbon-based nanomaterials15,16 and conjugated polymers17). Because of the localized surface plasmons associated with noble metal nanostructures (e.g., Au and Ag nanostructures), the optical cross sections of such plasmonic nanostructures are nearly 4 to 5 orders of magnitude higher than those of conventional organic dyes.18 At the localized surface plasmon resonance (LSPR) wavelength, plasmonic nanostructures exhibit large absorption and scattering cross sections in the near-infrared range. 19 Oscillating electrons undergo collisions with other electrons, and collisions with the lattice phonons, the surface of the nanostructures, and surface ligands further contribute to damping and dephasing of the surface plasmon (Figure 3B). All of these nonradiative processes generate heat.20 This highly efficient conversion of light into heat by plasmonic nanostructures makes them excellent photothermal materials.19 While these plasmonic nanostructures exhibit good stability and can be synthesized under mild aqueous conditions, their high cost hinders their real-world applications. For metal oxides and other inorganic semiconductors, electron−hole pairs are created upon exposure to sunlight when the energy of sunlight is higher than the band gap of semiconductors. Through the relaxation of electron−hole pairs to the band edge, heat is generated and released.21 Titanium oxides (TiOx), molybdenum disulfide (MoS2) nanoparticles, and MXenes (Ti3C2) are typical inorganic materials used for solardriven water purification.21 For carbon-based nanomaterials (e.g., carbon black, carbon nanotubes, graphene, graphene oxide (GO), and reduced graphene oxide (RGO)) and polymers (e.g., polydopamine (PDA) and polypyrrole), the closely spaced energy levels of the loosely held π electrons can effectively absorb light at different wavelengths, and heat is generated when the excited electrons relax to their ground state (Figure 3C).15 These carbon-based photothermal nanomaterials exhibit broad-band absorbance, chemical stability, low density, and low cost.6,11,14,15,22 In addition to its high photothermal conversion efficiency, PDA is biodegradable, which makes it highly attractive as an environmentally benign photothermal material. Polypyrrole has also been used because of its strong light absorption and photothermal conversion efficiency.23 Photothermal materials can be integrated into the top layer of membranes to harness photothermal effects, achieving solardriven localized heating at the surface.22 For light to heat

II. WORKING PRINCIPLES OF PHOTOTHERMAL MEMBRANES The photothermal effect is a direct conversion phenomenon in which photoexcitation of a material produces thermal energy (heat) (Figure 3A). Materials that exhibit strong photothermal effects upon sunlight illumination have attracted significant research attention for solar-enabled water purification.10 Various photothermal materials with high efficiencies of light-to-heat conversion have been developed, including B


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Figure 2. Schematics showing the utilization of photothermal membranes in different water treatment systems. (A) Biofouling-resistant photothermal RO/UF membranes. Reproduced from ref 11. Copyright 2018 American Chemical Society. (B) Photothermal SSG membranes. Reproduced with permission from ref 12. Copyright 2017 Elsevier. (C) PMD membranes. Reproduced with permission from ref 13. Copyright 2018 Royal Society of Chemistry.

a. Photothermal Bacterial Inactivation on RO and UF Membranes

Membrane fouling reduces the membrane process efficiency and quality of water produced and shortens the membrane’s life. To address the fouling issue, a commonly used RO membrane material, polyamide (PA), was modified by synergistic coupling with the photothermal and bactericidal properties of gold nanostars (AuNS), GO nanosheets, and hydrophilic poly(ethylene glycol) (PEG) (Figure 4A).27 The synthesized PA−GO−AuNS−PEG membrane exhibited high resistance to mineral scaling (CaCO3 and CaSO4) and organic fouling (HA). In the PA−GO−AuNS−PEG membrane, the GO nanosheets acted as templates for in situ AuNS growth, and the photothermal effects of AuNS by LSPR and GO by electron excitation−relaxation converted incident 808 nm laser illumination to thermal energy. Under resonant conditions, AuNS possesses high photothermal conversion efficiency of close to 100%.28 Dielectrically confined conduction electron oscillations (i.e., LSPR) in AuNS excited by incident light are partially damped in a nonradiative way (electron−electron and electron−phonon scattering), achieving a locally high temperature on the membrane surface. Through this photothermal effect, the resulting membrane effectively killed almost all nearsurface bacteria (Escherichia coli), unlike unmodified PA membranes (Figure 4D). Compared with other materials (e.g., organic dyes, carbon nanotubes, or graphene flakes), the ease of tuning the absorption maximum of shape-controlled Au nanostructures makes AuNS an attractive platform for photothermal applications. Furthermore, because of its hydrophilicity and circumneutral charge, the PEG modification increased the resistance to organic and inorganic fouling. Interestingly, the Au nanostructures also decreased organic foulant accumulation on the membranes, although the detailed mechanisms should be investigated systematically. This study showed that the incorporation of photothermal nanoparticles with surface modification can increase the resistance to inorganic, organic, and biofouling without compromising the membrane selectivity and water flux. While bactericidal activity was achieved under 808 nm NIR laser irradiation with AuNS and GO on a PA membrane, it would be even more beneficial if the membrane itself consisted largely of photothermal materials. Thus, a novel UF membrane was designed by incorporating RGO in a bacterial nano-

Figure 3. Working principles of photothermal membranes. (A) Schematic showing a photothermal membrane under sunlight. (B) Localized surface plasmon resonance induced photothermal effect of metal nanoparticles. (C) Solar-to-thermal conversion for carbonbased photothermal materials.

conversion, one of most important factors is broad-band sunlight absorbance because the solar spectrum at sea level ranges from 280 to 2500 nm.24 Another important factor affecting light absorbance is reflection. For almost all photothermal materials, especially inorganics, the refractive index is greater than 2, and thus, these materials exhibit >11% light reflection according to the Fresnel equation.25 To decrease their reflection, photothermal membranes are generally fabricated as nanoporous structures, which allow multiple reflections inside the pores and thereby achieve enhanced light absorbance efficiency (Figure 3A).22,26

III. WHAT WE HAVE LEARNED In this section, we describe our recent work on photothermal membranes and summarize the lessons learned. The introduction sequence is based on our membrane development sequence, starting with the most promising but expensive techniques and moving to more environmentally friendly and economical ones. C


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Figure 4. (A) Structure of a PA−GO−AuNS−PEG membrane. (B) Permeate and salt rejection of PA−GO−AuNS−PEG and PA membranes in CaSO4 (top, 10 mM NaCl and CaSO4) and HA (bottom, 10 mM NaCl and 10 mg/L HA) model foulants. (C) IR images showing photothermally enhanced surface temperature of membranes under 400 mW/cm2 NIR laser illumination. (D) Fluorescence images showing bactericidal properties. Reproduced from ref 27. Copyright 2015 American Chemical Society.

Figure 5. (A) Fabrication of the photothermal RGO/BNC UF membrane. (B) Bactericidal activity of the RGO/BNC membrane against E. coli: fluorescence (top) and SEM images (bottom). (C) Diffusion of model solutes through pristine BNC and RGO/BNC membranes to identify the pore size (left) and IR images showing photothermally enhanced surface temperature of membranes under 2.9 kW/m2 illumination (right). Reproduced from ref 11. Copyright 2018 American Chemical Society.

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Figure 6. (A) Scheme showing the fabrication of a bilayer evaporator based on RGO/BNC for SSG. RGO/BNC:BNC represents a bilayer film consisting of RGO-particle-filled BNC on a pure BNC layer. (B) Photographs of RGO/BNC hydrogels and aerogels. (C) Photographs of solar steam generation with RGO/BNC under a light intensity of 10 kW/m2. Reproduced with permission from ref 22. Copyright 2016 Wiley-VCH. (D) Facile preparation of a GO/wood-based interfacial evaporator. Insets: SEM images of wood cross section without (i) and with (ii) GO on the surface of the microporous structure. Reproduced from ref 15. Copyright 2017 American Chemical Society.

and can fit in conventional treatment units should be further investigated. Despite this challenge, the photothermal-materialcoupled membranes in our studies showed great promise for enhanced biofouling resistance, which would ensure long-term membrane stability and reduced cost for membrane replacement.

cellulose (BNC) matrix during bacterial growth. RGO was chosen as the photothermal material because of its broad-band solar absorption and high solar-to-thermal conversion efficiency,9 which generated a high surface temperature under irradiation (Figure 5C).11 BNC, a highly pure cellulose produced from bacteria, was used as the base supporting material because of its excellent mechanical strength, easy synthesis, high scalability, and low potential adverse environmental impacts. The RGO-incorporated BNC membrane exhibited high chemical stability under environmentally relevant pH conditions (pH 4−9) and strong mechanical stability under agitation/sonication and high pressure (100 psi), while most previously reported GO-based membranes would be produced by spin-coating and vacuum filtration, leaving their mechanical stability doubtful.29 In addition, under simulated sunlight with an intensity of 2.9 kW/m2, the RGO flakes in the BNC matrix converted the light energy efficiently to heat, and the high temperature of the membrane killed E. coli within 3 min. The easy and simple process for fabricating the RGO/BNC membrane benefits scalable synthesis, which is important for real-world applications. While these developments represent an innovative approach to harness photothermal activity for anti-biofouling, many current membrane modules, such as spiral-wound modules, are closed and well-packed, making penetration of light to the membrane difficult. To realize this implementation, membrane modules that can be combined with photothermal membranes

b. Photothermal Membranes for SSG

Photothermally induced steam generation at the water−air interface has emerged as a promising water treatment technique.6 Highly efficient photothermal interfacial evaporators share a few common features: (i) their materials that exhibit broad light absorption over visible and near-infrared wavelengths and efficient photothermal conversion are integrated within the membranes; (ii) their support materials have low thermal conductivity to minimize the heat transfer from the localized heating interface to the bulk water; and (iii) they have an open porous structure and hydrophilic nature. We made two interfacial evaporators utilizing RGO and GO photothermal materials with BNC and wood as their supporting bases, respectively. The bases facilitated water transport and provided thermal insulation. First, an RGO/BNC bilayer foam composed of a bottom layer of pure BNC and a top layer with RGO-incorporated BNC enabled excellent light-to-heat conversion and heat localization (Figure 6A).22 Under simulated solar illumination at 10 kW/m2, a steam generation efficiency of 83% was E


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Figure 7. Completely biodegradable solar steam evaporator based on PDA and BNC. (A) Bilayer PDA/BNC hydrogel. (B) Optical and SEM images of the surface and cross section of a PDA/BNC foam. (C) Vis−NIR extinction spectra of PDA particles with varying sizes. The inset is a picture of PDA particles in solution. (D) Transmittance and reflectance spectra of PDA/BNC hydrogel. (E, F) SSG performance. Reproduced with permission from ref 26. Copyright 2017 Royal Society of Chemistry.

Figure 8. (A) Artistic rendering of evaporation from a ce-MoS2/BNC solar evaporator under sunlight irradiation. Yellow: 2H phase of ce-MoS2. Red: 1T phase of ce-MoS2. White: BNC layer. (B) Photograph and (C) cross-sectional SEM image of a ce-MoS2/BNC bilayer aerogel. (D, E) Evaporation mass loss of ce-MoS2/BNC and bulk MoS2/BNC under (D) 0.76 and (E) 5.35 kW/m2 illumination and (F) solar evaporation efficiency. (G) Cell viabilities of MCF 10A cells and NIH/3T3 cells in the presence of ce-MoS2 and GO nanosheets after 24 h of incubation. 1X is the 4.5 mg/L concentration of ce-MoS2 and GO nanosheets in the solution. Reproduced with permission from ref 21. Copyright 2018 Elsevier.

achieved with the bilayer foam. The fabrication approach demonstrated in that work can be extended to other functional nanomaterials to realize composites with applications in energy harvesting, sensing, catalysis, and life sciences.30,31 Second, to develop a simpler and more cost-effective method, natural wood was modified by drop-casting of aqueous GO solution, and the product achieved highly efficient evaporation under light illumination (Figure 6B).15 Combined with the excellent photothermal properties of GO, the natural vessel structure, the hydrophilicity, and the low thermal conductivity of wood enabled the evaporator to efficiently pump saline water from the bottom to the evaporative surface. This work demonstrated that wood is

outstanding as a natural support material for interfacial evaporators and laid the groundwork for subsequent research efforts on wood-based interfacial evaporators.12,32,33 In SSG, the cost effectiveness and scalability of photothermal materials are important for real-world applications. Furthermore, the premature release of photothermal materials and the inevitable disposal of solar evaporators after use demand attention to the potential toxicity of the photothermal materials. Thus, interfacial evaporators made from renewable, biodegradable, and scalable materials hold the greatest promise for SSG applications. PDA in particular is flexible, scalable, and most importantly, completely biodegradable with specific enzymes. To take advantage of these distinctive properties, a F


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Figure 9. (A) Synthesis of the FTCS−PDA−PVDF membrane. (B) IR camera images of the FTCS−PVDF membrane under irradiation at (i) 7.0 and (ii) 0.75 kW/m2 and the FTCS−PDA−PVDF membrane under irradiation at (iii) 7.0 and (iv) 0.75 kW/m2 after 600 s of illumination. (C) Water flux and (D) efficiency of the solar-driven DCMD system using the FTCS−PDA−PVDF membrane with varying feed flow rates, using both pure water and 0.5 M NaCl saline water. The flow rate is the volume of feedwater that penetrates the membrane distillation modular per unit time. Reproduced with permission from ref 13. Copyright 2018 Royal Society of Chemistry.

MoS2, a phase transition from 2H (trigonal-prismatic coordination) to 1T (octahedral coordination) occurred. This phase transition enhanced the light absorption and photothermal conversion of the ce-MoS2. The solar evaporation efficiency of ce-MoS2/BNC was ∼19% higher than that of bulk MoS2/BNC (Figure 8F), which was attributed to the low-band-gap structure of the 1T phase of ce-MoS2. The high evaporation efficiency of ce-MoS2/BNC (∼81%, 5.35 kW/m2) is comparable to those of other reported photothermal materials (80−85%), including exfoliated graphite, RGO, graphene, carbon nanotubes, and AuNPs.6,16,22,35 In addition to its solar evaporation efficiency, the cytotoxicity of ce-MoS2 nanosheets was found to be lower than that of GO nanosheets with a similar size, explored using human mammary epithelial cells (MCF 10A) and mouse embryonic fibroblast cells (NIH/ 3T3). The low cytotoxicity of ce-MoS2 mitigates potential environmental risk from solar interfacial evaporators (Figure 8G). These findings showed that ce-MoS2 is a promising photothermal material for SSG, and it also can be used for other biomedical applications.

PDA-particle-loaded BNC bilayer evaporator was developed for SSG.26 The interfacial evaporator comprises a BNC bottom layer and a top BNC layer loaded with a high density of PDA particles (Figure 7A,B). PDA can absorb 99% of the photon energy in incident sunlight and convert it into heat within tens of picoseconds.34 To achieve maximum overlap between the light absorption of PDA particles and the solar spectrum, the size of the PDA particles was tuned, showing optimized light absorption at ∼1 μm (Figure 7C,D). For a control experiment, PDA was self-polymerized on BNC instead of using PDA particles, but its light absorption (∼87%) was much lower than that of PDA-particle-filled BNC (∼98%) because of the thin layer coating, which indicates the importance of PDA size optimization. The PDA/BNC interfacial evaporator exhibited excellent SSG performance under one sun (efficiency of ∼78%) due to its high light absorption, outstanding photothermal conversion, excellent heat localization, and rapid water transport (Figure 7E,F). Furthermore, the PDA particles were completely degraded by laccase enzymes. Hence, the biodegradable PDA-based interfacial evaporator can provide clean water without compromising its efficiency while exhibiting biodegradability and creating a low environmental impact. Recently, molybdenum disulfide has also received increased attention as a photothermal material because of its colloidal stability, good light absorption, and low toxicity. MoS2 nanosheets are more economical photothermal agents than noble metals. In one study, we utilized chemically exfoliated (ce) MoS2 as a highly efficient, scalable, and environmentally benign (low-toxicity) photothermal material for solar water purification (Figure 8).21 A pure BNC foam underneath the MoS2-incorporated BNC layer served as both a supporting layer and heat insulation layer (Figure 8A−C). During the exfoliation process through Li intercalation to synthesize ce-

c. Photothermal Membranes for Solar-Driven Membrane Distillation

Membrane distillation (MD) is another promising desalination technique. In a conventional MD process, hot feedwater generates a temperature difference between the two sides of a hydrophobic membrane.36 The water vaporizes on the hot feedwater−membrane interface, transports as vapor through the membrane pores, and condenses on the cold distillate side.37 In MD, the vapor pressure difference between the feed side and distillate side (ΔP) can be calculated by the following equation:36 ΔP = Pv,feed(S , T + ΔT ) − Pv,distillate(0, T ) G

(1)


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Accounts of Chemical Research Table 1. Photothermal Membrane Techniques, Strengths, Limitations, and Deployment Areas11,13,15,21,22,26,27,52,a

a The solar conversion efficiency (η) is defined as the ratio of the energy needed to generate water flux over the total energy input by solar irradiation (I, in kJ m−2 h−1) (same as the gained output ratio). The energy needed for water flux is calculated by multiplying the water flux (ṁ , in kg m−2 h−1) by the evaporation enthalpy of water (ΔHvap = 2454 kJ/kg). The current solar conversion efficiencies of SSG photothermal membranes are in the range of 72−94%, as cited from the literature published in 2018.16,17,21 For PMD, the solar conversion efficiencies are in the range of 21−45%.13,41,42 It should be noted that the solar conversion efficiency of a photothermal membrane for PMD is dependent on the temperature of the inlet feedwater.

were further treated with a simple fluorosilanization method using (tridecafluoro-1,1,2,2-tetrahydrooctyl)trichlorosilane (FTCS) to achieve hydrophobicity of the membrane. Facilitated by the broad light absorption and outstanding photothermal conversion properties of PDA, the membrane achieved a high solar conversion efficiency and water flux (efficiency of 45% and water flux of 0.49 kg m−2 h−1 under 0.75 kW/m2 irradiation with 0.5 M NaCl as feedwater) in a solardriven direct contact membrane distillation (DCMD) system. The membrane achieved water recovery rates of 50%−70% under irradiation at 7.0 kW/m2 with varied feed flow rates. In addition, the strong underwater adhesion of the PDA gives the membrane excellent robustness for long-term use. Compared with other membrane synthesis methods, including phase inversion or electrospinning, the simplicity and versatility of PDA polymerization to achieve proximal photothermal conversion activity makes the membrane even more attractive for future commercial applications.

in which Pv,feed(S, T + ΔT) is the partial vapor pressure of feedwater with salinity S at temperature T + ΔT, where ΔT is the temperature difference between the feed solution and the distillate. The minimum ΔT needed for MD provides a ΔP high enough to overcome the mass transfer resistance of a hydrophobic membrane, which is a function of the membrane’s properties, including its porosity, tortuosity, and thickness. MD can be operated at temperatures lower than boiling and at pressures lower than in RO,36 leading to decreased electricity input. As a non-pressure-driven process, the fouling on the MD membrane is reduced, thus the pretreatment of feedwater is often not needed in MD. MD is also highly compact and modular, complementing other desalination techniques. The use of renewable heat energies in MD, including waste heat from power plants,38 solar energy from solar collection systems,39 and geothermal heat,40 promotes MD as a sustainable desalination technique. In recent years, to further decrease the electricity input of MD and make it more accessible in remote areas where large power plants or heat energy sources are not available, a new MD configuration, PMD, has been developed.13,41−43 In the PMD process, photothermal materials are incorporated into the membranes, where they generate heat under solar or UV irradiation, thus providing a high trans-membrane temperature difference and high driving force for vapor transport. In the meantime, the temperature polarization effect, which is commonly observed in conventional MD systems and degrades MD’s overall performance,44 can also be alleviated in the PMD process by the increased membrane top surface temperature. For example, in our recently published work, inherently adhesive and robust PDA was coated on poly(vinylidene fluoride) (PVDF) membranes for solar-driven membrane distillation (Figure 9).13 The PDA-coated PVDF membranes

IV. COMPARISON OF PHOTOTHERMAL MEMBRANE APPLICATIONS FOR WATER PURIFICATION AND TREATMENT In this Account, we have summarized several photothermal membranes that we have developed for water purification applications, including RO, UF, SSG, and PMD (Table 1). In photothermal RO and UF membranes, the membrane surface locally heated by photothermal activity showed biofouling resistance, which helps to increase the membrane’s durability and reduces the membrane’s replacement frequency and cost. Photothermal materials thus can improve centralized RO and UF techniques. Nevertheless, centralized systems are difficult to install and maintain in rural areas because of their high H


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Figure 10. Comparison of membrane properties for (A) biofouling-resistant photothermal RO (PRO) membranes, (B) photothermal SSG membranes, and (C) PMD membranes. The arrow width represents the relative importance of the property. In each column, wider arrows indicate greater importance. In a row, a wider arrow suggests a higher-priority requirement of the property, but the arrow width does not convey any quantitative information on an absolute scale. Up/down arrows indicate desirable/undesirable for better membranes.

capital cost and intensive energy consumption.45,46 As potential alternatives for rural areas or less accessible areas, our newly developed photothermal membranes are attractive for decentralized water treatments. In particular, SSG and PMD utilize solar energy as the main source, and they can treat high-saline water or contaminated water to provide potable water in amounts of 2−12 L m−2 day−1 from SSG systems6,14,23 and 2−8 L m−2 day−1 from PMD systems13,41,43,47 under 8 h of sunlight irradiation. Furthermore, the cost of SSG to treat high-saline water (∼$1.5/m3 for 35 000 ppm NaCl) can be lower than that of a PV-RO system ($3.6−14.9/m3 for 1000−5800 ppm TDS) considering high installation and maintenance costs.48,49 As for the optimal physicochemical properties of photothermal membranes (Figure 10), all of the photothermal materials require excellent light absorption (e.g., >90%) and photothermal conversion efficiency to generate sufficient heat and ensure a high membrane surface temperature. To reduce the biological or colloidal fouling of RO/UF membranes, hydrophilic membrane surfaces with low roughness have commonly been employed.50,51 In contrast, for PMD membranes, (super)hydrophobicity can be achieved via increasing the surface roughness (e.g., anchoring SiO 2 nanoparticles43) and/or chemical modification to reduce the surface energy (e.g., fluorosilanization13), which inhibits direct water penetration and facilitates efficient vapor transport and high salt rejection. In terms of membrane thickness, a thick heat-insulating layer (e.g., BNC) helps to reduce the conductive heat loss and increase the thermal efficiency in SSG.6 For PMD, however, the membrane thickness needs be optimized to achieve both high thermal efficiency and permeability. Finally, increasing the membrane’s porosity can drastically improve the thermal efficiency of both SSG and PMD by decreasing the mass transfer resistance and the conductive heat loss from the heated membrane surface to the bulk water or cold distillate.

scalability, stability, and sustainability of photothermal membranes. For scalability, low-cost photothermal membranes with easy syntheses and high production rates should be prioritized. The price of each membrane can potentially be lowered with environmentally abundant materials. Scalable membrane syntheses and efficient production lines can lead to high production rates. For decentralized SSG/PMD systems, portable devices will be needed to provide water for household use in rural areas. On the other hand, compact multilayered 3D structure devices can increase water production rates by expanding membrane surface areas and enhancing heat recovery.53 For stability, efforts should be put into developing photothermal membranes with high robustness and longevity under extreme conditions. Compared with traditional water treatment processes, non-pressure-driven SSG and PMD possess the advantage of treating unconventional water sources such as hypersaline water from conventional and unconventional oil and gas recovery processes. These water sources have a far more complex composition than the simulated saline water in a lab. In their real applications, photothermal membranes can encounter reactive species, such as cations and anions (e.g., Ca2+, Mg2+, CO32−, and SO42−), toxic metals/ metalloids (e.g., Cd, As, Cr, Pb, etc.), dissolved organic matter (DOM), and reactive oxygen species (ROS) and reactive halogen species (RHS) generated from photolysis of nitrate or DOM.54,55 Potential fouling problems from mineral scaling/ organic fouling and photoinduced physicochemical transformations of photothermal materials with ROS/RHS should be addressed in the future, as should as their effects on the photothermal properties and water purification performance of the membrane. With a better understanding of membrane fouling mechanisms and potential physicochemical transformations of photothermal materials, the chemical and mechanical stability of photothermal membranes can be enhanced via innovative material or structural modifications. For sustainability, we need to develop photothermal membranes with higher water purification performance, increased energy efficiency, and lower environmental impacts. While most photothermal SSG studies have focused on material and configuration designs to achieve high solar evaporation efficiency, efficient condensation and collection of the vaporized water remains as a challenge. In fact, vapor-to-

V. OUTLOOK AND FUTURE DIRECTIONS While the new photothermal membranes discussed in this Account are promising for providing clean water to meet the needs of both developing and developed countries, challenges remain in translating such photothermal membranes from the laboratory scale to applied devices or utility-scale plants. The following future directions should be pursued to improve the I


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Accounts of Chemical Research liquid conversion on the collection surface can cause optical loss due to light reflection or scattering by the water droplets formed.49 In addition to steam collection, solar evaporators can be further improved by including photocatalytic activity to decompose dye56,57 and by generating electricity from salinity differences.58 For PMD, the low clean water production rates and relatively lower solar conversion efficiencies compared with other photothermal processes should be improved. To increase the thermal efficiency, PMD structures with heat recovery systems can be utilized or combined with low-grade heat sources, such as flowback water from unconventional oil and gas recovery systems. Last but not least, to ensure low environmental impacts of photothermal membranes, less toxic and biodegradable materials should be utilized. To minimize the potential harmful impacts of photothermal materials on local environments and human health, their biodegradability and toxicity should be extensively studied before their widespread use. Improving the sustainability of photothermal membranes lays the foundation for their real-world applications in both rural and urban communities and developing and developed countries.

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He obtained his Ph.D. in Polymer Materials Science and Engineering from Georgia Institute of Technology in 2009. His research interests include the design, synthesis, and self-assembly of plasmonic nanostructures for various biomedical and energy applications.

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ACKNOWLEDGMENTS The authors acknowledge support from the National Science Foundation Environmental Engineering Program (CBET1604542). The authors acknowledge Professor James C. Ballard for careful review of the manuscript.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Young-Shin Jun: 0000-0003-4648-2984 Xuanhao Wu: 0000-0001-6177-6089 Deoukchen Ghim: 0000-0003-2041-5820 Srikanth Singamaneni: 0000-0002-7203-2613 Notes

The authors declare no competing financial interest. Biographies Dr. Young-Shin Jun is a Professor in the Department of Energy, Environmental & Chemical Engineering (EECE) at Washington University in St. Louis (WUStL). She earned her Ph.D. from Harvard University in 2005 and conducted postdoctoral research at the University of California-Berkeley/Lawrence Berkeley National Laboratory. Her research interests include solid−water interfacial reactions and nucleation in environmental systems and the development of bioinspired membrane materials. Mr. Xuanhao Wu received his B.S. in Environmental Science and Technology from Fudan University in China in 2015. He is currently a Ph.D. candidate in EECE at WUStL. Mr. Deoukchen Ghim received his B.S. (2014) and M.S. (2016) in Chemical Engineering from the University of Seoul in the Republic of Korea. He is currently a Ph.D. candidate in EECE at WUStL. Dr. Qisheng Jiang received his B.S. in Materials Processing Engineering from the Beijing Institute of Technology in China in 2013 and his Ph.D. in Materials Science and Engineering at WUStL in 2018. Ms. Sisi Cao received her B.S. in Materials Science and Engineering from Hunan University in China in 2013 and her M.S. in Materials Science from WUStL in 2015. She is currently a Ph.D. student in MEMS at WUStL. Dr. Srikanth Singamaneni is a Professor in the Department of Mechanical Engineering and Materials Science (MEMS) at WUStL. J


Article

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