Recyclable Polydopamine-Functionalized Sponge for High-Efficiency

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Surfaces, Interfaces, and Applications

Recyclable polydopamine-functionalized sponge for high-efficiency clean water generation with dual purpose solar evaporation and contaminant adsorption Yue Zhang, Xiangyu Yin, Bo Yu, Xiaolong Wang, Qiuquan Guo, and Jun Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b10076 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 17, 2019

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ACS Applied Materials & Interfaces

Recyclable polydopamine-functionalized sponge for high-efficiency clean water generation with dual purpose solar evaporation and contaminant adsorption Yue Zhang†,‡, , Xiangyu Yin†,‡,, Bo Yu†,*, Xiaolong Wang†,*, Qiuquan Guo‡,* and Jun Yang‡ State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China †

Department of Mechanical and Materials Engineering, Western University, London, Ontario N6A 5B9, Canada

‡

ABSTRACT Solar desalination of seawater is an attractive and environmentally-friendly way to solve the long-standing water crisis. However, its efficiency is highly reliant on solar intensity. Additionally, increasing contamination in water makes it difficult to generate clean water through solo desalination process. To address, we propose a polydopamine (PDA) functionalized hybrid material with dual purpose solar evaporation and contaminant adsorption for high-efficient clean water production at all-weather condition. The hybrid material is fabricated by polymerization of dopamine onto commercial sponge in a facile, low-cost, and scalable manner. With excellent light absorption and chelation capabilities, the PDA film coated on sponge acts as both a photothermal material and adsorbent that allow to achieve clean water production with solar desalination when sunshine and with contaminant adsorption when cloudy or at night. Meanwhile, the solar evaporation and contaminant adsorption of PDA-sponge are synergized one another, resulting in the PDA-sponge a desirable material with capability of continuous clean water production at all-weather condition. The PDA-sponge is also highly recyclable with high retention rate of evaporation and adsorption efficiency even after 10 cycles. The promising PDA-based hybrid is believed to inspire new strategies for superior water treatment materials.

KEYWORDS: polydopamine, adsorption, photothermal effect, solar desalination, water purification

1 INTRODUCTION With increasing population and industrialization, clean water scarcity has become one of the most serious global challenges. More and more people suffered from malnutrition, sickness, and even death due to lack of drinkable water, while other aspects of human survival and development, such as energy production, agricultural and industrial activities, the quality of our environment and the economies of both developing and industrialized nations, are also strongly affected by water scarcity.1-3 More importantly, issues caused by water shortage will continue to worsen in the coming decades unless efficient new ways to produce clean water are applied. To this end, immense efforts have been sparked in developing advanced methods to alleviate water scarcity with less energy use and low adverse environmental effect.1,4-7 Desalination and wastewater purification are considered to be two effective methods of combating water shortage.8-12 Undrinkable seawater accounts for the most common resource on Earth, and even harvesting a tiny fraction would provide sufficient relief from water shortage. 1

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Currently, thermal desalination and reverse osmosis are two main methods being widely employed for desalination.9,13 Compared to conventional thermal or membrane technologies, solar-driven interface desalination has attracted significant attention due to its cleanness, sustainability, and energy conservation.14-19 With persistent efforts on enhancing solar efficiency, various photothermal desalination systems have been demonstrated to realize high-performance solar steam generation, such as, Au-paper, PPy-mesh, Al-AAO desalination systems and especially, the bilayered hybrid bio-foam composed of insulation layer and photothermal layer.20-35 However, these still exist some challenges for such systems which require urgent attention, e.g. limited reusability, difficult large-scale fabrication, expensive photothermal material, diminished efficiency with contaminated water, high dependence on solar intensity and low freshwater production in darkness or in regions with highly variable climates.8,20,22 On the other hand, water contamination caused by intentional or unintentional discharge of pollutants is another reason of water shortage, and its impact on the environment is of growing public concern.1,12,36-38 Currently, adsorption technique is considered to be one of the most straightforward techniques to remove pollutants from wastewater without producing any harmful by-product.10 Furthermore, an adsorption-based treatment method is less affected by the level of illumination compared to solar-driven methods, such as solar-driven desalination and solar degradation of pollutants. However, adsorption is typically not capable of salt ion removal.12 Therefore, a material that is simultaneously capable of both high-efficiency desalination and adsorption would overcome the individual limitations of both methods, providing an optimized system for around-the-clock clean water production. Nowadays, noble metal nanoparticles and carbon materials have been widely used in the field of solar-driven desalination due to their good photothermal capabilities.2,21,39-42 Among these materials, noble metal nanoparticles are generally stable, but do not possess the ability to adsorb contaminants. Desalination-capable carbon materials are also commonly used in water purification, but are constrained by their limited regeneration, low adsorption capacity for some contaminants, easy desorption at high temperatures and complex manufacturing processes.43-45 Therefore, it is necessary to figure out a cost-effective and scalable solution to fabricate material which owns dual desalination and wastewater purification abilities. Polydopamine (PDA) is famous for its universal adhesion, excellent biocompatibility and self-polymerization, which conveniently allows PDA-based materials to be prepared under mild conditions.46-50 Furthermore, the abundant functional groups such as catechol, amine and imine in PDA molecular structure are expected to be anchors for heavy metals ions and organic dyes through reversible electrostatic, bi-dentate chelating, or hydrogen bond interactions, which means that PDA-based adsorbents can be regenerated and maintain high adsorption capacity even after multiple adsorption-desorption cycles.51-55 In addition to its excellent adsorption characteristics, PDA has demonstrated broad optical adsorption over the visible and near infrared (NIR) field and excellent photothermal conversion characteristics, e.g. PDA is more efficient at converting light into heat compared to carbon nanotubes,49,56-58 revealing the suitability for solar-driven desalination. All these features make PDA a potential candidate for a novel water treatment mediator, capable of both high-efficiency desalination in sunlight and multi-contaminant adsorption under insufficient light conditions. Therefore, in this work, we fabricate a robust PDA-sponge hybrid material for all-weather clean water production with dual purpose solar evaporation and contaminant adsorption (Fig. 1a). The utilization of high-abundance, low-cost microporous sponge as the 3D porous platform to support the 2


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functional PDA molecules is another highlight of this work. With its interconnected pore structure and excellent mechanically stability, sponge can provide excellent reusability and a platform for large-scale photothermal evaporation. Furthermore, its rough surface can produce multi-scattering of incident solar light,20,21,26 which, combined with its excellent thermal insulation and floatability, allows for more incident light to be converted into heat at the air/water interface with minimal heat loss. The large specific surface area of microporous sponge also increases the contact area between its supported PDA and contaminants in wastewater, enhancing water purification capacity and speed. In turn, PDA increases the hydrophility of the sponge, promoting capillary rise of water to the hot region and facilitating rapid replenishment of the evaporated surface water.28 In short, the synergy between PDA and sponge offers a promising all-purpose solution to the challenging of water shortage. 2 EXPERIMENTAL SECTION 2.1 Materials Polyurethane sponge was supplied by Shaoxing Chengfeng Foam Co. Dopamine hydrochloride, tris(hydroxymethyl)aminomethane (Tris), NaCl, PbCl2, Cd(NO3)2, Rhodamine B (Rh B), and methylene blue (MB) were purchased from Sigma-Aldrich (Mississauga, ON). KCl, Na2SO4, MgCl2, NaHCO3, H3BO3, CaCl2, SrCl2 and NaF were obtained from Fisher Scientific Company, USA. Ethyl alcohol and HCl were obtained from Caledon laboratories limited (Georgetown, ON). All chemicals were used as received without further purification. The artificial seawater was prepared according to standard ASTM D 1141-98 (Standard practice for the preparation of substitute ocean water, 2013). The chemical composition of the artificial seawater is listed in Table S1. 2.2 Preparation of PDA-Sponge Hybrid Material Dopamine hydrochloride (2.0 mg mL-1) was dissolved in 10 mM Tris-HCl (pH=8.5) solution. A piece of clean polyurethane sponge was added to the above solution. The solution was stirred overnight at 25 C to achieve self-polymerization.46 Afterwards, the PDA-sponge was washed several times with deionized water to remove un-reacted chemicals. These self-polymerization and subsequent washing processes were repeated three times. Finally, the as-prepared PDA-sponge was dried in an oven at 60 C. 2.3 Characterization The morphology of the prepared PDA-sponge was examined by a field-emission scanning electron microscopy (SU8020 HITACHI, Japan). Energy dispersive X-ray spectroscopy (EDX) was also carried out on the Hitachi SU8020 scanning electronic microscope with a Bruker X-ray energy dispersive spectrometer using an accelerating voltage of 20 kV. Before SEM observation, all samples were fixed on aluminum stubs and coated with gold film. Absorptance spectrum in the wavelength interval of 200–2500 nm was measured on a Perkin Elmer Lambda 950 UV-Vis-NIR spectrometer with an integration sphere (module 150 mm). 2.4 Evaporation Rate Measurement via Solar Simulator To measure the in-situ weight change of seawater during the solar-driven evaporation process, a beaker full of water was placed on a 4 decimal electronic precision balance (OHAUS Adventure Pro, Parsippany, NJ, USA) connected to a computer for evaluation of the evaporation rate and solar-thermal conversion efficiency. A piece of PDA-sponge was placed on the surface of the water. A 300 W xenon lamp coupled with an AM 1.5G filter (Beijing Perfectlight Technology Co. Lt, PLS-SXE300UV) was used as the solar light source, and a strong light photometer (CEL-NP2000, China) was used to measure the light intensity. The solar light illuminated perpendicularly onto the floating PDA-sponge. The temperature of the steam and 3



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water at the lower surface were simultaneously measured with an infrared camera (FLIR T620) and thermocouple thermometer (Cole-Parmer, CN-91100-40, accuracy within ± 0.1 °C). The infrared camera was calibrated using the calibrated thermocouple before use. 2.5 Thermal Conductivity Measurement The thermal conductivity of dry and wet PDA-sponge is measured by sandwiching the material between two glass slides. The “sandwich” was placed between a heat source (ceramic plate heater) and a cool source (ice-water bath). The upper and lower surface temperature was monitored by a thermocouple. The Fourier equation was used to calculate the thermal conductivity using the formula:

(eq.

1). The conductivity of glass is 1.05 W m−1 K−1, ΔT is temperature difference, and Δx is the height of sample. The calculation of thermal conductivity was based on the assumptions that the sample and the glass slides were experiencing the same heat flux, and the emissivity coefficient of sample and glass slide was 0.9. 2.6 Adsorption Measurement The absorption tests were carried out in a temperature controlled water bath shaker using an agitation speed of 100 rpm. Rh B and MB were used as organic contaminants, and Cd2+ and Pb2+ were used as inorganic contaminants to evaluate the water purification performance of PDA-sponge. Firstly, a series of aqueous solutions containing different contaminants was prepared. Secondly, PDA-sponge was placed into 100 mL of each solution and shaken in a rotary shaker for 12 h to reach absorption equilibrium. In order to study the influence of illumination on the absorption capacity of PDA-sponge, shaking was performed in both light and dark conditions. For illuminated samples, the measured samples were illuminated by a Xe-lamp with a light intensity of 1 kW m-2 (= 1 sun). At fixed time intervals, the concentrations of the measurement samples were analyzed. The apparent concentrations of MB and Rh B were measured using an UV-Vis spectrometer (Cary 100 Bio UV–vis spectrophotometer) and calculated by absorbance at 664 nm (MB) and 554 nm (Rh B). The concentrations of metal ions were determined by an Inductively Coupled Plasma Mass Spectrometer (CP-MS, Agilent 7500cx). The absorption capacities of the absorbent were calculated according to the equation (eq. 2): 𝑞𝑒 =

(𝑐0 ― 𝑐𝑒) V

(eq. 2)

𝑚

where c0 and ce represent the initial and equilibrium concentrations (mg g-1) of contaminants, respectively, V is the volume of the solution, 50 mL, and m is the amount of the absorbent (mg). To further study the absorption isotherm, single contaminant absorption tests were conducted in solutions (100 mL) with different concentration levels: 100–500 mg L-1 for Cd2+ and Pb2+, and 100–800 mg L-1 for Rh B and MB. The absorption isotherm data were fitted to the following models: Langmuir model:

𝑐𝑒 𝑞𝑒

𝑐𝑒

1

(eq. 3)

= 𝑞𝑚 + 𝐾𝐿𝑞𝑚

where ce (mg L-1) is the equilibrium concentration of contaminants in solution, qe (mg g-1) is the amount absorbed at equilibrium, qm (mg g-1) is the maximum absorption capacity, and KL is a Langmuir absorption constant. 1

Freundlich model: ln 𝑞𝑒 = 𝑛ln 𝑐𝑒 + ln 𝐾𝐹

(eq. 4)

L-1)

where ce (mg is the equilibrium concentration of metal ion in solution, qe (mg g-1) is the amount absorbed at equilibrium, KF is a Freundlich constant, and 1/n is the Freundlich coefficient. The kinetics of each contaminant on the PDA-sponge was also determined. Initial concentration of MB was 500 mg L-1, while Rh B, Pb2+ and Cd2+ were 100 mg L-1. Wastewater samples were taken at fixed time intervals and the concentrations of contaminants were similarly measured. The absorption amount (qt) of a contaminant at a given time (t) was calculated by equation (eq. 5): 4


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𝑞𝑡 =

(𝑐0 ― 𝑐𝑡) V

(eq. 5)

𝑚

where c0 and ct represent the contaminant concentrations at an initial and given time, respectively, V is the volume of solutions (mL), and m is the amount of the absorbent (mg). The experimental data from the kinetic study was fitted to the following models: 𝑘1

Pseudo- first-order kinetic equation: lg (𝑞𝑒 ― 𝑞𝑡) = lg𝑞𝑒 ― 2.303𝑡 g-1)

(eq. 6)

where qt (mg is the amount of contaminant absorbed at time t (min), qe (mg absorption capacity, and k1 (min-1) is the rate constant.

g-1)

is the equilibrium

(eq. 7)

Pseudo-second-order kinetic equation:

where qt (mg g-1) is the amount of contaminant absorbed at time t (min), qe (mg g-1) is the equilibrium absorption capacity, and k2 (g mg-1 min-1) is the constant rate. For the desorption experiments, a 0.1 M HCl solution was used as the desorption agent for Pb2+ and 2+ Cd , while an ethanol-water (1:1) solution was used for MB and Rh B. 2.7 All-Weather Water Treatment Based on the high evaporation and absorption efficiency of PDA-sponge, we designed an integrated water treatment unit to achieve continuous production of clean water. 50 mL of polluted artificial seawater containing 250 mg L-1 MB, 50 mg L-1 Rh B, 50 mg L-1 Pb2+ and 50 mg L-1 Cd2+ was used to assess the performance of PDA-sponge (size: 5 × 5 × 1 cm, the weight of PDA is 15 mg). First, an absorption test was carried out in darkness for 12 h to stimulate static absorption at night. The concentrations of Pb2+, Cd2+, Cl-, Na+, Mg2+, Ca2+, K+ and SO42- in solution were analyzed every 6 h. Afterwards, the test was continued under solar illumination (5 kW m-2) to simulate daylight conditions. Following 2 hours of illumination, both the concentration of the aforementioned ions and weight of condensation were determined. 3 RESULTS AND DISCUSSION 3.1 Fabrication and Characterization of Microporous PDA-Sponge The fabrication of our proposed PDA-sponge hybrid material is schematically illustrated in Fig. 1b. First, cleaned sponge was immersed in a dopamine solution (Tris-buffer solution, pH 8.5) at room temperature. In this weakly alkaline solution, dopamine sequentially underwent non-covalent self-assembly, oxidative self-polymerization and intermolecular Michael addition to form a cross-linked polydopamine homopolymer.46 The detailed preparation procedures are described in the Experimental Section. This fabrication progress is facile, time-saving and cost-efficient. More important, through this fabrication process, a large-scale PDA-sponge sample (30 cm × 25 cm) can be obtained as shown in Fig. S1, which is an important guarantee of actual application. Due to the catechol group, the PDA coating adhered firmly to the sponge, darkening the appearance of the sponge while preserving its overall microporous morphology and structure (Fig. 1c). The increased N content on the PDA-sponge surface, shown by EDX results (Fig. S2), proves the formation of a high density, homogenous PDA layer on the sponge surface. Moreover, this PDA layer is found enhancing the hydrophility of sponge (Fig. S3), facilitating the rise of underlying water to the heating area via capillary action, which can compensate for the water evaporated during the desalination process resulting in efficiency improvement of solar desalination.21,29

5



Fig. 1 (a) Schematic illustration of PDA-sponge for all-weather water treatment. (b) Preparation of PDA-sponge. (c) SEM images of raw sponge and PDA-sponge. Insets are the optical photos before and after PDA functionalization. (d) UV-Vis-NIR absorbance spectra of raw sponge and PDA-sponge. As a potential material for localized solar-driven desalination, broadband and efficient light absorption is of utmost importance for our proposed PDA-sponge. Total solar absorbance of the PDA-sponge and the raw sponge was measured respectively via a UV-Vis-NIR spectrophotometer in the wavelength ranged from 200 to 2500 nm. As shown in Fig. 1d, the PDA-sponge exhibits a broader and stronger absorption than that of the raw sponge, especially in the visible and near infrared (NIR) region (appreciable overlap with the solar spectrum). We attribute the efficient and broadband light absorption capability of the PDA-sponge to the following synergistic effects: firstly, the PDA itself is capable of broad optical absorption over the visible and NIR region; secondly, the microporous structure of the sponge can further enhance the total absorption performance due to the reduced surface reflection and increased internal light scattering.20,26 The strong light absorbance ability of the PDA-sponge provides a solid foundation for focusing light on the surface of the PDA-sponge to achieve effective steam generation and desalination. Besides broad and strong light absorption and immediate water compensation, an ideal localized solar-driven desalination material should also feature high photothermal conversion and excellent insulation in order to produce steam at the water-air interface with minimal heat loss, as shown in Fig. 2a. The thermogenesis and evaporation performance of the PDA-sponge was first examined under simulated solar illumination at different intensities (1~5 sun, 1 sun = 1 kW m-2). Infrared images show that the initial water temperature was about 20 C (Fig. S4). After 15 minutes of illumination with the simulated sunlight of 5 kW m-2 (5 sun), a hot area formed on the upper surface of PDA-sponge and the water temperature at the material/air interface reached nearly 80 C (Fig. 2b). Meanwhile, only a small change in water temperature occurred in the raw sponge and the pure seawater (Fig. S5). The comparison suggests that PDA has excellent photothermal conversion capability. However, without a thermal-insulation layer, the converted heat would still dissipate into the bulk water and produce an adverse effect on the efficiency of water evaporation at the interface (Fig. S6a). On that note, sponge is known as a naturally good thermal insulator that can efficiently resist heat conduction.59 Infrared images in Fig. S6b show 6


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that after illumination under the same conditions (5 kW m-2, 15 min), the water temperature at the raw sponge/air interface increased slightly to about 32 C, while the bulk water underneath the sponge remained at room temperature. Combining the benefits of both PDA and sponge, our proposed PDA-sponge can efficiently absorb solar irradiation to locally heat water at the material/air interface, while minimizing heat loss (Fig. 2b) for efficient evaporation.26-33,60 3.2 Solar Steam Generation From the testing and analysis mentioned above, PDA-sponge satisfies all the requirements for solar-driven localized desalination.23,30,61 These include (i) strong and broad light absorption and high photothermal conversion capabilities, (ii) low thermal conductivity to minimize heat loss caused by heat transfer to the bulk water, (iii) floatability to enable contact with surface water so that the liquid–steam phase change can occur close to the air–water interface, (iv) microporous structure to offer steam escape channels, and (v) good hydrophility and capillarity to facilitate rapid replenishment of evaporated surface water. These characteristics, combined with its simple and scalable fabrication, low-cost, reusability and excellent durability, make PDA-sponge a very attractive material for desalination purpose. Therefore, we further investigated the evaporative performance of PDA-sponge by recording water mass change over time (schematic setup is shown in Fig. S7a). To simulate common real life conditions, the solar-driven desalination tests were first performed under low-intensity illumination of one-sun. Fig. 2c shows that the water mass loss gradually increases with time when exposed to one-sun illumination, and the PDA-sponge exhibits a larger change than that of the seawater only or raw sponge sample. The evaporation rate of PDA-sponge (1.18 kg m-2 h-1, 1 sun) is much higher than that of pure seawater (0.32 kg m-2 h-1, 1 sun). Note that the raw sponge sample exhibits a much slower evaporation rate than seawater, which is a result of both its poor hydrophility and good heat insulation properties. While under higher-intensity illumination, the water evaporation rate of PDA-sponge is significantly accelerated (Fig. 2d and Fig. S7b). This can be attributed to the excellent local photothermal capability and the efficient thermal insulation of PDA-sponge. Fig. S8 shows that the thermal conductivity of dry PDA-sponge and wet PDA-sponge is about 0.29 and 0.47 W m−1 K−1, respectively, both of which are lower than that of water (0.61 W m−1 K−1). Hence, the PDA-sponge can effectively suppress suppressed heat loss to bulk water, which is further confirmed by the large temperature gap between the steam and bulk water (Fig. 2e). In addition, the steam generation efficiency () was also used to assess the desalination performance of PDA-sponge, which can be estimated via the following formula:20

=

ṁ𝒉𝐋𝐕

(1)

𝑰

where ṁ is the mass flux, hLV is the total enthalpy of liquid-vapor phase change (sensible heat + phase change enthalpy) and I is the illumination intensity on the surface of sample. As shown in Fig. 2f, the steam generation efficiency of PDA-sponge is about 74.3%, 81.4%, and 92.1% under illumination of 1, 3, and 5 kW m-2, higher than that of most current solar desalination systems where the light absorbers are in direct contact with bulk water (Table S2). More recently, research work on reducing heat loss by confining water path to be two dimensional (2D) has become a hot spot in the area of solar-driven desalination. Taking advantages of this “2D water path”, high solar steam efficiency (>80%) can be achieved under normal one-sun illumination.26-28 This is also true for PDA-sponge, and when PDA-sponge is placed on thermal insulation foam (Fig. S9), it exhibits 85.4% steam generation efficiency under 1 sun illumination, which is comparable 7



to that of other materials reported in literatures as shown in Table S2. Although PDA-sponge can achieve excellent solar steam efficiency by utilizing special design, it does not own the capability of wastewater purification due to its limited contact area with wastewater. This deviated from our main purpose to achieve clean water production with solar desalination when sunshine and with contaminant adsorption when cloudy or at night. Thus, the full immersed PDA-sponge was also employed in this paper.

Fig. 2 (a) Schematic of clean water generation via solar desalination of PDA-sponge with excellent photothermal performance under solar irradiation. Solar irradiation increases local water temperature at the PDA-sponge/air interface, while the outstanding thermal insulation prevents heat conduction to the water below, inducing water evaporating to form clouds of condensed freshwater. (b) The surface (upper) and bulk (lower) water temperature distribution of the PDA-sponge after simulated solar illumination at 5 kW m-2 for 15 min. (c) Mass change of the seawater sample over time for the raw sponge sample and the PDA-sponge sample under 1 sun illumination. (d) Dependence of evaporation rate of seawater with and without PDA-sponge on illumination level. (e) Temperature change of the steam and bulk water over time in the PDA-sponge/seawater distillation system under 5 sun illumination. (f) Solar steam efficiency of the PDA-sponge/seawater distillation system at different illumination levels. 8


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Reusability is another important factor in determining the practical applications of a desalination material. After every desalination cycle, salt residue will remain in the structure of the material (Fig. S10a), thereby affecting long-term performance. The removal of residual salt from other desalting materials (free-standing nanoparticles film, AAM, etc.) is difficult due to their brittleness. In comparison, the mechanical stability of PDA-sponge provides it with excellent reusability. To prove this, 50 cycles of desalination was investigated. For each cycle, a PDA-sponge sample floating on seawater was first illuminated by simulated sunlight (1 kW m-2) for 1 h, followed by scrubbing of the PDA-sponge to remove residual salt (Fig. S10b). Fig. S10c shows that after a simple scrub, the PDA-sponge was ready to be used in the next cycle of desalination without further processing, and its desalting performance (measured by evaporation rate) remained relatively constant even after 50 recycles (Fig. S10d). 3.3 Wastewater Purification Seawater desalination is an efficient way to obtain clean water; however, it is sensitive to lighting conditions. As an alternative method, adsorption is less influenced by illumination and thus is especially applicable for clean water production in regions with variable climates.6,55,62 On that note, the abundant functional groups in PDA provide the PDA-sponge with strong adsorbing ability to remove contaminants, as shown in Fig. 3a. To verify this, adsorption tests were conducted first in the dark at room temperature (20 °C), and metal ions (Pb2+ and Cd2+) and organic dyes (Rhodamine B and methylene blue, abbreviated to Rh B and MB, respectively), commonly seen in involuntary industrial waste released into environment, were selected as representative offshore contaminants.1 To simulate serious pollution, wastewater with high concentrations of the aforementioned contaminants (at a level much higher than World Health Organization guidelines) was used in the experiments (Table S3). Equilibrium isotherms were used to probe the absorption process and uptake capacity of PDA-sponge. As shown in Fig. 3b, the raw sponge exhibited absorption of only Rh B, with a capability below 3 mg g-1. In contrast, the PDA-sponge was found strong absorption for all the contaminants (Rh B, MB, Pb2+ and Cd2+) with uptakes increased progressively with their concentration (Fig. 3c, d). The correlation coefficient r2 values of isotherms (Table S4) indicate that its adsorption behaviour closely follows the Langmuir model, which suggests that the adsorption of metal ions and dyes takes place at functional groups or binding sites.7,52,63 Based on model fitting results, the maximum adsorption capacity (qm) of PDA-sponge for MB and Rh B is 3090.45 and 2267.76 mg g-1, respectively, which are higher than all previously reported values from other adsorbents to the best of our knowledge.52,64,65 PDA-sponge also shows high adsorption capacity for Pb2+ and Cd2+ (Fig. 3c). The excellent adsorption capacity of PDA-sponge can be ascribed to the changes in surface charge of PDA-sponge, the aqueous chemistry of dye and metal salt molecules, and the large specific surface area of microporous PDA-sponge. In weakly alkaline solutions, the surface charge becomes negative due to the deprotonation of phenolic groups in PDA,66 which electrostatically attracts the positive charges in dye molecules and metal ions. Combined with the large specific surface area of the microporous sponge platform, the effective contact area between the supported PDA and contaminant molecules is greatly increased. The PDA-sponge adsorption kinetics of MB, Rh B, Pb2+ and Cd2+ was examined by investigating the concentration changes of collected samples over time (0 to 12 h) from each contaminant solution (Fig. 3e, f). Equilibrium is reached at around 200 minutes for all contaminants. The kinetics of the adsorption data was analysed using both pseudo-first-order and pseudo-second-order kinetic models. The mass changes of contaminants over time exhibit good agreement with the pseudo-second-order kinetic 9



model (Table S5). The pseudo-second-order model is based on the assumption that chemisorption is the rate determining step, which further confirms the adsorption mechanism of the PDA-sponge system.

Fig. 3 (a) Schematic of clean water generation via adsorption of PDA-sponge. (b) Adsorption behavior of Rh B on raw sponge. (c-h) Adsorption performance of PDA-sponge. Adsorption isotherm plots and curving fitting by Langmuir (solid line) and Freundlich (dot line) models of (c) metal ions (Pb2+ and Cd2+) and (d) organic dyes (Rh B and MB). Adsorption kinetic plots and curving fitting by pseudo-first-order (solid line) and pseudo-second-order (dot line) kinetic models of (e) metal ions (Pb2+ and Cd2+) and (f) organic dyes (Rh B and MB). (g) Reusability of PDA-sponge for MB removal. Inset is the photographs of original MB solution, and the MB solution after 10th adsorption in the darkness. (h) Removal behavior of PDA-sponge in high concentration of mixed wastewater. The mixed wastewater is 100 mL containing 500 mg L−1 MB, 100 mg L−1 Rh B, 100 mg L−1 Pb2+ and 100 mg L−1 Cd2+. The weight of PDA on PDA-sponge is 21±3 mg. Regeneration and ease of reuse is another criterion for assessing the practicality of a wastewater purification material. To investigate the reusability of PDA-sponge, its adsorption performance was evaluated by 12 h of use in dark conditions followed by the regeneration with a desorption agent; 0.1 M HCl solution was used as desorption agent for Pb2+ and Cd2+, while an 10


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ethanol-water (1:1) solution was used for MB and Rh B. Fig. 3g and Fig. S11 in the Electronic Supporting Information show that the regenerated PDA-sponge retains >90% of its original adsorption capacity after ten adsorption-desorption cycles. After the tenth cycle, the MB sample becomes noticeably lighter in colour (inset of Fig. 3g), which further confirms the high adsorption capacity and good cycle stability of PDA-sponge.

Fig. 4 (a) Photograph of contaminated artificial seawater treatment set-up via floating PDA-sponge in a sealed condensation bin under stimulated solar illumination. (b) Schematic design and working principle of all-weather clean water production with the dual purpose PDA-sponge. (c) Optical images of the contaminated seawater before and after purification for 6 h and 12 h in darkness, and (d) a further purification with subsequent solar-driven desalination under 5 sun illumination for 2 h. (e) Comparison of the concentration changes of contaminants treated for 12 h by PDA-sponge in dark. Inset is the photograph of contaminated artificial seawater at different time points. (f) The concentration changes of primary ions in contaminated artificial seawater after a purification and desalination process. The volume of contaminated artificial seawater is 50 mL, and it contains 250 mg L−1 MB, 50 mg L−1, Rh B, 50 mg L−1 Pb2+ and 50 mg L−1 Cd2+. The weight of PDA is 15±2 mg.

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To demonstrate the competitive adsorption performance of PDA-sponge with mixed pollutants, a high concentration pollutant solution (100 mL) consisting of 50 mg MB, 10 mg Rh B, 10 mg Pb2+ and 10 mg Cd2+ was tested in dark conditions at 20 °C and 40 °C, respectively. Fig. 3h shows the adsorption results of a PDA-sponge sample floating on the mixed pollutant solution for 12 h; the adsorption is highest for MB, followed by Rh B, Pb2+ and Cd2+ at both conditions. It is worth noting that at the higher temperature, the PDA-sponge shows a significant improvement in adsorption capacity. Given this, the adsorption behaviour of PDA-sponge for all contaminants (MB, Rh B, Pb2+ and Cd2+) was further studied under illumination (1 kW m-2). The results show that the PDA-sponge under illumination presents a significant improvement in adsorption capacity compared to that in dark conditions (Fig. S12, Table S6 and Table S7). A possible explanation is that illumination increases the temperature of water surrounding the PDA-sponge (Fig. 2b), which subsequently improves the activity of active groups in PDA45,67 and speeds up the random thermal motion of contaminant molecules and ions, thereby accelerating the coordination between PDA and contaminants. Besides, the higher pollutant concentration due to the evaporation of water under illumination also helps improve the adsorption capacity of the PDA-sponge. This illumination-favourable adsorption of PDA-sponge is much different from common carbon materials,62 providing it a wider range of temperature adaptation. 3.4. All-whether Clean Water Production Based on the above results, we can confirm that the PDA-sponge serves a dual purpose: seawater desalination and wastewater purification. Accordingly, we designed an integrated water treatment unit that allows the two functions to complement each other in order to achieve continuous production of clean water (Fig. 4a). Polluted artificial seawater containing 250 mg L−1 MB, 50 mg L−1, Rh B, 50 mg L−1 Pb2+ and 50 mg L−1 Cd2+ was used for testing, and PDA-sponge was placed on top of the seawater surface. As shown in Fig. 4b, during daytime, the PDA-sponge can distill clean water out of the contaminated solution via an evaporation-condensation process, while at night or on cloudy days, the strong adsorption of PDA-sponge will be in effect. Fig. 4c shows that through overnight adsorption, the colour of the polluted water became increasingly lighter, and nearly all organic dyes and most of the heavy metal ions were successfully removed from the solution (Fig. 4e). Following overnight absorption, solar-driven desalination was performed under illumination (solar intensity: 5 sun). It is worth mentioning that the evaporation rate of PDA-sponge (1.09 kg m-2 h-1, 1 sun) for the polluted seawater remained nearly unchanged compared to that of pure seawater (1.18 kg m-2 h-1, 1 sun) from previous results, which serves as a prerequisite for achieving simultaneous desalination and absorption. It is clear from Fig. 4d that the polluted water became colourless after about 2 h illumination, and all the residual organic dyes and metals ions left in the polluted water after overnight treatment were absorbed into the PDA-sponge during the illumination period (Table S8); this is because, as mentioned earlier, the solar-driven temperature rise of surface water increases its absorption capacity. More importantly, the concentrations of salt ions which could not be eliminated via absorption alone, such as Na+, K+ and Mg2+, were greatly reduced in water collected from the condensation bin, allowing it to meet the drinking water standard (Fig. 4f). In brief, the combination of high-efficiency desalination and absorption means PDA-sponge is particularly suitable for all-weather clean water production.

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4 CONCLUSIONS In summary, a recyclable, bi-functional and highly efficient water treatment material was fabricated by depositing PDA film onto commercial, low-cost sponge. Owing to its natural porous structure, high optical adsorption, excellent photothermal capability, low thermal conductivity, hydrophilicity, floatability and wide availability, PDA-sponge is a promising material for use in localized solar-driven desalination systems. In addition, the abundant functional groups in PDA endow the PDA-sponge with chemically adsorbing capabilities. Thus, by employing this material, we can concurrently achieve highly efficient solar-driven desalination and wastewater purification. Moreover, its water purification ability is enhanced by its concurrent solar-driven desalination since the resulting temperature rise is conducive for chemical absorption. Consequently, this complementation between desalination and absorption allows PDA-sponge to overcome the excessive dependence on solar radiation exhibited by conventional desalination materials. The material is also highly reusable and can retain high evaporation and adsorption efficiency even after being recycled 10 times. We believe that our proposed PDA-sponge holds great potential in inexpensive, all-purpose and inexpensive water treatment applications, and opens up new avenues in the development of multi-functional materials for the generation of clean water.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Photograph, SEM, EDX, wettability and Infra-red photos of PDA-sponge; Water mass change curves over time; Thermal conductivity and solar evaporation performance of PDA-sponge; Durability and reusability of PDA-sponge for contaminant removal; Adsorption isotherm and kinetic plots and parameters; Chemical composition of artificial seawater; Solar steam efficiency of PDA-sponge compared with previous reports; Pollutant residue.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] (B. Yu), [email protected] (X. Wang), [email protected] (Q. Guo) Author contribution  Y.Z. and X.-Y.Y. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS This research was supported by the National Key Research and Development Program of China (2016YFC1100401), NSFC (51775538 and 51573199), the West Light Program and Youth Innovation Promotion Association of CAS, the Natural Sciences and Engineering Research Council of Canada (NSERC: RGPIN-2016-05198) and the Research Accelerator Grant Program of the University of Western Ontario. 13



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Recyclable Polydopamine-Functionalized Sponge for High-Efficiency

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