Solar-Enabled Water Remediation via Recyclable Carbon Dot

Jul 5, 2019 - Scarcity of clean water, due to population growth, global warming, and depletion of natural freshwater sources, is among the most formid...
0 downloads 0 Views 3MB Size
Download PDF
Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

pubs.acs.org/journal/ascecg

Solar-Enabled Water Remediation via Recyclable Carbon Dot/ Hydrogel Composites Seema Singh,† Nitzan Shauloff,† and Raz Jelinek*,†,‡ †

Department of Chemistry and ‡Ilse Katz Institute for Nanotechnology, Ben Gurion University of the Negev, 1 Ben Gurion Ave, Beer Sheva 8410501, Israel

Downloaded via BUFFALO STATE on July 24, 2019 at 01:02:15 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: Scarcity of clean water, due to population growth, global warming, and depletion of natural freshwater sources, is among the most formidable environmental challenges facing humanity. Accordingly, development of cost-effective and widely applicable technologies for water remediation and purification is extremely important and highly sought. We present a new strategy for water purification using a composite material comprising carbon dots (C-dots) encapsulated within a porous hydrogel. The hydrogel matrix allows significant water uptake, while the embedded C-dots constitute effective photothermal mediators, absorbing solar energy for enhanced water evaporation. The C-dots further bestowed greater thermal and mechanical stability to the hydrogel host. The C-dot/hydrogel composite exhibited good operating parameters, including a water evaporation rate of 1.4 kg m2 h‑1 and solar-to-vapor conversion efficiency of 89%. It was applied for diverse water treatment applications, including water desalination and removal of heavy metal ions, detergents, and organic molecules from contaminated water. The C-dot/hydrogel construct is easily synthesized from inexpensive, biocompatible, and environmentally friendly building blocks, is recyclable, and may be employed in varied water purification applications. KEYWORDS: Carbon dots, Hydrogels, Water desalination, Carboxymethyl cellulose, Water purification

■

INTRODUCTION Maintaining access and availability of clean drinking water is among the most pressing environmental challenges in our time, both in developed and developing countries. Indeed, scarcity of freshwater on the one hand and water pollution on the other hand pose significant risks for human well-being, particularly in light of population growth, depletion of natural freshwater, and industrialization.1,2 The most widely employed purification technologies encompass either seawater desalination or water recycling;3−5 both have achieved significant progress over past few decades, although considerable hurdles still exist toward their wide and cost-effective adoption. Specifically, commercial seawater desalination is usually carried out through traditional approaches such as reverse osmosis, ion exchange, and thermal evaporation.6,7 These technologies, however, are generally energy intensive, technically complex, expensive, and large scale in their applicability.8 Similarly, water remediation technologies for removal of pollutants such as metal ions, detergents, and organic contaminants are also usually expensive and technically demanding.9,10 Solar-enabled purification strategies have been proposed as promising alternatives to traditional water remediation techniques.11 Such schemes are based on harnessing sunlight to heat and evaporate water, achieving purification through a much lower energy investment.12 Recent examples of solarinduced water purification techniques include the use of metal composites comprising gold, aluminum, titania, or indium.13−16 Other strategies have focused on the use of © XXXX American Chemical Society

nonmetallic constituents, such as silica nanoparticles, carbon sponges, and graphene for water treatment.17−19 Most such methods have had limited applicability, however, due to high fabrication costs, elaborate synthesis routes, and insufficient scalability. This work presents an innovative water purification strategy based upon a new composite material comprising carbon dots (C-dots) embedded within a hydrogel framework. Hydrogels are a class of porous matrixes formed by cross-linking of hydrophilic polymers.20 Because of their considerable internal surface area, hydrogels can absorb high amounts of water and other guest molecules; importantly, hydrogel frameworks are stable in aqueous solutions and do not dissociate.21,22 Hydrogels have been employed in diverse technologies and products, including hygiene products, contact lenses, artificial tissues, drug delivery, agricultural fertilizers, and others.23−27 Hydrogels have been specifically proposed as membrane-like barriers for seawater desalination, limiting passage of salt ions, thus generating purified water through application of external pressure.28 Water purification platforms based upon hydrogel composites containing reduced graphene oxide, black titania, or gold nanoparticles have been also reported.15,29,30 C-dots are a unique class of carbon-based nanomaterials, attracting considerable scientific and technological interest. CReceived: April 27, 2019 Revised: June 2, 2019 Published: July 5, 2019 A

DOI: 10.1021/acssuschemeng.9b02342 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Characterization Techniques. Atomic force microscopy (AFM) was applied in the tapping mode using an AC160 TS probe (CypherES, Oxford Instrument). Samples for AFM were prepared by dropcasting the dilute solution of C-dots on silicon wafer and drying at room temperature. UV−vis absorbance spectra were recorded on a Thermo Scientific Evolution 220 spectrophotometer. Scanning electron microscopy (SEM) images of hydrogel samples were acquired on a JEOL, SEM (Tokyo, Japan, JSM-7400F). For SEM analysis, hydrogel samples were freeze-dried, and the cross section was imaged at a magnification of 2500× at an acceleration voltage of 2 kV. Thermogravimetric analysis (TGA) was carried out using a Q500 TA Instruments (USA). Thermal analysis was performed by heating the sample from 30 to 800 °C at a heating rate of 10 °C/min under nitrogen flow. Rheology analysis yielding the storage modulus (G′) and loss modulus (G″) of the hydrogels was carried out on a rheometer (AR 2000, TA Instruments) at 25 °C by applying 0.5% strain with a frequency range of 0.1−100 rad/s. Samples for rheology analysis were prepared as a disk of diameter 20 mm and thickness 5 mm. Differential scanning calorimetry (DSC) was carried out on a DSC823e (Mettler Toledo, Switzerland) at a temperature range of 30−200 °C and linear heating rate of 5 K/min, under nitrogen flow. Optical absorbance of the C-dot/hydrogel composites in the solar spectrum range was measured on a Cary 5000 UV−vis-NIR spectrophotometer (Agilent, USA). For analysis, films of gels were prepared on quartz plates. Thermal imaging analysis for determination of C-dot/hydrogel temperatures before and after illumination was carried out by a thermographic camera (FLIR i7). Inductively coupled plasma optical emission spectrometry (ICP-OES) was employed for measuring ion concentrations in water (Spectro Acros, Germany). Surface tension was measured with a tensiometer (KSV Sigma 70, Finland). Gel Swelling. Equilibrium swelling of hydrogels was evaluated by immersing the completely freeze-dried hydrogel samples in distilled water, and water exhibiting salinity of 10 wt %. The gels were kept in water for 2 h to obtain the water swollen hydrogel. The equilibrium swelling was calculated by using eq 1.

dots can be synthesized from readily available carbonaceous building blocks, and they exhibit interesting optical properties, biocompatibility, and diverse applicability.31−33 Importantly, C-dots are promising and easily prepared substitutes to (often toxic) inorganic nanoparticles.34 Previous studies have reported incorporation of C-dots within porous matrixes, and such composite materials have been employed in sensing, drug delivery, solar energy applications, light emitting devices, and others.35−39 Here, we demonstrate that a chitosan/carboxymethyl cellulose (CMC) hydrogel encapsulating C-dots constitutes a generic and highly effective water purification platform. We show that the embedded C-dots facilitate enhanced absorbance of light in a broad spectral range, facilitating efficient photothermal heating and subsequent evaporation of gelincorporated water. The C-dot/hydrogel composite is easy to produce from readily available and environmentally benign reagents which are significantly less expensive than systems utilizing noble metals for photothermal conversion, and energy-intensive conventional techniques such as reverse osmosis and thermal evaporation.8,13 The C-dot/gel composite was stable, recyclable, and resilient even upon application of many purification cycles. We show that the C-dot/hydrogel composite could be implemented as a generic platform for water desalination and water remediation through removal of heavy metal ions, detergents, or organic pollutants. The C-dot/ hydrogel technology may be employed in diverse applications, from portable water purification kits to large-scale industrial platforms.

■

EXPERIMENTAL SECTION

Materials. Carboxymethyl cellulose (CMC) (800 cps), p-phenylenediamine (PPDA), and m-phenylenediamine (MPDA) were purchased from J&K Scientific, Israel. Chitosan (200−800 cps), CuSO4·H2O, NiCl2, AgNO3, CdCl2, and rhodamine 6G (R6G) were purchased from Sigma-Aldrich. Sodium hydroxide was purchased from Frutarom, Ltd. Sodium dodecyl sulfate (SDS) was obtained from Amresco. Epichlorohydrin (ECH) (99%) was supplied by Acros Organics, Israel. Synthesis of Carbon Dots. C-dots were synthesized according to previously reported methods.40 Briefly, PPDA (0.1 g) was dissolved in 10 mL of ethanol, and a transparent brown solution was obtained. The solution was then transferred to a poly(tetrafluoroethylene) (Teflon)-lined autoclave, and hydrothermal heating was carried out at 180 °C for 6 h. The crude brown product was centrifuged at 10 000 rpm. The supernatant was purified by silica column chromatography using ethyl acetate and ethanol as solvents, which were subsequently removed through vacuum drying to obtain the C-dot powder. C-dots were also synthesized from m-phenylelnediamine (MPDA) through the same method. Preparation of CMC/Chitosan/C-Dots Hydrogel. A 2.5 wt % stock solution of chitosan was prepared by dissolving in 0.1% acetic acid at 4 °C. A 2.5 wt % CMC solution was prepared in 9% NaOH solution. For preparation of CMC/chitosan/C-dots hydrogel, 65 mg of CMC was mixed with 35 mg of chitosan from the stock solutions. To this CMC/chitosan mixture, 5 mgs of C-dots were added and stirred for 6 h to obtain homogeneous dispersion of C-dots in gel. For cross-linking of the components, 375 μL of epichlorohydrin (ECH) was added slowly with stirring at 600 rpm for 1 h. This reaction mixture was heated at 50 °C for 8 h to obtain hydrogel. Similarly, other hydrogels were prepared by varying the concentration of C-dots from 5 mg to 20 mg and keeping the other reaction conditions constant. As a control, bare gels without C-dots were also prepared. The four hydrogels containing 0, 5, 10, and 20 mg of C-dots in 100 mg of CMC/chitosan matrix were denoted as bare gel, C-dot 5/gel, C-dot 10/gel, and C-dot 20/gel, respectively.

Q = Ws/Wd

(1)

where Q is equilibrium swelling, Ws and Wd are swollen and dry weight of the hydrogel, respectively. Water Recovery Measurements. To determine the water recovery of swollen hydrogels, irradiation of hydrogel samples was carried out with a solar illumination of 1 kW/m2 (AM 1.5 G) using a solar simulator (Sciencetech, AX-LA125, ASTM Class-AAA) at a wavelength range of 350−1800 nm. All hydrogel samples were illuminated for an hour, and the weight of the hydrogel before and after irradiation was measured through an electronic balance to analyze water recovery, using eq 2 (below). During irradiation, the hydrogel was covered with a glass assembly to collect the condensed water (Figure S10). The experiment was repeated for 50 cycles to evaluate the reusability of these hydrogels. water recovery (%) = (Wr /(Ws − Wd)) × 100

(2)

where Wr, Ws, and Wd are weight of recovered water through hydrogels after irradiation, weight of swollen hydrogel, and dry weight of hydrogels, respectively. Energy Efficiency for Solar-to-Vapor Generation. The efficiencies of solar steam generation from bare gel and C-dot/gel composites were evaluated through the following protocol. The swollen hydrogel samples were allowed to float on saline water (salinity 10 wt %) in a Petri dish for a continuous water supply to the gel. The gel/water assembly was placed on an electronic balance and illuminated at an intensity of 1 kW/m2. The weight change of gel + water upon irradiation was recorded for 1 h at time intervals of 10 min and was used to evaluate the rate of water evaporation and solar vapor generation efficiency by using eq 3 (below),41

η=

B

mh ̇ LV Coptqi

(3) DOI: 10.1021/acssuschemeng.9b02342 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. Carbon dot/hydrogel composite for water purification. (A) Scheme showing assembly of the C-dot/CMC/chitosan hydrogel composite. Sunlight absorbance by the embedded C-dots leads to heating/evaporation of hydrogel-incorporated water. (B) A representative atomic force microscopy (AFM) image of the p-phenylenediamine C-dots employed in the experiments. (C) Ultraviolet−visible (UV−vis) absorption spectrum of the C-dots.

Figure 2. Characterization of the C-dot/hydrogel composite. (A) Scanning electron microscopy (SEM) images of bare hydrogel (i) and C-dot/ hydrogel (ii); the pictures on the top right highlight the visual appearance of the gels. (B) Thermal gravimetric analysis (TGA) of the bare hydrogel (black) and hydrogel encapsulating 20 wt % C-dots (C-dot 20/gel, red). (C) Frequency sweep measurement of bare hydrogel (black) and C-dot 20/gel composite (red). in which η is the energy efficiency of solar-to-vapor generation, ṁ is mass flux, hLV represents enthalpy for liquid to vapor phase change for water in J, Copt is optical concentration on the surface of light absorber, and qi refers to solar irradiation intensity, i.e., 1 sun. Removal of Heavy Metal Ions, Surfactant, and Organic Dyes. To evaluate removal of heavy metal ions from water, 200 mg of dried C-dot/hydrogels were swelled in 5 mL of aqueous solutions containing 0.01 M CuSO4·H2O, NiCl2, AgNO3, and CdCl2, respectively. The swollen C-dot/hydrogel was irradiated, and water was collected, followed by measurement of metal ion concentrations through ICP-OES. Similarly, hydrogels were swelled in aqueous solutions containing rhodamine 6G (R6G, 100 mg/L), or sodium dodecyl sulfate (SDS; 0.05 M). Water purification in these cases was

carried out by irradiating the swollen hydrogel using a 100 W white light emitting diode (LED, Chanzon, China) operating in a wavelength range of 380−800 nm. Distance between the LED and C-dot/hydrogel samples was maintained to achieve an illumination intensity of 1 kW/m2 using a solarmeter.

■

RESULTS AND DISCUSSION Experimental Strategy. Figure 1A depicts the simple water purification strategy we developed. A porous CMC/ chitosan/C-dot hydrogel composite was prepared in a single step by covalent conjugation of chitosan, CMC, and C-dots using epichlorohydrin (ECH) as a cross-linker.42 The C-dot/ C

DOI: 10.1021/acssuschemeng.9b02342 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 3. Physical properties of water absorbed within the hydrogel. (A) Swelling of hydrogel in distilled water, and in saline water (10 wt %). (B) Enthalpy of water evaporation recorded through differential scanning calorimetry (DSC) experiments in C-dot/hydrogels comprising distinct Cdot concentrations. Weight-percent of the C-dots is indicated.

microscopy (TEM) analysis confirms internalization of the Cdots within the gel matrix (Figure S4). Thermal gravimetric analysis (TGA) depicted in Figure 2B underscores the contribution of the embedded C-dots to the thermal stability of the hydrogel. The TGA curves of the bare hydrogel, and hydrogel encapsulating 20% (w/w) C-dots, respectively, reveal two main degradation peaks, one at around 80 °C attributed to evaporation of water bound to the hydrogel network, and a second peak at approximately 250 °C corresponding to decomposition of the CMC/chitosan polymer.49−51 Importantly, Figure 2B demonstrates that the TGA curve recorded for the C-dot/hydrogel (red curve) exhibits an experimentally significant smaller weight change compared to the bare hydrogel (black curve), indicating greater thermal stability of the C-dot/hydrogel composite. This thermal resilience is ascribed to the extensive C-dot-mediated cross-linking within the composite gel.52 The rheology analysis in Figure 2C complements the TGA experiments, furnishing additional evidence for enhanced mechanical stability endowed by embedding C-dots within the hydrogel. Specifically, Figure 2C depicts the frequencydependent storage modulus (G′, reflecting gel elasticity) and loss modulus (G′′, gel viscosity) in the range of 0.1−100 rad/ s.53 The higher G′ values in the entire frequency range (Figure 2C) reflect the formation of a gel structure.26 Notably, Figure 2C reveals that the mechanical strength of the C-dot/hydrogel increased significantly compared to the bare hydrogel, as the G′ and G′′ values of the C-dot/hydrogel were higher than the corresponding parameters recorded for the bare hydrogel. This result is likely due to the more pronounced cross-linking within the C-dot/hydrogel framework, echoing the contribution of cross-linking in the TGA analysis (i.e., Figure 2B). Figure 3 examines the properties of the absorbed water within the C-dot/hydrogel framework. Figure 3A depicts the equilibrium swelling (Q) of C-dot/hydrogel composites comprising different C-dot weight ratios, upon addition of distilled water and saline water (10 wt %), respectively. Gel swelling analysis is important for assessing the capacity of the hydrogel to absorb and retain water.42,54 Figure 3A shows that lower Q values were recorded in the case of saline water as compared to pure water. This result is ascribed to the electrostatic repulsion between mobile salt ions and charged moieties within the hydrogel framework, thereby decreasing osmotic pressure and concomitant gel swelling.55 Somewhat lower gel swelling was apparent upon increasing the C-dot concentration within the hydrogels (Figure 3A), ascribed to cross-linking between the C-dots and the hydrogel network,

hydrogel allowed uptake of significant water quantity due to the porous structure and extensive internal surface area. The thrust of the water purification strategy was the observation that sunlight absorbed by the embedded C-dots resulted in efficient heating of the hydrogel-associated water, ultimately their evaporation and recondensation (Figure 1A, right). Figure 1B−C presents microscopic and spectroscopic characterization of the C-dots, synthesized from p-phenylenediamine (PPDA) as the carbonaceous precursor molecule.40 The atomic force microscopy (AFM) image in Figure 1B highlights the uniform, nanometer size distribution of the C-dots; a statistical analysis yielded sizes of 8 ± 2 nm (Figure S1C). The ultraviolet−visible (UV−vis) absorbance spectrum of the C-dots in Figure 1C reveals a peak at around 300 nm ascribed to the π−π* transition, and a broad absorbance at around 500 nm corresponding to Mie scattering and amine units upon the C-dots’ surface.43,44 The broad absorbance range in the visible spectral region underlies the water purification strategy developed here, since the light absorbed by the embedded C-dots is exploited for water heating and evaporation (i.e., Figure 1A). Comprehensive analytical characterization of the C-dots is provided in Figures S1 and S2. The C-dots in solution yielded fluorescence at ∼620 nm (maximum intensity) using a excitation wavelength of 510 nm (Figure S1A). Incorporation of C-dots within the hydrogel matrix gave rise to complete fluorescence quenching ascribed to self-quenching in the solid phase.45 Characterization of the C-Dot/Hydrogel Composite. Experimental parameters, particularly reaction temperature, duration, and concentrations of gel constituents (CMC, chitosan, C-dots), were optimized to increase gel swelling in water (e.g., higher concentration of absorbed water), enhance mechanical strength and resilience of the C-dot/hydrogel composite, and minimize dissolution in water. Figure S3A−B confirms the significance of the ECH cross-linker (e.g., scheme in Figure 1A). ECH reacted with the hydroxyl or amine groups of the CMC/chitosan to form oxirane moieties, further crosslinked to hydroxyl or amine groups in close vicinity.42,46 Importantly, C-dot/gel composite prepared without ECH and allowed to swell in water yielded a hydrogel from which the Cdots gradually leached (Figure S3A−B). Figure 2 presents physicochemical characterization of the Cdot/hydrogel. The scanning electron microscopy (SEM) images in Figure 2A highlight the fibrous structure of the hydrogel.47,48 Importantly, incorporation of the C-dots did not alter the porous organization of the hydrogel (Figure 2A i,ii); indeed, retaining gel porosity is critical for efficient water absorption and subsequent evaporation. Transmission electron D

DOI: 10.1021/acssuschemeng.9b02342 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 4. Light-induced heating of water absorbed within the C-dot/hydrogel composites. (A) Percentage light absorbance of C-dot and C-dot/ hydrogels in the range of 300 nm and 2500 nm recorded for C-dot (cyan), bare gel (black curve), 5% (w/w) C-dots (red), 10% C-dots (green), and 20% C-dots (blue). (B) Infrared (IR) images of bare hydrogel and 20% C-dot/hydrogel. The different colors reflect the temperature of water captured within the hydrogel; the temperatures recorded by the IR thermogram camera (within the crosses) are indicated above the images. (C) Extent of water heating (ΔT − difference between the temperature before and after irradiation) recorded in different illumination durations.

Figure 5. Water desalination using the carbon dot/hydrogel composite. (A) Percentage water recovery in different C-dot/hydrogels recorded upon solar illumination for 1 h. C-dot concentrations (weight %) are indicated. (B) Mass change of C-dot/gel + water system upon illumination of kW/ m2 at different irradiation times. (C) Measured salinity of water before and after solar illumination. (D) Water recovery efficiency of the C-dot 20/ gel composite recorded in consecutive swelling/evaporation cycles.

affecting reduced access of water molecules into the porous framework.56

The differential scanning calorimetry (DSC) data in Figure 3B illuminate the heat absorption profile of the hydrogelE

DOI: 10.1021/acssuschemeng.9b02342 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

Figure 6. Water remediation applications using the carbon dot/hydrogel. (A) Heavy metal ion removal efficiency following light illumination. (B) Surface tension of water containing sodium dodecyl sulfate (SDS) before and after illumination. (C) UV−vis absorbance spectra of water containing rhodamine-6-G (R6G), before and after illumination. In all cases, C-dot/hydrogels comprising 20 wt % C-dots were illuminated with 100 W white light emitting diode (LED).

spectral range examined, as compared to the bare hydrogel (blue curve vs black curve in Figure 4A). Importantly, more pronounced light absorbance by the C-dot/hydrogels was particularly apparent in the infrared region, in which heat energy is predominant. The increased absorbance can be attributed to C-dot self-assembly in the C-dot/gel matrix.60,61 C-dot-enhanced light absorbance, demonstrated in Figure 4A, underscores the feasibility of solar-induced heating and subsequent evaporation of water embedded within the hydrogel. To examine solar-mediated water heating in the Cdot/hydrogel composites, we applied solar-simulated illumination at 1 kW/m2 intensity in different time durations and measured the C-dot/hydrogel temperature using an infrared (IR) thermograph camera (Figure 4B). Essentially, the color variations in the images shown in Figure 4B reflect the gel temperatures as recorded by the thermal camera (the actual temperature color keys are shown in Figure S8). The thermal images in Figure 4B demonstrate the dramatic impact of the hydrogel-embedded C-dots upon water heating, complementing the absorbance profiles in Figure 4A. For example, after 60 min illumination using the solar simulator, the temperature of the water-saturated 20% (w/w) C-dot/hydrogel reached 58 °C, while the bare hydrogel, under the same illumination conditions, reached a much lower temperature of just 28 °C (Figure 4B). Lightinduced heating effects recorded in the thermal imaging experiments are quantitatively shown in Figure 4C. Echoing the thermal imaging data in Figure 4B, the curves depicted in Figure 4C underscore the contribution of embedded C-dots to water heating. Indeed, the graph in Figure 4C highlights the direct relationship between C-dot concentration and higher temperature of hydrogel-absorbed water. Despite the similar energy absorbance of the 10%-C-dot/gel and 20%-C-dot/gel composites as shown in Figure 4A, water heating and evaporation depend both upon water content, as well as

embedded water. The enthalpy values in Figure 3B, calculated from the raw DSC data (presented in Figure S5), correspond to the heat uptake required for evaporation of water molecules from the saturated hydrogel.57 The bar diagram in Figure 3B reveals that significantly lower water evaporation enthalpies were recorded upon increasing the concentration of the C-dots in the C-dot/hydrogel composites. Specifically, while the bare hydrogel exhibited water evaporation enthalpy of around 2050 J/g, the enthalpy decreased to 1940 J/g, 1740 J/g, and 1675 J/ g for the composites comprising 5% C-dots, 10% C-dots, and 20% C-dots, respectively. The lower enthalpies likely account for the incorporation and cross-linking of C-dots within the hydrogel, displacing water molecules that were physically bound to the hydrogel framework. As a consequence, a higher percentage of hydrogel-absorbed water molecules are unbound within the hydrogel pores (i.e., “free water”), accounting for the lower evaporation enthalpies recorded (Figure 3B).58,59 This interpretation is consistent with the swelling data in Figure 3A, depicting lower gel swelling induced by increasing C-dot concentration. Light-Induced Water Purification Using the C-Dot/ Hydrogel. Figures 4, 5, and 6 demonstrate that efficient water purification was accomplished by C-dot-mediated lightinduced heating of water absorbed within the C-dot/hydrogel. Figure 4A depicts percentage light absorbance by C-dot/ hydrogels exhibiting different concentrations of C-dots (the corresponding transmittance and reflectance spectra are shown in Figure S6). Light absorbance was recorded between 300 nm and 2500 nm, encompassing sunlight irradiation spectrum, designed to examine the extent of solar energy absorbance by the C-dot/hydrogel composites. Indeed, the absorbance data in Figure 4A reveal a direct correlation between the percentage of C-dots embedded within the hydrogel and extent of light absorbance. For example, the 20% C-dot/hydrogel sample displayed significantly higher light absorbance, in the entire F

DOI: 10.1021/acssuschemeng.9b02342 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering state of water inside the gel.62 Indeed, the water swelling analysis (Figure 3A) and DSC data (Figure 3B) indicate that the 20%-C-dot/gel composite contained a lower amount of water but a higher concentration of “free water” as compared to 10%-C-dot/gel (which embedded greater amount of bound water). Thus, the energy absorbed by 20%-C-dot/gel is utilized more efficiently for water heating as compared to the 10%-Cdot/gel composite. Overall, the data presented in Figure 4 confirm that water heating in the C-dot/hydrogel composites is due to light absorbance by the C-dots. The solar energy absorbed by the C-dots likely dissipates by nonradiative processes only, giving rise to the observed temperature increase.61,63 Figures 5−6 demonstrate practical applications of the Cdot/hydrogel composite for water purification. In the experiments summarized in Figure 5, C-dot/hydrogels were swelled in saline water (10 wt %), and recovery of purified water was measured following illumination in a solar simulator (1 kW/m2 intensity) for 60 min. As depicted in Figure 5A, while the calculated water recovery (e.g., percentage of water recovered in relation to water initially absorbed within the gel) was less than 15% in the case of the bare hydrogel, recovery reached 55% following solar illumination of the 20%-C-dot/hydrogel. The results outlined in the bar diagram in Figure 5A are consistent with both the temperature-increase data (Figure 4) and evaporation enthalpies determined in the DSC experiments (Figure 3B), confirming the significance of embedded C-dots for effective water recovery. Supporting this interpretation, lower water purification efficiencies were observed using a hydrogel matrix embedding C-dots synthesized from a different carbon precursor (m-phenylenediamine, MPDA), which exhibited lower absorbance in the wavelength range of solar irradiation (Figure S9). Figure 5B displays the evaporation rate of water, measured in a setup allowing continuous water uptake by the C-dot/ hydrogel (experimental setup outlined in Figure S11), upon irradiation with solar illumination of 1 kW/m2. Similar to data presented in Figure 4, the water evaporation rate was directly linked to the C-dot concentration, and highest for the 20%-Cdot/gel composite (determined as 1.4 kg/m2 h). We employed the water evaporation rates depicted in Figure 5B to calculate the efficiency of solar-to-vapor conversion (η) in the different C-dot/gel composites (eq 3 indicated in the Experimental Section). The energy efficiencies (η) values were 89%, 82%, 55%, and 35% for 20%-C-dot/gel, 10%-C-dot/gel, 5%-C-dot/ gel, and bare gel, respectively. Notably, the solar-to-vapor conversion efficiency of this biocompatible and cost-effective 20%-C-dot/gel composite is comparable to energy efficiency of photothermal absorbers reported previously.13,18,19 The effectiveness of water purification through application of the C-dot/hydrogel platform was further verified by determination of water salinity using inductively coupled plasma optical emission spectrometry (ICP-OES, Figure 5C). Notably, salinity of the recovered water (55% of absorbed water was recovered) was lower than 1%, which is the drinking water standard defined by the World Health Organization.64 We also assessed the reusage of the C-dot/hydrogel composite through application of consecutive swelling/evaporation cycles (Figure 5C). (For reusage of the C-dot/hydrogel composite after 1 h irradiation, the gel was dried in an oven to remove residual water). Indeed, the graph in Figure 5C demonstrates that water recovery efficiency was above 85% (in relation to the initial value) even after more than 50 cycles, underscoring the

practical applicability of the platform for water purification. The decrease in water desalination efficiency is attributed to accumulation of salt in within the hydrogel pores, adversely affecting water uptake and gel swelling. Figure 6 presents utilization of the C-dot/hydrogel system for water remediation applications, specifically removal of heavy metal ions (Figure 6A), detergents (Figure 6B), and organic dyes (Figure 6C). Figure 6A depicts the decontamination efficiency for Cu2+, Ni2+, Ag+, and Cd2+ respectively, which are among the most ubiquitous metal ion pollutants in drinking water.65 In the experiments highlighted in Figure 6A, the concentration of metal ions in water captured within a Cdot-20/gel prior to illumination at 1 kW/m2 intensity for 1 h was 0.01 M. Remarkably, less than 0.05% of the initial ion concentrations were present in the recovered water (after illumination). Figure 6B presents surface tension analysis of water containing 0.05 M sodium dodecyl sulfate (SDS) before and after illumination of the C-dot/hydrogel swelled in the contaminated water. Notably, while surface tension prior to remediation via the C-dot/hydrogel treatment was around 37 mN/m (accounting for the SDS surfactant film at the water surface), surface tension increased after illumination to approximately 72 mN/m, close to surface tension for pure drinkable water.66 We also evaluated remediation of water containing organic pollutants (Figure 6C). Figure 6C shows the UV−vis absorbance spectra of water containing rhodamine 6G (R6G) (0.5 μM), which is among the most common organic pollutants in industrial wastewater. As is apparent in Figure 6C, while R6G in untreated water yielded a significant UV−vis peak at around 530 nm (blue spectrum), this absorbance peak almost completely disappeared after illumination (red spectrum), indicating negligible concentration of R6G in the recovered water. Together, the experimental data in Figure 6 underscore the remarkable and versatile applicability of the C-dot/hydrogel platform for water remediation.

■

CONCLUSIONS This study presents construction of C-dot/CMC/chitosan hydrogels and their usage for water remediation. The composite material combines the high porosity of the hydrogel framework which allows significant uptake of water molecules and light absorbance by the C-dots in a broad spectra range, particularly encompassing the solar illumination spectrum. The C-dots constitute the core photothermal mediator in the system, converting solar irradiation to dissipated heat which efficiently raises the temperature of the hydrogel-absorbed water, resulting in evaporation and subsequent recondensation of purified water. Importantly, we also show that the embedded C-dots contributed to enhanced stability and mechanical strength of the hydrogel matrix, likely through cross-linking between surface moieties of the C-dots and the hydrogel framework. Overall, we demonstrate that the C-dot/ hydrogel facilitated effective purification of saline water, water contaminated with heavy metal ions, detergents, or organic dyes. The C-dot/hydrogel composite exhibits important practical advantages: recyclability, ease of preparation, and the use of inexpensive, biodegradable, and readily available building blocks. The technology may be implemented in diverse applications such as low-cost water desalination platforms and portable water purification kits. G

DOI: 10.1021/acssuschemeng.9b02342 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering

■

(12) Gao, M.; Connor, P. K. N.; Ho, G. W. Plasmonic Photothermic Directed Broadband Sunlight Harnessing for Seawater Catalysis and Desalination. Energy Environ. Sci. 2016, 9, 3151−3160. (13) Bae, K.; Kang, G.; Cho, S. K.; Park, W.; Kim, K.; Padilla, W. J. Flexible Thin-Film Black Gold Membranes with Ultrabroadband Plasmonic Nanofocusing for Efficient Solar Vapour Generation. Nat. Commun. 2015, 6, 10103−10111. (14) Liu, T.; Li, Y. Plasmonic Solar Desalination. Nat. Photonics 2016, 10, 361−362. (15) Liu, X.; Cheng, H.; Guo, Z.; Zhan, Q.; Qian, J.; Wang, X. Bifunctional, Moth-Eye-Like Nanostructured Black Titania Nanocomposites for Solar-Driven Clean Water Generation. ACS Appl. Mater. Interfaces 2018, 10, 39661−39669. (16) Zhang, L.; Xing, J.; Wen, X.; Chai, J.; Wang, S.; Xiong, Q. Plasmonic Heating from Indium Nanoparticles on a Floating Microporous Membrane for Enhanced Solar Seawater Desalination. Nanoscale 2017, 9, 12843−12849. (17) Polman, A. Solar Steam Nanobubbles. ACS Nano 2013, 7, 15− 18. (18) Zhu, L.; Gao, M.; Peh, C. K. N.; Wang, X.; Ho, G. W. SelfContained Monolithic Carbon sponge for Solar-Driven Interfacial Water Evaporation Distillation and Electricity Generation. Adv. Energy Mater. 2018, 8, 1702149. (19) Kim, K.; Yu, S.; An, C.; Kim, S.; Jang, J. H. Mesoporous ThreeDimensional Graphene Networks for Highly Efficient Solar Desalination Under 1 Sun Illumination. ACS Appl. Mater. Interfaces 2018, 10, 15602−15608. (20) Buwalda, S. J.; Vermonden, T.; Hennink, W. E. Hydrogels for Therapeutic Delivery: Current Developments and Future Directions. Biomacromolecules 2017, 18, 316−330. (21) Willner, I. Stimulli Controlled Hydrogels and Their Applications. Acc. Chem. Res. 2017, 50, 657−658. (22) Kopecek, J. Hydrogel Biomaterials: A Smart Future? Biomaterials 2007, 28, 5185−5192. (23) Calo, E.; Khutoryanskiy, V. V. Biomedical Applications of Hydrogels: A Review of Patents and Commercial Products. Eur. Polym. J. 2015, 65, 252−267. (24) Childs, A.; Li, H.; Lewittes, D. M.; Dong, B.; Liu, W.; Shu, X.; Sun, C.; Zhang, H. F. Fabricating Customized Hydrogel Contact Lens. Sci. Rep. 2016, 6, 34905−34914. (25) Lee, S. S.; Kim, H. D.; Kim, S. H. L.; Kim, I.; Kim, I.; Choi, J. S.; Jeong, J.; Kim, J. H.; Kwon, S. K.; Hwang, N. S. Self-Healing and Adhesive Artificial Tissue Implant for Voice Recovery. ACS Appl. Bio Mater. 2018, 1, 1134−1146. (26) Singh, S.; Mishra, A.; Kumari, R.; Sinha, K. K.; Singh, M. K.; Das, P. Carbon Dots Assisted Formation of DNA Hydrogel for Sustained Release of Drug. Carbon 2017, 114, 169−176. (27) Cheng, D.; Liu, Y.; Yang, G.; Zhang, A. Water- and FertilizerIntegrated Hydrogel Derived from the Polymerization of Acrylic Acid and Urea as a Slow Release N Fertilizer and Water Retention in Agriculture. J. Agric. Food Chem. 2018, 66, 5762−5769. (28) Höpfner, J.; Richter, T.; Kosovan, P.; Holm, C.; Wilhelm, M. Seawater Desalination via Hydrogels: Practical Realisation and First Coarse Grained Simulations. Prog. Polym. Colloid Sci. Intelligent Hydrogels 2013, 140, 247−263. (29) Zhou, X.; Zhao, F.; Guo, Y.; Zhang, Y.; Yu, G. A Hydrogel Based Antifouling Solar Evaporator for Highly Efficient Water Desalination. Energy Environ. Sci. 2018, 11, 1985−1992. (30) Jin, H.; Lin, G.; Bai, L.; Zeiny, A.; Wen, D. Steam Generation in a Nanoparticle-Based Solar Receiver. Nano Energy 2016, 28, 397− 406. (31) Wang, R.; Lu, K. Q.; Tang, Z. R.; Xu, Y. J. Recent Progress in Carbon Quantum Dots: Synthesis, Properties and Applications in Photocatalysis. J. Mater. Chem. A 2017, 5, 3717−3734. (32) Song, Y.; Zhu, S.; Shao, J.; Yang, B. Polymer Carbon Dots-A Highlight Reviewing Their Unique Structure, Bright Emission and Probable Photoluminescence Mechanism. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 610−615.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.9b02342.

■

Fluorescence spectra, AFM image, FTIR and XPS of Cdots; Images of ECH cross-linked C-dot/gel in water and C-dot/gel without ECH: TEM images; DSC graph of gels; spectrum of solar simulator and comparative table with standard; transmittance and reflectance graph of gels; thermal images of C-dot/gel composites; and image of glass assembly used for water collection (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Fax: (+) 972-8-6472943. ORCID

Seema Singh: 0000-0001-8132-5453 Raz Jelinek: 0000-0002-0336-1384 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We are thankful to Prof. Iris Visoly-Fisher and K. M. Anoop for their assistance with the solar simulator, and Dr. Arkadi Zilberman for help with thermal imaging. S.S. is grateful to the Marcus Fund of Ben Gurion University for a postdoctoral fellowship.

■

REFERENCES

(1) Bartram, J.; Brocklehurst, C.; Fisher, M. B.; Luyendijk, R.; Hossain, R.; Wardlaw, T.; Gordon, B. Global Monitoring of Water Supply and Sanitation: History, Methods and Future Challenges. Int. J. Environ. Res. Public Health 2014, 11, 8137−8165. (2) Elimelech, M.; Phillip, W. A. The Future of Seawater Desalination:Energy, Technology and the Environment. Science 2011, 333, 712−717. (3) Darre, N. C.; Toor, G. S. Desalination of Water: A Review. Curr. Pollution Rep. 2018, 4, 104−111. (4) Fujiwara, M.; Imura, T. Photo Induced Membrane Separation for Water Purification and Desalination Using Azobenzene Modified Anodized Alumina Membranes. ACS Nano 2015, 9, 5705−5712. (5) Boyjoo, Y.; Pareek, V. K.; Ang, M. A Review of Greywater Characteristics and Treatment Processes. Water Sci. Technol. 2013, 24, 81−88. (6) Shatat, M.; Riffat, S. B. Water Desalination Technologies Utilizing Conventional and Renewable Energy Sources. Int. J. LowCarbon Technol. 2014, 9, 1−19. (7) Manju, S.; Sagar, N. Renewable Energy Integrated Desalination: A Sustainable Solution to Overcome Future Fresh-Water Scarcity in India. Renewable Sustainable Energy Rev. 2017, 73, 594−609. (8) Ibrahim, A. G. M.; Rashad, A. M.; Dincer, I. Exergoeconomic Analysis for Cost Optimization of a Solar Distillation System. Sol. Energy 2017, 151, 22−32. (9) Mon, M.; Bruno, R.; Ferrando-Soria, J.; Armentano, D.; Pardo, E. Metal-Organic Framework Technologies for Water Remediation: Towards a Sustainable Ecosystem. J. Mater. Chem. A 2018, 6, 4912− 4947. (10) Bolisetty, S.; Peydayesh, M.; Mezzenga, R. Sustainable Technologies for Water Purification from Heavy Metals: Review and Analysis. Chem. Soc. Rev. 2019, 48, 463−487. (11) Lewis, N. S. Toward Cost Effective Solar Energy Use. Science 2007, 315, 798−801. H

DOI: 10.1021/acssuschemeng.9b02342 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering (33) Zhu, S.; Song, Y.; Zhao, X.; Shao, J.; Zhang, J.; Yang, B. The Photoluminescence Mechanism in Carbon Dots (Graphene Quantum dots, Carbon Nanodots, and Polymer Dots): Current State and Future Perspective. Nano Res. 2015, 8, 355−381. (34) Xiong, Y.; Schneider, J.; Ushakova, E. V.; Rogach, A. L. Influence of Molecular Fluorophores on The Research Field of Chemically Synthesized Carbon Dots. Nano Today 2018, 23, 124− 139. (35) Wang, Y.; Liang, Z.; Su, Z.; Zhang, K.; Ren, J.; Sun, R.; Wang, X. All-Biomass Fluorescent Hydrogels Based on Biomass Carbon Dots and Alginate/Nanocellulose for Biosensing. ACS Appl. Bio. Mater. 2018, 1, 1398−1407. (36) Li, W.; Liu, Q.; Zhang, P.; Liu, L. Zwitterionic Nanogels crosslinked by Fluorescent Carbon dots for Targeted Drug Delivery and Simultaneous Bioimaging. Acta Biomater. 2016, 40, 254−262. (37) Wang, H.; Chen, C.; Zhou, S. Carbon Based Hybrid Nanogels: A Synergistic Nanoplatform for Combined Biosensing, Bioimaging and Responsive Drug Delivery. Chem. Soc. Rev. 2018, 47, 4198−4232. (38) Hu, C.; Li, M.; Qiu, J.; Sun, Y.-P. Design and Fabrication of Carbon dots for Energy Conversion and Storage. Chem. Soc. Rev. 2019, 48, 2315−2337. (39) Yuan, F. L.; Yuan, T.; Sui, L. Z.; Wang, Z. B.; Xi, Z. F.; Li, Y. C.; Li, X. H.; Fan, L. Z.; Tan, Z. A.; Chen, A. M.; Jin, M. X.; Yang, S. H. Engineering Triangular Carbon Quantum Dots with Unprecedented Narrow Bandwidth Emission for Multicolored LEDs. Nat. Commun. 2018, 9, 2249−2259. (40) Jiang, K.; Sun, S.; Zhang, L.; Lu, Y.; Wu, A.; Cai, C.; Lin, H. Red, Green, and Blue Luminescence by Carbon Dots: Full-Color Emission Tuning and Multicolor Cellular Imaging. Angew. Chem., Int. Ed. 2015, 54, 5360−5363. (41) Awad, F. S.; Kiriarachchi, H. D.; AbouZeid, K. M.; Ö zgür, Ü .; El-Shall, M. S. Plasmonic Graphene Polyrethane Nanocomposites for Efficient Solar Water Desalination. ACS Appl. Energy Mater. 2018, 1, 976−985. (42) Jeong, D.; Joo, S. W.; Hu, Y.; Shinde, V. V.; Cho, E.; Jung, S. Carboxymethyl Cellulose-Based Superabsorbent Hydrogels Containing Carboxymehtyl β-cyclodextrin for Enhanced Mechanical Strength and Effective Drug Delivery. Eur. Polym. J. 2018, 105, 17−25. (43) Ding, H.; Yu, S.-B.; Wei, J.-S.; Xiong, H.-M. Full-Color Light Emitting Carbon Dots with a Surface-State-Controlled Luminescence Mechanism. ACS Nano 2016, 10, 484−491. (44) Qu, S.; Wang, X.; Lu, Q.; Liu, X.; Wang, L. A Biocompatible Fluorescent Ink Based on Water-Soluble Luminescent Carbon Nanodots. Angew. Chem., Int. Ed. 2012, 51, 12215−12218. (45) Wang, J.; Zhang, F.; Wang, Y.; Yang, Y.; Liu, X. Efficient Resistance Against Solid-State Quenching of Carbon dots Towards White Light Emitting Diodes by Physical Embedding into Silica. Carbon 2018, 126, 426−436. (46) Gericke, M.; Trygg, J.; Fardim, P. Functional Cellulose Beads: Preparation, Characterization, and Applications. Chem. Rev. 2013, 113, 4812−4836. (47) McBane, J. E.; Vulesevic, B.; Padavan, D. T.; McEwan, K. A.; Korbutt, G. S.; Suuronen, E. J. Evaluation of a Collagen-Chitosan Hydrogel for Potential Use as a Pro-angiogenic Site for Islet Transplantation. PLoS One 2013, 8, No. e77538. (48) Hata, Y.; Kojima, T.; Koizumi, T.; Okura, H.; Sakai, T.; Sawada, T.; Serizawa, T. Enzymatic Synthesis of Cellulose Oligomer Hydrogels Composed of Crystalline Nanoribbon Networks under Macromolecular Crowding Conditions. ACS Macro Lett. 2017, 6, 165−170. (49) Pu, S. Y.; Hou, Y. G.; Yan, C.; Ma, H.; Huang, H. Y.; Shi, Q. Q.; Mandal, S.; Diao, Z. H.; Chu, W. In Situ Co-Precipitation Formed Highly Water-Dispersible Magnetic Chitosan Nanopowder for Removal of Heavy Metals and Its Adsorption Mechanism. ACS Sustainable Chem. Eng. 2018, 6, 16754−16765. (50) George, J.; Ramana, K. V.; Bawa, A. S.; Siddaramaiah. Bacterial Cellulose Nanocrystals Exhibiting High Thermal Stability and Their Polymer Nanocomposites. Int. J. Biol. Macromol. 2011, 48, 50−57.

(51) Konwar, A.; Kalita, S.; Kotoky, J.; Chowdhury, D. Chitosan− Iron Oxide Coated Graphene Oxide Nanocomposite Hydrogel: A Robust and Soft Antimicrobial Biofilm. ACS Appl. Mater. Interfaces 2016, 8, 20625−20634. (52) Konwar, A.; Gogoi, N.; Majumdar, G.; Chowdhury, D. Green Chitosan−Carbon Dots Nanocomposite Hydrogel Film with Superior Properties. Carbohydr. Polym. 2015, 115, 238−245. (53) Liu, J.; Zhang, L.; Yang, Z.; Zhao, X. Controlled Release of Paclitaxel from a Self-Assembling Peptide Hydrogel Formed in situ and Antitumor Study in Vitro. Int. J. Nanomed. 2011, 6, 2143−2153. (54) Iyer, G.; Tillekeratne, L. M. V.; Coleman, M. R.; Nadarajah, A. Equilibrium Swelling Behavior of Thermally Responsive Metal Affinity Hydrogels, Part I: Compositional Effects. Polymer 2008, 49, 3737−3743. (55) Chang, C.; He, M.; Zhou, J.; Zhang, L. Swelling Behaviors of pH- and Salt-Responsive Cellulose-Based Hydrogels. Macromolecules 2011, 44, 1642−1648. (56) Hoffman, A. S. Hydrogels for Biomedical Applications. Adv. Drug. Delivery Rev. 2002, 43, 3−12. (57) Modrzejewska, Z. Characterization of Water State in Chitosan Hydrogel Membranes; Polish Chitin Society: Lodz, Poland, 2011; pp 49−60. (58) Ali, W.; Gebert, B.; Hennecke, T.; Graf, K.; Ulbricht, M.; Gutmann, J. S. Design of Thermally Responsive Polymeric Hydrogels for Brackish Water Desalination: Effect of Architecture on Swelling, Deswelling, and Salt Rejection. ACS Appl. Mater. Interfaces 2015, 7, 15696−15706. (59) Miyazaki, M.; Fujii, A.; Ebata, T.; Mikami, N. Infrared Spectroscopic Evidence for Protonated Water Clusters forming Nanoscale Cages. Science 2004, 304, 1134−1137. (60) Permatasari, F. A.; Fukazawa, H.; Ogi, T.; Iskandar, F.; Okuyama, K. Design of Pyrrolic-N-Rich Carbon dots with Absorption in the First Near-Infrared Window for Photothermal Therapy. ACS Appl. Nano Mater. 2018, 1, 2368−2375. (61) Li, D.; Han, D.; Qu, S. N.; Liu, L.; Jing, P. T.; Zhou, D.; Ji, W. Y.; Wang, X. Y.; Zhang, T. F.; Shen, D. Z. Supra-(Carbon Nanodots) with a Strong Visible to Near-Infrared Absorption Band and Efficient Photothermal Conversion. Light: Sci. Appl. 2016, 5, No. e16120. (62) Zhao, F.; Zhou, X.; Shi, Y.; Qian, X.; Alexander, M.; Zhao, X.; Mendez, S.; Yang, R.; Qu, L.; Yu, G. Highly Efficient Solar Vapour Generation via Hierarchically Nanostructured Gels. Nat. Nanotechnol. 2018, 13, 489−495. (63) Gan, Z.; Xu, H.; Fu, Y. Photon Reabsorption and Nonradiative Energy-Transfer-Induced Quenching of Blue Photoluminescence from Aggregated Graphene Quantum Dots. J. Phys. Chem. C 2016, 120, 29432−29438. (64) Safe Drinking-Water from Desalination; World Health Organization (WHO), 2011. (65) Richardson, S. D.; Ternes, T. A. Water Analysis: Emerging Contaminants and Current Issues. Anal. Chem. 2018, 90, 398−428. (66) Gascon, K. N.; Weinstein, S. J.; Antoniades, M. G. Use of Simplified Surface Tension Measurements to Determine Surface Excess: An Undergraduate Experiment. J. Chem. Educ. 2019, 96, 342− 347.

I

DOI: 10.1021/acssuschemeng.9b02342 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX