Solar-Enabled Water Remediation via Recyclable Carbon Dot/Hydrogel CompositesSeema 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

*S Supporting Information

ABSTRACT: Scarcity of clean water, due to population growth, global warming, anddepletion of natural freshwater sources, is among the most formidable environmentalchallenges facing humanity. Accordingly, development of cost-effective and widelyapplicable technologies for water remediation and purification is extremely importantand highly sought. We present a new strategy for water purification using a compositematerial comprising carbon dots (C-dots) encapsulated within a porous hydrogel. Thehydrogel matrix allows significant water uptake, while the embedded C-dots constituteeffective photothermal mediators, absorbing solar energy for enhanced waterevaporation. The C-dots further bestowed greater thermal and mechanical stabilityto the hydrogel host. The C-dot/hydrogel composite exhibited good operatingparameters, including a water evaporation rate of 1.4 kg m2 h‑1 and solar-to-vaporconversion efficiency of 89%. It was applied for diverse water treatment applications,including water desalination and removal of heavy metal ions, detergents, and organicmolecules from contaminated water. The C-dot/hydrogel construct is easily synthesized from inexpensive, biocompatible, andenvironmentally 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 isamong the most pressing environmental challenges in our time,both in developed and developing countries. Indeed, scarcity offreshwater on the one hand and water pollution on the otherhand pose significant risks for human well-being, particularly inlight of population growth, depletion of natural freshwater, andindustrialization.1,2 The most widely employed purificationtechnologies encompass either seawater desalination or waterrecycling;3−5 both have achieved significant progress over pastfew decades, although considerable hurdles still exist towardtheir wide and cost-effective adoption. Specifically, commercialseawater desalination is usually carried out through traditionalapproaches such as reverse osmosis, ion exchange, and thermalevaporation.6,7 These technologies, however, are generallyenergy intensive, technically complex, expensive, and largescale in their applicability.8 Similarly, water remediationtechnologies for removal of pollutants such as metal ions,detergents, and organic contaminants are also usuallyexpensive and technically demanding.9,10

Solar-enabled purification strategies have been proposed aspromising alternatives to traditional water remediationtechniques.11 Such schemes are based on harnessing sunlightto heat and evaporate water, achieving purification through amuch lower energy investment.12 Recent examples of solar-induced water purification techniques include the use of metalcomposites comprising gold, aluminum, titania, or indi-um.13−16 Other strategies have focused on the use of

nonmetallic constituents, such as silica nanoparticles, carbonsponges, and graphene for water treatment.17−19 Most suchmethods have had limited applicability, however, due to highfabrication costs, elaborate synthesis routes, and insufficientscalability.This work presents an innovative water purification strategy

based upon a new composite material comprising carbon dots(C-dots) embedded within a hydrogel framework. Hydrogelsare a class of porous matrixes formed by cross-linking ofhydrophilic polymers.20 Because of their considerable internalsurface area, hydrogels can absorb high amounts of water andother guest molecules; importantly, hydrogel frameworks arestable in aqueous solutions and do not dissociate.21,22

Hydrogels have been employed in diverse technologies andproducts, including hygiene products, contact lenses, artificialtissues, drug delivery, agricultural fertilizers, and others.23−27

Hydrogels have been specifically proposed as membrane-likebarriers for seawater desalination, limiting passage of salt ions,thus generating purified water through application of externalpressure.28 Water purification platforms based upon hydrogelcomposites 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. C-

Received: April 27, 2019Revised: June 2, 2019Published: July 5, 2019

Research Article

pubs.acs.org/journal/ascecgCite This: ACS Sustainable Chem. Eng. 2019, 7, 13186−13194

© 2019 American Chemical Society 13186 DOI: 10.1021/acssuschemeng.9b02342ACS Sustainable Chem. Eng. 2019, 7, 13186−13194

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dots can be synthesized from readily available carbonaceousbuilding blocks, and they exhibit interesting optical properties,biocompatibility, and diverse applicability.31−33 Importantly,C-dots are promising and easily prepared substitutes to (oftentoxic) inorganic nanoparticles.34 Previous studies havereported incorporation of C-dots within porous matrixes, andsuch composite materials have been employed in sensing, drugdelivery, solar energy applications, light emitting devices, andothers.35−39

Here, we demonstrate that a chitosan/carboxymethylcellulose (CMC) hydrogel encapsulating C-dots constitutes ageneric and highly effective water purification platform. Weshow that the embedded C-dots facilitate enhanced absorbanceof light in a broad spectral range, facilitating efficientphotothermal heating and subsequent evaporation of gel-incorporated water. The C-dot/hydrogel composite is easy toproduce from readily available and environmentally benignreagents which are significantly less expensive than systemsutilizing noble metals for photothermal conversion, andenergy-intensive conventional techniques such as reverseosmosis and thermal evaporation.8,13 The C-dot/gel compositewas stable, recyclable, and resilient even upon application ofmany purification cycles. We show that the C-dot/hydrogelcomposite could be implemented as a generic platform forwater desalination and water remediation through removal ofheavy 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 industrialplatforms.

■ EXPERIMENTAL SECTIONMaterials. Carboxymethyl cellulose (CMC) (800 cps), p-phenyl-

enediamine (PPDA), and m-phenylenediamine (MPDA) werepurchased from J&K Scientific, Israel. Chitosan (200−800 cps),CuSO4·H2O, NiCl2, AgNO3, CdCl2, and rhodamine 6G (R6G) werepurchased from Sigma-Aldrich. Sodium hydroxide was purchasedfrom Frutarom, Ltd. Sodium dodecyl sulfate (SDS) was obtainedfrom Amresco. Epichlorohydrin (ECH) (99%) was supplied by AcrosOrganics, Israel.Synthesis of Carbon Dots. C-dots were synthesized according to

previously reported methods.40 Briefly, PPDA (0.1 g) was dissolved in10 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 at180 °C for 6 h. The crude brown product was centrifuged at 10 000rpm. The supernatant was purified by silica column chromatographyusing ethyl acetate and ethanol as solvents, which were subsequentlyremoved through vacuum drying to obtain the C-dot powder. C-dotswere also synthesized from m-phenylelnediamine (MPDA) throughthe same method.Preparation of CMC/Chitosan/C-Dots Hydrogel. A 2.5 wt %

stock solution of chitosan was prepared by dissolving in 0.1% aceticacid at 4 °C. A 2.5 wt % CMC solution was prepared in 9% NaOHsolution. For preparation of CMC/chitosan/C-dots hydrogel, 65 mgof CMC was mixed with 35 mg of chitosan from the stock solutions.To this CMC/chitosan mixture, 5 mgs of C-dots were added andstirred for 6 h to obtain homogeneous dispersion of C-dots in gel. Forcross-linking of the components, 375 μL of epichlorohydrin (ECH)was added slowly with stirring at 600 rpm for 1 h. This reactionmixture was heated at 50 °C for 8 h to obtain hydrogel. Similarly,other hydrogels were prepared by varying the concentration of C-dotsfrom 5 mg to 20 mg and keeping the other reaction conditionsconstant. 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 100mg of CMC/chitosan matrix were denoted as bare gel, C-dot 5/gel,C-dot 10/gel, and C-dot 20/gel, respectively.

Characterization Techniques. Atomic force microscopy (AFM)was applied in the tapping mode using an AC160 TS probe (Cypher-ES, Oxford Instrument). Samples for AFM were prepared by drop-casting the dilute solution of C-dots on silicon wafer and drying atroom temperature. UV−vis absorbance spectra were recorded on aThermo Scientific Evolution 220 spectrophotometer. Scanning electronmicroscopy (SEM) images of hydrogel samples were acquired on aJEOL, SEM (Tokyo, Japan, JSM-7400F). For SEM analysis, hydrogelsamples were freeze-dried, and the cross section was imaged at amagnification of 2500× at an acceleration voltage of 2 kV.Thermogravimetric analysis (TGA) was carried out using a Q500 TAInstruments (USA). Thermal analysis was performed by heating thesample from 30 to 800 °C at a heating rate of 10 °C/min undernitrogen flow. Rheology analysis yielding the storage modulus (G′) andloss modulus (G″) of the hydrogels was carried out on a rheometer(AR 2000, TA Instruments) at 25 °C by applying 0.5% strain with afrequency range of 0.1−100 rad/s. Samples for rheology analysis wereprepared as a disk of diameter 20 mm and thickness 5 mm.Dif ferential scanning calorimetry (DSC) was carried out on a DSC823e(Mettler Toledo, Switzerland) at a temperature range of 30−200 °Cand linear heating rate of 5 K/min, under nitrogen flow. Opticalabsorbance of the C-dot/hydrogel composites in the solar spectrumrange was measured on a Cary 5000 UV−vis-NIR spectrophotometer(Agilent, USA). For analysis, films of gels were prepared on quartzplates. Thermal imaging analysis for determination of C-dot/hydrogeltemperatures before and after illumination was carried out by athermographic camera (FLIR i7). Inductively coupled plasma opticalemission spectrometry (ICP-OES) was employed for measuring ionconcentrations in water (Spectro Acros, Germany). Surface tensionwas measured with a tensiometer (KSV Sigma 70, Finland).

Gel Swelling. Equilibrium swelling of hydrogels was evaluated byimmersing the completely freeze-dried hydrogel samples in distilledwater, and water exhibiting salinity of 10 wt %. The gels were kept inwater for 2 h to obtain the water swollen hydrogel. The equilibriumswelling was calculated by using eq 1.

Q W W/s d= (1)

where Q is equilibrium swelling, Ws and Wd are swollen and dryweight of the hydrogel, respectively.

Water Recovery Measurements. To determine the waterrecovery of swollen hydrogels, irradiation of hydrogel samples wascarried out with a solar illumination of 1 kW/m2 (AM 1.5 G) using asolar simulator (Sciencetech, AX-LA125, ASTM Class-AAA) at awavelength range of 350−1800 nm. All hydrogel samples wereilluminated for an hour, and the weight of the hydrogel before andafter irradiation was measured through an electronic balance toanalyze water recovery, using eq 2 (below). During irradiation, thehydrogel was covered with a glass assembly to collect the condensedwater (Figure S10). The experiment was repeated for 50 cycles toevaluate the reusability of these hydrogels.

W W Wwater recovery (%) ( /( )) 100r s d= − × (2)

where Wr, Ws, and Wd are weight of recovered water throughhydrogels after irradiation, weight of swollen hydrogel, and dry weightof hydrogels, respectively.

Energy Efficiency for Solar-to-Vapor Generation. Theefficiencies of solar steam generation from bare gel and C-dot/gelcomposites were evaluated through the following protocol. Theswollen hydrogel samples were allowed to float on saline water(salinity 10 wt %) in a Petri dish for a continuous water supply to thegel. The gel/water assembly was placed on an electronic balance andilluminated 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 minand was used to evaluate the rate of water evaporation and solar vaporgeneration efficiency by using eq 3 (below),41

mhC qi

LV

optη =

(3)

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in which η is the energy efficiency of solar-to-vapor generation, m ismass flux, hLV represents enthalpy for liquid to vapor phase change forwater in J, Copt is optical concentration on the surface of lightabsorber, 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 ofdried C-dot/hydrogels were swelled in 5 mL of aqueous solutionscontaining 0.01 M CuSO4·H2O, NiCl2, AgNO3, and CdCl2,respectively. The swollen C-dot/hydrogel was irradiated, and waterwas collected, followed by measurement of metal ion concentrationsthrough ICP-OES. Similarly, hydrogels were swelled in aqueoussolutions containing rhodamine 6G (R6G, 100 mg/L), or sodiumdodecyl sulfate (SDS; 0.05 M). Water purification in these cases was

carried out by irradiating the swollen hydrogel using a 100 W whitelight emitting diode (LED, Chanzon, China) operating in awavelength range of 380−800 nm. Distance between the LED andC-dot/hydrogel samples was maintained to achieve an illuminationintensity of 1 kW/m2 using a solarmeter.

■ RESULTS AND DISCUSSION

Experimental Strategy. Figure 1A depicts the simplewater purification strategy we developed. A porous CMC/chitosan/C-dot hydrogel composite was prepared in a singlestep by covalent conjugation of chitosan, CMC, and C-dotsusing epichlorohydrin (ECH) as a cross-linker.42 The C-dot/

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 forcemicroscopy (AFM) image of the p-phenylenediamine C-dots employed in the experiments. (C) Ultraviolet−visible (UV−vis) absorption spectrumof 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-dot20/gel composite (red).

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hydrogel allowed uptake of significant water quantity due tothe porous structure and extensive internal surface area. Thethrust of the water purification strategy was the observationthat sunlight absorbed by the embedded C-dots resulted inefficient heating of the hydrogel-associated water, ultimatelytheir evaporation and recondensation (Figure 1A, right).Figure 1B−C presents microscopic and spectroscopic

characterization of the C-dots, synthesized from p-phenyl-enediamine (PPDA) as the carbonaceous precursor mole-cule.40 The atomic force microscopy (AFM) image in Figure1B highlights the uniform, nanometer size distribution of theC-dots; a statistical analysis yielded sizes of 8 ± 2 nm (FigureS1C). The ultraviolet−visible (UV−vis) absorbance spectrumof the C-dots in Figure 1C reveals a peak at around 300 nmascribed to the π−π* transition, and a broad absorbance ataround 500 nm corresponding to Mie scattering and amineunits upon the C-dots’ surface.43,44 The broad absorbancerange in the visible spectral region underlies the waterpurification strategy developed here, since the light absorbedby the embedded C-dots is exploited for water heating andevaporation (i.e., Figure 1A). Comprehensive analyticalcharacterization of the C-dots is provided in Figures S1 andS2. 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 hydrogelmatrix gave rise to complete fluorescence quenching ascribedto 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 inwater (e.g., higher concentration of absorbed water), enhancemechanical strength and resilience of the C-dot/hydrogelcomposite, and minimize dissolution in water. Figure S3A−Bconfirms the significance of the ECH cross-linker (e.g., schemein Figure 1A). ECH reacted with the hydroxyl or amine groupsof the CMC/chitosan to form oxirane moieties, further cross-linked to hydroxyl or amine groups in close vicinity.42,46

Importantly, C-dot/gel composite prepared without ECH andallowed to swell in water yielded a hydrogel from which the C-dots gradually leached (Figure S3A−B).Figure 2 presents physicochemical characterization of the C-

dot/hydrogel. The scanning electron microscopy (SEM)images in Figure 2A highlight the fibrous structure of thehydrogel.47,48 Importantly, incorporation of the C-dots did notalter the porous organization of the hydrogel (Figure 2A i,ii);indeed, retaining gel porosity is critical for efficient waterabsorption and subsequent evaporation. Transmission electron

microscopy (TEM) analysis confirms internalization of the C-dots within the gel matrix (Figure S4).Thermal gravimetric analysis (TGA) depicted in Figure 2B

underscores the contribution of the embedded C-dots to thethermal stability of the hydrogel. The TGA curves of the barehydrogel, and hydrogel encapsulating 20% (w/w) C-dots,respectively, reveal two main degradation peaks, one at around80 °C attributed to evaporation of water bound to the hydrogelnetwork, and a second peak at approximately 250 °Ccorresponding to decomposition of the CMC/chitosanpolymer.49−51 Importantly, Figure 2B demonstrates that theTGA curve recorded for the C-dot/hydrogel (red curve)exhibits an experimentally significant smaller weight changecompared to the bare hydrogel (black curve), indicatinggreater thermal stability of the C-dot/hydrogel composite. Thisthermal resilience is ascribed to the extensive C-dot-mediatedcross-linking within the composite gel.52

The rheology analysis in Figure 2C complements the TGAexperiments, furnishing additional evidence for enhancedmechanical stability endowed by embedding C-dots withinthe hydrogel. Specifically, Figure 2C depicts the frequency-dependent storage modulus (G′, reflecting gel elasticity) andloss modulus (G′′, gel viscosity) in the range of 0.1−100 rad/s.53 The higher G′ values in the entire frequency range (Figure2C) reflect the formation of a gel structure.26 Notably, Figure2C reveals that the mechanical strength of the C-dot/hydrogelincreased significantly compared to the bare hydrogel, as theG′ and G′′ values of the C-dot/hydrogel were higher than thecorresponding parameters recorded for the bare hydrogel. Thisresult is likely due to the more pronounced cross-linking withinthe C-dot/hydrogel framework, echoing the contribution ofcross-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 theequilibrium swelling (Q) of C-dot/hydrogel compositescomprising different C-dot weight ratios, upon addition ofdistilled water and saline water (10 wt %), respectively. Gelswelling analysis is important for assessing the capacity of thehydrogel to absorb and retain water.42,54 Figure 3A shows thatlower Q values were recorded in the case of saline water ascompared to pure water. This result is ascribed to theelectrostatic repulsion between mobile salt ions and chargedmoieties within the hydrogel framework, thereby decreasingosmotic pressure and concomitant gel swelling.55 Somewhatlower gel swelling was apparent upon increasing the C-dotconcentration within the hydrogels (Figure 3A), ascribed tocross-linking between the C-dots and the hydrogel network,

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 C-dot concentrations. Weight-percent of the C-dots is indicated.

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affecting reduced access of water molecules into the porousframework.56

The differential scanning calorimetry (DSC) data in Figure3B illuminate the heat absorption profile of the hydrogel-

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 watercaptured 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 uponsolar 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.

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embedded water. The enthalpy values in Figure 3B, calculatedfrom the raw DSC data (presented in Figure S5), correspondto the heat uptake required for evaporation of water moleculesfrom the saturated hydrogel.57 The bar diagram in Figure 3Breveals that significantly lower water evaporation enthalpieswere recorded upon increasing the concentration of the C-dotsin the C-dot/hydrogel composites. Specifically, while the barehydrogel exhibited water evaporation enthalpy of around 2050J/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, and20% C-dots, respectively. The lower enthalpies likely accountfor the incorporation and cross-linking of C-dots within thehydrogel, displacing water molecules that were physicallybound to the hydrogel framework. As a consequence, a higherpercentage of hydrogel-absorbed water molecules are unboundwithin the hydrogel pores (i.e., “free water”), accounting forthe lower evaporation enthalpies recorded (Figure 3B).58,59

This interpretation is consistent with the swelling data inFigure 3A, depicting lower gel swelling induced by increasingC-dot concentration.Light-Induced Water Purification Using the C-Dot/

Hydrogel. Figures 4, 5, and 6 demonstrate that efficient waterpurification was accomplished by C-dot-mediated light-induced 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 (thecorresponding transmittance and reflectance spectra are shownin Figure S6). Light absorbance was recorded between 300 nmand 2500 nm, encompassing sunlight irradiation spectrum,designed to examine the extent of solar energy absorbance bythe C-dot/hydrogel composites. Indeed, the absorbance datain Figure 4A reveal a direct correlation between the percentageof C-dots embedded within the hydrogel and extent of lightabsorbance. For example, the 20% C-dot/hydrogel sampledisplayed significantly higher light absorbance, in the entire

spectral range examined, as compared to the bare hydrogel(blue curve vs black curve in Figure 4A). Importantly, morepronounced light absorbance by the C-dot/hydrogels wasparticularly apparent in the infrared region, in which heatenergy is predominant. The increased absorbance can beattributed to C-dot self-assembly in the C-dot/gel matrix.60,61

C-dot-enhanced light absorbance, demonstrated in Figure4A, underscores the feasibility of solar-induced heating andsubsequent evaporation of water embedded within thehydrogel. To examine solar-mediated water heating in the C-dot/hydrogel composites, we applied solar-simulated illumina-tion at 1 kW/m2 intensity in different time durations andmeasured 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 thethermal camera (the actual temperature color keys are shownin Figure S8). The thermal images in Figure 4B demonstratethe dramatic impact of the hydrogel-embedded C-dots uponwater heating, complementing the absorbance profiles inFigure 4A. For example, after 60 min illumination using thesolar simulator, the temperature of the water-saturated 20%(w/w) C-dot/hydrogel reached 58 °C, while the barehydrogel, under the same illumination conditions, reached amuch lower temperature of just 28 °C (Figure 4B). Light-induced heating effects recorded in the thermal imagingexperiments are quantitatively shown in Figure 4C. Echoingthe thermal imaging data in Figure 4B, the curves depicted inFigure 4C underscore the contribution of embedded C-dots towater heating. Indeed, the graph in Figure 4C highlights thedirect relationship between C-dot concentration and highertemperature of hydrogel-absorbed water. Despite the similarenergy absorbance of the 10%-C-dot/gel and 20%-C-dot/gelcomposites as shown in Figure 4A, water heating andevaporation depend both upon water content, as well as

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 watercontaining rhodamine-6-G (R6G), before and after illumination. In all cases, C-dot/hydrogels comprising 20 wt % C-dots were illuminated with100 W white light emitting diode (LED).

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state of water inside the gel.62 Indeed, the water swellinganalysis (Figure 3A) and DSC data (Figure 3B) indicate thatthe 20%-C-dot/gel composite contained a lower amount ofwater but a higher concentration of “free water” as comparedto 10%-C-dot/gel (which embedded greater amount of boundwater). Thus, the energy absorbed by 20%-C-dot/gel is utilizedmore efficiently for water heating as compared to the 10%-C-dot/gel composite. Overall, the data presented in Figure 4confirm that water heating in the C-dot/hydrogel compositesis due to light absorbance by the C-dots. The solar energyabsorbed by the C-dots likely dissipates by nonradiativeprocesses only, giving rise to the observed temperatureincrease.61,63

Figures 5−6 demonstrate practical applications of the C-dot/hydrogel composite for water purification. In the experi-ments summarized in Figure 5, C-dot/hydrogels were swelledin saline water (10 wt %), and recovery of purified water wasmeasured following illumination in a solar simulator (1 kW/m2

intensity) for 60 min. As depicted in Figure 5A, while thecalculated water recovery (e.g., percentage of water recoveredin relation to water initially absorbed within the gel) was lessthan 15% in the case of the bare hydrogel, recovery reached55% following solar illumination of the 20%-C-dot/hydrogel.The results outlined in the bar diagram in Figure 5A areconsistent with both the temperature-increase data (Figure 4)and evaporation enthalpies determined in the DSC experi-ments (Figure 3B), confirming the significance of embeddedC-dots for effective water recovery. Supporting this inter-pretation, lower water purification efficiencies were observedusing a hydrogel matrix embedding C-dots synthesized from adifferent carbon precursor (m-phenylenediamine, MPDA),which exhibited lower absorbance in the wavelength range ofsolar 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), uponirradiation with solar illumination of 1 kW/m2. Similar to datapresented in Figure 4, the water evaporation rate was directlylinked to the C-dot concentration, and highest for the 20%-C-dot/gel composite (determined as 1.4 kg/m2 h). We employedthe water evaporation rates depicted in Figure 5B to calculatethe efficiency of solar-to-vapor conversion (η) in the differentC-dot/gel composites (eq 3 indicated in the ExperimentalSection). 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-vaporconversion efficiency of this biocompatible and cost-effective20%-C-dot/gel composite is comparable to energy efficiency ofphotothermal absorbers reported previously.13,18,19

The effectiveness of water purification through application ofthe C-dot/hydrogel platform was further verified by determi-nation of water salinity using inductively coupled plasmaoptical emission spectrometry (ICP-OES, Figure 5C). Notably,salinity of the recovered water (55% of absorbed water wasrecovered) was lower than 1%, which is the drinking waterstandard defined by the World Health Organization.64 We alsoassessed the reusage of the C-dot/hydrogel composite throughapplication of consecutive swelling/evaporation cycles (Figure5C). (For reusage of the C-dot/hydrogel composite after 1 hirradiation, the gel was dried in an oven to remove residualwater). Indeed, the graph in Figure 5C demonstrates thatwater recovery efficiency was above 85% (in relation to theinitial 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 toaccumulation of salt in within the hydrogel pores, adverselyaffecting water uptake and gel swelling.Figure 6 presents utilization of the C-dot/hydrogel system

for water remediation applications, specifically removal ofheavy metal ions (Figure 6A), detergents (Figure 6B), andorganic dyes (Figure 6C). Figure 6A depicts the decontami-nation efficiency for Cu2+, Ni2+, Ag+, and Cd2+ respectively,which are among the most ubiquitous metal ion pollutants indrinking water.65 In the experiments highlighted in Figure 6A,the concentration of metal ions in water captured within a C-dot-20/gel prior to illumination at 1 kW/m2 intensity for 1 hwas 0.01 M. Remarkably, less than 0.05% of the initial ionconcentrations were present in the recovered water (afterillumination).Figure 6B presents surface tension analysis of water

containing 0.05 M sodium dodecyl sulfate (SDS) before andafter illumination of the C-dot/hydrogel swelled in thecontaminated water. Notably, while surface tension prior toremediation via the C-dot/hydrogel treatment was around 37mN/m (accounting for the SDS surfactant film at the watersurface), surface tension increased after illumination toapproximately 72 mN/m, close to surface tension for puredrinkable water.66 We also evaluated remediation of watercontaining organic pollutants (Figure 6C). Figure 6C showsthe UV−vis absorbance spectra of water containing rhodamine6G (R6G) (0.5 μM), which is among the most commonorganic pollutants in industrial wastewater. As is apparent inFigure 6C, while R6G in untreated water yielded a significantUV−vis peak at around 530 nm (blue spectrum), thisabsorbance peak almost completely disappeared after illumi-nation (red spectrum), indicating negligible concentration ofR6G in the recovered water. Together, the experimental datain Figure 6 underscore the remarkable and versatileapplicability of the C-dot/hydrogel platform for waterremediation.

■ CONCLUSIONS

This study presents construction of C-dot/CMC/chitosanhydrogels and their usage for water remediation. Thecomposite material combines the high porosity of the hydrogelframework which allows significant uptake of water moleculesand light absorbance by the C-dots in a broad spectra range,particularly encompassing the solar illumination spectrum. TheC-dots constitute the core photothermal mediator in thesystem, converting solar irradiation to dissipated heat whichefficiently raises the temperature of the hydrogel-absorbedwater, resulting in evaporation and subsequent recondensationof purified water. Importantly, we also show that theembedded C-dots contributed to enhanced stability andmechanical strength of the hydrogel matrix, likely throughcross-linking between surface moieties of the C-dots and thehydrogel framework. Overall, we demonstrate that the C-dot/hydrogel facilitated effective purification of saline water, watercontaminated with heavy metal ions, detergents, or organicdyes. The C-dot/hydrogel composite exhibits importantpractical advantages: recyclability, ease of preparation, andthe use of inexpensive, biodegradable, and readily availablebuilding blocks. The technology may be implemented indiverse applications such as low-cost water desalinationplatforms and portable water purification kits.

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DOI: 10.1021/acssuschemeng.9b02342ACS Sustainable Chem. Eng. 2019, 7, 13186−13194

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■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acssusche-meng.9b02342.

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

■ AUTHOR INFORMATIONCorresponding Author*E-mail: razj@bgu.ac.il. Fax: (+) 972-8-6472943.ORCIDSeema Singh: 0000-0001-8132-5453Raz Jelinek: 0000-0002-0336-1384NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSWe are thankful to Prof. Iris Visoly-Fisher and K. M. Anoop fortheir assistance with the solar simulator, and Dr. ArkadiZilberman for help with thermal imaging. S.S. is grateful to theMarcus Fund of Ben Gurion University for a postdoctoralfellowship.

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