Environmental Applications of Three-Dimensional Graphene-Based ...

9 downloads 120877 Views 7MB Size Report
Dec 4, 2014 - amazing breakthroughs in research on graphene have provided .... be utilized to adsorb air pollution, including greenhouse effect gases, namely ...... (E) Illustration of the proposed reaction mechanism for hydrogen production.
Critical Review pubs.acs.org/est

Environmental Applications of Three-Dimensional Graphene-Based Macrostructures: Adsorption, Transformation, and Detection Yi Shen,†,§ Qile Fang,†,‡ and Baoliang Chen*,†,§ †

Department of Environmental Science, Zhejiang University, Hangzhou 310058, China Ningbo Institute of Materials Technology and Engineering, Chinese Academy of Sciences, Ningbo 315201, China § Zhejiang Provincial Key Laboratory of Organic Pollution Process and Control, Hangzhou 310058, China ‡

S Supporting Information *

ABSTRACT: Just as graphene triggered a new gold rush, three-dimensional graphene-based macrostructures (3D GBM) have been recognized as one of the most promising strategies for bottom-up nanotechnology and become one of the most active research fields during the last four years. In general, the basic structural features of 3D GBM, including its large surface area, which enhances the opportunity to contact pollutants, and its well-defined porous structure, which facilitates the diffusion of pollutant molecules into the 3D structure, enable 3D GBM to be an ideal material for pollutant management due to its excellent capabilities and easy recyclability. This review aims to describe the environmental applications and mechanisms of 3D GBM and provide perspective. Thus, the excellent performance of 3D GBM in environmental pollutant adsorption, transformation and detection are reviewed. Based on the structures and properties of 3D GBM, the removal mechanisms for dyes, oils, organic solvents, heavy metals, and gas pollutants are highlighted. We attempt to establish “structure−property−application” relationships for environmental pollution management using 3D GBM. Approaches involving tunable synthesis and decoration to regulate the micro-, meso-, and macro-structure and the active sites are also reviewed. The high selectivity, fast rate, convenient management, device applications and recycling utilization of 3D GBM are also emphasized.

1. INTRODUCTION Graphene (G), an allotrope of carbon, has triggered a new “gold rush” since its discovery by Novoselov and Geim.1 As the first two-dimensional (2D) atomic crystal available to us, graphene possesses many superior physical and chemical properties, such as high electronic and thermal conductivity,2,3 great mechanical strength,4 huge theoretical surface area5 and ready chemical functionalization,6,7 which together justify its nickname “miracle material”8 and make it highly attractive for numerous applications. The boom in nanotechnology and amazing breakthroughs in research on graphene have provided great promise for its wide application in the fields of electronics, photonics, composite materials, energy generation and storage, sensors and metrology, bioapplications, and the environment.8−10 Graphene nanomaterials and their derivatives have exhibited excellent performance in environmental pollutant removal, including that of heavy metals (Pb2+, Hg2+, Cd2+, Co2+, As (III, V), and Cr(VI)),11−17 anionic and cationic dyes,18−21 organic pollutants (phenolic compounds, pesticides, and polycyclic aromatic hydrocarbons (PAHs)),22−26 inorganic anionic pollutants,27,28 and gas pollutants.29,30 The broad applicability of graphene31,32 and its high efficiency in the control of various contaminants are mainly due to its large surface area, abundance of surface functional groups and fast electron © 2014 American Chemical Society

transfer, which make graphene a wonderful adsorbent and catalytic medium.9 Due to rapid increases in production and applications, the release of graphene into the environment poses a potential health risk. Recently, the adsorption, dispersion, toxicity and transformation of graphene in the aquatic environment has been well reviewed.31 Because of its unique 2D honeycomb lattice, graphene can act as a basic building block for graphitic materials of all other dimensionalities by being wrapped up into zero-dimensional (0D) fullerenes, rolled into one-dimensional (1D) nanotubes or stacked into 3D graphite.33 Assembly of graphene into 3D hierarchical architectures (Figure 1) has been recognized as one of the most promising strategies for “bottom-up” nanotechnology and become one of the most active research fields during the last four years.34 With their tremendous ascendency, 3D assembled macrostructures possess new collective physiochemical properties, which differ remarkably with respect to both the individual building block and the bulk material, and this further extends their application capabilities.35 It is worth noting that the greater part of the research dealing with 3D Received: Revised: Accepted: Published: 67

September 9, 2014 November 22, 2014 December 4, 2014 December 4, 2014 dx.doi.org/10.1021/es504421y | Environ. Sci. Technol. 2015, 49, 67−84

Environmental Science & Technology

Critical Review

Figure 1. (A) Proposed mechanism of GO self-assembly into 3D-GBM hydrogel.43 (B) A novel monolithic porous carbon constructed by the hydrothermal self-assembly of GO sheets with poly(vinyl alcohol) (PVA) as the linker in the formation process of a three-dimensional (3D) structure of r-EPGM (reduced expanded porous graphene macroform). SEM (a−c) and TEM (d−f) images of r-EPGM. On the left is a schematic of the spheroidal and hierarchical pore structure model for r-EPGM.46

graphene surface.46,48 This modifiable surface makes 3D GBM easy to be decorated. Decoration approaches endow 3D GBM with new physiochemical properties and can be summarized into three types, namely doped graphene sheets within 3D GBM,52,53 coconstruction with other carbonaceous materials,38,54 and incorporation of metal (oxides/sulfides) nanoparticles into 3D GBM.55,56 The synthesis of 3D macroporous architectures and aerogels built of carbon nanotubes and/or graphene was reviewed just recently.34 Although there have been a few reviews about 3D graphene architectures34,57−60 and applications using graphene-based composites,9,61−63 we noted that a comprehensive overview of the application of 3D GBM for environmental pollution management is still absent. The rapid growth of this field assures us that 3D GBM will be a new generation of materials in pollutant disposal with outstanding capacities and easy regulation. Thus, the aim of this review was to systematically categorize the environmental applications of 3D GBM, mainly focusing on the removal mechanism and detection role of 3D GBM for different types of pollutants, as summarized in Figure 2. Generally, the environmental applications of 3D GBM can be grouped into three aspects, namely the adsorption, transformation, and detection of environmental pollutants. Furthermore, 3D GBM devices, such as a column loaded with 3D GBM, have been practically applied in miniature-scale purification and detection. This review summarizes the structure−property relationships of 3D GBM derived from

GBM sprang up after the year 2013. Thus, 3D GBM remains at its initial stage but shows fascinating prospects. An analysis of the preliminary studies about its environmental application during the short period of 3D GBM development will serve to demonstrate the excellent removal capabilities of this new class of materials.36−44 3D GBM combines micro-, meso-, and macropores in such a way that the micro- and mesoporosity (see Figure 1B) provides high surface area while the macroporosity guarantees accessibility to this surface.34 A primary aspect is that the 3D network prevents aggregation and guarantees mass transport, consequently enhancing performance, offsetting the main disadvantage of nanoparticles (NPs) in environmental application.45 In addition, another important advantage is the integrated morphology of the graphene-based monolith, which makes it convenient for manipulation and collection in use and prevents the release of graphene nanosheets and their environmental risk. A great deal of work has been done to investigate the assembly of 3D GBM. In general, the synthesis of 3D GBM mainly started from precursor of graphene oxide (GO), and can be divided into three categories, including self-assembly approaches (Figure 1A), 43,46−48 template-directed approaches,49−51 and other approaches,36 all of which result in 3D graphene architectures with different structures and properties. What needs to be mentioned, even though the process of assembly accompanies with reduction of GO, there are still numerous functional groups on the partial reduced 68

dx.doi.org/10.1021/es504421y | Environ. Sci. Technol. 2015, 49, 67−84

Environmental Science & Technology

Critical Review

(BR),39,67 and rose bengal sodium salt (RB).39,67,97 The neutral dyes included acridine orange (AO).65 This new generation of materials provides excellent performance that is superior or comparable to the traditional and novel materials previously reported. For example, a 3D graphene oxide (GO) plus biopolymer composite gel can adsorb as much as 1100 and 1350 mg·g−1 of MB and MV, respectively.99 Another GO/ polyethylenimine (PEI) 3D GBM showed 800 mg·g −1 adsorption for amaranth,69 and it possesses the highest adsorption capacity under the same conditions compared with other carbon materials, such as powdered peanut hull (14.90 mg·g−1),100 poly(ether sulfones)/poly(ethylenimine) nanofibrous membrane (454.44 mg·g−1),101 and mesoporous carbon (520 mg·g−1).102 The extensive specific surface area and the interconnected 3D porous network of 3D GBM are thought to account for it because they allow the dye molecules to diffuse easily into the adsorbent with abundant adsorption sites. A GO/DNA hydrogel was synthesized in the year 2010 by Xu et al.92 and was first used to examine its dye-loading ability for safranin O. This composite hydrogel exhibited a total dyeloading capacity of 960 mg·g−1 on the GO component of the obtained 3D GBM, which is comparable to those of many carbon nanomaterials (210−785 mg·g−1 for dyes on the ordered mesoporous carbons).103 The high adsorption ability of the hydrogel was partially ascribed to the strong electrostatic interactions between the positively charged safranin O and the negatively charged GO and DNA. Afterward, the contribution of electrostatic interactions to dye adsorption onto 3D GBM was further confirmed by many studies.34,38,69,98,99 It is wellknown that the negative charge on the 3D GBM surface mainly originates from the deprotonation of oxygen-containing groups such as hydroxyl, carboxyl and carbonyl on graphene oxide nanosheets.44,104,105 Therefore, the content of these groups will significantly determine the negative charge density on 3D GBM and subsequently affect its adsorption capacity for cationic dyes.37,40 So far, multiple cationic dyes such as MV, MG, MB, Rh B, and fuchsin have been successfully treated by various 3D GBMs with outstanding adsorption capacities.36,44,99 In addition, the pH value of the solutions is another important factor for the removal of most cationic dyes using 3D GBM. Usually, the adsorbed amount increases with increasing pH values.38,44,99 At low pH values, the relatively high H+ concentration would compete strongly with cationic dyes for adsorption sites. Meanwhile, the hydroxyl and carboxyl groups of the 3D GBM would be protonated to form −OH2+ groups, which would lead to electrostatic repulsion and restrict dye molecules from coming into close proximity with the adsorbent surface. When the pH value increases, the competition between H+ and other cations becomes less significant, and the carboxyl and hydroxyl groups would be deprotonated to form −COO− and −O− groups and thereby enhancing the electrostatic attraction between the adsorbent surface and cationic dyes. As mentioned above, the negatively charged 3D GBM is more favorable for the adsorption of cationic (or basic) dyes37,106 than anionic dyes,37 which is due to the existence of electrostatic repulsion between oxygen-containing groups and the acidic groups of adsorbed anionic dyes. However, anionic dyes are also a big class of dye pollutant common in industry,64 thus some modification approaches have been attempted to get 3D GBM to adsorb anionic dyes by connecting positively charged functional groups to its graphene nanosheet. Chitosan (CS) chains with abundant -NH2 have been linked with graphene by Chen et al.64 via 3D GBM self-assembly (Figure

Figure 2. Environmental applications and roles of 3D GBM, and corresponding mechanisms for different types of pollutants.37,40,44,46,64−91

different synthesis and decoration schemes, mainly in order to provide a theoretical basis and technical guidance for their application in the special design of 3D GBM used in the removal of diverse pollutants.

2. ADSORPTION PROPERTIES AND MECHANISMS OF 3D GBM In general, its unique structure with large surface area and continuous pores enables 3D GBM to be an ideal agent for the removal of pollutants from contaminated water and air with excellent sorption capability and recyclability.64,92 In view of aqueous environments, 3D GBM is highly stable in water,93 ranging from a variety of harsh conditions such as strong acidic, salty aqueous solutions to even organic solvents such as N,Ndimethylformamide (DMF) and cyclohexane (CH).65 According to the pollutant type, the adsorption performance of 3D GBM can be categorized into three aspects including heavy metals, organic dyes, and oils and organic solvents. Other micropollutants such as the antibiotics tetracycline,94 fluoride,95 and chlorophenols96 can also be adsorbed by 3D GBM with high capacity. All this progress opens up a new platform for the use of 3D GBM in water purification. Moreover, 3D GBM can be utilized to adsorb air pollution, including greenhouse effect gases, namely CO2, tailpipe gases such as NOx and SOx, and other toxic gases such as acetone and formaldehyde, which are widely used in housekeeping and will cause harm to human health. 2.1. Organic Dye Adsorption. From an environmental point of view, the treatment and disposal of organic dyes from dye manufacturing and textile finishing are of great concern. Most dyes are dissolved and present as either cationic or anionic ions, and few are dispersive. Despite its short development history, 3D GBM has been used to treat different types of dyes (see details in Table S-1 of the Supporting Information (SI)). The cationic dyes included methylene blue (MB),36−42,46,64,66−68,97−99 malachite green (MG),65,67,97 rhodamine B (RhB),37,43,44,66,68,69 methyl violet (MV),36,41,65,99 brilliant green (BG),65,67,97 and rhodamine 6G (R6G).39,67,97 The anionic dyes included eosin Y,64 acid fushine,37 calcein (CA),67,97 methyl orange (MO),39,41,67,97 bordeaux red 69

dx.doi.org/10.1021/es504421y | Environ. Sci. Technol. 2015, 49, 67−84

Environmental Science & Technology

Critical Review

Figure 3. (A) Photographs of a graphene oxide−chitosan (GO−CS10) hydrogel. (B) SEM image of lyophilized GO−CS10 hydrogel. Removal of MB (C) and eosin Y (D) dyes from water by filtration. (C and D) Images of the apparatus used for filtration and the solution before and after filtration. The inserts (C and D) are the absorption spectra of the dye solution before and after filtration.64

3A and B). This was the first report that a GO-based adsorbent has high adsorption capacity (>300 mg·g−1) toward both cationic (MB) and anionic dyes (eosin Y) (Figure 3C and D). The MB adsorption capacity is much higher than that of CS beads (99 mg·g−1)107 and magnetic chitosan-GO composite (95.16 mg·g−1),108 and it is the first time that the GO-based material shows large adsorption capacity toward anionic dyes, significantly broadens the practical applications of GO-based adsorbents. An amine-rich 3D porous material generated via the incorporation of GO sheets and PEI was reported by Sui et al.69 This coconstructed material shows an excellent adsorption capacity for amaranth anionic dye of 800 mg·g−1, which is superior to other carbon materials, and it was thought that the strong affinity of protonated amine groups toward the sulfonated groups of the dye provides the driving force for the adsorption of anionic dye. In addition to electrostatic interactions, π−π stacking is considered another main interaction force for the adsorption of dyes onto 3D GBM.39,97 It is worth mentioning that in the early years of 2012, the π−π stacking was not recognized distinctly, so researchers usually put forward the existence of van der Waals interactions.44,66,109 Taking into account the π−π interactions, better adsorption capacity of the 3D GBM will be achieved for those dyes with benzene-containing structure enriched molecules such as MG, BR, BG, and CA.67,99 That may be one of the main reasons that the adsorption capacities for MV, MB and 4-nitrophenol (4-NP) (180−380 mg·g−1) are obviously higher than those for Congo red (CR) and MO (80− 150 mg·g−1) under the same conditions (1 mM initial concentration).41 The π−π stacking allows an organic dye with either neutral43,65 or negative37 charge to be adsorbed effectively onto 3D GBM. Of course, it is better to take both electrostatic interactions and π−π stacking into account to fully exploit the capacity for organic dye removal of 3D GBM. In the year 2012, Liu et al.36 first proposed that cationic dye adsorption onto a 3D GO sponge is through the two main driving forces of strong π−π stacking and anion−cation interactions. Then, the synergistic effect of these two forces was further confirmed and commendably used to realize high adsorption of dyes onto 3D GBM.37,40,46,64−69 Thus, the adsorption amounts of MG65 and MB40 reached as high as 242 and 833 mg·g−1, respectively. The 3D GBM synthesis method affects the quantity of the

functional groups on its surface and thus its adsorption mechanisms. 3D GBM synthesized via self-assembly beginning with GO nanosheets usually possesses more oxygen-containing functional groups,43,47 which are appropriate for dye removal via electrostatic interactions and π−π stacking, even those with partial reduction.47 Correspondingly, 3D GBM formed using the method of template-directed chemical vapor deposition (CVD) possesses most of the physicochemical properties of pristine graphene with little or no functional groups,49−51,110 and it can act as a stable adsorbent for dyes mainly through π−π stacking. The remarkable advantages of 3D GBM are its large surface area and 3D porous network structure, with which 2D graphene cannot compete. Its adsorption capacity is greatly improved by its high specific surface area, which increases the opportunity for dye molecules to contact the macrostructure. This unique porous structure and interconnected network determines the diffusion process of dyes into 3D GBM.92,109 The specific relationship between the dye diffusion process and the porous structure of 3D GBM has also been investigated. The adsorption process can be described as a successive process of dye diffusion through the boundary layer, intraparticle diffusion, and dye adsorption onto the graphene surface, namely the adsorbate molecules move slowly from larger pores to micropores.36,99 The high adsorption rate of 3D GBM for dyes is because its average pore diameter is large and available enough to facilitate the diffusion of dye.44,69 In addition to pore size, pore volume also influences diffusion. It is well-known that the aggregated GO nanosheets in a compact composite strongly limit the diffusion of adsorbate molecules.64 Thus, the porous structure should not be too compact or dense, and it can be properly tuned by adjusting the concentrations and weight ratio of the graphene and the cross-linking agent during the process of 3D GBM synthesis.99 In addition, other factors such as reducing the size of the hydrogel and agitating the solution can accelerate the diffusion of the adsorbate molecules.64 In addition, a column loaded with 3D GBM has been practically applied to dye adsorption in miniature-scale water purification (Figure 3C and D),64,65,111,112 and it is an attractive technique allowing nonstop operation.64 The gel-column can remove organic dyes just through filtration by means of gravity, then a nearly colorless filtrate was collected, and the 70

dx.doi.org/10.1021/es504421y | Environ. Sci. Technol. 2015, 49, 67−84

Environmental Science & Technology

Critical Review

Figure 4. (A) Snapshots of the process of removing n-decane (dyed with oil red O) floating on water using a graphene aerogel. (B) The relationship between Qwt (the ratio of the weight after full absorption to the weight of the aerogel) and the density (ρ) of the organic liquid absorbed by the aerogel (ρ = 6.0 mg·cm−3). (C) Snapshots of burning off an oil slick using aerogel as a wick.75 (D) Schematic for the separation of an oil−water mixture using RGO foam as a filter (a, b).70

absorption efficiency of pump oil to a 3D reduced graphene/ poly(vinyl alcohol) composite structure is 17 times greater than that of activated carbon (AC).46 Cong et al. first reported how macroscopic multifunctional graphene-based hydrogels can be used to absorb various oils and organic solvents with high efficiency,116 including cyclohexane, toluene, gasoline, paraffin oil, vegetable oil, and phenoxin, and its uptake capacity is approximately 12−27 times its weight. Superhydrophobic π−π stacking and capillary effects have been proposed as the main absorption mechanisms to explain this high performance. Subsequently, the hydrophobic π−π stacking interaction has been demonstrated by numerous studies as the dominant mechanism underlying the high capacity of 3D GBM for the absorption of oils and organic solvents.44,46,70−76 On this basis, the physicochemical properties of both the absorbent (namely 3D GBM) and the absorbates (oils and organic solvents) will elicit different effects on absorption capacity. With respect to 3D GBM, the hydrophobic degree of the planar graphene nanosheets and the amount of polar groups on their surface are critical for its absorption capacity for oils and organic solvents. In order to fully utilize the hydrophobic interactions and improve the absorption capacity for oil and organic solvents, some modification methods have been proposed. It is known that the hydrophobic surface of 3D GBM can be further modified via increasing the extent of its nanoscale roughness, which relates to the hydrophobic π−π stacking of 3D GBM.39,74,116,117 For example, the anchoring of a linking agent is utilized so that the dense distribution of polyvinylidene fluoride (PVDF) NPs on the surface of graphene sheets makes the graphene aerogel rougher at the nanoscale, which imparts superhydrophobicity to the obtained 3D GBM.71 Regulating the functional groups needs further investigation. Up to now, studies have confirmed that oxygencontaining groups of GO foams result in an affinity for oil that is weaker than that of reduced GO (RGO) foams.46,70 Although GO foams are hydrophilic, they still have higher absorption capacity than most other oil absorbents, which can be

characteristic adsorption peaks of dyes also disappeared in the UV−vis spectra (Figure 3C and D), indicating complete removal of the dye.64,65 In the entire filtration process, the adsorption capacity increased linearly with the permeation volume, indicating that the column was stable and its affinity for dye did not decrease during the adsorption process.64,111 Because of the in situ synthesis and application of 3D GBM, its structural stability is particularly important, and usually a linking agent is required to strengthen the 3D structure. After its application for dye adsorption, some simple regeneration methods are used for the consumed 3D GBM, including solvent washing (such as ethylene glycol,68 ethanol,44 and acidic ethanol113) and vacuum filtration.36 The adsorbed dye can be readily released from the 3D structure for recycling removal of dye without destroying the porous structure or adsorption capability of the 3D GBM.38,68 For example, the efficiency of dye removal was 100% for MB and >80% for RhB after three cycles.68 Obviously, 3D GBM has demonstrated its high capability as an organic dye scavenger offering high efficiency and recyclability, and hopefully it might be applied in contaminated water treatment. 2.2. Oil and Organic Solvent Absorption. In recent years, oil spills and organic solvent leakage from industry have caused catastrophic effects on marine and aquatic ecosystems. Therefore, the collection and removal of oil and organic pollutants from water has attracted worldwide attention. Traditionally, absorption treatment is considered to be a convenient and environmentally friendly method. It has been found that 3D GBM can absorb various oils (SI Table S-2) such as gasoline,70 olive oil,39,46,70 pump oil,39,46,70−73,97,114,115 and crude oil, 72,74 and organic solvents such as chloroform,66,70−73,113−115 nitrobenzene,73,114,115 tetrahydrofuran (THF),70,71,73,114 and DMF.70,97,113 Its high absorption capacity surpasses most of the materials that have been reported. For example, a graphene sponge can absorb significantly greater levels of toluene (54.7 times its own weight) than previously reported absorptive materials, for example, vaseline-loaded expanded graphite (10 times) by heat treatment, yielding the full release of absorbates (>99%),114 and a graphene/α-FeOOH aerogel maintains high absorption capacity (92% of the first cycle) after eight gasoline-absorbing and drying cycles.116 Thus, 3D GBM has a strong potential to become a versatile, efficient, and safe absorbent for oil/water separation, oil spill cleanup and organic solvent removal, and it should be scalable to industrial levels. 2.3. Heavy Metal Adsorption. Serious worldwide water pollution is caused by heavy metal ions, which are highly toxic pollutants in water resources. Recent reports have shown that 3D GBM has excellent adsorption ability for aqueous heavy metals and thus can be used as a promising adsorbent for water purification. Various heavy metals have been successfully treated by 3D GBM (SI Table S-3), including Cu 2 + , 3 7 , 6 4 , 7 7 , 7 8 , 9 3 , 1 1 9 − 1 2 1 Pb 2 + , 4 4 , 5 4 , 6 4 , 7 8 , 1 1 6 , 1 1 9 − 1 2 3 Cd2+,44,78,119−122 and Cr (VI).43,116,124 Its adsorption capacity is generally either comparable or superior to that of other ordinary materials. For instance, the calculated maximum adsorption capacity of graphene-based hydrogel is 139.2 mg·g−1 for Cr(VI) and 373.8 mg·g−1 for Pb2+,116 which are much higher than that of mesoporous γ-Fe2O3 (15.6 mg·g−1)125 and AC (69 mg·g−1)126 for Cr(VI), and that of exfoliated graphene sheets (40 mg·g−1)12 and chrysanthemum like (103.0 mg·g−1) and commercial bulk R-FeOOH (1.0 mg·g−1)127 for Pb(II). Graphene-carbon nanotube (CNT) aerogel shows excellent capability for aqueous Pb2+ removal with an adsorption amount of 230−451 mg·g−1,54 which is comparable to or higher than those of most carbon-based adsorbents reported in the literature such as CNTs (1.66−49.95 mg·g−1).128 From the perspective of macroscopic structure, the interconnected pore structure is beneficial for the diffusion process of metal ions,54,93,121 which usually follows an intraparticle diffusion model, including two steps of external diffusion and intraparticle diffusion.77,93 Similarly to dye adsorption, in the beginning, the removal rate of metal is much higher due to the large surface area of 3D GBM where the active sites are more readily available for metal ions.77 Then, because of interionic attraction and molecular association, the material’s surface forms a thick layer of hydration ions and the binding sites become exhausted, so the uptake rate is controlled by the transportation of adsorbate from the exterior to interior sites on the adsorbent. However, intraparticle diffusion is not the only rate-controlling step, certain other mechanisms such as complexes and/or ion-exchange may also control the adsorption rate. However, 3D GBM exhibits slightly lower adsorption rates compared with a graphene suspension. For

attributed to the hydrophobic planar graphene structure of GO nanosheets and the high porosity of the 3D network.46 Nevertheless, nitrogen-doping with aqueous ammonia can improve the hydrophilicity of 3D GBM,52 which may eliminate the high-efficiency absorption of oils and organic solvents, so the right selection of functional groups is important. The structure and properties of oils and organic solvents are also important attributes in the process of absorption. In view of structure, aromatic solvents have higher affinity for 3D GBM than aliphatic solvents,44,74,76 and this is attributed to the stronger hydrophobic π−π interactions that lead to higher absorption Morganic solvents/M3D GBM for toluene (322 g·g−1) and nitrobenzene (307 g·g−1) than for dodecane (288 g·g−1) or chloroform (270 g·g−1).115 The volume-based absorption capacity is not affected by variations in the density of the solvent. In addition to the benzene ring, other properties of organic absorbates also influence absorption,70,113 such as density, volatility and viscosity. Because a 3D GBM has a given porosity (or void volume), an increase in oil density means that a larger amount of the oil can be stored in the void space (Figure 4B).70,75,97,113,115 For volatile gasoline and diesel oil, the absorption values are lower because they are easily vaporized.70 High viscosity of the oil or organic solvent slows down the diffusion into the 3D structure.115 Due to its strongly hydrophobic π−π stacking interactions, 3D GBM displays a key characteristic of highly selective absorption of oil and organic solvents. Oil, even a high-density organic solvent like dichloromethane, lying below the water’s surface can be selectively absorbed by 3D GBM without water uptake.70,75,113,116 However, despite the high hydrophobicity of graphene nanosheets, the high cavity content and appropriate pore size of 3D GBM causes a certain amount of bulk water absorption, which deviates from the initial aim and renders oil absorption pointless. To avoid this unwanted phenomenon, Bi et al. applied a soot method, strongly adhering amorphous carbon NPs onto the interior region of a graphene sponge to completely repel water molecules without changing the overall microstructure.73 Furthermore, they also found that this modification made the surface of the graphene sponge selfcleaning, with water droplets slipping off completely when the surface is inclined at several degrees. Another remarkable characteristic of 3D GBM is its high absorption rate (Figure 4A). Once 3D GBM is placed on the surface of an oil−water mixture, the oil layer immediately starts shrinking and completely disappears after a few minutes.39,97,115,118 The average absorption rate for toluene and n-decane can reach as high as 68.8 g·(g·s)−172 and 27 g·(g· s)−1,75 respectively. The extremely rapid action of 3D GBM toward organic solvents is mainly due to the large surface66 contacting with the organic molecules and the abundant pore structures41,70,74,116 adopting and affluxing the organic molecules. In addition, 3D GBM can be used as an innovative filter device to separate oil from oil−water mixtures (Figure 4D).70,76 When an oil−water mixture is poured onto the top of 3D GBM, the oil is absorbed into the filter until the 3D GBM reaches its absorption saturation capacity, and then the excess oil flows out from the bottom of the 3D GBM and into the vessel. Of more importance, the water does not flow through the 3D GBM, instead it just accumulates to form a layer on top of the 3D GBM and is preserved.70 During the separation process, no external force is needed. 3D GBM with absorbed oil or organic solvent can be easily regenerated and recycled by 72

dx.doi.org/10.1021/es504421y | Environ. Sci. Technol. 2015, 49, 67−84

Environmental Science & Technology

Critical Review

example, the k2 values of a pseudo-second order model of Pb(II) adsorption by graphene hydrogel versus suspension are 0.21 × 10−3 and 0.41 × 10−3 g·(mg·min)−1, respectively, and this may be due to the limited diffusion of heavy metal ions into the pores of the monolithic material.44,93 Except for the diffusion process resulting from bulk effect, the interfacial reaction between 3D GBM and heavy metals is similar to that of graphene-based nanosheets. In most cases, the adsorption of heavy metals onto 3D GBM is well fitted to the Langmuir isotherm model with high R2 (0.9998−0.9999), which can be explained by the reason that the active sites are homogeneously distributed on the surface of 3D GBM,44,77,116 and the adsorption of metal ions takes place at the functional groups and surface binding sites to form a monolayer.44 However, Chen et al. reported an interesting phenomenon that the adsorption of Cu2+ and Pb2+ onto a graphene oxide-chitosan (GO−CS) hydrogel fits better with the Freundlich function than the Langmuir function.64 This deviation from the Langmuir function may result from the uneven surface of the GO−CS hydrogel with different types of adsorption sites. Most studies about Cu2+, Pb2+, and Cd2+ adsorption onto 3D GBM present experimental data consistent with a pseudo-secondorder kinetic model, which indicates that the adsorption of heavy metals onto 3D GBM is controlled by chemical adsorption involving covalent forces through sharing or exchange of electrons between adsorbent and adsorbate.44,77 The adsorption mechanisms of heavy metals onto 3D GBM are an increasing concern. In early 2012, ion exchange was recognized as the determinant mechanism in the adsorption of heavy metals onto 3D GBM.77 Subsequently, electrostatic adsorption instead of ion exchange was proposed as the main adsorption mechanism.44,122 In the process of ion exchange, protonated H3O+ molecules in the adsorption site are replaced by heavy metal ions. In the process of electrostatic adsorption, oxygen functional groups such as −COO− and −O−, which are negatively charged, attract the heavy metal cations. In both processes, the adsorption capacity is improved with increasing solution pH.44,77 At low pH levels, the H2O molecules and the species of the surface functional groups on 3D GBM are easily protonated to produce a positively charged surface, and the adsorption capacity of heavy metal ions is low due to the competition from H3O+ for adsorption sites and electrostatic repulsion. With increasing pH values, the covered H3O+ ions are deprotonated and leave more sites available on the 3D GBM surface for heavy metal ions. Meanwhile, the negative charge on the 3D GBM surface increases because the oxygencontaining functional groups become deprotonated, thus enhancing the electrostatic attraction between 3D GBM and heavy metal ions, eventually improving its adsorption capacity for heavy metals dramatically. However, preventing the pH from becoming high enough to form metal hydroxides is also important.44,77 Thus, selecting an optimal pH range has a significant effect on heavy metal uptake by 3D GBM.116,121 This pH regulation effect can be used to realize the regeneration of exhausted 3D GBM. Gao et al. used 0.1 mol· L−1 HCl solution and achieved full desorption of heavy metals from the 3D structure.44 In addition to solution pH, other environmental factors such as coexisting cations and temperature also impact adsorption capacity.93 Coexisting cations compete for the adsorption sites and simultaneously inhibit the ionization of oxygen functional groups on the 3D GBM surface, thus limiting their adsorption capacity for heavy metal ions. Because the adsorption of heavy

metal ions onto 3D GBM is an endothermic process, higher temperature improves performance. Taking advantage of electrostatic interactions, 3D GBM has been employed in electrolytic electrode devices that can be artificially controlled and operated to remove various heavy metal ions.78,79 Its electrical adsorption capacities are high at 434 mg·g−1 for Cd2+, 882 mg·g−1 for Pb2+, 1683 mg·g−1 for Ni2+, and 3820 mg·g−1 for Cu2+, values much higher than those of other graphene based adsorbents.78 Interestingly, anionic chromate (Cr2O72−) can also be adsorbed to graphene oxide hydrogel with high capacity (140 mg·g−1 on the GO component of the obtained hydrogel),43 so there may be other adsorption mechanisms such as physisorption93 and hydrogen bonding interactions44 participating in the adsorption process of heavy metals onto 3D GBM. The graphene nanosheets and functional groups might be the main active sites for physisorption and hydrogen bonding interactions. Nevertheless, there are still some mechanisms of removal and detection that are obscure. For example, a lot of researches recognized that the high adsorption of heavy metals on graphene nanosheets is attributed to the oxygen-containing functional groups, which form strong surface complexes with heavy metals,129,130 while this kind of statement is still absent in the study of 3D GBM. What’s more, cation-π interactions,131 which are strong between cations and the π face of the aromatic structure, have been widely recognized in systems of heavy metal cations and graphene,12,132,133 but they still have not been reported in the adsorption of heavy metals onto 3D GBM, and this needs to be investigated in depth to get a reasonable explanation. Theoretically, the mechanisms of the adsorption at the 3D GBM-liquid interfaces should be similar to those at the graphene-based nanosheets-liquid interfaces, so the researchers could refer to the study of graphene-based nanosheets on the road of 3D GBM. Actually, both 3D G and 3D GO structures possess large surface area and negatively charged oxygen-containing groups (such as hydroxyl and carboxyl groups), which makes them favorable for binding with positively charged molecules and ions. Thus, it is reasonable that they have high adsorption capacity for cationic dyes and heavy metal ions even without any other additives.37,121 In some cases, the addition of a crosslinking agent can further improve the adsorption performance to a higher level. For example, an increase of abundant functional groups such as catechol groups44 from dopamine serving as both reductant and surface functionalization agents in the synthesis of polydopamine-functionalized graphene hydrogel is expected to enhance the amount of active sites for heavy metal ions. The strong adsorption capacity of graphene/α-FeOOH hydrogel partially arises from synergistic effects of the electrostatic attraction, ion exchange and surface complexation. Surface complexion occurs between metal oxyhydroxides that develop from added Fe ions and the targeted heavy ion pollutants.116 In general, the addition of cross-linking to 3D GBM improves heavy metal adsorption. However, the consequence is not always in accordance. In some cases, increasing of chitosan induced a large decrease of adsorption capacity93 due to the more serious folding of GO sheets, which blocks more adsorption sites. Thus, the amount of additive modifying the 3D GBM needs to be well controlled even though the composition of 3D GBM has less influence on its adsorption capacity toward metal ions than dyes.64 2.4. Removal of Air Pollutants. Graphene and its derivatives have been regarded as attractive candidates for air 73

dx.doi.org/10.1021/es504421y | Environ. Sci. Technol. 2015, 49, 67−84

Environmental Science & Technology

Critical Review

Figure 5. Application of 3D graphene-based macrostructures in air pollution purification; the removal mechanisms include adsorption, preconcentration, separation, transformation, and mineralization. BTEX is an acronym that stands for benzene, toluene, ethylbenzene, and xylenes.

macropores do not really contribute to the observed adsorption capacity,82 and it is the micro- and mesopores present in the 3D network that act as efficient active sites for CO2 adsorption. Surface functional groups, such as the usual oxygencontaining groups and other decorated groups from additional components, are important in gas adsorption. On the one hand, the oxygen-containing functional groups of 3D GBM provide basic adsorption sites,69 and they also increase the polarity closest to the gas and form complexes with it.81,82 For example, GO (0.4 wt %) foams show the highest adsorption efficiency for acetone (100%), which is a kind of polar gas,81 followed by RGO foams (70%), CNT powder (60%), bamboo carbon (BC) and AC (