Recent Advances in Composites of Graphene and Layered DoubleHydroxides for Water Remediation: A Review

Hongwei Pang,[a, b] Yihan Wu,[a] Xiangxue Wang,[c, d] Baowei Hu,*[b] and Xiangke Wang*[a]

Chem. Asian J. 2019, 14, 2542 – 2552 T 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim2542

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Abstract: Composites of layered double hydroxides (LDHs)

and graphene (G) are exciting nanomaterials because oftheir unique surface structures and excellent physicochemi-

cal properties. Such materials offer the advantages of bothcomponents, that is, the large surface area and ample func-tional groups of graphene and the outstanding layeredstructure and ion-exchangeability of layered double hydrox-

ides, whilst effectively avoiding the coagulation of grapheneand the instability of pristine layered double hydroxides, andthey have been widely investigated for applications in water

remediation. This Minireview begins by summarizing the

most common methods for the synthesis of G@LDH compo-sites, including hydrothermal treatment, coprecipitation, and

in situ growth. Then, we review the adsorption and catalyticability of G@LDH materials in the removal of contaminantsfrom water, such as heavy metal ions, radionuclides, dyes,and other organic pollutants. Finally, we discuss the chal-

lenges and offer a perspective on the directions of future re-search of G@LDH composites.

1. Introduction

Typically, layered double hydroxides (LDHs) adopt a hydrotal-cite-like anionic lamellar structure, which is assembled through

noncovalent bonding interactions between the cationic hostlayers and the anionic guest interlayers.[1–3] LDH materials can

be expressed according to the general formula[M2++

1@pM3++p(OH)2]p++[(Aq@)p/q]p@·y H2O, in which M2++ and M3++ rep-

resent bi-/trivalent metal cations (e.g. , Mg2++, Ca2++, Ni2++, Al3++,

and Fe3++), Aq@ represents nonframework (in)organic q-valent in-terlayer anions (e.g. , NO3

@ , ClO4@ , and SO4

2@), p denotes the

molar ratio of M2++/(M2++++M3++), and y denotes the molaramount of intercalated water.[4–7] Different elemental composi-

tions and metal ratios can greatly influence the morphologyand crystal form of these composite materials, which directly

impact the basal spacing and interlayer region in the LDH, as

well as indirectly impacting their applications. Owing to theirparticular structural merits, such as large interlayer spacing,

broad chemical composition, and ion-exchange ability, LDHmaterials have been extensively investigated as catalysts or ad-

sorbents in the field of water remediation.[8–12] Indeed, over the

last decade, numerous efforts have been devoted to the modi-

fication of LDH materials to achieve better performance, withgreat advancements having been made.[13, 14] For example, sur-

face decoration, calcination, intercalation, and multiple compo-sition methods have all been employed to modify LDH materi-

als.[15–18] Yao et al. adopted a calcination method to improvethe adsorption capacity of glycerol-modified MgAl-LDH to-

wards methyl orange (MO),[19] whilst Yu et al. applied an LDH

material that had been intercalated with terephthalic acid andpyromellitic acid to decontaminate aniline from wastewater.[12]

However, the particle size, instability, lack of surface functionalgroups, and crystal morphology of pristine LDH still limit its

elimination efficiency, large-scale production, and applications.As a representative 2D sp2-hybridized carbon material, gra-

phene (G) has been extensively investigated in the fields of

(ad)sorption, photo-/electrocatalysis, ion exchange, and precip-itation since its discovery by Geim and Novoselov.[20] Graphene

oxide (GO) is a chemically modified product of graphene thatpossesses ample oxygen functional groups on both its basal

plane and at its periphery. G, GO, and their derivatives have ex-hibited excellent physicochemical properties and extraordinary

performance for water decontamination.[21–24] For instance, He

et al. fabricated a hydrogel from reduced GO (rGO) and appliedit to UVI decontamination.[25] A sorption amount of

134.2 mg g@1 UVI on rGO hydrogel was achieved at pH 4.0 andit retained 94.8 % of its adsorption ability after 10 sorption/de-

sorption cycles. Wu et al. used rhamnolipid-functionalized GO(RL-GO) for the elimination of methylene blue (MB) and ach-

ieved the removal of 529.1 mg g@1 MB at 298 K.[26] Although

the development of graphene-based materials has led to rela-tively low-cost graphene products, unmodified graphene ma-

terials remain limited in their practical applications, owing totheir aggregation and coagulation. Therefore, the design of

hybrid materials that combine graphene and other functionalnanomaterials is highly significant for multifunctional practical

applications.

Recently, G@LDH composites have been widely investigatedfor water remediation. Graphene provides ample functional

groups and can effectively promote the surface activity ofLDH, whilst LDH can effectively reduce the coagulation of gra-

phene, thereby affording these G@LDH materials great poten-tial for water environmental cleanup.[27, 28] Many G@LDH materi-

[a] H. Pang, Y. Wu, Prof. X. WangMOE Key Laboratory of Resources and Environmental Systems OptimizationCollege of Environmental Science and EngineeringNorth China Electric Power UniversityBeijing 102206 (P. R. China)E-mail : xkwang@ncepu.edu.cn

[b] H. Pang, Prof. B. HuSchool of Life SciencesShaoxing UniversityHuancheng West Road 508Shaoxing 312000 (P. R. China)E-mail : hbw@usx.edu.cn

[c] Dr. X. WangHebei Key Lab of Power Plant Flue Gas Multi-Pollutants ControlDepartment of Environmental Science and EngineeringNorth China Electric Power UniversityBaoding 071003 (P. R. China)

[d] Dr. X. WangGuangdong Provincial Key Laboratory of Environmental Pollution andHealthSchool of EnvironmentJinan UniversityGuangzhou 510632 (P. R. China)

The ORCID identification number(s) for the author(s) of this article can befound under :https ://doi.org/10.1002/asia.201900493.

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als have been synthesized for the elimination of heavy metalions, radionuclides, dyes, and organic pollutants from aqueous

solutions.[29–34] However, there are an insufficient number of re-views of the preparation methods and applications of G@LDH

composites in environmental remediation.This Minireview presents a brief summary of the existing

methods for the synthesis of G@LDH materials, including hy-drothermal synthesis, in situ growth, and coprecipitation, and

analyzes the strengths and weaknesses of these different

routes. Then, the application of G@LDH materials in the remov-al of serial aqueous contaminants (heavy metal ions, radionu-

clides, organic pollutants, and dyes) is discussed. Furthermore,the interaction mechanisms and elimination performance of

various G@LDH materials are briefly summarized and com-pared to assist in their practical application. Finally, the per-spectives and challenges facing G@LDH materials are discussed

for future investigation.

2. Synthetic Methods of G@LDH Materials

Typically, the formation of G@LDH materials involves a two-step synthesis. Firstly, graphene materials are prepared by

using a (modified) Hummer’s method and then dissolved inMilli-Q water. The suspension is dispersed by using ultrasound

prior to the next step. Commonly, metal cations are well-dis-persed within the LDH interlayer. Thus, G@LDH materials can

be formed from the reaction between metal ions and an alkali

source by using a range of different synthetic methods, includ-ing hydrothermal synthesis, coprecipitation, and in situ growth.

2.1. Hydrothermal Method

Hydrothermal treatment, also known as the “urea hydrolysismethod”, is a facile and environmentally friendly method forthe synthesis of G@LDH composites.[35–37] Compounds with uni-form morphology and excellent crystallinity can be obtained

by using the hydrothermal method.[38–40] Typically, a mixed so-

lution is used that contains precursor salts (nitrates, chlorides,sulfates, or hydroxides), urea, and graphene. After homogene-

ous mixing, the as-prepared suspension is transferred into aTeflon-lined stainless-steel autoclave and heated at the desired

temperature for an extended period of time. Huang et al. fabri-cated magnetic GO/MgAl-LDH (MGL) nanocomposites by using

this urea hydrothermal method and the formation processes is

shown in Figure 1 A.[41] They used MGL for the adsorptive re-moval of 192.3, 23.0, and 45.1 mg g@1 of PbII, CdII, and CuII, re-

spectively. Recently, hexamethylenetetramine (HMT) has been

Hongwei Pang received his B.Sc. from the Ag-ricultural University of Hebei (P. R. China) in2016, and is currently undertaking his gradu-ate project under supervision of Prof. XiangkeWang. His research interests include the syn-thesis of nanocomposites and the adsorptiveand reductive elimination of uranium.

Yihan Wu obtained her B.S. degree fromNorth China Electric Power University (P. R.China) in 2017 and is currently pursuing herM.S. at the same institution under the supervi-sion of Prof. Xiangke Wang. Her main inter-ests are in the design and synthesis of stablemetal–organic frameworks (MOFs) and theirapplications in environmental pollution treat-ment.

Xiangxue Wang received his Ph.D. in 2017from the University of Science and Technologyof China (P. R. China). He is currently an Assis-tant Professor at North China Electric PowerUniversity (P. R. China). His interests includethe synthesis of graphene-based nanomateri-als and their application in pollution manage-ment. He has published over 20 papers inpeer-reviewed journals.

Baowei Hu received his Ph.D. from the Xi’anUniversity of Technology (P. R. China) in 2010is currently a Professor at Shaoxing University(P. R. China). His research focuses on the ap-plication of nanostructured materials andnanotechnology for environment pollutiontreatment. He has published more than 50papers in peer-reviewed journals.

Xiangke Wang is a Professor at North ChinaElectric Power University (P. R. China), havingbeen a Research Fellow at the SUBATECH Lab-oratory (France) and the Karlsruhe ResearchCenter (Germany). His research focuses on thesynthesis of nanomaterials and their applica-tions in energy and environmental pollutionmanagement. He has published over300 papers in peer-reviewed journals, whichhave been cited more than 26 000 times, andhe currently has a H-index of 99. He has alsobeen listed as a highly cited researcher in thefield of Environmental and Engineering byThomson Reuters and Clarivate Analyticsfrom 2014–2018.

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used instead of urea as the alkali source for selected applica-tions, because HMT is more alkaline at high temperatures,

owing to the formation of ammonia.[42, 43] Wen et al. reported asynthesis of MgAl-LDHs and GO nanocomposites (LDHs/GO) by

using the hydrothermal method and their application in the ef-ficient removal of AsV from water solutions.[44] A homogeneous

solution of magnesium chloride, aluminum chloride, HMT, andGO was sealed in a 50 mL Teflon autoclave and the mixture

was heated at 140 8C for 12 h. The maximum removal capacityof AsV was 183.1 mg g@1, which was higher than that of thepristine LDH (129.7 mg g@1), owing to the introduction of GO.SEM and TEM images illustrated the stacking and aggregationof pristine LDH (Figure 1 B, C). After the introduction of GO, the

LDH/GO composite exhibited better dispersibility than theoriginal material and wrinkled GO was successfully attached

onto the surface of hexagonal platelet LDH (Figure 1 D, E).

2.2. Coprecipitation

Coprecipitation is another common method for the synthesisof G@LDH materials. Coprecipitation involves the condensation

of metal ions onto graphene materials and the formation of

brucite-like layered LDH under alkaline conditions by using acoprecipitation process.[45, 46] Typically, a graphene material is

first prepared and then dissolved in deionized water. Then, thissolution is added to a solution of the precursor salt in a certain

ratio (M2++/M3++). After ultrasonication, the mixed solution andthe alkaline solution are slowly added into a reactor. The simul-taneous addition of two solutions at a controlled pH value,

which is typically adjusted by using NaOH and Na2CO3, leadsto the coprecipitation of G@LDH. Although the crystallite sizes

are nonuniform, the coprecipitation method is used extensive-ly, owing to its greater simplicity and convenience compared

with other methods. Ruan et al. reported the synthesis of acomposite of reduced graphene oxide and a nickel chromium

LDH (rGO/NiCr-CO3-LDH) as a new sorbent for the efficient

elimination of methyl orange (MO).[47] The synthesis of rGO/NiCr-CO3-LDH is shown in Scheme 1 A. Briefly, GO was synthe-

sized by using a modified Hummer’s method and was mixedwith an aqueous solution of [Ni(NO3)2]·6 H2O and

Figure 1. A) Synthesis of the MGL composites.[41] B, C) SEM images of theLDHs (B) and LDHs/GO (C). D, E) TEM images of the LDHs (D) and LDHs/GO(E).[44]

Scheme 1. A) Synthesis of rGO/NiCr-CO3-LDHs by using coprecipitation.[47] B, C) In situ growth of the rGO/LDH composite (B)[51] and 3D sandwich-type compo-site rGO/LDH (C).[52]

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[Cr(NO3)3]·9 H2O (Ni2++/Cr3++ = 2:1). 1.0 m NaOH was used toadjust the pH value. Then, the mixed solution was added drop-

wise into a solution of NaCO3 under mechanical stirring at60 8C and the mixture was aged at 65 8C for 16 h. rGO/NiCr-

CO3-LDH exhibited a maximum elimination of MO of312.5 mg g@1 by using the Langmuir model.

2.3. In Situ Growth

In situ growth is a commonly employed method for the syn-thesis of G@LDH materials with specific structures.[48, 49] Typical-ly, the boehmite AlOOH is used as a precursor solution by hy-drolyzing aluminum isopropoxide. AlOOH gel can decrease the

surface roughness and improve the adhesive properties ofLDH.[31, 50] Then, the AlOOH gel and an as-prepared graphenematerial are mixed to synthesize G@AlOOH composites by

using a layer-by-layer technique. Next, a hydrothermal processis used to form the G@LDH material. For example, Tan et al.

modified LDH by using the in situ growth method to synthe-size a graphene/3D sandwich-structured LDH hybrid material

(rGO/LDH) for the capture of UVI from water.[51] Scheme 1 B

shows the formation process of the rGO/LDH composite. First,the as-obtained GO was mixed with an AlOOH sol under vigo-

rous agitation for 12 h. After centrifugation, the GO/AlOOHpowder was dissolved in Milli-Q water that contained urea and

[Ni(NO3)2] . The mixed solution was transferred into a Teflon-au-toclave and heated at 120 8C for 12 h. The BET specific area

and pore volume of LDH was greatly enhanced from85.0 m2 g@1 and 0.25 cm3 g@1 to 256.8 m2 g@1 and 0.66 cm3 g@1

after modification, respectively, which indicated that the intro-

duction of GO impeded the aggregation of pure LDH. Analo-gously, as shown in Scheme 1 C, Xu et al.[52] employed the

in situ growth method to fabricate a composite material ofrGO and a sandwich-type 3D LDH nanosheet array (rGO/LDH).

2.4. Other Methods

Several other methods have been developed for the fabrica-tion of G@LDH composites, such as exfoliation/restacking,

layer-by-layer assembly, and solvothermal and microwavemethods. However, these routes typically require harsh condi-tions, complex processes, or expensive chemicals, which havelimited their wider application.

The exfoliation/restacking method was investigated for the

synthesis of G@LDH materials because of the electrostatic at-traction between the LDH and G components.[53–55] The pres-

ence of surface functional groups make GO negativelycharged, whilst LDH is positively charged with replaceable

anionic interlayer structures.[21, 56] Huang et al. fabricated com-posites of rGO and CoAl-LDH nanosheets (rGO/CAN-LDH-NS)

by using the exfoliation/restacking method (Scheme 2 A).[56]

The layer-by-layer (LBL) method is analogous to the exfolia-tion/restacking method for the formation of G@LDH compo-

sites.[57, 58] Polymeric solutions, such as polyethyleneimine (PEI)and polyvinyl alcohol (PVA), were employed to provide a cat-

ionic surface and multiple cycles were required to completethe synthesis process.[59, 60] Scheme 2 B shows the LBL modifica-

tion process for the synthesis of (LDH/PVA/GO/PVA)n hybrid

composites.[59]

Solvothermal and microwave techniques are improvedmethods for the hydrothermal synthesis. In the solvothermal

reaction, deionized water is replaced by nonaqueous solventsto dissolve the precursor salts.[39, 61] Furthermore, microwave ir-

radiation, as a substitute for an autoclave, can effectively short-en the synthesis time of G@LDH materials compared with tra-ditional methods.[62, 63] Lonkar et al. synthesized a G/ZnAl-LDH

composite by using the microwave method (Scheme 2 C).[62]

Briefly, urea, GO, zinc nitrate, and aluminum nitrate (molar ratio

of Zn/Al = 3:1) were mixed together, transferred into a micro-wavable flask, and gradually heated to 150 8C under microwave

irradiation for 2 h.In summary, the hydrothermal synthesis requires high tem-

peratures and the precursor salts are sealed with G and ureaor HMT in a Teflon autoclave to achieve homogeneous G@LDHproducts. Coprecipitation demands the simultaneous additionof the as-prepared solution and an alkaline solution at the de-sired pH value to form the G@LDH composites. In situ growth

requires the introduction of AlOOH to improve the surfaceproperties of LDH, whilst the LBL method is also used to form

G@LDH materials. Previously reported G@LDH composites,along with their synthesis conditions and versatile applications,are listed in Table 1. However, inevitably, there are disadvantag-

es to each method, owing to the different fabrication condi-tions. Thus, the exploration of new routes with a focus on low

cost, mild/moderate conditions, and high efficiency is necessa-ry in the future.

Scheme 2. A) Synthesis of CAN-LDH-NS/rGO by using an exfoliation/restack-ing method.[56] B) Synthesis of (LDH/PVA/GO/PVA)n hybrid composites byusing a LBL modification process.[59] C) Formation of rGO/ZnAl-LDH by usinga microwave technique.[62]

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3. Applications of G@LDH Materials in WaterRemediation

The excellent adsorptive and catalytic properties of G@LDH

materials make them promising scavengers for the treatmentof water pollution, such as the elimination of heavy metal ions,

radionuclides, dyes, and organic contaminants. A comparisonof the elimination performance of G@LDH materials and other

materials is listed in Table 2.

3.1. Removal of Heavy Metal Ions

The presence of heavy metal contaminants in aqueous solu-

tions, such as CrVI, AsIII, and PbII, is highly problematic, owingto their high toxicity and nonbiodegradability.[6, 64] Once heavy

metal ions enter the natural environment, they can be readilystored, quickly accumulate, and transfer between organ-

isms.[65, 66] Owing to their large surface area, ample functionalgroups, and outstanding physicochemical properties, G@LDHmaterials have been studied for the removal of heavy metal

Table 1. Synthesis and applications of G@LDH composites.

Material Fabrication method Precursorsalt

Alkalisource

Synthesis conditions Application Sorption performance Reference

Treatment T[8C]

t[h]

[mg g@1] [%] T[K]

pH

MGL hydrothermal treatment nitrate NaOH heating 120 13 PbII, CuII, and CdII removal 192.3,45.1,23.0

298 [41]

LDH/GO hydrothermal treatment chloride HMT heating 140 12 UVI removal 129.9 298 4.5 [42]MGL hydrothermal treatment chloride HMT heating 140 12 AsV removal 73.1 298 4.0 [43]LDH/GO hydrothermal treatment chloride HMT heating 140 12 AsV removal 183.1 298 5.0 [44]rGO/NiCr-LDH

coprecipitation nitrate NaOH aging 65 16 methyl orange removal 312.5 306 [47]

rGO/LDH in situ growth nitrate urea heating[b] 120 12 UVI removal 277.8 298 4.0 [51]CG/MgAl-LDH

hydrothermal treatment nitrate urea heating 120 24 CrVI removal 172.6[c] [67]

MgAl-LDH/rGO

coprecipitation nitrate NaOH stirring 80 2 PbII removal 116.2 298 4.5 [68]

(rGO)/Ni/MMO

hydrothermal treatment chloride urea heating 190 48 methyl orange removal 210.8 298 4.5 [69]

GO@LDH hydrothermal treatment nitrate urea heating 110 8 UVI removal 159.9 298 5.0 [70]rGO/NiFe-CLDH

hydrothermal treatment nitrate NaOH heating 60 48 methylene blue degradation 93[d] 7.0 [71]

LDH/G coprecipitation nitrate NaOH stirring 95 24 rhodamine B degradation 93[d] [72]MGL mechanohydrothermal

treatmenthydroxide Mg(OH)2

Al(OH)3

heating 100 24 2,4-dichlorophenoxyacetic acidremoval

189.9[c] [73]

[a] Simplified for the purpose of comparison; [b] mixed with AlOOH prior to heating; [c] 1.0 g L@1 of the composite material ; [d] under visible light condi-tions. MMO = mixed metal oxide.

Table 2. Comparison between the removal performances of G@LDH composites and other materials.

Pollutant Material Removal mechanism Sorption performance Reference[mg g@1] [%] T [K] pH

PbII magnetite GO/LDH adsorption 192.3 298 [41]PbII MnFe2O4 adsorption 69.0 298 6.0 [79]CrVI calcined G/MgAl-LDH adsorption 172.6[a] [67]CrVI carbonaceous nanofibers adsorption 122.7 303 5.0 [80]UVI LDH/GO adsorption 129.9 298 4.5 [42]UVI FA@PEI adsorption 70.3 298 5.0 [65]SrII MgAl-LDH/GO (5 %) adsorption 213.4 298 6.0 [81]SrII magnetic PANI/GO adsorption 37.2 298 3.0 [82]methyl orange rGO/NiCr-LDH adsorption 312.5 306 [47]methyl orange sodium montmorillonite adsorption 24.0 303 [83]rhodamine B LDH/G photocatalysis 93[b] [72]rhodamine B MWCNT/WO3 photocatalysis 92[b] 5.5 [84]2,4-dichlorophenoxyacetic acid MGL adsorption 189.9[a] [73]2,4-dichlorophenoxyacetic acid organopalygorskite adsorption 179.7 298 [85]

[a] 1.0 g L@1 of the composite material ; [b] under visible light conditions. FA = fly ash, PANI = polyaniline, MWCNT = multiwalled carbon nanotubes.

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ions from aqueous solutions in recent years. For ex-ample, Yuan et al. prepared calcined G/MgAl-LDH

and applied it to the uptake of CrVI from aqueous en-vironments.[67] The high adsorption capacity of G/

MgAl-LDH (172.6 mg g@1), as determined by using aLangmuir model, was higher than those of calcined

MgAl-LDH (128.0 mg g@1) and G (21.6 mg g@1) alone,which demonstrated that the introduction of G in-duced a synergistic contribution between the com-

ponents. The mechanism for CrVI removal by G/MgAl-LDH is shown in Figure 2 A. Chemical adsorp-tion controlled the removal process and the syner-gistic reactions between the components enhanced

the CrVI-adsorption capacity. Wu et al. modified theG/MgAl-LDH composite (MGL) with Fe3O4 nanoparti-

cles and applied it to the elimination of AsV ions

from aqueous solutions.[43] Water-dispersible MGLwas prepared by using a traditional hydrothermal

synthesis with HMT as an alkali source. The elimina-tion of AsV with MGL was well-fitted to a Langmuir

model and the maximum removal was 73.1 mg g@1.MGL composites showed enhanced uptake ability

compared with pure LDH (37.7 mg g@1). The surface

area was greatly enhanced after the introduction ofGO and magnetic particles, owing to the presence of more

active sites for AsV elimination. Moreover, the magnetic deriva-tive of G@LDH could be readily separated from the aqueous

phase by using a permanent magnet (Figure 2 B). The out-standing performance made MGL a promising adsorbent for

real-world wastewater management.

3.2. Removal of Radionuclides

The wide utilization of nuclear chemistry inevitably leads tothe release of radionuclides into the environment. However,

once UVI, EuIII, SrII, ThIII, and other radionuclides have enteredthe environment, their toxicity, radioactivity, carcinogenesis,

and mutagenesis cause irreversible damage.[70, 74, 75] The lowcost, facile synthesis processes, and high removal capacities of

G@LDH materials give them great potential as sorbents for the

decontamination of radionuclides in practical applications. Yuet al. synthesized nanocomposites of GO and NiAl-LDH

(GO@LDH) by using the urea hydrothermal method.[70] The UVI-elimination behavior of GO@LDH was investigated under vari-

ous conditions. The maximum removal was 159.7 mg g@1,which was higher than those of GO (92.0 mg g@1) and pure

NiAl-LDH (69.0 mg g@1) alone. The effect of pH value and ion

strength on UVI elimination by GO@LDH are shown in Fig-ure 3 A, B. The pH-dependent and ion-strength-independent

uptake of UVI confirmed that the major interaction mechanismwas inner-sphere surface complexation.[76, 77] Sorption isotherms

of GO@LDH indicated that the removal process was dominated

by monolayer coverage. Linghu et al. employed surface com-plexation modeling (SCM) to study the removal of UVI by LDH

(Figure 3 C) and LDH/GO (Figure 3 D).[42] Their results demon-strated that the sorption of UVI onto LDH and LDH/GO involved

cation exchange at pH<4.0 and inner-sphere surface complex-ation at pH>5.0. According to the SCM analysis, UVI elimina-

tion on LDH/GO was well-fitted by a diffuse layer model (DLM),

with two inner-sphere surface complexes (SOUO2++ and

SOUO2(CO3)23@ species) and an ion-exchange complex (X2UO2).

The removal capacities of LDH and LDH/GO at pH 4.5 were99.0 and 129.9 mg g@1, respectively, and the reaction equilibri-um between UVI and the adsorbents was achieved in 6 h.

3.3. Uptake of Dyes

The presence of dye pollutants in water, as a result of dyeing,

the paper-making industry, and other effluent discharge, hasled to significant environmental issues. In addition to the visi-

ble consequences, the poor biodegradability of dye pollutants,owing to their complicated structures and xenobiotic proper-

ties, can result in biotoxicity and carcinogenicity.[69, 78] Typically,

adsorption and photocatalysis are employed for the elimina-tion of organic dyes from aqueous solution, because of the

high anion-exchange ability, flexible interlayer region, abun-dant functional groups, and unique structure of the G@LDH

composites. Zhao et al. reported that rGO-modified NiFe-calci-nated LDH (rGO/NiFe-CLDH) could be used for the photocata-

Figure 2. A) Mechanism for the removal of CrVI on calcined G/MgAl-LDH.[67]

B) Photograph of the magnetic separation of MGL.[43]

Figure 3. A, B) Effect of pH value (A) and ion strength (B) on the adsorption of UVI byGO@LDH.[70] C, D) SCM of UVI on LDH (C) and LDH/GO (D) at different pH values.[42]

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lytic decontamination of methylene blue (MB), a common in-

dustrial dye. The modified composites had excellent photoca-

talytic activity (93.0 %) and reusability (80 % removal percent-age after four cycles). The removal of MB by rGO/NiFe-CLDH

was mainly attributed to rGO and NiO, which were the key cat-alytically active substances. Under visible-light irradiation, the

electron donor (NiO) was excited, along with the generation ofphotoinduced electron–hole pairs. The electrons in the con-

duction band (CB) of NiO rapidly moved to rGO, which led to

the effective separation of electrons and holes. As a result, theproduction of photoinduced reactive species (COH and CO2@) in-

duced the degradation of MB (Scheme 3 A). Lan et al. reportedthe synthesis of a hybrid catalyst of a ZnCr-LDH and G (LDH/G)

by using a facile coprecipitation method and its application tothe degradation of rhodamine B (RhB).[72] The degradation per-

centage of RhB on LDH/G was 93 % and the RhB molecules

partly degraded into CO2 and H2O, which was ascribed to effi-cient photogenerated charge separation. Furthermore, even

after five cycles, about 88 % RhB photodegraded, thus indicat-ing the outstanding reusability and stability of LDH/G. The

mechanism for the photocatalytic elimination of RhB overLDH/G is shown in Scheme 3 B. The active COH radicals led to

the oxidation of organic pollutants, which were formed from

the reaction between photogenerated holes that remained inthe Cr 3d(t2g) orbital and OH@ in solution. The CO2

@ radicals

were formed from the elimination of excited electrons by ad-sorbed oxygen passed by G and then converted into reactiveCOH radicals, which participated in the degradation of RhB. The

p–p-stacking interactions between G and RhB accelerated the

removal process of photocatalytic degradation.

3.4. Decontamination of Other Organic Pollutants

Organic contaminants in water systems have received consid-

erable attention for many years. Hazardous organic matter iswidely used in industry and agriculture, such as pesticides,

pharmaceuticals, polychlorinated biphenyls (PCBs), and poly-

cyclic aromatic hydrocarbons (PAHs).[86, 87] Many organic pollu-tants in aqueous solution can have adverse effects on aquatic

organisms, human beings, and ecosystems, even at low con-centrations. Owing to the excellent surface properties and

unique structures of G and LDH, G@LDH materials and theirderivatives could be suitable candidates for organic decon-

tamination through adsorption and photocatalytic degrada-

tion.[47, 65] Zhu et al. synthesized binary ZnFe-LDH/rGO (CLDH/rGO) by using a hydrothermal method and modified it by calci-

nation at 450 8C under a nitrogen atmosphere.[88] CLDH/rGOwas studied for the elimination of paracetamol with different

rGO content (CLDH/rGO10, CLDH/rGO30, CLDH/rGO50, andCLDH/rGO90). Figure 4 A, B shows the photocatalytic degrada-

tion and adsorption performance of paracetamol by serial

CLDH/rGO composites. The photocatalysis process effectivelyincreased the removal of paracetamol compared with the ad-

sorption process. The introduction of rGO enhanced the ad-sorption process, whilst CLDH/rGO30 exhibited the highest

synergistic effect between CLDH and rGO. Zhang et al. modi-

Scheme 3. A) Mechanism for the removal of MB on rGO/NiFe-CLDH.[71] B) Mechanism for the photocatalytic elimination of RhB over LDH/G.[72] VB = valenceband, e@= electron, h++ = hole.

Figure 4. A) Photocatalytic degradation and B) adsorption of paracetamol by serial LDH composites.[88]

Chem. Asian J. 2019, 14, 2542 – 2552 www.chemasianj.org T 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim2549

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fied Mg/Al-LDH with Fe3O4 and GO by using a mechanohydro-thermal method for the decontamination of 2,4-dichlorophe-

noxyacetic acid (2,4-D).[73] The molar ratio of Mg/Al and Fe3O4

was controlled to be 3:1 and 4 wt %. Several materials were

fabricated with GO content of 1.10, 4.13, and 7.92 wt %, whichwere named MGL1, MGL2, and MGL3, respectively. The sorption

amount of 2,4-D on MGL3 (material loading: 1.0 g L@1) was189.9 mg g@1. The elimination mechanism involved the interca-lation of 2,4-D anions into LDH through an ion-exchange pro-

cess, whilst hydrophobic interactions and p–p interactions do-minated the interaction between GO and 2,4-D.

4. Conclusion and Perspectives

In this Minireview, the synthesis and applications of G@LDHmaterials for water remediation have been briefly introduced.

The introduction of graphene effectively mitigates the limita-tions of the pristine layered double hydroxides, and the excel-

lent physicochemical properties of the G@LDH compositesmake them potential scavengers for aqueous contaminants.

Traditional assembly routes of G@LDH materials, such as hy-

drothermal treatment, coprecipitation, and in situ growth, havebeen briefly illustrated and summarized. The applications of

G@LDH materials for the removal of heavy metal ions, radionu-clides, dyes, and other organic pollutants have been briefly in-

troduced. The excellent adsorptive and photocatalytic per-formance of (in)organic pollutants over G@LDH composites

has shown the great potential of these materials for the reme-

diation of aqueous solutions.The assembly and application of G@LDH materials for water

remediation is a new and emerging field that is worthy of in-tensive investigation. Individually, both G and LDH are exciting

as outstanding 2D structural materials. However, the construc-tion of monolayer G and LDH polymers that can adequately

achieve the same effect as G@LDH materials has not yet been

studied. To date, the study of G@LDH composites has re-mained at the molecular level, and their reaction mechanisms

with contaminants are not yet clear enough. The limitations ofG@LDH materials in terms of their purity, fine structure, and

electroresponsiveness still restrict their bulk production andpractical engineering applications. Innovative methods for the

synthesis of new G@LDH materials are still in demand in thisfield. In addition, water-decontamination experiments for

single-pollutant systems are mostly undertaken under labora-

tory conditions, whilst applications under natural conditionsthat involve multiple components have scarcely been investi-

gated. Thus, practical modification methods and versatile em-ployment require further investigation. Although the future in-

vestigation of G@LDH materials, not limited to water purifica-tion, still faces great challenges, the study of more-versatile

G@LDH composites would open up new research directions

and applications of these materials.For the decontamination of inorganic pollutants, such as

heavy metal ions and radionuclides, their adsorption and solid-ification by G@LDH is more efficient. Thus, surface modification

with more functional groups and higher specific surface areasis an efficient way of enhancing the sorption ability of G@LDH

composites. On the other hand, the elimination of organic pol-lutants typically involves adsorption and photocatalytic degra-

dation. For the adsorption of organic pollutants, surface com-plexation and p–p and hydrophobic interactions between the

pollutants and the G@LDH materials are typically the main in-teractions, whereas, for the photodegradation of these organic

compounds, the unique 2D layered structure and p-conjugat-ed system of G@LDH accelerate the transfer and separation of

photogenerated charge carriers, which is beneficial for the ef-

fective generation of reactive species (e.g. , COH and CO2@).These oxygen-containing active species and holes mainly takepart in photocatalytic reactions, and dominate the decontami-nation of organic pollutants. Besides the above-mentionedproperties, the stability and reusability of G@LDH compositesare also important for their applications in environmental pol-

lution management. Another challenge facing the application

of G@LDH materials in the decontamination of environmentalpollutants is their cost and we hope that the development of

new technologies should allow such G@LDH composites to besynthesized in large scale at lower cost. The advantages of gra-

phene and LDH will allow the application of G@LDH compo-sites for the treatment of real-world wastewater that contains

different types of organic and inorganic pollutants through dif-

ferent mechanisms, such as surface precipitation, adsorption,and photocatalytic degradation for a range of contaminants.

Acknowledgements

This work was supported by the National Key Research andDevelopment Program of China (2017YFA0207002), the Nation-al Natural Science Foundation of China (21876048 and21836001), the Guangdong Provincial Key Laboratory of Envi-ronmental Pollution and Health (GZKLEPH201815), and the

Fundamental Research Funds for the Central Universities(2018MS114 and 2018ZD11).

Conflict of interest

The authors declare no conflict of interest.

Keywords: adsorption · graphene · layered doublehydroxides · organic–inorganic hybrid composites · waterchemistry

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Manuscript received: April 10, 2019

Revised manuscript received: May 27, 2019

Version of record online: June 26, 2019

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