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Chemosphere 218 (2019) 845e859

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Chemosphere

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Review

Integrated adsorption and photocatalytic degradation of volatileorganic compounds (VOCs) using carbon-based nanocomposites: Acritical review

Weixin Zou a, b, c, Bin Gao c, **, Yong Sik Ok d, Lin Dong a, b, *

a School of the Environment, Nanjing University, Nanjing 210093, PR Chinab Jiangsu Key Laboratory of Vehicle Emissions Control, Center of Modern Analysis, Nanjing University, Nanjing 210093, PR Chinac Department of Agricultural and Biological Engineering, University of Florida, Gainesville, FL, 32611, USAd Division of Environmental Science and Ecological Engineering, Korea University, Seoul 02841, Republic of Korea

h i g h l i g h t s

� The advantages and disadvantages of carbon-supported hybrids were discussed.� Adsorption and photocatalytic models of VOC removal were reviewed.� The reaction processes and intermediates were proposed.� Major factors controlling adsorptive-photocatalytic reactions were discussed.

a r t i c l e i n f o

Article history:Received 6 September 2018Received in revised form6 November 2018Accepted 26 November 2018Available online 27 November 2018

Handling Editor: Jun Huang

Keywords:VOC abatementCarbon-based nanocompositeAdsorptionPhotocatalytic degradationModeling

* Corresponding author. School of the Environment210093, PR China. Tel.: þ86 25 83592290; fax: þ86 2** Corresponding author. Department of AgriculturaUniversity of Florida, Gainesville, FL, 32611, USA.

E-mail addresses: bg55@ufl.edu (B. Gao), donglin@

https://doi.org/10.1016/j.chemosphere.2018.11.1750045-6535/© 2018 Published by Elsevier Ltd.

a b s t r a c t

Volatile organic compounds (VOCs) are harmful for human and surrounding ecosystem, and a greatnumber of VOC abatement technologies have been developed during the past few decades. However, thesingle method has some problems such as high energy consumption, unfriendly environment, and lowremoval efficiency. Recently, the integration of adsorption and photocatalytic degradation of VOCs isconsidered as a promising one. Carbon material, with large surface area, high adsorption capacity, andfast electron transfer ability, is widely used in integrated adsorptive-photocatalytic removal of VOCs. It isthus crucial to digest and summarize recent research advances in carbon-based nanocomposites as theadsorbent-photocatalyst for VOC removal. To satisfy this need, this work provides a critical review of therelated literature with focuses on: (1) the advantages and disadvantages of various carbon-basednanocomposites for the applications of VOC adsorption and photocatalytic degradation; (2) modelsand mechanisms of adsorptive-photocatalytic removal of VOCs according to the material properties; and(3) major factors controlling adsorption-photocatalysis processes of VOCs. The review is aimed toestablish the “structure-property-application” relationships for the development of innovative carbon-supported nanocomposites and to promote future research on the integrated adsorptive and photo-catalytic removal of VOCs.

© 2018 Published by Elsevier Ltd.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8462. Carbon-based nanocomposites for VOC adsorption-photocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 846

2.1. ACFs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 847

, Nanjing University, Nanjing5 83317761.l and Biological Engineering,

nju.edu.cn (L. Dong).

W. Zou et al. / Chemosphere 218 (2019) 845e859846

2.2. CNTs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8472.3. Graphene and its derivatives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8472.4. AC . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8492.5. Biochar . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 849

3. Mechanisms and models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8503.1. Adsorption step . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8503.2. Photocatalysis step . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8523.3. Rate-determining step . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8533.4. Intermediate/regeneration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 853

4. Effects of environmental conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8544.1. Moisture content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8544.2. Temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8554.3. Light irradiation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8554.4. VOC properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 856

5. Conclusions and perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 856Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 857References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 857

1. Introduction

Volatile organic compounds (VOCs) such as alcohols, aldehydes,ketones, alkenes, and aromatic compounds, mainly coming fromexploitation, refining, storage, transport, and fossil fuels usage, areknown to have detrimental effects on ecological and environmentalfunctions (Iranpour et al., 2005; Qian et al., 2015). In general, theemitted VOC pollutants are not fixed in the originate medium,instead, they tend to move across and accumulate in differentenvironmental media including soil, water and air (Odum et al.,1997; Kesselmeier and Staudt, 1999; Ryu et al., 2003;Vandenbroucke et al., 2011). Highly-effective VOC eliminationtechniques for environmental remediation thus are of greatimportance and in urgent need.

Many VOCs abatement methods have been developed,including: 1) nondestructive (recovery) methods such as adsorp-tion, condensation, and membrane separation; 2) destructionmethods such as photocatalytic oxidation, incineration, ozonecatalytic oxidation, and biological degradation, etc. However, thesingle recovery or destruction remediation is not high-efficient, andthe hybrid treatments are found to be more attractive than indi-vidual techniques. Among them, the integration of adsorption andphotocatalytic degradation is one of the most promising technol-ogies for the abatement of low-concentration VOC, because of notonly its high removal efficiency, but also low energy consumptionand environmental friendliness (Pelizzetti et al., 1992; Yuana et al.,2011; Huang et al., 2016a; Huang et al., 2018c). In this integratedtechnology, the adsorption process can enrich VOC pollutants fromthe gaseous to solid phase to increase the performance of thephotocatalytic degradation; on the other hand, the photocatalysisprocess at room temperature and pressure oxidizes VOC pollutantsto CO2 and H2O, and thus regenerates the adsorbent for continuousVOC removal (Dibble and Raupp, 1992; Geng et al., 2010). All theseadvantages make adsorption-photocatalysis a suitable and prom-ising technology.

Generally, the photocatalysts including of metals, sulfides, ox-ides, phosphides, etc. are unstable and prone to agglomeration.With rich porous structure and large surface area, carbon is an ideamaterial to support the distribution and fixation of the nanosizedphotocatalysts (McEvoy and Zhang, 2016). Furthermore, the highlyadsorptive ability, rich porosity structure, black colour and the fastelectron transfer of carbon-based composites also keep the VOCmolecules in the proximity of the reaction active sites, enhancevisible light adsorption, promote the formation of radicals, prevent

the generation of by-products and inactivation of photocatalysts(Sunkara et al., 2010).

The overarching goal of this work is to provide a critical reviewof current research on the applications of carbon-based nano-composites on the integrated adsorptive and photocatalyticremoval of VOCs in the environment. Herein, it will first provide anoverview of carbon-based nanocomposites, according to theirvarious structures (such as specific surface area, pore size, surfacefunctional group, polarity, etc.), and the structurally-dependent-performance of VOC removal on carbon-based nanocompositesare proposed. Furthermore, the governing models and mechanismsof adsorptive-photocatalytic process are then discussed with anaim to explore the reaction process. After that, the effects of envi-ronmental conditions (such as moisture, temperature, and lightirradiation) as well as VOC characteristics (molecular polarity,molecule weight, and kinetic diameter) on the VOC removal aredescribed and analyzed to emphasize the importance of environ-mental complexity. Finally, prospects and challenges are alsosummarized and discussed at the end of this critical review.

2. Carbon-based nanocomposites for VOC adsorption-photocatalysis

At low concentration (ppb-ppm) under most of the environ-mental relevant conditions, conventional semiconductor photo-materials often have relatively terrible VOC adsorption ability.Moreover, in the absence of carbonmaterial, the generated reactionby-products are easily aggregated on the photocatalyst surface toreduce the catalytic efficiency (Tao et al., 2006a,b). Therefore,finding the optimal hybrid with high adsorption efficiencies andphotocatalytic abilities is crucial. When coupled with carbonaceousmaterials, multiple benefits are introduced: 1) providing adsorp-tion sites for VOC pollutants, 2) distributing and stabilizing pho-tocatalyst nanoparticles, 3) improving the photo-generated chargetransfer process via its unique electronic properties (Bradley, 2011).Generally, different carbonaceous materials have various structures(specific surface area, pore size) and surface chemical properties(function groups, hydrophilicity, hydrophobicity), which play animportant role in both of the photocatalyst combination andadsorption-photocatalytic VOC removal, and the correspondingrelationships are discussed as follows.

W. Zou et al. / Chemosphere 218 (2019) 845e859 847

2.1. ACFs

Activated carbon fibers (ACFs) prepared from fabric precursorsare a typical one-dimensional (1D) material, with large surface areaand more concentrated pore size distribution. The presence ofmicroporous in ACFs provides a peculiar adsorption force field andflows the VOC molecules towards the adsorption sites, leading tohigh adsorption potentials and capacity (Baur et al., 2015). How-ever, there are some shortcomings of ACFs in adsorption. On onehand, the microporous structure of ACFs is needed to optimize inorder to further improve the adsorption behavior, such as selec-tivity and rate. For example, the thermal treatment with differentcalcination temperatures was used to alter the pore structure ofACFs, and an increased of pore volume was observed at highertemperature treatment from the pore size distribution results(Fig. 1a), which was beneficial for larger molecule adsorption onACFs (Rong et al., 2003). On the other hand, the adsorption capacityfor polarizable VOC molecules is needed to improve (Li et al., 2017).ACFs is hydrophobicity because of few surface oxygen groups (hy-droxyl, carboxyl, carbonyl, etc.), and only nonpolar/weak polarVOCs (toluene, styrene, formaldehyde) could be removed by ACF-based nanocomposite (Table 1) (Shi et al., 2017; Tian et al., 2017;Liu et al., 2018). Therefore, Rong et al. (2002) used the oxidizationto alter the surface chemistry of ACF, and FT-IR result suggested thatmany polar groups were generated on ACF surface, mainly in theform of carboxylic acids (Fig. 1b), and the enhanced dipole inter-action and hydrogen bonds between ACF and formaldehyde led tobetter adsorption behavior. Liu et al. (2018) proposed that acidtreatment increased the surface hydroxyls and then more formal-dehyde absorbed on TiO2-ACF. Furthermore, the surface modifica-tion is able to enhance the binding between ACF and photocatalyst(Shi et al., 2012; Liu et al., 2018). Shi et al. (2012) used two coatingprocedures (hydrothermal treatment and dip-coating) to prepareTiO2/ACFs, and the generated OH� groups was beneficial for thebinding between TiO2 and ACFs by the hydrothermal treatment. Liuet al. (2018) employed acid treatment for TiO2-ACF and the CeTibond was formed at the interface, leading to the fast transfer of

Fig. 1. (a) Pore size distributions of the heat-treated and untreated ACFs (Rong et al.,2003). (b) FT-IR spectra of ACFs under oxidization at different temperatures andtimes (Rong et al., 2002). (c) Textural properties, UVevis adsorption spectra, time-series ratios of input to output concentrations of 2-ethyl-1-hexanol of thermallypost-treated and non-treated GO-TiO2 hybrids, and neat TiO2 (Chun and Jo, 2016). (d)Digital photographs, SEM and TEM results of TiO2/carbonaceous aerogel (Shi et al.,2016).

photo-generated electrons. On the basis of that, the larger pore sizeand more function groups on surface of ACFs are helpful for theintensive coupling interactions between carbon materials andphotocatalysts, and the synergistic effects enhance the reactionperformances of adsorption-photocatalysis.

2.2. CNTs

Carbon nanotubes (CNTs) in form of rolling up the graphenesheets into cylinders are another novel 1D carbon material.Different from other carbonmaterial, CNTs have the specific hollowstructure, which is beneficial for better stability of photocatalystnanoparticle and longer residence time of VOC molecule on ma-terial (Jo and Kang, 2015; Miao et al., 2018). Xu et al. (2010)observed that during the degradation of benzene, the TiO2 parti-cles maintained the nanosized dimension on the hollow structureCNTs composites. And in the adsorption-photocatalytic process ofstyrene removal, CNTs could prolong adsorption equilibrium,leading to the enough contact between styrene and CNT-TiO2nanocomposites, and then enhanced photocatalytic efficiency (Anet al., 2012). In addition, CNT has another important property,that is, it can either be metallic, semimetallic or semiconducting,depending on the helicity and diameter of the tube. The specialconductivity makes CNT as an efficient electron-transfer channel inthe adsorption-photocatalytic VOC removal (Li et al., 2011; Mirandaet al., 2014). In the presence of CNTs, photo-generated electronsmove freely towards CNTs surface with a lower Fermi level, andholes in valence band of photocatalyst are inclined tomigrate to thesurface and then oxidize the VOC pollutants. The VOCs removed byCNTs-nanocomposite are shown in Table 1 (Iijima, 1991; Yu et al.,2005, 2011; Gao et al., 2009; Xu et al., 2010; Zouzelka et al., 2016).

2.3. Graphene and its derivatives

With the two-dimensional layer of sp2 hybridized carbon atomsand one-atom thickness, graphene material has a great number ofadvantages than other carbons, such as high transmittance rate ofUVevisible lights, fast electrical and thermal conductivity, goodmechanical and tribological properties and corrosion resistance.And the delocalization of p network of the layers can efficientlysuppress the electron-hole recombination and the photogeneratedelectrons in graphene layers behave as massless Dirac fermions,leading to the enhanced photocatalytic performance (Geim andNovoselov, 2007; Allen et al., 2009; Low and Boonamnuayvitaya,2013; Chen et al., 2015). However, the single layer structure ofgraphene easily aggregates owing to strong van der Waals force, itszero bandgap makes graphene conductor not semiconductor, andthe hydrophobic graphene surface is difficult to deposit anddisperse metal oxides and adsorb VOC molecules (Jiang et al., 2011;Yu et al., 2014). On the basis of that, some treatment have beenapplied to modify graphene. Doping is an effective method, N-doped graphene could use as a substrate to disperse materials andenhance photocatalytic activity through fast charge transport(Wang et al., 2015a); Si-modified graphene provides the bondingbetween graphene and photocatalyst nanoparticals (Xiao et al.,2015).

Recently, the graphene derivatives have spurred immense in-terest (Cao et al., 2016; Zhang et al., 2018). The obtained grapheneoxide (GO) and reduced graphene oxide (rGO), with oxygen-containing function groups (such as hydroxyl, carboxylic,carbonyl groups) on surface, provide both anchor and dispersibilityof photocatalyst nanoparticles and adsorption-reactive sites forVOC molecules (Jiang et al., 2011; Zhai et al., 2015; Li et al., 2016; Joet al., 2017; Yu et al., 2018). Roso et al. (2017) investigated therelationship between adsorption-photocatalytic reactivity and the

Table 1Summary of adsorption and photocatalytic degradation of VOCs by typical carbon-based nanocomposites.

Carbon-based Nanocomposites BET porosity VOC initial concentration Adsorption efficiencies Photocatalytic efficiencies Ref.

TiO2-ACFs 441.3m2/g32 mm

toluene 460 ppm877 ppm1150 ppm

97%99%99%

81%a

62% a

57% a

(Li et al., 2017)

TiO2-ACFs 1020.6m2/g formaldehyde0.8 ppm

14.6% 83.6%b (Liu et al., 2018)

Au/TiO2-ACFs e styrene25± 1.5 ppm

e 91% b (Shi et al., 2017)

TiO2-ACFs 593.3m2/g10e30 mm

toluene115 ppm

99.4% 100% (Tian et al., 2017)

CNTs/TiO2 nanofiber 53m2/g4.1 nm

limonene 0.1 ppm0.7 ppm1.6 ppm

e 92% b

69% b

42% b

(Jo and Kang, 2015)

MnO2/MWCNT 93m2/g formaldehyde10 ppm

e 43% b (Miao et al., 2018)

CNTs/TiO2 e benzene 250 ppm e 64.6% a (Xu et al., 2010)CNTs/TiO2 sphere 55.2m2/g

0.215 cm3/gstyrene25± 1.5 ppm

95% 55.4% b (An et al., 2012)

MWCNT/TiO2 133m2/g acetone300± 20 ppm

e 50% b (Yu et al., 2011)

graphene/Fe3þeTiO2 105.7m2/g0.31 cm3/g11.56 nm

formaldehyde e 58% b (Low and Boonamnuayvitaya, 2013)

ZnO-rGO e acetaldehyde200 ppm

e 3.08mg�g�1catalyst

a (Chen et al., 2015)

N-doped grapheneeFe2O3 e acetaldehyde810 ppm

e 55% b (Wang et al., 2015a)

rGO-TiO2 105.1m2/g2.0e8.0 nm

formaldehyde0.5 ppm

e 88.3% b (Yu et al., 2018)

Ce-GO-TiO2 237.6m2/g4.54 nm

formaldehyde2.8 ppm

e 83.8% b (Li et al., 2016)

rGO-TiO2 e methanol4000± 200 ppm

e 80mg�g�1catalyst

b (Roso et al., 2017)

P25/graphene e benzene 156 ppm 8% 76.2% a (Zhang et al., 2010)GO-TiO2 113.8m2/g

0.47 cm3/g2-ethyl-1-hexanol0.8 ppm

e 55.1% a (Chun and Jo, 2016)

graphene hydrogel-AgBr@rGO 220m2/g bisphenol A20 ppm

87.2% 91.4%b (Chen et al., 2017)

TiO2/AC e formaldehyde1 ppm

33.9% 79.4% b (Lu et al., 2010)

TiO2/AC e aromatic compounds0.1 ppm

95% 90% b (Jo and Yang, 2009)

TiO2/AC 1025m2/g0.529 cm3/g

methanol 22.4 ppm e 53% b (Tao et al., 2006)

TiO2/AC 149m2/g0.25 cm3/g

propene 100 ppm e 60% b (Ouzzine et al., 2014)

TiO2/AC 848m2/g 2-propanol2000 ppm

93e94% 98% b (Horikoshi et al., 2013)

TiO2/AC 696m2/g0.42 cm3/g

acetone 175 ppm cyclohexane600 ppm

71%35%

85% a

e

(Selishchev et al., 2012)

TiO2/biochar 270.89m2/g0.1656 cm3/g

bisphenol A20 ppm

46.01% 21.41%b (Luo et al., 2015)

g-C3N4/biochar 43.9m2/g 2-Mercaptobenzothiazole10 ppm

e 90.5%b (Zhu et al., 2018)

biochar/Fe3O4 365m2/g0.54 cm3/g

carbamazepine30 ppm

e 50%b (Shan et al., 2016)

g-C3N4/biochar e p-nitrophenol32 ppm

90% 70% b (Pi et al., 2015)

a : mineralization efficiencies calculated based on the amount of CO2 produced.b : conversion efficiencies calculated from the amount of VOC reduced.

W. Zou et al. / Chemosphere 218 (2019) 845e859848

structures of GO, rGO, and few-layer graphene, and found that therGO-based nanocomposites shown the best elimination efficiencybecause of their deeper contact with the photocatalyst and lowermass-transfer limitations. In addition, the amount of functionalgroups on GO and rGO surface could be controlled to obtain thesuperior adsorption-photocatalytic activity (Zhang et al., 2010).Chun et al. (Chun and Jo, 2016) used various thermal conditions toinvestigate the relationship of the catalyst structures and reactionperformance, and proposed that different calcination temperatures(200, 300, 400, 500 �C) had effects on the surface oxygen content,specific surface area, visible light adsorption and activity of the GO-

TiO2 hybrids. Fig. 1c shown that the increased calcination temper-atures can enlarge surface area and pore volume, but reduce thevisible light adsorption, all of which affected the 2-ethyl-1-hexanolremoval (Chun and Jo, 2016). Therefore, in the adsorption-photocatalytic degradation VOC pollutants, the properties of ma-terials (specific surface area, pore volume, hydrophilicity/hydro-phobicity, light adsorption, charge transfer, etc.) should beconsidered and balanced comprehensively. The integratedadsorptive and photocatalytic removal of VOCs on graphene aredisplayed in Table 1.

Compared with the above 2D structural graphene and its

Fig. 2. Schematic illustration of the properties of carbon materials for adsorption-photocatalysts.

W. Zou et al. / Chemosphere 218 (2019) 845e859 849

derivatives, 3D graphene with porous structure has more lightutilization efficiency because of the light refraction and reflection inits interior structure (Chen et al., 2017), provides multidimensionalquality and charge transfer channel, highly decreases the aggre-gation of photocatalyst particles to expose more active sites, andimproves the adsorption property (Yang et al., 2017). Therefore, 3Dgraphene based materials possess potential application in the fieldof adsorption-photocatalytic field, but some problems, such as highcost, complex preparation, hard separation, etc., are worthy toinvestigate for industry application in further.

2.4. AC

AC is a traditional carbonaceous adsorbent widely used in manyfields for its high surface area, excellent porous structure, tunablesurface property and low cost (Ao and Lee, 2005). Table 1 sum-marizes the applications of AC-based nanocomposites in VOCremoval via the adsorption-photocatalysis process. The AC supportwith the superior adsorption capability can enrich VOC molecules,promote the transport of the pollutant to the nanosized photo-catalyst catalyst (Lu et al., 2010). In the coupled adsorption withphotocatalysis process, the concentrated pollutant moleculesaround the photocatalyst are degraded and thus the AC is in situregenerated (Jo and Yang, 2009). However, in general, the AC hasthe microporous with less than 2 nm, whereas, the particle size ofphotocatalyst is larger than 2 nm, on the basis of that, the syner-gistic photocatalytic oxidation of adsorbed VOC on carbonwould beprevented. Therefore, some methods are employed to modify thepore size of AC and enlarge the contact area of AC and photo-catalyst. For example, Sun et al. (Kang et al., 2011) used water vaporgasification and phosphoric acid modification on AC, and then theratio of mesopore to micropore increased; the modification of ACmorphology to sphere was confirmed to have improved smoothsurface, fluidity and mechanical strength (Ouzzine et al., 2014); themicrowave heating method was used to obtain a thin TiO2 coatedAC and the contact between AC and TiO2 was greatly enlarged(Horikoshi et al., 2013).

Like ACFs, AC is a natively nonpolar adsorbent with the strongaffiliation to hydrophobic VOC molecules with non-polarity orweak polarity (Selishchev et al., 2012; Zhang et al., 2017). Thedegradation efficiencies of VOCs with different polarities on AC-based photocatalysts are determined in comparison, that is,increasing the amount of AC in the nanocomposites negativelyinfluenced polar acetone degradation, but promoted the elimina-tion of nonpolar cyclohexane (Selishchev et al., 2012). It was foundthat part of methanol pollutant was directly desorbed withoutdegradation, due to the weak affiliation of AC to methanol (Taoet al., 2006a,b). On the basis of that, the surface functional groupsare modified on AC materials to have higher selectivity of certainadsorbates. Ma et al. (2011) used hexamethylene diamine tomodifyAC and amine compounds were generated on AC surface, and thenthe formaldehyde removal increased.

2.5. Biochar

Biochar, often produced from pyrolysis of biomass in an inertatmosphere, is an alternative low-cost carbonaceous adsorbent.Due to its rich surface function groups and stable structure, biocharhas been used in various applications for the removal of organiccontaminants (Zhang et al., 2012; Zhu et al., 2014; Rajapaksha et al.,2016). The surface function groups, pore structure, C/O ratio, andmineral types of biochar are strongly depended on the productionconditions including feedstock type, pyrolysis method, and pyrol-ysis temperature (Sun et al., 2014; Wang et al., 2015b). Despite ofthe promising potential applications, the commercial usage of

biochar is still limited, because its lower porosity and surface arealead to lower adsorption of organic pollutants compared with AC(Falco et al., 2013; Fang et al., 2018).

Some techniques are applied for the surface modification ofbiochar to improve its adsorption-photocatalytic performance (Luoet al., 2015; Lyu et al., 2017, 2018; Zhu et al., 2018). It is reported thatthe high-energy milling treatment not only makes the pristinebiochar particles turn into submicron-sized, but also generatesmore pore structures and surface functional groups, which lead tomore sorption sites and higher capacity (Shan et al., 2016). Ballmilling is reported to promote the formation of new chemicalbonds for the fixation of nanosized photocatalysts (Lyu et al., 2017,2018). Pi et al. (2015) used ball milling to design the g-C3N4/biocharinterfaces, and the enhanced composite interface accelerated thediffusion of pollutant molecules from biochar pores to g-C3N4. Inaddition, the pyrolysis treatment is employed to optimize the sur-face structures of biochar. Qin et al. (2017) used 300e700 �C to getdifferent biochar materials for 1,3-dichloropropene degradation,and found that there was a U-shaped relationship between pyrol-ysis temperature and degradation rate. The above structure-dependent-activity was attributed to the pore structures and sur-face hydroxyl radicals of the biochar. Inspired by the outstandingproperties, such as high porosity, low density, and hydrophobicity,the functionalized carbonaceous aerogels from biomass as the rawmaterial are prepared for adsorption-photocatalysis. Shi et al.(2016) used wintermelon as the raw material to obtained acarbonaceous aerogel (CA) and combined with TiO2, the Fig. 1dshown that the porous TiO2/CA possessed ultralight weight andhydrophobicity, and the SEM and TEM results shown that the TiO2/CAmaintained the porous architectures and TiO2 were anchored onthe CA surface, which were important for the pollutants into pho-tocatalytic active sites.

In summary, different carbon materials with various properties(pore size, morphology, specific surface area, surface hydrophilici-ty/hydrophobicity, conductivity, etc.) are displayed in Fig. 2 in

W. Zou et al. / Chemosphere 218 (2019) 845e859850

comparisons. The properties would have significant influences onthe stabilization of nanosized photocatalysts, the interactions be-tween carbons and photocatalysts, VOC molecule and lightadsorption, the photo-generated charge transfer, active radicalsgeneration, etc. Therefore, the comprehensive knowledge on theadvantages and disadvantages of carbon materials are indispens-able to industrial applications in further.

3. Mechanisms and models

The heterogeneous reactions including successive steps ofadsorption and photocatalysis process: (1) mass transport of theVOC pollutants from gas phase to the hybrid material surface; theadsorption of VOC molecules on carbon through surface in-teractions (i.e., hydrophobic effect, hydrogen bonds, p-p bonds andVan der Waals interactions); and the concentration differencepromotes the adsorbed VOCs diffuse to the photocatalyst; (2) whenthe illuminated photo energy is larger than the band gap of thephotocatalyst, electrons in valence band (VB) are transferred toconduction band (CB), pairs of electron-holes are thus generated;the generated holes and electrons at the surface possess theoxidation and reduction ability to form hydroxyl radical (�OH) andsuperoxide radical (�O2

�); and these radicals oxidize the VOC pol-lutants into CO2, H2O or other intermediate products (Zhao andYang, 2003; Huang et al., 2018a; Huang et al., 2018b); (3) theproducts are desorbed and the hybrid adsorbent/photocatalyst areregenerated. Both processes have a sequence of elementary reac-tion steps or a range of intermediates, which complicate the gov-erning mechanisms. The main elementary reaction steps can bewritten as follows:

S1: adsorption of VOC pollutant VOCsg / VOCsads (1)

S2: adsorption of O2 O2 / O2ads / 2Oads (2)

S3: reactive species under illumination S þ 2Oads þ hv / Xads* (3)

S4: first step for VOC oxidization VOCsads þ Xads* / I1ads þ Pads (4)

S5: second step for VOC oxidization I1ads þ Xads* / I2ads þ Pads (5)

S6: desorption of product Pads / Pg (6)

Where Xads* is donated as the active reaction radicals such as

�OH and �O2� species, Iads is the intermediate species adsorbed on

catalyst surface, and S is the composite surface. The mathematicalmodels thus have been made to correlate the various physico-chemical characteristics of carbon-based composites with theadsorption-photocatalytic VOC removal to understand the reactionmechanisms. In this part, adsorption, photocatalysis, the rate-determining step, intermediate/regeneration are discussed asfollowed.

3.1. Adsorption step

Various adsorption isotherm and kinetic models have beendeveloped to describe the interactions between VOCs and carbo-naceous adsorbents, most of which can be applied directly tosimulate the adsorption processes. Langmuir and Freundlichmodels are the most commonly used mathematical models, due tothe simplicity and easy interpretability (Kruk and Jaroniec, 2001).The Langmuir equation is displayed in Table 2, KL could be used tocalculate the dimensionless separation factor (RL), which analyzesthat whether the adsorption process is favorable or not, i.e., theprocess is irreversible (RL¼ 0), or favorable (0< RL< 1), or linear

(RL¼ 1), or unfavorable (RL> 1). Langmuir model relies on the hy-potheses that the adsorbent surface is homogenous, which has onlyone kind of adsorption site with uniform allotment of energy level.When the VOC molecule attaches on the site, no further adsorptiontake place at that spot. Therefore, Langmuir isotherm assumes thatthe maximum adsorption is occurred with the complete formationof monolayer adsorption of adsorbent molecules on surface and noshifting of adsorbate molecule is present in the surface plane(Vincent et al., 2009; Sharma et al., 2016). The Freundlich isothermis originally empirical, interpreted as adsorption sites with het-erogeneous surfaces or surfaces of varied affinities, and the varia-tion in adsorption energies are justified by the interactions of VOCand carbon-based materials or VOC intermolecular interactions(multilayer adsorption) (Basha et al., 2010). The Freundlich equa-tion is shown in Table 2, and the parameter n represents thebonding between the adsorbate and adsorbent, indicating differentadsorption processes, i.e., linear (n¼ 1), chemical (n< 1) or physical(n> 1) (Pezoti et al., 2016; Ammendola et al., 2017) On the basis ofthat, the Freundlich isotherm describes the surface heterogeneitywith the non-uniform distribution of energy level. Lee et al. (2006)used the Langmuir and Freundlich models to compare theadsorption behaviors of toluene and acetone on AC, respectively.The results suggested that the adsorption isotherms of both twovapors were well satisfied the Langmuir and Freundlich equations,and the maximum adsorption capacity (qmax) of toluene calculatedby Langmuir equation was three times higher than that of acetone(Fig. 3a), whichwas related to the AC surface and the vapor polarity,that was, activated carbon with nonpolarity and hydrophobicitywas beneficial for nonpolar toluene adsorption, not polar acetone(Lee et al., 2006). Shan et al. (2016) found that the differentadsorption processes of carbamazepine and tetracycline on bio-char/Fe3O4 and AC/Fe3O4 surface, the Freundlich and Langmuirmodels were fitted better on biochar/Fe3O4 and AC/Fe3O4, respec-tively. Because both magnetite and biochar materials could interactwith pollutant molecules via FeeO and p-p bonds. Different fromthe monomolecular adsorption of Langmuir isotherm, the Bru-nauereEmmetteTeller (BET) isotherm is applied for the multimo-lecular adsorption, which assumes that during the adsorption, thedevelopment of the multilayer region should be geometricallyrestricted (Pantuso et al., 2014). The BET equation is displayed inTable 2. Shayegan et al. (2018) proposed that the maximumadsorption capacity (Qm) from isotherm was lower than that ofLangmuir isotherm, because Langmuir isotherm considered thatthe adsorption process was amonolayer on active sites of adsorbentand no multi-layer adsorption was assumed. In addition, for themulti-components adsorption process, the Langmuir and Freund-lich models are not useful, and thus others are investigated for themulticomponent interaction (Qijin et al., 2012; Sidheswaran et al.,2012). Sidheswaran et al. (2012) developed the modified Freund-lich multi-component isotherm model for different VOCs on ACF,shown in Table 2, and the isotherm parameters were varied withthe competitive adsorption of the VOCs mixture on the ACF surface.

Furthermore, the pore structures of carbon-basedmaterials playan important role in the adsorption capacity, and thus it is neces-sary to use a model to study the relationship of pore and adsorptionmechanism. The DubinineRadushkevich (DeR) model is used todescribe the adsorption process with a Gaussian energy distribu-tion on a heterogeneous surface, and the surface heterogeneity isarisen from the pore structure of carbonaceous materials and theinteractions between adsorbate and adsorbent (Üner et al., 2016).The DeR equation (Table 2) based on the well-known Polanyi'spotential is widely applied to estimate VOC adsorption capacity onmicroporous carbon adsorbents, such as AC, ACF, etc. (Hung and Lin,2012; Jahandar Lashaki et al., 2012; Baur et al., 2015; Qin et al.,2017). Baur et al. (2015) investigated the toluene adsorption on

Table 2Summary of common models for gaseous adsorption-photocatalysis (Maudhuit et al., 2011; Pantuso et al., 2014; Shan et al., 2016; Sharma et al., 2016; Simonin, 2016; Chen et al., 2017).

Model Equation Mechanism

Isotherms Langmuir qe¼ qmKLP/(1þKLP) a homogeneous surface and single molecular layer adsorptionFreundlich qe¼ KFP1/n b heterogeneous surface and intermolecular interactionsFreundlich multi-component

qe¼ KiPiðPkj¼1

aijPjÞni�1 the multi-compounds where the monocomponent adsorption Freundlich isotherm

Dubinin-RadushkevichW ¼ Woexp [-KDR (RTln(

P0

P)b] c

the Polanyi potential theory, and applies when the adsorption process follows a pore filling mechanism

BETqe¼ qmB

P

P0/{(1� P

P0)[ 1þ ðB� 1Þ P

P0]}d

multilayer adsorption

Adsorption Kinetics pseudo first-orderlog (qe- qt)¼ log qe -

k12:303

tethe adsorption rate is proportional to the number of vacant adsorption sites

pseudo second-order t/qt¼ 1/k2qeb þ t/qe f the adsorption rate is proportional to the square of the number of vacant adsorption sites

Boyd's film-diffusion modelF¼ 1-

6p2

P∞n¼1

1n2

expð� n2BtÞg the gas film surrounding the adsorbent particle is the main resistance

interparticle diffusion modelqt/qe¼ 1-

6p2

P∞n¼1

1n2

exp

� n2p2tDc

r2p

!h

the constant diffusivity in spherical coordinate can be integrated with appropriate initial and boundary conditions.

Photocatalytic Kinetics Langmuir-Hinshelwood C¼ Ceexp�KadðkLH tþðCe�CÞÞ i both adsorption behavior and photocatalytic reaction rate based on the Langmuir mechanismfirst order C ¼ Ce exp-kt j the low VOC concentrationsecond order C ¼Ce/( kCetþ 1)k

power-law C ¼ A þ kt1/2 l the high VOC concentration

a qm: Langmuir maximum adsorption capacity; KL: Langmuir bonding term related to energy or net enthalpy of adsorption; P: the partial pressure.b KF: Freundlich affinity constant; n Freundlich linearity constant.c Wo: limiting pore volume; KDR: DR constant related to adsorption free energy; R: ideal gas constant; T: absolute temperature; P0: the saturation pressure.d qm: maximal adsorption capacity of the first adsorbed layer; B: BET constant; P0: the saturation pressure.e k1: the adsorption rate constant of the pseudo-first-order adsorption.f k2: the adsorption rate constant of the pseudo-second-order adsorption.g F: the fractional adsorption capacity at a given time (F¼ qt/qe). Bt: a mathematical function of F.h Dc: the intracrystalline diffusivity; rp: diffusion path length.i Ce: the concentration of VOC at equilibrium without light, kKH: the rate constant. Kad: equilibrium adsorption constant.j Ce: the concentration of VOC at equilibrium without light, k: the rate constant.k Ce: the concentration of VOC at equilibrium without light, k: the rate constant.l k: the rate constant, A: the constant.

W.Zou

etal./

Chemosphere

218(2019)

845e859

851

Fig. 3. (a) Langmuir and Freundlich isotherms of acetone and toluene vapors on ACand the corresponding of constants of isotherms (Lee et al., 2006). (b) Temporal evo-lutions of photocatalytic toluene and decane degradation as a single pollutant andmixture with 52± 3 ppb individual initial VOC concentrations (RH¼ 50%; T¼ 23 �C)(Debono et al., 2017). (c) Rate constant of indomethacin degradation reaction and theadsorption equilibrium constant as a function of the TiO2 content in TiO2/AC catalyst(Basha et al., 2010). (d) Possible pathways of photocatalytic degradation of tetra-bromobisphenol An under UV light irradiation on grapheneeTiO2 (Cao et al., 2015).

W. Zou et al. / Chemosphere 218 (2019) 845e859852

AC with different pore sizes, and DeR model provided the char-acteristic energy and the adsorption enthalpy of ultramicroporousand supermicroporous ACFs, revealing that the toluene was morestrongly adsorbed in ultramicroporous as compared to super-microporous, due to different overlap potential between the porewalls. In addition, from the limiting pore volume Wo of the D-Requation, it was suggested that the molecular size of VOCs might bean important factor to affect the adsorption capacity of carbons(Hung and Lin, 2012). In addition, the adsorption process could bedescribed from the mean free energy calculated from D-R model,the positive value suggests that the adsorption process is endo-thermic in nature, and the value between 1 and 8 kJ/mol is corre-sponded to physical adsorption (Üner et al., 2016).

Different from the above adsorption isotherms, adsorption ki-netics models are considered as a useful tool to determine thesorption rate of VOCs onto carbonaceous adsorbents. Some of themost commonly used adsorption kinetics models are also sum-marized in Table 2. Although these models are often considered assemi-empirical, they are still very useful to determine the gov-erning mechanisms and to compare the adsorption rate constant.The pseudo-first ordermodel assumes that the rate of occupation ofadsorption sites is proportional to the number of unoccupied sites,which is used for the simple physical adsorption, whereas thepseudo-second order kinetics model is fit for chemical adsorptionas well as physical adsorption (Tian et al., 2017). In addition, thepseudo-second order model is considered as a special kind ofLangmuir kinetics, proposing that the adsorbate concentration isconstant related to time, and the amount of adsorbate adsorbed atequilibrium determines the total number of binding sites (Pezotiet al., 2016). Chen at al (Chen et al., 2017) considered that the p -p interactions of bisphenol A and graphene made pseudo-second-order kinetic fit well. On the basis of that, besides the adsorptionenergy, the kinetic model also a method to determine whetherchemical adsorption or physical adsorption of VOCs happens oncarbons. However, sometimes neither of the two kinetics models isnot useful, and the additional investigations are still needed to

develop process-based mechanistic models for VOC adsorption onhybrid adsorption-photocatalysis materials. For the carbon mate-rial, it is found that the porous structure could introduce differentadsorption mechanisms at different adsorption times. In the initialtime of adsorption, the molecules enter pores of adsorbent, whichis the intraparticle and external diffusion-controlled mechanisms;at longer time, the molecules have completely entered into thepores and adsorbed onto the active sites of via the surface reactionmechanism (Simonin, 2016). For example, Fournel et al. (2010)proposed that due to the pore structure of ACF, there were twoadsorption kinetics in the VOC molecules on ACF-based materials,and the intraarticular mass transfer with the lower effectivediffusion coefficient was the limitation step of the VOC adsorptionkinetics.

Therefore, the adsorption models including isotherms and ki-netics are closely related to the properties of both adsorbents andadsorbates (e.g., surface area, pore structure, molecular weight,dielectric constant, boiling point, etc.), and the corresponding af-finities of carbon-based materials to VOCs, and we should usesuitable models according to the properties of specific carbons andVOC molecules.

3.2. Photocatalysis step

The photocatalysis process is a reaction between surfacereducible reactants and oxygen containing molecule, which is oftendescribed by the Langmuir-Hinshelwood (L-H), first-order, second-order, and Power law kinetics models, shown in Table 2 (An et al.,2010; Maudhuit et al., 2011; Huang et al., 2016b; Tian et al.,2017). L-H model is used to determine rate for a heterogeneouscatalytic mechanism based on some beliefs of the Langmuiradsorption isotherm, for example, the reactant molecules are inform of monolayer and chemisorbed reversibly on surface; theadsorption process involves one adsorbed species per surface site;the energy of adsorbed species is the same at any surface sites andis not affected by adsorption species on adjacent sites (Serpone,2007). Furthermore, the L-H model is influenced by some factors,including catalyst properties, light sources, mass transfer parame-ters, operational conditions, and kinetic parameters, which shouldbe taken into account (Maudhuit et al., 2011). In case of the lowinitial concentration of VOC pollutants, the L-H model can besimplified to the first-order reaction rate equation, whereas, in thehigh concentration, the semi-empirical power law reaction kineticscould well express (Tian et al., 2017). In addition, with multi-compound competition reaction onto the catalysts, the L-H modelalso could be developed to describe the experimental results.Debono et al. (2017) investigated the mixture effect on the photo-catalytic toluene and decane removal, Fig. 3b shown that the firstorder kinetic model was established in the photocatalytic degra-dation of toluene and decane as a single pollutant as well as inmixture, and the line slopes suggested that the reaction rates ofindividual degradation of toluene and decane were greatly inhibi-ted by the presence of VOCs mixture.

However, the L-H model does not consider of the intermediatesin the reaction, which limits its develop. For instance, in photo-catalyic trichloroethylene (TCE) degradation, the L-H model didn'tfit the TCE photocatalytic degradation, because the Cl� species wasgenerated to promote the reaction kinetic rate with irradiation time(Debono et al., 2017). Considering of the importance of in-termediates, it is expected to develop a model to predict the fate ofintermediates with reaction time, but for most VOC degradation,the reaction pathways of photocatalytic oxidization are not clear orknown, which limits the modeling method. Furthermore, thisabove issue is more challenging in case of the mixed compounds.

W. Zou et al. / Chemosphere 218 (2019) 845e859 853

3.3. Rate-determining step

The rate-determining step is the slowest step controlling theoverall reaction rate. Understanding the rate-determining step istherefore very important to the optimization of the adsorption-photocatalysis processes. Using a simplified approach, the rate-determining step of VOC adsorptive-photocatalytic removal canbe related to the rate constants of adsorption (kads) and photo-catalysis (kphoto). If kads> kphoto, the adsorption-photocatalysisprocess is mainly controlled by photocatalysis and vice versa.

From the above discussions about the reaction kinetics modelsof adsorption and photocatalysis, it is suggested that the rate con-stant values may change with many factors, which can affect therate-determining step of the adsorption-photocatalysis process(Mo et al., 2009). On one hand, the rate-determining step waspossibly controlled by the properties of carbon-based materials,such as adsorption capacity, photo-generated electron transferability, etc. For example, the reaction kinetics of indomethacinremoval was studied on the TiO2-AC photocatalytic adsorbents, andthe effects of the ration of TiO2 to AC on the rate constants ofadsorption and photocatalysis were determined, respectively(Fig. 3c). With the increasing content of TiO2 up to 10%, the pho-tocatalytic degradation rate was increased but then decreased,because the lower TiO2 content had high adsorption ability butpoorer degradation rate, the rate-determining step was the pho-todegradation of the adsorbed indomethacin and the increasedTiO2 content promoted the reaction; whereas above 10% TiO2

decreased the rate constant, because the reduced amount ofadsorbed indomethacinwas the rate-determining step (Basha et al.,2010).

On the other hand, it is considered that the rate constant valueswould change with the reaction conditions such as humidity, ox-ygen content, temperature, etc., further affecting the rate-determining step of the adsorption-photocatalysis process. In thecase of less humidity, the presence of water promotes the genera-tion of active hydroxyl radicals and then the increased photo-catalytic reaction rate of VOC degradation is the rate-determiningstep; whereas, too more humidity would occupy adsorption sitesand then the adsorption of VOC molecules becomes the rate-determining step (Jo and Yang, 2009). On the basis of that, it issuggested that the reaction kinetics of adsorption-photocatalysisprocesses are controlled by various factors, and thus may compli-cate the identification of the rate-determining step. Additional in-vestigations thus are required to explore the rate-determining stepof the integrated adsorptive and photocatalytic removal of VOCs bycarbon-supported nanocomposites.

Table 3Summary of intermediates of adsorption-photocatalytic degradation of VOCs.

VOCs Main reaction intermediates

toluene formic acid, benzaldehyde, benzyl alcohol, O2�, �OH

toluene formaldehyde, acetaldehyde, acetone, 2-butanone, o-cresol, benzaldehydetoluene benzaldehyde, benzene, benzyl alcohol, phenol, cresoltolueneisopropanol

hydrocarbonacetone

decane formaldehyde, acetaldehyde, propanal, acroleine, acetone, 2-butanone, etbenzene phenol, malonic acid, benzenediolacetylene carboxylic acids, COlimonene acetaldehyde, methacrolein, 2,3-dihydrofurane, formaldehyde, acetone, 2

ethanol, CO

3.4. Intermediate/regeneration

In theory, the VOC molecules would be adsorption-photocatalytic oxidized to carbon dioxide and water. However,actually, the reaction processes sometimes stop along the way, andthen yield undesired hydrocarbons and oxygen containing in-termediates compounds. The above mentioned toxic intermediatesnot only have harmful effects on the human health, but also causecatalyst deactivation by saturation and poison catalyst surface.Therefore, the study of intermediates generation is one of the mainconcerns for the application of adsorption-photocatalysis technol-ogy. A great number of the surface detection techniques are used tounderstand the possible reaction process, shown in Table 3 (VanDurme et al., 2007; Sleiman et al., 2009; Den and Wang, 2012;Thevenet et al., 2014; Ourrad et al., 2015). For example, Cao et al.(2015) used the liquid chromatography-mass spectrometry(LCeMS) to study the photocatalytic degradation process of tetra-bromobisphenol A on grapheneeTiO2, as well as the regenerationof the catalyst. The prepared hybrid adsorption-photocatalystsexhibited higher activity and photo-stability than that of TiO2,and the LCeMS shown the photodegradation intermediate speciesduring tetrabromobisphenol A removal, suggesting that the reac-tion pathways were involved debromination, substitution anddehydroxylation processes (Fig. 3d) (Cao et al., 2015). Debono et al.(2013) investigated the kinds and corresponding concentrations ofreaction intermediates during the decane photodegradation, themajority of the intermediates identified were carbonyl compoundsand alcohols, the temporal profiles of which consisted three phase:1) intermediate formation, 2) reaching a maximum concentration,3) decreased intermediate concentration, indicating that thedecane was degraded and the catalyst was regenerated during theprocess. In addition, the FT-IR technology is a useful method todetermine minimal intermediate species on the catalyst surface inthe adsorption-photocatalytic oxidation (Debono et al., 2011, 2013;Huang et al., 2018d). At present, a great number of in-situ tech-nologies have been employed to explore the adsorption-photocatalytic reaction process, but how to effectively control thegeneration of the toxic by-products and intermediate species by themean of adsorption-photocatalyst structures and reaction condi-tions deserves to be further investigated.

Different from the above intermediates, the active reactionradical species determined by electron spin resonance (ESR), suchas �OH, �O2

�, etc. could promote the oxidation of VOC moleculesduring adsorption-photocatalysis process (Sleiman et al., 2009;Guo et al., 2017). The generation of reaction radical species isclosely related to the surface adsorbed oxygen, H2O, reactants, etc.For example, the degradation of toluene was mainly oxidized byphoto-generated holes at dry conditions and by OH radicals in thepresence of water, respectively (Sleiman et al., 2009). Zou et al.

Detectiontechnology

Ref.

GC-MS, ESR (Van Durme et al.,2007)

FT-IR, GC-MS (Debono et al., 2011)GC-MS (Sleiman et al., 2009)FT-IR (Den and Wang, 2012)

hanol, methyl acetate, CO GC, HPLC, FT-IR (Debono et al., 2013)FT-IR (Jacoby et al., 1996)GC, HPLC (Thevenet et al., 2014)

-pentanone, acetic, 1-butanol, GC, HPLC, FT-IR (Ourrad et al., 2015)

W. Zou et al. / Chemosphere 218 (2019) 845e859854

(2016) proposed that the superoxide radical �O2� was mainly

coming from the oxygen in reaction, which oxidized pollutants andthen regenerated the Cu2O-rGO catalysts. For the trichloroethylenedegradation, the generated reactive chlorinated radicals from thereactant molecules made an increase in the abatement and thenregenerated the catalysts (Debono et al., 2017). Therefore, themodulation of reaction factors could efficiently increase theamount of active radical intermediate species and then promote theoxidation of VOC adsorbed on the materials.

4. Effects of environmental conditions

The discussions on the mathematical models show that themulti-factors synergy mechanism controls the adsorption-photocatalytic VOC degradation reaction in terms of thermody-namics and dynamics. Given the environmental complexity, such asmoisture content, temperature, illumination transmission, VOC

Table 4Summary of key factors affecting VOC adsorption/photocatalytic performances.

Factor VOCs Moisture Temperature VOC initial concen

Moisture 2-ethyl-1-hexanol 20% e 0.1 ppm50%80%

CH4 0% 25 �C e

25%75%

benzene 20e30% 19e25 �C 0.1 ppm50e60%80e90%ethylbenzene 20e30% 19e25 �C 0.1 ppm50e60%80e90%methanol 080% 55 �C 31.0 ppmdecane 050% e 0.8 ppm

Temperature CH4 0% 25 �C e

35 �C45 �C

1,3- dichloropropene 0% 20 �C 0.021 ppm e

25 �C30 �C35 �C40 �C

CH2Cl2 0% 7 �C 0.017mM27 �C47 �C67 �C

Light 1-propanol 10% 30 �C 400 ppm

Formaldehyde e e 3.3 ppm25mW/cm2

47mW/cm2

98mW/cm2

VOC 2-ethyl- 1-hexanol 20% e 0.1 ppm1.0 ppm1.5 ppm2.0 ppm

toluene e e 115 ppm230 ppm460 ppm690 ppm

benzene e e 23 ppmethylbenzenem-xylene e e 12 ppmo-xylene 10 ppmp-xylene 25 ppmpentane 0% e 90.2 ppmi-pentane 124 ppmhexane 107.5 ppmi-hexane 78.8 ppmheptane 104.8 ppm

properties, etc. the affecting factors for the integrated adsorption-photocatalysis on carbon-based nanocomposites are discussed inTable 4.

4.1. Moisture content

Moisture content plays a significant role in both of VOCadsorption and photocatalysis processes on the carbon-basednanocomposites. On one hand, the effect of moisture content onfor the adsorption process is described in Fig. 4a. It is shown that asmall amount of water, the polar molecule, is beneficial for themore hydrophilic VOCmolecules adsorption on carbon material viahydrogen bond interaction (Shayegan et al., 2018); whereas, athigher moisture, water would increase the competitive adsorptionof VOC gas, because adsorbed water fills the small pores in carbo-naceous adsorbent and interferes with the adsorption capacity ofVOC (Tao et al., 2006a,b; Sadasivam and Reddy, 2015; Chu et al.,

tration Light Removal efficiency Ref.

visible light 89% (Chun and Jo, 2016)85%70%

e 0.18mol kg�1 (Sadasivam and Reddy, 2015)0.08mol kg�1

0.55mol kg�1

UV lamp 88% (Jo and Yang, 2009)84%82%

UV lamp 99% (Jo and Yang, 2009)99%97%

UV lamp 65%50% (Tao et al., 2006)UV lamp 86%59% (Debono et al., 2013)e 0.18mol kg�1 (Sadasivam and Reddy, 2015)

0.12mol kg�1

0.11mol kg�1

55% ) (Qin et al. (2017)68%75%76%89%e 1.4mol kg�1 (Chiang et al., 2001)0.9mol kg�1

0.5mol kg�1

0.1mol kg�1

1.0mW/cm2 45%55%65% (Vincent et al., 2009)2.0mW/cm2

3.0mW/cm2

7mW/cm2 0% (Obee and Brown, 1995)2.1%4.2%10.6%visible light 89% (Chun and Jo, 2016)

80%68%60%

visible light 100% (Tian et al., 2017)100%87.1%65.5%e 35% (Boulamanti et al., 2008)

85%e 70% (Boulamanti et al., 2008)

55%50%

e 16% (Boulamanti and Philippopoulos, 2009)21%30%77%28%

Fig. 4. (a) The effect of moisture content on the adsorption process. (b) Evolution ofproduct CO2 concentration in the photocatalytic oxidation of toluene under dry andwet conditions (Debono et al., 2011). (c) Influence of RH on the adsorption-photocatalytic methanol degradation (Geng et al., 2010).

Fig. 5. (a) The effect of reaction temperature on the adsorption process of CH4 on coalcarbon material (Guan et al., 2018). (b) The effect of reaction temperature on thephotocatalytic degradation of formaldehyde and the photograph of photocatalyticdevices (Huang et al., 2017).

W. Zou et al. / Chemosphere 218 (2019) 845e859 855

2018). For example, Li et al. (2008) observed that relative moisturein reaction make a decrease of the breakthrough time of adsorbateformaldehyde on AC, but higher moisture inhibited the adsorptionof formaldehyde on AC, leading to more water molecules adsorb onthe active sites of AC and the reduced reaction performance. Chunet al. (Chun and Jo, 2016) compared the adsorption and photo-catalysis performances of 2-ethyl-1-hexanol over graphene oxi-deeTiO2 hybrids under different relative humidity (RH), and foundthat at a RH of 20% the degradationwas higher than that at other RHvalues of 75% and 80%.

On the other hand, for the photocatalytic reaction, the increasedadsorbed water as the photo-excited hole traps is helpful for hy-droxyl radicals to form, which not only directly attack VOC mole-cules, but also reduce the electronehole recombination (Sleimanet al., 2009). The photocatalytic toluene oxidation under dry andwet reaction conditions were compared, respectively, and the re-sults were shown that the wet reaction condition was morebeneficial for toluene photocatalytic oxidation to CO2 not otherintermediates, compared with dry condition (Fig. 4b) (Debonoet al., 2011). On the basis of that, water plays the positive role inthe photocatalytic process and then improves the mineralization ofVOC pollutants to CO2. Furthermore, water molecules couldcompete with reaction intermediates for adsorption onto photo-catalyst, and then prevent the catalyst deactivation (Debono et al.,2013). However, in adsorption-photocatalytic reaction, moremoisture content does not mean better. Geng et al. (2010) investi-gated the effect of RH on the reaction activity of adsorption-photocatalytic methanol removal, as displayed in Fig. 4c, theremoval efficiency was higher at RH¼ 20% than that at RH¼ 5% and35%. Therefore, the influences of moisture content on adsorption-photocatalytic reaction should be considered of both aspects(adsorption and photocatalysis). Less moisture content is beneficialfor the enhanced adsorption and photocatalytic mineralization ofVOC molecules on carbon-based materials, and thus increase thereaction performance; whereas, moremoisture would prevent VOCmolecule adsorption and decrease the activity.

4.2. Temperature

Temperature is one of the key factors in heterogeneous re-actions, affecting not only the adsorption of VOC gas on the mate-rial, but also the kinetic photocatalytic reaction (Mo et al., 2009) Inthe adsorption, it is mostly contributed to exothermic reaction, the

decreased temperature is helpful for the VOC adsorption on ad-sorbents, but reduce the diffusion rate of the adsorbate moleculesin the internal pores and the external boundary layer of theadsorbent particles (Mo et al., 2009; Zhao et al., 2018). Guan et al.(2018) found that with the increase of reaction temperature, theLangmuir volumes (VL) of CH4 on carbon were decreased. Fig. 5ashowed that when the temperature was higher than 323.15 K, theadsorption capacities were independent of the temperature andremained constant, which was the “critical temperature point” inthe adsorption process (Guan et al., 2018).Whereas, the subsequentphotocatalytic oxidation related to endothermic reaction, andincreasing reaction temperature would improve the activity (Taoet al., 2006a,b; Huang et al., 2017; Qin et al., 2017). In Fig. 5b, thephotocatalytic formaldehyde oxidation efficiency at 60 �C wasbetter than that at 30 �C, which indicated that the higher reactiontemperature had positive effect on the photocatalytic process(Huang et al., 2017). Therefore, balancing the adsorption and pho-tocatalytic performances via temperature is important to theenhanced overall reaction rates. For example, Qin et al. (2017)demonstrated that the 1,3-dichloropropene degradation increasedon biochar when the reaction temperature increased from 20 �C to40 �C, because they proposed that the photocatalysis was thecontrolled reaction during the process. At low temperature, thephotocatalytic oxidation reaction rate is much slower than that ofVOC molecules adsorption, the photocatalysis is the rate deter-mining step and thus the increased reaction temperature improvesthe adsorption-photocatalytic efficiency (Tao et al., 2006a,b).While, at a higher temperature VOCmolecules adsorption becomesslower, the adsorption is the rate determining step and thus theincreased reaction temperature limits the adsorption-photocatalytic efficiency (Mo et al., 2009). Furthermore, the tem-perature effect is closely relied on the kinetic diameter and masstransfer ability of VOCs molecules. With increasing temperatures,the reaction rate of toluene and butadiene removal were found todecrease, but for ethylene, acetaldehyde and formaldehyde, theremoval rates improved, because of different kinetic molecule di-ameters (Obee and Brown, 1995).

4.3. Light irradiation

The light irradiation have amain influence on the photocatalysisprocess, and less affecting the adsorption. The light wavelength and

W. Zou et al. / Chemosphere 218 (2019) 845e859856

intensity are the two important factors. On one hand, the lightwavelength is corresponded to the photo energy, if the photo-energy is less than the band energy of photocatalyst, electronscannot be excited to the conduction band, and then the VOC mol-ecules are not oxidized. On the other hand, the photocatalytic re-action rate is dependent on the light intensity. At low lightintensity, the reaction rate of photocatalytic process is proportionalto light intensity; with the increased light intensity, the reactionrates were power-law dependence and the zero-order dependenceon light intensity (Deng, 2018). Vincent et al. (2009) believed that atlow light irradiance the photo oxidation of 1-propanol was first-order kinetic, and the strong light intensity caused that the rateof electron-hole formation exceeded the rate of photocatalyticdegradation, leading to the electron-hole recombination and thehalf-order reaction kinetic order. Furthermore, the efficient use oflight in the complicated reaction environment is an important andrelevant issue, which is worthy of consideration and investigation.Dao et al. (2016) compared the photocatalytic activities at differentlight irradiation distances (from the catalyst surface to the lightsource), and proposed that the distance of the illumination had astrong influence on the photocatalytic performance, and longerdistance led to poorer activity. Therefore, the increased light irra-diation could greatly promote the VOC photo-degradation but havea loweffect on the concentration of by-products in reaction, and thelight management, such as light delivery, distribution, collectionand adsorption in the complicated system is the key for theadsorption-photocatalytic reaction performance.

4.4. VOC properties

VOC contaminants with various inlet concentrations and prop-erties, such as molecular dynamic size, boiling point, vapor pres-sure, etc. have an influence on the adsorption-photocatalyticreaction performance (Sidheswaran et al., 2012). The higher VOCconcentration leads to the decreased adsorption rates, increasedphotocatalytic reaction kinetics, and reduced pollutants minerali-zation to CO2 (Li et al., 2017). Because the increased initial con-centrations is not helpful for available adsorption sites of the VOCmolecules on the catalysts surface, reducing their adsorption rate;contrary to the adsorption kinetic, the photocatalytic reaction ratesof VOC are promoted due to themore reactants (Chun and Jo, 2016).Li et al. (2017) argued that more toluene concentration limits theadsorption process, from 98% at the toluene concentration(<1150 ppm) to 77% at 6900 ppm. Chun et al. (Chun and Jo, 2016)suggested that the higher photocatalytic efficiency of 2-ethyl-1-hexanol removal on graphene oxide-TiO2 with a lower reactionrate was attributed to the greater adsorption competition out-weighing the reaction rate effect. In summary, it could be proposedthat lower initial concentration is beneficial for the adsorption-photocatalyic VOC degradation than higher concentration.

In addition, the properties of VOC pollutants should be consid-ered for the adsorption-photocatalytic removal. Firstly, VOCs withhigher molecular weight are difficult to completely oxidize hy-drocarbons, owing to all the oxygen atoms required for totaloxidation. Secondly, the stereochemical structure of the VOC mol-ecules is an important factor. For example, the oxidation rates of thethree xylenes were differenced, resulting from the fact that o- andp-xyleneweremore stable thanm-xylene (Boulamanti et al., 2008).Aromatic VOCs compounds with the tertiary carbon atom in branchcould accelerate the reaction rate (Boulamanti and Philippopoulos,2009). Thirdly, the polarity of VOC molecules should be considered,due to the different affinity on carbons. Lee et al. (2006) demon-strated that the equilibrium adsorption capacity of toluene on ACwas higher than that of acetone, because hydrophobic AC was in-clined to the adsorption of weakly polar toluene. The aliphatic VOC

compounds with p electrons have stronger adsorption affinity tomany carbonaceous materials than that of polar compounds,leading to better reaction activity. Finally, according to the Dubinin-Radushkevich model, the kinetic size of VOC molecules is anaffecting factor. The o-xylene molecule with larger kinetic di-ameters has poorer adsorption capacity on the microspore space ofAC (Hung and Lin, 2012), which would reduce the photo-oxidation(Boulamanti and Philippopoulos, 2009). On the basis of that, theproperties of VOC, including molecular weight, stereochemicalstructure, the polarity, kinetic size, should be considered in com-bined with carbon-based materials in practice.

5. Conclusions and perspectives

The integration of adsorption and photocatalysis degradation isa promising technology for VOC removal because it is not onlyenvironmental friendly with low-energy consumption, but alsoeffective and renewable. This critical review provides a compre-hensive review on current development of carbon-supported nano-photocatalysts for adsorption-photocatalytic oxidation of VOCs.The main conclusions of this review are as follows: (1) Carbonmaterials with large specific surface area, rich porosity, uniqueelectronic properties and surface function groups, are recognized asideal supports to stabilize semiconductors and adsorb VOC mole-cules. (2) Different mathematical models of adsorption and pho-tocatalysis are used to determine the rate-determining step andreaction processes, and the main intermediates are discussed. (3)Environmental conditions such as moisture, temperature, lightirradiation and VOC properties play important role in controllingthe reaction process.

Although progresses have been made towards the potentialapplication of adsorption-photocatalytic degradation of VOCs bycarbon-based composites, several research questions are needed tobe addressed before the practical realization of this promisingtechnology. Some of the key challenges include: (1) The synthesesof stable and high-efficient carbon-based nanocomposites withtunable adsorption and photocatalytic abilities should be furtherexplored, such as the inactivation, regeneration, etc. Additionally,the carbon content has a U-type relationship with the visible lightadsorption, and thus selecting the optimal carbon content isimportant. (2) The pore size and the surface chemical structure ofcarbon materials should be optimized to enhance the binding ofphotocatalysts, and the 3D aerogel carbon based composites shouldbe further investigate to reduce the cost and preparation; (3) Car-bon nanomaterials and AC as the support for nanosized photo-catalysts have been explored in the literature. Low-cost carbonmaterials such as biochar have received much less attention in thesynthesis of hybrid adsorbent-photocatalyst. Additional researchefforts thus should be spent to explore and evaluate nano-composites based on low-cost carbons such as biochar. (4) Thestructures of carbonaceous materials affect the VOCs diffusion, thelight adsorption, the intermediate species removal, etc. for theadsorption-photocatalysis processes, and complicate the identifi-cation of the rate-determining step. However, at present, the ratedetermining step of the integrated adsorptive and photocatalyticremoval of VOCs by carbon-supported nanocomposites is still un-clear. Further experimental and modeling investigations thusshould be conducted. (5) How to effectively control the generationof the toxic by-products and intermediate species by the mean ofadsorption-photocatalyst structures and reaction conditions de-serves to be investigated. (6) The management of light irradiation,such as light delivery, distribution, collection and adsorption in thecomplicated system, and the use of solar light, as key challenges, areworthy of investigation in the application of adsorption-photocatalytic VOC removal.

W. Zou et al. / Chemosphere 218 (2019) 845e859 857

Acknowledgements

This work was partially supported by National Natural ScienceFoundation of China (Nos. 21707066, 21670182, 21573105).

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