photocatalytic degradation of recalcitrant organic pollutants in...
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Journal of Hazardous Materials 285 (2015) 398–408
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Journal of Hazardous Materials
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hotocatalytic degradation of recalcitrant organic pollutants in watersing a novel cylindrical multi-column photoreactor packed withiO2-coated silica gel beads
awei Lia, Qi Zhua, Chengjie Hana, Yingnan Yanga,∗, Weizhong Jiangb, Zhenya Zhanga,∗
Graduate School of Life and Environmental Sciences, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8572, JapanKey Laboratory of Agricultural Engineering in Structure and Environment, Ministry of Agriculture, China Agricultural University, Qinghua Donglu 17,aidian, Beijing 100083, China
i g h l i g h t s
A novel cylindrical multi-columnphotocatalytic reactor (CMCPR) wasdeveloped.Methyl orange, amoxicillin and 3-chlorophenol were degraded suc-cessfully in CMCPR.Electrical energy per order (EEO) wasused to evaluate the efficiency ofCMCPR.The CMCPR is high efficient, low-costand easily repeatable for water purifi-cation.
g r a p h i c a l a b s t r a c t
r t i c l e i n f o
rticle history:eceived 15 October 2014eceived in revised form 2 December 2014ccepted 10 December 2014vailable online 12 December 2014
eywords:hotocatalytic degradationecalcitrant organic pollutantsylindrical multi-column photoreactor
a b s t r a c t
A novel cylindrical multi-column photocatalytic reactor (CMCPR) has been developed and successfullyapplied for the degradation of methyl orange (MO), amoxicillin (AMX) and 3-chlorophenol (3-CP) inwater. Due to its higher adsorption capacity and simpler molecular structure, 3-CP compared with MOand AMX obtained the highest photodegradation (100%) and mineralization (78.1%) after 300-min pho-tocatalytic reaction. Electrical energy consumption for photocatalytic degradation of MO, AMX and 3-CPusing CMCPR was 5.79 × 104, 7.31 × 104 and 2.52 × 104 kW h m−3 order−1, respectively, which were lessthan one-thousand of those by reported photoreactors. The higher flow rate (15 mL min−1), lower initialconcentration (5 mg L−1) and acidic condition (pH 3) were more favorable for the photocatalytic degrada-tion of MO using CMCPR. Five repetitive operations of CMCPR achieved more than 97.0% photodegradation
iO2-coated silica gelater purification
of MO in each cycle and gave a relative standard deviation of 0.72%. In comparison with reported slurryand thin-film photoreactors, CMCPR exhibited higher photocatalytic efficiency, lower energy consump-tion and better repetitive operation performance for the degradation of MO, AMX and 3-CP in water.The results demonstrated the feasibility of utilizing CMCPR for the degradation of recalcitrant organicpollutants in water.
∗ Corresponding authors. Tel.: +81 29 853 4712; fax: +81 29 853 4712.E-mail addresses: [email protected] (Y. Yang),
[email protected] (Z. Zhang).
ttp://dx.doi.org/10.1016/j.jhazmat.2014.12.024304-3894/© 2014 Elsevier B.V. All rights reserved.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
Recalcitrant organic pollutants (ROPs), such as dyes, antibioticsand pesticides, have attracted increasing attention due to theirnature of persistence, biomagnification and harmful impacts onboth humans and the environment [1–6]. Effective removal of ROPs
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D. Li et al. / Journal of Hazard
rom aquatic environments is of vital importance not only for waterurification and conservation but also for maintaining human andcological health. Conventional physicochemical techniques, suchs adsorption, flocculation, reverse osmosis and extraction, merelyransfer the organic contaminants from one phase to another with-ut destroying them [7,8]. Classical biological treatment has proveno be inefficient for decomposing ROPs because of their complex
olecular structure, high toxicity and poor biodegradability [9–11].herefore, alternative advanced technologies are required to effec-ively remove ROPs from water.
Heterogeneous photocatalysis, with the merits of easy operationt ambient conditions and complete mineralization of hazardousontaminants, has shown great potential for decomposing ROPs inater [12–14]. Titanium dioxide (TiO2) is believed to be the mostromising photocatalyst because of its superior photocatalyticctivity, high chemical stability, inexpensiveness and nontoxicity15,16]. Irradiation of TiO2 by photons having energy that exceedsts band gap energy (3.2 eV) excites electrons (e−) from the valenceand to the conduction band, leaving holes (h+) behind in thealance band. The photogenerated electron–hole pairs can migrateowards the TiO2 surface and react with pre-adsorbed speciesH2O/OH− and O2) to form hydroxyl radical (•OH) and superox-de radical (O2
•−) [17]. These highly oxidative species (h+, •OH and2
•−) possess the potential to decompose nearby organic contam-nants on the TiO2 surface and even mineralize them into CO2, H2Ond inorganic ions.
Photoreactors used for liquid phase oxidation are typically basedn the slurry system, in which TiO2 nanoparticles are suspendedn an aqueous solution. Although this design has a simple reactoronfiguration and high surface area for adsorption and reaction,ts application is severely constrained by low-efficient utilizationf incident light and difficult separation of nano-sized TiO2 aftereaction [18,19]. Moreover, release of TiO2 nanoparticles into thenvironment leads to potential adverse effects on human and eco-ogical health. Interactions of TiO2 nanoparticles with exposed cellsnd organic pollutants disrupt cellular energy metabolism andncrease intracellular oxidative stress, DNA double strand breaksnd chromosomal damage, although nano-TiO2 alone shows noignificant cytotoxicity or genotoxicity [20,21]. In this case, twoarallel research routes have been proposed for solving the post-eparation issues of photocatalysts. One is concerned with theevelopment of a thin-film photoreactor by coating TiO2 film ontube or flatbed of glass, stainless steel or other materials [22–26].hin-film photoreactors have proven effective to degrade organicontaminants, whereas the coated films of TiO2 usually exhibitower photocatalytic efficiency than its nanoparticles owing to lim-ted contact area with contaminations [27]. Moreover, TiO2 filmsre difficult to be move out from the thin-film photoreactor foreactivation after long-term use. The other research route mainlyocuses on developing a packed-bed photoreactor via the loadingf nano-sized TiO2 on granular porous materials, such as zeolite,ctivated carbon and silica gel [28–30]. Amongst these, silica gel,ossessing high adsorption capacity, is the most preferable photo-atalyst carrier for optimum quantum efficiency [31,32] becauset is transparent to UV irradiation. Compared to thin-film sys-ems, photoreactors packed with TiO2-coated carriers have shownigher photocatalytic efficiency due to the lower mass transfer
imitation, larger surface area and easier separation/reactivationf photocatalysts [27,33]. Nevertheless, the problems with thesehotoreactors caused by the dense packing of small granular sil-
ca gel particles are the high resistance to solution flow and poorransmission of incident light [34]. Although these can be improved
o some extent by using larger silica gel beads as catalyst carriers35,36], the low illuminated surface area remains a bottleneck ofacked-bed photoreactors. The present study attempts to developnovel packed-bed photoreactor with high light-utilization effi-aterials 285 (2015) 398–408 399
ciency using TiO2-coated silica gel beads as photocatalysts for thedecomposition of ROPs in water.
Azo dyes, �-lactam antibiotics and organochlorine pesticidesare three common types of representative ROPs that are presentin different contaminated water bodies due to their widespreaduse in industry, agriculture and human life [37–39]. In this work,a novel cylindrical multi-column photocatalytic reactor (CMCPR)was developed for the decomposition of azo dye (methyl orange),�-lactam antibiotic (amoxicillin) and organochlorine pesticide (3-chlorophenol) in water. Electrical energy per order (EEO) was usedto calculate electrical energy consumption for the decompositionand compare with reported photoreactors. To optimize the devel-oped CMCPR, effects of different parameters, including flow rate,initial concentration and pH, on methyl orange degradation wereinvestigated. Finally, photodegradation of methyl orange usingCMCPR under the optimum conditions was repeated for five cyclesto evaluate its long-term operation performance.
2. Experimental
2.1. Chemicals and materials
Methyl orange (C14H14NaO3S) and 3-chlorophenol (ClC6H4OH,≥97.0%) were purchased from Wako Pure Chemical Industries,Ltd. (Osaka, Japan). Amoxicillin (C16H19N3O5S) was ordered fromSigma–Aldrich Co., Ltd. (Louisiana, USA). Ultrapure water obtainedfrom a SARTORIUS arium® 611 VF water purification system wasused to prepare all of the experimental solutions.
TiO2-coated silica gel beads (HQC21 [40]) obtained from ShintoV Cerax Company (Japan) were used as the photocatalysts to befixed in a self-designed cylindrical multi-column photoreactor. TheHQC21 TiO2 was prepared using the sol–gel technique and con-sisted of 100% anatase that was coated as a thin-film on the silicagel beads. The physicochemical properties of HQC21 are as follows:average diameter: 3 mm, BET surface area: 118 m2 g−1, microporevolume: 0.8 cm3 g−1, average micropore diameter: 15 nm, averagethickness of TiO2 film: 0.2 �m, average amount of TiO2:20% (w/w),zero point charge: 7.0.
2.2. Experimental apparatus
A novel cylindrical multiple-column photocatalytic reactor(CMCPR) was designed in this study (as shown in Fig. 1). The CMCPRconsists of 24 quartz glass tubes (external diameter: 10 mm, inter-nal diameter: 8 mm, length: 350 mm), each packed with 5 g ofTiO2-coated silica gel beads, and a UV black light lamp (Tokyo MetalBM-10BLB, length: 300 mm, diameter: 28 mm, power: 10 W, �max:365 nm) as the irradiation source. All of the TiO2-coated silica gelbead-packed columns were fixed on a cylindrical steel skeletoninserted axially with the UV black light lamp, and each columnwas connected with another one using silicon tube by a top-endmode. The average UV light intensity in the CMCPR is 7.5 W m−2.The external surface of the CMCPR was covered with aluminum foilto reflect UV light back into the reactor.
2.3. Experimental procedure
Six hundred (600) mL as-prepared MO, AMX and 3-CP aqueoussolutions were introduced separately into a reservoir (1000 mL)and circulated continuously through the CMCPR by a micro tubepump (MP-1000, EYELA, Japan). The circulation of each solutionwas conducted under UV irradiation and dark conditions, respec-
tively. Temperature of the whole laboratory was controlled at25 ± 1 ◦C by an air conditioner. Moreover, a mini air circulator wasalso used near the bottom of CMCPR to maintain constant localtemperature during the photocatalytic reaction. At a time interval
400 D. Li et al. / Journal of Hazardous Materials 285 (2015) 398–408
column photocatalytic reactor (CMCPR) and photocatalytic system.
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f 30 min, a 4 mL sample solution was withdrawn for analysis. Allxperiments were carried out in duplicate under the same condi-ions, and the mean values were used for further analyses. For theegeneration of TiO2-coated silica gel beads and the repetitive use ofeveloped photocatalytic reactor after each operation, the CMCPRas cleaned up by continuous irradiation and a series of washingssing deionized water.
.4. Analytic methods
The specific surface area and pore size distribution of TiO2-oated silica gel beads were determined by N2 adsorption usingBrunauer–Emmett–Teller (BET) specific surface area analyzer
SA3100, Coulter, USA). Morphological features of the TiO2-coatedilica gel bead and its silica gel carrier were analyzed by acanning electron microscope (SEM) (JSM-6330F, JEOL, Japan).bsorbance measurement of MO was performed using an UV–vispectrophotometer (SHIMADZU, UV1800) at 465 nm. The concen-ration of AMX was determined by a HPLC (JASCO, HIS-1500)quipped with a C18 reverse phase column (COSMOSIL, 5C18-AR-II,50 mm × 4.6 mm, 5 �m) at 60 ◦C. The mobile phase was a hybridolution of 60% KH2PO4 (0.025 M) in ultrapure water and 40% ace-onitrile at a flow rate of 0.50 mL min−1. Detection was performedt 228 nm with an UV/VIS detector (JASCO, UV-1570). The samePLC system was used for determining 3-CP concentration, withethanol/ultrapure water (60/40, v/v%) as the mobile phase at a
ow rate of 1.0 mL min−1. The corresponding column temperaturend detection wavelength were 40 ◦C and 275 nm, respectively.
. Results and discussion
.1. Reduction of MO, AMX and 3-CP in water by CMCPR
.1.1. Adsorption of MO, AMX and 3-CP on TiO2-coated silica geleads in CMCPR
Because the photocatalytic process is surface orientated, whichequires adsorption of organics onto the TiO2 surface [41], thedsorption capacity has a major influence on the efficiency ofhotodegradation [42]. The adsorptions of MO, AMX and 3-CP on
iO2-coated silica gel beads were evaluated separately by pass-ng each solution (25 mg L−1) through the CMCPR in the dark for00 min. As the dashed lines show in Fig. 2, the concentrations of 3-P and AMX were obviously reduced right after the solutions wereFig. 2. Percent reduction of MO, AMX and 3-CP in water using CMCPR in the dark(dashed lines) and under UV irradiation (solid lines).
introduced into CMCPR, and no significant reduction of MO wasobserved. The percent reductions of MO, AMX and 3-CP reached0.8%, 21.5% and 83.9%, respectively, in 240 min, and the values werealmost the same with up to 300-min dark reaction. The TiO2-coatedsilica gel beads were endowed with high adsorption capacity due totheir large BET surface area (118 m2 g−1) and the abundant porousstructure of the silica gel carrier (Fig. 3a). Thus, the decreased con-centration of MO, AMX and 3-CP in CMCPR in the dark can beattributed to their adsorption on TiO2-coated silica gel beads.
The adsorption capacities of these model compounds on TiO2-coated silica gel beads in CMCPR were different and in the order of3-CP > AMX > MO (Fig. 2). To interpret this observation, the effectof electrostatic interactions between the photocatalyst surface andthe organic molecules on the adsorption must be considered. Thezero point charge (pHzpc) of TiO2-coated silica gel beads was knownto be 7.0 [40], so that their surface is positively charged at pH < 7.0and negatively charged at pH > 7.0. The solution pH (7.8–7.5) of MOduring a 300-min dark reaction was higher than 7.0, implying thatthe surface of TiO2-coated silica gel beads was negatively charged.
Alternately, MO molecules are also negatively charged due to thedissociation of their sulfonic group ( SO3Na) into anions in theaqueous solution [43]. In this case, electrostatic repulsion betweenthe negatively charged surface of the TiO2-coated silica gel beads
D. Li et al. / Journal of Hazardous M
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ig. 3. SEM images of (a) silica gel beads, (b) TiO2-coated silica gel beads and (c)egenerated TiO2-coated silica gel beads.
nd MO anions resulted in the relatively low adsorption. As theolution pH (5.8–5.6) of AMX was higher than its pKa1 (2.68) butower than pKa2 (7.49), the AMX molecule is negatively chargedwing to the full dissociation of its carboxyl group ( COOH) [44].t this pH level, electrostatic attraction promoted the adsorption ofMX onto the positively charged surface of TiO2-coated silica gel
eads. The 3-CP is a weak acid with a pKa of 8.85 [45]. Althoughhe solution pH (6.7–6.2) of 3-CP in the 300-min dark reaction wasower than its pKa, the weakly acidic pH indicates that the phenolicydroxyl group ( OH) of the 3-CP molecule was partially dissoci-aterials 285 (2015) 398–408 401
ated. The negatively charged 3-CP molecule was easily adsorbedonto the positively charged surface of the TiO2-coated silica gelbeads because of the electrostatic attraction. Moreover, the adsorp-tion of the negatively charged 3-CP molecules on the TiO2-coatedsilica gel beads in return accelerated the dissociation of its phe-nolic hydroxyl group. That is helpful to the adsorption of 3-CP onthe TiO2-coated silica gel beads. In addition to the effect of electro-static interaction, the adsorption on microporous materials is alsoinfluenced by the molecular size of selected organic compounds[46]. The lower molecular size of 3-CP (5.7 Å) compared to MO(14.3 Å) and AMX (12.9 Å) (ChemBioDraw, Cambridge Soft.) is likelyto enhance the adsorption, as its molecules can enter 15-nm cavi-ties of the TiO2-coated silica gel beads more easily. Ultimately, allof these contributed to the highest adsorption of 3-CP, followed byAMX and MO, on the TiO2-coated silica gel beads in CMCPR.
3.1.2. Photocatalytic degradation of MO, AMX and 3-CP in waterusing CMCPR
The photocatalytic degradation of MO, AMX and 3-CP in waterwas performed by circulating each solution (25 mg L−1) throughCMCPR under UV irradiation (7.5 W m−2) for 300 min. The percentreduction of 3-CP reached 100% in 180 min, while that of MO andAMX was only 80.2% and 74.6% at the end of the 300-min UV irradia-tion. As reported in the literature [12,22,47], no obvious abatementof 3-CP, MO or AMX under UV irradiation in the absence of photo-catalysts was observed (data not shown). It can be seen from Fig. 3bthat, after loading nano-sized TiO2, many TiO2 clusters formedon the porous surface of the silica gel carrier, which enabled theTiO2-coated silica gel beads’ high photocatalytic activity. Therefore,reduced concentration of MO, AMX and 3-CP in CMCPR under UVirradiation was resulted by the photocatalytic degradation of thesemodel compounds.
As the solid lines shown in Fig. 2, when 3-CP was completelydecomposed after the 180-min photocatalytic reaction, there wereonly 65.7% of AMX and 54.7% of MO degraded. The percent degra-dation of MO was obviously lower than that of AMX in 240 minof photocatalysis, whereas, after 240 min, it gradually exceededthat of AMX. The inferred reason for this phenomenon is thatthe inequable photodegradation of AMX, MO and 3-CP was notonly resulted by the different adsorption on TiO2-coated silica gelbeads but also depended on their distinct molecular structures (seethe Supplementary material). Photodegradation of AMX involvescleavage of the �-lactam ring by the attack of •OH radicals onthe C N = bond (length: 1.5 Å) and a series of complex subsequentreactions to form the final products [44]. In terms of MO pho-todegradation, the destruction of its C N = bond (length: 1.3 Å) andN N bonds by the attack of •OH radicals leads to the fading ofthe dye [46]. The C N = bond of the AMX molecule is less stablethan that of the MO molecule due to its longer bond length, suchthat integrating with larger adsorption capacity contributed to thehigher percent degradation of AMX during the 240-min photo-catalysis. Accompanying with rapid decomposition of AMX, moreorganic intermediates were produced than those generated duringthe MO degradation. The intermediate products may compete withthe main substrate for consuming •OH radicals [12]. As a result, themuch intenser competing reactions in the decomposition of AMXleaded to its lower percent degradation after 240 min. On the otherhand, the basic pH of MO solution indicates that there are moreavailable OH− ions that can easily be oxidized to •OH radicals on theTiO2 surface. Due to the above-mentioned reasons, MO comparedwith AMX achieved higher percent degradation after 240-min pho-tocatalysis in the CMCPR. The photocatalytic degradation of 3-CP
occurs generally by the attack of •OH radicals on the C Cl bond,OH group or directly on the aromatic ring to form various inter-mediate products [12]. Therefore, the simple molecular structureof 3-CP, as well as the high adsorption capacity on TiO2-coated sil-
402 D. Li et al. / Journal of Hazardous Materials 285 (2015) 398–408
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ig. 4. Kinetic plots and apparent rate constant (kapp) values for the photocatalyticegradation of MO, AMX and 3-CP in water using CMCPR.
ca gel beads, was responsible for its highest percent degradationn CMCPR.
.2. Kinetic analysis of photocatalytic degradation
The photocatalytic degradation kinetics of many organic con-aminants have been analyzed using a pseudo first-order kinetic
odel [48], which can be expressed as:
n(C0
C) = kappt (1)
here C0 and C (mg L−1) are the concentrations of target organ-cs in solution at the initial time and time t (min), respectively;app (min−1) is an apparent rate constant for the photocatalyticegradation of organics. Fig. 4 illustrates the kinetic plots for pho-ocatalytic degradation of MO, AMX and 3-CP. The linear regressionoefficients (R2) of the kinetic plots were as high as 0.985, 0.927 and.995, respectively, which confirms that the photocatalytic degra-ation of MO, AMX and 3-CP using CMCPR follows the pseudorst-order mechanism. The calculated kapp values of photocatalyticegradation of MO, AMX and 3-CP were 5.3 × 10−3, 4.2 × 10−3 and0.5 × 10−3 min−1, respectively. The results are in good agreementith previous studies [43,44,49], which revealed the pseudo first-
rder mechanism for the photocatalytic degradation of MO, AMXnd 3-CP in water. Recently, the compliance of photodegradationf ROPs such as Direct Red 23 with the first order reaction has beenerified using a three-dimensional fitting kinetic model [50].
.3. Mineralization efficiency of MO, AMX and 3-CP
The detailed mechanisms for photocatalytic degradation of 3-P, MO and AMX have been reported in the literature [12,46,51].he main intermediate products of MO, AMX and 3-CP are listedn Table 1. Due to the potential hazards of yielded intermediatesTable 1), it is crucial to ensure total mineralization of organic con-aminants during their photodegradation process. Therefore, the
ineralization efficiencies of MO, AMX and 3-CP were investigatedy monitoring the changes of TOC in CMCPR.
The changes of TOC with photocatalysis of MO, AMX and 3-CPere described in Fig. 5a. The initial decrease of the TOC level dur-
ng photodegradation of AMX and 3-CP were resulted mainly byheir adsorption on TiO2-coated silica gel beads [36], whereas that
n MO degradation was caused primarily by photocatalysis. Uponontinuous UV irradiation, various intermediates are produced byeactions of photogenerated •OH radicals with pollutant moleculesn the solution. These intermediate products, in turn, can undergowater using CMCPR, and (b) the degradation and mineralization efficiency of MO,AMX and 3-CP after 300-min photocatalytic reaction in CMCPR.
further reaction with •OH radicals and be finally mineralized intoCO2, H2O and inorganic ions, leading to further decrease of the TOClevel. As shown in Fig. 5b, the mineralizations of MO, AMX and 3-CPexhibited lower efficiency than their degradations in CMCPR. Thisphenomenon was resulted by the fact that complete mineralizationof organic contaminants through the photocatalytic process usu-ally involves a series of complex intermediate reactions, therebytaking much longer than their degradation [46]. However, the dif-ferent mineralization efficiencies of MO, AMX and 3-CP stronglydepend on their distinct molecular structures (see the Supplemen-tary material). The 3-CP, which has a relatively simple molecularstructure, achieved the maximum mineralization efficiency (78.1%)after a 300-min photocatalysis in CMCPR. In comparison, a lowermineralization efficiency of MO (47.9%) followed by that AMX(35.7%) were obtained after 300-min photocatalytic reactions dueto their more complex molecular structures (see the Supplemen-tary material).
3.4. Electrical energy consumption for photocatalytic degradation
Electrical energy per order (EEO), accepted by the InternationalUnion of Pure and Applied Chemistry (IUPAC), is an importantfigure-of-merit for advanced oxidation processes on the use of elec-trical energy. In recent years, EEO has been used for calculation ofthe electrical energy consumption during the photocatalytic degra-dation of ROPs in water [35,48]. It is defined as the amount of
electrical energy in kilowatt hours (kW h) required to reduce theconcentration of a pollutant by one order of magnitude in a unitvolume of contaminated water [52]. The EEO values (usual units,
D. Li et al. / Journal of Hazardous Materials 285 (2015) 398–408 403
Table 1Main intermediate products generated during photocatalytic degradation of MO, AMX and 3-CP in water.
ROPs Intermediate products Molecular structure Toxicity information Reference
Methyl orange Benzenesulfonic acid Acute toxicity [46]
Hazardous to aquaticenvironmentCorrosive to metalsRespiratorysensitization
p-Hydroxybenzenesulfonicacid
Acute toxicity
Corrosive to metals
Dimethyl aniline Burns to skin
Phenylamine Acute toxicity
Hazardous to aquaticenvironmentMutagenic effect
4-hydroxyaniline Acute toxicity
Respiratorysensitization
p-Benzoquinone Acute toxicity
Strong irritant to skinand eyes
Amoxicillin (2R,4S)-2-((R)-((R)-2-amino-2-(4-hydroxyphenyl)acetamido)(hydroxy)methyl)-3-hydroxy-5,5-dimethylthiazolidine-4-carboxylicacid
– [51]
p-Hydroxybenzoic acid Acute toxicity
Hazardous to aquaticenvironmentStrong irritant to skinand eyes
(2S,5R,6R)-6-(hydroxyamino)-3,3-dimethyl-7-oxo-4-thia-1-azabicyclo[320]heptane-2-carboxylicacid
–
404 D. Li et al. / Journal of Hazardous Materials 285 (2015) 398–408
Table 1 (Continued)
ROPs Intermediate products Molecular structure Toxicity information Reference
(2S,4S)-2,3-dihydroxy-5,5-dimethylthiazolidine-4-carboxylicacid
–
(2S)-3-methyl-2-nitroso-3-sulfinobutanoicacid
–
3-Chlorophenol Chlorohydroquinone Acute toxicity [12]
Strong irritant to skinand eyes
Resorcinol Acute toxicity
RespiratorysensitizationStrong irritant to skinand eyes
Phenol Acute toxicity
Hazardous to aquaticenvironmentRespiratorysensitizationStrong irritant to skinand eyes
Maleic acid Acute toxicity
Corrosive to metalsStrong irritant to skinand eyes
Acetic acid Acute toxicity
Corrosive to metalsRespiratorysensitizationStrong irritant to skinand eyes
Toxicity data were obtained from material safety data sheets (MSDS) of the chemicals.
ous Materials 285 (2015) 398–408 405
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D. Li et al. / Journal of Hazard
W h m−3 order−1) for a completely flow-through photocatalyticystem can be calculated using the following equation:
EO = P
Flg(Ci/Ct)(2)
here P (kW) is the power, F (m3 h−1) is the water volume flowate in the flow-through system, and Ci and Ct (m3) are the con-entrations of target organics at the initial time and time t (h).ote that Eq. (2) implicitly assumes first-order kinetics, that is,
g (Ci/Ct) = lg (e)ln (Ci/Ct) = 0.4343 kappt, where t (min) is the reactionime in the reactor and kapp (min−1) is the first-order rate constant.onsequently, EEO can be expressed as
EO = P
0.4343 Fkappt(3)
The calculated EEO values for photocatalytic degradation ofO, AMX and 3-CP using CMCPR were 5.79 × 104, 7.31 × 104
nd 2.52 × 104 kW h m−3 order−1, respectively. The EEO valueor AMX degradation is higher than those for MO and 3-CPecomposition due to its more complex molecular struc-ure. The values of EEO were much lower than reportedhotoreactors, such as an Fe-doped TiO2 thin-film photore-ctor for MO degradation (EEO = 1.35 × 1010 kW h m−3 order−1)53], TiO2 slurry-suspension photoreactor for AMX degra-ation (EEO = 2.40 × 108 kW h m−3 order−1) [44] and TiO2/AC-P slurry-suspension photoreactor for 3-CP decomposition
EEO = 2.76 × 109 kW h m−3 order−1) [49]. The lower EEO values indi-ate clearly that the developed CMCPR is more energy-efficient forecomposition of ROPs in water.
.5. Comparison of CMCPR with reported photocatalytic reactors
Normally, degradation efficiency of different ROPs is highlyependent on the experimental conditions, such as the types ofarget organics, photocatalyst varieties, configuration of the pho-ocatalytic reactor, light irradiation source and reaction time. Inhis present work, a novel cylindrical multi-column photocatalyticeactor (CMCPR) was developed for decomposing different ROPsn water. The data given in Table 2 shows various earlier-reportedhotoreactors and CMCPR used for the degradation of MO, AMXnd 3-CP in water. In comparison with reported slurry and thin-lm photoreactors [49,53,54], the CMCPR not only achieved higheregradation efficiency but also consumed much less electricalnergy for both MO and 3-CP degradation (Table 2). In terms ofMX degradation, the reported slurry photoreactor obtained higheregradation efficiency due to its larger surface area for the adsorp-ion and reaction [44]. However, the CMCPR can effectively avoidhe tedious post-separation of nano-sized photocatalysts in thelurry-suspension system. Moreover, the corresponding EEO valueor AMX degradation using CMCPR was merely 1000th of that by theeported slurry photoreactor. In fact, the EEO values confirmed theower electrical energy consumption of CMCPR for photocatalyticegradation of different ROPs. Due to its high photocatalytic effi-iency and low energy consumption, the developed CMCPR seemso be a good option for decomposing ROPs in water.
.6. Effects of different operational parameters on photocatalyticfficiency of CMCPR
Using MO dye as the model compound of ROPs, a series of single-actor experiments were performed to investigate the effects of
ifferent operational parameters on photocatalytic efficiency ofMCPR. Several operational parameters of CMCPR including flowate, initial concentration and initial pH for the photocatalyticegradation of MO were examined.Fig. 6. Effect of flow rate on the photocatalytic degradation of MO in water usingCMCPR.
3.6.1. Effect of flow rateFlow rate plays an important role in the complete decompo-
sition of ROPs when using any continuous-flow photocatalyticreactor for water purification. The photocatalytic degradation ofMO (10 mg L−1) was performed using CMCPR with different flowrates, such as 3, 6, 9, 12 and 15 mL min−1. As seen from Fig. 6,the degradation efficiency of MO rose with the increase of flowrate. After 180 min of UV irradiation, the highest percent degra-dation (96.5%) and maximum kapp value (18.1 × 10−3 min−1) wereachieved with the flow rate of 15 mL min−1. Flow rate affects pho-tocatalytic reaction by changing the convective mass transfer andresidence time in the reactor. Convective mass transfer is the morepredominant factor that influences the photocatalytic reaction atlow flow rate, while residence time is the primary limiting factorin the case of high flow rate [35]. The enhancement of degrada-tion efficiency with increasing flow rate indicated that convectivemass transfer is the primary limiting factor in MO degradation usingCMCPR. Increasing flow rate could enhance the diffusion of dyemolecules and oxygen to the TiO2 surface thereby improving thephotocatalytic reaction [55]. A similar result has been reported byMerabet et al. [56], who used a circulating upflow photoreactor forindole degradation.
3.6.2. Effect of initial concentrationThe effect of initial MO concentration on the photocatalytic effi-
ciency of CMCPR was investigated in the range of 5–25 mg L−1 witha flow rate of 15 mL min−1. The results are illustrated in Fig. 7. Thepercent degradation of MO decreased from 98.2% to 55.0% after180 min of photocatalysis when increasing the initial concentrationfrom 5 to 25 mg L−1. The corresponding kapp value showed the samevariation trend and achieved the highest value (21.6 × 10−3 min−1)at the initial MO concentration, which was as low as 5 mg L−1.Because the amount of photocatalysts and the light intensity areconstant in CMCPR, the photogenerated oxidative species (h+, •OHand O2
•−) remain practically the same. Although increasing the ini-tial concentration enhances the adsorbed amount of MO on theactive surface of photocatalysts, the efficiency of photocatalyticdegradation decreases due to the lower ratio of oxidative speciesto dye molecules [56]. In addition, the reduction of incident pho-ton flux caused by the increasing initial concentration leads toless active photocatalytic site creation on the catalyst surface [35],resulting in lower photocatalytic degradation.
3.6.3. Effect of pH valueIn heterogeneous photocatalysis, pH is one of the major fac-
tors that influence the rate of the photocatalytic process [57]. The
406 D. Li et al. / Journal of Hazardous Materials 285 (2015) 398–408
Table 2Comparison of the developed CMCPR with reported photoreactors for the degradation of MO, AMX and 3-CP in water.
ROPs Concentration Catalyst Reactor type Power input Time Degradation kapp EEO Reference(mg L−1) (W) (min) (%) (×10−3 min−1) (kWh m−3 order−1)
MO 10 CdS–TiO2–Au Slurry 400 300 98.0 12.0 4.61 × 1010 [54]10 Fe-doped TiO2 Thin-film 40 180 35.3 3.4 1.35 × 1010 [53]10 TiO2-coated beads Immobilized 10 180 96.5 18.1 2.83 × 104 This study
AMX 104 Anatase TiO2 Slurry 6 300 70.9 7.0 2.4 × 108 [44]100 TiO2-coated beads Immobilized 10 300 57.7 2.6 1.97 × 105 This study
3-CP 40 TiO2/AC-HP Slurry 100 325 TiO2-coated beads Immobilized 10 1
0 30 60 90 120 150 180
0.0
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0
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0.9940.0045
5
10
15
20
25
0.986
0.972
0.956
0.947
0.0060
0.0090
0.0181
0.0216
R2k
app(min
-1)C
0(mg L
-1)
Fw
ewpwMNcbd(sa
F
ig. 7. Effect of initial concentration on the photocatalytic degradation of MO inater using CMCPR.
ffect of pH on the photocatalytic degradation of MO using CMCPRas examined at different pH values (i.e., 3, 5, 7 and 9). In thisresent experiment, the initial MO concentration and flow rateere 5 mg L−1 and 15 mL min−1, respectively. The initial pH of theO solution was adjusted by adding an appropriate amount ofaOH or HCl solution. As expressed in Fig. 8, the degradation effi-iency of MO increases with the decrease of initial pH, and theest result was obtained in the acidic solution. The percent degra-ations of MO after 90 min of photocatalytic reaction were 68.1%pH 9), 76.4% (pH 7), 85.6% (pH 5) and 97.3% (pH 3). The corre-
ponding kapp values were 18.7 × 10−3, 22.9 × 10−3, 27.3 × 10−3nd 40.0 × 10−3 min−1, respectively.
0 30 60 90 120 150 180
0.0
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3
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7
9 0.958
0.960
0.950
0.950
0.0187
0.0229
0.0273
0.0400
R2k
app(min
-1)pH value
ig. 8. Effect of pH on the photocatalytic degradation of MO in water using CMCPR.
00 38.7 20.0 2.76 × 109 [49]20 100 30.5 2.52 × 104 This study
As mentioned in Section 3.1.1 of this work, the surface ofthe TiO2-coated silica gel beads was positively charged in acidicmedium, whereas in basic medium it was negatively charged. Atacidic pH, the adsorption of MO on TiO2-coated silica gel beadswas facilitated by electrostatic attraction between the positivelycharged photocatalyst surface and MO anions. At alkaline pH, theelectrostatic repulsion between the surface of TiO2-coated silicagel beads and MO anions leads to a lower adsorption. The decol-orization rate of MO in CMCPR under dark conditions decreasesfrom 19.3% to 3.3% with increase of the initial pH from 3 to 9. Con-sequently, pH can influence the adsorption of MO on TiO2-coatedsilica gel beads via shifting the electrical potential of the photocata-lyst surface, thereby affecting the overall photocatalytic efficiency.Acidic pH is more favorable for the photocatalytic degradation ofMO using CMCPR due to higher adsorption of dye molecules onTiO2-coated silica gel beads.
In this section, effects of single factor including flow rate, ini-tial concentration and initial pH on the photocatalytic efficiency ofCMCPR were investigated. The experimental data were statisticallyanalyzed by one-way ANOVA, the p values of flow rate, initial con-centration and initial pH were 0.004560, 0.000760 and 0.000002,respectively. The p values are lower than 0.005, indicating all ofthe three operational parameters have significant effects on photo-catalytic efficiency of CMCPR for MO degradation. Results showedthat the higher flow rate (15 mL min−1), lower initial concentra-tion (5 mg L−1) and acidic condition (pH 3) were more favorable forthe photocatalytic degradation of MO using CMCPR. Nonetheless,response surface methodology (RSM) which has been used for oper-ational optimization of advanced oxidation process [58] should beemployed to optimize the developed CMCPR in the future research.
3.7. Repetitive operation performance of CMCPR
Repetitive operation performance is one of the most impor-tant parameters in the application of any developed photocatalyticreactor for decomposing ROPs in water [35,59]. The photocat-alytic degradation of 10 mg L−1 MO (600 mL) was repeated in upto five cycles to evaluate the repetitive operation performance ofCMCPR. As shown in Fig. 9, more than 97.0% of MO was decomposedafter 180 min of photocatalytic reaction in each operation. The fivecycles gave a relative standard deviation of 0.72%, indicating thatthe developed CMCPR has good repetitive operation performance.The surface morphology of TiO2-coated silica gel beads after fivecycles was nearly unchanged when compared with its originalform (Fig. 3b and c), implying that the immobilized photocatalystshave high stability. That contributed to the good reproducibilityof CMCPR for MO degradation. The good repetitive operation per-formance, as well as high photocatalytic efficiency and low energyconsumption, makes the CMCPR a promising alternative for the
degradation of ROPs in water. Photoreactor scale-up is a major chal-lenge in photochemical reaction engineering which using advancedoxidation technologies for the degradation of ROPs in water [60].Prior to the practical application of developed photoreactor for the
D. Li et al. / Journal of Hazardous M
0 3 6 9 12 15
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ig. 9. Five repetitive operations of the developed CMCPR for photocatalytic degra-ation of MO in water.
urification of ROPs contaminated water, the scale up of CMCPRhould be systematically investigated in the future research.
. Conclusions
The novel cylindrical multi-column photocatalytic reactorCMCPR) developed in this study has been successfully applied forhe degradation of methyl orange, amoxicillin and 3-chlorophenoln water. Compared with MO and AMX, the 3-CP obtained theighest photodegradation (100%) and mineralization (78.1%) after00-min photocatalytic reaction because of its higher adsorptionapacity and simpler molecular structure. Electrical energy con-umption for photocatalytic degradation of MO, AMX and 3-CPsing CMCPR were less than 1000th of those by reported photoreac-ors. The higher flow rate (15 mL min−1), lower initial concentration5 mg L−1) and acidic condition (pH 3) were more favorable forhe photocatalytic degradation of MO using CMCPR. Five repetitiveperations of CMCPR achieved more than 97.0% photodegradationf MO in each cycle and gave a relative standard deviation of 0.72%.he developed CMCPR shows high efficiency, low energy consump-ion and good repetitive operation performance for photocatalyticegradation of 3-CP, AMX and MO in water. Further studies on thetilization and scale up of CMCPR for the purification of practicalOPs contaminated water are under progress.
cknowledgements
This work was supported by Grant-in-Aid for Exploratoryesearch26670901 and Grant-in-Aid for Scientific ResearchA)22248025 from the Japan Society for the Promotion of ScienceJSPS).
ppendix A. Supplementary data
Supplementary data associated with this arti-le can be found, in the online version, atttp://dx.doi.org/10.1016/j.jhazmat.2014.12.024.
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