Chemical Engineering Journal 249 (2014) 160–166

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Chemical Engineering Journal

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Effects of TiO2 nanostructure and operating parameters on optimizedwater disinfection processes: A comparative study

http://dx.doi.org/10.1016/j.cej.2014.03.0791385-8947/� 2014 Elsevier B.V. All rights reserved.

⇑ Corresponding author. Tel.: +65 67906273; fax: +65 67910676.E-mail address: ddsun@ntu.edu.sg (D.D. Sun).

Darren Delai Sun a,⇑, You Wu b, Peng Gao a

a School of Civil and Environmental Engineering, Nanyang Technological University, Singapore 639798, Singaporeb Raffles Institution, 1 Raffles Institution Lane, Singapore 575954, Singapore

h i g h l i g h t s

� Hierarchical porous structure of 3DD-TiO2 did greatly improve photocatalytic activity.� Without and with UV illumination, the removal efficiency of E. coli by 3DD-TiO2 are 8.7% and 33.9%.� Optimum 3DD-TiO2 concentration is around 100 mg/L.� Humic acid and anions (SO4

2�) exhibit an inhibitory effect on water disinfection process.� Cations (Ca2+) enhance the disinfection rate.

a r t i c l e i n f o

Article history:Received 1 July 2013Received in revised form 18 March 2014Accepted 21 March 2014Available online 1 April 2014

Keywords:DisinfectionKineticsNanoribbonPhotocatalysisTiO2

a b s t r a c t

This paper reports the performance of bacterial disinfection using 3D dendritic titanium dioxide withnanoribbon structures (3DD-TiO2), which is newly developed by the researchers. Characterization of3DD-TiO2 using SEM and TEM shows that the diameter of nanoribbons is nearly 18 nm only, resultingin an aspect ratio of more than 50. The hierarchical porous structure of the 3DD-TiO2 would greatly favorthe improvement of photocatalytic activity via enlarging the specific surface area. In comparison with thesame-sized microspheres and commercial Degussa P25, which are both spherically structured, 3DD-TiO2

displays a consistently better performance under the tested environment. Without UV illumination, theremoval efficiency of Escherichia coli by 3DD-TiO2 is 6.4% and 8.7% higher than that of microspheres andP25, respectively. Under UV radiation, the disinfection kinetic constant k of the disinfection reaction using3DD-TiO2 is 23.2% and 33.9% higher than that of using microspheres and P25, respectively. A series ofoptimization studies have been carried out to identify the optimum operating parameters of a photocat-alytic process using 3DD-TiO2. The results tend to suggest: (a) optimum catalyst concentration is around100 mg/L; (b) the optimum temperature for bacteria inactivation ranges between 10 �C and 20 �C; (c)presence of humic acid and anions (SO4

2�) exhibits an inhibitory effect on water disinfection process;and (d) cations (Ca2+) enhance the disinfection rate. These optimum operating parameters could bedirectly referenced for full-scale applications in future.

� 2014 Elsevier B.V. All rights reserved.

1. Introduction

The ever increasing standards for providing high-quality drink-ing water have put more stress on the removal of contaminantsincluding pathogenic microorganisms [1]. Conventional watertreatment methods are typically chemical- and energy-intensiveand incur high capital costs to build infrastructure for large-scaletreatment [2]. Moreover, intensive chemical processes such asusing ammonia, hydrochloric acid, sodium hydroxide, ozone,

permanganate and chlorine compounds generate treatment resid-uals that cause pollution and contamination of freshwater sources[3]. In recent years, a great amount of effort has been channeledinto the research of photocatalytic disinfection of drinking water,which is considered a new method of cheap and low-energy disin-fection. In particular, titanium dioxide (TiO2) has been identified asone of the most effective photocatalysts for disinfection due to avariety of factors, including low cost, non-toxicity, chemical stabil-ity and high effectiveness under ultraviolet light.

TiO2 photocatalysts work via a strong oxidative process. Uponabsorbing photons with energy equivalent or larger amount of en-ergy than the band gap, photocatalysts form electron–hole pairs.

D.D. Sun et al. / Chemical Engineering Journal 249 (2014) 160–166 161

For TiO2, the band-gap (>3.0 eV for rutile phase and >3.2 eV foranatase phase) can be overcome with energy from photons fromUV radiation which has a wavelength ranging from 10 nm to400 nm, corresponding to photon energies from 3 eV to 124 eV.The electrons from the valence band will be excited into the con-duction band, forming electron–hole (e� h+) pairs in the TiO2 nano-particle. These charge carriers may either recombine or react withelectron donors and acceptors adsorbed onto the TiO2 surface. Po-sitive holes in the valence band interact with water molecules togenerate extremely reactive hydroxyl radicals which will theninactivate pathogens during the process of photocatalytic waterdisinfection [4]. The efficiency of the photocatalyst is dependenton the relative rate of recombination and reaction with electrondonors and acceptors [5,6].

The photocatalytic activity of TiO2 depends greatly on the size,shape and surface structural morphology. Commercially availableDegussa P25 TiO2 has become a standard for scientific research be-cause of a relatively well-defined nature (typically 70:30 anatase:rutile mixture) and superior photocatalytic activity than mostother commercial TiO2. More recently, nanomaterials have beenfound to possess superior biocidal properties due to their high spe-cific surface area as well as their special shape and morphology [3].This is supported by a recent study on antibacterial activity of sin-gle-walled carbon nanotubes (CNTs), which showed that the rate ofcell death was directly related to the diameter of CNTs since directcontact with CNTs causes membrane damage and thus loss of cellviability [7]. Hence, it is reasonable to assume that the TiO2 micro-spheres assembled by nanoribbons with small diameter may havehigh toxicity to bacteria cells as well. TiO2 microspheres have beensynthesized through various methodologies, including solvother-mal and hydrothermal reaction [8–11], electrochemical process[12] and template assisted calcinations method [13]. Liao et al.synthesized hierarchical TiO2 spheres consisting of nanorods andnanoparticles by a solvothermal method [8]; Tang and co-workersprepared rutile TiO2 microspheres with 100% exposed reactive(111) facets via a hydrothermal approach using TiCl3 as the pre-cursor [9]; Chen’s group reported that the synthesis of TiO2 micro-spheres with different nanostructures by electrochemical sparkdischarge spallation process [12]; and Bai et al. prepared carboncoated TiO2 nanorod microspheres through a facile calcinationmethod [13].

This work is aimed at achieving high efficiency bacterial inacti-vation using a novel 3D dendritic TiO2 (3DD-TiO2) microspherewith superfine nanoribbons, which have both of shape-dependentantibacterial activity and photocatalytic disinfection property. Theperformance of 3DD-TiO2, in term of Escherichia coli removal effi-ciency, is assessed when it is applied into the photocatalytic disin-fection process. The effects of temperature, organic and inorganicmatters on 3DD-TiO2 performance are also evaluated under vari-ous controlled test environments. This work could provide a prom-ising alternative nanomaterial for water disinfection.

2. Materials and methods

2.1. TiO2 nanomaterial synthesis

The 3DD-TiO2 nanostructures were synthesized through ahydrothermal process, according to the reported paper [14].Briefly, proper amount of titanium isoproxide (TTIP, >98%, Sig-ma–Aldrich) and HCl was mixed in bottle A for 30 min of stirring.After that, a certain amount of cetyltrimethyl ammonium bromide(CTAB, Sigma–Aldrich) was mixed with DI water in bottle B. After30 min, bottle B was added into bottle A and stirred for 1 h to formthe aqueous TTIP solution. After stirring, a certain amount of TTIPsolution and certain amount of ethylene glycol (EG, Sigma–Aldrich)

were put into a 125 mL autoclave. The total volume of reactionsolution was kept at 81 mL. The molar ratios of the starting mate-rials of TTIP aqueous solution were 100H2-

O:7HCl:0.03CTAB:0.05TTIP. The molar ratio of TTIP aqueoussolution to EG was 1:2. The hydrothermal reaction was carriedout at 150 �C for 20 h. The product was collected, and washed withethanol for 3 times, followed by drying at 80 �C for 24 h. The prod-uct was calcined at 550 �C for 2 h to remove any remaining organ-ics. Big TiO2 microspheres were synthesized as a control materialwith the molar ratio of TTIP aqueous solution to EG as 1:0. TiO2

P25 was purchased from Degussa Company, also used as a controlmaterial.

2.2. Characterization of TiO2 nanostructures

The morphology and size of the nanostructures were deter-mined by transmission electron microscopy (TEM) (JEOL 2010-Hmicroscope) operating at 200 kV, and scanning electron micros-copy (SEM) (JEOL 6340). The crystalline structures were obtainedby X-ray powder diffraction (XRD) patterns measured using Shi-madzu XRD-6000 X-ray diffractometer with monochromatedhigh-intensity Cu Ka irradiation (k = 1.5418 Å), operated at 40 kVand 30 mA.

2.3. Bacterial growth and exposure

E. coli (K12) was chosen as the model bacteria for antibacterialactivity tests. E.coli was cultivated in Luria–Bertani (LB) nutrientsolution at 37 �C for 18 h to get the exponential growth phase.The cells were harvested by centrifugation and washed with salinesolution (0.9% NaCl) to remove residual macromolecules. All glassapparatuses and solutions used in the experiments were auto-claved at 121 �C for 20 min to ensure sterility.

2.4. Experimental design

Bacterial cells were dissolved in saline solution containing100 lg/mL of TiO2 samples (sonicated for 1 h before use), with a fi-nal cell concentration around 107 cfu/mL. Disinfection reactionswere carried out in 200 mL glass beakers illuminated by a 5 WUV-A lamp of intensity 600 lW/cm2 (Jiangmen Shijin LightingCo., Ltd., China) and homogenized by magnetic stirrers, as shownin Fig. S1. After the disinfection reactions, samples were dilutedwith a gradient method and then applied uniformly on three LBculture medium plates per gradient solution. These plates wereincubated at 37 �C for 24 h. The results were obtained using thecolony counting method of E. coli growth on agar plates with serialdilution of each sample. The colony forming units were countedand compared with control plates to calculate percentage of cellviability (C/C0).

2.4.1. Effect of catalyst concentrationFirstly, prior to any experimentation, an optimal concentration

for bacterial disinfection for the photoreactor system must bedetermined to achieve the best disinfection efficiency. A compari-son was made between the efficiency of various 3DD-TiO2 concen-trations under UV radiation for 40 min. 3DD-TiO2 slurry wasintroduced to a 30 mL aqueous NaCl suspension of 107 cpu/mL ofE. coli at various concentrations.

2.4.2. Effect of nanostructureThe efficiency of disinfection activity of Degussa P25, micro-

spheres and 3DD-TiO2 were compared using disinfection. Basedon the optimal catalyst concentration, each type of TiO2 photocat-alyst was introduced at a fixed concentration of 100 mg/L into the

162 D.D. Sun et al. / Chemical Engineering Journal 249 (2014) 160–166

reaction slurry. Experiments were conducted both with and with-out UV radiation.

2.4.3. Effect of temperatureTo simulate different temperature conditions, the reaction

beakers were maintained at various temperatures ranging from10 �C to 60 �C using water baths. The same experiment was con-ducted for 3DD-TiO2 and P25 for comparison.

2.4.4. Effect of organic matterHumic acid (1415-93-6, Sigma Aldrich) was added into the

solutions to simulate the presence of organic matter in naturalwater. Humic acid was added at a concentration of 10 ppm intothe solution. The same experiment was conducted for 3DD-TiO2

and P25 for comparison.

2.4.5. Effect of IonsCa2+ and SO4

2�, which are predominant ions present in naturalwaters, were added into the solutions to simulate natural condi-tions. A chemical analysis of surface water [15] found Ca2+ andSO4

2� present at concentrations 24.85 mg/L and 24 mg/L respec-tively. CaCl2 and Na2SO4 were used as sources of Ca2+ and SO4

2�

ions.

3. Results

3.1. Characterization

As observed from Fig. 1a and b, the dimension of each 3DD-TiO2

was around 2–3 lm, including strands of 1D nanoribbons grownon the central TiO2 sphere. Each strand of nanoribbon has a lengthranging from 500 nm to 1.5 lm and diameter of roughly 25 nm.Further characterization was done using TEM to show images ofan individual 3DD-TiO2 (Fig. 1f) and the image of one strand ofTiO2 nanoribbon at the edge of the spheres (Fig. 1c). The overalldimensions of the 3DD-TiO2 are the same as that observed bySEM images in Fig. 1a and b, however, the exact diameter of thenanoribbon is nearly 18 nm, which is smaller than that observedby SEM due to the metal coating in measurement. The aspect ratioof the TiO2 nanoribbon is larger than 50. The high aspect ratio of3DD-TiO2 has three positive effects on the high disinfection activ-ity. Firstly, high aspect ratio of 3DD-TiO2 originated from the sharp

Fig. 1. High-magnification SEM and TEM images of TiO2 particles: (a) SEM image of 3DDimage of an enlarged part of one strand of TiO2 nanoribbon, (d) SEM image of TiO2 mhierarchical 3DD-TiO2.

ribbons can destroy the cell walls of E. coli when they directly con-tact with the bacteria. In addition, high aspect ratio of 3DD-TiO2

provides more reactive site during disinfection activity. Further-more, high aspect ratio of 3DD-TiO2 results in fast electron transferrate. Hence, the high aspect ratio of 3DD-TiO2 causes high photo-catalytic disinfection activity [16,17]. Typical hierarchical struc-tures can be observed for the 3DD-TiO2 in which: the distancesbetween different 3DD-TiO2 are in the range of micrometer scale,while the distances between different strands of TiO2 nanoribbonsare in the range of nanometer scale. Fig. S2 shows that the BET spe-cific surface areas of 3DD-TiO2 and smooth microspheres are66.76 m2/g and 27.65 m2/g, respectively, while the surface area ofP25 is about 45 m2/g) [9]. In addition, the inset of Fig. S2a shows3DD-TiO2 has a typical mesoporous microstructure with a poresize ranging from 1 nm to 10 nm. The hierarchical porous structureof the 3DD-TiO2 would greatly favor the improvement of photocat-alytic activity via enlarging the specific surface area, allowing morelight reflection and multiple scattering inside the interior, and pro-moting the electron transfer along the long 1D nanoribbon [18,19].Meanwhile, Fig. 1d shows the morphology of TiO2 microsphereswith size of 2–3 lm, which is similar with the overall dimensionsof our 3DD-TiO2, with smooth surfaces. Fig. 1e shows the typicalimage of Degussa P25. The individual particle size of P25 is around25 nm, but with certain aggregation. These two kinds of TiO2 areselected as control samples to monitor the shape-effect antibacte-rial activity of 3DD-TiO2.

X-ray diffraction was used to measure the crystal phase of the3DD-TiO2. From the XRD patterns in Fig. 2, the 3DD-TiO2 has astandard rutile structure (JCPDS file No. 21-1276) [20,21]. Thiscan be attributed to the acidic precursor reaction solution duringsynthesis of the nanomaterial [22,23].

3.2. Disinfection efficiency of 3DD-TiO2

3.2.1. Empirical disinfection mechanism and kineticsThe kinetics of the reaction was based on a pseudo-first-order

reaction kinetics as derived from the classical disinfection modelof the Chick–Watson equation [24]

lnNN0

� �¼ �K½N�nt ð1Þ

-TiO2, (b) SEM image of strands of 1D nanoribbons grown on a 3DD-TiO2, (c) TEMicrospheres, (e) SEM image of Degussa P25, and (f) TEM image of an individual

Fig. 2. XRD patterns of the synthesized hierarchical 3DD-TiO2 nanoribbon spheres.

Fig. 3. UV spectrophotometry results of treatment using various concentrations of3DD-TiO2.

Fig. 4. Comparison of results of treatment using P25, microsphere and 3DD-TiO2

without UV radiation.

D.D. Sun et al. / Chemical Engineering Journal 249 (2014) 160–166 163

where NN0

is the reduction in the concentration of bacteria, K is thedisinfection kinetic constant, N is the concentration of the disinfect-ing agent at time t, and n is the reaction order. It is noted that undera constant supplied irradiation, the concentration of the disinfectingagent can be considered to be constant with time for a fixed catalystconcentration and photoreactor lamp setup. Henceforth, the disin-fection kinetic constant, K, in Eq. (1) can be simplified into a first-or-der disinfection kinetic constant, k, as below.

lnNN0

� �¼ �kt ð2Þ

All first-order disinfection kinetic constants, k, presented in thismanuscript will be calculated by application of the classic Chick–Watson disinfection model.

Fig. 5. Comparison of results of treatment using P25, microsphere and 3DD-TiO2

under UV radiation. Inset depicts linearized plot of ln [survival fraction] versus time,according to the Chick–Watson model.

3.2.2. Effect of catalyst concentrationMalato et al. [3] reported that during photocatalytic disinfection

processes, initial bacterial inactivation rate increases when catalystconcentration of aqueous TiO2 slurry increases up to an optimalcatalyst concentration, whereby the disinfection rate becomesindependent of the slurry concentration due to light screening ef-fect. This optimal concentration has been found to depend on theirradiation system and the photoreactor geometry. Typically, fora reactor path length of several centimeters, as is the case for thispaper, the optimal catalyst concentration varies in the range ofseveral hundred milligrams per litre.

Fig. 3 shows the E. coli count at 10 min intervals using differentcatalyst concentrations. The results were obtained using the colonycounting method of E. coli growth on agar plates with serial dilu-tion of each sample. The efficiency at 100 mg/L is higher than thatat other concentrations, as evident from the faster decrease in cellviability. This could be attributed to the reduced percentage trans-mittance of UV radiation through the solution with an increase incatalyst concentration. As the amount of TiO2 increases, the in-crease in photon-catalyst interaction is reduced, causing the rateof disinfection activity to approach a maximum value. As such,we can consider 100 mg/L 3DD-TiO2 to be the optimum catalystconcentration.

3.2.3. Effect of nanostructureAs shown from Figs. 4 and 5, nanoribbon spheres are consis-

tently more efficient in bacteria disinfection both with and withoutUV radiation. Fig. 4 shows that about 80% of E. coli cells have been

killed by 3DD-TiO2, which is around 9% and 7% higher than that ofP25 and smooth microsphere, without light irradiation. The higherdisinfection activity of 3DD-TiO2 can be attributed to the sharp andlong nanoribbons which can destroy the walls of E. coli cells. Thisshows that it is intrinsically more efficient due to its novel mor-phology. Under UV radiation, k = 0.0494 min�1 using 3DD-TiO2

compared to 0.0401 min�1 and 0.0369 min�1 using microspheres

164 D.D. Sun et al. / Chemical Engineering Journal 249 (2014) 160–166

and P25 respectively (Table 1). The 3DD-TiO2 particles possessfavorable photocatalytic characteristics compared to microspheresand commercial P25 due to the higher surface area to volume ratio.The constituent nanoribbons units attached to the central TiO2

sphere allow for a much higher fraction of atoms to be located atthe surface of the particle, hence there is a significantly faster rateof electron transfer, which improves the disinfection activity.

Fig. 6. Results of treatment using P25 under various temperatures.

3.2.4. Effect of temperatureThe temperature of the photocatalytic system has a direct im-

pact on the rate of adsorption and desorption of materials, how-ever, current literature on the effect of temperature onphotocatalytic activity is lacking. As evident from Figs. 6 and 7,the rate of bacteria inactivation is fastest at 10 �C for both P25and 3DD-TiO2. There is a slight decrease in the rate from 20 �C to30 �C to 45 �C. The rate at 5 �C approximates that of 20 �C, however,due to electrical costs incurred in lowering the temperature, treat-ment at 20 �C is preferred over at 5 �C. At 60 �C, many cells diequickly from the high temperature, thus giving rise to the low cellviability especially at the start. A controlled experiment conductedat 60 �C without any TiO2 shows similarly high rates of cell death,hence it can be deduced that the low bacteria viability is largelydue to the high temperature instead of TiO2 photocatalytic activity.

The optimum range can thus be deduced to be from 10 �C to45 �C. Malato et al. reported that this could be attributed to thehigher activation energy required at low temperatures such thatdesorption of the final product becomes the rate limiting step.On the other hand, at higher temperatures, the exothermic stepof adsorption of reactants is not favored, limiting the rate of reac-tion [3].

Fig. 7. Results of treatment using 3DD-TiO2 under various temperatures.

3.2.5. Effect of organic matterHumic substances are generated from the decomposition of or-

ganic compounds and act as precursors to harmful disinfection by-products during bacteria disinfection by chlorination. They havealso been reported to encourage regrowth of bacteria in the ab-sence of residual disinfectant and are thus likely to have significantimpact on the disinfection properties of TiO2 [25]. As evident fromFig. 8, the efficiency of bacteria disinfection is greatly decreasedwith the introduction of the acid for both P25 and 3DD-TiO2. Thedisinfection kinetic constant 3DD-TiO2 drops from 0.0494 min�1

to 0.0438 min�1 in the presence of humic acid, causing an 11.3%decrease (Table 1). However, it should be noted that 3DD-TiO2 stillremains more efficient than P25 regardless of the addition of hu-mic acid.

Ng et al. [26] hypothesized that the effect of humic acid couldbe due to the competition for oxidizing species between humicacid and E. coli cells. Moreover, humic acid molecules have a largesurface area that easily blocks off and intercepts UV radiation inthe aqueous solution. They also reported that there is a decreasedamount of radiation available for activating TiO2 due to competi-tion from humic acid molecules for adsorption of UV photons.Some of the reactive oxygen species (ROS) generated will also be

Table 1Apparent first-order rate constants k of E. coli under different experimentalconditions.

Experimental condition k (min�1)

3DD-TiO2 (100 mg/L) 0.0494P25 (100 mg/L) 0.0369Microsphere (100 mg/L) 0.04013DD-TiO2 + humic acid 0.04283DD-TiO2 + Ca2+ 0.05333DD-TiO2 + SO4

2� 0.0427

Fig. 8. Results of treatment using P25 and 3DD-TiO2, with and without addition ofhumic acid. Inset depicts linearized plot of ln [survival fraction] versus time,according to the Chick–Watson model.

used for degradation of humic acid instead of deactivating E. colicells, further reducing the photocatalyst efficiency. ROS are gener-ated when TiO2 is exposed to UV radiation; accumulation of ROSmay lead to oxidative stress, which causes deactivation of cells. Acombination of the above factors is likely to cause the significantreduction in efficiency of TiO2 in bacteria disinfection.

Fig. 9. Results of treatment using 3DD-TiO2 with ions. Inset depicts linearized plotof ln [survival fraction] versus time, according to the Chick–Watson model.

D.D. Sun et al. / Chemical Engineering Journal 249 (2014) 160–166 165

3.2.6. Effect of ionsBased on Fig. 9, Ca2+ exerts a positive effect on photocatalytic

disinfection, as shown from the exponential decline in cell viabilityover time. Ca2+ is capable of enhancing TiO2 adsorption onto bacte-ria cells and disinfection of E. coli [27,28]. TiO2 has a point of zerocharge of 6.25, [29] therefore under neutral pH conditions, surfacecharge of the catalyst is negative. According to Li and McLandsbor-ough [30] the cell surface charge of E. coli is negative at pH 7, hencethe presence of cations adsorbed on the catalyst weakens its over-all negative charge and reduces the repulsion of E. coli. Ca2+ ionbridging [28] with TiO2 enhances surface charge attraction ofE. coli cells to the catalyst, which further promotes adsorption, thusleading to an increase in disinfection rate. K showed a 7.9% increasefrom 0.0494 min�1 to 0.0533 min�1 (Table 1).

In contrast, SO42� was observed to inhibit the photocatalytic

activity, leading to a 13.6% decrease in the value of k from0.0494 min�1 to 0.0427 min�1 (Table 1). It has been reported thatSO4

2� can act as scavengers of h+ and �OH to produce SO4� radicals

based on the following reactions [31]:

SO2�4 þ hþ ! SO�4 ð3Þ

SO2�4 þ �OH! SO�4 þ OH� ð4Þ

SO4�, is also a strong oxidative radical, which in theory assist in

photooxidation of E. coli. However, it is less reactive than the scav-enged h+ and �OH radicals, [31] thus it has a net inhibitive effect onthe disinfection kinetic.

4. Discussion

3DD-TiO2 has substantially higher photocatalytic activity com-pared to TiO2 microspheres and commercially available P25regardless of the physical conditions of the reactor. This can beattributed to the small size of the nanoribbons, which allows for di-rect interaction between TiO2 and bacteria. Further bacteria celldeath under UV irradiation is caused by damage of the cell walldue to attack by reactive hydroxyl radicals which are formed uponphotoexcitation of TiO2 molecules. Leakage of cell content occurs,leading to disruption of essential cell function such as respiratoryactivity.

This improved disinfection ability reduces the required concen-tration of reaction slurry and hence the amount of TiO2 needed formore effective water treatment. Costs of TiO2 production can becut significantly, further cementing 3DD-TiO2 as the most promis-ing disinfectant and which is feasible in commercial application.For cost effectiveness and minimal electricity costs, TiO2 concen-tration of 100 mg/L should be used and the treatment should beconducted between 10 �C and 20 �C. Concentrations higher than

100 mg/L will yield minimal increases in efficiency. However, re-sults will vary according to the photoreaction system pathway,as well as the amount of organic matter present in the water.

With the use of any nano-particles, there is a risk of nanoparti-cle contamination if particles are not properly recovered by post-treatment [32]. The use of dendritic nanoribbons, however, allowsthe catalyst to be easily separated from the solution due to rela-tively large size of the central micro-sphere. Fig. S3 shows thatmost of 3DD-TiO2 settled at the bottom of the centrifuge tube(right side of Fig. S3) after 30 min due to its relatively large size,while P25 could not be easily separated (left side of Fig. S3). Bey-doun et al. [33] attempted the use of magnetism to help retain orrecover the catalyst by attachment to magnetic particles. The addi-tion of foreign material may decrease the disinfection efficiencycompared to pure slurry reactors but contribute to better practical-ity overall [4]. Similar methods may be applied to the dendriticparticles to facilitate post-treatment particle separation. With this,the dentritic particle retains the properties of a nano-particlewhich has superior disinfection properties, yet can be easily re-trieved, preventing contamination.

There exist many other factors affecting the efficiency of waterdisinfection using 3DD-TiO2, including radiant flux, catalyst parti-cle size, concentration of oxygen in aqueous solution and pH. Toeffectively harness the full potential of the novel dendritic nanorib-bon structure, the combined effect of these factors must be fullyinvestigated. The interdependency of these factors makes it allthe more crucial to study the exact influence each exerts on theother. The size of aggregate nanoparticles in aqueous solutions,for instance, is affected by the pH, which then determines the capa-bility of the reaction slurry in transmitting UV radiation through-out the solution [3]. For future research, the effect of thesefactors with regards to different morphologies of TiO2 particles un-der solar illumination [34] must be analyzed to provide both thescientific and commercial sectors with an accurate picture of thenecessary details.

5. Conclusion

3D dendritic TiO2 microspheres with superfine 1D nanoribbons(3DD-TiO2) was introduced as a novel material for water disinfec-tion. Our results showed that this 3DD-TiO2 have substantiallyhigher disinfection activity compared to microspheres and com-mercialized P25 both in dark and UV irradiation conditions. Thiscan be attributed to the both of shape-dependent and high-effi-cient photocatalytic disinfection. The concentration of photocata-lyst used must be optimized for cost effectiveness, whilepresence of organic substances and differences in temperature ex-ert a significant impact on the disinfection rate. This implies thatthe settings of the process parameters should be carefully deter-mined in applying 3DD-TiO2 to water disinfection. The enhancedefficiency of the 3DD-TiO2 will undoubtedly be useful for commer-cial applications, as well as trigger research into more novel nano-structures to further strengthen the position of TiO2 as one of themost promising photocatalysts.

Appendix A. Supplementary material

Supplementary material associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.cej.2014.03.079.

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