23. - 25. 10. 2012, Brno, Czech Republic, EU

EVALUATION OF PHOTOCATALYTIC NANOPARTICLE RELEASE TO THE AQUATIC

ENVIRONMENT

Saioa ZORITAa, Josune OLABARRIETAa, Oihane MONZÓNa, Nerea RIOJAa, Pablo BENGURIAa,

Lorette SCIFOb

a Tecnalia-Environment Unit, c/Geldo, Parque Tecnológico de Bizkaia, Edificio 700, 48160 Derio, Spain

e-mail: saioa.zorita@tecnalia.com

b Tecnalia-France, Immeuble MIBI, 672 rue du Mas de Verchant, 34000 Montpellier, France

Abstract

Nanotechnology creates possibilities to produce new products with novel functionalities and improved

characteristics. Therefore the incorporation of engineered nanoparticles (ENP) in commercial products is

progressively increasing and so is the potential for release of these novel materials into the environment. In

fact, the occurrence and release of titanium nanomaterials in a wastewater treatment plant has already been

proven. This implies that nanomaterials will reach the aquatic environment and that they could represent a

risk to aquatic organisms and humans.

We have studied the discharge of nano-TiO2 (nTiO2) to water for two case studies related to photocatalytic

nanomaterials: A- The release of nTiO2 from two photocatalytic coatings under accelerated aging under a

water flow and B- The release of nTiO2 though a filtration pilot plant from tap water containing 2 mg/L

nanoparticles.

Case A. The influence of environmental parameters (water matrix, UV-light) was investigated while the

consequences of aging were evaluated under different endpoints, among them, the release of TiO2

nanoparticles. Despite that TiO2 emissions were observed for both coatings, only one of them presented

mechanical damages. TiO2 concentrations up to 150.5 μg/L were detected in the aging water.

Case B. Preliminary experiments were conducted at laboratory scale to determine optimum polymeric

material as well as pore size. Among the common commercial materials PVDF showed the best

performance. Two different industrial hollow fiber cartridges were tested with a pore size of 0.45 μm

(microfiltration) and 0.03 μm (ultrafiltration). TiO2 was detected at an average concentration of 51 μg/L in the

filtered water.

Keywords:

TiO2 water contamination, nanoparticles discharge, water, filtration, aging.

1. INTRODUCTION

Nanotechnology creates possibilities to produce new products with novel functionalities and improved

characteristics. Therefore the incorporation of ENP in commercial products is progressively increasing. ENP

embedded in products will inevitably enter the environment during their life cycle through e.g. product use or

weathering and disposal [1]. This implies that nanomaterials may represent a risk to environmental

organisms and humans.

The risks of ENP are associated with exposure levels and toxicity of ENM [2]. Once that ENP are released to

the environment their fate, transport, and transformation will determine their eco-/toxicity. Therefore it is vital

to quantify ENP release, to understand the factors affecting the release of ENM from products and their fate

when arriving to final disposal e.g. wastewater treatment plant. The knowledge in these areas will aid to

minimize ENP exposure as a result of safer produced materials and safer treatment of the ENP waste at

their end-of-life.

23. - 25. 10. 2012, Brno, Czech Republic, EU

The aim of this work is to estimate likely environmental nTiO2 concentrations in two case scenarios: A)

Release during use of a product: nTiO2 discharge from glass coatings under weathering and water flow

aging, and B) Release during end-of-life: “free” nTiO2 discharge from a filtration plant under likely operation

conditions.

2. EXPERIMENTAL

2.1 Case A-The release of n-TiO2 from two photocatalytic coatings under accelerated aging under a

water flow

Two photocatalytic coatings were investigated. One was a commercial product, Pilkington ActivTM (PA),

which had been suggested by Mills et al. [3] as a reference material for photocatalytic coatings. According to

manufacturer data and previous reports [3-4], it consists in a 12–15 nm thick nanocrystalline film of anatase

TiO2. It is deposited by an Atmospheric Pressure Chemical Vapour Deposition (APCVD) process on a glass

substrate heated to 615°C. The second material was an experimental coating, synthesized at low

temperature and deposited by dip-coating. It contained in average 50 wt.% of TiO2 nanoparticles (80%

anatase, 20% rutile) embedded in a siliceous matrix. An accelerated aging procedure was developed in

which 1 L water was circulated parallel to the surface of the coatings at 3 L/min for a period varying from 1 to

4 weeks. As shown in Fig. 1, the samples were placed in a Teflon® flume designed specifically for this

experiment and water was pumped on them from a reservoir, by a centrifugal pump. The water coming out of

the flume was recollected in the reservoir and re-circulated in the flume in a closed loop. Four water matrixes

(deionized water (DW), humic acids (HA), sodium chloride (NaCl) and humic acids plus sodium chloride (HA

+ NaCl)) were tested in order to evaluate the specific influence of each compound as well as their combined

action. All sample type/water combinations were probed in parallel in the dark and under UVA at ca. 1

mW/cm2. For each experiment, the samples were divided into four groups depending on the time they were

maintained in the flume: 1 week, 2 weeks, 3 weeks and 4 weeks. Each group comprised four replicates.

Figure 1. Experimental aging setup under water flow

The TiO2 was analyzed with an axial Varian Vista-MPX CCD simultaneous ICP-OES, with SPS 5 sample

preparation system. For each water, 2× 80 mL were digested under gentle heating with 2 mL nitric acid 65%

and 3 mL sulphuric acid 95%. When extremely low amounts of TiO2 were expected (DW and HA runs), 320

mL of the aging water were concentrated tenfold using a Speed-Vac Concentrator before being digested and

analyzed. The detection and quantification limits were 0.16 μg/L and 0.52 μg/L, respectively.

2.2 Case B-The release of n-TiO2 though a filtration pilot plant from tap water containing 2 mg/L NP

The pilot plant was manufactured to operate in dead-end and tangential (or cross flow) modes. The pilot

plant consisted on a 1000 L feed tank connected to tap water supply, a pump, two membrane modules that

could be operated simultaneously or individually, a dosing pump for the nanoparticles, a chemically

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enhanced backwash (CEB) and cleaning in place (CIP) systems and a Programmable Logic Controller (PLC)

as observed in Fig. 2. The pilot plant also included a 500 μm pre-filter, a flow meter, inlet and outlet pressure

controller, a thermometer and a manual valve to control the retentate water flow in cross flow filtration mode.

The plant was designed to treat a maximum of 1500 L/h.

Figure 2. Filtration pilot plant

Two PVDF hollow fibre membrane modules were tested: i) a 0.4 μm microfiltration module from Porous

Fibers with a surface area of 6.6 m2 giving a recommended filtrate flux between 50-100 L/m

2/h and ii) a 0.03

μm ultrafiltration module from Dow with a surface area of 33 m2 giving a recommended filtrate flux between

40-120 L/m2/h.

Experiments were performed in constant flow mode using tap water. Two different flows were tested in dead-

end mode: 800 L/h and 1300 L/h. In cross flow filtration the feed flow was set to 1300 L/h with a 300 L/h

retentate flow giving an average permeate flow of 1000 L/h. Twenty four hour filtrations were performed.

nTiO2 concentration in the inlet and outlet was evaluated by discrete and continuous sampling.

Since high fouling was observed with the microfiltration module backwashes were set every 30 min, acidic

cleaning with H2SO4 to combat inorganic fouling every 8 h and basic cleaning with NaOH to decrease

organic fouling every 12 h. For the ultrafiltration hollow fibre module acidic and basic cleaning cycles were

identical to microfiltration. However since the pressure was constant over longer periods of time with the

ultrafiltration system backwashes cleanings were set every 45 min.

The selected titanium dioxide nanoparticle was Aeroxide P25 from Evonik.Nanoparticle concentrations in the

permeate water were measured using inductively coupled plasma-optical emission spectroscopy (ICP-OES)

on a Varian 710 ICP-OES with an axial torch configuration. TiO2 NP samples were acidified with 2 % HNO3.

Samples were processed within one week. The method detection limit for nTiO2 was 1.5 μg/L.

3. RESULTS & DISCUSSION

3.1 Case A-The release of n-TiO2 from two photocatalytic coatings under accelerated aging under a

water flow

No difference was detected in sample weight before and after aging, suggesting there was no major loss of

material. This could moreover be confirmed by thickness measurements on the experimental coating, as no

23. - 25. 10. 2012, Brno, Czech Republic, EU

lower values were found for aged samples than for their respective control. Neither could any significant

variations in the TiO2 content of the coatings be evidenced by EDX. The efforts to assess TiO2 release

therefore focused on the elemental analysis of the water circulated in the flumes during the experiment. Fig.

3 shows the concentration measured for TiO2 in these waters as a function of water matrix and illumination.

Figure 3. Emissions of TiO2. Concentration of aging waters in TiO2 as measured by ICP-OES for (a)

experimental coatings and (b) Pilkington ActivTM

While the controls (not shown) exhibited concentrations below the detection limit of methodology (0.16 μg/L),

all the waters used for aging contained titanium dioxide to a lower or higher extent. TiO2 was then undeniably

lost from both coatings under several circumstances. Moreover the composition of the water seemed to have

a strong influence on TiO2 emissions. Indeed, if DW and HA resulted in relatively low amounts of TiO2

released over the 4 weeks of experiment, for both Pilkington ActivTM

and experimental samples, emissions

up to 5.9 μg/L (Pilkington ActivTM

, Fig. 3b) and 104.5 μg/L (experimental coatings, Fig. 3a) were recorded in

presence of NaCl. Besides, this effect was magnified when humic acids were added to NaCl, reaching the

highest TiO2 levels of the whole experiment: 30.8 μg/L and 150.5 μg/L for Pilkington ActivTM

and

experimental coating samples, respectively. At the other extreme, the lowest TiO2 losses were observed for

HA runs on experimental coatings and DW runs on Pilkington ActivTM

. In most cases, the samples irradiated

with UV-light released higher amounts of TiO2 than the samples aged under the same condition in darkness.

Although both coatings displayed similar tendencies of release, the magnitude of their emissions differed and

the loss of TiO2 was generally higher on experimental coatings than on Pilkington ActivTM

. One should

however put this result in perspective with the much lower initial TiO2 content of Pilkington ActivTM

. Taking

into account the exposed surface (10 samples, i.e. 7.5 × 10−3

m2), the cumulative release of TiO2 from the two

coatings during the 4 weeks of experiment is given in Table 1.

Table 1. Cumulative release of TiO2 over 4 weeks, expressed in mg/m2

Water typology Experimental coating Pilkington Activ

TM

Dark UVA Dark UVA

DW 0.73 1.72 0.06 0.12 HA 0.12 0.09 0.10 0.27

NaCl 3.60 13.93 0.79 0.24 HA+NaCl 10.50 20.07 2.49 4.11

It is interesting to see that on average, these values lay in the same range that experimental data obtained

by Kaegi et al. [5] from runoff water of fresh paints after rain events. Yet they generally remained lower,

consistently with the fact that paints have a very high TiO2 content and that efforts are made in photocatalytic

coatings to achieve a better immobilization of TiO2 nanoparticles. The ca. 10 mg/m2 of Ti emissions

(equivalent to 16.7 mg/m2 TiO2) accumulated over one month in Kaegi’s study are only overcome on

experimental coatings aged under the most aggressive conditions: HA + NaCl and UVA. In outdoor air

23. - 25. 10. 2012, Brno, Czech Republic, EU

applications (self-cleaning surfaces, air purification) the values calculated here would probably represents a

worst case scenario, as a continuous water flow seems exaggerated with respect to occasional rain events.

On the contrary in water treatment application, high water flows are likely, and our values may actually

underestimate real releases. Whatever the case, they have to be put in perspective with the fact that they

correspond to relatively fresh materials. Loosely bound particles probably account for a great part in the

observed releases and TiO2 emissions could decrease after the first weeks of use, as reported before for

paints [5]. Further investigations are then needed to evaluate the distribution of TiO2 emissions over time.

3.2 Case B-The release of n-TiO2 though a filtration

Preliminary experiments were conducted at laboratory scale to determine which material (PVDF, PTFE,

PES, Regenerated cellulose and nylon) performed best on the retention of nTiO2. PVDF showed a good

overall performance (good recovery, chemical stability) plus it is a commonly used material in filtration

industry. From laboratory studies it was observed that membrane pore (0.1-0.45 μm) was not a determinant

factor on the ENP release while water matrix highly influenced the NP transport across the membrane

(Figure 4). Water containing HAs and bicarbonate highly enhanced the release of nTiO2 compared to

distillated water (> 150 fold). However filtration using environmental waters (river, WWTP effluent) did not

provide as high nTiO2 concentration in permeate water as with synthetic water (HAs+NaHCO3).Quantified

nTiO2 permeate concentration ranged from 3 to 18 μg/L.

0

100

200

300

400

500

600

700

Batch 1 Batch 2

PVDF 0,22 μm

nT

iO2 c

on

cen

trati

on

in

perm

eate

wate

r (μ

g/L

) 7 ppm HAs

250 ppm NaHCO3

7 ppm HAs + 250 ppm NaHCO3

Figure 4. a) Laboratory scale filtration apparatus. b) Release of nTiO2. Filtration conditions: 75 mL water,

nTiO2 feed concentration: 1g/L, filtration pressure: 2 bar.

At pilot plant scale various experiments were performed as described in Section 2.2. nTiO2 concentration in

permeate water was above the limit of detection. An average of 51 μg/L nTiO2 concentration was calculated

from the results of all tested conditions. Filtration systems despite removing > 95% of ENP, the released

amount per day was significant. Permeate nTiO2 concentrations up to 108 μg/L were quantified under normal

operating conditions. This is equivalent to the release of 3.3 g nTiO2 per day from 31.2 m3 of treated water

(Feed concentration 2.1 mg/L nTiO2). Although similar results were obtained for all tested conditions (4 fold

difference between the maximum and minimum concentration), a slight increase in ENP release was

observed at higher processed flow rates and with ultrafiltration hollow fibre cartridge.

In a filtration processes, cleaning steps are necessary to avoid membrane fouling and maintain optimal

operating conditions. In this study the release of ENPs was only accounted for the filtered water. However

the required water for the cleaning steps becomes highly polluted with ENPs. These ENPs will enter the

environment unless this cleaning water is thereafter correctly treated.

a b

23. - 25. 10. 2012, Brno, Czech Republic, EU

One has to put these values in perspective. The only available study on the removal of TiO2 (nano and non

nano fraction ) from a secondary WWTP indicated that the presence of TiO2 in the inlet was one fourth of the

feed concentration we used [6]. In the same study it was observed that about 70-80% of the TiO2 was

removed with the sludge. Since nTiO2 does not easily undergo transformation or chemical reactions, the

release of these ENPs will be shifted to other compartments (eg. to soil if sludge is used for soil amendment,

to air if sludge is incinerated). This suggests that it is likely that the overall nTiO2 reaching the aquatic

compartment from a WWTP with membrane filtration as a tertiary treatment is inferior to the values

presented in this work.

4. CONCLUSIONS

Experimental information on the release of ENM during use or disposal of products is very scarce [1]. This

work assists at filling this gap and evidences the release of ENP in both of the case studies: through the

phase of A) use (glass coatings with self-cleaning properties) and B) end-of-life (free ENP that would reach a

water treatment facility with a filtration system).

The results obtained from these studies can aid to create more accurate exposure models that are

necessary to predict the risks associated with ENP.

ACKNOWLEDGEMENTS

Authors acknowledge the Basque Country Government and Diputación Foral de Bizkaia for financial

support.

LITERATURE

1. Gottschalk, F.; Nowack, B., The release of engineered nanomaterials to the environment. Journal of Environmental Monitoring 2011, 13, 1145-1155.

2. Gao, J.; Youn, S.; Hovsepyan, A.; Llaneza, V. L.; Wang, Y.; Bitton, G.; Bonzongo, J.-C. J., Dispersion and Toxicity of Selected Manufactured Nanomaterials in Natural River Water Samples: Effects of Water Chemical Composition. Environmental Science & Technology 2009, 43, 3322-3328.

3. Mills, A.; Lepre, A.; Elliott, N.; Bhopal, S.; Parkin, I. P.; O´Neill, S. A., Characterisation of the photocatalyst Pilkington ActivTM: a reference film photocatalyst? Journal of Photochemistry and Photobiology A: Chemistry 2003, 160, 213-224.

4. Chin, P.; Ollis, D. F., Decolorization of organic dyes on Pilkington Activ™ photocatalytic glass. Catalysis Today 2007, 123, 177-188.

5. Kaegi, R.; Ulrich, A.; Sinnet, B.; Vonbank, R.; Wichser, A.; Zuleeg, S.; Simmler, H.; Brunner, S.; Vonmont, H.; Burkhardt, M.; Boller, M., Synthetic TiO2 nanoparticle emission from exterior facades into the aquatic environment. Environmental Pollution 2008, 156, 233-239.

6. Kiser, M. A.; esterhoff, P.; enn, T.; ang, .; P rez-Rivera, J.; Hristovski, K., Titanium Nanomaterial Removal and Release from Wastewater Treatment Plants. Environmental Science & Technology 2009, 43, 6757-6763.