Fluorescence analysis of NOM degradation by photocatalytic oxidation and its potential to mitigate membrane fouling in drinking water treatment

Chemosphere 136 (2015) 140–144

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Technical Note

Fluorescence analysis of NOM degradation by photocatalytic oxidation and its potential to mitigate membrane fouling in drinking water treatment Bryan A. Nerger, Ramila H. Peiris, Christine Moresoli ⇑ Department of Chemical Engineering, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada

h i g h l i g h t s  Fluorescence-based monitoring of the photocatalytic oxidation of natural water.  Photocatalytic oxidation of NOM as a method to mitigate membrane fouling.  ZnO displayed higher extent of NOM degradation when compared to TiO2.  Photocatalytic oxidation treatment of river water considerably reduced UF membrane fouling.

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Article history: Received 22 August 2014 Received in revised form 26 March 2015 Accepted 30 March 2015 Available online 15 May 2015 Handling Editor: Shane Snyder Keywords: Fluorescence Membrane fouling Natural organic matter Photocatalytic oxidation Titanium dioxide Zinc oxide

a b s t r a c t This study examined the photocatalytic oxidation of natural organic matter (NOM) as a method to mitigate membrane fouling in drinking water treatment. ZnO and TiO2 photocatalysts were tested in concentrations ranging from 0.05 g L 1 to 0.5 g L 1. Fluorescence peaks were used as the primary method to characterize the degradation of three specific NOM components – fulvic acid-like humic substances, humic acid-like humic substances, and protein-like substances during photocatalytic oxidation. Fluorescence peaks and Liquid Chromatography–Organic Carbon Detection (LC–OCD) analysis indicated that higher NOM degradation was obtained by photocatalytic oxidation with ZnO than with TiO2. Treatment of the feed water by ZnO photocatalytic oxidation was successful in reducing considerably the extent of hydraulically reversible and irreversible membrane fouling during ultrafiltration (UF) compared to feed water treatment with TiO2. Fouling during UF of water subjected to photocatalytic oxidation appeared to be caused by low molecular weight constituents of NOM generated during photocatalytic oxidation. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The use of membrane filtration in drinking water treatment has been investigated extensively in recent years. However, membrane fouling, primarily caused by natural organic matter (NOM), severely limits the efficient operation of large-scale industrial systems by increasing operational costs and hydraulic resistance (Peiris et al., 2008). Several pre-treatment methods have been implemented to mitigate membrane fouling in drinking water treatment including coagulation (Kimura et al., 2005), flocculation (Wang et al., 2013), oxidation (Mozia et al., 2006), and bio-filtration (Peldszus et al., 2012). Most of these methods ⇑ Corresponding author. Tel.: +1 519 888 4567x35254; fax: +1 519 888 4347. E-mail addresses: [email protected] (B.A. Nerger), [email protected] (R.H. Peiris), [email protected] (C. Moresoli). http://dx.doi.org/10.1016/j.chemosphere.2015.03.089 0045-6535/Ó 2015 Elsevier Ltd. All rights reserved.

transfer organic matter and pollutants from the aqueous phase to a solid phase necessitating subsequent treatment of the solid phase (Huck and Sozan´ski, 2008; Liu et al., 2012). In addition, during the disinfection of water, residual NOM may react with chlorine-based disinfection chemical components to produce disinfection by-products (DBPs), which are potentially carcinogenic (Nieuwenhuijsen et al., 2009; Bond et al., 2014). To be considered in large-scale drinking water treatment processes, a pre-treatment method must be safe, cost effective, efficient, and tuneable (Gao et al., 2011). To that end, advanced oxidation processes (AOPs) represent a potential alternative to conventional NOM removal methods in drinking water pre-treatment (Matilainen and Sillanpää, 2010). AOPs break down pollutants and organic matter through the production of radicals such as hydroxyl radicals. These radicals lead to organic matter mineralization (Daneshvar et al., 2004; Hoffmann et al., 1995;

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Legrini et al., 1993; Matilainen and Sillanpää, 2010; Ohtani, 2010). NOM removal by AOPs also reduces the potential generation of harmful by-products during the disinfection stages of drinking water treatment (Legrini et al., 1993; Mills and Le Hunte, 1997; Liu et al., 2008). One common AOP, known as photocatalytic oxidation, employs metal oxide photocatalysts (e.g. TiO2, ZnO, and SnO2) which offer high efficiency, stability, low cost, tuneable properties, non-toxic properties, and large surface area (Chatterjee and Dasgupta, 2005). Owing to these advantages, photocatalytic oxidation has great potential for use in the production of drinking water by membrane filtration (Huang et al., 2008; Bai and Sun, 2011). In this study, we report on the feasibility of photocatalytic oxidation using TiO2 and ZnO catalysts as a method to mitigate NOM-related membrane fouling in drinking water treatment. The degradation of three specific NOM components – fulvic acid-like humic substances (HS), humic acid-like HS, and protein-like substances (PS) – was evaluated from the fluorescence characteristics of these components. 2. Materials and methods 2.1. Materials ZnO from Sigma–Aldrich Corporation (St. Louis, MO, USA) and anatase TiO2 from ACROS Organics (Geel, Belgium) were used as purchased. Water from the Grand River (GRW) (Kitchener, ON, Canada) was the source water. Typical characteristics of Grand River water include pH (7.5–8.0), turbidity (2.78–27.2 NTU), TOC (6.8– 8.1 mg L 1), DOC (6.3–8.0 mg L 1), conductivity (540– 588 mS cm 1), Ca2+ (54–62 mg L 1) and Mg2+ (19–23 mg L 1) as reported previously (Peiris et al., 2008). All water was filtered using a 200 lm filter (038A-2080; Keller Products, Inc., Acton, MA, USA) to remove any large particulate matter before use and was stored at 4 °C. No additional treatment was performed to the water before use. 2.2. Photocatalytic oxidation experiments The photocatalytic oxidation experiments were performed in a 2.0 L glass batch reactor situated in a fume hood. A 9-W UV-A lamp (Philips Pl-S9W/10) was used as an irradiation source. The UV lamp was placed in a 250 mL UV-A transparent borosilicate glass enclosure which was immersed in the water contained in the 2.0 L batch reactor. Borosilicate glass was chosen as a suitable alternative as compared to quartz. The glass enclosure was used to prevent direct contact between the water in the batch reactor and the UV lamp. Aluminum foil was wrapped around the reactor to prevent exposure of the water to ambient light sources and prevent direct eye contact with the UV light. A magnetic stirrer was used to ensure that a homogeneous solution was maintained. Each photocatalytic oxidation experiment was conducted with 2.0 L of GRW. GRW, relative to other sources of surface water, contains considerably higher HS levels (Peiris et al., 2008, 2010a, 2010b) and represented a suitable candidate for the evaluation of photocatalytic oxidation pre-treatment. Prior to the oxidation experiments, the catalyst was added and mixed in the raw water at room temperature. The catalyst concentrations tested were 0.05 g L 1, 0.25 g L 1, and 0.5 g L 1. After mixing, the water-catalyst solution was allowed to equilibrate at room temperature for 20 min. At this point the UV light was turned on to initiate photocatalysis. Samples were taken before and after treatment and placed directly in a UV-grade polymethylmethacrylate cuvette. Samples taken at the end of the photocatalytic oxidation (2 h) were filtered with a 0.45 lm syringe filter, VWR Inc., for the removal of suspended solids before fluorescence analysis.

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2.3. Fluorescence analysis Fluorescence analysis was performed with a Varian Cary Eclipse fluorescence spectrophotometer, equipped with a Peltier multicell holder (Varian, Palo Alto, CA). Fluorescence emission intensities were obtained at wavelengths between 300 nm and 600 nm (1 nm increments) and excitation between wavelengths of 250 nm and 380 nm (10 nm increments). Optimal instrument parameters (PMT voltage: 750 V, scan rate 600 nm min 1, and excitation and emission slit widths: 10 nm) for reproducible signals were previously determined (Peiris et al., 2009). All samples were maintained at room temperature (25 °C) during the analysis. The samples were not pH adjusted as previous work indicated that pH has no significant effect on the fluorescence measurements (Peiris et al., 2010a). All samples were mixed with a magnetic stirrer during fluorescence analysis. The fluorescence emission intensities for Milli-Q (Millipore) water were subtracted from all fluorescence emission intensities of the samples in order to reduce background noise and eliminate Raman scattering. 2.4. LC–OCD analysis Liquid Chromatography–Organic Carbon Detection (LC–OCD) analysis was performed with a DOC-Labor Dr. Huber (Karlsruhe, Germany) system (Huber and Frimmel, 1992) equipped with a Toyopearl HW-50S SEC column (Tosoh Bioscience, Tokyo, Japan). A detailed description of the methodology and parameters for LC–OCD analysis was previously reported (Peiris et al., 2013). 2.5. Membrane ultrafiltration Membrane ultrafiltration (UF) experiments were performed for GRW feed water treated with 0.5 g L 1 ZnO and TiO2. Prior to UF, the water was first filtered through 0.45 lm GN-6 Grid MetricelÒ membrane filters from Pall Corp. (Ann Arbor, MI) for the removal of the catalyst. In accordance with previous literature, microfiltration was selected as an appropriate method for catalyst removal (Xi and Geissen, 2001). The filtered water was then used directly as the feed for the UF flat sheet cross-flow filtration system. A detailed description of the UF set-up and the operating procedure was previously reported (Peiris et al., 2010a). Briefly summarized, flat sheet UF membranes with a 30 kDa molecular weight cut-off (Polysulfone-YMERSP3001; GE Osmonics) from Sterlitech Corp. (Kent, WA, USA) were conditioned before filtration as recommended by the supplier by immersion in 100 mL of 50% (v/v) ethanol solution for 24 h followed by rinsing with DI and Milli-Q water (resistivity 18.2 MX cm). The initial pure water flux at a trans-membrane pressure of 15 psi (103.42 kPa) was 2.4 L min 1 m 2. Filtration consisted of cycles of 1 h of permeation followed by 15 s of backwashing. The UF backwashing – which involved forcing permeate in the reverse direction through the membrane – removed the hydraulically reversible portion of foulants resulting in the partial recovery of the permeate flux. 3. Results and discussion 3.1. Fluorescence spectral features of GRW The effect of photocatalytic oxidation on the three major constituents of NOM – fulvic acid-like HS, humic acid-like HS, and PS – was characterized using peaks from fluorescence emission intensity profiles. These include the primary peak a at Ex/Em = 320 nm/420 nm corresponding to fulvic acid-like HS, a secondary peak b, appearing in the form of a shoulder around Ex/Em = 270 nm/450 nm and corresponding to humic acid-like

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HS, and a third peak c at Ex/Em = 280 nm/330 nm, corresponding to PS (Her et al., 2003). 3.2. Fluorescence analysis of photocatalytic oxidation pre-treatment The extent of NOM degradation, deduced from the decrease of the fluorescence intensity for peaks a, b, and c after photocatalytic oxidation of GRW, indicated that ZnO and TiO2 reduced considerably the fulvic acid-like HS, humic acid-like HS, and PS content of GRW (Table 1). ZnO photocatalytic oxidation displayed higher degradation of all NOM constituents when compared to TiO2 and all catalyst concentrations investigated (Table 1). It should be noted that differences in fluorescence peak signals were sufficient to clearly distinguish between the effect of catalyst type and catalyst concentration on the extent of degradation (Table 1). The fluorescence analysis was consistent as shown by the variability in the fluorescence peak intensity deduced from replicates at a given experimental condition which was generally less than 5% of the total peak intensity. The comparative degradation estimated from fluorescence measurements was supported by LC–OCD analysis which showed higher extents of HS and PS degradation for photocatalytic oxidation with ZnO as compared to TiO2 (Fig. 1). The LC– OCD analysis also showed significant increase in low molecular weight components of the water. This result can be attributed to the degradation of higher molecular-weight NOM constituents, which are broken down into lower molecular weight components during photocatalytic oxidation. The higher content of low molecular weight NOM components for the water subjected to ZnO compared to the water subjected to TiO2 (OC-signal at elution time between 45 and 52 min in Fig. 1) agrees with the higher degradation of NOM observed with ZnO deduced from fluorescence intensity (Table 1). The higher degradation of the three major NOM constituents, deduced from the intensity of peaks a, b, and c observed with ZnO can be attributed to the higher quantum efficiency as well as the lower UV opacity for ZnO (Shen Chan et al., 2011). The higher quantum efficiency means that a higher proportion of photons interacting with the ZnO particle create electron hole pairs which subsequently produce more hydroxyl radicals to break down NOM. On the other hand, the lower solution opacity of ZnO results in a higher number of UV light/photons interacting with ZnO particles and NOM in solution. In addition, the differences in NOM adsorption on ZnO and TiO2 particles could also lead to aggregation and sedimentation of the ZnO and TiO2 particles (Keller et al., 2010). Thus, ZnO could have improved stability and therefore improved photocatalytic activity. As ZnO and TiO2 concentration increased, differences in the three main fluorescence peaks were observed and could be related to the degradation of specific NOM components. The increased degradation with increasing catalyst concentration for fulvic

Fig. 1. LC-OCD spectra, organic carbon signal vs. elution time, of GRW untreated, and treated with 0.5 g L 1 ZnO and 0.5 g L 1 TiO2 (ultrafiltration operations presented in Fig. 2).

acid-like HS and humic acid-like HS suggests that the optimal catalyst concentration is higher than 0.5 g L 1 (Table 1: peaks a and b). In contrast, PS degradation (Table 1: peak c) was highest at 0.25 g L 1 catalyst concentration (highest decrease in fluorescence peak c intensity). The highest PS degradation observed with 0.25 g L 1 catalyst could likely reflect interactions between PS and catalyst particles. Previous studies with GRW and fluorescence analysis indicated strong interactions between PS and colloidal particulate matter (Peiris et al., 2012). One could envision that catalyst particles could act as colloidal particulate matter and interact with PS where an optimal catalyst and protein ratio would exist beyond which insufficient free PS would be available for photocatalytic oxidation, thus adversely affecting PS degradation efficiency. Future work will focus on developing a more detailed understanding of PS adsorption on ZnO and TiO2 catalyst particles during treatment by considering multiple fluorescence excitation– emission intensities. 3.3. Effect of photocatalytic oxidation pre-treatment on membrane fouling The role of photocatalytic oxidation with ZnO or TiO2 catalysts as a pre-treatment method to reduce membrane fouling during UF of water was examined for three sequential filtration cycles consisting of 1 h of permeation followed by 15 s of backwashing. The portion of the permeate flux not recovered after backwashing was considered to be the hydraulically irreversible membrane fouling. Permeate mass measurements were recorded throughout the

Table 1 Fluorescence peak intensity for GRW before and after 120 min of photocatalytic treatment. The relative decrease of the fluorescence peaks, D (%), represents the relative decrease of the peak intensity before and after 120 min of photocatalytic treatment. Catalyst type

a b c

Catalyst concentration (g L

1

)

Peak a intensity (a.u.)

Peak c intensity (a.u.)

Peak b intensity (a.u.)

t = 120 (min)

Da (%)

t = 120 (min)

Db (%)

t = 120 (min)

Dc (%)

ZnO

0.05 0.25 0.5

130.0 31.9 16.6

63.5 91.1 95.3

53.0 9.2 10.0

68.0 94.5 93.9

13.7 7.7 25.7

77.5 87.6 58.6

TiO2

0.05 0.25 0.5

321.5 310.8 261.8

9.8 12.9 26.6

188.0 162.9 148.3

13.7 1.5 10.3

56.1 28.9 43.4

9.7 53.5 30.2

Peak intensity at t = 0 was 356.6 a.u. Peak intensity at t = 0 was 165.4 a.u. Peak intensity at t = 0 was 62.1 a.u.

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of the interactions between NOM components and catalyst particles.

Acknowledgements The authors acknowledge Dr. Monica Tudorancea of the Department of Civil and Environmental Engineering, UW, for performing LC–OCD analysis, Professor William A. Anderson of the Department of Chemical Engineering, UW, for lending the UV lamp, and the Natural Sciences and Engineering Research Council of Canada (NSERC) and MITACS for financial support including an NSERC Undergraduate Student Research Award (USRA) to Bryan A. Nerger. References Fig. 2. Normalized flux profile for GRW untreated and treated with 0.5 g L 0.5 g L 1 TiO2.

1

ZnO or

experiment to calculate the permeate flux. Normalized flux, defined as the permeate flux at any given time divided by the initial flux at the beginning of the experiment, was used to estimate membrane fouling. The normalized permeate flux at the end of each sequential permeation and backwashing cycle was higher when the feed water was subjected to photocatalytic oxidation (Fig. 2), indicating fouling reduction. For GRW, the normalized permeate flux after three filtration cycles was 58.5% and 39.3% higher with the 0.5 g L 1 ZnO pre-treatment and 0.5 g L 1 TiO2 pre-treatment respectively as compared to GRW without pre-treatment. This fouling reduction can be attributed to the degradation of the HS and PS content, as discussed previously and confirmed through LC–OCD analysis (Fig. 1). Specifically, the more significant flux decline observed during UF for the TiO2 treated water as compared to ZnO treated water indicated a higher extent of fouling with the TiO2 treated water. This observation is consistent with the fluorescence and LC–OCD analysis where lower HS and PS degradation for TiO2 treated water was observed compared to ZnO treated water. The fouling observed for the treated water can be attributed to the residual HS and PS content present as low molecular weight constituents in the water after treatment (Fig. 1). 4. Conclusions This study examined the use of fluorescence peak intensities in order to evaluate ZnO and TiO2 photocatalytic oxidation pre-treatment as a method to mitigate membrane fouling when producing drinking water. The following conclusions can be drawn: 1. The fluorescence peak intensity was able to capture photocatalytic oxidation pre-treatment and provided simple, rapid, and accurate estimates of the degradation of two major NOM fractions, HS and PS, as confirmed using LC–OCD analysis. 2. ZnO is a suitable alternative catalyst to TiO2 for the degradation of humic substances (HS) and protein substances (PS), and displayed considerably higher degradation for three different catalyst concentrations tested. 3. ZnO photocatalytic oxidation of the feed water significantly reduced the hydraulically reversible and irreversible membrane fouling as compared to TiO2 photocatalytic oxidation. 4. Future work will investigate fluorescence excitation–emission matrices in order to develop a more in-depth understanding

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