The performance of different configurations of titanium dioxide-coated foam supports as photocatalysts in an enhanced solar disinfection system for drinking water treatment was evaluated, using the reduction of methylene blue, 1-4-dioxane, and Escherichia coli as performance indicators. Reactors with immobilized catalysts were able to match or surpass the performance of a suspension configuration due to effective mass transport and association between the analyte and the foam. Performance was related to the pore size of the foam, with the ideal pore size varying between target contaminants.
Solar disinfection (SODIS) is a well-known cost-effective mechanism for the inactivation of disease-causing organisms in water. The SODIS process involves the exposure of water in polyethylene terephthalate bottles to direct solar radiation for a minimum of 6 hours, after which the pathogen load is decreased and the quality of the water substantially improved. The inactivation caused by SODIS treatment is understood to involve several modes of action resulting from the absorption of short wavelength UV radiation and long wavelength infrared thermal radiation (Meierhofer & Wegelin, 2002; Blanco et al. 2009; Byrne et al. 2011). The integration of solar photocatalytic advanced oxidation into the SODIS method has been proposed as a robust approach for improving the microbial and chemical quality of the treated water (Lonnen et al. 2005; Shannon et al. 2008; Blanco et al. 2009; Byrne et al. 2011). The goal of this integration is to develop a small and inexpensive point-of-use photoreactor.
One aspect of photoreactor design is the photocatalyst configuration, which refers to the shape or form of the catalyst within the reactor. Variations include different sized particulate suspensions, immobilized thin films, and three-dimensional support structures that are coated by the photocatalytic material (Kisch & Macyk, 2002; Guiang et al. 2008; Li et al. 2008; Akhavan et al. 2009; Han et al. 2011; Zhang, 2012; Zhang et al. 2012). Traditionally, suspended systems have been common and are an efficient configuration, but the catalyst must be removed before consumption requiring the application of a chemical treatment or fine filter which may not be cheaply available and complicates the purification process (Gumy et al. 2006; Plantard et al. 2011). Therefore, immobilization of the photocatalyst remains an attractive alternative, particularly for small scale point-of-use applications.
One way to immobilize the photocatalytic material is to use a foam support. Foam is considered attractive because it has high porosity, high surface area, low density, and high permeability, all of which promote high efficiency reactions (Antoniou et al. 2009; Plesch et al. 2009; Plantard et al. 2011). A broad comparison of catalyst configurations including a wide range of foam pore sizes, suspension and fixed film reactors has not been empirically undertaken. Furthermore, in past investigations that have been performed with probe compounds, the relationship between configuration performance and target compound has not been described, particularly for biological contaminants and common environmental pollutants.
The following presents a case study in which the performance of different photocatalytic foams are compared in SODIS-like conditions. The study presented has the two-fold aim of: (i) predicting the best configuration for an immobilized catalyst in an enhanced SODIS reactor by evaluating the comparative performance of TiO2 coated foams of varying pore size, a TiO2 suspension and TiO2 fixed film photocatalyst configurations; and (ii) examining the relationship between target contaminant degradation and reactor configuration. For this second aim, the degradation for each reactor configuration was evaluated for methylene blue (MB) as a probe compound, Escherichia coli as an indicator of biological contaminants, and 1,4-dioxane as a common environmental pollutant.
MATERIAL AND METHODS
Photocatalytic material preparation and characterization
Materials were coated by an adaptation of the dip coating method described by Plesch et al. (2009) using an acidified mixture of 20% (by weight) Aeroxide P-25® TiO2 in distilled water and a heat treatment at 600 ̊C with a ramp rate of 2.5 °C/min.
Bench-scale solar simulator photocatalytic degradation experiments
The samples were kept in the dark for a 30 minute dark reaction before the lamp shutter was opened. For the 1,4-dioxane and MB samples one aliquot from each container was removed at 0, 5, 10, 20, 40, 80, 120 and 160 minutes. E. coli samples were removed at 0, 15, 30, 45, 60 and 75 minutes. Samples were not stirred continuously throughout the exposure in order to mimic SODIS reactor conditions; however, each sample was briefly stirred immediately prior to sampling to achieve an accurate sample with uniform distribution of contaminant prior to measurement. Individual foams were not re-used because the coating method was not optimized for these exploratory tests nor were the foams confirmed to be effective over longer periods of time.
Contaminant dosing and detection
One of the principal aims of this study was to investigate the relationship between photoreactor configuration with different types of contaminants to further describe the dependence of comparative reactor efficiency on the choice of test compound. We selected one representative compound from three categories: a probe compound to indirectly monitor hydroxyl radical (·OH) formation, a chemical pollutant, and a biological contaminant/pathogen. The probe compound selected was MB, one of the most commonly applied organic dyes, because of its ease of detection and extensive use in past literature (Wang & Ku, 2006; Natarajan et al. 2011; Sahoo & Gupta, 2012). Colored dyes are easily measured using spectrophotometric methods making them particularly good candidates for an initial investigation. 1,4-dioxane was chosen as the chemical pollutant because it is a recalcitrant organic that is not known to break down or be removed significantly by conventional methods. The pathogen selected was E. coli, a common contaminant in drinking water globally and a good indicator organism for predicting the presence of other micro-organisms. There is also a vast wealth of research that has been conducted on the treatment of E. coli using a variety of treatment methods (Acra et al. 1990; Galalgorchev, 1992; Cho et al. 2004; Lonnen et al. 2005).
MB samples (Fischer Scientific) were prepared by dissolving 200 mg/L of the dye into DI water. Two mL of this solution per liter of water sample was added to the photoreactor immediately prior to experimentation to achieve a starting concentration of 4 mg/L. Concentration was determined using a diode array UV-vis spectrophotometer at 663 nm. Suspension samples were spun down in a centrifuge before analysis to pellet and remove the TiO2.
Samples were dosed with 100 ppm 1,4-dioxane immediately prior to experimentation to achieve a concentration of 1 ppm. Suspension samples were filtered through a 0.45 μm Acrodisc syringe filter before analysis. A fixed concentration of an isoptic labeled surrogate internal calibration standard (1,4-dioxane-d8; Sigma Aldrich) was added to each aliquot after sampling. Dioxane and its surrogate standard in the water samples were extracted using liquid–liquid extraction with a solution of hexane-dichloromethane (80:20, v/v) followed by concentration through solid phase extraction with a C18 SPE cartridge and elution with acetonitrile (Luck & Hofmann, 2006). The eluted sample was collected in 2 mL amber vials and run on a gas chromatograph with a DB1701 column, showing a dioxane retention time of 8–9 minutes. The concentration of 1,4-dioxane was determined through correlation with standards and by relating the mass spectrometer response of 1,4-dioxane's quantification ion (m/z 88) to the response of the 1,4-dioxane-d8's quantification ion (m/z 96).
Bacterial preparation and enumeration were performed following ATCC guidelines and standard aseptic practices. A lyophilized E. coli stock culture (ATCC® 23631; Cedarlane Laboratories) was revived in LB broth (Sigma-Aldrich), grown into the logarithmic phase and used to prepare glycerol stocks. Stocks were reanimated the evening prior to experimentation and incubated overnight at 37 °C until they reached a concentration of 109 CFU/mL as confirmed by measuring the OD600. A portion of the stationary culture at 109 CFU/mL was transferred to a sterile autoclaved 50 mL centrifuge tube and spun down at 3,000 rpm for 10 minutes. The growth media was removed and the pellet suspended in 25 mL of a quarter strength Ringers solution. This process was repeated twice to ensure all growth media was removed from the solution. One mL of this solution was added to 25 mL ¼ Ringers to make a stock solution for inoculation. Immediately prior to experimentation, 1 mL of the stock solution was added to 75 mL ¼ Ringers in each photoreactor configuration, resulting in a starting concentration of ∼106 CFU/mL. At the indicated time intervals, the bacterial levels were enumerated using the following standard spread plate count method. First, 1 mL sample was taken from the culture and serially diluted with sterile phosphate buffer saline (PBS; Sigma-Aldrich) solution. One hundred μL of each dilution was pipetted into the middle of a sterile pre-poured LB agar 10 cm Petri plate (Sigma-Aldrich) and manually spread using a spin table and sterile spreader. Plates were incubated at 37 °C for 24 hours after which the colonies were counted.
RESULTS AND DISCUSSION
Characterization of photocatalytic coatings
Photocatalytic coatings of consistent thickness with relatively good adhesion were produced, although the mass of the coating varied between sample type and was likely dependent on its available surface area. For example, the finer pore size foams (20 and 40 PPI) supported a larger coating mass. None of the mass of catalyst was lost during each experiment, as verified by thoroughly rinsing the foams after use and re-taking their mass. Table 1 shows the average mass of catalyst applied to each support and the variation between samples. Suspension samples contained 1 g TiO2 for comparison.
|5 PPI Foam .||10 PPI Foam .||20 PPI Foam .||40 PPI Foam .||Fixed Film .|
|0.59 ± 0.06 g||0.62± 0.07 g||1.08± 0.06 g||1.16± 0.07 g||1.13± 0.05 g|
|5 PPI Foam .||10 PPI Foam .||20 PPI Foam .||40 PPI Foam .||Fixed Film .|
|0.59 ± 0.06 g||0.62± 0.07 g||1.08± 0.06 g||1.16± 0.07 g||1.13± 0.05 g|
The 20 and 40 PPI foams showed less cracks in the coating, although more clogged pores, pores in which an agglomeration of TiO2 accumulated, were identified. It is hypothesized that these clogs, as depicted in the bottom right hand corner of Figure 3(d), were formed by surface tension in the coating which allowed a bubble like structure to form and solidify in the pore during heat treatment. This is also supported by the partially blocked pore seen in upper right Figure 3(c), which has a structure resembling a popped bubble. It is likely that the greater portion of clogged pores also contributed to the large jump in average mass coating for the 20 and 40 PPI samples.
MB was the first compound used to investigate the performance of each of the photoreactor configurations: suspension, foams with 5, 10, 20 and 40 PPI, and fixed films.
Prior to testing photocatalytic degradation, the photolytic removal of 4 mg/L of MB was first measured in samples containing foams without TiO2 coating, as well as in a DI water only sample. These control experiments were performed with the aim of determining the extent of the photolytic degradation of MB to quantify the amount of light obscured by the foams. Some degradation of the MB dye is expected in the presence of simulated sunlight even without a catalyst as a result of direct photolysis by the high energy near UV wavelengths present at the far blue end of the radiation. The results of this control test showed direct photodegradation of MB varying between 24 and 42%. The least discolouration (24%) occurred in the samples containing the smallest pore size (40 PPI) foam, with the 20 and 40 PPI foams both showing a statistically significant reduction in photodegradation, which is attributed to the smaller pores blocking and reflecting more light. Small pores form a more compact structure, thereby enhancing this reflection and reducing photon interaction with the dye, preventing MB degradation. The photolytic degradation of MB was not significantly different between the control and the 5 and 10 PPI foam samples.
It is also interesting to note that after normalizing for mass the 5 PPI foam achieved a higher reduction of E. coli than the suspension. It is proposed that the E. coli might attach to the foam structure more efficiently than to a TiO2 suspension, coming into better contact with the TiO2 and therefore experiencing a greater level of inactivation during irradiation. This would suggest that foam pore size might affect two key parameters: the ability of the light to penetrate deep into the water sample, which is impaired by pore sizes that are small, such as 40 PPI in this study, and the ability to serve as attachment locations, with the 5 PPI foam being superior to a suspension in the case of treatment for E. coli.
This study investigated the comparative performance of a SODIS-like reactor with a TiO2 suspension to five configurations of immobilized TiO2 with three different contaminants in a photocatalytic batch reactor under simulated solar radiation. For MB, both before and after normalizing for mass of catalyst, reactors with the 5 and 10 PPI foams were among the top performers, with performance deteriorating with the 20 and 40 PPI foams. For 1,4-dioxane, the suspension, 5 PPI and 10 PPI foam were most effective. For E. coli, the 5 PPI foam resulted in maximum inactivation, showing greater inactivation after 80 minutes when compared to the TiO2 suspension and showing the fastest degradation kinetics expressed as per gram of TiO2. The 40 PPI foam was a poor performer for all contaminants. It is presumed that the very fine pore size of the foam prevented light from radiating through the water sample. This is supported by the decrease in photolytic degradation of MB observed in the control experiment. Overall, it was observed that TiO2 catalyst immobilized on a foam support could match the efficiency of a suspension. Performance was related to foam pore size and an optimum pore size of 5–10 PPI was observed; however, the optimum conditions were a function of the target contaminant. E. coli appeared to be the most sensitive to pore size, possibly due to lower mobility during exposure to light than the chemical contaminants. The results of this work are intended to guide future research into methods to improve SODIS effectiveness by using immobilized TiO2 on a foam support. The results suggest that a foam support with a pore size in the range of 5–10 PPI may provide significant treatment performance relative to a SODIS system that is free of the photocatalyst, while allowing the photocatalyst to be easily removed following treatment and potentially reused.
Funding for the work was generously provided by the Natural Sciences and Engineering Research Council Chair in Drinking Water Research, the Southern Ontario Water Consortium, and the Ontario Graduate Scholarship program.