Proceedings of the 13th International Conference on Environmental Science and Technology Athens, Greece, 5-7 September 2013
A HYBRID PHOTOCATALYSIS - ULTRAFILTRATION CONTINUOUS PROCESS FOR DEGRADATION AND REMOVAL OF REFRACTORY ORGANIC POLLUTANTS FROM WATER V.C. SARASIDIS, S.I. PATSIOS, K.V. PLAKAS and A.J. KARABELAS Chemical Process and Energy Resources Institute, Centre for Research and Technology - Hellas, P.O. Box 60361, 6th km Charilaou-Thermi road, Thermi, Thessaloniki, GR 57001, Greece e-mail:
[email protected]
EXTENDED ABSTRACT In natural water sources, Refractory Organic Compounds (ROC) are commonly encountered with concentrations ranging from a few mg/L to very low levels (pg/L to ng/L). These pollutants can be either products of chemical and biological transformation of animal and plant residues (Natural Organic Matter - NOM, e.g. polysaccharides, humic substances) or Synthetic Organic Compounds (SOC) from industrial, agricultural and other activities (e.g. industrial chemicals, pesticides, pharmaceuticals, etc). This complex mixture of heterogeneous organic compounds, due to its potential negative effect to public health, poses a serious problem in the treatment of potable water. Conventional water treatment methods (coagulation/flocculation, activated carbon, ion exchange, etc) are characterized by several shortcomings; indeed, most of them are ineffective for treating water containing ROC. Moreover, removal of ROC at very low concentrations can be quite costly. Therefore, the development of alternative treatment technologies, aiming to remove ROC has received great attention. Advanced Oxidation Processes (AOP) offer an attractive solution, already used for the treatment of water and wastewater containing ROC. In recent years, heterogeneous photocatalysis (e.g. using TiO2 particles) has gained considerable attention, as it involves a direct chemical degradation of the contaminant, rather than removal and transfer into another medium that needs further handling, as is the case in most conventional treatment processes. UV irradiation of the TiO2 semiconductor particles leads to the generation of highly oxidative species that degrade ROC to smaller intermediates and/or to complete mineralization. In this paper a novel method is described, which involves coupling the suspended-type TiO2 photocatalysis process with ultrafiltration membranes for particle separation. The Photocatalytic Membrane Reactor (PMR) is used to treat water containing three different types of organic compounds (Sodium Alginate - SA, Humic Acids - HA and DiCloFenac DCF), selected as representative compounds of NOM and SOC frequently encountered in water sources. The degradation rate and the removal efficiency of these compounds are assessed as a function of different process parameters, such as TiO2 concentration, feed-water pH, pollutant concentration and intensity of UV-A irradiation. Optimized treatment conditions result in satisfactory oxidation rates and rather high removal efficiencies, combined with stable membrane filtration performance. The encouraging results demonstrate that this novel hybrid system can operate continuously, under steady state conditions, for the efficient degradation of ROC, with no need for adding synthetic oxidants and no side-stream requiring further treatment. Keywords: photocatalytic membrane reactor, TiO2 nanoparticles, submerged ultrafiltration membrane, humic substances, polysaccharides, pharmaceuticals
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1.
INTRODUCTION
Refractory Organic Compounds (ROC) refer to a complex mixture of organic compounds that are commonly encountered in natural water sources, with concentrations ranging from a few mg/L to very low levels (pg/L to ng/L) [1]. ROC are usually recalcitrant to biological degradation and comprise Natural Organic Matter (NOM), which is the product of chemical and biological transformation of animal and plant residues, (e.g polysaccharides and humic substances) [2], and Synthetic Organic Compounds (SOC) from industrial, agricultural and other human activities (e.g. industrial chemicals, pesticides, Pharmaceutically Active Compounds - PhACs, Endocrine Disrupting Chemicals (EDC) etc.) [1]. Although ROC are usually present in small concentrations, there is public concern for their potentially adverse health effects, particularly in the case of potable water use. Specifically, evidence that NOM can act as a precursor for the formation of Disinfection By-Products (DBP), during the commonly performed chlorination of drinking water, and the potentially severe health effects associated with DBP, have led several countries to take measures to ensure control of DBP and their precursors (NOM) [2]. Furthermore, although SOC are usually present at low concentrations, many of them are considered toxic to humans particularly under long-term exposure; their negative cumulative effects are also of great concern when present as mixtures [3]. Advanced Oxidation Processes (AOP) have emerged as an attractive alternative for simultaneous removal and destruction of ROC [4-5], in contrast to conventional treatment technologies (e.g. coagulation, ion exchange, membrane filtration and activated carbon adsorption) that solely separate ROC from the water stream and “transfer” it to another phase or stream, which necessitates further treatment and/or disposal [6]. Among AOP, heterogeneous photocatalysis employing UV-A irradiation and semiconductor catalyst particles - commonly TiO2 - has a widely demonstrated efficiency for degrading a broad range of organic substances, mainly at experimental scale [4-5]. UV-A irradiation of TiO2 causes the promotion of electrons from the valence band to the conduction band, leading to the creation of highly oxidative holes on the valence band and formation of hydroxyl radicals. Thus, ROC can be degraded onto the TiO2 surface as well as by hydroxyl radicals in the fluid near the TiO2 particles [4]. Despite the large number of related experimental studies, there are insignificant full-scale applications of the heterogeneous photocatalysis technology. The successful application of suspended TiO2 photocatalytic treatment of water streams is constrained by a series of technical challenges mainly related to the effective recovery of the catalyst particles after water treatment [5]. A promising approach for separation and reuse of suspended TiO2 is the Photocatalytic Membrane Reactor (PMR) concept [7-8], involving the coupling of photocatalysis with a membrane separation process [9]. In this paper two novel systems are described, which involve coupling the suspended-type TiO2 nanoparticle photocatalysis process with ultrafiltration membranes, permitting a continuous, steady state operation. The PMR is used to treat water containing three different types of organic compounds (Sodium Alginate - SA, Humic Acids - HA and DiCloFenac - DCF), selected as representative compounds of NOM and SOC frequently encountered in water sources. The degradation rate and the removal efficiency of the aforementioned compounds are assessed as a function of several key process parameters, such as TiO2 concentration, feed-water pH, pollutant concentration and intensity of UV-A irradiation. Optimized treatment conditions are investigated concerning ROC mineralization as well as destruction of the aromatic rings of the tested compounds, manifested in the Specific UV254 Absorbance (SUVA254) measurements of the membrane permeate. The operation of the PMR is also evaluated with regard to the performance of the ultrafiltration membrane module. Special attention is paid to the attainment of steady-state operation with constant degradation rates and controlled membrane fouling phenomena.
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2.
MATERIALS AND METHODS
Titanium dioxide particles (Aeroxide® P25, Degussa-Evonik, Germany) were used as photocatalyst. The TiO2 nano-particles comprise 75% anatase and 25% rutile, have approx. 50 m2/g of active surface area, and an average primary particle size of 21 nm. In aqueous dispersions, TiO2 particles tend to aggregate and form fairly large agglomerates of size ranges depending on various parameters [10]. Organic compounds, used as representative ROC, include Sodium Alginate (Sigma-Aldrich Α2158), Humic Acids (Sigma-Aldrich H16752) and Diclofenac (Sigma-Aldrich D6899). Feed solutions were prepared using deionized water in the case of SA and HA tests and underground freshwater, without chlorination, in DCF tests. Calcium chloride dehydrate (CaCl2·H2O) supplied by Sigma-Aldrich, was added in deionized water to obtain 1 mM Ca2+ feed solution concentration. The pH of the suspension in the PMR was adjusted, by adding either HCl and H2SO4 or NaOH. The UF membranes made of hydrophilized Polyvinylidene fluoride (PVDF) with a nominal pore size of 0.04 μm were provided by Zenon Environmental Inc. Total Organ Carbon - TOC concentration of the feed, the PMR TiO2 suspension and the permeate was measured by a TOC analyzer (TOC-5000A, Shimadzu Co.). The suspension pH was monitored by a digital pH-meter (713 type, Metrohm Ltd.). DCF concentration was determined through reversed-phase HPLC using a Shimadzu (LC10AD VP) liquid chromatograph fitted with a XTerra MSC (Waters) column, (18.5μm, 150mm x 2.1mm) at 40oC, and coupled with a UV/Vis detector (SPD-10AVP) at 270 nm, after pre-concentration by solid-phase extraction (Discovery® DSC-18 SPE Tubes, 500 mg, 3 mL). Through SUVA254 measurements of feed and permeate samples, the degradation of HA aromatic rings was quantified; these rings are closely linked with the potential of organic matter for undesirable DBP formation during water chlorination. SUVA254 is defined as SUVA254 = (UV254/TOC) x 100. Measurements of UV254 absorbance of the feed and the permeate were taken with a UV/Vis spectrophotometer (UV-1700 Spectrophotometer, Shimadzu Co.) at a single wavelength (254 nm) with a 1 cm quartz cell. The UV-A light intensity of the UV-A lamps was measured by a portable radiometer (RM-12, Dr. Gröbel, UV-Elektronik GmbH, Germany) equipped with a UV-A sensor. Two laboratory scale pilot PMR systems (PMR1 and PMR2) were designed and constructed in the NRRE - CPERI Laboratory; in Figures 1 and 2 a schematic representation of these systems is provided. PMR1 is comprised of a cylindrical tank with a submerged UF membrane module of total surface area 0.47 m2. UV-A irradiation is provided by three 30W black light blue lamps (TLD 30W/08 BLB Philips), primarily emitting at 365 nm, hydraulically connected in series. The total effective volume of the PMR is 9 L. The reaction temperature is controlled at 20oC using a thermostatic bath (NesLab RTE17, Thermo Electron Co.). A piston pump (Fluid Metering Inc.) is used to withdraw permeate at the same, time averaged, flow rate as that of the feed, thus maintaining constant the working volume of the PMR. The operating permeate flux was kept constant at approx. 14 L/(m2·h). An online pressure transducer, connected to a computer, was used to monitor the Trans-Membrane Pressure (TMP), which allows to assess the fouling behaviour of the membrane module. Air was supplied into the membrane tank through a coarse bubble aerator placed at the core of the cylindrical membrane module; the aeration rate was approx. 0.02 Nm3/min. An automated periodic backwashing operation was implemented to mitigate membrane fouling. PMR2 is comprised of two tanks, i.e. a jacketed cylindrical vessel (made of anodized aluminum) with an effective volume of 2.3 L (photoreactor) and a Plexiglas vessel with an effective volume of 0.7 L, where a custom-made UF membrane module (of total surface area of 0.097 m2) is submerged. Four borosilicate glass tubes, in the form of sleeves closed at the immersed end, were fixed and properly sealed on holes of the top flange of the photoreactor. Four 24W black light lamps (Actinic BL PL-L 24W/10/4P Philips)
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emitting at wavelength 365 nm were employed as UV-Α light source of the system. The reaction temperature is controlled at 20oC using a thermostatic bath (NesLab RTE17, Thermo Electron Co.). A piston pump (Fluid Metering Inc.) is used to withdraw permeate at the same (time averaged) flow rate as that of the feed. The operating permeate flux was kept constant at approx. 15 L/(m2·h). An online pressure transducer, connected to a computer, was used to monitor the TMP and assess the membrane fouling behavior. Air was supplied by a small tube placed at the center of the photoreactor at a rate of 0.0012 Nm3/min. An automated periodic backwashing operation was implemented to mitigate membrane fouling. A magnetic stirrer provided sufficient mixing, thus ensuring uniformity of catalyst suspension throughout the photoreactor. A centrifugal pump (Iwaki Co. Ltd) was used to recirculate the TiO2 suspension through the photoreactor and the membrane tank at a volumetric rate of approx. 8 L/min.
Figure 1. Schematic diagram of the PMR01 system
Figure 2. Schematic diagram of the PMR02 system 3.
RESULTS AND DISCUSSION
3.1. Photocatalytic oxidation of Humic Acids The main experimental conditions and results concerning HA photocatalytic oxidation are summarized in Table 1. It appears that there is an optimum pH near 5.5 where the HA
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mineralization rate reaches a maximum for both HA feed concentrations (5 and 10 mg/L HA); this maximum rate is 9.56 and 14.13 mgTOC/h, respectively. The same trend holds for the HA mineralization efficiency that varies from 49.8% to 73.9% for 5 mg/L HA feed concentration, and from 49.6% to 62.6% for 10 mg/L HA feed concentration (Figure 3).
Experiment Nο
Table 1. Main experimental data concerning HA photocatalytic oxidation pH
[TiO2] (g/L)
Feed [TOC]* (mg/L)
Permeate [TOC]* (mg/L)
Backwashing mode
UV-A radiant power (W)
ΗA-01
3.9
0.75
2.38
0.81
1/9
15.1
ΗA-02
5.6
0.75
2.38
0.54
1/9
15.1
ΗA-03
6.7
0.75
2.62
1.25
1/9
15.1
ΗA-04
3.8
0.75
3.67
0.62
1/9
15.1
ΗA-05
5.5
0.75
4.27
1.03
1/9
15.1
ΗA-06
7.5
0.75
4.08
1.53
1/9
15.1
ΗA-07
5.7
0.75
2.31
0.56
1/15 15.1 * time average values
The effect of pH on the HA mineralization rate can be associated with several factors, including (i) the ζ-potential of the TiO2 particle surface, (ii) the agglomeration of TiO2 particles, and (iii) the formation potential of hydroxyl radicals [11]; the interplay of these factors seems to be complicated. The iso-electric point for Degussa P25 TiO2 is at pH = ~6.3 and the catalyst surface is positively charged in acidic conditions, thus promoting adsorption and subsequent oxidation of negatively charged molecules such as HA. However, TiO2 particles tend to agglomerate under acidic conditions [7] and the specific surface area of catalyst agglomerates is reduced, negatively affecting the photo-oxidation rates. The backwashing mode employed does not seem to affect the effectiveness of the photocatalytic mineralization process. Indeed, although Εxp. No. HA-02 and HA-07 were carried out under two different backwashing modes (1/9 and 1/15), with otherwise identical operating conditions, the HA estimated mineralization rates were quite close, (i.e. 9.56 and 9.47 mgTOC/h, respectively), and so were the mineralization efficiencies.
(a)
(b)
Figure 3. Effect of pH on (a) the HA mineralization rate and (b) the HA mineralization efficiency for two HA feed concentrations: 5 mg/L, and 10 mg/L; the numbers in parentheses at the base of the bars denote the measured time averaged pH.
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Concerning the reduction of the DBP formation potential of water streams containing HA, the PMR exhibits an excellent performance. The feed SUVA254 values vary between 4.73 and 6.44 m-1/(mg/L) whereas the permeate SUVA254 is below 0.36 m-1/(mg/L), except for Exp. No. HA-06, where it is quite higher [1.24 m-1/(mg/L)]. Thus, the overall SUVA254 removal efficiency is very high (over 95% in almost all cases); i.e. consistently higher than the corresponding HA overall removal efficiency. 3.2. Photocatalytic oxidation of Sodium Alginate The main experimental conditions and results concerning SA photocatalytic oxidation are summarized in Table 2. It is obvious that the catalyst concentration is an important parameter that can greatly affect the overall PMR process. Experiments were performed with six different TiO2 concentrations in the range 0.25 to 1.5 g/L.
Experiment Nο
Table 2. Main experimental data concerning SA photocatalytic oxidation
SA-01 SA-02 SA-03 SA-04 SA-05 SA-06 SA-07 SA-08 SA-09 SA-10 SA-11
pH
[TiO2] (g/L)
Feed [TOC]* (mg/L)
Permeate [TOC]* (mg/L)
Backwashing mode
6.8 6.6 7.2 6.7 6.9 6.8 6.5 6.8 6.5 6.7 6.3
0.25 0.50 0.75 1.00 1.25 1.50 0.25 0.50 0.75 0.75 1.00
2.38 2.09 2.19 2.50 2.49 2.46 2.47 2.66 2.23 2.55 2.61
1.60 0.87 0.49 0.68 0.73 0.86 1.49 1.09 0.58 0.73 0.73
1/5 1/5 1/5 1/5 1/5 1/5 1/9 1/9 1/9 1/15 1/15
UV-A radiant power (W)
17.7 17.7 17.7 17.7 17.7 17.7 17.7 17.7 17.7 17.7 17.7 * time average values
In Figure 4, one observes that an increase of catalyst loading up to 1 g/L leads to a steady increase of the SA mineralization rate, whereas the mineralization efficiency of SA reaches a maximum at 0.75 g/L TiO2. The maximum mineralization rate is approx. 10.37 mgTOC/h and the maximum mineralization efficiency is approx. 75%; these values are close to those for HA photo-oxidation under similar conditions. One might have expected this trend since the increase of the TiO2 catalyst concentration leads to an increase of the active sites on TiO2 surface, i.e., the surface area of the TiO2 available for degradation. However, further increase of TiO2 concentration appears to have a negative effect on percentage TOC removal. This particular trend is usually attributed [7] to possible UV-A light blocking, resulting from the increased turbidity of the higher TiO 2 suspension concentration. It is interesting to note that experiments performed with the same concentration of photocatalyst, but under different backwashing frequencies, exhibit practically the same mineralization efficiency.
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(b)
(a)
(g/L)
(g/L)
Figure 4. Effect of TiO2 concentration on (a) the SA mineralization rate and (b) the SA mineralization efficiency. Data refers to experiments with backwashing mode 1/5. 3.3 Photocatalytic oxidation of Diclofenac The main experimental conditions and results concerning DCF photocatalytic oxidation are summarized in Table 3.
Experiment Nο
Table 3. Main experimental data concerning DCF photocatalytic oxidation pH
[TiO2] (g/L)
Feed [TOC]* (mg/L)
Permeate [TOC]* (mg/L)
Backwashing mode
UV-A radiant power (W)
DCF-01
6.2
0.75
2.18
1.31
1/9
19.7
DCF-02
6.2
0.75
2.14
0.90
1/9
19.7
DCF-03
5.2
0.75
2.39
0.98
1/9
19.7
DCF-04
6.2
0.50
2.41
0.74
1/9
19.7
DCF-05
6.1
0.25
2.05
0.90
1/9
19.7
DCF-06
6.2
0.75
2.02
0.95
1/9
19.7
DCF-07
6.2
0.50
2.07
0.77
1/9 19.7 * time average values
Three different TiO2 concentrations have been used in these experiments: i.e. 0.25, 0.50 and 0.75 g/L. Figure 5 presents the TOC removal efficiency for various TiO 2 concentrations and different initial DCF concentration in the photoreactor [DCF0]. The mineralization efficiency varies between 39.8% and 75%, and it seems to achieve a maximum at 0.5 g/L TiO2 concentration. Mineralization efficiency refers to the complete destruction of both DCF molecule and its partial oxidized derivatives. The DCF alone is almost completely oxidized (> 99%) in all the experiments; its permeate concentration is approx. 0.01 mg/L for a feed concentration varying between 2 and 2.25 mg/L.
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100
TOC Removal (%)
90 80 70 60 50 40 30 0
0.2
0.4
0.6
0.8
1
TiO concentration (g/L) 2
Figure 5. Effect of TiO2 concentration on DCF mineralization efficiency; [DCF0] = 0 mg/L, [DCF0] = 8.0 mg/L και [DCF0] = 2.5 mg/L 3.4 Membrane filtration performance Two typical TMP vs. time diagrams are presented in Figure 6; the first (Fig. 6a) depicts the TMP temporal profile at the beginning (t = 1 h) of Exp. No. HA-02 and the second (Fig. 6b) at the end (t = 48 h) of the same experiment. It can be concluded that during the filtration operation there is hardly any fouling layer formation, neither at the beginning nor at the end of the experiment, as the TMP remains practically constant during the filtration stage and the two profiles are almost identical. Different backwashing schemes were employed both for HA and SA photoxidation and it was concluded that the TMP remained practically constant for the experimental period of approx. 48 h. This fact essentially proves that membrane fouling was insignificant, thereby allowing the successful continuous PMR operation under a moderate flux; i.e. 14 L/(m2·h).
Figure 6. Temporal variation of TMP during Exp. No. HA-02; the shaded areas designate backwashing periods. 4. CONCLUSIONS The presented encouraging results suggest that the novel hybrid processes described herein, operating in a continuous mode with no waste stream are capable of efficiently
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degrading ROC at typical concentrations encountered in common water treatment tasks. Further ongoing research is essential for paving the way towards practical applications. REFERENCES 1.
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