Membranes and Membrane Processes in Environmental Protection Monographs of the Environmental Engineering Committee Polish Academy of Sciences 2014, vol. 119, 173-184 ISBN 978-83-63714-18-5 APPLICATION OF SORPTION/PHOTOCATALYSIS/MEMBRANE SEPARATION IN TREATMENT OF WATER CONTAINING ESTROGENS AND XENOESTROGENS Mariusz DUDZIAK1 Abstract: The combined system of the sorption on activated carbon, photocatalysis, microfiltration and nanofiltration is the novel solution proposed to treat water containing estrogenic micropollutants. The combination of these techniques improves the purification effectiveness in comparison with the use of the single-stage processes. The efficiency of the combined process was evaluated on the basis of decomposition rates of 17β-estradiol and bisphenol A from various simulated water solutions. The hydraulic membrane capacity was also determined. It was shown, that the efficiency of estrogenic micropollutants decomposition was affected by the presence of natural organic matter in water (humic acids). At the presence of humic acids in water both, the decrease of micropollutants decomposition effectiveness was observed and the membrane blockage was more severe. The effectiveness of the treatment was improved by the addition of activated carbon. It was also an advantage considering the hydraulic membrane capacity. Keywords: combined system, sorption, photocatalysis, microfiltration, nanofiltration, estrogenic micropollutants INTRODUCTION Nowadays, the improvement of water treatment plants operation is based on the application of novel, highly effective methods and processes. Hence, the use of integrated systems combining conventional processes (coagulation, sorption on activated carbon) with advanced oxidation processes is observed [1]. One of the unit operation involved in such a modern system of treatment can also be pressure driven membrane processes (microfiltration, ultrafiltration, nanofiltration) [2]. Advanced oxidation processes assure the increase of the oxidation rate of lowmolecular weight and hardly chemically degradable organic compounds. One of the applied method is the heterogeneous photocatalysis with the use of UV/TiO2 in which the catalyst is in a form of a suspension. In the discussed method 1 Silesian University of Technology, Institute of Water and Wastewater Engineering, Konarskiego 18, 44-100 Gliwice, Poland, [email protected] 174 Dudziak M. the phenomenon of TiO2 catalyst activation with UV radiation leading to the formation of hydroxyl radicals which enable the oxidation of organic compounds is used [3]. Considering the necessity of catalyst particles separation the process is combined with low-pressure membrane techniques i.e. microfiltration or ultrafiltration [4,5]. The disadvantageous of such a solution is a possibility of low-molecular weight organic compounds (oxidation byproducts) permeation through the membrane, what decreases the efficiency of the treatment [2]. Moreover, the use of this type of membrane processes do not guarantee the removal of micropollutants, which cannot be completely eliminated via the photocatalysis, from water. Thus, novel configurations of water treatment processes which characterize with high efficiency of various contaminants types removal are developed and investigated. The aim of this study was to evaluate the efficiency of a water treatment system which combined sorption on activated carbon with photocatalysis, microfiltration and nanofiltration. The activated carbon was introduced to the photoreactor together with the catalyst. Next, after sorbent and catalyst particles separation in microfiltration, polishing of water via nanofiltration was applied. The efficiency of the process was determined on the basis of decomposition rates of particular estrogenic micropollutants chosen as a representatives of estrogenic and xenoestrogenic compounds. Additionally, the effectiveness of humic acids decomposition was investigated. The efficiency of hydraulic membrane capacity was also determined. EXPERIMENTAL Waters Simulated water solutions (pH = 7.0) prepared on the basis of deionized and tap waters with and without addition of humic acids (HA) were used. 17β-estradiol (E2), a natural estrogenic compound produced in living organisms [6] and bisphenol A (BPA) - an anthropogenic organic compound [6, 7] were added to all treated waters in the amount of 500 µg/L. Humic acids were used as representatives of high-molecular weight organic substances present in natural surface water [8]. The physicochemical characteristic of investigated waters is presented in Table 1. Standards of humic acids, 17β-estradiol and bisphenol A were supplied by Sigma-Aldrich (Poland). Application of sorption/photocatalysis/membrane separation… 175 Table 1. Physicochemical characteristic of the waters containing estrogenic micropollutants Type of waters Deionized water Tap water Tap water + 15 mgHA/L * pH 7.0* Conductivity, µS/cm 5.180 1064 Absorbance (UV=254 nm), 1/cm 0.000 0.004 1122 0.170 correction of water pH was made using of 0.1 mol/L HCl or 0.2 mol/L NaOH; HA humic acids. Chemical analysis The concentration of high-molecular weight organic compounds (humic acids) in water was determined via UV absorbance (λ=254 nm) using UV VIS Cecil 1000 spectrometer by Jena AG, while inorganic compounds (sum of the ions) via measurement of specific conductivity with the use multiparameter meter Inolab ® 740 by WTW. The micropollutants presence was determined using solid phase extraction (SPE) method proceeded with high performance liquid chromatography (HPLC). SupelcleanTM ENVI-18 (volume 6 mL, solid phase 1.0 g) tubes by Supelco were used in extraction. The bed of tubes was firstly conditioned with methanol (5 mL) and acetonitrile (5 mL) and washed with deionized water (5 mL). The extracted compounds were washed out of the tubes with the mixture of acetonitrile and methanol (1 mL) in the ratio 60:40 (v/v). The quantitativequalitative analysis of micropollutants in the obtained extract was made using HPLC with UV detector (λ=220 nm). Hypersil Gold C18 column by Polygen of length 25 cm, diameter 4.6 mm and sorbent granulation 5 µm was used. Acetonitrile by POCH was applied as the mobile phase. Photocatalysis with and without the addition of activated carbon The photocatalysis process was carried out at temperature 20ºC in Heraeus reactor equipped with medium-pressure immersed lamp of power 150 W (the exposure time varied from 5 to 60 min). Commercial titanium dioxide P25 by Degussa of dose 100 mgTiO2/L was used as a catalyst. Powdered activated carbon (CWZ-30 by Gryfskand, doses range 1-20 mgPAC/L) was added to the photocatalytic reactor together with the catalyst. In order to obtain reference results photocatalysis process was also performed without the addition of activated carbon Membrane separation The membrane separation process is based on microfiltration and nanofiltration processes. The nanofiltration process was preceded by the filtration of solution through 0.45 µm cellulose acetate membrane filter (microfiltration) by Millipore, what enabled the separation of catalyst and activated carbon particles. Volumetric 176 Dudziak M. permeate flux of the microfiltration membrane for deionized water (∆P = 0.1 MPa) was equal to 26.2·10-3 m3/m2·s. Commercially available NF-DK composite nanofiltration membrane by GE Osmonics (USA) of characteristic presented in Table 2 was used in the polishing step. The membrane filtration was carried out at transmembrane pressure of 0.1 MPa (microfiltration) or 2.0 MPa (nanofiltration) in a steel membrane cell (equipped with feed tank of volume 350 mL and flat sheet membrane cell of effective separation area 38.5 cm2) which enabled the performance of the process in the dead-end mode. Dead-end systems are not used very often. It is mainly due to the fact that filtration condition doesn't fit to crossflow systems used in industrial scale. However, the simplicity of the filtration modules causes that this is the quite popular tool in organic micropollutants separation mechanism analysis. Table 2. Properties of nanofiltration membrane (manufacturer date) Membrane NF-DK * Molecular weight cut-off, Da 150-300 Jv*, m /m2·s 19.8·10-6 3 Removal of MgSO4, % 98 volumetric permeate flux for deionized water at a transmembrane pressure 2.0 MPa: Jv=V/F·t where V - volume [m3], F - membrane area [m2], t - filtration time [s]. Combined treatment The study evaluating the efficiency of estrogenic micropollutants decomposition in the integrated photocatalysis-microfiltration-nanofiltration process were based on the treatment of water via photocatalysis (with and without the addition of activated carbon), next the separation of the catalyst (and activated carbon) particles was performed in microfiltration and, finally, nanofiltration was carried out. The hydraulic membrane capacity was also evaluated. The study results were compared with ones obtained during single-step nanofiltration water treatment. RESULTS AND DISCUSSION The results on the effectiveness of estrogenic micropollutants decomposition via photocatalysis without activated carbon addition are shown in Fig. 1. The results of photocatalytic oxidation of compounds present in deionized water were used as a reference once. Aditionally, it was determined that adsorption rate of bisphenol A and 17β-estradiol on the TiO2 particles (catalysis process) was slight and in both analyzed cases was lower than 5%. This phenomenon is commonly observed in the case of adsorption of organic micropollutants on TiO2 particles [9]. The highest decomposition of micropollutants was obtained during photocatalysis of tap water containing mainly inorganic compounds (determined via the measurements of specific conductivity). It was also observed, that the addition of humic acids to Application of sorption/photocatalysis/membrane separation… 177 treated water resulted in the decrease of photocatalysis effectiveness. Thus, it was concluded that the presence of inorganic compounds in water enhanced the oxidation of micropollutants, while of humic acids revealed an opposite effect. It was probably caused by the competitive action of high-molecular weight organic compounds on the catalyst surface, what limited the elimination of micropollutants. The competition phenomenon between various adsorbants (e.g. natural organic matter and phenol) of different concentration was also observed during water treatment with the use of activated carbon adsorption [8]. From among inorganic compounds on photocatalytic oxidation process intensification of micropollutants have an effect e.g. iron salts [10]. It was found, that the 17β-estradiol decomposition rate was higher than ones of bisphenol A. For example, 17β-estradiol and bisphenol A decomposition rates observed during treatment of tap water were equal to 63% and 48%, respectively. 100 90 Ef f iciency of decomposition, % 80 70 60 50 40 30 E2 BPA E2 BPA E2 10 BPA 20 0 Deionized water Tap water Water Tap water + HA Fig. 1. The influence of water composition on the effectiveness of estrogenic micropollutants decomposition during photocatalysis (UV irradiation time 5 min). The increase of the estrogenic micropollutants decomposition rate was obtained for integrated photocatalysis-activated carbon sorption configuration (Fig. 2). 178 Dudziak M. a Ef f iciency of bisphenol A decomposition, % 100 90 80 70 60 50 40 30 20 10 0 0 5 10 0 mgPAC/L 25 30 35 Time, min. 1 mgPAC/L 100 5 mgPAC/L 40 45 50 55 60 20 mgPAC/L 1 mgPAC/L 90 80 70 60 E2 BPA Abs. in UV 10 E2 20 E2 30 BPA 40 Abs in UV 50 Abs. in UV Ef f iciency of decomposition (Abs. decrease), % 20 BPA b 15 0 5 10 Time, min. 30 Fig. 2. Effect of activated carbon dosage (a) and UV irradiation time (b) on the effectiveness of estrogenic micropollutants decomposition (absorbance decrease) during photocatalysis (tap water + HA). It was found, that the efficiency of the process depended on the activated carbon dose (Fig. 2. a) and exposure time (Fig. 2. b). Such an effect was explained Application of sorption/photocatalysis/membrane separation… 179 by the sorption of low-molecular weight estrogenic compounds on activated carbon particles, which were present in the treatment system. Moreover, as it was observed by Li and Liu [11], the presence of activated carbon also improved photocatalysis process performance at certain conditions. According to the mechanism proposed by the authors TiO2 particles adsorb on the activated carbon specific surface (what was confirmed by SEM photos) and behave comparably to organic compounds particles. The oxidation reaction runs more effective when the catalyst is immobilized on the activated carbon than when it is in the form of a free suspension (system without the activated carbon addition). The author of [12] observed such a phenomenon during photocatalysis of high-molecular weight organic compounds (humic acids). The results of the study discussed in this paper also confirmed such a behavior (Fig. 2. b). However, the efficiency of high-molecular weight organic compounds decomposition was much lower than low-molecular weight micropollutants. This is the result of the fact that humic acids concentration was much higher in comparison to estrogenic micropollutants. As a result, decomposition of them was lower at the same photocatalysis conditions. Additionally, the curve trend of bisphenol A decomposition rate on the exposure time showed that the treatment effectiveness depended on process conditions (Fig. 2. a). During photocatalytic oxidation at the absence of activated carbon the bisphenol A decomposition rate increased linearly with the exposure time increase. In the case when activated carbon was added to the system, such a tendency was observed only within 30 minutes of the process. After that, exposition time was not important as earlier. The exposure time had no impact on bisphenol A removal when the highest dose of activated carbon (20 mgPAC/L) was applied. Similar dependences in the case of the photocatalitic decomposition of the 17β-estradiol was observed. Thus, it was concluded that at such conditions photocatalytic oxidation of contaminants was limited. It was probably caused by the turbidity increase and limitation of radiation within the solution. The phenomenon is known as a “screening effect” [13-16]. At a very high carbon doses intermolecular interactions and particles agglomeration limit the surface available for micropollutants. Finally, it results in the decrease of the water treatment effectiveness. The efficient water polishing was enabled by nanofiltration (Fig. 3). The results obtained for integrated process were compared with ones of single step nanofiltration. The efficiency of estrogenic micropollutants elimination via integrated photocatalysis-microfiltration-nanofiltration process with and without addition of activated carbon exceeded 99% and was higher than one of single-step nanofiltration. The explanation that the fact that single-step nanofiltration can not remove estrogenic micropollutants (in opposite to the integrated process) can be formulated based on the concentration differences between the solutions. In the case of the integrated system, part of the micropollutants were decomposed during the photocatalysis or adsorbed on the activated carbon. As a result, the concentration of the compounds in the solution before the nanofiltration was lower and effectiveness 180 Dudziak M. of the elimination higher. The impact of compound concentration on the elimination is very often phenomenon observed during nanofiltration [17]. Considering the elimination of high-molecular weight organic substances (determined by the measurements of UV absorbance) was high in both cases i.e. single-step nanofiltration and its integration with photocatalysis enhanced with activated carbon adsorption and microfiltration. The performance of the integrated process without the addition of activated carbon did not result in the effective elimination of low-molecular compounds (oxidation byproducts) which permeated through the membrane what decreased the treatment efficiency. It is about another than humic acids organic compounds containing aromatic ring in the particle reveal absorbance in UV254. The addition of the activated carbon to the photocatalytic reactor significantly limited the phenomenon. 90 80 70 60 50 40 E2 BPA Abs. in UV E2 BPA Abs in UV 10 E2 20 BPA 30 Abs. in UV Ef f iciency of decomposition (Abs. decrease), % 100 0 NF FK+MF+NF Process S+FK+MF+NF Fig. 3. Decomposition of estrogenic micropollutants (absorbance decrease) in the nanofiltration and integrated processes (tap water + HA, UV irradiation time in photocatalysis 5 min, activated carbon dosage 1 mgPAC/L). In Fig. 4 the comparison of membrane capacities obtained during simulated water nanofiltration (tap water with addition of humic acids) with and without its preceding with photocatalysis and microfiltration is shown. The photocatalysis process was performed with and without activated carbon addition (the dose 1 mgPAC/L). The deionized water filtration was used as a reference process. The volumetric permeate flux obtained for integrated photocatalysis (enhanced with activated carbon)-microfiltration-nanofiltration was higher than one of single-step nanofiltration. It is because the application of the advanced water treatment Application of sorption/photocatalysis/membrane separation… 181 preceding membrane filtration sufficiently limits the intensity of disadvantageous phenomena which usually accompany membrane filtration. Those are accumulation of humic acids on membrane surface which causes its blockage i.e. fouling [18]. Integrated photocatalysis-microfiltration-nanofiltration process realized without the activated carbon was also very profitable, taking into consideration membrane capacity. In the earlier works in this field [2] high effectiveness of the elimination low-molecular weight mycoestrogens (zearalenone and α-zearalenol) was proved. 24 Volumetric permeate f lux (J v ), 10-6 m3/m2·s 22 20 18 16 14 12 10 8 Deionized water 6 Simulated water (tap water with humic acids) 4 Simulated water af ter photocatalysis with microf iltration 2 Simulated water af ter photocatalysis with sorption and microf iltration 0 0 5 10 15 20 25 30 35 Time, min. 40 45 50 55 60 Fig. 4. Effect of water composition on nanofiltration membrane capacity. In the conclusion of this part of the work it should be emphasized that high capacity nanofiltration membrane in the integrated system is caused also by the fact that catalyst and activated carbon particles was separated in microfiltation process effectively. However, high fluctuation of the volumetric permeate flux was observed. In the case of the simulated water after photocatalysis process (determination conditions were given in Fig. 3 caption) volumetric permeate flux of the membrane during separation of the catalyst particles or both catalyst and activated carbon particles were decreased by appx. 47%. Additive of activated carbon (dosage 1 mgPAC/L) didn’t influence on this parameter. Those information are connected to one filtration cycle-time for taking 50% of the initial feed volume. Intensive membrane blockage was observed. This is one of the reasons to motivate further works in this area. 182 Dudziak M. CONCLUSIONS The high efficiency of the integrated photocatalysis (enhanced with activated carbon adsorption)-microfiltration-nanofiltration water treatment application to elimination low-molecular weight estrogenic micropollutants as well as highmolecular weight substances (humic acids) was revealed during this study. The results obtained for the integrated process were much more satisfactory than once of single-step photocatalysis. It was found that the introduction of activated carbon to the treatment system improved both, micropollutants elimination rate and membrane hydraulic capacity. The application of nanofiltration as a polishing step enabled the decrease of concentration of both, organic substances and investigated micropollutants, which were not completely eliminated during applied photocatalysis process configurations i.e. with and without its enhancement with activated carbon. Analysed integrated process shoud be improved in the field of the catalyst and activated carbon particles separation. ACKNOWLEDGEMENTS This work was supported by the Silesian University of Technology project no BK-256/RIE4/2013 and BK-266/RIE4/2014. REFERENCES 1. 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ZASTOSOWANIE SORPCJI/FOTOKATALIZY/SEPARACJI MEMBRANOWEJ W OCZYSZCZANIU WODY ZAWIERAJĄCEJ ESTROGENY I KSENOESTROGENY Mariusz DUDZIAK Streszczenie: Nowym rozwiązaniem oczyszczania wody zawierającej mikrozanieczyszczenia estrogeniczne może być połączenie procesu sorpcji na węglu aktywnym, fotokatalizy, mikrofiltracji i nanofiltracji. Ma to za zadanie zmniejszenie ograniczeń tych procesów realizowanych oddzielnie. Skuteczność łącznego procesu oceniona została na podstawie stopnia rozkładu 17β-estradiolu i bisfenolu A z różnych roztworów wody modelowej. Oceniono również wydajność hydrauliczną membrany. Wykazano, że skuteczność usunięcia mikrozanieczyszczeń estrogenicznych była związana z obecnością naturalnych substancji organicznych w wodzie (kwasy humusowe). Podczas oczyszczania wody zawierającej kwasy humusowe równocześnie zmniejszała się efektywność rozkładu mikrozanieczyszczeń oraz następowało intensywne blokowanie membrany. Wzrost skuteczności usuwania mikrozanieczyszczeń uzyskano, gdy do wody dodano węgiel aktywny. W tych warunkach poprawiła się również wydajność hydrauliczna membrany. Słowa kluczowe: system kombinowany, sorpcja, fotokataliza, mikrofiltracja, nanofiltracja, mikrozanieczyszczenia estrogeniczne.
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