Electrospun nanofibre membranes functionalised with TiO2 nanoparticles: Evaluation of humic acid and bacterial removal from polluted water
Introduction
TiO2 nanoparticles are known for their great availability, non-toxicity, stability, low cost and efficiency [1]. The exposure of TiO2 nanoparticles to UV light with an energy that matches their band gap energy leads to an excitation of electrons from the valence band to the conduction band, leaving holes in the valence band. The electrons react with molecular oxygen molecules producing superoxide radical anions while the holes react with water to produce hydroxyl radicals. These two reactive species can decompose organic materials [2], [3]. Immobilisation of TiO2 in a membrane photoreactor omits the need for photocatalyst separation in a subsequent process [4]. The longer the TiO2-particles are illuminated with UV light, the more organic material can be decomposed [2]. In literature an illumination time of 30 min to 6 h is commonly used [5], [6], [7], [8]. By incorporating TiO2 into membranes, extra functionality can be obtained which ensures the above mentioned photocatalytic and antimicrobial properties, as well as removal of turbid material by filtration. Kim et al. [9], for example, demonstrated the bactericidal action of TiO2, limiting bio-fouling on a membrane functionalised with TiO2 nanoparticles by using a reactor set-up that was illuminated with UV for 4 h per day.
Electrospinning was identified as a simple, rapid and inexpensive method [10], to produce such (TiO2) functionalised membranes [8]. Electrospinning is a production process for continuous nanofibres in a non-woven form, currently in the beginning phase of industrial scale production [5], [11]. These nanofibre membranes have small pore sizes and a large surface area to volume ratio compared to other non-woven membranes. Nanofibre membranes have a low density and interconnected open pore structure, making this non-woven membrane appropriate for a wide variety of filtration applications [12]. One of the potential applications of the nanofibres is water filtration as was initially tested in previous studies [13], [14], [15], [16], [17], [18]. An interesting feature of these microfiltration membranes is the high clean water permeability (CWP). For example, Daels et al. [8] obtained a CWP of 24 × 103 l/m2 h bar at a mean pore size of the membrane between 0.2 and 0.4 μm, which is very high compared to other commercial available microfiltration membranes (typical 3 × 103 l/m2 h bar) allowing high flux operations. Furthermore, successful incorporation of biocides was already performed on the electrospun nanofibre membranes, providing an antimicrobial effect [16], [19]. Also functionalisation with nTiO2 was proven successful by degrading methylene blue under UV irradiation [8]. Methylene blue is often used as a standard model compound for organic contaminants in waste water [6], [20], [21]. However, actual organic contaminants such as humic acids have a much more complex structure than methylene blue, with a variety of components including quinone, phenol, catechol and sugar moieties [22]. Therefore the results obtained in previous studies need to be verified with actual waste water. In particular, it seems interesting to study the treatment of secondary effluent of a wastewater treatment plant (WWTP).
Such secondary effluent is often discharged into surface waters, while there is an increased interest in water reuse. Humic acids represent a major fraction of natural organic matter in secondary effluent and consist of a complex mixture of macromolecular organic matter which is derived from the decomposition of plant and animal materials [22]. As these effluents cannot be reused without further treatment, numerous studies on advanced treatment of secondary effluent are initiated. The recalcitrant moieties in humic acids cannot be removed by biological treatment and introduce problems in further water treatment methods such as membrane filtration and disinfection processes. Humic acids are not hazardous for humans but reactions with halogen-based disinfecting agents (e.g. sodium hypochlorite) can form disinfection by-products [23] which are mutagenic and carcinogenic [24]. Humic acids are also a major contributor to the fouling of (micro)filtration processes [25], [26]. It is further a necessity to remove bacteria from water to enhance effluent quality and to avoid bacterial growth on membrane surfaces. Some bacteria may adhere to surfaces within the water system, excrete a matrix of polysaccharides and proteins (peptidoglycan), and form a biofilm on surfaces of industrial water systems, causing increased corrosion rates or bio-fouling on membrane surfaces [27]. Turbidity is an expression of the optical property of water that causes light to be scattered and is caused by the presence of suspended matter such as clay, (in)organic matter and microscopic organisms in water [28]. Suspended solids play an important role in defining the overall quality of secondary waste water, acting as a pollutant or by transporting other pollutants [29], [30]. Increase in suspended solids corresponds with a number of negative effects on freshwater ecosystems, including the variation of water quality and direct impact on planktonic and fish population [31]. Turbidity can be an indication for transport of nutrients which support microbial growth in the distribution system [32]. Also contaminants like viruses or bacteria can become attached to the suspended solids [28], [33]. In addition, coliform survival at high turbidity levels is possible if coliforms are embedded in suspended particles and chlorine is not able to come into contact with bacteria [28].
As humic acids, bacteria and particles pose a problem regarding secondary effluent quality in view of discharge of re-use, the objective of this study was to investigate the (photocatalytic) efficiency of (TiO2 functionalised) nanofibre membranes for secondary effluent treatment. More life-like circumstances than the standard methylene blue test [34] will be applied, by the use of a synthetic solution of synthetic humic acids, a bacterial solution and real secondary effluent. The synthetic humic acids were used in a higher concentrated (i.e. 60 mg/l), humic acid solution to simulate higher effluent with a darker, less transparent colour such as landfill leachate treatment plant effluent [35]. Contact tests were performed on this 60 mg/l synthetic humic acid solution and on a cell solution of Staphylococcus aureus. Contact and filtration experiments were performed on real secondary effluent. As such, the activity of functionalised nanofibre membranes was assessed by:
- (i)
Contact experiments on photodegradation of humic acids (secondary effluent and synthetic humic acids) with (nTiO2 functionalised) nanofibre membranes.
- (ii)
Contact experiments on photodegradation of S. aureus with (nTiO2 functionalised) nanofibre membranes.
- (iii)
Filtration of secondary effluent with (nTiO2 functionalised) nanofibre membranes under UV illumination.
Section snippets
TIO2 nanoparticles
The colloid solution consisting of TiO2 nanoparticles was prepared by acidic hydrolysis of TiCl4 in a manner analogous to the one proposed by Rajh at al. [36]. The solution of TiCl4 cooled down to −20 °C was added drop-wise to cooled water (at 4 °C) under vigorous stirring and then kept at this temperature for 30 min. The pH of the solution was between 0 and 1, depending on TiCl4 concentration. Slow growth of the particles was achieved by dialysis against water at 4 °C until the pH of the solution
Photodegradation contact tests
Contact tests on photodegradation with nTiO2 functionalised membranes were performed on (i) WWTP effluent, (ii) synthetic humic acids with a concentration of 60 mg/l and (iii) S. aureus (initial concentration 8 × 109 CFU/100 ml).
Conclusions
In this study, the photocatalytic activity and filtration performance of nanofibre membranes functionalised with TiO2 was demonstrated with different types of wastewater. Contact experiments showed extra removal on humic acids and S. aureus when using TiO2 functionalised membranes. Humic acid removal of 83% of secondary effluent was obtained after 2 h of illumination using a post-functionalised commercial TiO2 membrane which was the best performing membrane. Further, 60% degradation after 2 h of
Acknowledgements
Financial support from The Agency for Innovation by Science and Technology of Flanders (IWT-Tetra project contract numbers 060113, 080148 and 140218), the Ghent University Special Research Fund and Ministry of Education, Science and Technological Development of Republic of Serbia (Project No. 172056) is gratefully acknowledged.
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