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Bio-inspired multifunctional membrane for aquatic micro-pollutants removal Xiaotong Cao, Jianquan Luo, John M. Woodley, and Yinhua Wan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b10823 • Publication Date (Web): 21 Oct 2016 Downloaded from http://pubs.acs.org on October 22, 2016
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ACS Applied Materials & Interfaces
Bio-inspired Multifunctional Membrane for Aquatic Micro-pollutants Removal Xiaotong Cao,ab Jianquan Luo,a* John M. Woodleyc and Yinhua Wana* a.
State Key Laboratory of Biochemical Engineering, Institute of Process Engineering, University of the Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, China. b.
c.
Sino-Danish College, University of the Chinese Academy of Sciences, Beijing 100049, China.
Department of Chemical and Biochemical Engineering, Technical University of Denmark, 2800, Kgs. Lyngby, Denmark
ABSTRACT Micro-pollutants present in water have many detrimental effects on the ecosystem. Membrane technology plays an important role in the removal of micro-pollutants but there remain significant challenges such as concentration polarization, membrane fouling and variable permeate quality. The work reported here uses a multifunctional membrane with rejection, adsorption and catalysis functions to solve these problems. Based on mussel-inspired chemistry and biological membrane properties, a multifunctional membrane was prepared by applying ‘reverse filtration’ of a laccase solution and subsequent ‘dopamine coating’ on a nanofiltration (NF) membrane support, which was tested on bisphenol A (BPA) removal. Three NF membranes were chosen for the preparation of the multifunctional membranes based on the membrane properties and enzyme immobilization efficiency. Compared with the pristine membrane, the multifunctional membrane exhibited significant improvement of BPA removal (78.21±1.95%, 84.27±7.30% and 97.04±0.33% for NT103, NF270 and NF90 respectively), all of which are clearly superior to the conventional Fenton treatment (55.0%) under similar conditions, and comparable to soluble laccase coupled with NF270 membrane filtration (89.0%). The improvement would appear to be due to a combination of separation (reducing the enzymatic burden), adsorption (enriching the substrate concentration as well as prolonging the residence time) and finally, catalysis (oxidizing the pollutants and breaking the ‘adsorption saturation limits’). Furthermore, the synergistic effect of the polydopamine (PDA) layer on the enzymatic oxidation of BPA was confirmed, which was due to its enhanced adsorption and electron transfer performance. The multifunctional membrane could be reused for at least 7 cycles with an acceptable activity loss, demonstrating good potential for micro-pollutants removal. 1
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KEYWORDS: Multifunctional membrane; laccase immobilization; micro-pollutants; nanofiltration; adsorption; enzyme
1. INTRODUCTION Global water shortages and pollution greatly limit rapid urbanization and modernization of society. Thus effective water treatment technologies are required for water purification and restoration. For many years, membrane filtration has been regarded as one of the most promising technologies and has been widely used in water treatment due to several unique advantages such as energy saving, small plant footprint, continuous operation and high quality effluent.1-2 Nowadays, public concern about the environmental impact of micro-pollutants, including pharmaceuticals and personal care products, steroidal hormones, pesticides, as well as endocrine-disrupting chemicals, has grown due to their high resistance to conventional oxidation methods and negative effects on human health even at very low dosages.3-6 This not only provides new opportunities for membrane technology, but also brings some specific challenges.7 For example, Yoon and co-workers applied nanofiltration (NF) and ultrafiltration (UF) membranes to treat natural water containing 52 different types of micro-pollutants, and found that their retention was unexpectedly low due to the transport phenomenon associated with adsorption.8 Even utilizing a reverse osmosis (RO) membrane, the retention was still unsatisfactory because of the presence of hydrophobic adsorption between the micro-pollutants and the aromatic polyamide material.9-10 Aside from such adsorption of micro-pollutants in the membrane, there is also the problem of membrane fouling (associated serious flux decline), while the chemical cleaning process would wash those pollutants out from the membrane, resulting in a ‘secondary pollution’. Thus, relying on the retention ability of a conventional membrane alone would unfortunately never fulfill the stringent emission standards. On this basis we reasoned that membranes with multi-functional capacities would be needed in order to achieve a more effective removal of micro-pollutants. While it has been reported that adsorbents coupled with membranes (or membrane adsorbers) produced good micro-pollutant removal in short-term operations,11-13 nevertheless, after the occurrence of the inevitable ‘adsorption saturation’, the micro-pollutants’ rejection by the membrane were severely
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decreased, leading to an unsatisfactory permeate quality. Besides, the regeneration of adsorbents and membrane adsorbers would cause severe secondary pollution. In order to improve this situation we considered the integration of enzymatic catalysis with the membrane to help facilitate the micro-pollutants removal. It was confirmed that ligninolytic enzymes such as laccase (E.C. 1.10.3.2) and lignin peroxidase (E.C. 1.11.1.14) can eliminate compounds such as endocrine disruptors and pharmaceuticals effectively.14 Laccase is of great interest to researchers because it can oxidize both phenolic and non-phenolic compounds, as well as extremely recalcitrant environmental pollutants within a wide range of pH values, and the final products are usually non-toxic.15-16 However, since the applications of free enzymes are frequently limited due to inactivation, high costs for enzyme isolation/purification and serious product inhibition, enzyme immobilization is often favored. Using filtration membrane to retain enzyme, enzymatic membrane reactor can reuse enzymes in continuous mode, improve enzyme stability and extract the product to relieve the product inhibition effect.17-18 Indeed, recently it was reported that continuous oxidation of micro-pollutants (e.g. antibiotics and endocrine disruptors) by a reactive membrane with immobilized laccase has been achieved.19-21 Nevertheless it was found that the complex immobilization procedure resulted in a low enzyme activity and a full recycling mode was required to obtain a high level of micro-pollutant removal.19, 22 More recently, Luo and co-workers proposed a simple approach to immobilize enzyme in membranes using ‘reverse filtration’ of the enzyme solution followed by ‘dopamine coating’ on the membrane support,23 in order to entrap the enzyme in the membrane and minimize changes in enzyme as well as membrane structure.24 Dopamine can form a stable self-polymerized coating layer on various substrates under alkaline conditions in an air atmosphere25, which is inspired by the strong adhesion of mytilus edulis foot.26-27 This has been widely used for enzyme immobilization,28 and compared with other enzyme immobilization strategies (e.g. covalent bonding, entrapment, adsorption and crosslinking), the dopamine-based method is simple, facile, stable and shows less harm to enzyme activity as it is biocompatible and the non-covalent bonding is dominant.21,23-24 Besides, it was also reported that the polydopamine (PDA) layer improved the catalytic efficiency by adsorbing and enriching the substrate.29 On the other hand, it is well known that biological membranes are able to control substances entering into 3
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the cell for metabolism, and inspired by this, we reasoned that the enzyme of interest could be immobilized beneath a tight NF membrane since it may effectively reject the micro-pollutants and reduce the enzymatic burden. Based on the above, we therefore propose a novel concept using a bio-inspired multifunctional membrane, which combines membrane physical rejection and intrinsic adsorption, PDA adsorption and enzymatic catalysis functions, as illustrated in Fig. 1. Such a multifunctional membrane could be prepared using the simple method developed by Luo and co-workers.23 When such a membrane is applied to micro-pollutant removal, we hypothesized that the micro-pollutants would be partly retained by the NF membrane, then the residual pollutants would be adsorbed and enriched in the membrane and PDA layer (close to the enzymes), prolonging the contact time between substrate and enzyme, and finally, the immobilized enzyme would oxidize those micro-pollutants that enter into the membrane. After bio-oxidation, the adsorption sites would be liberated, avoiding the ‘adsorption saturation’ problem. If this hypothesis could be verified, the micro-pollutants in an aquatic environment could also be removed effectively and continuously by our multifunctional membrane. This work describes the preparation of a multifunctional membrane which can separate, adsorb and catalyze the pollutants simultaneously, and investigates its potential application for removal of bisphenol A (BPA), an endocrine disruptor widely found in aquatic environments, which has an irreversible detrimental effect on human.30 In hazardous waste landfill leachates, BPA concentrations can reach up to 17.2 mg·L-1.31 Laccase was chosen as the enzyme of interest due to its activity towards various micro-pollutants including BPA.5, 16 Suitable NF membranes for multifunctional membrane preparation were selected on the basis of their physical properties (e.g. permeability, rejection and adsorption capacity), as well as enzyme immobilization efficiency (e.g. enzyme loading and activity). The performance of nascent and multifunctional membranes was systematically compared in order to emphasize the benefit of the multi-functions. Moreover, by designing four membranes with different functions, the contribution of each function (i.e. rejection, adsorption and catalysis) to BPA removal and the synergistic mechanism of the multifunctional membrane were clarified. Finally, the stability and reusability of the prepared multifunctional membrane were also assessed. To the best of our knowledge, this is the first work investigating a multifunctional membrane combining separation, PDA/membrane adsorption and laccase 4
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catalysis functions together to remove micro-pollutants. The successful implementation of this work will open a new direction for multi-functionalization of NF membranes and provide an alternative method for continuous, stable and effective water treatment.
Fig 1 Schematic diagram of multifunctional membrane preparation and its immobilization/working mechanisms. Enzyme immobilization was carried out by reverse filtration of enzyme solution and then dopamine was self-polymerized at alkaline condition, where enzymes were immobilized via entrapment, covalent bonding as well as hydrophobic adsorption. When this multifunctional membrane was applied in water treatment, rejection, adsorption and catalysis functions worked simultaneously and a synergistic effect among them would facilitate the micro-pollutants removal.
2. EXPERIMENTAL 2.1 Materials A dead-end filtration cell (Amicon 8050, Millipore, USA) with an effective area of 13.4 cm2 was used in all the following filtration experiments. Laccase (EC 1.10.3.2, 60-70 KDa, 0.53U·mg-1) from Trametes versicolor was purchased from Sigma-Aldrich (Shanghai, China). Bradford reagent used for protein assay was supplied by Sigma-Aldrich. 2.6-dimethoxy phenol (DMP) was used as the assay substrate for laccase activity. DMP (154.16 g·mol-1) and BPA (228.29 g·mol-1) was purchased from Aladdin (Shanghai, China). All the enzyme solutions and BPA solutions were prepared using a 10 mM phosphate buffer (pH 5.3) and dopamine hydrochloride were prepared freshly using a 10 mM Tris-HCl buffer (pH 8.5). DMP solution was prepared using a 100 mM sodium citrate buffer (pH 4.3). All the
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chemical reagents used in experiments were of analytical grade and used directly without any further purification. Six commercial polymeric membranes (Table 1) were used in this work. Table 1 Basic properties of NF membranes used in this work
Membrane
MWCO Skin-layer material
Manufacturer
type
Ref.
(Da)
NT101
Polyamide (PA)
