Hyperbranched polymer membrane for catalytic degradation of polychlorinated biphenyl-153 (PCB-153) in water

https://doi.org/10.1016/j.reactfunctpolym.2018.12.019Get rights and content

Abstract

Catalytic membranes consisting of Fe/Pd bimetallic nanoparticles were fabricated via interfacial polymerisation for the degradation of a polychlorinated biphenyl (PCB-153). Characterisation techniques for the membranes included Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy, scanning electron microscopy (SEM) coupled with focused ion beam (FIB) and energy-dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM), contact angle analysis and X-ray photoelectron microscopy (XPS).Pure water flux studies were performed using a dead-end filtration cell system operated at 600 kPa. The efficiency of the fabricated membranes to dechlorinatePCB-153in water samples was monitored using GC–MS. The FTIR-ATR spectra illustrated that a polyamide film was successfully deposited onto the commercial polysulfone (PSf) membrane. SEM-FIB-EDS confirmed the presence and uniform dispersion of iron and palladium on the surface of the membranes. Performance evaluation studies of the membranes showed that the addition of the HPEI/Fe-Pd composite improved the hydrophilicity of the membrane from 73.3° to 25.3° for the modified membranes. A dechlorination efficiency of up to 93% was attainedand this was accompanied by the production of by-products such astetrachlorobiphenyl (PCB-52) and dichlorobiphenyl (PCB-2).

Introduction

Persistent organic pollutants (POPs) are synthetic organic compounds of anthropogenic origin. These compounds are highly resistant to chemical, photolytic and biological degradation. For the most part, POPs are insoluble in water and tend to accumulate, persist and bio-concentrate in living organisms at high trophic levels [1]. According to the Stockholm Convention, 29 POPs have been identified as having adverse effects on human health and the ecosystem [2]. These POPs are divided into three categories: (a) pesticides (aldrinand toxaphene); (b) industrial chemicals (polychlorinated biphenyls); and (c) unintentionally produced by-products (polychlorinated dibenzo-p-dioxins) [3].

Polychlorinated biphenyls (PCBs) are toxic organic compounds that are of great concern due to their adverse human health effects such as neurotoxicity and dermatological diseases. The PCBs are also known to be endocrine-disrupting compounds since they alter the endocrine system in humans and animals [4]. They are highly stable and resistant to environmental degradation and are thus categorised under persistent organic pollutants [5]. Conventional methods commonly used to treat PCB-contaminated water are inadequate to convert PCBs to less harmful or non-toxic compounds. The catalytic membrane reactor provides an alternative solution for the removal of these contaminants as it combines membrane-based separation and a catalytic chemical reaction in a single process. Due to their catalytic activities, the use of zero-valent iron nanoparticles has therefore been extensively studied to overcome the limitations observed in conventional water treatment technologies [6].

Over the past few decades, the use of nano-sized metal particles has become vitally important in the fields of catalysis, electrochemistry and biomedicine [[7], [8], [9]].Recently these nanoparticles have been used for the dechlorination of chlorinated organic compounds (COCs) from contaminated water [10].One of the most commonly used bimetallic systems in water treatment is iron and palladium (Fe/Pd). As stated by Han et al. (2016), catalytic reduction of compounds such as tetrachloroethene and PCBs involves an indirect reduction mechanism by way of reactive hydrogen species. In order to facilitate in-situ degradation of PCB 153, a local source of H2 should be present in close proximity to Pd. During the corrosion of iron in water, hydrogen is produced where synergy between Fe and Pd exists. In this bimetallic system the iron acts as a hydrogen source and catalyst support, whereas palladium serves as the catalyst for hydrogen activation and catalytic hydrodechlorination in the catalytic system [[11], [12], [13]]. Nanoscale Fe/Pd bimetallic particles are superior to the most commonly used reductant, namely zero-valent iron (Fe0), and various other iron-based bimetals (e.g. Ni/Fe, Cu/Fe, and Pt/Fe) for the dechlorination of several COCs, due to the large surface area of iron as well as the high hydrogenation activity of Pd [14]. However, unsupported nanoparticles are prone to agglomeration when used for the dechlorination of organic compounds in water. Agglomeration has been found to limit their activity and cause poor subsurface mobility in aqueous media and also hinders their recovery [15].Thus researchers have focused their attention on the synthesis and immobilisation of nanoparticles in porous matrices such as membranes modified with dispersing and capping agents (e.g. hyperbranched polyethyleneimine: HPEI) [16].

The function of HPEI in a membrane system is to act as both a hydrophilicity enhancer as well as a dispersing agent which prevents metal nanoparticle agglomeration. Furthermore, due to the chelating abilities of HPEI, the stability of nanoparticles may be effectively enhanced. This can result in the formation of well-dispersed nanoparticles confined by electrostatic interactions and/or complexation reactions between metal ions and nitrogen atoms of the amine groups [17,18].Following the encapsulation of the metal ions, they can be reduced via chemical reduction in aqueous medium to yield zero-valent encapsulated nanoparticles [19]. The nanocavities found in HPEI can also be used to entrap various contaminants [20].

Lin et al. [21] used HPEI-modified zero-valent iron nanoparticles to dechlorinate trichloroethylene (TCE), tetrachloroethylene (perchloroethyleneor PCE) and 1,2-dichloroethylene(1,2-DCE). After 2 h of treatment,97, 96, and 96% of TCE, PCE, and 1,2-DCE, respectively, were removed in a 100 mg/L initial solution. After the dechlorination reaction, PCE was converted to ethylene, while 1, 2-DCE was the major by-product after the degradation of TCE, which was subsequently dechlorinated to less toxic vinyl chloride and chloride-free ethylene.

Davenportet al. [22] modified a non-spongy polyvinylidene fluoride-polyacrylic acid membrane with Fe/Pd nanoparticles (PVDF-PAA-Fe/Pd) and used it for the dechlorination of trichloroethylene. In their study, ion exchange chromatography was used to validate the formation of chloride as a product of TCE dechlorination. Within the first 15 min of dechlorination, 40% of the TCE was adsorbed, indicating that the membrane had degraded and adsorbed the TCE at a high rate. Complete dechlorination was observed in the PVDF-PAA-Fe/Pd membrane, whereas no chloride formation was observed in the membrane without nanoparticles in spite of constant adsorption of TCE.

Zhang et al. [23] synthesised a PVDF membrane modified with Al2O3 and immobilised iron and palladium nanoparticles on the membrane (PVDF/Al2O3).The PVDF/Al2O3-Fe/Pd membrane was used to dechlorinate dichloroacetic acid and exhibited highdechlorination efficiency towards dichloroacetic acid. The authors obtained 97.71% dechlorination of dichloroacetic acid in the first 90 min with acetic acid being produced as a by-product. Adsorption studies revealed that negligible adsorption occurred between dichloroacetic acid and the components of the Al2O3/PVDF-Fe/Pd membrane, thereby illustrating that the removal of dichloroacetic acid occurred solely by catalytic dechlorination.

In this study, HPEI/PSf membranes were modified with Fe/Pd bimetallic nanoparticles for the dechlorination of PCB-153from water. To our knowledge, this is the first time that an HPEI/PSf-Fe/Pd membrane has been synthesised, characterised using extensive characterisation techniques and applied in the removal of PGB-153.

Section snippets

Materials

A commercially supplied (MICRODYN-NADIR) porous polysulfone (RM US100 P1016) membrane with a cut-off weight of 100 kD was purchased from Memcon (Pty) Ltd., Randburg, South Africa. Hyperbranched polyethyleneimine (HPEI, MW = 25,000 g/mol, with amine ratio of 1°:2°:3°, 34:40:26) was purchased from BASF South Africa (Pty) Ltd. FeCl2radical dot4H2O, K2PdCl4,NaBH4, analytical grade trimesoyl chloride (TMC), n-hexane, ethanol and methanol (≥99.9% for GC) were purchased from Sigma-Aldrich and used without any

FTIR-ATR analysis

The modification of the pristine PSf membrane with HPEI was confirmed by FTIR-ATR analysis as illustrated in Fig. 3. Absorption bands for polysulfone, which shows peaks at 2841 cm−1 and 1552 cm−1, were ascribed to single bondCH stretching and single bondCSO2stretching, respectively. These absorption bands were present in all the membranes. Three distinct bands were observed at 1727 cm−1, 1645 cm−1 and 1574 cm−1 corresponding to the Cdouble bondO stretching of the carboxylic acid (-COOH), amide-I (stretching of the carbonyl

Leaching studies

For all membranes, Fe:Pd ratios of ca. 4:1 were obtained. Leaching studies (Fig. 19) revealed minimal metal leaching after a period of four days, with all membranes displaying Fe and Pd leaching of <0.004 mg/L and < 0.45 mg/L, respectively. The low level of leaching further affirms the strong interaction of the nanoparticles with the HPEI macromolecule within the membrane matrix. As reported by Li et al. [73],the leaching of the metals is minimised due to the recapturing properties of the

Conclusion

Results obtained in this study confirmed that interfacial polymerisation between HPEI and TMC was successful. This was verified by FTIR-ATR spectroscopy, while SEM-EDS and SEM-FIB confirmed the successful reduction and immobilisation of the bimetallic nanoparticles on the membrane surface. The XPS analysis revealed the presence of Fe and Pd on the membrane surface, confirming their successful immobilisation and encapsulation in the HPEI. Contact angle results indicated an increase in

Acknowledgements

The authors would like to thank the Water Research Commission, South Africa (Project No. K5/2488/3), the Thuthuka National Research Foundation, South Africa (TTK150608118953) and the University of Johannesburg (Faculty of Science, Department of Applied Chemistry) for funding. This project was also partially funded by the National Research Foundation (NRF) of South Africa (Grant Number 93205).

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