Aqueous-phase synthesis of PAA in PVDF membrane pores for nanoparticle synthesis and dichlorobiphenyl degradation

Abstract

This paper deals with bimetallic (Fe/Pd) nanoparticle synthesis inside the membrane pores and application for catalytic dechlorination of toxic organic compounds form aqueous streams. Membranes have been used as platforms for nanoparticle synthesis in order to reduce the agglomeration, encountered in solution phase synthesis which leads to a dramatic loss of reactivity. The membrane support, polyvinylidene fluoride (PVDF) was modified by in situ polymerization of acrylic acid in aqueous phase. Subsequent steps included ion exchange with Fe²⁺, reduction to Fe⁰ with sodium borohydride and Pd deposition. Various techniques, such as STEM, EDX, FTIR and permeability measurements, were used for membrane characterization and showed that bimetallic (Fe/Pd) nanoparticles with an average size of 20–30 nm have been incorporated inside of the PAA-coated membrane pores. The Fe/Pd-modified membranes showed a high reactivity toward a model compound, 2,2′-dichlorobiphenyl and a strong dependence of degradation on Pd (hydrogenation catalyst) content. The use of convective flow substantially reduces the degradation time: 43% conversion of dichlorobiphenyl to biphenyl can be achieved in less than 40 s residence time. Another important aspect is the ability to regenerate and reuse the Fe/Pd bimetallic systems by washing with a solution of sodium borohydride, because the iron becomes inactivated (corroded) as the dechlorination reaction proceeds.

Introduction

Traditionally, membranes are used for the separations based on size exclusion, solution diffusion or Donnan exclusion [1]. However, membrane modification with novel, advanced functional groups [2] or layer-by-layer deposition of polyelectrolytes and the growth of polymer brushes [3] can extend their area of application toward advanced separations, development of biomaterials or catalysis. In the field of catalysis in particular, there is a growing interest in detoxification of organic chlorinated compounds from aqueous streams. Chlorinated organic compounds constitute a large group of pollutants of international concern due to their high toxicity, persistence and various sources of distribution in the environment.

In recent years, zero-valent nanoscale metal (especially iron) particles have attracted a growing attention in groundwater remediation of chlorinated solvents [4], [5]. However, iron nanoparticles tend to agglomerate in water rapidly to form micron size or larger aggregates, thus losing their reactivity. Various techniques have been employed to control the particle size, by using additives [6], [7] controlled seed mediated synthesis [8], [9] or coating with a protective layer [10]. For a more rapid and complete dechlorination, a second element is often added, resulting in creation of bimetallic nanoparticles. In these systems, the first metal (most commonly Fe, but also Mg, Sn, etc.) is an electron donor to degrade the organic compound and the second metal (usually a more noble metal such as Pd or Ni) promotes the reactivity through hydrogenation, acting as a catalyst. Several bimetallic systems that are used for dechlorination of toxic chlorinated organic compounds, have been reported in the literature: Fe/Ni [11], [12], Ni(B)/Fe(B) [13], Fe/Pd [14], [15], [16], [17], [18], [19], Mg/Pd [20], [21], [22], Sn/Pd [23].

In order to minimize the particle agglomeration and loss, another approach is to synthesize supported nanoparticles on activated carbon [24], [25], resins [23] or membranes, as substrates. Among these, membranes have a distinct advantage because of ease of functionalization and the ability to be operated under convective flow conditions (filtration), thus eliminating the mass transfer issues often encountered in diffusive (batch) mode operation. Membranes have received a special attention (including our research group) and have been successfully employed as platforms for nanoparticle synthesis. Membrane's open structure and high internal surface area ensure a high nanoparticle loading and easy active site accessibility. The most common membrane materials used for this purpose are: cellulose acetate [11], [26], hydrophilized polysulfone [27], polyacrylic acid-modified polyether sulfone [28], [29] and polyvinylidene fluoride [30], [31], [32]: these membranes have proven to be very efficient for the degradation of the organic chlorinated compounds from water.

In recent years there has been a great interest in our laboratory in PVDF membrane modification with PAA by dip coating or solvent phase in situ polymerization of acrylic acid, with subsequent Fe/Pd nanoparticle incorporation [30], [31], [33]. Dechlorination studies using a variety of toxic chlorinated organic compounds were performed and a mathematical model was developed. Currently we investigate a method of PAA modification of PVDF membranes using aqueous-phase polymerization (no organic solvent use) and study the nanoparticle longevity and stabilization by protecting the Fe/Pd nanoparticle surface.

The main goals of present study are: (i) synthesize bimetallic Fe/Pd nanoparticles within the pores of a functionalized membrane (polyacrylic acid-coated polyvinylidene fluoride) using green chemistry, (ii) characterize the membrane and nanoparticles characterization, (iii) study the reactivity toward a target toxic organic compound, 2,2′-dichlorobiphenyl, (iv) evaluate nanoparticle stability during operation (dechlorination) and regeneration, and (v) demonstrate operation under convective flow conditions.