Membrane-based bimetallic nanoparticles for environmental remediation: Synthesis and reactive properties

For example, significant changes of the topology found on the NP surface result in various facets, edges, corners, and defects [8], which could create additional reactive sites. Small metal particles having a high binding energy of their core electrons can influence the interaction between the surface sites with the reactants and products. The presence of secondary metal brings novel catalytic properties that are absent in the monometallic particles [9]. The deposition of a second metal can also enhance chemical reactivity by changing the electronic properties of the surface [10]. The creation and development of nanosized materials have brought important and promising techniques into the field of pollution control. Recently, various studies have been reported on the groundwater remediation through degradation of toxic chlorinated organic compounds (COCs) with nonimmobilized Feobased bimetallic NPs (Fe/Ni, Fe/Pd) [11-14]. In this case, COCs are reduced to nontoxic (or less) hydrocarbons in the presence of the second catalytic metal (Pd or Ni) by substitution of chlorine with hydrogen. Compared to the single zerovalent Fe system (such as iron filings), which has been used for decades for degradation of COCs, this catalytic hydrodechlorination technology has been developed because of enhanced reaction rate and elimination of toxic by-product formation as a result of the second, catalytic metal. Correlations between reactive properties and the NP structure, the distribution of first and second metals, composition of the second metal, and particle size (nano vs. bulk size) have not been clarified. The bimetallic NPs used in most of the studies were synthesized in the aqueous phase by reduction of metal cations with sodium borohydride [11-14]. The nanosized metals precipitated from solutions are extremely reactive as a result of the high surface energy, and they usually form aggregation without the protection of their surface [15, 161, The potential impact of noniminobilized NPs release in groundwater and the possible loss of reactivity arising from the surface fouling are not well documented in the literature. The common approaches used for the stabilization of the NPs include protection by capping ligands such as polymers or surfactants and dispersion in solid supports such as active carbon, metal oxides, zeolites, or polymer films [6]. However, immobilization of NPs in solid matrix may cause a diffusion resistance during the catalytic reaction because the reactants now have to diffuse to the NPs if the support has a dense structure. To overcome this problem, it is advantageous to synthesize and immobilize NPs in an open matrix. Microfiltration (MF) membranes are of great interest for this purpose because of the open structure and 100-500 nm pore size. it is essential for attaining highly efficient use of available sites as well as the easy accessibility to the NPs immobilized inside the membrane matrix. Membranes functionalized with bimetallic NPs offer great advantages for the catalytic reaction because diffusion limitation can be minimized under convective flow. In this paper, bimetallic NPs (Fe/Ni and Fe/Pd) were prepared directly inside a polyacrylic acid (PAA)/polyvinylidene fluoride (PVDF) membrane matrix. We investigated the role of PAA as a chelating (ion exchange) polymer for binding metal cations in the formation of NPs and in the dechlorination reaction of COCs. In this case, the issue of particle release into the solution phase can be avoided, and the NP surface fouling can be prevented because of the recapture of metal ions by PAA. We demonstrate the element distribution of first and second metals in the nano domain and the core/shell structure of bimetallic NPs. Our objective is not only to synthesize nanostructured bimetallic particles inside a membrane matrix for reductive dechlorination of toxic COCs, but also to understand and quantify the role of a second dopant metal, particle size (nano vs. bulk size) in the reactive properties of membranebased bimetallic NPs, and reaction pathway/reaction rate changes through reaction product distribution. The long-term performance of bimetallic NP reactivity was also investigated for the applications of environmental remediation.