Polymer supported inorganic nanoparticles: characterization and environmental applications

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Abstract

Nanoscale Inorganic Particles (NIPs) and their agglomerates offer excellent opportunities conducive to selective removal of a wide array of target compounds from contaminated water bodies. For example, (i) hydrated Fe(III) oxides or HFO particles can selectively sorb dissolved heavy metals like zinc, copper or metalloids like arsenic oxyacids or oxyanions; (ii) Mn(IV) oxides are fairly strong solid phase oxidizing agents; (iii) magnetite (Fe3O4) crystals are capable of imparting magnetic activity; (iv) elemental Zno or Feo are excellent reducing agents for both inorganic and organic contaminants. Very high surface area to volume ratio of these nanoscale particles offers favorable sorption and/or reaction kinetics. However, applications of NIPs in fixed-bed columns, in-situ reactive barriers and in similar flow-through applications are not possible due to extremely high pressure drops. Also, these NIPs are not durable and lack mechanical strength. Harnessing these inorganic nanoparticles and their aggregates appropriately within polymeric beads offers new opportunities that are amenable to rapid implementation in the area of environmental separation and control. While the NIPs retain their intrinsic sorption/desorption, redox, acid–base or magnetic properties, the robust polymeric support offers excellent mechanical strength, durability and favorable hydraulic properties in the flow-through systems. This paper discusses at length the preparation, characterization and environmental applications of two classes of polymer supported nanoparticles: (i) Hydrated Fe(III) Oxide (HFO) dispersed polymeric exchanger and their As(III), As(V), and heavy metals removal properties; (ii) Magnetically Active Polymeric Particles (MAPPs). The polymer supported nanoparticles are reusable and can be easily reprocessed over many cycles of operation.

Introduction

Abraham Warshawsky was one of the pioneers in conceiving, preparing and evaluating what is now popularly referred to as Solvent Impregnated Resins or SIRs [1], [2], [3]. From an engineering application viewpoint, SIRs constitute a desirable blend between liquid extractants and polymeric ion exchangers for selective removal of target compounds from the aqueous phase. It was well recognized that solid phase ion exchangers with covalently attached functional groups suffer from relatively slow intraparticle diffusion controlled kinetics. Also, synthesis or covalent attachment of novel or exotic functional groups into polymeric substrate is an involved process and that is why only a limited number of selective ion exchangers are available commercially. In contrast, liquid ion exchangers offer rapid kinetics but cause entrainment of organic solvent in the aqueous phase necessitating additional downstream treatment. Solvent Impregnated Resins or SIRs essentially constitute a healthy compromise between polymeric ion exchangers and liquid extractants. Characteristically, SIRs are porous polymer beads within which liquid extractants have been uniformly dispersed. In principle, the preparation of SIRs has three major components: first, the choice of an organic solvent or extractant with a specific affinity toward target solutes; second, a polymeric substrate or carrier within which the solvent phase can be appropriately impregnated; and third, the process of impregnation. In order to treat contaminated waters, the solvent/extractant should preferably be very hydrophobic with minimum water solubility. Polymeric carriers used to date are mostly polystyrene-based macroporous spherical beads. The solvent is retained within the pores of the polymeric carrier primarily through hydrophobic interaction and easily accessible to dissolved solutes from the aqueous phase. The application of SIRs has been investigated for analytical, hydrometallurgical and environment-related applications [1]. Minor leaching of the liquid extractant into the aqueous phase has been cited as a potential problem but providing a hydrophilic film around the polymer beads has nearly eliminated possible contamination of water by the hydrophobic solvent [4].

From a generic viewpoint, our ongoing research investigations pertaining to Polymer Supported Nanoparticles (PSNs) are scientifically analogous to SIRs; both of them attempt to integrate two separate phases for enhanced separation. A number of nanoscale inorganic particles (NIPs) offer favorable properties in regard to selective separation and/or chemical transformation of target contaminants. For example, (i) hydrated Fe(III) oxides or HFO particles can selectively sorb dissolved heavy metals like zinc, copper or metalloids like arsenic oxyacids or oxyanions; (ii) Mn(IV) oxides are fairly strong solid phase oxidizing agents; (iii) magnetite (Fe3O4) crystals are capable of imparting magnetic activity; (iv) elemental Zn° or Fe° are excellent reducing agents for both inorganic and organic contaminants [5], [6], [7], [8], [9], [10]. Fig. 1 depicts the properties of several NIPs. The methodology of preparation of these NIPs and their aggregates is environmentally safe, operationally simple and inexpensive. Extremely high surface area to volume ratio of these tiny particles (≈30 000 m2/m3) offers very favorable kinetics for selective sorption and oxidation–reduction reactions. Nevertheless, these particles cannot be used in fixed-bed columns, in underground reactive barriers or in any plug-flow type configuration due to excessive pressure drops and poor durability. On the contrary, many commercially available porous polymeric beads are very durable and offer excellent hydraulic properties in regard to pressure drops. The prices of these polymeric particles are quite competitive and the trend around the globe indicates that they will further decrease in the future. Ideally, it would be very desirable to develop a new class of polymeric/inorganic materials that combine the excellent hydraulic characteristics of spherical polymer beads with favorable sorption/redox/magnetic properties of inorganic nanoparticles. There remain a host of new opportunities to effectively utilize these hybrid particles for environmental separation and control. Conceptually, polymer-supported inorganic nanoparticles are analogous to SIRs in the sense that in both cases porous polymeric substrates are used to retain the active ingredients (i.e. organic extractants or inorganic nanoparticles) which are, however, permeable to the dissolved solutes from the aqueous phase. To this effect, we will confine our discussion to the following two classes of polymer-supported nanoscale particles;

  • (i)

    preparation and characterization of Magnetically Active Polymeric Particles (MAPPs);

  • (ii)

    Hydrated Fe(III) Oxide (HFO) dispersed polymeric ion exchanger, referred to as hybrid ion exchanger or HIX, and their heavy metals and arsenic removal properties.

It is worth mentioning that magnetic polymers have been previously used in the field of protein and biomolecule separation, water and wastewater treatment, desalination, color imaging and information storage [11], [12], [13], [14]. In all such applications, however, magnetization is imparted in the prepolymerization step and, as a result, the magnetized polymers are often proprietary products and overly expensive. In addition, in the area of bioseparation, magnetic polymers are used in once-through mode without regeneration and reuse. In contrast, the primary objective of the present work has been geared toward imparting magnetic activity into already available polymeric sorbent materials with specific sorption affinities toward various environmental contaminants, namely, heavy metals, metalloids, inorganic and organic ligands, chlorophenols and pesticides [15], [16], [17], [18]. Once magnetized, these polymeric particles can be immensely effective in scavenging target contaminants from the background of complex environmental matrices such as slurries or sediments with high-suspended solids content, viscous or radioactive liquid or a medium with high concentration of biomass. Also, for environmental monitoring or forensic purpose, Magnetically Active Polymer Particles (MAPPs) can be judiciously used to track down the origin of a particular contaminant present in natural water bodies.

Amorphous and crystalline Fe(III) hydroxides have long been known to possess selective sorption properties toward arsenites and arsenates [19], [20]. However, fine particle sizes of these precipitates are not suitable for application in fixed-bed columns. Granulated Ferric Hydroxides (GFH) have been recently prepared and successfully used in fixed-bed columns [21]. The use of these new materials is, however, limited due to their unavailability in the public domain. Fe(III) or Cu(II) loaded specialty chelating polymers exhibit arsenic removal properties through a polymeric ligand exchange mechanism [22], [23]. These chelating polymers are very expensive and hence, their applications in treating arsenic contaminated groundwaters cannot be economically viable. Activated alumina has been successfully used in remote villages of the Indian subcontinent to remove dissolved arsenic from contaminated groundwaters [24]. Activated alumina particles are chemically unstable and are partly lost during the regeneration step. Strong-acid cation exchangers with sulfonic acid functional groups are universally available and the least expensive polymeric sorbents. One specific objective of the study is to use a macroporous cation exchanger as the primary host to support nanoscale Hydrated Fe(III) Oxide (HFO) particles and investigate the arsenic sorption properties of the hybrid material.

Section snippets

Preparation of magnetically active polymeric particles (MAPPS)

It was recognized that if nanocrystals of magnetite, Fe3O4, could be irreversibly dispersed within the polymeric spherical beads, the non-magnetic polymeric sorbents would then be magnetically active and would respond to the magnetic field. The challenge in magnetization of polymeric beads lies in controlling the process conditions so that the formation of Fe3O4(s) is preferred over nonmagnetic Fe(OH)3(s) and Fe(OH)2(s). Under a reducing environment (i.e. absence of oxygen), Fe(OH)2(s) is the

Magnetization

The process of magnetization for different polymeric sorbent particles was carried out in a batch reactor as shown in Fig. 6. The entire process consisted of three major steps: first, introducing/loading Fe(II) within the sorbent at slightly acidic pH; second, slow oxidation of Fe(II) into crystalline magnetite (Fe3O4) at alkaline pH under a controlled redox environment; and third, drying of the resulting material. Specific steps followed in the laboratory were as follows: (1) Prepare 500 ml of

Magnetically Active Polymeric Particles (MAPPs): acquired magnetic activity

Table 1 includes the salient characteristics of the polymeric sorbents involved in the study of magnetization and the names of their manufacturers. They were obtained as spherical beads and their sizes ranged from 200 to 600 μm. These sorbents are capable of selectively removing a wide array of environmentally significant solutes, namely, hardness, heavy metals, radionuclides, anionic ligands, and metalloids. Dimensionless magnetic susceptibility (Xm) is a measure of the acquired magnetic

Concluding remarks

Nanoscale Inorganic Particles (NIPs) and their aggregates offer excellent opportunities conducive to selective removal of a wide array of target compounds from contaminated water bodies. Harnessing these inorganic nanoparticles and their aggregates appropriately within polymeric beads offers new opportunities that are amenable to rapid implementation in the area of environmental separation and control.

The polymer-supported nanoparticles are reusable and can be easily reprocessed over many

Acknowledgments

The study received partial financial support from EPA STAR Grant No. G9211010.

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