Stabilization of highly concentrated suspensions of iron nanoparticles using shear-thinning gels of xanthan gum
Introduction
Fe and Fe3O4 nanoparticles have recently generated a great interest in the fields of biomedical and environmental research.
In biomedicine iron oxide nanoparticles are used for the improvement of magnetic resonance imaging (MRI), for the hypothermal treatment of tumoral cells and for site-specific drug delivery (Berry, 2005, Berry and Curtis, 2003, Huh et al., 2005, Hilger et al., 2001, Liu et al., 2009, Pankhurst et al., 2003). In order to effectively deliver particles to the biological target particles' modification with biocompatible polymers, which determine steric stabilization, has been proposed (Berry, 2005).
In the field of groundwater remediation, nanoscale zerovalent iron (NZVI) presents several advantages over more traditional technologies: its high reactivity enables a more rapid degradation of contaminants, if compared to millimetric iron particles (Di Molfetta and Sethi, 2006, Zanetti and Fiore, 2005); it is able to treat otherwise recalcitrant contaminants; and its small size should facilitate the delivery of the suspension close to the source of contamination reducing the remediation time (Li et al., 2006, Tratnyek and Johnson, 2006). However, similarly to nanoparticles for biomedical applications, also delivery of NZVI for environmental remediation to the contamination target is not straightforward. Injection operations and transport in porous media are hindered by particle aggregation, lack of stability and phase separation even at very low iron concentrations (Schrick et al., 2004, Phenrat et al., 2007), whereas full scale applications of NZVI require the injection of highly concentrated suspensions (>10 g/L). The need of highly concentrated slurries, is due to the fact that dilute suspensions imply the introduction into the aquifer of a large volume of water, which can determine contaminant displacement and iron passivation (Gavaskar et al., 2005). Furthermore, high solid contents facilitate the transport of the slurry from the synthesis plant to the site of application.
The long term stability of a colloidal system is due to the sum of the interaction forces between particles (Elimelech et al., 1995; Israelavchvili, 1991). In the case of NZVI suspensions, attractive van der Waals forces and magnetic forces (Phenrat et al., 2007, Dalla Vecchia et al., in press) prevail over the repulsive ones, thus determining their instability. In highly concentrated and low viscosity suspensions the kinetics of agglomeration is very high and Brownian motion is responsible for bringing particles within the small range of the interaction forces (Elimelech et al., 1995).
The usual approach for minimizing particle aggregation and produce stable NZVI dispersions is to enhance the repulsive interaction forces, which can be achieved in three ways: by increasing the surface charge of nanoiron (electrostatic stabilization), by preventing colloids from approaching at close distances (steric stabilization) or by a combination of the two mechanisms (electrosteric stabilization). To date several compounds have been proposed as additives for the stabilization of NZVI, e.g. polyacrylate (Schrick et al., 2004, Kanel and Choi, 2007, Yang et al., 2007), triblock copolymers (Saleh et al., 2005, Saleh et al., 2007), polyvinyl alcohol-co-vinyl acetate-co-itaconic (Sun et al., 2007), guar gum (Tiraferri et al., 2007), carboxymethyl cellulose (He et al., 2007, He and Zhao, 2007), starch (He and Zhao, 2005), poly(4-styrenesulfonate) (Hydutsky et al., 2007) (Table S1). However, only one compound (the polyvinyl alcohol-co-vinyl acetate-co-itaconic proposed by Sun et al., 2007) is able to stabilize highly concentrated slurries (10 g/L).
From a kinetic point of view, stabilization can be achieved by reducing the Brownian motion and, consequently, the probability of the particles to collide. Among other factors, kinetic stabilization can be attained by increasing the viscosity of the liquid. This modification, that would also be beneficial in preventing the sedimentation of the particles, has not yet been reported on NZVI.
The aims of this study are: (i) to evaluate the possibility of stabilizing for more than 10 days highly concentrated iron slurries (>10 g/L) by increasing their viscosity through the addition of a biodegradable polymer, (ii) to identify the most suitable one and the required concentration, and (iii) to check the effectiveness of the process under variable solution ionic strength.
A 10 days' time scale, which is largely sufficient for the injection operations, was selected to permit the exploitation of transport mechanisms which take place after the injection and which are due to the natural groundwater movement, and to test the proposed method under severe conditions.
Section snippets
Nanoscale zerovalent iron source
Two colloidal dispersions of reactive nanoscale zerovalent iron were provided by Toda Kyogo Corp.: bare (Rnip-10DS) and polymaleic acid modified (Rnip-10E). Both the materials are composed of core–shell nanoscale particles dispersed in water. The core of the particles consists of Fe0 and the shell of Fe3O4. The composition of the materials provided by the supplier is summarized in Table S2. The average particle size is about 40 nm according to Nurmi et al. (2005), which report also a detailed
Viscosity of xanthan solutions
Xanthan solutions are non-Newtonian fluids characterized by a shear-thinning behaviour (Fig. 2), which results from the property of dissolved xanthan molecules to form aggregates through hydrogen bonding and polymer entanglement, which are progressively disrupted under the influence of applied shear stress (Phillips and Williams, 2000). Therefore xanthan solutions can be classified as ‘weak-gels’ (Mohammed et al., 2007). Viscosity of xanthan gels is stable over a wide range of temperatures
Discussion
In order to understand the sedimentation and aggregation behaviour of xanthan-NZVI dispersions, we describe a model for the interaction between xanthan and iron nanoparticles.
The structure of xanthan has been extensively investigated and it is well established that at high polymer concentration the molecules form a gel network through hydrogen bonding and polymer entanglement, which results in high viscosity at low shear rates and in a highly pseudoplastic flow (Phillips and Williams, 2000).
Conclusions
In this study we demonstrated that using 6 g/L of xanthan gum gels it is possible to avoid the sedimentation of 30 g/L of RNIP-10 iron nanoparticles and the aggregation of 15 g/L of RNIP-10 over a period of 10 days. The modified suspensions are stable for ionic strengths ranging from 6 × 10−3 to 12 mM. The improved stability of highly concentrated xanthan suspensions is caused by the formation of a polymer network. This mechanism should be distinguished from the steric and electrosteric stabilization
Acknowledgment
This work was conducted under the CIPE-C30 project funded by Regione Piemonte (Italy) and partially supported by the Lagrange Grant from Fondazione C.R.T. (Italy). The authors acknowledge Dr. M. Martin for the zeta potential measurements and for her precious advices, Prof. E. Santagata and D. Dalmazzo for the viscosity measurements and Prof. G. Rovero and Prof. C. Beltramo for the use of the Turbiscan Analyzer.
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