Aggregation and sedimentation of aqueous nanoscale zerovalent iron dispersions.

Nanoscale zerovalent iron (NZVI) rapidly transforms many environmental contaminants to benign products and is a promising in-situ remediation agent. To be effective, NZVI should form stable dispersions in water such that it can be delivered in water-saturated porous media to the contaminated area. Limited mobility of NZVI has been reported, however, attributed to its rapid aggregation. This study uses dynamic light scattering to investigate the rapid aggregation of NZVI from single nanoparticles to micrometer size aggregates, and optical microscopy and sedimentation measurements to estimate the size of interconnected fractal aggregates formed. The rate of aggregation increased with increasing particle concentration and increasing saturation magnetization (i.e., the maximum intrinsic magnet moment) of the particles. During diffusion limited aggregation the primary particles (average radius = 20 nm) aggregate to micrometer-size aggregates in only 10 min, with average hydrodynamic radii ranging from 125 nm to 1.2 microm at a particle concentration of 2 mg/L (volume fraction(phi= 3.2 x 10(-7)) and 60 mg/L (phi = 9.5 x 10(-6)), respectively. Subsequently, these aggregates assemble themselves into fractal, chain-like clusters. At an initial concentration of just 60 mg/L, cluster sizes reach 20-70 microm in 30 min and rapidly sedimented from solution. Parallel experiments conducted with magnetite and hematite, coupled with extended DLVO theory and multiple regression analysis confirm that magnetic attractive forces between particles increase the rate of NZVI aggregation as compared to nonmagnetic particles.

[1]  Mark R Wiesner,et al.  Velocity effects on fullerene and oxide nanoparticle deposition in porous media. , 2004, Environmental science & technology.

[2]  R. Mccurrie,et al.  Ferromagnetic materials : structure and properties , 1994 .

[3]  Paul G Tratnyek,et al.  Characterization and properties of metallic iron nanoparticles: spectroscopy, electrochemistry, and kinetics. , 2005, Environmental science & technology.

[4]  Raimund Bürger,et al.  Phenomenological foundation and mathematical theory of sedimentation–consolidation processes , 2000 .

[5]  D. Dunlop Hysteresis properties of magnetite and their dependence on particle size: A test of pseudo‐single‐domain remanence models , 1986 .

[6]  J. Coleman,et al.  Solubility of Mo6S4.5I4.5 nanowires in common solvents: a sedimentation study. , 2005, The journal of physical chemistry. B.

[7]  Xiao-yan Li,et al.  Permeability of fractal aggregates. , 2001, Water research.

[8]  Meejeong Kim,et al.  Gel barrier formation in unsaturated porous media. , 2002, Journal of contaminant hydrology.

[9]  T. Mallouk,et al.  Remediation of Cr(VI) and Pb(II) aqueous solutions using supported, nanoscale zero-valent iron , 2000 .

[10]  M. Morbidelli,et al.  Kinetics of gel formation in dilute dispersions with strong attractive particle interactions. , 2004, Advances in colloid and interface science.

[11]  James Farrell,et al.  Investigation of the Long-Term Performance of Zero-Valent Iron for Reductive Dechlorination of Trichloroethylene , 2000 .

[12]  Wei-xian Zhang,et al.  Nanoscale Iron Particles for Environmental Remediation: An Overview , 2003 .

[13]  D. Beke Intrinsic and Domain Magnetism in Nanocrystalline Materials , 1998 .

[14]  Alex I. Braginski,et al.  Fundamentals and technology of SQUIDs and SQUID systems , 2004 .

[15]  Robert F. Butler,et al.  Theoretical single‐domain grain size range in magnetite and titanomagnetite , 1975 .

[16]  C. Allain,et al.  The effects of gravity on the aggregation and the gelation of colloids , 1993 .

[17]  R. Chantrell,et al.  Agglomerate formation in a magnetic fluid , 1982 .

[18]  Marc Fermigier,et al.  Aggregation kinetics of paramagnetic colloidal particles , 1995 .

[19]  Thomas E. Mallouk,et al.  Delivery Vehicles for Zerovalent Metal Nanoparticles in Soil and Groundwater , 2004 .

[20]  Bruno Dufour,et al.  Adsorbed triblock copolymers deliver reactive iron nanoparticles to the oil/water interface. , 2005, Nano letters.

[21]  J. Vicente,et al.  Stability of Cobalt Ferrite Colloidal Particles. Effect of pH and Applied Magnetic Fields , 2000 .

[22]  P. G. de Gennes,et al.  Pair correlations in a ferromagnetic colloid , 1970 .

[23]  P. Bomans,et al.  Direct observation of dipolar chains in iron ferrofluids by cryogenic electron microscopy , 2003, Nature materials.

[24]  B. J. McCoy,et al.  Cluster aggregation and fragmentation kinetics model for gelation. , 2005, Journal of colloid and interface science.

[25]  Bruno Dufour,et al.  Surface Modifications Enhance Nanoiron Transport and NAPL Targeting in Saturated Porous Media , 2007 .

[26]  D. Sholl,et al.  TCE dechlorination rates, pathways, and efficiency of nanoscale iron particles with different properties. , 2005, Environmental science & technology.

[27]  A. Kim,et al.  Hydrodynamics of an ideal aggregate with quadratically increasing permeability. , 2005, Journal of Colloid and Interface Science.

[28]  B. Vincent,et al.  Structure Formation in Disperse Systems , 1987 .

[29]  K. Henn,et al.  Utilization of nanoscale zero‐valent iron for source remediation—A case study , 2006 .