Unique forms of manufactured nanomaterials, nanoparticles, and their suspensions are rapidly being created by manipulating properties such as shape, size, structure, and chemical composition and through incorporation of surface coatings. Although these properties make nanomaterial development interesting for new applications, they also challenge the ability of colloid science to understand nanoparticle aggregation in the environment and the subsequent effects on nanomaterial transport and reactivity. This review briefly covers aggregation theory focusing on Derjaguin-Landau-Verwey-Overbeak (DLVO)-based models most commonly used to describe the thermodynamic interactions between two particles in a suspension. A discussion of the challenges to DLVO posed by the properties of nanomaterials follows, along with examples from the literature. Examples from the literature highlighting the importance ofaggregation effects on transport and reactivity and risk of nanoparticles in the environment are discussed.

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.

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The surfaces of nanoscale zerovalent iron (NZVI) used for groundwater remediation must be modified to be mobile in the subsurface for emplacement. Adsorbed polymers and surfactants can electrostatically, sterically, or electrosterically stabilize nanoparticle suspensions in water, but their efficacy will depend on groundwater ionic strength and cation type as well as physical and chemical heterogeneities of the aquifer material. Here, the effect of ionic strength and cation type on the mobility of bare, polymer-, and surfactant-modified NZVI is evaluated in water-saturated sand columns at low particle concentrations where filtration theory is applicable. NZVI surface modifiers include a high molecular weight (MW) (125 kg/mol) poly(methacrylic acid)-b-(methyl methacrylate)-b-(styrene sulfonate) triblock copolymer (PMAA-PMMA-PSS), polyaspartate which is a low MW (2-3 kg/mol) biopolymer, and the surfactant sodium dodecyl benzene sulfonate (SDBS, MW = 348.5 g/mol). Bare NZVI with an apparent zeta-potential of -30 +/- 3 mV was immobile. Polyaspartate-modified nanoiron (MRNIP) with an apparent zeta-potential of -39 +/- 1 mV was mobile at low ionic strengths (< 40 mM for Na+ and < 0.5 mM for Ca2+), and had a critical deposition concentration (CDC) of approximately 770 mM Na+ and approximately 4 mM for Ca2+. SDBS-modified NZVI with a similar apparent zeta-potential (-38.3 +/- 0.9 mV) showed similar behavior (CDC approximately 350 mM for Na+ and approximately 3.5 mM for Ca2+). Triblock copolymer-modified NZVI had the highest apparent zeta-potential (-50 +/- 1.2 mV), the greatest mobility in porous media, and a CDC of approximately 4 M for Na+ and approximately 100s of mM for Ca2+. The high mobility and CDC is attributed to the electrosteric stabilization afforded by the triblock copolymer but not the other modifiers which provide primarily electrostatic stabilization. Thus, electrosteric stabilization provides the best resistance to changing electrolyte conditions likely to be encountered in real groundwater aquifers, and may provide transport distances of 10s to 100s of meters in unconsolidated sandy aquifers at injection velocities used for emplacement.

Rapid in situ degradation of chlorinated solvents present as nonaqueous phase liquids (NAPL) can be accomplished using reactive zerovalent nanoiron particles. Prior studies have shown that nanoiron transport in the subsurface is limited, and successful delivery of the nanoiron is essential for effective remediation. Here, the physical properties of bare and modified nanoiron are measured, and laboratory column reactors are used to compare the transport of three types of surface-modified nanoiron; triblock polymer-modified, surfactant-modified, and a commercially available polymer-modified nanoiron. The effect of particle concentration and solution ionic strength on the transport of each modified nanoiron is evaluated, and the filtration mechanisms for bare and modified particles are determined in microfluidic flow cells and quartz crystal microbalance (QCM) experiments that probe the particle-collector grain interaction. The effect of surface modification on nanoiron reactivity is evaluated in batch experiments. Transport of modified nanoiron does not directly correlate with-potential or colloidal stability, but rather correlates to particle-grain interactions. Filtration of bare nanoiron is caused by straining and subsequent clogging rather than by deposition to clean sand grains, suggesting that filter ripening models rather than clean bed filtration models should be used to describe nanoiron transport at high particle concentrations. Surface modification decreased nanoiron reactivity by two to four times, but as high as a factor of nine depending on the modifier used. Amphiphilic triblock copolymer modified nanoiron with a high hydrophobe/hydrophile ratio shows promise for in situ targeting of NAPL, but requires further optimization.

Reactive zero valent iron nanoparticles can degrade toxic nonaqueous phase liquids (NAPL) rapidly in contaminated groundwater to nontoxic products in situ, provided they can be delivered preferentially to the NAPL/water (oil/water) interface. This study demonstrates the ability of novel triblock copolymers to modify the nanoiron surface chemistry in a way that both promotes their colloidal stability in aqueous suspension and drives their adsorption to the oil/water interface. The ability of the copolymers to drive adsorption is demonstrated by the ability of copolymer-modified iron nanoparticles, but not the unmodified iron nanoparticles, to stabilize oil-in-water emulsions.

The effect of particle concentration, size distribution (polydispersity) and magnetic attractive forces (Fe(0) content) on agglomeration and transport of poly(styrene sulfonate) (PSS) modified NZVI was studied in water-saturated sand (d(p) = 300 microm) columns. Particle concentrations ranged from 0.03 to 6 g/L in 5 mM NaCl/5 mM NaHCO3 at a pore water velocity of 3.2 x 10(-4) m/s. Three NZVI dispersions with different intrinsic particle size distributions obtained from sequential sedimentation are compared. The influence of magnetic attraction (Fe(0) content) on NZVI agglomeration and deposition in porous media is assessed by comparing the deposition behavior of PSS-modified NZVI (magnetic) having different Fe(0) contents with PSS-modified hematite (nonmagnetic) with the same surface modifier. At low particle concentration (30 mg/L) all particles were mobile in sand columns regardless of size or magnetic attractive forces. At high concentration (1 to 6 g/L), deposition of the relatively monodisperse dispersion containing PSS-modified NZVI (hydrodynamic radius (R(H)) = 24 nm) with the lowest Fe(0) content (4 wt%) is low (attachment efficiency (alpha) = 2.5 x 10(-3)), insensitive to particle concentration, and similar to PSS-modified hematite. At 1 to 6 g/L, the attachment efficiency of polydisperse dispersions containing both primary particles and sintered aggregates (R(H) from 15 to 260 nm) of PSS-modified NZVI with a range of Fe(0) content (10-60%) is greater (alpha = 1.2 x 10(-2) to 7.2 x 10(-2) and is sensitive to particle size distribution. The greater attachment for larger, more polydisperse Fe(0) nanoparticles with higher Fe(0) content is a result of their agglomeration during transport in porous media because the magnetic attractive force between particles increases with the sixth power of particle/agglomerate radius. A filtration model that considers agglomeration in porous media and subsequent deposition explains the observed transport of polydisperse PSS-modified NZVI at high concentration.

Nanoscale zero-valent iron (NZVI) is used to remediate contaminated groundwater plumes and contaminant source zones. The target contaminant concentration and groundwater solutes (NO3-, Cl-, HCO3-, SO4(2-), and HPO4(2-)) should affect the NZVI longevity and reactivity with target contaminants, but these effects are not well understood. This study evaluates the effect of trichloroethylene (TCE) concentration and common dissolved groundwater solutes on the rates of NZVI-promoted TCE dechlorination and H2 evolution in batch reactors. Both model systems and real groundwater are evaluated. The TCE reaction rate constant was unaffected by TCE concentration for [TCE] < or = 0.46 mM and decreased by less than a factor of 2 for further increases in TCE concentration up to water saturation (8.4 mM). For [TCE] > or = 0.46 mM, acetylene formation increased, and the total amount of H2 evolved at the end of the particle reactive lifetime decreased with increasing [TCE], indicating a higher Fe0 utilization efficiency for TCE dechlorination. Common groundwater anions (5mN) had a minor effect on H2 evolution but inhibited TCE reduction up to 7-fold in increasing order of Cl- < SO4(2-) < HCO3- < HPO4(2). This order is consistent with their affinity to form complexes with iron oxide. Nitrate, a NZVI-reducible groundwater solute, present at 0.2 and 1 mN did not affect the rate of TCE reduction but increased acetylene production and decreased H2 evolution. NO3- present at > 3 mM slowed TCE dechlorination due to surface passivation. NO3- present at 5 mM stopped TCE dechlorination and H2 evolution after 3 days. Dissolved solutes accounted for the observed decrease of NZVI reactivity for TCE dechlorination in natural groundwater when the total organic content was small (< 1 mg/L).