Aggregation of nanoscale iron oxyhydroxides and corresponding effects on metal uptake, retention, and speciation: I. Ionic-strength and pH

Abstract

The capacity of nanosized particles to adsorb and sequester dissolved metals can be significantly impacted by the mechanism and extent of aggregation the particles have undergone, which in turn can affect the long-term fate and transport of potentially toxic metals in natural aqueous systems. Suspensions of monodisperse nanoscale iron oxyhydroxides were synthesized and subjected to increased pH (pH 8.0, 10.0) or ionic strength (0.1, 1.0 M NaNO₃) conditions to induce various states of aggregation prior to conducting macroscopic adsorption/desorption experiments with dissolved Cu(II) or Zn(II). The metal adsorption and retention capacities of the nanoparticle aggregates were compared to one another and to non-aggregated control nanoparticles, while the mode(s) of metal sorption to the nanoparticle surfaces were characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy analysis.

With increasing aggregation by both pH and ionic strength, the proportion of introduced zinc adsorbed to the iron oxyhydroxide nanoparticles progressively decreased from 45% on the monodispersed control particles to as low as 16% on the aggregates, while the proportion of introduced zinc retained upon desorption (obtained by lowering the suspension pH) increased from 7% on the control particles to as much as 17% on the aggregated particles. Copper exhibited a subtler trend of only slightly declining uptake (from 43% to 36%) and retention (from 35% to 30%) with increasing aggregation state. EXAFS analysis was consistent with the macroscopic results, showing relatively little change in Cu speciation between samples analyzed before and after the desorption step but significant increases in Zn–Fe interatomic distances and coordination numbers after desorption. This suggests the presence of both strongly- and weakly-bound zinc ions; the latter are likely affiliated with less stable, more distorted surface sorption sites and are thus more readily desorbed, resulting in the retention of zinc that is bound to more stable, less-distorted sorption sites. For both metals, inner-sphere bidentate sorption appears to dominate the sorption process to the nanoparticle aggregates, with potential structural incorporation into the aggregates themselves.

Introduction

Metal contamination is a serious concern for both human and environmental health. Even common metals such as zinc and copper, which are required dietary components, can cause negative health effects when they reach acute or chronic toxicity levels, disrupting ecosystems and causing sickness in humans. Such elevated levels can be found near anthropogenic sources of metal contamination including mining sites, which often feature low pH waters and high dissolved metal concentrations as a result of acid mine drainage (AMD) (Nordstrom and Alpers, 1999, Blowes et al., 2003). The onset of rainfall events following a prolonged dry season in these areas often produces a “first flush” of surface water runoff, where periodic pulses of acidic water result in elevated aqueous metal concentrations through sediment transport/deposition, mineral dissolution, and metal desorption processes, as has been documented in locations such as Iron Mountain, CA (Nordstrom et al., 2000, Druschel et al., 2004, Jamieson et al., 2005). The abundance of similarly contaminated mine sites throughout the world and the subsequent threat they pose to the health of surrounding inhabitants and ecosystems makes the effective remediation of these areas a high priority.

The correlation between the adsorption of dissolved metals to mineral particle surfaces and the corresponding long-term fate of such metals in natural aqueous systems is generally well known. The mineral–water interface can control the mobility and potential bioavailability of dissolved metals through adsorption processes which may lead over time to (co-)precipitation, surface precipitation, and/or structural incorporation into the solid phase, facilitating the sequestration of metals in sediments (Herbert, 1996, Ford et al., 2001, Grafe et al., 2004). Although the adsorption of dissolved metal species to minerals has been the focus of numerous studies, the tendency of metals to desorb and return to aqueous solution has been relatively less well assessed despite the fundamental importance of desorption processes in the long-term availability of metals in natural systems, particularly those that may undergo geochemical (e.g., pH, ionic strength) changes over time. Past studies have also established that most minerals have non-homogeneous surfaces involving multiple surface binding sites and strengths (Benjamin and Leckie, 1981, Hiemstra and van Riemsdijk, 1996, Villalobos et al., 2009), which has implications for the retention and sequestration of sorbed metals under varying geochemical conditions.

Iron oxyhydroxides are considered to be the dominant reactive mineral phase in marine and lake sediments (van der Zee and Roberts, 2003) due to their relative ubiquity, high surface area, and strong capacity for metal sorption (Dyer et al., 2004, Xu et al., 2006); such phases are particularly abundant in AMD environments, which are known for a characteristic red staining along riverbeds from freshly-precipitated iron hydroxides. Furthermore, iron oxyhydroxide phases have been observed in natural systems at the nanoscale (Penn et al., 2001, van der Zee and Roberts, 2003), likely contributing to their high reactivity with dissolved metals. However, under most natural aqueous conditions, nanoparticulate iron oxyhydroxides will rapidly aggregate following formation (Banfield et al., 2000, Gilbert et al., 2007, Yuwono et al., 2012). Despite the often repeated possibility of using nanoparticulate iron oxyhydroxides and other minerals as sorbents for dissolved metals, few studies have focused on the effects of aggregation on their sorption properties.

The aggregation of nanoparticles is likely to impact their adsorption and retention capacities in a variety of potential ways, including reducing available surface areas (leading to lowered adsorption) and aiding the structural incorporation of metals into nanoporous regions (decreasing desorption and increasing retention) (Kim et al., 2008). Additionally, the mechanism of aggregation, such as pH (Gilbert et al., 2007), ionic strength (Li and Xu, 2008), and temperature (Gilbert et al., 2009) is known to impact the formation of the resulting aggregates. These morphological variations caused by aggregation conditions may affect the sorption of metals, playing an important role in the binding strength of those metals and therefore their long-term stability in the resulting solid phases. Structural models for the complex interactions which may arise from sorption onto/into aggregates do not currently exist in the literature.

The purpose of this study is to examine the effects of different aggregation mechanisms on the adsorption and desorption behavior of metals onto/from iron oxyhydroxide nanoparticles, with the initial hypothesis that aggregation reduces adsorption but enhances the retention (i.e., reduces the desorption) of metals that are sorbed to nanoparticle aggregates. This involves a combination of macroscopic batch adsorption/desorption experiments conducted to quantify metal uptake and retention and extended X-ray absorption fine structure (EXAFS) spectroscopy to probe the local structural environment of the sorbed metals and determine the mode(s) of metal binding. Such structural information can then be used to (1) generate models for the uptake and retention of dissolved metals to nanoparticles and their aggregates; (2) explain the strength of metal binding to iron oxyhydroxide nanoparticles under a range of environmental conditions; and (3) assess the overall effectiveness of aggregated iron oxyhydroxide nanoparticles at metal retention in aqueous systems. Nanoparticle aggregation by increases in pH and ionic strength are explored in this, the first of a two-part study, with the second part focusing on aggregation as a function of temperature and time.