Article pubs.acs.org/est
Heteroaggregation of Cerium Oxide Nanoparticles and Nanoparticles of Pyrolyzed Biomass Peng Yi,†,‡ Joseph J. Pignatello,*,† Minori Uchimiya,§ and Jason C. White∥ †
Department of Environmental Sciences, The Connecticut Agricultural Experiment Station, New Haven, Connecticut 06511, United States ‡ Department of Civil, Environmental and Geomatics Engineering, Florida Atlantic University, Boca Raton, Florida 33431, United States § Agricultural Research Service, United States Department of Agriculture, 1100 Robert E. Lee Boulevard, New Orleans, Louisiana 70124, United States ∥ Department of Analytical Chemistry, The Connecticut Agricultural Experiment Station, New Haven, Connecticut 06511, United States S Supporting Information *
ABSTRACT: Heteroaggregation with indigenous particles is critical to the environmental mobility of engineered nanomaterials (ENM). We studied heteroaggregation of ceria nanoparticles (n-CeO2), as a model for metal oxide ENM, with nanoparticles of pyrogenic carbonaceous material (n-PCM) derived from pecan shell biochar, a model for natural chars and human-made chars used in soil remediation and agriculture. The TEM and STEM images of n-PCM identify both hard and soft particles, both Crich and C,O,Ca-containing particles (with CaCO3 crystals), both amorphous and “onion-skin” C-rich particles, and traces of nanotubes. Heteroaggregation was evaluated at constant n-CeO2, variable n-PCM concentration by monitoring hydrodynamic diameter by dynamic light scattering and ζ-potential under conditions where n-PCM is “invisible”. At pH 5.3, where n-CeO2 and n-PCM are positively and negatively charged, respectively, and each stable to homoaggregation, heteroaggregation is favorable and occurs by a charge neutralization−charge reversal mechanism (CNCR): in this mechanism, primary heteroaggregates that form in the initial stage are stable at low or high n-PCM concentration due to electrostatic repulsion, but unstable at intermediate n-PCM concentration, leading to secondary heteroaggregation. The greatest instability coincides with full charge neutralization. At pH 7.1, where n-CeO2 is neutral and unstable alone, and n-PCM is negative and stable alone, heteroaggregation occurs by a charge-accumulation, core−shell stabilization (CACS) mechanism: n-PCM binds to and forms a negatively charged shell on the neutral surface of the nascent n-CeO2 core, stabilizing the core−shell heteraggregate at a size that decreases with n-PCM concentration. The CNCR and CACS mechanisms give fundamental insight into heteroaggregation between oppositely charged, and between neutral and charged nanoparticles.
■
emissions,6 road runoff, industrial wastewater discharge, and municipal sewage sludge application,7 a widespread practice in the agricultural sector. The Organization for Economic CoOperation and Development includes n-CeO2 on its priority list
INTRODUCTION Cerium oxide nanoparticles (n-CeO2) are used as abrasives in the semiconductor industry,1 adsorbents for removing metal ions from water,2 additives in diesel fuel,3 components in microelectronics, ingredients in pharmaceuticals, UV blocking agents, and combustion catalysts in exhaust systems.4,5 The incorporation of n-CeO2 into agricultural chemicals has also been reported.4 n-CeO2 may enter the environment from direct application to soil (e.g., in agricultural chemicals), automotive © 2015 American Chemical Society
Received: Revised: Accepted: Published: 13294
July 21, 2015 October 1, 2015 October 13, 2015 October 13, 2015 DOI: 10.1021/acs.est.5b03541 Environ. Sci. Technol. 2015, 49, 13294−13303
Article
Environmental Science & Technology
et al.19 measured growth rate in hydrodynamic diameter (Dh) during heteroaggregation of positively charged hematite nanoparticles (HemNPs) at fixed concentration and negatively charged carbon nanotubes (CNTs) at varied concentration. As the CNT concentration increased, the growth rate rose to a peak level and then decreased. They attributed increasing heteroaggregation rate with increasing CNT concentration to the bridging of HemNPs by CNTs, and attributed the decrease of heteroaggregation rate at high CNT concentration to repulsion between nanotube arms that are attached to HemNPs.19 They also found that CNT−HemNP heteroaggregates can be broken up by ultrasonication.32 Several key aspects of heteroaggregation remain poorly understood. Little experimental work has been done on heteroaggregation between neutral and charged particles despite its potential importance.40 It is still unclear whether the surface charge of heteroaggregates plays an important role in determining heteroaggregation rates; thus, the mechanism of heteroaggregation remains ambiguous. Furthermore, the reversibility of heteroaggregation in the absence of external physical energy has seldom been examined. In this study, heteroaggregation of n-CeO2 with nanoparticles of pyrogenic carbonaceous material (n-PCM) represented by a pecan shell biochar is investigated. PCM from incidental sources, such as biomass burning and fuel combustion,41,42 is widespread in soil.43 Human-made PCMs such as activated carbon and biochar are of interest as soil amendments in agriculture and environmental remediation.44 Nanoparticulate forms of natural chars and biochars have been reported45,46 and are expected to have considerable effects on the fate and transport of many contaminants, but have not been well characterized. Here, the shape, elemental composition, atomic arrangement, and colloidal stability of n-PCM are characterized for the first time, to our knowledge. To study heteroaggregation, we use the techniques of dynamic light scattering (DLS) and ζ-potential measurement under conditions where the scattered light intensity is due almost solely to n-CeO2. The technique of making one particle invisible in a binary particle system, adopted from the work of Huynh et al.,19 permits deeper insight into the mechanism. Heteroaggregation was studied at pH 5.3, where n-CeO2 and n-PCM are oppositely charged, and at pH 7.1 where n-CeO2 is net neutral and n-PCM is negatively charged. Heteroaggregation was not studied at alkaline pH where repulsion was expected due to their mutual negative charge. The crucial role of ζ-potential of heteroaggregates in determining the growth rate of heteroaggregate hydrodynamic diameter is exhibited and a novel selfstabilization phenomenon is found in the neutral-charged binary particle system. Reversibility of heteroaggregation was also studied.
of engineered nanomaterials (ENM) requiring immediate testing for specified environmental end points related to physical− chemical behavior and toxicity.8 Adverse effects of n-CeO2 exposure include inhibition of crop plant germination,9 chronic inflammatory response in rats,10 and cytotoxicity and genotoxicity to human cell lines.6 Recently, it was reported that n-CeO2 can be absorbed by edible plants and transferred up the food chain.11,12 An ability to predict the fate and transport of n-CeO2 in the environment is crucial to assessing the risks of exposure. Aggregation is an important process that governs size distribution, dispersibility, solubility, deposition potential, bioaccumulation potential,10,11,13−15 and toxicity13,14 of ENM. Apart from its unique concerns, n-CeO2 may serve as a model for variably charged metal oxide nanoparticles, since it has an isoelectric point (pHIE) at pH ∼ 7.16,17 Previous studies of nCeO2 have mostly focused on its homoaggregation behavior in synthetic15 and natural15,18 waters. However, given the abundance of natural colloids and nanoparticles, heteroaggregation is expected to play a more important role than homoaggregation in the transport and bioavailability of nCeO2 in the environment.19−21 Quik et al.22 found that the degree of n-CeO2 sedimentation at low n-CeO2 concentration (1 mg/L) was higher in raw river water than in filtered river water, and attributed the high sedimentation in raw water to heteroaggregation with, or the deposition onto, particles larger than 0.2 μm. By contrast, homoaggregation appeared to dominate at higher n-CeO2 concentrations (10−100 mg/L). To our knowledge, there are no published studies quantifying size growth and surface charge properties of heteroaggregates of n-CeO2 with natural nanoparticles, nor on the mechanisms of their heteroaggregation. Nanoparticle heteroaggregation has been of fundamental interest due to its importance in sedimentation,23,24 mineral processing,24,25 materials manufacture,26,27 paper processing,24,28 water and wastewater treatment,28,29 and food engineering.30,31 Most studies have focused on heteroaggregation of oppositely charged particles,19,20,24,26,28,32−35 although a few focused on heteroaggregation of like-charge particles.25,36−39 Rollié et al.34 studied heteroaggregation of negatively charged polystyrene (PS) nanoparticles with positively charged Rhodamine-B labeled melamine−formaldehyde (MF-RhB) nanoparticles using flow cytometry. They found that with increasing MF-RhB concentration the formation rate of low-degree heteroaggregates increased to a maximum and then declined. This was attributed to electrostatic destabilization and restabilization, respectively. However, their ζ-potential results are contradictory to their model, showing that the heteroaggregates are highly negatively charged (−40 to −50 mV) at MF-RhB concentrations where heteroaggregation was fast, but nearly neutral at high MF-RhB concentrations where heteroaggregation was slow.34 The counterintuitive information may have been due to the interference of scattered light from both nanoparticles.34 Yates et al.24 visually observed supernatant turbidity after a 24 h sedimentation test of suspensions containing a fixed concentration of positively charged alumina particles and varied concentrations of negatively charged silica particles at pH 5. They found that the ζ-potential of suspended particles was relatively low in magnitude (−20 to +15 mV) at silica concentrations where sedimentation was greatest. They attributed the pattern in sedimentation to charge neutralization by silica particles. However, the greatest sedimentation did not occur at the point of neutrality, which is where it is expected to occur if a charge neutralization mechanism was operative. Huynh
■
EXPERIMENTAL SECTION Materials. Stock suspensions of 7.26 g/L n-CeO2 were made by adding the commercial powder (Sigma-Aldrich, #54484,
