Quantification of Heteroaggregation between Citrate-Stabilized Gold

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Quantification of Heteroaggregation between CitrateStabilized Gold Nanoparticles and Hematite Colloids Brian M. Smith, Daniel J. Pike, Michael O. Kelly, and Jeffrey A. Nason Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03486 • Publication Date (Web): 07 Oct 2015 Downloaded from http://pubs.acs.org on October 12, 2015

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Environmental Science & Technology

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Quantification of Heteroaggregation between

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Citrate-Stabilized Gold Nanoparticles and

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Hematite Colloids

4

Brian M. Smith, Daniel J. Pike, Michael O. Kelly and Jeffrey A. Nason*

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School of Chemical, Biological and Environmental Engineering, Oregon State University, 103

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Gleeson Hall, Corvallis, Oregon 97331, United States

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ABSTRACT: Collisions with and attachment to natural colloids (heteroaggregation) is likely to

9

significantly influence the fate, transport, and toxicity of engineered nanoparticles (ENPs). This

10

study investigated heteroaggregation between hematite (α-Fe2O3) colloids and citrate-capped

11

gold nanoparticles (Cit-AuNPs) using a novel approach involving time-resolved dynamic light

12

scattering and parallel experiments designed to quantify nanoparticle attachment and

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heteroaggregate surface charge. Experiments were performed in low ionic strength synthetic

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water at environmentally relevant pH in the presence and absence of Suwannee River Natural

15

Organic Matter (SRNOM). In the absence of SRNOM at pH values where Cit-AuNPs and

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hematite are oppositely charged, attachment efficiencies are high and Cit-AuNPs are capable of

17

destabilizing hematite following an “electrostatic patch” mechanism. Furthermore, maximum

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observed surface coverages were far below those predicted by geometry alone, a fact predicted

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by the random sequential adsorption (RSA) model that has significant implications for the

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estimation of heteroaggregate attachment efficiencies. At pH values where both particles are

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negative or in the presence of small amounts of SRNOM, attachment was minimal. Calculated

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attachment efficiencies using the measured surface coverages corroborate these findings. The

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calculation of attachment efficiencies and the identification of mechanisms governing

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heteroaggregation represents an important step towards predicting the transport, fate and

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toxicity of ENPs in the environment.

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INTRODUCTION

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The growing range of commercial and industrial applications for engineered nanoparticles

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(ENPs) [1], coupled with the potential for toxicity [2], has inspired a body of research devoted to

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understanding the fate and transport of ENPs in the environment [3-6]. Aggregation state is a

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key determinant of ENP reactivity and toxicity [7-10], and is central to understanding fate and

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transport in aquatic systems. ENPs may collide with and attach to like particles

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(homoaggregation) or unlike particles (heteroaggregation). In the environment, ENP

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concentrations will be much less than concentrations of naturally occurring colloids on both a

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mass and number basis [11-13]. Thus, the predicted collisions frequencies for ENP

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heteroaggregation with natural colloids are much higher than those for ENP homoaggregation

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[3, 5, 14]. As aggregates containing ENPs form and grow, the surface area available for reaction

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decreases and the processes of sedimentation and filtration are enhanced [9]. Aggregate size

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may also influence uptake and retention by biological organisms and cells [15]. Many factors


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influence the aggregation of colloids and ENPs, including ionic strength and composition, pH,

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capping agents present, and the concentration and type of natural organic matter (NOM) [9, 16-

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19].

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Charged particles are stable with respect to aggregation due, in part, to repulsive electrostatic

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forces that arise as particles approach one another. Particles are also stabilized by steric

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interactions between surface bound molecules, hydrophobic interactions and hydration forces

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[20]. Classic mechanisms of particle destabilization involve reducing or otherwise overcoming

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the electrostatic repulsive force between particles. These processes often involve charge

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neutralization by adsorption of oppositely charged ionic species or diffuse layer compression by

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increasing the ionic strength of the solution. Other mechanisms include the adsorption of high

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molecular weight polymers to induce interparticle bridging and sweep coagulation [21]. The

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extent to which particles are destabilized is described by the attachment efficiency ( α ), the

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probability that two colliding particles will attach. Determination of attachment efficiencies for

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homoaggregation of ENPs in various aquatic matrices has been the focus of many studies, such

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as those summarized by Petosa et al. [20]. However, a similar focus has not extended to the

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heteroaggregation process.

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Praetorius et al. [4] demonstrate a model for predicting the environmental fate of ENPs using

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a pseudo-first-order rate constant for heteroaggregation derived by multiplying the collision

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rate constant by α hetero , the attachment efficiency for heteroaggregation. Extending from the

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definition above, α hetero represents the probability that two unlike particles will attach upon

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close approach. If α hetero is known, the environmental fate of ENPs can be better predicted.



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However, the few recent studies that have investigated the ENP heteroaggregation with

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naturally occurring colloids [22-28] have not quantified the process in a way that can lead to an

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understanding of α hetero . Only Praetorius et al. [29], using a combined approach involving size

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distribution measurements and mathematical modeling, has estimated α hetero . Clearly,

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additional effort in this area is needed.

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The objectives of this work were to quantify ENP attachment to a naturally occurring colloid

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over a range of environmentally relevant conditions, elucidate the mechanisms governing

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heteroaggregation, and provide experimental estimates of α hetero . Citrate-capped AuNPs (Cit-

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AuNPs) and hematite (α-Fe2O3) colloids were used as a model system. The choice is justified by

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our solid understanding Cit-AuNPs homoaggregation in the presence and absence of NOM [16]

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and a desire to expand this work to better characterize the role that capping agents play in

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controlling ENP behavior. AuNPs are inert and can be readily produced with a range of capping

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agents [30-32]. Here, we investigate the heteroaggregation process using parallel techniques

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including time-resolved dynamic light scattering (TR-DLS) to quantify aggregation, a novel

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separation technique to quantify NP-colloid association and surface coverage, and zeta potential

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measurements of the heteroaggregates to probe surface charge characteristics.

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MATERIALS AND METHODS

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Hematite colloids. Hematite colloids were synthesized following established procedures [33,

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34] resulting in a stock concentration of 1.6 g/L as hematite (see Supporting Information (SI) for

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details). Hematite particle size was characterized using Transmission Electron Microscopy (TEM)

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(FEI Titan G2 80-200 TEM, Hillsboro, OR) and Dynamic Light Scattering (DLS) (90Plus Particle Size


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5

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Analyzer, Brookhaven Instruments, Holtsville, NY). The resulting particles were prolate

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spheroids with dimensions of approximately 80 × 120 nm as measured by TEM (SI Figure S1).

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The average hydrodynamic diameter measured by DLS ranged between 120-150 nm. Using the

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TEM dimensions and a solid bulk density of 5.26 g/cm3, a particle number concentration of the

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stock solution was estimated at 7.6 x 1011 particles/mL.

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The electrophoretic mobility of the hematite colloids was measured using a Brookhaven

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ZetaPALS (Holtsville, NY) and was converted to zeta potential using the Hückel equation

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adjusted using the Henry correction factor [35] (SI Section 5).

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Nanoparticles. Cit-AuNPs were purchased from NanoComposix, Inc. (12 nm Citrate

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NanoXact™ Gold) (San Diego, CA). Manufacturer reported specifications include an 11.8 + 0.8

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nm (one standard deviation) core diameter (TEM), a mass concentration of 51.6 mg/L as Au, and

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a particle concentration of 3.0 x 1012 particles/mL. As reported previously, the average

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hydrodynamic diameter as measured by DLS is 20 nm and the critical coagulation concentration

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(CCC) of in KCl at pH 6 is 56 mM [16]. Zeta potential of the Cit-AuNPs was measured as

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described above. Prior to use, excess citrate was removed by repeated centrifugal filtration and

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resuspension in DDI water; the final Cit-AuNP stock concentration was verified using UV-vis

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spectroscopy (SI Section 6 ).

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Experimental conditions and reagents. Heteroaggregation experiments were performed in

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aquatic media representative of surface and ground waters with low ionic strength and low

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organic content. Experimental conditions maintained a constant concentration of both KCl (1

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mM) and hematite (10 mg/L) while varying the NOM concentration from 0-1 mg C/L and the pH



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between values of 6.1, 8.0, and 10.0. Experiments were performed at approximately 25° C. All

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chemicals used were ACS reagent-grade. Stock solutions of HCl, KOH, and KCl were prepared in

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DDI water and filtered through a 0.2 µm nylon membrane syringe filter (VWR International,

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Radnor, PA). Suwannee River Natural Organic Matter (SRNOM) was purchased from the

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International Humic Substance Society (IHSS). A stock solution was prepared in DDI water and

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filtered through a 0.2 µm filter using methods described previously [32].

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Quantifying Cit-AuNP attachment to hematite. Cit-AuNP attachment to hematite colloids

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was quantified as follows. 6 mL samples were prepared in 15 mL polypropylene centrifuge

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tubes using the following order of addition: DDI water, KOH (for pH 8.0 and 10.0 samples),

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hematite colloids, KCl, SRNOM (if applicable), and Cit-AuNPs. The suspension was inverted and

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then left undisturbed for 30 minutes. Hematite and hematite-Cit-AuNP aggregates were

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subsequently separated from free Cit-AuNPs by filtration through a Nuclepore® track-etched

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membrane with 80 nm pores (Whatman). The filtrate and retentate (including the filter) were

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acidified separately by adding enough aqua regia (3:1 HCl to HNO3) to bring the final solutions to

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approximately 5% acid. Iron and gold were quantified in each sample using Inductively Coupled

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Plasma Optical Emission Spectrometry (ICP-OES) (Teledyne Leeman Prodigy, Hudson, NH).

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The electrophoretic mobilities of mixed systems containing both Cit-AuNPs and hematite

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were measured after allowing the particles to interact for approximately ten minutes.

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Heteroaggregate zeta potential was calculated as described above.

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Characterizing and quantifying aggregation. Time-resolved dynamic light scattering (TR-DLS)

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was used to investigate aggregation by tracking changes in the intensity weighted hydrodynamic


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diameter ( Dh ). An individual TR-DLS analysis consisted of Dh measurements at 15 second

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intervals for a period of 31 minutes. Homoaggregation experiments with hematite and Cit-

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AuNPs were performed as described previously [17]. Heteroaggregation experiments consisted

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of the following steps: (1) hematite particles were suspended in DDI water in 3.5 mL cuvettes;

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(2) size was measured by DLS; (3) KCl was added, and the size was measured again; (4) pH was

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adjusted using KOH, if applicable; (5) SRNOM was added, if applicable, and size was checked

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again; (6) Cit-AuNPs were added, the time was recorded, the cuvette was inverted, and the

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measurements were initiated; (7) pH was measured the end of each experiment.

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Hematite colloids dominated the scattering intensity in all experiments due to their larger size

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relative to Cit-AuNPs (scattering intensity ∝ Dh 6 [36]). Therefore, DLS measurements of

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systems containing both hematite and Cit-AuNPs represent the size of the hematite and/or

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hematite-Cit-AuNP aggregates. This was confirmed by independent verification of Cit-AuNP and

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hematite stability alone at these conditions (SI Figure S6).

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As described below in reference to Figure 5 and further verified via theoretical calculations (SI

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Section 15), initial attachment of Cit-AuNPs to hematite particles (formation of primary

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heteroaggregates) was rapid and essentially complete after approximately 75 s. Thus, the

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change in aggregate size due to the subsequent collisions between hematite colloids decorated

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with Cit-AuNPs (secondary aggregation) was quantified using a modification of previously

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established methods [17]. Briefly, the slope ( dDh dt ) was quantified from t = 75 s until

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Dh = 1.3 ⋅ Dh ,t →75s or 30 minutes, whichever was reached first. Aggregation was also quantified



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using the extent of aggregation at 30 minutes by averaging the final five measurements of a run

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( Dh ,30 ) and normalizing that value by the initial hydrodynamic diameter ( Dh ,0 ).

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RESULTS AND DISCUSSION

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Colloid and nanoparticle stability. Cit-AuNPs and hematite colloids were stable with respect

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to homoaggregation at all conditions examined in this work (SI Figures S7-S9). Intermediate

148

concentrations of SRNOM did induce hematite homoaggregation at pH 6.1 (SI Figure S11), but

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those conditions were not used in the evaluation of heteroaggregation phenomena. As shown

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in Figure 1a, hematite is positively charged at pH 6 and 8 and negatively charged at pH 10. In

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the presence of 1 mg C/L as SRNOM, hematite is negatively charged over the entire pH range

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and the CCC for homoaggregation in KCl at pH 6.1 increases, indicating adsorption and steric

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stabilization (SI Figure S12). Cit-AuNPs are negatively charged over the entire pH range in the

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presence and absence of SRNOM (Figure 1b), with SRNOM stabilizing the particles with respect

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to homoaggregation [16].

156


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157 zeta potential (mV)

75 (a)

50 25 0 -25 -50 -75

158 zeta potential (mV)

0 (b) -10 -20 -30 -40 3

159

4

5

6

7 pH

8

9

10

11

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Figure 1. Zeta potential of Hematite colloids (a) and Cit-AuNPs (b) in 1 mM KCl and the presence

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(●) and absence (○) of 1 mg C/L SRNOM. Error bars represent 95% confidence intervals.

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Cit-AuNP attachment to hematite. TEM images of samples containing Cit-AuNPs and

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hematite colloids unequivocally demonstrate the attachment of the Cit-AuNPs to the hematite

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colloids (Figure 2). The developed filtration protocol was used to quantify the extent of that

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process. Control experiments with only AuNPs or hematite demonstrated the efficacy of the

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filtration technique for separating free AuNPs from hematite colloids (SI Section 10). Free Cit-

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AuNP concentrations remaining after 30 minutes of interaction with hematite colloids are

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shown as a function of initial Cit-AuNP concentration in Figure 3Error! Reference source not

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found.. The 1:1 reference line represents the expected concentrations if no attachment of Cit-



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AuNPs to hematite (and subsequent removal by the filtration step) occurred. The data are

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presented in this way to clearly illustrate conditions under which free Cit-AuNPs exist in

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suspensions containing hematite colloids. Presentation of the data in terms of the directly

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measured attachment of Cit-AuNPs at each condition can be found in the SI (Figure S13-S15),

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along with mass balances on gold and iron for each experiment (SI Tables S1-S6).

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Figure 2. TEM images at varying magnifications of heteroaggregates of Cit-AuNPs (0.6 mg/L) and

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hematite colloids (10 mg/L) at pH 6.1 in 1 mM KCl.


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C

AuNP,free

@ 30 min. (mg/L)

11

2.0

(a)

1.5 1.0

1:1 line

0.5 0.0 0.0

0.5

1.5

AuNP,initial

C

AuNP,free

@ 30 min. (mg/L)

179

180

1.0 C

2.0

2.0 (mg/L)

2.5

3.0

(b)

1.5 1.0

1:1 line

0.5 0.0 0.0

0.5

1.0 1.5 2.0 CAuNP,initial (mg/L)

2.5

3.0

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Figure 3. Association of Cit-AuNPs with hematite at pH 6.1 (○), 8.0 (□), and 10.0 (◊) in the

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absence (a) and presence (b) of 1 mg C/L as SRNOM. All experiments were performed with 10

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mg/L hematite in 1 mM KCl. The solid black line represents a case where no AuNPs attach to the

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hematite. The dashed grey lines are eye guides with a slope of 1.

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At low Cit-AuNP concentration in the absence of SRNOM, all of the Cit-AuNPs were associated

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with the hematite at pH 6.1 and 8.0. This is expected due to the electrostatic attraction

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between the oppositely charged particles at these conditions. At higher Cit-AuNP

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concentrations, there was incomplete association of the added Cit-AuNPs with the hematite.

189

The Cit-AuNP concentration at which this incomplete association occurred decreased with



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decreasing pH (e.g., approximately 1.5 mg/L at pH 6.1 and 1.3 mg/L at pH 8). Above these

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concentrations, the slope of free vs. added Cit-AuNPs is approximately 1.0, suggesting that once

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the hematite colloids are coated to a critical extent the system is stabilized with respect to

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further AuNP attachment and all of the excess Cit-AuNPs remain free in suspension. At pH 10 in

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the absence of SRNOM, substantially fewer Cit-AuNPs were attached to the hematite colloids at

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each condition examined. Although hematite carries a net negative charge at pH 10, it is

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plausible that positively charged sites still exist and that Cit-AuNPs are attracted to those sites.

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The trend of decreasing attachment with increasing pH is consistent with the findings at pH 6.1

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and 8.

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In the presence of 1 mg C/L SRNOM, free Cit-AuNP concentrations track the 1:1 reference line

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(Figure 3b), indicating minimal attachment of Cit-AuNPs to the hematite colloids. This finding is

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not unexpected given the large body of research illustrating the stabilizing effect of NOM on

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natural colloids [37] and ENPs [17]. Repulsive electrostatic and steric forces between the Cit-

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AuNPs and the SRNOM-coated hematite at these conditions likely prevent attachment. In

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supporting experiments at varying NOM concentrations, the same effect was observed at

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SRNOM concentrations as low as 0.05 mg C/L (Figure S16), indicating that Cit-AuNPs attachment

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would be substantially diminished in most all natural aquatic systems.

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Heteroaggregate surface charge. Cit-AuNP attachment is also evidenced by measurements of

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heteroaggregate zeta potentials at each condition (Figure 4a). Heteroaggregate surface charge

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decrases from values indicative of bare hematite to values indicative of Cit-AuNPs with

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increasing Cit-AuNP concentration. Note that measurements were only made on suspensions


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verified to be absent of free Cit-AuNPs (i.e., only containing AuNP-hematite heteroaggregates)

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so that this result is a clear indication of Cit-AuNPs attachment to the hematite colloids. The

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behavior at pH 8.0 was similar to that at pH 6.1, but shifted to the left (lower Cit-AuNP

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concentrations), presumably due to the decreased surface charge of bare hematite at pH 8.0

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compared to pH 6.1.

zeta potential (mV)

80 (a)

60 40 20 0 -20 -40 0.0

0.5

1.0

1.5

2.0

2.5

3.0

216 Surface Coverage (θ)

0.14 0.12 0.10

θmax,pH = 6

(b)

θmax,pH = 8

0.08 0.06 0.04

θmax,pH = 10

0.02 0.00 0.0

0.5

1.0

1.5

2.0

2.5

3.0

217



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h

Aggregation Rate (dD /dt|

t->75s

)

14

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0.4 (c) 0.3 0.2 0.1 0.0 0.0

0.5 1.0 1.5 2.0 2.5 Cit-AuNP Concentration (mg/L)

3.0

219

Figure 4. Heteroaggregation experimental results in 1 mM KCl at pH 6.1 (○), 8.0 (□) and pH 10.0

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(◊) in the absence of SRNOM. (a) Fractional surface coverage of hematite by Cit-AuNPs; (b) zeta

221

potential of heteroaggregates; and (c) Rate of aggregation Error bars represent 95% confidence

222

intervals.

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Fractional surface coverage. The fraction of the total hematite surface area occupied by

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attached Cit-AuNPs ( θ ), was calculated at each condition using the mass-based measurements

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of Cit-AuNP attachment (SI Section 13). The results, presented in Figure 4b, clearly illustrate

226

that the surface coverage reaches a different maximum ( θ max ) at each pH and that θ max is

227

substantially less than 1.0. These results can be explained in the context of the Random

228

Sequential Adsorption (RSA) model where particles are only allowed to attach to a surface in

229

areas not occupied by previously deposited particles and are not allowed to rearrange after

230

attachment [38]. In the limit, this process results in a non-zero excluded area that is inaccessible

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for further attachment. As such, θ approaches a jamming limit that is dependent on the

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geometry of the system and the nature of the interactions between attached particles [39]. For


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example, the predicted θ max for hard, non-interacting spheres depositing on a flat surface is a

234

mere 0.547 [38].

235

In recent work with a similar system involving oppositely charged latex colloids of different

236

sizes, Sadowska et al. [40] compared experimental results to a model developed by Adamczyk

237

and Belouschek [41] that accounts for both the curvature of the collector and the repulsive

238

double layer interactions between neighboring particles on the collector surface. Applying a

239

similar analysis here, the maximum theoretical surface coverage ( θ max,theory ) can be calculated (SI

240

Section 14). Table 1 compares the results of the experimentally measured and theoretically

241

predicted values of θ max .

242 243

Table 1. Comparison of measured and predicted values of the maximum achievable surface

244

coverage ( θ max ) at each experimental condition.

pH

θ max,measured

θ max,theory

6

0.12

0.17

8

0.10

0.14

10

0.03

0.12

245 246

It has been noted that this analysis neglects interactions with the substrate which can

247

decrease accuracy for surfaces with high zeta potential [39]. The analysis also neglects the

248

possibility of preferential attachment only at specific sites distributed across the hematite



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surface. The fact that θ max,measured was less than predicted based on ENP-ENP interactions alone

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suggests that surface charge heterogeneity and/or interactions between the Cit-AuNPs and the

251

hematite surface may also play a role. The fact that θ max,measured decreases with increasing pH is

252

consistent with this hypothesis in that the concentration of favorable (positively charged)

253

attachment sites decrease with increasing pH and there are increasing unfavorable interactions

254

between Cit-AuNPs and the negatively charged regions of the hematite surface. Continued

255

work in this area is ongoing, focusing on the effects of nanoparticle/colloid size ratio, surface

256

charge heterogeneity and the role of NOM in controlling attachment. As the field moves

257

towards examining real-world colloidal material, it will be important to more fully understand

258

the influence of heterogeneity in size, shape and surface charge on ENP attachment.

259

Implications for estimating α hetero . Regardless of the underlying cause of the relatively sparse

260

θmax , these findings have significant implications on the modeling of heteroaggregation

261

processes and the estimation of α hetero . In their recent work on this topic, Praetorious et al. [29]

262

defined the global attachment efficiency ( α global ) for collisions between primary

263

heteroaggregates (e.g., colloids decorated with ENPs) as: 2

264

NP coll α global = f NP 2 ⋅ α homo + (1 − f NP ) ⋅ α homo + 2 ⋅ (1 − f NP ) ⋅ f NP ⋅ α hetero

265

Where f NP is the fraction of the hematite surface covered by nanoparticles (equivalent to θ

(1)

NP

266

coll in this work), α homo is the attachment efficiency for homoaggregation of ENPs, and α homo is the

267

attachment efficiency for homoaggregation of colloids. Here, f NP represents the probability


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with which each type of interaction (ENP-ENP, colloid-colloid, and ENP-colloid) will occur as two

269

primary aggregates approach one another. The reader is directed to the supporting information

270

of the original reference for a graphical representation of this concept. In that work, the

271

authors calculate f NP on the basis of geometrical considerations alone (total projected area of

272

all ENPs in suspension / total surface area of all suspended colloids). This formulation makes

273

several assumptions. First, it assumes that all ENPs rapidly attach to the colloid surfaces and

274

that no free ENPs remain. The present work has experimentally verified that assumption by

275

quantifying ENP attachment in a similar experimental system (i.e., oppositely charged particles).

276

Second, it is implicit that the defined relationship only be applied to systems with low surface

277

coverage, otherwise f NP has the potential to exceed 1.0 (i.e., a situation where there are more

278

ENPs than necessary to completely coat the colloid surfaces). This is also a reasonable

279

assumption for problems related to ENP fate and transport given the high probability that

280

colloids will vastly outnumber ENPs in natural systems. However, what the relationship fails to

281

recognize is the theoretical limit to surface coverage inherent in the RSA model and

282

experimentally observed in this work, as well as the implications of that fact on the estimation

283

of α hetero .

284

NP coll Assuming that α homo = α homo = 0 (both ENPs and colloids are stable with respect to

285

homoaggregation; the case in this work), Eq. 1 indicates that the global attachment efficiency

286

will approach zero as f NP approaches 1.0. On the contrary, f NP will approach the jamming



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limit ( θ max ) at increased ENP concentrations and α global should approach zero at that condition.

288

Here, we propose that the variable f NP in Eq. 1 be defined as follows:

289

290

f NP =

θ θ max

(2)

In this way, the actual probability of different types of collisions between primary aggregates

291

is more accurately represented. For example, when θ = θ max ( f NP = 1.0 according to Eq. 2)

292

interactions between primary aggregates will be dominated completely by α homo , because there

293

is no available surface area for colloid-colloid or ENP-colloid interactions. This is despite the fact

294

that there is surface area on the colloid that is not covered by an ENP (i.e., θ < 0 ). That

295

remaining surface area represents the excluded area inherent in RSA processes and shouldn’t be

296

considered as available (the assumption of the original definition of f NP [29]). This fact is

297

demonstrated below in the estimation of α hetero from this work.

NP

298

Secondary heteroaggregation. As evidenced above, Cit-AuNPs readily attach to hematite

299

colloids, forming primary heteroaggregates consisting of hematite colloids decorated with Cit-

300

AuNPs. Here we examine the influence of that initial attachment on the stability of the

301

suspension (i.e., the formation of secondary heteroaggregates consisting of two or more

302

primary aggregates). Alone, both Cit-AuNPs and hematite colloids were stable with respect to

303

homoaggregation at all experimental conditions (SI Figures S7-S9 and [16]). Results from several

304

representative heteroaggregation experiments at pH 6.1 are shown in Figure 5. At low Cit-AuNP

305

concentrations, there was no observable influence on hematite stability. However, the system


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306

aggregated dramatically at intermediate Cit-AuNP concentrations. At Cit-AuNP concentrations

307

greater than approximately 1.5 mg/L, there was a rapid increase in size (ca. 15 nm) indicative of

308

the formation of primary heteroaggregates. However, once formed, these aggregates were

309

stable. That this process appears complete after 75-100 seconds is consistent with calculations

310

of the characteristic reaction time for loss of AuNPs due to collision and attachment with

311

hematite (7.5 s, SI Section 15) and justifies the methodology outlined above for estimating the

312

rates of secondary aggregation by focusing on times after 75s. Rates of secondary aggregation

313

for a range of Cit-AuNP concentrations in the different aquatic matrices are summarized in

314

Figure 4c.

h

Hydrodynamic Diameter, D (nm)

315

316

700

Cit-AuNP Concentration 0.01 mg/L 0.1 mg/L 0.6 mg/L 1.0 mg/L 1.5 mg/L

600 500 400 300 200 100

0

500 1000 1500 Time after mixing (seconds)

2000

317

Figure 5. Representative aggregation profiles for mixtures of hematite (10 mg/L) and Cit-AuNPs

318

at concentrations ranging from 0.01 mg/L to 1.5 mg/L . Results are shown at pH 6.1 in 1 mM KCl

319

in the absence of SRNOM.

320



Page 20 of 41

20

321

In the absence of SRNOM, suspensions at pH 6.1 and 8 behaved similarly. The rapid

322

attachment of Cit-AuNPs to the hematite colloids upon addition induced substantial secondary

323

aggregation between the formed primary heteroaggregates. Secondary aggregation was

324

enhanced with increasing Cit-AuNP concentration to an optimum point, beyond which the rate

325

and extent of secondary aggregation decreased (restabilization). At the highest Cit-AuNP

326

concentrations, the formed primary heteroaggregates were stable with respect to further

327

secondary aggregation. At pH 6.1, maximum aggregation was observed at 0.6 mg/L Cit-AuNPs

328

and re-stabilization was observed at 1.5 mg/L. At pH 8.0, both the maximum rate of aggregation

329

and the point of re-stabilization occurred at lower Cit-AuNP concentrations (approximately 0.35

330

mg/L and 1.3 mg/L, respectively). Furthermore, the extents of aggregation at those conditions

331

were significantly larger than those reached by hematite homoaggregation under diffusion-

332

limited conditions (SI Figure S17). At pH 10, no change in size was observed, consistent with the

333

limited attachment of Cit-AuNPs at this condition where both particles types are negatively

334

charged.

335

In the presence of 1 mg C/L SRNOM at all three pH values, no aggregation of any type was

336

observed regardless of the Cit-AuNP concentration. Initial aggregation rates were not

337

significantly different than zero nor significantly different than aggregation rates in the absence

338

of SRNOM and Cit-AuNPs at a 95% confidence level (SI Table S9 and Figure S18). As shown in

339

Figure 1a, SRNOM coats the hematite particles and imparts a negative surface charge, a finding

340

in agreement with recent work by Chekli et al. [42]. As described above, SRNOM also stabilizes

341

Cit-AuNPs with respect to homoaggregation. At the conditions of these experiments, the


Page 21 of 41


21

342

combination of electrostatic and steric repulsive forces between the SRNOM molecules on

343

adjacent particles, be they hematite or Cit-AuNPs, prevent heteroaggregation.

344

Conceptual model for secondary heteroaggregation. Several qualitative aspects of the

345

observed behavior align with an adsorption and charge neutralization mechanism of particle

346

destabilization: (1) Cit-AuNP attachment to hematite was verified (Figure 2 and 3); (2) an

347

“optimal dose” of Cit-AuNPs that resulted maximum hematite destabilization was observed,

348

followed by re-stabilization at higher Cit-AuNP concentration (Figure 4c); (3) the surface charge

349

of hematite particles decreased upon attachment of Cit-AuNPs (Figure 4a) and (4) the

350

concentration of Cit-AuNPs required to reverse the charge of hematite at pH 8.0 was less than

351

that at pH 6.1 due to the lower initial hematite charge at pH 8.0 (Figure 4a). However, several

352

features cannot be explained by that mechanism. First, the maximum extent of aggregation

353

invoked by Cit-AuNP addition exceeded that reached by hematite destabilization through

354

double layer compression (SI Figure S17). This larger final size could simply be the result of

355

AuNPs being incorporated into the heteroaggregates, altering their stability, fractal dimension

356

and size, but may also suggest that the attached Cit-AuNPs enhance aggregation beyond simple

357

elimination of the electrostatic repulsive force between hematite particles. Secondly, the

358

concentrations of Cit-AuNPs that invoked maximum destabilization did not correspond to the

359

concentrations that resulted in heteroaggregates with no net surface charge as would be

360

predicted by a charge neutralization mechanism (Figure 4a,c). Additionally, substantial

361

aggregation was observed between heteroaggregates with significantly positive net surface



Page 22 of 41

22

362

charge. This significant positive charge should result in a substantial electrostatic repulsive

363

force, yet significant aggregation occurred.

364

Enhanced aggregation behavior observed for the destabilization of negatively charged colloids

365

by cationic polymers has been attributed to an “electrostatic patch” model [43-45] where

366

adsorbed cations incompletely cover the negatively charged particle surface. A positively

367

charged polymer-coated patch on one particle is attracted to a negatively charged, uncoated

368

patch on another particle, accounting for enhanced aggregation. The observed

369

heteroaggregation behavior is consistent with such a model, where destabilization occurs,

370

despite a net positive surface charge, through attractive interactions that develop between

371

negatively charged regions (i.e., attached Cit-AuNPs) and positively charged regions (i.e., bare

372

hematite surface) on adjacent heteroaggregates. Moreover, the observation that the maximum

373

rate of aggregation corresponds with approximately 30-40% of θ max (Figure 4b,c) is consistent

374

with the assertion of Healy and La Mer [46] that “half surface coverage” of a particle surface by

375

polymers will result in maximum flocculation. Finally, TEM images of the aggregates formed

376

under optimal conditions show gold nanoparticles bridging the gap between adjacent hematite

377

particles (Figure 2). These findings provide categorical evidence for the hypothesis by Praetorius

378

et al. [29] that ENP bridging is responsible for secondary heteroaggregation in such systems.

379

However, rather than heteroaggregate surface charge being the driver, evidence here suggests

380

that surface coverage in the context of the RSA model is the key controlling factor.


Page 23 of 41


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381

Heteroaggregate attachment efficiency. As an exercise to demonstrate the importance of

382

properly accounting for Cit-AuNP surface coverage, values of α hetero were estimated using Eq. 3,

383

which is a rearrangement of Eq. 1 [29]. 2

NP coll αglobal − f NP2 ⋅αhomo − (1 − f NP ) ⋅ αhomo

384

αhetero =

385

In this work, αglobal was calculated by normalizing the apparent rates of secondary

(3)

2 ⋅ (1 − f NP ) ⋅ f NP

386

heteroaggregation at each condition ( dDh dt t →75s ) by the apparent rate observed for the

387

homoaggregation of hematite colloids under diffusion limited conditions (0.414 nm/s) and

388

NP coll αhomo = αhomo = 0 (no homoaggregation).

389

both the value of f NP estimated by geometrical considerations alone [29] and the value

390

calculated using Eq. 2 (Table 2).

Calculations were performed for each trial using

391 392

Table 2. Calculated attachment efficiencies between Cit-AuNPs and hematite colloids in the

393

absence of SRNOM at varying pH using the different definitions of f NP .

Cit-AuNP pH

(mg/L)*

f NP calculated as surface

f NP calculated as shown in

area ratio [29]

Eq 2.

f NP

α hetero †

f NP

α hetero †

6.1

0.1-1

0.0005-0.05

17.3 ± 6.7

0.007-0.67

1.86 ± 0.38

8.0

0.1-1

0.0005-0.05

129 ± 12

0.10-0.99

2.39 ± 0.64



Page 24 of 41

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10.0

0.1

0.005

4.0

0.40

0.0089

394

†

values shown are mean values ± 95% confidence intervals

395

*

at Cit-AuNP concentrations outside this range either f NP =1.0 and α hetero has no physical

396

meaning or rates of aggregation were not significantly different than zero and α hetero could not

397

be calculated

398 399

As expected, estimates of f NP based on surface area ratios are dramatically lower than those

400

calculated according to Eq. 2. As a result, calculated values of α hetero are dramatically larger

401

when using that definition of f NP . It was expected that α hetero would be close to 1.0 at pH 6.1

402

and 8 in the absence of SRNOM because Cit-AuNPs and hematite are oppositely charged.

403

Calculated values of α hetero using the surface area based definition of f NP were 1-2 orders of

404

magnitude larger owing to the overestimate of f NP . This was even the case at pH 10 where

405

particles were observed to be quite stable and α hetero is expected to be much less than 1.0. On

406

the other hand, estimates using the procedure outlined in this work were much closer to the

407

expected values. This indicates a more accurate representation of the governing processes;

408

specifically, the proper representation of the probabiliites of different types of collisions

409

between primary heteroaggregates. Furthemore, consistent estimates for α hetero from

410

experiments with Cit-AuNPs concentrations varying over an order of magnitude is evidence that

411

the measured values of f NP are correct.


Page 25 of 41


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412

The fact estimates of α hetero were somewhat larger than 1.0 may be explained by the fact that

413

heteroaggregation rates were normalized by the rate of hematite homoaggregation. Because

414

the Cit-AuNPs and hematite are oppositely charged at pH 6.1 and 8 and the ionic strength is

415

relatively low, a greater attractive force exists between the primary heteroaggregates resulting

416

in faster aggregation than predicted. In modeling, it is common for best-fit values of the

417

attachment efficiency to incorporate aspects of DLVO interactions or other processes that may

418

not be explicitly included in the model. Perhaps with a more rigorous accounting of DLVO forces

419

between the oppositely charge particles (e.g., [47]), more refined estimates of α hetero could be

420

obtained. Nevertheless, as expected, α hetero is quite high at these conditions and attachment of

421

Cit-AuNPs to hematite colloids is rapid. On the other hand, at pH 10 where both particle types

422

are negatively charged, α hetero is very low. In the presence of SRNOM, attachment efficiencies

423

could not be quantified because no aggregation was observed, but attachment efficiencies can

424

be expected to be

Quantification of Heteroaggregation between Citrate-Stabilized Gold

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