Using Chromate to Investigate the Impact of Natural Organics on the

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Using Chromate to Investigate the Impact of Natural Organics on the Surface Reactivity of Nanoparticulate Magnetite Andrew L. Swindle,*,† Isabelle M. Cozzarelli,‡ and Andrew S. Elwood Madden§ †

Department of Geology, Wichita State University, 1845 Fairmount Avenue, Wichita, Kansas 67260, United States U. S. Geological Survey, 12201 Sunrise Valley Drive, Reston, Virginia 20192, United States § School of Geology and Geophysics, University of Oklahoma, 100 East Boyd Street, Norman, Oklahoma 73019, United States ‡

S Supporting Information *

ABSTRACT: Chromate was used as a chemical probe to investigate the size-dependent influence of organics on nanoparticle surface reactivity. Magnetite−chromate sorption experiments were conducted with ∼90 and ∼6 nm magnetite nanoparticles in the presence and absence of fulvic acid (FA), natural organic matter (NOM), and isolated landfill leachate (LL). Results indicated that low concentrations (1 mg/L) of organics had no noticeable impact on chromate sorption, whereas concentrations of 50 mg/L or more resulted in decreased amounts of chromate sorption. The adsorption of organics onto the magnetite surfaces interfered equally with the ability of the 6 and 90 nm particles to sorb chromate from solution, despite the greater surface area of the smaller particles. Results indicate the presence of organics did not impact the redox chemistry of the magnetite−chromate system over the duration of the experiments (8 h), nor did the organics interact with the chromate in solution. Brunauer−Emmett−Teller (BET) and scanning electron microscopy (SEM) results indicate that the organics blocked the surface reactivity by occupying surface sites on the particles. The similarity of results with FA and NOM suggests that coverage of the reactive mineral surface is the main factor behind the inhibition of surface reactivity in the presence of organics.

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INTRODUCTION Nanoparticles, those materials having dimensions between 1 and 100 nm, have been increasingly scrutinized over the past several years. Nanoparticles are ubiquitous in nature where they play key roles in the cycling of elements such as carbon, nitrogen, and iron,1 in microbial metabolic processes, in mineral weathering and precipitation,2 and in the transport of contaminants in subsurface and aqueous environments.3,4 Synthetic nanoparticles have shown potential as tools to remediate a wide variety of environmental contaminants such as heavy metals,5−7 radionuclides,8,9 and chlorinated organic compounds.10−13 Nanoparticles also have potential applications in a number of societally beneficial fields such as electronics, medicine, and water treatment.14,15 Much of the interest in nanoparticles stems from the fact that these materials often display physical and chemical properties, such as solubility, conductivity, magnetic properties, and aggregation kinetics, that vary based on particle size.16 While the fact that nanoparticles display size-dependent chemical and physical properties has been supported in numerous laboratory investigations,16 how these properties will impact the fate of nanoparticles in a chemically complex field setting is still largely unresolved. Swindle et al.17 conducted one of the first direct comparisons between the size-dependent surface reactivity of identical nanoparticles deployed in the field versus laboratory analogue conditions. They investigated the dissolution of magnetite nanoparticles of various sizes in the subsurface of the U. S. Geological Survey (USGS) research site at the Norman Landfill17 and also performed laboratory experiments of © 2015 American Chemical Society

magnetite dissolution in Fe(II)-bearing analogue groundwater. Three key observations of the previous study provide the relevant motivation and background for this study: (i) field reactivity for all particle sizes was minimal and was greatest for the largest particle sizes (90 nm > 44 nm > 6 nm), (ii) all sizes reacted extensively in the lab, with the smaller particles experiencing the greatest reactivity (6 nm ≈ 44 nm ≫ 90 nm), and (iii) an organic coating visible by electron microscopy formed on the mineral surfaces in the field experiments. Swindle et al.17 hypothesized that the dramatic difference between field and lab experiments was due to the lack of organics in the lab experiments, whereas in the field experiments an organic film quickly passivated the mineral surfaces and prevented dissolution. Furthermore, the reversal in the particle size dependence of dissolution between lab and field indicated that the adsorption of organics onto the particle surfaces had a size-dependent impact on the surface reactivity of the particles. Review articles summarizing the literature regarding the interactions of aquatic organic matter with nanoparticles and colloids focus on colloid stabilization, with limited discussion of effect on surface reactivity.18,19 In some cases, organics were shown to increase or decrease particle dissolution.18 Also, the particle size-dependent impacts of organics on surface reactions Received: Revised: Accepted: Published: 2156

October 2, 2014 January 16, 2015 January 21, 2015 January 21, 2015 DOI: 10.1021/es504831d Environ. Sci. Technol. 2015, 49, 2156−2162

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such as ion sorption20,21 is a topic not widely discussed in the current literature. Using a series of sorption experiments, this investigation builds on the earlier work of Swindle et al.17 by testing the hypothesis that organics in the leachate-impacted groundwater at the Norman Landfill site had a size-dependent impact on surface reactivity.

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minimum of three times to remove excess salts, and then stored in an anaerobic chamber. Surface areas and pore size distributions were determined for the 90 and 6 nm magnetite particles before and after reaction with 100 mg/L organic solutions from N2 sorption−desorption isotherms produced using a Quantachrome Nova 2000e surface area analyzer (details provided in the Supporting Information). Typical uncertainties between interlaboratory measurements of surface area and pore volume measurements by these methods are 4.6% and 3.2%, respectively.33 Reacted magnetite particles were also investigated using a Zeiss NEON 40 EsB field emission scanning electron microscope and a JEOL 2000FX transmission electron microscope to determine the geometric relationship between the magnetite particles and the adsorbed organics. Organic Stock Solutions. Suwannee River FA II and NOM were purchased from the International Humic Substance Society and used as received. Stock solutions were created by adding 100 mg of Suwannee River FA or NOM to a piperazineN,N′-bis(ethanesulfonic acid) (PIPES) buffer solution. The landfill leachate solution was prepared from impacted groundwater from the core of the leachate plume at the USGS Norman Landfill Site via lyophilization and dialysis against UP water (details in the Supporting Information). Historic groundwater data from the Landfill indicate that the nonvolatile dissolved organic carbon (NVDOC) concentration in the center of the leachate plume is ∼87 mg/L of C.34 However, analytical results of the leachate stock solution reported a NVDOC concentration of 12.06 mg/L, demonstrating components of the leachate were likely lost during dialysis. A filter analysis of the particulate matter indicated a particulate concentration of ∼650 mg/L; however, transmission electron microscopy (TEM) and SEM investigations revealed that the particulate fraction of the extracted leachate was a heterogeneous mixture of both organic and inorganic particles. Chromate Sorption Experiments. Triplicate chromate sorption experiments (50 mL each) were conducted in crimpsealed amber glass vials, which were initially loaded in an anaerobic chamber with a 95%/5% nitrogen/hydrogen atmosphere. The experimental pH was adjusted to 6.8 using a ∼10 mM PIPES buffer solution. Additional conditions are provided in Table 1. As this investigation sought to assess the impacts of organics on mineral surfaces, the magnetite,

MATERIALS AND METHODS

Experimental Design Rationale. For the purposes of these sorption experiments, the chemical probe needed to be an ionic species known to readily adsorb to magnetite and that displayed limited sorption capacity to dissolved organics. Ultimately, chromate was chosen as previous studies have found that it readily sorbs to magnetite surfaces6,22,23 where it is reduced from Cr(VI) to Cr(III) via electron transfer from structural ferrous iron.6,24 This reduction mechanism is highly dependent on magnetite stoichiometry (ratio of Fe(II)/ Fe(III)), and several investigations have indicated that magnetite particles with a higher ferrous iron content have greater contaminant-reducing capacity.25−27 The reduction of Cr(VI) to Cr(III)- and mixed Cr(III)−Fe(III) (hydr)oxides by reaction with Fe(II)-bearing mineral surfaces is likely one of the dominant processes in natural chromium cycling and provides a mechanism for the remediation of toxic Cr(VI)-bearing waters.28 Additionally, chromate was seen as an ideal chemical probe as previous investigations indicated that it does not readily react with dissolved organics in solution.29,30 A total of three organic reactants were selected for this study (Suwanee River fulvic acid II (FA), Suwanee River natural organic matter (NOM), and Norman Landfill leachate (LL)) to both provide opportunities for generalization using established standard organics and utilize site-specific organics from the field location of the previous field−lab nanoparticle comparison experiments.17 A discussion of the rationale behind the use of these standards is available in the Supporting Information. Chromate sorption experiments without added organics were conducted to establish baseline sorption values. Additional experiments were conducted with increasing concentrations of added organics (1, 50, and 100 mg/L, representative of natural waters that typically range up to 60 mg/L31) with the results compared to the baseline experiments. To investigate the role of organics on the important redox processes between the magnetite and chromate, chemical extractions and multiple analytical methods were used to track surface-bound and aqueous Cr(III) and Cr(VI). Thus, the inhibition of chromate sorption serves as a proxy for the impact of organics on the surface reactivity of the magnetite particles. The terms sorption and adsorption are used throughout this manuscript. For clarity, when these terms are used, sorption refers to the removal of chromate from solution via interaction with magnetite (through reductive precipitation or otherwise), while the term adsorption is used to describe the accumulation of organics on the surface of the magnetite particles. Particle Synthesis and Characterization. All water used in these experiments, both to prepare solutions and to rinse and store particles, was degassed, ultrapure (UP, 18.2 MΩ cm) water. All chemicals used in these experiments were reagentgrade. Two distinct sizes of magnetite particles (90 (±42) nm and 6 (±2) nm) were synthesized using established methods10,32 as detailed in the Supporting Information. Following synthesis, magnetite particles were magnetically separated, washed a

Table 1. Chromate Sorption Experimental Conditions experiment

magnetite size (nm)

FA (mg/L)

NOM (mg/L)

LL (mg/L)

CrO42− (mM)

SA (m2)

90-Cr 90-FA1 90-FA50 90-FA100 90-LL 6-Cr 6-FA1 6-FA50 6-FA100 6-NOM1 6-NOM50 6NOM100

90 90 90 90 90 6 6 6 6 6 6 6

0.0 1.0 50.0 100.0 0.0 0.0 1.0 50.0 100.0 0.0 0.0 0.0

0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 1.0 50.0 100.0

0.0 0.0 0.0 0.0 100.0a 0.0 0.0 0.0 0.0 0.0 0.0 0.0

0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

2 2 2 2 2 6/5b 6 6 6 5 5 5

a

Particulate concentration. DOC = 1.89 mg/L. bExperiment run with two different surface areas. 2157

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organics, and buffer were added and allowed to mix for ∼3 min prior to the addition of chromate. The amber vials were continuously agitated via an orbital shaker and sampled at regular intervals over the 8 h experiments for total Craq, Cr(VI)aq, and Cr(VI) associated with magnetite surfaces, with the amount of chromate sorbed onto magnetite determined by subtraction. Samples were measured for dissolved total chromium by flame atomic absorption spectrophotometry using a PerkinElmer AAnalyst 800 AAS, while Cr(VI) analysis was done with a Thermo Scientific GENESYS 6 UV−vis spectrophotometer (Supporting Information).

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RESULTS Impact of Organics on the Sorption of Chromate by Magnetite. Table 2 shows the surface-area normalized amount Table 2. Change in Chromate Sorption onto Magnetite Nanoparticles with the Addition of Organics μm of Cr sorbed/m2 grain size (nm) 6 6 6 6 6 6a 6a 6a 6a 90 90 90 90 90 a

organic (mg/L) FA 1 FA 50 FA 100 LL 2 NOM 1 NOM 50 NOM 100 FA 1 FA 50 FA 100 LL 2

0.5 h

% chg w/ org

8h

% chg w/ org

1.29 1.33 0.98 0.92 0.93 1.57 1.57 1.18 1.09 2.79 2.77 2.11 2.09 2.51

0 −3 24 29 28 0 0 25 31 0 1 24 25 10

1.52 1.53 1.26 1.22 1.19 1.84 1.83 1.36 1.22 3.86 3.83 3.07 3.03 3.36

0 −1 17 20 22 0 1 26 20 0 1 20 22 13

Figure 1. Plots showing Cr sorption per m2 of surface area versus time for (a) 90 nm magnetite particles in the presence of varying concentrations of FA and LL; (b) 6 nm magnetite particles in the presence of varying concentrations of FA and LL; (c) 6 nm magnetite particles in the presence of varying concentrations of NOM. Error bars correspond to the standard error calculated from triplicate experiments but are typically smaller than the symbols used in the plots.

experiments with organics to those without (Table 2, Figure 1b). Figure 1b illustrates the sorption of chromate onto the surface of the 6 nm magnetite particles. Similar to the 90 nm particles, the addition of 1 mg/L FA to the experiment had no significant impact on the total amount of Cr sorbed by the 6 nm particles. When the FA concentration was increased to 50 mg/L, the total amount of Cr sorbed by the 6 nm particles decreased by 17%, and when the FA concentration was raised to 100 mg/L, the total amount of Cr adsorbed was reduced by 20%. Figure 1b also shows that the presence of LL decreased the sorption capacity of the 6 nm magnetite particles to levels slightly below those observed in the experiments with 50 mg/L FA. Sorption experiments with the 6 nm particles and NOM were done using a second batch of synthesized particles. As each batch of particles was expected to vary slightly, the baseline experiment without organics was rerun to allow for an accurate assessment of the impact of NOM on chromate sorption. As the data show (Table 2), the second batch of 6 nm particles sorbed slightly more Cr; however, the addition of NOM decreased Cr sorption consistent with FA experiments (Table 2, Figure 1c). These data show that, at equivalent concentrations, NOM and FA have very similar impacts on the sorption of chromate by magnetite. Organics and Cr(VI) Reduction. Several investigators have reported that Cr(VI) is reduced to Cr(III) on the surface of magnetite particles (e.g., refs 6, 23, and 24). This redox process requires the oxidation of three ferrous iron atoms for every Cr(VI) reduced and simultaneously results in the oxidation of the magnetite surface as well as the precipitation of Cr3+ (hydr)oxides on the magnetite surfaces. These changes in the surface properties of the magnetite particles can influence the amount and rate of chromate removal from solution. Figure 2

Second batch of synthesized particles.

of chromium sorption for the first (0.5 h) and last (8 h) samplings, along with the percentage change in amount sorbed relative to the sample particles without added organics, for each of the magnetite−chromate sorption experiments. Results presented in Table 2 indicate that the presence of FA and NOM at concentrations of 50 mg/L and above and the stock LL at 2 mg/L DOC decreased the amount of chromate sorbed onto the surface of magnetite. Figure 1a illustrates the sorption of chromate onto the surface of the 90 nm magnetite particles over the 8-h experimental duration. Cr was readily sorbed to the magnetite surface in the absence of FA and when FA was at a low concentration (1 mg/L). Increasing the FA concentration to 50 mg/L resulted in a decrease of 20% in the total amount of Cr sorbed over the course of the experiment. However, increasing the FA concentration from 50 to 100 mg/ L had little additional influence, reducing the total amount of Cr sorbed by 22%. The percent change in sorption between the organic-free and the 50 and 100 mg/L FA and 2 mg/L LL decreased somewhat from the first sampling at 0.5 h to the last sampling at 8 h (Table 2). As demonstrated in Figure 1a, the impact of LL on the sorption of chromate onto magnetite varied over the course of the experiment but was generally equivalent to 50 mg/L FA. Results of sorption experiments with 6 nm magnetite were very similar to the 90 nm particle results when comparing 2158

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mg/L of NOM revealed size-dependent differences between the pore sizes available for reaction, in addition to sizedependent influences of the organics (Table 3). Table 3. Impact of Adsorbed Organics on Particle Surface Area Measurements particle 90 nm 90 nm + FA 6 nm 6 nm + FA 6 nma 6 nma + NOM

Figure 2. Plot showing Cr(III)/Crtotal associated with the 90 nm magnetite particle surfaces in the presence and absence of 100 mg/L of FA. Error bars correspond to the standard error calculated from triplicate experiments.

displays the ratio Cr(III)/Crtotal for Cr associated with the surface of the 90 nm magnetite particles in the presence of no FA and 100 mg/L of FA. The high value of this ratio indicates that most of the Cr associated with the magnetite surface has been reduced to Cr(III). The addition of 100 mg/L of FA has no appreciable effect on the ratio of reduced to total Cr, indicating that the organics are not significantly impacting the redox reactions between the magnetite and the chromate. Similarly, Figure 3 displays the ratio Cr(III)/Crtotal for Cr

a

mesopore volume (cc/ g)

micropore area (m2/g)

external surface area (m2/g)

total surface area (m2/g)

0.04 0.05

0.0 0.0

10 12

10 12

0.20 0.18

43 41

122 109

165 150

0.13 0.15

52 71

80 92

132 163

Second batch of particles.

Table 3 shows that aggregates of the 90 nm particles have no measurable microporosity (pore diameters < 2 nm) in contrast to the 6 nm particles in which >26% of the total surface area is contained within aggregate micropores. Analysis indicated that the 90 nm particles had a mesopore (pore width 2−50 nm) volume of 0.04 cc/g, while the mesopore volume of the 6 nm particles is an order of magnitude larger (0.20 cc/g). The adsorption of FA resulted in an increase in the total surface area and the mesopore volume of the 90 nm particles but made no contribution to the microporosity of the aggregates. In the case of the 6 nm particles, the adsorption of FA resulted in a 10% reduction in mesopore volume, a 5% reduction in micropore area, and a decrease of 11% in external surface area. On the other hand, the addition of NOM to the 6 nm particles increased the micropore area and the total surface area, indicating that the relative order of microporosity and total surface area can be described as NOM > 6 nm magnetite > FA ≫ 90 nm magnetite. Electron microscopy of the 90 (Figure 4) and 6 nm (Figure 5) particles both with and without adsorbed FA illustrates the

Figure 3. Plot showing Cr(III)/Crtotal associated with the 6 nm magnetite particle surfaces in the presence and absence of 100 mg/L of FA or NOM. Error bars correspond to the standard error calculated from triplicate experiments.

associated with the surface of 6 nm magnetite particles. Less Cr is reduced on the surface of the 6 nm particles, likely due to their lower ferrous iron content (Fe(II)/Fetotal of 0.23 vs Fe(II)/Fetotal of 0.31 for 90 nm magnetite); however, the addition of organics had no significant impact on the total amount of Cr reduced. These results indicate that the adsorbed organics in these experiments do not inhibit the transfer of electrons from ferrous iron atoms in the magnetite particles to chromate ions at the surface. A similar result was reported by Latta et al.,35 who determined that the transfer of electrons from aqueous ferrous iron to goethite was uninhibited by 20 mg/L of NOM. Chromate−organic control experiments revealed that FA and NOM did not interact with dissolved Cr concentrations and any decreases in Cr(VI) sorption and/or reduction are the result of magnetite−organic interactions rather than chromate−organic interactions (Supporting Information). Fulvic Acid Adsorption and Particle Surface Area. N2 gas adsorption measurements of the 90 and 6 nm magnetite particles before and after reaction with 100 mg/L of FA or 100

Figure 4. SEM images of 90 nm magnetite particles before (left) and after (right) mixing with 100 mg/L of FA. The arrows indicate FA that associated with the surfaces and macropore spaces of the particles. Scale bars are 200 nm.

dramatic difference in texture of the adsorbed organic matter in relation to the particles, in the spatial relationships of the organics to themselves and the magnetite. The untreated 90 nm magnetite particles formed loosely packed aggregates containing abundant macropores as well as some mesopores. Reaction with 100 mg/L of FA resulted in some of the macropore space in the aggregate structure being filled with FA (Figure 4). The 2159

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Impact of Organics on Surface Reactivity: Contrast between Field and Lab Observations. Iron oxides are known to readily adsorb natural organic matter,38 which acts to passivate the particle surfaces by occupying reactive surface sites39 and imparts a negative charge to the particle surface.40 The surface passivation of magnetite by organics can be seen in our results, which show that, as the concentration of organics in the experiment increases, the capacity for magnetite nanoparticles to sorb chromate decreases. Interestingly, FA and NOM concentrations of ∼50 mg/L resulted in significant decreases in chromate sorption capacity, although increasing the organic concentration to 100 mg/L had little additional effect. SEM images indicate that the organics adsorb to the magnetite surfaces as large globules. As the organics tend to aggregate on the particle surface rather than spread out uniformly, increasing the concentration from 50 to 100 mg/L did not result in a significant increase in the amount of magnetite surface area lost. In contrast to the observations of minerals exposed to organics in laboratory experiments, previous investigations have indicated that in natural waters mineral surfaces rapidly develop a relatively uniform coating of organic material, typically varying between 1 and 10 nm thick.17,37,40−42 In the laboratory setting, when the same materials are exposed to solutions using extracted organics, the thin coating fails to develop at concentrations less than ∼500 mg/L unless pH is held low (pH ∼2−4.8).43 Instead, the organics are deposited on the minerals as various sized aggregates.44−46 This is consistent with the magnetite−organic association observed in this investigation and indicates that the lack of a significant decrease in chromate sorption when organic concentrations are increased from 50 to 100 mg/L is due to self-aggregation of the organics rather than surface saturation of magnetite. Given the differences in mineral−organic interactions between natural and analogue systems, evidence supports that under natural conditions a uniform coating of organics may prove to have a greater impact on the surface reactivity of minerals than what was observed in this investigation. The development of a uniform coating would also be expected to significantly impact the reactive surface area contained within pore spaces, as the nonaggregated organics would be able to more freely enter these spaces. The reason for the discrepancy between mineral− organic adsorption under lab rather than field conditions is not known, although the loss of certain fractions of the organic pool during the isolation process used to create organic standards may play a role. Furthermore, in contrast to lab experiments with a fixed starting organic concentration, mineral nanoparticles exposed to flowing organic-rich surface waters and groundwaters such as those in ref 17 experience a continual delivery of new organics; the concentration of organics in solution is continually replenished. In the case of micropores, the development of a surface coating several nanometers thick would likely be sufficient to seal the pores from the surrounding solution, hindering the availability of surface sites in these spaces.47 Such a result was reported by Li et al.,47 who found that the adsorption of organic matter onto activated carbon lowered the materials’ reactive surface area and blocked pore sites, decreasing adsorption rates of atrazine. Size-Dependent Influence of Organics. The results of the sorption experiments indicate that the dissolved organics used in this investigation had a similar impact on chromate sorption by both the 6 and 90 nm particles. Interestingly, the

Figure 5. SEM images of 6 nm magnetite particles before (left) and after (right) mixing with 100 mg/L of FA. The arrow indicates FA that associated with the surfaces of the particles. Scale bars are 20 nm (left) and 100 nm (right).

BET measurements for the 90 nm particles revealed that the total surface area increased with the addition of FA, indicating that the surface area present on the organics more than compensated for the loss of magnetite surface area. The 6 nm magnetite particles, on the other hand, formed large, tightly packed aggregate structures lacking the large void spaces seen in the 90 nm particle aggregates (Figure 5). The scale of the images does not allow for accurate assessment of the distribution of FA into the pore spaces of the 6 nm particle aggregates. However, the data in Table 3 indicate that only a minor reduction in pore volume occurred. Furthermore, it is clear from Figures 4 and 5 that reaction with FA resulted in the adsorption of FA as large globules on the surface of the magnetite aggregates rather than as a uniform coating (see Supporting Information).

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DISCUSSION Impact of Organics on Surface Area. When measuring surface areas of nanoparticle aggregates, the total surface area measurement is a combination of external surface area (area not within pore spaces) and internal surface area (area contained in pores).36 Movement of ions into and out of micropores is limited as the small size of these pores increases the strength of particle-ion interactions. The adsorption of FA, NOM, and LL onto the magnetite surfaces was expected to reduce the capacity of the particles to adsorb chromate by competing for reactive surface sites. The data in Table 3 show that treatment with FA increased the surface of the 90 nm particles, while decreasing the surface area of the 6 nm particles. This is most likely due to the FA itself contributing to the surface area during the BET measurements. The adsorption of FA under these experimental conditions does not appear to greatly influence the meso- or microporosity of the particles; rather the FA occupies macropores between particles or adsorbs as large globules to the aggregate surfaces, thereby reducing the reactive magnetite surface area. Comparable results were reported by Yang et al.37 for the adsorption of humic acid (HA) onto aluminum and titanium oxides. The authors reported that exposure to 50 and 100 mg/ L of HA resulted in an increase in BET surface area for ∼150 nm α-Al2O3 from 4.73 to 5.50 and 7.24 m2/g, respectively. Conversely, exposure to 100 mg/L of HA decreased the BET surface area of ∼60 nm γ-Al2O3 from 208 to 195 m2/g. Exposure to 100 mg/L of HA resulted in a decrease in BET external surface area for ∼50 nm TiO2 (anatase) from 153 to 130 m2/g while having no impact on the micropore surface area.37 2160

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Notes

degree to which the presence of organics inhibited chromate sorption generally decreased over time (Table 2). While the exact mechanism for this is unclear, it may be related to preferential adsorption of organics onto external surfaces and into larger pore spaces, which would preferentially block the easily accessible reactive surface sites. These sites dominate the early stages of sorption onto particle surfaces, and once these sites become saturated, the reaction slows and becomes kinetically limited, as continued sorption is dependent upon the rate at which the metal ions can diffuse into the smaller pore spaces of the mineral aggregates. In that case, chromate sorption onto the reactive sites within the smaller pores would be less inhibited by the presence of organics. The chromate sorption experiments in this investigation were run for a total of 8 h; however, the trends displayed in Figure 1a−c indicate that, if run over a longer time scale, the difference between chromate sorption in the presence and absence of organics would continue to decrease. This is consistent with results reported by Chen et al.,48 who found that the adsorption of humic acids reduced the initial trichloroethene (TCE) reduction rate by nanoscale zerovalent iron (NZVI) by 23%, yet caused no significant decrease in the total amount of TCE reduced over the total ∼275 h experiments. However, we hypothesize that such a decrease in the importance of organics would not occur in the natural environment, where flowing waters would deliver fresh organics and continue to build up a barrier to mineral− solute interfacial reactions. Implications. Previous investigations into nanoparticulate magnetite chemistry have reported that surface reactivity of the mineral depends primarily on the Fe(II)/Fe(III) content (e.g., ref 26). The results of our investigation confirmed these previous studies by revealing that the surface area normalized amount of Cr sorption was much greater for the 90 nm particles, which were higher in Fe(II) content than the 6 nm particles. However, our results showed that complex dissolved organics reduced chromate sorption onto magnetite via blockage of the particle surface, regardless of the size or Fe(II) content of the particles. SEM images clearly show that the complete coverage of the magnetite particles by organics is not achieved in these experiments, contrary to previous investigations using natural waters.17,37,41,42,49 This contrast between the behavior of mineral−organic interactions in the lab and field implies that extracted organics may not be completely analogous to dissolved organics found in nature. This finding has significant consequences for investigations into the surface reactivity of natural and engineered nanoparticles in the environment and underscores the need for further research in the area of organic−mineral interactions using natural samples.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to acknowledge USGS scientists Jeanne Jaeschke for analytical support along with Jason Masoner and Kevin Smith for field support. This research was supported by the USGS Toxic Substances Hydrology Program and the USGS National Research Program. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U. S. Government.

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ASSOCIATED CONTENT

S Supporting Information *

Discussion of experimental rationale, particle synthesis, sampling/analysis protocol, chromate−FA control experiments, and results of chromate sorption in the presence of LL using different surface areas of 6 nm magnetite, as well as additional images. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

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Corresponding Author

*E-mail: [email protected]. 2161

DOI: 10.1021/es504831d Environ. Sci. Technol. 2015, 49, 2156−2162

Environmental Science & Technology

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