Article pubs.acs.org/est
Identifying Indicators of Reactivity for Chemical Reductants in Sediments Huichun Zhang†,‡ and Eric J. Weber*,† †
National Exposure Research Laboratory, U.S. Environmental Protection Agency, 960 College Station Road, Athens, Georgia 30605, United States ‡ Department of Civil and Environmental Engineering, Temple University, Philadelphia, Pennsylvania 19122, United States S Supporting Information *
ABSTRACT: To conduct site-specific exposure assessments for contaminants containing reducible functional groups, it is imperative to know the identity and reactivity of chemical reductants in natural sediments and to associate their reactivity with easily measurable sediment properties. For this purpose the reactivity, as defined by pseudofirst order reduction rate constants for p-cyanonitrobenzene (pCNB), was measured in twenty-one natural sediments of different origins that were incubated to attain both anoxic (less reducing) and anaerobic (microbially reducing) conditions. The reactivity of the anoxic sediments increased with pH and an increasing amount of Fe(II) added. A good electron balance between pCNB reduction and Fe(II) consumption was observed for anaerobic sediments of high solids loading (50 g/L), but not when solids loading was 5 g/L. Based on cluster and regression analysis, pCNB reactivity in the anaerobic sediments correlates strongly with aqueous Fe(II) concentrations for sediments with low organic carbon (OC) content (6.4%). These observations indicate surface-associated Fe(II) and reduced DOC are the predominant reductants in the anaerobic sediments, and that aqueous Fe(II) and DOC will serve as readily measurable indicators of pCNB reactivity in these systems.
■
INTRODUCTION Reductive transformation is an important fate process for several classes of priority pollutants such as nitroaromatic compounds (NACs)1−3 and halogenated organics4,5 in anoxic systems. The reaction kinetics for this process can vary significantly from one environmental system to another.3,5,6 The ability to estimate reduction rates of NACs in anoxic systems requires knowledge of both the molecular properties of the NAC and the reaction system of interest. Although much progress has been made in recent years to elucidate the pathways for contaminant reduction7−10 and the reactivity of the reductants2,11,12 in model systems, relatively little progress has been made in identifying the properties and reactivity of reductants in natural systems. Thus, to conduct site-specific chemical exposure assessments and to estimate half-lives for reductive transformations of contaminants in reducing sediments, it becomes imperative to elucidate the identity and reactivity of chemical reductants as a function of sediment properties and to associate the reactivity of a reducing environment to easily measurable indicators. To date, studies on reductive transformations have focused primarily on model or simulated systems. These studies have identified several abiotic reductants that will potentially serve as electron sources in natural systems: surface adsorbed Fe(II),5,6,13,14 secondary Fe(II) minerals,15,16 structure Fe(II) in clay minerals,17,18reduced dissolved organic matter (DOM)35,39 and © 2012 American Chemical Society
soluble Fe(II) complexes formed through chelation with DOM and low molecular weight organic ligands.19−21 Surfaceadsorbed Fe(II), which is formed after adsorption of aqueous Fe(II) by metal oxides and clay minerals, is believed to be one of the most reactive chemical reductants in natural sediments and aquifers.5,6,22 The much higher reactivity of surface-adsorbed Fe(II) is attributed to its much lower reduction potential than that of aqueous Fe(II).15,23 Although surface-adsorbed Fe(II) have been traditionally viewed as a stable species, recent studies demonstrated that electron transfer occurs between the adsorbed Fe(II) and the supporting Fe(III) oxides,11,24−26 or Fe(III)-containing clay minerals17,18 leading to the growth of an Fe(III) surface layer and the reduction of the structural Fe(III). Stable surface-adsorbed Fe(II) was only observed at high Fe(II) concentrations above site saturation.24 The pathway for electron transfer from the surface-adsorbed Fe(II) to contaminants still remains unclear. In addition, near-complete Fe atom exchange between aqueous phase and Fe(III) oxides11,24,26 casts questions regarding the extent of electron Special Issue: Rene Schwarzenbach Tribute Received: Revised: Accepted: Published: 6959
May 11, October October October
2010 15, 2012 19, 2012 22, 2012
dx.doi.org/10.1021/es302662r | Environ. Sci. Technol. 2013, 47, 6959−6968
Environmental Science & Technology
Article
of reactivity.47 Beyond measuring directly surface-associated Fe(II) or aqueous Fe(II), it is uncertain to what extent characterization of the bulk sediment would be required to identify indicators of reactivity. For example, recent studies have reported almost 6 orders of variation in the reactivity of NACs by different Fe(III)-bearing minerals indicating that some knowledge of the composition of the mineral phase is required for predicting reduction rates.8,35,46 The goals of this study were to provide further characterization of the predominant chemical reductant(s) in reducing sediments collected from a wide variety of sources to reflect a wide range in physicochemical properties, and more importantly, to identify readily measurable indicators of sediment reactivity as determined by reduction rates of pCNB. pCNB was selected as a chemical probe in this study due to its facile reduction in sediments and low tendency to partition to sediments.47 Reduction of pCNB occurs through the sequential formation of the N-nitroso intermediate (pCNN), which is typically not detected in anaerobic sediments, followed by formation of the hydroxylamine intermediate (pCHA), and ultimately the formation of the stable aromatic amine (pCNA).
transfer between the adsorbed Fe(II) and the supporting Fe(III) minerals. Moreover, it has been observed that during reductive transformation, though the reduction rate typically correlates with the amount of Fe(II) adsorbed, the presence of aqueous Fe(II) is necessary to sustain continuous reaction.3,11,27,28 Thus, it seems that aqueous Fe(II) is the primary electron source. The widespread occurrence of dissimilatory iron reducing bacteria (DIRB) leads to the continuous production of Fe(II) under reducing conditions.16,29,30 In addition to surface adsorbed Fe(II), various secondary Fe(II) minerals including green rust, magnetite, siderite, vivianite, and chukanovite can form from the reaction of aqueous Fe(II) with Fe(III) oxides16,28 and Fe(III)-containing clay minerals,17 and from the reduction of Fe(III) minerals by DIRB.16,29,30 The formation of these secondary Fe(II) minerals have been linked to facile reduction of contaminants such as NACs,1,16 RDX (a nitramine explosive),12,31,32 and chlorinated solvents.33,34 As will be discussed later, surface adsorbed Fe(II) and secondary Fe(II) minerals formed on the surface of Fe(III) minerals are hard to distinguish experimentally, and will be referred to as “surface-associated Fe(II)” hereafter. As compared to surface-associated Fe(II), structured Fe(II) in clay minerals is significantly less reactive, however, the abundance of clay minerals in most soils and sediments compensates for its lower reactivity toward reductive transformation of contaminants.2,35 Typically, lower Fe(II) contents in either magnetite or smectite have yielded lower rates of contaminant reduction.16,21,23,36 DOM can also function as a chemical reductant and/or electron shuttle for microbial-mediated reduction of iron minerals,37 and hence, affect the overall reducing capacity of the reducing environment. DOM can undergo microbial reduction under a variety of conditions including fermentation, halorespiration, humic-reducing, iron-reducing, methanogenesis, and sulfate-reducing conditions.38−40 Although studies of model systems have demonstrated that DOM can play an important role in reductive transformations,37,41 little evidence exists in the literature substantiating DOM as a significant chemical reductant in natural sediments. Our recent work with natural sediments has demonstrated the electron shuttling effect of microbially reduced DOM in the reduction of a surface bound azo compound in a number of anaerobic sediments.42 On the other hand, based on the comparison of reactivity patterns for the reduction of a series of nitrobenzenes in a methanogenic aquifer and appropriated model systems, it was concluded that surface-associated Fe(II) was the dominant chemical reductant despite the presence of significant concentrations of DOM.6 Additionally, Fe(II)−DOM complexes have been demonstrated to be a potentially important natural reductant through work with model DOM compounds19,20,43,44 and sediment porewater.21,45 Despite the level of understanding of the behavior of chemical reductants in model or simulated systems and the effort to compare their relative reactivity toward the same contaminants,12,35,46 little is known concerning either the contribution of each reductant to the overall reactivity of reducing sediments or the physiochemical properties of the sediments that are necessary to predict rates of electron transfer. Previous work in this laboratory demonstrated that aqueous Fe(II) must be present for significant reduction of p-cyanonitrobenzene (pCNB) to occur in a pond sediment under iron-reducing, sulfate-reducing and methanogenic conditions, which suggests a role for aqueous Fe(II) as an indicator
Toward this end, the effects of aqueous Fe(II) concentration and solution pH on the reduction kinetics of pCNB in sediments were monitored, as well as the electron balance between pCNB degradation and the consumption of aqueous and surface-associated Fe(II). The reactivity of twenty-one natural sediments of different origins and their corresponding supernatants was measured and correlated with physicochemical properties including BET surface area, cation-exchange capacity (CEC), texture, Fe speciation and composition, and sediment and aqueous organic carbon (OC) content. The stated goals of this study were accomplished by working with a set of sediments with varying physicochemical properties under both anoxic and anaerobic conditions.
■
MATERIALS AND METHODS Chemicals and Sediments. pCNB and p-cyanoaniline (pCNA) were purchased from Aldrich. Other chemicals including acetonitrile, FeCl2 • 4H2O, ferrozine, HCl (tracemetal-grade concentrated), 4-morpholinepropanesulfonic acid (MOPS), NaOH, and sodium acetate were obtained from Fisher or Sigma and were used as received. Stock solutions of pCNB and pCNA were prepared in acetonitrile. Twenty-one sediment and soil samples of different origins were collected and treated as follows: Cherokee Park pond sediment (CP) and Oconee River sediment (OR) were collected as described earlier;5,47 USDA P-1 stock (P1) and W-2 saprolite field sediments (SS)48and 8 soil samples from various locations including Alaska (AL), Arizona (AR), Colorado (CO), Georgia (GA), Ireland (IR), Louisiana (LO), Oklahoma (OK), and Pennsylvania (PA) were provided by Dr. John Washington. 6960
dx.doi.org/10.1021/es302662r | Environ. Sci. Technol. 2013, 47, 6959−6968
0.00038 0.0022 0.00020 ND 0.00027 0.0019 0.0025
0.00058 0.00034 ND 0.0011 0.0063 0.0017 ND
HA 0.051 ± 0.006 IR 0.10 ± 0.02 LO 0.040 ± 0.005 OR 0.0020 ± 0.0010 OH1 0.052 ± 0.004 OH2 0.031 ± 0.003 OH3 0.39 ± 0.01
OH4 0.044 ± 0.003 OK 0.0082 ± 0.0004 P1 0.0081 ± 0.0010 PA 0.13 ± 0.03 PL 0.24 ± 0.01 RA 0.018 ± 0.003 SS 0.0000 ± 0.0001
Hagerstowna Ireland Louisiana Oconee River Ohio 0−4 cm Ohio 12−16 cm Ohio 24−26 cm
Ohio 36−43 cm Oklahoma USDA P1 Stock Pennsylvania R Pledgera Rainsa USDA Saprolite
6961
8.5 2.7 3.3 2.0 81.3 1.7 21.5
59.0 0.0 2.6 0.7 4.7 5.2 9.8
0.1 17.6 19.5 22.4 5.1 7.6 4.4
17.57 2.90 6.12 ± 0.34 18.96 ND ND 2.95
ND 23.40 8.62 0.96 ± 0.38 15.27 ± 0.81 12.56 21.14 ± 0.64
1.28 ± 0.08 14.26 ND 7.19 ± 0.38 17.98 ND 7.43
13.5: 61.9: 24.6 57.6: 35.8: 6.6 64.5: 21.3: 13.9 13.5: 46.0: 40.5 12.2: 22.7: 65.1 55.0: 35.8: 9.2 50.10: 48.03: 1.87
16.9: 70.7: 12.4 31.6: 39.6: 28.8 3.1: 79.0: 17.9 97.2: 0.01: 2.9 31.4: 44.6: 24.0 40.3: 38.0: 21.6 5.7: 73.1: 21.2
77.3: 17.3: 5.4 4.2: 87.4: 8.4 47.6: 36.8: 15.6 18.9: 76.3: 4.8 54.9: 32.5: 12.6 71.9: 24.6: 3.5 52.5: 31.0: 16.50
sand: silt: Clay
214 69 45 111 73 6 0
209 57 134 2 276 235 221
62 88 498 427 74 89 94
± ± ± ± ± ± ±
± ± ± ± ± ± ±
± ± ± ± ± ± ±
13 12 9 6 3 1 0
9 1 6 1 26 15 27
7 7 15 16 4 29 17
aqueous phase Fe(II) (μM)
32.8 8.5 3.3 14.4 36.3 2.9 0.0
14.2 10.7 6.3 0.1 30.5 29.3 29.8
4.4 16.3 47.9 23.5 9.2 13.2 30.3
± ± ± ± ± ± ±
± ± ± ± ± ± ±
± ± ± ± ± ± ±
10.1 0.3 0.6 1.4 14.9 0.1 0.0
0.7 2.2 1.2 0.1 1.8 0.3 9.6
0.3 1.4 5.8 2.3 0.5 2.2 10.9
ferrozine: Fe(II) (μmol/g)
195 39 22 167 153 7.8 30
70 78 92 8 205 185 255
68 90 351 92 120 85 157
± ± ± ± ± ± ±
± ± ± ± ± ± ±
± ± ± ± ± ± ±
11 23 3 19 7 0.1 1
4 11 7 4 41 20 57
4 12 79 8 2 81 15
214 42 25 180 188 8.5 66
134 85 97 18 210 234 248
125 99 455 94 119 222 153
± ± ± ± ± ± ±
± ± ± ± ± ± ±
± ± ± ± ± ± ±
15 27 8 15 11 0.2 1
14 14 7 2 8 30 5
1 12 90 5 1 103 15
0.5 N HCl 0.5 N HCl total Fe Fe(II) (μmol/g)d (μmol/g)e
465 59 28 238 229 34 216
128 130 118 13 502 295 511
374 141 452 108 273 118 268 12 32 8 1 134 75 5
± ± ± ±
35 22 5 39
±8 ± 13
± ± ± ± ± ± ±
± 27 ±9 ± 34
± 117 ±2 ±2
6 N HCl Fe(II) (μmol/g) f
1558 153 170 353 450 533 2774
458 151 337 110 991 678 939
699 509 1163 1836 964 631 803
T otal Fe (ppm)h
90 18 14 14 28 197 33
ND 5504 15445 5787 ± 471 15525 17811 24682±1500
± 1011 20050 ± 21 5222 7791 ± 61 17670 ±9 ND ±7 ND ± 583 42729
± ± ± ± ± ± ±
± 219 24420 ± 433 ±7 28979 ± 129 ± 71 ND 31265 ± 149 24896 ± 17 ND ± 95 21146±1566
6 N HCl total Fe (μmol/g)g
3.11 0.92 2.02 11.00 4.16 4.56 0.07
3.08 16.64 3.41 0.03 2.55 2.21 3.90
0.52 4.14 8.93 1.45 3.72 4.13 6.16
% OC
10.8 9.8 9.4 ND 17.8 11.6 6.3
20.7 78.0 20.9 5.7 17.3 12.1 12.1
7.6 9.1 94.3 14.6 17.5 18.8 47.1 1.1 11.7 3.7 0.2 4.0 3.0 1.5
0.9 1.0 5.3 0.2 2.6 5.4 12.3
± 1.3 ± 0.2 ± 0.3
± 0.4 ± 0.2 ± 1.0
± ± ± ± ± ± ±
± ± ± ± ± ± ±
DOC (mg/L)
0.41 1.19 1.27 ND 0.62 1.14 0.46
0.54 1.17 1.01 0.53 0.26 0.43 0.44
0.72 0.79 0.89 0.30 0.66 0.57 0.87
0.03 0.12 0.18 0.01 0.06 0.03 0.02
0.02 0.08 0.04 0.01 0.05 0.09 0.23
± 0.05 ± 0.09 ± 0.04
± 0.01 ± 0.10 ± 0.02
± ± ± ± ± ± ±
± ± ± ± ± ± ±
SUVA254 nm (L/mg-C)i
0.04 0.37 0.33 ND 0.13 0.30 0.02
0.12 0.73 0.31 0.02 0.02 0.05 0.06
0.22 0.12 0.48 0.03 0.25 0.13 0.29
0.01 0.11 0.10 0.02 0.01 0.02 0.01
0.07 0.01 0.02 0.01 0.04 0.01 0.05
± 0.01 ± 0.01 ± 0.02
± 0.01 ± 0.03 ± 0.01
± ± ± ± ± ± ±
± ± ± ± ± ± ±
SUVA350 nm (L/mg-C) j
a Data from ref 49. bPseudofirst-order rat constants of sediment supernatants. cNot determined. dWeak acid extractable Fe(II). eWeak acid extractable total Fe. fStrong acid extractable Fe(II). gStrong acid extractable total Fe. hTotal Fe by acid digestion. i(A254 nm/DOC (mg C/L)) × 23.03. j(A350 nm/DOC (mg C/L)) × 23.03.
± 0.0003 ± 0.0002 ± 0.0001
± 0.00010 ± 0.00010
± 0.00010 ± 0.0001 ± 0.0003
± 0.00010 ± 0.0017 ± 0.00010
0.0001 0.00023 ± 0.00004 0.003 0.000069 ± 0.00001 0.01 0.0032 ± 0.0008 0.010 NDc 0.001 0.00011 ± 0.00003 0.0012 0.00080 ± 0.00010 0.006 0.00073 ± 0.00020
± ± ± ± ± ± ±
0.0012 0.041 0.12 0.081 0.013 0.0096 0.025
AL AR BU CP CO CN GA
CEC (1/h) associated BET supernatantb (m2/g) (mEq/100g)
Alaska H Arizona B Burtona Cherokee Park Colorado Coltona Georgia
sup
sediment
k
k anaer (1/h g of sediment) sediment
Table 1. Sediment Properties and Pseudo-First Order Rate Constants for the Reduction of pCNB in the Anaerobic Sediment Suspensions (10 g/L) and Associated Supernatants
Environmental Science & Technology Article
dx.doi.org/10.1021/es302662r | Environ. Sci. Technol. 2013, 47, 6959−6968
Environmental Science & Technology
Article
Figure 1. (a) Formation of [soluble Fe(II)] or [ferrozine extractable Fe(II)] in anoxic CP sediment (5g/L, pH7); (b) Reduction kinetics of pCNB in anoxic sediments. The points are the experimental results, and the lines are the pseudofirst order kinetic fits; (c) Effect of pH on the reduction kinetics of pCNB in anoxic CP sediments (5 g/L); and (d) Effect of the sediment loading on the reaction kinetics of pCNB in anoxic sediments. Reaction conditions: anoxic sediment 5g/L, pH 7.0 buffer.
Four river sediments from Old Woman Creek in Ohio (OH1 − OH4) were collected at depths of 0−4, 12−16, 24−28, and 36−43 cm, respectively and were provided by Dr. Y.-P. Chin. The above samples were wet sieved (1 mm), air-dried, and stored for future use at room temperature. Burton, Colton, Hagerstown, Pledger, and Rains soil samples were provided by Dr. Dharni Vasudevan49 and were used as received. Table 1 lists the properties of the sediments. Kinetic Experiments. Batch experiments with anaerobic sediments were performed in a COY anaerobic chamber (2− 5% H2). The reaction setup is similar to that described previously47 and is reported in detail in SI Text S2. Briefly, a typical batch experiment consisted of a 65 mL serum bottle charged with 60 mL of 10 mM sodium acetate, 25 mM pH 7.0 MOPS buffer and 0.05−2.5 g of sediment. The bottles were sealed with Teflon-faced gray butyl septa, moved outside of the chamber, and placed upside down on a slowly reciprocating tabletop shaker at room temperature for 5 w. At the end of the 5-w incubation, anaerobic conditions had been established and the bottles were moved back into the chamber and the addition of pCNB stock solution marked time zero for the kinetic studies. The bottles were then placed on a rotator inside the chamber (49 rpm/min). Three 0.5 mL suspension samples were withdrawn at appropriate time intervals. The first two samples were filtered and analyzed for pCNB, its reduction products, and aqueous Fe(II). The third sample was analyzed for ferrozine extractable Fe(II), which is a measure of surfaceassociated Fe(II).4 Rate constants for the reduction of pCNB (kanaer) were calculated based on pseudofirst-order kinetics (typical R2 > 0.95). For batch experiments with anoxic sediments, serum bottles were similarly prepared but without the addition of sodium acetate, and were kept inside the chamber for 3 days prior to
the addition of pCNB stock solution. Experiments with the anoxic sediments were conducted with different MOPS buffers at four different pH’s to study the effect of pH on reaction kinetics. To study the effect of Fe(II) on the reaction kinetics of anoxic sediments, a desired amount of ferrous solution was added into each sediment-containing serum bottle and equilibrated for 24 h before the addition of pCNB stock solution. To study the effect of Fe(II) on the reaction kinetics of anaerobic sediments, in addition to adding Fe(II) to the suspension as described above, the sediment suspension was centrifuged; a portion of the supernatant was removed replaced with an equal volume of 10-mM sodium acetate in pH 7.0 buffer and equilibrated for 24 h before the addition of pCNB stock solution. Analytical Procedures. A detailed description of the sediment analysis is included in Text S3 in SI. pCNB and pCNA were analyzed by an Agilent 1100 HPLC with a Restek C18 column (150 × 4.6, 5 μm). The HPLC mobile phase consisted of nanopure water and acetonitrile at a flow rate of 1 mL min−1. Gradient elution was run to separate pCNB and reaction products. pH values of the sediment suspensions were measured with an Orion 250A+ advanced pH meter. UV absorbance of the sediment supernatants was measured with an Agilent 8453 UV-vis spectrometer at 254 and 350 nm. DOC of anaerobic sediment supernatants (without Na acetate addition during incubation) was analyzed by Shimadzu TOC 5050A instrument. The ferrozine method50 was used for aqueous Fe(II) and total Fe analysis. Note that ferrozine extractable Fe(II) is a measure of the surface-complexed Fe(II).4 Further details on sediment analyses, including determination of %OC, CEC, total Fe, and weak and strong acid extractable Fe, are provided in SI Text S2. 6962
dx.doi.org/10.1021/es302662r | Environ. Sci. Technol. 2013, 47, 6959−6968
Environmental Science & Technology
Article
Figure 2. (a) Adsorption isotherms of Fe(II) in anoxic sediments (Cw = equilibrium aqueous Fe(II) concentration, Cs = adsorbed Fe(II) concentration, Cs = initial Fe(II) concentration − Cw); (b) effect of Fe(II) addition on the reduction kinetics of pCNB in anoxic CP sediment. Reaction conditions: 5 g/L anoxic CP, OR, or P1 sediment, Fe(II) added between 0 and 500 μM, 25 mM pH 7 buffer, 25 μM pCNB.
Data Analysis. Minitab Release 15 statistical software trial version51 was used to conduct multiple linear regression (including best subsets regression, multiple stepwise regression, and linear regression) and cluster analysis. Multiple linear regressions were conducted to predict the response of dependent variables (i.e., ksed) from a group of potential predictor variables (sediment properties). Intercorrelated sediment properties were not included as predictor variables for the regression analysis due to potential confounding effects. Based on the Pearson correlations (chose only one property if the correlation between two properties is greater than 0.70, see more discussion later), we chose to use sediment properties including BET, % clay, soluble Fe(II), 6N−Fe(II), 6N-total Fe, %OC or DOC, and SUVA254 nm as the best seven independent predictors.
log kanoxic for NACs was found to be independent of pH in a clay mineral system.9 Second, when varying the sediment loadings between 0 and 1100 m2/L for anoxic CP, OR, and P1 sediments, the pCNB reduction rate constants increase roughly linearly (Figure 1d). Third, anoxic sediment slurries were treated with varying amounts of Fe(II) while keeping the sediment loading constant (5 g/L). Initially, the sorption of Fe(II) was measured in CP, OR, and P1 sediment slurries. As shown in Figure 2a, sorption of Fe(II) can be well-described by the Langmuir equation. A similar sorption isotherm was previously observed for sorption of Fe(II) on goethite.27 The anoxic CP sediment exhibited the lowest maximum sorption for Fe(II). This is likely related to its high Fe(II) content (as shown for anaerobic conditions in Table 1) that inhibits further sorption of Fe(II) to the sediment surface. Similar behavior has been observed for Fe(II) uptake by magnetite containing different fractions of structural Fe(II).16,23 Examination of the effect of aqueous Fe(II) concentration on pCNB reduction kinetics revealed that kanoxic versus [aqueous Fe(II)] for anoxic CP sediment also obeyed the Langmuir equation (Figure 2b). These results are consistent with the premise that surfaceassociated Fe(II) is the predominant reductant in the anoxic sediments. Reduction Kinetics of pCNB in Anaerobic Sediments. Amendment of the sediments with sodium acetate and incubation for 5 w prior to treatment with pCNB provided dynamic, microbially reduced (anaerobic) sediment slurries. A typical kinetic trend for pCNB reduction kinetics in the anaerobic sediments is similar to that shown in Figure 1b. The pseudofirst-order rate constants (kanaer) for all twenty-one sediments are summarized in Table 1. Overall there is a 2- to 390-fold increase in the reactivity of anaerobic vs anoxic sediments. We attribute this increase in reactivity to the microbial-mediated reduction of Fe(III) minerals resulting in a greater concentration of surface-associated Fe(II) and reduced DOM.42 To test the role of surface-associated Fe(II) on the reductive reactivity of the sediment suspensions, between 33 and 100% of the supernatant in anaerobic CP sediment was replaced with the reaction medium (i.e., 10-mM sodium acetate in pH 7.0 buffer) to vary the aqueous and ferrozine extractable Fe(II) concentrations. The measured pCNB reduction rate constants in these sediment suspensions were found to increase linearly with increasing [aqueous Fe(II)] or [ferrozine extractable Fe(II)] with R2 of 0.96 and 0.90 (Figure S1 in SI). Increasing the concentration of
■
RESULTS AND DISCUSSION Reductive Degradation of pCNB in Anoxic Sediments. For anoxic sediments (i.e., 3 day incubation period without the addition of sodium acetate), reducing conditions were not fully developed as evidenced by the limited reduction of iron (i.e., the concentration of aqueous Fe(II) is an order of magnitude less than the concentration of aqueous Fe(II) measured in the anaerobic sediments) (Figure 1a). We hypothesize that surfaceassociated Fe(II) is the most reactive chemical reductant in these anoxic sediments, and that an increase in aqueous Fe(II) concentration will result in an increase in surface-associated Fe(II) concentration, and hence the reducing activity. With increase in soluble Fe(II) concentration, an increase in surfaceassociated Fe(II) concentration was observed in the anoxic CP sediment during a 10-day incubation period (Figure 1a). Control experiments without sediments (i.e., Fe(II) with pCNB) showed that pCNB was stable through the study period (up to 3 w). In contrast, significant pCNB degradation was observed in the presence of all sediments except SS, with kanoxic (calculated based on pseudofirst-order kinetics) varying between 5.0 × 10−6 and 0.012 h−1 (Supporting Information (SI) Table S1). Figure 1b shows a typical kinetic trend for the reduction of pCNB in anoxic sediment slurries. The hypothesis that surface-associated Fe(II) is the predominant reductant in anoxic sediments was tested in three experiments. First, pCNB reduction was monitored in anoxic CP sediment at four different pH values between 6.5 and 7.1. Similar to studies of Fe oxides,6,46 a good linear correlation was observed between log kanoxic and pH (Figure 1c). Note that 6963
dx.doi.org/10.1021/es302662r | Environ. Sci. Technol. 2013, 47, 6959−6968
Environmental Science & Technology
Article
Figure 3. Time course of (a) pCNB and the reduction products and (b) soluble Fe(II) in 50 g/L of anaerobic CP sediment; (c) In the above reactor, time course of soluble Fe(II) consumed (black triangles) and the amount of electrons accepted by pCNB to form pCHA and pCNA (red circles); and (d) time course of Fe(II) species and the amount of electrons accepted by pCNB in 5 g/L of anaerobic CP sediment. Reaction conditions: 25 mM pH 7 buffer, 25 μM pCNB.
control (triangles in Figure 3c). Because it takes 4 e− to reduce pCNB to pCHA and an additional 2 e− to form pCNA, we can calculate the amount of e− consumed within the initial reaction period of 10 min (circles in Figure 3c). By comparison, the average loss of aqueous Fe(II) (i.e., e− donated) agrees well with the amount of electrons accepted by pCNB and its reduction intermediates. Previous studies of the reduction of NACs by Fe(II)/magnetite3 or Fe(II)/goethite demonstrated that the number of electrons transferred to the nitrobenzenes accounts for >95% of decrease in aqueous Fe(II) concentration. The good electron balance observed for the sediment system suggests that Fe(II) is the ultimate electron donor in this reaction system. The second experimental system is similar to the first one except that the loading of anaerobic CP sediment was lowered to 5 g/L. As show in Figure 3d, the number of electrons accepted by pCNB increases over the studied reaction period (up to 25 h); however, the amount of aqueous Fe(II) consumed increases only slightly within 30 h; and the amount of ferrozine extractable Fe(II) consumed stays negligible during the reaction course. Similar trends were observed for all other sediments under the identical reaction conditions (data not shown). A poor electron balance between pCNB reduction and Fe(II) consumption is also observed in OR sediment under iron-reducing, sulfate-reducing, and methanogenic conditions.47 This is likely due to the facile regeneration of aqueous Fe(II) from microbial-mediated reduction of Fe(III) minerals6 and the additional pathway for electron transfer through the microbialmediated reduction of DOM.42 Sediment Physicochemical Properties. The primary goal of this work was to determine if there are readily measurable physicochemical properties of the reducing sediments
aqueous Fe(II) by 0.2 and 1.0 mM in the original anaerobic CP suspension, however, did not significantly increase the rate constant for pCNB reduction (0.081 ± 0.010 h−1 per g of sediment). We speculate that the sediment surface was saturated with Fe(II) in the original anaerobic CP sediment. The addition of Fe(II) had no effect on the amount of surface-associated Fe(II) formed, and hence, the pCNB reduction rate. Note that 47 ± 1 μM of aqueous Fe(II) was measured in the CP sediment suspension 24 h after its supernatant was entirely replaced with the reaction medium. This indicates facile regeneration of Fe(II) species by DIRB once aqueous Fe(II) is removed from the system.52 The predominant role of surface-associated Fe(II) as the reductant in the anaerobic sediments was further substantiated by measuring the electron balance between pCNB reduction and aqueous Fe(II) consumption in two experimental systems. In the first experiment, pCNB was dosed to 25 μM in the 50 g/L of anaerobic CP sediment slurries. pCNB disappeared within 2 min, with concurrent formation of pCNA. A significant amount of an intermediate also formed within the first two minutes and then slowly disappeared (Figure 3a). Based on previous work,53 the intermediate was most likely p-cyanohydroxyaniline (pCHA). Due to the lack of an authentic standard, the pCHA concentration was tentatively approximated by comparing its UV-absorbance A at 273 nm to that of pCNB at time zero (i.e., [pCHA]t = ApCHA at time t/ApCNB at time 0 × [pCNB]0). Overall a good mass balance (>91%) was achieved for the loss of pCNB. In addition, a rapid consumption of aqueous Fe(II) was observed after the addition of pCNB (Figure 3b); no change in [Fe(II)] was observed in the controls. The consumption of aqueous Fe(II) was obtained by subtracting the aqueous Fe(II) concentration in the reaction mixture from that in the sediment 6964
dx.doi.org/10.1021/es302662r | Environ. Sci. Technol. 2013, 47, 6959−6968
Environmental Science & Technology
Article
Figure 4. Scatter plots and correlation between kanaer and (a) DOC and (b) [Fe(II)] for all anaerobic sediments (10 g/L). Data for sediments with relatively low %OC ( 0.70, SI Table S2) among the various forms of iron speciation (i.e, aqueous Fe(II), ferrozine extractable Fe(II), weak acid extractable Fe(II) and total Fe, and strong acid extractable 6965
dx.doi.org/10.1021/es302662r | Environ. Sci. Technol. 2013, 47, 6959−6968
Environmental Science & Technology
Article
work suggests that it is not necessary to analyze iron mineral composition to predict sediment reactivity for pCNB reduction in anaerobic sediments. We speculate that sediments have been modified by continual microbial-mediated iron reduction resulting in the coverage of Fe-containing minerals with surface-associated Fe(II). Most likely, as pointed out in Gorski and Scherer 200957 for the reactivity of magnetite, the presence of a specific Fe(II) reductant in reducing sediments may not be as important as the presence of either aqueous Fe(II) or DIRB that can effectively regenerate the predominant Fe(II) reductant(s). Although regeneration of reactive Fe(II) species by either adsorption of Fe(II) or reduction by DIRB is believed to be a dominant factor in the reductive reactivity of aqueous mineral suspensions and aquifer columns,3,6,58,59 the regeneration of Fe(II) by DIRB is likely more important in anaerobic sediments. The use of aqueous Fe(II) and DOC concentrations as readily measurable indicators of sediment reactivity, coupled with the application of quantitative structural-activity relationships that are based on molecular descriptors such as oneelectron reduction potentials to account for relative reactivities,60 will reduce the uncertainty associated estimating the environmental concentrations of NACs and their transformation products in reducing environments. Further work is necessary to determine how the relative contributions of Fe(II) reductants and reduced DOM will vary with NAC reactivity and solution pH, as indicated by the significant increase in pCNB reduction in anoxic CP sediments (i.e., an 2-order magnitude increase in ln kanoxic in CP sediment over a 0.5 range in pH). Furthermore, it is necessary to determine the relevance of these results with respect to chemicals containing other types of functional groups that are susceptible to abiotic reduction (e.g., aromatic azos and halogenated aliphatics).
reductant is misleading. Cluster analysis of the data set indicates that the data set is significantly skewed toward lower %OC and DOC concentrations. Although there is significant variation in the %OC of the sediment data (i.e., ranging from 0.07 to 16.4%), only 4 of the 19 sediment samples (i.e., Burton, GA, IR, and PA) have a medium to high %OC (>6.2%); whereas the other 15 sediments contain relatively low %OC (
