ARTICLE pubs.acs.org/est
Short-Term Fe Cycling during Fe(II) Oxidation: Exploring Joint Oxidation and Precipitation with a Combinatorial System Justina M. Burns, Preston S. Craig, Timothy J. Shaw, and John L. Ferry* Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States
bS Supporting Information ABSTRACT: The net oxidation of Fe(II)aq by dioxygen initiates a suite of reactions including the oxidation of multiple Fe(II) complexes, generation of secondary oxidants, Fe(III) reduction, and precipitation of insoluble products. This work reports application of a multifactorial strategy to describe the oxidation of Fe(II) under conditions that correspond to those found where Fe(II)-rich groundwaters mix rapidly with overlying oxygenated waters. Response surfaces were constructed describing the relationship of the net oxidation process with mixtures of the common ligands chloride (Cl-), bromide (Br-), total carbonate (CO32-), Fe(II), and Suwannee River natural organic matter (SRNOM) at pH 8.00. Response surfaces were generated in the presence and absence of added phosphate, representing conditional end members corresponding to geographical locations where Fe(III) precipitation is respectively forced and unconstrained. Comparison of net Fe(II) oxidation rates in the presence and absence of phosphate then enabled resolution of the relative contributions of Fe(II) oxidation and Fe(III) reduction to the overall process. The differences between the two surfaces demonstrated the importance of Fe(II) regeneration on the rapid (min) time scale during net oxidation. The minimum Fe(II) concentration necessary to initiate measurable cycling is reported. The presence of reactive oxygen species was evaluated through the use of probes added to the center point condition of the experimental matrix. Analysis of the statistical significance of the Fe(II)-factor relationships demonstrated that over the conditional scale of the experiments complexation of Fe(II) by the selected ligands did not correlate to the experimental outcome.
’ INTRODUCTION The one electron oxidation of aqueous Fe(II) by dioxygen is thermodynamically favored and rapid under many environmental conditions. This reaction is the initiation step for the formation of several reactive oxygen species (ROS) and occurs wherever the discharge of anoxic groundwater introduces Fe(II) to oxygenated waters in rivers, subterranean estuaries, hydrothermal vents, etc.1-7 Reactive species generated in these environments include O2- 3 , HO 3 , H2O2, carbon centered radicals, and peroxyl radicals (Figure 1).8,9 Some of these are capable of the one electron reduction of Fe(III) complexes, and the published data indicate that Fe may cycle many times between the Fe(II) and Fe(III) oxidation states during the net oxidation process.9-11 This means a general description of the kinetics for net Fe(II) loss needs to account for Fe(II) oxidation and feedback from concurrent Fe(III) reduction (eq 1): ½dFeðIIÞ ¼ kox ½FeðIIÞ½oxidants dt net
∑
-
∑kred ½FeðIIIÞ½reductants
ð1Þ
where kox and kred are the generic rate terms for the rate constants of the summed Fe(II) oxidation and Fe(III) reduction reactions r 2011 American Chemical Society
respectively. The term “oxidants” includes dioxygen and various ROS and “reductants” includes superoxide and (speculatively) various carbon centered radicals. Fe cycling is ultimately limited in environmental and biological systems by the loss of Fe(III)aq through precipitation as sparingly soluble complexes such as Fe2(CO3)3, FePO4, and Fe(OH)3/oxyhydroxides.12-17 In this study, contrasting combinatorial data sets were generated that described the rate of Fe(II) oxidation in the presence or absence of added phosphate (PO43-, representing total phosphate species). The overall experimental conditions were mixtures of Cl-, Br-, Fe(II), (CO32-)tot, and Suwannee River natural organic matter (SRNOM), collectively interrogated by experiment in the presence and absence of added phosphate. All of the factors were selected on the basis of previously published observations that individually, the factors interacted with the net Fe(II) oxidation process and/or were also reactive with secondary oxidants such as HO 3 .9,10,14,17-20 Factor ranges were chosen to encompass the saline and fresh water end members as well as the intermediate conditions encountered during the mixing of Received: August 11, 2010 Accepted: February 10, 2011 Revised: January 24, 2011 Published: March 01, 2011 2663
dx.doi.org/10.1021/es102748p | Environ. Sci. Technol. 2011, 45, 2663–2669
Environmental Science & Technology
ARTICLE
Figure 1. The oxidation state of Fe can cycle rapidly between Fe(II) and Fe(III) during the relatively slow net Fe(II) oxidation (L = ligand forming a water-soluble Fe complex, P = precipitating ligand forming an insoluble Fe complex).
Table 1. Design Points for the Five-Factor Central Composite Design Used in All Experimentsa factor concentration levelsb
factor (units) coded factor levels x1: [Cl-] (mM) x2: [Br-] (μM) x3: [Fe(II)]o (μM)
-2
-1
0
1
2
0.00
155
388
621
776
0.00
209
525
841
1050
110
164
200
20.0
55.8
x4: [CO32-]tot (mM)
0.30
0.87
1.73
x5: [SRNOM] (mg C/L)
0.00
3.19
8.00
2.58 12.8
3.15 16.0
a
M1 and M2 were identical save for the addition of 12 mM PO43- to M2. b Denotes initial concentrations.
the same in permeable sediments and estuaries. This phenomenon is often observed at locations where organic-rich groundwaters mix with overlying oxygenated waters, including the Chesapeake Bay, the Carolina and Florida coasts, Patos Lagoon, and in fluidized Amazonian muds.5,7,1-24 Phosphate was added to selectively precipitate Fe(III) from solution and thereby reduce the rate of Fe(III) reduction.17,25-27 Comparison of the two data sets demonstrated evidence for short-term Fe cycling governed kinetically by the speciation of resulting Fe(III). Fluocyanobenpyrazole (FCBP) and 1,3-dicyanotetrachlorobenzene (DCTCB) were used as probes for the presence of HO 3 , in accordance with a method reported previously.9 The effects of each factor or factor-factor interaction on the net rate of Fe(II) oxidation were quantified.28-30
’ EXPERIMENTAL SECTION Materials. All salts (99%þ), solvents, and acids were acquired from Fisher Scientific. FerroZine reagent (98%) was purchased from VWR. Suwannee River NOM (SRNOM) was acquired from the International Humic Substances Society (IHSS) (see Supporting Information Tables 1-4 for elemental composition). Fluocyanobenpyrazole (FCBP or fipronil) was from O-Chem, and 1,3-dicyanotetrachlorobenzene (DCTCB or chlorothalonil) was from TCI America. All reagents were used as received. All solutions were made in Barnstead E-pure (18 MΩ cm-1) water, which had been distilled to remove residual H2O2. Experimental Design. Experimental parameter space was constructed from the variables Cl-, Br-, Fe(II), (CO32-)tot, and SRNOM, patterned using the circumscribed Box-Wilson experimental design with concentrations bracketing the range representing the freshwater and saltwater end members (Table 1).9,28,31,32 The design required six replicate experiments at the center point and three replicate experiments under all
Figure 2. The addition of 12 mM total phosphate increased the apparent rate of Fe(II) oxidation at pH 8.00 (n = 6). Nominal [Cl-]o = 388 mM, [Br-]o = 525 μM, total [CO32-]o = 1.73 mM, SRNOM = 8.00 mg C/L, [Fe(II)]o = 110 μM.
other conditions. The overall experimental matrix contained a series of 43 experimental conditions in the defined parameter space (Design Expert version 7.0.2, Stat-Ease Inc., Minneapolis, MN), for a total of 132 experiments including replicates. Two experimental matrices were performed: matrix 1 (M1) with no added phosphate, and matrix 2 (M2) made up in 12 mM phosphate. Solutions were prepared for both using a customized J-Kem Eclipse fluid handling station (relative volumetric standard deviation F
% impact
β0
intercept
β1 β2
ClBr-
-16.4 4.24
0.94 0.94
303.27 20.23
